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WO2017068350A1 - Methods of making metal oxide catalysts - Google Patents

Methods of making metal oxide catalysts Download PDF

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Publication number
WO2017068350A1
WO2017068350A1 PCT/GB2016/053275 GB2016053275W WO2017068350A1 WO 2017068350 A1 WO2017068350 A1 WO 2017068350A1 GB 2016053275 W GB2016053275 W GB 2016053275W WO 2017068350 A1 WO2017068350 A1 WO 2017068350A1
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WIPO (PCT)
Prior art keywords
titania
catalyst
metal oxide
metal
graphene oxide
Prior art date
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Ceased
Application number
PCT/GB2016/053275
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French (fr)
Inventor
Salvador ESLAVA
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
University of Bath
Ip2ipo Innovations Ltd
Original Assignee
University of Bath
Imperial Innovations Ltd
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Publication of WO2017068350A1 publication Critical patent/WO2017068350A1/en
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    • B01J37/02—Impregnation, coating or precipitation
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    • B01J21/00—Catalysts comprising the elements, oxides, or hydroxides of magnesium, boron, aluminium, carbon, silicon, titanium, zirconium, or hafnium
    • B01J21/06—Silicon, titanium, zirconium or hafnium; Oxides or hydroxides thereof
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    • C01B13/32—Methods for preparing oxides or hydroxides in general by oxidation or hydrolysis of elements or compounds in the liquid or solid state or in non-aqueous solution, e.g. sol-gel process
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Definitions

  • the present invention concerns methods of making a metal oxide catalyst. More particularly, but not
  • this invention concerns methods of making a metal oxide catalyst, in particular a transition metal oxide catalyst, using graphene oxide as a sacrificial support.
  • the invention also concerns the metal oxide catalyst produced thereby and the use of the metal oxide catalyst in catalytic reactions.
  • Metal oxide catalysts have found many uses in chemical and energy producing fields, including use as photocatalysts . It is expected that, by 2050, the world energy demand will double, due to the growth in human population and the industrial development of heavily populated countries. In parallel, fossil fuel resources are dwindling, becoming more expensive, and contributing to the increase in carbon dioxide (CO2) emissions which exacerbate global warming. Alternative energy resources, for example artificial photosynthesis, offer an
  • UV and visible light absorbing semiconductor photocatalysts comprising metal oxides such as T1O2, ZnO, Fe 2 0 3 , WO3, Cu 2 0 and various combinations forming
  • heterojunctions have been shown to photocatalytically generate solar fuels from water and/or carbon dioxide.
  • further research and progress is necessary to increase the current low yields of product formation.
  • the low efficiency of these photocatalytic systems has commonly been attributed to the lack of absorption of visible light, low surface area, fast recombination of photogenerated charge carriers and back reactions of reduced species to form C0 2 and/or H 2 0.
  • a method of making a metal oxide catalyst comprises:
  • the metal oxide catalyst produced may have an improved catalytic activity, in particular photocatalytic activity, when compared to metal oxide catalysts which are produced by other methods or which are commercially available, for example, Aeroxide ® P25 titania catalyst.
  • the enhanced catalytic activity of the obtained metal oxide results from the combination and synergy of the metal precursor and graphene oxide flakes which provide a higher surface area and better dispersability in water suspensions of the metal oxide.
  • the one or more transition metals may be present in the metal precursor in any oxidation state, including zero oxidation state.
  • the metal may be present in the metal precursor as metal oxide, inorganic or organic metal salt, or metal alloy.
  • the metal precursor of the present invention comprises a metal selected from the group consisting of Titanium, Zinc, Zirconium, Tungsten, Iron and combinations thereof. More preferably, the metal precursor comprises Titanium, for example a titanium oxide .
  • the metal precursor may be any metal precursor suitable for producing the relevant metal oxide catalyst after calcination.
  • the metal precursor may be a metal chloride, a metal oxide, a metal hydroxide, a metal nitrate or combinations thereof .
  • the metal precursor may be
  • R alkoxide
  • x, y, z integers
  • crystalline nanostructured metal oxides requires finding simple methods or strategies to achieve their full oxidation and crystallization into the required metal oxide, while obtaining the desired morphology and
  • the metal precursor comprises
  • the polyoxotitanium cages have a degree of condensation as defined by y/x of from 0.50 to 1.60, preferably from 0.80 to 1.60, more preferably from 1.00 to 1.60. It is believed that a higher degree of condensation as defined by y/x of from 0.50 to 1.60, preferably from 0.80 to 1.60, more preferably from 1.00 to 1.60. It is believed that a higher degree of condensation as defined by y/x of from 0.50 to 1.60, preferably from 0.80 to 1.60, more preferably from 1.00 to 1.60. It is believed that a higher degree of
  • condensation may help the oxidization and condensation of the polyoxotitanium cages to the fully condensed titania (T1O 2 ), which may lead to an improved catalytic
  • a preferred example of the polyoxotitanium cages suitable for the method of the present invention is [TiisOie (OEt) 32] .
  • [Tii 6 Oi S (OEt ) 32] has a degree of condensation of 1 and therefore is likely to form highly condensed titania using the method of the present invention .
  • step a) comprises dissolving the metal precursor in the solvent.
  • the concentration of the metal precursor is from 0.01 g/cm 3 to 5 g/cm 3 , preferably from 0.01 g/cm 3 to 3 g/cm 3 , more preferably from 0.03 g/cm 3 to 1 g/cm 3 , such as 0.05 g/cm 3 to 0.8 g/cm 3 , for example 0.1 g/cm 3 .
  • step a) comprises dispersing graphene oxide in the solvent.
  • the amount of graphene oxide is from 1 wt% to 10 wt%, preferably from 1 wt% to 8 wt%, more preferably from 1 wt% to 5 wt%, such as 2 wt% to 4 wt%, for example of 3 wt%, of the final solid weight of the graphene oxide supported metal precursor before calcination.
  • the dispersion of graphene oxide is further facilitated by means of, for example,
  • the dispersed graphene oxide solution is treated by alternating sonication and stirring for at least 3 hours. It is believed that dispersing graphene oxide evenly in the solution may help the anchoring of the metal precursor to the graphene oxide .
  • Graphene oxide is known as being in the shape of flakes or sheets and has a high surface area. It has, in general, a two-dimensional structure. Graphene oxide flakes may have a wide size distribution. For example, the graphene oxide flakes may have a longest lateral dimension of from 2 to 100 ⁇ by scanning electron microscopy (SEM) . The most abundant longest lateral dimension may be, for example, from 2 to 20 ⁇ ,
  • the most abundant maximum lateral size of the graphene oxide flakes is in the range of from 2 to 4 pm by SEM.
  • the metal precursor is anchored on graphene oxide flakes by the substitution of the labile terminal groups with the hydroxyls on graphene oxide basal planes and carboxylic groups on graphene oxide edges . It has been found that anchoring on graphene oxide helps proper supporting and/or arrangement of the metal precursor on two dimensions, avoiding dominant arrangement into globular structures during calcination where much
  • Graphene oxide advantageously helps to control the nucleation and growth of the nanocrystals , lowering the size of the metal oxide nanocrystals. Moreover, graphene oxide may help to shape the aggregates of these nanocrystals into the shape of two-dimensional flakes which may be beneficial in minimizing the light shielding and therefore is an advantageous shape for catalysis, in particular photocatalysis . Without wishing to be bound by theory, it is
  • the type of solvent may be important in helping the anchoring of the metal precursor to the graphene oxide flakes and the formation of the two- dimensional graphene oxide supported metal precursor.
  • the choice of the solvent may depend on, for example, the solubility of the metal precursor, the dispersability of graphene oxide, and the polarity of the solvent.
  • the solvent has a boiling point of from 40 °C to 120 °C, preferably from 40 °C to 100 °C, more
  • the boiling point of the solvent is neither too low that the solvent may start evaporating during preparation of the graphene oxide supported metal precursor, nor too high that graphene oxide may reduce during the removal of the solvent and lose anchoring sites.
  • Suitable solvents include, but are not limited to, tetrahydrofuran (THF) and toluene.
  • THF tetrahydrofuran
  • the solvent is THF.
  • the solvent is anhydrous, for example, anhydrous THF.
  • the mixing comprises mixing the metal precursor, for example, polyoxotitanium cages, in the solvent with graphene oxide in the solvent dropwise.
  • the mixing may be accompanied by stirring for better contact between the metal precursor and graphene oxide flakes.
  • the mixed solution is further treated by means of, for example, stirring to help anchoring of the metal
  • the mixed solution is further stirred for at least 10 minutes.
  • Step a) of the present invention may be performed at or about room temperature .
  • step b) the solvent is removed by evaporation.
  • the solvent may be removed by heat
  • step b) of the present invention is performed at a temperature at or about the boiling point of the solvent.
  • the heat treatment of step b) is performed at a temperature in the range of from 40 °C to 100 °C.
  • the removal of the solvent is controlled at a slow speed to help maintaining the shape and structure of the graphene oxide supported metal precursor.
  • step b) of the present invention may be performed in an oven.
  • the solvent may be removed by reduced pressure .
  • the graphene oxide supported metal precursor is substantially free of solvent after step b) of the present invention.
  • the graphene oxide supported metal precursor contains less than 5 wt%, preferably less than 3 wt%, more preferably less than 1 wt%, for example less than 0.1 wt%, of solvent after step b) .
  • the complete removal of solvent is beneficial in reducing globular aggregation of the metal oxide catalyst particles during calcination.
  • step c) of the present invention is performed in an oxidizing atmosphere, for example, in air.
  • Graphene oxide is a sacrificial support in the present invention and is removed during calcination by oxidization, decomposition and gasification.
  • the calcination of step c) of the present invention does not comprise combustion of graphene oxide.
  • the temperature may be increased stepwise to a plateau temperature and then held at the plateau temperature for a period of time before the calcination is completed.
  • the plateau temperature is, in general, higher than the temperature normally used for the treatment of graphene oxide in the existing methods of preparing metal oxide composite catalysts.
  • the plateau temperature is in the range of from 350 °C to 600 °C, preferably from 350 °C to 500 °C, more preferably from 400 °C to 500 °C, for example the plateau
  • step c) The plateau temperature of the calcination of step c) helps the formation of the fine particle of the metal oxide catalyst during the
  • Control of the process is found to reduce the risk of agglomeration of the metal oxide catalyst.
  • the temperature is increased at a ramp rate of less than 20 °C per minute, more preferably less than 15 °C per minute, most preferably less than 12 °C per minute, for example at 10 °C per minute.
  • the temperature is held at the plateau temperature for a period of time. The length of the time depends on the amount of the graphene oxide supported metal precursor, the nature of the metal precursor and the plateau temperature.
  • the temperature is held at the plateau temperature for at least 30 minutes, or at least 45 minutes, or at least 50 minutes, for example at least 1 hour.
  • the metal oxide catalyst produced is essentially free of graphene oxide.
  • no more than 30 wt% preferably no more than 20 wt%, more preferably no more than 10 wt%, most preferably no more than 5 wt%, for example no more than 1 wt% of the
  • the catalyst is a photocatalyst , for example, a photocatalyst for the one of the reactions to convert water to hydrogen and oxygen.
  • the catalyst is a metal oxide catalyst where the metal is selected from the group consisting of Titanium, Zinc, Zirconium, Tungsten, Iron and combinations thereof.
  • the catalyst produced contains less than 5 wt%, preferably less than 3 wt%, more preferably less than 2 wt% most preferably less than 1 wt%, for example less than 0.1 wt% graphene oxide, based on the total solid weight of the metal oxide catalyst after
  • Graphene oxide is used as a sacrificial support in the method of the present invention and helps the formation of the metal oxide particles in the shape of two-dimensional flakes with nanometer thickness, thereby increasing the surface area of the metal oxide catalyst and improving the catalytic activity.
  • the metal oxide catalyst produced by the method of the present invention preferably has an improved
  • the metal oxide catalyst produced by the method of the present invention is a titania catalyst.
  • hydrogen production rate of the titania catalyst of the present invention could be more than 50 ⁇ g' ⁇ -h -1 , preferably more than 100 ⁇ g ⁇ h -1 , more preferably more than 150 ⁇ g ⁇ tr 1 , for example more than 200 mol g- 1 h _1 .
  • the improved catalytic activity is
  • the metal oxide catalyst produced by the method of the present invention has a BET surface area of no less than 40 m 2 g" 1 , preferably no less than 50 m 2 g" 1 , preferably no less than 55 m 2 g- 1 , for example no less than 60 m 2 g- 1 .
  • the metal oxide catalyst produced by the method of the present invention is a titania catalyst.
  • the titania catalyst produced by the method of the present invention has a higher anatase-rutile ratio when compared to commercially available titania photocatalysts . It has been generally believed that rutile-anatase titanias give better catalytic performance over single-anatase titanias for reasons such as possible slower recombination
  • the titania catalyst produced by the method of the present invention which is a purer anatase titania, exhibits improved performance over mixed-phase titanias, such as Aeroxide ® P25.
  • the titania catalyst of the present invention contains no less than 80 wt%, preferably no less than 85 wt%, more preferably no less than 90 wt%, most preferably no less than 95 wt% of anatase.
  • the improved performance of the anatase titania catalyst may be due to factors such as morphology, particle size, dispersability, and/or surface area, which may have a profound effect on the final performance of the titania catalyst.
  • the metal oxide catalyst produced by the method of the present invention preferably has a size in the range of from 5 to 15 nm, preferably from 5 to 13 nm, for example from 5 to 11 nm, as measured by transmission electron microscopy (TEM) .
  • TEM transmission electron microscopy
  • nanoparticles are much smaller than metal oxide catalysts produced by some conventional methods.
  • the smaller particle size, in combination with the higher surface area and the two-dimensional flake shape of the metal oxide particles, is believed to contribute to the
  • most of the metal oxide catalyst produced by the method of the present invention is in the form of flakes.
  • the flakes are, in general, of a two- dimensional shape with a thickness of from 5 to 15 nm, preferably from 5 to 13 nm, for example from 5 to 11 nm.
  • the thickness of the metal oxide catalyst flakes may be larger due to stacking of graphene oxide supports or of resulting flakes, resulting in a thickness of less than 50 nm.
  • the catalytic reaction is the photocatalytic reduction of water with a hole scavenger, for example methanol or ethanol, preferably methanol.
  • a hole scavenger for example methanol or ethanol, preferably methanol.
  • a titania catalyst comprising titania, preferably no less than 80 wt% titania, more preferably no less than 90 wt% titania, most preferably no less than 95 wt% titania.
  • the titania comprises no less than 85 wt% of anatase titania and the titania has a size in the range of from 5 to 15 nm by TEM.
  • the titania is in the form of flakes.
  • the titania catalyst of the present invention comprises less than 5 wt%, preferably less than 3 wt%, more preferably less than 2 wt%, for example less than 1 wt% graphene oxide or graphene oxide in its reduced form.
  • catalysts are metal oxide catalysts generally, and is not limited to photocatalyst or titania catalysts.
  • Figure 2a shows SEM images of GO spread on Si0 2 /Si wafer
  • Figure 3 shows hydrogen production rates of the titania catalysts and P25 from methanol solutions
  • Figure 4 shows SEM images of the titania catalysts and P25 (scale bar 5 m for 4a-4e and 200 nm for 4f) ;
  • Figure 5 shows TEM images of the titania catalysts
  • FIG. 6 shows the diffuse reflectance UV-visible
  • Figure 7 shows X-ray diffraction of the titania
  • Figure 8 shows Raman spectra of titania catalyst
  • Figure 1 shows its compact structure, which exhibits four ⁇ 4 - ⁇ , eight ⁇ 3 - ⁇ , four ⁇ 2 - ⁇ , and sixteen ⁇ 2 -0 ⁇ bridging ligands and sixteen OEt terminating ligands connected to sixteen octahedral titanium atoms.
  • Two orthogonal blocks of eight TiOe octahedra compose the titanium oxo core [TiigOiel .
  • Graphene oxide was prepared by oxidation and exfoliation of graphite, using a modified Hummer's method (Hirata, M. ; Gotou, T. ; Horiuchi, S . ; Fujiwara, M . ; Ohba, M. Carbon 2004, 42, 2929-2937) . Briefly, 100- 500 ⁇ graphite particles (Sigma Aldrich) were oxidized in a mixture of sulfuric acid, potassium permanganate and sodium nitrate. After reaction overnight under vigorous stirring, doubly deionized water and hydrogen peroxide (30%) were slowly added to stop the reaction. The GO was then washed by centrifuging and redispersing in doubly deionized water at least ten times. Finally, the GO was freeze-dried for storage. Elemental analysis (wt%) found: C 42.6, H 0.9, N 1.0 (by elemental
  • Figure 2a shows the ⁇ images of GO spread on
  • headspace were purged with argon for 30 min, with a gas flow of approx. 100 mL min -1 to remove any air.
  • the argon flow passing through the reactor was lowered to 5 mL min -1 and directly connected to a gas
  • GC chromatograph
  • DID discharge ionization detector
  • irradiation source consisted of a 75 W Xe lamp
  • Figure 3 shows the hydrogen production rate vs . time measured at the outlet of the photocatalytic reactor and Table 1 lists the averaged hydrogen production rate.
  • the averaged hydrogen production rate was 5.5 ⁇ g ⁇ h "1 for the titania prepared from ⁇ ⁇ (OEt) 32 cages, and 2.0 ⁇ g ⁇ tr 1 for the Ti (OEt ) 4-based
  • titania prepared from ⁇ ⁇ (OEt) 32 cages.
  • the improved photocatalytic performance of titania prepared from ⁇ ⁇ (OEt) 32 cages may be
  • the averaged hydrogen production rate measured was 42.8 pmol g ⁇ tr 1 .
  • the titania catalysts resulting from calcining a composite of Tii 6 0i6 (OEt) 32 cages and GO has an averaged hydrogen production rate of 223.8 pmol g- ⁇ -h" 1 , which is five times more than that of P25 titania and forty times more than that of titania prepared from only TiieOie (OEt) 32 cages, or over one hundred times more than that of titania prepared from only Ti(OEt)4 (Table 1) .
  • the titania is prepared with Ti(OEt) 4 and GO, averaged hydrogen production rate was only 7.4 mol g -1 ! -1 .
  • the results clearly show that the combination of TiieOis (OEt) 32 cages and GO for the production of titania is especially beneficial for the improved catalytic performance of the titania catalysts.
  • iisOie (OEt) 32 cages which more than doubles reaching 62 m 2 g- 1 .
  • P25 titania has a surface area of 55 m 2 g _1 , slightly lower. Comparison of the surface areas with the hydrogen production indicates that the higher the surface area the higher the hydrogen
  • SEM images were obtained by field emission gun scanning electron microscopy (FEG-SEM) LE01525 with an acceleration voltage of 5 kV.
  • FEG-SEM field emission gun scanning electron microscopy
  • titania particles were deposited on highly doped Si-n ++ wafer and GO on 300nm Si0 2 coated n-type Si wafer.
  • Titanias prepared with ⁇ 6 (OEt ) 32 cages or Ti(OEt) 4 have a particle size in the micrometer range, with large particles with a diameter > 5 ⁇ ( Figures 4a and 4b) .
  • the addition of GO results in much smaller titania particle sizes ( Figures 4c and 4d) , especially for the titania prepared with Tii 6 0i6 (OEt ) 32 cages and GO ( Figures 4c and 4f) .
  • the SEM image of P25 ( Figure 4e) is also included for comparison.
  • TEM (transmission electron microscopy) imaging was carried out (by a JEOL JEM1200EXII) to reveal further insights into the shape and size of titanias .
  • TEM images were analysed by ImageJ to calculate average particle size.
  • TEM images of the different titanias are shown in Figure 5.
  • Particles prepared with Ti(OEt)4 are too large for TEM, but a few edges could be imaged.
  • TEM shows that Ti (OEt) 4-based titania consists of bulky particles containing rough surfaces with surface features below 5nm ( Figure 5a) .
  • titania prepared with Tii S Oi6 (OEt) 32 cages consists of globular aggregates of 12+3 nm nanoparticles ( Figure 5b) .
  • nanoparticles are 3 times smaller than P25 titania particles (shown in Figure 5d for comparison) .
  • the smaller particle sizes and consequently higher surface areas are in line with the higher photocatalytic hydrogen production rates.
  • their arrangement in two-dimensional flakes can help to minimize the light shielding and improve this way the photocatalytic performance.
  • TEM shows that the resulting particles are very diverse including coated GO flakes, bulky
  • Ti(0Et) (OEt/Ti:4).
  • the addition of GO to the titania synthetic route also leads to more transparent and purer titanias, when being added to both precursors, Tii 6 Oie (OEt ) 32 cages and Ti(OEt) 4 .
  • the titania prepared with TiisOie (OEt) 32 cages and GO obtains the most
  • hydroxyls provide anchoring sites for metallic oxo alkoxo cages, which can facilitate its uniform
  • GO is shown to decompose easily at 450 °C in 1 h in air, probably due to its two-dimensional shape with atomic thickness.
  • Other carbon supports will not imprint two-dimensional shape, lack anchoring groups, and could require higher temperatures and/or longer calcination times for the combustion, which could compromise the surface area, particle size and dispersability in water.
  • titania formation i.e., the nucleation and growth of the metal oxide, for example, titania, or in other words, the oxidation of the metal precursor such as polyoxotitanium cages and crystallization
  • O/Ti the degree of condensation
  • cages with high degree of condensation include Tii 7 0 2 4 (C ⁇ Pr) 20 and
  • titania catalyst produced by the method of the present invention which is a pure anatase titania, outperforms mixed-phase P25 titania in the photocatalytic decomposition of water.
  • other factors such as morphology, particle size, dispersability, and surface area can have a profound effect on the final performance of titania catalyst.
  • Tii60i6 (OEt) 32 cages and GO. It shows that under the current calcination conditions, essentially all GO have gasified to oxidized species such as C02 and CO. The absence of graphene derivatives is further confirmed by Raman spectroscopy ( Figure 8, For a clearer comparison, the origin of the y axis of each distribution has been shifted by the indicated values) . However, when GO is used with Ti(OEt) 4 , TEM shows higher survival of GO probably due to the higher organic content in Ti(OEt)4 compared to ⁇ (OEt) 32 , which must favor a less oxidizing atmosphere during calcination. According to elemental analysis and SEM/TEM imaging no more 30% of the original GO survives in the Ti(OEt) 4 - GO based titania.
  • GO is used as a sacrificial support for the synthesis of the metal oxide, for example, the titania catalysts. It appears that GO was not needed in its reduced form to obtain a final titania with enhanced photocatalytic performance.
  • GO' S role is to support or template the formation of nanostructured ( ⁇ 10 nm) particles from ii e Oi6 (OEt) 32 decomposition and their arrangement on flakes or layers, which maximizes the available surface for light irradiation and photocatalysis . It is
  • TiisOis (OEt) 32 cages have anchored on GO flakes by the substitution of the labile terminal ethoxides with the hydroxyls on GO basal planes and carboxylic groups on GO edges.
  • Anchoring on GO ensures proper supporting/arrangement of the cages on two dimensions, avoiding dominant arrangement into globular structures during calcination where much resulting titania would be shielded.
  • GO can therefore be seen as promoting a heterogeneous nucleation of titania when in THF suspension leading eventually to a high density of very small crystals.
  • GO offers control at two levels.
  • each particle is given equal weighting irrespective of its size.
  • agglomerates, or particles in the micrometer range with main size distribution centered between 5 and 50 um.
  • P25 titania nanoparticles are aggregated to
  • Titania prepared with Tii 6 0i6 (OEt ) 32 cages contains particles or agglomerates with a wide distribution centered at 15 ⁇ and smaller particles or agglomerates down to 0.25 ⁇ .
  • the resulting titania agglomerate or particle size decreases from 15 to 5 um, while keeping particles down to 0.25 ⁇ .
  • titania prepared with Ti(OEt)4 has no particles below 1 ⁇ , indicating that Tii S Oi6 (OEt) 32 is a better precursor than Ti(OEt) 4 for the formation of finer titania.
  • Adding GO to Ti(OEt) 4 has also a positive effect: it lowers the minimum particle size from 1 ⁇ to 0.25 m. There are however some agglomerates with size distribution centered at 45 ⁇ . All titanias, except P25 titania, have a few
  • the Z -potential, or electrokinetic potential is related to the colloidal properties of particle
  • Titania prepared from both Ti(OEt) 4 and

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Abstract

The present invention relates to a method of making a metal oxide catalyst plus a catalyst produced thereby and its use in catalytic reactions. The catalyst may be a transition metal oxide catalyst and the method may involve the use of graphene oxide as a sacrificial support. The present invention also relates to a titania catalyst.

Description

Methods of making metal oxide catalysts
Field of the Invention The present invention concerns methods of making a metal oxide catalyst. More particularly, but not
exclusively, this invention concerns methods of making a metal oxide catalyst, in particular a transition metal oxide catalyst, using graphene oxide as a sacrificial support. The invention also concerns the metal oxide catalyst produced thereby and the use of the metal oxide catalyst in catalytic reactions.
Background of the Invention
Metal oxide catalysts have found many uses in chemical and energy producing fields, including use as photocatalysts . It is expected that, by 2050, the world energy demand will double, due to the growth in human population and the industrial development of heavily populated countries. In parallel, fossil fuel resources are dwindling, becoming more expensive, and contributing to the increase in carbon dioxide (CO2) emissions which exacerbate global warming. Alternative energy resources, for example artificial photosynthesis, offer an
opportunity to exploit alternative inexhaustible energy sources. UV and visible light absorbing semiconductor photocatalysts comprising metal oxides such as T1O2, ZnO, Fe203, WO3, Cu20 and various combinations forming
heterojunctions have been shown to photocatalytically generate solar fuels from water and/or carbon dioxide. However, further research and progress is necessary to increase the current low yields of product formation. The low efficiency of these photocatalytic systems has commonly been attributed to the lack of absorption of visible light, low surface area, fast recombination of photogenerated charge carriers and back reactions of reduced species to form C02 and/or H20. To make artificial photosynthesis economically viable, significant
improvements in the properties of the photocatalysts are needed.
Due to the challenging multielectron requirement of the overall water splitting reaction (H2O → ¾ + ½02) , most research on photocatalytic systems has focused on the study of either the individual oxidation or reduction of H2O, by using chemical electron or hole scavengers, respectively. One approach to connect both photocatalytic reactions is through the use of an electron mediator, such as the Fe3+/Fe2+ and I03 ~/I~ redox couples, in what is usually called the Z-scheme. This approach, which
resembles the principles of operation of dye sensitised solar cells, has tremendous potential and justifies the research on photocatalysts for half reactions.
In known photocatalytic composites or hybrids involving graphene oxide, the latter is typically reduced to reduced graphene oxide by solvothermal treatment at temperatures of around 200 °C.
It is one of the aims of the present invention to provide new methods of making metal oxide catalysts having improved properties, especially photocatalysts having improved properties. It is also one of the aims of the present invention to provide an improved method of producing titania catalysts with small particle size, high surface area, two-dimensional arrangement, and high dispersability in aqueous suspensions, from
polyoxotitanium cages. The resulting titania catalysts have shown improvement in catalytic performance when compared to the standard P25 titania in solar hydrogen generation with methanol as a hole scavenger. Summary of the Invention
According to a first aspect of the invention there is provided a method of making a metal oxide catalyst. The method comprises:
a) contacting a metal precursor with graphene oxide in the presence of a solvent to provide a graphene oxide supported metal precursor, wherein the metal precursor comprises one or more transition metals;
b) removing the solvent;
c) calcining the graphene oxide supported metal precursor in an oxidizing atmosphere to remove the graphene oxide and generate the metal oxide catalyst .
It has been found that, surprisingly, when the metal precursor is supported on graphene oxide flakes and is calcined, the metal oxide catalyst produced may have an improved catalytic activity, in particular photocatalytic activity, when compared to metal oxide catalysts which are produced by other methods or which are commercially available, for example, Aeroxide® P25 titania catalyst. Without wishing to be bound by theory, it is believed that the enhanced catalytic activity of the obtained metal oxide results from the combination and synergy of the metal precursor and graphene oxide flakes which provide a higher surface area and better dispersability in water suspensions of the metal oxide.
The metal precursor of the present invention
comprises one or more transition metals. The one or more transition metals may be present in the metal precursor in any oxidation state, including zero oxidation state. For example, the metal may be present in the metal precursor as metal oxide, inorganic or organic metal salt, or metal alloy.
Preferably, the metal precursor of the present invention comprises a metal selected from the group consisting of Titanium, Zinc, Zirconium, Tungsten, Iron and combinations thereof. More preferably, the metal precursor comprises Titanium, for example a titanium oxide .
The metal precursor may be any metal precursor suitable for producing the relevant metal oxide catalyst after calcination. In some embodiment of the present invention, the metal precursor may be a metal chloride, a metal oxide, a metal hydroxide, a metal nitrate or combinations thereof . In another embodiment of the present invention, the metal precursor may be
polyoxometallic cages. Optionally, the polyoxometallic cages may be single metal polyoxometallic cages with organic ligands such as alkoxides or carboxylates (with a general formula of [MxOy(OR)z], in which R=alkoxide, x, y, z = integers) . They have been previously used as well defined precursors in the formation of metal oxides in photovoltaics and catalysis. Alternatively, the
polyoxometallic cages may be heterometallic oxo cages containing Titanium and other metals such as Fe, Cu, Co, or Ni (with a general formula of TixOy (OR) zMraXn (R = alkoxide, M = main group transition metal or lanthanide; X = anion such as a halide, x, y, z, m, n= integers) . Exploiting the full potential of these polyoxometallic cages as precursors for the preparation of highly
crystalline nanostructured metal oxides requires finding simple methods or strategies to achieve their full oxidation and crystallization into the required metal oxide, while obtaining the desired morphology and
textural properties for the final application.
Optionally, the metal precursor comprises
polyoxotitanium cages . Polyoxotitanium cages have a general formula of TixOy(OR)z (in which R=alkoxide, x, y, z = integers) .
Preferably, the polyoxotitanium cages have a degree of condensation as defined by y/x of from 0.50 to 1.60, preferably from 0.80 to 1.60, more preferably from 1.00 to 1.60. It is believed that a higher degree of
condensation may help the oxidization and condensation of the polyoxotitanium cages to the fully condensed titania (T1O2), which may lead to an improved catalytic
performance of the titania catalysts produced. It is also believed that a higher degree of condensation favors the formation of fine titania particles at a lower
temperature. A preferred example of the polyoxotitanium cages suitable for the method of the present invention is [TiisOie (OEt) 32] . [Tii6OiS (OEt ) 32] has a degree of condensation of 1 and therefore is likely to form highly condensed titania using the method of the present invention .
Preferably, step a) comprises dissolving the metal precursor in the solvent. Optionally, the concentration of the metal precursor is from 0.01 g/cm3 to 5 g/cm3, preferably from 0.01 g/cm3 to 3 g/cm3, more preferably from 0.03 g/cm3 to 1 g/cm3, such as 0.05 g/cm3 to 0.8 g/cm3, for example 0.1 g/cm3.
Preferably, step a) comprises dispersing graphene oxide in the solvent. Optionally, the amount of graphene oxide is from 1 wt% to 10 wt%, preferably from 1 wt% to 8 wt%, more preferably from 1 wt% to 5 wt%, such as 2 wt% to 4 wt%, for example of 3 wt%, of the final solid weight of the graphene oxide supported metal precursor before calcination. Preferably, the dispersion of graphene oxide is further facilitated by means of, for example,
sonication and/or stirring. In one embodiment of the present invention, the dispersed graphene oxide solution is treated by alternating sonication and stirring for at least 3 hours. It is believed that dispersing graphene oxide evenly in the solution may help the anchoring of the metal precursor to the graphene oxide .
Graphene oxide is known as being in the shape of flakes or sheets and has a high surface area. It has, in general, a two-dimensional structure. Graphene oxide flakes may have a wide size distribution. For example, the graphene oxide flakes may have a longest lateral dimension of from 2 to 100 μιη by scanning electron microscopy (SEM) . The most abundant longest lateral dimension may be, for example, from 2 to 20 μιτι,
optionally from 2 to 10 μηα by SEM. In one embodiment of the present invention, the most abundant maximum lateral size of the graphene oxide flakes is in the range of from 2 to 4 pm by SEM.
Without wishing to be bound by theory, it is
believed that the metal precursor is anchored on graphene oxide flakes by the substitution of the labile terminal groups with the hydroxyls on graphene oxide basal planes and carboxylic groups on graphene oxide edges . It has been found that anchoring on graphene oxide helps proper supporting and/or arrangement of the metal precursor on two dimensions, avoiding dominant arrangement into globular structures during calcination where much
resulting metal oxide would eventually be shielded.
Without wishing to be bound by theory, it is believed that graphene oxide flakes help to promote a
heterogeneous nucleation of metal oxide when the metal precursor is supported, which leads to the production of a high density of very small particles of metal oxide catalyst. Graphene oxide advantageously helps to control the nucleation and growth of the nanocrystals , lowering the size of the metal oxide nanocrystals. Moreover, graphene oxide may help to shape the aggregates of these nanocrystals into the shape of two-dimensional flakes which may be beneficial in minimizing the light shielding and therefore is an advantageous shape for catalysis, in particular photocatalysis . Without wishing to be bound by theory, it is
believed that the type of solvent may be important in helping the anchoring of the metal precursor to the graphene oxide flakes and the formation of the two- dimensional graphene oxide supported metal precursor. The choice of the solvent may depend on, for example, the solubility of the metal precursor, the dispersability of graphene oxide, and the polarity of the solvent.
Optionally, the solvent has a boiling point of from 40 °C to 120 °C, preferably from 40 °C to 100 °C, more
preferably from 40 °C to 80 °C, for example from 60 °C to 80 °C. Advantageously, the boiling point of the solvent is neither too low that the solvent may start evaporating during preparation of the graphene oxide supported metal precursor, nor too high that graphene oxide may reduce during the removal of the solvent and lose anchoring sites. Suitable solvents include, but are not limited to, tetrahydrofuran (THF) and toluene. Preferably, the solvent is THF. Preferably, the solvent is anhydrous, for example, anhydrous THF.
Preferably, step a) of the present invention
comprises mixing the metal precursor, for example, polyoxotitanium cages, in the solvent with graphene oxide in the solvent dropwise. Optionally, the mixing may be accompanied by stirring for better contact between the metal precursor and graphene oxide flakes. Preferably, the mixed solution is further treated by means of, for example, stirring to help anchoring of the metal
precursor to graphene oxide. In one embodiment of the present invention, the mixed solution is further stirred for at least 10 minutes. Step a) of the present invention may be performed at or about room temperature .
Preferably, in step b) the solvent is removed by evaporation. The solvent may be removed by heat
treatment. Preferably, step b) of the present invention is performed at a temperature at or about the boiling point of the solvent. For example, the heat treatment of step b) is performed at a temperature in the range of from 40 °C to 100 °C. By doing so the removal of the solvent is controlled at a slow speed to help maintaining the shape and structure of the graphene oxide supported metal precursor. Optionally, step b) of the present invention may be performed in an oven. Alternatively or additionally, the solvent may be removed by reduced pressure .
Advantageously, the graphene oxide supported metal precursor is substantially free of solvent after step b) of the present invention. For example, the graphene oxide supported metal precursor contains less than 5 wt%, preferably less than 3 wt%, more preferably less than 1 wt%, for example less than 0.1 wt%, of solvent after step b) . The complete removal of solvent is beneficial in reducing globular aggregation of the metal oxide catalyst particles during calcination.
The calcination of step c) of the present invention is performed in an oxidizing atmosphere, for example, in air. Graphene oxide is a sacrificial support in the present invention and is removed during calcination by oxidization, decomposition and gasification. Preferably, the calcination of step c) of the present invention does not comprise combustion of graphene oxide. Without wishing to be bound by theory, it is believed that calcination under carefully controlled conditions could reduce the risk of compromising important properties of the metal oxide catalyst produced which may affect its catalytic activity, for example the surface area, the particle size and the dispersabxlxty of the metal oxide catalyst in water. During the calcination of step c) of the present invention, the temperature may be increased stepwise to a plateau temperature and then held at the plateau temperature for a period of time before the calcination is completed. In the calcination of step c) of the present invention, the plateau temperature is, in general, higher than the temperature normally used for the treatment of graphene oxide in the existing methods of preparing metal oxide composite catalysts. Preferably, in the calcination of step c) of the present invention the plateau temperature is in the range of from 350 °C to 600 °C, preferably from 350 °C to 500 °C, more preferably from 400 °C to 500 °C, for example the plateau
temperature is 450 °C. The plateau temperature of the calcination of step c) helps the formation of the fine particle of the metal oxide catalyst during the
decomposition and/or oxidization of graphene oxide.
Control of the process is found to reduce the risk of agglomeration of the metal oxide catalyst. Preferably, during the calcination of step c) of the present
invention, the temperature is increased at a ramp rate of less than 20 °C per minute, more preferably less than 15 °C per minute, most preferably less than 12 °C per minute, for example at 10 °C per minute. Once the temperature reaches the plateau temperature, the temperature is held at the plateau temperature for a period of time. The length of the time depends on the amount of the graphene oxide supported metal precursor, the nature of the metal precursor and the plateau temperature. Optionally, the temperature is held at the plateau temperature for at least 30 minutes, or at least 45 minutes, or at least 50 minutes, for example at least 1 hour. After the calcination of step c) of the present invention, the metal oxide catalyst produced is essentially free of graphene oxide.
Advantageously, in order to avoid light shielding, no more than 30 wt%, preferably no more than 20 wt%, more preferably no more than 10 wt%, most preferably no more than 5 wt%, for example no more than 1 wt% of the
original graphene oxide survives in the metal oxide catalyst produced. Optionally, the catalyst is a photocatalyst , for example, a photocatalyst for the one of the reactions to convert water to hydrogen and oxygen. Optionally, the catalyst is a metal oxide catalyst where the metal is selected from the group consisting of Titanium, Zinc, Zirconium, Tungsten, Iron and combinations thereof.
Preferably, the catalyst produced contains less than 5 wt%, preferably less than 3 wt%, more preferably less than 2 wt% most preferably less than 1 wt%, for example less than 0.1 wt% graphene oxide, based on the total solid weight of the metal oxide catalyst after
calcination. Graphene oxide is used as a sacrificial support in the method of the present invention and helps the formation of the metal oxide particles in the shape of two-dimensional flakes with nanometer thickness, thereby increasing the surface area of the metal oxide catalyst and improving the catalytic activity.
The metal oxide catalyst produced by the method of the present invention preferably has an improved
catalytic activity. In one embodiment of the present invention, the metal oxide catalyst produced by the method of the present invention is a titania catalyst.
When used in the photoreduction of water with methanol as hole scavenger, hydrogen production rate of the titania catalyst of the present invention could be more than 50 μπιοΐ g'^-h-1, preferably more than 100 μιηοΐ g^h-1, more preferably more than 150 μτηοΐ g^tr1, for example more than 200 mol g-1h_1. The improved catalytic activity is
believed to be a result of the surface area, the particle size, and the shape and the size of particle agglomerates in the reacting suspensions of the titania catalyst.
The metal oxide catalyst produced by the method of the present invention has a BET surface area of no less than 40 m2g"1, preferably no less than 50 m2g"1, preferably no less than 55 m2g-1, for example no less than 60 m2g-1. In the catalytic decomposition of water into hydrogen and oxygen, it has been observed that higher surface area is usually associated with higher hydrogen production, although the relation is not linear. It is believed that other factors, such as nanoparticle size and aggregate shape and size may also influence the final performance of the metal oxide catalyst. In one embodiment of the present invention, the metal oxide catalyst produced by the method of the present invention is a titania catalyst. The titania catalyst produced by the method of the present invention has a higher anatase-rutile ratio when compared to commercially available titania photocatalysts . It has been generally believed that rutile-anatase titanias give better catalytic performance over single-anatase titanias for reasons such as possible slower recombination
resulting from charge stabilization by electron transfer between phases and/or the smaller band gap of rutile which may allow more photoexcitation by visible light. However, it has been found that, surprisingly, the titania catalyst produced by the method of the present invention, which is a purer anatase titania, exhibits improved performance over mixed-phase titanias, such as Aeroxide® P25. The titania catalyst of the present invention contains no less than 80 wt%, preferably no less than 85 wt%, more preferably no less than 90 wt%, most preferably no less than 95 wt% of anatase. The improved performance of the anatase titania catalyst may be due to factors such as morphology, particle size, dispersability, and/or surface area, which may have a profound effect on the final performance of the titania catalyst.
The metal oxide catalyst produced by the method of the present invention preferably has a size in the range of from 5 to 15 nm, preferably from 5 to 13 nm, for example from 5 to 11 nm, as measured by transmission electron microscopy (TEM) . These metal oxide
nanoparticles are much smaller than metal oxide catalysts produced by some conventional methods. The smaller particle size, in combination with the higher surface area and the two-dimensional flake shape of the metal oxide particles, is believed to contribute to the
improved catalytic activity.
Advantageously, most of the metal oxide catalyst produced by the method of the present invention is in the form of flakes. The flakes are, in general, of a two- dimensional shape with a thickness of from 5 to 15 nm, preferably from 5 to 13 nm, for example from 5 to 11 nm. The thickness of the metal oxide catalyst flakes may be larger due to stacking of graphene oxide supports or of resulting flakes, resulting in a thickness of less than 50 nm.
According to a second aspect of the invention, there is also provided the use of the metal oxide catalyst of the present invention in a catalytic reaction.
Optionally, the catalytic reaction is the photocatalytic reduction of water with a hole scavenger, for example methanol or ethanol, preferably methanol.
According to a third aspect of the invention, there is also provided a titania catalyst comprising titania, preferably no less than 80 wt% titania, more preferably no less than 90 wt% titania, most preferably no less than 95 wt% titania. The titania comprises no less than 85 wt% of anatase titania and the titania has a size in the range of from 5 to 15 nm by TEM. Preferably, the titania is in the form of flakes. Advantageously, the titania catalyst of the present invention comprises less than 5 wt%, preferably less than 3 wt%, more preferably less than 2 wt%, for example less than 1 wt% graphene oxide or graphene oxide in its reduced form.
The invention is described herein mainly with reference to photocatalysts , for example, titania
catalysts. However, the invention extend to metal oxide catalysts generally, and is not limited to photocatalyst or titania catalysts.
It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa..
Detailed Description
Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows the structure of Tii60i6 (OEt) 32,·
Figure 2a shows SEM images of GO spread on Si02/Si wafer
(scale bar 5μπι) ;
Figure 2b Histogram of the maximum lateral size
distribution of GO mats (over 163 counts) ;
Figure 3 shows hydrogen production rates of the titania catalysts and P25 from methanol solutions; Figure 4 shows SEM images of the titania catalysts and P25 (scale bar 5 m for 4a-4e and 200 nm for 4f) ;
Figure 5 shows TEM images of the titania catalysts and
P25 (scale bar 50nm) ;
Figure 6 shows the diffuse reflectance UV-visible
spectra of the titania catalysts and P25;
Figure 7 shows X-ray diffraction of the titania
catalysts and P25;
Figure 8 shows Raman spectra of titania catalyst
prepared according to one embodiment of the present invention and graphene oxide; Figure 9 shows laser beam scattering particle size
distribution in suspension of the titania catalysts and P25.
By way of example only, certain possible variation will now be described. Examples
1) Preparation of the metal precursor
ίιεθιε (OEt ) 32 cages were solvothermally prepared by controlled hydrolysis of Ti(OEt)4 and isolated by crystallization in mm- and cm-size crystals as reported previously by Fornasieri G (Journal of American
Chemistry Society, 2005, 127, 4869-4878) . Figure 1 shows its compact structure, which exhibits four μ4-οχο, eight μ3-οχο, four μ2-οχο, and sixteen μ2-0^ bridging ligands and sixteen OEt terminating ligands connected to sixteen octahedral titanium atoms. Two orthogonal blocks of eight TiOe octahedra compose the titanium oxo core [TiigOiel .
Briefly, 14 mL of Ti(OEt)4 (Alfa Aesar, 99+%), 14 mL of anhydrous ethanol (Sigma Aldrich) , and 600 μΐ* of doubly deionized water were placed in a 45 mL Teflon lined stainless steel autoclave and heated to 100 °C for 14 days . The mixture was slowly cooled to room temperature to provide colourless crystals of
TiiSOi6 (OEt) 32 cages (~60% yield) . Theoretical elemental analysis (wt%) for Ce4Hi6o048Tii6 is calculated as : C
31.2, H 6.5. Elemental analysis carried out on a Thermo Scientific Flash 2000 configured for %CHN found: C
31.3, H 6.5. 2) Preparation of graphene oxide
Graphene oxide (GO) was prepared by oxidation and exfoliation of graphite, using a modified Hummer's method (Hirata, M. ; Gotou, T. ; Horiuchi, S . ; Fujiwara, M . ; Ohba, M. Carbon 2004, 42, 2929-2937) . Briefly, 100- 500 μτη graphite particles (Sigma Aldrich) were oxidized in a mixture of sulfuric acid, potassium permanganate and sodium nitrate. After reaction overnight under vigorous stirring, doubly deionized water and hydrogen peroxide (30%) were slowly added to stop the reaction. The GO was then washed by centrifuging and redispersing in doubly deionized water at least ten times. Finally, the GO was freeze-dried for storage. Elemental analysis (wt%) found: C 42.6, H 0.9, N 1.0 (by elemental
analysis carried out on a Thermo Scientific Flash 2000 configured for %CHN) .
Figure 2a shows the ΞΕΜ images of GO spread on
Si02/Si wafer. Flakes had a wide size distribution, with most abundant longest lateral dimension being between 2 and 4 μιτι. A histogram with the flake size distribution is shown in Figure 2b. 3) Preparation of titania photocatalysts
Four different types of titania particles were prepared from two different precursors, and with and without the addition of GO, but using the same
experimental procedure . For the preparation of the first titania, 1 g of Tii6OiS (OEt ) 32 cages was dissolved in 10 mL of anhydrous tetrahydrofuran (THF) . 0.031 g of GO (3% of final solid weight) was dispersed in 10 mL of THF by alternating sonication and stirring for 3 h. Both samples were mixed dropwise under vigorous
stirring, and kept under stirring for 10 min. The solvent was then evaporated under stirring in an oil bath at 70 °C. The mixture was further dried in an oven at 70 °C overnight. Finally, the mixture was heated with a ramp rate of 10 °C min-1 to 450 °C and kept at this temperature for 1 h. The same procedure was carried out to prepare a second titania sample, using Ti(OEt)4 (1.31 mL) instead of Tii60i6 (OEt ) 32 cages. The same procedure was followed to prepare a third and a fourth titania with TiisOie (OEt) 32 cages and Ti(OEt)4, respectively, but without the addition of GO.
4) Characterization of titania catalysts
a) Photocatalytic tests
100 mg of sample were placed in a 180 mL quartz reaction vessel with a mixture of 20 mL of methanol and 80 mL of distilled water. The suspension was first sonicated for 30 min and then kept under stirring for the remaining of the experiment. The liquid and
headspace were purged with argon for 30 min, with a gas flow of approx. 100 mL min-1 to remove any air. Next, the argon flow passing through the reactor was lowered to 5 mL min-1 and directly connected to a gas
chromatograph (GC, Perkin Elmer Clarus 580GC) for monitoring the gas evolution in the reactor by a 1 μΐ. gas sampling valve automated system. The GC used helium as carrier gas, a 5 A molecular sieve column, and a discharge ionization detector (DID) . The photocatalytic reaction was carried out by irradiating laterally the suspension with a solar simulator for 3 h. The
irradiation source consisted of a 75 W Xe lamp
(Hamamatsu RC0020) . In order to simulate the solar spectrum, an AM 1.5G filter was placed before the sample and the light intensity was adjusted to 100 mW cm-2 using neutral density filters.
Figure 3 shows the hydrogen production rate vs . time measured at the outlet of the photocatalytic reactor and Table 1 lists the averaged hydrogen production rate. The averaged hydrogen production rate was 5.5 μιτιοΐ g^h"1 for the titania prepared from Τί ιεθιβ (OEt) 32 cages, and 2.0 μπιοΐ g^tr1 for the Ti (OEt ) 4-based
titania. The improved photocatalytic performance of titania prepared from Τί ιεθιε (OEt) 32 cages may be
attributed to the cage's higher degree of condensation (0/Ti:l) when compared to Ti(OEt)4 (O/Ti:0), that favors the oxidization/condensation to titania T1O2 (0/Ti:2). A commercially available and widely studied
photocatalyst , P25 titania, was also tested in
photocatalytic tests under the same conditions as an comparative example, the averaged hydrogen production rate measured was 42.8 pmol g^tr1.
The titania catalysts resulting from calcining a composite of Tii60i6 (OEt) 32 cages and GO has an averaged hydrogen production rate of 223.8 pmol g-^-h"1, which is five times more than that of P25 titania and forty times more than that of titania prepared from only TiieOie (OEt) 32 cages, or over one hundred times more than that of titania prepared from only Ti(OEt)4 (Table 1) . However, if the titania is prepared with Ti(OEt)4 and GO, averaged hydrogen production rate was only 7.4 mol g-1!!-1. The results clearly show that the combination of TiieOis (OEt) 32 cages and GO for the production of titania is especially beneficial for the improved catalytic performance of the titania catalysts.
Table 1. Properties measured on titania catalysts
Figure imgf000021_0001
b) Surface areas Surface areas were calculated using the Brunauer- Emmett-Teller (BET) method and are shown in Table 1. The lowest surface area values are reported for the titanias prepared with either Tii6Ois (OEt) 32 cages or Ti(OEt) 4, below 30 m2g_1, in agreement with their relatively poor performance in photocatalytic hydrogen production. The addition of GO increases the surface area, especially on the titania prepared with
iisOie (OEt) 32 cages which more than doubles reaching 62 m2g-1. For comparison, P25 titania has a surface area of 55 m2g_1, slightly lower. Comparison of the surface areas with the hydrogen production indicates that the higher the surface area the higher the hydrogen
production although the relation is not linear so other factors such as nanoparticle size and aggregate shape and size influence the catalytic performance of the titania catalyst. c) SEM and TEM images
SEM images were obtained by field emission gun scanning electron microscopy (FEG-SEM) LE01525 with an acceleration voltage of 5 kV. For better contrast in SEM, titania particles were deposited on highly doped Si-n++ wafer and GO on 300nm Si02 coated n-type Si wafer.
SEM images of the different titanias are shown in Figure 4. Titanias prepared with ΤίιεΟΐ6 (OEt ) 32 cages or Ti(OEt)4 have a particle size in the micrometer range, with large particles with a diameter > 5 μιη (Figures 4a and 4b) . However, the addition of GO results in much smaller titania particle sizes (Figures 4c and 4d) , especially for the titania prepared with Tii60i6 (OEt ) 32 cages and GO (Figures 4c and 4f) . The SEM image of P25 (Figure 4e) is also included for comparison.
TEM (transmission electron microscopy) imaging was carried out (by a JEOL JEM1200EXII) to reveal further insights into the shape and size of titanias . TEM images were analysed by ImageJ to calculate average particle size. TEM images of the different titanias are shown in Figure 5. Particles prepared with Ti(OEt)4 are too large for TEM, but a few edges could be imaged. TEM shows that Ti (OEt) 4-based titania consists of bulky particles containing rough surfaces with surface features below 5nm (Figure 5a) . On the other hand, titania prepared with TiiSOi6 (OEt) 32 cages consists of globular aggregates of 12+3 nm nanoparticles (Figure 5b) . When GO is used with Tii6Oie (OEt) 32 cages, the diameter of particles decreases down to 8+3 nm and they are mostly arranged as flakes similar in shape to the original GO flakes (Figure 5c) . These titania
nanoparticles are 3 times smaller than P25 titania particles (shown in Figure 5d for comparison) . The smaller particle sizes and consequently higher surface areas are in line with the higher photocatalytic hydrogen production rates. Moreover, their arrangement in two-dimensional flakes can help to minimize the light shielding and improve this way the photocatalytic performance. On the other hand, when GO is added to Ti(OEt)4, TEM shows that the resulting particles are very diverse including coated GO flakes, bulky
particles with nanostructured roughness and a few GO bundles (Figure 5e) . The presence of these GO bundles can explain the high light absorbance of this titania sample (see Figure 6 below) , which was not expected for the measured carbon content (0.5 wt%, Table 1) . d) Diffusion reflectance ultraviolet-visible
spectroscopy
Diffuse reflectance ultraviolet-visible spectroscopy was carried out on a UV/Vis spectrophotometer Perkin Elmer Lambda 35 with an integrating sphere. Figure 6 shows the UV-visible diffuse reflectance of the
different titanias prepared and P25 titania. It is assumed that most of the decrease in reflectance is due to absorption, rather than optical interference due to scattering. All samples absorb in the UV range due to the wide bandgap of titania. P25 titania is the most transparent in the visible range. The other titanias absorb in the visible range due to the carbon content (Table 1) . The absorbance spectra reveal that the calcination of Tii60i6 (OEt) 32 cages results in a more transparent titania than when using Ti(OEt)4 as a precursor for the catalysts. These results agree with the elemental analysis data (Table 1) , showing a purer titania when prepared from Τίιεθιβ (OEt ) 32 cages. This may be explained by the advantageous higher condensation degree in TiisOie (OEt ) 32 cages (0/Ti:l) compared to that in Ti(OEt)4 (O/Ti:0), or by the lower organic content in iisOie (OEt) 32 cages (0Et/Ti:2) compared to that in
Ti(0Et) (OEt/Ti:4). The addition of GO to the titania synthetic route also leads to more transparent and purer titanias, when being added to both precursors, Tii6Oie (OEt ) 32 cages and Ti(OEt)4. The titania prepared with TiisOie (OEt) 32 cages and GO obtains the most
transparent titania in the visible range, only overcome by P25 titania. This indicates that supporting the ii60i5 (OEt) 32 cages on GO, known for being two- dimensional and with high surface area, substantially facilitates the air exposure of titania precursors to air and therefore their oxidation, condensation and conversion to titania during the calcination.
It is demonstrated that the use of GO as sacrificial support could provide a viable strategy to simply calcine precursors for catalyst synthesis and obtain a metal oxide or a mixed metal oxide with nanostructured dimensions and optimized light exposure. It shows that it may be possible to synthesize two-dimensional materials with enhanced properties . The many oxygen groups in GO (epoxides, carboxylic groups, and
hydroxyls) provide anchoring sites for metallic oxo alkoxo cages, which can facilitate its uniform
dispersion/attachment on two-dimensional flakes and limit grain growth during the calcination. According to one embodiment of the present invention, GO is shown to decompose easily at 450 °C in 1 h in air, probably due to its two-dimensional shape with atomic thickness. Other carbon supports will not imprint two-dimensional shape, lack anchoring groups, and could require higher temperatures and/or longer calcination times for the combustion, which could compromise the surface area, particle size and dispersability in water. Therefore, there is an important equilibrium to attain between the sacrifice of the support and the metal oxide formation, for example, titania formation (i.e., the nucleation and growth of the metal oxide, for example, titania, or in other words, the oxidation of the metal precursor such as polyoxotitanium cages and crystallization) . The use of a cage with a high degree of condensation (O/Ti) favours the formation of fine titanias at low
temperature. Examples of other cages with high degree of condensation include Tii7024 (C^Pr) 20 and
Tii8027 (OH) (OtBu)i7. It is also equally important to tailor the chemistry of the cage and substrate (GO in this case) to promote the anchoring/support . e) Powder X-ray Diffraction Patterns and Raman
spectra
Powder X-ray diffraction (XRD) patterns in the 2Theta range 10-70° were measured on a Bruker D2 PHASER desktop diffractometer using Cu-Κ radiation, with a total integration time of 1232s. The result is shown in Figure 7. According to the result, the crystalline phase of titanias prepared with ii60i6 (OEt) 32 cages and from Ti(OEt)4 is pure anatase phase, unlike P25 titania which is approx. 80% anatase, 20% rutile. It is
generally believed that P25 titania and other rutile- anatase titanias give better photocatalytic performance over single anatase titanias due to two factors (1) the slower recombination resulting from charge
stabilization by electron transfer between phases and (2) the smaller band gap of rutile allows more
photoexcitation by visible light. However, it is noted here that the titania catalyst produced by the method of the present invention, which is a pure anatase titania, outperforms mixed-phase P25 titania in the photocatalytic decomposition of water. This evidences that other factors such as morphology, particle size, dispersability, and surface area can have a profound effect on the final performance of titania catalyst. Practically no GO is found on the SEM or TEM inspection of titania prepared with a mixture of
Tii60i6 (OEt) 32 cages and GO. It shows that under the current calcination conditions, essentially all GO have gasified to oxidized species such as C02 and CO. The absence of graphene derivatives is further confirmed by Raman spectroscopy (Figure 8, For a clearer comparison, the origin of the y axis of each distribution has been shifted by the indicated values) . However, when GO is used with Ti(OEt)4, TEM shows higher survival of GO probably due to the higher organic content in Ti(OEt)4 compared to Τίιεθιε (OEt) 32 , which must favor a less oxidizing atmosphere during calcination. According to elemental analysis and SEM/TEM imaging no more 30% of the original GO survives in the Ti(OEt)4 - GO based titania.
SEM images, Raman spectroscopy and XRD practically show no signs of GO or its reduced version (reduced GO, RGO) in the titania prepared with Tii60i6 (OEt) 32 and GO which outperforms the rest of titanias tested. Wrinkled flakes are easily observed by SEM in those composites or hybrids and Raman signals assigned to C sp2. RGO is not completely transparent, so it compromises the light irradiation of the photocatalyst composites or hybrids, but it is reported that using limited amounts (<3%wt.) the shielding effect is overcome by minimized electron- hole recombination. In the present invention, GO is used as a sacrificial support for the synthesis of the metal oxide, for example, the titania catalysts. It appears that GO was not needed in its reduced form to obtain a final titania with enhanced photocatalytic performance. GO' S role is to support or template the formation of nanostructured (<10 nm) particles from iieOi6 (OEt) 32 decomposition and their arrangement on flakes or layers, which maximizes the available surface for light irradiation and photocatalysis . It is
believed that TiisOis (OEt) 32 cages have anchored on GO flakes by the substitution of the labile terminal ethoxides with the hydroxyls on GO basal planes and carboxylic groups on GO edges. Anchoring on GO ensures proper supporting/arrangement of the cages on two dimensions, avoiding dominant arrangement into globular structures during calcination where much resulting titania would be shielded. GO can therefore be seen as promoting a heterogeneous nucleation of titania when in THF suspension leading eventually to a high density of very small crystals. Here it is demonstrated that GO offers control at two levels. On one hand, it controls the nucleation and growth of the nanocrystals, lowering its size down to 8+3 nm. On the other hand, it shapes the aggregates of these nanocrystals into 2 -dimensional flakes, an advantageous shape for photocatalysis. f) Particle size distributions of titanias in
suspension after photocatalytic tests Together with the surface area and particle size, the shape and size of particle agglomerates in the reacting suspensions are also critical in determining the photocatalytic activity because they strongly affect the light scattering and absorption properties. The particle size distributions of titanias in
suspension after the photocatalytic tests were measured by laser beam scattering technique using a Malvern Mastersizer 2000 particle size analyzer. The number and volume weighted particle size distributions are shown in Figures 9a and 9b, respectively. The size range of analysis for laser beam scattering is typically above 0.2 μι. The number-weighted particle size distribution shows that titanias prepared with Tii6OiS (OEt) 32 cages contain in suspension particles or agglomerates above 0.25 μιτι particles with a maximum at 0.36 μπι (Figure 9a) . Titania prepared with Ti(OEt)4 shows, however, all particles above 1 um with a maximum at 2.2 μπι. Adding
GO to Ti(OEt)4 reduces the particle or agglomerate size, obtaining comparable results to those on titanias prepared with Τχιεθΐδ (OEt ) 32 cages. On the other hand, P25 titania shows a bimodal distribution centered at 0.41 and 2.2 μτη. These results are in agreement with the SEM and TEM imaging.
In a number-weighted distribution each particle is given equal weighting irrespective of its size.
However, in a volume-weighted distribution the
intensity is proportional to (radius)3, so larger particles hold higher weight than smaller ones in the distribution. Figure 9b shows the volume-weighted particle or agglomerate size distributions in
suspension. All titanias, including P25, contain
agglomerates, or particles, in the micrometer range with main size distribution centered between 5 and 50 um. P25 titania nanoparticles are aggregated to
agglomerates with a broad size distribution centered at 6 μτη. Titania prepared with Tii60i6 (OEt ) 32 cages contains particles or agglomerates with a wide distribution centered at 15 μιη and smaller particles or agglomerates down to 0.25 μπι. When GO is added to Tii60i6 (OEt ) 32 cages, the resulting titania agglomerate or particle size decreases from 15 to 5 um, while keeping particles down to 0.25 μπι . On the other hand, titania prepared with Ti(OEt)4 has no particles below 1 μτη , indicating that TiiSOi6 (OEt) 32 is a better precursor than Ti(OEt)4 for the formation of finer titania. Adding GO to Ti(OEt)4 has also a positive effect: it lowers the minimum particle size from 1 μπι to 0.25 m. There are however some agglomerates with size distribution centered at 45 μτη . All titanias, except P25 titania, have a few
agglomerates above 100 um.
The Z -potential, or electrokinetic potential, is related to the colloidal properties of particle
suspensions and the particle size. Suttiponparnit et al . in Nanoscale Res. Lett. 2011, 6:27 demonstrated in a set of titanias with different particle sizes that the Z -potential becomes more positive for smaller particles at any pH. Z -potentials of titanias in methanol aqueous suspensions were measured by a Malvern Zetasizer Nano ZS .
Titania prepared from both Ti(OEt)4 and
Τίι60ΐ6 (OEt) 32 cages in the 20%vol. methanol aqueous suspensions (neutral pH) have a Z-potential around -30 mV, which becomes more positive when GO is used in the synthesis (Table 1) . In the case of titania
prepared from Τίιεθιε (OEt) 32 and GO, the Z-potential is around 0 mV, like that of P25 titania. This shows a clear effect of GO as sacrificial support on the final colloidal properties of resulting titania and in decreasing the particle size. Whilst the present invention has been described and illustrated with reference to particular
embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein.

Claims

Claims
1. A method of making a metal oxide catalyst
comprising:
a) contacting a metal precursor with graphene oxide in the presence of a solvent to provide a graphene oxide supported metal precursor, wherein the metal precursor comprises one or more transition metals;
b) removing the solvent;
c) calcining the graphene oxide supported metal precursor in an oxidizing atmosphere to remove the graphene oxide and generate the metal oxide catalyst.
2. The method of claim 1, wherein the metal precursor comprises a metal selected from the group consisting of
Titanium, Zinc, Zirconium, Tungsten, Iron, and
combinations thereof.
3. The method of claim 2, wherein the metal precursor comprises Titanium.
4. The method of claim 3, wherein the metal precursor comprises polyoxotitanium cages.
5. The method of claim 5, wherein the poloxotitanium cages comprise TiieOis (OEt) 32.
6. The method of any one of claims 1 to 5 , wherein in step b) the solvent is removed by heat treatment.
7. The method of claim 6, wherein the heat treatment is performed at a temperature in the range of from 40 °C to 100 °C.
8. The method of any one of claims 1 to 7, wherein the solvent is anhydrous tetrohydrofuran .
9. The method of any one of claims 1 to 8 , wherein the graphene oxide supported metal precursor contains less than 1 wt% of solvent after step b) .
10. The method of any one of claims 1 to 9, wherein the calcination of step c) is performed in air.
11. The method of any one of claims 1 to 10, wherein in step c) the temperature is increased by a ramp rate to a plateau temperature and the plateau temperature is in the range of from 350 °C to 600 °C.
12. The method of claim 11, wherein the plateau
temperature is in the range of from 350 °C to 500 °C.
13. The method of claim 11 or 12, wherein the ramp rate is less than 15 °C per minute.
14. The method of any one of claims 1 to 13, wherein after step c) the metal oxide catalyst produced contains essentially no graphene oxide.
15. A metal oxide catalyst produced by the method of any one of claims 1 to 14.
16. A metal oxide catalyst of claim 15, wherein the metal oxide catalyst is a catalyst selected from the group consisting of titania, zinc oxide, zirconia, tungsten oxide, iron oxide and combinations thereof, for example, the metal oxide catalyst is a titania catalyst.
17. A metal oxide catalyst of claim 15 or 16, wherein the metal oxide catalyst is in the form of flakes.
IS. A titania catalyst comprising titania, wherein the titania comprises no less than 85 wt% of anatase titania and wherein the titania has a size in the range of from 5 to 15 nm as measured by TEM.
19. The titania catalyst of claim 18, wherein the titania is in the form of flakes.
20. The titania catalyst of claim 18 or 19, wherein the titania catalyst comprises less than 1 wt% graphene oxide or graphene oxide in its reduced form.
21. Use of the metal oxide of any one of claims 15 to 17 or use of the titania catalyst of any one of claims 18 to
20 in a catalytic reaction.
22. Use of the metal oxide of any one of claims 15 to 17 or use of the titania catalyst of any one of claims 18 to 20 according to claim 21, wherein the catalytic reaction is the photocatalytic reduction of water with a hole scavenger .
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