SE1450912A1 - Method for modifying palladium-gold catalyst, in particularfor hydrodechlorination of tetrachloromethane, and palladium-gold catalyst with suitable structure of the active metal phase produced thereby - Google Patents

Abstract

Abstract The invention relates to a method for modifying palladium-gold catalyst, in particular forhydrodechlorination of tetrachloromethane, characterised in that a) after prior reduction ofthe initial supported palladium-gold Catalyst, palladium not bound to the Pd-Au alloy is beingremoved using nitric(V) acid, wherein the said alloy is the active metal phase of thesupported palladium-gold catalyst, and subsequently b) the cata|yst's metal active phase isreduced in flow of a HZ/Ar reducing mixture with formation of the modified supportedpalladium-gold catalyst, wherein the amount of palladium and gold in the Catalyst resultsfrom the Au/Pd atomic ratio of 1.44 before the washing step, whereas for the catalyst obtained after modification the Au/Pd atomic ratio ranges from 1.50 to 2.00. ln addition, the present invention provides a palladium-gold catalyst obtained withthe method disclosed above, so that it comprises a suitable structure of the metal phase, inwhich palladium is contained exclusively in the cata|yst's Pd-Au alloy, which means that thecatalyst is free from palladium that is not bound to gold, and the Au/Pd atomic ratio in a so modified supported palladium-gold catalyst is from 1.50 to 2.00. Fig. 3 (8 claims)

Description

Method for modifying palladium-gold catalyst, in particular for hydrodechlorinationof tetrachloromethane, and palladium-gold catalyst with suitable structure of the active metal phase produced thereby The subject matter of the present invention is a method for modifying palladium-goldcatalyst, in particular for hydrodechlorination of tetrachloromethane, and palladium-goldCatalyst with suitable structure of the active metal phase produced thereby. ln particular, theinvention relates a method for improving the activity of bimetallic Pd-Au/C catalysts andenhancing their resistance to poisoning during hydrodechlorination of tetrachloromethane byremoving from these catalysts the undesired monometallic palladium phase, i.e., palladium particles which do not form the Pd-Au alloy.

Catalytic CCL; hydrodechlorination to hydrocarbons is a promising method fordisposal of CCI4, which is a toxic and carcinogenic contaminant of air and water. Obtaining of an active, long-lived Catalyst is a key issue for practical application of the process.

A key role, both in the development of practical applications of heterogeneouscatalysis, and the development of theoretical concepts in this field, is played by metals,which represent the active phase of heterogeneous catalysts. One of the mostcomprehensively studied and reported metals is palladium, which displays exceptionalcatalytic performance in a series of very diverse reactions (e.g., CO oxidation, alkane andcycloalkane dehydrogenation, Fischer-Tropsch Synthesis, isomerisation and hydrogenolysisof hydrocarbons, hydrodesulphurisation, hydrodechlorination, selective hydrogenation of unsaturated hydrocarbons, and many others). lt has been shown for many reactions that alloys of two active metals (or alloys of anactive metal with an inactive one) have a favourable effect on the course of catalysedreactions. This can be exemplified by rhodium-stabilised platinum gauzes used as catalysts inoxidation of ammonia to nitrogen oxide. Alloy catalysts are, for instance, used in the refiningindustry (promotion of platinum reforming catalysts by lr, Re, Ge, Sn) and in petroleumindustry (promotion of palladium catalysts for selective hydrogenation of unsaturated hydrocarbons using Sn, Au, Cu, and Pb). From the abundant group of alloy catalysts, Pd-Au catalysts are distinguished by their exceptional activity in many diverse reactions, often ofgreat industrial importance, as exemplified by: vinyl acetate Synthesis [Y. F. Han, J. H. Wang,D. Kumar, Z. Yan, D. W. Goodman, J. Catal., 2005, 232, 467], low temperature CO oxidation[F. Gao, Y. L. Wang, D. W. Goodman, J. Am. Chem. Soc., 2009, 131, 5734; F. Gao, Y. L. Wang,D. W. Goodman, J. Phys. Chem. C, 2009, 113, 14993; and J. Xu, T. White, P. Li, C. H. He, J. G.Yu, W. K. Yuan, Y. F. Han, J. Am. Chem. Soc., 2010, 132, 10398], direct H2O2 Synthesis from H2and 02 [J. K. Edwards, B. E. Solsona, P. Landon, A. F. Carley, A. Herzing, C. J. Kiely, G. J.Hutchings, J. Catal., 2005, 236, 69; B. E. Solsona, J. K. Edwards, P. Landon, A. F. Carley, A.Herzing, C. J. Kiely, G. J. Hutchings, Chem. Mater., 2006, 18, 2689; J. K. Edwards, A. F. Carley,A. A. Herzing, C. J. Kiely, G. J.Hutchings, Faraday Discuss., 2008, 138, 225; and J. K. Edwards,E. Ntainjua, A. F. Carley, A. A. Herzing, C. J. Kiely, G. J. Hutchings, Angew. Chem., Int. Ed.,2009, 48, 8512], hydrogenation of hydrocarbons [A. Sárkány, O. Geszti, G. Sàfrán, Appl.Catal., A, 2008, 350, 157; B. Pawelec, A. M. Venezia, V. La Parola, E. Cano-Serrano, J. M.Campus-Martin, J. L. G. Fierro, Appl. Surf. Sci., 2005, 242, 380; and A. Hugon, L. Delannoy, J.-M. Krafft, C. Louis, J. Phys. Chem. C, 2010, 114, 108231, acetylene trimerisation [A. F. Lee, C.J. Baddeley, C. Hardacre, R. M. Ormerod, R. M. Lambert, G. Schmid, H. West, J. Phys. Chem.,1995, 99, 6096; C. J. Baddeley, R. M. Ormerod, A. W. Stephenson, R. M. Lambert, J. Phys.Chem., 1995, 99, 5146; C. J. Baddeley, M. Tikhov, C. Hardacre, J. R. Lomas, R. M. Lambert, J.Phys. Chem., 1996, 100, 2189; and J. Storm, R. M. Lambert, N. Memmel, J. Onsgaard, E.Taglauer, Surf. Sci., 1999, 436, 259.1, hydrodechlorination of chlorine-containingcontaminants polluting groundwater [IVL O. Nutt, K. N. Heck, P. Alvarez and M. S. Wong,Appl. Catal., B, 2006, 69, 1151.

According to the definition given by Ponec and Bond [V. Ponec, G.C. Bond, Catalysisby Meta/s and Alloys, Elsevier, Amsterdam, 4, 1995], an alloy is deemed any metallic systemcomposed of two or more components, irrespective of how the atoms of each componentare distributed with respect to each other. Atomic radii of metals, crystal structure andthermodynamic aspects determine the type of the alloy formed by the metals. lf, forinstance, two metals differ in atomic radii by not more than about 10 - 15%, they have thesame crystal structure and when the a|loy's enthalpy of formation is very small, the metals would form a continuous series of solid solutions - single phase alloys, in the entire concentration range. For instance, this is the case for Pd-Ag and Pd-Au systems where both metals form alloys with the properties of ideal solution. ln catalytic hydrodechlorination of organic compounds, includingtetrachloromethane, palladium shows very high initial activity that drops drastically due tothe catalyst deactivation during the catalysed process. From the literature review it can beconcluded that modification of a monometallic catalyst with another metal can bothenhance the catalyst's resistance to deactivation during the reaction [B. Coq, S. Hub, F.Figueras, D. Tournigant, Appl. Catal A: General, 101, 41, 1993], and have a favourable effecton the selectivity to the desired reaction product [R. Ohnishi, I. Suzuki, M. lchikawa, Chem.Lett., 841, 1991; R. Ohnishi, W.-L. Wang, lvl. lchikawa, Appl. Catal. A: General, 113, 29, 1994;and E.I. DuPont de Nemours & Co., US Patent 5.629.462] Gold-promotion of catalystscontaining Pd, Ru, Rh, Os, lr, and Pt resulted in enhanced lifetime and mechanical strength,and prevented from sintering of the catalysts' metal phases. [E.l. DuPont de Nemours & Co.,US Patent 5.447.896] For bimetallic catalysts for hydrodechlorination of halogenatedhydrocarbons, the metals most frequently added to Pd are as follows: Au [E.|. DuPont deNemours & Co., US Patent 5.629.462, and S. Morikawa, S. Samejima, M. Yositake, S.Tatsematsu, European Patent 0347830 A2, 1989], Ag [B. Coq, S. Hub, F. Figueras, D.Tournigant, Appl. Catal A: General, 101, 41, 1993; R. Ohnishi, W.-L. Wang, M. lchikawa,Appl. Catal. A: General, 113, 29, 1994; and B. Heinrichs, P. Delhez, J.-P. Schoebrechts, J.-P.Pirard, .l. Catal., 172, 322, 1997], and Fe [B. Coq, S. Hub, F. Figueras, D. Tournigant, Appl.Catal A: General, 101, 41, 1993,' N. Lingaiah, M. A. Uddin, A. Muto, Y. Sakata, Chem.Commun., 1657, 1999; and W. X. Zhang, C. B. Wang, H. L. Lien, Catal. Tod., 40, 387, 1998].The effects of the following metals were also studied: K [B. Coq, J. M. Cognion, F. Figueras, D.Tournigant, J. Catal., 141, 21, 1993], TI [R. Ohnishi, W.-L. Wang, M. lchikawa, Appl. Catal. A:General, 113, 29, 1994], Co [B. Coq, S. Hub, F. Figueras, D. Tournigant, Appl. CatalA: General,101, 41, 1993], Cu [R. Ohnishi, W.-L. Wang, M. lchikawa, Appl. Catal. A: General, 113, 29,1994], Rh [Bodnariuk, P., Coq, B., Ferrat, G., Figueras, F., J. Catal., 116, 459, 1989], and Sn[Bodnariuk, P., Coq, B., Ferrat, G., Figueras, F., J. Catal., 116, 459, 1989].

Favourable changes in catalytic performance of palladium alloys with another metal(usually much less active in a given reaction) as compared with those of palladium are explained by geometrical and/or electronic factors. The literature on catalytic activity of Pd- Au alloys has been comprehensively reviewed by Feng and Goodman [G. Feng, D. W.Goodman, Chem Soc Rev.]. For a Pd-Au system, the diluting of the component which is activein the catalytic reaction (Pd) by the inactive component (Au) is of essential importance forcatalytic performance. The diluting usually results in a poorer adsorption of reactants andintermediates on the catalyst, which in turn may increase the selectivity to the desiredreaction products. Very often, this enhancement of selectivity is achieved at the expense ofthe alloy catalyst activity, which is lower than that of the monometallic palladium catalyst.Not always, however, adding gold to palladium results in a lower catalytic activity of thelatter, and in some cases an increase in Pd activity was observed following Pd-Au alloyformation. This is explained by a lower poisoning of bimetallic Pd-Au catalysts by reactants and reaction products.

For supported palladium catalysts, which are used in practice for hydrodechlorinationof organic compounds, the most important factors determining the properties of these catalysts are as follows: 0 composition of the Pd-Au metallic phase (whereas the surface compositionmay differ from that inside the crystallites [R. Bouwman, W. M. H. Sachtler, J. Catal., 19,127, 1970.]), segregation of its components, dispersion of metal particles (most frequently10 + 60 % metal atoms represent the surface), crystallite size (mostly 2 + 20 nm), location of the metal in pores of the support; 0 support type and structure -in most cases the supports of palladium catalystsare various types of active carbons; in addition, e.g., AlzOg, Si02, CaC03, BaSO4 are used. Thefollowing support parameters are important: particle size (usually 1 + 100 pm), specificsurface area (100+ 1500 mz/g), pore structure (pore volume and distribution), acid-base properties; 0 concentration of the metallic phase - for the most of palladium catalysts it isin the range of 0.5 + 5 wt% of metal, although it happens that much higher concentration must be used; 0 method of preparation.

The primary problem in preparing bimetallic catalysts is to achieve a close contact of the two components, allowing for formation of bimetal particles. The method and conditionsof preparation have significant effect on the nature of the association between Pd and theother metal forming the active phase of the support catalyst, and on the special distributionof the catalyst's active sites. ln addition, the method of preparation should meet otherrequirements: the catalyst obtained after preparation should show a uniform distribution ofthe metal particles of strictly controlled size on the support and display a high activity and selectivity, as well as a long lifetime in the reaction conditions.

The following essential steps of preparation can be distinguished: 0 suitable preparation of the selected support (purification, etching, drying, calcination), 0 simultaneous or two-stage deposition of precursors of both metals on the support, whereas the deposition can be non-selective or selective, 0 transformation of precursors into the final active phase (drying, calcination, reduction). ln the non-selective deposition of precursors, the interaction between twoprecursors is significantly weaker than that between them and the support, so the palladiumand gold precursors are located in separate places on the support surface. Therefore,surface diffusion of precursors during the catalyst activation is necessary for the formationof bimetallic Pd-Au particles. Usually it is difficult to achieve sufficiently good intermixing ofboth metal components, and as a result, both monometallic particles and a broad range ofdifferently composed bimetallic particles can be found on the surface of the reduced catalyst.

The simplest and at the same time the most frequently used method for producingbimetallic Pd-Au support catalysts is to impregnate the support using salts of both metals(simultaneously or consecutively), foilowed by calcination and reduction of the catalyst. Thisis, however, a non-selective method, with the primary drawback being the non-homogeneityof the active phase with respect to the size of metal particles, as well as to their compositionand shape. As a result, not only the particles of Pd-Au alloy phases (with differentcompositions of both metals) are present on the support, but also monometallic Pd and Au particles [Xu, T. White, P. Li, C. H. He, J. G. Yu, W. K. Yuan, Y. F. Han, J. Am. Chem. Soc., 2010, 5 132, 10398; J. K. Edwards, B. E. Solsona, P. Landon, A. F. Cariey, A. Herzing, C. J. Kiely, G. J.Hutchings, J. Catal., 2005, 236, 69; B. E. Solsona, J. K. Edwards, P. Landon, A. F. Carley, A.Herzing, C. J. Kiely, G. J. Hutchings, Chem. Mater., 2006, 18, 2689; J. K. Edwards, A. F. Carley,A. A. Herzing, C. J. Kiely, G. J. Hutchings, Faraday Discuss., 2008, 138, 225; and J. K. Edwards,E. Ntainjua, A. F. Carley, A. A. Herzing, C. J. Kiely, G. J. Hutchings, Angew. Chem., Int. Ed.,2009, 48, 8512,' M. Venezia, V. La Parola, G. Deganello, B. Pawelec J. L. G. Fierro, J.Catal.,2003, 215, 317].

Apart from impregnation-based techniques a number of other methods forproducing bimetallic Pd-Au support catalysts are used, for instance: deposition of precursorsusing ion exchange method and chemical grafting method, reversed micelle method, metal-organic complex decomposition, ”sol-gel” method, application of ,,polyol” processes,photolytic reduction, direct and indirect redox reaction. Combination of each of thesemethods with further processing of catalysts (drying, calcination, reduction, sintering) allowsto some extent for control over the size, composition and structure of Pd-Au alloy nanoparticles on the Catalyst support.

Problem: ln spite of the development of methods for producing supportedpalladium-gold catalysts, it happens very often that even if highly advanced preparationtechniques are used, apart from the Pd-Au alloy phase of the desired composition, thecatalyst contains palladium particles, which contribute to a poorer catalytic performancewhen the catalyst is used in hydrodechlorination of tetrachloromethane. Modification ofsupported palladium catalysts by gold significantly improves their catalytic performance inhydrodechlorination of tetrachloromethane, the necessary conditions to achieve high activity and resistance to poisoning are yet as follows:- sufficiently good homogeneity of the Pd-Au alloy, as the catalyst active phase;- absence of palladium particles in the catalyst, apart from the alloy.

The method disclosed in the present patent application allows for significant andeffective improvement of the catalyst performance, if the method employed to prepare thecatalyst does not result in a fully satisfying intermixing of palladium with gold and formation of the Pd-Au alloy as the active phase of the catalyst.

According to the present invention, the method for modifying palladium-gold Catalyst, in particular for hydrodechlorination of tetrachloromethane, is characterised in that a) after prior reduction of the initial supported palladium-gold catalyst, palladium notbound to the Pd-Au alloy is being removed using nitric(V) acid, wherein the said alloy is the active metal phase of the supported palladium-gold catalyst, and subsequently b) the catalyst's metal active phase is reduced in flow of a HZ/Ar reducing mixture with formation of the modified supported palladium-gold Catalyst, wherein the amount of palladium and gold in the catalyst results from the Au/Pdatomic ratio of about 1.44 before the washing step, whereas for the catalyst obtained after modification the Au/Pd atomic ratio ranges from 1.50 to 2.00.

Preferably, in the method according to the present invention, a palladium-goldcatalyst on a very pure active carbon support, more preferably synthetic active carbon ofmesoporous structure and trace elements content <10 ppm, including phosphor and sulphur <2 ppm, is used as the initial supported palladium-gold catalyst.

Preferably, in the method according to the present invention, a 8 - 10% nitric(V) acidsolution, more preferably a 10% nitric(V) acid solution, is used in washing palladium from the catalyst.

Preferably, in the method according to the present invention, the process of washing palladium from the catalyst lasts for 35 - 45 hours, preferably 40 hours.

Preferably, in the method according to the present invention, the nitric(V) acidsolution per catalyst mass ranges from 30 to 50 g acid solution per 1 g of the catalyst, more preferably 40 g per 1 g of the catalyst.

Preferably, in the method according to the present invention, prior to drying andreduction steps, the catalyst is purified by washing with distilled water, preferably deionised water, followed by drying with stirring for 20 - 40 hours, preferably 24 hours, attemperature 30°C - 40°C, and then for 2 hours in argon flow of 40 to 150 cm3/hour,preferably 100 cm3/hour, preferably at temperature from 110° to 130°C, and more preferably at 120°C.

Preferably, in the method according to the present invention, the reduction is carried out in a stream of reduction mixture H2 in Ar, preferably from 30 to 70% H2 in Ar, and morepreferably 50% H2 in Ar, at a temperature increase from 1°C to 8°C/minute, more preferably4°C/minute from 20 to 390°C, with temperature 390°C maintained for 2 hours, and reduction mixture flow from 40 to 150 cm3/hour, preferably 100 cm3/hour. ln addition, the present invention provides a palladium-gold catalyst obtained withthe method disclosed above, so that it comprises a suitable structure of the metal phase, inwhich palladium is contained exclusively in the cata|yst's Pd-Au alloy, which means that thecatalyst is free from palladium that is not bound to gold, and the Au/Pd atomic ratio in a so modified supported palladium-gold catalyst is from 1.50 to 2.00.

The present invention is now explained more in detail in preferred embodiment, withreference to the accom panying figures, wherein: Fig. 1 shows a flow reactor for preparation of supported bimetallic Pd-Au catalysts; Fig. 2 shows changes in catalytic activity of the catalysts during CCI.,hydrodechlorination, reaction temperature 90°C, Hz/CCL; partial pressureratio "13.4, and Fig. 3 shows diffraction profiles of a 6.1% PdflAusg/C catalyst reduced at 390°C, before and after washing procedure with nitric(V) acid.

Preferred embodiment Reagents and equipment Catalyst support: Síbunit synthetic active carbon of verV High purity, specific surface area387 mz/g, pore volume 0.75 cma/g, average pore size “7 nm, dominant share of mesopores. manufacturer - Boreskov Institute of Catalysis, Novosibirsk.Reagents and gases used for preparation and testing of catalysts:- PdClz (precursor of the metal phase), analytical purity. Manufacturer - POCh Gliwice - Ni-i4AuCl4*H2O (precursor of the metal phase), spectral purity. Manufacturer -Johnson Matthey, Chemicals Limited - HCI, 35-38%, density 1.19 g/cma, analytical purity. Manufacturer - Chempur, Piekary šlaskie8 - HF, 40%, density 1.13g/cm3, analytical purity. Manufacturer - POCh Gliwice - HNOg, 65%, density 1.53 g/cm3, analyticai purity. Manufacturer - Chempur, Piekary šlaskie- Hydrogen, purity 99.999% - Argon, purity 99.999% - Tetrachloromethane, analytical purity. Manufacturer - POCh Gliwice.

Preparation of catalysts:1. Preparation of Catalyst support: - preparation of Sibunit active carbon in a mixture of 5% HF and 15% HCI for 3 hours, 500cm3 acid mixture per 60 g carbon was used; - washing the acids out of the sample with hot distilled water until no Cl* ions were found inthe filtrate; - drying in air at 50°C for 20 hours., and then for 20 hours at 60°C.2. Preparation of monometallic 3 wt% Pd/Csibuni, catalyst.

The method for preparation procedure - capillary impregnation (the amount of theimpregnating solution matched the pore volume of the support used). The impregnatingsolution was a H2PdCl4 solution, obtained by dissolving PdClg (in an amount following fromthe assumed 3 wt% metal content in the cataiyst) in distilled water and adding concentratedHCI, so as to have the CF/Pd” molar ratio of 3.89. Following impregnation, the preparationwas dried for twenty-four hours in a device allowing for mixing and simultaneously forwarming it up with a radiant lamp. After further drying at 120°C for 8 hours, the Catalyst wasreduced under fluidized bed conditions in a 50% Hz/Ar flow, at linearly increasingtemperature 4°C/min from 20°C to 390°C, and then the temperature 390°C was maintained for 2 hours.

Usually, the palladium concentration in a so prepared monometallic palladiumcatalyst is from 1 to 5 wt%. ln the preferred embodiment, a catalyst with 3% palladium content was used. 3. Preparation of a bimetallic 6.1 wt% PduAusg/Csibunn catalyst (Pd/Au atomic ratio =41/59).

The method of preparation comprised a contact of gold salt (in aqueous solution)with the reduced parent palladium 3 wt% Pd/Csibuni, Catalyst. The catalyst was prepared in the reactor depicted in Fig. 1.

A weighted portion (about 5 g) of initial 3 wt% Pd/Csibunit catalyst was placed in areactor used for preparation of the Catalyst, flushed in argon flow (100 cms/min, roomtemperature, 1 hour), and then the temperature was increased to 120°C at a rate of2°C/min. The employed gas flow resulted in bead fluidisation. The temperature 120°C andthe argon flow were maintained for another 1 hour, and subsequently, the gas flowingthrough the reactor was changed to hydrogen and the Catalyst was reduced in a hydrogenflow for 1 hour at 120°C. After completed reduction, the gas was again changed to argon,and the catalyst was cooled down to room temperature in argon flow. Under continuousmixing of the reactor content with argon, the palladium catalyst was poured over withdistilled water (three times more water than the mass of the catalyst), and then aNH4AuC|4*H2O aqueous solution (0.124 wt%) [C. Micheaud, M. Guerin, P. Marecot, C. Geron,J. Barbier, J. Chim. Phys.,93, 1394, 1996] was dropped in for 10 min, in an amount matchingthe assumed content of gold in the bimetallic catalyst. After the gold salt solution wasentirely added drop by drop, the reaction was continueci for another 10 min., and theprogress of the reaction was observed as a gradual decolourisation of solution in the reactor.Subsequently, the catalyst was filtered off, washed with distilled water, pre-dried (identicallyas the initial palladium catalyst) and reduced in a 20% Hz/Ar flow, flow rate 50 cma/min, atlinearly increasing temperature (4°C/min) from 20°C to 390°C. The reduction temperature390°C was maintained for another 2 hours, and finally the catalyst was cooled down to room temperature in argon flow.

Determination of the dispersion of the metal phase dispersion and the mixing level of the Pd-Au alloy To determine the dispersion of the metal phase deposited on the support the following chemisorption measurements were performed:- pulse measurements (H2 pulses in argon), in a flow apparatus, - steady-state measurements, using CO as the adsorbate, in a Micromeritics ASAP2020 analyser.

The aforementioned measurements were complemented with the X-ray diffractionmeasurements of catalysts that allowed for estimation of the size of the metal phasepartícles and the mixing level of the Pd-Au alloy. The diffraction profiles (in the range of 26from 20° to 90°, with a 0.05°/15 s interval) were obtained with a diffractometer (Rigaku- Denkí, Japan) with a CuKa (Philips, Holland) lamp as the radiation source, using a nickel filter.

The sizes of the metal particles, calculated from the chemisorption measurements, and the conclusions from the x-ray studies are shown in Table 1: Table 13% Pd/Csib Pd41AU59/C5ibH2 chemisorption Crystallites Pd 8.3 nm Crystallites Pd-Au 18.4 nmCO chemisorption Crystallites Pd 5.6 nm Crystallites Pd-Au 11.8 nmCoexistence of a few Pd-Au alloycrystallites Pd “'4n m, fraction phases of different composition,X-ray diffractionwith lower dispersion, very with the dominant Pd15-Au85studiesweakly visible phase and a very weakly visible,highly dispersed palladium phase The above data indicate that in spite of employing a selective method for preparationof the bimetallic catalyst, contributing to formation of the Pd-Au alloy phase, the intermixingof both metals was not ideal, and additionally, some fraction of palladium did not form the alloy.

Characterisation of catalytic performance of catalysts in hydrodechlorination of tetrachloromethane After reduction, the catalysts were tested in hydrodechlorination oftetrachloromethane in the gas phase. The reaction was carried out at 90°C underatmospheric pressure in a flow apparatus described earlier [M. Bonarowska, J. Pielaszek,VA. Semikolenov, Z. Karpiriski, J. Catal., 209 (2002) 528, and M. Bonarowska, J. Pielaszek, W.Juszczyk, Z. Karpiriski, J. Catal., 195 (2000) 3041, at a reactants flow rate 30 cma/min and 11 HzfCClq partial pressure ratio "13.4 (601/447 kPa). The test reaction lasted for "TO hours.The reaction products were analysed with a gas chromatograph (HP5890 series ll, Hewlett Packard, USA) with flame ionisation detector.

The initially catalytically very active palladium catalyst (conversion within the first fivehours close to 100%) deactivated very rapidly due to poisoning. After 70 hours of reaction,the conversion dropped down to a very low level ”3%, whereas the final selectivity to hydrocarbons desired in the reaction was "30%.

Modification of the palladium catalyst by gold, according to the preparationprocedure disclosed above, resulted in a significant improvement of its catalyticperformance. Initially high Conversion of "100% was maintained for about 10 hours, butafter that time the catalyst was deactivated as well: that final conversion dropped down to30%, with the selectivity to hydrocarbons of about 70%. The conversion changes reported for both catalysts are shown in Fig. 2.

The present inventlon relates to removal of palladium particles remaining outside thealloy from a bimetallic catalyst (containing a suitably composed Pd-Au alloy as the activephase), as it is the palladium not bound in the alloy which causes the catalyst deactivationand poisoning by undesired reaction products during the hydrodechlorination oftetrachloromethane. Because the catalyst's high activity and poisoning resistance result fromthe presence of the homogeneous Pd-Au alloy as the catalyst active phase, the assumed procedure of palladium removal does not result in removal of the alloy.

Palladium, which does not form the alloy, was removed from the catalyst by mixingthe 6.1% PdluAusg/Csib catalyst (reduced earlier at 390°C according to procedure disclosedabove) with a 10% nitriclv) acid solution, wherein 20 cms acid solution was used per 0.5 gcatalyst. Washing palladium from the catalyst Iasted for 40 hours, followed by the removalof the liquid phase, and the catalyst was washed with many portions of distilled water untilneutral reaction of the filtrate was obtained. After pre-drying (twenty-four hours, radiant lamp and stirring), and primary drying in argon flow (2 hours, 120°C), the catalyst was reduced in a 50% Hz/Ar flow at 390°C for 2 hours, in a reactor of a volume of about 3 cm3.

Following the Washing and reduction procedure of palladium, the catalyst was tested in the hydrodechlorination reaction of tetrachloromethane. lt was found that the method 12 for the catalyst treatment assumed in testing essentially improved its catalytic activity. Theinitial high conversion of "100% was maintained for about 45 hours (almost five times longerthan for the catalyst before washing), and after 70 hours of reaction the catalyst was onlyslightly deactivated, as the final conversion was as high as “94%, with preserved selectivityto hydrocarbons at the level about ”70%. When the disclosed procedure was applied to amonometallic 3% Pd/Csib palladium catalyst, its catalytic activity after 70 hours reaction wasvirtually the same as for a catalyst, which was not washed with the acid, i.e., "3%, with avery similar level of selectivity to hydrocarbons ”30%. The conversion changes reported for both catalysts are shown in Fig. 2.

The effectiveness of removing palladium which does not form the Pd-Au alloy fromthe catalyst was confirmed by the analysis of the x-ray profiles. Fig. 3 compares he profilesfor a 6.1% Pd41Au5g/Csib catalyst reduced at 390°C, washed and not washed with the nitric(V)acid solution. Apart from the experimental data from XRD measurements, Fig. 3 shows the x-ray profiles decomposed into a sum of Pearson Vll analytical functions. It is clearly seen thatthe procedure of washing palladium from the bimetallic catalyst used in testing is effective:the diffraction profile of the catalyst obtained after washing does not reveal the palladium phase, which was noticeable for this catalyst after preparation and reduction at 390°C.

For the monometallic 3% Pd/Csii, palladium catalyst, both x-ray profiles, before andafter washing with nitric(V) acid, were virtually the same, proving that the washingprocedure used in testing slightly reduced the amount of palladium in the catalyst, but didnot result in significant changes of crystallite sizes of the metal. Therefore, comparableactivities of the 3% Pd/Csib catalyst before and after washing with nitric(V) acid are understandable.

X-ray studies on the 6.1% PdflAusg/Csib catalyst were complemented with the analysisof metal phase composition using the flame atomic absorption spectroscopy (FAAS). Theanalysis showed that in catalyst before washing with the nitric(V) acid solution, the Au/Pdatomic ratio was 1.44, whereas, the ratio increased to 1.62 for the catalyst after 40 hours washing with HNO; solution, due to washing palladium out of the catalyst.

A very clear enhancement in catalytic activity of the 6.1% Pd41Au59/Csib catalyst afterwashing with nitric(V) acid used in testing is strictly correlated with the disappearance of the highly dispersed palladium phase in the catalyst. Therefore, the proposed method oftreating13 the supported bimetallic Pd-Au catalysts, where a satisfactory mixing level of both metalscannot be achieved in the preparation procedure, may be a very effective, simple in usemethod for improving catalytic performance of these systems. Fig.3 shows the diffractionprofiles of a 6.1% Pd41Au59/C Catalyst reduced at 390°C, before and after washing procedurewith nitric(V) acid.

Acknowledgement This work was financed by the Polish National Science Centre (NCN) under the DEC-2011/01/ B/ST5/03888 research project.

The fees related to protection of the invention have been financed from the funds ofthe project NanOtechnology, Biomaterials and aLternative Energy Source for ERA integration FP7-REGPOT-CT-2011-285949-NOBLESSE. 14

Claims (6)

Claims

1. Method for modifying palladium-gold catalyst, in particular for hydrodechlorination of tetrachloromethane, characterised in that a) after prior reduction of the initial supported palladium-gold catalyst, palladiumnot bound to the Pd-Au alloy is being removed using nitric(V) acid, wherein thesaid alloy is the active metal phase of the supported palladium-gold catalyst, and subsequently b) the catalyst's metal active phase is reduced in flow of a Hz/Ar reducing mixture with formation of the modified supported palladium-gold catalyst, wherein the amount of palladium and gold in the Catalyst results from the Au/Pdatomic ratio of about 1.44 before the washing step, whereas for the catalyst obtained after modification the Au/Pd atomic ratio ranges from 1.50 to 2.00.

2. Method according to claim 1, characterised in that a palladium-gold catalyst on avery pure active carbon support, more preferably synthetic active carbon ofmesoporous structure and trace elements content <10 ppm, including phosphor and sulphur <2 ppm, is used as the initial supported palladium-gold catalyst.

3. Method according to claim 1 or 2, characterised in that a 8 - 10% nitric(V) acidsolution, more preferably a 10% nitric(V) acid solution, is used in washing palladium from the catalyst.

4. Method according to claim 1 or 2, or 3, characterised in that the process of washing palladium from the catalyst lasts for 35 - 45 hours, preferably 40 hours.

5. Method according to any of claims 1 to 4, characterised in that the nitric(V) acidsolution per catalyst mass used ranges from 30 to 50 g acid solution per 1 g of the catalyst, more preferably 40 g per 1 g of the catalyst.

6. Method according to any of claims 1 to 5, characterised in that prior to dryingand reduction steps, the catalyst is purified by washing with distilled water,preferably deionised water, followed by drying with stirring for 20 - 40 hours,preferably 24 hours, at 30°C - 40°C, and then for 2 hours in argon flow of 40 to 150 cm3/hour, preferably 100 cm3/hour, preferably at temperature from 110" to130°C, and more preferably at 120°C. Method according to any of claims 1 do 6, characterised in that the reduction is carried out in a stream of reduction mixture H2 in Ar, preferably from 30 to 70%H2 in Ar, and more preferably 50% H2 in Ar, at a temperature increase from 1°Cto 8°C/minute, more preferably 4°Clminute from 20 to 390°C, with temperature390°C maintained for 2 hours, and reduction mixture flow from 40 to 150 cm3/hour, preferably 100 cm3/hour. Palladium-gold catalyst obtained with the method according to one of theforegoing claims 1 to 7, characterised in that it comprises a suitable structure ofthe metal phase, in which palladium is contained exclusively in the catalysfls Pd-Au alloy, which means that the catalyst is free from palladium that is not boundto gold, and the Au/Pd atomic ratio in a so modified supported palladium-gold Catalyst is from 1.50 to 2.00. 16