GB1586398A - Pellet type oxidation catalyst - Google Patents

Description

(54) IMPROVED PELLET TYPE OXIDATION CATALYST (71) We, GENERAL MOTORS CORPORATION, a Company incorporated under the laws of the State of Delaware, in the United States of America, of Grand Boulevard, in the City of Detroit. State of Michigan, in the United States of America (Assignees of LOUIS HEGEDUS and JACK CURTIS SUMMERS) do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention is an improvement in or a modification of the invention the subject of our prior application for letters patent No. 1882/77 now British Patent 1,537,732, and relates to oxidation catalysts of the pellet type having high catalytic performance over their required lifetime in an operating environment which includes precursor compounds of lead and phosphorus which are known catalyst poisons. More particularly, the catalyst of the present invention is intended to oxidize the unburned hydrocarbon compounds and carbon monoxide in vehicle emissions. Numerous catalysts have been proposed and are known for oxidizing the unburned constituents in vehicle exhaust gas and, in fact. devices of both the particulate and monolith type are today in use on motor vehicles for the purpose of cleaning up the exhaust to levels required bv U.S. Federal and State standards. However, as future U.S. standards require lower levels of unburned hydrocarbons, a greater demand will be placed on the known catalysts for higher conversion efficiencies while still remaining effective over a lifespan as long as 50,000 miles and beyond.

Applicants have conducted extensive theoretical and laboratory studies, with results verified by engine dynamometer real time testing, and have developed the improved catalyst according to the invention. as compared to known catalysts. wherein a larger macroporous volume and radius is provided for higher diffusivity of the exhaust gas in the catalyst with resultant higher catalytic activity (and also more rapid poisoning of the catalyst). and having a larger surface area to slow down the rate of poison penetration.

Penetration of the support by the catalytically active metallic material is provided to a greater depth in order to achieve the catalyst lifetime required.

As previously noted, catalysts having oxidizing capability are well known in the prior art and such catalysts include those having special physical structure. U.S. patent 3,907.512 issued September 23 1975 to Ziegenhain et al discloses an alumina product having a pore volume greater than about 1.0 cc/g. and a surface area greater than about 275 m-/g. and having a macropore diameter of from 0 to 10.000 A. the patentee noting that larger diameter macropore material has little value for catalytic purposes. U.S. patents 3.388.077 and 3.259.589 issued to Hoekstra on June 11. 1968 and to Michalko on July 5. 1966 respectivelv. disclose catalysts for treatment of combustible waste gas wherein an organic acid such as citric acid is used in controlled amounts with the solution of metallic catalytic material in order to produce a finite zone of the catalytic material either on or within the surface of the alumina support. The patentee however does not teach any pore size characteristics, surface area or depth of penetration of the catalytic material.

It is recognized in the art that the bulk of catalyst activity is lost due to lead and phosphorus poisoning. On the basis of extensive laboratory testing and analysis we have found that the poison penetrates the pellet in a sharp front indicating that the rate of poisoning reaction is much faster than the rate of diffusion of the phosphorus and lead containing poison precursors into the pellet. It was also found that the rate of poison penetration increases with increasing diffusivity and decreases with increasing surface area of the pellet support. Likewise, it was determined that the diffusivity of the reactants and poison precursors increases with increasing macroporous volume and radius. Finally the impregnation depth of the catalytically active materials was identified as important to the lifetime of the catalyst in that the penetrating poison front should not reach the end of the impregnated zone of the support before obtaining the required operating lifetime of the catalyst.

Accordingly, the present invention comprises the further characterization of the feature of the macropore volume of the support structure additional to the features of the support claimed in the aforesaid parent patent application.

In order to establish a common understanding as to the results achieved by the invention and as to the parameters defining the improved catalyst of the invention, the following information describes the testing techniques used with respect to the catalysts shown in Tables 1A and 1B hereinafter.

Support surface area is measured by the N2 absorption BET technique: which is well known in the art.

The penetration of the support pellet by the catalytically active materials is determined by microscopic examination of a cross-section of the catalyst against a micron scale. In the case of noble metal materials, the catalyst is treated in a boiling solution of stannous chloride (SnCl2) which turns the catalytic material black for good visualization.

The support pore structure, pore radius and pore volume, is readily determined by the mercury penetration technique using pressures up to and over 60,000 psig, this being well known in the art as mercury porosimetry. In our program, tests were made on an Aminco Porosimeter with the results plotted to show the cumulative volume of mercury penetration per gram.of sample on the vertical axis, the porosimeter pressure being shown on the horizontal axis on a logarithmic scale. In this regard, two types of catalytic support are typically in use in automotive applications, namely, bimodal and monomodal supports.

Bimodal supports have two types of pores, micropores and macropores. The porosimeter curve for the bimodal support shows two steps. The inflection point between the ends of the first step starting from the origin of the curve represents the break-point between the macropores and the micropores in the support, the portion of the curve to the left of the inflection point representing the macropore structure, that to the right representing the micropore structure. The pore radius for the two types of pores can be directly calculated from the pressure applied in the test for any given pressure. The cumulative macro and micropore volumes in cc/g. may be read directly from the curve. The pore radius and volume information is then used to calculate the integral average values for the pore radii.

The integral averaging for the macropores proceeds from 0 to the macropore volume as discussed above while that for the micropores proceeds from the macropore volume to the total volume. The details of the test and of curve analysis and calculations are fully set forth in, among others," Chemical Engilzeeri)lg Kinetics" by J. M. Smith, McGraw-Hill Publishing Company, New York, Second Edition, 1970.

Real-time engine dynamometer aging of catalysts is accomplished on a 350 CID engine having a 4 barrel carburettor and having a water brake, operating at an average speed of 50.5 mph. to approximate the vehicle driving schedule obtained in complying with the U.S.

Federal Appendix "D" Schedule consisting of eleven 3.7 mile laps of stop-and-go driving with lap speeds varying from 30 to 70 mph., cycles being repeated for 50,000 miles. This is described in greater detail as the "Simulated Federal Schedule" in Society of Automotive Eiigineers (SAE) paper No. 730558. title "Engine Dynamometers for the Testing of Catalytic Converter Durability" given at the SAE meeting in Detroit, Michigan, May 14-18 1973. The catalyst tested was loaded into a standard 260 cubic inch underfloor converter and the engine was run on a commercial type "lead-free" fuel having approximately 10 mg. lead/litre (Pb/l) and 0.5 mg. phosphorus/litre (P/l). Periodic testing of the inlet and outlet gas streams at the converter, as well as of many other parameters of interest, is made during the course of the 1000 hour test to monitor converter performance. The details of these tests are set forth in SAE paper No. 730557, titled "An Engine Dynamometer System for the Measurement of Converter Performance" given at the above noted meeting.

Accelerated durability testing was performed using a 350 CID, 4 barrel carburettor, engine dynamometer at steady state high temperature conditions (566"C. bed inlet temperature), the catalyst being loaded into a 1000 cubic centimeter converter with the engine being run on a fuel containing 105 mg. Pb/l. and 5 mg. P/1., experiments having shown that 35 hours exposure to this fuel exhaust in the 1000 cm converter results in poison concentrations and distributions similar to the effects of 50,00 miles exposure to lead-free fuel as described above for the simulated Federal Schedule.

Shown in TABLES 1A and 1B is a comparison of the physical properties measured for six different catalysts made in accordance with the process disclosed in our aforesaid co-pending patent application as compared with those measured for two typical commercial catalysts.

The comparative performance of catalytic action for certain of the catalysts is shown in accelerated durability curves of Figure 1 and in the 1,000 hour real-life charts shown in Figures 2-5. The catalysts having platinum/palladium (Pt/Pd) are used in the weight ratio of 5:2 = Pt:Pd and in a total amount of approximately 0.05 troy oz./260 cubic inch converter, the same total applying to catalyst C, platinum only.

In determining the crush number of a catalyst, a representative sample of 10 pellets of the catalyst was taken, each of said 10 pellets being subjected to a crushing force in pounds between two flat plates, the load at failure being the crush number for each pellet and the crush number shown for the catalyst being the average crush number for the 10 pellets. The crush number is expressed in TABLES 1A and 1B in terms of fresh - libs, which term indicates the load in pounds weight required to crush fresh catalyst, i.e., unused catalyst prior to loading in a converter.

TABLE 1A Commercial CATALYST A B C surface area (m2/g.) 94 131 130 pellet density (g/cm3 1.132 1.052 1.052 macroradius* ( ) 6,270 10,872 10,872 microradius* (A) 90 73 73 macrovolume (cm3/g) 0.140 0.170 0.170 microvolume (cm3/g) 0.461 0.497 0.497 solid density (g/cm ) 3.54 3.53 3.53 impregnated depth (microns) 39 103 132 Pt - wt. % 0.035 0.043 0.060 Pd - wt. % 0.014 0.019 - pellet shape spherical spherical spherical pore volume (cm3/g) 0.601 0.667 0.667 (total porosity) crush number 16.4 14 14 (fresh - lbs.) *integral averaged value' TABLE 1B Commercial CATALYST D E F G H surface area (m2/g.) 125 118 125 119 119 pellet density (g/cm3) 1.159 0.970 1.103 0.910 0.760 macroradius*( ) 187.000 20,325 -- 93.300 37.593 (monomodal) 104 microradius* ( ) 94 95 -- 145 192 macrovolume (cm3/g 0.025 0.179 -- 0.260 0.440 microvolume (cm3/g) 0.0579 0.569 -- 0.553 0.590 solid density (g/cm3) 3.38 3.53 3.43 3.50 3.50 impregnated depth (microns) 227 152 36 100 137** Pt - wt. % 0.033 0.046 0.037 0.113 0.117 Pd - wt. % 0.014 0.0200 0.015 0.0499 0.053 Pellet shape spherical spherical cylindrical spherical spherical pore volume (monomodal) (cm3/g) 0.604 0.748 0.615 0.813 1.03 (total porosity) crush number 16 11.5 -- 11.9 9 (fresh - lbs.) *integral averaged value **average value In Figure 1 are plotted the results of accelerated durability testing on catalysts A, B and C and it can be seen that the commercial type catalyst hydrocarbon conversion diminished rapidly after about 30 hours under this rigorous test whereas catalysts B and C had not yet reached the point where the poison layer front passed the catalyst material zone after 35 hours of testing. It is also to be noted that the hydrocarbon conversion is significantly higher for the catalysts B and C, than that for the commercial type catalyst A.

A comparison of the characteristics of catalysts B and C with those of catalyst A shows that the catalysts B and C have larger macropore radius and volume values which, it is theorised, cause increased diffusivity of the reactants in the catalyst pellet. This also causes an increase in the diffusivity of the lead and phosphorus precursors which is, however, compensated for by the larger surface area of the improved catalysts. With regard to the earlier breakthrough of the reactants in catalyst A, note its thinly impregnated depth of 39 microns as compared to 103 and 132 microns for catalysts B and C, respectively.

Similar results may be observed with respect to the catalysts D and E, Figures 3 and 4, respectively, as compared to catalysts A and F, Figures 2 and 5, respectively. These data present the results of real-time testing under the Simulated U.S. Federal Schedule described above over a 1,000 hour period at a speed of 50.5 mph.

With respect to catalyst D, it should be noted that whereas the macroporous volume is at the bottom end of the range specified in the claims of our co-pending application 0.025 cm3/g., the macroporous radius is quite large, in excess of 187,000 A, this balance being a viable combination. In accordance with the invention disclosed in our co-pending application a catalyst support having a macroradius greater than 10,000 A, preferably of from 15,000 to 25,000 A, will meet most diffusivity and conversion requirements when combined with a macrovolume of from 0.020 to 0.200 cm3/g., preferably from 0.150 to 0.190 cm3/g. and having a surface area of at least 100 m2/g. up to 250 m2/g., preferably 110 to 150 m2/g. The depth of penetration of the catalyst support by the catalytically active metallic material should be from at least 90 microns to 250 microns, a range of from 100 to 245 being preferred to achieve the desired 50,000 mile lifetime of the catalyst. We have since discovered that these diffusivity and conversion requirements are also met when the catalyst support has a macrovolume of from 0.200 cm3/g to a macrovolume at which the catalyst is not able to sustain the load and vibration forces to which it is subjected when used in a catalytic converter in an automobile, e.g., without crushing.

A catalyst crush number, as described above, of 7 has been established as being the lower limit for commercially used catalysts according to the invention, this figure being based on the fact that the load on the pellets in a catalytic converter, by reason of maximum weight of catalyst, in about 6 pounds. However, it should be noted that lower density (higher macrovolume) supports will have a correspondingly lower total weight in a converter and accordingly a lower crush number is appropriate for such materials. Crush numbers substantially lower than those listed in TABLES 1A and 1B would be acceptable as long as the catalyst is able to sustain the load and vibration forces to which it is subjected when used in a catalytic converter.

As can be seen in TABLE 1B, catalysts of the present invention, G and H, have substantially higher macrovolumes, 0.26 and 0.45 cm3/g., respectively, and substantially lower pellet density than the other catalysts listed. The crush numbers for catalysts G and H are also lower, as might be expected in view of the lower pellet density, 11.9 and 9, respectively. As noted above, the greater macrovolume results in greater diffusivity of exhaust gases in the catalyst with the increased surface area acting to trap the catalyst poisons for a resultant higher catalyst activity. Also as described above, the increased depth of catalyst penetration assures desired catalyst performance over the necessary lifespan of the catalyst.

From the standpoint of performance, TABLE II shows the results of steady state testing on an engine dynamometer, the results indicating improved catalyst performance as disclosed herein.

TABLE II Catalyst G H HC CO HC CO 600 second efficiency (%) 96.8 99.5 97.5 99.4 205 second breakthrough (%) 20 12 20 12 seconds to 50% efficiency (light-off) 24 19 26 21 EPA predicted efficiency (%) 91.5 82.6 91.9 82.6 Table II illustrates the conversion efficiency of the catalyst tested in terms of percentage and time taken in seconds for the catalyst to achieve a conversion level of 50%. The test is run over a ten-minute period at the end of which it is considered that the catalyst bed has reached a steady state, warmed-up condition. The 600 second efficiency is the percentage constituent conversion achieved at the end of this ten-minute period. The 205 second breakthrough figure represents the percentage amount of the constituent present not converted after 205 seconds of operation of the tested converter, and is determined be calculation from data taken during the test. The seconds to 50% efficiency (light-off) is the observed time in seconds that it takes the tested converter to obtain 50% conversion of the specified constituents in the exhaust gases fed to the converter. The EPA predicted efficiency is a percentage conversion efficiency figure for the tested catalyst obtained by calculation from data obtained during the testing of the catalyst.

In carrying out the invention, the porous refractory support may be selected from the well known refractory ceramic forms of alumina (including its forms in various states of hydration, i.e., a alumina, bayerite, gibbsite, and boehmite). The pellet catalyst may be formed in various shapes such as spherical or ball, extrudate forms as cylindrical and hollow cylinders, granules, and rings. The preferred forms are those of spheres formed by well known methods such as by a rotating disc, and a solid cylinder formed by extrusion. The sphere radius in the catalyst samples was about 0.159 cm. The cylinder dimensions were about 0.318 cm. in height and diameter.

Control over the pore size and pore volume may be obtained by adding a combustible organic material to the support material when the support composition is being formed. By controlling the size and quantity of such combustible organic materials, the desired pore characteristics and surface area are obtained on firing of the support. Surface area is also affected by temperature and time of final calcining. Also, the desired depth of penetration by the catalytically active material may be obtained by treating the fired catalyst support with a dilute acid wash to obtain a controlled surface etch of the desired depth by dissolving free sodium oxide (Na,O). By way of example, the following is a description of a technique used successfully in obtaining a catalyst support having the desired characteristics described herein.

In forming the catalyst "B", an alumina trihydrate commercially available as the Bayer Process product, e.g., bayerite, and gibbsite, is ground to an average particle size of 10 microns, 90% in the 5-15 micron range. This alumina trihydrate contains from 0.15 to 0.5% by weight Na2O, necessary to enable control of the penetration depth of catalytically active material.

The ground material is then flash calcined to form amorphous alumina exiting the calciner at a temperature of from 450-550"F. (232-288"C.). The calcined alumina was disc balled to form spheres having an average radius of 0.159 cm. on final firing. The spraying slurry was also provided with Saran microballoons, plastic hollow spheres of from 8-30 microns diameter, about 20 microns average diameter used in this instance, available commercially from Dow Chemical Co. Saran is a trade name for a vinyl chloride vinylidene chloride copolymer. The amount of said microballoons may be from 0.25 to 0.5% by weight of the balled alumina. 0.5% being used in this instance. The amount of water used in the balling process should be such as to provide sufficient for full absorption in and saturation of the product. in this instance, an amount which is about 40% of the product weight.

The balled alumina is then stored in a moisture tight container to allow rehydration to the monohydrate, boehmite, form whereupon it is soaked in water at 180-2000F. (82-93" C.) for 5 to 10 minutes to assure complete rehydration. Excess water is then drained from the product, the product dried, i.e. at 225-4000 F. (107-204" C.), and then calcined at a temperature of about 1700 F. (927 C.) where it is held for from half to 1 hour to develop the desired surface area, e.g., about 130 m2/g., and the desired pore structure, the microballoons being readily burned out.

The calcined alumina is treated by soaking in a dilute acid to remove free Na2O (unbonded in the alumina lattice) and thus to assure the desired depth of penetration of the catalytically active platinum/palladium, and help achieve the desired surface area. The penetration depth was found to be proportional to the increase in acid concentration which may be from about 0.03 to 0.15 wt. % acid, the time varying from about 1 to 5 minutes. In this instance, the alumina was soaked in a dilute hydrochloric acid solution, 0.05% by weight HCL, for a one minute period, excess solution being separated off and the product dried at about 400" F. (204"C.) for about one hour. The resulting support pellets are then sprayed with a catalyst solution of chloroplatinic acid and palladium chloride, or any mixture of any of the possible soluble salts well known in the art, such as to result in a catalyst having a weight ratio of Pt:Pd = 5:2 and a total metal content in a 260 cu. in. converter of 0.05 troy oz. The resultant catalyst is air dried at a temperature of 600-700" F.

(316-371" C.).

From the foregoing description it can be seen that applicants have provided an improved pellet catalyst having a pore structure, surface area and depth of penetration of catalytic material such as to enable the required catalyst life time to be achieved at conversion rates meeting U.S. regulatory requirements.

Claims (4)

WHAT WE CLAIM IS:

1. A noble metal pellet-type oxidation catalyst comprising a porous alumina support member having a surface area of at least 100 m2/g. up to 250 m2/g., a pore structure characterized by a macropore volume greater than 0.200 cm3/g. up to a macropore volume at which the catalyst is not able to sustain the load and vibration forces exerted thereon when in use in a converter in a motor vehicle, and a macropore radius greater than 10,000 A"; and a catalyst material consisting of platinum or platinum/palladium deposited on said support to a depth of at least 90 microns up to 250 microns.

2. A noble metal pellet type oxidation catalyst comprising a porous alumina support member having a surface area of at least 100 m2/g. up to 250 m2/g. a pore structure characterized by a macropore volume greater than 0.200 cm3/g. up to a macropore volume at which the crush number, as hereinbefore defined, of the catalyst has a value of 7, and a macropore radius greater than 10,000 A; and a catalyst material consisting of platinum or platinum/palladium deposited on said support to a depth of at least 90 microns up to 250 microns.

3. A noble metal pellet-type oxidation catalyst according to claim 1 or 2 in which the surface area is 119 m-/g.. the macrovolume is 0.260 cm3/g., and the depth of penetration is 100 microns.

4. A noble metalpellet-type oxidation catalyst according to claim 1 or 2 in which the surface area is 119 m-/g. the macrovolume is 0.440 cm3/g., and the depth of penetration is 137 microns.