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Al2O3 catalysts for the combustion of methane at low concentrations
ELSEVIER
Applied CatalysiSB: Environmental 5 (1994) 149-164
Investigation of Pt/&Q, and Pd/A& catalysts for the combustion of methane at low concentrations R. Burch *, P.K. Loader Catalysis Research
Group, Chemistry Department, University of Reading, Whiteknights Reading, RG6 2ADUK
Received24 May 1994;revised 2 August 1994;accepted2 August 1994
Abstract Pt/Al,O, and Pd/A1203 catalysts have been preparedfromchlorine-freeprecursors andinvestigated for the combustion of methane under lean, stoichiometric, and rich conditions using dilute mixtures. It has been found that under lean conditions, and at low conversions under stoichiometric or rich conditions, Pd/A1,03 is a more effective catalyst. However, at higher conversions with stoichiometric or rich mixtures Pt/Al,O, is a more active catalyst. This change over between Pd/Al,O, and Ptl Al,O, is associated with a ‘light-off’ effect observed with Pt/Al,O, catalysts. Various possible explanations for these effects are discussed. With the Pt/Al,O, catalysts there is no evidence of a particle size effect, which is in contrast to reports in the literature that the methane combustion reaction is structure sensitive. Reasons for these variations, including the possible inhibition of activity by chlorine used in many catalyst preparations, are discussed. It is concluded that platinum can be a more effective catalyst than palladium for methane combustion under real conditions and that, in consequence, platinum may play an important role in multimetallic catalysts for emission control for natural gas vehicles. Keywords:
Exhaust;Methanecombustion; Natural gas vehicles; Palladium/alumina; Platinum/alumina
1. Introduction Natural gas is a relatively clean source of hydrocarbons and so is of interest as an environmentally friendly fuel for motor vehicles. The use of natural gas also leads to a decrease in the emissions of SO, [ 11. However, legislation on emissions still requires that the exhaust gases be further cleaned up, and these regulations are now beginning to include the removal of trace amounts of methane, which comprises about 95% of natural gas. *Corresponding author. Tel. (+44-734)
318451, fax. (t44-734)
311610.
0926-3373/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO926-3373(94)00037-9
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Methane is the least reactive of the hydrocarbons and therefore the most difficult to oxidise. It has been found from tests on both automobile and stationary combustion engines that the traditional three-way catalysts used for controlling the emissions from gasoline powered vehicles are not able to remove all the methane. Thus, whilst > 95% of NO, and carbon monoxide were removed from the exhaust from a stationary combustion engine [ 21, only 50% of the methane was removed, On automobile engines [ 31 the conversion of methane was < 15%. The low conversion of methane occurs because the air:fuel ratio required for optimum performance in the natural gas system is different from gasoline [ 41. Under these conditions, threeway catalysts will require temperatures higher than are seen in exhaust gases in order to oxidise the methane 151. It is clear, therefore, that new catalysts need to be designed which can oxidise methane efficiently under the conditions present in the exhaust of natural gas powered systems. Such catalysts will also need to retain the high levels of carbon monoxide and NO, conversion seen on the three-way catalysts. In tests under simulated and real exhaust conditions [ 51, Pd-Pt catalysts were found to give superior methane conversion to conventional Pd-Pt-Rh three-way catalysts, as well as other metals. Although platinum catalysts are superior to palladium for the oxidation of higher alkanes and alkenes [ 6,7], the combustion of methane is easier over palladium under similar, oxygen rich, conditions. However, when platinum and palladium catalysts were compared under a range of conditions [3], platinum was found to give higher conversions at O,:CH, ratios less than 2.4 (2: 1 is stoichiometric) . Such fuel-rich mixtures may be found in the systems in which methane is used as a fuel so it is possible that platinum may have an important role in emission control for natural gas vehicles. Therefore, a more detailedunderstanding of platinum catalysts for methane combustion under realistic conditions of temperature and gas composition is required. This paper presents the results of a study of Pt/Al,O, catalysts for methane combustion in various gas mixtures, and compares their behaviour with that seen for Pd/Al,O, under similar experimental conditions,
2. Experimental 2.1. Catalystpreparation A series of Pt/Al,O, catalysts were prepared by dry impregnation. The A&O3 (Azko Chemie CK300, surface area 185 m* g-‘) was crushed and sieved to give particle sizes between 250 and 850 pm and was then dried at 500°C for 1 h. The required mass of di-nitroso di-amino platinum solution (Johnson Matthey) was made up to 2 g with doubly de-ionised water (i.e. 2 cm3 total solution). To this solution was added approximately 1 g of the support. The resulting slurry was shaken before being dried in an oven at 120°C for 2 h. The dried catalyst was then
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Table 1 Characterisation of Pt/Al,O, catalysts Catalyst
0.5% Pt l%Pt 2% Pt 4% Pt 4% Pd
Fresh
Used
Dispersion (%)
Average diameterinm
84 68 53 48 5.2
1.4 1.7 2.2 2.4 21.8
Dispersion (%)
Average diameter/nm
30
3.7
calcined at 500°C for 2 h. and cooled in a desiccator. The catalysts prepared contained 0.5, 1, 2, and 4% Pt by weight. A 4% Pd/A1203 catalyst was prepared from palladium nitrate (Johnson Matthey) using the same technique as for the platinum catalysts except that calcination was performed for 17.5 h as opposed to 2 h. The metal dispersion of the catalysts was determined by hydrogen chemisorption, assuming an H/Pt ratio of 1. Average particle diameters were calculated assuming spherical geometry and a value for the concentration of metal atoms on the surface of 1.25 X 10” atoms rnM2and 1.27 X 10” atoms mm2 for platinum and palladium, respectively [ 81. The dispersions and average particle sizes for the fresh catalysts are given in Table 1. 2.2. Catalyst testing Catalysts were tested in a micro-reactor flow system operating at atmospheric pressure. The catalyst ( 100 mg), supported by a glass wool plug, was placed centrally in a silica reactor (5 mm i.d., 30 cm long). A thermocouple was placed in the centre of the catalyst bed to monitor the temperature of the bed. The reactor was mounted centrally in a vertical tube furnace heated by a Eurotherm 812 temperature controller, and the temperature of the furnace monitored using a chromelalumel thermocouple situated on the wall at the mid-point of the furnace. The gases used were 2% CH4/N2, air and nitrogen (all from B.O.C.). The flows of these gases were controlled to give a total flow of 200 cm3 min- ’ and a partial pressure of reactants (CH,+ 0,) of 9.1 Torr. Three different O,/CH, ratios were used, namely 5: 1 (oxygen-rich), 2: 1 (stoichiometric) , and 1: 1 (methane-rich). The catalyst was heated to 300°C (as measured by the thermocouple in the catalyst bed) in the reaction mixture. The temperature was then increased to 550°C in steps of 25°C. The catalyst was held at each temperature for 10 min. (during which time three samples were taken for analysis) or until a steady value of methane conversion was reached. The temperature was then lowered to 300°C in 25°C steps and the same testing procedure carried out. This cycle was repeated. The product gases were analyzed by gas chromatography. Samples were taken using a lo-port sampling valve (Valco) . Analysis was performed using a Perkin
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Elmer 8410 gas chromatograph with a 2 m Porapak QS column operating at 60°C. Products were detected using a flame ionisation detector operating at 350°C A methanator (Ni/ZrO, catalyst at 350°C) was used in order to allow the detection of carbon monoxide and carbon dioxide. In all cases the activity of the catalyst is defined as the percent conversion of methane into all products. Conversions of methane were reproducible to within 0.1% at low conversions. 3. Results 3.1. 02/CH45:1 (2000 ppm CH,) All the catalysts showed the same trends. Representative results for the 4% Pt catalyst are shown in Fig. 1. A smooth increase in conversion with temperature is observed. There is a small decrease in activity from the first heating to the first cooling run. Results for the second heating and cooling runs were the same as for the first cooling run. The hysteresis from initial to steady state reached a maximum of 25°C for all the catalysts. The steady state activities for the platinum catalysts for this gas mixture are shown in Fig. 2. As expected, increasing the platinum loading gives an increase in conversion for a given temperature. Only the 4% Pt catalyst gives significant conversion below 400°C and complete conversion of methane is not seen for these catalysts even at 550°C the highest being 92.5% for the 4% Pt catalyst, Arrhenius plots have been constructed for conversions below 204/o,assuming that the kinetics of the reaction are first order in methane and independent of oxygen concentration. The calculated Arrhenius parameters are shown in Table 2. % CH4 Conversion
Temperature (C) Fig. 1. Methane conversion as a function of temperature for a 4%Pt/Al,O, catalyst using a 02:Ci.IJ ratio of 5: 1. (A) First heating run, (0) first cooling run, ( A ) second heating run, (e) second cooling run.
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% CH4
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Conversion
100,
Temperature (C) Fig. 2. Steady-state methane conversions as a function of temperature for various Pt/A&O, catalysts using a O,:CH, ratio of 5:l. (A) O.S%Pt, (0) l.O%Pt, ( A ) 2.O%Pt, (0) 4%Pt.
The catalysts all showed similar behaviour when exposed to a 2:l 02/CH4 mixture. Typical results are shown for the 4% Pt catalyst in Fig. 3. It was found that on heating there was a sudden increase in activity at a fixed reaction temperature. For example, on the first heating for the 4% Pt catalyst, the conversion on reaching 450°C was 26%, but this rose to a steady value of 53% over a period of a few minutes. A hysteresis is seen between the two heating runs and between heating and cooling. Both cooling runs gave the same results. This is in contrast to the results obtained with the 5:l mix where a hysteresis was seen only between the initial heating run and subsequent cooling and heating runs. The activities seen for the four catalysts during the second heating are shown in Fig. 4. Due to the light-off in activity, almost complete conversion is seen at 475°C for all except the 0.5% Pt catalyst. However, even this sample gives complete combustion at 550°C. It is of note that light-off does not occur at the same temperature for the four catalysts but at about the same conversion level, ca. 15%. Table 2 Parameters calculated from Arrhenius plots Catalyst
E,/M mol-I OZ/CH,
InA~O,/CH, S:f
E,/kJ mot-’ OJCH, 2:l
In A O,/CH, 2:l
15.1 i: 17.6& 14.5* 12.7i
1015 104& 91.44 9?J.1*
14.51 2.0 15.5_+ O,f 13.6, 1.1 15.85 2.2
5:l 0.5% Pt 1% Pt 2% Pt 4% PI
1os+ 121+ loo+ 83.85
I.5 10 5 5
0.3 2.5 1.0 1.3
10 1.0 5.0 11
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CH4 Conversion
,
I
550
Temperature (C) Fig. 3. Methane conversion as a function of temperature for a 4%Pt/Al,O, catalyst using a 02:CHJ ratio of 2:L (A) First heating run, (0) first cooling run, ( A ) second heating run, (0) second cooling run.
Arrhenius plots showed good linearity up to the point at which light-off occurs, where there is a discontinuity. Therefore, Arrhenius parameters were calculated using data obtained below the light-off temperature. These values are shown in Table 2. o/oCH4 Conversion
Temperature (C) Fig. 4. Methane conversions, during the second heating run, as a function of temperature for various using a 02:CHJ ratio of 2:l. (A) 0.5%Pt, (0) l.O%Pt, ( A) 2.0RPt, i*j 44cPt.
catalysts
Pt/hl,O,
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Temperature (C) Fig. 5. Methane conversion as a function of temperature for a 4%Pt/Alz03 catalyst using a 02:CH4 ratio of 1:l. (0) First heating run, ( * ) first cooling run, ( A ) second heating run, (0) second cooling run.
3.3. O,/CH, 1:l (6000ppm CH,) Results are shown for the 4% Pt catalyst in Fig. 5. As with the 2: 1 mix, light-off was observed. Again, there is a hysteresis between the heating and cooling curves which is sustained for at least two heating/cooling cycles. The activities of the four catalysts for the second heating are shown in Fig. 6. Light-off again occurs at a given conversion level for all the catalysts as opposed to a given temperature. The conversion required for light-off is ca. 3%, much lower than with the 2: 1 02/CH4 mix. It is also clear from Fig. 6 that the conversion levels % CH4 Conversion 100
7
go80
7060-
40 -. 30 SO20-
I:“’
‘“li&dLu &IO
350
400
I
I
450
500
I
550
Temperature (C) Fig. 6. Methane conversions, during the second heating run, as a function of temperature for various Pt/Ai,O, catalysts using a O,:CH, ratio of 1:l. (A) 0.5%Pt, (0) l.O%Pt, ( A ) 2.0%Pt, (0) 4%Pt.
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Table 3 Effect of O,/CH, ratio on the conversion of methane at low temperatures Catalyst
TempYC
OZ/CHJ .5:1
OJCH, 2: I
0,/C& 1:I
0.5% Pt
325 350 31.5 400 350 315 400 425 350 375 300 425 300 325 350 375
0
0.3 0.7 1.6 3.0 1.2 2.5 5.0 9.3 1.8 3.7 6.9 13.7 0.9 1.9 3.9 7.7
OS 1.1 2.1 4.7 1.5 2.9 59” 70” 2.0 50” I ”
l%Pt
2% Pt
4% Pt
0.3 0.7 1.7 0 0.3 1.3 3.8 0 0.7 2.1 5.2 0.7 1.4 3.5 5.8
1.2 2.4 69 62’
’ Light-off had occurred at this point.
off after light-off, although almost complete conversion is seen at 550°C. Carbon monoxide formation was observed for conversions above 40%, the selectivity to carbon monoxide reaching 45% at 100% CH4 conversion. As light-off occurred at low conversions, it was not possible to construct sensible Arrhenius plots for these experiments. From a comparison of Figs. 2,4, and 6, the effect of changing the gas composition can be seen. It is apparent that on going from an oxygen-rich to a methane-rich mixture the catalysts become more active at a given temperature, although at the highest temperatures the oxygen limitation seen with the 1:1 mix leads to values for conversion being lower than with the 2: 1 mix. These differences are mainly due to the light-off seen for the 1:1 and 2: 1 mixes, although at temperatures below these points the same trend is still seen, as is shown in Table 3, i.e., the conversion increases in the order 5:1<2:1< 1:l. 3.4. Particle size effects with platinm
catalysts
In order to determine whether any correlation existed between catalyst activity and particle size, the rate of methane conversion was calculated per surface platinum atom, again assuming first order kinetics with respect to methane. These specific rates were calculated from initial results using the values obtained on fresh catalysts, and, in one case, from steady state results using the metal dispersion obtained from hydrogen chemisorption on the used sample (see Table i >. The used sample was the 4% Pt catalyst which had been given two heat-cool cycles in the 5: 1 gas mix, as shown in Fig. 1. Plots of specific rate versus average particle size are shown for
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Rate/ (~Molecdes CH4 is /surface Pt atom)
0.09 o.os-
/--
0.07 0.06’ 0.05 0.04L I 0.030.02;
.
l
.
/ ’
l
* .
Average Pt Particle Size inm Fig. 7. Specific rate of methane oxidation as a function of platinum particle size at various temperatures. (A) 425”C, (0) 45O”C,( A ) 475”C, (0) 5OO”C,( * ) 525”C, ( X ) 550°C.
the oxygen-rich mixture in Fig. 7. It is clear that the specific rate is independent of particle size in the range 1.4 to 3.7 nm at all temperatures in the range 425 to 500” C. At higher temperatures there may be a small increase in rate with increasing particle size. However, overall these results clearly indicate no particle size effect for this gas mixture. Similar results were obtained using the 2: 1 gas mixture. 3.5. Palladium vs platinum The 4% Pd/A1,03 catalyst was tested using 5:l and 1:l 02/CH4 gas mixtures. The results obtained are shown in Table 4, where the results for the 4% Pt catalyst Table 4 Comparison of 4% Pt/Al,O, and 4% Pd/Alz03 catalysts Temp./“C
OJCH, 5: 1 Pt
300 325 350 375 400 425 450 475 500 525 550
1.2 2.2 4.2 6.6 14.0 22.0 35.3 49.2 65.8 81.6 93.8
02/CH4 1:l Pd 23.0 40.6 82.5 94.5 98.5 100
Pt
Pd
0.6 1.2 4.0 59.0 67.9 78.8 88.1 94.6 97.9 99.0 99.4
9.1 18.2 33.0 50.7 59.2 68.1 77.6 86.2 91.1 93.1 94.5
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are also shown for comparison. In the oxygen-rich mixture the palladium catalyst gives 23% conversion at 300°C compared to 1.2% for the platinum catalyst. Complete conversion is achieved at 450°C on palladium, at which temperature platinum gives only 35% conversion. Under fuel-rich conditions the palladium catalyst is again superior at lower temperatures, giving 33% conversion at 350°C as opposed to 4% for the platinum catalyst. However, above 350°C the platinum catalyst exhibits light-off. A similar increase is not seen on the palladium catalyst so higher conversions are achieved on the platinum catalyst above this temperature.
4. Discussion There are few datain the literature with which to directly compare the conversions seen on the platinum catalysts used in this study. This is because most other work has used gas mixtures with far higher concentrations of reactants. However, some of the general trends observed here have been noted in other work and will now be discussed. 4.1. Light-off on platinum catalysts A major feature seen on the platinumcatalysts is the light-offin activity observed with both the stoichiometric and methane-rich gas mixtures. Similar behaviour has been observed previously on platinum catalysts. Thus, Cullis and Willatt [ 91 have found a doubling in conversion between 369 and 374°C on a 3% Pt/ThO, catalyst, and an eightfold increase between 337 and 360°C for a 2.7% Pt/Al,O, catalyst for a CH,/O, ratio of 2.2. Trimm and Lam [lo] found a jump in activity at 540°C for a 0.4% Pt/Al,O, catalyst in a 1:l CH,/O, reaction mixture. In both cases a discontinuity was seen in Arrhenius plots, leading to an apparent change in the activation energy to a lower value. Such a discontinuity was seen in the Arrhenius plot obtained in this work for the stoichiometric gas mix. The exact cause of the light-off phenomenon is not clear. One possibility is that it is due to local heating of metal particles within the catalyst bed. A detailed explanation of the effect is given by Schwartz et al. [7] in terms of the relative rates of heat generation at the surface due to the exothermic reaction, and heat loss from the surface by conduction and convection. There is, however, some debate as to whether or not such local heating effects really do occur, Studies have been reported [ 1l-151 in which various techniques have been used to measure both the temperature of the support and of the metal particles during both the CO/O2 and CO/H, reactions. Whilst some studies have found temperature differences up to 190°C as well as hot spots, on platinum andPd/SiO;? catalysts [ 11,121, others find little or no change [ 13-151. The lack of a temperature difference has been attributed [ 131 to a rapid dissipation on heat from metal particles to the support, Therefore, whilst local heating of metal particles can occur, the lifetime of this effect may be
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small, so conditions which lead to the local heating of metal particles will need to be maintained in order for the effect of light-off to be seen. As will be discussed later, this may involve the presence of specific concentrations of reactants on the surface, which will give rise to an enhanced reaction rate. The maximum extent of any local heating of the platinum particles, if it occurs at all in the present work, was derived from the results obtained using the stoichiometric gas mixture (Fig. 4). The activities at temperatures above the light-off point were re-calculated using the activation energies obtained from the data below the light-off temperature. Comparison of the calculated and observed conversions showed that the platinum particles appear to be between 75 and 100°C hotter than the average temperature measured by the thermocouple located in the catalyst bed (e.g. for 4% Pt, 99% conversion is calculated to occur at 550°C but is actually observed at 450°C). Clearly a local heating effect could explain our results. However, a closer inspection reveals that the origins of these effects are rather more complicated. It is clear from Figs. 4 and 6 that light-off does not occur at a particular temperature but at a specific conversion. This is ca. 15% for the stoichiometric mix and ca. 3% for the methane-rich mix. These correspond to conversions of 600 and 180 ppm CH4, respectively. In addition, no light-off is seen at all for the oxygen-rich mixture. Thus, the total amount of methane reacted, and therefore the total amount of heat generated on the metal particles at light-off, does not appear by itself to explain the light-off phenomenon. On the other hand it is possible that when specific concentrations of methane and oxygen are adsorbed together on the surface the combustion reaction will be favoured. For example, we will show [ 161 that the rate of methane combustion goes through a sharp maximum before declining again, as the surface concentration of oxygen goes from zero to monolayer coverage. It seems possible that the surface coverage of adsorbed methane and adsorbed oxygen may change as the reaction proceeds and that an optimum State may be reached at the point at which light-off is observed, whilst at lower conversions there may be too much oxygen on the surface for adsorption of methane to be effective. The kinetics of methane combustion have been reported previously [ 171 to be approximately first order in methane and zero order in oxygen. It therefore seems reasonable to assume that the ease with which methane can approach, and dissociatively adsorb, on the (partially oxidised) platinum surface will be an important factor. A similar idea has been proposed for the combustion of other hydrocarbons on platinum and palladium filaments by Schwartz et al. [ 71 who suggest that fractional coverage of the surface by both reactants is a requirement for optimum catalyst performance in the combustion reaction. As the initial amount of oxygen present is less for the methane-rich mixture ( 02:CH4 = 1: 1) than for the stoichiometric mixture ( 02:CH4 = 2: 1)) it would be expected that the required surface concentrations of adsorbed methane and adsorbed oxygen would be reached at lower conversion in the former case. This is indeed what has been observed. On the other hand, for the oxygen-rich mixture
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(O,:CH,=5:1) there may be too much oxygen present at all conversions and so no optimum surface coverage by both reactants is possible and light-off is not observed. It is also worth noting that no light-off is seen on the palladium catalyst, This may be because the amount of oxygen taken up by palladium is too great, even under these conditions, for the required surface concentration of methane to be reached. Such a phenomenon has also been proposed for other hydrocarbons by Schwartz et al. [7], who suggest that the stable oxide layer present on palladium prevents the formation of a necessary fractional coverage of hydrocarbon, Whilst the above discussion suggests that specific surface concentrations of the two reactants are necessary for increased methane combustion, it is still not clear how much of the increase in the rate of reaction seen during light-off is due to this effect and how much is due to the local heating which is caused by it. For both the stoichiometric and methane-rich gas mixtures, where light-off is seen, there is a hysteresis between the heating and cooling curves. This could be because once the platinum particles are hot it is necessary to increase the rate of cooling, by lowering the background (catalyst bed) temperature before the platinum particles can attain their ‘normal’ temperature. On the other hand, it is also possible that the favourable surface concentrations of reactants, once present, can be maintained for longer during cooling, it being harder for oxygen to re-cover the surface of the metal particles. However, the two effects - local heating, and surface coverage by adsorbed species - are interrelated and it is not possible to distinguish between them with any certainty. Comparison of Figs. 2, 4, and 6 shows that the activity of platinum catalysts increases on going from an oxygen-rich to a methane-rich system, although at higher temperatures the oxygen deficiency in the 1:l mix leads to slightly lower conversions than for the 2:l mixture. Whilst the major differences are due to the temperature at which light-off occurs, conversions below these temperatures are also higher for more methane-rich mixtures, as is shown in Table 3. Thus, under conditions in which platinum is likely to be oxidised to a greater extent the catalyst becomes less active. This indicates that a more oxidised platinum surface is less active than a less oxidised surface. Similar conclusions have been reached before. Hicks et al. [ 181 have found that dispersed platinum which forms PtO, on oxidation is far less active than larger crystallites on which a layer of chemisorbed oxygen forms. This can be attributed to the differing reactivities of the surface oxygen species. Indeed, Niwa et al. [ 191 has related the reactivity of platinum catalysts to the ease of reduction of the platinum using temperature-programmed reduction. Further to this, Drozdov et al. [ 61 have found from measurements of the heat of adsorption of oxygen on reduced and oxidised platinum surfaces, that oxygen is bound more weakly on platinum metal than on the oxide, Therefore, a less oxidised form of platinum will be more active and platinum catalysts will be less active under conditions in which oxide formation is favoured i.e. an oxygen-rich gas mixture.
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In contrast to this, palladium metal adsorbs oxygen more strongly than the oxidised form, so palladium will be more active under conditions favouring oxide formation [ 61. Thus, one would expect palladium to be more active under oxygenrich conditions. This is the case, as is seen by the results presented in Table 4. The superiority of palladium for methane combustion under oxygen-rich conditions has been reported before, both by Drozdov et al. [ 61 and by Yao [ 171. In the latter case, in a mixture containing 12,500 ppm CH, and with Oz/CH4 = 4, palladium was found to give conversions up to 75% at temperatures up to 475°C whereas platinum gave conversions of only 10%. In our experiments, under methane-rich conditions the palladium catalyst is superior up to the point at which light-off occurs on the platinum catalyst (375°C). At these lower conversions, there will still be an excess of oxygen, so conditions favouring oxide formation will apply. Therefore, palladium would be expected to be the better catalyst. Under conditions which promote light-off in platinum catalysts, our results show that these catalysts are superior to palladium and may therefore be a useful component in exhaust catalysts. This has also been found to be the case by Oh et al. [ 31 who tested both platinum and palladium catalysts at 550°C with changing 02/ CH4 ratio. They find that for ratios <2.4 the platinum catalyst is superior. In addition, both the platinum and palladium catalysts showed a large drop in conversion around the stoichiometric point. For the palladium catalyst this appears to be in contrast to expected results, as palladium should be better in oxygen-rich conditions. This effect is attributed [ 31 to there being too much oxygen in the mix for methane to be adsorbed on the surface in sufficient quantities. This again indicates that the concentrations of reactants on the catalyst surface are important in methane combustion over these metals, and it is in agreement with the previous discussion regarding the origin of the light-off seen in the present work over platinum catalysts. 4.2. Production of carbon monoxide It was observed that carbon monoxide was produced on all catalysts in the methane-rich gas mix. The gas mix used contained only half the oxygen required for complete combustion of methane so the production of carbon monoxide is expected. Assuming that all the hydrogen from the methane is oxidised to water, carbon monoxide production should begin at 50% conversion. At 100% conversion the selectivity to carbon monoxide should be 50%. It was found that some carbon monoxide is formed at below 50% conversion. At high conversion the carbon monoxide selectivity is only slightly lower than expected. This indicates that the carbon monoxide is not being further oxidised. This is surprising since complete carbon monoxide combustion has been seen to occur on platinum catalysts at ca. 200°C [ 31. This may be due to localised areas on the catalyst having insufficient oxygen for complete combustion to occur. Another possibility is that the carbon
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dioxide produced is undergoing a reverse water-gas shift reaction with hydrogen produced from the incomplete methane combustion, to give carbon monoxide and water. If this is the case then the oxygen used to oxidise the carbon monoxide will not be available for methane combustion so giving an oxygen deficiency and thus less than complete methane conversion. However, almost complete combustion is seen at 550°C so this may not be the only additional mechanism for methane conversion. Indeed, it has been concluded by Oh et al. [ 31 from work in which carbon dioxide was added to the reactant stream, that the water-gas shift reaction is not important over platinum catalysts. Other possible reactions which may be promoted by 3-way exhaust catalysts [2] are between methane and either water or carbon dioxide, to give carbon monoxide and hydrogen, Either of these processes would explain the observation of carbon monoxide in the reaction products. 4.3. The ejjfect of particle size From the results shown in Fig. 7 it is apparent that there is no change in the specific activity of surface platinum atoms in the range of particle sizes studied in this work (1.4 to 3.7 nm diameter). In contrast, both Hicks et al. [ 181 and Otto [ 201 have found the specific activity of surface platinum atoms on small platinum crystallites to be lower than on particulate platinum. However, this effect has been found [ 181 to depend on the support, the catalyst precursor, and catalyst pretreatment, and not directly on the dispersion of the catalyst. In further contrast to our results, Briot et al. [21] have found that a catalyst treated in reaction mixture (O,/CH, = 4) at 600°C for 14 h is far more active than the fresh catalyst. The aged catalyst was found to have particles with an average diameter of 12 run, compared to 2 nm for the fresh sample. Similarly, Drozdov et al. [ 61 found that a catalyst calcined at 9OO”C,with 3% dispersion exhibits a rate of methane oxidation more than 7 times that for a catalyst with 96% dispersion. However, in both cases the more active catalysts were made by severe heat treatments of ch2oriize-cont~ailting samples, whereas in our work the catalysts were deliberately prepared chlorine-free, The heat treatments will cause the loss of some of the surface chlorine in the catalysts. Cullis and Willatt [ 22 3 have found that the activity of platinum catalysts for combustion is inhibited by the introduction of chlorine-containing organic compounds. Clearly, chlorine acts as a poison for the combustion reaction, presumably by blocking sites for adsorption of reactants. Thus, it possible that it is the removal of chlorine which leads to much of the enhanced activity observed by Briot et al. and by Drozdov et al. rather than the increase in particle size. Indeed, the latter may also be caused by chlorine removal, as it has been found that Pt/Al,O, catalysts prepared from chlorine-containing precursors have better dispersions than those prepared from chlorine-free precursors [ 231. This has been found [ 24,251 to be due to an interaction between platinumchlorine species and the support. Therefore, removal of chlorine may cause the loss of some metal-support interaction and thus allow migration of platinum to form
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larger particles. If this leads to the formation of particulate platinum, it is possible that the effect of chlorine removal and the effect of platinum particle growth could be confused. Thus, removal of chlorine may lead to platinum particle growth and removal of chlorine may lead to a higher activity. It would not necessarily be correct, therefore, to associate higher activity with larger platinum particles. In summary, our results indicate that for small platinum particles the methane combustion reaction is not structure sensitive.
5. Conclusions Platinum catalysts have been found to become more active as the reactant mixture is changed from oxygen-rich to methane-rich. The methane combustion reaction on small platinum particles is not structure sensitive. In stoichiometric, or methanerich mixtures, platinum catalysts experience light-off which may be due to local heating effects or to variations in the surface concentrations of adsorbed methane and adsorbed oxygen. In oxygen-rich mixtures oxygen prevents sufficient methane from being adsorbed and so light-off is not observed. Due to these light-off effects, platinum catalysts are superior to palladium in a fuel-rich gas mixture, whereas palladium is superior in an oxygen-rich atmosphere. Thus, platinum does have a role as a component in catalysts for emission control for natural gas vehicles.
Acknowledgements We are grateful to British Gas for providing financial support for this research and for giving permission to publish these results.
References [l] J.M. der Kinderen, W.J.J. van der Waal, G. Heijkoop and J.W. Geus, Proceedings of the 1989 International Gas Research Conference, Tokyo, 1989, p. 1373. [2] T. Fowler, D. Lander and D. Broomhall, Fuel, 70 (1991) 499. [3] S.H. Oh, P.J. Mitchell and R.M. Siewert, J. Catal., 132 (1991) 287. [4] H. Windawi, Platinum Metal Rev., 36 (4) (1992) 185. [5] R.M. Siewert and P.J. Mitchell, European Patent 0 468 556 Al (1991). [6] V.A. Drozdov, P.G. Tsyrulnikov, V.V. Popovskii, N.N. Bulgakov, E.M. Moroz and T.G. Galeev, React. Kinet. Catal. Lett., 27 (2) (1985) 425. [7] A. Schwartz, L.H. Holbrook and H. Wise, J. Catal., 21 (1971) 199. [8] J.R. Anderson, Structure of Metallic Catalysts, Academic Press, New York, 1975. [9] CF. Cullis and B.M. Willatt, J. Catal., 83 (1983) 267. [lo] D.L. Trimm and C.-W. Lam, Chem. Eng. Sci., 35 (1980) 1405. [ 1l] D.R. Kember and N. Sheppard, J. Chem. Sot., Faraday Trans. 2,77 (1981) 1321. [ 121 D.J. Kaul and E.E. Wolf, J. Catal., 91 (1985) 216. [ 131 S. Sharma, D. Boecker, G.J. Maclay and R.D. Gonzalez, J. Catal., 110 (1988) 103.
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[ 141 R.J. Matyi, J.B. Butt and L.H. Schwartz, J. Catal., 91 (1985) 185.
[ 151 J.C. Frost, P. Meehan, S.R. Morris, R.C. Ward and J. Mayers, Catal. Lett., 2 (1989) 97. [ 161 R. Burch and PK. Loader, in preparation. [ 171 Y.F. Yu Yao, Ind. Eng. Chem. Prod. Res. Dev., 19 ( 1980) 293. [ 181 R.F. Hicks, H. Qi, M.L. Young and R.G. Lee, J. Catal., 122 (1990) 280. [ 191 M. Niwa, K. Awano and Y. Murakami, Appl Catal., 7 (1983) 317. [20] K. Otto, Langmuir, 5 (1989) 1364. 1211 P. Briot, A. Auroux, D. Jones and M. Primet, Appl. Catal., 59 (1990) 141. [22] CF. Cullis and B.M. Willatt, J. Catal., 86 (1984) 187. [23] J. Volter, G. Lietz, H. Spindler and H. Lieske, J. Catal., 104 ( 1987j 375. [24] H. Lieske, G. Lietz, H. Spindler and J. Volter, J. Catal., 81 (I) (1983) 8, [25] G. Lietz, H. Lieske, H. Spindler, W. Hanke and J. Volter, J. Catal., 81 (1) (1983) 17.
Applied CatalysiSB: Environmental 5 (1994) 149-164
Investigation of Pt/&Q, and Pd/A& catalysts for the combustion of methane at low concentrations R. Burch *, P.K. Loader Catalysis Research
Group, Chemistry Department, University of Reading, Whiteknights Reading, RG6 2ADUK
Received24 May 1994;revised 2 August 1994;accepted2 August 1994
Abstract Pt/Al,O, and Pd/A1203 catalysts have been preparedfromchlorine-freeprecursors andinvestigated for the combustion of methane under lean, stoichiometric, and rich conditions using dilute mixtures. It has been found that under lean conditions, and at low conversions under stoichiometric or rich conditions, Pd/A1,03 is a more effective catalyst. However, at higher conversions with stoichiometric or rich mixtures Pt/Al,O, is a more active catalyst. This change over between Pd/Al,O, and Ptl Al,O, is associated with a ‘light-off’ effect observed with Pt/Al,O, catalysts. Various possible explanations for these effects are discussed. With the Pt/Al,O, catalysts there is no evidence of a particle size effect, which is in contrast to reports in the literature that the methane combustion reaction is structure sensitive. Reasons for these variations, including the possible inhibition of activity by chlorine used in many catalyst preparations, are discussed. It is concluded that platinum can be a more effective catalyst than palladium for methane combustion under real conditions and that, in consequence, platinum may play an important role in multimetallic catalysts for emission control for natural gas vehicles. Keywords:
Exhaust;Methanecombustion; Natural gas vehicles; Palladium/alumina; Platinum/alumina
1. Introduction Natural gas is a relatively clean source of hydrocarbons and so is of interest as an environmentally friendly fuel for motor vehicles. The use of natural gas also leads to a decrease in the emissions of SO, [ 11. However, legislation on emissions still requires that the exhaust gases be further cleaned up, and these regulations are now beginning to include the removal of trace amounts of methane, which comprises about 95% of natural gas. *Corresponding author. Tel. (+44-734)
318451, fax. (t44-734)
311610.
0926-3373/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO926-3373(94)00037-9
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Methane is the least reactive of the hydrocarbons and therefore the most difficult to oxidise. It has been found from tests on both automobile and stationary combustion engines that the traditional three-way catalysts used for controlling the emissions from gasoline powered vehicles are not able to remove all the methane. Thus, whilst > 95% of NO, and carbon monoxide were removed from the exhaust from a stationary combustion engine [ 21, only 50% of the methane was removed, On automobile engines [ 31 the conversion of methane was < 15%. The low conversion of methane occurs because the air:fuel ratio required for optimum performance in the natural gas system is different from gasoline [ 41. Under these conditions, threeway catalysts will require temperatures higher than are seen in exhaust gases in order to oxidise the methane 151. It is clear, therefore, that new catalysts need to be designed which can oxidise methane efficiently under the conditions present in the exhaust of natural gas powered systems. Such catalysts will also need to retain the high levels of carbon monoxide and NO, conversion seen on the three-way catalysts. In tests under simulated and real exhaust conditions [ 51, Pd-Pt catalysts were found to give superior methane conversion to conventional Pd-Pt-Rh three-way catalysts, as well as other metals. Although platinum catalysts are superior to palladium for the oxidation of higher alkanes and alkenes [ 6,7], the combustion of methane is easier over palladium under similar, oxygen rich, conditions. However, when platinum and palladium catalysts were compared under a range of conditions [3], platinum was found to give higher conversions at O,:CH, ratios less than 2.4 (2: 1 is stoichiometric) . Such fuel-rich mixtures may be found in the systems in which methane is used as a fuel so it is possible that platinum may have an important role in emission control for natural gas vehicles. Therefore, a more detailedunderstanding of platinum catalysts for methane combustion under realistic conditions of temperature and gas composition is required. This paper presents the results of a study of Pt/Al,O, catalysts for methane combustion in various gas mixtures, and compares their behaviour with that seen for Pd/Al,O, under similar experimental conditions,
2. Experimental 2.1. Catalystpreparation A series of Pt/Al,O, catalysts were prepared by dry impregnation. The A&O3 (Azko Chemie CK300, surface area 185 m* g-‘) was crushed and sieved to give particle sizes between 250 and 850 pm and was then dried at 500°C for 1 h. The required mass of di-nitroso di-amino platinum solution (Johnson Matthey) was made up to 2 g with doubly de-ionised water (i.e. 2 cm3 total solution). To this solution was added approximately 1 g of the support. The resulting slurry was shaken before being dried in an oven at 120°C for 2 h. The dried catalyst was then
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Table 1 Characterisation of Pt/Al,O, catalysts Catalyst
0.5% Pt l%Pt 2% Pt 4% Pt 4% Pd
Fresh
Used
Dispersion (%)
Average diameterinm
84 68 53 48 5.2
1.4 1.7 2.2 2.4 21.8
Dispersion (%)
Average diameter/nm
30
3.7
calcined at 500°C for 2 h. and cooled in a desiccator. The catalysts prepared contained 0.5, 1, 2, and 4% Pt by weight. A 4% Pd/A1203 catalyst was prepared from palladium nitrate (Johnson Matthey) using the same technique as for the platinum catalysts except that calcination was performed for 17.5 h as opposed to 2 h. The metal dispersion of the catalysts was determined by hydrogen chemisorption, assuming an H/Pt ratio of 1. Average particle diameters were calculated assuming spherical geometry and a value for the concentration of metal atoms on the surface of 1.25 X 10” atoms rnM2and 1.27 X 10” atoms mm2 for platinum and palladium, respectively [ 81. The dispersions and average particle sizes for the fresh catalysts are given in Table 1. 2.2. Catalyst testing Catalysts were tested in a micro-reactor flow system operating at atmospheric pressure. The catalyst ( 100 mg), supported by a glass wool plug, was placed centrally in a silica reactor (5 mm i.d., 30 cm long). A thermocouple was placed in the centre of the catalyst bed to monitor the temperature of the bed. The reactor was mounted centrally in a vertical tube furnace heated by a Eurotherm 812 temperature controller, and the temperature of the furnace monitored using a chromelalumel thermocouple situated on the wall at the mid-point of the furnace. The gases used were 2% CH4/N2, air and nitrogen (all from B.O.C.). The flows of these gases were controlled to give a total flow of 200 cm3 min- ’ and a partial pressure of reactants (CH,+ 0,) of 9.1 Torr. Three different O,/CH, ratios were used, namely 5: 1 (oxygen-rich), 2: 1 (stoichiometric) , and 1: 1 (methane-rich). The catalyst was heated to 300°C (as measured by the thermocouple in the catalyst bed) in the reaction mixture. The temperature was then increased to 550°C in steps of 25°C. The catalyst was held at each temperature for 10 min. (during which time three samples were taken for analysis) or until a steady value of methane conversion was reached. The temperature was then lowered to 300°C in 25°C steps and the same testing procedure carried out. This cycle was repeated. The product gases were analyzed by gas chromatography. Samples were taken using a lo-port sampling valve (Valco) . Analysis was performed using a Perkin
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Elmer 8410 gas chromatograph with a 2 m Porapak QS column operating at 60°C. Products were detected using a flame ionisation detector operating at 350°C A methanator (Ni/ZrO, catalyst at 350°C) was used in order to allow the detection of carbon monoxide and carbon dioxide. In all cases the activity of the catalyst is defined as the percent conversion of methane into all products. Conversions of methane were reproducible to within 0.1% at low conversions. 3. Results 3.1. 02/CH45:1 (2000 ppm CH,) All the catalysts showed the same trends. Representative results for the 4% Pt catalyst are shown in Fig. 1. A smooth increase in conversion with temperature is observed. There is a small decrease in activity from the first heating to the first cooling run. Results for the second heating and cooling runs were the same as for the first cooling run. The hysteresis from initial to steady state reached a maximum of 25°C for all the catalysts. The steady state activities for the platinum catalysts for this gas mixture are shown in Fig. 2. As expected, increasing the platinum loading gives an increase in conversion for a given temperature. Only the 4% Pt catalyst gives significant conversion below 400°C and complete conversion of methane is not seen for these catalysts even at 550°C the highest being 92.5% for the 4% Pt catalyst, Arrhenius plots have been constructed for conversions below 204/o,assuming that the kinetics of the reaction are first order in methane and independent of oxygen concentration. The calculated Arrhenius parameters are shown in Table 2. % CH4 Conversion
Temperature (C) Fig. 1. Methane conversion as a function of temperature for a 4%Pt/Al,O, catalyst using a 02:Ci.IJ ratio of 5: 1. (A) First heating run, (0) first cooling run, ( A ) second heating run, (e) second cooling run.
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Conversion
100,
Temperature (C) Fig. 2. Steady-state methane conversions as a function of temperature for various Pt/A&O, catalysts using a O,:CH, ratio of 5:l. (A) O.S%Pt, (0) l.O%Pt, ( A ) 2.O%Pt, (0) 4%Pt.
The catalysts all showed similar behaviour when exposed to a 2:l 02/CH4 mixture. Typical results are shown for the 4% Pt catalyst in Fig. 3. It was found that on heating there was a sudden increase in activity at a fixed reaction temperature. For example, on the first heating for the 4% Pt catalyst, the conversion on reaching 450°C was 26%, but this rose to a steady value of 53% over a period of a few minutes. A hysteresis is seen between the two heating runs and between heating and cooling. Both cooling runs gave the same results. This is in contrast to the results obtained with the 5:l mix where a hysteresis was seen only between the initial heating run and subsequent cooling and heating runs. The activities seen for the four catalysts during the second heating are shown in Fig. 4. Due to the light-off in activity, almost complete conversion is seen at 475°C for all except the 0.5% Pt catalyst. However, even this sample gives complete combustion at 550°C. It is of note that light-off does not occur at the same temperature for the four catalysts but at about the same conversion level, ca. 15%. Table 2 Parameters calculated from Arrhenius plots Catalyst
E,/M mol-I OZ/CH,
InA~O,/CH, S:f
E,/kJ mot-’ OJCH, 2:l
In A O,/CH, 2:l
15.1 i: 17.6& 14.5* 12.7i
1015 104& 91.44 9?J.1*
14.51 2.0 15.5_+ O,f 13.6, 1.1 15.85 2.2
5:l 0.5% Pt 1% Pt 2% Pt 4% PI
1os+ 121+ loo+ 83.85
I.5 10 5 5
0.3 2.5 1.0 1.3
10 1.0 5.0 11
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CH4 Conversion
,
I
550
Temperature (C) Fig. 3. Methane conversion as a function of temperature for a 4%Pt/Al,O, catalyst using a 02:CHJ ratio of 2:L (A) First heating run, (0) first cooling run, ( A ) second heating run, (0) second cooling run.
Arrhenius plots showed good linearity up to the point at which light-off occurs, where there is a discontinuity. Therefore, Arrhenius parameters were calculated using data obtained below the light-off temperature. These values are shown in Table 2. o/oCH4 Conversion
Temperature (C) Fig. 4. Methane conversions, during the second heating run, as a function of temperature for various using a 02:CHJ ratio of 2:l. (A) 0.5%Pt, (0) l.O%Pt, ( A) 2.0RPt, i*j 44cPt.
catalysts
Pt/hl,O,
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Temperature (C) Fig. 5. Methane conversion as a function of temperature for a 4%Pt/Alz03 catalyst using a 02:CH4 ratio of 1:l. (0) First heating run, ( * ) first cooling run, ( A ) second heating run, (0) second cooling run.
3.3. O,/CH, 1:l (6000ppm CH,) Results are shown for the 4% Pt catalyst in Fig. 5. As with the 2: 1 mix, light-off was observed. Again, there is a hysteresis between the heating and cooling curves which is sustained for at least two heating/cooling cycles. The activities of the four catalysts for the second heating are shown in Fig. 6. Light-off again occurs at a given conversion level for all the catalysts as opposed to a given temperature. The conversion required for light-off is ca. 3%, much lower than with the 2: 1 02/CH4 mix. It is also clear from Fig. 6 that the conversion levels % CH4 Conversion 100
7
go80
7060-
40 -. 30 SO20-
I:“’
‘“li&dLu &IO
350
400
I
I
450
500
I
550
Temperature (C) Fig. 6. Methane conversions, during the second heating run, as a function of temperature for various Pt/Ai,O, catalysts using a O,:CH, ratio of 1:l. (A) 0.5%Pt, (0) l.O%Pt, ( A ) 2.0%Pt, (0) 4%Pt.
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Table 3 Effect of O,/CH, ratio on the conversion of methane at low temperatures Catalyst
TempYC
OZ/CHJ .5:1
OJCH, 2: I
0,/C& 1:I
0.5% Pt
325 350 31.5 400 350 315 400 425 350 375 300 425 300 325 350 375
0
0.3 0.7 1.6 3.0 1.2 2.5 5.0 9.3 1.8 3.7 6.9 13.7 0.9 1.9 3.9 7.7
OS 1.1 2.1 4.7 1.5 2.9 59” 70” 2.0 50” I ”
l%Pt
2% Pt
4% Pt
0.3 0.7 1.7 0 0.3 1.3 3.8 0 0.7 2.1 5.2 0.7 1.4 3.5 5.8
1.2 2.4 69 62’
’ Light-off had occurred at this point.
off after light-off, although almost complete conversion is seen at 550°C. Carbon monoxide formation was observed for conversions above 40%, the selectivity to carbon monoxide reaching 45% at 100% CH4 conversion. As light-off occurred at low conversions, it was not possible to construct sensible Arrhenius plots for these experiments. From a comparison of Figs. 2,4, and 6, the effect of changing the gas composition can be seen. It is apparent that on going from an oxygen-rich to a methane-rich mixture the catalysts become more active at a given temperature, although at the highest temperatures the oxygen limitation seen with the 1:1 mix leads to values for conversion being lower than with the 2: 1 mix. These differences are mainly due to the light-off seen for the 1:1 and 2: 1 mixes, although at temperatures below these points the same trend is still seen, as is shown in Table 3, i.e., the conversion increases in the order 5:1<2:1< 1:l. 3.4. Particle size effects with platinm
catalysts
In order to determine whether any correlation existed between catalyst activity and particle size, the rate of methane conversion was calculated per surface platinum atom, again assuming first order kinetics with respect to methane. These specific rates were calculated from initial results using the values obtained on fresh catalysts, and, in one case, from steady state results using the metal dispersion obtained from hydrogen chemisorption on the used sample (see Table i >. The used sample was the 4% Pt catalyst which had been given two heat-cool cycles in the 5: 1 gas mix, as shown in Fig. 1. Plots of specific rate versus average particle size are shown for
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Rate/ (~Molecdes CH4 is /surface Pt atom)
0.09 o.os-
/--
0.07 0.06’ 0.05 0.04L I 0.030.02;
.
l
.
/ ’
l
* .
Average Pt Particle Size inm Fig. 7. Specific rate of methane oxidation as a function of platinum particle size at various temperatures. (A) 425”C, (0) 45O”C,( A ) 475”C, (0) 5OO”C,( * ) 525”C, ( X ) 550°C.
the oxygen-rich mixture in Fig. 7. It is clear that the specific rate is independent of particle size in the range 1.4 to 3.7 nm at all temperatures in the range 425 to 500” C. At higher temperatures there may be a small increase in rate with increasing particle size. However, overall these results clearly indicate no particle size effect for this gas mixture. Similar results were obtained using the 2: 1 gas mixture. 3.5. Palladium vs platinum The 4% Pd/A1,03 catalyst was tested using 5:l and 1:l 02/CH4 gas mixtures. The results obtained are shown in Table 4, where the results for the 4% Pt catalyst Table 4 Comparison of 4% Pt/Al,O, and 4% Pd/Alz03 catalysts Temp./“C
OJCH, 5: 1 Pt
300 325 350 375 400 425 450 475 500 525 550
1.2 2.2 4.2 6.6 14.0 22.0 35.3 49.2 65.8 81.6 93.8
02/CH4 1:l Pd 23.0 40.6 82.5 94.5 98.5 100
Pt
Pd
0.6 1.2 4.0 59.0 67.9 78.8 88.1 94.6 97.9 99.0 99.4
9.1 18.2 33.0 50.7 59.2 68.1 77.6 86.2 91.1 93.1 94.5
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are also shown for comparison. In the oxygen-rich mixture the palladium catalyst gives 23% conversion at 300°C compared to 1.2% for the platinum catalyst. Complete conversion is achieved at 450°C on palladium, at which temperature platinum gives only 35% conversion. Under fuel-rich conditions the palladium catalyst is again superior at lower temperatures, giving 33% conversion at 350°C as opposed to 4% for the platinum catalyst. However, above 350°C the platinum catalyst exhibits light-off. A similar increase is not seen on the palladium catalyst so higher conversions are achieved on the platinum catalyst above this temperature.
4. Discussion There are few datain the literature with which to directly compare the conversions seen on the platinum catalysts used in this study. This is because most other work has used gas mixtures with far higher concentrations of reactants. However, some of the general trends observed here have been noted in other work and will now be discussed. 4.1. Light-off on platinum catalysts A major feature seen on the platinumcatalysts is the light-offin activity observed with both the stoichiometric and methane-rich gas mixtures. Similar behaviour has been observed previously on platinum catalysts. Thus, Cullis and Willatt [ 91 have found a doubling in conversion between 369 and 374°C on a 3% Pt/ThO, catalyst, and an eightfold increase between 337 and 360°C for a 2.7% Pt/Al,O, catalyst for a CH,/O, ratio of 2.2. Trimm and Lam [lo] found a jump in activity at 540°C for a 0.4% Pt/Al,O, catalyst in a 1:l CH,/O, reaction mixture. In both cases a discontinuity was seen in Arrhenius plots, leading to an apparent change in the activation energy to a lower value. Such a discontinuity was seen in the Arrhenius plot obtained in this work for the stoichiometric gas mix. The exact cause of the light-off phenomenon is not clear. One possibility is that it is due to local heating of metal particles within the catalyst bed. A detailed explanation of the effect is given by Schwartz et al. [7] in terms of the relative rates of heat generation at the surface due to the exothermic reaction, and heat loss from the surface by conduction and convection. There is, however, some debate as to whether or not such local heating effects really do occur, Studies have been reported [ 1l-151 in which various techniques have been used to measure both the temperature of the support and of the metal particles during both the CO/O2 and CO/H, reactions. Whilst some studies have found temperature differences up to 190°C as well as hot spots, on platinum andPd/SiO;? catalysts [ 11,121, others find little or no change [ 13-151. The lack of a temperature difference has been attributed [ 131 to a rapid dissipation on heat from metal particles to the support, Therefore, whilst local heating of metal particles can occur, the lifetime of this effect may be
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small, so conditions which lead to the local heating of metal particles will need to be maintained in order for the effect of light-off to be seen. As will be discussed later, this may involve the presence of specific concentrations of reactants on the surface, which will give rise to an enhanced reaction rate. The maximum extent of any local heating of the platinum particles, if it occurs at all in the present work, was derived from the results obtained using the stoichiometric gas mixture (Fig. 4). The activities at temperatures above the light-off point were re-calculated using the activation energies obtained from the data below the light-off temperature. Comparison of the calculated and observed conversions showed that the platinum particles appear to be between 75 and 100°C hotter than the average temperature measured by the thermocouple located in the catalyst bed (e.g. for 4% Pt, 99% conversion is calculated to occur at 550°C but is actually observed at 450°C). Clearly a local heating effect could explain our results. However, a closer inspection reveals that the origins of these effects are rather more complicated. It is clear from Figs. 4 and 6 that light-off does not occur at a particular temperature but at a specific conversion. This is ca. 15% for the stoichiometric mix and ca. 3% for the methane-rich mix. These correspond to conversions of 600 and 180 ppm CH4, respectively. In addition, no light-off is seen at all for the oxygen-rich mixture. Thus, the total amount of methane reacted, and therefore the total amount of heat generated on the metal particles at light-off, does not appear by itself to explain the light-off phenomenon. On the other hand it is possible that when specific concentrations of methane and oxygen are adsorbed together on the surface the combustion reaction will be favoured. For example, we will show [ 161 that the rate of methane combustion goes through a sharp maximum before declining again, as the surface concentration of oxygen goes from zero to monolayer coverage. It seems possible that the surface coverage of adsorbed methane and adsorbed oxygen may change as the reaction proceeds and that an optimum State may be reached at the point at which light-off is observed, whilst at lower conversions there may be too much oxygen on the surface for adsorption of methane to be effective. The kinetics of methane combustion have been reported previously [ 171 to be approximately first order in methane and zero order in oxygen. It therefore seems reasonable to assume that the ease with which methane can approach, and dissociatively adsorb, on the (partially oxidised) platinum surface will be an important factor. A similar idea has been proposed for the combustion of other hydrocarbons on platinum and palladium filaments by Schwartz et al. [ 71 who suggest that fractional coverage of the surface by both reactants is a requirement for optimum catalyst performance in the combustion reaction. As the initial amount of oxygen present is less for the methane-rich mixture ( 02:CH4 = 1: 1) than for the stoichiometric mixture ( 02:CH4 = 2: 1)) it would be expected that the required surface concentrations of adsorbed methane and adsorbed oxygen would be reached at lower conversion in the former case. This is indeed what has been observed. On the other hand, for the oxygen-rich mixture
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(O,:CH,=5:1) there may be too much oxygen present at all conversions and so no optimum surface coverage by both reactants is possible and light-off is not observed. It is also worth noting that no light-off is seen on the palladium catalyst, This may be because the amount of oxygen taken up by palladium is too great, even under these conditions, for the required surface concentration of methane to be reached. Such a phenomenon has also been proposed for other hydrocarbons by Schwartz et al. [7], who suggest that the stable oxide layer present on palladium prevents the formation of a necessary fractional coverage of hydrocarbon, Whilst the above discussion suggests that specific surface concentrations of the two reactants are necessary for increased methane combustion, it is still not clear how much of the increase in the rate of reaction seen during light-off is due to this effect and how much is due to the local heating which is caused by it. For both the stoichiometric and methane-rich gas mixtures, where light-off is seen, there is a hysteresis between the heating and cooling curves. This could be because once the platinum particles are hot it is necessary to increase the rate of cooling, by lowering the background (catalyst bed) temperature before the platinum particles can attain their ‘normal’ temperature. On the other hand, it is also possible that the favourable surface concentrations of reactants, once present, can be maintained for longer during cooling, it being harder for oxygen to re-cover the surface of the metal particles. However, the two effects - local heating, and surface coverage by adsorbed species - are interrelated and it is not possible to distinguish between them with any certainty. Comparison of Figs. 2, 4, and 6 shows that the activity of platinum catalysts increases on going from an oxygen-rich to a methane-rich system, although at higher temperatures the oxygen deficiency in the 1:l mix leads to slightly lower conversions than for the 2:l mixture. Whilst the major differences are due to the temperature at which light-off occurs, conversions below these temperatures are also higher for more methane-rich mixtures, as is shown in Table 3. Thus, under conditions in which platinum is likely to be oxidised to a greater extent the catalyst becomes less active. This indicates that a more oxidised platinum surface is less active than a less oxidised surface. Similar conclusions have been reached before. Hicks et al. [ 181 have found that dispersed platinum which forms PtO, on oxidation is far less active than larger crystallites on which a layer of chemisorbed oxygen forms. This can be attributed to the differing reactivities of the surface oxygen species. Indeed, Niwa et al. [ 191 has related the reactivity of platinum catalysts to the ease of reduction of the platinum using temperature-programmed reduction. Further to this, Drozdov et al. [ 61 have found from measurements of the heat of adsorption of oxygen on reduced and oxidised platinum surfaces, that oxygen is bound more weakly on platinum metal than on the oxide, Therefore, a less oxidised form of platinum will be more active and platinum catalysts will be less active under conditions in which oxide formation is favoured i.e. an oxygen-rich gas mixture.
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In contrast to this, palladium metal adsorbs oxygen more strongly than the oxidised form, so palladium will be more active under conditions favouring oxide formation [ 61. Thus, one would expect palladium to be more active under oxygenrich conditions. This is the case, as is seen by the results presented in Table 4. The superiority of palladium for methane combustion under oxygen-rich conditions has been reported before, both by Drozdov et al. [ 61 and by Yao [ 171. In the latter case, in a mixture containing 12,500 ppm CH, and with Oz/CH4 = 4, palladium was found to give conversions up to 75% at temperatures up to 475°C whereas platinum gave conversions of only 10%. In our experiments, under methane-rich conditions the palladium catalyst is superior up to the point at which light-off occurs on the platinum catalyst (375°C). At these lower conversions, there will still be an excess of oxygen, so conditions favouring oxide formation will apply. Therefore, palladium would be expected to be the better catalyst. Under conditions which promote light-off in platinum catalysts, our results show that these catalysts are superior to palladium and may therefore be a useful component in exhaust catalysts. This has also been found to be the case by Oh et al. [ 31 who tested both platinum and palladium catalysts at 550°C with changing 02/ CH4 ratio. They find that for ratios <2.4 the platinum catalyst is superior. In addition, both the platinum and palladium catalysts showed a large drop in conversion around the stoichiometric point. For the palladium catalyst this appears to be in contrast to expected results, as palladium should be better in oxygen-rich conditions. This effect is attributed [ 31 to there being too much oxygen in the mix for methane to be adsorbed on the surface in sufficient quantities. This again indicates that the concentrations of reactants on the catalyst surface are important in methane combustion over these metals, and it is in agreement with the previous discussion regarding the origin of the light-off seen in the present work over platinum catalysts. 4.2. Production of carbon monoxide It was observed that carbon monoxide was produced on all catalysts in the methane-rich gas mix. The gas mix used contained only half the oxygen required for complete combustion of methane so the production of carbon monoxide is expected. Assuming that all the hydrogen from the methane is oxidised to water, carbon monoxide production should begin at 50% conversion. At 100% conversion the selectivity to carbon monoxide should be 50%. It was found that some carbon monoxide is formed at below 50% conversion. At high conversion the carbon monoxide selectivity is only slightly lower than expected. This indicates that the carbon monoxide is not being further oxidised. This is surprising since complete carbon monoxide combustion has been seen to occur on platinum catalysts at ca. 200°C [ 31. This may be due to localised areas on the catalyst having insufficient oxygen for complete combustion to occur. Another possibility is that the carbon
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dioxide produced is undergoing a reverse water-gas shift reaction with hydrogen produced from the incomplete methane combustion, to give carbon monoxide and water. If this is the case then the oxygen used to oxidise the carbon monoxide will not be available for methane combustion so giving an oxygen deficiency and thus less than complete methane conversion. However, almost complete combustion is seen at 550°C so this may not be the only additional mechanism for methane conversion. Indeed, it has been concluded by Oh et al. [ 31 from work in which carbon dioxide was added to the reactant stream, that the water-gas shift reaction is not important over platinum catalysts. Other possible reactions which may be promoted by 3-way exhaust catalysts [2] are between methane and either water or carbon dioxide, to give carbon monoxide and hydrogen, Either of these processes would explain the observation of carbon monoxide in the reaction products. 4.3. The ejjfect of particle size From the results shown in Fig. 7 it is apparent that there is no change in the specific activity of surface platinum atoms in the range of particle sizes studied in this work (1.4 to 3.7 nm diameter). In contrast, both Hicks et al. [ 181 and Otto [ 201 have found the specific activity of surface platinum atoms on small platinum crystallites to be lower than on particulate platinum. However, this effect has been found [ 181 to depend on the support, the catalyst precursor, and catalyst pretreatment, and not directly on the dispersion of the catalyst. In further contrast to our results, Briot et al. [21] have found that a catalyst treated in reaction mixture (O,/CH, = 4) at 600°C for 14 h is far more active than the fresh catalyst. The aged catalyst was found to have particles with an average diameter of 12 run, compared to 2 nm for the fresh sample. Similarly, Drozdov et al. [ 61 found that a catalyst calcined at 9OO”C,with 3% dispersion exhibits a rate of methane oxidation more than 7 times that for a catalyst with 96% dispersion. However, in both cases the more active catalysts were made by severe heat treatments of ch2oriize-cont~ailting samples, whereas in our work the catalysts were deliberately prepared chlorine-free, The heat treatments will cause the loss of some of the surface chlorine in the catalysts. Cullis and Willatt [ 22 3 have found that the activity of platinum catalysts for combustion is inhibited by the introduction of chlorine-containing organic compounds. Clearly, chlorine acts as a poison for the combustion reaction, presumably by blocking sites for adsorption of reactants. Thus, it possible that it is the removal of chlorine which leads to much of the enhanced activity observed by Briot et al. and by Drozdov et al. rather than the increase in particle size. Indeed, the latter may also be caused by chlorine removal, as it has been found that Pt/Al,O, catalysts prepared from chlorine-containing precursors have better dispersions than those prepared from chlorine-free precursors [ 231. This has been found [ 24,251 to be due to an interaction between platinumchlorine species and the support. Therefore, removal of chlorine may cause the loss of some metal-support interaction and thus allow migration of platinum to form
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larger particles. If this leads to the formation of particulate platinum, it is possible that the effect of chlorine removal and the effect of platinum particle growth could be confused. Thus, removal of chlorine may lead to platinum particle growth and removal of chlorine may lead to a higher activity. It would not necessarily be correct, therefore, to associate higher activity with larger platinum particles. In summary, our results indicate that for small platinum particles the methane combustion reaction is not structure sensitive.
5. Conclusions Platinum catalysts have been found to become more active as the reactant mixture is changed from oxygen-rich to methane-rich. The methane combustion reaction on small platinum particles is not structure sensitive. In stoichiometric, or methanerich mixtures, platinum catalysts experience light-off which may be due to local heating effects or to variations in the surface concentrations of adsorbed methane and adsorbed oxygen. In oxygen-rich mixtures oxygen prevents sufficient methane from being adsorbed and so light-off is not observed. Due to these light-off effects, platinum catalysts are superior to palladium in a fuel-rich gas mixture, whereas palladium is superior in an oxygen-rich atmosphere. Thus, platinum does have a role as a component in catalysts for emission control for natural gas vehicles.
Acknowledgements We are grateful to British Gas for providing financial support for this research and for giving permission to publish these results.
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