Enahncement of the reaction of nitric oxide and carbon monoxide by hydrogen and water over platinum and rhodium-containing catalysts

A. Frennet and J.-M. Bastin (Eds.) Catalysis and Automotive Pollution Control 111 Studies in Surface Science and Catalysis, Vol. 96 9 1995 Elsevier Science B.V. All rights reserved.

123

ENHANCEMENT

OF THE REACTION OF NITRIC OXIDE AND CARBON MONOXIDE BY HYDROGEN AND WATER OVER PLATINUM AND RHODIUM-CONTAINING CATALYSTS

R. D t ~ m p e l m a n n a, N . W . C a n t a a n d D.L. T r i m m b

~School of Chemistry, Macquarie University, Sydney NSW 2109, Australia bSchool of Chemical Engineering and Industrial Chemistry, University of New South Wales, Kensington NSW 2033, Australia

ABSTRACT

The addition of hydrogen or water to a stoichiometric mixture of CO and NO over supported platinum and rhodium catalysts can significantly increase the conversion of both NO and CO. Theselectivity of NO reduction is also affected in a remarkable manner. With Pt/AI203 at 210~ for example, addition of"~900 ppm 1-12increases the NO conversion from 6% to 48% and that of CO from 5% to 33%. The nitrogen atom of NO is converted quite selectively to NH3 while the oxygen atom forms CO2 rather than water. Addition of ~1400 ppm water also enhances both CO and NO conversion over Pt/AI203 but N2 rather than NH3 is the favoured product. Hydrogen and water also produce significant rate enhancements with Rh/AI203 and Rh/CeO2/AI203but the product distributions are somewhat different from those with Pt/A1203. Possible reasons for the enhanced activities and the observed selectivities are discussed.

1.INTRODUCTION

There is considerable literature concerning catalysis of the NO + CO and NO + H2 reactions over Rh and Pt in various forms. A general conclusion is that the latter reaction is substantially faster than the former with Rh [1,2] and especially Pt [1-4] under equivalent conditions. With respect to NO removal, the presence of CO inhibits the NO + H2 reaction [1-6], see also [7]. Rather surprisingly there appears to be no definitive studies of the mixed NO, CO, 1-12reaction system [7] even though all three gases are simultaneously present in automobile exhaust gases.

124 There is also little detailed work on the effect of water on the catalysed reaction of CO with NO. One might expect any effects to arise via hydrogen formation through the water gas shitt reaction (WGSR) between CO and 1-120. However, appreciable formation of NH3 has been found with CO, NO and water mixtm-es under conditions of little or no WGS activity [8] which indicates that water can act in other ways. In this context it may be noted out that current promoters such as ceria are associated with a high WGS activity [9,10] and that water may also act as an reoxidant of reduced ceria [11] with hydrogen evolution [12]. Promotional effects by both hydrogen [13-15] and water [15] have been reported for the oxidation of CO by oxygen. The exact mechanism is unclear. The present work describes the effect of hydrogen and of water on the NO + CO reaction over Rh and Pt at temperatures typical for the warm-up phase of catalytic converters. The data obtained demonstrate pronounced effects on reaction rates and product distributions which have apparently not been reported both in previous laboratory studies of the binary systems (NO + CO and NO + 142) and when using simulated exhaust gas mixtures.

2.EXPERIMENTAL

Catalyst: The catalysts were prepared by incipient wetness of powdered A1203 (Condea alumina washeoat grade, surface area ~140mE/g) with aqueous H2PtCI6 and RhC13 solutions to yield nominal contents of 1 wt% Pt and 0.53% Rh (same molar content). The slurries were dried at 50~ under mild vacuum in a rotary evaporator, dried fitrther at 100~ overnight and subsequently calcined for 4 h at 500~ The eeria containing catalyst was prepared by first impregnating with a solution of Ce(NO3)3, followed by drying and calcination as above, and then impregnated with the rhodium salt. The resulting powders were pressed, crushed and a sieve fraction of 106-180 mm was used in subsequent experiments. The pretreatment consisted of a temperature programmed reduction (1% Hz/He, 9~ up to 500~ followed by 30 min equilibration with 2000 ppm of CO and NO at 500~ and cooling in CO/NO to reaction temperature. Reactor: The experiments were performed in a continuous flow tubular reactor (Pyrex, 8 mm OD, 5 mm ID). The catalyst sample was held by a plug of quartz wool on top of a thermoeouple. All gases and gas mixtures were of high purity grade and not fiuther pmified. Calibrated mass flow controllers assured the desired concentrations. Analysis: The analyses were performed on-line by infrared spectroscopy (IR) and mass spectrometry (MS). The former method used a dispersive infrared spectrometer (Perkin-Elmer 580B) with a multiple path cell (2.4m total path length) and a control

125 computer. A routine was developed by which the absorbances of CO (2117 cml), NO (1877 crnl), N20 (1300 crn-1), NH3 (965 cm1) and CO2 (679 crn-1) were repeatedly acquired and stored on a cycle time of ~ 7 minutes. The mass spectrometer (VG300SX) was used for the detection of H~ (rn/e =2), CO2 (m/e =22) and NO (m/e =30). Signals at m/e =44 can stem from either N20 or CO2. Therefore, carbon dioxide was reliably measured by use of its doubly charged ion at m/e =22, which has a low background signal and is not interfered with N20 (no signal at m/e = 22 detected). In one instance (Figure 1) CO was also calculated from the MS signal at m/e =12 knowing the fragmentation patterns of CO and CO,. Formation of N2 and H~O were calculated by nitrogen and hydrogen balances. The gases CO, NO and N20 were calibrated through use of diluted standard mixtures. Known concentrations of CO~ and NH3 were obtained by passing CO and excess 02, or NO and excess H~, over the catalyst at 300~ In the latter case, concurrent nitrogen formation was measured by C~ analysis and the absence of N~O demonstrated by IR. Conditions: The catalyst weight was 75 mg and the flow rate 100 ml/min (STP) giving a GHSV of 50,000 h 1 (STP). The effect of addition of hydrogen and/or water on the reaction of CO and NO was determined under isothermal conditions. Each experiment was performed in the sequence a,b,c,d,a,c,b,d,a with a = only CO and NO, 2000 ppm each; b = a + ~ 890 ppm H2; c - a + ~ 1400 ppm (0.14%) H~O ; d = a + 900 ppm H2 and 1400 ppm H20. Steady-state reaction rates were obtained by nmning the corresponding experimental conditions for at least 21 minutes (3 IR analysis of the 5 gases) and taking the average of the last two measurements. Periods as long as one hour were required under some conditions to achieve a steady-state. The sequences were executed over several days at random temperatures in the range 190 - 290~

3.RESULTS 3.1 Example run Figure 1 provides an example of the changes in concentration observed during the first half of the standard sequence - in this case for reaction over Pt/A1203 at 290~ It should be noted that the data are not internally normalized, e.g. MS versus IR data, which would be impossible without some assumptions. The apparently negative N2 concentrations in some cases simply reflect the limits of analysis under conditions of small formation of N2. The conclusions which follow are based not solely on Figure 1 but on the examination of many data sets for a range of conditions. As shown in the top panel, introduction of H2 (a --~ b) induces a drop in NO signal (i.e. enhanced NO conversion) and an increase in CO2 formation as detennined by both MS and IR. Thus CO conversion has increased as well. Substitution of H2 by

126 1-120 (19 ~ c) gives ~ 2000 ppm CO~ (i.e. complete CO conversion) and the NO signal drops to zero. Addition of both Hz and 1-120 (c ~ d) gives a similar result while deletion of both (d ~ a) returns the concentrations close to the original values, after a delay. The subsequent a ~ c step results in similar steady state concentrations to those seen alter the successive a ~ b ~ c steps. The distribution between the various nitrogen containing products (lower panel) is also interesting.

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Fig. 1 Concentration changes upon addition of H2 and~or H20 using Pt/Al203at 2~'~C. a =2000tTrn CO+NO, b.= a +890tlxn H~ c = a +1 4 0 0 ~ H20, d= a +H2+H20 Ammonia is dominant when hydrogen alone is included (b) whereas nitrogen dominates on addition of water alone (r Both NH3 and N2 are formed when 142 and 1-120 are included together (d). It should be noted here that the quantifies of NH3

127 detected (~ 540 ppm) are sufficient to account for most of the 890 ppm of H2 added. Conversely, very tittle hydrogen can be converted to water (< 60 ppm by H balance). The total carbon balance traces in the top panel reveal interesting transient effects. Addition of water (b --> e or a --> e) results in periods during which the carbon balance is in excess of the input quantity (2000 ppm). This indicates that a surface species has been reacted to produce carbon dioxide. Peaks in ammonia evolution occur over the same periods and the calculated N2 concentrations are then strongly negative as expected if the quantifies of NH3 (plus N20) being produced exceed that expected from the NO conversion. Deletion of water (d --> a) is followed by a period of deficient carbon balance (i.e. uptake to form a surface species). The significance of these transient effects is commented upon later. It should be stressed here that both the steady state concentrations and the transient effects were quite reproducible in response to other step-changes in the input concentration in the remainder of the standard sequence (i.e. e --> b --> d --->a). 3.2 Effects of temperature Similar sequences of experiments to those illustrated in Figure 1 were carried out at other temperatures and with the Rh/A1203and Rh/CeO2//0203 catalyst. Figure 2 shows concentration versus temperature plots extracted from the data set. The method of data collection results in some scatter (since determinations at different temperatures may be long separated in time) but minimises bias (since measurements with different compositions at the same temperature are close in time). The following conclusions may be drawn 1. With Pt/A1203 inclusion of H2 and/or H20 greatly enhances CO2 production (i.e. CO conversions) at all temperature (top panel, left). The enhancement by H2 exceeds that by H20 below 250~ but the reverse is true at higher temperatures. A similar pattern of enhanced CO conversion is evident with Rh/A1203 (top panel, fight) but its extent is less pronounced because of the higher intrinsic activity of rhodium for the reaction of CO with NO. 2. The principal nitrogen-containing product over Pt/A1,O3 when H2 is added is ammonia (second panel, left). However enhancement by water occurs largely with nitrogen formation (third panel, left). There is moderate N20 formation under all conditions (bottom panel left) and it is the most important nitrogen containing product for the reaction of CO and NO alone. 3. With Rh/A1203 enhancement by H2 (or by H2 and H,O together) gives more N,O than NH3 or N2 at the lowest temperatures but the relative amounts of each tend to equalise in terms of nitrogen content above 240~ As with Pt/A1,O3 nitrogen is the favoured product when water alone is added (third panel, right). Experiments with a catalyst containing 6% ceria (Rh/CeOJA1203), not shown, exhibit similar responses to the addition of H2 and H,O as the undoped Rh/A1,O3.

128

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129 Differences in activity and selectivity between the two catalysts were also minor indicating that eeria has little effect on reaction characteristics under the conditions used here.

4.DISCUSSION Tables 1 and 2 summarise the most striking findings of the present work for the effect of H2 and HzO on the reaction of CO and NO. With Pt/A1203 at 210~ addition of hydrogen (Table 1) increases the conversion of NO by a factor of eight and that of CO similarly. Hydrogen conversion is close to complete. Over 90% of this hydrogen reacts with the nitrogen atom of NO to form ammonia, the dominant nitrogencontaining product, rather than with the oxygen atom to give water. Conversely the oxygen from NO is converted to CO2 with very high selectivity. In stoichiometric terms the reaction can be represented as CO + NO + 1.5H2

CO2 + NH3

(1)

At the same temperature, hydrogen enhances NO conversion over Rh/AlzO3 to a similar extent. However the conversion of 1-12is incomplete and the reaction tends to produce less ammonia and more NzO and N2. As a result the NO conversion is increased to a greater extent than the CO conversion. The increase in CO conversion on hydrogen admission appears very peculiar and to our knowledge has not been reported previously. Assuming that the reaction of NO proceeds by dissociation of adsorbed NO on vacant sites (*) i.e. *NO + * ~ * N + * O

(2)

then the adsorbed oxygen is seen to prefer to react with adsorbed carbon monoxide to form carbon dioxide O* + *CO ~

CO2 + 2*

(3)

rather than with hydrogen to form water O* + H* --~ *OH + * 9O H + H* ~ 1-120 + 2*

(4a) (4b)

The most probable explanation is that steps (3), (4a) and (4b) are all fast but that the quantity of adsorbed CO is much greater than that of hydrogen. As a result, the probability of an individual O* reacting according to (3) is very high. This is similar

130 to the situation prevailing during the reaction of CO/HJO2 mixtures on platinum group metals [13,14] where it is possible selectively to oxidise CO in a large excess of H2 because the former predominates on the metal surface. There is no simple explanation for the enhanced NO conversion observed when hydrogen is present. Step (2) above is generally thought to be the initial step [16,17]. It seems tmlikely that hydrogen could act by removing adsorbed O since the concentration of this species is believed small due to its efficient removal by reaction with adsorbed CO. In any case this reaction should lead to water which is not the favoured hydrogen containing product. Removal of adsorbed N to form ammonia (as observed) with creation of vacant sites could increase the rate of reaction (2) but only in situations where the nitrogen coverage was relatively high. This is just conceivable with rhodium [7] but is most tmlikely with platinum where a very large fraction of the surface is occupied by carbon monoxide [ 17,18]. However, removal of either adsorbed O or N by hydrogen may lead only temporarily to a large increase in vacant sites, because thus sites would be rapidly occupied by CO. A different situation arises if the removal of N or O itself is rate detennining, which seems rather unlikely. Another possibility is that NO dissociation is "hydrogen assisted' as proposed by Hecker and Bell [19] for the reaction between NO and 1-12i.e. *NO + H* ~

(5)

*N + *OH

Table 1 Enhancement by hydrogen Catalyst Inlet composition (temperature)

Conversion

(%)

NO

CO

Selectivities (%) of underlined atoms to NI-I~ a C0__2 N_N2 r NI-'I3 d b

Pt/A1203 210~

2000 ppm CO + NO + 890 ppm HE

6 47

5 33

97

100 96

e 0

75

Rh/ml203

2000 ppm CO + NO + 890 ppm H2

16 69

12 35

82

100 89

44 45

13

210~

a selectivity of the hydrogen atom, H ~ NH3 = 3 N H 3 / ( H 2 consumed) b selectivity of the oxygen atom, N(O) ~ CO2 = CO2/(NO consumed - N20 formed) selectivity of the nitrogen atom, (N)O ~ N2 = 1-(NH3+2N20)/(NO consumed) d selectivity of the nitrogen atom, (N)O ~ NH3 = NH3/(NO consumed) e not measured accurately at low conversions but N20 dominant

131 This path is favoured by recent theoretical calculations [20] which show that the activation energy is less than that for unassisted dissociation (reaction (2)) on both Pt (25 versus 54 kJ/mol) and Rh (13 versus 29 kJ/mol). An apparent problem with step (5) in CO/NO/H, mixtt~es is that it produces OH which one might expect to react to form water which is a minor product especially with Pt. A rapid reaction between CO and OH, or between water and support-bound isocyanate species as discussed later, would be required to produce CO2. "Hydrogen assisted' NO dissociation could also proceed via *NO + H* ~

(6)

*NH + *O

Although high activation energies were calculated for this reaction 20 (Pt: 96 kJ/mol, Rh: 67 kJ/mol) it would produce directly the adsorbed oxygen required for reaction with CO (3). In recent discussions on the NO + HE reaction Hirano et al. [7] found no experimental support for hydrogen assisted dissociation of NO. They argued, as an alternative, that NO dissociation requires empty sites adjacent to adsorbed NO molecules and that hydrogen could replace some weakly bound NO. They thought that this might provide additional sites for dissociation because of the small size of hydrogen. In summary there are a variety of possible explanations for the rate enhancement caused by hydrogen but none are very convincing with the current state of knowledge. Table 2 Enhancement by water

Catalyst (temperature)

Inlet composition

Conversion (%)

Selectivities ~ (%) of N-atom of NO to

NO

CO

N2

N20 b

NH3

Pt/A1203 290~

2000 ppm CO + NO + 1400 ppm H20

24 98

21 100

c

83 r

-

77

10

12

Rh/A1203 270~

2000 ppm CO + NO + 1400 ppm H20

100 100

75 100

69 98

30 1

2

a for

N2 and NH3 defined as in Table 1 N 2 0 = 2N20/(NO consumed) not measured accurately, but selectivity to N20 is at least 83%

b (N)O ~

Water can enhance the reaction of CO with NO to a similar extent as shown in Table 2. The effects are apparent at somewhat higher temperatures and are again

132 more pronounced with Pt/A1203 than with Rh/AI203. It is most unlikely that the promotion arises through the water gas shift reaction since this would produce hydrogen and large amounts of ammonia would then be expected on the basis of the results in Table 1. Experimentally very little ammonia is observed using Pt/AI203 at temperatures above 270~ Nitrogen is the dominant nitrogen-containing species and the main reaction during water promotion can be represented simply as 2CO + 2NO ~ N, + 2CO2

(7)

With Rh/A1,O3 the principal effect of water is to favour nitrogen production at the expense of nitrous oxide CO + 2NO ~

(8)

CO2 + NzO

CO conversion is increased as a result of either reaction (7) or (8). The formation of nitrogen over Pt and Rh catalysts at temperatures below 327 ~ is now thought to proceed via a surface N20 intermediate under conditions where there are vacant sites [21 ]. It is difficult to see how this could be accelerated by water. One faint possibility was suggested by Muraki et al. [15] to explain water promotion of the reaction between CO and 02 on platinum catalysts. They suggested that water adsorbed in competition with carbon monoxide in such a way as to reduce the strong inhibitory effects present when carbon monoxide coverages are very high. It is difficult to see how this is possible given that the heat of adsorption of CO on Pt(111) for example, (134 kJ/mol) is very much greater than that of H,O (46 kJ/mol) [20]. The heat of adsorption of CO falls very steeply at high coverages (which might allow competition) but even then it is hard to see how this would increase the number of vacant sites as required to increase the rate of NO dissociation, reaction (2). Alternative explanations assuming that reaction is confined to the metal would require the supposition of'catalytic cycles' such as H20 *NO *N *N20 *CO

+ + + + +

2* ~ *H ~ *NO ~ *H ~ *OH~

*OH N* *N20 N2 CO2

+ +

H* *OH

+ +

*OH+ * H* + *

(9a) (9b) (9e) (9d) (9e)

in which a small quantity of adsorbing water results in the dissociation of many NO molecules. Reaction (9e) would need to be fast to prevent buildup of *OH. Recent calculations [22] indicate that the activation energy for reaction (9e) is low on platinum group metals (e.g. 4kJ/mol for Pd). While this cycle is very speculative, it

133

can account for some of the differences between 1-12and 1-120promotion. Dissociation of water according to reaction (9a) would be slower than hydrogen dissociation necessitating a higher temperature, as observed experimentally. As a consequence the concentrations of adsorbed H and OH would be lower because of faster removal by reactions (9b), (9d) and (9e), leading to low ammonia formation - again as observed. A final possibility which warrants mention is the potential involvement of isocyanates (NCO) as intermediates. Such species are very unstable on platinum group metals [23] but their spillover to supports is well documented [24]. Support bound isocyanates react readily with water even at room temperature to give ammonia and carbon dioxide [24]. Some evidence for the presence of NCO and their diffusion to the support is provided by the carbon balance and NH3 traces in Figure 1. The periods of "excess' carbon and ammonia evolution after water introduction are equivalent to about 30 mmol. For comparison the total quantity of Pt is 3.9 mmol and the number of support sites is 150 mmol (assuming 1019 sites/m2). Thus the data is consistent with the view that roughly 20% of the alumina surface is covered by isocyanate groups which are hydrolysed to NH3 and CO, on introduction of water. The corresponding periods of carbon deficit observed on deletion of water are almost step changes, indicating that the spillover and transport processes required to refill empty sites are quite fast at 290~ It is unlikely that hydrolysis of isocyanates is a major product route under steady state conditions with water present since little ammonia is formed. The process could act as a way of converting water to ammonia during enhancement by hydrogen but more direct experimental methods, such as infrared spectroscopy, are needed to evaluate this possibility.

5.CONCLUSIONS

1. Hydrogen enhances the rate of the reaction of CO and NO on alumina supported platinum and rhodium. The major pathway involves combination of hydrogen with the nitrogen of NO to form ammonia while the oxygen released is taken up by CO. 2. Water promotes the reaction of CO with NO. It does not seem to participate directly in the reaction but affects the characteristics of the reaction in a way which favours nitrogen production. 3. A variety of explanations can be advanced to explain the effects of hydrogen and water but none are very convincing given current knowledge. 4. Support bound isocyanate species may be involved in transient effects seen when water is introduced and subsequently deleted.

134

ACKNOWLEDGMENT

This work was supported by a grant from the Australian Research Council. The financial support of the Swiss National Foundation for one of the authors (R.D.) is gratefi~y acknowledged.

REFERENCES

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

T.P. Kobylinski and B.W. Taylor, J. Catal., 33 (1974) 376. H. Shinjoh, H. Muraki and Y. Fujitani, Stud.Surf.Sci.Catal., 30 (1987) 187. K.C. Taylor and R.L. Klimisch, J. Catal., 30 (1973) 478. M. Shelefand H.S. Gandhi, Ind. Eng. Chem. Prod. Res. Develop., 11 (1972) 393. J.H. Jones, J.T. Kummer, K. Otto, M. Shelef and E.E. Weaver, Env. Sci.Technol., 5 (1971) 790. L. Heezen, V.N. Kilian, R.F. van Slooten, R.M. Wolf and B.E. Niewenhuys, Stud.Surf.Sci.Catal. 30, (1987) 381. H. Hirano, T. Yamada, K.I. Tanaka, J. Siera and B.E. Nieuwenhuys, in: New Frontiers m Catalysis, Guczi et al. (eds.), Proc. 10th Int. Congress on Catalysis, Elsevier, (1993) 345, see also the included Question/Answer section. M.L. Unland, J. Phys. Chem., 77 (1973) 1952. G. Kim, Ind. Eng. Chem. Prod. Res. Dev., 21 (1982) 267. B. Harrison, A.F. Diwell and C. Hallett, Platinum Metals Rev., 32 (1988) 73. R.K. Herz, Ind. Eng. Chem. Prod. Res. Dev., 20 (1981) 451. C. Padeste, N.W. Cant and D.L. Tfimm, Catal. Lett., 18 (1993) 305. S.E. Oh and R.M. Sinkevitch, J. Catal., 142 (1993) 254. J.R. Stetter and K.F. Blurton, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 214. H. Muraki, S. Matunaga, H. Shinjoh, M.S. Wainwright and D.L. Tfimm, J.Chem. Tech. Biotechnol., 52 (1991) 415. B.E. Nieuwenhuys, Surf. Sci., 126 (1983) 307. R.M. Lambert and C.M. Comrie, Surf. Sci., 46 (1974) 61. R.F. van Slooten and B.E. Nieuwenhuys, J. Catal., 122 (1990) 429 W.C. Hecker and A.T. Bell, J. Catal., 92 (1985) 247. E. Shustorovich and A.T. Bell, Surf. Sci., 289 (1993) 127. H. Hirano, T. Yamada, K.I. Tanaka, J. Siera, P. Cobden and B.E Nieuwenhuys, Surf. Sci., 262 (1992) 97. E. Shustorovich and A.T. Bell, Surf. Sci., 253 (1993) 386. J. Rask6 and F. Solymosi, J. Catal., 71 (1981) 219. F. Solymosi,L. V61gyesiand J. Rask6,Z. Phys. Chemie (Neue Folge), 120 (1980) 79.

135 24 25 26 27 28 29 30 31

32

M.L. Unland, J.Catal., 31 (1973) 459. R.J.H. Voorhoeve and L.E. Trimble, J.Catal., 54 (1978) 269. D.A. Lorimer and A.T. Bell, J.Catal., 59 (1979) 223. M. Shelef and H.S. Gandhi, Ind.Eng.Chem.Prod.Res.Develop., 13 (1974) 80. W.F. Egelhoff, in: The chemical physics of solid surfaces and heterogeneous catalysis, D.A. King and D.P. Woodn~(Eds.), Elsevier, Vol. 4 (1982) 397. J.J. Jones, J.T. Kummer, K. Otto, M. Shelef and E.E. Weaver, Env.Seienee & Teeh. 5 (1971) 790.. H. Shinjoh, H. Muraki and Y. Fujitani, in: Catalysis and Automotive Pollution Control, A. Crueq and A. Frennet (Eds.), Elsevier, 187 (1987). L. Heezen, V.N. Kilian, R.F. van Slooten, R.M. Wolf and B.E. Niewenhuys, in: Catalysis and Automotive Pollution Control, A. Crueq and A. Frennet (Eds.), Elsevier, 381 (1987). Y. Amenomiyaand T. Tagawa, 8th Proe.Int.Cong.Ca_ta!.,Bedin,Vol. 2 (1984) 557.