NO/H2 reaction

Tolia, A. A., Williams, C. T., Weaver, M. J. et al. (1995) Surface-enhanced Raman spectroscopy as an in situ real-time probe of catalytic mechanisms at high gas pressures. The NO-H2 reaction on rhodium , Langmuir, 11, 3438. 

Previous kinetic investigations dealing with the NO + H2 reaction over supported noble metal-based catalysts showed different kinetic features according to the nature of the support [29,53-58], Initially, this reaction has been described in the absence of oxygen on Rh deposited on silica and alumina by the following mechanism [29],... 

Engelmann-Pirez, M., Granger, P. and Leclercq, G. (2005) Investigation of the catalytic performances of supported noble metal based catalysts in the NO + H2 reaction under lean conditions, Catal. Today, 107, 315. 

Dhainaut, F., Pietrzyk, S. and Granger, P. (2007) Kinetics of the NO + H2 reaction over supported noble metal based catalysts Support effect on their adsorption properties, Appl. Catal. B 70, 100. 

Shestov, A.A., Burch, R. and Sullivan, J.A. (1999) A transient kinetic study of the mechanism of the NO + H2 reaction over Pt/Si02 catalysts 2. Characteristic features of SSITKA profiles, 186, 362. 

The methods discussed so fer are in principle applicable under all pressure and temperature conditions. Modern surface analysis tools used in the study of clean, well-defined surfeces, however, require high-vacuum conditions, thus limiting their application to reactions that oscillate under these conditions. Currently these include only the CO/O2, the CO/NO, and the NO/H2 reactions. Another possibility is the use of UHV methods on samples that have been introduced into a high-vacuum system after stopping the oscillatory reaction. This, however, violates the in situ measurement requirement. 

The role of NO during the CO/NO reaction is analogous to that played by oxygen in the CO/O2 reaction discussed above. Different sticking coefficients have been measured on the two forms of the (100) surface for NO as well 319). A model similar to that of Imbihl et al. (52) for CO/O2 has been proposed by the same group for NO/CO on Pt(lOO) (144). The hex 1 X 1 phase transition was modeled in the same manner, and oscillations and multiple steady states similar to experiments were predicted. As discussed in Section IV,B,1, this model could also display oscillations at lower temperatures, at which the phase transition was not involved. This general model of the hex 1 x l phase transition may also be applicable to the recently discovered oscillations in the NO/H2 reaction on Pt(lOO) (2/5). 

Oscillations in the rate of CO2 production have been observed for many supported metal catalysts and single-crystal surfaces. Similar oscillations have been observed for most of the reactions discussed in this review. An example is shown in Fig. 10 for the NO + H2 reaction on Pt(lOO) (46). 

Fig. 10. Oscillatory behavior of the NO + H2 reaction on Pt(lOO) [reproduced with permission from Cobden et al. (46)].

Kobylinski and Taylor (75) studied the NO i CO and NO H2 reactions on supported noble metals and found that the activity for the first reaction increases in the order Pt < Pd < Rh < Ru and that for the second reaction Ru < Rh < Pt < Pd. The first reaction is slower than the second only for Ru was the order reversed. Ru is an excellent catalyst for the NO reduction with a minimum of NH3 production. However, Ru forms volatile oxides under operating conditions resulting in an unacceptable catalyst loss. The most efficient catalyst appears to be Rh (7). 

Many techniques have been applied to examine the reaction pathways of the NO-CO and NO-H2 reactions and to elucidate the reaction mechanisms. In this section some of the relevant results are discussed with emphasis on the reaction mechanism. Both the NO-CO and NO-H2 reactions are discussed here since it is likely that the mechanisms of N2 and N2O formation are independent of the type of reducing agent. Note that CO dissociation is not considered to be involved in the mechanism. However, dissociative adsorption of NO into N and O adatoms is an important process on the relevant metals, as discussed in Section 11.A. Possible mechanisms of N2, N2O, and NH3 formation can then be evaluated on the basis of the following hypothetical mechanisms involving all the possible elementary steps in which NOaus, Nads, Oads, COads, and Hads can participate ... 

Lambert and Comrie (.1.1) investigated the CO + NO reaction on Pt(lll) and (110) surfaces and concluded that the reaction proceeds by a L-H mechanism between Oads and molecular CO ads - NO dissociation is also the prime step in the NO + H2 reaction on a Pt foil at a pressure of 10 mbar (7). NH, was found to be the major product at temperatures lower than 600 K and N2 was the major product at temperatures higher than 600 K when the NO/H2 ratio was —1/5. 

Attempts to form a N layer on the Pt(lOO) surface by means of the NO-H2 reaction were unsuccessful. The NO and H2 partial pressures were varied from 10 to 10 mbar and the temperature from 400 to 600 K. Formation of a nitrogen overlayer was not observed under these conditions. 

Information from some studies (Ref. 17) suggests an inverse dependence of the rate of NO decomposition with respect to the oxidizing atmosphere over noble metal catalyst. From the facts mentioned above, it is reasonable that the rate of NO-H2 reaction was positive order with respect to H2 partial pressure but negative order with respect to NO partial. pressure with Rh catalysts. 

Fig. 3.17. Rate of N2 desorption as a function of time during the NO-H2 reaction on Pt(l 0 0) at PN0=3 y I0 9 bar and T=A6Q K (a) period-1 oscillations at FWPh2=1 (b) period-2, (c) period-4, and (d) and aperiodic oscillations at FWPii2=l-4. (V. P. Zhdanov, Impact of surface science on the understanding of kinetics of heterogeneous catalytic reactions. Surface Scence, 500 (2002) 966).

Figure 6. Modelled oscillations for the NO + H2 reaction on Pt 100 atT = 434 K, Pno = 1.1x10 mbar, PH2 = 2x10-5 mbar. Local adsorbate coverages, reaction rates and the fraction of the surface in the (1x1) phase are shown29.

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 fester 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, H2 reaction system [7] even though all three gases are simultaneously present in automobile exhaust gases. 

PtO.25RhO.75 (100) alloy, and Pt/Rh(100) and Rh/Pt(100) bimetal surfaces underwent chemical restructuring at T > 400 K by heating in ca. 10" Torr of NO or O2, and the surfaces were covered with a common hybrid overlayer of Rh-O/Pt-layer. The Rh-O/Pt-layer gives a characteristic p(3xl) LEED pattern and is active for the reaction of NO + H2. As a result, Pt-Rh(lOO) alloy and Pt/Rh(100) and Rh/Pt(100) bimetal surfaces have almost equal catalytic activity. Pt(l 10) surface was poorly active compared to Pt(lOO) surface for NO + H2 reaction, but it was shown that Rh/Pt(110) and Rh/Pt(100) have almost equal catalytic activity. Therefore, it is concluded that a common Rh-O/Pt hybrid overlayer prepared by chemical restructuring is responsible to the prominent catalytic activity of the Pt-Rh catalysts for NOx reduction. 

In fact, when NO + H2 reaction was performed on Pd(lOO) and Rli(lOO) surfaces, accumulation of N(a) intennediates occurs on theses surfaces although Pd(lOO) and Rli(lOO) surfaces are inactive for the chemisorption of N2 [1,4]. However, no accumulation of N(a) intennediates was attained on Pt(lOO) and on Pd(llO) surfaces although they catalyze the NO + H2 reaction. It is noteworthy that the accumulation of N(a) does not occur on Pd(llO) surface but it does on the Pd(lOO) surface [4]. This fact may suggest that NO + H2 reaction will be a structure sensitive reaction. 

From the view point of the hybrid surface formation, NO + H2 reaction on Pt(llO) and Rh/Pt(110) surfaces are very interesting, because Pt(110) surface is poorly active compared to Pt(lOO) surface as shown in Fig. 4, that is, this reaction is a typical structure sensitive reaction. To compare the Rh/Pt(110) surface to Rh/Pt(100), we prepared Rh/Pt(110) surface by depositing Rh ions on a missing row p(lx2) Pt(l 10) surface. Rh deposited Pt(l 10) surface did not give any LEED spots because too thick Rli layer was deposited. Rli atoms were removed from... 

In the present paper we describe the performance of Pt/CoOjj/Si02 catalysts. We have made a series of 5 w% Pt/CoO,/Si02 catalysts, with different cobalt oxide loadings. It was earlier found that the addition of 3 w% C03O4 resulted in a catalyst that oxidizes CO already at room temperature (12). In the present paper we report results of comparative studies of the CO/NO and NO/H2 reactions over Pt/CoO /Si02, Pt/Si02 and CoO /Si02 catalysts. 

Cobalt oxide should be partially reduced to provide for the O vacancies required to dissociate NO molecules. Partially reduced active CoO centers play an important role to account for the improved selectivity and activity of Pt/CoO,/Si02 catalyst in NO/H2 reactions. Oxidation of the Pt/CoO /Si02 catalyst leads to a higher on-set temperature of the reaction, but the selectivity to N2 increases during the reaction cycle due to the formation of the active CoO centers. 

Figure 7 shows clearly the positive effect of the addition of cobalt oxide to a Pt catalyst on the NO conversion. It was found earlier that when the coverage of NO is high, more NjO is formed than at low NO coverage (4, 11). Nj is formed when the surface of the catalyst is covered with much N. Hence, at a lower CO/NO ratio, more NjO formation can be expected. Reaction 5 is favored at lower temperatures (4, 11, 21, 22). This explains why the temperatures of NO conversion are lower for CO NO = 1 2.5. Figure 7a shows a maximum around 300°C for the CoO catalyst. This has also been found for the NO/H2 reaction over the same catalyst. The maximum corresponds to the maximum in N2O formation. Hence, most likely, the maximum in conversion observed for the CO/NO reaction around 300°C is also caused by N2O formation. Thus it seems that the main reaction product over the oxidized cobalt oxide catalyst is N2O. 

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