NO CO reaction

The undoubtedly structure-sensitive reaction NO -r CO has a rate that varies with rhodium surface structure. A temperature-programmed analysis (Fig. 10.8) gives a good impression of the individual reaction steps CO and NO adsorbed in relatively similar amounts on Rh(lll) and Rh(lOO) give rise to the evolution of CO, CO2, and N2, whereas desorption of NO is not observed at these coverages. Hence, the TPRS experiment of Fig. 10.8 suggests the following elementary steps  

Breaking of the N-O bond by the rhodium surface is the most essential step in the catalytic reduction of NO (see also Chapter 7). Although rhodium is sufficiently reactive to achieve this (even without promoters), dissociation can nevertheless be severely impeded if the surface coverage is too high (as Fig. 7.12 shows). In the low coverage regime, however, such effects play no role. 

Reijei, R.A. van Santen, andJ.W. Niemantsverdriet, J. Chem. Rhys. 101 (1994) 100S2 M. Hopstaken J.W. Niemantsverdriet J. Phys. Chem. B104 (2000) 30S8 J. Chem. Phys. 113 (2000) S4S7 

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Oh S H, Fisher G B, Carpenter J E and Goodman D W 1986 Comparative kinetic studies of CO-O2 and CO-NO reactions over singie crystai and supported rhodium cataiysts J. Catal. 100 360... 

The same trends regarding the effect of sulfur have been reported for NO adsorption on Pt(lOO)90 and Rh(100).6 In the case of Pt(100) dissociative adsorption is completely inhibited upon formation of a p(2x2) overlayer at a sulfur coverage equal to 0.25, while the binding strength of molecularly adsorbed NO is lowered by more than 50 kJ/mol, as calculated by analysis of NO TPD data. Due to this complete inhibition of dissociative adsorption, the CO+NO reaction is completely deactivated, although it proceeds easily on sulfur free Pt(100). In the case of Rh(100) a sulfur coverage of only 0.08 suffices to completely inhibit NO dissociation at 300 K. 

Table 10.4 lists the rate parameters for the elementary steps of the CO + NO reaction in the limit of zero coverage. Parameters such as those listed in Tab. 10.4 form the highly desirable input for modeling overall reaction mechanisms. In addition, elementary rate parameters can be compared to calculations on the basis of the theories outlined in Chapters 3 and 6. In this way the kinetic parameters of elementary reaction steps provide, through spectroscopy and computational chemistry, a link between the intramolecular properties of adsorbed reactants and their reactivity Statistical thermodynamics furnishes the theoretical framework to describe how equilibrium constants and reaction rate constants depend on the partition functions of vibration and rotation. Thus, spectroscopy studies of adsorbed reactants and intermediates provide the input for computing equilibrium constants, while calculations on the transition states of reaction pathways, starting from structurally, electronically and vibrationally well-characterized ground states, enable the prediction of kinetic parameters. 

Tolia, A. A., Williams, C. T., Takoudis, C. G. et al. (1995) Surface-enhanced Raman spectroscopy as an in-situ real-time probe of catalytic mechanisms at high gas pressures. The CO-NO reaction on rhodium , J. Phys. Chem., 99, 4599. 

Rainer, D. R., Vesecky, S. M., Koranne, M. et al. (1997) The CO + NO reaction over Pd A combined study using single-crystal, planar-model-supported, and high-surface-area Pd/Al203 catalysts , J. Catal., 167, 234. 

Figure 4.12. Formation mechanism of NCO groups in a CO + NO reaction on Ag/Al203 catalysts proposed by [133],...

Granger, P., Delannoy, L., Lecomte, J.J. et al. (2002) Kinetics of the CO + NO Reaction over Bimetallic Platinum-Rhodium on Alumina Effect of Ceria Incorporation into Noble Metals, J. Catal., 207, 202. 

The specific role of OSC materials in NO activation and NO dissociation has largely been confirmed by many authors over Pt-Rh [87,88] and Pd catalysts [89,90] or even over bare OSC oxides [91]. By EPR, Lecomte et al. [87] evidence the presence of 02 superoxide species over a Pt—Rh/Al203 catalyst modified by ceria. The formation of these species could be closely related to the performance of the Pt—Rh/Ce02—A1203 catalyst in CO+NO reaction. 

Kinetic aspects of the CO + NO reaction and related N20 formation/transformation... 

A particular attention on the mechanisms for the formation of N20 over noble metals has been paid in our laboratory [37-40]. It was previously found that an enhancement in the initial selectivity towards the production of N2 (Table 10.1) during the CO + NO reaction can be related to an increase in the relative rate of step (13) over supported Pt-based catalysts [33], Unexpectedly, Rh exhibits a poor selectivity towards the formation of N2 at low conversion and low temperature, which has been mainly related to a stronger NO adsorption on Rh than on Pt and Pd. 

Oh, S.H. (1990) Effects of cerium addition on the CO-NO reaction kinetics over alumina-supported rhodium catalysts, J. Catal. 124, 477. 

Subsequent experiments on the same system aimed to determine the stability of the isocyanate species and to measure the reactivity of the Pd(lll) model catalyst for the CO + NO reaction.125 When exposing the sample to different CO/NO ratios (2 and 1.5) at room temperature, peaks were obtained which corresponded to threefold NO, atop NO, and threefold CO, with the higher CO/NO ratio leading to a greater amount of CO binding. When the samples were flashed to 650 K and cooled back to 300 K in the presence of the reaction mixtures, isocyanate was formed. However, as is apparent from Figure 10.25, an increase in the CO/NO ratio strongly favored isocyanate formation. 

Figure 10.24 Reaction pathways of the CO + NO reaction on Pd(111). (Reprinted from Ozensoy, E. et al., J. Am. Chem. Soc., 124, 8524-8525, 2002. Copyright 2002. With permission from American Chemical Society.)...

Figure 12c. C3H6 conversion in CO +NO reaction over perovskites [61, 62], Conditions ...

A complication to the above is when competing reactions occur. For example, when NO is added to the CO + 02 feed, reduction of NO by CO can occur as well as CO oxidation. In this case, a rate expression for the CO-NO reaction would first be determined from experiments with CO and NO (as well as C02 and water), but no 02 in the feed before considering mixtures of CO, NO and 02. When modelling the latter, terms for 02 inhibition may be necessary in the kinetics expression for the CO-NO reaction as well as for NO inhibition in the CO oxidation expression (as already mentioned earlier). Simultaneous reaction of C3H6 and CO can be handled in a similar way. 

The results in Figure 2.15 reveal that the CO+NO reaction is obviously structure-sensitive. On Rh(100), the first signal that appears is that of CO2 at 300 K. This implies that at, or somewhere below, this temperature the NO has already dissociated into N and O atoms. In Chapter 4 (when we have a technique that can monitor reactions on the surface) we will see that NO already starts to dissociate at around 200 K on Rh(100). Note that all NO has dissociated, because desorption of NO or any other form of NOx is not observed. When most of the oxygen is consumed, CO desorption competes with CO2 formation in the temper-... 

Table 4.2 Activation energy (Ea) for several elementary reaction steps involved in the CO + NO reaction on Rh(lll) [37] and Rh(100) [38].

The rate of the reaction (86-90) is about two orders of magnitude slower than the O2/C reaction, consistent with the greater strength of the NO bond than that in O2. The CO/CO2 ratio in the products of the reaction increases with increasing temperature (86, 87). At low temperatures (850 K), a stable chemisorbed oxygen compled (86) forms and inhibits the reaction. At AFBC temperatures, however, it has been observed that the reaction is accelerated in the presence of oxygen (91). This latter result may be a consequence of the increase in the CO concentration within a char particle as the 0 concentration is raised. Because the O2/C reaction is so much faster than the NO/C or the carbon catalyzed CO/NO reaction (86, 91), the situation exists in which the effectiveness factor for the O2/C reaction is small and little O2 penetration into char occurs at a time when the effectiveness factor for the NO reduction reactions are near unity. Additional NO reduction reactions that may occur are the CO/NO reaction catalyzed by bed solids (90 - 92) and the reduction of NO by sulfite-containing, partially sulfated limestone (93). 

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