Reduction of NO by CO and

In Section II.A, it was shown that the decomposition of NO proceeds on Pt, Pd, and Rh surfaces. However, it is not an efficient process and probably contributes only slightly to the NO removal. Exhaust gas contains the reducing gases CO, H2, and various hydrocarbons, with H2 produced by the water gas shift reaction 

Unfortunately, due to the presence of various gases in the exhaust, dini-trogen is just one of a variety of reaction products that can be formed in the catalytic converters used in automobiles. The main undesirable reaction products are N2O and NH3, which arc formed especially under reducing conditions. Some of the processes that have been proposed involve adsorbed isocyanate (NCO) and adsorbed HNCO as intermediates. The formation of NCOads complexes has been observed on supported metals by IR spectroscopy. It has been estabfished that the presence and the stability of the NCO groups depend on the support (74). It is most likely that the isocy anate species resides mainly on the support and its role is merely that of a spectator. 

The reduction of NOj, by he on noble metal surfaces has not been investi- 

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  

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We open the Sect. 1.5.1 by illustrating the chemistry of magnesium oxide and their defects. This is also the support material of choice of the cluster deposition experiments presented here. These comprise three catal3ffic reactions, the oxidation of CO, the reduction of NO by CO, and the polymerization of acetylene. These studies are reviewed in Sects. 1.5.2-1.5.5. 

The use of the cobalt triad carbonyls as catalysts continues to provide many papers for this report. Publications cover the silylformylation of 1-Hexyne catalyzed by diodium-cobalt carbonyl clusters the formation of hydroxycarbene cobalt carbonyl derivatives, the use of rhodium cluster carbonyls in the water-gas shift reaction Rh4(CO) 2> and Co3Rh(CO)] 2 catalysts for the hydrosilation of isoprene, cyclohexanone and cyclohexenone catalytic reduction of NO by CO and the carbonylation of unsaturated compounds The chemistry of iridium carbonyl cluster complexes has been extended by making use of capping reactions with HgCl2and Au(PPh3)Q... 

Voorhoeve RJH, Remeika JP, Trimble LE, Cooper AS, Disalvo FJ, Gallagher PK. Perovskite-like Lai. K MnOB and related compotmds SoUd state chemistry and the catalysis of the reduction of NO by CO and H2. J Sohd State Chem 1975 14 395-406. 

The oxidation of HC and CO must proceed in balance with the reduction of NO by CO, HC, or H2. For the NO removal reaction, a reductant is required. First NO is adsorbed on the catalyst surface and dissociates forming N2 which leaves the surface, but the O atoms remain. CO is required to remove the O atoms to complete the reaction cycle (53). 

The strength and interrelation of catalysis, classical promotion and electrochemical promotion is illustrated in Fig. 2.3. The reaction under consideration14 is the reduction of NO by CO in presence of 02. This is a complex reaction system but of great technological importance for the development of efficient catalytic converters able to treat the exhaust gases of lean burn and Diesel engines. 

The reduction of NO by CO on Pt/p"-Al203 is another system exhibiting spectacular electrochemical promotion behaviour.7,23,24 Electrochemical supply of Na+ on the Pt catalyst surface can cause the rates of C02 and N2 formation (rCo2 and rN2)to increase by 48% and 1300%, respectively, over their values on a clean surface.23... 

An in situ infrared investigation has been conducted of the reduction of NO by CH4 over Co-ZSM-5. In the presence of O2, NO2 is formed via the oxidation of NO. Adsorbed NO2 then reacts with CH4. Nitrile species are observed and found to react very rapidly with NO2, and at a somewhat slower rate with NO and O2. The dynamics of the disappearance of CN species suggests that they are reactive intermediates, and that N2 and CO2 are produced by the reaction of CN species with NO2. While isocyanate species are also observed, these species are associated with A1 atoms in the zeolite lattice and do not act as reaction intermediates. A mechanism for NO reduction is proposed that explains why O2 facilitates the reduction of NO by CH4, and why NO facilitates the oxidation of CH4 by O2. 

In situ infrared observations show that the primary species present during the reduction of NO by CH4 over Co-ZSM-5 are adsorbed NO 2 and CN. When O2 is present in the feed NO2 is formed by the homogeneous and catalyzed oxidation of NO. In the absence of O2, NO2 is presumed to be formed via the reaction 3 NO = NO2 + N2O. The CN species observed are produced via the reaction of methane with adsorbed NO2, and transient response studies suggest that CN species are precursors to N2 and CO2. A mechanism for the SCR of NO is proposed (see Figure 10). This mechanism explains the means by which NO2 is formed from adsorbed NO and the subsequent reaction sequence by which adsorbed NO2 reacts with CH4 and O2 to form CN species. N2 and CO or CO2 are believed to form via the reaction of CN with NO or NO2. CH3NO is presumed to be formed as a product of the reaction of CH4 with adsorbed NO2. The proposed mechanism explains the role of O2 in facilitating the reduction of NO by CH4 and the role of NO in facilitating the oxidation of CH4 by O2. 

Finally, a group from General Motors has explored the mechanistic importance of the N20 + CO reaction as an intermediate step during the reduction of NO by CO on noble metal exhaust catalysts [87,88]. Quasi-linearization of the non-linear NO + CO reaction system by identifying a critical kinetic parameter revealed that, indeed, the rate of the N20 + CO conversion as an intermediate step in the overall NO + CO conversion can be two to three orders of magnitude faster than the isolated N20 + CO reaction. This suggests that the observed suppression of N20 production at higher temperatures may be due to its fast reaction with adsorbed CO once produced, and that, contrary to the accepted wisdom, the formation of N20 and its subsequent reaction with CO can make a major contribution to the kinetics of the reduction of NO by CO in three-way catalytic converters. The validity of the theoretical results was verified by both... 

Gopinath, C. S. and Zaera, F. (2000) Transient kinetics during the isothermal reduction of NO by CO on Rh(lll) as studied with effusive collimated molecular beams , J. Phys. Chem. B, 104, 3194. 

Kondarides, D.I., Chafik, T. and Verykios, X.E. (2000) Catalytic reduction of NO by CO over rhodium catalysts. 2. effect of oxygen on the nature, population, and reactivity of surface species formed under reaction conditions, J. Catal., 191, 147. 

Between 100 and 200°C, the selectivity to N20 is very high (70-85%) and decreases with the temperature in accordance with most of the studies on the reduction of NO by CO. 

The reduction of NO by CO was investigated in detail by Oh et al. in closer conditions than those encountered in automotive converters weaker NO and CO concentrations, close to the stoichiometry, more realistic metal content (<1%) [74,75], The main conclusions of Oh s works are ... 

The structure of supported rhodium catalysts has been the subject of intensive research during the last decade. Rhodium is the component of the automotive exhaust catalyst (the three-way catalyst) responsible for the reduction of NO by CO [1], In addition, it exhibits a number of fundamentally interesting phenomena, such as strong metal-support interaction after high temperature treatment in hydrogen [21, and particle disintegration under carbon monoxide [3]. In this section we illustrate how techniques such as XPS, STMS, EXAFS, TEM and infrared spectroscopy have led to a fairly detailed understanding of supported rhodium catalysts. 

The studies discussed above deal with highly dispersed and therefore well-defined rhodium particles with which fundamental questions on particle shape, chemisorption and metal-support interactions can be addressed. Practical rhodium catalysts, for example those used in the three-way catalyst for reduction of NO by CO, have significantly larger particle sizes, however. In fact, large rhodium particles with diameters above 10 nm are much more active for the NO+CO reaction than the particles we discussed here, because of the large ensembles of Rh surface atoms needed for this reaction [28]. Such particles have also been extensively characterized with spectroscopic techniques and electron microscopy we mention in particular the work of Wong and McCabe [29] and Burkhardt and Schmidt [30], These studies deal with the materials science of rhodium catalysts that are closer to the ones used in practice, which is of great interest from an industrial point of view. 

At low temperature, the dissociation of adsorbed NO species occmring over reduced perovskite and yielding N2O and N2 was recognized as the rate determining step for catalytie reduction of NO by CO. The dimeric species of NO, such as N2O2, can be an intermediate, the formation of which involves the N-N bond formation and N-0 bond eleavage[40]. Two parallel reaetions for chemisorbed NO dissociation occurring over a redueed surfaee with N2O and N2 as the respective products were assumed ... 

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. 

Several different homogeneous catalyst systems for the reduction of NO by CO have been described to date (183-185, 187, 189), and in all cases, the reduction follows (113) with nitrous oxide as the reduced N-containing product, this despite the fact that reduction to N2 is more favored thermodynamically. The reason for adherence to the stoichiometry of (113), rather than (114), for example, may relate to the fact that N20, once formed, is a very poor ligand. 

The three catalyst systems discussed in this section for the reduction of NO by CO underscore the mechanistic complexity of a reaction which is stoichiometrically simple. Extensive bond reorganization is required in the reduction via (113), and each of the catalyst systems appears to proceed by a different mechanism. While two of the three systems possess a common feature in terms of C02 formation, each appears to be different with respect to N20 production. The systematic development of new homogeneous... 

Baraldi and co-workers [52] have described a wealth of dynamic XPS studies on surface reactions, including adsorption, dissociation, desorption, and even catalytic reactions, such as the epoxidation of alkenes [60], and the reduction of NO by H2 and CO [61]. 

As in the case of CO oxidation, the reduction of NO by CO depends on the heterogeneity of the supported model catalyst. The effect of size and shape of the metal particles has been addressed with Pd/MgO(l 00) model catalysts [88, 90, 91, 168]. 

Pt-zeolite catalysts were also employed for the selective reduction of NO by HCs and were very active at lower reaction temperatures compared to Fe-, Cu- and Co-zeolites, but little, if any, literature was found for the Pt-exchanged zeolite catalysts deahng with the water tolerance. Iwamoto and coworkers conducted a comparative investigation of Pt- and Cu-MFI and Fe-MOR for their performance in NO reduction by C2H4. Pt-MFI-97 was found to be more active than the other two metal-exchanged zeolite catalysts at low temperatures and its activity for NO conversion at 200"C is hardly alfected by the addition of 8.6% H2O into the feed gas stream (Table 6), whereas the conversion of N2O formed during the course of the reaction slightly decreases by less than 10 /o. In... 

It was also found that isocyanate species adsorbed on Rh are significantly stabilized over the W -doped Rh/Ti02 catalyst and are present on the catalyst surface under all experimental conditions that lead toward formation of N2O in the gas phase. As a result, reduction of NO by CO over doped catalysts occurs at lower temperatures ( e50°C) compared to the undoped catalyst mainly due to expansion of the temperature window of N2O formation. 

Hydrocarbons in the exhaust react with oxygen and with NO. Although these reactions have had much less attention than oxidation of CO and reduction of NO by CO, reactions of hydrocarbons play an important role in the overall reaction mechanism of the three-way converter, particularly because the converter is by no means a homogeneously mixed reactor, see Fig. 5.15 [57]. Hence, zones exist where e.g. ethylene and nitrogen atoms are coadsorbed on the noble... 

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