NO reduction by CO

Perovskite-like materials have been extensively investigated as potential catalysts for the reduction of NO by CO (Table 25.3). Among these, copper and iron lanthanides were considered as very active toward the NO + CO reaction. 

The following different routes for NO and CO elimination have been proposed [56,57,64]  

What route is followed depends on the reaction temperature at low temperatures route 2 predominates, whereas at high temperatures the NO reduction proceeds via route 1 [56,57]. 

For comparison piuposes, the catalytic activity of several perovskites in NO + CO reaction is presented in Table 25.4, adopted from Ref. [64]. The activity of Rh/Al203 has also been included as a reference value. Obviously, some perovskites exhibit satisfactory activity compared to Rh/Al203. Good stability and SO2 tolerance are also often reported for perovskite materials. 

Nanoscale Fe-based perovskites (LaFei ,(Cu,Pd)03) have been found to address considerable activity towards NO reduction by CO [61]. Their superior catalytic performance was ascribed to the facilitation of anion vacancies generation after Cu incorporation, which in turn determines the NO adsorp-tion/dissociation. Furthermore, the enhanced reducibility of Cu-doped LaFe03 perovskites results in oxygen vacancies regeneration and CO oxidation promotion [61]. Advanced preparation methods, such as reverse microemulsion, can also result to physicochemical characteristics (nanosized perovskites with higher surface areas) and catalytic performance (more active crystal phases) modifications [57]. 

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Figure 2.3. Catalysis (0), classical promotion ( ), electrochemical promotion ( , ) and electrochemical promotion of a classically promoted (sodium doped) ( , ) Rh catalyst deposited on YSZ during NO reduction by CO in presence of gaseous 02.14 The Figure shows the temperature dependence of the catalytic rates and turnover frequencies of C02 (a) and N2 (b) formation under open-circuit (o.c.) conditions and upon application (via a potentiostat) of catalyst potential values, UWr, of+1 and -IV. Reprinted with permission from Elsevier Science.

Most studies of the effect of alkalis on the adsorption of gases on catalyst surfaces refer to CO, NO, C02, 02, H2 and N2, due to the importance of these adsorbates for numerous industrial catalytic processes (e.g. N2 adsorption in NH3 synthesis, NO reduction by CO). Thus emphasis will be given on the interaction of these molecules with alkali-modified surfaces, especially transition metal surfaces, aiming to the identification of common characteristics and general trends. 

This observation is directly related to the observed dramatic electrochemical promotion of NO reduction by CO and C3H6 in presence of 02 on Rh/YSZ upon electrochemical O2 supply to the Rh catalyst surface (Fig. 2.3 and Chapters 4 and 8). 

Figure 4.26. Transient response of the rate of CO2 formation and of the catalyst potential during NO reduction by CO on Pt/p"-Al2C>396 upon imposition of fixed current (galvanostatic operation) showing the corresponding (Eq. 4.24) Na coverage on the Pt surface and the maximum measured (Eq. 4.34) promotion index PINa value. T=348°C, inlet composition Pno = Pco = 0.75 kPa. Reprinted with permission from Academic Press.

Figure 4.40. Effect of catalyst potential on the rates of formation of C02) N2 and N20 and on the selectivity to N2 during NO reduction by CO on Pt/(3 -Al20j.96 Reprinted with permission from Academic Press.

Subsequent elegant work by Lambert and coworkers61 has shown that, while under UHV conditions the electropumped Na is indistinguishable from Na adsorbed by vacuum deposition, under electrochemical reaction conditions the electrochemically supplied Na can form surface compounds (e.g. Na nitrite/nitrate during NO reduction by CO, carbonate during NO reduction by C2FI4). These compounds (nitrates, carbonates) can be effectively decomposed via positive potential application. Furthermore the large dipole moment of Na ( 5D) dominates the UWr and O behaviour of the catalyst-electrode even when such surface compounds are formed. 

Figure 6.4. Examples for the four types of global classical promotion behaviour. Work function increases with the x-axis. (a) Steady-state (low conversion) rates of ethylene oxide (EtO) and C02 production from a mixture of 20 torr of ethylene and 150 torr of 02 for various Cs predosed coverages on Ag(lll) at 563 K19 (b) Rate of water-gas shift reaction over Cu(l 11) as a function of sulphur coverage at 612 K, 26 Torr CO and 10 Torr H202° (c) Effect of sodium loading on NO reduction to N2 by C3H6 on Pd supported on YSZ21 at T=380°C (d) Effect of sodium loading on the rate of NO reduction by CO on Na-promoted 0.5 wt% Rh supported on Ti02(4% W03).22...

Figure 8.64. Transient effect of a constant negative applied potential on the on the rates of C02, N2 and N20 formation, on the NO conversion and nitrogen selectivity during NO reduction by CO on Rh/YSZ.69 Reprinted with permission from Elsevier Science.

Electrochemical Promotion of a Classically Promoted Rh Catalyst for NO Reduction by CO in Presence of 02... 

Figure 8.65. Dependence of the catalytic rates and turnover frequencies of C02 on the reaction temperature and on the catalyst potential for the initially sodium free Rh/YSZ catalyst (labeled C2) during NO reduction by CO in presence of gaseous 02. Reprinted with permission from Elsevier Science.

Recent NEMCA investigations have shown that J3"-A1203, a Na+ conductor, can be used as an active catalyst support to dramatically enhance the rate and selectivity of several enviromentally important reactions such as NO reduction by CO, H2 and C3H6, all catalyzed by Pt. Sodium supply to the catalyst has been found to enhance not only the catalytic activity, but also product selectivity to nitrogen. 

This is manifest in the kinetics depicted in Figures 9.15 and 9.16 which bear interesting similarities and some subtle differences with those of NO reduction by CO discussed in the previous section (Figs. 9.13 and 9.14). 

Here we discuss the results obtained for the model reaction of C2H4 oxidation on Ir02, Pt and Rh but similar conclusions are reached when using other model reactions such as CO oxidation or NO reduction by CO.25 27 The three systems shown in Figure 11.3 were used to compare ... 

F.A. Alexandrou, V.G. Papadakis, X.E. Verykios, and C.G. Vayenas. The promotional effect ofNa on the NO reduction by CO on supported Pt, Pd and Rh catalysts in Proc. 4thlntnl. Congress on Catalysis and Automotive Pollution Control2, 1-16 (1997). 

As already described (1) the inhibition of the NO reduction by CO due to carbon deposits in mixtures containing hydrocarbons depends on the support, with the most acidic supports leading to higher amounts of carbon deposits. 

The role of the metal in the NO reduction by CO is clearly shown by CO, NO and CO-NO (1 1 mixture) adsorptions at room temperature and at reaction temperatures... 

Here we describe an EP study of catalytic NO reduction by CO and by propene over Pt/P alumina. The latter process is especially significant for the removal of NO in oxidising environments, a key problem that must be overcome for pollution abatement from lean-bum gasoline engines. 

Compared to CO, these reactions were much less studied over TW catalysts. Kobylinski and Taylor [67] have compared the NO reduction by CO and by H2. Their main results are summarized in Tables 8.10 (light-off activity) and 8.11 (selectivity). 

Pt and Pd are by far the most active metals in NO reduction by H2 while Rh and Ru present the highest activity in NO reduction by CO. However, when the two reducers are injected together (last column), CO tends to impose its behavior in NO reduction. This is due to a strong adsorption of CO, which inhibits the reduction by H2. 

Pt-Mo/y-AljO catalyst (1) lowers the activity but increases the selectivity of the Pt catalyst for the NO reduction by H2, and (2) increases the activity for the NO reduction by CO and the activity and selectivity for the NO reduction by CO + H2> Based on the TPR and IR data, we attribute these results to a strong interaction between Pt and Mo oxides on the y-A170, support. Such interaction facilitates the removal of the surface... 

NO reduction by CO, 28 162 olefin oxidation, 27 241,242 water-gas shift, 28 118,119 coordination number, 30 265 -copper alloy, 26 75 -copper alloy films thermodynamic properties of, 22 118 -copper oxide, 27 90,91 -manganese oxide, 27 91,92... 

Peter, SD Garbowski, E Perrichon, V Primet, M. NO reduction by CO over aluminate-supported perovskites. Catal. Lett., 2000, Volume 70, 21-33. 

Fomi, L Oliva, C Barzetti, T Selli, E Ezerets, AM Vishniakov, AV. FT-IR and EPR spectroscopic analysis of Lai-xCcxCoOs perovskite-like catalysts for NO reduction by CO. Appl. Catal, B Environmental, 1997, Volume 13, Issue 1, 35-43. 

By analyzing energy barriers for product desorption under ammonia synthesis, CO hydrogenation, and NO reduction by CO, we can refine the models further. For these three processes, the reaction conditions are very different. The ammonia synthesis process is weakly exothermic, whereas the CO hydrogenation reaction has... 

From the analysis of the ammonia synthesis, CO hydrogenation and NO reduction by CO in [56] we deduce that the barrier for generalized desorption is on the order of... 

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