Reduction of NO with CO

Thermally activated (450 °C) coprecipitated Sn02-CuO gels show high activity and selectivity for N2 formation, at low temperature (150 °C), in the catalytic reduction of NO with CO.98 Independently, Hungarian workers have found that Sn02, containing small amounts (0.1—1 mole%) of Cr203 deposited on the surface, has a similar effect on the reaction.99... 

Reduction of NO with CO or H2 was found to be an interesting example of intrafacial catalytic process (30). If this reaction is conducted over a transition-metal oxide, the reaction rate appears to be related primarily to the thermodynamic stability of the oxygen vacancies adjacent to a transition metal ion. Associative as well as dissociative adsorption of NO have been reported on perovskite oxides (14, 22, 80, 174) (see also Section VI,B) the adsorption on the reduced oxides is stronger than in the oxidic compounds. Dissociative adsorption takes place at moderate temperatures as in NO reduction over Lao.gsBao.isCoOs at 100°C with the subsequent formation of N2 and N20 (73). 

Similar product compositions (90% N2 plus N20 and NH3) with high (>80%) conversions were attained on Ni-Fe-Zr alloys. Pt70Zr30 was also shown to be active in the reduction of NO with CO at 523 K, giving a 9 1 N2-N20 mixture at 93% conversion (55). 

In a series of papers published by Swedish and Japanese researchers, they used supported and massive perovskites of the general formula Lai xSrxM 2l Cul,Rul,03 (M = A1, Mn, Fe or Co) as catalysts for the reduction of NO with CO. Skoglundh et al. (1994) prepared nine alumina washcoated mixed oxide catalysts containing La, Sr, Cu and Ru. The solids obtained were thoroughly characterized using several techniques. They were tested in two different reactant streams (i) N0/C0/C3H6/02/N2 (feed stream A) and (ii) NO/CO/N2 (feed stream B). 

Most researchers have shown convincing evidence of the fact that N2O is an intermediate in the reduction of NO with CO. 

The use of supported mixed oxides in both catalytic combustion and NO reduction are challenging applications. In the case of high-temperature catalytic combustion there is an unsatisfied need to obtain solids able to maintain high combustion activity during extended operation, i.e., high-surface-area structurally stable solids. Another hot subject that deserves exploration is the use of washcoated highly dispersed mixed oxides for the selective reduction of NO with CO and hydrocarbons. These studies should be conducted in the presence of both SO2 and H2O to evaluate the potential practical application of these catalysts. 

Fig. 60. Reduction of NO with CO over Rh/Ce ) 6Zro 402 reaction profile vs. temperature obtained after an initial period of 60 min at 300 K (reaction conditions NO(l%) and CO(3%) in He, heating/cooling rate 1 Kmin W/ F= 1 x 10 3 gcil,il ysl ml"1 min). Selectivity in N20 formation (squares) and CO conversion (triangles) measured on a freshly reduced catalyst NO conversion measured in run-up experiment on a freshly reduced catalyst (solid circles) and run-down experiment on an aged catalyst (open circles). (Fomasiero et al. 1998.)...

Pt/MnOx/SiOj and Pt/CoOySi02 catalysts are efficient catalysts for the reduction of NO with CO and the oxidation of CO with O2 at low temperature, with a much lower on-set temperature for the reactions compared with Pt/Si02. An oxidative pretreatment lead to a shift to higher light-ofif temperatures for 62 and NO conversion. For a pure CoOySi02 catalyst primarily NjO was formed following an oxidation step. 

Fig. 7 shows the results of catalytic tests in the reduction of NO with CO, in the absence of O2, for reduced CuAPO-34 catalyst and suggests that this is a very active catalyst with a high selectivity (>80%) at all temperatures in the 300-550°C range. This is a very interesting result especially if the activity of CuAPO-34 is compared with that of a copper exchanged ZSM-5 catalysts. In fact, the reaction rate measured at 400°C on CuAPO-34 was 1.16 pmol s g, a value about 20 times greater than that observed on a Cu-ZSM5-40 catalyst (40 indicates the SI/Al ratio) with a similar Cu content (1.72% wt.) in the same experimental conditions. 

LaFei.xCOxOs (x = 0 or 0.4) perovskites were obtained by nanocasting using Fluka 05120 and Black Pearls 2000 conunercial carbons, which act as hard template during the gel formation and subsequently limiting the particle growing during the perovskite formation. Perovskites with the same nominal composition were also conventionally prepared. The results clearly showed that both commercial carbons was efficient to prepare perovskites with nanosized nature and specific surface area substantially improved. In addition, catalytic tests in the reduction of NO with CO evidenced the better activity of the nanocast perovskites. 

The catalytic tests (not shown) revealed that the nanocast perovskites were more active in the reduction of NO with CO than those conventionally prepared. Considering that the catalyst tests were carried out using the same contact time and that nanocast and uncast LaFeOs perovskites have the same composition, the observed higher activity of the former must be related with a higher number of accessible active sites and therefore with its higher specific surface area. 

R.K.C. de Lima, M S. Batista, M. Wallau, EA. Sanches, Y.P. Mascarenhas, E.A. Urquieta-Gonzalez, 2009, High specific surface area LaFeCo perovskites - Synthesis by nanocasting and catalytic behavior in the reduction of NO with CO, Appl. Catal. B Environ., 90, 3-4, 441-450. 

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