A Study of NO+CO Reaction over Various Supported Catalysts in the Presence of O2and H2O

Abstract

NO and CO are the main gases emitted into the atmosphere from both stationary and mobile combustion systems. These toxic gases represent major form of pollutants and contribute mainly to the formation of smog, acid rain and ozone depletion in the stratosphere. The simultaneous elimination of these two toxic gases leading to non-toxic compounds such as N2 and CO2 is considered an efficient process. Although the studied reaction is spontaneous, it is well known that the presence of a catalyst is necessary. For the catalytic reduction of NOx, reducing agents such as carbon monoxide (CO), hydrogen (H2), ammonia (NH3), urea ((NH2)2CO) and different hydrocarbons are usually being employed. Many supports such as alumina, titania, zirconia, ceria were usually used for dispersing the active phase for the elimination of nitrogen oxide. It is well known that supports play an important role in determining the nature and number of active sites, and consequently in the activity of the catalysts. The metal oxides are advantageous for the stability of the catalyst support and/or for enhancement of the catalyst activity under dynamic operations. Among the transition metals studied, copper show a potential activity in the NO+CO reaction and have been extensively studied. Sakurai et al. tested catalytic activities of cupric oxide and alumina-supported copper oxides in NO reduction by CO. Metal oxides such as Ce and Ni are commonly employed O2 storage component, which are frequently added to the catalyst to enhance the catalytic active sites. These oxides improve the performance of the catalyst, further making it less sensitive to temporary variations in the air-fuel ratio by getting reversibly oxidized or reduced at typical operating temperatures. Loof et al. stated that at higher temperatures, the O2 storage capacity in catalysts containing Pt, Rh, Ni and Ce supported on Al2O3, Ni is found to be the dominant storage metal and Ce predominates over Pt and Rh. The reduced surface of Ce in contact with reduced noble metals is the most effective state of the catalyst and greater activity is detected with decreasing CeO2 crystallite size. Addition of cerium oxide play an important role in providing oxygen storage by transforming between Ce2O3 under reducing conditions and CeO2 under oxidizing conditions. It is further believed that doped ceria inhibits the deactivation of active components in the reaction due to the sintering of the metals and surface deterioration of the support. Garcia et al. pointed out that the Cu-Ce interaction affected the reactivity of the catalysts towards NO thereby increasing the amount of adsorbed species which in turn lead to high NO adsorption at low temperature. The NO dissociation peaks were shifted towards low temperature upon addition of ZrO2 onto CuO/TiO2, indicating NO decomposition activity was higher in CuO-ZrO2/ TiO2 than by CuO/TiO2. Hernandez et al. suggested that the presence of acid-base sites on Pt/ZrO2-CeO2 seemed to be important for the NO+CO reaction. The Cu-Ce synergism for NO reduction with CO in the presence of O2, H2O and SO2 has been investigated by Wen and He. The results suggested that the synergistic effect between Cu and Ce ions could enhance the performance of catalyst both in activity and resistance to H2O and SO2. Redox Mechanism involving lattice oxygen/oxygen vacancy participation was proposed for CO oxidation on CeO2. Sayle et al. conducted atomic simulation of a CO oxidation mechanism on CeO2 and suggested that the surface oxygen of CeO2 is more active than the bulk oxygen. In the present study the importance of (i) method involved in preparation of catalyst, (ii) metal support interaction of catalyst and (iii) Cu-Ce synergistic effect in the NO reduction by CO are discussed.