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 N 2 and CO 2 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 NO x , reducing agents such as carbon monoxide (CO), hydrogen (H 2 ), ammonia (NH 3 ), urea ((NH 2 ) 2 CO) and different hydrocarbons are usually being employed.1,2 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. 3Among the transition metals studied, copper show a potential activity in the NO+CO reaction and have been extensively studied. [4][5][6] Sakurai et al. tested catalytic activities of cupric oxide and alumina-supported copper oxides in NO reduction by CO.7 Metal oxides such as Ce and Ni are commonly employed O 2 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 O 2 storage capacity in catalysts containing Pt, Rh, Ni and Ce supported on Al 2 O 3 , Ni is found to be the dominant storage metal and Ce predominates over Pt and Rh. 8 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 CeO 2 crystallite size.9 Addition of cerium oxide play an important role in providing oxygen storage by transforming between Ce 2 O 3 under reducing conditions and CeO 2 under oxidizing conditions. 10 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.11 The NO dissociation peaks were shifted towards low temperature upon addition of ZrO 2 onto CuO/TiO 2 , indicating NO decomposition activity was higher in CuO-ZrO 2 / TiO 2 than by CuO/TiO 2 .12 Hernandez et al. suggested that the presence of acid-base sites on Pt/ZrO 2 -CeO 2 seemed to be important for...