Supported and un-supported zinc and chromium oxide catalysts for lower temperature CO oxidation: A review

Carbon monoxide (CO) is one of the most poisonous gasses present in the environment and lower temperature complete oxidation of CO is very important processes for human health protection. The automobile vehicles exhaust is one of the major sources of CO production into the environment and application of catalytic converter in automobile vehicles reduces the CO gasses emission. There are various types of catalysts are used in the catalytic converter for CO oxidation purposes among them the zinc oxides (ZnO) and chromium oxides (Cr2O3) are one of the oldest and best-known catalysts. The small modification of ZnO and Cr2O3 catalysts led to increases their specific surface area and decreases their mean crystallite size. The small addition of various metal ions into the hexagonal ZnO and Cr2O3 catalysts surfaces the formation of highly dispersed metal oxide for low-temperature CO oxidation. The catalytic oxidation of CO over ZnO and Cr2O3 catalysts could proceed via a Mars-van Krevelen mechanism, which involves the redox process. The lattice oxygen present in catalyst surface being consumed upon reaction for CO oxidation and replaced by oxygen in air. The ZnO and Cr2O3 catalysts are one of the best catalysts from lower cost, thermally activity and selectivity point of view. The certain attention has been paid on recovery of ZnO and Cr2O3 catalysts from reaction systems and their reuse for several times without losses of catalytic activity in CO oxidation. This review paper represents important information about the pure and substituted ZnO and Cr2O3 catalysts for CO emissions control