Recent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble Versus Inexpensive Metals and Their Reaction Mechanisms

Abstract

Preferential oxidation (PROX) of CO is applicable because of its low cost, ease of implementation, and low loss of H2 during purification. Mo–SiO2 and Cr–SiO2 photocatalysts utilized charge separation at metal=O bonds, and the PROX selectivity of CO was high. The CO PROX rates using semiconductor-based photocatalysts were comparable to those of photocatalysts (380 μmol h−1 g cat −1 ) and were also selective (100 %). These photocatalysts are advantageous because they do not activate H2 in comparison to noble metal catalysts. Below 473 K using noble metals, Ru, Rh, Pt, or Au supported on reducible metal oxides exhibited excellent CO thermal-PROX rates of ~4900 μmol h−1 g cat −1 ; however, the CO PROX selectivity of ~48 % was insufficient because of H2 activation on noble metals in nature. CO adsorbed onto TiO2 and O2 was stabilized at the interface between a Ti site and an Au atom. Weakly adsorbed water increased the effective number of active sites by stabilizing Au–OOH or Au–COOH. The PROX rates of CO using Au/TiO2-based catalysts under dark conditions increased under UV–visible light by the effect of charge separation and surface plasmon resonance and the promoted electron transfer to the adsorbed O2. In the case of CuO–CeO2 catalysts, CO adsorbed onto CuO and reacted with lattice O atoms at the boundary between CuO and CeO2 to form CO2 at an O vacancy, which was subsequently filled with an O2 molecule. The combination of Cu or Co with a reducible metal oxides also provided performance comparable to or higher than that of CuO–CeO2 owing to adequate standard reduction potential for Cu2+, Co3+, Mn3+/4+, and Ce4+. Finally, binary metal–organic framework consisting of oxyhydroxide Ti clusters interlinked by organic ligands and Cu oxyhydroxide ligands showed superior CO PROX performance (76 % conversion and 99 % selectivity) to that achieved using CuO–CeO2 owing to effective dispersion of Cu–O–Ti connection in microporous crystallites. Further progress is needed to alleviate the activity loss in the presence of moisture and/or CO2 based on suggestions that steric hindrance of some types of microporous crystallites would suppress the blocking of moisture or CO2.