Preferential oxidation of carbon monoxide over Pt–FeO_x/CeO₂ synthesized by two-nozzle flame spray pyrolysis

Pt-FeO_x-CeO₂ catalysts for the preferential oxidation of CO (CO-PROX) with controllable segregation between the Pt/FeO_x and CeO₂ components were synthesized by two-nozzle flame spray pyrolysis. This was achieved by flame-spraying the two active components independently in two interfacing flames. By adjusting the intersection distance between the two aerosol flames, it was possible to tune the morphology and the reducibility of the catalysts. Intimate interactions between Pt/FeO_x and CeO₂ (without oversintering the two components) achieved at the greatest flame distance gave reducibility at the lowest temperature, as corresponding to the formation of oxygen vacancies around the Pt (from H-spillover). Maximum CO conversion (>99.5%) for the two-nozzle synthesized catalysts was achieved below 90 °C, which is 30 °C lower than for mechanically mixed Pt/FeO_x and CeO₂. While high CO₂ selectivity was attained at lower temperatures, it was limited by the adsorption of reactive oxygen. The oxygen vacancies on the Pt-FeO_x-CeO₂ catalyst were gradually oxidized at low temperatures due to unconsumed O₂, and this led to a decrease in CO conversion with time on stream. In contrast, complete consumption of O₂ and excess of dissociated H₂ sustained the high oxygen vacancies content at high temperatures and hence high activity (although nonselective) was achieved. We further identified the loss of CO₂ selectivity at high temperature as originating from the weakened CO adsorption compared to that of H₂ in the presence of oxygen vacancies.