Carbon monoxide hydrogenation on Co-Rh/Nb2O5 catalysts

Carbon monoxide hydrogenation activities and product distributions were investigated here for the first time on a series of seven Co-Rh/Nb2O5 catalysts: two monometallics Co and Rh supported on Nb2O5 and five bimetallics Co-Rh supported on Nb2O5 with similar Co (similar to 1.9 wt%) and variable Rh loadings (0.3-2.3 wt%). Catalytic performances at atmospheric pressure and 493 K were evaluated after low temperature reduction (LTR, 533-573 K) and after high temperature reduction (HTR, 773 K). Temperature-programmed reduction characterization revealed that the reduction temperature of the dominant Co phase on calcined catalysts, Co3O4, strongly decreased as the Rh/Co bulk atomic ratio increased, while the reduction temperature of the Rh2O3 phase (363 to 419 K) was not strongly influenced by the presence of Co3O4. It was observed that the activity decay effect caused by metal-support interaction was remarkably inhibited on the bimetallics with respect to the monometallics by comparing reaction rates after LTR and after HTR. The addition of Rh to the Co monometallic catalyst significantly altered the product distribution. An unusual promotion of the selectivity to long chain hydrocarbons was observed. This promotion was more intense after HTR on the bimetallic catalysts, reaching similar to 56% in the diesel fraction on the bimetallic catalyst with higher Rh concentration. Alcohol selectivity was enhanced up to 3.5 and 5.4% for ethanol and propanol, respectively, on the bimetallic catalyst with lower Rh concentration. The total CO hydrogenation reaction rate and the selectivity for methane were approximately constant as the Rh concentration on the bimetallics increased, suggesting that the metal surface area did not vary considerably on these catalysts. This agrees with hydrogen adsorption measurements on the bimetallic catalysts and with the XPS surface structural characterization of the calcined Co-Rh/Nb2O5 catalysts, which revealed that the specific surface area of active metal (Rh + Co) precursor oxide did not vary considerably as the Rh concentration increased. (C) 1999 academic Press.