Preferential oxidation of CO on Pt-Nb₂O₅/Al₂O₃ catalysts. Study of the Nb promotion effect and kinetic analysis

The effect of Nb addition to Pt-based catalysts applied to the preferential oxidation of CO to CO2 (PROX reaction) was studied. The activity of platinum niobia-promoted catalysts was measured during PROX in a hydrogen rich flow. A unique effect of niobium as a catalyst was observed, which could either act as a promoter or as inhibitor depending on niobia content. The most interesting result occurred when platinum was supported on bulk Nb2O5 in which case the CO activity was significantly inhibited. Increasing the Nb content increased the oxidation states of Pt and Nb affecting also their binding energies. A shift of the linear CO band frequency to higher values was observed as Nb loading was increased, and this band was an spectator species because it did not change as CO conversion increases. The reduced Pt Nb-promoted catalysts were in a partially Pt+2 oxidation state. The addition of a small amount of Nb promoted CO conversion on the 1%/Pt/Al2O3 catalyst. A 100% CO conversion were obtained at 170°C on 1%Pt/5%Nb-Al2O3, whereas the un-promoted catalyst did not show full conversions below 200°C. Kinetic parameters were provided for the promoted Pt/5%Nb-Al2O3 and Pt/Al2O3 catalysts. A kinetic model was proposed, which suggested that carbonate-like species acted as a barrier for the spillover mechanism, which in turn benefited the selectivity of the catalysts. This is an abstract of a paper presented at the 2007 AIChE Annual Meeting (Salt Lake City, UT 11/4-9/2007).