Structural and catalytic differences in the effect of Co and Mo as promoters for Pt-based aqueous phase reforming catalysts

Reaction rates (site time yields, normalized to CO chemisorption sites) and product selectivity were determined for PtCo, PtMo, and Pt supported on multi-walled carbon nanotubes for aqueous phase reforming of glycerol. The bimetallic PtCo and PtMo catalysts had 4.6x and 5.4x higher glycerol consumption rates than Pt, and 3.9x and 0.6x rates in H-2 formation compared to Pt. Hydrogen generation selectivity was similar on Pt and PtCo, but PtMo had an increase in selectivity to C-O cleavage products, which reduced H-2 yield at conversions over 60% (85-90% hydrogen yield for Pt and PtCo, 65% for PtMo). X-ray absorption spectroscopy and scanning transmission electron microscopy results indicate that PtCo adopts multiple mono- and bimetallic structures (Pt shell/Co core, well-mixed alloy, Pt only), which maintain surface Pt sites that are selective to hydrogen generation while adding a promotional metal that increases reaction rates. This is in contrast to a previously characterized PtMo catalyst which introduced surface sites capable of deoxygenation reactions (in the form of Pt Mo metallic or Pt MoO/OH acid pairs) which result in the observed decrease in selectivity. (C) 2014 Elsevier B.V. All rights reserved.