Effect of molybdenum addition on supported platinum catalysts for the water–gas shift reaction

The effect of molybdenum on water–gas shift (CO + H2O ↔ CO2 + H2) turnover frequencies was investigated for 2 wt% platinum, up to 11 wt% molybdenum, alumina and silica supported catalysts. The maximum PtMo turnover frequencies were observed for Mo weight loadings of 1.4% and 1.2% when supported on Al2O3 and SiO2, respectively. Above these loadings, the exposed Pt surface area as measured by hydrogen chemisorption was reduced. The hydrogen chemisorption values and X-ray absorption spectroscopy of the reduced catalysts indicate the formation of PtMo bimetallic particles that are surface rich in Mo, yet increases in Pt–Mo coordination do not correlate with changes in turnover frequency, and most of the Mo is present in the form of Mo-oxide. The decrease in TOF from the optimum Mo loading is attributed to differences in surface coverages of intermediates that are observed in the changing reaction orders and ascribed to the interplay between the PtMo alloy metal function and the promotion of water activation by MoOx promotion of the support. The decreased Pt dispersion and CO binding energy of Pt–Mo catalysts result in CO reaction orders as high as 0.8 at 300 °C, and Mo addition resulted in apparent activation energies which were 20–40 kJ/mol lower than the Mo free samples. The promoted catalysts achieved higher TOF than Pt/CeO2 when calculated at or below 250 °C with 6.8% CO, 8.5% CO2, 21.9% H2O, 37.4% H2, and balance Ar at 1 atm total pressure.

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► PtMo/Al2O3 and SiO2 turnover frequencies are comparable to Pt/CeO2.
► The increase in Pt TOF with Mo is lower on SiO2 than on Al2O3.
► H2 reduction forms PtMo bimetallic particles that are surface rich in Mo.