Kinetic characteristics of oxygen-enhanced water gas shift on CeO2-supported Pt–Cu and Pd–Cu bimetallic catalysts

• Pd–Cu/CeO2 is most active and stable under practical OWGS/WGS conditions.
• Addition of O2 to WGS removes CO to free up the active surface for H2O adsorption.
• O2 does not change chemistry of H2O chemisorption, but increases the probability.
• The OWGS/WGS rate is determined by the balance of surface reactants.
• The Langmuir–Hinshelwood model can be used to describe the OWGS reaction.

Our laboratory has developed a new approach to enhance water gas shift (WGS) at low temperature by adding a small amount of O2 over CeO2-supported bimetallic catalysts, which is called oxygen-enhanced water gas shift (OWGS). In the present study, the activities of bimetallic and monometallic catalysts were comparatively evaluated and the origin for better performance in the bimetallic catalysts was sought by a kinetic study. The CeO2-supported Pt–Cu and Pd–Cu catalysts showed not only higher activity, but also higher stability for about 70 h under practical OWGS condition, which was corroborated by little change in FT-IR spectra of surface species before and after the long-duration reaction. In the kinetic analysis, O2 addition to WGS significantly increased the reaction order in CO for all the catalysts tested while the reaction order in H2O changed little upon O2 addition to WGS. The catalysts with relatively low CO order such as Pt or Pd had relatively high H2O order while the catalysts with high CO order such as Cu had low H2O order. Such a trend shows that the reaction rate is determined by the balance between the two reactants on the surface and O2 addition to the feed changes this balance by removing some CO to make more sites open for H2O adsorption and activation. In the presence of the product gases, H2O activation becomes more rate-limiting, so that the combination of noble metal and copper on CeO2 is more effective with the use of added oxygen to remove the CO strongly adsorbed on the active sites.

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