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

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.