(758g) Design of Cr-Free Promoted Cu-FeOx Catalysts for High Temperature Water-Gas Shift

Industrial H₂ production proceeds via steam reforming of CH₄ (CH₄ + H₂O → CO + 3H₂) followed by high and low temperature water-gas shift (WGS) catalytic reactions to control the final desired CO/H₂ ratio (CO + H₂O → CO₂ + H₂). The current high temperature WGS (HT-WGS) catalyst contains carcinogenic hexavalent Cr that is of great environmental concern. Although much research has been performed in recent years on finding a non-toxic promoter to replace the toxic hexavalent Cr, the fundamentals of such promoted catalysts are still lacking and prevent the development of a design strategy for Cr-free HT-WGS catalysts.

Performance behavior of four different promoters (Ca, Ge, Co and Ni ) as substitutes for Cr in HT-WGS Cu-FeO_x catalysts were compared with the industrial Cr-promoted Cu-FeO_x catalyst for HT-WGS. The dynamics of the promoter surface distribution (before and after reverse HT-WGS reaction) in the catalysts were determined by in situ High Sensitivity-Low Energy Ion Scattering (HS-LEIS) spectroscopy. In the fresh dehydrated catalysts, all the promoters were found to be uniformly dispersed throughout the outermost surface layers (~2nm). After activation under the reverse HT-WGS reaction conditions, however, the distribution of the promoters dramatically altered. While Ca and Ge surface segregated, Cr, Co and Ni migrated into the bulk iron oxide phase to form Fe_3-xM_xO₄ solid solutions (M represents promoter). CO Temperature Programmed Reduction (TPR) studies were performed to understand the effect of the surface distribution of promoters on catalytic HT-WGS reaction performance. The CO-TPR quantitatively chemical probes the number of reactive surface and bulk lattice oxygen atoms as well as the reaction kinetics for CO oxidation to CO₂. The Ca and Ge promoted Cu-FeO_x catalysts significantly retarded both the low temperature surface reduction kinetics and amount of removable oxygen atoms relative to Cr promotion. The Co and Ni promoted Cu-FeO_x catalysts, however, enhanced the amount of reducible surface oxygen, but retarded the surface reduction kinetics relative to the Cr promotion. The above observations imply that the surface segregation of promoters (Ca and Ge) significantly retards catalyst surface reduction, whereas, promoters forming bulk solid solution (Cr, Co and Ni) with iron oxide improve the redox ability of the catalysts for the HT-WGS reaction.

The in situ characterization studies demonstrate, for the first time, the dynamics of promoted Cu-FeO_x catalysts and their effect on number of reducible catalyst surface oxygen atoms and redox kinetics during CO-TPR (the rate-determining-step for HT-WGS). The promotion mechanisms of the different cations (Cr, Ca, Ge, Co and Ni) on the HT-WGS reaction will be discussed. These new insights are guiding the rational design of Cr-free promoted Cu-FeO_x catalysts for the HT-WGS reaction.