Effects of prereduction at moderate temperatures on Rh/CeO2 on the interactions of D2, H2 and CO with Rh/CeO2

Abstract

Temperature regimes for the uptake of hydrogen by reaction with various surface species on Rh/CeO₂ catalysts obtained from an RhCl₃ precursor are identified by temperature-programmed reduction (TPR). Evidence is obtained from TPD for the presence of hydrogen having different energies of desorption from Rh/CeO₂ after prereductions at 373 and 653 K [low-temperature reduction (LTR) and moderate-temperature reduction (MTR), respectively]. ¹H NMR and H–D isotope exchange studies at room temperature are used to gain insights into the nature and relative amounts of ‘weakly’ and ‘strongly’ chemisorbed forms of hydrogen (H₂ and D₂) detectable at Rh/CeO₂ surfaces. After prereduction at various temperatures over LTR Rh/CeO₂, a weakly chemisorbed form, associated with metallic rhodium surfaces, is observed to promote H–D isotope exchange with D₂ at 295 K. However, such exchange is strongly inhibited by prior outgassing at 473 K or by reduction of Rh/CeO₂ at temperatures in the range 473–653 K (MTR). Conversely, the latter prereductions are shown by TPD studies to enhance greatly the amount of hydrogen strongly chemisorbed by the CeO₂ support. Evidence for MTR-induced inhibition of access to metallic rhodium surfaces also emerges from studies using carbon monoxide to probe the accessibility of various rhodium sites. However, ‘direct’ blockage of rhodium surface sites by hydrogen species strongly chemisorbed thereon is considered to be less probable than ‘indirect’ inhibitions arising initially from electronic effects associated with MTR-induced formation of oxygen vacancies and surface hydrides located at Rh/ceria microinterfaces. Together these eventually progress to decoration/encapsulation of the rhodium particles.