Evidence of strong metal–oxide interactions in promoted Rh/SiO 2 on CO hydrogenation: Analysis at the site level using SSITKA

Abstract

It has been suggested that the behavior of Group VIII metal catalysts supported on transition metal oxides can be significantly affected by pretreatment conditions due to strong metal–oxide interactions (SMOI). However, the origins for the SMOI effect are still in debate. In this research, SMOI of Rh and vanadium oxide (as a promoter) supported on SiO 2 were studied at the site level for the first time, which provides an insight into the modification of surface properties after high temperature reduction. H 2 chemisorption, Fischer–Tropsch synthesis (FTS), and SSITKA (steady-state isotopic transient kinetic analysis) were used to probe the SMOI effects. The catalytic properties of the catalysts for CO hydrogenation were investigated using a differential fixed bed reactor at 230 °C and 1.8 atm, while for SSITKA, a reaction temperature of 280 °C and an excess of H 2 was used to maximize methane production. The addition of V to Rh/SiO 2 suppresses H 2 chemisorption, and high reduction temperature further decreases H 2 chemisorption on Rh/V/SiO 2 but has little effect on Rh/SiO 2. As reduction temperature increases, the activity for CO hydrogenation on Rh/SiO 2 remains essentially unchanged, but the activity of Rh/V/SiO 2 decreases significantly. SSITKA shows that the concentration of surface reaction intermediates decreases on Rh/V/SiO 2 as the reduction temperature increases, but the activities of the reaction sites increase. The results suggest that Rh being covered by VO x species is probably the main reason for the decreased overall activity induced by high reduction temperature, but more active sites appear to be formed probably at the Rh–VO x interface.