Abstract
The location of the promoter element in rhodium on alumina and silica catalysts promoted by vanadium oxide has been studied by various techniques. Our results prove that an intimate contact between the active component, rhodium, and the promoter, vanadium oxide, is present in most catalysts studied. For the silica-supported systems, temperature programmed reduction and diffuse reflectance infrared spectroscopy pointed to the formation of a mixed oxide (RhVO 4) during calcination. Reduction of this oxide phase resulted in a vanadium oxide layer on top of the metal particle, as could be concluded from carbon monoxide chemisorption experiments. CO chemisorption was suppressed in the Rh/V 2O 3/SiO 2 catalysts, while transmission electron microscopy showed that the rhodium particle size was not influenced by the addition of vanadium oxide. This indicates that the suppression of CO chemisorption is not due to a decrease of metal particle size, but due to covering of the metal particle. Infrared spectroscopy showed that the amount of linearly bonded and bridge-bonded CO was almost completely suppressed, while the amount of gem-dicarbonyl species remained unaffected. No suppression of hydrogen chemisorption was observed. From this and TPD experiments it could be concluded that hydrogen adsorption occurs both on the exposed Rh atoms, as well as on the vanadium oxide patches partly covering the surface Rh atoms, pointing to the formation of hydrogen bronzes. Temperature programmed reduction experiments showed that RhVO4 was not formed in Rh/V 2O 3/Al 2O 3 during calcination. For V/Rh < 1.0, Rh 2O 3 and V 2O 3 particles exist separately on the support, due to the strong interaction between V 2O 5 and Al 2O 3. Only for catalysts with a V/Rh value around 7.0 (near-monolayer of vanadium oxide on alumina), oxidation at 898 K resulted in the formation of RhVO 4. For these catalysts, Rh2O3 might be positioned on top of the vanadium oxide layer after calcination. Almost all adsorbed CO was present in the form of the gem-dicarbonyl species and only a minor suppression of CO adsorption was observed.
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
