The structure of a rhodium/titania catalyst in the strong metal-support interaction state as determined by EXAFS

Abstract

Reduction of a highly dispersed 2.85 wt % Rh/Ti02 catalyst at 473 K after previous calcination at 623 K resulted in EXAFS whose primary contributions are due to nearest rhodium (average coordination number of 3.1 and distance of 2.67 A) and oxygen neiphbors (coordination 2.5 and distance 2.71 A). These oxygen neighbors originated at the metalsupport interface. The average rhodium-rhodium coordination number did not change in the SMSI state produced by reducing the catalyst at 673 K. However, the average coordination distance contracted by 0.04 A with an accompanying decrease of the Debye-Waller factor of the Rh-Rh bond of 0.0012 A2. This is due tQ the fact that in the SMSI state the surface of the metal particles is not covered with chemisorbed hydrogen. The SMSI state leads to a structural reorganization of the support in the vicinity of the rhodium metal particles. This can be concluded from the appearance of a Rh-Ti bond at 3.42 A in the SMSI state coupled with the fact that the average coordination number of the rhodiumsupport oxygen bonds does not increase. Other types of rhodium-oxygen bonds could not be detected with EXAFS in this state. Thus, these results provide no evidence for coverage of the metal particle by a suboxide of TiOz in the SMSI state.

X‐ray Absorption Spectroscopy of an Fe‐Promoted Rh/TiO2 Catalyst for Synthesis of Ethanol from Synthesis Gas

“…The coordination number (N = 1.1) and bond length (r RhÀTi = 2.53 ; Table 3, case A) match very well the EXAFS analysis reported by Sakellson et al [20] on Rh/TiO 2 reduced at 775 K. However, works by Koningsberger and co-workers have assigned the same feature to the coordination of the reduced Rh particles to oxygen anions of the support. [18,19] It is not possible to confidently assign the peak to Ti or O based solely on the quality of the fit, since both elements have similar forms of back-scattering and phase shift functions. [20] The arguments against the coordination of Rh to Ti are as follows: 1) the peak in the FT attributed to RhÀTi coordination is seen for samples reduced at low temperature as well as high temperature; 2) the appearance of a similar peak is seen in the EXAFS of highly disperse Rh clusters on Al 2 O 3 ; 3) partial overlap of the RhÀRh and RhÀTi peaks complicates the extraction of phase shift functions from standard compounds as well as fitting of the experimental data.…”

“…[16,17] Prior extended X-ray absorption fine structure (EXAFS) spectroscopy studies of monometallic Rh/ TiO 2 catalysts, and supported Rh catalysts in general, were initiated primarily by the pioneering studies of the groups of Koningsberger, Haller, and others. [18][19][20][21][22][23][24][25][26] Thus, it is instructive to discuss the major conclusions from earlier studies to provide a basis for the present investigation.…”

Preparation and Activity of Small Rhodium Metal Particles on Fishbone Carbon Nanofibres

“…The phenomenon of interface hydrogen has repeatedly been observed with several metals, such as Rh, Ir, and Pt, on oxidic carriers (γ -Al 2 O 3 , TiO 2 , zeolite-Y) by Koningsberger and co-workers (43)(44)(45)(46). Reduction of the catalysts at low temperature, i.e., below 300…”

Exafs evidence for direct Rh-Tan+ bonding and coverage of the metal particles in a Rh/Ta2O5 SMSI catalyst

“…The fact that the rhodium metal particles in Rh/Ta 205 are covered to a larger extent than the metal particles in Rh/TiO2 can be explained in several ways. First of all, the metal particles in Rh/TiO 2 are very small [1,2] and coverage has up to now only been reported in literature for larger metal particles. Another reason might be the fact that the Rh/Ta205 sample was reduced at a much higher temperature (858 K) than the Rh/TiO 2 samples (723 and 773 K).…”

“…In two recent studies we reported on the structure of a titania supported rhodium catalyst in the normal and the strong metal support interaction (SMSI) state [1,21. From these studies it became evident that during reduction up to 723 K alloy formation (Which is one of the proposed explanations for SMSI) had not taken place.…”