Structure of rhodium/titania in the normal and the SMSI state as determined by extended x-ray absorption fine structure and high-resolution transmission electron microscopy

Abstract

Extended X-ray absorption fine structure (EXAFS) and high-resolution transmission electron microscopy (HRTEM) have been used to study the structure of a Rh/Ti02 catalyst. After reduction in H, at 473 K (when the catalyst is in the normal state) the metal particles contain on the average five rhodium atoms and are situated preferably on edges of the Ti0, crystallites but also on [ l o l l and to a lesser extent on [OOl] anatase crystal faces. Reduction in H2 at 723 K leads to the SMSI state. Besides oxygen neighbors from the support, the rhodium metal atoms in the metal-support interface have Ti"' neighbors at 3.4 and 4.3 A. These distances and their coordination numbers fit well with a model in which the metal particles rest on a TiO, suboxide. This indicates that the supporting oxide near the metal particle has been reduced to a suboxide of TiO,. In the SMSI state no indication for coverage has been found with either EXAFS or HRTEM. On the contrary, exposing the catalyst in the SMSI state to oxygen at 100 K resulted in changes in the EXAFS spectrum due to physisorption of oxygen. Consequently, in the SMSI state the particles are either not covered or are incompletely covered with TiO,. Since a Rh/AI2O3 catalyst under the same conditions became partly oxidized, it is evident that for the Rh/Ti02 catalyst oxidation has been suppressed. This is most probably the result of an electronic influence from the reduced supporting oxide. Even after oxygen admission at room temperature, the rhodium particles on the TiO, support remain in the metallic state. The TiO, suboxide in the vicinity of the metal particles starts to reoxidize and the metal-support interaction becomes weaker. IntroductionIn heterogeneous metal catalysis the support is used to provide a large surface area to facilitate the preparation of well-dispersed catalysts and to prevent sintering of the small supported metal particles, in order to preserve their state of high dispersion. It is often found that support materials modify the chemical reactions of the metal catalyst. Examples are shape selectivity induced by a zeolitic support and bifunctional catalysis of metal particles dispersed on an acidic support, where the metal component catalyzes the hydrogenation/dehydrogenation reactions and the acidic support facilitates isomerization of olefinic compounds. In addition, the support may have a more direct influence on the chemical properties of supported metal particles, especially after reduction at high (>650 K) temperature. Thus, it is well-known that, for metals dispersed on certain transition-metal oxides, the capacity to adsorb hydrogen or carbon monoxide drastically diminishes when the catalyst is reduced at high, rather than low temperatures (<650 and usual >450 K) even though the particle size remains unchange~l-~ Non-transition-metal oxides like A1,03 and SiOz do not influence the capacity to adsorb gasses; the decrease in adsorption after reduction at high temperature of the metal particles dispersed on these supports can be accounted for by ...

The effect of Fe–Rh alloying on CO hydrogenation to C2+ oxygenates

“…The EXAFS data did not conclusively indicate Fe-Rh bonding, but did show Rh bonded to Ti or lattice-bound oxygen with bond lengths of 2.53 Å and 2.48 Å, respectively. This conclusion was supported by other studies that found evidence of either Rh-Ti bonding or Rh bonding to lattice-bound oxygen in the TiO2 [8][9][10]. Subsequent transmission electron microscopy (TEM) studies suggest intimate contact of Rh and Fe through the simultaneous detection of Rh and Fe in the same particles studied using spot scans of energy-dispersive spectroscopy (EDS) [4].…”

“…The need to develop alternative sources of liquid fuels has led to renewed interest in developing catalysts for the efficient conversion of synthesis or ''syn'' gas (CO + H2), derived from biomass, coal, and natural gas, to simple alcohols and higher oxygenates [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Currently, the only industrially applied process involves syngas conversion to methanol over a Cu-based catalyst (CuZnO/Al2O3) at temperatures above 500 K [1].…”

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

“…Although coverage of the metal particles by reduced support species was not observed, it could not be completely excluded either. The EXAFS after oxygen adsorption showed that oxidation was suppressed, although the metal particles were covered with adsorbed oxygen [2]. A Rh/A1203 catalyst was oxidized under these conditions [2,3].…”

“…The EXAFS after oxygen adsorption showed that oxidation was suppressed, although the metal particles were covered with adsorbed oxygen [2]. A Rh/A1203 catalyst was oxidized under these conditions [2,3]. Thus, if the metal particles in the Rh/TiO 2 sample in ref.…”

“…Thus, if the metal particles in the Rh/TiO 2 sample in ref. [2] were covered with a TiO x suboxide, this coverage was not complete.…”