Titania metal-support interactions

At the end of the seventies, scientists at Exxon discovered that metal particles supported on titania, alumina, ceria and a range of other oxides, lose their ability to chemisorb gases such as H2 or CO after reduction at temperatures of about 500 °C. Electron microscopy revealed that the decreased adsorption capacity was not caused by particle sintering. Oxidation, followed by reduction at moderate temperatures restored the adsorption properties of the metal in full. The suppression of adsorption after high temperature reduction was attributed to a strong metal-support interaction, abbreviated as SMSI [2]. 

S. H. Overbury, L. Ortiz-Soto, H. G. Zhu, B. Lee, M. D. Amiridis, and S. Dai, Comparison of Au catalysts supported on mesoporous titania and silica Investigation of Au particle size effects and metal-support interactions, Catal. Lett. 95(3-4), 99-106 (2004). 

The focus of these studies has been on identifying mild activation conditions to prevent nanoparticle agglomeration. Infrared spectroscopy indicated that titania plays an active role in dendrimer adsorption and decomposition in contrast, adsorption of DENs on silica is dominated by metal-support interactions. Relatively mild (150° C) activation conditions were identified and optimized for Pt and Au catalysts. Comparable conditions yield clean nanoparticles that are active CO oxidation catalysts. Supported Pt catalysts are also active in toluene hydrogenation test reactions. 

Salama, T. M., Hattori,H., Kita,H., Ebitani, K., and Tanaka, T., X-ray adsorption spectroscopic and electron paramagnetic resonance studies on the strong metal-support interaction of platinum supported on titania dispersed on silica, J. Chem. Soc. Faraday Trans. 89(12), 2067 (1993). 

In the Au/Al203/NiAl(100) system, hemispherical particles occur even at low coverage,7 unlike the situation with titania size distribution was narrow, and particles were stable to 600 K, implying low mobility of adsorbed atoms. Paradoxically, on alumina large particles migrate and coalesce faster than small ones, presumably because the metal-support interaction is weaker but with Au/FeO the diffusivity of atoms is higher due to a lower concentration of surface defects. 

Metal-support interactions have been recently reviewed by Bond (93), who drew special attention to catalysts that gave evidence for strong metal-support interactions (SMSI). This condition was first observed in 1978 by Tauster et al. (94) for Pt on titania catalysts. The catalysts seemed to lose their capacity for H2 and CO chemisorption but nevertheless retained and enhanced their activity for only two types of reaction methanation and Fischer-Tropsch synthesis. Since then a considerable number of papers devoted to SMSI studies have been published all over the world. 

Burch and Flambard (113) have recently studied the H2 chemisorption capacities and CO/H2 activities of Ni on titania catalysts. They attributed the enhancement of the catalytic activities for the CO/H2 reaction (after activation in H2 at 450°C) to an interfacial metal-support interaction (IFMSI). This interaction is between large particles of Ni and reduced titanium ions the Ti3+ is promoted by hydrogen spillover from Ni to the support, as pictured in Fig. 8. The IFMSI state differs from the SMSI state since hydrogen still chemisorbs in a normal way however, if the activation temperature is raised to 650°C, both the CO/H2 activity and the hydrogen chemisorption are suppressed. They define this condition as a total SMSI state. Between the temperature limits, they assumed a progressive transition from IFMSI to SMSI. Such an intermediate continuous sequence had been... 

Therefore, at least on titania, transition metals promote the spillover of hydrogen to the support this is a necessary step in the reduction of the support (and hence modification of the global solid s catalytic properties). In other words, hydrogen spillover is a prerequisite in each of these recently recognized metal-support interactions (SMSI and IFMSI). Evidently these very specific metal-support interactions are, from the point of view of the spillover phenomena, merely the reduction of more or less easily reducible metal oxides, as mentioned in the preceding subsection. 

When considering metal-support interaction effects, the whole set of Electron Microscopy data presented in the previous section point out some important differences between the behaviour of noble metal catalysts supported on ceria and that of titania-supported catalysts. Much higher reduction temperatures are required in the case of ceria-type supports to observe nanostructural features similar to those described for the so called SMS I efTect. 

To summarise the results concerning the study of reversibility of metal-support interaction states, we could first state that the classic reoxidation treatment at 773 K does not allow the recovery of the NM/Ce02 catalysts from the decorated or alloyed states. The noble metal/ceria phase separation may only be achieved upon reoxidation at temperatures well above 773 K. This observation represents an additional major difference between titania and ceria supported noble metal catalysts. Moreover, the likely regeneration of NM/CcOi catalysts reduced at 773 K by reoxidation at 773 K would actually prove, in good agreement with earlier HREM studies on the reduced catalysts (117,194), that the observed deactivation effects are not due to decoration or alloying phenomena, rather consisting of purely electronic effects (105). 

Santos J, Phillips J, Dumesic J (1983) Metal support interactions between iron and titania for catalysts prepared by thermal-decomposition of iron pentacarbonyl and by impregnation. J Catal 84 147... 

Beard BC, Ross PN (1986) Pt-Ti alloy formation from high-temperature reduction of a titania-impregnated Pt catalyst - implications for strong metal support interaction. J Phys Chem 90 6811... 

Titania-supported Metals. - After reduction at 473 K, platinum-group metals supported on Ti02 chemisorbed both hydrogen and carbon monoxide in quantities indicative of moderate-to-high dispersion, but following reduction at 773 K chemisorption was drastically lowered e.g., H/Mt <0.01 for Pt, Ir, and Rh, 0.05-0.06 for Pd and Ru, and 0.11 for Os). Agglomeration, encapsulation, and impurities were eliminated as possible causes and a strong metal-support interaction (SMSI) was proposed. Titania is not unique in its SMSI properties and 11 oxides used to support iridium were classified as follows ... 

Although this review has dealt with the interactions of metals with reduced oxide surfaces, metal-support interactions are certainly not limited to these. Evidence for metal-support interaction involving non-reduced surfaces exists even in the metal/ titania system. Enhanced hydrogenolysis activities have been found for low-temperature-reduced Rh/titania (7) and Ru/titania (49). These effects presumably involve interaction with Ti4+ ions. 

We have presented evidence that the strong metal-support interactions observed with titania and niobia are due to an oxide layer over the metal catalyst. This layer interacts chemically with the metal, as evidenced by the fact that the titania layers on Pt, Rh, and Pd do have slightly different properties. The fact that the methanation rates for a titania-covered and a niobia-covered Pt foil are identical indicates that the reason for enhanced methanation activity on the oxide-covered surface is likely due to geometric and not electronic cons id er at ions. 

A number of investigations have indicated that so-called "strong metal-support interactions (SMSI)" are caused by the migration of partially reduced oxide species onto the surface of titania supported metal particles (1 8). While this is an attractive theory in that it can account for the observed modifications in chemical properties of the metal particles, there is no agreed mechanism by which the postulated transport processes occur. 

In a second set of experiments, overlapping thin films of nickel and titania were heated to 1100 K in the presence of hydrogen in a controlled atmosphere scanning transmission electron microscope (STEM). During reaction the metal film was observed to restructure into discrete particles which eventually moved and attacked the titania substrate. We believe that this behavior provides direct evidence that both the mobility of reduced titania and metal species are involved in the initiation of strong metal-support interactions. 

The results of the present study demonstrate that migration of both titania and metal species may be involved in the initiation of strong metal-support interactions during treatment in hydrogen at temperatures in excess of 770 K. 

Recently titania appeared as a non-conventional support for noble metal catalysts, since it was found to induce perturbations in their H2 or CO adsorption capacities as well as in their catalytic activities, This phenomenon, discovered by the EXXON group, was denoted "Strong Metal-Support Interactions" (SMSI effect) (1) and later extended to other reducible oxide supports (2). Two symposia were devoted to SMSI, one in Lyon-Ecully (1982) (3) and the present one in Miami (1985) (4) and presently, two main explanations are generally proposed to account for SMSI (i) either the occurence of an electronic effect (2,5-13) or (ii) the migration of suboxide species on the metal particles (14-20). The second hypothesis was essentially illustrated on model catalysts with spectroscopic techniques.lt can be noted that both possibilities do not necessarily exclude each other and can be considered simultaneously (21). 

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