Titania metal supports

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]. 

Common catalyst compositions include oxides of chromium or molybdenum, or cobalt and nickel metals, supported on silica, alumina, titania, zirconia, or activated carbon. 

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. 

This method has not been used very much to prepare bimetallic particles. It is always performed at fixed pH as in Section 4.2.3, i.e. the same recipe was applied without attempt to understand the underlying chemistry. Platinum-gold and palladium-gold (Pt or Pd Au = 5 95) have been deposited this way on titania-silica supports.66 After calcination at 673 K, the particles are small (<5nm), and no separate platinum or palladium metal particles are found. With Pd-Au/CeC>2 (Pd Au = 2 to 100) calcined at 673 K, however, only large gold particles (>8nm) and small palladium particles (<3nm) were found at gold loadings above lwt.%.184... 

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. 

Since natural sunlight can only penetrate a few microns depth, the use of thin films of titania applied to ceramic or metallic supports as maintenance free decontamination catalysts for the photocatalytic oxidation of volatile organic compounds is of interest for the abatement or control of these emissions. The sol-gel technology can be readily incorporated as a washcoating step of the catalyst supports that may be subsequently heat-treated to fix the titania to the support. The surface area, porosity and crystalline phases present in these gels is important in controlling their catalytic activity. Furthermore, the thermal stability and development of porosity with heat-treatment was important if the sol-gel route is to be used as a washcoating step to produce thin films. 

Due to its high photocatalytic activity towards the complete mineralisation of VOCs [7,8] titania in its anatase form is normally used. Using ceramic monoliths with high titania content (50%) the total oxidation of chlorinated organic compounds at low temperature has been demonstrated [9]. However, since the photons from natural light may only penetrate a few microns into the catalyst surface the use of a wash-coating technique, where only a thin active film of titania is applied to the ceramic or metallic support can be considered as an ideal technique to produce maintenance free photocatalytic reactors. 

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). 

A significant change in catalytic activity occurs when a metal-supported on a reducible oxide such as titania is heated in hydrogen at relatively high temperatures. For one thing these high temperature reduced (HTR) materials show a dramatic decrease in hydrogen and carbon monoxide chemisorption, a... 

Impregnation has been used to prepare a number of catalysts having different metal support combinations. Highly loaded nickel catalysts supported on alumina, titania, silica, niobia and vanadium pentoxide were prepared by adsorption of nickel nitrate from an ammoniacal solution onto the support material. The supported salts were dried at 120°C and calcined at 370°C before reduction to the supported metallic nickel. It was found that the ease of reduction depended on the crystallinity of the support. Amorphous or poorly crystalline supports made the reduction of the nickel oxide more difficult than on crystalline supports. As examples of its generality, this procedure was also used to prepare... 

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... 

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