Titania-supported catalysts reduction temperature effects

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. 

The reversibility is a major characteristic feature of the SMSI effect (300-302). In the case of NM/TiOj, reoxidation at about 773 K, followed by a reduction at low temperature, 473 K, is known to be effective for recovering the catalysts from the SMSI state (300-302,323). Probably by analogy with these earlier studies on titania-supported noble metal systems, similar reoxidation temperatures (773 K) have also been applied to NM/Ce02 catalysts for recovering their chemisorptive and/or catalytic properties from the deactivated state (133,144,221). Data commented below, in which the nanostructural changes of Rh and Pt catalysts in a redox cycle have been followed, prove, nevertheless, that drastic differences are also observed in the reversibility behaviour of ceria based systems, and also that more severe treatments are required to recover this family of catalysts from their corresponding interaction states. 

Titania-supported vanadia catalysts have been widely used in the selective catalytic reduction (SCR) of nitric oxide by ammonia (1, 2). In an attempt to improve the catalytic performance, many researchers in recent years have used different preparation methods to examine the structure-activity relationship in this system. For example, Ozkan et al (3) used different temperature-programmed methods to obtain vanadia particles exposing different crystal planes to study the effect of crystal morphology. Nickl et al (4) deposited vanadia on titania by the vapor deposition of vanadyl alkoxide instead of the conventional impregnation technique. Other workers have focused on the synthesis of titania by alternative methods in attempts to increase the surface area or improve its porosity. Ciambelli et al (5) used laser-activated pyrolysis to produce non-porous titania powders in the anatase phase with high specific surface area and uniform particle size. Solar et al have stabilized titania by depositing it onto silica (6). In fact, the new SCR catalyst developed by W. R. Grace Co.-Conn., SYNOX , is based on a titania/silica support (7). 

Figure 6 shows the activity results for the sono and classic series after reduction treatments at 623 K. It is clear that the addition of titania to the gels promotes an increase of the activities. The sonogel-supported catalysts are more active than the classics. The increase in the reduction temperature induces a depletion in the rate of hydrogenolysis, but such inhibition effect is lower in magnitude to that observed for a Rh/TS reference catalyst Rh/gel catalysts showed selectivities toward ethane higher than 80%. 

Supported metals are used extensively in heterogeneous catalysis. In the present investigation platinum is loaded onto titania and titania-alumina supports to study the SMSI effects in detail. The catalysts were characterized by X-ray Diffraction(XRD), Stepwise Temperature Programmed Reduction (STPR) and chemisorption measurements. All the samples exhibit eharacteristic behaviour showing SMSI effect after HTR, though there is only moderate interaction in the mixed oxide sample. From STPR studies, the reducibility of platinum and the support in supported platinum systems is shown to depend on the extent of the interaction at the interface. 

The surfaces of the as-received materials are covered by O and/or OH species, as indicated by the peak at 1.098 GkHz Remarkably, these surfaces were effectively cleaned simply by holding the electrode potential within the electrochemical double-layer region. This mild surface-reduction procedure should be contrasted to the rigorous and often technically demanding methods employed at the solid-gas interface, which usually involve several cycles of high-temperature calcination and reduction, limiting the choice of catalyst support to, typically, non-conductive oxides such as alumina, titania, and silica. 

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