Supports surface reduction

It appears, however, that support surfaces are not always as refractory to reduction as chemical intuition would dictate. An important new finding is that lanthanum oxide undergoes reduction, in the presence of a supported metal, to "LaO", and the properties of Pd/lanthana are similar in several respects to those of metal/titania (43,44). Even with alumina supports, surface reduction has been found in some instances (45-471. The reason for this anomalous behavior is not fully understood, although sulfur has been found capable of promoting the reduction (46). A recent report has described suppressed H2 chemisorption on Rh/zirconia (4 ) although this was not found in an earlier study of Ir/zirconia (2). One may suspect differences in surface reducibility between the supports used in the two cases. 

The first one is the direct synthesis of metallic nanoclusters, not via formation of (hydro)oxides and their reduction in gas-phase, because the successive reduction for formed (hydro)oxides sometimes results in the size growth of metal particles due to the aggregation and/or sintering. The second one is the use of precisely designed metal complexes, which are well adsorbed on the support surfaces, as shown in Figure 1. 

Figure 9.7 Transmission electron microscopy of rhodium particles on a model titania support after reduction in H2 at 200 °C (top) and the same catalyst in the SMSI state after reduction at 500 °C (bottom). An amorphous overlayer on the surface of the SMSI catalyst is clearly discerned (from Logan etal. [25]).

Carbon-supported platinum (Pt) and platinum-rathenium (Pt-Ru) alloy are one of the most popular electrocatalysts in polymer electrolyte fuel cells (PEFC). Pt supported on electrically conducting carbons, preferably carbon black, is being increasingly used as an electrocatalyst in fuel cell applications (Parker et al., 2004). Carbon-supported Pt could be prepared at loadings as high as 70 wt.% without a noticeable increase of particle size. Unsupported and carbon-supported nanoparticle Pt-Ru, ,t m catalysts prepared using the surface reductive deposition... 

The internal mass-transfer effects can be reduced, however, by decreasing the particle dimensions of the porous support containing the biocatalyst. Particle-diameter decrease results in a reduction of the distance from the outer support surface that the substrate must cross and, consequently, also results in a decrease of the substrate concentration gradient. 

A related problem is the reduction in support surface area. This is especially a problem in the case of titania, where the anatase polymorph is only stable under oxidative regeneration conditions from about 400°C to 750°C. The addition of Si, Zr and Ta as promoter elements may avoid or diminish surface collapse of the support oxide. 

The use of highly dispersed catalysts from soluble salts of molybdenum is another approach to the reduction of catalyst amount because of their excellent activity despite their higher price. Recently, metal carbonyl compounds, such as Fe(CO)5, Ru3(CO)i2, and Mo(CO)6 have been investigated as metal cluster catalysts. Preparation involved their deposition and decomposition on catalyst support surfaces (71-73). 

Data is shown in Figs. 7a and 7b for oxygen reduction on carbon steel in room temperature 0.6M NaCl. iL increases with co0 5 as predicted. Hence if the corrosion rate is determined by the mass transport of oxygen to the disk surface to support oxygen reduction, then the corrosion rate will increase as a function of the rotation rate, co, raised to the 0.5 power and linearly with dissolved oxygen concentration. The diffusion boundary layer thickness, 8d, may be calculated from Fick s first law after iL is determined. Recall that 8 = nFDCJiL for one dimensional diffusion at the steady state. This leads to the following expression for the diffusional boundary layer thickness ... 

In Section V below we show that spillover can induce catalytic activity on the support. The nature of the active site created on the support may result from the surface reduction, or the adsorbed hydrogen may be a center and site for reaction (123). On the other extreme, spiltover hydrogen has been shown to inhibit ortho-para conversion over sapphire and ruby surfaces... 

For example, chromium trioxide reacts with an —OH group pending from the support surface yielding a centre precursor [193]. This is activated by partial reduction of Cr6+ with agents without detachable H (CO, monomer etc.) [194],... 

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