Hydrogen spillover, reduction

In addition, if we accept the conventional mechanism of Brpnsted acid site generation by hydrogen spillover from Pt sites, as proposed for Pt-S04=/Zr02, we must recognize that this would be in fact a reduction process (zerovalent hydrogen yielding -OH groups). 

For both series of metal oxides the apparent activation energies of the catalyzed (spillover) reduction reactions were found to be similar to those of the noncatalyzed reductions. Generally, the effect of the Pt was to increase the available reactive hydrogen and/or to increase the rate of the nucleation (pre-exponential factors). Thus, this catalysis increases the availability of H but does not ("classically ) decrease the activation energy. 

The activation energy E of the dissociation of H2 is the same in both cases, although Ex and E2, the heats of adsorption of H, differ, with , > E2. In other words, the SMSI state is observed with a support of high electronic conductivity. In this state the work function of the support is smaller than that of the supported metal. If the TiOz support is not initially a conductor, it becomes one by reduction with H2 in the presence of Pt at high temperature. This reduction may not be limited to the surface. For low reduction temperatures, surface OH" groups and Ti3+ ions are formed by hydrogen spillover, as shown previously (100). But this formation of Ti3+ and OH" is not the source of SMSI, since the electrons are not transferred into the conductivity bands to be trapped by Pt. 

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. 

It has been shown in the previous sections that the addition of small amounts of a transition metal to various metal oxides lowers the temperature required for their reduction by H2. This phenomenon has been attributed to hydrogen spillover. It follows that a partial reduction of the host oxide can induce or modify the catalytic activity of the host material. 

The influence of spillover species on an acceptor phase can be in the extreme either subtle or profound. Many of the phenomena associated with hydrogen spillover are as subtle as the influences of type-2 hydrogen on the activity of ZnO (189) or as significant as bulk reduction, bronze formation, or catalytic activation. The effects may be similar to the exposure of a surface to a hydrogen plasma. 

The CO-H2 synthesis properties of metal/titania catalysts have been found in several studies to be essentially unaffected by the temperature of reduction, in contrast to the strong effect this factor has on chemisorption properties. This problem has focused attention on the special nature of the metal-titania contact perimeter. Reduction of titania, undoubtedly through hydrogen spillover, begins there. It is important to note that Ti J+ cations are produced by reduction temperatures as low as 473K, as shown by 02 adsorption/H20 decomposition measurements (36) or by temperature programmed reduction. In the latter study, the amount of-TiJ+ produced at temperatures below 503K was equivalent to a TiJ+/Pt atom ratio of 0.6 (37). 

On the basis of these results the following mechanisms are deduced (i) Partial reduction of the support adjacent to the platinum via hydrogen spillover, resulting in a special deactivation for MCP hydrogenolysis and in a change of product distribution. This... 

First and the most obvious conclusion, which can be drawn from the experiments described above, is that the SMSI state of Pt and other noble metals dispersed on a Ti02 support can be induced, probably more effectively, by means of the reduction with Ti metal. Consequently, the hydrogen spillover is not the only mechanism leading to SMSI. 

---