Work function of the support

For the same catalyst, the activation energy is drastically decreased and the reaction rate enormously increased with increasing electron concentration and decreasing work function of the support. This behavior... 

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. 

Catalyst Work function of the support [eV] TON 10 ofCO > Semiconductor type of support... 

Weckhuysen et al. [6,33] have recently published several UV-Vis DRS works devoted to investigate the surface chemistry of supported chromium catalysts as a function of the support composition. The same authors [34] have also tried... 

An important consideration for the electronics of semiconductor/metal supported catalysts is that the work function of metals as a rule is smaller than that of semiconductors. As a consequence, before contact the Fermi level in the metal is higher than that in the semiconductor. After contact electrons pass from the metal to the semiconductor, and the semiconductor s bands are bent downward in a thin boundary layer, the space charge region. In this region the conduction band approaches the Fermi level this situation tends to favor acceptor reactions and slow down donor reactions. This concept can be tested by two methods. One is the variation of the thickness of a catalyst layer. Since the bands are bent only within a boundary layer of perhaps 10-5 to 10 6 cm in width, a variation of the catalyst layer thickness or particle size should result in variations of the activation energy and the rate of the catalyzed reaction. A second test consists in a variation of the work function of the metallic support, which is easily possible by preparing homogeneous alloys with additive metals that are either electron-rich or electron-poor relative to the main support metal. 

However, equation (3) still contains two unknowns the work function of the metal and the work function of the gold deposit. A multilaboratory study has shown that neither carbon nor gold provide a suitable internal standard for catalyst supports (34). Alternatively, the binding energy scale has been referenced to a core level of the support metal cation, for example, the Si 2p peak of silica. This is no improvement. The work function of a high surface area, amorphous catalyst support has never been measured. 

This brings us to the rear support-film interface. What sort of barrier exists at this junction Are the electron exchange kinetics voltage-dependent at this interface The effect of changing the work function of the substrate on the current-voltage curves (in the dark and under illumination) has been investigated for Ti02 nanocrystalline films [344]. The onset potential for the photocurrent is found to be the... 

A last practical question how can one easily measure the absolute potential of a support The answer is simple by simply depositing a metal (any metal) electrode on it and measuring the work function of the backspillover-modified electrode [140]. 

There is similar electrochemical promotion behavior of Rh films on YSZ and similar metal-support interaction-induced behavior of dispersed Rh on different supports for the model reaction of C2H4 oxidation on Pt (Figure 40). In particular, there are very similar p values (p ep]y[sj e 120) upon increasing the potential and work function of the Rh film or upon increasing the work function (or absolute potential) of the support of the dispersed Rh catalyst (Figure 40). 

The same reaction was also investigated over Rh nanoparticles dispersed over pure and doped Ti02 and other porous supports of known work functions [136]. It was established that the influence of electrochemical promotion on kinetic parameters of the model reaction is identical to the influence of metal-support interactions, under conditions at which the change of the work function of the catalyst is the same, regardless of the means by which the alteration in the work function is achieved. 

Interesting phenomena observed gave rise to some speculations which have lacked sound physical justification examples are the speculations on a massive transfer of electrons from (or to) the support to (or from) the metal, the ideas about changes of the work function of the metal by such a transfer or by the.field of the support, etc. However, these ideas have received considerable attention and unfortunately they have tended to shadow other pieces of information concerning the metal-support interaction. 

Although the most well established function of the support is to disperse the metal and to retard the sintering, recent works on metal-support interaction collected in (1) have shown that the chemi-sorptive and catalytic activity of group VIII metals were considerably lowered when the metal was supported on reducible oxide, such as Ti02, and reduced at high temperature. The objective of the work reported in this paper was to further investigate the role of the support on the adsorptive and on the catalytic properties of platinum. 

Simultaneously, because of the good physical and thence electrical contact between both solid phases, there is an alignment of their Fermi levels which demands an electron migration - even limited -from the support to the metal in agreement with the respective values of the work functions of the metal and of the reduced support. 

Interactions at room temperature When CO is first introduced (Fig.l), a increases irtfantaneously and then remains independent of P 0. The fact that a does not decrease means that CO does not dissociate on titania nor at the interface, otherwise the filling of anionic vacancies by atomic oxygen (Eq,-6) would have decreased substantially a by consuming free electrons. The sharp initial increase, on the contrary, shows that CO chemisorb on Pt with a donor effect probably due to the creation of dipoles as proposed for H. chemisorption which renders ohmic the electrical contact between the metal and its semiconductor support (26, 17, 28)Accordlng to these authors, the creation of a dipole layer decreases the work function of the metal which approaches the electron affinity of the semiconductor, thus suppressing the Schottky barrier. Presently CO adsorbs as a donor molecule on Pt decreasing 0, which allows elec-... 

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