Metal surface sites

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

Methanatlon Studies. Because the most effective way to determine the existence of true bimetallic clusters having mixed metal surface sites Is to use a demanding catalytic reaction as a surface probe, the rate of the CO methanatlon reaction was studied over each series of supported bimetallic clusters. Turnover frequencies for methane formation are shown In Fig. 2. Pt, Ir and Rh are all poor CO methanatlon catalysts In comparison with Ru which Is, of course, an excellent methanatlon catalyst. Pt and Ir are completely inactive for methanatlon In the 493-498K temperature range, while Rh shows only moderate activity. 

Methanation studies suggest the presence of mixed metal surface sites with a Ru ensemble requirement of about four. 

Can this demand for a significant number of metal active sites be further quantified by a general expression in terms of cathode potential demand The answer is, in principle, yes, although the dependence of the relative populations of metal surface sites and oxidized surface sites on cathode potential could depend on (H20)/r-oh d somewhat different way, depending on the degree to which the... 

Still another way to characterize metal surface sites by a chemical reaction is with the unique molecules (+)— and (—)—apopinene (Fig. 1.5).25-28 The apopinenes are an enantiomeric pair of molecules with a double bond steri-cally hindered on one side by a gem-dimethyl group. During hydrogenation, each enantiomer may hydrogenate to the saturated symmetrical apopinane or isomerize to its enantiomer, which will have the same reactivity on a symmetrical surface (Scheme 1.1). 

SOH represents any surface site unassociated with any species of M, SOM a metal/surface-site complex and x the apparent ratio of moles of protons released or consumed per mole of adsorbate removed from solution. 

In surface-complexation models, the relationship between the proton and metal/surface-site complexes is explicitly defined in the formulation of the proposed (but hypothetical) microscopic subreactions. In contrast, in macroscopic models, the relationship between solute adsorption and the overall proton activity is chemically less direct there is no information given about the source of the proton other than a generic relationship between adsorption and changes in proton activity. The macroscopic solute adsorption/pH relationships correspond to the net proton release or consumption from all chemical interactions involved in proton tranfer. Since it is not possible to account for all of these contributions directly for many heterogeneous systems of interest, the objective of the macroscopic models is to establish and calibrate overall partitioning coefficients with respect to observed system variables. 

It is not currently possible to examine the configuration of the adsorbed species unambiguously. However, since thermodynamic arguments do not require a specific model at the molecular level, it is still possible to analyze equilibrium data within a thermodynamic context. Most surface reactions are inferred from experimental observations of reaction stoichiometries and perhaps only in a limited range of T. Consequently, the choice of specific surface species is dependent on two considerations (1) the need to explain the observed measurements in terms of reaction stoichiometries, and (2) the selection of a model to allow the representation of metal/ surface site interaction intensities. 

To what extent is the macroscopic proton release the direct expression of the metal/surface site reactions Table V compares the macroscopic proton coefficients (Xp ) ) with the coefficient expected if only the Cd(II) surface reactions are considered is the proton coefficient determined by considering the mole fraction of Cd(II) surface species and their formation reactions (Figure 14b). For example, when pSOH is 2.84, y = 0.11 x 1 + 0.89 x 2 = 1.89. At high alumina concentrations pSOH 2.14-2.53) the single surface reaction required to fit the data sets a limiting proton release of 2.0. 

With the ability to obtain information about the concentrations of various types of metal surface sites in complex metal nanocluster catalysts, HRTEM provides new opportunities to include nanoparticle structure and dynamics into fundamental descriptions of the catalyst properties. This chapter is a survey of recent HRTEM investigations that illustrate the possibilities for characterization of catalysts in the functioning state. This chapter is not intended to be a comprehensive review of the applications of TEM to characterize catalysts in reactive atmospheres such reviews are available elsewhere (e.g., 1,8,9 )). Rather, the aim here is to demonstrate the future potential of the technique used in combination with surface science techniques, density functional theory (DFT), other characterization techniques, and catalyst testing. 

Fig. 41 Experimentally determined surface oxygen coverage, 0OX, as a function of Pt electrode potential during a cathodic scan from 1.0 V and the associated logJoRR versus Vcath dependence calculated (for ambient conditions) from the combined effects of enhancement of the rate of the ORR at Pt metal surface sites with increase in cathode overpotential and cathodic stripping of a blocking surface oxygen species between 1.0 and 0.6 V [88]. The logJoRR versus Vcath plot shown, as calculated from Eq. 36, exhibits a gradual change of dV/d logj ( Tafel slope ) from around 40 mV/decade at 1.0 V to 120 mV/decade at potentials lower than 0.65 V [88].

Hydrous metal surface sites may act as general catalysts or as specific catalysts. Weak acidic sites and weak basic sites are found on surfaces that can promote reactions by donating protons (general acid catalysis) or hydroxide ions (general base catalysts). Surface sites may also exhibit nucleophilic or electrophilic character. 

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