The bifunctional model for methanol electro-oxidation suggests that competent catalysts should contain at least two types of surface elements: those that bind methanol and activate its C-H bonds and those that adsorb and activate water. Our previous work considered phase equilibria and relative Pt-C and M-O (M ) Ru, Os) bond strengths in predicting improved activity among single-phase Pt-Ru-Os ternary alloys. By addition of a correlation with M-C bond strengths (M ) Pt, Ir), it is possible to rationalize the recent combinatorial discovery of further improved Pt-Ru-Os-Ir quaternaries. X-ray diffraction experiments show that these quaternary catalysts are composed primarily of a nanocrystalline face-centered cubic (fcc) phase, in combination with an amorphous minor component. For catalysts of relatively high Ru content, the lattice parameter deviates positively from that of the corresponding arc-melted fcc alloy, suggesting that the nanocrystalline fcc phase is Pt-rich. Anode catalyst polarization curves in direct methanol fuel cells (DMFC's) at 60°C show that the best Pt-Ru-Os-Ir compositions are markedly superior to Pt-Ru, despite the higher specific surface area of the latter. A remarkable difference between these catalysts is revealed by the methanol concentration dependence of the current density. Although the rate of oxidation is zero order in [CH 3 OH] at potentials relevant to DMFC operation (250-325 mV vs RHE) at Pt-Ru, it is approximately first order at Pt-Ru-Os-Ir electrodes. This finding implies that the quaternary catalysts will be far superior to Pt-Ru in DMFC's constructed from electrolyte membranes that resist methanol crossover, in which higher concentrations of methanol can be used.

This article confirms the physical significance of a new method for determining adsorption energy distributions in porous materials. The premise of the model is that an adsorption isotherm consists of several equilibrium processes corresponding to adsorption into a distinct pore size regime. A distribution type equilibrium constant, K, involving the gas and adsorbate in a pore of capacity n describes each process. In order to define the n's and K's for all the processes involved, isotherms are collected at several temperatures to minimize the ratio of unknowns to knowns. In this article the model is extended to a series of adsorptives and it is shown that the resulting K's, ΔH's, and n's are not meaningless empirical fit parameters but have the meaning suggested by the model. The ΔG's vary linearly with ΔH for each pore size regime and both correlate linearly with the square root of the van der Waals a parameter, a 1/2. In addition to providing strong support for the physical significance of the parameters, these correlations enable prediction of the K values for adsorption of a new adsorptive by a characterized adsorbent given the a parameter of the adsorptive. The correlations show that the strongest binding corresponds to the adsorptive selecting pores from the distribution available that match its molecular dimensions. The n's for the different adsorptives provide insight into the pore distribution in the solid and about the pores utilized in the adsorption of different adsorptives. Prediction of the K's from a and estimating n's from molecular diameters is suggested as a way to attain the long-range goal of predicting the total isotherm for a new adsorptive from molecular properties. The practical application of this information for use in separations is illustrated. The concept of effective pressure, P eff, is introduced for catalysis to allow comparison of the concentrating effect of different microporous solids.

Equilibria involving three probe gases adsorbed on two porous carbonaceous supports are measured at various temperatures. A novel analysis of the data is offered which uses multiple process equilibria to calculate adsorption equilibrium constants for the interaction of the gas with the solid. Equilibria involving three distinct processes are found. The equilibrium constants (K 1,ads , K 2,ads , and K 3,ads ) are obtained as well as the capacity of the solid for each type of process (n 1 , n 2 , and n 3 ), in millimoles of adsorptive per gram of solid. The first process, K 1,ads , involves adsorption of the gas in the solid's micropores which are of molecular dimensions. The second process, K 2,ads , involves adsorption in the larger micropores. The third process, K 3,ads , involves adsorption by the remaining surface. Multilayer formation is likely involved in some processes. The temperature dependencies of the K ads 's produce the enthalpy of adsorption for these processes. This analysis is important for, in contrast to BET analyses, it provides thermodynamic data for different adsorptives that can be interpreted in terms of those molecular properties that facilitate probe-solid interactions and can provide a quantitative definition of solid reactivity.