Abstract:A recently-developed Monte Carlo method is used to simulate the energetics of n-alkanes from butane to decane in a variety of different all-silica zeolite structures (MFI, MOR, FAU, RHO, LTA, and FER). Where possible, the predicted values of the heat of adsorption are compared to experimental data and are generally found to be in good agreement. On the basis of the energetic data, the graphs of heat of adsorption as a function of mean pore diameter appear to show a maximum between 4 and 5 Å. However, close inspection of the location and conformation of the alkanes in small pore zeolites reveals that the molecules adopt highly coiled conformations localized exclusively in regions of maximum void volume. In the case of the small pore zeolites studied heresRHO and LTAsthese maximum void volumes are the R-cages and the alkanes "feel" a larger pore diameter than that generally used to characterize the zeolite (that of the channels). It is necessary to obtain information on the location and conformations of sorbed molecules to fully understand the trends in the heat of adsorption as a function of pore diameter.

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The bonding of atomic oxygen on Pt(111) and Rh(111) was examined using density functional theory in order to understand their different chemical properties. The oxygen-surface interactions were modeled by bonding atomic oxygen to 10-atom clusters of Pt and Rh designed to model the (111) surface. Density functional theory was applied using the local density and generalized gradient approximations; results were obtained for both double-and triple-basis sets. Optimized geometries and binding energies were computed and favorably compared to available experimental values. Interestingly, the ionic bonding in the two cases is nearly the same, based on the similarities in the charge on oxygen. The Hirshfeld charges on oxygen were -0.225 and -0.207 for Rh 10 -O and Pt 10 -O , respectively, using the double-basis set. A more detailed analysis of the covalent bonding using crystal orbital overlap populations indicated that the 2p orbitals of oxygen interact in a greater bonding fashion with both the sp and d orbitals of Rh than with those of Pt. Additional calculations with adsorbed hydroxyl on these metal clusters show differences in covalent bonding similar to that of oxygen. In this case, however, differences in ionic bonding play a role; oxygen in hydroxyl has a greater charge on Pt than Rh. This leads to smaller differences in the interaction energies of hydroxyl on Rh and Pt compared with oxygen, resulting in differences in chemical reactivity between the two metals, especially with respect to reactions involving hydrogen transfer.