CO Adsorption Site Preference on Platinum: Charge Is the Essence

The adsorption of CO on transition-metal surfaces is a key step in catalysis and a reference system for surface science and computational catalysis. Here, the change in CO site preference with coverage, from top to bridge and back to top, is analyzed using charge transfer and chemical bonding. The relative stability of top and bridge sites is related to the variation in the surface platinum charge with CO coverage. Both the Pt–C σ* (Pauli repulsion) and the C–O π* (back-donation) occupancies increase with platinum charge; however, destabilizing Pauli repulsion dominates over stabilizing back-donation, and adsorption weakens with increasing surface charge. CO at the top sites is more sensitive to Pauli repulsion, leading to a change in site preference from top to bridge with increasing platinum charge and, consequently, with increasing CO coverage. The higher back-donation at the bridge sites eventually switches the site preference back to top near monolayer coverage

Role of Surface Hydroxyl Species in Copper-Catalyzed Hydrogenation of Ketones

A comprehensive, coverage-dependent mean-field microkinetic model is developed for the hydrogenation of carbonyl compounds on Cu(111). In the model, hydrogenation by surface hydrogen, surface hydroxyl species, and adsorbed water molecules is considered, including a reaction pathway via keto–enol tautomerization. The model parameters were calculated by VdW-DF2 density functional theory and account for inter- and intraspecies repulsion. Accounting for these coverage effects changes the surface from being completely covered with 25% oxygen atoms and 75% hydroxyl groups to a surface with 65% free sites. Including coverage effects also surprisingly increases the calculated turnover frequency from 6 × 10^–5 to 2 × 10^–3 s^–1. In the dominant reaction path, the carbonyl group is hydrogenated to an alkoxy intermediate by surface hydrogen, followed by a proton transfer from either a surface hydroxyl species or an adsorbed water molecule to form the alcohol product. The addition of small amounts of water suffices to open this pathway. The pathway in which acetone is converted to 2-hydroxypropylene via keto–enol tautomerization is kinetically irrelevant under the considered conditions. Regeneration of the hydroxyl groups is the rate-controlling step in the mechanism, suggesting an alternative role for the reducible oxide promoters which are often encountered for Cu-based carbonyl hydrogenation catalysts

Ethylene Hydrogenation over Pt/TiO₂: A Charge-Sensitive Reaction

Controlled charge transfer between a support and small metal particles provides unique opportunities to tune the activity of supported metal catalysts, as first proposed by Schwab [G. M. Schwab et al., <i>Angew. Chem</i>. <b>1959</b>, <i>71</i>, 101–104]. By controlling the thickness of polycrystalline anatase TiO₂ films, the TiO₂ carrier concentration can be manipulated by an order of magnitude. When 1 nm Pt particles are deposited on these TiO₂ films, the variation in the charge transfer between the TiO₂ support and the Pt particles is found to dramatically increase the ethylene hydrogenation activity. The sensitivity of ethylene hydrogenation to charge transfer was anticipated from the large effect of the Pt charge on the ethylene and ethylidyne adsorption energy, e.g., compared to CO and H. Our results demonstrate that the controllable Schwab effect provides a powerful tool to tune catalytic activity. An even larger effect can be expected for supported sub-nanometer clusters, and for the selectivity of hydrogenation reactions

Evaluating the Structure of Catalysts Using Core-Level Binding Energies Calculated from First Principles

X-ray photoelectron spectroscopy (XPS) is a powerful and popular surface characterization technique, and the measured shifts in the core electron binding energies are sensitive to the chemical structure and local environment of the surface species. C 1s binding energies were calculated with density functional theory (DFT) for 17 structures including eight well-characterized structures on a Co(0001) surface and nine on a Pt(111) surface, while B 1s binding energies were calculated for six well-characterized structures and compared with experimental values. DFT calculations describe the 2.8 eV variation in the C 1s binding energies on Co surfaces, the 4.2 eV variation in the C 1s binding energies on Pt surfaces, and the 5.5 eV variation in the B 1s binding energies in the test sets with average deviations of 85, 73, and 53 meV, respectively. The shift in the C 1s and the B 1s binding energies can be correlated with the calculated charges, though only within homologous series. To illustrate how binding energy calculations can help elucidate catalyst structures, the nature of the resilient carbon species deposited during Fischer–Tropsch synthesis (FTS) over Co/γ-Al₂O₃ catalysts was studied. The catalysts were investigated using XPS after reaction, and the measured C 1s binding energies were compared with DFT calculations for various stable structures. The XPS peak at 283.0 eV is attributed to a surface carbide, while the peak at 284.6 eV is proposed to correspond to remaining waxes or polyaromatic carbon species. Boron promotion has been reported to enhance the stability of Co FTS catalysts. Again, the combination of XPS with DFT B 1s binding energy calculations helped identify the nature and location of the boron promoter on the Co/γ-Al₂O₃ catalyst