With rising atmospheric CO2 levels, there has been increasing interest in artificial photosynthetic schemes for converting this greenhouse gas into valuable fuels and small organics. Photoelectrochemical schemes for activating the inert CO2 molecule, however, operate at excessive overpotentials and thus do not convert actual light energy to chemical energy. Here we describe the selective conversion of CO2 to methanol at a p-GaP semiconductor electrode with a homogeneous pyridinium ion catalyst, driving the reaction with light energy to yield faradaic efficiencies near 100% at potentials well below the standard potential.

Pyridinium and its substituted derivatives are effective and stable homogeneous electrocatalysts for the aqueous multiple-electron, multiple-proton reduction of carbon dioxide to products such as formic acid, formaldehyde, and methanol. Importantly, high faradaic yields for methanol have been observed in both electrochemical and photoelectrochemical systems at low reaction overpotentials. Herein, we report the detailed mechanism of pyridinium-catalyzed CO(2) reduction to methanol. At metal electrodes, formic acid and formaldehyde were observed to be intermediate products along the pathway to the 6e(-)-reduced product of methanol, with the pyridinium radical playing a role in the reduction of both intermediate products. It has previously been thought that metal-derived multielectron transfer was necessary to achieve highly reduced products such as methanol. Surprisingly, this simple organic molecule is found to be capable of reducing many different chemical species en route to methanol through six sequential electron transfers instead of metal-based multielectron transfer. We show evidence for the mechanism of the reduction proceeding through various coordinative interactions between the pyridinium radical and carbon dioxide, formaldehyde, and related species. This suggests an inner-sphere-type electron transfer from the pyridinium radical to the substrate for various mechanistic steps where the pyridinium radical covalently binds to intermediates and radical species. These mechanistic insights should aid the development of more efficient and selective catalysts for the reduction of carbon dioxide to the desired products.

The enantioselective surface chemistry of chiral R-2-bromobutane was studied on the naturally chiral Cu(643)R&S and Cu(531)R&S surfaces by comparing relative product yields during temperature-programmed reaction spectroscopy. Molecularly adsorbed R-2-bromobutane can desorb molecularly or debrominate to form R-2-butyl groups on the surfaces. The R-2-butyl groups react further by beta-hydride elimination to form 1- or 2-butene or by hydrogenation to form butane. Temperature-programmed reaction spectroscopy was used to quantify the relative yields of the various reaction products. At low coverages of R-2-bromobutane on Cu(643)R&S and Cu(531)R&S, the surface chemistry is not enantioselective. At monolayer coverage, however, the product yields indicate that the R-2-bromobutane decomposition reaction rates are sensitive to the handedness of the two chiral surfaces. The impact of surface structure on enantioselectivity was examined by studying the chemistry of R-2-bromobutane on both Cu(643)R&S and Cu(531)R&S. The selectivity of R-2-bromobutane desorption versus debromination is enantiospecific and differs significantly from Cu(643) to Cu(531). The selectivity of the R-2-butyl reaction by beta-hydride elimination versus hydrogenation is only weakly enantiospecific and is similar on both the Cu(643) and Cu(531) surfaces. These results represent the first quantitative observations of enantioselectivity in reactions with well-known mechanisms probed using a simple adsorbate on naturally chiral metal surfaces.

The evaluation of different surface coatings used in alkali metal atomic magnetometers is necessary for the improvement of sensitivity of these devices. A method to measure the polarization lifetime of alkali atoms in the region between substrates with different coatings was developed to determine the effectiveness of the coating at preserving alkali spin polarization as well as chemical compatibility and high-temperature stability. Multiple coatings can be compared under identical experimental conditions, using an experimental geometry that allows surface characterization before and after evaluation of the polarization lifetime. Multilayered, cross-linked octadecyltrichlorosilane films, alkyltrichlorosilane monolayers, and octadecylphosphonic acid monolayers were evaluated using this approach.