Reflection absorption infrared spectroscopy (RAIRS) was used to simultaneously monitor gas-phase and surface species in the presence of ambient pressures of acetylene and hydrogen over a single-atom alloy (SAA). The alloy consisted of isolated Pd atoms at surface coverages in the range of 0.0028–0.085 ML in a Cu(111) surface. When C2H2(g) is present, but not H2(g), the RAIR spectra are similar for Cu(111) with and without Pd, independent of C2H2(g) pressure for temperatures between 180 and 500 K. The addition of H2(g) leads to different RAIR spectra depending on the presence of Pd. With a C2H2:H2 ratio of 1:100 and a SAA-Pd/Cu(111) surface with less than 1% Pd, C2H2(g) is converted to C2H4(g) at 400 K at total pressures up to 10 Torr. From the rate of change in the gas-phase IR peaks, a range of initial turnover frequencies was estimated, which depend on which sites are assumed to be active for hydrogenation. Postreaction surface analysis with Auger electron spectroscopy (AES) showed a significant carbon coverage, which decreased with increasing Pd coverage. The combined RAIRS and AES results suggest that, by increasing reactivity for ethylene formation, Pd also limits the amount of carbon that is deposited, while also changing the extent of oligomer formation.

Single-atom alloys can be formed by depositing a low coverage of active metal onto the surface of another metal. Few experimental techniques permit verification that the deposited metal exists as single-atom sites on the host metal surface. Here we use polarization-dependent reflection absorption infrared spectroscopy (PD-RAIRS) to characterize the surface of a Pd/Cu(111) single-atom alloy through the use of CO as a probe molecule. In the presence of 1 × 10–2 Torr of CO at 300 K, significant coverages of CO are only achieved when Pd is present on the surface. The Pd coverage at the surface is determined with RAIRS from the C–O stretch peak areas while the total amount of Pd in the first few atomic layers is measured with Auger electron spectroscopy. Isolated Pd atoms in Cu(111) are revealed with RAIRS by the C–O stretch peak of CO bound on top of a Pd atom. The appearance of C–O stretch peaks due to bridge-site CO occurs at Pd coverages where Pd atoms start to agglomerate. An isosteric heat of adsorption for CO of 32 kJ/mol was measured with PD-RAIRS for CO in dynamic equilibrium with the Pd/Cu(111) surface and provides new information on the bonding properties of isolated Pd atoms in the Cu(111) host.

Acetylene hydrogenation was monitored at ambient pressure with polarization-dependent reflection absorption infrared spectroscopy (RAIRS), which permitted gas phase and surface species to be simultaneously monitored as C 2 H 2 (g) was converted first to C 2 H 4 (g) and then to C 2 H 6 (g). Experiments in which an acetylene-covered surface was hydrogenated with 1.0 × 10 −2 Torr H 2 between 120 and 300 K indicated that vinyl is the intermediate species to ethylene formation and that the addition of one H to acetylene is the rate-limiting step of the reaction. At a C 2 H 2 (g)/H 2 (g) ratio of 1:100, the reaction was monitored from 300 to 370 K and separately in a constant pressure and constant temperature reaction at 370 K. Ethylidyne and di-σ-ethylene were observed on the surface in both reactions and were found to be spectator species in the hydrogenation of ethylene to ethane. A minor hydrogenation pathway involves a third species, which is best assigned to an ethylidene intermediate. A small coverage of π-ethylene was also present during the annealing experiment at 350 K but disappeared at 370 K, indicating that it is also an intermediate in the reaction. In a separate experiment to compare acetylene hydrogenation with ethylene hydrogenation at 370 K, spectra were acquired with a C 2 H 4 (g)/H 2 (g) ratio of 1:10. Ethylene hydrogenation proceeds approximately three times faster when starting with ethylene as compared with hydrogenation of the ethylene produced by acetylene hydrogenation. This indicates that the surface is covered with different intermediates when it is first exposed to acetylene. The results presented here demonstrate a simple way to use polarization-dependent RAIRS to distinguish surface species from gas phase reactants and products. The method should be applicable to a wide range of catalytic reactions over metal surfaces and offers new opportunities for operando studies in catalysis.

Emissive semiconductor nanocrystals can be efficient FRET donors to chemically responsive organic fluorophores; conjugating the two creates a ratiometric (or self-calibrating) sensor. Given that the organic component engenders chemical and/or biological sensitivity, it is natural to assume that the NC-dye response is reflective of the photochemical properties of the neat dye. In the case of a pH sensor like fluorescein, the dye emission can be analytically described by the Henderson-Hasselbalch equation. Our research demonstrates that the ratiometric sensing response of CdS/ZnS-fluorescein pH sensors, however, does not directly reflect the Henderson-Hasselbalch relationship and is difficult to predict. The reasons for this complexity are the alternations of the dye microenvironment within the nanoscale architecture and the nature of energy transfer from the NC donor to the acceptor dye.

The midway point: The dehydrogenation of ethylamine on Pt(1 1 1) generates a stable surface intermediate, which was identified by reflection absorption infrared spectroscopy and density functional theory calculations as aminovinylidene. It is a novel surface species with a π‐bond delocalized across its three atomic centers.