Reflection absorption infrared spectroscopy and temperature programmed desorption were used to study the adsorption of acrolein, its partial hydrogenation products, propanal and 2-propenol, and its full hydrogenation product, 1-propanol on...

The hydrogenation of acrolein on the Ag(111) and Pd/Ag(111) single-atom alloy (SAA) surfaces was studied using temperature-programmed reaction spectroscopy. On Ag(111), the percent conversion of acrolein increased with increasing atomic hydrogen coverage. At an acrolein coverage of 0.17 ML followed by a low exposure to atomic hydrogen, the highest selectivity to 2-propenol was reached at 19%. However, increasing the acrolein coverage caused the selectivity to 2-propenol to decrease and the selectivity to propanal to increase to a maximum of 92%. In all cases, increasing atomic hydrogen exposure decreased the selectivity to 2-propenol. The selectivity to 1-propanol was constant at only 1%. Activity and conversion on the Ag(111) and Pd/Ag(111) SAA surfaces were compared for Pd coverages of 0.2, 0.5, and 0.9%. With increasing Pd coverage, the percent conversion increased while the selectivity to 2-propenol decreased. The selectivity toward 1-propanol only increased from 1 to 3% for the 0.9% Pd/Ag(111) SAA surface.

We have used reflection absorption infrared spectroscopy (RAIRS) and temperature programmed reaction (TPR) to study the selective hydrogenation of acetylene on both a clean Ag(111) surface and on a Pd/Ag(111) single-atom-alloy surface. The partial hydrogenation of acetylene to ethylene is an important catalytic process that is often carried out using PdAg alloys. It is challenging to study the reaction with ultrahigh vacuum techniques because H2 does not dissociate on Ag(111), and while H2 will dissociate at Pd sites, H-atom spillover from Pd to Ag sites does not generally occur. We bypassed the H2 dissociation step by exposing the surfaces to atomic hydrogen generated by the hot filament of an ion gauge. We find that hydrogen atoms react with acetylene to produce adsorbed ethylene at 85 K, the lowest temperature studied. This is revealed by the appearance of a RAIRS peak at 950 cm−1 due to the out-of-plane wagging mode of adsorbed ethylene when acetylene is exposed to a surface on which H atoms are pre-adsorbed. The formation of both ethylene and ethane are detected with TPR, but no acetylene coupling products, such as benzene, were found. From quantitative analysis of the TPR results, the percent conversion and selectivities to ethylene and ethane were determined. Low coverages of Pd enhance the conversion but do so mainly by increasing ethane formation.

Reflection absorption infrared spectra (RAIRS) were simulated with density functional theory calculations for five different rotational conformers of 1-propanol on the Ag(111) surface for comparison to the corresponding experimental spectra. The experimental RAIR spectra were obtained by adsorbing multilayers of 1-propanol at 90 K and then annealing the surface to 180 K to desorb the multilayer and produce the most stable 1-propanol monolayer structure on Ag(111). The multilayer spectrum features a broad O–H stretch characteristic of hydrogen bonding between the molecules as well as broad and complex peaks in the C–H stretch and C–H deformation regions. After annealing to 180 K, the O–H stretch peak disappears, and the remaining peaks are unusually sharp. Comparison of the experimental and simulated spectra indicates that 1-propanol adsorbs as only one of the five conformers.