Structure and decomposition pathways of D-(−)-tartaric acid on Pd(111)

Abstract

The adsorption of chiral D-(−)-tartaric acid is studied on a Pd(111) substrate using a combination of temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS). It is found that reaction at room temperature occurs predominantly via deprotonation of the carboxylic acid groups. Bitartrate species form at ~300K at low coverages while monotartrate species predominate at higher coverages since the removal of the second proton is inhibited by surface crowding. It also appears that the bitartrate can rehydrogenate on heating to reform some monotartrate species. The hydrogens deriving from the carboxylate group desorbs at ~315K, and the mono- and bitartrate species are stable to ~390K where they decompose to evolve hydrogen, carbon dioxide and some water. Carbon monoxide is also formed and evolves in a desorption-rate limited state at ~450K. Biacidic, second-layer D-(−)-tartaric acid adsorbs at higher coverages and initially desorbs in a state at ~329K shifting to higher temperatures as the second-layer coverage increases, indicative of attractive interactions between adsorbates. A decomposition intermediate is detected by dosing D-(−)-tartaric acid at low temperatures (~100K) and heating to ~300K or by dosing at ~300K and heating to ~320K, and is characterized by modes at ~1313, 1261, 1202 and 1116cm−1 assigned to δCH modes and a νCOalc vibration suggesting that the intermediate may form by the removal of the COO group.