Reaction of Trimethylaluminum with Water on Pt(111) and Pd(111) from 10^–5 to 10^–1 Millibar

The reaction between adsorbed trimethylaluminum (TMA) and water was studied on Pt(111) and Pd(111) surfaces. Upon exposure to TMA at approximately 10^–5 mbar, C- and Al-containing species appeared on both surfaces, as observed by X-ray photoelectron spectroscopy (XPS). On both surfaces, the adsorbed Al oxidation state observed by XPS was closest to metallic. Density functional theory (DFT) calculations suggest that decomposition to methyl aluminum (Al-CH₃; “MMA”) or atomic Al is thermodynamically favorable. The formation of a Pd–Al alloy was observed on Pd(111), but Pt–Al alloy formation was not observed on Pt(111). Following TMA adsorption, each surface was exposed to water vapor at 400 °C either at a pressure of 7 × 10^–6 mbar (UHV-XPS) or at 0.1 mbar (in situ XPS). The substrate and water dosing conditions determined the ability of each surface to remove residual carbon: on Pt(111), carbon from the TMA precursor was removed from Pt(111) during 0.1 mbar water exposure at 400 °C, whereas carbon was not removed after the 7 × 10^–6 mbar water exposure. On Pd(111), however, carbon-containing fragments of TMA were removed at both water pressures. XPS also revealed another effect of water dosing conditions: the as-deposited Al was only fully oxidized to Al₂O₃ during water exposure at 0.1 mbar, whereas mixed hydroxide-containing and metallic Al species persisted after exposure to water at 7 × 10^–6 mbar on both surfaces.