PdH α-phase is associated with residual oxygen as revealed by in situ 1H NMR measurements and DFT-NMR estimations

An experimental and DFT study was conducted to investigate the chemical nature of alpha and beta PdH phases in Pd/ TiO2 through the detailed analysis of their 1H NMR chemical shifts. alpha-PdH states are identified with shielded 1H NMR resonances at frequencies around -30 ppm from TMS. In situ NMR measurements revealed that the appearance of this shielded peak was only observable if PdO was partially reduced. DFT estimations revealed similar NMR chemical shifts due to hydrogen adsorbed at the interface between the metal and surface segregated, i.e. the top, oxide layers of palladium. For fully reduced Pd, DFT estimations relaxed into a single beta phase, similar results were obtained through quantitative in situ NMR measurements. NMR chemical shift DFT calculations over beta phase revealed a linear dependency of 1H chemical shifts for 0.02 <= H/Pd <= 1, due to the anionic character of hydrogen atom within the local electronic structure. This finding was also supported by the experimental data. Anionic character hydrogen atom is due to well localized core hydrogen 1s orbitals at low concentrations which are dispersed as H content increases. These results indicate that the presence of trace oxygen can be involved in the alpha phase of PdH.