The interaction of hydrogen with a Pd(100) surface

Abstract

The interaction of H 2 with Pd(110) was studied by simultaneous measurements of work function (Δφ), thermal desorption spectra (TDS) and LEED. Upon adsorption at 130 K, (2x1), (1x2) and streak phases form in sequence, the first two being associated with maximum coverages of 1 and 1.5 ML respectively. The (1x2) phase, which is reconstructed, converts to the (2x1) at 180 ≤ T ≤ 210 K with the migration of hydrogen to subsurface sites. These ordered surfaces exhibit a broad, asymmetric desorption peak, which we believe consists of two distinct desorption states, which we label β 1 and β 2. At much higher exposures, low temperature desorption peaks α 1 (∼160 K) and α 2 (∼230 K) appear. The α 1 peak originates from decomposition of layers of palladium hydride near the surface, the α 2 from the desorption of hydrogen from the reconstructed (1x2) phase, with consequent lifting of the reconstruction. The α 2 phase is thus a surface adsorbed species. The absence of a change in Δφ during β 1 desorption (low temperature side of β peak) leads us to postulate that the β 1 desorption is “fed” from subsurface sites which are either populated at 130 K or during the thermal desorption experiment. The β 2 state exhibits second-order kinetics and a large work function change on desorption. It originates from surface adsorbed species. The normal isotope effect observed in filling the α 1 states suggests that bulk diffusion is not limiting its rate of formation. A more likely explanation is layer by layer growth of palladium hydride, the driving force being lower for D 2 than H 2 at equal pressures because of the higher decomposition pressure of the deuteride.