CO-Assisted Subsurface Hydrogen Trapping in Pd(111) Films

We use low-energy electron microscopy to image CO displacing adsorbed H from the Pd(111) surface. Quantitative electron diffraction reveals that upon codosing atomic H and CO the latter assists the absorption of hydrogen, which ends up trapped between the first and second palladium layers, and blocks its desorption. Density functional calculations reproduce this effect, which is found to originate from the antibonding character of the interaction between the adsorbed hydrogen and the CO π states. SECTION: Surfaces, Interfaces, Catalysis A classic system of gas−metal interactions is hydrogen and palladium. This system lies at the heart of applications ranging from purifying hydrogen to catalysis. The interplay of H and carbon monoxide on Pd is also of great interest because of their role in gas sensors, hydrogen purification, and heterogeneous catalysis. In the latter, Pd is used in selective hydrogenation reactions, CO oxidation, methanol synthesis, and steam reforming of alcohols to obtain hydrogen and CO. H and CO coadsorption on Pd surfaces has been studied with a wealth of surface-sensitive techniques, including thermal desorption, electron diffraction, vibrational spectroscopies, scanning tunneling microscopy, and theoretical simulations based on density functional theory (DFT). (See refs 11. and 13 for a literature review.) The results reveal a complex phase diagram strongly dependent on pressure and deposition temperature as well as on the order in which the species are adsorbed. As a general rule, CO and H on Pd surfaces repel each other, so that coadsorption at low temperatures gives pure domains of H and CO that compress as their respective coverages increase. In sequential dosing, the first-dosed species blocks adsorption of the second species. However, when a H precovered surface is raised to ∼125 K, CO adsorption leads to the complete removal of the adsorbed H. Part of the H desorbs after recombination and another portion dissolves into the Pd interior, leaving pure CO superstructures on the surface. Absorbed H in Pd is known to affect the rate and selectivity of catalytic reactions, and theory suggests that it is most strongly bound between the topmost two Pd layers, a proposal that has some experimental support. How much H is displaced into Pd by CO, its interior location, and whether its concentration can be controlled are open questions. Here we use low-energy electron microscopy (LEEM) to study CO-induced absorption of H into ultrathin palladium films. Spectral information contained in electron reflectivity is used to image CO displacing adsorbed H. To overcome the limits of dosing H2 under ultrahigh vacuum conditions, 19,22 we use atomic H to increase the absorbed H concentration. Quantitative electron diffraction and DFT-based theory reveal that coadsorption of CO and atomic H at 170 K efficiently traps the later at octahedral sites between the first two Pd layers. The experiments were carried out in a Elmitec LEEM III with a base pressure below 10−10 Torr. LEEM is a nonscanning microscopy technique where a beam of electrons reflected from a surface is magnified by lenses, giving a lateral resolution of ∼10 nm, all while the sample is heated or while dosing gases at chamber pressures below ∼10−5 Torr. Pd(111) films were grown on a Ru(0001) crystal by molecular beam epitaxy during LEEM observation. Films between 2 and 20 atomic layers thick were studied to explore finite-size effects in H absorption. H2 and CO were separately dosed from gas reservoirs using leak valves. Atomic hydrogen and CO were codosed from a cracker consisting of a hot tungsten filament in a water-cooled tube through which H2 was leaked. (The cracker also generated CO, giving a flux estimated to be ∼10−3 times the H flux.) Gas dosing was performed at 170 K, the lower limit of the sample manipulator, and exposures are given in Langmuirs (1 L = 10−6 Torr × s, with the pressure measured by an ionization gauge). Figure 1a shows a LEEM image of a typical Pd film, which consists of atomically flat terraces separated by atomic steps. Information about the concentration and type of adsorbate Received: November 2, 2011 Accepted: December 12, 2011 Published: December 12, 2011 Letter