Effects of non-local exchange on core level shifts for gas-phase and adsorbed molecules

Density functional theory calculations are often used to interpret experimental shifts in core level binding energies. Calculations based on gradient-corrected (GC) exchange-correlation functionals are known to reproduce measured core level shifts (CLS) of isolated molecules and metal surfaces with reasonable accuracy. In the present study, we discuss a series of examples where the shifts calculated within a GC-functional significantly deviate from the experimental values, namely the CLS of C 1s in ethyl trifluoroacetate, Pd 3d in PdO and the O 1s shift for CO adsorbed on PdO(101). The deviations are traced to effects of the electronic self-interaction error with GC-functionals and substantially better agreements between calculated and measured CLS are obtained when a fraction of exact exchange is used in the exchange-correlation functional

Significant quantum effects in hydrogen activation

Dissociation of molecular hydrogen is an important step in a wide variety of chemical, biological, and physical processes. Due to the light mass of hydrogen, it is recognized that quantum effects are often important to its reactivity. However, understanding how quantum effects impact the reactivity of hydrogen is still in its infancy. Here, we examine this issue using a well-defined Pd/Cu(111) alloy that allows the activation of hydrogen and deuterium molecules to be examined at individual Pd atom surface sites over a wide range of temperatures. Experiments comparing the uptake of hydrogen and deuterium as a function of temperature reveal completely different behavior of the two species. The rate of hydrogen activation increases at lower sample temperature, whereas deuterium activation slows as the temperature is lowered. Density functional theory simulations in which quantum nuclear effects are accounted for reveal that tunneling through the dissociation barrier is prevalent for H2 up to ∼190 K and for D2 up to ∼140 K. Kinetic Monte Carlo simulations indicate that the effective barrier to H2 dissociation is so low that hydrogen uptake on the surface is limited merely by thermodynamics, whereas the D2 dissociation process is controlled by kinetics. These data illustrate the complexity and inherent quantum nature of this ubiquitous and seemingly simple chemical process. Examining these effects in other systems with a similar range of approaches may uncover temperature regimes where quantum effects can be harnessed, yielding greater control of bond-breaking processes at surfaces and uncovering useful chemistries such as selective bond activation or isotope separation

CO Adsorption on Clean and Oxidized Pd(111)

The adsorption of CO on clean and oxidized Pd(111) surfaces has been investigated using a combination of high-resolution core level spectroscopy (HRCLS), reflection absorption infrared spectroscopy (RAIRS), and density functional theory (DFT) calculations. The HRCLS and RAIRS measurements reveal that CO adsorbs on Pd(111), Pd(5)O4 and PdO(101) at 100 +/- 10 K and that the CO coverage decreases with increasing oxidation state of Pd for the same CO exposures of 10 Langmuirs. Based on the DFT calculations, the CO layer on clean Pd(111) was found to include molecular adsorption in both hollow and bridge sites, whereas CO occupies a combination of bridge and atop sites on the Pd5O4 and PdO(101) surfaces

Intrinsic Ligand Effect Governing the Catalytic Activity of Pd Oxide Thin Films

High-pressure X-ray photoelectron spectroscopy, mass spectrometry, and density functional theory calculations have been combined to study methane oxidation over Pd(100). The measurements reveal a high activity when a two-layer PdO(101) oriented film is formed. Although a one-layer PdO(101) film exhibits a similar surface structure, no or very little activity is observed. The calculations show that the presence of an oxygen atom directly below the coordinatively unsaturated Pd atom in the two-layer PdO(101) film is crucial for efficient methane dissociation, demonstrating a ligand effect that may be broadly important in determining the catalytic properties of oxide thin films

Dissociative Adsorption of Hydrogen on PdO(101) Studied by HRCLS and DFT

High-resolution core-level spectroscopy (HRCLS) and density functional theory (DFT) calculations have been used to investigate the adsorption and dissociation of hydrogen on a PdO(101) film grown on Pd(111). Energy-dependent measurements of the O 1s and Pd 3d(5/2) binding energies enable identification of surface components that correspond to undercoordinated Pd and O atoms. HRCLS data obtained at 110 K, after hydrogen exposure at the same temperature, reveal hydrogen adsorption and formation of Pd-H and O-H groups. Adsorption at room temperature results instead in complete reduction of the oxide. The experimental results are supported by the DFT calculations of core-level shifts and barriers for water formation