62 research outputs found
Oxidation of Hydrocarbons at Surface Defects: Unprecedented Confirmation of the Oxomethylidyne Pathway on a Stepped Rh Surface
Computational Investigation of Thermochemistry and Kinetics of Steam Methane Reforming on Ni(111) under Realistic Conditions
Visualization of Compression and Spillover in a Coadsorbed System: Syngas on Cobalt Nanoparticles
Competitive adsorption and lateral pressure between surface-bound intermediates are important effects that dictate chemical reactivity. Lateral, or two-dimensional, pressure is known to promote reactivity by lowering energetic barriers and increasing conversion to products. We examined the coadsorption of CO and H2, the two reactants in the industrially important Fischer-Tropsch synthesis, on Co nanoparticles to investigate the effect of two-dimensional pressure. Using scanning tunneling microscopy, we directly visualized the coadsorption of H and CO on Co, and we found that the two adsorbates remain in segregated phases. CO adsorbs on the Co nanoparticles via spillover from the Cu(111) support, and when deposited onto preadsorbed adlayers of H, CO exerts two-dimensional pressure on H, compressing it into a higher-density, energetically less-preferred structure. By depositing excess CO, we found that H on the Co surface is forced to spill over onto the Cu(111) support. Thus, spillover of H from Co onto Cu, where it would not normally reside due to the high activation barrier, is preferred over desorption. We corroborated the mechanism of this spillover-induced displacement by calculating the relevant energetics using density functional theory, which show that the displacement of H from Co is compensated for by the formation of strong CO-Co bonds. These results may have significant ramifications for Fischer-Tropsch synthesis kinetics on Co, as the segregation of CO and H, as well as the displacement of H by CO, limits the interface between the two molecules. © 2013 American Chemical Society
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Synergetic effects of the Cu/Pt{110} surface alloy: enhanced reactivity of water and carbon monoxide
We have used synchrotron-based high-resolution X-ray photoelectron spectroscopy in combination with ab initio density functional theory calculations to investigate the characteristics of water and CO adsorption on the bimetallic Cu/Pt{110}-(2 x 1) surface at a Cu coverage near 0.5 ML. Cu fills the troughs of the reconstructed clean surface forming nanowires, which are stable up to 830 K. Their presence dramatically influences the adsorption of water and CO. Water adsorption changes from intact to partially dissociated while the desorption temperature of CO on this surface increases by up to 27 K with respect to the clean Pt{110} surface. Ab initio calculations and experimental valence band spectra reveal that the Cu 3d-band is narrowed and shifted upward with respect to bulk Cu surfaces. This and electron donation to surface Pt atoms cause the increase in the bond strength between CO and the Pt surface atoms. The pathway for water dissociation occurs via Cu surface atoms. The heat of adsorption of water bonding to Cu surface atoms was calculated to be 0.82 eV, which is significantly higher than on the clean Pt{110} surface; the activation energy for partial dissociation is 0.53 eV (not corrected for zero point energy)
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