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Selective substitution: In the oxidation of HCl with oxygen to give Cl2 and water, RuO2(110) serves as a stable, active model catalyst for the Sumitomo process (see picture; Ru in red and blue). The stability of the catalyst is related to the selective replacement of undercoordinated bridging surface O atoms (Obr) by Cl atoms (Clbr). The chlorination of RuO2(110) is self‐limiting, in that chlorine incorporation terminates when all bridging O atoms are replaced.

Density functional theory (DFT) calculations and temperature programmed desorption (TPD) experiments were performed to study the adsorption of hydrogen on the Co(111) and Co(100) surfaces. On the Co(111) surface, hydrogen adsorption is coverage dependent and the calculated adsorption energies are very similar to those on the Co(0001) surface. The experimental adsorption saturation coverage on the Co(111)/(0001) surface is θ max ≈ 0.5 ML, although DFT predicts θ max ≈ 1.0 ML. DFT calculations indicate that preadsorbed hydrogen will kinetically impede the adsorption process as the coverage approaches θ = 0.5 ML, giving rise to this difference. Adsorption on Co(100) is coverage independent up to θ = 1.00 ML, contrasting observations on the Ni(100) surface. Hydrogen atoms have low barriers of diffusion on both the Co(111) and Co(100) surfaces. A microkinetic analysis of desorption, simulating the expected TPD experiments, indicated that on the Co(111) surface two TPD peaks are expected, while on the Co(100) only one peak is expected. Low coverage adsorption energies of between 0.97 and 1.1 eV are obtained from the TPD experiment on a smooth single crystal of Co(0001), in line with the DFT results. Defects play a important role in the adsorption process. Further calculations on the Co(211) and Co(221) surfaces have been performed to model the effects of step and defect sites, indicating that steps and defects will expose a broad range of adsorption sites with varying (mostly less favorable) adsorption energies. The effect of defects has been studied by TPD by sputtering of the Co crystal surface. Defects accelerate the adsorption of hydrogen by providing alternative, almost barrierless pathways, making it possible to increase the coverage on the Co(111)/(0001) surface to above θ = 0.50 ML. The presence of defects at a high concentration will give rise to adsorption sites with much lower desorption activation energies, resulting in broad low temperature TPD features.

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Experiments that provide insight into the elementary reaction steps of C x H y adsorbates are of crucial importance to better understand the chemistry of chain growth in Fischer−Tropsch synthesis (FTS). In the present study we use a combination of experimental and theoretical tools to explore the reactivity of C 2 H x and C 3 H x adsorbates derived from ethene and propene on the close-packed surface of cobalt. Adsorption studies show that both alkenes adsorb with a high sticking coefficient. Surface hydrogen does not affect the sticking coefficient but reduces the adsorption capacity of both ethene and propene by 50% and suppresses decomposition. On the other hand, even subsaturation quantities of CO ad strongly suppress alkene adsorption. Partial alkene dehydrogenation occurs at low surface temperature and predominantly yields acetylene and propyne. Ethylidyne and propylidyne can be formed as well, but only when the adsorbate coverage is high. Translated to FTS, the stable, hydrogenlean adsorbates such as alkynes and alkylidynes will have long residence times on the surface and are therefore feasible intermediates for chain growth. The comparatively lower desorption barrier for propene relative to ethene can to a large extent be attributed to the higher stability of the molecule in the gas phase, where hyperconjugation of the double bond with σ bonds in the adjacent methyl group provides additional stability to propene. The higher desorption barrier for ethene can potentially contribute to the anomalously low C 2 H x production rate that is typically observed in cobalt-catalyzed FTS.

High-resolution core-level shift spectroscopy and temperature-programmed reaction experiments together with density functional theory calculations reveal that the oxidation of HCl with oxygen producing Cl 2 and water proceeds on the chlorine-stabilized RuO 2 (110) surface via a one-dimensional Langmuir-Hinshelwood mechanism. The recombination of two adjacent chlorine atoms on the catalyst's surface constitutes the ratedetermining step in this novel Deacon-like process.