Hydrogen Adsorption on Co Surfaces: A Density Functional Theory and Temperature Programmed Desorption Study

Abstract

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.

Adsorption and decomposition of H2O on cobalt surfaces: A DFT study

“…So, previous studies were mainly focused on hcp-Co surfaces [2,11,34,35]. Nevertheless, it is worth noting that many supported cobalt catalysts are usually synthesized to comprise cobalt crystallites with average diameters less than 100 nm, which are stable as fcc-Co [36]. Therefore, a number of work have been devoted to explore the catalytic properties of fcc-Co [36][37][38][39] recently.…”

“…Nevertheless, it is worth noting that many supported cobalt catalysts are usually synthesized to comprise cobalt crystallites with average diameters less than 100 nm, which are stable as fcc-Co [36]. Therefore, a number of work have been devoted to explore the catalytic properties of fcc-Co [36][37][38][39] recently. For instance, Valero and Raybaud [39] concentrated their efforts on the origin of cobalt catalyst deactivation due to the deposition of O and C species on fcc-Co surfaces, based on thermodynamic model calculations.…”

DFT study of the coverage-dependent chemisorption of molecular H 2 on neutral cobalt dimers

“…It is also an important process in the development of novel materials for H2-sensors, as small amounts of cobalt have been found to increase the sensitivity of potential materials for H2-sensors [14] and for trace-H2 detection [15][16][17][18]. There are several experimental studies of the reaction of H2 and D2 with extented cobalt surfaces [19][20][21]. They find that the adsorption of H2 is dissociative at temperatures as low as 90K and that defects and steps have the capability of multiple atomic hydrogen (Ha) chemisorption rather than molecular hydrogen adsorption [20].…”

“…They find that the adsorption of H2 is dissociative at temperatures as low as 90K and that defects and steps have the capability of multiple atomic hydrogen (Ha) chemisorption rather than molecular hydrogen adsorption [20]. The studies on hydrogen chemisorption on Co(0001) surfaces showed that the maximum hydrogen coverage is ~0.5ML and only after sputtering, a denser H-overlayer of 0.75 ML, is obtained [21]. In practically most reported cases [19][20][21] a H2 molecule dissociates during the adsorption on cobalt surfaces and nanoparticles, where the H atoms bind at the three-fold (3f) hollow sites.…”

“…The studies on hydrogen chemisorption on Co(0001) surfaces showed that the maximum hydrogen coverage is ~0.5ML and only after sputtering, a denser H-overlayer of 0.75 ML, is obtained [21]. In practically most reported cases [19][20][21] a H2 molecule dissociates during the adsorption on cobalt surfaces and nanoparticles, where the H atoms bind at the three-fold (3f) hollow sites. A detailed STM and DFT study revealed that a denser H-(1x1) phase is found on Co nanoparticles deposited on copper surfaces in which step edges are important in activating the dissociation of H2, through spillover from the copper substrate [20].…”