Alkali-induced enhancement of surface electronic polarizibility

Effect of potassium on carbon adsorption on the Co(0001) surface

The effect of potassium on carbon adsorption and deposition on the Co(0001) surface was studied on the basis of theoretical calculations. Thermodynamically, the surface C_n species is expected, and C₂ dimer may be a critical elementary unit. With the increase of carbon coverage, a fraction of the carbon atoms may diffuse into the subsurface. But kinetically, the formation of C_n species is more favorable, and there is no driving force for carbon to migrate into the subsurface. As the surface carbon concentration increases, the adsorbed carbon atoms turn into carbon chains and then into graphene sheets parallel to the surface. Potassium promoter has little effect on the most stable adsorption configurations of carbon atoms but increases the adsorption energy of carbon species, which can be explained by the decreasing of the surface work function resulting from the electron effect of potassium promoter. The potassium promotes carbon deposition and carbonization of the cobalt surface to a certain extent. These results could support some useful information for the carbon deposition and cobalt carbide formation.

Potassium adsorption behavior on hcp cobalt as model systems for the Fischer-Tropsch synthesis: a density functional theory study

Potassium (K), an important impurity in syngas from biomass, can have a large influence on the activity and selectivity of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts in Biomass to Liquids (BTL) processes. In this work, the potassium adsorption behavior on hcp cobalt was systematically studied using density functional theory. The surface energy calculations and Wulff construction of the equilibrium shape of hcp cobalt showed it is dominated by 10 facets. The interaction of K with these facets has been investigated. The results showed that the stepped facet (10-12) has the highest K adsorption energy of -2.40 eV. The facets (0001), (10-10), (10-11), (10-15), and (21-30) also showed relatively high K adsorption energies in the range of -2.28 to -2.34 eV. The corrugated facets exhibited comparatively lower K adsorption energies (-2.04 to -2.18 eV), and would be less favorable for K adsorption. It was also found that the adsorption properties depend on coverage, where the K adsorption energy decreased with increasing coverage. Diffusion energy barrier calculations indicated that K was mobile on typical facets (0001) and (10-11) with very low diffusion barriers (<0.15 eV). On stepped facets, although K could move freely along the same step (diffusion barrier <0.01 eV), diffusion from one step to another had a significantly higher barrier of 0.56 eV. This suggested that K atoms would be mobile to some extent during FTS reaction conditions, and tend to occupy the most favorable sites independent of their initial position. The results obtained in this work provide valuable information on the interaction of K with cobalt surfaces, relevant for practical cobalt catalysts and their application in BTL processes.

Comparative vibrational study on alkali coadsorption with CO and O on Ni(111) and Cu(111)

High-resolution electron energy loss spectroscopy was used to investigate alkali (Na, K) coadsorption with CO and O on Cu(111) and Ni(111). Measurements provided new insights in these systems. A CO-induced weakening of the alkali-substrate bond was revealed on both substrates. The effect is more pronounced for the Na+CO/Ni(111) system. Submonolayers of alkalis were found to promote the preferential population of the subsurface site for O/Cu(111) but not for O/Ni(111).