The adsorption of hydrogen on iron; A surface orbital modified occupancy — bond energy bond order calculation

Abstract

A Surface Orbital Modified Occupancy — Bond Energy Bond Order (SOMO-BEBO) model calculation of hydrogen adsorption on iron is presented. This calculation represents a novel approach to the CFSO-BEBO method in that the calculation is correlated in a consistent way with the thermal desorption spectra of the hydrogen-iron system. Heats of molecular adsorption calculated are −32.88, −35.68 and −49.57 kJ/mol for the iron (110), (100), and (111) surfaces, respectively. Heats of dissociative adsorption calculated are −54.40, −75.30 and −87.90 kJ/mol for the three states on the iron (111) surface; −51.21 and − 73.62 kJ/mol for the two states on the iron (100) surface; and −63.78 kJ/mol for the one state on the iron (110) surface. Activation energies for dissociative adsorption were found to be small or zero for the iron (111) surface while non-zero activation energies of 49.27 and 45.05 kJ/mol were calculated for the iron (100) and (110) surfaces, respectively. The FeH single-order bond energy has been calculated to be 298.2 kJ/mol. The radius of the hydrogen surface atom has been estimated to be 1.52 × 10 −10 m consistent with the expected size of an H − ion. The elimination of certain surface sites for molecular adsorption as a result of the ferromagnetism of iron is suggested by the calculation. The reason for the absence of well defined LEED patterns for hydrogen adsorption on the iron (111) and (100) surfaces [Bozso et al., Appl. Surface Sci. 1 (1977) 103] is explained on the basis of the size of the H − surface ion. The adsorption of hydrogen on the iron (110) surface is consistent with a relatively stable, small-sized H + 2 surface ion giving, therefore, a regular LEED pattern and a positive surface potential upon adsorption of hydrogen on this surface.