Chemisorption of hydrogen and electrical effects on transition metals
Harkins, C. Girvin
Master of Science
Measurements of the changes in electrical resistance of evaporated pure titanium metal films caused by chemisorption of hydrogen on the surface and the absorption of hydrogen into the metal are described. Resistance measurements were conducted at 273, 298, 373, and 418 K. Titanium films of the order of 3 thick exhibited a linear decrease in resistance with the amount of hydrogen absorbed. The rate of decrease at 298 and 373 K was less than the sharp drop in resistance observed at 418 K. This behavior is due to an increase in electron concentration caused by absorption of hydrogen atoms in the Ti bulk with a simultaneous contribution of carriers (electrons) to the d-band. Iron films, the order of 12 thick, show a decrease in resistance at both low and high Hg coverages withtthe formation of a solid solution of H (bcc) iron. The slow observed resistance change is associated with the magnitude (small) of the effective charge of the hydrogen atom (.26e) that enters the conduction band. Palladium films of the order of 2 A thick, increase in resistance on chemisorption of hydrogen, pass through a maximum and then decrease.. This initial increase may be attributed to the formation of a negative anionic (H") layer that effectively reduces the number of carriers in the conduction band. The second effect (resistance decrease) may be ascribed to the diffusion of hydrogen atoms into the lattice together with the filling up of the d-holes at the Fermi level and the s-p band. Calculated values for titanium films were slightly higher than unity for films of the order of 14-16 A but were about fivefold higher for thicker films of the order of 4 A, suggesting that attainment of maximum coverage had not been achieved for these particular experiments. The much lower values for the thinner films suggest that no drastic change in bonding type occurs. For iron, lvalues were less than unity for thinner films, and larger for thicker films. This information is shown in Table 2, column 8 values for thinner films suggest that a strict one-to-one ratio of metal atoms to hydrogen molecules do not occur in the sorption process, therefore, a small fraction of molecular hydrogen participates in bonding by dissociative absorption into the bulk. Z calculated for palladium resulted in values less than unity with a minor change in Pd-H bonding as the concentration of hydrogen within the lattice increase.