Hydridization and Catalysis by Lanthanide Films

Hydrogen absorption to give the dihydrides MH₂₊₁ containing interstitial hydrogen H_i has been studied for the metals Gd, Dy, Er, Yb and Lu in the form of films deposited in ultra-high vacuum on glass. Film areas were determined by Kr adsorption, and hydrogen content, in particular interstitial hydrogen H_i, characterized by gas uptake, temperature programmed desorption, electrical conductivity and work function measurements by the diode method. The catalytic activity of the dihydride films for the H_2+D_2 -> 2HD reaction was studied at a pressure of 1.1 Torr over 175-579 K, and at 273 K over 0.19-6.2 Torr. Arrhenius plots for the rate constant show a low temperature low activation energy region changing over at a temperature T_c to a higher temperature higher activation energy regime, with T_c on average for the five metals about 50 K below the temperature T_max at which the interstitial hydrogen H_i has disappeared. The suggested mechanisms are begin{align*}tag{1}T < T_c:D_2 + H_i square_s -> (D_2H_i) square_s->square_sD_i + HD, tag{2} T > T_c:D_2 + H_2 + 4\ square_s -> 2(D_i square_s) (H_i square_s) -> 4 square_s + 2HD, end{align*} (2) where H_i square_s, D_i square _s, denotes a hydrogen, deuterium, atom held on a surface octahedral site in the f.c.c. metal sublattice. These mechanisms agree with the observed approximate first-order pressure dependency down to 77 K. The rate constants at both 273 K (under T_c) and 573 K (over T_c) decrease over Gd, Dy, Er, to Yb, and rise again to Lu, and this is discussed in terms of the metal-hydrogen, H_i square_s, or D_i square_s, bond strength.