2819.     ---------------.  [RIDEAL, E. K.]  Mechanism of Catalytic Hydrogenation.  Chem. and Ind., 1943, pp. 335-338; Chem. Abs., vol. 38, 1944, p. 15.

        An address on the development and extension of Sabatier’s ideas by Rideal over the last 20 yr. is given.  In agreement with Sabatier, and contrary to the views of H. S. Taylor, Rideal concludes that a metallic catalyst is covered with a monolayer of chemisorbed H₂, which is adsorbed without appreciable energy of activation.  Further changes in H₂ retention with temperature are due to solution of H₂ in the metal, or activated diffusion.  The methods of photoelectric thresholds, or better, the change in contact potential caused by the presence of a monolayer were used by Rideal to study the chemisorbed layer.  The ortho-para H₂ conversion at a metallic surface is due to interaction between a chemisorbed and a van der Waals adsorbed species and, thus, can be used as an indicator for chemisorbed H atoms.  This is an aid in explaining the mechanism of certain hydrogenation reactions.  The theory of intermediate compounds formation best explains such hydrocarbon catalysis reactions as the exchange of H and D in C₂H₄ and C₃H₆, and the migration of the double bond in higher olefins under conditions when exchange occurs.  Although the mechanism by which H₂ is added across a double bond at the surface of Ni is as yet uncertain, present evidence favors the view that catalytic hydrogenation proceeds through the addition in 1 step of an undissociated molecule of H₂.  Organometallic intermediate compounds explain also the catalytic cyclization reactions of heptane, octane, and the Fischer-Tropsch synthesis.  The main reactions occurring in the latter are due to van der Waals adsorbed H₂ alone rather than chemisorbed H₂, which in this synthesis causes formation of large quantities of CH₄.

                    ----------.  See abs. 643, 808, 1407, 1519, 1520, 3510.