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 Sabatierís 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.