1124
V. KESAVULU AXD H. AUSTINTAYLOR
Vol. 64
SITES FOR HYDROGEN CHEMISORPTION O S ZIXC OXIDE BY
v. KESAVULU AMD H. AUSTIXT A Y L O R
Wm. H . Nichols Laboratory, New York University, A'ew York 55, S. 1.. Received March 7, 1060
The adsorptio; of hydrogen on zinc oxide shows maxima around 60 and 300. Repeated adsorptions a t 300" enhance the adsorption a t 60 . This is attributed to a reduction of ZnO producing highly polarized zinc atoms located in the negative potential wells between three oxide ions. These highly polarized atoms are considered Type A sites responsible for the 60" maximum. Oxidation of the surface is shown to eliminate these sites, which can be regenerated by reduction. Water produced during the reduction has been proved by mass-spectrometric analysis. By a series of heating and cooling experiments it has been shown that the Type A sites are independent of the Type B sites responsible for the high temperature maximum. Type A sites can be degassed effectively a t 150'. A study of the kinetics on Type A sites alone becomes possible. The energy of activation on Type A is zero. An accounting is offered for the HzD2 exchange on Type A sites. A mechanism for Type B chemisorption is proposed and the effect of free electron density in the crystal on both types A and B is considered. The over-all picture is a synthesis of the geometric and electronic views of the catalyst surface.
H. S. Taylor's2 original concept of "active centers" in chemisorption and catalysis has undergone considerable change. The geometrical interpretation, of faces, edges and corners with varying degrees of saturation, has given way to an electronic interpretation. Sites are viewed as centers on the surface where the exchange of electrons with the interior is energetically favorable. As applied to the chemisorption of a gas on a semiconductor, this electronic interpretation implies the migration of mobile quasi-free electrons or of positive holes to the surface and then a binding of the reactant in a charged or polarized state. This presupposes the presence of electrons in the conduction band or of holes in the valence band. The chemisorptire property of a solid shoiild be closely related to its electrical conductivity, which is determined by the density of free electrons and holes in the crystal. Efforts to establish such a rclationship have not always been successful. The realization that a site which is active for the adsorption of one gas need not necessarily be artive for all gases implies a specificity in such a relationship. The problem reduces to a recognition of the sites on a particular surface which are active for the adsorption of a particular gas. The study of chemisorption developed here presents itself as a powerful tool for such recognition and leads to a synthesis of the geometric and electronic points of view so far as zinc oxide and hydrogen are concerned. Preliminary Experiments.-An adsorption study, involving a continuous variation of the catalyst temperature, was devised as :t rapid means of scanning the characteristics of a catalyst over a wide range of temperature. The use of this technique for determining the behavior of ZnO toward h j drogen chemisorption revealed that an irrevrrsiblr change was taking place in the characteristics of the catalyst around 60". Examination showed that this change was brought about by a chemical reduction of the catalyst surface. At the same time, scarcely any change in the surface behavior for hq drogen adsorption around 300' was observable. The apparatus was a conventional constant volume systrm in whicih pressure changes were measured on a dibutyl phthalate nianomctt~r. Hydrogen was purified by passage through a heated palladium thimble. The zinc oxide ponder, ZnO-I, 13 41 g . , made by heating ~ i n coxalate :\t 450" in a vurrent of air. The cat:tl\st chamher was firs6 i o o l d bv : t stream of nitrogen coolcd by liqnid nitrogcw. .kfter the tcniprr:ttu1e frll lielow -80" a known volume of Ltlniittcd to the reactlon vcxicl. I3y tontlol(1) I h s t r n c t t d from a d1,wItation r11 t h o Department of Ciimiiitrv aubnlitted tiy V. I
