3930
YUNG-FANG Yu YAO
Adsorption of Polar Molecules on Metal Oxide Single Crystals
by Yung-Fang Yu Yao Scientific Laboratory, Ford Motor Company, Dearborn, Michigan
(Received June 9, 1966)
Micro single crystals of NiO, a-Cr2O3, and a-A1203exposing only or predominantly one of their crystalline faces are prepared. Isotherms and heats of absorption of H20, CHIOH, CH3NH2, and CHaCN on these crystals were determined at 23O. All of these compounds chemisorb on the crystal surfaces to give a heat of adsorption of 20-30 kcal./mole. The stepwise heat curves obtained are attributed to adsorption on cationic and anionic surfaces with the initial higher heat plateau assigned to the cationic surface. Based on a simplified electrostatic interaction model, the adsorption bond energy of H 2 0 to the cations of the (111) faces of NiO and the (0001) faces of Cr203and A1203were calculated as a function of bond distance. The maximum bond energies agree reasonably well with the experimental values for all three oxides. The equilibrium bond distances thus calculated are also of the expected order of magnitude. CHIOH and CH3CN behave like H 2 0 and are held to the surface primarily by electrostatic force. Coordination bond formation by donations of electrons from the amino N to the d band of the cations also contributes to the adsorption bond energy of CH3NHzon NiO, Cr203,and Fe20abut not on The heats of adsorption of the four adsorbates on the oxygen ions are approximately equal and are less than that on the cations. Results on the crushed X i 0 and the more heteroplanar Fe203are also included to show the effect of crystal face on chemisorption.
Introduction The interaction between organic molecules and surfaces of ionic solids such as metal oxides plays an important role in various surface phenomena. Very little is known about the nature and the energy of interaction or the stereochemistry of the adsorbed phase. Most experimental work in this field has been confined to either commercial catalysts or metal oxides prepared by thermal decomposition of their hydroxides or some unstable salts. The surfaces of these solids are not well defined. With the exception of some work on the physical adsorption of inert gases on halide crystals exposing a single crystalline f ace,'I2 scarcely any work has been conducted on ionic single crystals with one or two crystalline faces predominantly exposed. In this study, single crystals of N O , a-A1203, aCr203,and a-Fe203 with a very limited number of their crystalline faces exposed have been prepared. These crystals exhibit no change in surface area after repeated evacuation a t 900'. Calorimetric heats and adsorption isotherms of several polar organic compounds and The Journal
of
Physical Chemistry
water on these surfaces have been determined in order to obtain some information on the nature of the chemisorption bond.
Experimental Section The Preparation of the Crystals. Since this is the first time such crystalline materials have been used as adsorbents, some detailed description of their preparation and properties will be given. (1) NickeZ Oxide. This crystal was prepared by mixing NiO powder (J. T. Baker, C.P. grade, 15 g.) with anhydrous Na4B207 (Matheson Coleman and Bell, reagent grade, 28 g.) in a platinum crucible and firing in a muffle furnace a t 1130' for 20 hr. It was then gradually cooled down to 750' in 8 hr. The Na4B207 was leached out in hot dilute nitric acid, and the NiO crystals were recovered. The glittering green crystals were octahedrons as seen under the microscope (Figure la), and X-ray diffraction gave typical NiO patterns. The crystals were sieved; the ones passing 350 mesh (1) D. M. Young, Trans. Faraday SOC.,48, 548 (1952). (2) T. Hayakawa, Bull. Chem. SOC..Japan, 30, 236, 343 (1957).
ADSORPTION OF POLAR MOLECULES ON METALOXIDESINOLE CRYSTALS
Figure 1.
sieve were used for adsorption studies, and the larger ones were crushed with a Wig-*Bug and designated as sample NiO (C) for some supplementary adsorption studies. Emission spectral analysis of the final sample showed 0.01% B, 0.01% Na, and no Pt. (.8) a-CrsOs. These crystals were prepared by firing KCr207(B and A reagent grade) in a gold crucible at 900' for 3 hr. followed hy cooling to 700' in 2 hr. The crystalline platelets were recovered by hot dilute HNO8 treatment. The crystals, as seen under the microscope (Figure lb), were green lustrous thin hexagonal platelets. The X-ray diffraction pattern of a crushed sample identified it as a-Cr20a,and that of the uncrushed platelets showed a single line indicating that the (OOO1) faces of the crystals were parallel with the plane of the platelets. Emission spectral analysis showed
