H. C. Yao and M. Shelef
2480
Nitric Oxide a dl Carbon Monoxide Chemisorption on Cobalt-Containing Spinels I
6.Yao and M. Shelef"
Scientii'ic Research Staff, Ford Motor Company,Dearborn, Michigan 48 121 (ReceivedMay 28, 1974) Publication costs assisted by the Ford Motor Company
The chemisorption of nitric oxide and carbon monoxide was investigated on Co304 and other cobalt-containing normal spinels, such as CoA1204and ZnCo204. On Co304 the chemisorption of NO and CO is very similar, with that of the NO being somewhat stronger. Monolayer coverage by NO at 25' is attained at 500 Torr and by CO at 1000 Torr. The NO can coadsorb on a surface covered with preadsorbed CO. Preadsorptiom of NO prevents the coadsorption of CO. The uptake of NO and CO by ZnCozOd is the same as by @03011. On the other hand, CoA1204is almost totally inert for the adsorption. The divalent cobalt ions in CoAlzO4 were found by ion-scattering spectroscopy to be largely shielded from the surface. Chemisorption of @O on two crystallographic modifications of cobaltous oxide has shown that the COO with the NaC.1 structure is active in chemisorption while the modification having the ZnS structure is not. It is concluded that it is the tetrahedral coordination of the cobaltous ions in CoA1204that is responsible for their inactivity i n chemisorption rather than their oxidation state.
Introductisin Studies of niiric oxiide chemisorption on a series of transition metal oxide^^-^ and on platinum6 have been reported previously. This communication deals with the chemisorption of NO and CO on Co304 and other cobalt-containing spinels. The chemisorption behavior of this oxide is of particular interest on several accounts. First, this oxide is the most active of the base metals of the first transition series in the catalysis of oxygen-transfer reaction^.^-^^ Secondly, it rapidly becomes deactivated by the solid-state interaction with the most common catalyst support, A1203.*J1 Thirdly, both the active oxide, C0304, and the inactive product of its interac1,ion with A1203, CoA1204 (cobalt aluminate), have the same spinel structure. By examining the chemisorbing surfacer; by ion-scattering spectroscopy and the chemisorbed species by infrared spectroscopy, along with the chemisorptbn measurements, some insight has been gained into the deactivation of Cos04 by interaction with A1203to form CoA1204. Since C0304 bas a ijpinel structure (C02+c023+04),with the divalent and trivalent cobalt ions in differently coordinated sites within the crystal, one objective of this study was to elucidate whether these cobalt ions behave differently with respect to chemisorption and catalytic activity. The other cobalt-containing spinels, such as Co2+A1204and ZnC02~+04,with cobalt ions of only one oxidation state and one type of coordixlation, were used in the chemisorption study for comparison with C O ~ + C O ~ ~ + O ~ . Finally, carbon monoxide chemisorption measurements were also performed cm two different samples of cobaltous oxide, one of the commonly observed NaCl structure, with the cobaltous ions HI octahedral coordination, and the other of the zinc blende-wurtzite structure, with the cobaltous ions in the tetrahedral coordination. xperirnentsl Sii?~ti01~ A. Adsorbents. Cobalt oxide (C0304) was prepared by precipitation from an aqueous C o ( N 0 3 ) ~solution with ammonia. The solid was heated to 400' for 8 hr. The resulting black powder was examined by X-ray diffraction. It had a spinel crystalline structure without impurity lines and had The Journal of Physical Chernisfry,Vol. 78, Alo, 24, 1974
a BET area of approximately 20 m2/'g. CoA1204 was purchased from City Chemical Co. and was preheated to 800' in air prior to use. The heated sample had a BET area of 11 m21g. ZnCo204 was prepared by dissolving 2nCO3 and Co(NO3)z in "03 solution, using a 1% excess of Zn over the stoichiometric amount. This precaution was taken to assure that no excess of free Cos04 would be present in the end product. The solution was evaporated to near dryness and heated to 400' for 8 hr. The sample was then degassed under vacuum a t 800' for 3 hr. The BET area of the sample was 6.2 m2/g. Both COA1204 and ZnCozOd were examined by X-ray diffraction and showed a spinel crystalline structure. ZnO was prepared by the decomposition of ZnCO3 (Allied Chemical, reagent grade) at about 400' and degassed under vacuum a t 400' for 3 hr. The BET area was 19.3 m2/g. The cobaltous oxide of the usual NaCl structure was prepared by the decomposition of C o c o 3 in uacuv at 300' for 5 hr and further degassed at 400' for 3 hr. The COOwith the zinc blende-wurzite structure was prepared by decomposition of cobalt acetate in U ~ C U Oat 280" for 20 hr according to the procedure of Redman and Steward.12 X-ray patterns of the COO confirmed a zinc blende (3H) structure with minor wurtzite (2N)stacking. B. Adsorbates. Nitric oxide was purified by several freezing-evaporation cycles and two-thirds of the middle fraction was used in the chemisorption experirnents.l CP grade carbon monoxide and ultra-pure grade argon (99.9% minimum) were used without further purification. C. Execution of Measurements. The volumetric measurements were performed in a conventional constant volume adsorption apparatus equipped with a precision pressure gauge linked with a fused quartz Bourbon capsule having a pressure range of 0-1000 Torr (Texas Instruments, Model 145). The mass spectrometric analyses of the gas phase above the adsorbent were carried out by a CEC614 mass spectrometer equipped with a batch-inlet system and operating a t a pressure of 0.1 Torr at the high-pressure side of the gold leak of the analyzer. The infrared spectra were taken with a Perkin-Elmer Model 1810 spectrometer.
Nitric
Oxide and Carbon Monoxide Ghemisorption
The ion-scattering spectra were taken by a 3M ion scattering spectrometer using a beam of He+ ions accelerated to 1000 eV from a static atmosphere of helium a t a pressure of 6 X 10-5 Torr. For the ad
