Chemisorption on zinc oxide

Because zinc oxide is a relatively well-understood oxide semiconductor, we shall first review its properties as a hydrogenation catalyst in the catalytic hydrogen-deuterium exchange reaction. Since the latter essentially measures the rate of reversible chemisorption of hydrogen at equilibrium, data on the hydrogen chemisorption will be included in this survey. Any theory of hydrogen chemisorption on zinc oxide must explain all the following well-established facts. 

Butanol, reaction over reduced nickel oxide catalysts, 35 357-359 effect of ammonia, 35 343 effect of hydrogen, 35 345 effect of pyridine, 35 344 effect of sodium, 35 342, 351 effect of temperature, 35 339 over nickel-Kieselguhr, 35 348 over supported nickel catalysts, 35 350 Butanone, hydrogenation of, 25 103 Butene, 33 22, 104-128, 131, 135 adsorption on zinc oxide, 22 42-45 by butyl alcohol dehydration, 41 348 chemisorption, 27 285 dehydrogenation, 27 191 isomerization, 27 124, 31 122-123, 32 305-308, 311-313, 41 187, 188 isomerization of, 22 45, 46 isomers... 

Parravano and Boudart (1S6) have depicted chemisorption of hydrogen on zinc oxide, as occurring through heterolytic splitting on a pair of adjacent zinc-oxygen sites, followed by proton-transfer, e.g.,... 

Reversible chemisorption and hydrogen-deuterium exchange on zinc oxide have been observed (Taylor et al., 136,137 , Harrison and McDowell,... 

It is important to note that the principal features of hydrogen chemisorption, which are summarized above, apply equally well to other adsorbents than zinc oxide, for instance to chromium oxide. A satisfactory theory therefore must not depend on specific properties of zinc oxide. In this connection let us recall the important experiments of Pace and Taylor (14) and Kohlschiitter (15), who found that the slow rates of chemisorption of hydrogen and deuterium on chromium oxide, zinc oxide-chromium oxide, and nickel on kieselguhr, were identical within experimental error. It would be interesting to perform such an experiment on zinc oxide because it permits one to make a decision on the nature of the slow activated step (16). 

Morrison and Miller (33) have made some direct measurements on the adsorption of oxygen on zinc oxide powder (Fig. 4), which seem in accord with the conductivity measurements of Bevan and Anderson on sintered zinc oxide. The reversible region of chemisorption was shown to be above about 45O C, corresponding to the reversible region of conductivity versus oxygen pressure found by Bevan and Anderson to be above about 500°C. 

Taylor and Strother (44) were the first investigators to observe that there were two types of chemisorption of hydrogen on zinc oxide. Their results are shown in Fig. 9. Physical adsorption occurs at low temperatures there is a peak for one type of chemisorption (it will be assumed to be Type A adsorption) at about 350°K, and for Type B adsorption (again assumed) at about 500°K. 

Each of the various processes of adsorption may have desorptions of the reverse forms, for example, dissociative adsorption may have as its reverse, associative desorption. However, the process of chemisorption may not be reversible [ 1.2.2(c)]. Desorption may lead to species other than that adsorbed, for example, ethane dissociatively adsorbed on clean nickel gives little or no ethane upon desorption, 1-butene dissociatively adsorbed to methylallyl and H on zinc oxide gives mainly 2-butenes upon desorption, and some W03 may evaporate from tungsten covered with adsorbed oxygen. 

The method was used for the study of the chemisorption of hydrogen on nickel and on zinc oxide, and Keier and Roginskil could demonstrate the heterogeneous character of the surfaces of these adsorbents 309). They could also demonstrate the heterogeneous character of the surface of charcoal for the chemisorption of hydrogen 310). 

Table 1. Chemisorption of Oxygen on Zinc Oxide between 100 ° and 250 °C...

It is evident from examples like these that the investigation of electron transfer in catalysis is dependent on the availability of test reactions of well-known acceptor or donor type. Lately, it has become clear that sometimes the same reaction can exert both functions, depending on the conditions. Thus, the carbon monoxide oxidation is a donor reaction on most p-conducting catalysts, like nickel oxide 13) when the chemisorption of carbon monoxide governs the reaction rate. However, on zinc oxide, the chemisorption of the acceptor oxygen is rate-determining. 

That the adsorptions involved were actually due to chemisorption was indicated by measurements of nitrogen adsorption on the zinc oxide surfaces studied. These measurements showed that already at 56° C. the van der Waals adsorption of nitrogen was too small to measure on zinc oxide surfaces with surface areas by the BET method in the neighborhood of 5-25 sq. meters/g. The van der Waals adsorption of hydrogen would be still smaller and lower by one or more orders of magnitude than the quantities of gas involved in the desorption and readsorption processes. ... 

Figure P6.3 Typical adsorption rate curve for the adsorption of hydrogen on zinc oxide at 1 atm. [Adapted with permission from H. S. Taylor and S. C. Liang, The Heterogeneity of Catalyst Surfaces for Chemisorption, J. Am. Chem. Soc., 69, 1306 (1947). Copyright 1947 by the American Chemical Society.]...

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