Adsorption of hydrogen and H 2-D 2 exchange reaction on alumina

Abstract

The chemisorption of hydrogen on alumina was investigated over a wide range of temperatures (−196–450 °C). At least five different states of chemisorbed hydrogen on alumina were detected by the temperature-programmed desorption as peaks with the peak maxima at about −40, 25, 100, 200, and 320 °C. These peaks (or types) are referred to as H(I), H(II), H(III), H(IV), and H(V), respectively. Except for H(IV), each type of chemisorption was separately investigated in more detail by measuring either the isotherms or the rates of adsorption and desorption. H(I) is the weakest chemisorption which occurs rapidly and reversibly at −75 °C and fits the dissociative Langmuir isotherm. At room temperature, H(II) and H(III) are the major chemisorption: The former is a reversible chemisorption which is equilibrated in 30 min, while the latter is a slow irreversible chemisorption which continues up to 70 hr at room temperature. H(V), the strongest chemisorption, is too slow to be detected at temperatures below 250 °C, but it is the major adsorption at higher temperatures. The total number of active sites for chemisorption was found to be 1.9 × 10 13 sites/cm 2. The experimental results indicate that at least four types of chemisorption, H(I), H(III), H(IV), and H(V), share all or part of the sites which are the surface defects of alumina. Isotopic exchange of hydrogen and deuterium was also studied in connection with the above results. Two kinds of reactions were carried out: (1) surface-exchange reaction between various types of preadsorbed H 2 on alumina and D 2 in the gas phase; and (2) equilibration reaction of gaseous mixtures of H 2 and D 2 but without preadsorption. The results showed that at low temperatues the isotopic-exchange reaction occurred only through hydrogen chemisorbed as type H(I) and did not involve the other types of chemisorption.