Studies of the hydrogen held by solids. Part 17.—The ortho-para H2 Conversion and H2–D2 exchange reactions over a transition alumina

Abstract

The rates of the H₂–D₂ exchange, the o-p H₂ and p-o D₂ conversions were measured on the surfaces of a very pure alumina, with and without a portion of the sites selectively poisoned with CO₂. These data were supplemented with results obtained from surface dehydroxylated to different extents. When the alumina was dehydroxylated below 525°, the rate of the ortho-para H₂ conversion at 0° was only a little higher than the rate of the H₂–D₂ exchange. Both rates fell in a commensurate manner as the surface was poisoned with CO₂(in a way which eliminated the strongest sites first) to less than 1 % of their initial values at about 16 × 10¹² CO₂/cm². At –195°, the exchange reaction was immeasurably slow, while both conversion reactions (H₂ and D₂) had comparable and easily measurable rates, the D₂ conversion being a little faster. These conversion rates fell as the surface was poisoned with CO₂ to less than 1 % of their initial values at 18 × 10¹² CO₂/cm². Both reactions were also poisoned by lowering the dehydroxylation temperature below 300°. After pretreatment at 800°, the site density for the conversion reactions increased to about 30 × 10¹² CO₂/cm²; that for the exchange reaction fell to about 9 × 10¹²CO₂/cm².

Arrhenius plots for the H₂–D₂ exchange were linear on poisoned and unpoisoned surfaces and indicated an activation energy of about 2.3 kcal/mol. The Arrhenius plots for the conversion reactions were non-linear and varied with the extent of poisoning with CO₂. In all cases, however, they converged with the plots for the exchange reaction above –80°, but curved to lower slopes (lower activation energies) at lower temperatures. Evidently, the conversion and exchange reactions both proceeded by the same dissociative mechanism at high temperatures but only the paramagnetic component of the conversion reaction was significant near the temperature of liquid nitrogen. An extensive search was made, but no e.p.r. signal could be found even though the site density determined by CO₂ poisoning was ∼10¹⁹/g. It is suggested, therefore, that the conversion reactions are catalyzed by interaction with exposed ²⁷Al nuclei.