Role of chemisorption in simple catalytic reactions

Abstract

Hydrogen and oxygen sorption and their interaction have been studied on evaporated films of Pt, Pd, Rh, Ni, Fey Mo, Cu and Mn. Hydrogenation of cyclopropane has been followed on films of Ni, and sorption and interaction of hydrogen and nitrogen have been measured on films of Fe. Oxygen adsorbed on nickel does not react at 78°K with hydrogen adsorbed subsequently, but restricts the hydrogen adsorption roughly proportional to the degree of the surface coverage by oxygen. At room temperature there is extensive interaction of the preadsorbed oxygen with subsequently admitted hydrogen only if part of the surface had been left uncovered by oxygen, otherwise atomization of H2 in the gas phase is necessary. Platinum behaves in the same way even at 73°K. With molybdenum there is no interaction between the preadsorbed oxygen and admitted hydrogen at 273°K even if hydrogen is present in the adsorbed state, atomization is again necessary.Products of the interaction at 273°K differ with various metals in a characteristic way. With Ni and Mo the reaction is stopped after all oxygen had already reacted and the reaction cannot be repeated on the same surface at this temperature. With Pt, Pd and Rh, however, the sequence of oxygen and hydrogen sorption can be repeated without limitation. Cu behaves differently. There is no hydrogen adsorption and no interaction with oxygen even on surfaces not fully precovered by oxygen. We conclude that hydrogen enters the reaction with oxygen always in the adsorbed state. There seems no connection between the efficiency of the metal in this reaction and the presence or absence of hydrogen causing a " positive " effect in the change of the film conductivity. Two conditions are required if a metal is to be active in the 024-H~ reaction : (i) all reactants must be adsorbed, (ii) their heat of adsorption has to be low.In hydrogenation of cyclopropane on nickel a fast reaction takes place only on a very small portion of the surface, while on the remaining part the dissociated and dehydrogenated species are removed slowly. Thus, various adsorbed particles play a different role in the catalytic reaction. Further results show that adsorption is a necessary prerequisite for a catalytic reaction, but itself does not represent a sufficient condition. Tn all systems studied, adsorption of both components is largely competitive.

Solid-state sensors for trace hydrogen gas detection

“…According to Ponee et al 71 the reaction between hydrogen and oxygen at the Pd surface at 0 °C goes to completion producing desorbable water, in agreement with the last step, reaction (39d), ofthe four-step mechanism (39). However, according to Lundstrom et af.,33 the details of the waterproduction reactions on the palladium layer are still not known.…”

Edge‐Defined, Film‐Fed Growth (EFG) Technique for the Preparation of α‐Al2O3:C Crystal with Highly Sensitive Thermoluminescence Response

“…During the growth process, divalent ions of carbon impurity are supposed to replace trivalent cations of Al, which leads to the formation of hole‐trapping centers 5 . It had been reported that hydrogen has a strong decarbonizing effect at high temperatures, 12 and the divalent carbon ions in the crystal decrease after annealing in H 2 , which leads to a decline in absorption coefficient at 206 nm. Moreover, the carbon in the as‐grown crystal volatilizes in the form of CO or CO 2 after annealing in air; hence, the crystal that was first annealed in air and then in hydrogen shows a very low absorption coefficient at 206 and 256 nm.…”

Site requirements in the kinetics of alkane transformations catalysed by metals

“…They agreed, from their results, as Roberts [19], Beeck and Ritchie [20] and other authors [21], that hydrogen atom was localized on a surface metallic atom. Some authors, as Ponec et al [22], on the base of this conclusion, proposed that, by the measure of the quantity of chemisorbed hydrogen, the area determination of the surface atoms could be possible. It was the first time that surface atoms and their coverage by adsorbed species was taken in consideration.…”

“…These functions (equations (22) and (23)) were only dependent on the thermodynamics of hydrogen chemisorption and on the number Z of potential sites involved in the chemisorption process of the hydrocarbon. The values for Z were found between 7±2 and 8±2 for methane [34] and ethane [48] chemisorption on rhodium, respectively.…”