The calorimetric determination of the heats of adsorption of oxygen on evaporated metal films

Abstract

A Beeck-type calorimeter has been used to measure the integral heat of adsorption of oxygen on evaporated films of titanium, chromium, manganese, iron, cobalt, nickel, niobium, molybdenum, rhodium, palladium, tantalum, tungsten, platinum and aluminium. The variation of the heat with the extent of the adsorption has also been determined, except with palladium, platinum and aluminium, for which the amounts of oxygen adsorption were too small to allow this to be done. The heats of adsorption reported are based on a careful determination of the heat capacity of the calorimeter and the overall error is not more than ± 5 kcal/mole. Some investigations of calorimetric behaviour have been made. The surface areas of the films have been measured before and after an adsorption by the B.E.T. method; krypton isotherms at — 196 °C were used for this purpose. From these data, it has been possible to estimate the number of atoms of oxygen adsorbed per surface metal atom. In most cases, this quantity, together with the heat of adsorption, can be related to the oxygen/metal ratios and the heats of formation of certain oxides; this observation is remarkable in view of the limited extent of the adsorption. In the cases of rhodium, palladium and platinum the adsorption stops short of a monolayer and is probably true chemisorption rather than oxide formation. For those metals on which oxide formation occurs, the decrease in heat of adsorption with increasing coverage has been interpreted, in general, in terms of an increase in the oxidation number of surface metal ions by electron transfer within the adsorbed layer. Some theoretical correlations between the heat of adsorption and the atomic and the bulk properties of the species concerned have been examined and found of little value; an empirical relation with the metallic radius is given.

Dynamics and Thermodynamics of Pd–PdO Phase Transitions: Effects of Pd Cluster Size and Kinetic Implications for Catalytic Methane Combustion

“…These smaller clusters in their metallic state are expected to bind chemisorbed oxygen more strongly at their surfaces than larger clusters because of their lower surface coordination, as confirmed from the higher temperatures required to desorb chemisorbed O* atoms from Pd clusters (750 K for 1.6 nm vs 700 K for 9.1 nm clusters) . The stronger binding to O* and the more negative free energies for PdO formation (Figure b) found in these smaller clusters than the bulk structures appear to follow the linear correlation between the O* binding strengths [ Q O*–Pd = (−Δ H O*–Pd ) for oxygen dissociation, Step 1 of Scheme ] and heats of bulk oxide formation [ Q Pd–PdO = (−Δ H Pd–PdO ° ) = −(Δ G Pd–PdO ° + TΔ S Pd–PdO ° )], established previously by varying the metal identity across bulk transition metals. , The instabilities of small metallic clusters have also been shown from an increase in the heat of cluster adhesion on oxide surfaces [e.g., for 1–4 nm Pb on MgO(100)] to values much larger than predicted with the Gibbs–Thomson model by microcalorimetry . These enthalpic instabilities in small clusters lead, in turn, to their greater tendency for bulk oxidation (Figure ).…”

Promoting Catalytic Oxygen Activation by Localized Surface Plasmon Resonance: Effect of Visible Light Pre‐treatment and Bimetallic Interactions

“…Light pre-treatment and reaction:C atalytic oxygen activation by Au/TiO 2 ,P t/TiO 2 ,a nd AuPt/TiO 2 ,w ith or without light pre-treatment, was studied by evaluating formic acid oxidation in a7 0mL glass spiral reactor,a sd etailed elsewhere. [22] If molecular O 2 is adsorbed on the catalysts in an aqueous system and then activated or reduced to OOH ads and OH ads , [26][27][28][29][30][31][32] it can readily oxidize formic acid as detailed by the below Equations (1) to (6 The 50 %f ormic acid oxidation rate (R 50 )w as used to define catalyst performance as ab asis for investigating the effect of light pretreatment. In atypical light pre-treatment experiment, a1gL À1 catalyst suspension was adjusted to pH 3.0 AE 0.05 by using 0.5 m perchloric acid after which it was transferred into ag lass spiral reactor.p Ha djustment to 3.0 AE 0.05 was designed to minimize carbonate formation in the solution during formic acid oxidation.…”

Interaction of Reactive Gases with Platinum Aerosol Particles at Room Temperature: Effects on Morphology and Surface Properties

“…Based on the exposed platinum surface, a release of thermal energy of 4.8 × 10 −17 J can be calculated. For oxygen, the adsorption heat is supposed to be even higher [24]. Furthermore, we assume a certain van der Waals (vdW) energy between two primary particles that are separated from each other by a distance of 4 Å, e.g., by the disappearance of platinum oxide when hydrogen is added.…”