Metal-adsorption calorimetry

Mo-V-Te and Mo-V-Te-Nb mixed-metal oxide catalysts have been characterized by means of C3H8-TPR and NH3 adsorption calorimetry. All samples were strongly heterogeneous, with initial adsorption heats of = 100-80 kJ moT for the Mo-V-Te samples. Introducing an Nb component into the catalysts slightly decreased the initial adsorption heats to = 60 kJ moT but drastically increased the surface density of weak acid sites (<30kJ moT ) [83]. 

Numerous examples of applications of DSC to the study of the stabiUty of binary, ternary and quaternary metal oxides can be found in the Uterature, and we will not focus on this type of systematic characterization. Fewer studies have been performed to determine the acid-base character of mixed oxides using adsorption calorimetry of probe molecules. 

Ru-HPA metal-acid bifunctional catalysts on various supports (siUca, graphite, KL zeolite) have also been characterized by NH3 adsorption calorimetry, which revealed heterogeneous acid site strength distributions varying in the order HPA-SiOj > HPA-graphite > HPA-KL [106]. 

Surface Thermodynamics Small Molecule Adsorption Calorimetry on Metal Single Crystals... 

Studies of metals or supported metals by adsorption calorimetry are not as extensive as for metal oxides. Several reviews have been published [6,131]. Many recent studies deal with measurements of integral and differential heats of adsorption of H2, CO, O2 and hydrocarbons, because these molecules are involved in numerous commercial catalytic processes. Microcalorimetric methods provide an effective means of measuring the strengths of adsorbate-surface interactions, not only on clean metal surfaces, but also on metal surfaces that have been e osed to reaction conditions. 

Catalytic reactions involving secondary alcohols on metal oxides are thought to proceed through mechanisms involving a cooperative action of acidic and basic sites. For the studied zeolites, a quasi-linear correlation was established between the alkene selectivities and the ratio of basic to acidic sites, as determined by adsorption calorimetry (Figure 13 see also Figure 6). 

The interaction energies of NH3 with Cu" and Ag" metal cations dispersed in ZSM-5 zeolites have been studied by means of adsorption calorimetry [295]. Two different families of coordinatively unsaturated metal cations were evidenced. A (rather heterogeneous) fraction of the reversibly bound ligands were found to be more energetic, their interaction with the sites giving rise to a heat comprised in the 100-150 kj mol interval. The other ligands interacted more weakly with the metal sites, with a nearly constant heat of ca. 50 kJ mol [295]. 

S. Cemy, Adsorption calorimetry on filaments, vacuum-evaporated films and single crystals of metals. Thermochim. Acta 312, 3-16 (1998)... 

It is true, however, that many catalytic reactions cannot be studied conveniently, under given conditions, with usual adsorption calorimeters of the isoperibol type, either because the catalyst is a poor heat-conducting material or because the reaction rate is too low. The use of heat-flow calorimeters, as has been shown in the previous sections of this article, does not present such limitations, and for this reason, these calorimeters are particularly suitable not only for the study of adsorption processes but also for more complete investigations of reaction mechanisms at the surface of oxides or oxide-supported metals. The aim of this section is therefore to present a comprehensive picture of the possibilities and limitations of heat-flow calorimetry in heterogeneous catalysis. The use of Calvet microcalorimeters in the study of a particular system (the oxidation of carbon monoxide at the surface of divided nickel oxides) has moreover been reviewed in a recent article of this series (19). 

H2 adsorption on high surface area Rh/Ce02 catalysts has been studied by calorimetry [77]. As the reduction progresses, the mean heat of adsorption and the amount of hydrogen adsorbed on the metal, as determined by volumetry and calorimetry, decrease considerably. The initial heats were around 37kJ mol" . 

The strength of interaction of hydrocarbons with metallic surfaces serves as a probe to investigate the role of desorption and adsorption of these species in a variety of catalytic reactions (Table XXIX). Ostrovskii and Medvedkova (242) used calorimetry and gas chromatography to measure heats of adsorption of C4-C8 hydrocarbons over Co before and after use in Fischer-Tropsch synthesis. For the fresh samples, the adsorption of aliphatic hydrocarbons was reversible, and the initial heat of adsorption increased with increasing chain length of the adsorbate. It appeared that the saturated hydrocarbons were adsorbed with the C—C bond axis parallel to the surface, and the contributions from the CH3 and CH groups were calculated as cHj = -4.63 and mol". With these values, the experimental... 

Fewer data, particularily calorimetric, are available for evaluating the influence of temperature on metal cation adsorption. The enthalpy of Cd(II) adsorption onto rutile was determined using isoperibol solution calorimetry and a value of +10 kJ/mole was found (6). A recent variable temperature study (25) allows enthalpies for Cd(II), Zn(II), and Ni(II) adsorption onto hematite (synthesized in the presence of 0.86% Si) to be calculated using equation (8). These data are summarized in Table IV. 

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