Oxygen chemisorption measurement

Oxygen chemisorption measurements were performed in the above flow apparatus using He as carrier gas (30 Ncm -min" ). Prior to chemisorption measurements, catalyst samples (0.25 - 1.00 g) were treated "in situ" for 15 min in a flow of CH4/02/He (Pch4 02 He 2 1 7) reaction mixture at 550-... 

Hydrogen and oxygen chemisorption measurements were performed in a conventional glass system at 22°C. Before a H2 chemisorption measurement, the catalyst was reduced, or oxidized and reduced, at temperatures and during times to be specified under Results. These temperatures were reached with a heating rate of 5°C min-1. 

One of the best ways of characterizing a supported catalyst is determination of dispersion and effective surface area of the catalyticaUy active component. The dispersion of metal oxide catalysts can be determined by selective chemisorption of oxygen at appropriate temperatures [14-16]. The dispersions obtained from oxygen chemisorption measurements on various catalysts are presented in table 1. The N2 BET surface areas of various samples are also shown in this table. As can be noted, dispersion for 20 wt% catalyst is similar, within experimental limitations, irrespective of their origin. The BET surface area measurements also reveal that both the preparation methods yield similar type of catalysts in terms of physico-chemical characteristics. These catalysts were further evaluated for selective oxidation of / -methox doluene to p-... 

Chemisorption measurements, combined with oxygen uptake, TPR, and pulsed CO-O2 experiments were employed to determine the source of large differences in dynamometer sweep performance of a series of Pt/Rh, Pd/Rh and Pd-only TWCs after dynamometer aging. The following observations have been made ... 

Besides CO, hydrogen, oxygen, and other gases can also be used in chemisorption, provided suitable conditions are applied for sample pretreatment and chemisorption measurements (Overbury et ai, 1975 Bartley et al., 1988 Scholten et al., 1985 and Van Delft et ai, 1985). 

The existence of Pt-Ba interactions has been confirmed by FT-1R analysis of CO chemisorption measurements. A band characteristic of CO linearly adsorbed on Pt sites is observed in all cases however, the intensity of the band decreases on increasing the Ba loading (due to the decrease of the Pt dispersion) and shifts towards lower energy (from 2072 to 2049 cm ) according to the increase in the system basicity. Hence the data indicate a strong interaction between Pt and the basic oxygen anions of the Ba phase, thus suggesting that the exposed Pt sites and the Ba component are in close proximity [98]. 

Another way of investigating structure is through the classical method on metals of varying catalyst particle size. The key to this method is to measure active catalyst surface areas in order to determine changes in turnover rates with ensemble size. In recent years several chemisorption techniques have been developed to titrate surface metal centers on oxides (25). In this volume Rao and Narashimha and Reddy report on the use of oxygen chemisorption to characterize supported vanadium oxide. 

Surface Bond Energies Thermochemical data are very scant in the area of oxygen chemisorption (57). These data would be of great value for interpreting spectroscopic and kinetic data and for the analysis of reaction mechanisms. The vast majority of the available data are for low oxidation state systems (55). Although calorimetry offers a means for direct measurements, for analysis of reaction pathways it is necessary to have detailed values for many types of species (M-OH, MO-H, M-OR, M-R, M-O, M-H), and these are usually... 

A similar relationship can be observed with promoted M0S2. Each family of catalysts has its own linear correlation, which cannot be compared to each other directly because of the corrosivity problem. More recently, low-temperature oxygen chemisorption has been claimed to be more reliable, but it also lacks a well-determined stoichiometry (52). Oxygen chemisorption has also been applied to tungsten and rhenium sulfides, as well as promoted molybdenum and tungsten sulfides. In the isotropic class, it has been applied only to ruthenium sulfide, in which case it gives approximately the same result as a BET measurement due to the isotropic nature of this sulfide (41). 

Chemisorption measurements have shown that ethylene does not adsorb on pure silver, but only on a silver surface which has been preoxidized [339]. Complete coverage with an oxygen monolayer, however, also seems to destroy the capacity to adsorb ethylene, as was demonstrated by Force and Bell [114,116] (favouring the idea of adsorption on silver). Consequently, partial oxygen coverage seems to be a necessary condition for catalytic activity. 

In carbon materials, active surface sites only represent a fraction of the total surface area, called active surface area (ASA). Knowledge of the nature and concentration of the active sites is of paramount importance for a better understanding of the kinetics involved in heterogeneous gas-solid reactions. However, although ASA reveals interesting information about the sample, there is a need for a reliable and standardised method for its estimation. The aim of this work is to compare ASA determination by the usual methods (i.e., gravimetry, TPD) with another method, which is based on pressure measurements in order to perform an oxygen chemisorption isotherm (OCI). The results showed that the OCI method seems to be a valuable and alternative method for ASA determination, as it avoids the main potential source of errors Inherent in the usual methods. 

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