Chemisorption stoichiometry

Therefore, it is especially noteworthy that calculation of the surface structure and nature by the chemisorption data of probe molecules, such as the total number of surface metal atoms, may produce prodigious errors, sometimes as high as 100%, which of course are transferred to related properties such as 

Metal Gas Temperature/° C Pressure/torr Chemisorbed stoichiometry 

In the following section, we take fused iron catalyst for ammonia s mthesis as an example to introduce the idiographic applications of chemisorption in the study of catalyst. 

2 Physical and chemical adsorption experiment on iron eatalyst 

The first use of chemisorption in the study of heterogeneous catalysis was introduced by Emmett during the study of iron-based catalyst for ammonia S3mthesis which used the chemical adsorption of CO and CO2 to measure the surface area of active Fe iron and promoters of K2O and AI2O3. He obtained the following instructive revelation Although content of promoters is very little, they cover most of the surface of the catalyst, which shows that the promoters tend to occupy the surface phase. Since then, many researchers have used chemisorption to study the effects of various components in the traditional iron catalyst, as well as the relationship between the mutative trends of various component and changes in activity 

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C. O2 uptidces were determined in a pulse mode (Vq2 puise pmol) at the same temperature of the pretreatment by using a TCD connected to a DP 700 Data Processor Carlo Erba Instruments). The number of reduced sites was calculated by assuming the chemisorption stoichiometry 02/"reduced site" of 1/2. 

Further problems can arise because of uncertainties concerning the stoichiometry of the adsorption reaction. For most metals it is assumed that the surface stoichiometry with H2 is H/M = 1. However, there is evidence especially for very small metal particles (of the order of 1 -5 nm) that the stoichiometry can exceed H/M = 1. For quantitative measurements of surface area it is necessary to establish the chemisorption stoichiometry and structure. In practice it is usually possible to achieve approximate estimate of the surface area by some other independent method (for example, from particle size analysis by X-ray line broadening or by TEM). In the case of CO, the CO/M ratio is generally taken as 1.0, but the true value may depend on the particle size and on the particle morphology. With N2O the N2O/M ratio at monolayer coverage is usually assumed to be 0.5, but once again there is no certainty about the validity of this particular assumption. 

They demonstrate unusual CO chemisorption stoichiometries (e.g., CO/Rh = 2.6 on [Rhg]-NaY). CO forms Rh lCOlig directly with small metal ensembles in the zeolite cages (55, 237). 

For chemisorption study, the adsorption isotherm (first isotherm) was obtained in each case by plotting the amount of O2 adsorbed at 300 °C against the O2 equilibrium pressure, Peq, up to 250 mm Hg. The resorption isotherm (second isotherm) was measured after outgassing for one hour at the same temperature. The total chemisorption values were taken as the difference between the first and the second isotherms. To minimize adsorption on the support and maximize adsorption on the supported complex, the total chemisorption values on pure support were subtracted from the total chemisorption producing the so called net adsorption values (a ) on the supported complex as expressed in m mol O2 g" CoPc. These values were used to calculate the degrees of dispersion (D), the chemisorption stoichiometries and specific surface areas of supported CoPc phase [18,19],... 

The region of the cyclic voltammogram, corresponding to anodic removal of Hathermal desorption spectra of platinum catalysts. However, unlikely the thermal desorption spectra, the cyclic-voltammetric profiles for H chemisorbed on Pt are usually free of kinetic effects. In addition, the electrochemical techniques offer the possibility of cleaning eventual impurities from the platinum surface through a combined anodic oxidation-cathodic reduction pretreatment. Comparative gas-phase and electrochemical measurements, performed for dispersed platinum catalysts, have previously demonstrated similar hydrogen and carbon monoxide chemisorption stoichiometries at both the liquid and gas-phase interfaces (14). 

Chemisorption data are reported in Table 1. The average Pd particle size has been calculated by the procedure recently developed by our group [7] for the monometallic Pd sample. For bimetallic samples, in the absence of any information about a possible influence of Au on Pd/CO chemisorption stoichiometry, only the experimental CO/Pd values have been reported. As expected, Pd dispersion decreases with the increase of treatment temperature, but no evident role seems to be played by Au neither on Pd sintering nor on Pd dispersion. This allows to exclude the occurrence of Au surface enrichment and also strong electronic interactions in the alloy. 

H2 uptake and the metal dispersion (D) of the fresh catalysts were evaluated hy H2-TPD measurements in the range 193-923K, after reduction at 773K for Ih, assuming the chemisorption stoichiometry H/Nisurf of 1/1 [10]. 

The measurement of the metallic surface area in a multi-component system as a bimetallic supported catalyst or an alloy is feasible by selective chemisorption on the metallic phase. The chemisorption stoichiometry is defined with reference to the adsorbate related to the metallic element [8]. Therefore, the chemisorption process is very different if the adsorbed gas molecule is dissociated or not. The two kinds of chemisorption involve different energetic behaviours and different theoretical models define them associative and dissociative adsorption. In the first case, the gas is adsorbed without fragmentation in the second case, the gas molecule is adsorbed after its decomposition in one or more fragments. Hydrogen, for example, is always adsorbed in its dissociated form. 

In the iron catalyst, the surface area of the active phase a-Fe was measured by selective CO chemisorption at the temperatirre of hquid nitrogen (77.4 K). If the chemisorption stoichiometry of CO adsorbed for iron is one, and the surface atom density of iron (Cm,Pe) is 1.5406 x 10 , thus the sm-face area (Spe) of a-Fe is ... 

The basic surface areas of base metals and alkaline earth metal oxides represented by K2O were measured by selective CO2 chemisorption at the temperature of dry ice and alcohol (195.2K). If the chemisorption stoichiometry of CO2 adsorbed in K2O is two, and the surface atom density of K (Cm,K) is 1.4136 x 10 , thus the... 

The exposed fraction of potassium atoms is calculated to be 68.49%-94.33% by CO2 chemisorption according to the content of K2O and the chemisorption stoichiometry of two. Because the amount of adsorbed CO2 includes that from K2O, and from CaO and other basic oxides, and the chemisorption stoichiometry of CO2 may deviate from two. As a result, using CO2 chemisorption quantity to calculate the exposed fraction of potassium atom is not very accurate, but most of the basic oxides being exposed on the surface of catalyst are, as the sparks fly upward. 

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