Hydrogen chemisorption stoichiometry

Chemisorption measurements (Quantachrome Instruments, ChemBET 3000) were conducted in order to determine the metal (Co) dispersion. Therefore, the nanomaterial catalysts were reduced under a hydrogen flow (10% H2 in Ar) at 633 K for 3 h. The samples were then flushed with helium for another hour at the same temperature in order to remove the weakly adsorbed hydrogen. Chemisorption was carried out by applying a pulse-titration method with carbon monoxide as adsorbing agent at 77 K. The calculation of the dispersion is based on a molar adsorption stoichiometry of CO to Co of 1. 

The activities in FT reaction (expressed as turnover rates, Vt of CO transformed to hydrocarbons and oxygenates) of bulk and supported tungsten carbides are compared to that of a rhodium catalyst (3.5 wt%) supported on alumina (Table 18.6). Its dispersion (94%) has been measured by hydrogen chemisorption by assuming unity stoichiometry of adsorbed hydrogen on Rh. 

If one could disregard the complicated influence of poisons on mass transfer processes, it would be possible to state in a first approximation that catalyst activity for a selected reaction is a monotonic function of the surface area occupied by the active component. The problem that arises is the measurement of the catalytic surface area in the presence of a support material. In the case of Pt such a measurement is relatively simple, done by hydrogen chemisorption (56, 57) or titration (55), although even in this case there are uncertainties associated with surface stoichiometry (59, 60). These problems become more complicated when Pd, or other noble metals are incorporated at the same time, and still more so, when the catalysts have been contaminated (61). 

Hydrogen chemisorption is not yet a routine characterization method for supported metal clusters because stoichiometries of chemisorption on the clusters are not well known and are different from those of chemisorption on metal crystallites [13]. Chemisorption of CO is also of limited value because CO typically reacts with supported clusters, leading to changes in cluster structure. Research is needed to clarify these matters. 

Chemical Measurements. All the catalysts in Table I showed a suppression in hydrogen chemisorption to varying extent. Such a suppression manifested itself either as an overestimation of crystallite size from chemisorption data or as a lower adsorption stoichiometry of H/Ni,, than what would be expected of a comparable Ni/Si02 catalyst. In addition, the suppression was more severe with increasing reduction temperature (8-10). As mentioned... 

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). 

CO chemisorption at 308 K was used to measure the metal dispersion and a stoichiometry ratio of M/CO (M = Pd or Pt) = 1 1 was assumed in the calculation for both Pt and/or Pd. Due to the possible formation of Pd hydride and the uncertainty on the metal to hydrogen ratio, hydrogen chemisorption was not used. Our previous results on Pd/MPS catalysts showed that the hydrogen concentration during reduction has little influence on the dispersion obtained. 

Second, the stoichiometry of adsorption must be known in order to calculate surface concentrations. This is extremely difhcult to establish. Hydrogen, for example, adsorbs as M-H species over crystalline planes. This is conhrmed with parallel hydrogen chemisorption and BET measurements on nonsupported nickel. However, the stoichiometry of the bond M Hn appears to increase for low coordination sites (see Chapter 3) so that overall values for very small crystallites may be greater than one. Carbon monoxide is even more troublesome. Several modes coexist linear Ni-CO, bridged, Ni CO, and subcarbonyl, Ni(CO), so that some assumptions are inherent in its use. ... 

The use of H2-O2 titration is a method of enhancing the sensitivity relative to a standard hydrogen chemisorption measurement which involves the use of hydrogen to titrate a chemisorbed layer of oxygen.63 The enhanced sensitivity results from an increased hydrogen consumption as the stoichiometry is raised to 3 H 1 Pt... 

Platinum and chlorine (samples made with chloride precursors) contents of the catalyst samples were determined with X-ray fluorescence spectroscopy (XRF) (Phillips PW 1480 spectrometer). BET surface areas of catalysts were within 5% that of the silica support material. Platinum dispersion was measured with hydrogen chemisorption in a volumetric set-up, using a procedure described elsewhere [3]. Stoichiometry of H/Pt = 1 was assumed for calculating the platinum dispersion [4]. Transmission electron microscopy (TEM) (Phillips CM 30, 300kV) was used to check the platinum particle size in some of the catalysts. Average platinum particle size was determined based on analysis of about 100 platinum crystallites. 

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. 

The number of surface atoms can be determined by chemisorption of probe molecules (H2, O2...), knowing the stoichiometry of the adsorbed species. As an example, in the case of Pt, the stoichiometry of irreversibly adsorbed hydrogen (H/Pts) and oxygen (0/Pts) at room temperature are both close to 1/1 [108-111]. Knowing the total number of atoms (elemental analysis) and the number of irreversibly adsorbed H and O, the dispersion of the particles (D = Pts/Pt) is then easily obtained. Note that the dispersion of these particles decreases when their size increases (Fig. 5). 

Chemisorption has been applied to numerous catalytic systems including the TMS catalysts, and valuable information on the active surface area or catalytic sites densities has been obtained. However, it is important to keep in mind that the interpretation of the results is subject to the assumption that the stoichiometry of the chemisorption is known. It is well known that on metal surfaces, dispersion is calculated using one hydrogen atom per metal atom. Consequently, dispersion higher than 100% is not unusual for highly dispersed catalysts with particle sizes below 15 A, reflecting the existence of metal atoms associated with more than one hydrogen. This assumption is also made for TMS catalysts. 

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