Chemisorption of probe molecules

Volume 57B Spectroscopic Analysis of Heterogeneous Catalysts. Part B Chemisorption of Probe Molecules edited by J.L.G. Fierro... 

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). 

B.J.H. methods) (iii) the average diameter (T.E.M.) and/or the dispersion (chemisorption of probe molecule) of the metallic particle. EXAFS will also provide average coordination numbers, which decrease sharply as the particle size decreases. 

The size and morphology are characteristic parameters of metal particles. It is possible to determine them by various techniques transmission electron microscopy (TEM) [105-107], X-ray photoelectron spectroscopy (XPS) [108], X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAES) [109, 110], thermoprogrammed oxidation, reduction or desorption (TPO, TPR or TPO) and chemisorption of probe molecules (H2, O2, CO, NO) are currently used. It is therefore possible to know the particles (i) size (by TEM) [105-107], extended X-ray absorption fine structure (EXAES) [109, 110]), (ii) structure (by XRD, TEM), (iii) chemical composition (by TEM-EDAX, elemental analysis), (iv) chemical state (surface and bulk metal atoms by XPS [108], TPD, TPR, TPO) and... 

Raman spectroscopy has been used frequently to investigate the chemisorption of probe molecules (Cooney et al., 1975 Weber, 2000). Several groups reported variable Raman cells in which the temperature of the sample and the environment can be controlled so that catalytic reaction conditions can be simulated (Abdelouahab et al., 1992 Brown et al., 1977 Chan and Bell, 1984 Cheng et al., 1980 Lunsford et al., 1993 Mestl et al., 1997a Vedrine and Derouane, 2000). In these investigations, conversion and selectivity values were not measured simultaneously with the spectra. The developments of these Raman experiments have been reviewed elsewhere (Banares, 2004 Knozinger and Mestl, 1999 Vedrine and Derouane, 2000). 

Spectroscopic Characterization of Heterogeneous Catalysts. Part A. Methods of Surface Analysis. Part B. Chemisorption of Probe Molecules, J.L.G. Fierro. Ed.. Elsevier (1990). (Part A on surface structure methods, surface groups on oxides. X-ray, Mdssbauer Part B on Infrared, NMR, EPR, thermal desorption,. ..)... 

Spectroscopic Analysis of Heterogeiieous Catalysts. P rt B Chemisorption of Probe Molecules... 

A problem, different in nature, that needs additional attention refers to the characterization of the active centers involved in adsorption and catalytic processes and particularly to the estimation of the number of metallic centers and the exposed surface in supported and unsupported perovskites. A number of chemical and physical methods have been used for metals and oxides, and those based on selective chemisorption of probe molecules seem to be the most promising for this purpose (307). However, while considerable progress has been made for supported metals, no method has been accepted for oxides. This has been caused by the comparatively complex nature of these latter compounds where oxide ions and metal ions of different oxidation states may be present. As probe molecules, O2, CO, and NO were the most frequently used (307) the 02 chemisorption presents the problems inherent to any method based on gas adsorption at low temperatures (a large fraction of physisorbed gas accompanying the chemisorption). On the other hand, its symmetric character renders this molecule unamenable to study by IR spectroscopy. Nonetheless, this method has been used with some success by Weller et al. (308-310) on simple oxides, and its possible application to perovskites and other mixed oxides should be explored. Previous chemisorption work... 

Experimental studies show that covalent oxides in usual conditions present neither Lewis acidity nor significant basicity. This can be evidenced by the lack of any significant chemisorption of probe molecules with acidic or electrophylic character. In particular, they are unable to chemisorb carbon dioxide in the form of carbonates or sulfur dioxide in the form of sulfites. 

---