Probe molecules chemisorption

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. 

Optical second harmonic generation (SHG), which is the conversion of two photons of frequency u to a single photon of frequency 2co, is known to be an inherently surface-sensitive technique, because it requires a noncentrosymmetrical medium. At the interface between two centrosymmetrical media, such as the interface between two liquids, only the molecules which participate in the asymmetry of the interface will contribute to the SHG [18]. SHG has been used as an in-situ probe of chemisorption, molecular orientation, and... 

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... 

The adsorption microcalorimetry has been also used to measure the heats of adsorption of ammonia and pyridine at 150°C on zeolites with variable offretite-erionite character [241]. The offretite sample (Si/Al = 3.9) exhibited only one population of sites with adsorption heats of NH3 near 155 kJ/mol. The presence of erionite domains in the crystals provoked the appearance of different acid site strengths and densities, as well as the presence of very strong acid sites attributed to the presence of extra-framework Al. In contrast, when the same adsorption experiments were repeated using pyridine, only crystals free from stacking faults, such as H-offretite, adsorbed this probe molecule. The presence of erionite domains in offretite drastically reduced pyridine adsorption. In crystals with erionite character, pyridine uptake could not be measured. Thus, it appears that chemisorption experiments with pyridine could serve as a diagnostic tool to quickly prove the existence of stacking faults in offretite-type crystals [241]. 

A common method for determining the existence and the densities of different active sites is selective chemisorption. However, ignorance of the nature of the active sites makes it impossible to choose suitable probe molecules. Instead, a variation of the selective chemisorption technique can be used that makes use of the reactivity of the active sites. 

Lunsford et al. (202) used trimethylphosphine as a probe molecule in their 31P MAS NMR study of the acidity of zeolite H-Y. When a sample is activated at 400°C, the spectrum is dominated by the resonance due to (CH3)3PH+ complexes formed by chemisorption of the probe molecule on Bronsted acid sites. At least two types of such complexes were detected an immobilized complex coordinated to hydroxyl protons and a highly mobile one, which is desorbed at 300°C. (see Fig. 45)... 

As the chemisorption technique is very convenient, this layer is widely used for optical and optoelectronic devices. Among a number of chemisorption layer techniques, the use of compounds with carboxyl functional group is most prevalent for preparation of the chemisorption layer of probe molecules on the surface of anodic oxidized aluminum. As the probe molecules are arranged on the solid surface directly by using this technique, the chemisorption layer may possess a lower diffusion barrier for oxygen. Thus, highly sensitive devices for PSP can be accomplished by using a chemisorption layer. In this section, the fluorescence probes for PSP based on the chemisorption layer are introduced. 

Enthalpy changes on adsorption and desorption of probe molecules on catalyst surfaces may also be followed by differential thermal analysis (DTA) (67) although this method has been used only sporadically in the past. The experimental techniques have been described by Landau and Molyneux (67) very recently. As an example, Bremer and Steinberg (68) observed three endothermic peaks during the desorption of pyridine from a MgO-Si02 catalyst these peaks were assigned as three different chemisorption states of pyridine. 

Nevertheless, C02 is an extremely valuable probe molecule because the infrared spectra of the chemisorbed species respond very sensitively to their environments. Thus, the frequency separation of the typical band pairs of the carbonate structures may be taken as a measure of the local asymmetry at the chemisorption site. The application of 13C-FT-NMR should be extremely valuable for a still more extensive study of the nature of sites by C02 adsorption. Due to the very detailed information on the structure of sites on oxide surfaces that can be obtained by C02 chemisorption studies, this compound should in some cases also be applicable as a specific poison. A very careful study of the type of interaction with the surface, however, has to be undertaken for each particular system before any conclusive interpretation of poisoning experiments becomes meaningful. 

The aim of specific poisoning is the determination of the chemical nature of catalytically active sites and of their number. The application of the HSAB concept together with eight criteria that a suitable poison should fulfill have been recommended in the present context. On this basis, the chemisorptive behavior of a series of hard poisoning compounds on oxide surfaces has been discussed. Molecules that are usually classified as soft have not been dealt with since hard species should be bound more strongly on oxide surfaces. This selection is due to the very nature of the HSAB concept that allows only qualitative conclusions to be drawn, and it is by no means implied that compounds that have not been considered here may not be used successfully as specific poisons in certain cases. Thus, CO (145, 380-384), NO (242, 381, 385-392, 398), and sulfur-containing molecules (393-398) have been used as probe molecules and as specific poisons in reactions involving only soft reactants and products (32, 364, 368). 

The problem with sulfide catalysts (hydrotreatment) is to determine the active centres, which represent only part of their total surface area. Chemisorption of O2, CO and NO is used, and some attempts concern NIL, pyridine and thiophene. Static volumetric methods or dynamic methods (pulse or frontal mode) may be used, but the techniques do not seem yet reliable, due to the possible modification (oxidation) of the surface or subsurface regions by O2 or NO probe molecules or the kinetics of adsorption. CO might be more promising. Infrared spectroscopy, especially FTIR seems necessary to characterise co-ordinativcly unsaturated sites, which are essential for catalytic activity. CO and NO can also be used to identify the chemical nature of sites (sulfided, partially reduced or reduced sites). For such... 

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). 

The active site on the surface of selective propylene anmioxidation catalyst contains three critical functionalities associated with the specific metal components of the catalyst (37—39) an CC-H abstraction component such as Bi3+, Sb3+, or Te4+ an olefin chemisorption and oxygen or nitrogen insertion component such as Mo6+ or Sb5+ and a redox couple such as Fe2+/Fe3+ or Ce3+/ Ce4+ to enhance transfer of lattice oxygen between the bulk and surface of the catalyst. The surface and solid-state mechanisms of propylene ammoxidation catalysis have been determined using Raman spectroscopy (40,41), neutron diffraction (42—44), x-ray absorption spectroscopy (45,46), x-ray diffraction (47—49), pulse kinetic studies (36), and probe molecule investigations (50). 

The same types of catalysts used for HDS are also used for hydrodenitrogena-tion, and ammonia is used as a probe molecule to characterize these catalysts. An IINS investigation of NH3 on partially desulfided RuS2 (25) showed that the chemisorption was dissociative, forming NH2 groups on the coordinatively unsaturated ruthenium sites. 

Much of our effort involves studies of the chemical behavior of dusters not only as a function of size, but also as a function of metal type, charge state (neutral, cationic or anionic), and reagent molecule. There are two different operating conditions for which we probe the chemisorption of molecules onto clusters as a function of duster size. The first is such that the rate of reaction is kinetically controlled. Here we obtain information about the rate at which the first reagent molecule chemisorbs onto the otherwise bare cluster. In the second case, chemisorption studies are carried out under near steady-state conditions. In this instance we attempt to determine how many molecules a particular size cluster can bind, i.e. the degree of saturation. 

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,. ..)... 

For supported metal catalysts, no simple calculation is possible. A direct measurement of the metal crystallite size or a titration of surface metal atoms is required (see Example 1.3.1). TWo common methods to estimate the size of supported crystallites are transmission electron microscopy and X-ray diffraction line broadening analysis. Transmission electron microscopy is excellent for imaging the crystallites, as illustrated in Figure 5.1.5. However, depending on the contrast difference with the support, very small crystallites may not be detected. X-ray diffraction is usually ineffective for estimating the size of very small particles, smaller than about 2 nm. Perhaps the most common method for measuring the number density of exposed metal atoms is selective chemisorption of a probe molecule like H2, CO, or O2. 

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