Adsorption of probe molecules

The adsorption of probe molecules followed by different techniques allows one to prove the metal-metal interaction for catalysts prepared by redox reactions. For example, by chemisorption measurements, a decrease in the total amount of adsorbed H2 was observed with an increasing germanium content introduced by catalytic reduction on parent rhodium catalysts [81]. As TEM characterization showed a comparable mean particle size for all the catalysts, such evolution suggests that Ge covers the Rh surface [41]. These results were consistent with those 

450°C) and then reduction (hydrogen, 8h, 500°C). Reprinted from C. L Pieck, P. Marecot,). Barbier, Appl. Catal. A, 1996, 143, 283-298, Copyright (1996), with permission from Elsevier. 

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Hydrogen adsorption from solution Oxygen adsorption from solution Underpotential deposition of metals Adsorption of probe molecules from solution ... 

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

The FT-IR measurements were carried out in a Nicolet IRTF spectrometer. Samples (self-supported discs) were activated in situ under vacuum (10 6 mbar) at 450°C before adsorption of probe molecules. Adsorption was carried out at room temperature the physisorbed species were removed at RT (C02) and 150°C (2,6-dimethylpyridine) under vacuum. 

Adsorptions of probe molecules followed by Infrared were also carried out in order to estimating the acidity and the basicity of aluminas. Carbon dioxide and 2,6-dimethylpyridine were respectively used for the basicity and the acidity. Figure 3 reports the results obtained. 

Acidity of both zeolites was investigated by adsorption of ammonia, pyridine, d3-acetonitrile and pivalonitrile ((CH3)3CCN) used as probe molecules followed by FTIR spectroscopy. All samples were activated in a form of self-supporting wafers at 450 °C or 550 °C under vacuum for 1 h prior to the adsorption of probe molecules. 

Note that in all the examples discussed so far, infrared spectroscopy gives its information on the catalyst in an indirect way, via hydroxyl groups on the support, or via the adsorption of probe molecules such as CO and NO. The reason why it is often difficult to measure the metal-oxide or metal-sulfide vibrations of the catalytically active phase in transmission infrared spectroscopy is that the frequencies are well below 1000 cm-1, where measurements are difficult because of absorption by the support. Infrared emission and Raman spectroscopy, discussed later on in this chapter, offer better opportunities in this respect. 

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

Physicochemical methods, i.e. adsorption of probe molecules followed by varied analytical techniques (gravimetry, chromatography, calorimetry, spectroscopic techniques, etc.) are currently used for estimating more precisely the concentration of the potential active sites.[34 36] However, very few methods are well adapted for this purpose most of the methods employed for the characterization of the acidity of solid catalysts lead to values of the total concentrations of the acid sites (Brpnsted + Lewis) and to relative data on their strength, whereas few of them discriminate between Lewis and Brpnsted acid sites. It is however the case for base adsorption (often pyridine) followed by IR spectroscopy, from which the concentrations of Brpnsted and Lewis sites can be estimated from the absorbance of IR bands specific for adsorbed molecules on Brpnsted or Lewis sites. 

Many techniques have been developed to study coking, both chemical (adsorption of probe molecules having different sizes (2), solvent extraction (3)) and physical (X-ray diffraction (4), electron microscopy (5), IR (6), NMR (7)). Among these techniques 129-Xe NMR applied to microporous systems such as zeolites (8), coupled with HRADS techniques makes it possible to clarify the location of the coke and the blocking of the zeolite micropores. 

Acidity and basicity are relative properties. Many compounds are amphoteric and behave as acids or as bases according to a partner. Metal oxides are classified as acidic, amphoteric or basic. Experimentally, this classification corresponds to the adsorption of probe molecules[7, 8]. NH3 is a base probe molecule that reacts with the electron deficient metal atoms (Lewis acid) or the protons adsorbed on the hydrated surface, CO2 is usually considered as acidic and thus it is expected to adsorb more strongly on basic sites. According to this classification, Ti02 belongs to an amphoteric species and MgO to a basic species. A general difficulty for such classifications is that the order can vary with the choice of the probe. The Hard and Soft Bases and Acids theory[9, 10] responds to the necessity to refine the model with a second scale it is better to couple... 

The PL spectra were actually very sensitive to the overall surface structure and this allowed the study of the behavior of each type of luminescence center upon thermal treatment or adsorption of probe molecules. These studies have also shown that, by the way of an energy-transfer process, emission can arise from surface sites that are not necessarily those that absorbed light in the first step of the PL phenomenon. For instance, at 300 K, the energy absorbed by 5- and 4-coordinated sites is transferred to the 3-coordinated ones, whilst at 77 K, the energy-transfer process is largely suppressed and the original emission profiles of 4- and 5-coordinated centers can be observed [41]. 

The adsorption of probe molecules may provide evidence of the existence of metal-oxygen bonds at the surface of a metal oxide. In Figure 3.14, the subtraction... 

Heats of adsorption of probe molecules have frequently been measured at room temperature [8] however, the results obtained from such measurements can sometimes be of questionable accuracy as a result of non-equilibrium conditions and non-specific adsorption. 

Instead of TPD, microcalorimetry of adsorption shows the heat evolved during the adsorption of probe molecules, usually ammonia, on acid sites [91-94]. This measurement can determine the distribution of adsorption enthalpies but cannot differentiate between adsorption on Lewis and Br0nsted acid sites. 

A munber of studies have been addressed to define the location and nature of the active species [2,5], Likewise, the assessment of textural and chemical properties induced by the exchanged cations becomes crucial to the understanding of the applicability of these materials in a given process. A large variety of methods can be used to evaluate these surface properties. Among them, N2 adsorption, IR spectroscopy and adsorption of probe molecules are widely used. 

A principal motivation for the study of probe molecule adsorption on catalyst surfaces is to develop correlations of catalyst surface properties with catalytic behavior. In addition, measurements of the heats of adsorption of probe molecules can provide essential information about reaction mechanisms if the probe molecules are chosen to resemble possible reaction intermediates of the catalytic cycle. 

To characterize the model catalysts in detail it is, however, necessary to identify the adsorption sites of the clusters. One possibility is to study the adsorption of probe molecules on the deposited clusters by means of thermal desorption (TDS)... 

The structures of typical aluminophosphates, namely A1P04-11[6] and VPl-5[7] were generated from the reported crystal structures, adsorption of probe molecules such as water and ammonia, as well as the template organic molecules and larger molecules were... 

Dispersive and specific interactions are considered to contribute independently to the adsorption of probe molecules at the adsorbent surface. It was presented that the adhesion of the fibre-matrix interface depends clearly on the measured strength of acid/base interactions of both fibre and polymer-matrix. Fowkes [2,3] indicated also that the surface of fillers can be chemically modified to enhance acid-base interaction and increase adsorption. 

Figure 4. Specific Adsorption of Probe Molecules on Titania as a Function of Carbon Content...

Figure 6a Specific Adsorption of Probe Molecules on TiC>2 Type Ila IOM-CMS Materials versus Wt% Ti02...

Adsorption of probe molecules 13-0-04 AIPO4 deNOx catalysts 30-P-31... 

It has been well-known for a long time that besides their cation sensitivity, far-infrared spectra of microporous materials can imdergo remarkable changes upon adsorption of probe molecules [366]. Disregarding the far-infrared normal modes of the adsorbed molecules themselves, the expectations with respect to this spectral region are twofold. 

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