Selective chemisorptions

2 Metal Dispersion by Chemisorption and Titration Selective Chemisorption. - This is the most frequently used technique for determining the metal area in a supported catalyst and depends on finding conditions under which the gas will chemisorb to monolayer coverage on the metal but to a negligible extent on the support. Various experimental methods, conditions, and adsorbates have been tried and studies made of catalyst pre-treatment and adsorption stoicheiometry, viz, the (surface metal atom)/(gas adsorbate) ratio, written here as Pts/H, Bh jQO,etc., and reviews to about 1975 are available. A summary is given in Table IV of ref. 2 of methods used to confirm the various adsorption stoicheiometries proposed, sometimes from infrared studies. These include chemisorption on metal powders of known BET area or, more satisfactorily, one of the instrumental methods reviewed in Section 3 for the determination of crystallite size distributions. For many purposes, a relative measurement of metal dispersion is sufficient, conveniently expressed as the ratio (number of atoms or molecules adsorbed)/(totfl/ number of metal atoms in the catalyst), e.g., H/Ptt. 

The application of selective chemisorption to supported Pt catalysts is well established but there have been valuable additional studies of the use of hydrogen in the pulse-flow technique and of CO adsorption using TPD and carbon monoxide. Recently the usual assumption about the stoicheiometry for hydrogen adsorption, Ptg/H = 1 has been questioned. For the Council of Europe Pt-Si02 catalyst, where a weak metal-support interaction was postulated, 1.75 hydrogen atoms per surface metal atom were found at 300 K in two adsorbed forms (the formation of jSa was activated). Recent work on selective chemisorption applied to metals of catalytic interest other than platinum will now be examined. 

---

As this field is very wide, we will discuss first the gases that can be used to study metal dispersion by selective chemisorption, and then some specific examples of their application. The choice of gases, is, of course, restricted to those that will strongly chemisorb on the metal, but will not physically adsorb on the support. Prior to determining the chemisorption isotherm, the metal must be reduced in flowing hydrogen details are given elsewhere. The isotherm measurement is identical to that used in physical adsorption. 

Both of the surface area techniques described in this article are well established. However, the determination of total surface area by physical adsorption using the BET equation is a very general method of wide applicability. The use of selective chemisorption to determine the surf e area of metals is much newer, and has only... 

The SMSI effect in Mn-promoted Ru/Ti02 catalysts was studied in more detail making use of the SSIMS technique, as well as with TEM, and selective chemisorption experiments. The SSIMS technique revealed the presence of TiO c forming two new surface sites, TiO -Ru and TiO-Mn. These species were found to be located at the immediate vicinity of the Ru nanoparticles. These new surface sites were considered to alter the electronic properties of the Ru metal surface and, as a consequence, the product selectivity. 

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. 

It should be realized that selective chemisorption titrates only atoms on the surface, but that the surface composition can be changed upon adsorption (75). A difficulty in the interpretation of chemisorptive titration is the possible presence of electronic effects, which can alter the chemical nature of the titrated atoms. Adsorbates can also show strong interactions with each other on pure metals, and decreases in this interaction can cause an increase in the number of molecules adsorbed per active metal atom exposed. The surface composition of active metal atoms deduced from these measurements will then be too high. 

It is obvious that chemisorptive titration is most ideally applied under such conditions of pressure and temperature that only adsorbates bonded to one surface atom exist. If, in addition, there are surface species bonded to several surface atoms, selective chemisorption applies if... 

Experimental evidence of surface enrichment in Pt3Sn stems from AES (//) and selective chemisorption (14). Both techniques indicate surface enrichment of tin, the element with the lower heat of sublimation. Table I shows that AES yields lower values of surface enrichment than surface titration. This is not surprising because AES scans not only the atoms of the surface layer, but also those of lower-lying layers. However, a quantitative comparison of AES and chemisorption data (14) shows that the results can only be matched if enrichment occurs by inversion of the outer layers, i.e., if depletion of tin atoms occurs in layers next to the outer layer enriched in tin atoms. 

Procedures aimed at reducing or detoxifying aflatoxins and/or their effects have been reviewed by Phillips et al. (35), and include technological procedures for food and feeds and chemical degradation, as well as biocontrol and microbial inactivation, dietary modification and chemoprotection, and reduction in toxin bioavailability via selective chemisorption with clay. 

A notable exception are chemisorbed complexes in zeolites, which have been characterized both structurally and spectroscopically, and for which the interpretation of electronic spectra has met with a considerable success. The reason for the former is the well-defined, although complex, structure of the zeolite framework in which the cations are distributed among a few types of available sites the fortunate circumstance of the latter is that the interaction between the cations, which act as selective chemisorption centers, and the zeolite framework is primarily only electrostatic. The theory that applies for this case is the ligand field theory of the ion-molecule complexes usually placed in trigonal fields of the zeolite cation sites (29). Quantum mechanical exchange interactions with the zeolite framework are justifiably neglected except for very small effects in resonance energy transfer (J30). ... 

There has been considerable interest in the development and application of phyllosilicate clays (HSCAs) to selectively chemisorb aflatoxins in aqueous suspensions, including milk, reduce the uptake of aflatoxin by blood and its distribution to body organs such as the liver, reduce the transmission of aflatoxin Mi to milk in lactating animals and to decrease the toxic effect of aflatoxin to many animal species (Phillips et al., 1995). It is believed that the HSCAs when added to feeds act by the selective chemisorption of the aflatoxin in the gastrointestinal tract of the animal resulting in a marked reduction in the bioavailability of the aflatoxins. 

At the higher metal level (2.0-4.5% Ni with up to 2% Sb) used to study artificially contaminated materials, XRD results have shown the formation of Ni-Sb alloys (NiSb x<0.08) whereas XPS data have indicated that a non-reducible antimony oxide, a well dispersed reducible Sb phase together with reducible Sb (that form an alloy with reducible Ni), were present. Selective chemisorption data for unsupported Ni-powders showed that one surface structure can effectively passivate 2-3 Ni atoms with respect to H2 chemisorption. XPS examination confirmed that Sb segregates at the surface of Ni particles where it can drastically affect the electron properties of neighboring Ni atoms thus reducing their activity. 

The surface of the catalyst is, therefore, practically covered with acetylene only, and when a mixture of the two gases is hydrogenated it is only acetylene that is converted to ethylene until practically all acetylene has disappeared. The selective hydrogenation of nonsubstituted and substituted acetylene and ethylene mixtures may be ascribed to the selective chemisorption of the gases (391). 

Carbon may also be used as a catalyst without an active component supported on it. This function may overlap with the widespread application of high surface area carbons and sorbents for selective chemisorption processes which are not considered here. The application of carbon as a catalyst in its own right has been reviewed [211-213], All applications are based on the... 

Selective chemisorption of H2 is probably the most widely practised means of assessing dispersion, particle... 

The chemical composition can be measured by traditional wet and instrumental methods of analysis. Physical surface area is measured using the N2 adsorption method at liquid nitrogen temperature (BET method). Pore size is measured by Hg porosimetry for pores with diameters larger than about 3.0 nm (30 A) or for smaller pores by N2 adsorp-tion/desorption. Active catalytic surface area is measured by selective chemisorption techniques or by x-ray diffraction (XRD) line broadening. The morphology of the carrier is viewed by electron microscopy or its crystal structure by XRD. The active component can also be measured by XRD but there are certain limitations once its particle size is smaller than about 3.5 nm (35 A). For small crystallites transmission electron microscopy (TEM) is most often used. The location of active components or poisons within the catalyst is determined by electron microprobe. Surface contamination is observed directly by x-ray photoelectron spectroscopy (XPS). 

John Sinfelt feels "that the most generally valuable tool for a long time has been the measurement of adsorption isotherms, including the BET method for determination of total surface area and various selective chemisorption methods for determining the amount of surface associated with particular component. The BET method has been widely used by catalytic chemists for almost half a century for the characterization of catalytic materials. It is among the foremost developments in surface science during the twentieth century". 

Selective chemisorption methods have been used with success for the determination of metal surface area and particle size in supported catalysts, and for titration of acid sites on silica-alumina and zeolite catalysts. The chemisorption methods are sometimes neglected in the quest for a more physical description of the catalyst surface, possibly with the penalty of missing an important and quantitative piece of information about the catalyst surface. 

C-H Bond Activation. We have also examined the chemisorption of various hydrocarbons on different transition metal clusters. In this section we describe results obtained for methane activation on neutral clusters. First, we note that under our experimental conditions (low pressure, near room temperature, short contact time) methane activation readily occurs only on specific type and size metal clusters. For instance, we detect no evidence that methane reacts with iron(12), rhodium(26) or aluminum(2Z) clusters, whereas as shown in figure 5 strong size selective chemisorption is... 

Beckler, R. K. and M. G. White, Polynuclear Metal Complexes as Model Mixed Oxide Catalysts Selective Chemisorption of NH3 and NO , J. Catal, 109, pp. 25-36 (1988) Beckler, R. K. and M. G. White, Polynuclear Metal Complexes as Model Mixed Oxide Catalysts Isomerization Activity , J. Catal, 110, pp. 364-374 (1988). Coulier, L., V. G. Kishan, J. A. R. van Veen, and J W. Niemantsverdriet, Surface science models for CoMo Hydrodesulfurization Catalysts the Influence of the support on hydrodesulfurization acidity , J. Vac Scl Technol A. 19, Issue 4, 1 July/August 2001, pp 1510-5. 

Kenvin, J. C. and M. G. White, Selective Chemisorption and Oxidation Reduction Kinetics of Supported CuO Prepared from Copper Acetylacetonates on Cab-O-... 

---