Chemisorption, on supported metals

A large variety of problems related to the nature of the adsorption processes have been studied by infrared spectroscopy. The most extensive and productive application of this method has been in studies of chemisorption on supported-metal samples. Spectra of physically adsorbed molecules have provided important information on the interaction of these molecules with the surface of the adsorbent. Experimental developments have reached a state where it is evident that the infrared techniques are adaptable to practically all types of samples which are of interest to catalytic chemists. Not only are the infrared techniques applicable to studies of chemisorption and physical adsorption systems but they add depth and preciseness to the definitions of these terms. 

II. Chemisorption on Supported-Metal Samples 1. Experimental Considerations... 

Chemisorption on supported metals is necessarily more complex than on massive metals there are a greater diversity of types of gold atom at the surface of particles, because in addition to those found on stepped or kinked surfaces, some are close to the support surface and may be influenced by it, and there is the strong possibility of adsorption on the support as well. At this stage we are not concerned, except incidentally, with the co-adsorption of carbon monoxide and oxygen, since this is intimately connected with their reaction, and this is discussed later (Chapter 6). Figure 5.7 shows diagrammatically the various locations for carbon monoxide molecules on Au/TiC>2 for which there is some spectroscopic evidence. 

Figure 3.16. Hydrogen chemisorption on supported metals plots of H/Mtot versus (12-N) where N is the first-Hshell coordination numb determined by EXAFS (a) Pt/Al203 (b) RI1/AI2O3 (c) Ir/Si02 and Ir/Al203 (d) curve for estimating dispersion D from (12-JV) (see text).

The articles by J. R. Anderson, J. H. Sinfelt, and R. B. Moyes and P. B. Wells, on the other hand, deal with a classical field, namely hydrocarbons on metals. The pattern of modem wTork here still very much reflects the important role in the academic studies of deuterium exchange reactions and the mechanisms advanced by pioneers like Horiuti and Polanyi, the Farkas brothers, Rideal, Tw igg, H. S. Taylor, and Turkevich. Using this method, Anderson takes ultrathin metal films with their separated crystallites as idealized models for supported metal catalysts. Sinfelt is concerned with hydrogcnolysis on supported metals and relates the activity to the percentage d character of the metallic bond. Moyes and Wells deal with the modes of chemisorption of benzene, drawing on the results of physical techniques and the ideas of the organometallic chemists in their discussions. 

There is a wealth of information available on CO chemisorption over single-crystal and polycrystalline platinum surfaces under ultrahigh-vacuum conditions research efforts in this area have gained a significant momentum with the advent of various surface analysis techniques (e.g., 2-8). In contrast, CO chemisorption on supported platinum catalysts (e.g., 9, 10, 11) is less well understood, due primarily to the inapplicability of most surface-sensitive techniques and to the difficulties involved in characterizing supported metal surfaces. In particular, the effects of transport resistances on the rates of adsorption and desorption over supported catalysts have rarely been studied. 

In all of the spectral studies of gases chemisorbed on supported metals, a background spectrum has been recorded prior to the chemisorption of the gas. The spectrum of the chemisorbed gas is then obtained by subtracting out the background by means of an automatic per cent transmission recorder. The background contains bands due to the surface hydroxyls and Si-0 or Al-0 bands of the carrier material. It is necessary that both the... 

Relatively few studies have focused on influence of the acid/base properties of the support on the chemisorption of reactants on supported metal clusters. A NMR study by Tong et al.23 showed that the stretching frequency of CO chemisorbed on zeolite supported Pt particles correlates with the surface local density of states (LDOS) of the Pt. The LDOS also showed a correlation with the faujasite framework acidity, but an explanation of this correlation is lacking. Several infrared studies on similar supported Pt catalysts show that the mode of CO... 

FiC5. 14. NMR spectra of Ru/SiOi. The initial number of silanol protons has been reduced by exchange with deuterium. Both traces are difference spectra with respect to the state after initial evacuation. The continuous line represents a sample under 20 Torr of H, gas, and the dashed line represents a sample after pumping away the reversible hydrogen. There is both reversible and irreversible spillover to the support (signal at. 3 ppm), and ther e is rever sible and irreversible chemisorption on the metal (sigiral at 65 ppm). [Reproduced with permission from Uner et al. (47). ... 

Selective chemisorptions of hydrogen and carbon monoxide have also been used to determine the surface area of other Group VIII metals, especially by Yates et al. (6). Development of methods applicable to other metals is only a matter of ingenuity and from now on every investigation of catalysis on supported metals must include a determination of the surface area of the metal. 

A fast as well as a slow component was observed in the room temperature adsorption of H2 and 02 on Rh on rutile and anatase Ti02 catalysts. Both components of the H2 adsorption decreased in magnitude with increasing catalyst reduction temperature, while the fast 02 adsorption increased in magnitude and the slow 02 adsorption stayed constant. The fast H2 adsorption proved to be due to chemisorption on the metal and the slow H2 adsorption to spillover from the metal to the support. The fast 02 adsorption was due to chemisorption on the metal as well as on the support, with the concurrent reoxidation of Ti ions formed during H-spillover. The slow 02 adsorption was caused by corrosive chemisorption of the metal particles. 

The reducibility of Ti02 has in recent years been studied with ESR and NMR techniques (21-23). In the course of our studies (21) we noticed that support reduction is a relatively slow process and that, as a consequence, hydrogen chemisorption on a metal on Ti(>2 catalyst has a fast component, due to adsorption on the metal surface, and a slow component, due to spillover of H atoms from metal to support and subsequent support reduction. We have studied the time dependence of H2 chemisorption, as well as that of (>2 chemisorption, in more detail and the results of this study are presented In this paper. 

H2 chemisorption. Both Rh/R-Ti02 and Rh/A-Ti02 show a decrease in H2 chemisorption when the reduction and evacuation temperature is increased, while at the same time the slope of the chemisorption vs. In t curve decreases. The decrease in H2 chemisorption is of course due to the gradual transition of the Rh particles into the SMSI state. Whatever the explanation for this state, an electronic interaction between metal particles and support or a covering of the metal particles by the support, in this SMSI state the metal particles are unable to adsorb H2. The decreased slope of the H/Rh-ln t curve can be explained in several ways, such as slow H2 chemisorption on Rh because of an activated process, dependence on metal dispersion, or an effect related to the support. The experiments in which H2 chemisorption was started around 200°C proved that the time dependence is indeed due to a slow adsorption at room temperature, but the experiment with Rh/Si(>2 showed that there is no kinetic limitation in the H2 chemisorption on the metal part of the catalyst. In accordance with this conclusion, no effect of rhodium dispersion on the time dependence of the H2 chemisorption was observed for catalysts in the normal state (cf. Figure 1 curves A, B and F). 

H2 chemisorption on noble metals (NM)/Ce02 has been extensively studied IS) and special attention was generally given to the so called strong metal-support interaction (SMSI), e.g. the suppression of H2 and CO chemisorption after a high temperature (usually 773 K)... 

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. 

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