Metal oxide surface, dissociative chemisorption

Another example of the interaction of water with a relatively simple metal oxide surface is provided by the water vapor/a-Al203(0001) system (Figure 7.9(a)). Oxygen Is synchrotron radiation photoemission results indicate that significant dissociative chemisorption of water molecules does not occur below 1 torr p(H20) [149]. However, following exposure of the alumina (0001) surface to water vapor above this threshold p(H20) , a low kinetic energy feature in the Is spectrum grows quickly,... 

Anodier example of dissociative chemisorption is the heterolytic cleavage of hydrogen on metal oxide surfaces. The reaction of hydrogen wifli a zinc oxide surface produces a zinc-hydride bond and a proton boimd to an oxygen center (Eq. 5-25) [T3 9]. 

The relative ease with which hydrogen chemisorbs on the surface of a metal oxide surface mainly depends on the chemical nature of the oxide and on the O-vacancies. Thus, hydrogen adsorbs dissociatively on a perfect titanium oxide surface [10,11]. The energetically most favorable mode for the adsorption of atomic hydrogen is the adsorption on the outermost O atom, accompanied by the reduction of a Ti atom. In this mode, protons are formally adsorbed while an equivalent amount of Ti(IV) atoms are reduced to Ti(III). Theoretical calculations have demonstrated that H adsorption is less favorable on a defective surface than on a perfect surface. However, the best adsorption mode for the atomic chemisorption on a defective surface is heterolytic adsorption, which involves two different adsorption sites one H+/0= and one H on the surface. This adsorption mode is best on irreducible oxides such as MgO however, it is less favorable than adsorption on the perfect Ti02 surface [10]. The heat of atomic adsorption in all cases is very weak and dissociation onto the surface is unlikely. The molecular adsorption (physisorption), thus, remains the most stable system. 

Schematic representation of the cross section of the surface layer of a metal oxide. , Metal ions O, oxide ions. The metal ions in the surface layer (a) have a reduced coordination number. They thus behave as Lewis acids. In the presence of water the surface metal ions may first tend to coordinate H20 molecules (b). For most of the oxides dissociative chemisorption of water molecules (c) seems energetically favored.

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

Motivated by the application to catalytic oxidation processes [1], much research has been conducted to determine and elucidate the mechanism(s) of the dissociative chemisorption of 02 on metal surfaces, in particular, Pt single crystal surfaces. 

The electron affinities of clusters behave in a similar manner. This fact, undoubtedly, has a role to play in the chemistry exhibited by nanometals that has been reported in the literature recently. For example, it has been shown that Au atoms (Gold is a noble metal in the bulk state) supported on a TiOa substrate shows a marked size effect in their ability to oxidize the diatomic gas CO to CO2 via a mechanism involving O2 dissociative chemisorption and CO adsorption (Valden et al., 1998). Small Ni particles have also been found to dissociate CO (Doering et al., 1982). Smaller nanoparticles of Ag can dissociate molecular oxygen to atomic oxygen at low temperatures, whereas in the bulk state, the species adsorbed on the Ag surface is O2 (Rao et al., 1992). 

The presence of solution can dramatically affect dissociative chemisorption. In the vapor phase, most metal-catalyzed reactions are homolyticlike, whereby the intermediates that form are stabilized by interactions with the surface. Protic solvents, on the other hand, can more effectively stabilize charge-separated states and therefore aid in heterolytic activation routes. Heterolytic paths can lead to the formation of surface anions and cations that migrate into solution. This is directly relevant to methanol oxidation over PtRu in the methanol fuel cell. The metal-catalyzed route in the vapor phase would involve the dissociation of methanol into methoxy or hydroxy methyl and hydrogen surface intermediates. Subsequent dehydrogenation eventually leads to formation of CO and hydrogen. In the presence of an aqueous media, however, methanol will more likely decompose heterolytically into hydroxy methyl (—1) and intermediates. 

Selective oxidation of CO in hydrogen over different catalysts has been extensively examined. Most research to date has occurred with formulations that include a precious metal component supported on an alumina carrier. The catalyst-mediated oxidation of CO is a multistage process, commonly obeying Langmuir-Hinshelwood kinetics for a single-site competitive mechanism between CO and 02. Initially, CO is chemisorbed on a PGM surface site, while an 02 molecule undergoes dissociative chemisorption either on an adjacent site or on the support in order for surface reaction between chemisorbed CO and O atoms to produce C02. 

Illustration of the surface of a metal oxide where small circles are the metal atoms and large circles are oxygen atoms. (A) Surface metal atoms are not totally coordinated. (B) In water, surface metal ions coordinate water molecules. (C) Dissociative chemisorption leads to a hydroxylated surface. From Schindler and Stumm (1987). 

In the adsorbed phase, in which the particles are mobile because of surface diffusion, the chemical reaction of the metal-oxide formation takes place via dissociative chemisorption of the oxygen. This means that, since the reaction takes place in the adsorption phase and the various reaction partners can have different condensation coefficients, the ratio of the rates of incidence is a necessary but not adequate criterion. According to Ritter [293, 298], the chemisorption rate of oxygen is the critical step for the completion of the reaction ... 

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