Probing Surface Redox Properties

The heats of reduction of oxide samples can be determined by studying the adsorption of hydrogen, CO and various hydrocarbons on the fully oxidized catalysts [27, 59]. The extent of reduction of the catalyst surface can be evaluated in particular using H2. The measurement of hydrocarbon (e.g. propene, propane, acrolein, etc.) adsorption heats is complicated by the subsequent reaction of the adsorbed species or by incomplete desorption of the products [60]. 

In the case of CO reduction, the catalyst-oxygen bond energy has to be calculated by subtracting the heat of formation of CO2. 

However, it is known that, in the absence of processes other than plain surface coordination, CO acts as a weak Lewis base and can interact with the strongest surface Lewis acid sites. NO can also be employed either as a probe to identify Lewis acid sites or as a reducing agent. However, NO may disproportionate into N2O and oxygen and it is also very likely to form nitrosyl complexes in the presence of transition metal ions [60]. 

The heats of oxidation of the reduced oxides can be further measured using O2 adsorption. Large variations of the reoxidation heat can be sometimes observed when any further oxidation is limited by the diffusion of oxygen into the reduced portion of the particle [61]. 

Formosinho, H. Burrows, Chemical Kinetics From Molecular Structure to Chemical Reactivity (Elsevier, Amsterdam, 2007) 

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Capacitive currents are generated by events occnrring within very small distances from an electrode surface and their magnitude is controlled by the microscopic area of the electrode. In most electrochemical experiments designed to probe the redox properties of solution phase reactants, the timescale of the measurement is often snch that the diffnsion layer is several times larger than the critical dimension (e.g., radins) of the microelectrode. For example, the depletion layer thickness, (5, can be estimated as ... 

In addition to the acidic and basic properties mentioned previously, oxides and halides can possess redox properties. This is particularly true for solids containing transition metal ions because the interactions with probe molecules such as CO, H2, and O2 can lead to electron transfer from the surface to the adsorbed species and to the modification of the valence state of the metal centers. An important role in surface redox processes involving CO is played by the most reactive oxygen ions on the surface (e.g., those located at the most exposed positions such as corners), which can react with CO as follows ... 

Section 5.3 considered NMR spectroscopic approaches to the bulk characterization of oxides and oxidation catalysts. Gatalytic activity is, however, intrinsically linked with the nature of the catalyst surface and hence a number of techniques have been developed in order to probe this. As discussed in Section 5.1, two of the most significant parameters impacting on catalyst activity are the acid-base characteristics of a surface and the redox properties of the material, and NMR techniques exist to probe both of these characteristics. One of the most common techniques to probe surface structure is GP-MAS NMR, in particular CP from hydrogen to the nucleus of interest-either the metal or the oxygen of the metal oxide. Historically, the source of surface H species has often been those naturally present on the catalyst surface, as chemisorbed hydroxyls or physisorbed water. As such, much of the work in this area involves the study of supports such as Si02. Applications of CP-MAS and other spectroscopic approaches to the study of oxide surfaces are outlined in the following sections. 

Formic acid is a popular molecule for probing the catalytic properties of metal oxides [23-28], The selectivity of its decomposition has frequently been used as a measure of the acid-base properties of oxides. This is a tempting generalization to make oxides that produce dehydration products (H2O and CO) are described as acidic oxides, while their basic counterparts produce dehydrogenation products (H2 + CO2). It has been shown that in many cases the product selectivity is better connected to the surface redox behavior of the oxide [29], Thus, more reducible surfaces produce higher yields of CO2, Consequently, particular attention has been paid in surface science studies to the interaction between adsorbed formate ions (the primary reaction intermediate) and surface metal cations, as well as to the participation of lattice oxygen anions in the surface reaction mechanism,... 

The objective of this work was to compare the catalytic properties of Pt-Re/Al203-Cl catalysts prepared either by the surface redox reaction or by classical coimpregnation. According to Augustine and Sachtler [15-16], the interactions between platinum and rhenium were probed by using hydrogenolysis of cyclopentane as a test reaction. 

The heat of adsorption measured e erimentally depends upon the imposed conditions, such as the temperature of the adsorption which means the temperature of the calorimeter, the sample pretreatment, the diffusion problem and the quality of the probe, among others. The probe will depend on what has to be determined the acid-base character of the catalyst, the redox properties or the surface distribution of metal particles on a supported catalyst... 

Temperature-prograimned reduction, oxidation and desorption (TPR, TPO, TPD), belong probably to the most widely used in situ techiuques for the characterization of oxidation catalysts and are discussed in more detail in Section 19.4. While TPD (with ammonia as the probe molecule) is frequently used to examine surface acid sites, TPR and TPO (with H2 or O2, respectively) provide information on the redox properties of oxide catalysts being crucial for their performance in catalytic oxidation reactions. Important information on reaction mechanisms can be obtained when the catalysts are heated in the presence of reactants combined with mass spectrometric product analysis. This is called temperature-programmed reaction spectroscopy (TPRS). As far as reaction mechanisms and kinetics are concerned, transient techniques which reflect the response of the catalytic system to a sudden change of reactant are inevitable tools. Two such techiuques, namely the temporal analysis of products (TAP) reactor and steady-state isotopic transient kinetic analysis (SSITKA) will be described in more detail in Section 19.5. 

Organometallic probes of surfaces. So far the chemical means to characterize surfaces at a molecular level have been based mainly on the spectroscopic methods used with molecular probes, including CO, NO, O2, C02 and organic bases. These molecules give information concerning the acid-base and redox properties of the surfaces of oxides and sulfides. They also probe the coordination properties of ensembles of metal atoms at the surfaces of monometallic or bimetallic particles. 

The chemisorption of an organometallic molecule at the surface of an oxide is a new tool to chemically characterize the reactivity of the functional groups of the surface towards the metal and its ligands. These probe molecules allow evaluation, for example, of the acid-base (nucleophilic) character of the 0 or OH groups, the electrophilic character of cations or surface OH groups, the redox properties of the surface, etc. 

Very often, the electrode-solution interface can be represented by an equivalent circuit, as shown in Fig. 5.10, where Rs denotes the ohmic resistance of the electrolyte solution, Cdl, the double layer capacitance, Rct the charge (or electron) transfer resistance that exists if a redox probe is present in the electrolyte solution, and Zw the Warburg impedance arising from the diffusion of redox probe ions from the bulk electrolyte to the electrode interface. Note that both Rs and Zw represent bulk properties and are not expected to be affected by an immunocomplex structure on an electrode surface. On the other hand, Cdl and Rct depend on the dielectric and insulating properties of the electrode-electrolyte solution interface. For example, for an electrode surface immobilized with an immunocomplex, the double layer capacitance would consist of a constant capacitance of the bare electrode (Cbare) and a variable capacitance arising from the immunocomplex structure (Cimmun), expressed as in Eq. (4). 

CV investigations of 6-mercaptopurine and 8-mercaptoquinoline SAMs on pc-Au electrodes have been presented by Madueno et al. [186] and He etal. [187], respectively. Several model electrode reactions involving various redox probes were studied using such modified electrodes. Baunach and Kolb etal. [188] have deposited copper on disordered benzyl mercaptan film on Au(lll) surfaces. They have also studied the behavior of benzyl mercaptan SAM on Au(lll) in H2SO4 solution using CV and STM. Structural and electrical properties of SAMs based on tetrathiafulvalene derivatives on Au(lll) were investigated. These mono-layers were disordered, or at least loosely... 

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