Carbon monoxide adsorbed state

Although most of the carbon monoxide adsorbed is rather weakly bound, some is rather strongly bound. Thus, about 20% of that irreversibly held at —78° survives helium flushing at 25° and some residual adsorption persists beyond 200°. However, flushing to the original temperature of activation restores the ehromia to its initial adsorptive state. The fraction held at 25° is relatively independent of the temperature of activation (Fig. 7). 

Several cases involve the adsorption of reactants and products (shielding or blocking effects), but one interesting situation in electrocatalysis is that of the electrooxidation of an organic fuel such as methanol. It involves the formation of a carbon monoxide adsorbed residue that follows a progressive oxidation. We can first state that the reaction occurs without an electrocatalytic influence of the... 

The electrochemical performance of carbon monoxide adsorbed on platinum basal planes has been extensively studied with the help of various physical techniques such as LEED, AES, and in situ infrared spectroscopy of the surface species [68,69]. The common feature of these studies is the use of a non-adsorbing anion electrolyte such as perchloric acid. The problem arises from the coincident potential region for adsorbed carbon monoxide oxidation and desorption that only on carbon monoxide surface saturation, we can neglect the influence of the hydrogen-adsorbed states. 

In a liquid state high-resolution NMR study of adsorption on colloidal palladium, [33] it was found that under 3 atm. of 99% CO, the resonance which would correspond to carbon monoxide adsorbed on the 7.0 nm crystalline palladium colloid stabilized in methanol solution with PVP could not be directly... 

Equation (7.17) to equation (7.19) suggest that for a PtRu alloy catalyst there are two catalyst sites that can be occupied. Carbon monoxide adsorbs onto platinum, while the hydroxide ions adsorb onto ruthenium. Therefore, to understand the rates of reaction mathematically, the coverage of platinum and ruthenium by molecules should be considered. Enback and Lindbergh [64] developed a steady-state model to simulate the reaction kinetics of CO poisoning in the presence of a PtRu/C catalyst. The reaction kinetic parameters were obtained from fitting the model predictions to the experimental measurements and are listed in Table 7.2. The following equation set was used to describe their mathematical model ... 

Figures 17 and 18 show the two predominant binding states of carbon monoxide adsorbed on solid surfaces [15,16]. These are the bridge sites and top sites. Unlike atoms that always occupy the high symmetry sites with three-fold or four-fold coordination, carbon monoxide prefers two-fold or one-fold coordination on most metal surfaces at low coverages. As the coverage of CO is increased, a surface structure with CO in both top and bridge sites, such as that shown in figure 19, may form. In this case, because of repulsive interaction between molecules, the molecules in top sites move sideways to occupy a pseudo hexagonal structure site that is most stable at these high coverages. Here we notice a balance of the adsorbate-substrate and adsorbate-adsorbate interaction that clearly controls the location of molecules on surfaces.

Takasu Y, Matsuda Y, Toyoshima I. 1984. A photoelectron spectroscopic study of the effect of particle-size on the adsorbed state of carbon-monoxide over supported palladium catalysts. Chem Phys Lett 108 384-387. 

Exepriments in the gas phase have supplied us with considerable knowledge on the state of adsorbed carbon monoxide on platinum. 

Figure 7. Steady-state oxygen Auger signal on Pt at 483 K as a function of CO pressure for several fixed O pressures. Triangles enclosed in brackets ((A)) indicate the presence of a detectable concentration of adsorbed carbon monoxide. The fixed oxygen pressures (torr) were 10 8 (QJ,...

For the transfer of redox electrons (inner-sphere electron transfer) in which redox particles are adsorbed on a thin superficial film that covers a metal electrode, the transfer current of redox electrons is not always decreased but rather increased by the presence of the thin film. Such an increase in the reaction ciurent will occur, if the film acts as a reaction catalyst providing the adsorbed state of redox particles favorable for the redox electron transfer. For example, the anodic oxidation of carbon monoxide is catalyzed by the presence of an anodic oxide film on... 

The fact that surface structure, in particular steps and coordinatively unsaturated sites, has an influence on the state and reactivity of carbon monoxide is entirely in keeping with the empirical correlation (Fig. 6) between heat of adsorption, electron binding energies, and molecular state. Elegant studies by Mason, Somorjai, and their colleagues (32, 33) have established that with Pt(lll) surfaces, dissociation occurs at the step sites only, and once these are filled carbon monoxide is adsorbed molecularly (Fig. 7). The implications of the facile dissociation of carbon monoxide by such metals as iron, molybdenum, and tungsten for the conversion of carbon monoxide into hydrocarbons (the Fischer-Tropsch process) have been emphasized and discussed by a number of people (32,34). 

We were interested in the change in the oxidation state of Pd (II), incorporated in the zeolite, during heat treatment in oxygen or in vacuo. Hydrogen and carbon monoxide interactions were also studied. The experiments involved two techniques ESR, which provides direct identification of palladium in an ionic state, and IR spectroscopy, which gives information on the superficial structure of the exchanged zeolite and on the adsorbed species. 

IRES Versus Other Reflection Vibrational Spectroscopies. In order to achieve a sensitivity sufficient to detect absorption due to molecules at submonolayer coverages, some sort of modulation technique is highly desirable. Two candidates for modulation are the wavelength and the polarization state of the incident light. The former has been successfully applied to single crystal studies by Pritchard and co-workers (5j, while the latter is the basis of the Toronto ellipsometric spectrometer and of the technique employed by Bradshaw and coworkers (6) and by Overend and co-workers (7). The two different techniques achieve comparable sensitivities, which for the C-0 stretching mode of adsorbed carbon monoxide amounts to detection of less than 0.01 monolayer. Sensitivity, of course, is very much a function of resolution, scan rate, and surface cleanliness. 

In this chapter, recent results are discussed In which the adsorption of nitric oxide and its Interaction with co-adsorbed carbon monoxide, hydrogen, and Its own dissociation products on the hexagonally close-packed (001) surface of Ru have been characterized using EELS (13,14, 15). The data are interpreted In terms of a site-dependent model for adsorption of molecular NO at 150 K. Competition between co-adsorbed species can be observed directly, and this supports and clarifies the models of adsorption site geometries proposed for the individual adsorbates. Dissociation of one of the molecular states of NO occurs preferentially at temperatures above 150 K, with a coverage-dependent activation barrier. The data are discussed in terms of their relevance to heterogeneous catalytic reduction of NO, and in terms of their relationship to the metal-nitrosyl chemistry of metallic complexes. 

The application of surface-enhanced Raman spectroscopy (SERS) for monitoring redox and other processes at metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)63+/2+ adsorbed electrostatically to chloride-coated silver, and Fe(CN)63 /" bound to gold electrodes the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 

The idea that a metal atom in the zero oxidation state is both a soft acid and soft base can be used to explain surface reactions of metals. Soft bases such as carbon monoxide and olefins are strongly adsorbed on surfaces of the transition metals. Bases containing P, As, Sb, Se, and Te in low oxidation states are strongly adsorbed, blocking the active sites (Pearson, 1966). The clean surfaces are incomplete solids, in that the surface atoms have no nearest neighbors in one of the three-dimensional coordinate system. This means that there are atomic orbitals, both filled and empty, which are not being used to form surface orbitals. 

We may conclude that the chemisorption of carbon monoxide on gold surfaces is generally somewhat weak it cannot for example lift the reconstruction of the Au(110)(l x 2) surface to restore the normal (1 x 1) phase.61 It prefers atoms of low CN and it desorbs below room temperature. Nevertheless, under the appropriate conditions considerable information can be obtained about its adsorbed state. 

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