Adsorption structure sensitivity

D. Uner, M. Uner, Adsorption calorimetry in supported catalyst characterization adsorption structure sensitivity on Pt/y-Al203. Thermochim. Acta 434, 107-112 (2005)... 

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

Clavilier J, Feliu JM, Aldaz A. 1988. An irreversible structure sensitive adsorption step in bismuth underpotential deposition at platinum electrodes. J Electroanal Chem 243 419-433. 

Hammer B, Nielsen OH, Nprskov JK. 1997. Structure sensitivity in adsorption CO interaction with stepped and reconstructed Pt surfaces. Catal Lett 46 31. 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

Both the data on hydrogen adsorption and formic acid oxidation show pronounced structural sensitivity, thus confirming a paramount role of surface structure in electrocatalytic reactions. It can be concluded that each crystallographic orientation represents a distinct electrochemical (chemical) entity. The investigation of stepped surfaces seems to be necessary to reach an understanding of these systems on a molecular level. Hydrogen adsorption shows dependences on the terrace orientation, step orientation, and step density. All the... 

Asymmetric diarylmethanes, hydrogenolytic behaviors, 29 229-270, 247-252 catalytic hydrogenolysis, 29 243-258 kinetics and scheme, 29 252-258 M0O3-AI2O3 catalyst, 29 259-269 relative reactivity, 29 255-257 schematic model, 29 254 Asymmetric hydrogenations, 42 490-491 Asymmetric synthesis, 25 82, 83 examples of, 25 82 Asymmetry factor, 42 123-124 Atom-by-species matrix, 32 302-303, 318-319 Atomic absorption, 27 317 Atomic catalytic activities of sites, 34 183 Atomic displacements, induced by adsorption, 21 212, 213 Atomic rate or reaction definition, 36 72-73 structure sensitivity and, 36 86-87 Atomic species, see also specific elements adsorbed... 

Adsorption at Low Pressure (P < 10" Torr). The adsorption of propene has been studied with thermal desorption spectroscopy (TDS) on all of the different forms of the (100) and (111) surfaces and under several different conditions of exposure. For exposures at low pressure (P< 10 Torr), no selective oxidation is observed. For small exposures (< 5 L) at low-temperature (100K-120K), four propene desorption states are observed from the Ci O(lll) surface comparecf to two desorption states from the Cu9O(100)-Cii surface. These TDS results are shown in Figure 3, and give a cfear indication of a structure-sensitive interaction of propene with Cu20. 

On the face of it, the structural sensitivity of high-silica zeolites to the adsorbed species is not altogether surprising. The fact that physical adsorption can cause structural changes in an adsorbent is well known. Bangham et... 

Adsorption of CO on Ni(7 9 11). Since the azimuthal angle distributions are sensitive to subtle differences between surface structures, it is of interest to examine the role of larger surface irregularities such as surface steps on the measured quantities. For example, suppose the orientation of the primary ion beam in the SIMS experiment is fixed at different azimuthal angles with respect to the step edge. If the ejection process is structure-sensitive, then changes in yield and cluster formation probabilities should be observed as the ion bombards "up" or... 

Catalytic reactions at a metal surface involve a subtle and delicate balance of adsorption forces. Too weak an adsorption and the catalyst will have low activity, too strong and the surface becomes poisoned by adsorbed reactants or products. Consequently, quite small changes in the nature of a metal surface may result in significant variations in catalytic properties. Structure sensitivity is known to exist. There is good evidence that the selectivity and activity of a metal catalyst are affected by changes in structure and/or electronic properties. 

A final interesting observation both for Ni311 312 and Ru310 catalysts, is that small metal particles disappear during the course of the CO/H2 reaction. This effect has been attributed to the exothermicity of the reaction. The release of heat into a small particle could raise its temperature well above the reaction temperature. This in turn could result in major reconstruction of the particle to give a surface structure which best suited the adsorption of these particular reactants. How this would be reflected in terms of structure sensitivity is an interesting question. 

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