Real surface adsorption

As computer power continues to increase over the next few years, there can be real hope that atomistic simulations will have major uses in the prediction of phases, phase transition temperatures, and key material properties such as diffusion coefficients, elastic constants, viscosities and the details of surface adsorption. 

Such information can be obtained from cyclic voltammetric measme-ments. It is possible to determine the quantity of electricity involved in the adsorption of hydrogen, or for the electrooxidation of previously adsorbed CO, and then to estimate the real surface area and the roughness factor (y) of a R-C electrode. From the real surface area and the R loading, it is possible to estimate the specific surface area, S (in m g ), as follows ... 

For microporous materials the 5bet values obtained are usually much higher than the real surface area, because in the region where the BET equation is applied (this equation assumes multilayer adsorption but not condensation) conden.sation already takes place. 

In reality, the adsorption of gas particles on a real surface can be simultaneously influenced by inhomogeneity of the surface and interaction between absorbed particles. Presumably, it is the nature of a specific absorbate-adsorbent pair that controls the major mechanism in each case. 

The determination of the specific surface area of a zeolite is not trivial. Providers of zeolites typically give surface areas for their products, which were calculated from gas adsorption measurements applying the Brunauer-Emmet-Teller (BET) method. The BET method is based on a model assuming the successive formation of several layers of gas molecules on a given surface (multilayer adsorption). The specific surface area is then calculated from the amount of adsorbed molecules in the first layer. The space occupied by one adsorbed molecule is multiplied by the number of molecules, thus resulting in an area, which is assumed to be the best estimate for the surface area of the solid. The BET method provides a tool to calculate the number of molecules in the first layer. Unfortunately, it is based on a model assuming multilayer formation. Yet, the formation of multilayers is impossible in the narrow pores of zeolites. Specific surface areas of zeolites calculated by the BET method (often termed BET surface area) are therefore erroneous and should not be mistaken as the real surface areas of a material. Such numbers are more related to the pore volume of a zeolite rather than to their surface areas. 

The Langmuir and BET models incorporate an assumption that the energy of adsorption is the same for all surface sites and not dependent on degree of coverage. Since in reality the energy of adsorption may vary because real surfaces are heterogeneous, the Freundlich adsorption model (see Chap. 2) [37] attempts to account for this ... 

The electrode roughness factor can be determined by using the capacitance measurements and one of the models of the double layer. In the absence of specific adsorption of ions, the inner layer capacitance is independent of the electrolyte concentration, in contrast to the capacitance of the diffuse layer Q, which is concentration dependent. The real surface area can be obtained by measuring the total capacitance C and plotting C against Cj, calculated at pzc from the Gouy-Chapman theory for different electrolyte concentrations. Such plots, called Parsons-Zobel plots, were found to be linear at several charges of the mercury electrode. ... 

Almost all tests carried out to study the starting process of atmospheric corrosion have been performed in a surface without corrosion products however, in real conditions, the metal is covered with corrosion products after a given time and these products begin to play its role as retarders of the corrosion process in almost all cases. Corrosion products acts as a barrier for oxygen and contaminants diffusion, the free area for the occurrence of the corrosion is lower however, the formation of the surface electrolyte is enhanced. Only in very polluted areas the corrosion products accelerate the corrosion process. Water adsorption isoterms were determined to corrosion products formed in Cuban natural atmospheres[21]. Sorption properties of corrosion products (taking into account their salt content-usually hygroscopics) determine the possibilities of surface adsorption and the possibility of development of corrosion process... 

Although the Bilangmuir isotherm is an ideal model (in the sense that real surfaces with exactly the characteristics described by the Bilangmuir model do not exist), it is often successful in describing the adsorption of enantiomers on chiral stationary phase. The reason is that, because of their... 

An analog of the platinized platinum electrode is the black or gray nickel electrode, which under certain experimental conditions can be used as the hydrogen electrode instead of the platinum electrode [65]. This electrode can be obtained by electrochemical deposition of nickel under the experimental conditions described in [65]. The real surface areas of these electrodes significantly surpass their geometric surface areas. In addition, potential-dependent adsorption of hydrogen occurs on the nickel surface and the measurement of the hydrogen capacity of the electrode in alkaline medium offers a tool for the determination of the real surface area [66]. 

Determination of the Real Surface Area of Pt Electrodes by Hydrogen Adsorption Using Cyclic Voltammetry 82... 

Variants of preparation have been proposed [135, 248] including sintering [391] or co-electrodeposition of the precursors [138, 407], and aluminization of the surface of Ni at high temperature whose nature has a definite effect on the resulting electrocatalytic activity [408]. The main features of Raney Ni have been evaluated, including the pore size distribution and the real surface area [93, 135]. It has been found that the composition of the precursor alloys and their particle size have important influence on the adsorption properties of the resulting Raney metal, hence on its electrocatalytic properties [409]. 

Alumina, silica and many other metal oxides are insulators. However, recent experiments indicate that the surfaces of these insulators are mainly ionic (Masel, 1996). The pristine or freshly cleaved surfaces of single crystals of these oxides (cleaved under ultrahigh vacuum) are fairly inert and do not have significant adsorption capacities for even polar molecules such as CO and S02 (Masel, 1996 Henrich and Cox, 1994). However, the surface chemistry and adsorption properties are dominated by defects on real surfaces. For example, oxide vacancies on alumina expose the unsaturated aluminum atoms, which are electron acceptors, or Lewis acid sites. 

Eq. 6 allows us to calculate the total integral heat of adsorption on an energetically homogeneous surface using density functional theory or other simulation method. To calculate the heat of adsorption on a real surface we must first determine the distribution of adsorptive... 

Monatomic steps at real surfaces do significantly influence not only the nucleation act, but also the spreading and overlapping of 2D islands, which determine the shape of potentiostatic current transients. Neglecting the adsorptive contribution in eq. (3.64), i.e., ep Qcd, and assuming a barrierless nucleation and growth of a condensed 2D Mcads phase only at steps with a regular pattern characterized by a... 

These facts obviously raise the question of what constitutes the best computational model of a small catalytic particle. As catalysis is often a local phenomenon, a cluster model of the reactive or chemisorption site may give quite a reasonable description of what happens at the real surface [1,3,30]. However, the cluster should still be large enough to eliminate cluster edge effects, and even then one must bear in mind that the cluster sizes employed in many computational studies are still much smaller than real catalytic particles (say 10-50 versus 50-1000 atoms, respectively). Hence, a slab model of a stepped surface may provide a much more realistic model of the active site of a catalytic nanoparticle. Hammer [31,32] has carried out quite extensive DFT-GGA slab calculations of N2 and NO dissociation at stepped Ru and Pd surfaces, showing how the dissociation energy is significantly lower at the low-coordination step sites compared to terrace sites. The special reactivity of step sites for the dissociation of NO and N2 has been demonstrated in several recent surface-science studies [33,34]. Also, the preferential adsorption of CO on step sites has been demonstrated in UHV [35], under electrochemical conditions [36], as well as by means of DFT-GGA slab calculations [37]. 

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