Modification of the surface structure

It has been demonstrated in earlier sections that the catalytic activity of nickel oxide in the room-temperature oxidation of carbon monoxide is related to the number and the nature of the lattice defects on the surface of the catalyst and that any modification of the surface structure influences the activity of the solid. Changes of catalytic activity resulting from the incorporation of altervalent ions in the lattice of nickel oxide may, therefore, be associated not only with the electronic structure of the semiconductor (principle of controlled valency ) (78) but perhaps also with the presence of impurities in the oxide surface or a modification of the surface structure because of this incorporation. In order to determine the influence of dopants on the lattice defects in the surface of the solid and on its catalytic activity, doped nickel oxides were prepared under vacuum at a low temperature (250°). Bulk doping is not achieved and, thence, one of the basic assumptions of the electronic theory of catalysis (79) is not fulfilled. 

In some cases, the surface can be effectively protected during transfer. The use of a drop of pure water to protect Pt surfaces has been mentioned above. Hubbard and co-workers [43] have shown that iodine adsorption can also be used to protect Pt surfaces. Any type of protection is likely to be specific to a particular substrate and must be studied carefully to ensure that it does not involve modification of the surface structure. 

The specific surface area can be increased by modification of the surface structure. Chemically treated surfaces should provide a reactive surface that is insensitive... 

An attempt has been made, on the basis of gravimetric and spectroscopic investigations of the kinetics of chemisorption of Me3SiCl on silicas of differing degrees of hydration, to explain the chemistry involved in the modification of the surface structure according to the equation ... 

A high value of AX entails an appreciable modification of the surface structure as a consequence of strong interactions between the phases and vice versa, and thus the inner-layer capacitances have to correlate with AX (or A ) values. A good correlation between the experimental inner-layer thickness l = 4 7rCi and AX has been demonstrated for various... 

Mass transfer in falling film has been considered mainly with the dependence of liquid-side mass transfer. Recent research activities are concentrated on the potential increase of mass transfer in the liquid film to enhance the reactor performance. For this purpose, different modifications of the surface structure of the reaction plates are proposed. 

ZrOj-based materials have attracted considerable interest in recent decades. In a catalytic sense, they appear to have some advantages in areas of practical application over traditional oxides, such as Si02 and AljO, [1-4], More significant is the fact that additives can bring about a strong modification of the surface structure of zirconia [5-8], in which case substitution of Zr with dopant cations results in a rise in anion vacancy concentrations and conductivity. Indeed this is the basis of its redox properties and the catalytic use of stabilized zirconia. 

The sol-gel procedure is different from all other chemical pretreatment processes in that it relies on the formation of chemical bonds at the interface to aid in the production of a strong durable bond, rather than a modification of the surface structure of the substrate [25]. 

Reconstruction A modification of the surface structure occurring upon a crystal cleavage when the surface atoms are shifted along the surface with respect to their positions in the bulk. 

The modification of the surface structure produced by the rupture of the Ah 0-Al moieties interaction is intrinsically endothermic, and caused the measured heat for the AI2O3/CO interaction to be lower than what expected for a plain a -coordination. This result does justify the dramatically low heat of interaction measured within the calorimetric cell, and does explain the lack of correlation between the energetic and vibrational parameters [1, 20, 34, 35]. 

Here we describe AFM results, which, in combination with X-ray photoelectron spectroscopy (XPS) data, support the notion that the macromolecular network of amber can be resolved with subnanometer resolution. Amber samples of 4x5x6 mm were cut from a big piece of Baltic amber, fractured by means of a metal wedge (Figure 19.7), mounted on the sample holder, and immediately transferred into the UHV chamber, in which the AFM measurements were carried out at a pressure of 3.5 x 10 mbar using loading forces of 2.5-3 nN. No obvious modifications of the surface structure could be detected within the scaimed areas upon repetitive imaging. For experimental details, see Ref. [97]. 

The most versatile method to prepare such hollow capsules is self-assembly [203-205, 214, 215]. Owing to their amphiphilic nature and molecular geometry, lipid-based amphiphiles can aggregate into spherical closed bilayer structures in water so-called liposomes. It is quite reasonable that the hollow sphere structure of liposomes makes them suitable as precursors for the preparation of more functional capsules via modification of the surfaces with polymers and ligand molecules [205, 216, 217]. Indeed, numerous studies based on liposomes in this context have been performed [205, 209, 213]. 

Fig. 8.1. Local modification of the electronic structure in the gap. Charge-density contours of a system with an A1 tip and an A1 sample. (a) Free Al(ll 1) surface, (b) At a tip-sample distance 8 bohr (4.2 A) (c) 7 bohr (3.6 A) and (d) 5 bohr (2.6 A). (Reproduced from Ciraci et al., 1990a, with permission.)...

These structural changes are accompanied by significant reactivity modifications of the surface vanadia species. The addition of the surface potassium oxide species decreases the reducibility of the surface vanadia species in temperature programmed reduction (TPR) studies and the TOF for methanol oxidation.23,50 The most likely reason for this behavior is that the surface potassium oxide species is intimately coordinated to the bridging V-O-Support bond and retards its participation in these redox processes. Thus, all oxidation reactions, involving one surface vanadia site as well as dual surface vanadia-acidic sites, will be retarded by the surface potassium oxide additive. The basic properties of the surface potassium oxide additive may also affect the product selectivity by... 

Generally, LEED experiments are conducted on specified faces of single crystals. When this is done, the diffraction pattern produced consists of a series of spots with a location, shape, and intensity that can be interpreted in terms of the surface structure. We focus attention on what can be learned from the location and shape of the spots since the study of intensity is beyond the scope of this book. It is generally assumed that the surface examined by LEED is an extension of an already-known bulk crystal structure. The correctness of this assumption can be tested, and results are often expressed in terms of modifications of the three-dimensional structure at the surface. Before we turn to the LEED patterns below, we must first figure out how they are read. 

Fig. 3. Two STM images of the nickelfl 1 1) surface with 2% (right) and 7% (left) gold coverage, respectively. Au is imaged as dark depressions in the surface. The nickel atoms surrounding the gold appear brighter owing to a local modification of the electronic structure, indicating a changed chemical reactivity of these. Adapted from Reference (79).

The oxidation of carbon monoxide on nickel oxide has often been investigated (4, 6, 8, 9, II, 16, 17, 21, 22, 26, 27, 29, 32, 33, 36) with attempts to correlate the changes in the apparent activation energy with the modification of the electronic structure of the catalyst. Published results are not in agreement (6,11,21,22,26,27,32,33). Some discrepancies would be caused by the different temperature ranges used (27). However, the preparation and the pretreatments of nickel oxide were, in many cases, different, and consequently the surface structure of the catalysts—i.e., their composition and the nature and concentration of surface defects— were probably different. Therefore, an explanation of the disagreement may be that the surface structure of the semiconducting catalyst (and not only its surface or bulk electronic properties) influences its activity. 

Nature of Active Sites. There is no apparent correlation between the increase of catalytic activity and a modification of the electronic structure of nickel oxide, since the electrical properties of both catalysts are identical. It is probable that local modifications of the nickel oxide surface are responsible for the change of its activity and of the reaction mechanism. It should be possible to associate these structural modification with local modifications of the height of the Fermi level, but it would be difficult to explain the results by the electronic theory of catalysis which considers only collective electrons or holes. A discussion based only on the influence of surface defects seems, therefore, to be more straightforward. 

These results lead us to propose that the adsorption mode of the molecule of prenal and consequently the hydrogenation selectivity strongly depend upon structural and electronic modifications of the surface. 

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