Strained overlayers

Pt surfaces tend to restructure into overlayers with an even higher density of Pt atoms than the close-packed (111) surface [21]. The Pt atoms are closer to each other on the reconstructed surfaces than in the (111) surface. The overlap matrix elements and hence the bandwidth are therefore larger, the d bands are lower and consequently these reconstructed surfaces bind CO even weaker than the (111) surface. The reconstructed Pt surfaces are examples of strained overlayers. The effect of strain can be studied theoretically by simply straining a slab. Examples of continuous changes in the d band center and in the stability of adsorbed CO due to strain are included in Figure 4.10. The effect due to variations in the number of layers of a thin film of one metal on another can also be described in the d band model [22,23]. 

Figure 2.33, for example, illustrates how the removal of a row of Cu atoms on the Cu(llO) surface releases the stress in the strained overlayer of adsorbed atomic oxygen, thus enhancing surface reconstruction. 

This expression indicates that the change in hybridization energy is opposite and proportional to the shift of the d band center. Thus, if the d band shifts upwards the hybridization energy increases and vice versa. Strain and the associated shift of the d band can be brought about by growing the desired metal pseudomorfically on another material with a different lattice constant. The term pseudomorfic means that the overlayer grows with the same lattice constant as the substrate. The overlayer may thereby be strained or compressed depending on the lattice constants of the two materials. 

On solid medium, Azospirilhim and Escherichia coli strains were plated on LB agar with l%(w/v) pectin and after six days of incubation, the plates were overlayed with 2% (w/v) solution of hexadecyltrimetyl ammonium bromide (HTAB) (Plazinski and... 

The A. irakense strain is clearly able to hydrolyze pectin on solid medium. A clear halo was observed after incubation for seven days and overlayed with HTAB, indicating the degradation of pectin, whereas non-hydrolyzed polymer is precipitated. For the other Azospirilhim species, we were not able to demonstrate pectinolytic activity in this way. 

Figure 4. Isoenzyme profiles of wild type and mutant strains. Detection of PG activity was by ruthenium red staining on pectate-agarose overlay gels after lEF. WT indicates wild type and the numbers refer to specific mutant isolates.

The in situ activity overlays exhibit, for N402 strain, clearing and darker zones when polygalacturonic acid (PGA) and pectin, respectively, were used as substrates. These zones correspond to a proteins with a molecular mass of around 63-65 kDa... 

The first deals with small islands of silver on a ruthenium substrate. One may look at this sample as a, perhaps somewhat far-fetched, model of a supported catalyst or a bimetallic surface. As metal layers are almost never in perfect registry with the substrate, they possess a certain amount of strain. Goodman and coworkers [46] used these strained metal overlayers as model systems for bimetallic catalysts. Here we look first at the electronic properties of the Ag/Ru(001) system as studied by UPS. 

Structural, Catalytic, Electronic, and Electrochemical Properties of Strained-Copper Overlayers on Ruthenium(0001)... 

That CO chemisorption is perturbed on strained-layer Ni is not surprising in view of CO chemisorption behavior on other metal overlayer systems. For example, on Cu/Ru it has been proposed that charge transfer from Cu to Ru results in decreased occupancy of the Cu 4s level. This electronic modification makes Cu more nickel-like , and results in an increase in the binding energy... 

The catalytic activity of strained-layer Ni on W(llO) for methanation and ethane hydrogenolysis has been studied as a function of Ni coverage. The activity per Ni atom site for methanation, a structure-insensitive reaction, is independent of the Ni coverage and similar to the activity found for bulk Ni. The activation energy for this reaction is lower on the strained-metal overlayer, however, very likely reflecting the lower binding strength of CO on the bimetallic system. 

Figure 1. The "spot test. Each petri plate contains, in a thin overlay of top agar, the tester strain TA98 and, in the cases of plates C and D, a liver microsomal activation system (S-9 Mix). Mutagens were applied to 6-mm filter-paper discs which were then placed in the center of each plate (A) spontaneous revertants (B) furyl-furamide (AF-2) (1 fig) (C) aflotoxin Bi(l fig) (D) 2-aminofluorene (10 fig). Mutagen-induced revertants appear as a ring of colonies around each disc (3).

Figure 5.10 Agar overlay screening procedure to screen microbial populations for hydantoinase activity (Morin, Hummel and Kula, 1986). A screen for dihydropyiidinase activity based on Schiff base formation with PDMB (upper panel) led to the identification of numerous strains with D-hydantoinase activity, which in combination with a D-carbamoylase is employed to produce D-amino acids.

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