Oxygen, chemisorption removal

The first STM evidence for the facile transport of metal atoms during chemisorption was for oxygen chemisorption at a Cu(110) surface at room temperature 10 the conventional Langmuir model is that the surface substrate atoms are immobile. The reconstruction involved the removal of copper atoms from steps [eqn (1)], resulting in an added row structure and the development of a (2 x 1)0 overlayer [eqn (2)]. The steps present at the Cu(llO) surface are... 

Influence of Metal Ions on Oxygen Chemisorption and Ignition of Chars. We have carried out extensive studies of the influence of metal ions in wood on pyrolysis mechanisms (5.6) and this approach has now been extended to oxygen chemisorption of the chars. The metal ions occur in wood predominantly as the counterions of the uronic acid components of the hemicelluloses (12). We have shown that they can be almost completely removed by very mild acid treatment without any other major change in the chemistry of the wood. Table II shows that the major metal ions in cottonwood are Ca, K and Mg. The acid-washing process removed 98X of the metal ions in... 

Oxygen Chemisorption.—Most studies on Pt have used polycrystalline wires, foils, or specially cut single-crystal faces. In all cases, samples are initially heavily contaminated with graphitic surface layers. Pignet, Schmidt, and Jarvis, for example, showed by AES that heating at 400 °C in 1 atm of oxygen effectively removed surface C and S and that the resulting surface was not... 

Perhaps the most fascinating detail is the surface reconstruction that occurs with CO adsorption (see Refs. 311 and 312 for more general discussions of chemisorption-induced reconstructions of metal surfaces). As shown in Fig. XVI-8, for example, the Pt(lOO) bare surface reconstructs itself to a hexagonal pattern, but on CO adsorption this reconstruction is lifted [306] CO adsorption on Pd( 110) reconstructs the surface to a missing-row pattern [309]. These reconstructions are reversible and as a result, oscillatory behavior can be observed. Returning to the Pt(lOO) case, as CO is adsorbed patches of the simple 1 x 1 structure (the structure of an undistorted (100) face) form. Oxygen adsorbs on any bare 1 x 1 spots, reacts with adjacent CO to remove it as CO2, and at a certain point, the surface reverts to toe hexagonal stmcture. The presumed sequence of events is shown in Fig. XVIII-28. 

Prior to inclusion of PVP-protected Pt nanoparticles the SBA-15 silica is calcined at 823K for 12h to remove residual templating polymer. Removal of PVP is required for catalyst activation. Due to the decomposition profile of PVP (Figure 6), temperatures > 623 K were chosen for ex situ calcination of Pt/SBA-15 catalysts. Ex-situ refers to calcination of 300-500 mg of catalyst in a tube furnace in pure oxygen for 12-24 h at temperatures ranging from 623 to 723 K (particle size dependent) [13]. Catalysts were activated in He for 1 h and reduced at 673 K in H2 for 1 h. After removal, the particle size was determined by chemisorption. Table 2 is a summary of chemisorption data for Cl catalysts as well as nanoparticle encapsulation (NE) catalysts (see description of these samples in proceeding section). 

Physical adsorption is a readily reversible process, and alternate adsorption and desorption stages can be carried out repeatedly without changing the character of the surface or the adsorbate. Chemisorption may or may not be reversible. Often one species may be adsorbed and a second desorbed. Oxygen adsorbed on charcoal at room temperature is held very strongly, and high temperatures are necessary to accomplish the desorption. CO and/or C02 are the species that are removed from the surface. Chemical changes like these are prima facie evidence that chemisorption has occurred. 

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