Hydrogen adsorption reduction temperature dependence

Gomez-Sainero et al. (11) reported X-ray photoelectron spectroscopy results on their Pd/C catalysts prepared by an incipient wetness method. XPS showed that Pd° (metallic) and Pdn+ (electron-deficient) species are present on the catalyst surface and the properties depend on the reduction temperature and nature of the palladium precursor. With this understanding of the dual sites nature of Pd, it is believed that organic species S and A are chemisorbed on to Pdn+ (SI) and H2 is chemisorbed dissociatively on to Pd°(S2) in a noncompetitive manner. In the catalytic cycle, quasi-equilibrium ( ) was assumed for adsorption of reactants, SM and hydrogen in liquid phase and the product A (12). Applying Horiuti s concept of rate determining step (13,14), the surface reaction between the adsorbed SM on site SI and adsorbed hydrogen on S2 is the key step in the rate equation. 

The above results for Pt supported on three different types of difficult to reduce supports show that high temperature reduction usually causes large decreases in hydrogen adsorption capacity. However, the reduction temperature required for hydrogen adsorption suppression depends on the support. Hydrogen adsorption capacities of catalysts reduced at high temperatures can be restored by oxygen treatment followed by low temperature reduction, i.e. the processes responsible for adsorption suppression are reversible. 

In the case of contact of hydrated silica surface with air (water vapor), the structure of adsorbed water is determined not only by the adsorbent surface (i.e., phase boundary of silica/water) but also by the phase boundary of water/air. Appearance of these phase boundaries leads to reduction of the free energy of the interfacial water accompanied by lowering of its freezing temperature. The information on a structure of the adsorption complexes at a surface of oxide adsorbents can be obtained from temperature dependences of chemical shift of protons of interfacial water molecules. It is necessary to take into account that 5h is defined by strength of the hydrogen bonds between water molecule and active surface sites and depends on the amounts of hydrogen bonds per water molecule. 

Of interest, k. values at 800°C were higher than those at 700°C and 900°C. Presumably, hydrogen reduction involves both initial adsorption of hydrogen followed by reactions between the adsorbed hydrogen and the surface oxides. Increased temperatures would presumably decrease hydrogen adsorption, and k. apparently depends to some extent on adsorption. 

The experiment was carried out in a reaction cell shown in Fig. 3.3 with inner walls covered by a zinc oxide film having thickness 10 pm [20]. The surface area of the measuring film on the quartz plate was about 1/445 of the total film area on the wall of the vessel. The results of direct experimental measurements obtained when the adsorbent temperature was -196 C and temperature of pyrolysis filament (emitter of H-atoms) 1000°C and 1100°C, are shown on Fig. 3.4. One can see a satisfactory linear dependence between parameters A r (the change in film conductivity) and APh2 (reduction of hydrogen pressure due to adsorption of H-atoms), i.e. relations... 

Kubelkova and coworkers studied the interaction of CO with platinum in a NaX zeolite [141]. They decomposed PtlNHj I]] on the zeolite under various conditions and observed distinct CO vibrational spectra for PtO-CO, Pt -CO, Pti -CO and [Pt(CO)2] complexes after CO adsorption at room temperature. Subsequent reduction with hydrogen resulted in characteristic particle size distributions with distinct CO vibrational frequencies depending on whether the clusters (-1.2 nm) were inside the supercages or outside the cages (-4.0nm). 

In this chapter, recent results are discussed In which the adsorption of nitric oxide and its Interaction with co-adsorbed carbon monoxide, hydrogen, and Its own dissociation products on the hexagonally close-packed (001) surface of Ru have been characterized using EELS (13,14, 15). The data are interpreted In terms of a site-dependent model for adsorption of molecular NO at 150 K. Competition between co-adsorbed species can be observed directly, and this supports and clarifies the models of adsorption site geometries proposed for the individual adsorbates. Dissociation of one of the molecular states of NO occurs preferentially at temperatures above 150 K, with a coverage-dependent activation barrier. The data are discussed in terms of their relevance to heterogeneous catalytic reduction of NO, and in terms of their relationship to the metal-nitrosyl chemistry of metallic complexes. 

In the case of alumina supported rhenium (12), the nature of the supported metal also seems highly dependent on pretreatment of the catalyst. Increasing the precalcination temperature from 500 to 700 C, evidently increased the amount of exposed, fully reduced Re shown subsequently (after reduction in hydrogen at 500 C) by adsorption of CO. Additional types of reduced Re adsorption sites were also apparently present on the sample precalcined at 700 C. 

The present work was undertaken to examine this possibility by trying a new method of low-temperature catalyst preparation. The method studied involves the adsorption of metal precursors on supports and the reduction by sodium tetrahydroborate solution for the preparation of supported platinum catalysts. The adsorption and reduction of platinum precursors are carried out at room temperature and the highest temperature during the preparation is 390 K for the removal of solvent. The activities of the catalysts prepared were examined for liquid-phase hydrogenation of cinnamaldehyde under mild conditions. Our attention was directed to not only total activity but also selectivity to cinnamyl alcohol, since it is difficult for platinum to hydrogenate the C=0 bond of this a, -unsaturated aldehyde compared to the C=C bond [2]. We examined the dependence of the catalytic activity and selectivity on preparation variables including metal precursor species, support materials and reduction conditions. In addition, the prepared catalysts were characterized by several techniques to clarify their catalytic features. The activity of the alumina-supported platinum catalyst prepared by the present method was briefly reported in a recent communication [3]. 

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