Oxide catalysts oxygen adsorption

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

Baker WS, Pietron JJ, Teliska ME, et al. 2006. Enhanced oxygen reduction activity in acid by tin-oxide supported Au nanoparticle catalysts. J Electrochem Soc 153 A1702-A1707. Blizanac BB, Lucas CA, Gallagher ME, et al. 2004a. Anion adsorption, CO oxidation, and oxygen reduction reaction on a Au(lOO) surface The pH effect. J Phys Chem B 108 625-634. 

Luo, J., Zhang, Q., Garcia-Martinez, J. and Suib, S.L. (2008) Adsorptive and acidic properties, reversible lattice oxygen evolution, and catalytic mechanism of cryptomelane-type manganese oxides as oxidation catalysts. Journal of the American Chemical Society, 130, 3198-3207. 

The desorption of oxygen which was suggested as the slowest step on CuO catalyst is the rupture of a M-0 bond in its nature and the adsorption of nitrous oxide through oxygen end which was suggested as the slowest step on MgO catalyst is the formation of a M-0 bond. Therefore, the results we have obtained in the present study appears to be consistent to the conclusion proposed by Vijh. 

Materials NaGeX zeolite was kindly supplied by Dr. G. Poncelet (Universite Catholique de Louvain) and the mixed tin-antimony oxide catalysts (SnSbO) by I.C.I. Ltd. The H-Z is the acidified form of commercially available Norton mordenite. The ZSM-5 and ZSM-11 zeolites were synthesized following the patent literature (15,16). 1-Butene (Prochem) was a natural abundance compound, while methanol (95 % l C, British Oxygen Corporation (B.O.C.)), ethanol (95 % C, B.O.C.) and ethylene ( 90 % C, Prochem) were JC-enriched compounds. For the latter a 30 % v/v dilution was realized prior to adsorption. 

As alluded to before, the adsorption of atoms and molecules may also induce segregation in alloys. Upon revisiting the thermodynamic behavior of the improved Cu-Ag alloy catalysts for ethylene epoxidation synthesized by Linic et al, (section 2.1) Piccinin et al. calculated that, while in the absence of oxygen Cu prefers to stay in the subsurface layers, oxygen adsorption causes it to segregate to the surface which then phase-separates into clean Ag(lll) and various Cu surface oxides under typical industrial conditions (Fig. 7). This casts doubt on the active state of the previous Cu-Ag catalysts being a well-mixed surface Ag-Cu alloy. 

Vayenas and co-workers31,32 used cyclic voltammetry to investigate oxygen adsorption and ethylene oxidation over a Pt catalyst-electrode. A cathodic... 

Iron molybdates, well known as selective methanol oxidation catalysts, are also active for the propene oxidation, but not particularly selective with respect to acrolein. Acetone is the chief product at low temperature (200°C), whereas carbon oxides, besides some acrolein, predominate at higher temperatures [182,257], Firsova et al. [112,113] report that adsorption of propene on iron molybdate (Fe/Me = 1/2) at 80—120°C causes cation reduction (Fe3+ -> Fe2+) as revealed by 7-resonance spectroscopy. Treatment with oxygen at 400°C could not effect reoxidation (in contrast to similarly reduced tin molybdate). The authors assume that this phenomenon is related to the low selectivity of iron molybdate. 

The source of this discrepancy is unknown to us. Equation (349) is, undoubtedly, adequate for the description of the reaction kinetics on an iron-chromium oxide catalyst. The fact that in one of the works (124) magnetite without the addition of chromium oxide served as a catalyst can hardly be of consequence since a study of adsorption-chemical equilibrium (344) on an iron-chromium oxide catalyst (7% Cr203) (52) led to the value of the average energy of liberation of a surface oxygen atom that practically coincides with that found earlier (50) for an iron oxide catalyst with no chromium oxide. It may be suspected that in the first work (124) the catalyst was poisoned with sulfur of H2S that possibly was contained in unpurified C02... 

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