Adsorbed state of oxygen

In relation to gas detection mechanism of the Pd-SnC>2 sensor, the adsorbed state of oxygen was studied by temperature programmed desorption (TPD) technique. 

In the present work, factors influencing the adsorbed state of oxygen on the Pd-Sn02 sensor have been investigated by the TPD technique. Relations between the TPD results and the conductivity changes of the sensor have also been examined. 

It is known that the nature of oxygen adsorbates on Sn02 and on Ag-Sn02 are considerably modified by coexistence of water adsorbates 10, 11). Since the practical sensors are usually exposed to an atmosphere containing moisture under their operating conditions, it would be necessary to know how water influences the adsorbed state of oxygen on the present sample. 

The adsorbed state of oxygen is strongly dependent upon various factors adsorption temperature, heating and cooling history, and coexistence of water adsorbates. 

Adsorbed state of oxygen. Research on the adsorbed state of oxygen plays an important role in the reactive mechanism study of catalytic oxidation of hydrocarbon molecules and ammonia oxidation. In general, the adsorptive states of oxygen in the transition metal oxides and metal catalysts are the negative ion type, such as O, O, and 0 etc. Of course, there are also molecular O2, as well as unstable O3 etc. 

A variety of adsorbed states of oxygen can be transformed according to the following equation. 

A different aspect of AES concerns shifts in the observed peak energies that are due to chemical shifts of atomic core levels. Thus one is able to distinguish between atoms in different chemical environments (in a way analogous to XPS). In particular, studies of different oxidation states of oxygen adsorbed on metals have shown chemical shifts that grow with increasing oxygen valency. 

In a second group of metal oxides, which are not easily reduced, the oxygen is strongly bound and the catalyst is generally in a fully oxidized state. Thus O2- is not reactive, but an adsorbed form of oxygen, much more weakly bound, is active. This leads only to combustion. Quite a number of these metals are non-transition metal oxides. 

Oxidation reactions undoubtedly constitute one of the most intensively studied areas in catalysis, particularly as regards their application in chemical industry. In this context the selectivity of the reaction to desired, partially oxygenated products is of over-riding significance, the most stable end-products always being CO2 and H2O. ZnO is of interest as a catalyst for this class of reactions not, unfortunately, as a good, selective catalyst but rather as a useful model system. We have already seen that an appreciable amount is known about the adsorption properties of hydrocarbons on ZnO and it will become apparent that the adsorbed states of O2 and of intermediate oxygenated hydrocarbon species are also relatively well characterized. 

The main research tool for the adsorbed state of negative oxygen ions is electron paramagnetic resonance (EPR). 

G.B. Fisher, S.J. Schmieg, The molecular and atomic states of oxygen adsorbed on Rh(lOO)—adsorption. J. Vacuum Sci. Technol. a-Vacuum Surf Films 1(2), 1064—1069... 

The effect of increasing pressure is to move the average hydrocarbon content towards the heavier species, but increasing temperature seems to favour the production of lighter species. The final proportions are also determined by the state of the catalyst, and the physical anangement of tire reactor. The formation of the oxygenated compounds could also involve reactions between the H2O content of tire gas in the form of adsorbed OH radicals and hydrocarbon radicals since the production of these molecules is also well beyond the thermodynamic expectation. 

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