Oxygen adsorbed platinum

Watanabe M, Shibata M, Motoo S. 1985. Electrocatalysis hy ad-atoms. PartXn. Enhancement of carbon monoxide oxidation on platinum electrodes by oxygen adsorbing ad-atoms (Ge, Sn, Pb, As, Sb and Bi). J Electroanal Chem 187 161-174. 

These effects are most striking on silver since it is, itself, a very unreactive surface. There is every reason to expect, however, that oxygen will behave similarly on other metals. More complex reaction behavior will, of course, be observed as the intrinsic reactivity of the metal increases. Oxygen adsorbed on platinum should show similar properties. In fact the formation of surface OH groups from HjO and 0(a) was recently reported 145). The ability of platinum itself to break C-H and C-C bonds complicates oxidation mechanisms, but future work should provide a greater understanding of the relative role of surface oxygen in oxidation catalysis. 

The sensor compositions usually include noble metals, particularly palladium and platinum, since they are found to increase sensitivity through catalysing the reaction between the oxygen adsorbate and reducing gases. In this role the additive is said to be acting as a chemical sensitizer . 

It is shown below that a number of molecular species are important in the interaction of oxygen with platinum surfaces, and these are precursors to the dissociative state. The interception of these adsorbed states in the electrochemical process are important in determining the catalytic mechanism of electroreduction, and we also show below that surface defects play an important role in their reactivity and surface lifetimes. 

Also, for platinum, a room-temperature titration of oxygen adsorbed on the metal with gaseous hydrogen (24-26) has become a very valuable method, particularly when interference by the support is to be feared, e.g., for catalysts with small amounts of metal (27). 

Fio. 12. Isotopic exchange of molecular oxygen with the oxygen adsorbed on platinum 1—200 2—250°. 

Fio. 13. The change of activation energy of the oxygen adsorbed on platinum film with the growth of the depth of exchange. 

The rate of exchange of molecular oxygen on the silver film at 350° C is ten times less than that on the platinum film. The energy of activation is 19 kcal/mole. Oxygen adsorbed on the silver film is uniform. 

The O2/H2O system is very slow so that the exchange current a I equilibrium is extremely low (10 /10 A cm 2) as a consequence, any other reaction at the electrode will hamper its study and that could be the reaction of impurities or other redox reactions involving the electrode itself. The so-called noble metals are not really inert and do interact with oxygen a platinum surface in contact with an O saturated solution adsorbs oxygeti as an electronically conducting monolayer but can be further oxidized to PIO, PtO . A detailed analysis of these phenomena, which falls outside the scope of the present review, can be found elsewhere [311. A platinum electrode, when a complete electronically conducting monolayer of I l—O is formed at the surface of the metal, behaves as an ideally inert electrode in such conditions, rest potentials dependent on pO2 and pH can be measured during a few hours, close to... 

Methane oxidation at mild or low temperatures can be catalyzed by platinum group metals. Palladium is one of the most efQdent metals (1) and has been studied over mai supports (2-6). This particular metal, when supported on alumina, b ins to show an increase in its activity between 350 and 420°C. At these conditions a general increase in the active spedes particle size is observed. Piimet and Briot (7,8) defined two states for the Pd/Al203 supported catalyst a state I, obtained after simple reduction and a state n after the catalyst had reacted at 600°C for 14 h under 02/CH4=4A. State II was more active than state I and showed a lower binding oietgy of oxygen with palladium. However, the state of the active phase was not clear. The diffoences in activity, also observed by others, have also been related to the formation/decomposition of PdO (9), to the oxygen adsorbed on metallic Pd (2), to the modification of Pd surface spedes (3), and to the reconstruction of PdO crystallites (4, 10). One of the hypotheses for the activation of the Pd catalysts was the establishment of an epitaxy between the metal and the support (8, 11). 

Values of the VSIP are experimentally based and theoretically considered as the opposite of the diagonal Hamiltonian matrix elements. Valence orbitals are of the Slater form for simplicity. The oxygen molecule-platinum site interactions are represented as an adsorbed ensemble that is characterized by its specific VSIP. 

The Most Likely Intermediate Species Involved in the Oxygen Electroreduction Mechanism on Pt(111) and Pt( 100) Clusters The adsorption configuration study for the oxygen adsorbates on platinum, described above, provides the possibility of exploring further mechanistic aspects of the oxygen electroreduction, particularly in relation to the possible species involved in the reaction that account for the different behavior of Pt(l 11) and Pt(100) electrode surfaces. 

NO form a number of higher oxides, such as N02 and N03 that compete with adsorbed NO and atomic oxygen for platinum surface sites. Several atomistically detailed models of the NO oxidation reaction based on DFT-derived parameters for the reaction kinetics have been reported [58,59]. These models are successful in describing the sensitivity of the reaction kinetics to surface coverage. They are somewhat limited in terms of the surface species and the reaction steps considered. 

Kalish and Burstein (60) found that the amount of oxygen adsorbed by a platinum layer adjacent to the surface was about one hundred times that required for a monolayer. Temkin and Kul kova (61) have noted a similar phenomenon for oxygen adsorption on silver. The amount of... 

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