Oxygen adsorption modes

Figure 9.5. Possible reaction pathways as a function of the oxygen adsorption mode on platinum (Pauhng adsorption model and Griffith adsorption model ).

Electrocatalyzed oxygen reduction proceeds always in the adsorbed state. Figure 17 depicts the three different modes of oxygen adsorption on a metal or metal oxide surfaces, which are supposed to be of relevance for cathodic oxygen reduction (113). So-called Griffith adsorption (I), which has been observed by Gland and co-workers (114) on Pt(l 11) surfaces due to interac-... 

During the period of selective reduction of nitrobenzene to nitrosobenzene, the most likely intermediate is a surface species of which only one of the two oxygen atoms interacts with the oxide surface, possibly as shown in structure II. Direct evidence for this adsorption mode could not be obtained by FT-IR spectroscopy, most probably because the lifetime of this mode is too short to allow its observation. 

Table 8 Bond lengths (in A) between chemically active surface metal (M) and oxygen (0) atoms in different metal oxides. Large differences in M-M and M-0 distances between the different surfaces may be an important factor for the relative stability of different adsorption modes which typically involve different surface atoms in the surface-adsorbate bonding. It can be noted that two binding structures were proposed for TiC>2 anatase (101), depending on environmental influences.

Fig. 16. (a) Adsorption modes of molecular oxygen [342], (b) Different oxygen reduction pathways. (c) Coordination sphere of a surface cation at the oxide-electrolyte interface [345]. 

Hence, the study of the different adsorption modes of oxygen is an interesting way to bring a deeper understanding of ORR mechanism on a catalyst. The Electrochemieal Quartz Crystal Miero-balance (EQCM) is a partieularly convenient analytieal method to study adsorption - desorption of species on a eatalyst smfaee. 

Analysis performed with the EQCM technique enabled to investigate the possible adsorption mode of oxygen on a catalyst and then to develop a hypothesis for the ORR mechanism. However, this method is less efficient for catalysts with multiple and complex oxide formation steps, like gold or platinum catalysts, since it is difficult to observe separated adsorption steps with the micro-balance. " ... 

Nanocrystalline MgO also has a large capacity for adsorbing acid gases such as S02, C02, HC1, HBr, and S03 at near stoichiometric proportions. Work by Klabunde has demonstrated that nanocrystalline magnesium oxide has an enhanced surface reactivity over that anticipated from surface area alone. Nanocrystalline MgO chemisorbed 6 molecules of S02 per nm2, whereas larger microcrystalline MgO only chemisorbed 1.8 molecules per square nanometer (Stark and Klabunde, 1996). The proposed mechanism for the difference in adsorption capacities is due to a monodentate-type adsorption mechanism of S02 via the sulfur atom in the instance of nanocrystalline MgO, whereas microcrystalline MgO favors a bidentate adsorption mode through sulfur and an oxygen atom. 

A simplified flow diagram for a VPSA system to produce oxygen is illustrated in Figure 16. The adsorbent is contained in the three vertical vessels. The beds are cycled so that one is in use in the adsorption mode while another is being depressured and the third is being desorbed or regenerated. Both two- and three-bed systems are used. 

The zeolite beds which are not in the adsorption mode are regenerated. First, the two beds are equalized in pressure. One of the beds is then regenerated at a pressure of 4.2 psia. The vacuum blower is used to evacuate the bed under regeneration. Oxygen product is used to pressurize the regenerated bed before it is placed in service for the adsorption cycle. The three beds are cycled so that a nearly continuous stream of oxygen product is produced. 

O2 reduction pathways were affected to a great extent by the O2 adsorptiOTi modes on catalyst surfaces. There are three adsorption models for molecular oxygen adsorption (Fig. 11.3) ... 

By recording an electrochemical response, typically the current at the SECM tip, sample, or both, as the tip is scaimed in close proximity to a surface, micro- to nano-spatially resolved chemical reactivity images of the surface, and quantitative data for analyzing heterogeneous electron transfer rates [3,4, 13, 54, 55], as well as diffusion profiles [56] and adsorption/desorption phenomena [57] can be acquired. For visualization of the ORR activity of catalysts, various operation modes of SECM including SG-TC [48, 49, 58], TG-SC [59, 60], and the oxygen competition mode may be used [61]. 

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