Disordered surface layer particles size

The most obvious choice to determine phases that may be present in the molybdena catalyst is XRD. Matching of diffraction lines obtained for the catalyst with those of pure bulk compounds gives unequivocal identification of phases present. This is one of the few techniques that yields positive results. The absence of matching diffraction lines, however, is not proof that the phase in question is not present in the catalyst. The XRD technique is limited to particle sizes of above approximately 40 A for oxides or sulfides, lower sized particles giving no discernible pattern over that of the broad alumina pattern. Thus, the presence of a highly dispersed phase, either as small crystallites or as a surface compound of several layers thickness will not be detected. Also, if the phase is highly disordered (amorphous), a sharp pattern will not be obtained, although some broad structure above that of the alumina may be detected. It is a moot point as to whether such a case is considered as a separate phase or a perturbation of the alumina structure. Ratnasamy et al. (11) have examined their CoMo/Al catalyst from the latter point of view, with particular emphasis on the effect of calcination temperature. 

The second point concerns the surface mobility of atoms on small particles at low temperatures (close to ambient). From the work of Listvan106 on Au clusters it appears that surface mobility of Au occurs at room temperature (see also refs. 102 and 107). In this work it is proposed that a small particle consists of a crystalline core covered with a few disordered layers of mobile surface atoms. If such mobility is real it raises important questions about the relevance of bulk structures to surface structures in small particles. LEED experiments clearly show108 109 that for a bulk solid such a surface film does not exist at, or near, room temperature. However, the situation for small particles is less clear, and several theoretical treatments109 110 have emphasized that the solid-liquid transition should always appear smeared out when the particle size decreases. Catalysis depends on surface effects, so may be less dependent on particle size or overall morphology than might be anticipated. 

In the electrodes for PAFC, the Vulcan XC-72 carbon black is most widely used catalyst support material [95]. The oxidation of Vulcan carbon black in the presence of phosphoric acid at 191 °C showed that the disordered central part of carbon particles was oxidized while the outer crystalline part remained intact [96]. Among the attempts to improve the oxidation resistance of Vulcan carbon black, the most widely used method is the heat treatment which increases the level of graphitization on the carbon surface [97]. The heat treatment of Vulcan carbon black at the temperature of 2200 °C which reduced the surface area of Vulcan from 240 to 80 m /g improved oxidation resistance more than twofold [98, 99]. Other highly graphitic carbon materials such as CNT [100] and graphene [101] have been used as support materials because of their high surface area and electrical conductivity. When selecting the carbonsupport material, the oxidation resistance is the critical property for carbon supports to enhance the durability of HT-PEMFC MEAs however, the surface area, shape, and size of support material should also be considered to achieve the desired dispersion of Pt particles as well as the pore structure within the catalyst layer. 

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