Electrocatalysts impregnation-reduction

The impregnation-reduction method has been frequently used for the synthesis of PtSn supported on inorganic carriers such as SiOg, AlgOs, or SAPO, but this approach has rarely been employed for the synthesis of carbon supported electrocatalysts. ° In general, the metal content in those samples is ca. 1-2 wt%, well below the demands of a state of the art fuel-cell electrocatalyst. A number of routes have been explored for the synthesis of carbon supported bimetallic PtSn samples. In general, they lead to materials composed of a wide range of phases, such as metallic and/or oxide Pt, Sn oxides, or PtSn solid solutions of different stoichiometry. 

Recently, Lamy and co-workers [4,5] described that PtSn/C electrocatalysts were more active than PtRu/C electrocatalysts for ethanol oxidation. For electrocatalysts prepared by co-impregnation-H2 reduction and Bonneman methods, they found that the optimum tin composition was in the range of 10-20 at.%. In these conditions, the electrode activity was enhanced and the CO-intermediates coming from ethanol dissociative chemisorption were reduced. Xin and co-workers [6-9] prepared PtRu/C and PtSn/C electrocatalysts by a polyol method and tested for ethanol oxidation. It was observed that the addition of some elements, like W, could improve the PtRu/C electrocatalyst activity. However, the activities of the PtRu/C electrocatalysts were inferior to those of PtSn/C electrocatalysts. It was also found that PtSn/C electrocatalysts with Pt Sn atomic ratios of 60 40 and 50 50 were more active than electrocatalysts with 75 25 and 80 20 atomic ratios. Thus, it seems that the performance of PtSn/C electrocatalysts depends greatly on their preparation procedure. 

Pt-doped carbon aerogels have been used successfully in the preparation of cathode catalyst layers for oxygen reduction reaction (ORR) in PEMFC systems [83-86]. Thus, different Pt-doped carbon aerogels with a Pt content of around 20 wt% were prepared by impregnation [83]. Results obtained with these Pt catalysts were compared with others supported on carbon blacks Vulcan XC-72 and BP2000, which are commonly used as electrocatalysts. The accessibility of the electrolyte to Pt surface atoms was lower than expected for high-surface-area... 

CoPc modified carbon paste electrodes were reported by Chicharo et al. to show good catalytic activity towards the measurement of triazolic herbicides sueh as amitrole at low oxidation potential (+0.4 V, Table 7.1) in basic media, a detection limit of 0.04 jig mL was obtained using a injection system . A screen-printed carbon electrode which was impregnated with CoPc electrocatalyst, was employed in conjunction with acetylcholinesterase by Hartley and Hart for the reduction of organophosphate pesticides The detection limits were of the order of 10 and 10 ... 

Carbon can be used [7,11,62,70—73] as an electrocatalyst for the O2 reduction in alkaline electrolytes (compare also section 5 in chapter VIII). The performance which is not so good as that of silver (see Fig. 79) appears adequate for certain purposes, for instance, in small zinc-air cells. Activation procedures [72,73] which are not of an electrochemical nature improve the performance of carbon oxygen electrodes. The performance rapidly becomes poor with decreasing pH below pH <14. In acid solution, the impregnation of carbon with platinum metals or other electrocatalysts is required. The data [73] in Table 8... 

The diagram for carbon in Fig. 83 displays a very small domain of stability. It is thermodynamically possible for carbons to be easily oxidized to carbon dioxide, carbonic acid, and carbonates. Reduction of carbon may lead to the formation of methane, methyl alcohol and other organic substances. However, the energetically possible reactions are strongly irreversible [2] and do not occur under normal conditions of pressure and temperature. Schmidt [24] reported a corrosive destruction of carbon electrodes when a critical potential was exceeded during the reduction of O2. The carbon electrodes were not impregnated with metallic electrocatalysts. The critical potential depended upon the extent to which an oxygen layer was present (compare section 5 in chapter VIII). 

---