Controlled Crystal Face Catalysts

While area-specific ORR activities on low-index single-crystal surfaces (i.e., (Ill), (100), and (110) surfaces) of pure Pt are roughly comparable, Stamenkovic et al. have shown that the ORR activity on PtjNi( 111) is 9-fold higher than that of Pt( 111) 

FIGURE 13.9 Scanning electron micrograph of PtjNi nano-octahedra forming a supercrystal on a Si substrate, showing the uniform size and shape of the octahedra. Reprinted with permission from Ref. [87]. American Chemical Society. 

ELECTROCATALYST DESIGN IN PROTON EXCHANGE MEMBRANE FUEL CELLS 

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Since in some cases the previously described alloy, dealloyed, and controlled-crystal-face catalysts also develop porous/hollow structures, it is of particular interest to determine to what extent the hollow structure affects the high ORR activities seen in those catalysts. Focus points for future research should include (i) developing scalable synthesis techniques and (ii) determining whether the surface and bulk diffusion rates of Pt in these hollow structures, relative to the fuel cell life, are sufficiently slow for this type of catalyst to be practical. 

Control of fhe precursor phase is an imporfanf aspecf in fhe formafion of an active catalyst. The topotactic transformation to the pyrophosphate phase leaves many of fhe feafures of fhe precursor unchanged. Controlling the morphology and preferentially exposing the desired crystal faces have the potential to increase the activity of fhe catalyst or its selectivity to MA. 

Ohno, T. Higo, T. Murakami, N. Saito, H. Zhang, Q. Yang, Y. Tsubota, T. Photocatalytic Reduction of C02 over Exposed-Crystal-Face-Controlled Ti02 Nanorod Having a Brookite Phase with Co-Catalyst Loading. Appl. Catal., B, 2014, 152-153, 309— 316. 

A summary of catalyst s mthesis methods investigated for both activity and potential to control the crystal face population(s) is shown in Table 1. Of these, the methodology indicated as Shows Promise appears to provide a wide range of structures over only a moderately small range of tested parameters. Due to this unexpected result, additional efforts were directed to this method. 

The widespread use of rhodium for use in vehicle emission control (Chapter 8) has led to the use of NO as an adsorbate for the determination of Rh dispersion.NO Rh(to ) ratios of up to ca. 2 1 have been reported " suggesting that a surface stoichiometry of up to two is possible. This is consistent with FTIR studies which confirm the presence of gem-dinitrosyl species for highly dispersed catalysts in addition to linear bound species. Bridge bonded NO (NO Rh = 1 2) is found on the Rh (111) surface," although the presence of sites capable of multiple NO adsorption on this crystal face appears to depend on the presence of co-adsorbates. Good... 

Superimposed on the effect of carbon hybridization in affecting carbon shape is that of the catalyst particle. The catalyst particle influences the shape of the resultant carbon structure by (i) its size, (ii) its interaction with a support or template (when used), and (iii) its shape as reflected by the crystal faces exposed to the reactants. Control of the catalyst particle s morphology thus becomes a key determinant in generating the morphology of SCMs [14],... 

It is a pity that open surface of a-Fe single crystal is possibly obtained only under rigorously controlled laboratory conditions. In industrial production, we are ignorant of how the (111) surface of iron exposed fully on the siu face of a-Fe crystallites during the manufacture and reduction of catalysts and also how the stability of the (111) surface is retained in the reactor. According to the studies of chemisorptions of CO and CO2, more than 50% of a-Fe surface is covered by K2O or KOH. Studies by Ertl et indicated that when there are K2O or KOH present on the a-Fe... 

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