Nonnoble metal catalysts

The relatively high cost and lack of domestic supply of noble metals has spurred considerable efforts toward the development of nonnoble metal catalysts for automobile exhaust control. A very large number of base metal oxides and mixtures of oxides have been considered, especially the transition metals, such as copper, chromium, nickel, manganese, cobalt vanadium, and iron. Particularly prominent are the copper chromites, which are mixtures of the oxides of copper and chromium, with various promoters added. These materials are active in the oxidation of CO and hydrocarbons, as well as in the reduction of NO in the presence of CO (55-59). Rare earth oxides, such as lanthanum cobaltate and lanthanum lead manganite with Perovskite structure, have been investigated for CO oxidation, but have not been tested and shown to be sufficiently active under realistic and demanding conditions (60-63). Hopcalities are out-... 

At least two catalytic processes have been used to purify halogenated streams. Both utilize fluidized beds of probably nonnoble metal catalyst particles. One has been estimated to oxidize >9000 t/yr of chlorinated wastes from a vinyl chloride monomer plant (45). Several companies have commercialized catalysts which are reported to resist deactivation from a wider range of halogens. These newer catalysts may allow the required operating temperatures to be reduced, and still convert over 95% of the halocarbon, such as trichlorethylene, from an exhaust stream. Conversions of C-l chlorocarbons utilizing an Englehardt HDC catalyst are shown in Figure 8. For this system, as the number of chlorine atoms increases, the temperatures required for destruction decreases. 

The reaction can be carried out in either the liquid or vapor phase, but the vapor phase process is more common. Catalysts used in the liquid phase are palladium, palladium with platinum, or nickel, or cobalt suspended in a solvent such as methanol, ethanol, or mineral oil. In the vapor phase process, a nonnoble metal catalyst is used. Metals such as copper or chromium with copper are used in either a fixed or fluidized bed reactor. 

Figure 3.27. Tafel plots of the polarization curve for the best non-noble metal catalyst obtained on Norit compared to a catalyst from E-Tek loaded with Pt at 10 wt%. The GDE currents are expressed in A/mg metal. Dark circle reported state-of-the-art activity at 900 mV for H2/saturated O2 at 65°C, 100 kPa, for a cathode with 0.4 mg Pt/cm. The nonnoble metal catalyst was made by adsorbing iron acetate (0.2 wt% Fe) on pretreated Norit. This material was heat treated at 900°C in H2 Ar NH3 (1 1 2). The pretreated Norit was obtained by refluxing Norit in HNO3 (according to Figure 10 in ref. [113] reproduced with permission of The Electrochemical Society).

For non-noble metal ORR catalysts, the definition of catalytic activity is different from that of Pt-based catalysts. For a nonnoble metal catalyst, the similar preparation procedure for CL and ORR measurement steps using rotating disk electrode technique to that for Pt-based catalyst have been widely used in literature. However, due to both the ORR onset and half-wave potentials catalyzed by non-noble metal catalysts are much lower than those of Pt-based catalysts, it is difficult or impossible to observed ORR current density at 0.9 V vs RHE. A current density at other lower potentials may be used to define the catalyst activity for the purpose of comparison. In this case, Eqn (3.7) may still usable except the electrode potential is not 0.9 V, instead of... 

Pt-based catalysts are two necessary approaches at the current technology stage. It is believed that non-noble metal electrocatalysts is probably the sustainable solution for PEM fuel cell commercialization. In the past several decades, various nonnoble metal catalysts for ORR have been explored, including non-pyrolyzed and pyrolyzed transition metal nitrogen-containing complexes, transition metal chalcogenides, conductive polymer-based catalysts, metal oxides/carbides/nitrides/ oxynitrides/carbonitrides, and enzymatic compoimds. The major effort in non-noble metal electrocatalysts for ORR is to increase both the catalytic activity and stability. 

In Chapter 4, the fundamentals of ORR including thermodynamics and electrode kinetics are presented. The ORR kinetics including reaction mechanisms catalyzed by different electrode materials and catalysts including Pt, Pt alloys, carbon materials, and nonnoble metal catalysts are discussed based on literature in terms of both experiment and theoretical approaches. It is our belief that these fundamentals of ORR are necessary in order to perform the meaningful characterization of catalytic ORR activity using both RDE and RRDE methods. 

Garsuch A, MacIntyre K, Michaud X, Stevens DA, Dahn JR (2008) Fuel cell studies on a nonnoble metal catalyst prepared by a template-assisted synthesis route. J Electrochem Soc 155 (9) B953-B957... 

---