Fuel cells will undoubtedly find widespread use in this new millennium in the conversion of chemical to electrical energy, as they offer very high efficiencies and have unique scalability in electricity-generation applications. The solid-oxide fuel cell (SOFC) is one of the most exciting of these energy technologies; it is an all-ceramic device that operates at temperatures in the range 500–1,000 °C. The SOFC offers certain advantages over lower temperature fuel cells, notably its ability to use carbon monoxide as a fuel rather than being poisoned by it, and the availability of high-grade exhaust heat for combined heat and power, or combined cycle gas-turbine applications. Although cost is clearly the most important barrier to widespread SOFC implementation, perhaps the most important technical barriers currently being addressed relate to the electrodes, particularly the fuel electrode or anode. In terms of mitigating global warming, the ability of the SOFC to use commonly available fuels at high efficiency, promises an effective and early reduction in carbon dioxide emissions, and hence is one of the lead new technologies for improving the environment. Here, we discuss recent developments of SOFC fuel electrodes that will enable the better use of readily available fuels.

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Advanced anodes for high-temperature fuel cells

Catalytic removal of NO

Developments in water gas shift (WGS) catalysis, especially during the last decade, are reviewed. Recent developments include the development of 1 chromium‐free catalysts that can operate at lower steam to gas ratios and 2 more active catalysts that can operate at gas hourly space velocities above 40,000 h−1. A current challenge is to develop catalysts for use in fuel cell applications. Precious metal catalysts supported on partially reducible oxide supports (Pt‐ceria, Pt‐titania, Au‐ceria, etc.) are the current front runners. A critical review of the mechanism of the WGS reaction is also presented.

Water Gas Shift Catalysis

Selective catalytic reduction with NH3 (NH3-SCR) is the most efficient technology to reduce the emission of nitrogen oxides (NOx) from coal-fired industries, diesel engines, etc. Although V2O5-WO3(MoO3)/TiO2 and CHA structured zeolite catalysts have been utilized in commercial applications, the increasing requirements for broad working temperature window, strong SO2/alkali/heavy metal-resistance, and high hydrothermal stability have stimulated the development of new-type NH3-SCR catalysts. This review summarizes the latest SCR reaction mechanisms and emerging poison-resistant mechanisms in the beginning and subsequently gives a comprehensive overview of newly developed SCR catalysts, including metal oxide catalysts ranging from VOx, MnOx, CeO2, and Fe2O3 to CuO based catalysts; acidic compound catalysts containing vanadate, phosphate and sulfate catalysts; ion exchanged zeolite catalysts such as Fe, Cu, Mn, etc. exchanged zeolite catalysts; monolith catalysts including extruded, washcoated, and metal-mesh/foam-based monolith catalysts. The challenges and opportunities for each type of catalysts are proposed while the effective strategies are summarized for enhancing the acidity/redox circle and poison-resistance through modification, creating novel nanostructures, exposing specific crystalline planes, constructing protective/sacrificial sites, etc. Some suggestions are given about future research directions that efforts should be made in. Hopefully, this review can bridge the gap between newly developed catalysts and practical requirements to realize their commercial applications in the near future.

Selective Catalytic Reduction of NOx with NH3 by Using Novel Catalysts: State of the Art and Future Prospects.

An overview is given of the selective catalytic reduction of NOx by ammonia (NH3‐SCR) over metal‐exchanged zeolites. The review gives a comprehensive overview of NH3‐SCR chemistry, including undesired side‐reactions and aspects of the reaction mechanism over zeolites and the active sites involved. The review attempts to correlate catalyst activity and stability with the preparation method, the exchange metal, the exchange degree, and the zeolite topology. A comparison of Fe‐ZSM‐5 catalysts prepared by different methods and research groups shows that the preparation method is not a decisive factor in determining catalytic activity. It seems that decreased turnover frequency (TOF) is an oft‐neglected effect of increasing Fe content, and this oversight may have led to the mistaken conclusion that certain production methods produce highly active catalysts. The available data indicate that both isolated and bridged iron species participate in the NH3‐SCR reaction over Fe‐ZSM‐5, with isolated species being the ...

https://www.tandfonline.com/doi/pdf/10.1080/01614940802480122

The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal‐Exchanged Zeolite Catalysts

Characterization of model automotive exhaust catalysts: Pd on ceria and ceria–zirconia supports

Evidence for Oxidation of Ceria by CO2

The relation between deactivation of CuZSM-5 in the selective reduction of NO and dealumination of the zeolite

The extent of the selective catalytic reduction (SCR) of nitric oxide to dinitrogen in the presence of excess oxygen is enhanced by the oxygen on several zeolite-based catalysts and using different reductants. When the catalyst is Cu-ZSM-5 and the reductant is a hydrocarbon, an NO2 intermediate has been suggested by several investigators. This work shows that at short residence times, with excess reductant and in the absence of oxygen, the NO2 itself is reduced only back to NO. Thus, for the selective reduction of NO2 to N2 (N-pairing) strongly oxidizing conditions are required, same as for the complete reduction of NO. In the presence of excess oxygen the activity of Cu-ZSM-5 in the NO + O2 reaction to form NO2 parallels the SCR in every respect. It is higher over Cu-ZSM-5 than on Cu/Al2O3 or on H-ZSM-5. The coppercontaining zeolite is also active in the decomposition of NO2 back to NO and O2 while the other catalysts are much less active. The inhibiting effect of water on the NO + O2 catalytic reaction is also parallel to the effect on SCR. This evidence strengthens the notion of an NO2 intermediate.

H.-W. Jen

Papers

Characterization of model automotive exhaust catalysts: Pd on ceria and ceria–zirconia supports

Evidence for Oxidation of Ceria by CO2

The relation between deactivation of CuZSM-5 in the selective reduction of NO and dealumination of the zeolite

NO2 formation over Cu-ZSM-5 and the selective catalytic reduction of NO

In Situ ESR Monitoring of CuH-ZSM-5 Up to 500°C in Flowing Dry Mixtures of No(NO2), C3H6(C2H5OH), and Excess O2