Reformate-Tolerant Catalyst Stability

The reformate gas contains up to 12% CO for SR and 6 to 8% CO for ATR, which can be converted to H2 through the WGS reaction. The shift reactions are thermodynamically favored at low temperatures. The equilibrium CO conversion is 100% at temperatures below 200°C. However, the kinetics is very slow, requiring space velocities less than 2000 hr1. The commercial Fe-Cr high-temperature shift (HTS) and Cu-Zn low-temperature shift (LTS) catalysts are pyrophoric and therefore impractical and dangerous for fuel cell applications. A Cu/CeOz catalyst was demonstrated to have better thermal stability than the commercial Cu-Zn LTS catalyst [37], However, it had lower activity and had to be operated at higher temperature. New catalysts are needed that will have higher activity and tolerance to flooding and sulfur. 

The MCFC has some disadvantages, however the electrolyte is very corrosive and mobile, and a source of CO2 is required at the cathode (usually recycled from anode exhaust) to form the carbonate ion. Sulfur tolerance is controlled by the reforming catalyst and is low, which is the same for the reforming catalyst in all cells. Operation requires use of stainless steel as the cell hardware material. The higher temperatures promote material problems, particularly mechanical stability that impacts life. 

Solid oxide catalysts such as hexaaluminates and perovskites, in which an active metal catalyst is incorporated into a coke-resistant lattice, are effective for liquid hydrocarbon reforming due to their thermal stability over a broad-range of temperature. However, sulfur tolerance of those materials has yet to be demonstrated. 

Rather than increasing the operating S/C ratio, it is more desirable to develop reforming catalysts that are inherently more carbon-tolerant than Ni [19, 35, 37-43], For example, it has been suggested that Ru and Rh do not facilitate the formation of carbon deposits because of poor carbon solubility in these metals [30, 44]. However, Ru and Rh are prohibitively expensive. It has also been shown that the promotion of Ni with alkaline earth metals such as Mg suppresses carbon-induced catalyst deactivation [18, 36]. There have also been reports that by selectively poisoning the low-coordinated Ni sites with small amounts of sulfur, the carbon-induced deactivation of Ni can be suppressed [9, 45]. In addition, the patent literature is rich with multiple examples where numerous additives, including those mentioned below in this text (e.g., Sn and Au), have been suggested to promote the stability of Ni catalysts [46]. 

The catalysts are required to promote the desired reactions to occur at an appreciable rate. The catalysts used in PEM fuel cells are typically Pt-based due to the high stability and reactivity of Pt. Pt alloys may also be introduced to further increase kinetic activity, improve stability, and improve tolerance to contaminants on the anode for use in reformed fuel. The high cost of Pt necessitates a maximum utilization of the Pt. For this reason, Pt is typically in the form of very small particles of approximately 2 to 8 nm diameters, supported on larger carbon particles. The carbon particles provide a high surface area support structure to enhance the dispersion of the catalyst particles as well as providing an electrical and thermal pathway from the reaction sites... 

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