Poisoning tolerance

CO2 is generally considered as a diluent to H2 in the reformate with PtRu as the anode catalyst. But if the fuel cell condition leads to the reversed water gas shift (RWGS) to occur (please refer to Section 5.2.1), trace amounts of CO may be able to form according to Reactions 4.11 and 4.12  

A study reported that the presence of 40% CO2 lowered the fuel cell performance by 10% (Rajalakshmi 2004). Thermodynamically, there is no doubt that some CO can form. Assuming a gas mixture from steam reforming CH4 containing 75% H2 and 25% CO2 vol is fully saturated with water at 27°C, the mixture will contain 3.5% H2O, 72% H2, and 24% CO2 vol. Using the reaction 

Voltage loss caused by CO poisoning is fully recoverable when CO-free H2 is subsequently used. 

H2S is a worse poison than CO to the anode, and it can severely poison the anode even at ppb levels. It forms a S layer on the surface of the catalyst according to Reaction 4.11  

And thus the poisoning is not recoverable when switching to S-fee H2. Fortunately, H2S and all the S-containing compoimds should have been removed prior to the fuel processing steps and thus the fuel stream is not expected to contain any of it before entering the fuel cell. 

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Catalyst contamination from sources such as turbine lubricant and boiler feed water additives is usuaUy much more severe than deactivation by sulfur compounds in the turbine exhaust. Catalyst formulation can be adjusted to improve poison tolerance, but no catalyst is immune to a contaminant that coats its surface and prevents access of CO to the active sites. Between 1986 and 1990 over 25 commercial CO oxidation catalyst systems operated on gas turbine cogeneration systems, meeting both CO conversion (40 to 90%) and pressure drop requirements. 

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

Platinum-free electrocatalysts for fuel cells could be designed when Pd on carbon electrocatalysts promoted with nanocrystal oxide particles like C03O4, Mn30 and NiO, were used [62], In terms of activity and poison tolerance, the latter were significantly superior. 

In the case of ethanol, Pd-based electrocatalysts seem to be slightly superior to Pt-based catalysts for electro-oxidation in alkaline medium [87], whereas methanol oxidation is less activated. Shen and Xu studied the activity of Pd/C promoted with nanocrystalline oxide electrocatalysts (Ce02, C03O4, Mn304 and nickel oxides) in the electro-oxidation of methanol, ethanol, glycerol and EG in alkaline media [88]. They found that such electrocatalysts were superior to Pt-based electrocatalysts in terms of activity and poison tolerance, particularly a Pd-NiO/C electrocatalyst, which led to a negative shift of the onset potential ofthe oxidation of ethanol by ca 300 mV compared... 

Although the above-discussed studies have defined sulfur-poisoning tolerances for conventional nickel catalysts used in steam reforming of natural gas and naptha, they have not considered in sufficient detail the kinetics of poisoning at above-threshold concentrations nor the effects of catalyst and/or gas compositions on rate of deactivation and tolerance level. Nor is there any previous report on the effects of sulfur on product distribution (i.e., relative rates of production of H 2, CO, CH4) in steam reforming of hydrocarbons. 

This term refers to the sensitivity of a catalyst to poisoning under specified conditions. Two other terms are typically used to describe poisoning susceptibility. Poisoning resistance is the degree to which a catalyst resists deactivation, i.e., a catalyst which deactivates slowly is more resistant to poisoning than one that deactivates rapidly. Poisoning tolerance is defined typically as either the ultimate amount of poison a catalyst can adsorb and... 

The effect of a fourfold change in catalyst volume on catalyst deterioration is depicted in Figure 4. The smallest converter (1000 cm3) had a rapid initial loss in activity followed by a milder loss rate. The milder loss rate paralleled the rate of activity loss of the larger converters. The larger converter was clearly better for poison tolerance. 

Pd-Ni (Pd4oNieo) [26] Dealloying a ternary AlysPdjoNiis alloy under free corrosion conditions Vulcan -0.4 V(vs. Hg/HgO) Better E and current dcmsily than nanoporous Pd/vulcan. This is supported by the high /f/Zb ratio of the Pd4oNi o, indicating a better poisoning tolerance... 

Modern sensors have been modified with a heating circuit to be more poison tolerant to the P and Si found in the engine exhaust and to improve the operating temperature range of the O2 sensor during driving, particularly in cold start (23). 

The catalyst is typically evaluated for its composition, size, size distribution, shape, activity, durability, poisoning tolerance, and mass loss. 

Bouwman P, Teliska ME, Lyons K. Increased poisoning tolerance of Pt-FePO oxygen reduction catalysts. In Proton conducting membrane fuel cells IV. Van Zee et al., editors. Electrochenucal Society Proceedings 2004. 

Bouwman, P.J., Teliska, M.E., Swider Lyons, K. 2006. Increased poisoning tolerance of Pt-FePo oxygen reduction catalysts. Proton Conducting Membrane Fuel Cell IV Proceedings of the International Symposium. 222-6. 

The addition of a nanocrystalline oxide, Ce02, C03O4, Mns04 and NiO [102], In203 [103], 1102 [104], Mn02 [105], or [106] has been observed to promote the activity of Pd catalyst toward AOR significantly. These oxides improved the poisoning tolerance of metal catalysts also. 

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