CO poisons

Co3(P04)2 8H20 CO poisoning Copoly(amide-imides) Copolyamides... 

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Negleeting CO desorption, as in the standard ZGB model, the CO-poi-soned state is irreversible sinee there is no possibility of removing CO from the surfaee. So, CO desorption has to be eonsidered in order to avoid the fully CO-poisoned state. The adsorption and desorption of X then drives the system from a state with high eoneentration of adsorbed CO to the reaetive state and baek. This proeess ean be understood with the aid of Fig. 8. At low X eoverage only the reaetive state is stable. Inereasing X eoverage eauses site bloeking and eonsequently the adsorption of both CO and O2 is redueed. 

Proton Exchange Membrane 0-85 Can operate at ambient temperature High power density Sensitive to CO-poisoning Need for humidification Transportation Distributed Power... 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

The catalysts at the anode can be made less sensitive to CO poisoning by alloying platinum with other metals such as ruthenium, antimony or tin[N.M. Markovic and P.N. Ross, New Flectro catalysts for fuel cells CATTECH 4 (2001) 110]. There is a clear demand for better and cheaper catalysts. Another way to circumvent the CO problem is to use proton-exchange membranes that operate at higher temperatures, where CO desorbs. Such membranes have been developed, but are not at present commercially available. 

CO poison can probably allow observation of the v(CH2) modes of the first products of the polymerization. 

The lower total activity for Rh electrodes may be partly due to increased CO poisoning and slower CO electro-oxidation kinetics compared with Pt electrodes, as demonstrated by the number of voltammetric cycles required to oxidize a saturated CO adlayer from Rh electrodes (see Section 6.2.2) [Housmans et al., 2004]. In addition, it is argued that the barrier to dehydrogenation is higher on Rh than on Pt, leading to a lower overall reaction rate [de Souza et al., 2002]. These effects may also explain the lower product selectivity towards acetaldehyde and acetic acid, which require the dehydrogenation of weakly adsorbed species. 

However, the Pt anode is seriously poisoned by trace amounts of carbon monoxide in reformates (fuel gas reformed from hydrocarbon), because CO molecules strongly adsorb on the active sites and block the HOR [Lemons, 1990 Igarashi et ah, 1993]. Therefore, extensive efforts have been made to develop CO-tolerant anode catalysts and cell operating strategies to suppress CO poisoning, such as anode air-bleeding or pulsed discharging. 

Figure 10.1 shows 4 at various electrodes as a function of CO poisoning time at 26 °C. For the pure Pt electrode, the value of 4 decreases and reaches nearly zero after 30 minutes. In contrast, the Pt-Fe, Pt-Ni, Pt-Co, and Pt-Mo alloys retain high HOR activity for a prolonged period of time the reduction in 4 is negligibly small. Such CO tolerance of these alloys was found to be almost independent of the composition for example, alloying Pt with only 5 at% Fe resulted in excellent tolerance. However, Pt alloys with Ti, Cr, Cu, Ge, Nb, Pd, In, Sb, W, An, Pb, or Bi showed complete CO poisoning after a short time, while the combination of Pt with Mn, Zn, Ag, or Sn exhibited only limited CO tolerance. 

Besides using Pt-Ru or Pt-Co alloy anodes, CO poisoning can be mitigated by elevating the operating temperature. However, temperature dependencies of the HOR rates in the presence of CO with relevance to PEFC operation have been scarcely reported. One of the difficulties is correction of the change in H2 concentration [H2] in the... 

Waszczuk P, Wieckowski A, Zelenay P, Gottesfeld S, Coutanceau C, Leger JM, Lamy C. 2001b. Adsorption of CO poison on fuel ceU nanoparticle electrodes from methanol solutions A radioactive labeling study. J Electroanal Chem 511 55-64. 

Formulating a hypothesis Though it seems far-fetched, CO poisoning is hypothesized to be the cause of the fish kill. 

Pt (5 wt%) supported on platelet and ribbon graphite nanofibers exhibited similar activities to those observed by Pt (25 wt°/o) on carbon black [138], This phenomenon was attributed to the crystallographic orientations adopted by the catalyst particles dispersed on graphitic nanofiber structures [139]. Also, the electrocatalysts supported on CNFs were less susceptible to CO poisoning than Pt supported on carbon black. 

CO Poison (50 ppm per stack) Poison Poison (<0.5%) Fuef Fuel Fnel... 

As discussed previously, both temperature and CO concentration have a major influence on the oxidation of H2 on Pt in CO containing fuel gases. Benjamin et al. (35) derived Equation (5-12) for the voltage loss resulting from CO poisoning as a function of temperature... 

Alberti et al. investigated the influence of relative humidity on proton conductivity and the thermal stability of Nafion 117 and compared their results with data they obtained for sulfonated poly(ether ether ketone) membranes over the broad, high temperature range 80—160 °C and RHs from 35 to 100%. The authors constructed a special cell used in conjunction with an impedance analyzer for this purpose. Data were collected at high temperatures within the context of reducing Pt catalyst CO poison-... 

The organoactinide surface complexes exhibited catalytic activities comparable to Pt supported on sihca [at 100% propylene conversion at —63°C, >0.47s (U) and >0.40 s (Th)], despite there being only a few active sites (circa 4% for Th, as determined by CO poisoning experiments and NMR spectroscopy) [92]. Cationic organoactinide surface complexes [Cp An(CH3 ) ] were proposed as catalytic sites. This hypothesis could be corroborated by the use of alkoxo/hydrido instead of alkyl/hydrido surface ligands, which led to a marked decrease of the catalytic activity, owing to the oxophilic nature of the early actinides [203, 204]. Thermal activation of the immobihzed complexes, support effects, different metal/ligand environments and different olefins were also studied. The initial rate of propylene conversion was increased two-fold when the activation temperature of the surface complexes under H2 was raised from 0 to 150°C (for Th 0.58 0.92 s" ). 

PEM fuel cells operate at relatively low temperatures, around 80°C. Low temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. However, they require that a noble-metal catalyst (typically platinum) be used to separate the hydrogen s electrons and protons, adding to system cost. The platinum catalyst is also extremely sensitive to CO poisoning, making it necessary to employ an additional reactor to reduce CO in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel. This also adds cost. Developers are currently exploring platinum/ruthenium catalysts that are more resistant to CO. 

Occupants of the premises shall then be examined. If they are experiencing CO poisoning symptoms—that is, headaches, nausea, confusion, dizziness, and other flu-like symptoms—an EMS crew shall be notified immediately and the occupants evacuated and administered oxygen. 

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