Combustion catalyst layer

Pt is, however, an expensive and limited resource. For a 60 kW fuel cell vehicle, the cost of Pt would be over 2,400 at current cost and loading of Pt. Even worse, replacing combustion engines in all existing vehicles by fuel cell drive systems at no penalty in power would exceed the known reserves of Pt. Catalyst layer design, therefore, strives to reduce the Pt loading markedly at no penalty in the fuel cell voltage. 

Air from the compressor is split into two streams primary air is premixed with the fuel and then fed to the catalyst, which is operated under O2 defect conditions secondary air is used first as a catalyst cooling stream and then mixed with the partially converted stream from the catalyst in a downstream homogeneous section where ignition of gas-phase combustion occurs and complete fuel burnout is readily achieved. The control of the catalyst temperature below 1000 Cis achieved by means of O2 starvation to the catalyst surface, which leads to the release of reaction heat controlled by the mass transfer rate of O2 in the fuel-rich stream and of backside cooling of the catalyst with secondary air. To handle both processes, a catalyst/heat exchanger module has been developed, which consists of a bundle of small tubes externally coated with an active catalyst layer, with cooling air and fuel-rich stream flowing in the tube and in the shell side, respectively [24]. 

The active layer must provide the required activity, selectivity and thermochemical stability properties. Different active phases can be adopted depending on the operating constraints and the fuel type. In the following we will mainly focus on CH4 (i.e. the main constituent of natural gas) as the reference fuel for GT applications. In this respect, the combustion catalysts that have been most extensively investigated for configurations based on lean combustion concepts are PdO-based systems and metal-substituted hexaaluminates. 

Catalytic experiments were done with 75 ml of a catalyst and FAV=10 000 m /h/kgcat-Combustion of toluene was chosen as a model volatile organic compound and non-steady-state reaction condition (linear increase 3.5 °C/min in reaction temperature) was applied in the catalyst activity evaluation. Concentration of toluene in air was 1 g /m. Temperatures of gaseous reaction mixture entering and leaving the catalyst layer were measured by thermocouples. Catalytic activities expressed as the inlet temperatures Tso or T90, at which 50 or 90 % conversions of toluene were achieved, were taken as a measure of the catalytic activity. 

Monoliths made of metal foils can also be used as substrates in combustion catalysts [19, 20]. The metal is generally an iron- or nickel-based steel containing small amounts of aluminum. The aluminum diffuses to the surface on heating and oxidizes to form an adherent alumina layer. This alumina layer gives the alloy high oxidation resistance and is essentially self-healing as it arises from diffusion from the bulk material. It also provides good adhesion for the alumina washcoat. 

Arai, H., K. Eguchi and M. Machida, 1989, Cation substituted layered hexaaluminates for a high-temperature combustion catalyst, in Proc. MRS Int. Meeting on Advanced Materials, Vol. 2, p. 243. Arai, H., K. Eguchi, M. Machida and T. Shiomitsu, 1991, Catal. Sci. Technol. 1, 195. 

Yoshida et al. [173] designed an integrated methanol fuel processor from silicon and Pyrex glass substrates for a power equivalent of 10 W. It contained functional layers for steam reforming, evaporation, and combustion. Commercial Cu/ ZnO catalyst served for reforming and the Pt/TiOa combustion catalyst was prepared by a sol-gel method. A power density of 2.1 W/cm was determined for the device. 

The combustion catalyst was impregnated onto the a-alumina layer on the Fecralloy surface (see Section 10.2.1). The catalysts were reduced prior to testing, which does not seem very viable for a later practical application. The anode off-gas was mixed upstream of the reactor by laboratory equipment and then further mixed with air in a separate micro-mixer. The steam reforming side of the reactor was operated at an... 

Extended periods at open circuit and idle conditions should be avoided to improve catalyst layer stability, hi a fuel-ceU hybrid vehicle, this can be accomphshed by utilizing the energy-storage system (e.g., batteries or capacitors) in a manner analogous to that employed in hybrids that use internal combustion engines. Owing to parasitic power requirements, fuel-cell power plants are inefficient at extremely low power therefore, the time spent at these conditions should be minimized to maximize efficiency and life. 

Lead compounds were not found on the surrounding activated coating layer, rather only associated with the precious metal. The Pt sites are less poisoned by lead than are Pd or Rh sites because the Pt sites are protected by the sulfur in the fuel. Fuel sulfur is converted to SO2 in the combustion process, and Pt easily oxidizes SO2 to SO on the catalyst site. The SO reacts with the lead compounds to form PbSO, which then moves off the catalyst site so that lead sulfate is not a severe catalyst poison. Neither Pd nor Rh is as active for the SO2 to SO reaction, and therefore do not enjoy the same protection as Pt. 

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