Active Catalyst Layer

In order to obtain adjacent active and inactive channels which provide internal heat exchange capability as described in Section 12.3.3, the metal foils are generally coated on a single side with the active catalyst layer. Deposition of the active layer is typically effected by spraying methods [18, 59-61]. 

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. 

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An unstabilized high surface area alumina siaters severely upon exposure to temperatures over 900°C. Sintering is a process by which the small internal pores ia the particles coalesce and lose large fractions of the total surface area. This process is to be avoided because it occludes some of the precious metal catalyst sites. The network of small pores and passages for gas transfer collapses and restricts free gas exchange iato and out of the activated catalyst layer resulting ia thermal deactivation of the catalyst. 

U.S. producers of, 4 748t Activated carbon adsorption, as advanced wastewater treatment, 25 909 Activated catalyst layer, 70 40-42 Activated charcoal, 73 461 Activated coke, for SO and NO removal, 77 720... 

Alumina, 2 345t 5 582. See also Activated alumina Aluminum oxide (alumina) Bauxite(s) Calcined alumina Fused alumina Tabular alumina in the activated catalyst layer, 10 41 adsorption capacity vs. years of service, 1 630... 

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]. 

Although this reaction network has been studied extensively, its mechanism is still under debate (10). In this study, a single-pellet reactor was used, and the pellet was prepared mechanically by pressing the active catalyst layer between two alumina layers. In this way a step-type catalyst pellet was produced, which approximated a Dirac-type catalyst distribution. 

Figure 13.9 Schematic diagram of a polymer electrolyte fuel cell (A) gas manifolding, (B) porous graphite block, (C) active catalyst layer (dispersed Pt and Teflon binder), and (D) polymer electrolyte.

Vs - volume of the active catalyst layer Xd - liquid reactant conversion z - axial coordinate Eg - bed porosity... 

Aligned multiwall CNT arrays were synthesized as a basis for a microstructured catalyst, which was then tested in the Fischer-Tropsch reaction in a microchannel reactor [269]. Fabrication of such a structured catalyst first involved MOCVD of a thin but dense A1203 film on a FeCrAlY foam to enhance the adhesion between the catalyst and the metal substrate. Then, multiwall CNTs were deposited uniformly on the substrate by controlled catalytic decomposition of ethene. Coating the outer surfaces of the nanotube bundles with an active catalyst layer results in a unique hierarchical structure with small interstitial spaces between the carbon bundles. The microstructured catalyst was characterized by the excellent thermal conductivity inherent to CNTs, and heat could be efficiently removed from the catalytically active sites during the exothermic Fischer-Tropsch synthesis. 

SD is routinely used to deposit thin films and has proven benefits from economies of scale in the metallization of plastics. The technique has already been used to create enhanced and unique MEAs for H2 -air proton exchange membrane fuel cell (PEMFC) systems. In this project, JPL is pursuing the use of SD to create DMFC membrane electrode assembly structures with highly electro-active catalyst layers that will reduce the amount and cost of the Pt-alloy catalyst at the fuel cell anode. 

Loss of catalytic activity Catalyst layer fracture... 

The general layout of a cell includes a proton-conducting polymer electrolyte membrane (PEM), sandwiched between the anode and the cathode. Each electrode compartment is composed of (i) an active catalyst layer (CL), which accommodates finely dispersed nanoparticles of Pt that are attached to the surface of a highly porous and electronically conductive support, (ii) a gas diffusion layer (GDL), and (iii) a flow field (FF) plate that serves at the same time as a current collector (CC) and a bipolar plate (BP). This plate conducts current between neighboring cells in a fuel cell stack. At the cathode side, usually a strongly hydrophobic microporous layer (MPL) is inserted between CL and GDL, which facilitates the removal of product water from the cathode CL. The central unit including PEM and porous electrode layers, excluding the bipolar plates, is called the membrane electrode assembly (MEA). 

Gas concentration distributions in gas flow channel, gas diffusion, and active catalyst layers. 

On the other hand, while chlorides accumulate near the top of the catalyst, they are more mobile and can be detected in significant concentrations, up to 0.05%, at all levels in a deactivated bed. Although reasonable hves of at least two years can often be achieved in the presence of chloride there is more rapid movement of the peak in temperature profile, and the concentration of carbon monoxide in the outlet gas increases more rapidly. Surface chlorides, which are formed by reaction with zinc oxide, are mobile and sinter the catalyst surface. Chlorides are also soluble in condensed steam and can be washed down onto lower, more active catalyst layers. 

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