Catalyst material

As can be seen from Eigure 11b, the output voltage of a fuel cell decreases as the electrical load is increased. The theoretical polarization voltage of 1.23 V/cell (at no load) is not actually realized owing to various losses. Typically, soHd polymer electrolyte fuel cells operate at 0.75 V/cell under peak load conditions or at about a 60% efficiency. The efficiency of a fuel cell is a function of such variables as catalyst material, operating temperature, reactant pressure, and current density. At low current densities efficiencies as high as 75% are achievable. 

J. C. Downing and K. P. Goodboy, "Qaus Catalysts and Alumina Catalyst Materials and Their AppHcation," ia L. D. Hart, ed.. Alumina Chemicals Handbook, American Ceramic Society, WesterviUe, Ohio, 1990. 

Rhenium oxides have been studied as catalyst materials in oxidation reactions of sulfur dioxide to sulfur trioxide, sulfite to sulfate, and nitrite to nitrate. There has been no commercial development in this area. These compounds have also been used as catalysts for reductions, but appear not to have exceptional properties. Rhenium sulfide catalysts have been used for hydrogenations of organic compounds, including benzene and styrene, and for dehydrogenation of alcohols to give aldehydes (qv) and ketones (qv). The significant property of these catalyst systems is that they are not poisoned by sulfur compounds. 

Electroless Electrolytic Plating. In electroless or autocatalytic plating, no external voltage/current source is required (21). The voltage/current is suppHed by the chemical reduction of an agent at the deposit surface. The reduction reaction must be catalyzed, and often boron or phosphoms is used as the catalyst. Materials that are commonly deposited by electroless plating (qv) are Ni, Cu, Au, Pd, Pt, Ag, Co, and Ni—Fe (permalloy). In order to initiate the electroless deposition process, a catalyst must be present on the surface. A common catalyst for electroless nickel is tin. Often an accelerator is needed to remove the protective coat on the catalysis and start the reaction. 

Binders. To create needed physical strength in catalysts, materials called binders are added (51) they bond the catalyst. A common binder material is a clay mineral such as kaolinite. The clay is added to the mixture of microparticles as they are formed into the desired particle shape, for example, by extmsion. Then the support is heated to remove water and possibly burnout material and then subjected to a high temperature, possibly 1500°C, to cause vitrification of the clay this is a conversion of the clay into a glasslike form that spreads over the microparticles of the support and binds them together. 

Catalyst Material used to activate resins to promote hardening. For polyesters, organic peroxides are used primarily. For epoxies, amines and anhydrides are used. 

The reactor volume is calculated from Mj and the bulk density of the catalyst material, (-r ) depends not only on composition and temperature, but also on the nature and size of the catalyst pellets and the flow velocity of the mixture. In a heterogeneous reaction where a solid catalyst is used, the reactor load is often determined by the term space velocity, SV. This is defined as the volumetric flow at the inlet of the reactor divided by the reaction volume (or the total mass of catalyst), that is... 

The table also gives the catalyst material, the operating temperature range, the maximum (for A>1) or minimum (for A<-1) measured A value and the maximum (for p>l) or minimum for (prate enhancement, p, value. It also provides the maximum measured promotion index, PIi5 value. An asterisk in the p column indicates that electrochemical promotion causes also a change in product selectivity. 

I. Material cost minimization The main consideration here is the problem of efficient catalyst material utilization which requires the use of thin (e.g. 10 nm thick) catalyst electrodes or dispersed catalysts.7... 

Diesel and gasoline exhaust gas purification has drawn increasing attention. Catalyst materials containing different noble metals, such as Pd, Pt, and Rh,... 

Analytical electron microscopy (AEM) permits elemental and structural data to be obtained from volumes of catalyst material vastly smaller in size than the pellet or fluidized particle typically used in industrial processes. Figure 1 shows three levels of analysis for catalyst materials. Composite catalyst vehicles in the 0.1 to lOim size range can be chemically analyzed in bulk by techniques such as electron microprobe, XRD, AA, NMR,... 

Figure 1. Three levels of analysis for catalyst materials, a) bulk analysis of an entire catalyst pellet, b) surface analysis and depth profiling from the surface inward, c) analytical electron microscopy of individual catalyst particles too small for analysis by other techniques.

Analysis of catalyst materials using digital x-ray Images of elements in an AEM offers a method of quickly finding significant areas for analysis. Indeed, Important elements acting as promoters or poisons may not be located at obvious image features at all. 

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

Operating pressure 1.2 bar Catalyst material formation 3oTi7oOx Si02 suspension + impregnation finally anodic oxidation... 

Catalyst material liAlOj (sol-gel) Device inner volume 650 mm ... 

Reactor type Chip reactor with thin-film sensors and membrane Catalyst material Pt... 

Catalyst material Sputtered silver Operating pressure 1 atm... 

Catalyst material Pt (sputtered) Pt/y-alumina (sol-gel) 2.5 pm Glass plate thickness 1 mm... 

The imdoubted advantage of mini fixed-bed micro reactors is that they foUow a widely accepted processing path and in principle can use all of the commercial catalysts, if they can be crushed to a size much below the micro-channel diameter. Hence catalyst material flexibility is a major driver. 

Catalyst material Pd/AfOj Pt/AfOj Rh/AfOj Meander net centerline length -2.2 mm... 

Catalyst material Activated carbon Post channels at outlet width 25 pm... 

Catalyst material Platinum Housing b diameter length 22 mm 40 mm... 

Catalyst material Pt/Sn on alumina/silica filament Specific surface area lOSm m ... 

Similar findings were made by BASF in studies investigating an undisclosed gas-phase reaction in capillaries made of quartz, catalyst material and reactor-wall material [105]. The dimensions were chosen in such a way that they match the of surface-to-volume ratio of a fixed-bed reactor used previously for the same reaction. A quartz capillary shows no conversion, whereas reactor-wall material actually has a greater activity than the catalyst itself Hence BASF came to the, at first sight, surprising conclusion that in their production process it was the reactor wall, and not the catalyst, which catalyzes the reaction. The reactor wall was 70 times more active than the catalyst it needs a temperature increase of about 100 °C to have both at equal conversion. 

Reactor type Micro-channel reactor in disk housing Catalyst material Pd... 

Micro channel depth 100-200 pm Total catalyst material 20 mg... 

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