Catalyst wash-coated

Several authors, [3], [4], have shown that a non homogeneous flow distribution over the monolith cross sectional area can cause both a decline of conversion efficiency and a premature ageing of the catalyst wash coat due to localized poisoning and thermal degradation. 

Ryu, J-H, Lee, K-Y, La, H, Kim, H-J, Yang, J-I, Jung, H. Ni catalyst wash-coated on metal monohth with enhanced heat-transfer capability for steam reforming. J. Power Sources 2007 171 499-505. 

Materials that are routinely coated with catalyst wash-coats are ceramics such as cordierite, which is the construction material of ceramic monoliths, metals such as Fecralloy, the construction material of metallic monoliths (see Section 6.2) and stainless steel [57]. The amount of catalyst material that can be coated onto a monolith ranges between 20 and 40 g m , while plate heat-exchangers may even take up more catalyst when coated prior to the sealing procedure, because the access to the channels is better. 

Ferrandon and Krause investigated the effect of the catalyst support on the performance of rhodium catalysts wash-coated onto cordierite monoliths for autothermal reforming of gasoline [245]. The samples contained 2 wt.% rhodium on gadolinium/ceria, and lanthanum-stabilised alumina [245]. The latter sample showed higher activity and superior selectivity. Only 30 ppm of light hydrocarbons (C > 1) were detected. However, the sample also had higher surface area and rhodium dispersion. Both samples showed stable performance for more than 50-h test duration. 

To add surface area, the supports are uniformly coated with a slurry of gamma-alumina and recalcined under moderate conditions. The wash coat acts to accept the active metals, typically low levels of platinum and palladium, in a conventional impregnation process. In the United States in passenger car apphcations the spherical catalyst is used almost exclusively, and methods have been developed to replace the catalyst without removing the converter shell when vehicle inspection reveals that emission standards are not met. 

The most widely used exhaust control device consists of a ceramic monolith with a thin-waHed open honeycomb stmcture. The accessible surface of this monolith system is iacreased by applyiag a separate coatiag, a wash coat, of a high surface area material such as gamma-alumiaa with the catalyticaHy active species impregaated iato this washcoat. The catalyst aeeds to oxidize hydrocarboas, coavert CO to CO2, and reduce NO. The whole system forms a catalytic converter that, suitably encased, is placed between the engine and the muffler/silencer unit. 

The reactor configuration we propose, shown in Figs. 1 and 2, allows rapid heat transfer along the axial direetion of the reactor by conduction through the wall made of high conductivity metal such as copper or aluminum. The catalyst can be packed into the honeycomb cells or wash coated on the walls of the cells. 

Figure 2.66 Cross-section of a micro channel coated with a catalyst layer (left) (source INM, Saarbrucken, Germany) and typical surface morphology of wash-coat catalyst carriers (right).

Zeolite catalysts and adsorbents have also been incorporated into monolithic contactors by several routes, including extruded zeoHte/binder composites [70], wash-coated ceramic monoliths [71] and corrugated thin-sheet monoliths [72]. 

The catalyst bed was a coated wall reactor using commercial CuZnAl catalyst. An alumina sol was used to enhance the adhesion of the catalyst to the channel walls. After the shims were washed thoroughly, the alumina adhesion layer was deposited using an alumina sol (NYACOL AL20DW colloidal alumina, PQ Corporation) and then dried at 60 °C. To decrease the surface tension of the wash-coat solvent, small amounts of 2-propanol were added to a catalyst slurry of ICI Synetix 33—5 catalyst, with 20 wt % alumina sol and water. The catalyst was calcined at 350—400 °C for 2 h after air-drying. Before testing, the catalyst was reduced by flowing H2/N2 over it at 280 °C. 

To process the methanol and water mix, a CuZnAl catalyst was wash-coated onto the microchannel walls. The alumina was deposited by dipping the plates into a 20% alumina suspension, which also included a stabilizer and a binder. After any excess was wiped off, the plates were calcined at 600 °C for 1 h in air. Air was removed from the pores by placing the calcined plate in a vacuum. The alumina wash-... 

The catalyst, used in the form of a ceramic honeycomb monolith, is constituted, as in mobile applications, by a noble metal and an absorber element, such as potassium, deposited on a Y-AI2O3 wash-coat layer. In the oxidation and absorption cycle, the SCON Ox catalyst works by simultaneously oxidizing CO and UHCs to CO2 and H2O, while NOj, are captured on the adsorber compound. Catalyst regeneration is accomplished by passing a controlled mixture of regeneration gases across the surface of the catalyst in the absence of oxygen. 

Figure 7-5 Porous catalyst pellets consisting of spheres or grains pressed together into spheres or cylinders. Reactants must diffuse into the pellet in the space between spheres or grains and products must diffuse out of the pellet for reaction to occur. At the right is shown a monolith catalyst in which a ceramic is coated with a wash coat (gray) of porous catalyst.

Figure 7-19 Sketch of a catalytic tube vrall reactor Mtii gases floMng dowi the tube and reaction occurring on the w s of the tube, which are coated with a wash coat of catalyst (dots).

This is exactly the situation we considered previously for diffusion in porous catalysts so we multiply r" in the porous wash coat by the effectiveness factor t to obtain... 

The SCR catalyst designed for FCCU regenerator flue gas service is a homogenous monolith, typically made from 1 mm thick material. Some catalysts are extruded clays that receive a wash coat of titanium dioxide before impregnation of the vanadium and tungsten metals. Another type involves painted plates of expanded metal... 

Since 1981, three-way catalytic systems have been standard in new cars sold in North America.6,280 These systems consist of platinum, palladium, and rhodium catalysts dispersed on an activated alumina layer ( wash-coat ) on a ceramic honeycomb monolith the Pt and Pd serve primarily to catalyze oxidation of the CO and hydrocarbons, and the Rh to catalyze reduction of the NO. These converters operate with a near-stoichiometric air-fuel mix at 400-600 °C higher temperatures may cause the Rh to react with the washcoat. In some designs, the catalyst bed is electrically heated at start-up to avoid the problem of temporarily excessive CO emissions from a cold catalyst. Zeolite-type catalysts containing bound metal atoms or ions (e.g., Cu/ZSM-5) have been proposed as alternatives to systems based on precious metals. 

Catalysts prepared by the wash-coating method were first used to check the reproduction of the measured values. For this reason, six elementary metal salts (platinum, zirconium, molybdenum, nickel, silver, and rhodium) were dissolved and impregnated onto a titer-plate. The catalysts were pre-reduced inside the reactor with 5% hydrogen in 95% nitrogen at 250 °C. The results were recorded first before the pre-reduction and then after the pre-reduction. The repeated measurements indicated good reproducibility in both cases. The conversion of methane with the rhodium catalyst is better after the pre-reduction. Methane conversion after 18 h runtime was still stable. 

Fig. 4.34 Semi-automated continuous coating apparatus for catalyst and precursor preparation. The procedure is demonstrated here using an unstructured metal foil unwound from a roll. The wash-coat slurry is fed through a slot die onto the metal foil. After coating, the foil is sent through a drying compartment and wound up.

The catalyst (Pd/ZnO) was introduced by wash coating prior to the mounting, which was performed by electron beam welding. Calcination and reduction of the catalyst were performed after the welding procedure. The total heating power of the six heating cartridges was 1.5 kW. Bores were introduced for temperature determination at various positions. The feed inlet temperature was set to 140 °C. 

Self-made Cu/Zn catalysts were prepared by introducing y-alumina wash coats with an average thickness of 10 pm as carrier material into the micro channels. Their BET surface area was determined as 72 m2 g 1 and the average pore diameter was 45 nm. The active components, Cu and Zn, were introduced by wet impregnation at two different load levels (8 and 16 wt.%) and weight ratios (3 1 and 1 1) [26],... 

In a later study, Pfeifer et al. [30] prepared Pd/Zn catalysts by both pre- and postimpregnation of wash-coated zinc oxide particles with palladium and compared their performance in methanol steam reforming. The catalytic performance of the samples was tested at a 250 °C reaction temperature, 3 bar pressure, a S/C ratio of two and 250 ms residence time. The WHSV amounted as 0.3 Ndm3 (min gcat) 1. The thickness of the coatings was calculated to 20 pm. The formation of the PdZn alloy was proven to occur at temperatures exceeding 200 °C by XRD measurements. 

The [MSR 6] reactor type (see below) was applied for methanol steam reforming over Cu/Ce02/Al203 catalysts by Men et al. [34, 35], Wash coating of the alumina was performed, followed by subsequent impregnation steps with ceria and copper salt solutions. At 250 °C reaction temperature and a water/methanol molar ratio of 0.9, the copper/ceria atomic ratio was varied from 0 to 0.9, revealing the lowest conversion for pure ceria and a sharp maximum for a ratio of 0.1 (see Figure 2.13). 

The amount of wash coat which was deposited in the testing reactors was in the same range, between 14 and 17 mg, for the rhodium, platinum and palladium samples tested. The platinum sample was calcined after impregnation at a lower temperature of 450 °C, all other samples at 800 °C. The reason for this will be explained below. The content of the active noble metal was around 5 wt.%. All noble metal-containing samples were laboratory-made catalysts. A commercial a-alumina-based catalyst containing 14 wt.% Ni was added for comparison, as nickel catalysts are applied in industrial steam reforming [52],... 

Pt/y-alumina wash-coat catalyst introduced by co-impregnation was used for butane combustion. After brief heating for 10 s, the reaction ignited and proceeded... 

Germani et al. [82] wash-coated Cu/ZnO catalyst on to micro channels and compared their performance with that of conventional monoliths for the low-temperature water-gas shift. Up to six plates could be put into a stack-like reactor heated by cartridges, which had a maximum operation temperature of600 °C (see Figure 2.47). The reactor had capabilities for measuring the inlet and outlet temperature of the gases via thermocouples. 

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