Powder catalyst modeling

The studies reported in the literature have suggested that the surface tension of Cu depends on its surrounding environment it is higher in vacuum and varies as vacuum > H2 > CO. Well-rounded particles are likely to form when the surface tension is low. In CO, the surface tension is lowered to the extent that the Cu prefers to spread out as sheets rather than as three-dimensional spherical particles. Experiments carried out on real (practical) powder catalysts are consistent with the data from the model systems. As in the model systems, sintering by Cu particles is dominant, the particles growing to several tens of nanometres. The type and extent of sintering depend on the exact composition of the bimetallic catalyst. For Cu > Ru, ETEM studies show the sintering of Cu to be primarily by particle coalescence. 

Altogether, the data reported in this section indicate a very good predictive quality of the model simulations this implies in the first place that the SCR kinetics estimated over powdered catalyst were successfully validated at this bigger scale. However, the excellent agreement between monolith data and model predictions based on intrinsic kinetics also confirms the accurate model description of physical phenomena, specifically external and intraporous mass transfer, which were not significant in the microreactor runs over the powdered catalyst, but played an important role in the monolith runs, as pointed out by the direct comparison in Fig. 44. 

Attenuated total reflection IR spectroscopy can be used to investigate model as well as powder catalysts. 

In this paper, we will report the electronic and catalytic reactivities of the model VC/V(110) surface, and our attempt to extend them to VC powder catalysts. By using high-resolution electron energy loss spectroscopy (HREELS) and NEXAFS techniques, we observed that the surface properties of V(110) could be significantly modified by the formation of vanadium carbide some of the experimental results on these model surfaces were published previously.3-5 We will discuss the selective activation of the C-H bond of isobutane and the C=C bond of isobutene on V(110) and on VC/V(110) model systems. These results will be compared to the catalytic performances of vanadium and vanadium carbide powder materials in the dehydrogenation of isobutane. 

These electronic properties in turn give rise to some unique catalytic properties for vanadium carbide. Compared to metallic vanadium, vanadium carbide shows an enhancement in the activation of the C-H bond of alkanes and a reduction in the interaction with the C=C bond of alkenes. The surface reactivity of VC/V(110) can be generally described as similar to those of Pt group metals, although the VC/V(110) surface might have an even higher activity towards the activation of C-H bonds. The dehydrogenation of isobutane on VC powder catalysts will be compared to the reactivities of the VC/V(110) model surfaces. 

This paper covers primarily the spectroscopic characterization of the VC/ V(110) model surface and VC powder materials. These results are organized as follows section 24.3.1 uses the VC/V(110) surface as a model system to briefly describe the electronic properties of metallic vanadium and vanadium carbide. Section 24.3.2 extends the fundamental information obtained on the VC/V(110) model system to VC powder catalysts by comparing the NEXAFS results of these two types of materials. Finally, section 24.3.3 uses the dehydrogenation of isobutane as an example to compare the catalytic properties of the VC/V(110) surface and the VC powder catalysts. 

The VC/V(110) appears to be a reliable model system for the VC powder catalysts, as suggested by the NEXAFS results. This is further supported by the correlation of the reactivities of iso-butane and isobutene on VC/V(110) to link the dehydrogenation activity of isobutane on VC powder catalysts. 

Kuipers and co-workers [34] developed this model further into the randomly oriented layer model. These authors argued that powdered catalysts contain a... 

The correlation of spectroscopic data between model and real catalysts has always been a concern in catalyst characterization. Weiher et al. (2005) tried to address this issue with a cell design that was compatible with both model catalysts (e.g., submonolayer amounts of metals deposited on a silicon wafer) and real catalysts such as high-surface-area supported metals. Moreover, they also wished to have a design in which plug-flow conditions existed for the powder catalyst experiments. 

In fundamental catalysis studies catalysts are quite often tested under conditions which differ widely fi om the industrial practice of a continuous process, e.g., tests are carried out in batch using model feedstocks, in stirred reactors, with powdered catalyst or single pellets at conversions that are quite different from those in practice (e.g., differential conversions). While such tests can yield valuable... 

The CO + NO reaction on various kinds of Pd catalysts was investigated by Rainer et al (112). Kinetics data for Pd/A Os and planar model Pcl/Al2O3/Ta(110) catalysts were compared with those for CO ) NO on Pd(lll), (100), and (110) surfaces. Figure 20 is a comparison of the Arrhenius plots measured at partial pressures of about 1 mbar in each reactant tor the model catalysts and at a CO pressure of 5.9 and a NO pressure of 6.8 mbar (in a helium carrier) for the Pd/A Os powder catalyst. 

Fig. 20. Arrhenius plots for the rate of the NO-CO reaction on various kinds of Pd catalysts. The planar model and single-crystal surface data were taken in a batch reactor (1 mbar of each reactant) and the powder catalyst in a flow reactor (Pco = 5.9 mbar. Pno = 6.8 mbar) [adopted with permission from Rainer et al.

Amsterdam developed models for this purpose which take the randomness of powdered catalyst samples explicitly into account (refs. 8,9. ... 

Arrhenius plots are shown in Figure 19 for CO2 production from a CO + NO reaction mixture for model oxide-supported Pd, Pd single crystals, and Pd/Al203 powder catalysts. For the powder and model catalysts, a pronounced increase in activity is seen with an increase in cluster size or loading. The larger clusters display the characteristics of the less open (111) plane. 

Figure 19 CO + NO reaction Arrhenius plots for single-crystal, model planner-supported, and Pd/Al203 powder catalysts. The powder catalyst data were taken in the flow reaction mode (4.4/5.2 CO/NO ratio, steady state), and the model catalyst and single-crystal data were acquired for a batch reaction mode in 1 Torr of each reactant. (From Ref. 32.)...

The second scale which determines the relation between the selectivity and conversion is the diffusion of the reactants through the catalyst poes. Model calculations conducted by McCarty indicated that at 10 atm die coupling of methyl radicals occurs preferentially inside the pores in a particle of 25 mm in diameter. The effect of this time scale is shown in Figure 10(a) in terms of the intraphase and interphase profiles for methane and ethane inside a catalyst pore. Clearly, higha C2 selectivities are obtained on catalysts with an open pore structure and low surface area. A majority of the literature results have been obtained using powdered catalysts in which diffusional effects are not in rtant however, such effects could be relevant at high pressure in fixed-bed reactors requiring the use of catalysts in a pelletized form. 

Fig. 3.16. CO oxidation on Au/Ti02 powder catalysts prepared by deposition precipitation (from [73]) on the left and on a model catalyst Au/TiO2(110) curve (a), panels (b) and (c) represents band measured by STS and the proportion of particles presenting a bandgap, as a function of particle size (from [74])...

A complete range of metastable cerium-zirconium mixed metal oxide powders (CexZr(i.x)Oy, 0 < X < 1) were prepared through a similar hydroxide precipitation technique reported by Hori, et al. [11]. Cerium (IV) ammonium nitrate and zirconium oxynitrate precursors are completely dissolved in de-ionized water with mild heat and precipitated through the addition of excess ammonium hydroxide (-100 vol%). The ceria-zirconia is thoroughly washed with excess distilled water and allowed to evaporate to dryness overnight. The ceria-zirconia system is calcined in atmosphere for 1 hour at 773 K and subsequently milled into a fine powder. The model ceria-zirconia catalysts are prepared from the ground cerium-zirconium oxide powders using a 13 mm diameter pellet die and hydraulic press. 

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