Conventional Active Metals

The progress of sulfiding of the NiW/AC and NiW/Al203 catalysts could be monitored by XRD, MOS and EXAFS techniques. These techniques confirmed the coexistence of the Ni-W-S and NiS-WO S phases. The presenee of the latter phase was more evident on the y-Al203 support than on AC. This resulted from the stronger interaetion of the W oxide with the former support. In addition to these phases, a separate Ni sulfide was also identified. The study also showed that the conversion of the W oxide to sulfide phase inereased when sulfidation was conducted at a high pressure. In every case, it was easier to sulfide Mo oxide than W oxide. 

Earlier studies pointed to some differences between the morphology of M0S2 in Mo/AC compared with the Mo/Al203-supported catalysts. While using XPS, the former study showed that in the sulfided Mo/AC catalysts with Mo loadings above 3 wt.%, M0S2 was present in the form of tiny three-dimensional 

% Binding energies, eV Intensity ratios M0S2 size nm 

For the Co/AC catalyst sulfided at 373 K, van der Kraan et al. observed the MOS doublet that coincided with the Co-Mo-S doublet in the CoMo/AC catalyst. However, after sulfiding of the Co/AC at 673 K, the doublet disappeared. At the same time, in CoMo/AC catalyst sulfided at 673 K the Co-Mo-S doublet was present, although slightly changed. In spite of the doublet similarities, the activity of the Co/AC sulfided at 373 K for H2—D2 exchange was much lower than that of the CoMo/AC catalyst sulfided at 673 K. It is believed that the higher activity must result from the presence of the Co-Mo-S active phase. Therefore, the doublet in the Co/AC sulfided at 373 K may be attributed to some unidentified phase. 

It was noted earlier that depending on sulfiding temperature, Type-I and Type-II Co-Mo-S active phases can be formed in the y-Al203-supported catalysts. For the latter formed at higher sulfiding temperatures, the interaction with y-Al203 was not evident. On the basis of MOS results, Topsoe concluded that the active Co-Mo-S phase in the CoMo/AC catalyst resembled Type-II phase in the y-Al203-supported catalysts. It appears that such a phase is facilitated when the interaction with catalyst support is minimal. 

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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. 

Reetz et al. have used N-(octyl)4Br-stabilized Pd colloids (typical size, e.g., 3nm) as precursors to form so-called cortex-catalysts, where the active metal forms an extremely fine shell of less than lOnm on the supports (e.g., AI2O3). Within the first 1-4 s, the impregnation of AI2O3 pellets by dispersed nanostructured metal colloids leads to the time-dependent penetration of the support which is complete after 10 s. Cortex catalysts were reported to show a threefold higher activity in olefin hydrogenation than conventionally prepared catalysts of the same metal loading (5% Pd on AI2O3) [388]. 

Except Ru (not usable in TWC because of the volatility of its oxide [68]), the most active metal is the rhodium. This has been largely confirmed by further studies so that Rh may be considered as a key-component of TWC for NO reduction [69,70], As far as Pd is concerned, it seems that the active site is composed of Pd"+ —Pd° pairs, which may explain the higher activity of Pd in N0+C0+02 mixture (T5( 200°C) [71]. A detailed kinetic study by Pande and Bell on Rh catalysts has evidenced a significant support effect [72], The kinetic data were represented by a conventional power law expression ... 

The preparation of catalysts usually involves the impregnation of a support with a solution of active metal salts. The impregnated support is then dried, calcined to decompose the metal salt and then reduced (activated) to produce the catalyst in its active form. Microwaves have been employed at all stages of catalyst preparation. Beneficial effects of microwave heating, compared with conventional methods, have been observed especially in the drying, calcination, and activation steps. 

Recently, Chaudhari compared the activity of dispersed nanosized metal particles prepared by chemical or radiolytic reduction and stabilized by various polymers (PVP, PVA or poly(methylvinyl ether)) with the one of conventional supported metal catalysts in the partial hydrogenation of 2-butyne-l,4-diol. Several transition metals (e.g., Pd, Pt, Rh, Ru, Ni) were prepared according to conventional methods and subsequently investigated [89]. In general, the catalysts prepared by chemical reduction methods were more active than those prepared by radiolysis, and in all cases aqueous colloids showed a higher catalytic activity (up to 40-fold) in comparison with corresponding conventional catalysts. The best results were obtained with cubic Pd nanosized particles obtained by chemical reduction (Table 9.13). 

Catalysts. - Group VIII metals, conventional base metal catalysts (Ni, Co, and Fe) as well as noble metal catalysts (Pt, Ru, Rh, Pd) are active for the SR reaction. These are usually dispersed on various oxide supports. y-Alumina is widely used but a-alumina, magnesium aluminate, calcium aluminate, ceria, magnesia, pervoskites, and zirconia are also used as support materials. The following sections discuss the base metal and noble metal catalysts in detail, focusing on liquid hydrocarbon SR for fuel cell applications. 

The properties of these new materials as catalyst support were tested on Fischer-Tropsch process (CO-H2 reaction) in a fixed bed differential reactor. Three materials were tested a) CON, a conventional activated carbon b) SC-155 (G40.60) and c) C-155 (G20.20). All of them were previously iron doped until 5% metallic iron wt/wt was reached. The test conditions were Reaction temperature =270°C H2/CO ratio=3, pressure = latm. The main properties of the tested catalyst supports and their performance in the first hour test are shown in Table 2. SC-155 (G40.60) and C-155 (G20.20) were selected for this test in order to compare materials with near the same specific surface area but with different structural composition, and CON was selected because it is of common use and has very different texture characteristics respect to the other two materials. 

Anyhow, at 25 °C, an ideal gas at 1 atm is 0.041 M. Condensed matter with small molecules (or metals such as silver and gold) can be up to 100 M. Hence, at their boiling points, most substances show an activity coefficient in the gaseous state (comparing with the molarity of the condensed matter and not the conventional activity a = 1 of pure substances) of the order of magnitude 1000. In view of the almost ideal nature of the gaseous state, it would perhaps be more appropriate to say that the condensed matter has/ 10 3 relative to the vapour at 1 atm. 

Synovec and Yeung developed a highly selective and sensitive laser-based CD detector for both conventional and microbore liquid chromatography [30] and applied it to the analysis of complex mixtures of optically active metal complexes. As expected limits of detection are very low. For example for the determination of (+) trisfethylenenediamine) cobalt (HI) using an Ar-ion laser source, a microbore column, and a CD detector operating at 488 nm, the measured limit of detection is 5.6 ng. 

Instead of nickel, other catalytically active metals are used as well. Rhodium and ruthenium, for example, show an activity that is about ten times higher than that of nickel, platinum and palladium [6], The addition of small amounts of copper to the conventional nickel catalyst is reported to improve the activity of nickel at high temperatures [13],... 

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