Single-metal catalysts

The activity of single metal catalysts as a function of mass... 

HYDROGENOLYSIS OF THE LOWER ALKANES ON SINGLE METAL CATALYSTS RATES, KINETICS, AND MECHANISMS... 

Even when other mechanisms are operative, a master impregnation variable would appear to be solution pH, and in particular, final pH. The two main experiments of the SEA method, the pHinitiai-pHfinai plot and the pH-uptake survey might be used for many systems to diagnose the type of adsorption mechanism and to determine the optimal impregnation conditions. These features of the SEA method will be reviewed for a single metal catalyst system, and then applied to composite oxide surfaces for the synthesis of promoted and bimetallic catalysts. 

A sampling of support PZCs measured in our own lab, and the suitability of adsorbing cations and/or anions is shown in Table 3.2 and is similar to the 2 offered by Brunelle three decades ago [Ij. This information is useful in designing a well-dispersed single-metal catalysts as shown above, or in the extension of electrostatic adsorption to promoted and bimetallic catalysts to be discussed next. 

Single-Metal Catalysts Metal oxides such as Ce02 [17, 46], Zr02 [47], and Ti02 [48]which are non-soluble in the reaction medium (heterogeneous) have been used... 

Metals often serve as catalysts and Lewis acids possessing powerful activation ability toward Lewis base sites of many substrates. Cascade reactions promoted by a single metal catalyst have been smdied extensively, and fruitful achievements were reported during the past two decades. However, the number of cascade transformations utilizing two or more distinct metal catalysts is very limited. Considering the differing ability of various metals for the activation of diverse substrates, cascade reactions that use two or even more metals as catalysts will enable novel transformations and be capable... 

The synthesis of spiroacetals was improved by using new simple and readily accessible Rh(COD)2 complexes that allowed excellent conversions and an overall reduction of reaction times [33]. Moreover, very recently a dual metal catalytic system (Rh(l) and lr(I)) was successfully utilized for these reactions on alkynediols, and in some cases it works more efficiently than the single metal catalyst [34]. The combination of the two metal complexes ([Rh(bpm)(CO)2]BAr4 and [Ir (bpm)(CO)2]BAr4 (bpm = bis(l-pyrazolyl)methane, BAt4 = tetrakis[3,5-bis (trifluoromethyl)phenyl]borate) acted cooperatively to promote an efficient dual activation pathway for both the 5-exo and 6-endo cyclization in which the Rh(I) preferentially promotes the 6-membered ring formation, while the Ir(I) in preference promotes the 5-membered cyclization of alkynediols. 

There are only a few weU-documented examples of catalysis by metal clusters, and not many are to be expected as most metal clusters are fragile and fragment to give metal complexes or aggregate to give metal under reaction conditions (39). However, the metal carbonyl clusters are conceptually important because they form a bridge between catalysts commonly used in solution, ie, transition-metal complexes with single metal atoms, and catalysts commonly used on surfaces, ie, small metal particles or clusters. 

Catalysts vary both in terms of compositional material and physical stmcture (18). The catalyst basically consists of the catalyst itself, which is a finely divided metal (14,17,19) a high surface area carrier and a support stmcture (see Catalysts, supported). Three types of conventional metal catalysts are used for oxidation reactions single- or mixed-metal oxides, noble (precious) metals, or a combination of the two (19). 

Meta/ Oxides. The metal oxides aie defined as oxides of the metals occurring in Groups 3—12 (IIIB to IIB) of the Periodic Table. These oxides, characterized by high electron mobiUty and the positive oxidation state of the metal, ate generally less active as catalysts than are the supported nobel metals, but the oxides are somewhat more resistant to poisoning. The most active single-metal oxide catalysts for complete oxidation of a variety of oxidation reactions are usually found to be the oxides of the first-tow transition metals, V, Cr, Mn, Fe, Co, Ni, and Cu. 

Base Metal Catalyst - An alternate to a noble metal catalyst is a base metal catalyst. A base metal catalyst can be deposited on a monolithic substrate or is available as a pellet. These pellets are normally extruded and hence are 100% catalyst rather than deposition on a substrate. A benefit of base metal extruded catalyst is that if any poisons are present in the process stream, a deposition of the poisons on the surface of the catalyst occurs. Depending on the type of contaminant, it can frequently be washed away with water. When it is washed, abraded, or atritted, the outer surface is removed and subsequently a new catalyst surface is exposed. Hence, the catalyst can be regenerated. Noble metal catalyst can also be regenerated but the process is more expensive. A noble metal catalyst, depending on the operation, will typically last 30,000 hours. As a rule of thumb, a single shift operation of 40 hours a week, 50 weeks a year results in a total of 2,000 hours per year. Hence, the catalyst might have a 15 year life expectancy. From a cost factor, a typical rule of thumb is that a catalyst might be 10%-15% of the overall capital cost of the equipment. 

Small diameter, single-wall nanotubes have been synthesized with metal catalysts by maintaining a dc arc (30 V, 95 A) between two electrodes in —300 Torr of He gas.[21,22] The metal catalyst (cobalt[22] or... 

Most of the NO reducing catalysts in pellet or monolithic form begin to lose their activity at 2000 miles and fail to be effective at 4000 miles. This lack of durability may well be connected to the usage of the NO bed for oxidation purposes during the cold start, which exposes the NOx catalysts to repeated oxidation-reduction cycles. Better catalyst durability can be anticipated in the single bed redox catalyst with a tightly controlled air-to-fuel ratio, since this oxidation-reduction cycle would not take place. Recent data indicates that the all metal catalysts of Questor and Gould may be able to last 25,000 miles. 

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