Synergism catalyst activity

Deactivation rates and aged catalyst properties have been investigated as a function of time on stream for iron-based Fischer-Tropsch catalysts in the presence/absence of potassium and/or silicon. There is a synergism in activity maintenance with the addition of both potassium and silicon to an iron catalyst. The addition of silicon appears to stabilize the surface area of the catalyst. Catalysts containing only iron or added silicon with or without potassium consist mainly of iron oxide at the end of the run. However, iron carbides are the dominant phase of the iron catalyst with added potassium alone. Catalyst surface areas increase slightly during synthesis. The bulk phase of the catalyst does not correlate to the catalyst activity. The partial pressure of water in the reactor is lower for potassium-containing catalysts and is not a reliable predictor of catalyst deactivation rate. 

In view of this need, we discuss here a variety of electrocatalytic topics, ranging from basic and microscopic concepts to phenomenological principles. Thus, the origin of electrodic reactions, electrosorption, and electrode kinetics are introduced briefly for the benefit of the nonelectrochemist. Since electrocatalytic reactions take place at the electrode surface, attention is given to recent efforts to link catalyst activity with microscopic surface properties. These include surface crystallographic orientation, crystallite size and distribution, adsorbate-adsorbent-support synergism, multiple adsorption states, identification of surface intermediates, and electrocatalytic surface reaction mechanisms. 

Direct conversion of methane to ethane and ethylene (C2 hydrocarbons) has a large implication towards the utilization of natural gas in the gas-based petrochemical and liquid fuels industries [ 1 ]. CO2 OCM process provides an alternative route to produce useful chemicals and materials where the process utilizes CO2 as the feedstock in an environmentally-benefiting chemical process. Carbon dioxide rather than oxygen seems to be an alternative oxidant as methyl radicals are induced in the presence of oxygen. Basicity, reducibility, and ability of catalyst to form oxygen vacancies are some of the physico-chemical criteria that are essential in designing a suitable catalyst for the CO2 OCM process [2]. The synergism between catalyst reducibility and basicity was reported to play an important role in the activation of the carbon dioxide and methane reaction [2]. 

The same group, in a previous work, reported on the realization of a hybrid anode electrode [197]. An appreciable improvement in methanol oxidation activity was observed at the anode in direct methanol fuel cells containing Pt-Ru and Ti02 particles. Such an improvement was ascribed to a synergic effect of the two components (photocatalyst and metal catalyst). A similar behavior was also reported for a Pt-Ti02-based electrode [198]. Another recent study involved the electrolysis of aqueous solutions of alcohols performed on a Ti02 nanotube-based anode under solar irradiation [199]. 

In the NO-SCR by NH3, we note the highest reduction activity and selectivity on catalyst containing both vanadium and molybdenum than catalysts issued containing Mo or V, only. Furthermore, it should be underlined that a higher efficiency is obtained with ZSM-5 as host structure than samples issued from USY and MOR. Where a higher loss of porous volume were observed. On the basis of characterization data it has been suggested that the observed synergism in the SCR reaction is related to the existence of electronic interaction between the V and Mo species. In particular, it has been proposed that the presence of such electronic interactions modifies the catalysts redox properties, which have been claimed an essential property in the NO-SCR by NH3 reaction. 

Catalyst solutions of Fe and Ru or Fe/Ru mixed metal carbonyls were found to exhibit two orders of magnitude higher activity than either the Ru or Fe carbonyl catalysts alone. A weaker synergistic behavior was observed for Fe/Ir mixed metal carbonyls, which were 50% more active. Rh carbonyl catalysts were the most active ( 590 mol H2/mol catalyst per day), but did not display a synergism in combination with other metal carbonyls. 

Rh, are the base of active catalysts for CO hydrogenation and the hydroformylation of olefins. The presence of several promoters modifies their catalytic behavior and synergic effects on the base-metal have been observed Table 8.5 illustrates several examples in which homonuclear or heteronuclear carbonyl compounds have been used in the preparation of Co- or Co-Rh-based catalysts for the CO hydrogenation and/or hydroformylation reactions. 

A highly interesting class of catalysts is represented by bimetallic systems, which in many important catalytic processes show improved activity or selectivity compared with catalysts involving only one metal. Understanding their better performance is still a challenge. One metal can tune and/or modify the catalytic properties of the other metal as the result of both electronic or/and structural effects. Several mechanisms for synergism can be proposed, but it is difficult to assess their relative importance. It is clear that each metal can play a very important role in proper circumstances [41]. 

Despite the large amount of data collected in this field, options seem to be restricted to coatings containing Ni or Co, and Mo, with some additives apparently necessary to impart the wanted properties. It follows that the problem of cathode activation is only in part a question of synergism, while the microscopic structure of the catalyst appears to be of greater importance. Thus, Raney Ni still stands out in the group of possible catalysts [110, 531]. The role of Al (or Zn) does not seem to be simply that of sacrificial components. Residual Al present in the structure plays probably a role which can be similar to that of Cd in Ni-Mo. For this reason mixed Raney metals show great promise [415]. Raney Ni-Co (10%) exhibit a very extended... 

It appears probable, and will be substantiated below, that the effects of C02 and the synergic promotion of one catalyst component by another are but two facets of the formation of an active catalyst through the interactions among its components and the surrounding gas phase. It is the purpose of this article to review the recent observations and interpretations concerning the distribution of phases and elements, the physicochemical state of the catalyst components, the mode of activation of the reactants, and to show that a consistent picture is emerging that elucidates the function of the copper-based catalysts in the synthesis mechanism. 

Further, since the activity of zinc oxide has been found to be higher than that of other oxides such as alumina or chromia, no single-component catalyst, oxide, or metal is known at the present time that would effectively catalyze methanol synthesis at low temperatures and pressures, i.e., below 250°C and 100 atm. It is the synergic promotion in multicomponent catalysts that brings about greatly enhanced activity at low temperatures. 

The bimetallic catalysts exhibit higher catalytic activity at lower temperatures in comparison with pure Pt and Rh ones, as shown in Fig. 2. Otherf workers have also observed this synergism for Pt-Rl bimetallic catalysts.s The rate constants found by the RF-GC technique such as those in Table 1, are very close to thosm determined experimentally by the frequency response method " for the adsorption of CO on Pt/Si02. The values of the estimated activation energies for CC dissociative adsorption, given in Table 2, are low. ... 

Isooctane 1.5% Pt/ Ceo gGdo.20i 9 750 3 0.5-3.5 Catalyst calcination temperature plays a crucial role for catalytic activity and sulfur tolerance. Catalyst calcined at 800 °C exhibit a better performance and sulfur tolerance due to improved catalyst stability and synergism Lu et al.135... 

This section has focused on liquid—liquid biphasic catalysis in which catalysts are supported in different solvents to the substrates and products. Considerable efforts have also been directed to supporting homogeneous catalysts on solid supports including silica, alumina, and zeolites as well as functionalized dendrimers and polymers.33 It has also been found that synergic effects sometimes prevail between particles embedded in the support and the tethered molecular catalyst, increasing the activity of the catalyst.34... 

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