Role in catalyst performance

Esters such as ben2oates and phthalates are also used in the preparation of high activity catalysts for olefin polymeri2ation. They appear to function as electron donors in the catalyst complex, and play a significant role in catalyst performance (115). 

An interesting aspect is the relative net electron donicities of carbene hgands. In a series of N-stabilised carbenes [ClNPr ), SIMes, Imes], Herrmann et al. showed that unsaturated NHC are better net donors than saturated NHC and sometimes can even outperform the acyclic and more basic carbene ligand ClNPr l [57,104,107] (see Figure 1.21). A similar observation was made by Dorta et al. [19]. However, the absolute differences between the net donicities of these different carbene ligands are very small and are unlikely to play a major role in catalyst performance [19]. 

While STEM methods were valuable in investigating the morphology and the structure of OER catalysts, they lacked the ability to discriminate between different valence states of Ir and Ru overlayers or altered chemical environments at the interface with Pt-NSTF whiskers. Therefore, the interfacial region, which plays a critical role in catalyst performance and durability, was also characterized by XPS [17,26],... 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

In summary, the basicity and the strong NiO-MgO interactions in binary NiO/MgO solid solution catalysts, which inhibit carbon deposition and catalyst sintering, result in an excellent catalytic performance for C02 reforming. The characteristics of MgO play an important role in the performance of a highly efficient NiO/MgO solid-solution catalyst. Moreover, the NiO/MgO catalyst performance is sensitive to the NiO content a too-small amount of NiO in the solid solution leads to a low activity, and a too-high amount of NiO to a low stability. CoO/MgO solid solutions have catalytic performances similar to those of NiO/MgO solid solutions, but require higher reaction temperatures. So far, no experimental information is available regarding the use of a FeO/MgO solid solution for CH4 conversion to synthesis gas. 

Most industrial reactors and high pressure laboratory equipment are built using metal alloys. Some of these same metals have been shown to be effective catalysts for a variety of organic reactions. In an effort to establish the influence of metal surfaces on the transesterification reactions of TGs, Suppes et collected data on the catalytic activity of two metals (nickel, palladium) and two alloys (cast iron and stainless steel) for the transesterification of soybean oil with methanol. These authors found that the nature of the reactor s surface does play a role in reaction performance. Even though all metallic materials were tested without pretreatment, they showed substantial activity at conditions normally used to study transesterification reactions with solid catalysts. Nickel and palladium were particularly reactive, with nickel showing the highest activity. The authors concluded that academic studies on transesterification reactions must be conducted with reactor vessels where there is no metallic surface exposed. Otherwise, results about catalyst reactivity could be misleading. 

Rb+- and Cs+-impregnated X zeolites were found to exhibit the highest activity and selectivity in these transformations. A CsX zeolite treated with boric acid, for example, gave better than 50% overall selectivity in the formation of styrene and ethylbenzene (410°C, 60% conversion).275 Treatment of these catalysts with copper or silver nitrate resulted in further improvements in catalyst performance.276 The promoting role of these metals was suggested to be their involvement in dehydrogenation of methyl alcohol. 

While the intrinsic activity and selectivity of a catalyst establish its performance in the absence of mass transfer effects, it is well known that the placement of the active components and access to these components by reactants can play a major role in the performance of practical catalysts. One of the challenges for reaction engineers is to develop models for predicting the distribution of active components in a catalyst and the effects of this distribution, together with the pore size distribution and particle size and shape, on the performance of a catalyst. 

It is well known that catalyst support plays an important role in the performance of the catalyst and the catalyst layer. The use of high surface area carbon materials, such as activated carbon, carbon nanofibres, and carbon nanotubes, as new electrode materials has received significant attention from fuel cell researchers. In particular, single-walled carbon nanotubes (SWCNTs) have unique electrical and electronic properties, wide electrochemical stability windows, and high surface areas. Using SWCNTs as support materials is expected to improve catalyst layer conductivity and charge transfer at the electrode surface for fuel cell oxidation and reduction reactions. Furthermore, these carbon nanotubes (CNTs) could also enhance electrocatalytic properties and reduce the necessary amount of precious metal catalysts, such as platinum. 

Nafion content in the catalyst layer plays an important role in electrode performance. Incorporation of Nafion ionomer into carbon-supported catalyst particles to form the catalyst layer for the gas diffusion electrode can establish a three-dimensional reaction zone, which has been proven by cyclic voltammetric measurements. An optimal Nafion content in the catalyst layer of the electrode may minimize the performance loss that arises from ohmic resistance and mass transport limitations of the electrode [6],... 

The above modeling study showed therefore the importance of having a proper knowledge of the pore texture and catalyst distribution of the catalytic filter, since they can seriously affect its performance. This suggests, in line with Ref. 40, the need of a proper characterization of the porous structure of the catalytic filters, concerning pore connectivity, pore size distribution, presence of deadend pores, etc., since each of these features might play a primary role in reactor performance. On the basis of such characterization work, valuable information could be drawn in order to choose or optimize the preparation routes. 

The nature of support also plays a critical role in catalytic performance. Acidic supports such as A1203 favors the dehydration of ethanol to produce ethylene, which leads to carbon deposition on the catalyst surface.242,252,254,259 The carbon deposition can be minimized over basic supports such as Mg0.235, 241,258,259 However, these supports also favor condensation of alcohols to higher oxygenates.260 Interestingly,... 

The conditions and procedures of precipitation play a significant role in catalyst morphology, texture, pore structure, physical strength, and consequently the performance (activity, selectivity, and stability) of the catalyst. By merely changing the sequence of solution addition, catalyst components can be precipitated simultaneously or sequentially. The methods of precipitation most often used are constant pH coprecipitation, sequential precipitation, acid-to-base precipitation, and base-to-acid precipitation. 

The question arises, why do bi- or multi-phasic catalysts generally show better activity and selectivity than the active phase alone The aim of this paper is to answer this question by exploring the role of interfacial effects. We shall examine first how the thermodynamic and structural properties of one phase influence its interactions, not only with the gaseous reactants, but also with coexisting solid phases as a result of its bulk, surface, and defect structure. We will also examine the conditions necessary for these interactions and set up a structural classification of the main components of mild oxidation catalysts. This will lead finally to a discussion of the role of interfacial effects in catalyst performance using some illustrative examples. Thermodynamic and Structural Properties of Single Phase Catalysts... 

The most efficient method for NO removal from stationary and mobile sources is catalytic reduction with ammonia, hydrocarbons, CO or H2. Modified zeolites are active catalysts in these processes. For Cu-ZSM-5 especially high activity and stability have been reported. In this work the properties of copper-containing ZSM-5 zeolites prepared by wet or solid state ion-exchange have been investigated. The Bronsted acidity of the Cu -exchanged samples was much lower than that of the parent zeolites, and they had high activity in selective reduction with ammonia, propene or propane. A comparison of Cu-ZSM-5 activity in the decomposition of NO and in the reaction of NO with propene or propane revealed that the hydrocarbons as well as the nitrogen oxides play important role in the performance of NO reduction catalysis. 

It is well known that catalytic activity is strongly dependent on the shape, size, and distribution of the metal particles [6], Furthermore, support also plays a vital role in the performance of a catalyst. Carbon is an excellent support in many ways its high specific surface area is necessary for high metal loading its pore structure is suitable for mass transfer and its high conductivity of graphitization can reduce resistance in electron transportation. In research on catalyst structures, XRD has played an effective role in determining catalyst composition. 

The electrodes in PEMFCs, including both cathode and anode, are multi-component and gas porous matrices, also called gas diffusion electrodes (GDEs). A GDE is generally composed of a reaction layer (the CL) and an accessorial layer (the GDL). The GDL, adjacent to the catalyst layer and the flow field, plays an important role in PEMFC performance and has several functions (1) transportation... 

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