Stability of catalysts

The stability of catalyst is one of the most important criteria to evaluate its quality. The influence of time on stream on the conversion of n-heptane at SSO C is shown in Fig. 5. The conversion of n-heptane decreases faster on HYl than on FIYs with time, so the question is Could the formation of coke on the catalyst inhibit diffusion of reactant into the caves and pores of zeolite and decrease the conversion According to Hollander [8], coke was mainly formed at the beginning of the reaction, and the reaction time did not affect the yield of coke. Hence, this decrease might be caused by some impurities introduced during the catalyst synthesis. These impurities could be sintered and cover active sites to make the conversion of n-heptane on HYl decrease faster. 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

Techniques for attaching such ruthenium electrocatalysts to the electrode surface, and thereby realizing some of the advantages of the modified electrode devices, have been developed.512-521 The electrocatalytic activity of these films have been evaluated and some preparative scale experiments performed. The modified electrodes are active and selective catalysts for oxidation of alcohols.5 6-521 However, the kinetics of the catalysis is markedly slower with films compared to bulk solution. This is a consequence of the slowness of the access to highest oxidation states of the complex and of the chemical reactions coupled with the electron transfer in films. In compensation, the stability of catalysts is dramatically improved in films, especially with complexes sensitive to bpy ligand loss like [Ru(bpy)2(0)2]2 + 51, 519 521... 

The use of water instead of organic solvents is key to attaining the goal of environmentally benign chemical synthesis. In this context, organic reactions in water are now of great interest and much research effort has been devoted to pursuing efficient reactions in water [1-5]. Unique reactivity and selectivity have been often observed in aqueous media, but one of the big issues is the stability of catalysts in water. Many active catalysts are not stable in water but decompose in the presence of even a small amount of water. To overcome this, we searched for efficient catalysts that are stable and can work well in aqueous media. 

Amination of i-butanol to diisobutylamine was investigated on vanadium modified granulated Raney nickel catalyst in a fixed bed reactor. The addition of 0.5 wt.% V to Raney nickel improved the yield of amines and the stability of catalyst. Factorial experimental design was used to describe the conversion of alcohol, the yield and the selectivity of secondary amine as a function of strong parameters, i.e. the reaction temperature, space velocity and NHs/i-butanol molar ratio. Diisobutylamine was obtained with 72% yield at 92% conversion and reaction parameters P=13 bar, T=240°C, WHSV=1 g/g h, and molar ratios NH3/iBuOH= 1.7, H2/NH3= 1.9. 

The conditions encountered in studying the stability of catalysts under electrochemical load are very complicated. Stability depends strongly on the potential and on the nature of the working substance. For example, pure CoTAA, when used as an oxygen catalyst at potentials of about 800 mV, is active only for a period of some hours. If, however, it is used in the anode for the oxidation of formic acid at 350 mV, it will give more than 6 months (4000 hours ) continuous service under the same conditions. 

Stability of catalysts not affected by drying temperatures, but is affected by sulfiding. Sulfided catalyst fouled by different mechanism.81 ... 

Figure 1 Effect of nickel loading on stability of catalysts for C02 reforming CtyCH4= 4.

In this paper the origins of the difference in stability of catalysts on different support materials are discussed. Catalysts supported on y-alumina and on titania (anatase) have been tested in the oxidation of 1-butene to butanone. Fresh and spent catalysts have been investigated by means of temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to reveal the causes for deactivation. 

Discussion Point DPS The Sharpless enantioselective catalyst system has some substantial disadvantages with respect to other enantioselective systems name at least two. Enantioselective oxidation is to some extent underdeveloped with respect to e.g. hydrogenation can you establish logical connections with i) the stability of catalysts in the reaction medium ii) the nature of the oxidants used and their environmental impact Hi) the number of reactions that can successfully be accomplished. 

Ga.s Stability of catalyst Environmental SO2 oxidation and NO selective catalytic reduction V2O5 addition favorable CuO/y-Al203-monohth (168)... 

D L Tnmm, Thermal stability of catalyst supports. Catalyst Deactivation V (C H Bartholomew, and J B Butt, Eds ), Elsevier, Amsterdam, 1991, p 29... 

M.R. Sad, N.S. Figoli, J.N. Beltramini, E.L. Jablonski,4t.A. Cazzeroni, and J.M. Parera, Evaluation of Activity, Selectivity, and Stability of Catalysts for Naphtha Reforming, J. Chem. Teclmol., Biotechnol., 30 U980) 374. 

Stability of Catalysts with Different Pore Volume and External Stu ace Area... 

The introduction of microporous and mesoporous supports with well-dispersed Ti has allowed for some detailed kinetic studies into the PO and water generation mechanisms primarily as a result of the outstanding stability of catalysts prepared from these materials. Prior to the synthesis of stable catalysts, a number of reaction mechanisms were proposed based on observed trends in reactivity, DPT calculations [63,64,76,78], and analogs to liquid-phase epoxidation reactions over Ti-based catalysts [14,15,89] rather than kinetic analysis. The first proposed mechanisms were constructed for the Au/Ti02 and Au/Ti02/Si02 system. 

Mechanical strength and thermal stability of catalyst particles are always of concern to process designers. In some cases it may be the most critical feature. This was emphasized, for e.xample, in steam reforming. Strong pellets with good thermal resistance are required. Catalyst designers use mixed oxides fired at high temperatures to form ceramic compounds, Particles must be preformed and active components added later. 

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