Deactivation and Poisoning

A model for the changes in reactivity for a reaction on a catalytic surface in the presence of adsorbed inactive atoms. The model is based on a mean field description of the formation of partly disordered structures for the adsorbed atoms. 

If other gases than the reactants and products of the catalytic reaction are present and if these foreign atoms are chemisorbed much more weakly than the intermediates of the catalytic reaction, the presence of the foreign gas will be of no consequence for the kinetic of the catalytic reaction. In this case the foreign gas behaves as an inert. 

If the foreign gas is chemisorbed much more strongly than the intermediates, the surface will to a large extent become covered by adsorbed foreign atoms. In this case the foreign gas behaves as a poison and the consequences for the kinetics will mainly be a large decrease in the reaction rate. 

In the present chapter we formulate a simple model for the changes in reactivity of the catalytic reaction under the conditions of a partial poisoning by adsorption of foreign atoms. For the understanding of the kinetic phenomena an approximate but transparent model is preferable. 

A number of different processes contribute to the loss of catalytic activity. 

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Thus it is clear that both metal and acid sites can be poisoned and deactivated rather quickly in hydrocarbon conversion reactions and at very mild experimental conditions. 

In the following, catalyst poisoning and deactivation through coking and metals deposition will be considered, but not fouling by salt deposition, etc. 

The questions of scale-up take us closer to industrial realities. A common problem for fixed-bed reactors is how to decrease the tendency of catalyst poisoning and deactivation. Since this seems to be unavoidable, one has to And methods to restore the catalytic activity. This problem affects all fixed-bed reactors, and consequently it will not degrade the cross-flow reactors compared to other fixed-bed reactors. 

The monolith catalysts are the least tested in pilot scale, however they have the advantage that they offer good mechanical strength and have high catalytic activity. On the other hand their cost is considerably higher and they arc more prone to poisoning and deactivation than dolomite and related catalysts. Because of their cost, the most important operational variable is the life of the catalyst. 

To improve the resistance of the catalyst towards poisoning and deactivation the tresh catalyst was immersed into a slurry of CaS04 x V2 H2O, 30 weight-% in H2O. The nets were blown clean with compressed air, and then left to harden. This procedure was repeated until 130 g/m of gypsum were deposited on the nets. 

Depending on the reaction tem xrature and atmosphere, either or S(VI) can be the dominant species (Fig.l). S(VI) is the main form when the feed gas is highly oxidizing and when the temperature is high, as shown in the poisoned and deactivated cataiysts fnmi... 

Catalytic total oxidation of volatile organic compounds (VOC) is widely used to reduce emissions of air pollutants. Besides supported noble metals supported transition metal oxides (V, W, Cr, Mn, Cu, Fe) and oxidic compounds (perovskites) have been reported as suitable catalysts [1,2]. However, chlorinated hydrocarbons (CHC) in industrial exhaust gases lead to poisoning and deactivation of the catalysts [3]. Otherwise, catalysts for the catalytic combustion of VOCs and methane in natural gas burning turbines to avoid NO emissions should be stable at higher reaction temperatures and resists to thermal shocks [3]. Therefore, the development of chemically and thermally stable, low cost materials is of potential interest for the application as total oxidation catalysts. 

Measurement of pore size distribution has now become an essential feature of particle characterization. Questions of pore didusional resistance, pore mouth poisoning, and deactivation control can only be answered if accurate descriptions of both size and shape are available over the whole pore size range. Historically, macroporcs have been measured with mercury porosimeters and mesopores with nitrogen adsorption-desorption isotherms. 

First, sulfur is removed from the hydrocarbon stream (usually natural gas), in order to prevent catalyst poisoning and deactivation with the use of a guard bed. Steam is mixed in the main stream in a fixed steam to carbon molar basis. The steam reform reactor (SRR) is a multitubular catalyst filled furnace reactor where the hydrocarbon plus steam are converted into syngas at high temperatures (700°C - 850 C) according to the following reaction ... 

Electrochemical methods are not as widely used in process analysis as spectroscopic methods and methods based on measurement of physical rather than chemical properties of the material to be analyzed or characterized. Calibration of the analyzer is always a problem and no general procedure can be recommended or applied because it depends on the particular application and analytical method. Poisoning and deactivation of the active surface of the electrode shortens the lifetime of the sensors. Accumulation of static electricity on the body of the sensor creates extra noise. 

Dumesic JA Selective poisoning and deactivation of acid sites on sullated ilrconla catalysts for n-butane Isomsrlzation. J... 

There are many different ways to combine the three main ingredients (metallocene molecule(s), MAO, and the silica) chemically to obtain the final supported metallocene catalysts. One of the most recommended procedures in the patent literature is to first treat the silica with MAO and then combine it with metallocene molecules. No matter which procedure is used, one must be aware that the resulting final heterogeneous metallocene catalyst is very reactive towards moisture, leading to its destruction, and to polar molecules that act as a very effective poison and deactivate the catalyst in an irreversible manner. The long-term storage of different batches of supported catalyst under inert atmosphere, in a temperate environment that excludes excessive heat, and with no exposure to light is essential for its constant activity and reliability. 

In addition, some fuel cells will require deep-desulfiuized fuels. For example, methanol-based fuels for on-board fuel cell applications require the use of a fuel with sulfur content <1 ppmw in order to avoid poisoning and deactivation of the reformer catalyst. To use gasoline or diesel commercial fuels, which are the ideal fuels for fuel cells because of their high energy density, ready... 

Yaluris, G., Larson, R.B., Kobe, J.M., Gonzales, M.R., Fogash, K.B., and Dumesic, J.A. Selective poisoning and deactivation of acid sites on sulfated zirconia catalysts for n-butane isomerization. J. Catal 1996,158, 336-342. 

Thirdly, cobalt-based catalysts are poisoned and deactivated by chemical poisoning (e.g. sulfur compounds in most industrial syngas... 

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