Deactivation by poisoning

If a fraction a of the total catalyst surface has been deactivated by poison, the pore-mouth poisoning model assumes that a cylindrical region of length (aL) nearest the pore mouth will have... 

The authors ascribed the high WGS rates of the Pt/FSM-16 catalysts to confinement effects which increased the activities of Pt surface atoms, as well as to anisotropic morphological effects. Based on infrared studies, the authors identified unidentate formate species as intermediates in Au/NaY, and though the species was also observed on Au/Na Mordenite, the catalyst was found to deactivate by poisoning from a carbonate species. Only stable carbonates were observed on Au/Na-ZSM-5. The authors proposed a mechanism for Au/NaY, depicted in Scheme 94. 

If the avoidance of deactivation by poisoning is difficult, could a partially deactivated catalyst be regenerated by chemical or physical means In the oil and chemical industry, catalyst reactivation is a standard procedure, done sometimes in situ or after removal from the reactor. Logistically, it does not seem to be feasible to remove an automotive... 

Pore mouth deactivation by poisoning or fouling is more likely when precursor molecules are large and the pores are narrow and long. These factors make the Thiele modulus for poison deposition, h, large. In such instances the poison precursor molecules will reside in the vicinity of the pore mouth longer and be more likely to lie down there. 

Deactivation by Poisoning. Deactivation by this mechanism occurs when the poisoning molecules become irreversibly chemisorbed to active sites, thereby reducing the number of sites available for the main reaction. The poisoning molecule, P, may be a reactant and/or a product in the main reaction, or it may be an impurity in the feedstream. 

In many large-scale reactors, such as those used for hydrotreating, and reaction systems where deactivation by poisoning occurs, the catalyst decay is relatively slow. In these continuous-flow systems, constant conversion is usually necessary in order that subsequent processing steps (e.g., separation) are not upset. One w to maintain a constant conversion with a decaying catalyst in a packed or fluidized bed is to increase the reaction rate by steadily increasing the feed temperature to the reactor. (See Figme 10-26.)... 

The colloidal nickel catalyst cannot be removed by conventional filtration techniques nor have effective means of deactivation by poisoning been found. Ziegler claims that addition of colloidal iron will poison the nickel catalyst. The use of iron and other potential nickel poisons has been studied in some detail. Salts of Cd, Cu, Cr, Fe, Hg, Se, V, and Zn along with phenylacetylene and sulfur dichloride have been tested as nickel deaotivators. Iron, cadmium, and copper salts seemed effective in limiting alkylation between olefins and triethylaluminum... 

Formation of byproducts may partly be caused by the outer surface acidity of the zeolites. If so, deactivation of the outer surface should lead to better selectivities. The outer surfaces of H-mordenite and HB were deactivated by poisoning with triphenylphosphine. For checking outer surface deactivation we have developed a chemical probe molecule [6]. In this way, it was found that the poisoning with triphenylphosphine resulted in a totally inactive outer surface within the time scale of the reactions. 

Many ideas come to mind when one tries to think about this elusive variable. Professor Petersen and I wrote a book about it, and we thought we really had an idea. Unfortunately, the older I get, the more fleeting the idea seems. However, I do think that we can pin down the elusive a in some cases. One case is deactivation by poisoning. This is because the catalyst is subject to a specific chemical deactivation thus for the classical academic reaction A B we have a parallel scheme such as... 

Deactivation by Poisoning and the Improvement of Three Way Catalysts for Natural Gas-Fueled Engines... 

Catalyst deactivation by poisoning (chlorine) was modeled by the following set of equations ... 

Recent results showed that Au/Ce02 catalysts are subject to deactivation by poisoning or blockage of the active sites by either carbonates, formates or hydrocarbons [217,234,235]. These species appeared to be formed by CO and H2, and their formation was facilitated by oxygen-deficient sites on ceria [217]. However, regeneration of the WGS activity can be achieved by calcination 95% of the initial activity is recovered by heating in air at 673 K [217,234,235]. 

The porosity of the beads used is the result of a lot of optimization, and is formed both by macropores with pore diameters exceeding 0.1 pm and by micropores with a pore diameter less than 20 nm. The micropores give the high BET surface area, whereas the macropores assure a high intraparticle mass transfer rate as well as a resistance against deactivation by poisoning. 

Figure 4 Deactivation by poisoning of active metal sites oy of Component "X".

In the investigation of catalyst deactivation by poisoning, the distribution of the active centers, the stoichiometry, and diffusion are of decisive importance. In the following, poisoning of the most important classes of catalysts, i.e., metals, semiconductors, and acidic insulators, is discussed. 

If there is no deactivation by poisoning, then m = 0. This could then be a process of deactivation by sintering. With the simplifying assumption n = 1, Equation 5-88 becomes ... 

Sueh exponential equations can sometimes also describe deactivation by poisoning, provided the concentration of the catalyst poison is constant Examples are the hydrogenation of ethylene on copper catalysts (poisoning by CO) and the dehydrogenation of alkanes on Cr/Al203 catalysts. 

The relevance of interphase gradients distinguishes between two different classes of problems, and this is reflected on the type of boundary condition at the pellet s surface. It is known that specifying the value of the concentration (or temperature) at the surfece (Dirichlet boundary condition) may not be realistic, and thus finite external transfer effects have to be considered (in a Robin-type boundary condition) [72]. Apart from these, a large number of additional effects have also been considered. Some examples include the nonuniformity of the porous pellet structure (distribution of pore sizes [102], bidisperse particles [103], etc.), nonuniformity of catalytic activity [104], deactivation by poisoning [105], presence of multiple reactions [106], and incorporation of additional transport mechanisms such as Soret diffusion [107] or intraparticular convection [108]. 

This chapter discusses the local (i.e., up to the particle size) effects of deactivation by poisoning and by coking. The effect on the reactor scale is dealt with in Chapter 11. 

Sahimi and Tsotsis [1985] have simulated catalyst deactivation by poisoning in structure models of catalysts, discussed in Chapter 3. 

So far in this chapter, the catalyst activity has been assumed uniform throughout a pellet. Nonuniform activity can result from poor impregnation, deliberate partial impregnation, or from deactivation by poisoning species. In such cases, the intrinsic kinetics can be represented by ... 

When fuel contains heavier hydrocarbons than methane, or it is biofuel, or contains alcohols, the feedstock often contains additional compounds such as sulphur and phosphorus, that is, fertiliser impurities. In the petrochemical industry, gas-borne reactive spedes (i.e., sulphur, arsenic, chlorine, mercury, zinc, etc.) or unsaturated hydrocarbons (i.e., acetylene, ethylene, propylene and butylene) may act as contaminating agents (Deshmukh et al, 2007). These impurities cause catalyst deactivation by poisoning. The effect of a poison on an active surface is seen as site blockage or atomic surface structure transformation (Babita et a/., 2011). Therefore, it is important to choose poisoning-resistant catalyst materials. For example, nickel is not the most effective MSR catalyst although it is widely used in industry due to its low market price compared to ruthenium and rhodium. Both Ru and Rh are more effective in MSR and less carbon is formed in these systems, than in the case of Ni. However, due to the cost and availability of precious metals, these are not widely used in industrial applications. 

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