Sulfur poisoning mechanism

In this paper, we discuss the sulfur-poisoning mechanism of Pd(R)/Al203 catalysts according to the chemical states and the relative quantity of sulfur on/in the catalysts and explore the approaches and procedures to regenerate poisoned catalysts. 

In an extension of the work by Foley et Tsai et al.43 undertook the determination of the causes of SO2 deactivation of supported metal catalysts in the reduction of NO with NH3. They placed foils of Ni, Ru, Pd and Pt in the beds of catalyst particles of these metals supported on AI2O3, then, after reaction, they removed and examined the foils by Auger electron spectroscopy (AES). This determination of the surface composition and depth of penetration of the elements enhances the interpretation of the sulfur-poisoning mechanism. 

It was shown in laboratory studies that methanation activity increases with increasing nickel content of the catalyst but decreases with increasing catalyst particle size. Increasing the steam-to-gas ratio of the feed gas results in increased carbon monoxide shift conversion but does not affect the rate of methanation. Trace impurities in the process gas such as H2S and HCl poison the catalyst. The poisoning mechanism differs because the sulfur remains on the catalyst while the chloride does not. Hydrocarbons at low concentrations do not affect methanation activity significantly, and they reform into methane at higher levels, hydrocarbons inhibit methanation and can result in carbon deposition. A pore diffusion kinetic system was adopted which correlates the laboratory data and defines the rate of reaction. 

Deactivation is due primarily to two mechanisms formation of carbon-containing deposits and sulfur poisoning. Carbon deposition may be minimized by the addition of alkali metals, optimization of metal cluster size, and use of oxygen ion-conducting supports. Sulfur poisoning is usually irreversible and there are few reports of catalysts that are tolerant of sulfur levels typical of commercial fuels. 

What is the mechanism of sulfur poisoning of metals Is it due to a geometrical blocking or does it involve electronic effects that may propagate many atomic distances away from the adsorption site ... 

Is the mechanism of sulfur poisoning a function of the metal, of the reaction and/or of reaction conditions can the dependence of the system on any of these variables be predicted a prioril... 

What are the rates of sulfur adsorption and of sulfur poisoning and can they be predicted Can catalyst life in commercial catalyst applications be predicted based on poisoning mechanisms and models ... 

What are the mechanisms of sulfur removal from surfaces and can these mechanisms be used in the development of regeneration techniques for sulfur-poisoned catalysts ... 

Whether such bulk gettering effects are transient or steady state is unclear. The mechanisms of such effects and whether they are due to bulk or surface phenomena is also unknown. Thus it is clear that bulk thermodynamic information used in a cursory way does not help in rationalizing observed sulfur-poisoning behavior. Furthermore, since most metals of catalytic interest do not form stable bulk sulfides under typical reaction conditions, the observed severe poisoning by sulfur suggests that surface rather than bulk thermodynamics may be required. 

The experimental techniques utilized in the study of sulfur poisoning are markedly more critical to obtaining quantitative, basic data than in any other type of reaction study. In most previous studies of sulfur-poisoned catalysts, rate data were affected by experimental complexities to a sufficient extent that a basic understanding of poisoning rates and mechanisms was not possible. Most of these difficulties occurred because of ... 

Poisoning of iron catalysts during ammonia synthesis by sulfur compounds has received relatively little attention (154, 240-244). Nevertheless, the previous work provides information on the poisoning mechanism and interesting examples of how oxide promoters may influence the sulfur poisoning behavior of a catalytic metal. 

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