Poison selective catalytic

Oxidation/selective catalytic reduction, poisons, 5 258t Oxidation states... 

A special type of selective catalytic reaction is the poisoning of the catalytic formation of ammonia by oxygen or oxygen-containing gases,... 

Crude oil contains about 0.01% metals and up to 5% sulfur present in large aromatic structures. These levels are highly dependent on the origin of the crude. For example, California crude is relatively low in sulfur but higher in metals than crude from Kuwait. Any process to remove them must be economical with little destruction of the hydrocarbons and minimum consumption of H2. The catalyst is Co, Mo/A1203 with particles a few mm in diameter. Although sulfur is usually a poison for catalytic reactions it is used here in a positive function to control selectivity. It is presulfided to decrease activity towards excessive consumption of H2 that leads to unwanted saturation of aromatic molecules. 

The external versus internal surface has also been investigated in the case of Co-H-zeolites active in the selective catalytic reduction of NO by methane [20]. The highly hindered nitrile, ortho-toluonitrile (oTN) has been used, co-adsorbed with CO and NO. The pre-adsorption of oTN poisons the external cationic sites of Co-H-ZSM5. Successive adsorption of CO shows that some of the Co ions are on the inner surface and some on the outer surface. The coadsorption of oTN and NO shows that trivalent Co ions are actually located in the internal cavity surface of Co-H-ZSM-5 while divalent Co ions distribute almost equally in the internal and external surface. 

The major concern in applying selective catalytic reduction is deactivation or poisoning of the catalyst. One cause of deactivation may be catalyst poisons present in the flue gases. 

Hydroxyethyl)-pyridine was dehydrated to 2-vinyl-pyridine in liquid phase over solid acid catalysts, with very high selectivity and fairly good reaction rate at relatively low reaction temperature (160°C). The catalytic activity is well correlated with the presence on the catalyst surface of medium to weak Bronsted acid sites. The analysis of coke left behind onto the catalyst and the effect of partial poisoning of catalytic activity by CO2 indicate that the reaction takes place through two mechanisms, involving either a Bronsted acid site or a couple of acid-base sites. 

R. T. Yang, J. P. Chen, M. A. Buzanowski, and J. E. Cichanowicz", Catalyst Poisoning in the Selective Catalytic Reduction Reaction", 1989 Joint Symposium on Stationary Combustion Control, San Francisco, CA, 1989. 

In many cases the effect of poisoning on catalytic surfaces can be represented as a linear function of the fraction of total surface poisoned ( non-selective poisoning). 

Trace metal concentrations in herbaceous biomass materials are the final area of concern, particularly with the emphasis on mercury emissions management and the possible concern for selective catalytic reduction catalyst deactivation or poisoning as a result of cofiring straws and other herbaceous biomass fuels [1,17]. The database ftM" herbaceous materials is not extensive. Table 5.10 presents a general range of values. 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

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