Alkali metal poisoning, catalyst

Pt-Re-alumina catalysts were prepared, using alumina containing potassium to eliminate the support acidity, in order to carry out alkane dehydrocyclization studies that paralleled earlier work with nonacidic Pt-alumina catalysts. The potassium containing Pt-Re catalyst was much less active than a similar Pt catalyst. It was speculated that the alkali metal formed salts of rhenic acid to produce a catalyst that was more difficult to reduce. However, the present ESCA results indicate that the poisoning effect of alkali in Pt-Re catalysts is not primarily due to an alteration in the rhenium reduction characteristics. 

The presence of alkali salts may create other problems even in systems where deposition of hot vapors is not an issue. Alkali salts can be corrosive to metal surfaces and can poison catalysts such as those in tar cracking and synthesis gas applications. 

The production of sulphuric acid by the contact process, introduced in about 1875, was the first process of industrial significance to utilize heterogeneous catalysts. In this process, SO2 was oxidized on a platinum catalyst to S03, which was subsequently absorbed in aqueous sulphuric acid. Later, the platinum catalyst was superseded by a catalyst containing vanadium oxide and alkali-metal sulphates on a silica carrier, which was cheaper and less prone to poisoning. Further development of the vanadium catalysts over the last decades has led to highly optimized modem sulphuric acid catalysts, which are all based on the vanadium-alkali sulphate system. 

To prevent damage to downstream equipments and poisoning of downstream catalysts, the following contaminants have to been removed from the flue gas particulate (ash, char, and fluid bed material) causing erosion, alkali metals (sodium and potassium compounds) responsible for hot corrosion, tars (high molecular weight hydrocarbons and refractory aromatics), and catalyst poisoning species (H2S, HC1, NH3, and HCN). 

Molybdenum In its pure form, without additions, it is the most efficient catalyst of all the easily obtainable and reducible substances, and it is less easily poisoned than iron. It catalyzes in another way than iron, insofar as it forms analytically easily detectable amounts of metal nitrides (about 9% nitrogen content) during its catalytic action, whereas iron does not form, under synthesis conditions, analytically detectable quantities of a nitride. In this respect, molybdenum resembles tungsten, manganese and uranium which all form nitrides during their operation, as ammonia catalysts. Molybdenum is clearly promoted by nickel, cobalt and iron, but not by oxides such as alumina. Alkali metals can act favorably on molybdenum, but oxides of the alkali metals are harmful. Efficiency, as pure molybdenum, 1.5%, promoted up to 4% ammonia. 

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. 

The effect of steaming and of extensive poisoning by alkali metal ions is not limited to Y-type zeolites, as Lago et al. (12) have observed similar phenomena with mildly steamed H-ZSM-5. The activity for hexane cracking increased by about a factor of four upon mild steaming of the catalyst. Selective Cs poisoning indicated that the concentration of a more active site in the steamed sample was only about 6% of the tetrahedral framework aluminum. These sites exhibited a specific activity 45-75 times greater than that of a normal site in H-ZSM-5. 

Other impurities such as HCl, alkali metals and some vapour-phase metals, causing corrosion of the gas turbine, heat exchangers or poisoning catalysts of the catalytic tar crackers. 

Within the Fischer-Tropsch research ECN Biomass concentrates on the definition of the gas cleaning with respect to the typical B R in urities, like NHj, HCl, HCN, H]S, COS, tars (heavy organic molecules), soot, and alkali metals, Traces (< ppm) of these compounds can already be a poison for the Fischer-Tropsch catalysts. For the implementation of B R and Fischer-Tropsch ECN its strategy is on the demonstration of integrated systems to reduce the time necessary to realise a first full-scale installation for conversion of biomass and residue, gas cleaning, and Fischer-Tropsch synthesis. To achieve this ECN focuses on two lines of development ... 

The promoting effect of the addition of alkali on the catalytic performance of many transition-metal-based catalysts is experimentally well known, but there is no general agreement on its theoretical explanation. The same holds for the opposite effect the poisoning of catalysts by, e.g., the adsorption of sulphur. 

In summary, alkali promotion of supported metal catalysts is an interesting subject that does have important technological implications in those cases where the presence of alkali has a pivotal influence on the surface chemistry of the metal phase. Fundamental studies of such systems are certainly justified. However, we should maintain a sense of proportion. Alkalis find relatively limited use as promoters in practical catalysis—indeed in some cases they act as powerful poisons. And we should not lose sight of the fact that what is actually present at the surface of the working catalyst is not an alkali metal, but some kind of alkali surface compound. This chapter deals with the application of alkali promoters to catalysis by metals, as opposed to catalysis by oxides, and, in particular, the technique of electrochemical promotion (EP), which enables us to address some pertinent issues. 

Other components that poison the reformer catalysts include silica and alkali metals. Silica may act by blocking the pore mouth of the catalyst. The alkali metals reduce the turnover frequency of the catalyst. ... 

Figure 14 clearly shows the effect of the addition of potassium to the catalyst on the activity and of the effect of the presence of WO3 in the catalyst thereon. The poison resistance to both alkali metal oxides and arsenious oxide is significantly increased by the presence of WO3 in the WO3/V2O5 on Ti02 catalyst. The reason for this effect is that the Bronsted acid sites protect the catalyst for the attack of alkali. Moreover, WO3 inhibits the oxidation of ammonia and the oxidation of SO2 as well. 

Chen and Yang [118] reported that the strength of the poison of alkali metals oxides follows the order, Cs > Rb > K > Na > Li for the SCR catalysts mentioned. 

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