Poisoning noble metals catalysts

CO formed through the decarbonylation of aldehydes may poison noble metal catalysts,... 

Sulfur from SO2 can poison noble metal catalysts by its strong bonding with the metal, forming the metal sulfide and even penetrating into the bulk metal l 3. When alumina is used as the catalyst support, irreversible deactivation can result from the formation of Al2(S04)3 with concurrent substantial reduction in surface area and pore volume " . Similar activity loss with decreased surface area and pore volume accompanying sulfur accumulation in the catalyst can result from the formation and deposition of sulfates of ammonia, particularly at lower operating temperatures, but these effects can usually be reversed by heating 2,47... 

Each precious metal or base metal oxide has unique characteristics, and the correct metal or combination of metals must be selected for each exhaust control appHcation. The metal loading of the supported metal oxide catalysts is typically much greater than for nobel metals, because of the lower inherent activity pet exposed atom of catalyst. This higher overall metal loading, however, can make the system more tolerant of catalyst poisons. Some compounds can quickly poison the limited sites available on the noble metal catalysts (19). 

Base Metal Catalyst - An alternate to a noble metal catalyst is a base metal catalyst. A base metal catalyst can be deposited on a monolithic substrate or is available as a pellet. These pellets are normally extruded and hence are 100% catalyst rather than deposition on a substrate. A benefit of base metal extruded catalyst is that if any poisons are present in the process stream, a deposition of the poisons on the surface of the catalyst occurs. Depending on the type of contaminant, it can frequently be washed away with water. When it is washed, abraded, or atritted, the outer surface is removed and subsequently a new catalyst surface is exposed. Hence, the catalyst can be regenerated. Noble metal catalyst can also be regenerated but the process is more expensive. A noble metal catalyst, depending on the operation, will typically last 30,000 hours. As a rule of thumb, a single shift operation of 40 hours a week, 50 weeks a year results in a total of 2,000 hours per year. Hence, the catalyst might have a 15 year life expectancy. From a cost factor, a typical rule of thumb is that a catalyst might be 10%-15% of the overall capital cost of the equipment. 

Sulfur poisons catalytic sites in the fuel cell also. The effect is aggravated when there are nickel or iron-containing components including catalysts that are sensitive to sulfur and noble metal catalysts, such as found in low temperature cell electrodes. Sulfur tolerances are described in the specific fuel cell sections of this handbook." In summary, the sulfur tolerances of the cells of interest, by percent volume in the cleaned and altered fuel reformate gas to the fuel cells from published data, are ... 

Catalytic hydrogenation of thiophene poses a problem since noble metal catalysts are poisoned, and Raney nickel causes desulfurization. Best catalysts proved to be cobalt polysulfide [425], dicobalt octacarbonyl [426], rhenium heptasulfide [5i] and rhenium heptaselenide [54]. The last two require high temperatures (230-260°, 250°) and high pressures (140, 322 atm) and give 70% and 100% of tetrahydrothiophene (thiophane, thiolene), respectively. 

Divalent sulfur is a poison for most noble metal catalysts so that catalytic hydrogenation of sulfur-containing compounds poses serious problems (p. 10). However, allyl phenyl sulfide was hydrogenated over tris trisphenyl-phosphine)rhodium chloride in benzene to give 93% yield of phenyl propyl sulfide [674. ... 

PEM fuel cells operate at relatively low temperatures, around 80°C. Low temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. However, they require that a noble-metal catalyst (typically platinum) be used to separate the hydrogen s electrons and protons, adding to system cost. The platinum catalyst is also extremely sensitive to CO poisoning, making it necessary to employ an additional reactor to reduce CO in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel. This also adds cost. Developers are currently exploring platinum/ruthenium catalysts that are more resistant to CO. 

Studies of the deactivation of ATR catalysts show that the sulfur present in conventional fuels is responsible for rapid deactivation of both Ni-based and noble metal catalysts. At some conditions, sulfur appears to selectively poison the sites responsible for the SR reaction(s). 

Reductions with noble metal catalysts proceed smoothly (at 20°C) when the bases are in the form of hydrochlorides the free bases tend to poison the catalyst. A pyridine ring is reduced more easily than a benzene ring thus, 2-phenylpyridine gives 2-phenylpiperidine (384), quinoline gives 1,2,3,4-tetrahydroquinoline (385) and acridine gives 9,10-dihydroacridine (386). 

The first isomerization catalysts, used in the 1930s, were Friedel-Crafts type catalysts, i.e. A1C13, but owing to their sensitivity to poisons and their corrosive nature they were quickly replaced by noble metal catalysts.2 The first noble metal catalysts consisted of platinum supported on an inert support, for example carbon, and were less corrosive and less sensitive. However, the operating temperature required for these catalysts was very high (600-750 K). 

Noble metal catalysts are known to be easily poisoned by sulfur. Tungsten carbide appeared to show noble metal characteristics, yet was found to be sulfur resistant. The n-heptane isomerization reaction described above was repeated with the two catalysts unsupported W2C and 0.3% Pt/Al203. 

Clean Air Act redirected the interest toward the inherently more active noble metal catalysts and stimulated an interest in detailed examination of the poisoning phenomena. 

Recent evaluations of S02 oxidation over noble metal catalysts (Pt, Pd, and Rh) have given some information on one particular secondary reaction. It was observed in car tests that S03 formation under the conditions of automobile exhaust is highly vulnerable to catalyst deactivation either by thermal sintering or by poisoning (78, 79). At the same time, the data indicated a lesser sensitivity of CO and hydrocarbon oxidation to catalyst aging. The results were confirmed in laboratory experiments (80). This is one example of preferential suppression of an undesirable side reaction. Obviously, the importance of a given poison on the different secondary reactions will vary widely with catalyst formulation and operating conditions. 

All the surface processes on automotive catalysts which have been tested for the effects of lead poisoning are affected by the access of lead to the catalyst surface. The effect will differ, though, for different surface processes. Oxidation of hydrocarbons has been found repeatedly to be more vulnerable than oxidation of carbon monoxide to lead poisoning (10, 19, 25). The initial oxidation activity of noble metal catalysts, never exposed to poisons, is higher for CO than for hydrocarbons (54). Therefore, it is best to use the effect of lead on hydrocarbon oxidation for assessing the susceptibility of a given oxidation catalyst to this type of poisoning. 

Hydrocarbon oxidation on base metal catalysts is also susceptible to lead poisoning, especially if the catalysts are exposed to relatively high temperatures, for at least part of their service time. It was noted above that lead retention, especially on base metal catalysts, also increases with temperature up to a certain point. This behavior is shown by the results of Yao and Kummer (81) in Fig. 18. One should note that the hydrocarbon used for testing catalyst activity, namely propylene, was quite reactive. With a less reactive test hydrocarbon one could expect a still sharper effect. The comparison with a reference production noble metal catalyst, given in Fig. 18, is quite instructive. 

There is no evidence in vehicle operation that the oxidation activity of noble metal catalysts suffers from poisoning by SOz (24, 28, 84), although Hunter claims (43) that Pt can be poisoned below 900°F. In contrast, severe deactivation of base metal catalysts has been observed in many instances. 

In certain instances of poisoning, especially in the case of base metal catalysts, the deactivation can be simply explained by the formation of new bulk solid phases between the base metal and the poison. Examples are the formation of lead vanadates (14) in vanadia catalysts, or of sulfates in copper-chromite and other base metal catalysts (81). These catalyti-cally inactive phases are identifiable by X-ray diffraction. Often, the conditions under which deactivation occurs coincide with the conditions of stability of these inert phases. Thus, a base metal catalyst, deactivated as a sulfate, can be reactivated by bringing it to conditions where the sulfate becomes thermodynamically unstable (45). In noble metal catalysts the interaction is assumed to be, in general, confined to the surface, although bulk interactions have also been postulated. 

B. Angele, and K. Kirchner (1980) The poisoning of noble metal catalysts by phosphorus compounds. I. Chemical processes, mechanisms, and changes in the catalyst, Chem. Eng. Sci 35 2089-2091... 

The main causes of the deactivation of diesel catalysts are poisoning by lubrication oil additives (phosphorus), and by SOx, and the hydrothermal instability. The SCR by HC is less sensitive to SOx than the NO decomposition. The Cu-based catalysts are slightly inhibited by water vapor and SOx, and suffer deactivation at elevated temperature. Noble metal catalysts such as Pt-MFI undergo low deactivation under practical conditions, are active at temperatures below 573 K but the major and undesired reduction product is N20 (56). 

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