Poisoning supported palladium catalysts

However, by partially poisoning the palladium catalyst supported on calcium carbonate with lead acetate and quinoline (the Lindlar catalyst ) it is possible to reduce alkynes to... 

Other partially poisoned catalysts have long been used in the laboratory. Supported palladium catalysts, poisoned with lead (Lindlar catalysts), sulfur, or quinoline, are used for the hydrogenation of acetylenic compounds to cis-olefins. Another... 

In the Rosenmund reaction (Eq. 5-84), acid chlorides are hydrogenated to aldehydes. The catalyst is a supported palladium catalyst (5 % Pd/BaS04) poisoned by sulfur compounds such as quinoline, tiourea, or thiophene to prevent further reduction of the aldehyde. 

The supported palladium catalyst known to be the most active for total methane oxidation was the subject of considerable amount of research [1-9]. However, no agreement about the mechanism reaction was observed in the literature [1-8]. The Langmulr-Hinshelwood [1-4], the Eley-Rideal [5-7], and the Mars-Van Krevelen [8], mechanisms were proposed for the total oxidation of methane on the supported palladium catalysts. This diversity is explained by the variation of the active surface in each case. Indeed, according to Burch et al.[9], the active sites can be modified by the pre-treatment conditions, by the particle size, by the support nature and by the presence of some poisons such as chlorides. Others difficulties result from the fact that it is not confirmed if the active site is a partial or a total oxidized palladium particle. In addition, little is known about the reactive oxygen form. Indeed, it is not yet established if the reactive oxygen is a chemisorbed molecular or ionic form or a lattice oxygen ion. The aim of this paper is to identify the palladium oxidation state under catalytic stream, to study the reactive form of oxygen and to propose a mechanism of the reaction. 

We have studied the hydrogenolysis of 2-(perfluorohexyl)ethane thiocyanate to 2-(perfluorohexyl)ethane thiol. It was discovered that perfluoroalkyl thiocyanates can be reduced to thiols and co-product hydrogen cyanide with molecular hydrogen in the presence of a carbon-supported palladium-tin catalyst. This result is surprising since it is known that palladium and other gronps 8 to 10 metal catalysts are poisoned by the product thiol, traces of hydrogen snlfide byprodnct, and the hydrogen cyanide co-product. For that reason, we characterized the catalyst to understand why it was so robust under conditions that would normally poison snch a catalyst. 

The most selective and widely used catalyst is palladium, usually on an alumina support. A bimetallic palladium catalyst has also been developed.310 Palladium is more selective and less sensitive to sulfur poisoning than are nickel-based catalysts. Additionally, sulfides can also be employed. 

It also seems that if Pt and Pd are impregnated on the same support, the catalyst poisons almost as fast as palladium, whereas Pt itself appears to retain a residual oxidation activity suggesting that alloy formation occurs with Pd at the surface. However, Pd appeared to resist sintering better in this application and therefore impregnation of the two metals into separate layers of the support was advantageous. For example, both the steady state and light-off performance were improved by impregnating the outer shell with Pt and the inner shell with Pd. 

Catalytic experience tells us that frequently only a small fraction of the sites at the surface of the metal participate in the reaction. Recently, Krai measured the metal surface area of palladium catalysts supported on carbon, their specific activity for various hydrogenation reactions, and their poisoning by thiophene (44). By combining the classic technique of poisoning with the measurement of metal surface area. Krai was able to show that the Taylor ratio, i.e., the fraction of active sites in each particular reaction, changed from unity to about 10 . With a Taylor ratio of unity, the reaction would be called facile. Otherwise, we deal with a structure-sensitive or demanding reaction. 

On the other hand, cerium has been shown to be an effective oxygen reservoir, enhancing the activity of many oxidation catalysts. Due to this property, cerium oxide is also considered to potentially enhance the thioresistance of the catatysts. This aspect is of great practical importance, since the use of palladium catalysts is hindered by the poisonous effect of sulphur compoimds, often present in off gases. Most works dealing with ceria-zirconia catalysts have been carried out with catalysts prepared by coprecipitation methods, whereas in this work an ahemative procedure, based on the incipient wetness technique is used to incorporate ceria to the zirconia support. The aim is to maintain the advantages of zirconia supports, especially the thermal stability. 

Palladium was added to unmodified zirconium oxide and supports A and B by incipient wetness, using an aqueous solution of Pd(N03)2 as precursor, in order to obtain solids with 1% wt palladium in all the cases. The solids produced according to this procedure will be called here catalysts U, A and B, respectively. PdCfe was not used as precursor, as chlorine anions are considered poisons for the catalysts, so a wash step is necessary to eliminate them when PdCb is us as precursor. On the other hand, it is known that Pd(N03)2 leads to catalysts with higher crystallite size and lower metal dispersion, although they are more active for the oxidation of methane [7]. The catalysts thus prepared were dried overnight at 100 C after impregnation, and then calcined in air at 550°C for 2 h. 

For special purposes, e.g., for partial reduction of triple to double bonds138 or of cumulenes to polyenes,139 the catalyst may be deactivated by partial poisoning. Double bonds are not hydrogenated in presence of these catalysts. Lindlar138 gives the following directions for preparation of a palladium catalyst deactivated by lead acetate and supported on calcium carbonate ... 

Similar deactivation is observed with bismuth and copper salts. A palladium catalyst poisoned with quinoline-sulfur and support on barium sulfate is used in the Rosenmund-Saytzeff reaction (p. 67). 

The parameters that affect the degradation of supported platinum and palladium automotive exhaust catalysts are investigated. The study includes the effects of temperature, poison concentration, and hed volume on the lifetime of the catalyst. Thermal damage primarily affects noble metal surface area. Measurements of specific metal area and catalytic activity reveal that supported palladium is more thermally stable than platinum. On the other hand, platinum is more resistant to poisoning than palladium. Electron microprobe examinations of poisoned catalyst pellets reveal that the contaminants accumulate almost exclusively near the skin of the pellet as lead sulfate and lead phosphate. It is possible to regenerate these poisoned catalysts by redistributing the contaminants throughout the pellet. 

Figure 4 shows the total conversion of ethanol as a function of temperature as measured by gas chromatography. Except for the silica catalysts, the platinum catalysts exhibit equal or lower light-ofif temperatures than the supported catalysts with palladium as active material (compare with Figure 7). The platinum on alumina and platinum on titania catalysts are more active than the other catalyst combinations. The conversion curves for the Pd and Pt on ceria catalysts practically coincide, which implies that ceria would be a more suitable support material for a palladium catalyst than for a platinum catalyst. The activities of the silica catalysts are low. This observation is consistent with recent results in another research project using the same type of silica sol (Zwinkels et al, 1994). According to these experiments, it is crucial to reduce the alkali content to a very low level in the support, since sodium increases the mobility of silica, which poisons the active platinum and palladium sites. Platinum is apparently more sensitive to this phenomenon than palladium.

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