Effects of Lead Catalysts

Since the initial work of Onto et al. (1) a considerable amount of work has been performed to improve our understanding of the enantioselective hydrogenation of activated ketones over cinchona-modified Pt/Al203 (2, 3). Moderate to low dispersed Pt on alumina catalysts have been described as the catalysts of choice and pre-reducing them in hydrogen at 300-400°C typically improves their performance (3, 4). Recent studies have questioned the need for moderate to low dispersed Pt, since colloidal catalysts with Pt crystal sizes of <2 nm have also been found to be effective (3). A key role is ascribed to the effects of the catalyst support structure and the presence of reducible residues on the catalytic surface. Support structures that avoid mass transfer limitations and the removal of reducible residues obviously improve the catalyst performance. This work shows that creating a catalyst on an open porous support without a large concentration of reducible residues on the Pt surface not only leads to enhanced activity and ee, but also reduces the need for the pretreatment step. One factor... 

As an example, when automotive catalytic mufflers and converters were introduced many years ago, the automobile industry required the petrochemical industry to eliminate lead from gasoline since lead degraded and reduced the effectiveness of the catalyst and caused the destruction of the gasoline. One set of industrial compounds that can harm catalysts are halogens, a family of compounds that include chlorine, bromine, iodine, and fluorine. Bromine, while not prevalent in industry, is present in chemical plants. Freons are fluorine compounds. Silicone is another compound that is deleterious to catalysts. It is used as a slip agent, or a lubricant, in many industrial processes. Phosphorous, heavy metals (zinc, lead), sulfur compounds, and any particulate can result in shortening the life of the catalyst. It is necessary to estimate the volume or the amount of each of those contaminants, to assess the viability of catalytic technologies for the application. 

An unusual effect of the catalyst concentration on product rates has been observed. Increasing the concentrations of both the iodide promoter and the ruthenium catalyst precursor while holding their ratio constant leads to... 

The effects of the catalyst on burning rate and flame reaction indicate that the super-rate burning phenomena observed in the combustion of HMX-CMDB propellants are fundamentally the same as the combustion phenomena of catalyzed double-base propellants. This implies that the lead catalysts act on the combustion of HMX to produce super-rate burning. 

Significant improvement of the activity of selectivity of Pd on Si02 could be achieved in the hydrogenation of acetylene by adding Ti, Nd, or Ce oxides to the catalyst.395 The metal oxides modify both geometrically and electronically the Pd surface. They retard the sintering of the dispersed Pd particles, suppress the formation of multiply bound ethylene, and facilitate the desorption of ethylene. The beneficial effect of lead in the hydrogenation of 1,3-butadiene over a Pd-Pb-on-... 

Examination of the spent pellets from these early catalyst beds showed already then the deleterious effect of lead on the surface (17,18), the covering of the pellet exterior by impervious lead containing layers (14), the interaction of lead with base metal active components (14), etc., as the causes of activity decrease. 

In connection with the separate effects of lead and its scavengers, several investigators have observed that without the use of scavengers, lead retention on the catalyst drops off markedly. On reflection, this could well be anticipated, since the volatile compounds with scavengers are the main lead transport entities from the locus of combustion to the catalyst. 

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. 

The effect of lead on hydrocarbon oxidation over noble metal catalysts has been variously presented as a relation between catalyst activity and lead content in gasoline (19, 22), or of lead supplied to the engine (31, 32), etc. The most meaningful correlation is between hydrocarbon activity and lead deposit on the catalyst. Two examples of such correlations, one for laboratory-tested samples and the other for fleet-tested catalysts, will follow. 

In order to explain the effects of the catalyst composition we postulate that two major routes to coke exist, viz. (i) radical reactions giving rise to condensation of the aromatic structures which ultimately lead to thermal coke and (ii) dehydrogenation reactions which... 

In this paper, the effect of lead adatoms on the activity of platinum catalysts, in liquid phase hydrogenation of olefinic compounds, is presented. 

The test-reaction, used in order to evaluate the effect of lead, was the hydrogenation of olefinic diacids (maleic, methylmaleic and dimethyl maleic acids), carried out in an aqueous solution of 0.5 M sulphuric acid. The characterization and the modification of the catalyst were performed in the same solution, before starting the hydrogenation experiments. More experimental details were reported in (13,14). 

Oxide catalysts are known to be effective for oxidation reactions. In this study, we wanted to produce carbon monoxide through partial oxidation of the biomass, as this could be expected to lead to a conversion of carbon monoxide into hydrogen via the water-gas shift reaction. An oxidization of the tarry product is also expected. By these two effects, improvement of the efficiency of the gasification is expected. Oxide catalyst is expected to enhance the oxidation reaction needed for this scenario. Since oxide catalyst is considerably cheaper than nickel catalyst, its use would make the whole gasification process more economical. Hence, we decided to examine the effect of oxide catalysts on gasification with partial oxidation using cellulose as a model compound. 

While examining the effect of modified catalysts on aldaric acid formation it was found that Pt/C catalysts that were treated with lead salts were extremely selective for the oxidation of the hydroxy group next to the initially formed carboxylic acid to give 2-keto gluconic acid. 2.83 xhese catalysts also promote the selective oxidation of a number of other a-hydroxy acids. For example, lactic acid has been oxidized to pyruvic acid in greater than 95% yield (Eqn. 21.27).83 Similar results have been observed with lead doped Pd/C catalysts. 2,75... 

Fukuda and Kusama [11] successfidly partially hydrogenated butynediol by enq>loying Pd-CaCOj and quinoline as poison. In later work of Fukuda [3], the combination effect of lead acetate and quinoline on Pd/BaCOs gave partial hydrogenation as weU. hi order to selectively produce m-butenediol, Chaudhari et al. [12] reported the use of Lindlar catalyst doped with zinc acetate that gave partial reduction with selectivity as high as 99.8% toward m-butenedioL... 

Giacomazzi, R.A., Homfeld, M.F., The Effect of Lead, Sulfur, and Phosphorous on the Deterioration of Two Oxidizing Bead-Type Catalysts SAE Meeting, Detroit, May, 1973, SAE 730595. 

In addition, the presence of sulphur during the ageing of autocatalysts is known to lead to a significant deterioration in performance under certain conditions. Specifically, the rich-side NOx performance of Pd-based catalysts is often significantly reduced by rich ageing treatments in the presence of sulphur. Such poisoning can occur in at least two ways. First of all, it is possible that the number of active sites is lowered by the retention of reduced sulphur species on the active Pd sites. Secondly, it is possible that the formation of reduced sulphide species within the bulk of the Pd leads to a decrease in the turnover frequency of the active sites. The formation of such species within the bulk of Pd particles has been recently demonstrated [9], and it is possible that the electronic modification of the Pd particles induced by these sulphide species could reduce the NO dissociation probability on the Pd sites and hence reduce the effectiveness of the catalyst within the NOx reduction reaction. 

EFFECT OF LEAD ON VEHICLE CATALYST SYSTEMS IN THE EUROPEAN ENVIRONMENT... 

A test programme was initiated to investigate the effect of lead on Three Way Catalyst (TWC) with high temperature excursions to simulate autobahn running. The programme was completed in three stages Laboratory, Dynamometer and Vehicle tests. 

The dynamometer ageing test results demonstrated that fuel with 10 mg/1 is unacceptable. However, if the effect of lead deposition is assumed to be linear then maximum Pb levels of approx 5 mg/1 can result in legal emission levels being achieved at 80K km when catalysts experience a modest duty cycle as described in this paper. 

The vehicle durability tests have consistently shown that substantial catalyst deactivation takes place with fuel at 10 mg/1 Pb. Lead levels of 3 mg/1 and below allow catalyst systems to function satisfactorily during 80K km of AMA drive cycle which would indicate that even if a customer continually drives at low speed (which gives max lead deposition condition) then catalyst deterioration due to lead will be minimal. The test results indicate that the effect of lead on the HEGO sensor is more critical than its 6ffect on the catalyst. 

Since the test schedule started the lead levels in unleaded fuel available at the pumps in Europe (Germany, Switzerland, Austria) has been monitored. Against expectation lead levels have dropped rapidly to an average of 2mg/l. This level, if maintained, will ensure that the effect of lead on catalyst systems will be negligible up to 80K km. 

Effect of Lead on Vehicle Catalyst Systems in the European Environment ,... 

The new generation of fixed bed catalysts made fi om activated base metals (Metalyst ) is best described as an extremely durable collection of empty eggshells composed of high surface area activated metal. This technology offers an increase in volumetric activity at lower catalyst bulk densities without the presence of expensive, but not utilized, materials. In other words, the amount of metal in the reactor is reduced by as much as 40 to 77% leading to a more effective use of metal (1). In this paper, the effectiveness of this catalyst technology will be compared to others for the hydrogenation of various compounds (2, 3,4). 

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