Resistance to poisons

Rhenium catalysts are exceptionally resistant to poisoning from nitrogen, sulfur, and phosphorus, and are used for hydrogenation of fine chemicals. 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

In 1916, calcium arsenate [7778-44-1] dusted by airplane was used to control the boU weevil however, throughout many developments in effective insecticides, such as organophosphates, the boU weevils became resistant to poisons that were formerly effective (see Insectcontroltechnology). 

Meta/ Oxides. The metal oxides aie defined as oxides of the metals occurring in Groups 3—12 (IIIB to IIB) of the Periodic Table. These oxides, characterized by high electron mobiUty and the positive oxidation state of the metal, ate generally less active as catalysts than are the supported nobel metals, but the oxides are somewhat more resistant to poisoning. The most active single-metal oxide catalysts for complete oxidation of a variety of oxidation reactions are usually found to be the oxides of the first-tow transition metals, V, Cr, Mn, Fe, Co, Ni, and Cu. 

A catalyst with a substantially improved resistance to poisoning by phosphoms ia catalytic oxidatioa appHcatioas was developed. la part, the catalyst ia this program peroiitted printers to use photolighographic technology without payiag an unreasonable cost ia terms of frequeat replacement of oxidatioa catalysts. 

Platinum and rhodium sulfided catalysts are very effective for reductive alkylation. They are more resistant to poisoning than are nonsulfided catalysts, have little tendency to reduce the carbonyl to an alcohol, and are effective for avoidance of dehydrohalogenation in reductive alkylation of chloronitroaromatics and chloroanilines (14,15). Sulfided catalysts are very much less active than nonsulfided and require, for economical use, elevated temperatures and pressures (300-2(KX) psig, 50-l80 C). Most industrial reductive alkylations, regardless of catalyst, are used at elevated temperatures and pressures to maximize space-time yields and for most economical use of catalysts. 

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

Attention has been given to the synthesis of bimetallic silver-gold clusters [71] due to their effective catalytic properties, resistance to poisoning, and selectivity [72]. Recently molecular materials with gold and silver nanoclusters and nanowires have been synthesized. These materials are considered to be good candidates for electronic nanodevices and biosensors [73]. 

Au/C was established to be a good candidate for selective oxidation carried out in liquid phase showing a higher resistance to poisoning with respect to classical Pd-or Pt-based catalysts [40]. The reaction pathway for glycerol oxidation (Scheme 1) is complicated as consecutive or parallel reactions could take place. Moreover, in the presence of a base interconversion between different products through keto-enolic equilibria could be possible. 

Prominent co-catalysts for the Pt-on-carbon anode catalyst in the oxidation of polyhydric alcohols are Ru or Ce02 [54, 60]. Their increased resistance to poisoning with mainly CO during operation is associated with the existence of a bifunctional mechanism (Scheme 11.6). 

It has long been recognized that the addition of impurities to metal catalysts can produce large effects on the activity, selectivity, and resistance to poisoning of the pure metal For example, the catalytic properties of metals... 

For a catalyst the desired properties are high and stable activity high and stable selectivity controlled surface area and porosity good resistance to poisons... 

But most of the issues involve the catalyst system itself. The catalyst must be active and selective for the fuel of choice, stable, and resistant to poisoning and attrition while subjected to variations in flow, temperature, and pressure." For successful operation at commercial scale, the reforming process must be able to achieve high conversion of the hydrocarbon feedstock at high space velocities, as well as high H2 and CO selectivities. The reforming catalyst has to meet performance targets (see Table 1) as identified by U.S. DOE before it becomes feasible for use in the fuel reformers of transportation fuel cell... 

All of these problems are related to the performances of the catalysts used in coal liquefaction. Very active, durable, recoverable, and regenerable catalysts are most wanted in the primary liquefaction stage, where catalyst poisons from asphaltenes and minerals are most severe. Multifunctional catalysts should be designed by selecting supports with specific functions, such as strong but favorable interactions with catalytic species, resistance to poisons, and improved properties to allow easy recovery, while maintaining high activity. 

TMS catalysts fell into a special category due to their exceptional resistance to poisons. In fact, the presence of sulfur compounds, the most common poison of metallic and oxide catalysts, does not decrease their catalytic activity, but is needed to maintain high activity. Sulfide catalysts are also very resistant to carbon deposition, which is illustrated by their use for converting residual oils. Arsenic, as well as nickel and vanadium contained in heavy petroleum fractions, are some of the few substances that cause significant deactivation, and this only occurs by physical blockage of pore structure in supported catalysts. 

In 1949, the development of a catalyst based on a combination of platinum and an acidic component (e.g. A1203, A1C13) allowed the use of lower reaction temperatures than with the early catalysts.6 However, problems were still encountered with chlorine corrosion. In the 1960s, Universal Oil discovered that the addition of rhenium to a bifunctional Pt/Al203 catalyst resulted in slower deactivation by carbon deposition, and other dopants have since been found to modify the catalyst acidity and resistance to poisons, e.g. Cl, Sn, Ir. More recently, catalysts based on zeolites and noble metals have been shown to be more resistant to nitrogen and sulphur compounds, while giving a high activity and selectivity to branched alkanes. 

A detailed understanding of the properties of early transition metal carbides is of practical importance because they often demonstrate unique catalytic advantages over their parent metals in activity, selectivity and resistance to poisoning.1,2 We have chosen vanadium carbide films, produced on a vanadium (110) single crystal surface, as model systems to understand the fundamental aspects related to the electronic and catalytic properties of carbide materials. One of the main advantages of using the single crystal surface is that it enables one to apply powerful surface science spectroscopies to determine the electronic and catalytic modifications... 

This cathode (called TWAC) exhibits a low Tafel slope up to a few kA m-2 so that the reduction in overpotential, compared to the traditional cathodes, reaches 0.2 V at 3 kA m 2 which is almost the same as that achieved with a Rh-activated cathode. The resistance to poisoning by Fe impurities is also improved this is probably related to its exceptionally high surface area, the roughness factor being of the order of 104. 

Some predictions beyond the theory of electrocatalysis for pure metals seem indeed possible. It is, however, necessary to stress again that the applicability of a cathode depends on the impact of many factors, the most outstanding ones being the intrinsic stability and the resistance to poisoning. This is probably still the weak point of cathodes. Their life-time appears to be lower than for anodes, although the deactivation process for cathodes is slower and less abrupt than for anodes. 

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