Promoter catalyst-poison-resistant

Catalyst-poison-resistant promoters protect the active phase against poisoning by impurities, either present in the starting materials or formed in side reactions. 

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. 

The catalyst used is a promoted nickel oxide on an alumina support [4], manufactured in cylindrical form. Because of the highly oxidised state of the nickel, the catalyst is resistant to classic poisons and extended lifetimes have been observed in industrial installations. 

Cheekatamarla and Lane [62, 63] studied the effect of the presence of Ni or Pd in addition to Pt in the formulation of catalysts for the ATR of synthetic diesel. For both metals, a promotional effect with respect to catalytic activity and sulfur poisoning resistance was found when either alumina or ceria was used as the support. Surface analysis of these formulations suggests that the enhanced stability is due to strong metal-metal and metal-support interactions in the catalyst. 

Certain types of catalyst uranium oxide and chromium oxide may be used as a promoter. This is reported to give a higher resistance to catalyst poisoning by sulfur components and a lower tendency to form carbon deposits. 

A problem may occur when higher-molecular-weight materials are employed as feedstocks and result in coke formation and deposition. When an alkali-promoted catalyst is employed, corrosion and fouling problems in the reformer (or even in equipment downstream of the reformer because of the tendency of the alkali to migrate) may occur with some frequency. However, coke formation can be eliminated by the use of a proprietary alkali-free catalyst that has an extremely high activity and resistance to poisoning. 

Electronic promoters, for example, the alkali oxides, enhance the specific activity ofiron-alnmina catalysts. However, they rednce the inner snrface or lower the thermal stability and the resistance to oxygen-containing catalyst poisons. Promoter oxides that are rednced to the metal during the activation process, and form an alloy with the iron, are a special group in which cobalt is an example that is in industrial use. Oxygen-containing compounds such as H2O, CO, CO2, and O2 only temporarily poison the iron catalysts in low concentrations. Sulfur, phosphorus, arsenic, and chlorine compounds poison the catalyst permanently. 

Vanadia supported on silica(with titania) promoted by Fe and CTu oxides was studied for SCR of NOx by Bjorklund et al.28 Both Fe and Cu (as oxides) enhanced the activity however, the resistance to deactivation by SO2 differed as the Fe-promoted catalyst became slightly more active with time on stream and the Cu-promoted catalyst decreased in activity to less than half the initial activity with time on stream. The activity for SCR was related to the concentration of as inferred from the solid electrical conductivity. In this case different promoters were shown to change dramatically the ability of a potential poison to deactivate the SCR catalyst Nikolov, Klissurski, and Hadjiivanov29 also studied the deactivation of vanadia/titania. A combination of ESCA, XRD, and IR were employed to characterize the surface and bulk compositions. They concluded that deactivation involved the transformation of the active anatase titania to inactive rutile. Further, there was a concomitant decrease in total surface area and a loss of phosphate promoters for the selective oxidation of xylenes to phthalic anhydride. 

Sintering of Supported Metals- The ability of a supported metal catalyst to resist or to succumb to sintering or deactivation by potential poisons depends on several factors the support, the reaction environment, and/or the presence of promoters. Moreover there are differences in these effects depending on the active metal. 

The development of these catalysts occurred in an atmosphere of tight secrecy, and little has been published on those efforts. All three were of the particulate type one contained noble metal(s) while the other two were noble-metal promoted base-metal catalysts (16). The UOP noble metal catalyst was provided several years later to GM and to Ford for evaluation with unleaded fuel and for other studies, and it can be deduced that it was supported on low-density (about 0.32 g/mL apparent bulk density) 1/8 inch spheres (17) and may have contained about 0.47 troy ounces of platinum per cubic foot (ca. 0.16 weight percent), with the platinum concentrated in a subsurface shell some distance below the exterior surfaces of the spheres for improved poison resistance (18) The Cyanamid catalyst was later studied by Ford (16, 19) it apparently was an extrudate (1/8" diameter x 1/8" long) of about 0.67 g/mL ABD, with about 125 ppm (weight) of palladium and 5 weight % each of CuO and 2 5 Si02 95% AI2O3 support of about... 

Weng, D., Li, J., Wu, X., etal (2008). Promotional Effect of Potassium on Soot Oxidation Activity and S02-poisoning Resistance of Cu/Ce02 Catalyst, Catal Commun., 9, pp. 1898-1901. 

Catalysts based on zeolites have also been used successfully to remove NOX from the emissions of large and small-scale stationary sources. The zeolite can be extmded directly as a monohth or applied as a washcoat on preformed cordi-erite supports. The use of several zeolites—including wide-pore mor-denite— for this reaction has been patented. Up to 95% NOX conversion can be achieved with mordenite, with no promoters, and some zeolites are stable at temperatures up to 600°C. Long catalyst lives have been achieved in retrofitted coal based SCR units with low sulfur trioxide formation and good poisons resistance. Spent zeolite catalysts can be disposed of in approved landfill sites because they have negligible heavy-metal content. ... 

The objective of the symposium is to promote a scientific approach of the phenomenon of catalyst deactivation which will contribute to the development of catalysts less subject to structural transformations and more resistant to poisons and coke formation. 

Although metals or even promoted metals have very low sulfur tolerances in synthesis reactions, other materials, such as metal oxides, nitrides, borides, and sulfides, may have greater tolerance to sulfur poisoning because of their potential ability to resist sulfidation (18). The extremely low steady-state activities of Co, Ni, and Ru metals in a sulfur-contaminated stream actually correspond to the activities of the sulfided metal surfaces. However, if more active sulfides could be found, their activity/selectivity properties would be presumably quite stable in a reducing, H2S-containing environment. This is, in fact, the basis for the recent development of sulfur active synthesis catalysts (211-215), which are reported to maintain stable activity/ selectivity properties in methanation and Fischer-Tropsch synthesis at H2S levels of 1% or greater. Happel and Hnatow (214), for example, reported in a recent patent that rare-earth and actinide-metal-promoted molybdenum oxide catalysts are reasonably active for methanation in the presence of 1-3% H2S. None of these patents, however, have reported intrinsic activities... 

It should be emphasized here that even though sulfur tolerances (steady-state activities in the presence of sulfur impurities), of promoted and unpromoted nickel catalysts are extremely low, their sulfur resistances (rates of activity loss in the presence of sulfur impurities), can vary greatly with catalyst configuration, composition, and support. Thus it may be possible to extend significantly nickel catalyst life in the presence of sulfur poisons through addition of promoters or use of novel supports (23,99,100,113, 114, 161, 194, 225-227). This is in fact the basis of two recent patents (225, 226). 

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