Poisoning studies, bimetallic catalysts

Sodium or potassium severely poisons Pt-Re catalysts but the manner in Which the alhali metal operates is not apparent. The present study was designed to use ESCA to determine the valence state of Re in Pt-Re bimetallic catalysts. The valence state would be determined in san les that had been reduced and transferred to the instrument without exposure to an oxidizing atmosphere. Catalysts with and without potassium would be examined. 

Bimetallic Ni-based catalysts were also studied for SR of higher hydrocarbons in order to avoid the carbon formation and sulfur poisoning problems of conventional Ni catalysts.Murata et prepared a series of bimetallic catalysts by adding alkali and alkaline-earth metals to Ni catalyst supported on zirconia and alumina for SR of i-Cg and methylcyclohexane (MCFI). The performance of various bimetallic catalysts for SR of i-Cg and MCH are summarized in Figures 21a and 21b, respectively. It was reported that the stability of Ni/Zr02 is considerably improved by the addition of alkaline-earth metals (M), particularly strontium, to the catalyst with an M/Ni ratio of 0.5 by... 

A brief overview of bimetallic catalysts is presented. Electronic vs. ensemble effects are discussed, and literature is reviewed on single crystal bimetallics, and supported bimetallic clusters. Bimetallic cluster compounds are considered as models. Structural considerations, effects of potential poisons, particles from bimetallic cluster compounds, and catalytic activity/selectivity studies are briefly reviewed and discussed. 

Frequently, studies of bimetallic catalysts are carried out primarily with an eye toward effects of poisoning and catalysts lifetime. Afterall, these are the most important parameters that are the most sensitive and important from an industrial viewpoint. 

Table II. A Brief Summary of Literature Relating Generally to Reactivity—Selectivity-Poisoning Studies of Bimetallic Catalysts...

Rates of deactivation of Ni and Ni bimetallic catalysts as a result of poisoning by 10-ppm H2S during methanation were investigated in a series of studies by Bartholomew and co-workers (23, 113, 161, 194). Effects of catalyst composition and geometry, gas composition and reaction temperature on the rate of deactivation were considered. Deactivation rates were found to be relatively insensitive to temperature and quite sensitive to gas and catalyst composition (194). In fact, the rates of deactivation were 2-3 times more rapid in a H2-rich mixture (H2 /CO = 99), compared to a normal synthesis (H2/CO = 3-4) mixture. 

The recent accomplishments of near-edge X-ray absorption spectroscopy in catalysis studies are already quite impressive, in particular if one considers the limited availability of suitable X-ray spectrometers. Developments of catalytic interest have concerned the Shell Higher Olefin process, size effects, metal-support interaction, mono- and bimetallic catalysts (in particular the PtRe/Al203 system), the reactivity of supported metal catalysts, dynamical and in situ catalyst studies, and a variety of oxide and sulfide catalysts. Other catalytic problems are now coming within easy experimental reach, such as the study of sulfur poisoning and the nature of coking. 

The understanding of the interaction of S with bimetallic surfaces is a critical issue in two important areas of heterogeneous catalysis. On one hand, hydrocarbon reforming catalysts that combine noble and late-transition metals are very sensitive to sulphur poisoning [6,7]. For commercial reasons, there is a clear need to increase the lifetime of this type of catalysts. On the other hand. Mo- and W-based bimetallic catalysts are frequently used for hydrodesulphurization (HDS) processes in oil refineries [4,5,7,8]. In order to improve the quality of fuels and oil-derived feedstocks there is a general desire to enhance the activity of HDS catalysts. These facts have motivated many studies investigating the adsorption of S on well-defined bimetallic surfaces prepared by the deposition of a metal (Co, Ni, Cu, Ag, Au, Zn, A1 or Sn) onto a single-crystal face of anodier metal (Mo, Ru, Pt, W or Re) [9-29]. 

From the studies described above, one can expect that sulphur alters (or poisons) the properties of catalysts that combine gold and transition metals by inducing a reduction in the degree of wetting of the surface of the transition metal by gold. This effect can explain changes induced by sulphur on the activity and selectivity of bimetallic catalysts used for hydrocarbon reforming [7,22,30]. 

The impact of CO on the PEMFC anode performance has been widely studied experimentally and by modelling, and maity mitigation methods have been proposed. In recent years, the impacts of H2S and NHj have become an important subject of research. It is well accepted that the major impact of CO and H2S contaminants on the hydrogen-fed PEMFC anode is a kinetic effect due to poisoning of the electrocatalyst, while NHj mainly affects the ionomeric membrane by reducing ionomer conductivity.In both cases, significant performance degradation can be induced. In order to enhance CO tolerance, bimetallic catalysts such as Pt-Ru, Pt-Mo and Pt-Sn have been proposed however, the... 

Bartholomew et al. (1979) studied H2S poisoning of a Ni bimetallic catalyst on alumina for methanation activity. Poisoning tests were conducted at 1... 

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