Sulfur resistance

The main cause of anode wear is electrochemical oxidation or sulfur attack of anodic surfaces. As copper is not sufficiently resistant to this type of attack, thin caps of oxidation and sulfur-resistant material, such as platinum, are bra2ed to the surface, as shown in Eigure 15a. The thick platinum reinforcement at the upstream corner protects against excessive erosion where Hall effect-induced current concentrations occur, and the interelectrode cap protects the upstream edge from anodic corrosion caused by interelectrode current leakage. The tungsten undedayment protects the copper substrate in case the platinum cladding fails. 

The saturated, cleaned raw synthesis gas from a Texaco partial oxidation system is first shifted by use of a sulfur resistant catalyst. Steam required for shifting is already present ia the gas by way of the quench operation ia the generator. The shifted gas is then processed for hydrogen sulfide and carbon dioxide removal followed by Hquid nitrogen scmbbiag. 

The 17% ferritic steels are easier to fabricate than the martensitic grades. They are used extensively in equipment for nitric acid production. The oxygen- and sulfur-resistant 30% chromium steel can be used at temperatures up to 1150°C but only for lightly loaded and well-supported furnace items because of its poor creep and brittlement properties when equipment is down to ambient temperatures [18]. 

Catalysts used in hydrotreatment (hydrodesulfurization, HDS) processes are the same as those developed in Germany for coal hydrogenation during World War II. The catalysts should be sulfur-resistant. The cobalt-molybdenum system supported on alumina was found to be an effective catalyst. 

F. Moeller If you want to realize a catalyst which can hold some ppm s of sulfur, you should realize that you need not just one catalyst. You need a catalyst which does the job at all temperatures. We know of catalysts that can have some sulfur in the feed gas at relatively high temperature levels. That s no problem. But if you go to the low temperatures that you need to get specification SNG so that you can put your gas into your pipeline, then you have a real problem. So if you look for a sulfur-resistant catalyst, it must be resistant over the whole process sequence. Personally, I cannot foresee a catalyst which will work at lof temperature and be insensitive to sulfur. 

We report the discovery of a new Pd-Sn catalyzed hydrogenolysis reaction to produce thiol product in high yields. The relationship between catalyst activity and surface characterization (chemisorption, ESCA, and in situ temperature-dependent XRD,) has aided om understanding of the reasons why these catalysts are sulfur resistant, extremely active, and activated at certain temperatures and pressures. The predominant mode of deactivation appears to be the formation of Pd-CN species rather than the formation of Pd-S species on the surface of the catalyst. 

The use of carbon-based catalysts offers certain advantages over metal catalysts due to their availability, durability, and low cost. In contrast to the metal-based catalysts, carbon catalysts are sulfur resistant and can withstand much higher temperatures. Muradov [98,99] screened a variety of carbon materials and demonstrated that the efficient catalytic methane decomposition can be accomplished over high surface area carbons at temperatures... 

The by-product, representing a relatively reduced form of sulfur, is a reasonable model for the sulfur impurities in the synthesis gas obtained from sulfur-rich coal. This sodium sulfide test of sulfur resistance of water gas shift catalyst systems generated in basic solutions is a very severe test since the quantities of sulfur involved are much larger than those likely to be found in synthesis gas made from any sulfur-rich coals. 

Cheng, Z, Zha, S, and Liu, M. Stability of materials as candidates for sulfur-resistant anode of solid oxide fuel cells, J Electrochem Soc, 2006 153 A1302-A1309. 

For Pd-Ag membranes the manufacture of defect-free thin layers, reproducible in production, and stability are major issues. In addition, sulfur resistance and the interaction of the membrane with the catalyst material is important. 

Alloying the nickel of the anode to improve tolerance for fuel contaminants has been explored. Gold and copper alloying decreases the catalytic activity for carbon deposition, while dispersing the anode with a heavy transition metal catalyst like tungsten improves sulfur resistance. Furthermore, ceria cermets seem to have a higher sulfur tolerance than Ni-YSZ cermets [75],... 

Sulfur and carbon monoxide can be killers (literally) with hydrogenation catalysts. It will poison them, making them completely ineffective. Some sulfur often shows up in the benzene feed, carbon monoxide in the hydrogen feed. The alternatives to protect the catalyst are either to pretreat the feed and/or the hydrogen or to use a sulfur resistant catalyst metal like tin, titanium, or molybdenum. The economic trade-offs are additional processing facilities and operating costs vs. catalyst expense, activity, and replacement frequency. The downtime consequences of catalyst replacement usually warrarit the more expensive treatment facilities. 

Commercial SCR catalysts are made of homogeneous mixtures of titania, tungsta and vanadia (or molybdena). Titania in the anatase form is used as a high surface and sulfur-resistant carrier to disperse the active components. Tungsta or molybdena is employed in large amounts (10 and 6% w/w, respectively) to increase the surface acidity and the thermal stability of the catalyst and to limit the oxidation of SO2. Vanadia is responsible for the activity in the reduction of NO, but it is also active in the oxidation of SO2. Accordingly, its content is kept low, usually below 1-2% w/w. 

Another difference between Co and Fe is their sensitivity towards impurities in the gas feed, such as H2S. In this respect, Fe-based catalysts have been shown to be more sulfur-resistance than their Co-based counterparts. This is also the reason why for Co F-T catalysts it is recommended to use a sulphur-free gas feed. For this purpose, a zinc oxide bed is included prior to the fixed bed reactor in the Shell plant in Malaysia to guarantee effective sulphur removal. Co and Fe F-T catalysts also differ in their stability. For instance, Co-based F-T systems are known to be more resistant towards oxidation and more stable against deactivation by water, an important by-product of the FTS reaction (reaction (1)). Nevertheless, the oxidation of cobalt with the product water has been postulated to be a major cause for deactivation of supported cobalt catalysts. Although, the oxidation of bulk metallic cobalt is (under realistic F-T conditions) not feasible, small cobalt nanoparticles could be prone to such reoxidation processes. 

More recently, Koizumi et al. observed that Mn has an additional beneficial effect in unsupported Fe-based F-T catalysts. These authors studied the sulfur resistance of Mn-Fe catalysts and they observed superior catalysts stabilities, especially when the catalysts were pre-reduced in CO. This group also used IR spectroscopy in combination with CO as a probe molecule to compare Fe and Mn-Fe catalysts. It was found that the addition of Mn led to the appearance of several well-resolved bands upon CO adsorption. The appearance of the bands arising from bridged-bonded CO on Fe indicated that the size of the Fe particles were clearly larger than in the case of the unpromoted catalysts. They attributed the decreased reactivity towards H2S to the observed increase in Fe particle size. 

The hydrogenation activity of W2C/Zeolite Y was evaluated in the synthesis of amines. The standard conditions reported earlier were followed. Although traditionally a Ni catalyst is used for hydrogenation, there is a need for a hydrogenation catalyst that is more sulfur resistant than Ni. It was found that the W2C/Zeolite Y may act as a hydrogenation catalyst for acetonitrile. Acetonitrile conversion was found to be high at 573-723 K, but the selectivity was low. A mixture of ethylamine, diethylamine and hydrocarbons was formed, Table 22.2. In this reaction, about 15% of the products could not be accounted for and further work is needed to investigate this reaction. 

Noble metal catalysts are known to be easily poisoned by sulfur. Tungsten carbide appeared to show noble metal characteristics, yet was found to be sulfur resistant. The n-heptane isomerization reaction described above was repeated with the two catalysts unsupported W2C and 0.3% Pt/Al203. 

Based on the investigation of reaction, heat transfer and mass transfer of the KD306-type sulfur-resisting methanation catalyst [9-11], the non-isothermal one-dimensional and two-dimensional reaction-diffusion models for the key components have been established, and solved using an orthogonal collocation method in this paper. The scope is to study the catalyst intraparticle reaction-diffusion processes that involve parallel, non-first order, equilibrium-restrained reactions. 

The shift from carbon monoxide to carbon dioxide generally occurs in two steps - first a High Temperature Shift Conversion and then a Low Temperature shift conversion. In some cases the two steps may be combined in one isothermal or adiabatic step called Medium Temperature Shift Conversion. When the feed gas to the CO conversion is not desulfurized, the CO conversion is called Sour Gas Shift and a special type of sulfur-resistant catalyst is used166. 

Hydrogenolysis of adsorbed thiophene (poison) is the rate determining step. Sulfur resistance was Pt > Pt-Ir > Pt-Re.65 66 ... 

Effect of alloying on the sulfur resistance of bimetallic Pt-based catalysts T.F. Garelto, A. Borgna and C.R. Apestegufa ... 

Although nickel is not the most active metal, for reasons of cost and stability, commercial catalysts are based on this metal. These catalysts are in general supported on alumina and contain relatively high metal loadings. There is, however, interest in the development of more thermally stable, sulfur resistant, and possibly even regenerable catalysts in the future. It may be for these reasons that zeolite-based methanation catalysts have recently attracted more interest. 

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