Sulphur tolerance

Nowadays, most of the car manufacturers are interested to replace noble metals by cheaper active phases or at least to significantly lower their content without altering their tolerance to poisoning effects. Recent developments using gold and silver as active components specifically for low temperature applications could be promising, but their sulphur tolerance is questionable in those temperature conditions [12-14], Apparently, such an aspect may exclude an extensive development of those silver-based catalysts for NO abatement processes for stationary sources. 

Now, regarding the SCR with hydrocarbons in 02 excess, numerous investigations have shown a low activity below 200°C. However, it was found that H2 can promote the reduction of NO below 200°C on molybdenum and sodium-modified Pt/Si02 and Pt/Al203 catalysts [103]. Such a promotional effect also observed on silver-based catalysts originates extensive investigations in this field and offers new perspectives in the developments of non-noble metal-based catalysts. However, further developments of that variety of catalysts seem to be questionable due to their low sulphur tolerance. 

Advanced water-gas shift reactors using sulphur-tolerant catalysts to produce more hydrogen from synthesis gas at lower cost. 

Wang, L., Murta, K., Inaba, M. (2004). Development of novel highly active and sulphur-tolerant catalysts for steam reforming of liquid hydrocarbons to produce hydrogen. Appl. Catalysis A General 257,443-47. 

Breen, J.P., Burch, R., Hardacre, C., Hill, C.J., Krutzsch, B., Bandl-Konrad, B., Jobson, E., Cider, L., Blakeman, P.G., Peace, L.J., Twigg, M.V., Preis, M. and Gottschling, M. (2007) An investigation of the thermal stability and sulphur tolerance °fAg/y-Al203 catalysts for the SCRofNOx with hydrocarbons and hydrogen. Appl. Catal. B Environ., 70, 36-44. 

One feature of particular interest is the resistance that gold-based systems might display against sulphur poisoning. Only a little work has been carried out in this area to date, but gold on titania catalyses the Claus reaction and is 5-10 times more effective than titania itself, indicating that gold can be sulphur-tolerant ... 

FIGURE 4.6 Sulphur tolerance test of Pt202S04 catalyst in the presence of various H2S amounts. Gas composition 1 l%C0-7%C02-26%H20-26%H2-He contact time 0.04 g s ml . (Taken from Figure 9 of I. Valsamakis, M.F. Stephanopoulos, Appl. Catal. B Environ. 106 (2011) 255.)... 

Rh nanoparticles, or Pd-Pt bimetallic catalysts, analyzing the intermediates and final products of the process, and sulphur tolerance of precious metals as well as the effect of some chelating agents. ... 

A schematic of the conventional WGSR is shown in Fig. 7.2. For the conventional coal-to-hydrogen process, the WGSR takes place after the filtration unit and before the acid gas removal unit. For this reason, a sulphur tolerant catalyst must be used for the WGSR. 

Peters, T.A., Kaleta, T., Stange, M. and Bredesen, R. (2013) Development of ternary PdAgTM alloy membranes with improved sulphur tolerance. Journal of Membrane Science, 429,448-458. 

It may be advantageous to carry out the WGS reaction on the raw S5mgas from gasification of coal and heavy hydrocarbons by a so-called sour shift catalyst. This allows removal of CO2 and H2S in the same wash system (see Section 1.5.3). It requires a catalyst that is sulphur-tolerant and capable of working at low H2O/C ratios. The conventional iron-based HTS catalyst can operate in the presence of sulphur, but it requires addition of significant amounts of steam to eliminate the problem of the carbide formation reaction. This problem is solved by using a molybdenum sulphide-based catalyst [168] [232]. The catalyst is promoted and is based on alumina support. It requires the presence of... 

Sammells AF, Nicholson SB, Ang PGP (1980) Development of sulphur-tolerant components for the molten carbonate fuel ceU. J Electrochem Soc 127(2) 350-357... 

Impurities always decrease the cell efficiency, hence the clean-up system is required for it. The most common impurities are sulphur compounds, halides, nitrogen compounds etc. The sulphur tolerance of MCFC is influenced by temperature, pressure, cell components, gas composition etc. The anode has relative tolerance for <10 ppm of H2S, whereas <1 ppm SO2 is also acceptable. For increasing pressures, these concentration limits decrease, whereas for increasing temperatures these limits tend to increase. Sulphur poisons the catalytic reaction sites for the water gas shift reaction and tends to block the electrochemical sites by chemisorption of Ni surface. The following set of reactions takes place when nickel combines with H2S ... 

In reaction (1.39), the anode is sulphided and then gets reduced to nickel. The poisoning due to H2S interferes with the water gas shift reaction equilibrium. Hence, chromium is used in the anodes as it acts as a sulphur-tolerant catalyst. As CO2 is required at the cathodes, it is supplied by anode gas recychng. Hence, the sulphur may also contaminate the cathode as it may be present in the exhaust stream of the anode. The sulphur upon entering the cathode may react with the carbonate ions to produce alkali sulphates which are transported towards the anode via the electrolyte. On reaching the anode, the sulphate S04 is reduced to S. ... 

Anode Pt, Ni-Zr02 Cermet, Ni-YSZ Cermet ( 150 pm thick by EVD or Slurry deposition) High porosity Electronic conductivity Large triple phase boundary sulphur tolerance... 

It should be noted that Ni free electrodes with very high sulphur tolerance has been reported. He et al. studied H2S tolerance of sohd oxide fuel cells (SOFCs) with composite anodes containing Cu, Ce02 and YSZ. H2S levels up to 450 ppm had no effect on anode performance when this anode was operated an SOFC on H2 800 °C. At higher H2S concentrations, anode deactivation was observed. This was attributed to the reaction of the Ce02 with H2S to form 02028 [58]. 

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