H2S oxidation

The removal of hydrogen sulfide from industrial off-gases is another process where the adsorptive and catalytic properties of porous carbons can be combined advantageously, particularly when the H2S concentration is low. Hydrogen sulfide can either be converted into sulfur dioxide or into elemental sulfur  

Katoh et al. [145] showed that H2S, methanethiol, and dimethylsulfide could be removed simultaneously from gaseous streams at room temperature over wet activated carbon fibers, while Dalai et al. [142] reported on the oxidation of methanethiol over an activated carbon. More recently, an activated carbon prepared from a cellulosic precursor by CO2 activation was found to exhibit outstanding performance in the oxidation of H2S (1000 ppm) in a hydrogen stream at 423 K namely, 100% conversion for more than 10 hours, and 100% selectivity to sulfur [146]. The selectivity aspects in the oxidation of H2S to sulfur were addressed in a paper by Bashkova et al. [147]. These authors reported 

There have also been attempts to nse CNFs and MWCNTs as a catalyst in this process [148,149], The highest activities and selectivities (toward snlfnr) were obtained on CNFs with the graphene layers oriented perpendicularly to the axis (platelet-CNFs) and on MWCNTs. However, the carbon materials were contaminated with remains of the metal catalysts nsed for their synthesis (Ni, Ni-Cn), which may affect the catalytic properties observed. 

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W C, A Tempcz)rrk, R C Hawley and T Hendrickson 1990. Semianalytical Treatment of Solvation for Molecular Mechanics and Dynamics. Journal of the American Chemical Society 112 6127-6129. ensson M, S Humbel, R D J Froese, T Matsubara, S Sieber and K Morokuma 1996. ONIOM A Multilayered Integrated MO + MM Method for Geometry Optimisations and Single Point Energy Predictions. A Test for Diels-Alder Reactions and Pt(P(t-Bu)3)2 + H2 Oxidative Addition. Journal of Physical Chemistry 100 19357-19363. 

H2 Oxidizing substances and preparations which exhibit highly exothermic reactions when in contact with... 

Fig. 4.30 Suggested anodic behaviour of electrodeposited Sn-35Ni alloy 1, observed curve 2a, H2 evolution 2b, H2 oxidation 3, true anodic curve (after Clarke and Elbourne " )...

S. Neophytides, D. Tsiplakides, P. Stonehart, M.M. Jaksic, and C.G. Vayenas, Non-Faradaic Electrochemical enhancement of H2 oxidation in alkaline solutions, J. Phys. Chem. 100, 14803-14814 (1996). 

There is a third real reason for deviations from Eq. (5.18) in the case that a non-conductive insulating product layer is built via a catalytic reaction on the catalyst electrode surface (e.g. an insulating carbonaceous or oxidic layer). This is manifest by the fact that C2H4 oxidation under fuel-rich conditions has been found to cause deviations from Eq. (5.18) while H2 oxidation does not. A non-conducting layer can store electric charge and thus the basic Eq. 5.29 (which is equivalent to Eq. (5.18)) breaks down. 

Figure 6.8. Example of rule G3 (volcano-type behaviour) Effect of Ph2(=Pd) (a), Po2 (=Pa) (b) and of potential UWR and AO (c) on the rate of H2 oxidation on Pt /graphite (a and b) and Pt/black (c) in aqueous 0.1 M KOH solutions.72,73 Note that under the pH2, Po2 conditions of Fig. 6.7c the open-circuit rate is positive order in H2 (Fig. 6.8a) and negative order in 02 (Fig. 6,8b) and that the orders are reversed with the applied positive potential (Uwr=1 -2 V). At this potential the rate passes through its maximum (volcano) value (Fig. 6.8c). Reprinted with permission from McMillan Magazines Ltd (ref. 72) and from the American Chemical Society (ref. 73).

The variation in quasireference electrode in presence of reactive gas mixtures. This is due to its high catalytic activity for H2 oxidation. Nevertheless the agreement with Eq. (7.11) is noteworthy, as is also the fact that, due to the faster catalytic reaction of H2 on Pt than on Ag and thus due to the lower oxygen chemical potential on Pt than on Ag,35 the work function of the Pt catalyst electrode is lower than that of the Ag catalyst-electrode over the entire UWr range (Fig. 7.8b), although on bare surfaces O0 is much higher for Pt than for Ag (Fig. 7.8b). 

Figure 9.23. Schematic diagram of the apparatus (a, left) and of the electrochemical cell-reactor (b, right) used for H2 oxidation on Pt/Nafion.35 Reproduced by permission of The Electrochemical Society, Inc.

Consequently, the observed non-faradaic rate enhancement is due to the acceleration of the catalytic rate of H2 oxidation on the Pt catalyst-electrode. 

Figure 10.2. NEMCA in H2 oxidation on Pt/graphite in 0.1 M KOH Steady-state effect of applied positive (anodic) current (I) on the increase in the rates of hydrogen ( ) and oxygen (O) consumption Ph2=0.8 kPa, po2 L25 kPa r 2 (=rg =rc°)=2,38xl0 7 mol/s is the open-circuit catalytic rate Fv=540 cm3/min at STP. Reprinted with permission from Nature, McMillan Magazines Ltd.3...

H2 Oxidation on Pt Fully Dispersed on C Electrodes in Aqueous Alkaline Solutions... 

The electrochemical promotion of H2 oxidation at room temperature using aqueous alkaline solutions and finely dispersed Pt/graphite electrodes has been already described in section 10.2. Faradaic efficiency, A, values up to 20 and p values up to 5 were obtained. The dispersion of the Pt catalyst was of the order of 50%.12,13... 

Eigure 3.9 shows temperatures for 50% conversion (T o) of CH3OH and its decomposed derivatives over Pt/y-Al203, Pd/y-Al203, and Au/a-Pe203 catalysts [52]. Eor MeOH oxidation, palladium is more active than platinum, while gold lies in between. These three catalysts are similarly active for the oxidation of HCHO and HCOOH. Catalytic oxidation at temperatures below 0°C can proceed over palladium and platinum for H2 oxidation, while it happens over gold for CO oxidation. 

Figures lb and Ic report the conversions of CO and H2 as functions of reaction temperature for the different catalysts. In line >vith general literature indications over all the investigated systems CO and H2 oxidize at markedly lower temperatures than CH4 Tio% ranging from 393 to 523 K and from 393 to 573 K have been observed for CO and H2 respectively, to be compared with 773-823 K required by CH4.

In the case of H2 oxidation the two investigated classes of catalysts show different behaviors. Again perovskite type catalysts calcined at 973 K show higher combustion activity than hexaaluminates calcined at 1573 K, but characteristic values of parent activation energy (5-7 Kcal/mole) have been calculated for perovskite catalysts that are markedly lower than... 

Veser G., Experimental and theoretical investigation cf H2 oxidation in a high-temperature catalytic microreactor, Chem. Eng. Sd. 56 (2001) 1265-1273. 

Chattopadhyay, S., Veser, G., Detailed simulations of catalytic ond non-catalytic ignition during H2-oxidation in a micro-channel reactor isothermal case, in Proceedings of the ChemConn-2001, pp. 1-6 (December 2001), Chennai,... 

The kinetics of H2 oxidation has been investigated on a Ni/YSZ cermet nsing impedance spectroscopy at zero dc polarization. The hydrogen reaction appears to be very complex. The electrode response appears as two semicircles. The one in the high-freqnency range is assumed to arise partly from the transfer of ions across the TPB and partly from the resistance inside the electrode particles. The semicircle observed at low freqnencies is attributed to a chemical reaction resistance. The following reaction mechanism is suggested ... 

Sasaki K, Mo Y, Wang JX, Balasubramanian M, Uribe E, McBreen J, Adzic RR. 2003. Pt submonolayers on metal nanoparticles—Novel electrocatalysts for H2 oxidation and O2 reduction. Electrochim Acta 48 3841-3849. 

Breiter MW. 2003. Reaction mechanisms of the H2 oxidation/evolution reaction. In Vielstich W, Gasteiger HA, Lamm A editors. Handbook of Fuel Cells—Fundamentals, Technology and Apphcations. Volume 2. Chichester Wiley. 

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