Measurement of H2S consumption

H2 TPR measurements are used to probe the reducibility and may reveal more information on the nature of vanadium and molybdenum species. The assignment of the TPR peaks has been based on the literature study [9, 10] but also by using two reference samples V1-Z15 and Mol-Z15 prepared by solid-state ion exchange. TPR thermograms of V-Mo-Zeolite systems can be divided into two zones of H2 consumption (/) Mo-Zeolites exhibit two reduction peaks at 600 and 850°C corresponding to the reduction of Mo6+ into Mo4+ through the Mo5+ step and to the reduction of Mo4+ into Mo°, respectively while (ii) V-Zeolites led to a broad asymmetric feature around 710°C, which has been previously attributed to the reduction of V5+ into V3+. Finally, the TPR profiles of V-Mo-Zeolite catalysts seem more like a superposition of both Mo and V-catalysts TPR profiles. 

The TPR profiles of calcined LDHs precursors show two peaks of H2 consumption (Fig.l). The first peak around 570 K corresponds to the release of NO3 anions as NO2, and their subsequent reduction to NO and N2O as identified by mass spectrometry [9]. The second peak with maxima at 705,920 and 1000 K for HA, HC and HG samples respectively, corresponds to the reduction of NiO particles. These experiments show that the reducibility of the nickel oxide particles decreases when the Mg content increases. This could be compared with the decrease of the Ni crystal size measured by XRD in HC and the lack of detection of these particles in HG samples. This behaviour has been attributed to the formation of excess Ni aluminate and Ni spinel type phases decreasing the size of the mixed oxide particles and hindering their reducibility [6]. 

Figure 9.26 shows the steady state effect of applied current I on the induced changes, ArH2(=rH2 -r 2) and Ar0(=ro-io )> in the rates of consumption of H2 and O respectively, where the superscript o always denotes open-circuit conditions. The dashed lines in Fig. 9.26 are constant Faradaic efficiency, A, lines. The maximum measured A values are near 40 at low current densities. This value is in excellent qualitative agreement with the following approximate expression which can predict the magnitude of A in NEMCA studies ... 

One rather unfortunate aspect of the M + hydrocarbon (and M + OX) reactions mentioned thus far is that the products of the reactions were not detected directly, but were instead inferred via the pressure and temperature dependencies of the measured rate constants for metal reactant consumption and by comparison to ab initio calculations. Exceptions are the reactions of Y, Zr + C2H4 and C3H6, for which the Weisshaar group employed the 157 nm photoionization/mass spectrometry technique to identify the products of the reaction as those resulting from bimolecular elimination of H2.45 47 95... 

The PHSS method of real-time H2S measurement allows for investigating the potentially complex H2S kinetic responses of organs, tissues, cells, and mitochondria as levels of 02 and NO as well as metabolic state are adjusted within physiological limits. Kinetic changes in H2S concentration continuously reported by the PHSS, which are not seen with other H2S measurement techniques, suggest potentially complex interactions of H2S production and consumption mechanisms. H2S may likely exist as a cellular pool of free and labile persulfides able to rapidly respond to redox challenges with production and consumption pathways that operate to maintain the pool. This possible scenario reinforces the need for the PHSS as a valuable tool to provide a continual report of H2S throughout the course of an experimental treatment or to accurately determine H2S levels in situ. 

Figure 5.5 Activition of an [NiFe] hydrogenase such as that from D.gigas, by incubation with H2, as measured by different assay methods.These are production of H2 with the low-potential donor methyl viologen consumption of H2 with the high-potential acceptor dlchlorolndophenol and Isotope exchange of H2 with H2O. The proportions of the enzyme in three different states, unready, ready and active, are indicated by the shading.

In this study we performed initial rate experiments, reacting H2S with lepidocrocite (23). The consumption of H2S was measured continuously by using a pH2S electrode cell (25). To avoid interferences of pH buffer solutions with the iron oxide surface, the pH was stabilized by using a pH-stat that added appropriate amounts of HC1 to the solution. The added volume, which was also continuously monitored, provided information about the amount of protons consumed during the reaction. Dissolved iron was measured only in some runs. 

Catalysts were characterized by TPR. H2-consumption of the samples was monitored by measuring the thermal conductivity of the effluent gas passed through a dry-ice trap to collect water produced. 20 mg of metal oxide was calcined in O2 flow at 723K for 3h... 

In order to find v (53), we measured the rate of methane consumption at PcoJ,H2/JBCH4f>H2o K that is approximately equal to r+ and the rate... 

The sulfidation mechanism was investigated by temperature-programmed sulfidation, as the oxidic catalyst was heated in a flow of H2S and ff2, and the consumption of IH S and ff2 and the evolution of H20 were measured continuously (13). ft was found that IH S is taken up and H20 given off, even at room temperature, indicating a sulfur-oxygen exchange reaction. This conclusion was confirmed by quick extended X-ray absorption fine structure (QEXAFS) studies (Fig. f, phase 2), which also demonstrated... 

Temperature programmed reduction (TPR) was followed both by thermogravimetry and conventionally, i.e. by measuring the change in composition of a 5% H2 in Ar mixture. Slight reduction occurred between 570 and 670 K, due perhaps to the presence of small amounts of Ni(OH)2 or basic carbonate reduction was rapid above 670 K but H2 consumption continued until at least 1000 K. In isothermal studies, high degrees of... 

The fact that O atom consumption exceeded H2S consumption by 10-20 % was taken as further evidence for such a chain mechanism. The measured rate coefficient for the overall reaction of 0-1- H2S was found to be 1.2 x 10 l.mole" .sec The ratio of hydrogen produced to H2S consumed gives the ratio of the rate of the chain mechanism, reactions (29) and (30), to the overall rate, reactions (28), (29) and (30)... 

The technique based on studies of H2/O2 mixtures close to the second explosion limit described in Chapter 1 has also proved to be an extremely useful source of rate data. Measurements of the relative rate of consumption of hydrogen and an organic compound added to the mixture in trace amounts gives rate constants for the reactions of H and OH with the additive and provides information on the reactions of the radicals derived from such additives. Although the technique is limited to operating over... 

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