N2O reduction

Attard GA, Ahmadi A. 1995. Anion-surface interactions. Part 3 N2O reduction as a chemical probe of the local potential of zero total charge. J Electroanal Chem 389 175-190. 

Climent V, Attard GA, Feliu JM. 2002a. Potential of zero charge of platinum stepped surfaces a combined approach of CO charge displacement and N2O reduction. J Electroanal Chem 532 67-74. 

Indirect evidences for N2O adsorption are reported in a study [99] of the effect of Pb, Cd, and T1 adatoms on the reduction process of N2O at polycrystalline Pt electrodes. N2O reduction was studied in detail owing to its use as a probe... 

It was demonstrated that there is a direct correspondence between the potential of zero total charge and the maximum rate of N2O reduction. 

Pb) and metal oxide (ZnO, In203, Sn02, doped Fc203, NiO) electrodes in N2O reduction was compared in [106, 107]. 

Yoshinari, T. (1980). N2O reduction by Vibrio succinogenes. Appl. Environ. Microbiol. 39,... 

There has been a long controversy in the literature concerning the lowest O2 concentrations where denitrification can occur (Robertson and Kuenen, 1984). Some believe it can occur at aerobic-anaerobic interfaces (Christensen and Tiedje, 1988 Bonin et al., 1989), while others believe that low O2 conditions inhibit synthesis of the essential enzymes for denitrification (Payne, 1976 Kapralek et al., 1982). It has also been speculated for some time that anoxic microsites may allow for the occurrence of denitrification in aerobic environments (Jannasch, 1960). Laboratory studies indicate that there are differential effects of O2 on the different steps of denitrification, whereby the NO3- reduction step is less sensitive than N( >7 or N2O reduction (Bonin and Raymond, 1990). 

As shown in Table 9.9, Direct Catalyst Decomposition is an economic option for N2O reduction in an existing nitric add plant. The cost per tonne of CO2 equivalents removed range from 0.4 to 1.0 EUR (based on 2001 costs) -depending on the temperature and the space velodty (i.e., catalyst loading)221. 

Figure 2.1 Overview of processes, which influence the N2O distribution in the ocean. The dashed arrows indicate N2O reduction during N2 fixation (see e.g. Yamazaki et al. (1987)). Please note that NO is not an obligate intermdiate of the nitrification sequence.

Yamazaki, T., Yoshida, N., Wada, E., and Matsuo, S. (1987). N2O reduction by Azotobacter vinelandii with emphasis on kinetic nitrogen isotope effects. Plant Cell Physiol. 28(2), 263-271. 

In the acetylene inhibition technique, acetylene is added to a water sample, which inhibits the reduction of N2O to N2 (Sorensen, 1978). The accumulation of N2O is then measured using gas chromatography and an electron capture detector and the denitrification rate is taken to be equal to the total N2O flux. One potential problem is incomplete inhibition of N2O reduction to N2, particularly in the presence of hydrogen sulfide, a compound commonly found under anaerobic conditions. Another potential problem with the technique is that acetylene also inhibits nitrification, a process that often supplies the NOs and N02 substrates for denitrification. To inhibit nitrification is to inhibit denitrification if it is at aU substrate limited (Hynes and Knowles, 1978). 

Then there are wider questions about the possible limiting roles of various trace elements in key biogeochemical processes Do trace elements other than iron limit phytoplankton growth and primary production Is the composition of phytoplankton assemblages controlled by trace elements Are various processes in the cycle of nitrogen limited by trace elements (e.g., N2 fixation by iron N2O reduction by copper) What are the links between trace elements and the reduced sulfur cycle in surface seawater ... 

Chen, R, Gorelsky, S. I., Ghosh, S., Solomon, E. 1. (2004). N2O reduction by the pa-sulfide-bridged tetranuclear Cuz cluster active site. Angewandte Chemie International Edition, 43, 4132—4140. 

For a better understanding of the first steps of the reaction of NO over Cu-ZSM-5 and Fe-ZSM-5 zeolites the following measurements were performed (i) the products of the gas phase interactions were followed by MS, (ii) the valence state and coordination of transition metal ions in zeolites by ESR spectroscopy. Catalysts were prepared both by conventional and solid-state ion-exchange methods and pretreated in vacuum, in oxidative and in reductive atmosphere. The conventional ion-exchanged samples are more active in NO decomposition than the solid-state exchanged ones. Over the reduced catalysts the first step consists in N2O formation and the oxidation of Cu Cu (Fe Fe ) followed by N2O reduction to N2 (in these conditions O2... 

Figure 3 Schematic mechanisms of the reactions investigated (a) direct N2O decomposition, (b) N2O reduction with CO or CjHg. Atomic O results from the activation of N2O over the extraframework iron site.

The results presented evidence possibilities of tailoring uniform iron sites in FeMFI zeolites, under specific synthesis and activation conditions. Preparation of steam-activated Fe-silicalite containing mainly isolated iron species in extraffamework positions is essential to derive stmcture-activity relationships in various N2O conversion reactions over iron zeolite catalysts. The activity of the cluster-free Fe-silicalite was significantly higher in N2O reduction with CaHg and CO. However, some level of association of iron species leads to higher activities in direct N2O decomposition. Due to the intrinsic reaction mechanism, this result demonstrates the sensitivity of reactions for the form of the iron species in Fe-zeolites, rather than the existence of a unique active site. 

This means that those organisms in soil active in denitrification have N2O as a freely diffusable, obligate intermediate of denitrification. Since the cells did not seem to sequester pools of N2O, factors that stimulate N2O production or reduce N2O reduction would be expected to cause an increase in the N2O emitted from denitrifying cells. This interpretation helps explain why a number of factors affect N2O production (see the section on factors affecting N2O production). 

A detailed study of N2O reduction on a variety of single-crystal Pt-group metal electrodes shows that N2O reduction current maxima occurred exactly at Claviher s Eq=o- Therefore the N2O reduction data have been used for obtaining the Eq values (Table 5). The importance of a local value of Eq=o has been emphasized, especially with respect to reconstructing metal surfaces such as Pt(lOO) and Pt(llO), which can be prepared in a variety of crystallographic states [5, 65-68]. 

Heyden et al. suggested that hydrated and dehydrated monomolecular iron sites in Fe-ZSM-5 are responsible for N2O decomposition. They proposed that Z [FeO]+ is a key intermediate. Furthermore, water strongly adsorbs to give Z Fe(OH)2 "h This deactivates the Z [FeO]+ site. The activation energy for N2O decomposition in the presence of water increases steeply compared with the anhydrous situation, because water has to desorb from Z Fe(OH)2 in order for N2O reduction to occur. Hydration and subsequent dehydration of the oxy-iron complex may provide an alternative explanation for the oscillatory reaction found by El-Malki et al. shown in Fig. 4.28. If the reaction is not isothermal, the temperature fluctuations arising from the exothermic N2O decomposition reaction may lead to fluctuation in the water adsorption. This may provide an alternative explanation of the oscillatory kinetic behavior in the Fe +-ZSM-5 system. 

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