Methane Oxidation Activity Research Articles

Surface tuned La0.9Ca0.1Fe0.9Nb0.1O3-δ based anode for direct methane solid oxide fuel cells by infiltration method

It has been found in our previous study that La0.9Ca0.1Fe0.9Nb0.1O3-δ-Sm0.2Ce0.8O1.9 (LCFNb-SDC) anode shows high catalytic activity and stability for solid oxide fuel cells (SOFCs) under H2 and CO atmospheres. In this study, LCFNb-SDC is further tuned into a promising anode for direct methane SOFCs through surface modification with Ni. The LCFNb, SDC and Ni materials are found to be thermal-mechanically and chemically compatible with each other up to 850°C. Although LCFNb-SDC anode shows poor catalytic activity for methane oxidation, further modification of such anode with a small amount of Ni particles significantly improves the cell performance in CH4. The maximal power densities of 1031 and 729mWcm−2 in H2 and CH4 have been achieved for the electrolyte supported SOFC with LCFNb-SDC-Ni|SSZ (Sc0.2Zr0.8O2−δ)|LSM (La0.8Sr0.2MnO3) configuration at 850°C, respectively. Besides, the same cells also exhibit good stabilities in both H2 (500h) and CH4 (100h) with no obvious performance degradation and carbon deposition. Surface tuned LCFNb anode by impregnating SDC and a small amount of Ni is thus highly promising as an anode for direct methane SOFCs to deliver favorable performance with reasonable stability.

Bacteria as Greenhouse Gases Reducing Agents from Paddy Plantation

High methane oxidation activity of local isolated methanotrophic bacteria have a potent as methane gases reducing agent while combined with nitrogen fixing bacteria as paddy biofertilizer. The aim of the research was to evaluate the effectiveness of the bacteria as methane gases reducing agent and biofertilizer in paddy plantation. The research was arranged in a completely randomized design consisted of fertilizer types and watering system treatments with four replicates. The research showed that paddy shoot length was not affected by the treatment. On the other hand, both plant freshand dry weight, as well as the number of productive tiller were affected by interaction of fertilizer types and watering system. Fertilizer types affected grain per panicle and methane flux after fertilization. In the end of paddy vegetative stage, bacterial fertilizers were capable to reduce methane emission in different rate. The different result in methane flux was likely due to the interaction between soil local microorganisms and soil chemical component.

Study of methane oxidation over alumina supported Pd–Pt catalysts usingoperandoDRIFTS/MS andin situXAS techniques

Methane oxidation over Pd–Pt/Al2O3 model catalysts calcined at three different conditions is investigated using operando diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry, and in situ X-ray absorption spectroscopy while cycling the feed gas stoichiometry between lean (net-oxidising) and rich (net-reducing) conditions. When calcined in air, alloy Pd–Pt nanoparticles are present only on catalysts subjected to elevated temperature (800 ◦C) whereas calcination at lower temperature (500◦C) leads to segregated Pt and Pd nanoparticles on the support. Here, we show that the alloy Pd–Pt nanoparticles undergo reversible changes in surface structure and composition during transient methane oxidation exposing a PdO surface during lean conditions and a metallic Pd–Pt surface (Pd enriched) under rich conditions. Alloyed particles seem more active for methane oxidation than their monometallic counterparts and, furthermore, an increased activity for methane oxidation is clearly observed under lean conditions when PdO has developed on the surface, analogous to monometallic Pd catalysts. Upon introducing rich conditions, partial oxidation of methane dominates over total oxidation forming adsorbed carbonyls on the noble metal particles. The carbonyl spectra for the three samples show clear differences originating from different surfaces exposed by alloyed vs. non-alloyed particles. The kinetics of the noble metal oxidation and reduction processes as well as carbonyl formation during transient methane oxidation are discussed.

Case study of a modern lean-burn methane combustion catalyst for automotive applications: What are the deactivation and regeneration mechanisms?

One way to lower CO2 and other harmful emissions of the transportation sector is the development of natural gas fueled vehicles. Availability of natural gas is good, and it is easy to apply to stoichiometric and lean-burn engines, which makes it ready-to-use technology. The main concern in the field is a sulfur poisoning of the exhaust gas after treatment system. We aim to clarify mechanisms of sulfur poisoning and regeneration of a lean-burn methane oxidation catalyst. Overall, it is concluded that sulfur itself is not the only reason for the deactivation of methane oxidation catalyst, but it is a joint effect of water vapor and sulfur species. The irreversible sulfur poisoning deteriorates oxygen mobility and hinders water desorption, which inhibits low temperature methane oxidation activity. The regeneration of sulfur poisoned catalyst takes place stepwise: PdSO4→PdSO3+0.5O2→Pd+SO2+0.5O2. The formation of metallic palladium makes the catalyst vulnerable for sintering, which leads to deactivation during long-term regeneration.

Solid-State Ion-Exchanged Cu/Mordenite Catalysts for the Direct Conversion of Methane to Methanol

The selective oxidation of methane to methanol is a highly challenging target, which is of considerable interest to gain value-added chemicals directly from fuel gas. Copper-containing zeolites, such as Cu/mordenite, have been currently reported to be the most efficient catalysts for this reaction. In this work, it is shown that solid-state ion-exchanged Cu/mordenites exhibit a significantly higher activity for the partial oxidation of methane to methanol than comparable reference catalysts, i.e., Cu/mordenites prepared by the conventional liquid-phase ion exchange procedure. The efficiency of these Cu/mordenites remained unchanged over several successive cycles. From temperature-programmed reduction (TPR) measurements, it can be concluded that the solid-state protocol accelerates Cu exchange at the small pores of mordenite: those are positions where the most active Cu species are presumably located. In situ ultraviolet–visible (UV-vis) spectroscopy furthermore indicates that different active clusters inc...

Oleylamine-modified impregnation method for the preparation of a highly efficient Ni/SiO2 nanocatalyst active in the partial oxidation of methane to synthesis gas

In this research, a novel modified wet impregnation method has been successfully developed to synthesize 5% Ni/SiO2 nanocatalyst with high catalytic activity and stability for the partial oxidation of methane. Oleylamine was used as a capping agent in the impregnation solution to improve Ni dispersion and interaction with silica surfaces. The product was analyzed and characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, N2 physisorption measurement and transmission electron microscopy (TEM) and temperature- -programmed H2 reduction (H2-TPR). Partial oxidation of methane over the modified catalyst was performed in a continues-flow fixed-bed reactor under atmospheric pressure at 700?C. The modified catalyst showed 91% CH4 conversion, 86% H2 yield and 95% CO selectivity, and these results almost remained constant within 5 h reaction on stream. The excellent catalytic performance of the catalyst was reasonably attributed to the small and uniform distribution of Ni nanoparticles on the support, and structural characterization confirmed this conclusion.

Assessing the relative stability of copper oxide clusters as active sites of a CuMOR zeolite for methane to methanol conversion: size matters?

Copper-containing zeolites exhibit high activity in the direct partial oxidation of methane into methanol at relatively low temperatures. Di- and tricopper species have been proposed as active catalytic sites, with recent experimental evidence also suggesting the possibility of the formation of larger copper oxide species. Using density functional theory based global geometry optimization, we were able to identify a general trend of the copper oxide cluster stability increasing with size. For instance, the identified ground-state structures of tetra- and pentamer copper clusters of CunOn2+ and CunOn-12+ stoichiometries embedded in an 8-ring channel of mordenite exhibit higher relative stability compared to smaller clusters. Moreover, the aluminium content and localization in the zeolite pore influence the cluster's stability and its geometrical motif, which offers a perspective of tuning the properties of copper-exchanged zeolites by creating copper oxide clusters of a given structure and size. With the activity of the cluster towards methane being connected to its stability, such tuning will potentially allow the design of catalysts with engineered properties.

Effect of ethylene glycol on structure, thermal stability, oxygen storage capacity, and catalytic CO and CH4 oxidation activities of binary CeO2–Al2O3 and ternary CeO2–ZrO2–Al2O3 cryogels

Porous and homogeneous cryogels of binary ceria-alumina (CA) and ternary ceria-zirconia-alumina (CZA) were prepared from cerium nitrate, zirconium (IV) dinitrate oxide hydrate, and aluminium sec-butoxide by a one-pot sol–gel technique in an aqueous system and subsequent freeze drying, and effect of ethylene glycol (EG) used for chelation of cerium ions on structure, thermal stability, reducibility, oxygen storage capacity, and catalytic carbon monoxide and methane oxidation activities of the catalysts was investigated. EG did not significantly affect surface area, pore volume, mean pore size, and surface morphology of the catalysts after calcination and subsequent oxidative/reductive atmosphere heating at 900 °C. Whereas use of EG resulted in much finer particles of cerium oxides and ceria-zirconia solid solution for CA and CZA, respectively, distributed throughout alumina with high dispersion. X-ray diffraction (XRD) and Raman spectra suggested that while a cubic CeO2 phase was observed for CA after the oxidative heating regardless of whether or not EG was employed, finely-divided cerium oxides or crystalline CeAlO3 were respectively obtained after the reductive heating depending on whether EG was respectively used or not used. As for CZA, while a pseudo-cubic t″ phase was seen after the reductive heating regardless of whether EG was used or not, mixed phases of pseudo-cubic t″ and cubic or a cubic one were respectively observed after the oxidative heating depending on whether EG was respectively employed or not employed. The solid solubility of zirconia in ceria was improved by EG for CZA. The oxygen storage capacity and catalytic CH4 and CO oxidation activities were also enhanced by employing EG for both catalysts, which was ascribed fundamentally to the finer particles of cerium oxides and ceria-zirconia solid solution for CA and CZA, respectively, although CO oxidation on CA was unimproved by EG after the reductive heating.

Stable and active monolithic palladium catalyst for catalytic oxidation of methane using nanozeolite silicalite-1 coating on cordierite

An active, stable, and efficient monolithic catalyst for low temperature catalytic oxidation of methane is of great importance in various combustion applications. Mesoporous silicalite-1 zeolite coatings on cordierite monoliths are prepared by a sucrose-modified steam-assisted crystallization (SAC) method. Addition of 4 mol% of sucrose to the SAC system reduces the silicalite-1 crystal size from 2–8μm to 200–250nm. Further addition of sucrose degrades the zeolite crystallinity, and reduces the surface area and micropore volume. The nanozeolite coating possesses good mechanical stability with a weight loss of 0.5wt%. The Pd catalyst supported on the silicate-1 coated monolith exhibits excellent stability and catalytic methane oxidation activity. CH4 conversion was >99.4% after 72h time-on-stream at 450°C. The T50 and T90 for 1% CH4 were 286°C and 347°C, respectively, and complete conversion of CH4 was achieved at about 400°C.

Archaea catalyze iron-dependent anaerobic oxidation of methane

Anaerobic oxidation of methane (AOM) is crucial for controlling the emission of this potent greenhouse gas to the atmosphere. Nitrite-, nitrate-, and sulfate-dependent methane oxidation is well-documented, but AOM coupled to the reduction of oxidized metals has so far been demonstrated only in environmental samples. Here, using a freshwater enrichment culture, we show that archaea of the order Methanosarcinales, related to "Candidatus Methanoperedens nitroreducens," couple the reduction of environmentally relevant forms of Fe3+ and Mn4+ to the oxidation of methane. We obtained an enrichment culture of these archaea under anaerobic, nitrate-reducing conditions with a continuous supply of methane. Via batch incubations using [13C]methane, we demonstrated that soluble ferric iron (Fe3+, as Fe-citrate) and nanoparticulate forms of Fe3+ and Mn4+ supported methane-oxidizing activity. CO2 and ferrous iron (Fe2+) were produced in stoichiometric amounts. Our study connects the previous finding of iron-dependent AOM to microorganisms detected in numerous habitats worldwide. Consequently, it enables a better understanding of the interaction between the biogeochemical cycles of iron and methane.

Improved enrichment culture technique for methane-oxidizing bacteria from marine ecosystems: the effect of adhesion material and gas composition.

Cultivation of microbial representatives of specific functional guilds from environmental samples depends largely on the suitability of the applied growth conditions. Especially the cultivation of marine methanotrophs has received little attention, resulting in only a limited number of ex situ cultures available. In this study we investigated the effect of adhesion material and headspace composition on the methane oxidation activity in methanotrophic enrichments obtained from marine sediment. Addition of sterilized natural sediment or alternatively the addition of acid-washed silicon dioxide significantly increased methane oxidation. This positive effect was attributed to bacterial adhesion on the particles via extracellular compounds, with a minimum amount of particles required for effect. As a result, the particles were immobilized, thus creating a stratified environment in which a limited diffusive gas gradients could build up and various microniches were formed. Such diffusive gas gradient might necessitate high headspace concentrations of CH4 and CO2 for sufficient concentrations to reach the methane-oxidizing bacteria in the enrichment culture technique. Therefore, high concentrations of methane and carbon dioxide, in addition to the addition of adhesion material, were tested and indeed further stimulated methane oxidation. Use of adhesion material in combination with high concentrations of methane and carbon dioxide might thus facilitate the cultivation and subsequent enrichment of environmentally important members of this functional guild. The exact mechanism of the observed positive effects on methane oxidation and the differential effect on methanotrophic diversity still needs to be explored.

Deactivation Aspects of Methane Oxidation Catalysts Based on Palladium and ZSM-5

Catalytic methane oxidation is used in exhaust gas after treatment to reduce methane emissions in stationary combustion processes as well as in natural gas vehicles. Pd/zeolite catalysts provide higher activity than the commonly used Pd/Al2O3, but suffer from rapid deactivation under reaction conditions and in the presence of steam. This work presents a detailed study of deactivation of 1 wt% Pd/H–ZSM-5 catalyst in reaction conditions in dry and wet feed including characterization of both the active phase and the support. Initially well-dispersed PdO particles are not stable and sinter immediately after the exposure to the reaction mixture even in dry feed and stabilize at lower dispersion level. Despite the reasonable methane oxidation activity exhibited by the large particles, the initial PdO particles could potentially provide a 40 °C lower methane oxidation temperature. In the presence of water in the feed at moderate temperature (450 °C), support degradation governs the deactivation of the catalyst, which is evidenced by a significant decrease in BET surface area and generation of extra-framework aluminium species. These results suggest that active and stable catalysts could be obtained by careful choice and design of the support material with the aim to guarantee high dispersion of PdO.

Microbial community composition and methanotroph diversity of a subarctic wetland in Russia

This study assessed the microbial diversity, activity, and composition of methane-oxidizing communities of a subarctic wetland in Russia,with mosaic cover of Sphagnum mosses and lichens of the genera Cladonia and Cetraria. Potential methane-oxidizing activity of peat sampled from lichen-dominated wetland sites was higher than that in the sites dominated by Sphagnum mosses. In peat from lichendominated sites, major bacterial groups identified by high-throughput sequencing of the 16S rRNA genes were the Acidobacteria (35.4-41.2% of total 16S rRNA gene reads), Alphaproteobacteria (19.1-24.2%), Gammaproteobacteria (7.9-11.1%), Actinobacteria (5.5-13.2%), Planctomycetes (7.2-9.5%), and Verrucomicrobia (5.1-9.5%). The distinctive feature of this community was high proportion of Subdivision 2 Acidobacteria, which are not char- acteristic for boreal Sphagnum peat bogs. Methanotrophic community composition was determined by mo- lecular analysis of the pmoA gene encoding particulate methane monooxygenase. Most (-80%) of all pmoA gene fragments revealed in peat from lichen-dominated sites belonged to the phylogenetic lineage represented by a microaerobic spiral-shaped methanotroph, "Candidatus Methylospira mobilis." Members of the genus Methylocystis, which are typical inhabitants of boreal Sphagnum peat bogs, represented only a minor group of indigenous methanotrophs. The specific feature of a methanotrophic community in peat from lichen-dominated sites was the presence of uncultivated USCa (Upland Soil Cluster alpha) methanotrophs, which are typical for acidic upland soils showing atmospheric methane oxidation. The methanotrophic community composition in lichen-dominated sites of a tundra wetland, therefore, was markedly different from that in bo- real Sphagnum peat bogs.

Biotic Interactions in Microbial Communities as Modulators of Biogeochemical Processes: Methanotrophy as a Model System.

Microbial interaction is an integral component of microbial ecology studies, yet the role, extent, and relevance of microbial interaction in community functioning remains unclear, particularly in the context of global biogeochemical cycles. While many studies have shed light on the physico-chemical cues affecting specific processes, (micro)biotic controls and interactions potentially steering microbial communities leading to altered functioning are less known. Yet, recent accumulating evidence suggests that the concerted actions of a community can be significantly different from the combined effects of individual microorganisms, giving rise to emergent properties. Here, we exemplify the importance of microbial interaction for ecosystem processes by analysis of a reasonably well-understood microbial guild, namely, aerobic methane-oxidizing bacteria (MOB). We reviewed the literature which provided compelling evidence for the relevance of microbial interaction in modulating methane oxidation. Support for microbial associations within methane-fed communities is sought by a re-analysis of literature data derived from stable isotope probing studies of various complex environmental settings. Putative positive interactions between active MOB and other microbes were assessed by a correlation network-based analysis with datasets covering diverse environments where closely interacting members of a consortium can potentially alter the methane oxidation activity. Although, methanotrophy is used as a model system, the fundamentals of our postulations may be applicable to other microbial guilds mediating other biogeochemical processes.

Effect of biomass concentration on methane oxidation activity using mature compost and graphite granules as substrata

Reported methane oxidation activity (MOA) varies widely for common landfill cover materials. Variation is expected due to differences in surface area, the composition of the substratum and culturing conditions. MOA per methanotrophic cell has been calculated in the study of natural systems such as lake sediments to examine the inherent conditions for methanotrophic activity. In this study, biomass normalised MOA (i.e., MOA per methanotophic cell) was measured on stabilised compost, a commonly used cover in landfills, and on graphite granules, an inert substratum widely used in microbial electrosynthesis studies. After initially enriching methanotrophs on both substrata, biomass normalised MOA was quantified under excess oxygen and limiting methane conditions in 160ml serum vials on both substrata and blends of the substrata. Biomass concentration was measured using the bicinchoninic acid assay for microbial protein. The biomass normalised MOA was consistent across all compost-to-graphite granules blends, but varied with time, reflecting the growth phase of the microorganisms. The biomass normalised MOA ranged from 0.069±0.006μmol CH4/mg dry biomass/h during active growth, to 0.024±0.001μmol CH4/mg dry biomass/h for established biofilms regardless of the substrata employed, indicating the substrata were equally effective in terms of inherent composition. The correlation of MOA with biomass is consistent with studies on methanotrophic activity in natural systems, but biomass normalised MOA varies by over 5 orders of magnitude between studies. This is partially due to different methods being used to quantify biomass, such as pmoA gene quantification and the culture dependent Most Probable Number method, but also indicates that long term exposure of materials to a supply of methane in an aerobic environment, as can occur in natural systems, leads to the enrichment and adaptation of types suitable for those conditions.

Effect of Location and Distribution of Al Sites in ZSM-5 on the Formation of Cu-Oxo Clusters Active for Direct Conversion of Methane to Methanol

A series of Cu/ZSM-5 materials were synthesized and tested for the selective oxidation of methane to methanol reaction in a three stage reaction. The efficiency of the catalysts is related to the ability of the zeolite framework to stabilize multinuclear Cu-oxo species, namely dicopper and tricopper oxo clusters. Spectroscopy characterization by EXAFS showed that the exchange with moderate Cu loadings led to preferential formation of trinuclear Cu complexes [Cu3(μ–O)3]2+ in HZSM-5. The concentration of Al pairs in ZSM-5 is found to limit the maximum concentration of multinuclear Cu-oxo species that can be formed. Above such maximum, inactive Cu species including Cu oxide nanoparticles are formed. Conversely, it was found that at low loadings the Cu speciation in Cu/ZSM-5 occurs as a mixture of Cu monomers and dimers. Furthermore, it was found that not only the structure of Cu-oxo clusters is relevant for the activation of methane, but also the local environment in which the cluster is embedded. Comparison of methane to Cu stoichiometries achieved for Cu/ZSM-5 and Cu/MOR systems containing the same type of active [Cu3(μ–O)3]2+ cluster shows that approximately 50 % of these clusters are inactive on ZSM-5. While MOR stabilizes the trinuclear clusters in highly constrained 8-MR side pockets, the possibility of ZSM-5 to stabilize part of these clusters in less constrained local environments might be the reason for a lower activity in methane oxidation.

Fate of methane in aquatic systems dominated by free-floating plants

Worldwide the area of free-floating plants is increasing, which can be expected to alter methane (CH4) emissions from aquatic systems in several ways. A large proportion of the CH4 produced may become oxidized below the plants due to the accumulation of CH4 as a result of a decrease in the diffusive water-atmosphere flux and the entrapment of part of the ebullitive CH4, in combination with suitable conditions for methane oxidizing (MOX) bacteria in the aerobic rhizosphere. We used a set of essays to test this hypothesis and to explore the effect of different densities for three widespread free-floating species: Azolla filiculoides, Salvinia natans, and Eichhornia crassipes. The gas exchange velocity, proportion of CH4 bubbles trapped by the plants, occurrence of radial oxygen loss from roots, and MOX rates on the roots were assessed. We subsequently used the outcome of these experiments to parameterize a simple model. With this model we estimated the proportion of the produced CH4 that is oxidized, for different plant species and different densities. We found that in a shallow (1 m) system up to 70% of the CH4 produced may become oxidized as a result of a strong decrease in gas exchange combined with high MOX activity of the rhizosphere microbiome. As floating plants also are likely to increase CH4 production by organic matter production, especially when their presence induces anaerobic conditions, the overall effect on CH4 emission will strongly depend on local conditions. This explains the contrasting effects of floating plants on CH4 emissions in literature as reviewed here. As the effect of floating plants on CH4 emissions, including the high MOX rates we show here, can be substantial, there is an urgent need to consider this impact when assessing greenhouse gas budgets.

Photocatalytic oxidation of methane over silver decorated zinc oxide nanocatalysts.

The search for active catalysts that efficiently oxidize methane under ambient conditions remains a challenging task for both C1 utilization and atmospheric cleansing. Here, we show that when the particle size of zinc oxide is reduced down to the nanoscale, it exhibits high activity for methane oxidation under simulated sunlight illumination, and nano silver decoration further enhances the photo-activity via the surface plasmon resonance. The high quantum yield of 8% at wavelengths <400 nm and over 0.1% at wavelengths ∼470 nm achieved on the silver decorated zinc oxide nanostructures shows great promise for atmospheric methane oxidation. Moreover, the nano-particulate composites can efficiently photo-oxidize other small molecular hydrocarbons such as ethane, propane and ethylene, and in particular, can dehydrogenize methane to generate ethane, ethylene and so on. On the basis of the experimental results, a two-step photocatalytic reaction process is suggested to account for the methane photo-oxidation.

The influence of gas composition on Pd-based catalyst activity in methane oxidation − inhibition and promotion by NO

The individual influence, as well as the combined effect of H2O and NO on the activity of Pd/Al2O3, PtPd/Al2O3 and PtPd/CeAl2O3 catalysts in complete methane oxidation under lean conditions were investigated. Under temperature-programmed ramping experiments the activity was severely inhibited in the presence of 5vol.% H2O in the reaction mixture. We propose that this is due to blocking by both water and hydroxyl species. Under the influence of NO without water in the gas flow, it was found that the methane oxidation activity was partly suppressed, due to blocking of active sites. Indeed TPD performed after ramping experiments showed NOx storage on the catalyst. Contrary to the negative effect of NO in the dry case, the promotional NO effect on the activity was observed when water was co-fed, comparing the case with only water presence. The promotional NO effect was confirmed with isothermal experiments, where e.g. the methane conversion decreased from initial 96% to 25% after 10h of exposure in CH4O2H2O mixture at 450°C over the Pd/Al2O3 sample, while the decrease was only from 88% to 60% when catalyst was exposed to CH4O2H2ONO mixture. We propose that the reason is that the NO reacts with the hydroxyl species to form HNO2, which reduces the water deactivation effect.

Palladium catalyst supported on stair-like microstructural CeO2 provides enhanced activity and stability for low-concentration methane oxidation

Octahedral CeO2 microparticles possessing a stair-like microstructure comprising self-assembled nano-sized rectangular blocks were synthesized by a hydrothermal synthetic route. The samples were characterized by X-ray diffraction, scanning/transmission electron microscopy, and nitrogen adsorption/desorption techniques. The structural functions of the as-synthesized CeO2 as support for the Pd catalyst were investigated by the catalytic oxidation of low-concentration methane. The results showed that the unique stair-like structure of CeO2 improved the stability of the Pd catalyst for continuous conversion of low-concentration methane, which suggests that the as-synthesized CeO2 has potential application in the removal of low-concentration methane from ventilation air and other oxidations.