Water Partial Pressure Research Articles

Persistent hot spots of CO2 and CH4 in coastal nearshore environments

AbstractNearshore environments are typically supersaturated with the potent greenhouse gases methane and carbon dioxide, due to intense remineralization of the elevated supply of organic carbon in these systems. These environments are characterized by overlapping biogeochemical gradients and heterogeneous morphology, and the overall spatial variability in nearshore greenhouse gas concentrations remains unclear. We measured surface water partial pressures of carbon dioxide and methane synoptically with water quality parameters in the coastal Baltic Sea, covering two ice‐free seasons. The high‐frequency flow‐through data revealed sites with recurring very high partial pressures of carbon dioxide and methane (i.e., hot spots) scattered around the 50 km × 40 km study area, exceeding overall partial pressure averages by 455 μatm (CH4) and 2396 μatm (CO2). High partial pressures were linked with elevated inputs of allochthonous and autochthonous organic matter, underpinning the major role of organic enrichment of coastal environments in global carbon cycling.

Time‐resolved DRIFT Spectroscopy Study of Carbonaceous Intermediates during the Water Gas Shift Reaction over Au/Ceria Catalyst

AbstractChemCatChem homepage for more articles in the collection. The mechanism of the low‐temperature water gas shift reaction (LTWGS) on an Au/CeO2 catalyst was investigated by means of in situ diffuse reflectance infrared (DRIFT) spectroscopy. Under steady‐state LTWGS reaction (373–623 K), the catalyst is partially reduced, and signals from carbonate/formate dominates the infrared spectra. Time‐resolved pulse of CO experiment under a constant partial pressure of water at 423 K indicates that Ce4+ can be reduced to Ce3+ and that formate (HCOO) species cannot be directly related to the CO2 production. Further information was obtained by performing modulation excitation spectroscopy (MES) experiments coupled with a phase‐sensitive detection (PSD) method. Under periodic modulation of the CO partial pressure while keeping the H2O concentration constant, most of the intense bands of carbonate and formate remained constant, indicating that these species are only spectators. The same is observed for the concentration of Ce3+. Conversely, signals in‐phase with the conversion of CO to CO2 are observed and assigned to carboxyl [C(O)OH] and carboxylate (CO2δ−) species, while some monodentate formate (m‐HCOO) also changes but at a lower rate. A plausible associative reaction mechanism where carboxyl/carboxylate are key intermediates is postulated.

Microkinetic Simulations of Methanol-to-Olefin Conversion in H-SAPO-34: Dynamic Distribution and Evolution of the Hydrocarbon Pool and Implications for Catalytic Performance

The dominating hydrocarbon pool species (HCPs) in zeolites for methanol-to-olefin (MTO) conversion have been the subject of intense debate for decades due to the diversity of structures and the complexity of reaction networks. We performed microkinetic simulations in a three-site model to study the MTO conversion in industrially relevant H-SAPO-34 zeolite under a wide range of operating conditions. The energetics of 229 and 342 elementary reaction steps were employed, respectively, in the aromatic-based and olefin-based cycles. The dynamic distribution and evolution of the retained aromatic or olefinic HCPs and the origin of olefin products were revealed with respect to reaction conditions. We corroborate that the olefin-based cycle dominates the MTO conversion in H-SAPO-34 under most reaction conditions, and the contribution of the aromatic-based cycle increase with increasing temperature, decreasing pressure, and/or decreasing water partial pressure. The paring route, dedicated highly to propene formation, prevails in the aromatic-based cycle; the side chain route, favoring exclusively ethene formation, only prevails at extremely lower temperatures, higher pressures, and higher water contents. The inherent activity of each aromatic HCP via the paring cycle increases, while its population retained in H-SAPO-34 usually decreases, with the methylation degree remarkably from tetramethylbenzene to hexamethylbenzene. The contents of higher methylbenzenes increase with decreasing water partial pressure, leading to the enhanced contribution of the aromatic-based cycle. The olefin-based cycle contributes to the formation of both ethene and other olefins, and the product distribution is drastically sensitive to reaction conditions. Ethene predominantly comes from the cracking of C5 and C6 species, and propene comes from C5 to C7 species. The olefin-based cycle shifts from the interconversion of C2–C7 olefins toward C2–C5 ones with increasing temperature and water partial pressure to enhance ethene formation. Under industrially relevant conditions, the conversion rate of methanol via the olefin-based cycle is 40-fold greater than that via the aromatic-based cycle (6.6 vs 0.15 s–1), and the former agrees unexpectedly with the experimental value in low-silica AlPO-34 (7.0 s–1). This work thus solves the puzzle of dominating HCPs as a function of reaction conditions in H-SAPO-34 and provides mechanistic insights into the kinetic behaviors, which are the basis for the optimization of the MTO catalyst and process.

Corrosion Behaviors of N80 and 1Cr Tubing Steels in CO2 Containing Downhole Environment—A Case Study of Underground Gas Storage in LiaoHe Oil Field

In order to evaluate the effect of chromium content in carbon steel on the corrosion resistance of carbon steel materials, the corrosion behavior of 1Cr and N80 steels was investigated in this study by immersion weight loss method under three different CO2 partial pressure and temperature conditions in formation water for 72, 168 and 336 h. Detailed material surface morphological characterization was performed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and confocal laser scanning microscopy (CLSM). The results show that the pitting corrosion of N80 carbon steel is serious at medium temperature and low CO2 partial pressure (50 °C, 0.30 MPa), and the corrosion rate is significantly higher than that of 1Cr steel. However, at high-temperature and high CO2 partial pressure (100 °C/0.63 MPa and 114 °C/0.73 MPa), 1Cr steel is more inclined to the mesa corrosion dominated by local corrosion characteristics, and the corrosion rate is seriously higher than that of N80 steel with uniform corrosion. From the experimental results, we can know the corrosion resistance of carbon steel and 1Cr steel is not only affected by the corrosion environment, but also depends on the formation process of the product film, as well as its compactness and integrity characteristics. At low-temperature and low CO2 partial pressure, 1 wt.% chromium content can provide a certain degree of corrosion resistance, while high temperature and high partial pressure can broaden the application window of carbon steel N80 and weaken the corrosion inhibition effect of chromium.

Isobutanol to isobutene: Processes and catalysts

The present review aims at summarizing the scientific literature data and patents concerning the isobutanol, preferably from a fermentation process, selective dehydration reaction to produce isobutene (isobutylene). The benefits and drawbacks of the conditions used on the process design and catalyst performances are illustrated, analyzed and discussed with much objectivity. Thermodynamic equilibria of the various reactants and products are provided together with the limiting step rate. Isobutanol selective dehydration to isobutene can be done under low or high pressure conditions. This choice impacts not only the downstream recovery and purification of isobutene, but also the stability of the catalyst and the reactor volume. Due to the strong inhibition of the alumina catalysts by steam, the reaction under pressure requires longer contact times and catalyst volume. The alumina catalysts need to have a good hydrothermal stability to avoid a rapid deactivation. Isobutene purification is also addressed. The impact of the water partial pressure on the catalytic dehydration is described and shown to affect the side reactions. Mass transfer limitations and kinetics of the reaction are well documented. Relevant advices are provided to perform the reaction without undesired products.

Probing Local Environments of Oxygen Vacancies Responsible for Hydration in Sc-Doped Barium Zirconates at Elevated Temperatures: In Situ X-ray Absorption Spectroscopy, Thermogravimetry, and Active Learning Ab Initio Replica Exchange Monte Carlo Simulations

Proton-conducting oxides, specifically heavily Sc-doped barium zirconate perovskite, have attracted attention as electrolytes for intermediate-temperature protonic ceramic fuel cells because of their high proton conductivity and high chemical stability against carbon dioxide in that temperature regime. Hydration is a key reaction for incorporating protons by filling oxygen vacancies, VO, with hydroxyl groups and activating proton conduction in the perovskite. However, probing the local environment of oxygen vacancies responsible for hydration is challenging because the behavior depends on the temperature and water partial pressure, which necessitates in situ observations and calculations of the local environments at elevated temperatures. To obtain such information, we combined in situ X-ray absorption spectroscopy (XAS) for both the Sc and Zr K-edges, thermogravimetry, X-ray diffractometry, and active learning ab initio replica exchange Monte Carlo (RXMC) simulations in undoped and 20–40 at % Sc-doped barium zirconates at and below 800 °C. The presence of oxygen vacancies adjacent to Sc and Zr in the dehydrated samples and the hydration of these oxygen vacancies under a wet atmosphere were probed by in situ XAS for Sc and Zr pre-edges at elevated temperatures. Here, the microscopic hydration linearly responds to the macroscopic degree of hydration. RXMC sampling further supports the presence of Sc-VO-Zr and Sc-VO-Sc environments. An initial hydration occurs in the Sc-VO-Zr environment at and above 600 °C, but the Sc-VO-Sc environment contribution is greater at higher degrees of hydration. The Zr-Vo-Zr environment is the least abundant among them for the whole temperature range examined and thus has a negligible impact.

Spatial and temporal variability of the sources and sinks of carbonate system in the southwest bay of Bengal from 2014 to 2020

This study presents a synthesis of surface water partial pressure of CO2 (pCO2) and nutrient measurement in the southwest Bay of Bengal (swBoB) from 2014 to 2020 and characterizes the spatial and temporal variability. pCO2 rates found to be high (1191 μatm) during the 2015 monsoon and low (176 μatm) in the summer season during the month of May 2015. The inter-annual CO2 fluxes varied from −4.79 to 9.97 mmol Cm−2d−1. The significant a negative CO2 flux (−4.79 mmol Cm−2d−1) was recorded during the summer season in the year 2019, whereas a positive significant CO2 flux (9.97 mmol Cm−2d−1) was observed during the monsoon in 2014. Major physical parameters are at their highest during summer owing to increased high solar radiation during cloud-free circumstances, reduced or inadequate riverine flux, and a lack of vertical mixing of the water column, which results in the lowest nutrients concentration, Dissolved Oxygen (DO), Dissolved Inorganic Nitrogen (DIN), Dissolved Organic Carbon (DOC), chlorophyll-a, Particulate Organic Carbon (POC), pCO2, that leads to negative CO2 flux to the atmosphere. In contrast during monsoon season colossal discharge of freshwater high DO, DIN, DOC, chlorophyll-a, POC, pCO2 as results source CO2 flux to the atmosphere. Statistical analysis the correlation coefficient depicts Total Alkalinity (TA), DIC, POC, DIN, and DO found a positive correlation with pCO2 and fCO2 during the monsoon season. In the swBoB, pCO2 had a negative relationship with sea surface temperature (SST), sea surface salinity (SSS), and pH because CO2 solubility changes with SST and increases in cold water rather than warm water. In this study, we examine the association between all carbonate variables and the SSS and SST to better understand seasonal fluctuations.

Electrical and sensing properties of indium-doped barium cerate

Systematic analysis of electrical characteristics of BaCe0.75In0.25O3-δ (BCI25) sintered sample was performed in a dry and a wet argon atmosphere in the 250 °C–700 °C temperature range. The water vapor sensing properties of BCI25 porous film and its response and recovery times were investigated under different conditions of temperature and water vapor concentration. The 30 μm thick film obtained from the powder calcined at 1050 °C exhibited sensitivity comparable to that of the sintered sample with a significantly shorter response and recovery times. While the sensitivity of the film gradually decreased with a decrease in partial pressure of water vapor (p(H2O)), a noticeable sensitivity was still observed at p(H2O) of 200 Pa. Decrease in conductivity depended logarithmically on the partial pressure of water with the slope of 0.52 that is close to the theoretical value. After several cycles, the reusability test proved an almost unchanged ratio between the impedance value in the dry and the wet Ar atmosphere (p(H2O) = 2.34 kPa), which implied that BCI25, having good stability and sensitivity, is a promising high-temperature humidity sensor.

Operando monitoring of gas drying by adsorption on supported ionic liquids: Determination of velocity of adsorption front by microwaves

For adsorption-based gas drying to reach very low dew points, ionic liquids (ILs) immobilized on solid porous supports are promising materials. Prior investigations with the IL 1-ethyl-3-methylimidazolium methanesulfonate ([EMIM][MeSO3]) show high absorption capabilities for water (vapour) and low regeneration (desorption) temperatures (< 100 °C). Due to the negligible evaporation losses of this IL combined with a high thermal stability and low melting point, [EMIM][MeSO3] is ideal for immobilization on highly porous supports. In this work, the IL covers at least a monolayer of the substrate (here silica), thereby filling about 30 % of the pores with the IL. Water then mainly (ab)sorbs in the IL and practically not adsorbs on the surface of silica. In the presence of water, the dielectric losses of the supported IL increase strongly and can therefore be measured by microwave perturbation methods. For this purpose, a quartz tube containing the fixed-bed of the supported IL was placed in the radial center of a microwave resonator operating in a mode so that the electric field strength varies in axial direction along the bed. During sorption experiments, the electrical signals measured with a network analyzer allowed to track the sorption front, to calculate the velocity of the moving front, and to detect the breakthrough of water vapor. The validation of this method was accomplished by simultaneously measuring the humidity of the carrier gas at the in- and outlet of the packed bed by humidity sensors. The water partial pressure (carrier gas N2), the temperature, and the gas velocity were varied in a range relevant for gas drying (5–15 mbar; 40–60 °C; 0.05–0.30 m s−1). The results demonstrate that microwave-derived parameters, like resonant frequency or quality factor, are suitable to locate the sorption front along the fixed-bed and to calculate the front velocity and the breakthrough time.

Highly selective CO2 photoreduction to CO on MOF-derived TiO2.

Metal-Organic Framework (MOF)-derived TiO2, synthesised through the calcination of MIL-125-NH2, is investigated for its potential as a CO2 photoreduction catalyst. The effect of the reaction parameters: irradiance, temperature and partial pressure of water was investigated. Using a two-level design of experiments, we were able to evaluate the influence of each parameter and their potential interactions on the reaction products, specifically the production of CO and CH4. It was found that, for the explored range, the only statistically significant parameter is temperature, with an increase in temperature being correlated to enhanced production of both CO and CH4. Over the range of experimental settings explored, the MOF-derived TiO2 displays high selectivity towards CO (98%), with only a small amount of CH4 (2%) being produced. This is notable when compared to other state-of-the-art TiO2 based CO2 photoreduction catalysts, which often showcase lower selectivity. The MOF-derived TiO2 was found to have a peak production rate of 8.9 × 10-4 μmol cm-2 h-1 (2.6 μmol g-1 h-1) and 2.6 × 10-5 μmol cm-2 h-1 (0.10 μmol g-1 h-1) for CO and CH4, respectively. A comparison is made to commercial TiO2, P25 (Degussa), which was shown to have a similar activity towards CO production, 3.4 × 10-3 μmol cm-2 h-1 (5.9 μmol g-1 h-1), but a lower selectivity preference for CO (3 : 1 CH4 : CO) than the MOF-derived TiO2 material developed here. This paper showcases the potential for MIL-125-NH2 derived TiO2 to be further developed as a highly selective CO2 photoreduction catalyst for CO production.

Influence of scandium codoping on the stability of barium cerate electrolyte

Proton (H+) conducting electrolytes significantly outperform oxide ion (O2–) conducting electrolytes in terms of performance while also costing less to operate the cell. This makes them popular and potential electrolytes in future electrochemical devices such as intermediate temperature solid oxide fuel cells (IT-SOFCs). The standard solid-state reaction method has been utilized in the synthesis of scandium (Sc) co-doped barium zirconium cerate ceramics, BaCe0.8-xZr0.2ScxO3-δ. The powder X-ray diffraction spectra confirmed that the prepared samples are of single phase and have orthorhombic symmetry. The samples were analyzed for thermal stability by employing thermogravimetric analysis (TGA). All samples are stable up to 800 °C, according to TGA performed on dried and pre-hydrated samples—however, the samples under higher partial pressure of water exhibit lower degree of stability. Raman analysis was used to determine the stability of perovskite phase of the studied compositions both before and after TGA. This study showed that most initial phases either persisted or remained stable when the samples were treated at high vapor pressure. The thermal stability increased as the amount of Sc-substitution increased. Also, to investigate how the presence of grains and grain boundaries influenced the total resistance of samples with different Sc concentrations, complex impedance spectroscopy (CIS) was performed.

Chemical denudation dynamic based on hydrochemical measurements in the area of Drienovecká jaskyňa Cave basin (Slovak karst, Slovakia)

Denudation is a fundamental process for the development of karst relief. It has different intensity in different geological and climatic condition and beside this many other partial factors influence the intensity and dynamic of the process during the year. In karst systems, there is often a positive correlation between water partial pressure of CO2 and denudation. The Drienovecká jaskyňa Cave spring represents typical active fluviokarstic spring and its catchment area is represented by Wetterstein and Waxenec limestones, and Drienovec conglomerates. This catchment study area is located at the contact of the conti-nental and oceanic climate, in terms of altitude at the border between the lowland and moun-tain climate. During three hydrological years we measured main karst water parameters that helped us bring first results on chemical denudation in the catchment area. The water from the spring is of calcium-magnesium-bicarbonate type. Hydrogeochemical analyses in-dicate that it is mostly water from an open hydrogeological structure. Water of the cave spring is oversaturated with respect to calcite, which is documented by the saturation index of calcite, whose values are higher than the solubility product of calcite. During the studied period, the dynamic of the chemical denudation rate is clearly observable with peaks in spring months in year 2014 and 2016. The average value of denudation rate for the entire period is 21.06 mg Ca2+. Factors that influence chemical denudation include, for example, type of carbonate rock, CO2 partial pressure, water and air temperature, chemical compo-sition of the aqueous solution, activity of soil organisms, vegetation and others. Water from melted snow can also play a significant role in changes in water chemistry and thus chemical denudation. URL: https://www.gcass.science.upjs.sk/

Energetics of formation and stability in high pressure steam of barium lanthanide cobaltite double perovskites.

This study concerns energetics of formation and the stability in high water partial pressure of BaLnCo2O6-δ, (Ln = La, Pr, Nd, and Gd) (BLnC) and BaGd1-xLaxCo2O6-δ, where x = 0.2, 0.5, and 0.7 (BGLC) double perovskite cobaltites. Those materials are extensively studied due to their potential applications as a positrode in electrochemical devices. Therefore, their stability under such conditions is a key issue. All investigated materials are thermodynamically stable relative to binary oxides and exhibit strongly exothermic enthalpies of formation. Moreover, BaGd0.3La0.7Co2O6-δ and BaGd0.8La0.2Co2O6-δ remain the main perovskite structure up to 3 bars of water vapor at 400 °C. At higher steam pressure, reaching 10 bar at 300 °C, the partial decomposition to constituent oxides and hydroxides was observed. The BGLC compounds exhibit higher negative formation enthalpies in comparison to single-Ln compositions, which does not translate into higher chemical stability under high steam pressures since the BLnC series retained the main perovskite structure at higher temperatures as well as in higher water vapor pressures.

A Low-Cost Carbon Dioxide Monitoring System for Coastal and Estuarine Sensor Networks

As the interest in marine or ocean-based carbon dioxide removal (mCDR) increases, so does the need for more spatially resolved measurement, reporting, and verification (MRV) of the CO2 sequestered (and for understanding the impacts of these environmental manipulation experiments). The mCDR/MRV community can borrow technology from the past decade’s exponential growth of interest in ocean acidification and monitoring. However, available sensors are insufficient to meet the growing demand for spatially resolved mCDR due to technological complexities, availability, and cost. For instance, in a recent review, there were few commercially available autonomous solutions for pH and pCO2 monitoring and none for total alkalinity or total dissolved inorganic carbon), and the majority of the pH and pCO2 sensors and analyzers cost well above $10,000. Proper MRV of mCDR requires high spatiotemporal resolution monitoring of the inorganic carbon system. Here we describe our efforts toward a low-cost CO2 flux or ∆pCO2 (the difference between air and water partial pressure of CO2) monitoring system intended for use in distributed sensor arrays in coastal, estuarine, and blue carbon research and in mCDR approaches.

Effects of Water Masses and Circulation on the Surface Water Partial Pressure of Carbon Dioxide in Summer in Eastern Beibu Gulf, China

A gulf is a typical ecological zone where carbon cycle is jointly affected by complex environmental factors and strong human activities, and the Beibu Gulf has complex water masses and circulation structures. In this study, we used underway, continuous observational data of the surface water partial pressure of carbon dioxide (pCO2), temperature (SST), salinity (SSS), dissolved oxygen (DO), and chlorophyll α along with vertical profile observations of temperature, salinity, carbonate system parameters and nutrients to determine the spatiotemporal variations and research effects of water masses and circulation on summer pCO2 in the eastern part of Beibu Gulf. In the summers of 2011 and 2014, the mean pCO2 in the eastern part of Beibu Gulf was 417 μatm and 405 μatm, respectively, and the mean sea–air CO2 flux was 3.3 mmol m−2 d−1 and 1.6 mmol m−2 d−1, respectively. In the summer of 2011, the northern part of the Beibu Gulf was controlled by a cyclonic circulation, and pCO2 at the center of the cyclonic circulation increased by more than 15 μatm to a mean value of more than 10 μatm above that of the surrounding waters. The southern part of the Beibu Gulf was affected by an anticyclonic circulation and western coastal water masses, with a high temperature, low salinity, low pCO2, and downwelling surface waters. In the summer of 2011, the mean pCO2 was approximately 17 μatm lower than that in the surrounding waters, and no clear downwelling was observed in summer 2014. The eastern part of Beibu Gulf was a source of atmospheric CO2 in the summer, only the region affected by the northern coastal water in the eastern part of Beibu Gulf was a sink of atmospheric CO2, and pCO2 had distinctly different spatiotemporal distributions under the influence of complex water masses and circulation structures.

Local Hydrophobicity of Atomically Dispersed Fe-N-C Catalysts and Its Impact on the Oxygen Reduction Reaction in Membrane Electrode Assembly

Non-precious metal electrocatalysts with atomically dispersed transition metal sites have shown promising performance for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. The Fe-N-C catalysts, with single iron atom coordinated by nitrogen atoms (FeNx) on carbonaceous supports, exhibits outstanding ORR activity as compared to its other metal counterparts (M-N-C). [1] The Fe-N-C catalysts synthesized via sacrificial support method (SSM) showed superior ORR performance and this family of Fe-N-C electrocatalysts is so far the only commercially available PGM-free catalysts for fuel cell application. [2] For membrane electrode assembly (MEA) performance, the pore structure [3] and surface hydrophobicity [4] of the carbon matrix are of particular significance, because the high current throughput requires good oxygen transport and well-controlled water management on cathodes. In this talk, we present the local hydrophobicity of SSM-based Fe-N-C electrocatalysts [1] and its impact on the ORR macrokinetics in MEA. Specifically, nitrogen physisorption and water vapor physisorption were used to study the pore morphology and the local surface hydrophobicity, respectively. As shown in Figure 1, the sacrificial templated Fe-N-C catalyst showed a large amount of mesoporosity while its surface was very hydrophobic, as indicated by the little water uptake through whole pressure range. Such porosity and local hydrophobicity features were further correlated to its macrokinetic performance in a study of water partial pressure dependence under MEA configuration. Reference [1] T. Asset and P. Atanassov, Joule, 2020, 4, 33.[2] https://pajaritopowder.com [3] Y. Huang, Y. Chen, M. Xu, T. Asset, P. Tieu, A. Gili, D. Kulkarni, V. de Andrade, F. de Carlo, H. S. Barnard, A. Doran, D. Y. Parkinson, X. Pan, P. Atanassov, and I. Zenyuk, Materials Today, 2021, 47, 53.[4] L. Liu, S. J. Tan, T. Horikawa, D. D. Do, D. Nicholson and J. Liu, Adv Colloid Interface Sci, 2017, 250, 64. Figure 1

Seasonal, Diurnal, and Tidal Variations of Dissolved Inorganic Carbon and pCO2 in Surface Waters of a Temperate Coastal Lagoon (Arcachon, SW France)

We report on diurnal, tidal, and seasonal variations of dissolved inorganic carbon (DIC), water partial pressure of CO2 (pCO2), and associated water–air CO2 fluxes in a tidal creek of a temperate coastal lagoon with 70% of intertidal flats, during eight tidal/diurnal cycles and two consecutive years covering all seasons. Surface waters of the lagoon were always slightly oversaturated in CO2 with respect to the atmosphere with an average pCO2 value of 496 ± 36 ppmv. Seasonally, subsurface water pCO2 values were controlled by both temperature and biological/tidal advection effects that compensated each other and resulted in weak annual variations. High-resolution temporal pCO2 records reveal that the highest fluctuations (192 ppmv) occurred at the tidal/diurnal scale as a result of biological activity, advection from the tidal flat, and porewater pumping that all contributed to water pCO2 and carbonate chemistry variations. Total alkalinity (TA) versus salinity plots suggest a net production of alkalinity in the lagoon attributed to benthic carbonate dissolution and/or anaerobic degradation of organic matter. We specifically highlighted that for the same salinity range, during flooding, daytime pCO2 were generally lower than nighttime pCO2 values because of photosynthesis, whereas during ebbing, daytime pCO2 were higher than nighttime pCO2 values because of heating. Waters in the lagoon were a relatively weak CO2 source to the atmosphere over the year compared to other estuarine and lagoon waters elsewhere, and to sediment-air fluxes measured simultaneously by atmospheric Eddy Covariance (EC) in the Arcachon lagoon. Because of low values and small variations of the air-sea pCO2 gradient, the variability of fluxes calculated using the piston velocity parameterization was greatly controlled by the wind speed at the diurnal and, to a lesser extent, seasonal time scales. During the emersion, the comparison of these pCO2 data in the tidal creek with EC fluxes measured 1.8 km away on the tidal flat suggests high heterogeneity in air-sea CO2 fluxes, both spatially and at short time scales according to the inundation cycle and the wind speed. In addition to tidal pumping when the flat becomes emerged, our data suggest that lateral water movement during the emersion of the flat generates strong spatial heterogeneity in water–air CO2 flux.

On the effect of electrode material in electrical conductivity relaxation - An alternative interpretation of the two-fold relaxation behavior in La5.4WO11.1-δ

Oxygen and proton transport properties of La5.4WO11.1-δ are investigated using electrical conductivity relaxation. Chemical diffusion coefficients and surface exchange coefficients of both hydrogen and oxygen presumed to govern conductivity relaxation are extracted from fitting the two-fold conductivity relaxation observed after hydration/dehydration steps at fixed oxygen partial pressure to the model equations. Surprisingly, the kinetic parameters obtained from fitting are found to depend on the magnitude of the current used in the measurements. A two-fold relaxation behavior with characteristics depending on the magnitude of the current is also observed after oxidation/reduction steps under dry conditions. Furthermore, at fixed temperature, oxygen and water partial pressures, the conductivity is found to exhibit relaxation behavior after a current step, while the apparent steady-state conductivity is found to depend on the applied current. The observations are attributed to the use of gold electrodes in the experiments and associated interfacial capacitances. It is obvious from this study that the use of partially ion-blocking electrodes in conductivity and conductivity relaxation measurements on mixed ionic-electronic conducting oxides with prevailing ionic conduction may lead to erroneous results and interpretations.

The water assisted vinylene mechanism for cobalt Fischer-Tropsch synthesis assessed by multi-catalyst modelling of kinetics and deactivation

The paper describes development of a mechanism and a consistent rate expression for Fischer-Tropsch (FT) synthesis over cobalt-based catalysts. The developed mechanism relies on a two-step hydrogen assisted activation of CO. The carbon atom of CO is first hydrogenated by surface hydrogen to formyl; followed by the rate-limiting step whereby the oxygen atom is hydrogenated by adsorbed water. The produced CH* monomer is incorporated into the growing chain giving vinylene intermediate. The vinylene intermediate is either terminated to an olefin by adding hydrogen to the α-carbon atom or propagates by adding hydrogen to the β-carbon position. The resulting expression for CO consumption, the Fischer-Tropsch rate, can respond positively or negatively to the partial pressure of water, in agreement with experimental observations. A special feature is that the chain propagation probability does not depend on the partial pressure of hydrogen. The resulting kinetic model is tested on several cobalt catalysts supported on alumina; spanning from γ-alumina with average pore sizes ranging from 6.1 to 18.3 nm to α-alumina with a wide pore structure; and with cobalt particle sizes from 8 to 19 nm. Water was added sequentially to the syngas feed, causing enhanced deactivation, for testing the water response on activity and selectivity. A deactivation model comprising sintering and cobalt oxidation, and the FT-kinetics, describe the observed CO conversions with great precision for all catalysts. Selectivities are also well described, but with slight deviations at least partly due the effect of deactivation. Trends in some of the kinetic parameters are rationalized in terms of cobalt crystallite and pore sizes.

Investigation of Hydrogen Electrode Formation on the Surface of Noble Metals (Pd, Ir, Au) Exposed to Nitrogen Atmosphere: A Comprehensive Study Using Scanning Kelvin Probe

The importance of hydrogen evolution reaction (HER) in engineering and energy conversion related applications grows stronger and it is important to develop better catalysts to enhance the efficiency of the catalytic process. The introduction of computational chemistry (e.g., DFT) into the field of electrochemistry allows general prediction and understanding of activity trends. Great progress is made in modelling the electrochemical double layer under potentiostatic control [1]. However, a reliable and feasible experimental methodology providing in-depth experimental data for improving the theoretical model is lacking, as the electrochemical interface is usually buried under bulk electrolyte, which prevents most important surface analytical techniques to be applied for its investigation. However, it could be recently shown that a hydrogen electrode forms on a palladium surface even in nitrogen atmosphere, even at very low humidity [2] . The electrochemical double layer of this “hydrogen electrode in the dry” is confined just 1-2 monolayers of adsorbed water. However, it is still fully polarizable by adjusting the hydrogen activity from the backside of the electrode and it is even possible to measure full current-potential dependencies for reactions such as e.g. oxygen reduction [3]. Hence, this could be revolutionary new approach for investigating electrochemical electrodes in operando by standard surface analytical techniques. For this however, it needs to be investigated in how far this approach can be also applied beyond palladium. Since the potential of such “electrodes in the dry” cannot be measured with standard reference electrode a Kelvin probe is used instead [2,3]. Basically, the Kelvin probe measures the Volta potential difference between sample and probe (ψ KP - ψ S), and upon suitable calibration such measurement provides the electrode potential of the sample.Evers et.al has proposed the “Hydrogen electrode in dry” concept based on adjusting the hydrogen activity (cathodic charging) from one side of a palladium sample and measuring the work function on the other side using Kelvin probe [2]. In this work, the hydrogen electrode formation in dry (<0.1% R.H) as well as humid condition (>95% R.H) for Palladium (Pd), iridium (Ir) and gold (Au) was carried out using scanning Kelvin probe method by electrochemical charging of hydrogen from entry side and subsequently measuring the Volta potential on exit side (see fig.1). The measured potential on the exit side can be related to electrode potential. Palladium showed a 1:1 correlation between applied potential and measured potential at higher hydrogen activities (between 0 to 300 mVSHE) by establishing electrochemical equilibrium in both conditions, in accordance with [1]. Though the partial pressure of water is very low (0.23mbar) in dry condition, still the hydrogen electrode formation occurred on the palladium. In the case of Ir, it shows 1:1 correlation only in humid condition and not in dry condition. Further, the hydrogen electrode formation on Ir at different partial pressures of water was investigated by controlling the humidity on the exit side of the sample and the obtained results can be attributed to slight differences of thickness of the water layer in the monolayer and even sub-monolayer range. A linear decrease in the potential was observed when increasing the humidity from low to high humidity and the correlation between potential applied from the back and potential measure in the “dry” shows a decrease in responsiveness with decreasing humidity. No correlation was observed for iridium in dry conditions and it can be attributed to presence of insufficient amount of adsorbed water. For gold no correlation was observed in both the conditions. However, a slight behavior was observed at very high activities of hydrogen. This may be due to the initial hydrophobicity of gold, which has first to overcome by a formation of a double layer. On the whole, the effect of water layer adsorption on different metals and its influence on the establishment of a hydrogen electrode at a dry surface was studied.