Water Partial Pressure Research Articles

Strain Effects on the Adsorption of Water on Cerium Dioxide Surfaces and Nanoparticles: A Modeling Outlook.

Nanocrystalline ceria exhibits significant redox activity and oxygen storage capacity. Any factor affecting its morphology can tune such activities. Strain is a promising method for controlling particle morphology, whether as core@shell structures, supported nanoparticles, or nanograins in nanocrystalline ceria. A key challenge is to define routes of controlling strain to enhance the expression of more active morphologies and to maintain their shape during the lifespan of the particle. Here, we demonstrate a computational route to gain insights into the strain effects on particle morphology. We use density functional theory to predict surface composition and particle morphology of strained ceria surfaces, as a function of environmental conditions of temperature and partial pressure of water. We find that adsorbed molecular water is not sufficient to shift stability and as such under all compressive and tensile strains studied, the most stable particle is of octahedral shape, similarly to the unstrained case. When dissociative water is involved at the surfaces of the particle, then the most stable particle morphology changes under high water coverage and tensile strain to cuboidal or truncated cuboidal shapes. This shift in shape is due to strain effects that affect the strength of water adsorption.

A two-stage sorption strategy to improve heat storage performance of salt/porous matrix composites

Composites combining salt hydrates and porous matrix like metal–organic frameworks (MOFs) are used to enhance the performance of thermochemical storage materials. However, the mechanism for the enhancement and the strategy for the selection of salt hydrates and porous matrix are not clear. The heat storage performance is determined by water adsorption dynamics, capacity and their sorption energy of the single components. The single components (salts or matrix) normally have either fast water adsorption dynamics or high-water uptake capacity, but not both of them. In this study, we investigate the enhancement of adsorption in composite materials, and a novel strategy is then proposed to combine the advantages from adsorbents with higher adsorption dynamics at low water partial pressure with those with a high capacity at high water partial pressure to promote the water adsorption capacity and kinetics, hence efficient energy storage. Two types of composites have been used to demonstrate the two-stage strategy developed in this study, in which SrCl2 and SrBr2 are incorporated into MOF MIL-101(Cr) to enhance adsorption and heat storage. In these composites, SrCl2 and SrBr2 can be regarded as the moisture pumps, capturing water vapour from air to improve the adsorption dynamics, while MIL-101(Cr) with high adsorption capacity functions as a water reservoir to take the water accumulated by the salts. Through this two-stage design, the water sorption capacity of the composites is 2–3 times higher than the water uptake of either salts or MOF within 120 min. As SrCl2 has a faster water adsorption kinetics than SrBr2, SrCl2/MIL-101(Cr) reaches an uptake of 0.73 g/g, however, the water sorption capacity of SrBr2/MIL-101(Cr) is 0.54 g/g. The composite samples also present good heat storage capacities of 1462 J/g for SrCl2/MIL-101(Cr) and 1526 J/g for SrBr2/MIL-101(Cr), and good cycling stability after 15 repeating adsorption/desorption cycles.

Effects of water on the degradations in the Ni-YSZ anode of the direct ammonia solid oxide fuel cells

Direct ammonia solid oxide fuel cells (DA-SOFCs) are promising next-generation energy conversion devices. Despite their possibilities, the electrochemical performance of DA-SOFCs degrade severely over long-term operation. However, the degradation mechanisms are still unveiled. Here, we reveal that the degradations of DA-SOFCs are significantly initiated by the presence of the water, which is the by-product of the SOFC operation, inside the fuel electrode. We measured the stability of the DA-SOFCs by varying the fuels and water partial pressure in the fuel. Thorough surface characterizations verified that Ni becomes favorable for oxidation when ammonia is present in the water, resulting in a reduction of the catalyst reactivity for NH3 decomposition and electric conductivity. Furthermore, we conducted a density functional theory calculation and found that N* adsorbed onto Ni decomposed from NH3 facilitates the H2O decomposition and results in nickel oxidation.

Diagnosis of Polymer Electrolyte Fuel Cell: Should We Rely Only on Electrical Quantities?

Polymer electrolyte fuel cells (PEMFC) will play a central role in the establishment of a de-carbonized society. They are seen as a key technology to empower electric heavy-duty vehicles. However, the lifespan is still below the fixed standards for their commercialization. A way to increase their durability is to complement them during operation with a mitigation strategy enabling to avoid the arise of faulty states accelerating degradation processes. Nowadays, most of the proposed fault identification and isolation architectures rely on monitoring of the performance through electrochemical impedance spectroscopy (EIS). Nevertheless, the analysis of impedance data is often complicated, since the contribution of many processes in the EIS spectra are coupled with each other and, for this reason, difficult to identify. This constitutes a limiting factor for its employment as diagnostic tool. In the last years, in order to overcome such issues, the use of nonelectrical periodic inputs and/or outputs has been advanced. The idea motivating this research direction is the possibility to detect the contribution of selected processes on the performance of PEMFCs separating them from the others. Novel frequency response techniques involving back pressure, temperature and partial pressure have been developed [1].In this contribution, the so-called concentration frequency response analysis (CFRA) is considered [2, 3]. It consists on perturbing the cathode side of the cell with a feed characterized by a periodic partial pressure of oxygen and water, and the detection of the electric response, voltage or current. A test station has been designed and constructed in order to apply this technique and investigate its capabilities. A model based analysis of the measured CFRA spectra indicates the possibility to decouple the contributions of the water transport into Nafion and oxygen mass transport on the performance of the cell depending on the chosen concentration input and electrical output. In this way, the analysis and identification of the related power losses is simplified with respect to classic EIS.Moreover, the performance of a fault identification and isolation architecture employing CFRA as monitoring technique have been evaluated and compared to EIS. For such purpose, an identifiability analysis of parameters related to faulty states and obtained by model fitting to spectra of each technique has been performed. The results clearly show that the employment of CFRA based on water pressure input offers the most reliable parameter estimation and, consequently, constitutes the most preferable diagnosis method among the others analyzed.

Effect of anion α-functionalization on the water affinity of thermo-responsive phosphonium acetate-derived ionic liquids

This study presents a facile method for assessing the effect of anion functional group substitution on the water affinity of phosphonium-acetate ionic liquids. By combining the same cation with three different α-substituted carboxylate anions, the effect of anion functional group substitution on the IL miscibility with water and hygroscopicity was experimentally evaluated and rationalized using COSMO-RS sigma profile analysis. Lower critical solution temperature (LCST) phase separation experiments and water vapor partial pressure measurements revealed that there is a stark α-substitution dependency on the affinity of water for acetate [AcO], pivalate [PivO], and trifluoroacetate [TFA] derived tributyl(octyl)phosphonium [P4448] ILs. It was found that the IL water affinities varied as: [P4448][AcO] > [P4448][PivO] > [P4448][TFA]. With the lowest water partial pressure and no LCST driven phase separation, [P4448][AcO] exhibited a high affinity for water. On the other hand, [P4448][PivO] and [P4448][TFA] exhibited LCST-driven phase separation and room temperature immiscibility, respectively, with higher water partial pressures, as a result of relatively weak water affinities. COSMO-RS sigma profile analysis revealed that the aliphatic carboxylate anions should incorporate apolar interactions commensurate with or greater than the polar hydrogen bond acceptor interactions for phase immiscibility of the corresponding IL with water. It was found that the relative magnitudes of apolar and polar interactions, assessed using the anion sigma profile peak intensity ratio (Ir), are directly correlated with the IL water affinity, and inversely correlated with the solution water activity coefficient. Therefore, it was concluded that Ir forms a quantitative link between the structural changes at the molecular level and the bulk water affinity exhibited by the ionic liquid as a whole.

Mixed proton and oxide ion conduction, phase stability, and conducting mechanisms in the Sr2CeO4-based materials

Sr2CeO4 possesses structural characteristic of one-dimensional (1D) edge-sharing CeO6 octahedral chains, exhibits superior phase and chemical stability compared to the perovskite SrCeO3. There have been previous attempts to improve electrical conductivity through proton conduction by acceptor-doping of Yb on the Ce site. However, its electrical conductivity remained low, for instance, at 500 °C, the reported value was 3.16 × 10−7 S/cm, hence limiting its practical applications. In this paper, we synthesized four new rare earth doped Sr2Ce0.95M0.05O3.975 (where M = Y, Dy, Er, Tb) by the solid state reaction method. We thoroughly investigated their phases, electrical properties, and stability. Additionally, for the first time, we investigated the proton and oxide ion conduction mechanisms in Sr2CeO4-based materials using a bond-valence-based method. This analysis revealed a clear 1D conduction along the c-axis within the CeO6 tetrahedral chains and 2D conduction within the a-c plane, respectively. The findings revealed that all doped materials showed obvious proton conduction, alongside partial oxide ion conduction. Notably, the Er-, Dy-, and Y-doped samples demonstrated significantly improved conductivity under wet condition (with a water partial pressure of ∼0.03 atm) at 500 °C, reaching approximately ∼ 10−4 S/cm. This is nearly three orders of magnitude higher than the previously reported Yb-doped Sr2CeO4.

Benzene Oxidation in Air by an Amine-Functionalized Metal-Organic Framework-Derived Carbon- and Nitrogen-Loaded Zirconium Dioxide-Supported Platinum Catalyst.

To learn more about the behavior of amine (NH2)-functionalized metal-organic framework (MOF)-derived noble metal catalysts in the removal of aromatic volatile organic compounds in air, benzene oxidation at low temperatures has been investigated using 0.2-, 0.8-, and 1.5%-platinum (Pt)/Universitetet i Oslo (UiO)-66-NH2. The benzene conversion (XB) of x%-Pt/UiO-66-NH2-R under dry conditions (175 °C) was 23% (x = 0.2%) < 52% (x = 0.8%) < 100% (x = 1.5%): 'R' suffix denotes reduction pretreatment using a hydrogen (10 vol %) and nitrogen mixture at 300 °C for the generation of metallic Pt (Pt0) sites and simultaneous partial MOF decomposition into carbon- and nitrogen-loaded zirconium dioxide. The prominent role of reduction pretreatment was apparent in benzene oxidation as 1.5%-Pt/UiO-66-NH2 did not exhibit catalytic activity below 175 °C (dry condition). The promotional role of moisture in benzene oxidation by 1.5%-Pt/UiO-66-NH2-R was evident with a rise in the steady-state reaction rate (r) at 110 °C (21 kPa molecular oxygen (O2)) from 1.3 × 10-3 to 5.0 × 10-3 μmol g-1 s-1 as the water (H2O) partial pressure increased from 0 to 1.88 kPa. In contrast, the activity was lowered with increasing RH due to catalyst poisoning by excess moisture (r (110 °C) of 6.6 × 10-04 μmol g-1 s-1 at 2.83 kPa H2O (21 kPa O2)). Kinetic modeling suggests that XB proceeds through the Langmuir-Hinshelwood mechanism on the Pt/UiO-66-NH2-R surface (dissociative O2 chemisorption and the involvement of two oxygen species in benzene oxidation). According to the density functional theory simulation, the carbon and nitrogen impurities are to make the first XB step (i.e., hydrogen migration from the benzene molecule to the substrate) energetically favorable. The second hydrogen atom from the benzene molecule is also extracted effectively, while the oxygen derived from O2 facilitates further XB. The Pt0 sites dissociate the O2 and H2O molecules, while the product of the latter, i.e., free hydrogen and hydroxyl, makes the subsequent XB steps energetically favorable.

Positronium lifetime measurement using a clinical PET system for tumor hypoxia identification

Positronium (Ps) imaging has been recently spotlighted for its potential applications in nuclear medicine. Among them, we expect that the Ps lifetime can be a sensor for tumor hypoxia; hence, we are aiming to achieve Ps lifetime imaging, Quantum PET (Q-PET), based on whole gamma imaging (WGI). A linear correlation was previously shown between the Ps lifetime and the oxygen partial pressure (pO2) in water by using a bench-top detector system. However, nobody has ever measured the Ps lifetimes for pO2 values of 10 mmHg and 40 mmHg, corresponding to hypoxic cancer cells and healthy tissue cells, respectively. Therefore, this study aimed at measuring the Ps lifetime of 22Na solutions each having such tiny differences in the pO2 value. For possible extension toward imaging, we used VRAIN, a clinical brain-dedicated time-of-flight (TOF) PET system. The pO2 values of the 22Na solutions were adjusted to 0, 10, 40, and 160 (air) mmHg by adjusting the inflow of nitrogen gas. Each 22Na solution was placed near the center of the detector geometry and measured. Three different energy windows (EWs) for 1275 keV gamma detection were compared (1100–1500 keV, 900–1500 keV and 700–1500 keV). Due to the increased number of events, the error values (σ: standard deviation) of the calculated Ps lifetimes decreased as the EW widened. The optimized EW was determined to be 700–1500 keV, which showed the lowest error value. The Ps lifetime values gradually became shorter as the pO2 values increased. The Ps lifetime values at 0 mmHg and 160 mmHg were 1.9389 ± 0.0025 ns and 1.9104 ± 0.0024 ns at the EW of 700–1500 keV. The difference between the lifetime values was more than three times the errors (±3σ), and we considered it was a statistically significant difference. The Ps lifetime values of 10 mmHg and 40 mmHg were 1.9360 ± 0.0026 ns and 1.9291 ± 0.0024 ns at the same condition, and the difference could be distinguished with an error value of more than ± 1σ.

High temperature corrosion and oxide scale formation of nickel in molten NaOH at various basicity levels

The corrosion behavior of alloy Ni 201 in molten sodium hydroxide (NaOH) at 600 °C was investigated at varying basicity levels of the molten NaOH. The ability for Ni 201 to form passivating oxides was investigated after immersion tests varying from 70 to 340 h under atmospheres of argon and argon with different partial pressure of water. Morphology and thicknesses of the corrosion products were characterized by Scanning Electron Microscopy (SEM) and crystallography of the corrosion products by X-ray Diffraction (XRD). Dynamic polarizations were made to investigate the effects of basicity and electrochemical potential.The results showed that Ni 201 corroded at a reduced rate in molten acidic NaOH compared to neutral NaOH due to the formation of NiO. The oxide scales formed on Ni 201 in acidic NaOH were shown to grow non-parabolically and did not result in full corrosion protection as the oxide scales showed crack development over time.

Stream CO2 emissions are overestimated without consideration of diel water and atmospheric CO2 variability

AbstractStreams are significant emitters of carbon dioxide (CO2) to the atmosphere that are influenced by diel CO2 dynamics. However, we know little about diel CO2 variability within streams, the diel dynamics of CO2 in the air above streams, and the consequences for emission calculations. We studied five pre‐alpine streams by equipping three sites per stream in close proximity (~ 1 km apart) with automatic logging stations that continuously recorded water and air CO2 partial pressures (pCO2) for 2–4 d. All streams and sites showed increased pCO2 at night and decreased pCO2 during the day, however, with fourfold higher diel amplitudes for atmospheric pCO2 compared to the water. Calculating diffusive CO2 fluxes with fixed compared to dynamic measured atmospheric CO2 resulted in negligible to 431% lower estimates. We might thus currently overestimate fluvial CO2 emissions and should include diel water and air CO2 variability to more accurately assess stream CO2 emissions.

Updated climatological mean ΔfCO2 and net sea–air CO2 flux over the global open ocean regions

Abstract. The late Taro Takahashi (Lamont-Doherty Earth Observatory (LDEO), Columbia University) and colleagues provided the first near-global monthly air–sea CO2 flux climatology in Takahashi et al. (1997), based on available surface water partial pressure of CO2 measurements. This product has been a benchmark for uptake of CO2 in the ocean. Several versions have been provided since, with improvements in procedures and large increases in observations, culminating in the authoritative assessment in Takahashi et al. (2009a, b). Here we provide and document the last iteration using a greatly increased dataset (SOCATv2022) and determining fluxes using air–sea partial pressure differences as a climatological reference for the period 1980–2021 (Fay et al., 2023, https://doi.org/10.25921/295g-sn13). The resulting net flux for the open ocean region is estimated as -1.79±0.7 Pg C yr−1, which compares well with other global mean flux estimates. While global flux results are consistent, differences in regional means and seasonal amplitudes are discussed. Consistent with other studies, we find the largest differences in the data-sparse southeast Pacific and Southern Ocean.

High-temperature water vapor sensors based on rare-earth-doped barium cerate

In this novel study, BaCe0.9Y0.1O3–δ, BaCe0.9Eu0.1O3–δ, BaCe0.9Nd0.1O3–δ, and BaCe0.9Dy0.1O3–δ powders were synthesized by the self-combustion method and processed into thick porous films (60–70 % porosity) to investigate the water vapor sensing properties in the 400–700 °C temperature range. All samples showed a stable response value to water vapor in the whole temperature range, expressed as impedance ratio in dry and wet argon (ZdryAr/ZwetAr), and were able to detect 0.03 vol% of water vapor at 550 °C within the impedance range of 103 Ω at 100 Hz. The response values increased with the partial pressure of water and decreased with the temperature, whereas the maximal value of 3.41 reached the BaCe0.9Eu0.1O3–δ sample at 550 °C and p(H2O) = 4.28 kPa. The average response time was several seconds and only slightly changed with the material type and experimental conditions. The recovery time depended on temperature and the ZdryAr/ZwetAr ratio, whereas the increase in the gas flow rate from 100 cm3/min to 200 cm3/min significantly reduced the recovery time for the BaCe0.9Eu0.1O3–δ sample from 230 s to 55 s at 550 °C and p(H2O) = 2.14 kPa. All the samples exhibited stability and a high degree of reversibility after multiple exchanges of wet and dry atmospheres at different temperatures. Yet, their tendency to deteriorate in the presence of CO2 might challenge a potential application in aggressive environments.

Study of the influence of the gas circulation ratio on the production of C5–C18 alkenes in the Fischer–Tropsch synthesis

The process of producing C5+ hydrocarbons, including unsaturated ones, on a zeolite-containing catalyst Сo-Al2O3 /SiO2 /ZSM-5/Al2O3 in flow and flow-circulation modes of operation at a temperature of 250 °C, a pressure of 2.0 MPa, GHSV 1000 h–1 has been studied , H2 /CO ratio = 1.70 in the source gas and circulation ratios of 4, 8 and 16. It was determined that the process indicators (selectivity and productivity for C5+ products) pass through a maximum at a circulation ratio of 8. The use of gas circulation in comparison with flow synthesis mode allows you to regulate the composition of products. An increase in the circulation ratio in the range of 4–16 leads to an increase in the proportion of formed olefins with a hydrocarbon chain length containing 5–20 carbon atoms, from 53.9 wt.% up to 65.7 wt.%. The use of a zeolite-containing catalyst intensifies the formation of C8–C12 alkenes in comparison with the Co-Al2O3 /SiO2 catalyst by 3,3 times – the content increases from 13,5 wt.% up to 44.2 wt.% at similar values of circulation ratio, pressure and H2 /CO ratio = 1.70 in the source gas. It was found that as the circulation ratio increases, the rate of deactivation of the zeolite-containing catalyst decreases, which may be caused by a decrease in the partial pressure of water in the reaction volume.

The Effect of Reduction Pretreatments on the Size of Supported Pt and Pd Nanoparticles Prepared by Strong Electrostatic Adsorption

Supported catalyst synthesis involves pretreatment (drying, reduction) of metal complexes to form metal nanoparticles. This study has been undertaken to explore the effect of reduction temperature, heating rate, and water partial pressure on final particle size of Pt and Pd supported on a total of four carbon and oxide supports. Supported nanoparticles were synthesized by strong electrostatic adsorption (SEA) and dry impregnation (DI); the former method was hypothesized to yield greater nanoparticle stability in thermochemical reducing environments stemming from the strong interaction of the precursor with the support during impregnation. Reduced samples were characterized by in-situ and ex-situ XRD and STEM. The DI-derived samples generally showed an expected increase of particle size with increased reduction temperature, and severe particle coalescence in humid hydrogen, while the SEA-derived samples did not sinter at the elevated reduction temperatures (up to 500 °C) and were remarkably stable in the humid reducing environment.Graphical

Improved control-oriented polarization characteristic modeling for proton exchange membrane water electrolyzer with adaptive hunting game based metaheuristic optimization

The proton exchange membrane water electrolyzer has garnered increasing attention for the large-scale consumption of intermittent renewable energy to produce high purity “green hydrogen”, contributing to the realization of futural carbon neutrality goals. The necessity of control-oriented polarization characteristic modeling with accurate parameter identification remains a significant challenge for the performance evaluation of this type of water electrolyzer. In this paper, an improved control-oriented polarization characteristic modeling for proton exchange membrane water electrolyzer with adaptive hunting game based metaheuristic optimization is proposed to foster collaborative performance enhancement between model improvement and optimizer investigation. The inspiration behind model improvement lies in the nonlinear mechanism linkage between partial pressures of liquid water supplying and anode gas separator. Then, an adaptive hunting game based metaheuristic optimizer is proposed to address the inherent local multi-extremum distribution problem in the established model. Experimental data from a commercial water electrolyzer system is used to validate the effectiveness and practicability of the proposed model-optimizer pair. Detailed comparisons reveal that the improved model development, with its consideration of critical mechanisms, ensures enhanced metaheuristic efficiency in the designed optimizer, both in terms of robust intensity evaluation and efficient quasi-Newton based local search. Statistically, the collaboration performance enhancement reaches up to 7.33% when compared with the other model-optimizer.

The limit of macroscopic homogeneous ice nucleation at the nanoscale.

Nucleation in small volumes of water has garnered renewed interest due to the relevance of pore condensation and freezing under conditions of low partial pressures of water, such as in the upper troposphere. Molecular simulations can in principle provide insight on this process at the molecular scale that is challenging to achieve experimentally. However, there are discrepancies in the literature as to whether the rate in confined systems is enhanced or suppressed relative to bulk water at the same temperature and pressure. In this study, we investigate the extent to which the size of the critical nucleus and the rate at which it grows in thin films of water are affected by the thickness of the film. Our results suggest that nucleation remains bulk-like in films that are barely large enough accommodate a critical nucleus. This conclusion seems robust to the presence of physical confining boundaries. We also discuss the difficulties in unambiguously determining homogeneous nucleation rates in nanoscale systems, owing to the challenges in defining the volume. Our results suggest any impact on a film's thickness on the rate is largely inconsequential for present day experiments.

Enhanced performance of reversible solid oxide fuel cells by densification of Gd0.1Ce0.9O2−δ (GDC) barrier layer/La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF6428)-GDC oxygen-electrode interface

In reversible solid oxide cells (RSOCs) with yttria-stabilized zirconia (YSZ) electrolyte and LSCF6428-based oxygen electrode, a GDC barrier layer is used to prevent inter-diffusion of constituents. However, the performance of such RSOC is limited to the achievable density of GDC at prevailing temperatures of traditional fabrication methods. In this work, the GDC barrier layer was densified via infiltration with polyvinylpyrrolidone (PVP) chelated metal precursors sol and subsequent sintering at 1200 °C. The NiO-YSZ fuel electrode-supported RSOCs with YSZ electrolyte, porous or infiltrated GDC barrier layer and LSCF6428-GDC composite oxygen electrode were fabricated. The RSOCs were analyzed with respect to operating temperature (650–800 °C), water partial pressure at fuel electrode (pH2OFE=0.03–0.5 atm), oxygen partial pressure at oxygen electrode (pO2AE=0.02–0.21 atm) and current loading. The performance evaluation, using current-voltage curve and electrochemical impedance spectroscopy (EIS), showed that the RSOC with an infiltrated GDC barrier layer exhibited a 32–48% increase in performance in fuel cell (FC) mode in the 650–800 °C range, compared to that of the RSOC with a porous GDC layer. Similarly, in electrolysis cell (EC) mode, comparing current density at 1.3 V, chosen as the reference voltage for ease of comparison, based on a thermoneutral voltage of 1.285 V in electrolysis cell (EC) operation at 750 °C[1], the RSOC with an infiltrated GDC barrier layer exhibited values of over 1.2 A∙cm−2, in contrast to the RSOC with a porous GDC barrier layer, which showed 0.8 A∙cm−2. In the case of the RSOC with infiltrated GDC, a durability test was conducted at 750 °C under an EC current density of 1A∙cm−2for 100 h. Our experimental results demonstrate that the RSOC with an infiltrated GDC barrier layer not only exhibits enhanced performance but also maintains stability. Comparative analysis was conducted using Distribution of Relaxation Time (DRT) analysis of Electrochemical Impedance Spectroscopy (EIS) data, to further understand the differences between the cells.

Urbanization and weather dynamics co-dominated the spatial-temporal variation in pCO2 and CO2 fluxes in small montanic rivers draining diverse landscapes

Rivers have been widely reported as important CO2 emitters to the atmosphere. Rapid urbanization has a profound impact on the carbon biogeochemical cycle of rivers, leading to enhanced riverine CO2 evasions. However, it is still unclear whether the spatial–temporal patterns of CO2 emissions in the rivers draining diverse landscapes dominated by urbanization were stable, especially in mountainous areas. This study carried out a two-year investigation of water environmental hydrochemistry in three small mountainous rivers draining urban, suburban and rural landscapes in southwestern China, and CO2 partial pressure (pCO2) and fluxes (fCO2) in surface water were measured using headspace equilibrium method and classical thin boundary layer model. The average pCO2 and fCO2 in the highly urbanized river were of 4783.6 μatm and 700.0 mmol m−2 d−1, conspicuously higher than those in the rural river (1525.9 μatm and 123.2 mmol m−2 d−1), and the suburban river presented a moderate level (3114.2 μatm and 261.2 mmol m−2 d−1). It provided even clearer evidence that watershed urbanization could remarkably enhance riverine CO2 emissions. More importantly, the three rivers presented different longitudinal variations in pCO2, implying diversified spatial patterns of riverine CO2 emissions as a result of urbanization. The urban land can explain 49.6–69.1% of the total spatial variation in pCO2 at the reach scale, indicating that urban land distribution indirectly dominated the longitudinal pattern of riverine pCO2 and fCO2. pCO2 and fCO2 in the three rivers showed similar temporal variability with higher warm-rainy seasons and lower dry seasons, which are significantly controlled by weather dynamics, including monthly temperature and precipitation, but seem to be impervious to watershed urbanization. High temperature-stimulated microorganisms metabolism and riched-CO2 runoff input lead much higher pCO2 in warm-rainy seasons. However, it showed more sensitivity of pCO2 to monthly weather dynamics in urbanized rivers than that in rural rivers, and warm-rainy seasons showed hot moments of CO2 evasion for urban rivers. TOC, DOC, TN, pH and DO were the main controls on pCO2 in the urban and suburban rivers, while only pH and DO were connected with pCO2 in the rural rivers. This indicated differential controls and regulatory processes of pCO2 in the rivers draining diverse landscapes. Furthermore, it suggested that pCO2 calculated by the pH-total alkalinity method would obviously overestimate pCO2 in urban polluted rivers due to the inevitable influence of non-carbonate alkalinity, and thus, a relatively conservative headspace method should be recommended. We highlighted that urbanization and weather dynamics co-dominated the multiformity and uncertainty in spatial–temporal patterns of riverine CO2 evasions, which should be considered when modeling CO2 dynamics in urbanized rivers.

A Revolutionary Approach to Study the Hydrogen Electrode Formation on Different Noble Metals Exposed to Monolayer to Sub-Monolayers Range of Water Using Scanning Kelvin Probe

The inadequacy of modern surface analytical tools to probe the electrochemical double layer (EDL) which governs the catalytic reactions at the interface hampers its in-depth investigation. The reason for this is that the EDL is usually buried under the bulk electrolyte. Though great progress was made in computational modeling the EDL [1,2], at the experimental level similar progress is necessary. One approach is to study the EDL of electrodes covered by electrolyte of nano-scale thickness. Till date, such studies of electrodes covered by electrolyte in the monolayer to sub-monolayer range are scarce, although they were at the center of the birth of electrochemical surface science [3]. Recently the “hydrogen electrode in the dry” concept which was established on palladium surface under dry nitrogen conditions could be a novel approach to understand that facet of the electrode interface activities [4]. The amount of water in this case is confined to just 1-2 monolayers to adsorbed water or even below. Similarly, this approach was successful applied for studying the current-potential relationships for oxygen reduction reaction exposed to different amounts of water ranging from micro meter to monolayers of thickness [5]. In this study, this concept was explored to other noble metal systems such as Pd, Au, Pt, and Ir to understand the hydrogen electrode formation.The present study focuses on hydrogen electrode formation on palladium, iridium, gold, and platinum exposed to inert conditions carried out by scanning Kelvin probe method (see fig) at different hydrogen activities. The activities of hydrogen on the surface was defined by electrochemically (entry side from a=0 to a=0.2) cathodic hydrogen charging method. Firstly, the hydrogen activity of 0.2 was defined on the palladium side by increasing the cathodic potential stepwise by potentiostatic hydrogen charging and measuring the corresponding potential on the exit side by Kelvin probe. This potential on palladium in both wet (>95% rh) and dry (<0.1% rh) conditions follows a 1:1 correlation with the applied entry potential, in accordance with the Nernst equation. Iridium samples show 1:1 in humid conditions, but not in dry conditions, i.e. in presence of a much thinner layer of adsorbed water molecules. At intermediate partial pressures of water, a linear response with slope lower than 1 was observed, that increased with increasing humidity. Whereas, gold samples show no correlation in both wet and dry conditions. Additionally, hydrogen electrode formation on emersed surface was checked to understand the effect of an already existing EDL on the formation of the hydrogen electrode. Also, ellipsometry spectroscopy measurements were carried out to determine the water layer thickness at different partial pressures of water. Overall, the current experimental approach of measuring the potential of these different “dry” electrodes gives more insights about hydrogen electrode formation on noble metals at different hydrogen activities, effect of adsorbed anions, and the effect of different amounts of adsorbed water.

Structures and reactivity of monomeric MoO[formula omitted] moieties supported on ZrO[formula omitted] (111) slab: A DFT study

Supported molybdenum oxides have been widely employed as catalysts for variety of reactions including upgrading of bio-oil via hydrodeoxygenation. Herein, we perform spin-polarized density functional theory calculations to investigate the structures and reactivity of monomeric MoOx species supported on ZrO2 in the absence and presence of hydroxyl(s). Monomeric molybdenum moieties were found to have a mono-oxo (one molybdenyl), di-oxo (two molybdenyls) or a pero-oxo (one molybdenyl and Mo coordinated to a peroxide) configuration. Based on the formation energies, we find that mono-oxo moieties are the most stable among the three. This is also true for the moieties in the presence of hydroxyls on the support. Further, the computed vibrational frequencies of a mono-oxo moiety on a hydroxylated support is a closer match to the peaks observed in experiments. A thermodynamic analysis of the molybdenum oxide moieties on a hydroxylated support indicated that the dehydroxylation is favoured at high temperatures and low partial pressures of water. On adsorption of hydrogen on the oxygen of the support, hydrogen loses its electron and reduces Mo of the monomeric moiety without making an oxygen vacancy. We studied the activity of this reduced site for deoxygenation of anisole to benzene. The overall barriers for this reaction are comparable to previously computed barriers for deoxygenation of bio-oil model compounds on an oxygen-vacant molybdenum oxide surfaces, suggesting that this may be an important mechanistic pathway during a hydro-deoxygenation process.