High Hydrogen Partial Pressure Research Articles

A High-Performance Nanoreactor for Carbon–Oxygen Bond Hydrogenation Reactions Achieved by the Morphology of Nanotube-Assembled Hollow Spheres

Hydrogenation of carbon–oxygen bonds is extensively used in organic synthesis. However, a high partial pressure of hydrogen or the presence of excess hydrogen is usually essential to achieve favorable conversions. In addition, because most hydrogenations are consecutive reactions, the selectivity is difficult to manipulate, leading to an unsatisfactory distribution of products. Herein, a copper silicate nanoreactor with a nanotube-assembled hollow sphere (NAHS) hierarchical structure is proposed as a solution to these problems. In the case of dimethyl oxalate (DMO) hydrogenation, the NAHS nanoreactor achieves remarkable catalytic activity (the yield of ethylene glycol is 95%) and stability (>300 h) when the H2/DMO molar ratio is as low as 20 (in comparison to typical values of 80–200). For further investigation, nanotubes and lamellar-shaped Cu/SiO2 catalysts with similar surface areas of active sites of NAHSs were investigated as contrasts. By a combination of high-pressure hydrogen adsorption and Monte ...

Hydrogen production from solid feedstock by using a nickel membrane reformer

The Heatpipe Reformer technology allows the generation of hydrogen-rich, pressurized synthesis gas from solid feedstock like lignite or biomass. The resulting high hydrogen partial pressure and thus driving force makes it suitable for membrane separation. This work promotes the application of hydrogen permeable membranes as hydrogen separators directly in the reformer. This should allow a high hydrogen yield due the shift of the gasification reactions to the product side when hydrogen is removed continuously. The material of choice for this task is nickel as it combines good hydrogen permeability with good mechanical properties at the operation temperature of biomass gasification of 800°C.The experimental section presents measurements with a bundle of nickel membranes used for the demonstration of the shift of different gas mixtures to the product side by hydrogen removal. Hydrogen removal enhanced CO and CH4 conversion at an operation temperature of 800°C. A high purity of at least 99.9% was achieved by the highly selective solution-diffusion process of the separation. The experimental data was also used for an energy balance of the membrane process to allow a proper membrane layout in terms of membrane area per hydrogen production.As a last step, the membrane bundle was applied directly in the Heatpipe Reformer, an allothermal pressurized gasifier. It produced 200 mlmin−1 of hydrogen and showed no signs of degradation or fouling. This proof of concept showed the suitability of nickel membranes for hydrogen separation under gasification conditions.

An ab initio study of hydroxylated graphane.

Graphene-based derivatives with covalent functionalization and well-defined stoichiometry are highly desirable in view of their application as functional surfaces. Here, we have evaluated by ab initio calculations the energy of formation and the phase diagram of hydroxylated graphane structures, i.e., fully functionalized graphene derivatives coordinated with -H and -OH groups. We compared these structures to different hydrogenated and non-hydrogenated graphene oxide derivatives, with high level of epoxide and hydroxyl groups functionalization. Based on our calculations, stable phases of hydroxylated graphane with low and high contents of hydrogen are demonstrated for high oxygen and hydrogen partial pressure, respectively. Stable phases of graphene oxide with a mixed carbon hybridization are also found. Notably, the synthesis of hydroxylated graphane has been recently reported in the literature.

Microbial electrosynthesis of butyrate from carbon dioxide: Production and extraction

To date acetate is the main product of microbial electrosynthesis (MES) from carbon dioxide (CO2). In this work a tubular bioelectrochemical system was used to carry out MES and enhance butyrate production over the other organic products. Batch tests were performed at a fixed cathode potential of −0.8V vs SHE. The reproducibility of the results according to previous experiments was validated in a preliminary test. According to the literature butyrate production could take place by chain elongation reactions at low pH and high hydrogen partial pressure (pH2). During the experiment, CO2 supply was limited to build up pH2 and trigger the production of compounds with a higher degree of reduction. In test 1 butyrate became the predominant end-product, with a concentration of 59.7mMC versus 20.3mMC of acetate, but limitation on CO2 supply resulted in low product titers. CO2 limitation was relaxed in test 2 to increase the bioelectrochemical activity but increase pH2 and promote the production of butyrate, what resulted in the production of 87.5mMC of butyrate and 34.7mMC of acetate. The consumption of ethanol, and the presence of other products in the biocathode (i.e. caproate) suggested that butyrate production took place through chain elongation reactions, likely driven by Megasphaera sueciensis (>39% relative abundance). Extraction and concentration of butyrate was performed by liquid membrane extraction. A concentration phase with 252.4mMC of butyrate was obtained, increasing also butyrate/acetate ratio to 16.4. The results are promising for further research on expanding the product portfolio of MES.

Biohydrogen Production by an Ethanol Type Bacterium Oligosphaera ethanolica

The effects of glucose and products inhibition on the growth of an ethanol type fermentation bacterium Oligosphaera ethanolica 8KG-4T in the phylum Lentisphaerae was investigated. Glucose concentration below 3.8 g L-1 was completely converted, while 30 g L-1 glucose completely inhibited the growth and hydrogen production of the pure culture. The addition of hydrogenotrophic methanogen Methanospirllum hungatei improved approximately 2 times higher growth rate compared to that of the pure culture. The coculture with M. hungatei tolerated up to 38 g L-1 glucose and 23% of the glucose was consumed. The addition of 20 mM acetate slightly inhibited the growth and hydrogen production, and 200 mM acetate allowed slow growth and hydrogen production of the pure culture. Compared with acetate, ethanol exhibited more significant inhibition on growth and hydrogen production of O. ethanolica. The addition of 100 mM ethanol had 45% inhibition on the growth and glucose consumption during two weeks of incubation. Inhibition gradually increased with increasing ethanol concentration up to 300 mM, when complete inhibition of glucose consumption occurred. Under high hydrogen partial pressure, the growth and glucose consumption of the pure culture was prolonged. The hydrogen potential with carbohydrates was evaluated. The maximum hydrogen yields were 49.6, 55.7 and 30.7 ml H2/g-xylose in 1, 5 and 10 g L-1, respectively.

The Influence of Hydrogen Concentration in Sparging Gas on Hydrogen Production and Consumption via Anaerobic Fermentation

Abstract. The main problem of hydrogen production via anaerobic fermentation is the very low production rate due to severe hydrogen-consuming reactions. Hydrogen partial pressure is a major factor influencing reactions of hydrogen production and consumption. To study the influence of H2 concentration on hydrogen production and consumption, H2 was mixed with N2 at initial concentrations of 20%, 40%, 60%, 80%, and 100% to sparge the fermentation broth. The results showed that H2 concentration had significant influence on these reactions, changing the pathway of glucose hydrolysis and acidification. With increasing H2 concentration in the sparging gas, the pathway of glucose acidogenesis hydrolysis transferred from mixed acid production (propionic acid and butyric acid) to propionic acid production. When the H2 concentration was 20% and the N2 concentration was 80%, the production rates of propionic acid and butyric acid were highest (0.76 and 0.41 mmol mmol-1 glucose consumed d-1, respectively) and accounted for 100% of total soluble metabolic products. When the H2 concentration in the sparging gas was more than 40%, the production rates of propionic acid and butyric acid decreased, and glucose degradation was inhibited. In addition, due to propionic acid type fermentation and high hydrogen partial pressure, acetogenesis was not feasible thermodynamically, resulting in a low production rate of H2 during the whole fermentation process. When the initial H2 concentration in the sparging gas was 40%, the production rate of H2 was highest, (0.42 mol mol-1 glucose consumed d-1). In addition, the theoretical production rate of H2 calculated from the production of soluble metabolic products was less than the actual value. This resulted from the co-existence of homoacetogenesis and acetic acid oxidation due to the bidirectionality of homoacetogens, establishing a new symbiotic relationship. Keywords: Anaerobic fermentation, Hydrogen consumption, Hydrogen partial pressure, Hydrogen production, Sparging.

A Novel F420-dependent Thioredoxin Reductase Gated by Low Potential FAD: A TOOL FOR REDOX REGULATION IN AN ANAEROBE

A recent report suggested that the thioredoxin-dependent metabolic regulation, which is widespread in all domains of life, existed in methanogenic archaea about 3.5 billion years ago. We now show that the respective electron delivery enzyme (thioredoxin reductase, TrxR), although structurally similar to flavin-containing NADPH-dependent TrxRs (NTR), lacked an NADPH-binding site and was dependent on reduced coenzyme F420 (F420H2), a stronger reductant with a mid-point redox potential (E'0) of -360 mV; E'0 of NAD(P)H is -320 mV. Because F420 is a deazaflavin, this enzyme was named deazaflavin-dependent flavin-containing thioredoxin reductase (DFTR). It transferred electrons from F420H2 to thioredoxin via protein-bound flavin; Km values for thioredoxin and F420H2 were 6.3 and 28.6 μm, respectively. The E'0 of DFTR-bound flavin was approximately -389 mV, making electron transfer from NAD(P)H or F420H2 to flavin endergonic. However, under high partial pressures of hydrogen prevailing on early Earth and present day deep-sea volcanoes, the potential for the F420/F420H2 pair could be as low as -425 mV, making DFTR efficient. The presence of DFTR exclusively in ancient methanogens and mostly in the early Earth environment of deep-sea volcanoes and DFTR's characteristics suggest that the enzyme developed on early Earth and gave rise to NTR. A phylogenetic analysis revealed six more novel-type TrxR groups and suggested that the broader flavin-containing disulfide oxidoreductase family is more diverse than previously considered. The unprecedented structural similarities between an F420-dependent enzyme (DFTR) and an NADPH-dependent enzyme (NTR) brought new thoughts to investigations on F420 systems involved in microbial pathogenesis and antibiotic production.

Octahedral Ni-nanocluster (Ni85) for Efficient and Selective Reduction of Nitric Oxide (NO) to Nitrogen (N2)

Nitric oxide (NO) reduction pathways are systematically studied on a (111) facet of the octahedral nickel (Ni85) nanocluster in the presence/absence of hydrogen. Thermodynamic (reaction free energies) and kinetic (free energy barriers, and temperature dependent reaction rates) parameters are investigated to find out the most favoured reduction pathway for NO reduction. The catalytic activity of the Ni-nanocluster is investigated in greater detail toward the product selectivity (N2 vs. N2O vs. NH3). The previous theoretical (catalyzed by Pt, Pd, Rh and Ir) and experimental reports (catalyzed by Pt, Ag, Pd) show that direct N-O bond dissociation is very much unlikely due to the high-energy barrier but our study shows that the reaction is thermodynamically and kinetically favourable when catalysed by the octahedral Ni-nanocluster. The catalytic activity of the Ni-nanocluster toward NO reduction reaction is very much efficient and selective toward N2 formation even in the presence of hydrogen. However, N2O (one of the major by-products) formation is very much unlikely due to the high activation barrier. Our microkinetic analysis shows that even at high hydrogen partial pressures, the catalyst is very much selective toward N2 formation over NH3.

Effect of hydrogen addition on the deposition of titanium nitride thin films in nitrogen added argon magnetron plasma

The properties and performance of thin films deposited by plasma assisted processes are closely related to their manufacturing techniques and processes. The objective of the current study is to investigate the modification of plasma parameters occurring during hydrogen addition in N2 + Ar magnetron plasma used for titanium nitride thin film deposition, and to correlate the measured properties of the deposited thin film with the bulk plasma parameters of the magnetron discharge. From the Langmuir probe measurements, it was observed that the addition of hydrogen led to a decrease of electron density from 8.6 to 6.2 × (1014 m−3) and a corresponding increase of electron temperature from 6.30 to 6.74 eV. The optical emission spectroscopy study reveals that with addition of hydrogen, the density of argon ions decreases. The various positive ion species involving hydrogen are found to increase with increase of hydrogen partial pressure in the chamber. The thin films deposited were characterized using standard surface diagnostic tools such as x-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), x-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). Although it was possible to deposit thin films of titanium nitride with hydrogen addition in nitrogen added argon magnetron plasma, the quality of the thin films deteriorates with higher hydrogen partial pressures.

Elucidating the field influence on the energetics of the methane steam reforming reaction: A density functional theory study

To help realize lower operating-temperatures for the highly endothermic Ni-catalytic methane steam reforming (MSR) process, we focused on elucidating the influence of an applied electric field on the energetics of the said reaction. Two aspects were considered in this study: the electric field effects on (i) the adsorption and electronic properties of the MSR-involved species, and (ii) the overall MSR energy profile. Our results show that for Ni-based MSR processes, a positive field strengthens the adsorption of the reactants, promotes product desorption, impedes coke formation, lowers the overall energy profiles and consequently, reduces the temperature requirements for the overall MSR-on-Ni reaction. Based on our phase diagram obtained from first principles, we show that CO can be obtained from the dehydrogenation of COH and CHO at moderate hydrogen partial pressure values with a negative field, while methanol is formed on the surface via hydroxyl oxidation of CH3 at high hydrogen partial pressures and positive field values. This investigation suggests ways to facilitate the MSR reforming reaction in the presence of an electric field and also points toward a number of elementary reactions that need to be considered for establishing microkinetic model studies.

Rapid and catalyst-free van der Waals epitaxy of graphene on hexagonal boron nitride

Recently, hexagonal boron nitride (h-BN) has been shown to act as an ideal substrate to graphene by greatly improving the material transport properties thanks to its atomically flat surface, low interlayer electronic coupling and almost perfect reticular matching [1]. Chemical vapour deposition (CVD) is presently considered the most scalable approach to grow graphene directly on h-BN. However, for the catalyst-free approach, poor control over the shape and crystallinity of the graphene grains and low growth rates are typically reported [2–5]. In this work we investigate the crystallinity of differently shaped grains and identify a path towards a real van der Waals epitaxy of graphene on h-BN by adopting a catalyst-free CVD process. We demonstrate the polycrystalline nature of circular-shaped pads and attribute the stemming of different oriented grains to airborne contamination of the h-BN flakes. We show that single-crystal grains with six-fold symmetry can be obtained by adopting high hydrogen partial pressures during growth. Notably, growth rates as high as 100 nm/min are obtained by optimizing growth temperature and pressure. The possibility of synthesizing single-crystal graphene on h-BN with appreciable growth rates by adopting a simple CVD approach is a step towards an increased accessibility of this promising van der Waals heterostructure.

Structure and stability of clean and adsorbate covered intermetallic PdGa surfaces: A first principles study

In this work, using ab initio density functional theory based calculations we have studied the structure and stability of clean and hydrogen covered low-indexed (100) and (110) surfaces of intermetallic PdGa. We find that for the clean (100) surface, the stability of the surface terminations is independent of the surface preparation condition. On the contrary, at least three different types of surface terminations can be stabilized for the (110) surface by tuning the surface preparation conditions. Upon adsorbing molecular and atomic hydrogen on these surfaces, we find that: (a) at 450K hydrogen adsorbs only at high hydrogen partial pressure and (b) the relative stability of the different surface terminations is unaffected by the presence of adsorbates.

Hydrogen permeation through palladium membranes and inhibition by carbon monoxide, carbon dioxide, and steam

Palladium membranes are being developed for the separation of hydrogen from syngas in industrial applications. However, syngas constituents carbon monoxide, carbon dioxide, and steam are known to adsorb at the membrane surface and inhibit the permeation of hydrogen. The current study combines an experimental study and modelling approach in order to investigate and quantify the inhibition effects. Experiments have been performed with a 2.8μm thick palladium membrane (surface area 174cm2) on a tubular alumina support, including systematic variation of the concentrations of carbon monoxide, carbon dioxide, and steam at 22bar total pressure and 350–450°C. Carbon monoxide and steam inhibit hydrogen permeation. No significant effect has been found for carbon dioxide, except indirectly by carbon monoxide produced in situ from carbon dioxide. A constriction resistance model has been derived, explicitly relating the decrease in surface coverage by adsorbed hydrogen to the ensuing decrease in transmembrane flux. Very high surface coverages by inhibiting species θi>0.995 are predicted. The results highlight that inhibition effects are greatly reduced at high hydrogen partial pressures due to competitive adsorption. Due to the lateral diffusion of permeating hydrogen atoms in the metallic membrane, the thickness of the palladium membrane strongly determines the extent to which surface coverage by non-hydrogen species causes a decrease in hydrogen transmembrane flux. Depending on the operating conditions, membranes are predicted to have an optimal minimum thickness below which an increased intrinsic permeance is offset by an increased impact of inhibition.

Biomass hydrolysis inhibition at high hydrogen partial pressure in solid-state anaerobic digestion

In solid-state anaerobic digestion, so-called ss-AD, biogas production is inhibited at high total solids contents. Such inhibition is likely caused by a slow diffusion of dissolved reaction intermediates that locally accumulate. In this study, we investigated the effect of H2 and CO2 partial pressure on ss-AD. Partial pressure of H2 and/or CO2 was artificially fixed, from 0 to 1 557mbars for H2 and from 0 to 427mbars for CO2. High partial pressure of H2 showed a significant effect on methanogenesis, while CO2 had no impact. At high PH2, the overall substrate degradation decreased with no accumulation of metabolites from acidogenic bacteria, indicating that the hydrolytic activity was specifically impacted. Interestingly, such inhibition did not occur when CO2 was added with H2. This result suggests that CO2 gas transfer is probably a key factor in ss-AD from biomass.

Evidence of syntrophic acetate oxidation by Spirochaetes during anaerobic methane production

To search for evidence of syntrophic acetate oxidation by cluster II Spirochaetes with hydrogenotrophic methanogens, batch reactors seeded with five different anaerobic sludge samples supplemented with acetate as the sole carbon source were operated anaerobically. The changes in abundance of the cluster II Spirochaetes, two groups of acetoclastic methanogens (Methanosaetaceae and Methanosarcinaceae), and two groups of hydrogenotrophic methanogens (Methanomicrobiales and Methanobacteriales) in the reactors were assessed using qPCR targeting the 16S rRNA genes of each group. Increase in the cluster II Spirochaetes (9.0±0.4-fold) was positively correlated with increase in hydrogenotrophic methanogens, especially Methanomicrobiales (5.6±1.0-fold), but not with acetoclastic methanogens. In addition, the activity of the cluster II Spirochaetes decreased (4.6±0.1-fold) in response to high hydrogen partial pressure, but their activity was restored after consumption of hydrogen by the hydrogenotrophic methanogens. These results strongly suggest that the cluster II Spirochaetes are involved in syntrophic acetate oxidation in anaerobic digesters.

Technical and economic feasibility of adapting an industrial steam reforming unit for production of hydrogen from renewable ethanol

The steam reforming of ethanol could be a solution to reduce CO2 emissions in industrial hydrogen plants, given its renewable character. To adapt this process into the Repsol refineries, a scheme with pre-reforming section has been considered. Ethanol steam reforming has been tested using a commercial nickel catalyst at industrial pre-reforming conditions, particularly at high pressure (25 bar). In addition, a new “Ethanol-to-shift” process using a commercial water gas shift (WGS) catalyst to convert ethanol is proposed.The pilot plant tests show that complete ethanol conversion is reached at low space velocity (WHSV < 1 h−1) or at high temperature (400 °C), with predominance of methane formation. The commercial nickel catalyst is stable for at least 530 h due to the use of high hydrogen partial pressure to inhibit coke deposition. Then, it has been proven that it is a technically feasible process for industrial scale. The WGS catalyst (Fe–Cr), however, does not achieve full conversion even at very high temperature.An economic analysis indicates that the steam reforming process would only be viable with a low price of ethanol (around 350 €/t), which is far from the current market price.

An investigation on the effect of high partial pressure of hydrogen on the nanocrystalline structure of silicon carbide thin films prepared by radio-frequency magnetron sputtering

The aim of the study reported in this paper is to investigate the role of the high partial pressure of hydrogen introduced during the growth of nanocrystalline silicon carbide thin films (nc-SiC:H). For this purpose, we report the preparation as well as spectroscopic studies of four series of nc-SiC:H obtained by radio-frequency magnetron sputtering at high partial pressure of hydrogen by varying the percentage of H2 in the gas mixture from 70% to 100% at common substrate temperature (TS=500°C). The effects of the dilution on the structural changes and the chemical bonding of the different series have been studied using Fourier transform infrared and Raman spectroscopy. For this range of hydrogen dilution, two groups of films were obtained. The first group is characterized by the dominance of the crystalline phase and the second by a dominance of the amorphous phase. This result confirms the multiphase structure of the grown nc-SiC:H thin films by the coexistence of the SiC network, carbon-like and silicon-like clusters. Furthermore, infrared results show that the SiC bond is the dominant absorption peak and the carbon atom is preferentially bonded to silicon. The maximum value obtained of the crystalline fraction is about 77%, which is relatively important compared to other results obtained by other techniques. In addition, the concentration of CHn bonds was found to be lower than that of SiHn for all series.Raman measurements revealed that the crystallization occurs in all series even at 100% H2 dilution suggesting that high partial pressure of hydrogen favors the formation of silicon nanocrystallites (nc-Si). The absence of both the longitudinal acoustic band and the transverse optical band indicate that the crystalline phase is dominant.

Effect of Hydrogen in Size-Limited Growth of Graphene by Atmospheric Pressure Chemical Vapor Deposition

Analysis of graphene domain synthesis explains the main graphene growth process. Size-limited graphene growth caused by hydrogen is studied to achieve efficient graphene synthesis. Graphene synthesis on Cu foils via the chemical vapor deposition method using methane as carbon source is limited by high hydrogen concentration. Results indicate that hydrogen affects graphene nucleation, the growth rate, and the final domain size. Considering the role of hydrogen as both activator and etching reagent, we build a model to explain the cause of this low graphene growth rate for high hydrogen partial pressure. A two-step method is proposed to control the graphene nucleation and growth rate separately. Half the time is required to obtain similar domain size compared with single-step synthesis, indicating improved graphene synthesis efficiency. The change of the partial pressure and transmission time between the two steps is a factor that cannot be ignored to control the graphene growth.

Influence of Reaction Parameters on the First Principles Reaction Rate Modeling of a Platinum and Vanadium Catalyzed Nitro Reduction

This paper describes the influence of key reaction parameters on the development of a rate model which can be used to forecast starting material conversion independent of scale. A nitro reduction was examined via first principles reaction progress modeling. The reaction parameters, most notably hydrogen partial pressure and agitation rate, influenced the choice of rate model. At lower hydrogen partial pressures, the reaction rate was influenced by gas to liquid mass transfer, hydrogen pore diffusion, and the rate of the surface reaction during the course of the overall reaction. No single model could be generated to explain the rate observations at lower hydrogen partial pressures. At higher hydrogen partial pressures, a kinetic reaction model was used to generate an equation to forecast the substrate concentration as a function of time and reaction parameters. This reaction model was confirmed to be independent of scale provided that the mass transfer coefficient exceeded a minimum threshold value. The m...

Sulphide mineral reactions in clay-rich rock induced by high hydrogen pressure. Application to disturbed or natural settings up to 250 °C and 30 bar

As abiotic hydrogen redox reactivity is kinetically limited, most of the possible redox reactions induced by hydrogen remain insignificant at low temperature, even at a geologic time scale. This is the case for sulphate and carbonate reduction, but some H2 induced redox reactions may be significant at low temperature conditions, in particular the pyrite reduction into pyrrhotite. In this experimental study, the geochemical impact of hydrogen in a clay-rich rock containing 1–2wt.% framboidal pyrite is evaluated under mid-hydrothermal condition (90 to 250°C) and with hydrogen partial pressure ranging from 3 to 30bar. This study demonstrates that the main geochemical perturbation induced by hydrogen in a claystone host-rock formation is the destabilisation of pyrite, which leads to the production of sulphide. Two different reaction mechanisms can be distinguished as a function of temperature and hydrogen pressure. When temperature is lower than 150°C and the hydrogen partial pressure is below 6bar, pyrite solubility controls the sulphide concentration at low values. By contrast, at higher temperature or at higher hydrogen partial pressure, the rate and extent of the reaction are driven by pyrrhotite precipitation. A complete replacement of pyrite into pyrrhotite occurs within a short interval of time at T>90°C and P(H2)>10bar. The pH of the media is also a critical parameter controlling the extent of the reaction as alkaline conditions may promote pyrrhotite precipitation at lower temperature and hydrogen pressure. These conclusions have a direct application in the safety assessment of nuclear waste repositories, but may also be extended to other contexts such as the underground storage of hydrogen, and, given the wide range of temperatures used in the experiments, can even be applied to hydrothermal-hosted submarine systems where hydrogen is naturally produced by olivine serpentinization.