H2 Partial Pressure Research Articles

Engineering stable active Cu0-Cuσ+ sites via in situ surface reconstruction over Cu/CeO2 catalyst for the hydrogenation of carbon-oxygen bonds

Cu/CeO2 catalysts have been wildly used for the hydrogenation of carbon-oxygen bonds reactions due to their excellent catalytic performance. It is widely accepted that the Cu0-Cuσ+ interface could be the primary active sites during the reaction, but how to form and maintain highly dispersed such species remains challenging under the reaction conditions of high temperature and H2 partial pressure. In this work, we proposed an effective strategy to construct stable Cu0-Cuσ+ interface sites over the Cu/CeO2 catalysts by in situ N2O-involved oxidation treatment. It was evidenced that the sequential oxidation-reduction by N2O and H2 broke the grain boundaries of Cu nanoparticles and therefore facilitated the formation of the active Cu0-Cuσ+ sites, leading to an enhanced activity. However, a rapid deactivation was observed after this temporary enhancement due to the aggregation of copper species. We further introduced some additional cerium species during catalyst preparation. The 16Cu/CeO2-1Ce catalyst prepared by post-impregnation of 1 wt% cerium achieved a stable performance of 90.6% MA conversion after the N2O-involved oxidation treatment, which was much higher than that of 26.2% over the 16Cu/CeO2 catalyst. It was demonstrated that the strong interaction between the formed CeO2 and reconstructed Cu species prevented the metal sintering and increased the fraction of the Cu0-Cuσ+ sites. This method that combined in situ N2O-involved oxidation treatment and cerium modification may offer potential in engineering stable active sites for high-performance catalysts in the hydrogenation of carbon-oxygen bonds.

Anaerobic dihydrogen consumption of nutrient-limited aquifer sediment microbial communities examined by stable isotope analysis

ABSTRACT The biogeochemical consequences of dihydrogen (H2) underground storage in porous aquifers are poorly understood. Here, the effects of nutrient limitations on anaerobic H2 oxidation of an aquifer microbial community in sediment microcosms were determined in order to evaluate possible responses to high H2 partial pressures. Hydrogen isotope analyses of H2 yielded isotope depletion in all biotic setups indicating microbial H2 consumption. Carbon isotope analyses of carbon dioxide (CO2) showed isotope enrichment in all H2-supplemented biotic setups indicating H2-dependent consumption of CO2 by methanogens or homoacetogens. Homoacetogenesis was indicated by the detection of acetate and formate. Consumption of CO2 and H2 varied along the differently nutrient-amended setups, as did the onset of methane production. Plotting carbon against hydrogen isotope signatures of CH4 indicated that CH4 was produced hydrogenotrophically and fermentatively. The putative hydrogenotrophic Methanobacterium sp. was the dominant methanogen. Most abundant phylotypes belonged to typical ferric iron reducers, indicating that besides CO2, Fe(III) was an important electron acceptor. In summary, our study provides evidence for the adaptability of subsurface microbial communities under different nutrient-deficient conditions to elevated H2 partial pressures.

Research on V2O3 preparation by stepwise catalysis solution-phase hydrogen reduction from shale V-rich solution

Solution-phase hydrogen reduction reaction (SpHR) was introduced to overcome the problems of ammonia nitrogen pollution, high energy consumption, and long production flow of traditional reduction roasting. From φ-pH diagram, the increase of reaction temperature, vanadium concentration and H2 partial pressure is beneficial to the preparation of V2O3. High purity V2O3 could not be prepared by single addition catalysis and synchronous catalysis in SpHR process, and the Ni + NiSO4 stepwise catalysis can effectively eliminate the problem of nickel wrapped in vanadium precipitates during the precipitation process. Under the optimal conditions of Ni + NiSO4 stepwise catalysis of nickel powder dosage 5 g/L, NiSO4 dosage 8 g/L, nickel powder catalytic time 90 min, NiSO4 catalytic time 30 min, the precipitate with nickel content 11.15 % was obtained, and Ni catalyst can be separated by magnetic separation with nickel recovery ratio of 99.52 %. Ni2+ in SpHR precipitates to Ni(OH)2 and then disintegrates and reduces to Ni, and Ni + NiSO4 stepwise catalysis includes two steps of pre-catalysis and secondary catalysis. After fourth cyclic experiment, the vanadium content of V2O3 product obtained was 67.46 % (i.e. 99.21 % V2O3 purity met the requirements of GB 40301–2021), the total recovery was 98.68 %, and the vanadium loss ratio was stable at 1.22 %. The process of SpHR has no ammonia nitrogen wastewater or waste gas discharge, and the price of V2O3 with purity more than 99 % on the market is about 260,000 ¥/t. The stepwise catalysis SpHR has remarkable environmental benefits and economic benefits.

Light-driven ammonia synthesis under mild conditions using lithium hydride.

Photon-driven chemical processes are usually mediated by oxides, nitrides and sulfides whose photo-conversion efficiency is limited by charge carrier recombination. Here we show that lithium hydride undergoes photolysis upon ultraviolet illumination to yield long-lived photon-generated electrons residing in hydrogen vacancies, known as F centres. We demonstrate that photon-driven dehydrogenation and dark rehydrogenation over lithium hydride can be fulfilled reversibly at room temperature, which is about 600 K lower than the corresponding thermal process. As light-driven F centre generation could provide an alternative approach to charge carrier separation to favour chemical transformations that are kinetically or thermodynamically challenging, we show that light-activated lithium hydride cleaves the N≡N triple bond to form a N-H bond under mild conditions. Co-feeding a N2/H2 mixture with low H2 partial pressure leads to photocatalytic ammonia formation at near ambient conditions. This work provides insights into the development of advanced materials and processes for light harvesting and conversion.

Catalytic Metal‐Gated Nano‐Sheet Field Effect Transistor and Nano‐Sheet Tunnel Field Effect Transistor Based Hydrogen Gas Sensor‐ A Design Perspective

AbstractIn this work, for the first time, the catalytic metal gate (CMG) based nanosheet Field Effect Transistor (NSFET) and nanosheet Tunnel Field Effect Transistor (NSTFET) are proposed for nano‐scale device dimensions, and the different design aspects of catalytic metal gate (CMG)‐based transduction for Hydrogen (H2) sensing are extensively investigated using numerical device simulation. The influence of applied biasing conditions and structural parameter specifications on the sensing performance of CMG‐NSFET and CMG‐NSTFET are methodically analyzed from device electrostatics and carrier transport mechanisms. Furthermore, the relative maximum sensitivity variations with H2‐partial pressure, ambient temperature, and the presence of ambient Oxygen are comprehensively studied. The study reveals that compared to CMG‐NSFET, the CMG‐NSTFET demonstrates a high immunity against bias and doping variations, with a notably higher (> 50%) sensitivity within low H2 partial pressures (10−15 – 10−10 Torr). Next, the sensing performance of CMG‐NSTFET is systematically optimized through a band‐gap and gate‐stack engineering approach, leading to a 180% to 650% sensitivity improvement from lower (10−15 Torr) to higher (10−5 Torr) range of H2 partial pressure. Finally, the performance of optimized CMG‐NSFET and CMG‐NSTFET are extensively benchmarked against other reported nanostructured CMG‐FET and TFET‐based H2 gas sensors, exhibiting a notably higher sensitivity in the proposed sensors.

Efficient Low‐temperature Hydrogen Production by Electrochemical‐assisted Methanol Steam Reforming

AbstractMethanol steam reforming (MSR) provides an alternative way for efficient production and safe transportation of hydrogen but requires harsh conditions and complicated purification processes. In this work, an efficient electrochemical‐assisted MSR reaction for pure H2 production at lower temperature (~140 °C) is developed by coupling the electrocatalysis reaction into the MSR in a polymer electrolyte membrane electrolysis reactor. By electrochemically assisted, the two critical steps including the methanol dehydrogenation and water‐gas shift reaction are accelerated, which is attributed to decreasing the methanol dehydrogenation energy and promoting the dissociation of H2O to OH* by the applied potential. Furthermore, the reduced H2 partial pressure by the hydrogen oxidation and reduction process further promotes MSR. The combination of these advantages not only efficiently decreases the MSR temperature but also achieves the high rate of hydrogen production of 505 mmol H2 g Pt−1 h−1 with exceptionally high H2 selectivity (99 %) at 180 °C and a low voltage (0.4 V), and the productivity is about 30‐fold than that of traditional MSR. This study opens up a new avenue to design novel electrolysis cells for hydrogen production.

Efficient Low-temperature Hydrogen Production by Electrochemical-assisted Methanol Steam Reforming.

Methanol steam reforming (MSR) provides an alternative way for efficient production and safe transportation of hydrogen but requires harsh conditions and complicated purification processes. In this work, an efficient electrochemical-assisted MSR reaction for pure H2 production at lower temperature (~140 °C) is developed by coupling the electrocatalysis reaction into the MSR in a polymer electrolyte membrane electrolysis reactor. By electrochemically assisted, the two critical steps including the methanol dehydrogenation and water-gas shift reaction are accelerated, which is attributed to decreasing the methanol dehydrogenation energy and promoting the dissociation of H2 O to OH* by the applied potential. Furthermore, the reduced H2 partial pressure by the hydrogen oxidation and reduction process further promotes MSR. The combination of these advantages not only efficiently decreases the MSR temperature but also achieves the high rate of hydrogen production of 505 mmol H2 g Pt -1 h-1 with exceptionally high H2 selectivity (99 %) at 180 °C and a low voltage (0.4 V), and the productivity is about 30-fold than that of traditional MSR. This study opens up a new avenue to design novel electrolysis cells for hydrogen production.

Fundamentals of Gas Permeation in PEM Water Electrolyzers Operated at High Pressures

The high cost of clean hydrogen (H2) remains a major hurdle to drive large scale adoption and achieve zero emission goals. For large scale deployment using low cost renewable electricity, polymer electrolyte membrane water electrolyzers (PEMWEs) show high potential based on their high faradaic efficiency and high operating current density. Operation of PEMWEs at high differential pressures can significantly reduce the per kilogram price of H2 and increase the overall energy efficiency while also eliminating the necessity to compress H2 during storage and transportation.1 Thinner membranes reduce the voltage losses from ohmic resistance and achieve high faradaic efficiencies. However, a higher crossover of hydrogen from the cathode through the membrane to the anode is observed for thinner membranes at high cathode operating pressures. As Ir/IrO2 (anode catalyst) is inefficient in oxidizing H2 1, this increased crossover of H2 not only reduces the PEMWE efficiency but also results in formation of an explosive gas mixture. Some initial studies also suggest critical effect of crossover H2 on durability of the anode catalyst. Addition of a gas recombination catalyst (GRC) in the membrane is a highly useful strategy to oxidize permeating H2 and prevent the formation of such explosive gas mixtures. Despite the importance, fundamentals of gas recombination reaction mechanism along with the effects of GRC and its properties on it is not well established.In this study, effect of the GRC and its properties (loading, particle size, distribution, and location) on H2 permeation will be elucidated using a high pressure (up to 30 bar) electrolyzer setup coupled with online gas chromatography (Figure 1). The role of H2 and O2 partial pressures and operating current densities (simulated by varying the flow rates) will also be closely examined. Effects caused by the addition of anode and cathode catalyst layers on H2 permeation will also be presented in addition to long-term membrane durability results.Figure 1. Decreased H2 permeation rate observed with GRC (Membrane A) at different cathode (H2) operating pressures at 80 °C using ex-situ electrolyzer setup. Deactivation of the GRC reaction is observed when switching the anode gas from O2 to N2. Figure 1

Excessive Ethanol Oxidation Versus Efficient Chain Elongation Processes

PurposeChain elongation is a metabolic feature that consists of the elongation of short-chain fatty acids to longer and more valuable acids when ethanol is available. To lower the operational costs, the process can also be performed using mixed microbial cultures. However, certain microorganisms in the mixed cultures can use the ethanol provided in competing reactions, which is usually termed excessive ethanol oxidation (EEO). Although minimizing ethanol use is essential, there is a lack of studies analyzing the extent, causes, and solutions to excessive ethanol oxidation processes.MethodsTo address this knowledge gap, ethanol, and acetic acid mixtures, at a molar ratio of 5 to 2, were fermented, and the following were analyzed: the fermentation profile at different (1) pH and (2) headspace gas compositions, (3) a 16S analysis of the headspace gas composition fermentations, and (4) a thermodynamic analysis of the reactions involved.Results and Conclusions: All fermentations, except the ones at the lowest pH (5.3), exhibited a significant EEO activity that reduced the yield of chain-elongated products. It was demonstrated that neither the inhibition of methanogenic activity nor the increased H2 partial pressure is an efficient method to inhibit EEO. It was also shown that CO2 can act as an electron acceptor for EEO, promoting the growth of acetogenic bacteria. In the absence of CO2, sulfate was used as an electron acceptor by sulfate-reducing bacteria to facilitate EEO. Methods such as low pH operation with in-line extraction, and the use of alternative sulfur salts, are proposed to increase the ethanol use efficiency in chain elongation processes.Graphical

Rates and reversibility of CO2 hydrogenation on Cu-based catalysts

Kinetics of reaction pathways involved in the conversion of CO2 to methanol and CO on Cu/ZnO/Al2O3 are resolved using in situ chemical titration, steady-state kinetic measurements, and mathematical formalisms for reversibility to probe salient species governing methanol selectivity and yield during CO2 hydrogenation. Across a range of H2:CO2 = 1:1 to H2:CO2 = 80.5:1, active site density determined from in situ chlorine uptake remained invariant; hence, observed trends in rates can be interpreted as only arising from reaction kinetics and not from changing active site density. Kinetic and thermodynamic contributions to rates are decoupled to evaluate forward and reverse rates of methanol synthesis and reverse water-gas shift (RWGS) reactions. These kinetic analyses show that the forward rates of methanol synthesis exhibit persistent first order dependence on hydrogen pressure and are inhibited by water more significantly than the forward rates of RWGS. In contrast, the reverse rates of methanol synthesis and RWGS are both inhibited by H2. Consequently, without any modifications to the Cu/ZnO/Al2O3 catalyst formulation, methanol selectivity can be increased to > 80 % by increasing inlet H2 partial pressure and methanol yield can be enhanced by ∼ 20 % by adding water adsorbents even under conditions far from equilibrium. The kinetic treatments presented herein demonstrate a dearth of H* species during catalysis, provide thermodynamic constraints precluding sequential RWGS and CO hydrogenation as the pathway for methanol synthesis, reveal PH2 and PH2O as salient in determining methanol selectivity and yield by impacting both the forward and reverse rates of CO2 hydrogenation on Cu/ZnO/Al2O3, and explicate the fundamentals of novel sorption-enhanced methanol synthesis, which not only alleviates equilibrium constraints but also alters the intrinsic rate at which the system approaches equilibrium.

Hydrogen-Driven Microbial Redox Reactions in Deep Geosystems

In the subsurface, biotic and abiotic processes can generate and consume hydrogen. Hydrogen has a low reduction potential and is thus a highly energetic electron donor when involved in sulfate, carbon dioxide or ferric iron reduction. Although known as important drivers for the deep biosphere, the contributions of different processes to hydrogen turnover in different geosystems still are not well understood. In context with the ongoing transformation to renewable energy resources, underground H2 storage (UHS) in deep porous or salt cavern systems came into focus. In situ microbial and geochemical reactions that consume H2 are highly relevant topics in deep biosphere research, and also are still a major uncertainty during UHS. Consequently, we studied the potential microbial hydrogen oxidation rates – combined with the possible production of metabolic products like H2S, acetic acid or CH4 - in formation fluids from natural gas fields and salt caverns, thereby considering the importance of in situ pressure and temperature conditions, fluid chemistry and mineral composition. In addition, more defined experiments were conducted with selected pure cultures representing important metabolic groups of deep biosphere microorganisms. Several original formation fluids showed immediate H2 consumption. Microorganisms oxidized hydrogen at relevant in situ pressure conditions (up to 100 bar) and tolerated dynamically changing pressure and temperature conditions. The microbial hydrogen oxidation rate was strongly dependent on H2 partial pressures and the availability of e.g., sulfate as a terminal electron acceptor. High-throughput sequencing of 16S rRNA gene amplicons indicated hydrogen oxidation by sulfate reducing bacteria to be the presumed process in the studied porous rock reservoir fluids. In addition, hydrogen turnover by methanogenic and acetogenic as well as iron-reducing microorganisms was investigated. Also, the importance of biotic reactions in relation to abiotic hydrogen turnover processes at mineral surfaces will be discussed.

Rumen Lachnospiraceae isolate NK3A20 exhibits metabolic flexibility in response to substrate and coculture with a methanogen.

Hydrogen (H2) is the primary electron donor for methane formation in ruminants, but the H2-producing organisms involved are largely uncharacterized. This work integrated studies of microbial physiology and genomics to characterize rumen bacterial isolate NK3A20 of the family Lachnospiraceae. Isolate NK3A20 was the first recognized isolate of the NK3A20 group, which is among the ten most abundant bacterial genera in 16S rRNA gene surveys of rumen microbiota. NK3A20 produced acetate, butyrate, H2, and formate from glucose. The end product ratios varied when grown with different substrates and at different H2 partial pressures. NK3A20 produced butyrate as a major product using glucose or under high H2 partial pressures and switched to mainly acetate in the presence of galacturonic acid (an oxidized sugar) or in coculture with a methanogen. Growth with galacturonic acid was faster at elevated H2 concentrations, while elevated H2 slowed growth with glucose. Genome analyses revealed the presence of multiple hydrogenases including a membrane-bound Ech hydrogenase, an electron bifurcating butyryl-CoA dehydrogenase (Bcd-Etf), and an Rnf complex that may be involved in modulating the observed metabolic pathway changes, providing insight into H2 formation in the rumen. IMPORTANCE The genus-level NK3A20 group is one of the ten most abundant genera of rumen bacteria. Like most of the rumen bacteria that produce the hydrogen that is converted to methane in the rumen, it is understudied, without any previously characterized isolates. We investigated isolate NK3A20, a cultured member of this genus, and showed that it modulates hydrogen production in response to its growth substrates and the hydrogen concentration in its environment. Low-hydrogen concentrations stimulated hydrogen formation, while high concentrations inhibited its formation and shifted the fermentation to more reduced organic acid products. We found that growth on uronic acids, components of certain plant polymers, resulted in low hydrogen yields compared to glucose, which could aid in the selection of low-methane feeds. A better understanding of the major genera that produce hydrogen in the rumen is part of developing strategies to mitigate biogenic methane emitted by livestock agriculture.

Hydrogen production and geometry optimization of ethanol steam reforming combining water gas shift reaction in a crossflow membrane tube reactor

Crossflow tube reactors are a novel design showing excellent H2 production and separation potential. A membrane reactor enables H2 production and separation in the same unit and helps the reaction overcome the equilibrium conversion limitations. This work investigates the performance of ethanol steam reforming (ESR) followed by a water gas shift reaction (WGSR) for H2 production and separation. Crossflow catalytic tubes are designed in ESR and WGSR, and Pd-based membrane tubes are installed behind the WGSR to permeate H2 and enhance the chemical reaction. The effects of inlet temperature, steam-to-ethanol molar (S/E) ratio, and reaction pressure on the system performance are investigated. The results showed that a higher inlet temperature lowers H2 yield, and 600 °C is the most suitable temperature for this system. The S/E ratio at 3 leads to the highest H2 recovery. Higher S/E ratios may increase ethanol conversion but result in more steam in the reactor, lowering the H2 partial pressure and recovery. Two-stage design optimization is performed for the WGSR catalytic membrane tubes. In the first stage with parametric sweep, the H2 yield and recovery are improved by 1.25% and 22.27%, respectively. In the second stage via the Nelder-Mead method, the H2 yield and recovery are further improved by 6.79% and 18.76%, respectively. The two-stage optimization in total intensifies 8.8% H2 yield and 45.22% H2 recovery. A comparison between ESR + WGSR/without membrane and ESR + WGSR + membrane with optimization suggests the H2 yield is substantially lifted by 22.26%, and only 0.34 times catalyst is applied in the system. The results show that an optimized crossflow membrane reactor has excellent prospects for effective hydrogen generation and separation.

Steerable isobutyric and butyric acid production from CO2 and H2 by Clostridium luticellarii.

Clostridium luticellarii is a recently discovered acetogen that is uniquely capable of producing butyric and isobutyric acid from various substrates (e.g. methanol), but it is unclear which factors influence its (iso)butyric acid production from H2 and CO2 . We aimed to investigate the autotrophic metabolism of C. luticellarii by identifying the necessary growth conditions and examining the effects of pH and metabolite levels on product titers and selectivity. Results show that autotrophic growth of C. luticellarii requires the addition of complex nutrient sources and the absence of shaking conditions. Further experiments combined with thermodynamic calculations identified pH as a key parameter governing the direction of metabolic fluxes. At circumneutral pH (~6.5), acetic acid is the sole metabolic end product but C. luticellarii possesses the unique ability to co-oxidize organic acids such as valeric acid under high H2 partial pressures (>1 bar). Conversely, mildly acidic pH (≤5.5) stimulates the production of butyric and isobutyric acid while partly halting the oxidation of organic acids. Additionally, elevated acetic acid concentrations stimulated butyric and isobutyric acid production up to a combined selectivity of 53 ± 3%. Finally, our results suggest that isobutyric acid is produced by a reversible isomerization of butyric acid, but valeric and caproic acid are not isomerized. These combined insights can inform future efforts to optimize and scale-up the production of valuable chemicals from CO2 using C. luticellarii.

Potential-Dependent Competitive Adsorption of Electrolytes Kinetically Inhibits High-Current Hydrogen Oxidation in Non-Aqueous Media

Performing electrochemical transformations in non-aqueous media can unlock new reactivities and selectivities unattainable in aqueous media. For instance, non-aqueous media (e.g. acetonitrile (MeCN), dimethylformamide (DMF), dimethylsulfoxide (DMSO)) allow the dissolution of organic compounds and enable water-incompatible organic electrosynthesis reactions. On the other hand, the lithium-mediated nitrogen reduction reaction (NRR) carried out electrochemically in tetrahydrofuran (THF) is one of the most promising routes towards electrifying the Haber-Bosch reaction and thus eliminating the need for hundreds of atmospheres of pressure and high temperatures. However, many cathodic non-aqueous electrochemical reactions either do not operate with well-defined anodic reactions (e.g. unproductive solvent and/or electrolyte oxidation) or are often coupled with the sacrificial oxidation of a dissolving metal electrode (e.g. magnesium, zinc). Hence, the development of a non-sacrificial anodic reaction is essential for engineering practical, sustainable non-aqueous electrochemical transformations. In this study, we investigate the hydrogen oxidation reaction (HOR) in a variety of non-aqueous solvents (MeCN, DMF, DMSO, THF) and buffers (AcO-/AcOH, Cl-/HCl, etc.) whose base serves as the proton acceptor for HOR. The reason why HOR can be sustainable is because it produces protons, which can be paired with a number of proton-demanding electro-reductions such as NRR, carbon dioxide reduction (CDR) to form oxalic acid or carboxylic acids, and more. Hydrogen is also stoichiometrically cheaper and less greenhouse-emitting than sacrificial metals, making HOR both cheaper and greener. First, we demonstrate a fabricated gas diffusion electrode (GDE) architecture that supports high H2 transport-limited current densities in non-aqueous solvents at ambient pressure. We find that the maximum HOR current density obtained is not H2 transport-limited but instead depends on the kinetics influenced by the pKa and steric profile of the buffer electrolytes present. By also varying the buffer electrolyte concentration and H2 partial pressure and conducting voltametric studies in the absence of H2, we demonstrate that the HOR current density reaches a maximum due to a competitive adsorption mechanism of the buffer electrolytes, limiting the surface concentration of adsorbed hydrogen atoms from H2 dissociative adsorption and thus inhibiting HOR. Based on these findings, we outline strategies for mitigating HOR inhibition via this competitive adsorption mechanism to unlock higher HOR current densities.

Metatranscriptomic Analysis Reveals the Coexpression of Hydrogen-Producing and Homoacetogenesis Genes in Dark Fermentative Reactors Operated at High Substrate Loads.

Microbial communities in dark fermentation continuous systems are affected by substrate type, concentration, and product accumulation (e.g., H2 and CO2). Metatranscriptomics and quantitative PCR (qPCR) were used to assess how high organic loading rates (OLR) from 60 to 160 g total carbohydrates (TC)/L-d modify the microbial community diversity and expression of key dark fermentative genes. Overall, the microbial communities were composed of H2-producing bacteria (Clostridium butyricum), homoacetogens (Clostridium luticellarii), and lactic acid bacteria (Enteroccocus gallinarum and Leuconostoc mesenteroides). Quantification through qPCR showed that the abundance of genes encoding the formyltetrahydrofolate synthetase (fthfs, homoacetogens) and hydrogenase (hydA, H2-producing bacteria) was strongly associated with the OLR and H2 production performance. Similarly, increasing the OLR influenced the abundance of the gene transcripts responsible for H2 production and homoacetogenesis. To evaluate the effect of decreasing the H2 partial pressure, silicone oil was added to the reactor at an OLR of 138 and 160 g TC/L-d, increasing the production of H2, the copies of genes codifying for hydA and fthfs, and the genes transcripts related to H2 production and homoacetogenesis. Moreover, the metatranscriptomic analysis also showed that lactate-type fermentation and dark fermentation simultaneously occurred without compromising the reactor performance for H2 production.

Microbial hydrogen consumption leads to a significant pH increase under high-saline-conditions: implications for hydrogen storage in salt caverns

Salt caverns have been successfully used for natural gas storage globally since the 1940s and are now under consideration for hydrogen (H2) storage, which is needed in large quantities to decarbonize the economy to finally reach a net zero by 2050. Salt caverns are not sterile and H2 is a ubiquitous electron donor for microorganisms. This could entail that the injected H2 will be microbially consumed, leading to a volumetric loss and potential production of toxic H2S. However, the extent and rates of this microbial H2 consumption under high-saline cavern conditions are not yet understood. To investigate microbial consumption rates, we cultured the halophilic sulphate-reducing bacteria Desulfohalobium retbaense and the halophilic methanogen Methanocalculus halotolerans under different H2 partial pressures. Both strains consumed H2, but consumption rates slowed down significantly over time. The activity loss correlated with a significant pH increase (up to pH 9) in the media due to intense proton- and bicarbonate consumption. In the case of sulphate reduction, this pH increase led to dissolution of all produced H2S in the liquid phase. We compared these observations to a brine retrieved from a salt cavern located in Northern Germany, which was then incubated with 100% H2 over several months. We again observed a H2 loss (up to 12%) with a concurrent increase in pH of up to 8.5 especially when additional nutrients were added to the brine. Our results clearly show that sulphate-reducing microbes present in salt caverns consume H2, which will be accompanied by a significant pH increase, resulting in reduced activity over time. This potentially self-limiting process of pH increase during sulphate-reduction will be advantageous for H2 storage in low-buffering environments like salt caverns.

No acetogen is equal: Strongly different H2 thresholds reflect diverse bioenergetics in acetogenic bacteria.

Acetogens share the capacity to convert H2 and CO2 into acetate for energy conservation (ATP synthesis). This reaction is attractive for applications, such as gas fermentation and microbial electrosynthesis. Different H2 partial pressures prevail in these distinctive applications (low concentrations during microbial electrosynthesis [<40 Pa] vs. high concentrations with gas fermentation [>9%]). Strain selection thus requires understanding of how different acetogens perform under different H2 partial pressures. Here, we determined the H2 threshold (H2 partial pressure at which acetogenesis halts) for eight different acetogenic strains under comparable conditions. We found a three orders of magnitude difference between the lowest and highest H2 threshold (6 ± 2 Pa for Sporomusa ovata vs. 1990 ± 67 Pa for Clostridium autoethanogenum), while Acetobacterium strains had intermediate H2 thresholds. We used these H2 thresholds to estimate ATP gains, which ranged from 0.16 to 1.01 mol ATP per mol acetate (S. ovata vs. C. autoethanogenum). The experimental H2 thresholds thus suggest strong differences in the bioenergetics of acetogenic strains and possibly also in their growth yields and kinetics. We conclude that no acetogen is equal and that a good understanding of their differences is essential to select the most optimal strain for different biotechnological applications.

Active Site Identification for Glycerol Hydrodeoxygenation over the Oxygen Modified Molybdenum Carbide Surface

Density functional theory and microkinetic reactor modeling were used to investigate the hydrodeoxygenation (HDO) mechanism of glycerol on the oxygenated Mo2C catalyst surface to understand the activity and product selectivity under practically relevant reaction conditions. Reactor simulations with multiple active site models predicted that a fully oxygenated surface with acid–base (OH,O) pairs is active for glycerol dehydration and it can selectively cleave one C–O bond to produce 3-hydroxypropanal (HPA). The acid sites are not directly involved in the C–O bond scission process; however, surface oxygen vacancy formation is promoted in the presence of acid sites. The rate-limiting C–O bond cleavage process occurs on the exposed Mo sites with a concerted β-hydrogen transfer to the nearest conjugate base (Mo–O) via an E2 elimination mechanism. Dehydration of HPA to acrolein was observed at longer residence times. Our analysis revealed that reaction conditions, such as temperature and partial pressure of H2, can be tuned to promote further deoxygenation of HPA and acrolein to produce propanal and propylene.

Butanol as a major product during ethanol and acetate chain elongation.

Chain elongation is a relevant bioprocess in support of a circular economy as it can use a variety of organic feedstocks for production of valuable short and medium chain carboxylates, such as butyrate (C4), caproate (C6), and caprylate (C8). Alcohols, including the biofuel, butanol (C4), can also be generated in chain elongation but the bioreactor conditions that favor butanol production are mainly unknown. In this study we investigated production of butanol (and its precursor butyrate) during ethanol and acetate chain elongation. We used semi-batch bioreactors (0.16L serum bottles) fed with a range of ethanol concentrations (100-800mM C), a constant concentration of acetate (50mM C), and an initial total gas pressure of ∼112kPa. We showed that the butanol concentration was positively correlated with the ethanol concentration provided (up to 400mM C ethanol) and to chain elongation activity, which produced H2 and further increased the total gas pressure. In bioreactors fed with 400mM C ethanol and 50mM C acetate, a concentration of 114.96 ± 9.26mM C butanol (∼2.13gL-1) was achieved after five semi-batch cycles at a total pressure of ∼170kPa and H2 partial pressure of ∼67kPa. Bioreactors with 400mM C ethanol and 50mM C acetate also yielded a butanol to butyrate molar ratio of 1:1. At the beginning of cycle 8, the total gas pressure was intentionally decreased to ∼112kPa to test the dependency of butanol production on total pressure and H2 partial pressure. The reduction in total pressure decreased the molar ratio of butanol to butyrate to 1:2 and jolted H2 production out of an apparent stall. Clostridium kluyveri (previously shown to produce butyrate and butanol) and Alistipes (previously linked with butyrate production) were abundant amplicon sequence variants in the bioreactors during the experimental phases, suggesting the microbiome was resilient against changes in bioreactor conditions. The results from this study clearly demonstrate the potential of ethanol and acetate-based chain elongation to yield butanol as a major product. This study also supports the dependency of butanol production on limiting acetate and on high total gas and H2 partial pressures.