H2 Formation Research Articles

Tetrabutylammonium Borohydride: A Sacrificial Reductant in Organic Electrosynthesis

In this work, tetrabutylammonium borohydride (TBAB) is investigated as a terminal reductant in reductive organic electrosynthesis as replacement for sacrificial anodes. Sacrificial anodes are typically based on easily oxidized metals, such as Mg, Zn, or Al, and are consumed during the reaction, resulting in stoichiometric metal waste. In contrast, oxidation of TBAB enables the use of inert anodes and results in anodic H2 formation, effectively serving as the inverse of cathodic proton reduction. Our results indicate that TBAB oxidation at carbon-based electrodes can replace the use of sacrificial anodes in several organic electrosynthetic reactions, including Birch reduction, hydrodesulfurization of thioethers, hydrodeoxygenation of alcohols1, and cross-electrophile couplings.References Villo, P.; Lill, M.; Alsaman, Z.; Soto Kronberg, A.; Chu, V.; Ahumada, G.; Agarwala, H.; Ahlquist, M.; Lundberg, H. Electroreductive Deoxygenative C–H and C–C Bond Formation from Non-Derivatized Alcohols Fueled by Borohydride Oxidation. ChemElectroChem. 2023, accepted for publication (a previous version can be found in ChemRxiv, DOI: 10.26434/chemrxiv-2023-tw9l1-v2.) Figure 1

H i in Molecular Clouds: Irradiation by FUV Plus Cosmic Rays

We extend the analytic theory presented by Sternberg et al. and Bialy & Sternberg for the production of atomic hydrogen (H i) via far-ultraviolet (FUV) photodissociation at the boundaries of dense interstellar molecular (H2) clouds to also include the effects of penetrating (low-energy) cosmic rays (CRs) for the growth of the total H i column densities. We compute the steady-state abundances of the H i and H2 in one-dimensional gas slabs in which the FUV photodissociation rates are reduced by depth-dependent H2 self-shielding and dust absorption and the CR ionization rates are either constant or reduced by transport effects. The solutions for the H i and H2 density profiles and the integrated H i columns depend primarily on the ratios I UV/Rn and ζ/Rn, where I UV is the intensity of the photodissociating FUV field, ζ is the H2 CR ionization rate, n is the hydrogen gas density, and R is the dust surface H2 formation rate coefficient. We present computations for a wide range of FUV field strengths, CR ionization rates, and dust-to-gas ratios. We develop analytic expressions for the growth of the H i column densities. For Galactic giant molecular clouds (GMCs) with multiphased (warm/cold) H i envelopes, the interior CR zones will dominate the production of the H i only if s−1, where M GMC is the GMC mass, and including attenuation of the CR fluxes. For most Galactic GMCs and conditions, FUV photodissociation dominates over CR ionization for the production of the H i column densities. Furthermore, the CRs do not affect the H i-to-H2 transition points.

Plasma-based dry reforming of CH4: Plasma effects vs. thermal conversion

In this work we evaluate the chemical kinetics of dry reforming of methane in warm plasmas (1000–4000 K) using modelling with a newly developed chemistry set, for a broad range of parameters (temperature, power density and CO2/CH4 ratio). We compare the model against thermodynamic equilibrium concentrations, serving as validation of the thermal chemical kinetics. Our model reveals that plasma-specific reactions (i.e., electron impact collisions) accelerate the kinetics compared to thermal conversion, rather than altering the overall kinetics pathways and intermediate products, for gas temperatures below 2000 K. For higher temperatures, the kinetics are dominated by heavy species collisions and are strictly thermal, with negligible influence of the electrons and ions on the overall kinetics. When studying the effects of different gas mixtures on the kinetics, we identify important intermediate species, side reactions and side products. The use of excess CO2 leads to H2O formation, at the expense of H2 formation, and the CO2 conversion itself is limited, only approaching full conversion near 4000 K. In contrast, full conversion of both reactants is only kinetically limited for mixtures with excess CH4, which also gives rise to the formation of C2H2, alongside syngas. Within the given parameter space, our model predicts the 30/70 ratio of CO2/CH4 to be the most optimal for syngas formation with a H2/CO ratio of 2.

Pyrite copper nickel sulfide for stable hydrogen evolution reaction in alkaline media

The alkaline hydrogen evolution reaction is a promising solution to meet future energy demand due to the sustainable production of hydrogen via water electrolysis. The development of copper nickel sulfide (CuNiS) electrocatalysts for hydrogen evolution reaction (HER) relies on robust active sites without degradation. However, the performance of CuNiS depends on the nature of sulfur precursors and complexing agents since the copper and sulfur oxidation states determine the adsorption and release of H2. The CuNiS synthesized with thioacetamide and 3-mercaptopropionic acid as sulfur source and complexing agent, respectively, at 100°C for 10h delivered a lower overpotential of −96 mV to achieve a current density of −10 mA cm−2 in 1 M KOH electrolyte. The positive and negative shifts in binding energy peaks of Ni and S, respectively, caused charge redistribution and charge transfer between metals and S. The transfer of electrons from Ni to Cu atoms decreases the hydrogen binding energy and accelerates the Had desorption process, which leads to the subsequent formation of H2. Moreover, the negative shift in S improves the electron occupation, which helps with proton adsorption and the release of H2.

Dehydrogenation of Aqueous Formic Acid by Iridium and Ruthenium Complexes with Nitrogen‐rich Diamino‐bis(1H‐1,2,4‐triazole)

AbstractThe development of efficient catalysts for the dehydrogenation of formic acid (FA) through molecular level control and precise regulation remains challenging. Herein, we report the newly developed complexes of iridium and ruthenium with nitrogen‐rich ligand for aqueous FA dehydrogenation. The Cp*Ir complex 1 (Cp*=cyclopentadienyl) and Ru(p‐cym) complex 2 (p‐cym=p‐Cymene) were synthesized and fully characterized. The single crystal structures of these complexes have a typical piano‐stool structure. A turnover frequency of 12468 h−1 was achieved for FA dehydrogenation at 90 °C by using complex 1 bearing 3,3’‐diamino‐5,5’‐bis(1H‐1,2,4‐triazole) (DABT). The effects of pH, temperature, concentration of formic acid and amount of catalyst on the reaction kinetics were studied in detail. Kinetic isotope effect (KIE) experiments and 1H NMR detection of reaction revealed that a plausible mechanism involves the decarboxylation of formate and the formation of H2. More importantly, the influence of H2O and H+ was involved in the rate‐determining step. This study demonstrated the effectiveness of nitrogen‐rich ligand for FA dehydrogenation, which would contribute to the development of new and effective hydrogen release systems.

Pt nanoclusters modified porous g-C3N4 nanosheets to significantly enhance hydrogen production by photocatalytic water reforming of methanol

For the use of green hydrogen energy, it is crucial to have efficient photocatalytic activity for hydrogen generation by water reforming of methanol under mild conditions. Much attention has been paid to g-C3N4 as a promising photocatalyst for the generation of hydrogen. To improve the separation of photogenerated charge, porous nanosheet g-C3N4 was modified with Pt nanoclusters (Pt/g-C3N4) through impregnation and following photo-induced reduction. This catalyst showed excellent photocatalytic activity of water reforming of methanol for hydrogen production with a 17.12 mmol·g−1·h−1 rate at room temperature, which was 311 times higher than that of the unmodified g-C3N4. The strong interactions of Pt–N in Pt/g-C3N4 constructed effective electron transfer channels to promote the separation of photo-generated electrons and holes effectively. In addition, in-situ infrared spectroscopy was used to investigate the intermediates of the hydrogen production reaction, which proved that methanol and water eventually turn into H2 and CO2via formaldehyde and formate. This study provides insights for understanding the photocatalytic hydrogen production in the water reforming of methanol.

Solid-liquid-vapour equilibrium conditions of tetra-iso-amyl ammonium bromide (TiAAB) semiclathrates formed from H2

The tetra-iso-amyl ammonium bromide (TiAAB) is synthesized according to Menschutkin's reaction and the fundamental characterization of TiAAB is conducted using ESI mass spectrum and 13C NMR spectra, respectively. Using an isochoric equilibrium cell the H2 semi-clathrates were formed in the presence of three different TiAAB solutions; a 15 wt% solution, a 30 wt% solution, and a 45 wt% solution. Using TiAAB allows for the formation of H2 semi-clathrates at much milder P-T conditions as compared to other quaternary ammonium salts such as TBAB, TBAN, TBPB, and TBAF. The mixed H2/TiAAB semi-clathrates were found to form over a narrow range of temperatures from 303 K to 304 K, and the pressure ranged from atmospheric pressure to above 4 MPa. The experimental results obtained in the current work contribute to developing a method to store hydrogen gas with mixed H2/TiAAB semi-clathrate.

Tailoring the *CO and *H Coverage for Selective CO2 Electroreduction to CH4 or C2H4.

In the electrochemical CO2 reduction reaction (CO2RR), the coverages of *CO and *H intermediates on a catalyst surface are critical for the selective generation of C1 or C2 products. In this work, we havesynthesized several CuxZnyMnz ternary alloy electrocatalysts, including Cu8ZnMn, Cu8Zn6Mn, and Cu8ZnMn2, by varying the doping compositions of Zn and Mn, which are efficient in binding *CO and *H adsorbates in the CO2 electroreduction process, respectively. The increase of *H coverage allows to promotion of the CH4 and H2 formation, while the increase of the *CO coverage facilitates the production of C2H4 and CO. As a result, the Cu8ZnMn catalyst presented a high CO2-to-CH4 partial current density (-418±22mAcm-2) with a Faradaic efficiency of 55±2.8%, while the Cu8Zn6Mn catalyst exhibited a CO2-to-C2H4 partial current density (-440±41mAcm-2) with a Faradaic efficiency of 58±4.5%. The study suggests a useful strategy for rational design and fabrication of Cu electrocatalysts with different doping for tailoring the reduction products.

Numerical analysis of the enrichment of CH4/H2 in ammonia combustion in a hot co-flow environment

A numerical investigation is conducted using OpenFOAM to study the characteristics of ammonia flames enriched with methane and hydrogen, influenced by a diluted co-flow (exhaust gas). The study aims to explore various flame characteristics, including fuel consumption, ignition delay time, lift-off height and position, temperature, and CO and NO emissions while maintaining a constant energy input of 5 kW. Non-stabilized flames are identified by analyzing the OH peak in the computational domain among ammonia flames enriched with CH4/H2 for different dilutions of O2 (6 %, 9 %, and 12 %). The study is structured into three stages of numerical simulations corresponding to stabilized flames: (1) no reaction, (2) reaction in the absence of O2, and (3) reaction with O2 to see the sole impact of the hot co-flow and local turbulence mixing on the primary cracking of ammonia. Furthermore, the rates of H2 and N2 formation resulting from the utilization of ammonia in the presence of hot co-flow are thoroughly examined. Additionally, the behavior of ammonia flames and CH4/H2-enriched ammonia flames under preheated air conditions (23 % O2) is analysed. Stable flames are observed in the co-flow at 12 % O2, displaying characteristics of Moderate or Intense Low-oxygen Dilution (MILD) combustion. Conversely, ammonia flames under preheated air conditions exhibit conventional flame behavior. Pure ammonia flames exhibit stronger temperature gradients and higher levels of CO and NO emissions. As the enrichment of NH3 with CH4/H2 increases, peak temperatures decrease while lift-off heights increase, indicating a transition towards more distributed flame structures. Notably, the instance with 40 % H2 enrichment exhibits lower NO emissions and a higher rate of ammonia consumption along with more distributive flame characteristics.

Catalytic CO2 Reduction with Heptacoordinated Polypyridine Complexes: Switching the Selectivity via Metal Replacement.

The discovery of molecular catalysts for the CO2 reduction reaction (CO2 RR) in the presence of water, which are both effective and selective towards the generation of carbon-based products, is a critical task. Herein we report the catalytic activity towards the CO2 RR in acetonitrile/water mixtures by a cobalt complex and its iron analog both featuring the same redox-active ligand and an unusual seven-coordination environment. Bulk electrolysis experiments show that the cobalt complex mainly yields formate (52 % selectivity at an applied potential of -2.0 V vs Fc+ /Fc and 1 % H2 O) or H2 (up to 86 % selectivity at higher applied bias and water content), while the iron complex always delivers CO as the major product (selectivity >74 %). The different catalytic behavior is further confirmed under photochemical conditions with the [Ru(bpy)3 ]2+ sensitizer (bpy=2,2'-bipyridine) and N,N-diisopropylethylamine as electron donor, where the cobalt complex leads to preferential H2 formation (up to 89 % selectivity), while the iron analog quantitatively generates CO (up to 88 % selectivity). This is ascribed to a preference towards a metal-hydride vs. a metal-carboxyl pathway for the cobalt and the iron complex, respectively, and highlights how metal replacement may effectively impact on the reactivity of transition metal complexes towards solar fuel formation.

Photo-Electro-Biochemical H2 Production Using the Carbon Material-Based Cathode Combined with Genetically Engineered Escherichia coli Whole-Cell Biocatalysis.

Abio/bio hybrids, which incorporate biocatalysts that promote efficient and selective material conversions under mild conditions into existing catalytic reactions, have attracted considerable attention for developing new catalytic systems. This study constructed a H2 -forming biocathode based on a carbon material combined with whole-cell biocatalysis of genetically-engineered-hydrogenase-overproducing Escherichia coli for the photoelectrochemical water splitting for clean H2 production. Low-cost and abundant carbon materials are generally not suitable for H2 -forming cathode due to their high overpotential for proton reduction; however, the combination of the reduction of an organic electron mediator on the carbon electrode and the H2 formation with the reduced mediator by the redox enzyme hydrogenase provides a H2 -forming cathodic reaction comparable to that of the noble metal electrode. The present study demonstrates that the recombinant E. coli whole cell can be employed as a part of the H2 -forming biocathode system, and the biocathode system wired with TiO2 photoanode can be a photoelectrochemical water-splitting system without external voltage assistance under natural pH. The findings of this study expand the feasibility of applications of whole-cell biocatalysis and contribute to obtaining solar-to-chemical conversions by abio/bio hybrid systems, especially for low-cost, noble-metal-free, and clean H2 production.

Effect of supports on the kind of in-situ formed ZnOx species and its consequence for non-oxidative propane dehydrogenation

Non-oxidative propane dehydrogenation (PDH) to propene is the basis of various large-scale processes suffering however from high costs or environmental incompatibility of currently applied Pt- or Cr-containing catalysts. Herein, we demonstrate that active and selective catalysts can be obtained from cheap and commercially available Zr- or Ti-based supports and ZnO without producing any liquid or solid waste. Catalytically active species formed in situ under PDH conditions are composed of isolated ZnOx as concluded from X-ray absorption spectroscopic analysis. The kind of support affects the geometry of such species that is probably decisive for catalyst activity. ZnOx on the surface of LaZrOx revealed the highest Zn-related TOF of propene formation. However, the following activity order in terms of space time yield of propene formation (STYC3H6) at 550 °C and about 50% equilibrium propane conversion using a feed with 40 vol% propane was obtained: ZnO//TiZrOx > ZnO//SiZrOx > ZnO//LaZrOx > ZnO//TiO2. The best-performing catalyst showed STYC3H6 of 2 kg kgcat−1 h−1 and was durable in 8 PDH/regeneration cycles. Temporal analysis of products with submillisecond resolution suggests that H2 formation should be the rate-determining step in the course of the PDH reaction.

Modelling the dynamics of hydrogen synthesis from methane in nanosecond‐pulsed plasmas

AbstractA chemical kinetics model was developed to characterise the gas‐phase dynamics of H2 production in nanosecond‐pulsed CH4 plasmas. Pulsed behaviour was observed in the calculated electric field, electron temperature and species densities at all pressures. The model agrees reasonably with experimental results, showing CH4 conversion at 30% and C2H2 and H2 as major products. The underlying mechanisms in CH4 dissociation and H2 formation were analysed, highlighting the large contribution of vibrationally excited CH4 and H2 to coupling energy from the plasma into gas‐phase heating, and revealing that H2 synthesis is not affected by applied pressure, with selectivity remaining unchanged at ~42% in the 1–5 bar range.

Hybridization of the thermo-chemical process of polyvinyl chloride in the production of bioethanol using horse hay

The improper disposal of polyvinyl chloride (PVC) is hazardous to the environment. To rectify the inappropriate disposal of PVC, this study proposes a sustainable method to valorize hydrogen chloride (HCl) (from the thermolysis of PVC) in bioethanol (BE) production. Prior to saccharification, lignocellulosic biomass (horse hay) was treated with HCl. The optimized parameters for acid treatment were determined from a series of saccharification tests. In detail, the sugar recovery yield was 75 % after 2 % HCl, 120 °C, and 2 h treatment of horse hay. In addition, this study suggests that a nitrogen source is required during fermentation to enhance BE productivity. To valorize the residual materials from the saccharification of horse hay and dechlorination of PVC, they were pyrolyzed. Co-pyrolysis of the residues was performed in a CO2 gas environment. The gas-phase reaction of CO2 and volatiles stemming from the two residues was observed, which directly led to the enhanced generation of carbon monoxide (CO). These observations are attributed to the simultaneous redox mechanisms of CO2 and volatiles. To accelerate the reaction rate of the redox mechanisms, catalytic co-pyrolysis of the residues over Co/SiO2 was performed, and such attempts were confirmed by the enhanced formation of H2 and CO. This study highlights that the hybridization of the thermochemical process of PVC in the production of BE could be a viable approach for waste valorization.

Boosting reactivity of water-gas shift reaction by synergistic function over CeO2-x/CoO1-x/Co dual interfacial structures

Dual-interfacial structure within catalysts is capable of mitigating the detrimentally completive adsorption during the catalysis process, but its construction strategy and mechanism understanding remain vastly lacking. Here, a highly active dual-interfaces of CeO2-x/CoO1-x/Co is constructed using the pronounced interfacial interaction from surrounding small CeO2-x islets, which shows high activity in catalyzing the water-gas shift reaction. Kinetic evidence and in-situ characterization results revealed that CeO2-x modulates the oxidized state of Co species and consequently generates the dual active CeO2-x/CoO1-x/Co interface during the WGS reaction. A synergistic redox mechanism comprised of independent contribution from dual functional interfaces, including CeO2-x/CoO1-x and CoO1-x/Co, is authenticated by experimental and theoretical results, where the CeO2-x/CoO1-x interface alleviates the CO poison effect, and the CoO1-x/Co interface promotes the H2 formation. The results may provide guidance for fabricating dual-interfacial structures within catalysts and shed light on the mechanism over multi-component catalyst systems.

Water Activation Triggered by Cu-Co Double-Atom Catalyst for Silane Oxidation.

High-performance catalysts sufficient to significantly reduce the energy barrier of water activation are crucial in facilitating reactions that are restricted by water dissociation. Herein we present a Cu-Co double-atom catalyst (CuCo-DAC), which possesses a uniform and well-defined CuCoN6 (OH) structure, and works together to promote water activation in silane oxidation. The catalyst achieves superior catalytic performance far exceeding that of single-atom catalysts (SACs). Various functional silanes are converted into silanols with up to 98 % yield and 99 % selectivity. Kinetic studies show that the activation energy of silane oxidation by CuCo-DAC is significantly lower than that of Cu single-atom catalyst (Cu-SAC) and Co single-atom catalyst (Co-SAC). Theoretical calculations demonstrate two different reaction pathways where water splitting is the rate-determining step and it is accelerated by CuCo-DAC, whereas H2 formation is key for its single-atom counterpart.

Water Activation Triggered by Cu−Co Double‐Atom Catalyst for Silane Oxidation

AbstractHigh‐performance catalysts sufficient to significantly reduce the energy barrier of water activation are crucial in facilitating reactions that are restricted by water dissociation. Herein we present a Cu−Co double‐atom catalyst (CuCo‐DAC), which possesses a uniform and well‐defined CuCoN6(OH) structure, and works together to promote water activation in silane oxidation. The catalyst achieves superior catalytic performance far exceeding that of single‐atom catalysts (SACs). Various functional silanes are converted into silanols with up to 98 % yield and 99 % selectivity. Kinetic studies show that the activation energy of silane oxidation by CuCo‐DAC is significantly lower than that of Cu single‐atom catalyst (Cu‐SAC) and Co single‐atom catalyst (Co‐SAC). Theoretical calculations demonstrate two different reaction pathways where water splitting is the rate‐determining step and it is accelerated by CuCo‐DAC, whereas H2 formation is key for its single‐atom counterpart.

Engineering the active sites by tuning the Ni and Mn composition in hierarchical heterostructured composites for electrocatalytic water splitting

Hydrogen production via electrochemical water splitting is a potentially clean and sustainable energy technology. Composition tuned binary NiMn oxides of Ni0.75Mn0.25O and Ni0.8Mn0.2O as an anode and cathode, respectively, demonstrate an excellent bifunctional electrocatalytic performance with a long term stability of 77 hours at a higher current density of 200 mA cm−2 toward overall water splitting (OWS) in an alkaline 1M KOH electrolyte solution. An anion exchange membrane H-type electrolytic cell requires a cell voltage of 1.74 V in order to generate a current density of 10 mA cm−2. The manganese oxide-modified nickel oxide nanosheets expedite the O–O bond formation via synergistic interaction between Ni3+ (t2g6eg1) and Jahn–Teller active Mn3+ (t2g3eg1). Moreover, due to their single electron occupancy in the eg orbital, the higher Mn3+ and NiO active sites could readily adsorb OH− ions from the electrolyte and promote a higher oxidation state, thereby accelerating the OER reaction. During OER, the energy and occupancy of these antibonding orbitals result in strong σ-bonds between these orbitals and oxygen-related adsorbate. Furthermore, the highest concentration of Ni(OH)2 attracts H+ ions, promoting the formation of H2.

Numerical investigation of sonochemical production in a single bubble under dual-frequency acoustic excitation

A numerical computation study about sonochemical activity of an oxygen bubble oscillations in a dual-frequency acoustic field is presented in this paper. The variation of excitation frequency causes the change of magnitude and duration of pressure acting on the bubble, affecting the instantaneous bubble radius, internal bubble temperature as well as chemical reactions. The numerical results indicate the formation of O2, HO2 •, •OH, H•, O, H2 and H2O2 in an O2 bubble during oscillations. The main oxidant is •OH radical. The influence of several parameters (e.g. couples of frequencies, the total pressure amplitude and the ratio of pressure amplitudes of the two waves) on the molar yields of free radicals is analyzed. There is an optimum bubble collapse temperature (around 5648 K) for the •OH production. Moreover, the corresponding frequency intervals of enhanced effect and weakened effect under different coupled frequencies (i.e. 40, 140, 213, 355 and 515 kHz) are also investigated.

Insights into corn straw gasification with K2CO3 and CaO additives: From the aspect of micro, macro to mechanism

This study provided a total study on the corn straw gasification with K2CO3 and CaO additives from the aspect of micro, macro and mechanism. During the devolatilization process, K2CO3 was beneficial to enhance the depolymerization and fragmentation of CS thus yielding more char, while CaO showed the inhibition. The CO and COOH cracking and reforming was promoted under the K2CO3 to generate more CO2, while CaO can adsorb CO2 to form CaCO3 and then remained in the char. Moreover, K2CO3 was beneficial for the decomposition lignin and hemicellulose to generate more CO and the thermal devolatilization of methoxyl groups to yield more CH4. In addition, K2CO3 and CaO all can improve the C-C/C-O-O formation. When the K2CO3/CaO mixture was utilized, the co-existence of K2CO3 and CaO exhibited a synergy on catalyzing the cracking and reforming of CO, C-C and C-O-C, resulting in a higher CO yield during devolatilization process. Moreover, K2CO3 and CaO exhibited a synergy on CO2 and char formation, thus improving the reaction between these two substance to form more CO duing the solid-gas reaction process. The CaO in char could also adsorb the CO2 to form CaCO3 following the CaO carbonation. Since this reaction was exothermic, the gasification process was enhanced and water gas shift reaction was driven to generate more CO2 and H2 formation. The K2CO3 in char played a positive effect on the reaction between char and H2 to yield more CH4. The synergy between K2CO3 and CaO reached an optimum under the mixing ratio of 4:2. The generated bimetallic carbonate K2Ca(CO3)2 during devolatilization process was postulated to be the reason for the synergy. Under this condition, the highest LHV of gas products of 6.68 MJ/Nm3, lowest tar content of 3.69 g/Nm3 and highest cold gas efficiency (CGE) of 86.49% were achieved