Enzymatic Hydrogen Electrosynthesis at Enhanced Current Density Using a Redox Polymer

High-temperature tolerant enzymes offer multiple advantages over enzymes from mesophilic organisms for the industrial production of sustainable chemicals due to high specific activities and stabilities towards fluctuations in pH, heat, and organic solvents. The production of molecular hydrogen (H2) is of particular interest because of the multiple uses of hydrogen in energy and chemicals applications, and the ability of hydrogenase enzymes to reduce protons to H2 at a cathode. We examined the activity of Hydrogen-Dependent CO2 Reductase (HDCR) from the thermophilic bacterium Thermoanaerobacter kivui when immobilized in a redox polymer, cobaltocene-functionalized polyallylamine (Cc-PAA), on a cathode for enzyme-mediated H2 formation from electricity. The presence of Cc-PAA increased reductive current density 340-fold when used on an electrode with HDCR at 40 °C, reaching unprecedented current densities of up to 3 mA·cm−2 with minimal overpotential and high faradaic efficiency. In contrast to other hydrogenases, T. kivui HDCR showed substantial reversibility of CO-dependent inactivation, revealing an opportunity for usage in gas mixtures containing CO, such as syngas. This study highlights the important potential of combining redox polymers with novel enzymes from thermophiles for enhanced electrosynthesis.

Catalytic Role of Refractory Interstellar Grain Analogs on H2 Formation

Refractory dust grains have an important role to play in the chemistry of star and planet-forming regions. Their surfaces interact with interstellar gas and act as a catalyst for the formation of simple and complex molecules in space. Several mechanisms have been invoked to explain how molecular hydrogen is formed in reactions on dust grain surfaces in different regions of space. In this article, we give an overview of our understanding of the laboratory experiments, conducted over the last 20 years, that deal with H2 formation on interstellar grain analogs in space simulated conditions.

The in-situ formation of molecular and warm ionised gas triggered by hot galactic outflows

Molecular outflows contributing to the matter cycle of star forming galaxies are now observed in small and large systems at low and high redshift. Their physical origin is still unclear. In most theoretical studies only warm ionised/neutral and hot gas outflowing from the interstellar medium is generated by star formation. We investigate an in-situ H2 formation scenario in the outflow using high-resolution simulations, including non-equilibrium chemistry and self-gravity, of turbulent, warm, and atomic clouds with densities 0.1, 0.5 and 1 cm−3 exposed to a magnetised hot wind. For cloud densities ≳ 0.5 cm−3 a magnetised wind triggers H2 formation before cloud dispersal. Up to 3 per cent of the initial cloud mass can become molecular on ∼10 Myr time scales. The effect is stronger for winds with perpendicular B-fields and intermediate density clouds (nc ∼ 0.5 cm−3). Here H2 formation can be boosted by up to one order of magnitude compared to isolated cooling clouds independent of self-gravity. Self-gravity preserves the densest clouds well past their ∼15 Myr cloud crushing time scales. This model could provide a plausible in-situ origin for the observed molecular gas. All simulations form warm ionised gas, which represents an important observable phase. The amount of warm ionised gas is almost independent of the cloud density but solely depends on the magnetic field configuration in the wind. For low density clouds (0.1 cm−3), up to 60 per cent of the initially atomic cloud mass can become warm and ionised.

Photocatalytic H2 Production from Naphthalene by Various TiO2 Photocatalysts: Impact of Pt Loading and Formation of Intermediates

This work presents a comparative study of the efficiency of two commercial TiO2 photocatalysts, Aeroxide P25 (ATiO2) and Sachtleben Hombikat UV100 (HTiO2), in H2 production from an aqueous solution of naphthalene. The TiO2 photocatalysts were platinized by the photodeposition method varying the platinum content of the suspension to 0.5, 1.0, and 5.0 wt%. A full physicochemical characterization for these materials was performed, showing no structural effects from the deposition method, and confirming a well dispersion of nanosized-Pt0 particles on the surface of both photocatalysts. Pristine ATiO2 shows around 14% higher photocatalytic fractional conversion of naphthalene than pristine HTiO2 after 240 min of irradiation, while both materials exhibit negligible activity for H2 formation. The 0.5 wt% Pt- HTiO2 increases the photocatalytic fractional conversion of naphthalene from 71% to 82% and produces 6 µmol of H2. However, using a higher Pt content than the optimal platinization ratio of 0.5 wt% dramatically inhibits both processes. On the other hand, regardless of the fractional ratio of Pt, the platinization of ATiO2 results in a decrease in the fractional conversion of naphthalene by 4% to 33% of the pristine value. Although the presence of Pt islands on the surface of the ATiO2 is essential for the H2 evolution, no dependency between the Pt ratio and the H2 formation rate was observed since all the platinized materials show a similar H2 formation of around 3 µmol. Based on the EPR results, the higher photocatalytic activity of the Pt-HTiO2 is attributed to the efficient charge carrier separation and its larger surface area. The recyclability test confirms that the inhibition of the photocatalytic process is related to the deactivation of the photocatalyst surface by the adsorption of the photoformed intermediates. A strong relationship between the photocatalytic activity and the kind of the aromatic compounds was observed. The H2 evolution and the photooxidation of the aromatic hydrocarbons exhibit higher photonic efficiencies than that of their corresponding hydroxylated compounds over the Pt-HTiO2.

Stochastic on-lattice simulation of H2 formation on interstellar grains

We realized stochastic model evaluating efficency of recombination H2 in interstellar medium based on the approach of the continious-time random walk on two-dimentional lattice. This method allows to model inhomogeneous surfaces. We estimate recombination efficiensy as a function of model parameters. The influence of uncertainty of diffusion/desorption energy ratio on molecular hydrogen recombination was considered also.

Computational study on the reactivity of imidazolium-functionalized manganese bipyridyl tricarbonyl electrocatalysts [Mn[bpyMe(Im-R)](CO)3Br]+ (R = Me, Me2 and Me4) for CO2-to-CO conversion over H2 formation

DFT calculations are employed to investigate the selectivity of imidazolium-functionalized manganese electrocatalysts for CO2 reduction over H2 formation, and the role of the substituents on the imidazolium moiety during catalysis.