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The simultaneous photocatalytic H2 evolution with environmental remediation over semiconducting metal oxides is a fascinating process for sustainable fuel production. However, most of the previously reported photocatalytic reforming showed nonstoichiometric amounts of the evolved H2 when organic substrates were used. To explain the reasons for this phenomenon, a careful analysis of the products and intermediates in gas and aqueous phases upon the photocatalytic hydrogen evolution from oxalic acid using Pt/TiO2 was performed. A quadrupole mass spectrometer (QMS) was used for the continuous flow monitoring of the evolved gases, while high performance ion chromatography (HPIC), isotopic labeling, and electron paramagnetic resonance (EPR) were employed to understand the reactions in the solution. The entire consumption of oxalic acid led to a ~30% lower H2 amount than theoretically expected. Due to the contribution of the photo-Kolbe reaction mechanism, a tiny amount of formic acid was produced then disappeared shortly after the complete consumption of oxalic acid. Nevertheless, a much lower concentration of formic acid was generated compared to the nonstoichiometric difference between the formed H2 and the consumed oxalic acid. Isotopic labeling measurements showed that the evolved H2, HD, and/or D2 matched those of the solvent; however, using D2O decreased the reaction rate. Interestingly, the presence of KI as an additional hole scavenger with oxalic acid had a considerable impact on the reaction mechanism, and thus the hydrogen yield, as indicated by the QMS and the EPR measurements. The added KI promoted H2 evolution to reach the theoretically predictable amount and inhibited the formation of intermediates without affecting the oxalic acid degradation rate. The proposed mechanism, by which KI boosts the photocatalytic performance, is of great importance in enhancing the overall energy efficiency for hydrogen production via photocatalytic organic reforming.

Transition metal sulfides are an extraordinarily vital class of semiconductors with a wide range of applications in the photocatalytic field. A great number of recent advances in photocatalytic transformations of lignocellulosic biomass, the largest renewable carbon resource, into high-quality fuels and value-added chemicals has been achieved over metal sulfide semiconductors. Herein, the progress and breakthroughs in metal-sulfide-based photocatalytic systems for lignocellulose valorization with an emphasis on selective depolymerization of lignin and oxidative coupling of some important bioplatforms are highligted. The key issues that control reaction pathways and mechanisms are carefully examined. The functions of metal sulfides in the elementary reactions, including CO-bond cleavage, selective oxidations, CC coupling, and CH activation, are discussed to offer insights to guide the rational design of active and selective photocatalysts for sustainable chemistry. The prospects of sulfide photocatalysts in biomass valorization are also analyzed and briefly discussed. ispartof: Advanced Materials vol:33 issue:50 ispartof: location:Germany status: published

Photocatalytic selective organic transformations provide an efficient synthetic alternative for several industrially relevant chemicals, using solar rather than thermal energy. However, in most cas...

In this study, the usage of biomass-based materials as co-catalysts was tested for simultaneous sugar industry wastewater treatment/hydrogen production by catalytic wet air oxidation and the final product distribution in gas and liquid effluents were investigated. For this purpose, biochar (BC) and graphene oxide-like biomass derivative (BGO) were synthesized from corncob and coupled with LaCoO3 (LC). LaCoO3-Biochar (LC/BC) catalyst was determined as the most compatible one with pure LC in terms of removal efficiencies which were approximately 100% for total saccharide (TSC), 90% of total organic carbon (TOC) and chemical oxygen demand (COD) removal, respectively. This shows that biochar is a promising alternative to replace metal catalysts. The catalyst reusability was examined and the fresh and used catalysts were characterized by SEM-EDX, XRD, and BET analyses. There were slight decreases in catalytic activities of all catalysts including LC which were associated with the chemical and morphological alterations supported by the characterization analyses as a result of formation of acidic intermediates some of which were identified as formic acid and acetic acid in the liquid effluent. The formation of CO2 was determined as proportional with TOC reduction but the behaviour for H-2 production was the opposite as a result of total oxidation in which the final products are CO2 and H2O. Although the amount of H-2 produced was relatively lower for the selected catalyst LC/BC with higher treatment performance, the produced H-2/initial sucrose ratio equal to 0.109 was considered as promising., Ege University Scientific Research Project Coordination [17-MUH-040], This work was supported by Ege University Scientific Research Project Coordination (Grant number: 17-MUH-040). The authors would like to thank for the analyses in characterization studies which were performed in Ege University Central Research Test and Analysis Laboratory Application and Research Center, Middle East Technical University Central Research Laboratory, and Recep Tayyip Erdo.gan University Central Research Laboratory Application and Research Center. The authors would also like to thank to Ege University Chemistry Department for gas product analyses and Ege University Chemical Engineering Department for liquid product analyses which are gratefully acknowledged.

International audience; Despite the report of several structural and functional models of the [NiFe]-hydrogenases, it is still unclear how the succession of electron and proton transfers during H-2 production catalysis are controlled in terms of both sequence (order of the chemical or redox steps) and sites (metal and/or ligand). To address this issue, the structure of the previously described bioinspired [NiFe]-hydrogenase complex [(LNiFII)-Ni-N2S2-F-II Cp(CO)](+) ((LNiFeCp)-Fe-II-Cp-II, with L-N2S2 = 2,2'-(2,2'-bipyridine-6,6'-diy1)bis(1,1'-diphenylethanethiolate) and Cp = cyclopentadienyl) has been fine-tuned by modifying exclusively the Fe site. In [(LNiFeCp)-Ni-N2S2-Fe-II-Cp-II* (CO)](+) ((LNiFeCp)-Fe-II-Cp-II*, with Cp* = pentamethylcyclopentadienyl), the Cp- ligand has been replaced by Cp*- to change both the redox and structural properties of the overall complex as a consequence of the steric hindrance of Cp*-. The (LNiFeCp)-Fe-II-Cp-II* complex acts as an efficient electrocatalyst to produce H-2. Density functional theory (DFT) calculations support a CEEC cycle, following an initial reduction. The initial protonation leads to the cleavage of one thiolate-iron bond and the next reduction to the generation of a bridging Fe-based hydride moiety. Interestingly, the second protonation step generates a species containing a terminal Ni-based thiol and a bridging hydride. In the presence of CO, the electrocatalytic activity of (LNiFeCp)-Fe-II-Cp-II* for H-2 production is markedly inhibited (about 90% of loss), while only a partial inhibition (about 30% of loss) is observed in the case of (LNIFeCp)-I-II-Cp-II. DFT calculations rationalized this effect by predicting that interactions of the one- and two-electron-reduced species for (LNiFeCp)-Fe-II-Cp-II* with CO are thermodynamically more favorable in comparison to those for (LNiFeCp)-Fe-II-Cp-II.

Hydrogen production by the water gas shift reaction (WGS) is equilibrium limited. In the current study, we demonstrate that the overall efficiency of the WGS can be improved by co-feeding methanol and removing CO2 in situ. The thermodynamics of the water gas shift and methanol reforming/WGS (methanol-to-shift, MtoS) reactions for H2 production alone and with simultaneous CO2 adsorption (sorption-enhanced, SEWGS and SEMtoS) were studied using a non-stoichiometric approach based on the minimisation of the Gibbs free energy. A typical composition of the effluent from a steam methane reformer was used for the shift section. The effects of temperature (450–750 K), pressure (5–30 barg), steam and methanol addition, fraction of CO2 adsorption (0–95%) and energy efficiency of the shift systems have been investigated. Adding methanol to the feed facilitates autothermal operation of the shift unit, with and without CO2 removal, and enhances significantly the amount of H2 produced. For a set methanol and CO input, the MtoS and SEMtoS systems show a maximum productivity of H2 between 523 and 593 K due to the increasing limitation of the exothermic shift reaction while the endothermic methanol steam reforming is no longer limited above 593 K. The heat of adsorption of CO2 was found to make only a small difference to the H2 production or the autothermal conditions.

Background: Under low O-2 concentration ( hypoxia) and low light, Chlamydomonas cells can produce H-2 gas in nutrient-replete conditions. This process is hindered by the presence of O-2, which inactivates the [FeFe]-hydrogenase enzyme responsible for H-2 gas production shifting algal cultures back to normal growth. The main pathways accounting for H-2 production in hypoxia are not entirely understood, as much as culture conditions setting the optimal redox state in the chloroplast supporting long-lasting H-2 production. The reducing power for H-2 production can be provided by photosystem II (PSII) and photofermentative processes during which proteins are degraded via yet unknown pathways. In hetero- or mixotrophic conditions, acetate respiration was proposed to indirectly contribute to H-2 evolution, although this pathway has not been described in detail.Main body: Recently, Jurado-Oller et al. (Biotechnol Biofuels 8: 149, 7) proposed that acetate respiration may substantially support H-2 production in nutrient-replete hypoxic conditions. Addition of low amounts of O-2 enhanced acetate respiration rate, particularly in the light, resulting in improved H-2 production. The authors surmised that acetate oxidation through the glyoxylate pathway generates intermediates such as succinate and malate, which would be in turn oxidized in the chloroplast generating FADH(2) and NADH. The latter would enter a PSII-independent pathway at the level of the plastoquinone pool, consistent with the light dependence of H-2 production. The authors concluded that the water-splitting activity of PSII has a minor role in H-2 evolution in nutrient-replete, mixotrophic cultures under hypoxia. However, their results with the PSII inhibitor DCMU also reveal that O-2 or acetate additions promoted acetate respiration over the usually dominant PSII-dependent pathway. The more oxidized state experienced by these cultures in combination with the relatively short experimental time prevented acclimation to hypoxia, thus precluding the PSII-dependent pathway from contributing to H-2 production.Conclusions: In Chlamydomonas, continuous H-2 gas evolution is expected once low O-2 partial pressure and optimal reducing conditions are set. Under nutrient-replete conditions, the electrogenic processes involved in H-2 photoproduction may rely on various electron transport pathways. Understanding how physiological conditions select for specific metabolic routes is key to achieve economic viability of this renewable energy source.

© 2019 Elsevier B.V. A plasma-assisted chemical looping system for the production of H2 (PCLH) was investigated in this study. This system allows the partial oxidation of CH4 at mild temperatures (573–773 K). Four active oxygen carriers: Fe2O3, NiO-impregnated Fe2O3 (NiO/Fe2O3), SrFeO3−δ and NiO-impregnated SrFeO3−δ (NiO/SrFeO3−δ) were compared, each working both as a packed material for the plasma reactor and an oxygen source for the partial oxidation of CH4. Similar conversions of CH4, and low yields of H2 were obtained in Fe2O3 and SrFeO3−δ. It was concluded that in these cases, H2 was mainly produced from direct cracking of CH4 by plasma. In contrast, when using NiO/Fe2O3 and NiO/SrFeO3−δ, substantial production of H2 was achieved. It is proposed that there is a synergistic effect between the catalyst and the oxygen carrier; the presence of the metallic Ni phase was responsible for catalysing the production of H2, and the oxygen from the support helped prevent the build-up of coke. As a result, the activity of Ni was continuously maintained for H2 production. The chemical loop is closed with the oxygen carriers being regenerated in air with plasma and then used in the next looping cycle. The high H2 production capability in NiO/Fe2O3 was repeatable; whilst, NiO/SrFeO3−δ deactivated in the second and third cycles. Amongst the temperatures studied, NiO/Fe2O3 at 673 K resulted in the best performance for H2-rich gas production. A further increase in the operating temperature led to a total combustion of CH4.

When scaling up photo-electrochemical processes to larger areas than conventionally studied in the laboratory, substrate performance must be taken into consideration and in this work, a methodology to assess this via an uncomplicated 2 dimensional model is outlined. It highlights that for F-doped SnO2 (FTO), which is ubiquitously used for metal oxide photoanodes, substrate performance becomes significant for moderately sized electrodes (5 cm) under no solar concentration for state of the art Fe2O3 thin films. It is demonstrated that when the process is intensified via solar concentration, current losses become quickly limiting. Methodologies to reduce the impact of substrate ohmic losses are discussed and a new strategy is proposed. Due to the nature of the photo-electrode current-potential relationship, operation at a higher potential where the photo-current saturates (before the dark current is observed) will lead to a minimum in current loss due to substrate performance. Crucially, this work outlines an additional challenge in scaling up photo-electrodes based on low conductivity substrates, and establishes that such challenges are not insurmountable.

Hydrogen evolution by the catalytic decomposition of formic acid in solution is a key reaction for hydrogen storage for which palladium is one of the most efficient catalysts. Based on density functional theory (DFT) computations, we explain why the presence of an anionic promoter renders palladium more active and more selective for formic acid dehydrogenation. The promotion is well-captured by modeling the anion by a negatively charged surface. This promotional effect can be traced back to the modulation of the electric field at the catalyst surface, with a strongly contrasted action on the energy of the various species along the competing pathways through the electrostatic interaction between the electric field and the surface dipole moment. As a result, both the reaction kinetics and selectivity are markedly improved. This opens the door to a rational design of catalytic systems using promoters.

Trace amount of Ru-doped Ni/Mg(Al)O catalyst was prepared from hydrotalcite precursor and was tested in the reforming of propane under daily start-up and shut-down (DSS) operation. Mg(Ni,Al)O periclase derived from the hydrotalcite was dispersed in an aqueous solution of Ru nitrate, followed by calcination and reduction. Finely dispersed Ni–Ru was supported on Mg(Al)O periclase particles. The activity as well as the sustainability in the propane reforming was compared with the commercial Al 2 O 3 -supported Ni and Ru catalysts. The DSS operations of partial oxidation, steam reforming and autothermal reforming were carried out under steam or air purging conditions. The Ru doping suppressed the surface oxidation of Ni particles on the Ni/Mg(Al)O catalyst, resulting in the high activity as well as the high sustainability in all reforming reactions except under steam–air combined purging. The deactivation was effectively suppressed by increasing either reaction temperature or Ru doping even under steam–air combined purging. Ni particles also suffered by sintering, but the sintered Ni particles were redispersed and the activity was sustained during DSS operations. Air-purged partial oxidation was the most effective for the redispersion of the sintered Ni particles. This likely indicates that the finely dispersed Ni particles were continuously regenerated by reversible reduction–oxidation between Ni 0 and Ni 2+ via Mg(Ni)Al periclase assisted by hydrogen spillover from Ru or NiRu alloy.

A catalytic hollow fibre membrane reactor (CHFMR) was developed in this study for combined steam methane reforming (SMR) and water gas shift (WGS) reaction. This is achieved by incorporating a Ni/SBA-15 catalyst into a plurality of micro-channels with open entrance from inner surface of Al 2 O 3 hollow fibres, followed by coating of a 3.3 µm Pd membrane on the outer surface of the hollow fibre using an electroless plating method. In addition to systematic characterizations of each reactor component, i.e. Ni/SBA-15 catalyst, micro-structured ceramic hollow fibre and Pd separating layer, the effect of how the reactor was assembled or fabricated on the catalytic performance was evaluated. Electroless plating of the Pd membrane impaired the catalytic performance of the deposited Ni/SBA-15 catalyst. Also, the over-removal of hydrogen from the reaction zone was considered as the main reason for the deactivation of the Ni-based catalyst. Instead of mitigating such deactivation using “compensating” hydrogen, starting the reaction at higher temperatures was found more efficient in improving the reactor performance, due to a better match between hydrogen production (from the reaction) and hydrogen removal (from the Pd membrane). An effective methane conversion of approximately 53%, a CO 2 selectivity of 94% and a H 2 recovery of 43% can be achieved at 560 °C. In order for a more significant “shift” phenomenon, alternative methodology of fabricating the reactor and more coke resistant catalysts are recommended.

Microstructured alumina hollow fibers, which contain a plurality of radial microchannels with significant openings on the inner surface, have been fabricated in this study and used to develop an efficient catalytic hollow fiber reactor. Apart from low mass-transfer resistance, a unique structure of this type facilitates the incorporation of Ni-based catalysts, which can be with or without the aged secondary support, SBA-15. In contrast to a fixed bed reactor, the catalytic hollow fiber reactor shows similar methane conversion, with a gas hourly space velocity that is approximately 6.5 times higher, a significantly greater CO2 selectivity, and better productivity rates. These results demonstrate the advantages of dispersing the catalyst inside the microstructured hollow fiber as well as the potential to reduce the required quantity of catalyst.

Photocatalytic systems for the reduction of aqueous protons are strongly pH-dependent, but the origin of this dependency is still not fully understood. We have studied the effect of different degrees of acidity on the electron transfer dynamics and catalysis taking place in a homogeneous photocatalytic system composed of a phosphonated ruthenium tris(bipyridine) dye (RuP) and a nickel bis(diphosphine) electrocatalyst (NiP) in an aqueous ascorbic acid solution. Our approach is based on transient absorption spectroscopy studies of the efficiency of photo-reduction of RuP and NiP correlated with pH-dependent photocatalytic H2 production and the degree of catalyst protonation. The influence of these factors results in an observed optimum photoactivity at pH 4.5 for the RuP-NiP system. The electron transfer from photo-reduced RuP to NiP is efficient and independent of the pH value of the medium. At pH 4.5, the efficiency of the system is limited by the poor protonation of NiP, which inhibits its ability to reduce protons to hydrogen. We have therefore developed a rational strategy utilising transient absorption spectroscopy combined with bulk pH titration, electrocatalytic and photocatalytic experiments to disentangle the complex pH-dependent activity of the homogenous RuP-NiP photocatalytic system, which can be widely applied to other photocatalytic systems.

Daily startup and shutdown (DSS) operations using several purge gases were applied for Ni-loaded Mg(Al)O periclase catalyst to test the sustainability in steam reforming of CH 4 . Thirteen wt% Ni loaded Mg(Al)O catalysts were prepared by using Mg(Ni)–Al hydrotalcite as the precursor with varying Mg/Al ratios. H 2 O/N 2 (100/25 ml min −1 ), O 2 /N 2 (40/10 ml min −1 ) and CO 2 /H 2 O/N 2 (40/15/25 ml min −1 ) were tested as the purge gas in imitation of steam, air and spent gas, respectively. Use of air as the purge gas resulted in a quick deactivation of all Ni loaded catalysts due to the oxidation of Ni metal on the catalyst surface. Spent gas was the most inert for the DSS operation causing no significant deactivation of Ni loaded catalysts. Steam, the most convenient purge gas for DSS operation of PEFC, revealed evident deactivation depending on the (Mg + Ni)/Al ratio in Ni/Mg(Al)O catalysts; use of the (Mg + Ni)/Al ratio of 3/1 resulted in a quick deactivation with steam purge, although this ratio was the most profitable for the steady state operation with the Ni/Mg(Al)O catalysts. The most stable operation was achieved with the ratio of 6/1 in steam as the purge gas. The deactivation took place mainly by the oxidation of Ni metal by steam as the purge gas during the DSS operation between 200 and 700 °C. It is likely that Mg(Al)O on the Ni/Mg(Al)O catalysts was hydrated by steam to form Mg(OH) 2 on the catalyst surface, resulting in the oxidation of Ni metal finely dispersed on Mg(Al)O periclase.

Effects of the additions of noble metal, i.e. Rh, Pd and Pt, on Ni/Mg(Al)O catalyst have been investigated for the daily start-up and shut-down (DSS) operation under steam purging in the steam reforming of CH 4 . Mg 2.5 (Ni 0.5 )-Al hydrotalcite was prepared by coprecipitation and calcined to form Mg 2.5 (Al,Ni 0.5 )O periclase. When the powders of the periclase were dipped in an aqueous solution of the nitrate of Rh(III), Pd(II) or Pt(II), the hydrotaclite was reconstituted on the surface of Mg 2.5 (Al,Ni 0.5 )O particles due to a “memory effect,” resulting in the formation of highly dispersed noble metal-Ni supported catalysts after the calcination followed by the reduction. The addition of noble metal on Ni resulted in a decrease in the reduction temperature of Ni 2+ in Mg 2.5 (Al,Ni 0.5 )O periclase and an increase in the amount of H 2 uptake on the Ni 0 over the Ni/Mg 2.5 (Al)O catalyst. When Rh-, Pd- and Pt-Ni 0.5 /Mg 2.5 (Al)O catalysts were tested for the DSS-like operation under steam purging, the deactivation due to the Ni metal oxidation by steam was effectively suppressed by hydrogen spillover from noble metal to Ni. Especially, only 0.05 wt% of noble metal loading was enough to suppress effectively the deactivation during the DSS-like operation in the case of using both Rh and Pt.

Ethanol steam reforming has been investigated in the low temperature range, focusing not only on H-2 productivity, but also on catalyst stability, very critical parameters under such conditions. Different supports (SiO2 and ZrO2), active phases (Ni, Co, Cu) and reaction temperature (300-500 degrees C) have been employed. Ni confirmed the best performing active phase to promote ethanol decomposition and reforming already at low reaction temperature. However, stability towards coking remains a key problem. The support plays a key role from this point of view. Indeed, the stabilization of the active phase in very dispersed form allowed to reach stable catalyst performance with time-on-stream. SiO2, thanks to no Lewis acidity and sufficiently strong metal-support interaction, demonstrated an interesting support for Ni under the selected operating conditions. (C) 2013 Elsevier B.V. All rights reserved.

Analysis of the complete genome of Thermococcus sp. strain AM4, which was the first lithotrophic Thermococcales isolate described and the first archaeal isolate to exhibit a capacity for hydrogenogenic carboxydotrophy, reveals a proximity with Thermococcus gammatolerans , corresponding to close but distinct species that differ significantly in their lithotrophic capacities.

International audience; The dinuclear nickel–manganese complex [Ni(xbsms)Mn(CO)3(H2O)]+ (H2xbsms = 1,2-bis(4-mercapto-3,3-dimethyl-2-thiabutyl)benzene) is reported as a bio-inspired mimic of the active site of NiFe hydrogenases catalyzing hydrogen evolution from trifluoroacetic acid in DMF with an overpotential requirement of 860 mV. This is higher than that displayed by Ni–Ru systems [Canaguier et al., Chem.–Eur. J., 2009, 15, 9350–9364] but similar to that found for related noble metal free Ni–Fe mimics [Canaguier et al., Chem. Commun. 2010, 46, 5876–5878]. A combined electrochemical and theoretical (DFT) study suggests a heterolytic mechanism for hydrogen evolution from a hydride derivative. The structure of the active intermediate, with a bridging hydride ligand between Ni and Mn, resembles that of the Ni–C active state of NiFe hydrogenases.

Effect of the addition of small amount of noble metal, i.e . Ru, Rh, Pd, Ir or Pt, in the Ni/Mg(Al)O catalyst on the catalytic activity for the partial oxidation of propane has been investigated. Mg 2.5 (Ni 0.5 )–Al hydrotalcite was prepared by co-precipitation and was calcined to form Mg 2.5 (Al,Ni 0.5 )O periclase. When the powders of the periclase were dipped in an aqueous solution of the nitrate of Ru(III), Rh(III), Pd(II), Ir(III) or Pt(II), a reconstitution of the hydrotaclite took place on the surface of Mg 2.5 (Al,Ni 0.5 )O particles due to a “memory effect”. The calcination followed by the reduction of the dipped samples produced highly dispersed noble metal-Ni supported catalysts. The loading of noble metal gave rise to a decrease in the reduction temperature of Ni 2+ to Ni 0 on Mg 2.5 (Al,Ni 0.5 )O periclase, an increase in the amount of H 2 uptake on the Ni metal and moreover a decrease in the particle size of Ni metal formed on the noble metals-Ni/Mg 2.5 (Al)O catalyst. When the noble metals-Ni 0.5 /Mg 2.5 (Al)O catalysts were tested in the temperature-cycled operation of propane partial oxidation between 400 and 700 °C, the deactivation due to both Ni oxidation and coke formation on the catalyst was effectively suppressed by the loading of noble metals. The combination of Ru and Ni 0.5 /Mg 2.5 (Al)O was the most effective; the high sustainability against both Ni oxidation and coke formation was obtained only with 0.1 wt% of Ru loading. A dipping of 1.0 g of the powders of Ni 0.5 /Mg 2.5 (Al)O periclase in a 5 ml of Ru(III) nitrate-aqueous solution was enough to reconstitute the hydrotalcite on the surface of the powder particles, leading to the formation of the Ru–Ni bimetal loaded catalyst with the high activity as well as the high sustainability.