Excess H2 Research Articles

Influence of metal nuclearity and physicochemical properties of ceria on the oxidation of carbon monoxide

The redox properties of ceria make it suitable as a catalyst or support in oxidation reactions. Ceria-supported transition metal nanoparticles or isolated single atoms provide a metal-support interface that reduces the energy cost to remove interfacial oxygen atoms, providing active oxygen species that can participate in Mars van Krevelen oxidation processes. CO oxidation is a key probe reaction to test the reducibility of ceria-supported catalysts and is also practically important in the elimination of CO at relatively low temperatures in various applications. Preferential oxidation of CO (PROX) in excess H2 controls the CO concentration to ultra-low levels to prevent poisoning of hydrogen oxidation electrocatalysts. The reactivity of catalysts in CO oxidation and selectivity towards CO over H2 in PROX is dependent on the type and dispersion of metal species, the structural and chemical properties of CeO2, and the synthetic preparation methods of the catalysts. In this review, we summarize recently published works on catalytic CO oxidation and PROX reactions on ceria-supported metal nanoparticles and single atoms. We summarize the reactivity on different supported metals, and on different CeO2 surfaces with the same metal. We summarize the most likely reaction mechanisms as suggested by density functional theory calculations. The factors contributing to selectivity towards CO oxidation in PROX reactions on various supported metals are also discussed.

Diallyl trisulfide, a H2 S donor, inhibits cell growth of human papillary thyroid carcinoma KTC-1 cells through a positive feedback loop between H2 S and cystathionine-gamma-lyase.

Diallyl trisulfide (DATS), derived from garlic, is a well-known hydrogen sulfide (H2 S) donor. H2 S has recently emerged as a novel gasotransmitter involved in the regulation of cancer progression. The present study demonstrated that DATS along with other two H2 S donors, NaHS and GYY4137, significantly inhibited papillary thyroid carcinoma KTC-1 cells growth. DATS treatment triggered a rapid H2 S generation within 5 min in KTC-1 cells. Iodoacetamide, a potent thiol blocker reagent, partially rescued the cell membrane damage and ultimate cell death induced by DATS, indicating H2 S contributed to the apoptosis-inducing efficacy of DATS on thyroid cancer cells. Specifically, DATS treatment significantly upregulated the expression and enzymatic activity of cystathionine gamma-lyase (CTH), one of H2 S-producing enzymes, which was responsible for endogenous H2 S generation. After DATS treatment, H2 S quickly permeated cell membranes and activated NF-κΒ/p65 signaling pathway in KTC-1 cells. Nuclear translocated NF-κB bound to the promoter of CTH to enhance its transcription. These evidences proved that exogenous H2 S elevated CTH expression. CTH, in turn, catalytically generated a much higher level of endogenous H2 S. This positive feedback sustained excess H2 S production, which resulted in PTC cells growth inhibition. These findings may shed light on the development of novel H2 S-based antitumor agents.

Kinetics of aromatics hydrogenation on HBEA

Zeolite HBEA catalyzes hydrogenation of aromatic hydrocarbons—methyl-substituted benzenes (benzene and toluene), alkenyl-substituted benzenes (styrene), and polycyclics (naphthalene)—in presence of excess H2 at high-temperatures (573–748 K) with rates that depend linearly on aromatic and H2 pressures. The observed kinetic behavior can be rationalized based on a sequence of elementary steps where the first hydrogenation step of the adsorbed benzenic intermediate is rate-determining while subsequent hydrogenation and desorption steps are quasi-equilibrated and H+ is the most abundant surface species. Styrene hydrogenation exhibits the highest rates among the aromatics considered and results exclusively in ethylbenzene synthesis; in contrast, benzene/toluene and naphthalene hydrogenation results in formation of their triply-hydrogenated five-membered ring and doubly-hydrogenated ring open analogs, respectively. Based on independent studies involving co-reaction of cyclohexene and 1-methyl-1-cyclopentene with H2, we infer their facile interconversion and hydrogenation to methylcyclopentane implying that conversion of benzene to methylcyclopentane likely occurs via intervening formation of both five- and six-membered ring intermediates. Taken together, these studies demonstrate feasibility of aromatics hydrogenation and propensity of benzenic rings in these hydrocarbons to undergo ring reduction or ring opening during their activation with H2 on Brønsted acid zeolites.

Photocatalyzed preferential oxidation of CO under simulated sunlight using Au-transition metal oxide-sepiolite catalysts.

In the present study a series of Au-transition metal oxides supported on a clay mineral such as sepiolite were tested in the preferential oxidation of CO in an excess of H2 under simulated solar light irradiation and in the absence of light, at 30 °C and atmospheric pressure. Transition metal oxides (ZnO, Fe2O3, NiO, MnO2, and Co3O4) were dispersed over the sepiolite surface where, subsequently, Au nanoparticles with an average particle size between 2 and 3 nm were successfully deposited-precipitated. The obtained photocatalysts were characterized by XRD, XRF, DRUV-Vis, N2 adsorption-desorption and HRTEM in order to evaluate the optical, structural and chemical properties of the prepared samples. Despite the low amount of gold (nominal 1.0 wt%), the catalysts exhibited an outstanding behavior under light irradiation, with reaction rates between 4.5 and 5.2 mmol COox gcat-1 h-1 for the Au-NiSep, Au-CoSep and Au-ZnSep samples. These photocatalysts exhibited a high dispersion of the respective transition metal oxides over the sepiolite support and the presence of low-coordinated hemispherical gold nanoparticles. The superior photocatalytic efficiency of these samples was ascribed to the reduction of the electron-hole pair recombination of photogenerated charge carriers by the excitation of the localized surface plasmon resonance of the Au nanoparticles. The BET surface area and the gold particle size seemed to be relevant factors affecting the catalytic performance.

Active galactic nuclei winds as the origin of the H2 emission excess in nearby galaxies

ABSTRACT In most galaxies, the fluxes of rotational H2 lines strongly correlate with star formation diagnostics [such as polycyclic aromatic hydrocarbons (PAHs)], suggesting that H2 emission from warm molecular gas is a minor by-product of star formation. We analyse the optical properties of a sample of 309 nearby galaxies derived from a parent sample of 2015 objects observed with the Spitzer Space Telescope. We find a correlation between the [O i]λ6300 emission-line flux and kinematics and the H2 S(3) 9.665 $\mu\mathrm{ m}$/PAH 11.3 $\mu\mathrm{ m}$. The [O i]λ6300 kinematics in active galactic nuclei (AGNs) cannot be explained only by gas motions due to the gravitational potential of their host galaxies, suggesting that AGN-driven outflows are important to the observed kinematics. While H2 excess also correlates with the fluxes and kinematics of ionized gas (probed by [O iii]), the correlation with [O i] is much stronger, suggesting that H2 and [O i] emissions probe the same phase or tightly coupled phases of the wind. We conclude that the excess of H2 emission seen in AGNs is produced by shocks due to AGN-driven outflows and in the same clouds that produce the [O i] emission. Our results provide an indirect detection of neutral and molecular winds and suggest a new way to select galaxies that likely host molecular outflows. Further ground- and space-based spatially resolved observations of different phases of the molecular gas (cold, warm, and hot) are necessary to test our new selection method.

A two-step approach to the hydrothermal gasification of carbohydrate-rich wastes: Process design and economic evaluation

A two-step approach to hydrothermal gasification of carbohydrate-rich wastes and wastewaters is a promising route for H2 production and simultaneously as a water clean-up technology. Experimental data and kinetic models are used to further develop the process for industrial scale. In this work, a preliminary process design is conducted in order to assess the market potential of the two-step process. For stabilisation, two cases are considered: the use of excess vs. stoichiometric H2, and for gasification, the utilisation of sequential reactors for gasification housing Pt and Ru catalysts is compared to a single reactor with Pt alone. A total of four options are conceptually designed and economically evaluated. Using state-of-the-art insights and process techniques and the current market scenario, a minimum H2 selling price of 3.4 $ kg−1 was obtained.A sensitivity study showed that the feedstock price, concentration and quantity, played a crucial role in the selling price of H2. These variables are all correlated and are dependent on the industry from where the feedstock is obtained. Industrial wastewater streams rich in carbohydrate residues and associated with gate fees were found to be promising feedstock for the process.Further advancement in the areas of catalyst development (hydrothermal stability, affordability) as well as increased H2 yields are necessary in order to improve the economics of this process on industrial scale.

Flexible nanoporous activated carbon cloth for achieving high H2, CH4, and CO2 storage capacities and selective CO2/CH4 separation

Innovative tools are needed for the synthesis of smart, new, efficient, and safe nanoporous carbon materials for energy gas storage. Here, a flexible nanoporous activated carbon cloth was developed. Polypyrrole nanoparticles were polymerized in dispersed form on commercial viscose rayon cloth fiber surfaces. Then, the material was carbonized and activated by physical and chemical activation methods applied individually. Chemical activation conditions were varied and optimized. This produced a high porosity flexible nanoporous carbon textile with a surface area of ~2000 m2 g−1, total pore volume of 0.85 cm3 g−1, and high nitrogen content. The new flexible nanoporous carbon cloth achieved superior H2 and CH4 storage capacities and CO2 capture compared to so-far-reported activated carbon cloth, and values were comparable to or higher than those reported for powder activated carbons. Excess H2 uptake values were 4.0 and 0.173 wt% at 77 K and 298 K at 20 bar, respectively, and CH4 storage amounted to 7.5 mmol g−1 at 20 bar and 298 K, which is among the highest reported values for porous carbon materials. CO2 uptake values were 4.2 and 14.3 mmol g−1 at 1 and 20 bar at 298 K, respectively, which are values superior to those previously reported for activated carbon cloth. Hence, the flexible nanoporous activated carbon cloth is effective for greenhouse gas (i.e., CO2) uptake during post- and pre-combustion conditions. Separation selectivity for CO2/CH4 binary mixtures was evaluated based on the ideal adsorbed solution theory (IAST) model and a high value of 15.9 was achieved.

CuO-CeO2 nanocomposite catalysts produced by mechanochemical synthesis

Mechanochemical synthesis based on ball-milling of individual oxides was applied as a one-step preparation technique for CuO-CeO2 catalyst for preferential CO oxidation in H2 excess. The mechanical energy dose transferred to the original powder mixture determines both the catalyst composition and activity. It is found that after 90 min of milling (corresponding to a dose of 372 kJ mol–1), a mixture of 10 wt.% CuO-CeO2 powder exhibits a CO conversion of 97% at 423 K. Four active oxygen states, which are not observed in case of pure CeO2, were detected in the nanocomposite lattice and attributed to the presence of Cu in surface sites as well as in subsurface bulk sites of CeO2, in nearest neighbor and next nearest neighbor positions. Correspondingly, oxidation of CO to CO2 was found to occur in a two-stage process with Tmax = 395/460 K, and oxidation of H2 to H2O likewise in a four-stage process with Tmax = 426/448/468/516 K. The milled powder consists of CeO2 crystallites sized 8–10 nm agglomerated to somewhat larger aggregates, with CuO dispersed on the surface of the CeO2 crystallites, and to a lesser extent present as Cu2O.

Gas phase selective hydrogenation of phenylacetylene to styrene over Au/Al2O3

AbstractBACKGROUNDTrace quantities of phenylacetylene can poison styrene polymerisation catalysts. The phenylacetylene content must be less than 10 ppm and selective hydrogenation (to styrene) is viewed as a viable process solution. High styrene selectivities have been achieved in batch liquid phase operations while a switch from conventional batch liquid to continuous gas phase reaction presents process advantages in terms of higher throughput and enhanced productivity. We aim to provide the first direct comparison of Au/Al2O3 and Pd/Al2O3 in gas phase continuous catalytic hydrogenation of phenylacetylene.RESULTSTemperature programme reduction (TPR) generated metal particles at the nano‐scale (mean size = 3.0–4.3 nm), with evidence of electron donation from the aluminium oxide (Al2O3) carrier. Pd/Al2O3 exhibited a greater specific hydrogen (H2) uptake capacity than Au/Al2O3 under reaction conditions to deliver appreciably higher turnover frequencies (TOF) for reaction in excess H2. Stepwise hydrogenation predominated over Au/Al2O3 with 100% selectivity to styrene at 353 K where an increase in temperature favoured subsequent hydrogenation to ethylbenzene. Under the same conditions, Pd/Al2O3 was non‐selective, activating styrene to generate ethylbenzene with a greater contribution of direct phenylacetylene hydrogenation to ethylbenzene at higher temperature.CONCLUSIONKinetic analysis has revealed stepwise phenylacetylene hydrogenation in excess H2 over Au/Al2O3 with 100% selectivity to styrene. Stepwise hydrogenation also prevailed over Pd/Al2O3 at the lower temperature but surface activation of styrene coupled with enhanced H2 dissociation generated significant ethylbenzene. Decreasing inlet H2/phenylacetylene (to 1 mol/mol) over Pd/Al2O3 lowered rate where the activity/selectivity profile overlapped that exhibited by Au/Al2O3 in excess H2. © 2019 Society of Chemical Industry

Adaptation of anaerobic culture to bioconversion of carbon dioxide with hydrogen to biomethane

Biogas is composed of energetically useable methane (CH4) ranging from 50 to 70% and carbon dioxide (CO2) taking up 30–50%. To upgrading biogas to biomethane for better exploitation of its energetical potential, many different physico-chemical processes are commercially applied. Besides these methods, biological methods for the conversion of CO2 with electrolytic H2 to biomethane based on the activity of hydrogenotrophic methanogens are also currently being researched. This study deals with the adaptation of anaerobic culture to H2 and CO2 as substrates under thermophilic and mesophilic conditions. Thermophilic cultures initially showed slower adaptation, but CO2 conversion efficiency was finally by 12.7% higher compared to the mesophilic culture under the same conditions of the H2 loading rate. Thermophilic conditions were favourable also for homoacetogens producing higher concentration of acetate from CO2 and H2 under thermophilic condition compared to mesophilic condition in the excess of H2. Maximum capacity of the used thermophilic suspended biomass for bioconversion of CO2 was the H2 loading rate 2 L/(L.d) resulting in CO2 bioconversion efficiency of 84% (median) in contrast with 40% (median) for mesophilic culture. The system was limited by the concentration of biomass round 4 g/L.

Alumina-supported cobalt phosphide as a new catalyst for preferential CO oxidation at high temperatures

Novel alumina-supported cobalt phosphide catalysts (designated as CoP-3, CoP-10, CoP-20 and CoP-40) prepared from the precursors with Co loadings of 3, 10, 20 and 40 wt% by H2-temperature-programmed reaction were investigated as potential catalysts for preferential CO oxidation (PROX) in excess H2 at high temperatures. It was found that the catalytic activities of these Co2P/γ-Al2O3 catalysts were related to their Co loadings. The CoP-10 catalyst showed the best PROX performance in temperature range of 220–240 °C, which was attributed to its optimal microstructures (high surface area, small particle size and big amount of active site).

H2 Binding, Splitting, and Net Hydrogen Atom Transfer at a Paramagnetic Iron Complex.

While diamagnetic transition metal complexes that bind and split H2 have been extensively studied, paramagnetic complexes that exhibit this behavior remain rare. The square planar S = 1/2 FeI(P4N2)+ cation (FeI+) reversibly binds H2/D2 in solution, exhibiting an inverse equilibrium isotope effect of KH2/ KD2 = 0.58(4) at -5.0 °C. In the presence of excess H2, the dihydrogen complex FeI(H2)+ cleaves H2 at 25 °C in a net hydrogen atom transfer reaction, producing the dihydrogen-hydride trans-FeII(H)(H2)+. The proposed mechanism of H2 splitting involves both intra- and intermolecular steps, resulting in a mixed first- and second-order rate law with respect to initial [FeI+]. The key intermediate is a paramagnetic dihydride complex, trans-FeIII(H)2+, whose weak FeIII-H bond dissociation free energy (calculated BDFE = 44 kcal/mol) leads to bimetallic H-H homolysis, generating trans-FeII(H)(H2)+. Reaction kinetics, thermodynamics, electrochemistry, EPR spectroscopy, and DFT calculations support the proposed mechanism.

Texture formation in chemical vapor deposition of Ti(C,N)

The growth mechanism of Ti(C,N) coatings produced by chemical vapor deposition was investigated as a function of the TiCl4/CH3CN molar ratio in excess of H2. The depositions were carried out at a total pressure of 8 kPa, using single crystalline (0 0 l) α-Al2O3 substrates. The Ti(C,N) coatings were characterized by X–ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The investigated coatings were between 6 and 13 µm thick. The reaction orders of TiCl4 and CH3CN were determined to 0 and 1, respectively, showing that CH3CN is the rate-determining reactant.The preferred orientation of the deposited Ti(C,N) was investigated, showing that molar ratios TiCl4/CH3CN higher than 2.5 lead to 〈2 1 1〉/〈3 1 1〉 oriented coatings. A formation mechanism for the 〈2 1 1〉/〈3 1 1〉 orientations is suggested. Such high ratios lead to the formation of Ti {1 1 1} twinning planes, which provide surface sites that can facilitate fast dissociation of the strong cyanide bond, and thereby cause faster growth in the 〈2 1 1〉/〈3 1 1〉 directions. Coatings deposited at lower molar ratios show a pronounced 〈1 1 1〉 out-of-plane orientation, characterized by a {1 1 1} rocking curve yielding values for full width at half maximum (FWHM) below 0.5°.

Au nanoparticles supported on nanorod-like TiO2 as catalysts in the CO-PROX reaction under dark and light irradiation: Effect of acidic and alkaline synthesis conditions

Gold nanoparticles precipitated-deposited on titania nanostructures (1.0 wt% nominal loading) were studied in the preferential CO oxidation in excess of H2 at room temperature and atmospheric pressure, both in dark and under simulated solar light irradiation. Titania supports were synthesized by means of two hydrothermal methods markedly acid and basic, giving rise to rutile nanorods and anatase deformed nanorods structures, respectively. Characterization techniques such as N2 physisorption, XRD, XPS, DRUV-vis, HRTEM and XRF were performed in order to study the chemical, structural and optical properties of the catalysts. Well defined rutile nanorods structures were obtained from the acidic treatment allowing a regular distribution of gold nanoparticles and resulting quite active in the CO-PROX reaction. In particular the sample from the acidic synthetic approach calcined at 700 °C displayed the best results as it was highly selective to CO2 under both dark and simulated solar light irradiation.

Hydrogen storage in MIL-88 series

To be selected as sorbents for gas storage, metal–organic frameworks (MOFs) must be stable to avoid collapsed in humid media. MIL-88 series (abbreviated as MIL-88s) including MIL-88A, B, C, D satisfies high flexibility and stability; it, therefore, may become a suitable candidate for hydrogen storage based on the adsorption. In this work, the grand canonical Monte Carlo simulations for the pressures below 100 bar showed that in MIL-88 series MIL-88D exhibits the highest absolute and excess gravimetric H2 capacities of 5.15 wt% and 4.03 wt% at 77 K, and 0.69 wt% and 0.23 wt% at 298 K, respectively. Meanwhile, MIL-88A has the highest absolute and excess volumetric H2 uptakes of 50.69 g/L and 44.32 g/L at 77 K, and 6.97 g/L and 2.49 g/L at 298 K. These results are comparable to the best MOFs for hydrogen storage to date. It was shown that the hydrogen uptakes depend on the special surface area and the pore volume of the MIL-88s, apart from depending on the type of the ligand. By utilizing the van der Waals dispersion-corrected density functional theory (DFT) calculations, we elucidated the interaction between the H2 molecule and the MIL-88 series. The adsorption energy, as well as the isosteric heat of adsorption, revealed that the H2—MIL-88C interaction is strongest despite its lowest storage capacity. This observation implies an implicit role of electronic structure on the H2 adsorption capacities at the considered conditions. However, at the low temperature, the DFT calculations could elucidate the preferred adsorption sites of hydrogen molecule on the surface of MIL-88s. Besides, we also found that the interaction is dominated by the bonding state of the H2 molecule and the p orbitals of the O and C atoms of the MIL-88s. The most substantial overlap between the electronic density of states (DOS) of the MIL-88C and the DOS of the H2 molecule leads to the most robust interaction between the H2 molecule and the MIL-88C.

Nucleophilic versus Electrophilic Reactivity of Bioinspired Superoxido Nickel(II) Complexes.

The formation and detailed spectroscopic characterization of the first biuret-containing monoanionic superoxido-NiII intermediate [LNiO2 ]- as the Li salt [2; L=MeN[C(=O)NAr)2 ; Ar=2,6-iPr2 C6 H3 )] is reported. It results from oxidation of the corresponding [Li(thf)3 ]2 [LNiII Br2 ] complex M with excess H2 O2 in the presence of Et3 N. The [LNiO2 ]- core of 2 shows an unprecedented nucleophilic reactivity in the oxidative deformylation of aldehydes, in stark contrast to the electrophilic character of the previously reported neutral Nacnac-containing superoxido-NiII complex 1, [L'NiO2 ] (L'=CH(CMeNAr)2 ). According to density-functional theory (DFT) calculations, the remarkably different behaviour of 1 versus 2 can be attributed to their different charges and a two-state reactivity, in which a doublet ground state and a nearby spin-polarized doublet excited-state both contribute in 1 but not in 2. The unexpected nucleophilicity of the superoxido-NiII core of 2 suggests that such a reactivity may also play a role in catalytic cycles of Ni-containing oxygenases and oxidases.

Hydrogen peroxide metabolism and functions in plants.

Contents Summary 1197 I. Introduction 1198 II. Measurement and imaging of H2 O2 1198 III. H2 O2 and O2·- toxicity 1199 IV. Production of H2 O2 : enzymes and subcellular locations 1200 V. H2 O2 transport 1205 VI. Control of H2 O2 concentration: how and where? 1205 VII. Metabolic functions of H2 O2 1207 VIII. H2 O2 signalling 1207 IX. Where next? 1209 Acknowledgements 1209 References 1209 SUMMARY: Hydrogen peroxide (H2 O2 ) is produced, via superoxide and superoxide dismutase, by electron transport in chloroplasts and mitochondria, plasma membrane NADPH oxidases, peroxisomal oxidases, type III peroxidases and other apoplastic oxidases. Intracellular transport is facilitated by aquaporins and H2 O2 is removed by catalase, peroxiredoxin, glutathione peroxidase-like enzymes and ascorbate peroxidase, all of which have cell compartment-specific isoforms. Apoplastic H2 O2 influences cell expansion, development and defence by its involvement in type III peroxidase-mediated polymer cross-linking, lignification and, possibly, cell expansion via H2 O2 -derived hydroxyl radicals. Excess H2 O2 triggers chloroplast and peroxisome autophagy and programmed cell death. The role of H2 O2 in signalling, for example during acclimation to stress and pathogen defence, has received much attention, but the signal transduction mechanisms are poorly defined. H2 O2 oxidizes specific cysteine residues of target proteins to the sulfenic acid form and, similar to other organisms, this modification could initiate thiol-based redox relays and modify target enzymes, receptor kinases and transcription factors. Quantification of the sources and sinks of H2 O2 is being improved by the spatial and temporal resolution of genetically encoded H2 O2 sensors, such as HyPer and roGFP2-Orp1. These H2 O2 sensors, combined with the detection of specific proteins modified by H2 O2 , will allow a deeper understanding of its signalling roles.

Novel cobalt nitride-induced oxygen activation on Pt-based catalyst for catalytic oxidation

Cobalt nitride as a new type of O2 activator was used to modify Pt-based catalyst for catalytic oxidation. The nitrided Co/Pt/γ-Al2O3 catalyst system was highly efficient for preferential CO oxidation in excess H2 at low temperatures, which was attributed to a noncompetitive dual-site reaction pathway.

Methylation of the N-terminal histidine protects a lytic polysaccharide monooxygenase from auto-oxidative inactivation.

The catalytically crucial N-terminal histidine (His1) of fungal lytic polysaccharide monooxygenases (LPMOs) is post-translationally modified to carry a methylation. The functional role of this methylation remains unknown. We have carried out an in-depth functional comparison of two variants of a family AA9 LPMO from Thermoascus aurantiacus (TaLPMO9A), one with, and one without the methylation on His1. Various activity assays showed that the two enzyme variants are identical in terms of substrate preferences, cleavage specificities and the ability to activate molecular oxygen. During the course of this work, new functional features of TaLPMO9A were discovered, in particular the ability to cleave xyloglucan, and these features were identical for both variants. Using a variety of techniques, we further found that methylation has minimal effects on the pKa of His1, the affinity for copper and the redox potential of bound copper. The two LPMOs did, however, show clear differences in their resistance against oxidative damage. Studies with added hydrogen peroxide confirmed recent claims that low concentrations of H2 O2 boost LPMO activity, whereas excess H2 O2 leads to LPMO inactivation. The methylated variant of TaLPMO9A, produced in Aspergillus oryzae, was more resistant to excess H2 O2 and showed better process performance when using conditions that promote generation of reactive-oxygen species. LPMOs need to protect themselves from reactive oxygen species generated in their active sites and this study shows that methylation of the fully conserved N-terminal histidine provides such protection.

Role of hydrogen absorption in supported Pd nanocatalysts during CO-PROX: Insights from operando X-ray absorption spectroscopy

The nature of the active phase (metallic vs. oxidic, metal phase vs. concentrated hydride/ dilute solid solution with hydrogen) in heterogeneous catalysis by supported metals is still a matter of high debate. Here, we have monitored for the first time oxide-supported Pd nanocatalysts (average particle size 4.5 nm) during both CO oxidation (in H2-free atmosphere) and preferential oxidation of CO in H2 excess (PROX) by operando X-ray absorption spectroscopy. Under our conditions, the CO conversion in the absence of H2 is around 30% at 150 °C and reaches 100% at 200 °C, whereas in the presence of H2 the conversion reaches a maximum of 15% (at 250 °C), in agreement with our previous works using a conventional flow-fixed bed reactor. The active phase for CO oxidation below 200 °C is metallic Pd, whereas it is a solid solution of Pd with hydrogen during PROX below 300 °C. This work provides a direct evidence of the presence of subsurface/bulk hydrogen as a probable cause of the low PROX performance of supported Pd catalysts.