Mechanistic understanding of the thermal-assisted photocatalytic oxidation of methanol-to-formaldehyde with water vapor over Pt/SrTiO3.

Anaerobic thermal-assisted photocatalytic methanol conversion in the gas phase in the presence of water vapor has been suggested as an interesting way to generate formaldehyde as a valuable coupled product in addition to H2 production. Here, the reaction mechanism and photocatalyst deactivation are investigated in detail using in situ diffuse reflectance infrared fourier transform (DRIFTS) and electron paramagnetic resonance (EPR) spectroscopy. EPR shows that paramagnetic oxygen vacancies are not involved in the reaction mechanism over undoped SrTiO3. Instead, on an optimized 0.1 wt% Pt/SrTiO3 photocatalyst, methoxy species are formed by dissociative adsorption of methanol leading to formaldehyde formation while the formation of CO, CO2 (via a formate intermediate) and methyl formate occurs through three concurrent reactions from formyl species. Our findings suggest that CO adsorbed on Pt is a spectator species not perturbing the reaction kinetics, and deactivation is shown to be strongly correlated with the accumulation of formate groups on SrTiO3, which is more pronounced at high reaction temperatures. The mechanistic understanding provided here forms the basis for the further optimization of photocatalysts to increase methanol conversion and improve formaldehyde selectivity.

Manganese Oxide / Silica catalysed Decarboxylative Ketonization of Aliphatic Carboxylic Acids in a Continuous Flow Reactor.

The sustainable synthesis of long carbon chain molecules from carbon dioxide, water and electricity relies on the development of waste-free, highly selective C-C bond forming reactions. An example for such a power-to-chemicals process is the industrial-scale fermentation for the production of hexanoic acid. Herein, we describe how this product is transformed into 6-undecanone via decarboxylative ketonization using a heterogeneous manganese oxide / silica catalyst. The reaction reaches full conversion with near-complete selectivity when carried out in a continuous flow reactor, requires no solvent or carrier gas, and releases carbon dioxide and water as the only by-products. The reactor was operated for several weeks with no loss of reactivity, producing 7 kg of 6-undecanone from 10 g of catalyst and achieving a productivity of 1.135 kg per litre of reactor volume per hour. 6-Undecanone and other long-chain ketones accessible this way can be hydrogenated to industrially meaningful alkanes, or converted into valuable fatty acids via a hydrogenation / elimination / isomerizing hydrocarboxylation sequence.

Efficient Atomically Dispersed Co/N-C Catalysts for Formic Acid Dehydrogenation and Transfer Hydrodeoxygenation of Vanillin.

Atomically dispersed catalysts have gained considerable attention due to their unique properties and high efficiency in various catalytic reactions. Herein, a series of Co/N-doped carbon (N-C) catalysts was prepared using a metal-lignin coordination strategy and employed in formic acid dehydrogenation (FAD) and hydrodeoxygenation (HDO) of vanillin. The atomically dispersed Co/N-C catalysts showed outstanding activity, acid resistance, and long-term stability in FAD. The improved activity and stability may be attributed to the high dispersion of Co species, increased surface area, and strong Co-N interactions. XPS and XAS characterization revealed the formation of Co-N3 centers, which are assumed to be the active sites. In addition, DFT calculations demonstrated that the adsorption of formic acid on single-atom Co was stronger than that on Co13 clusters, which may explain the high catalytic activity. The Co/N-C catalyst also showed promising performance in the transfer HDO of vanillin with formic acid, without any external additional molecular H2.

The Catalytic Effect of Iron and Alkali and Alkaline Earth Metal Sulfates Loading Series on the Conversion of Cellulose-Derived Hydrochars and Chars.

The catalytic effect of minerals on biomass conversion was studied focusing on Fe as well as alkali and alkaline earth metals as the metallic inorganic elements typically present in minerals found in biomass. A mineral-free reference hydrochar and an analogous char material based on cellulose were systematically doped with sulfates of the different metallic inorganic elements in various amounts via impregnation, thereby excluding differences originating from the counterion and the carbon matrix. Thermogravimetric reactivity measurements were performed in diluted O2 and CO2, and the derivative thermogravimetry curves were fitted using the random pore model. This procedure enabled a quantification of the apparent activation energy decrease due to doping as well as the influence of doping on the carbon structural parameter. Fe sulfate was always among the most active minerals, and alkali metal sulfates were typically more active than alkaline earth metal sulfates. The only exception was the high activity of very small Ca sulfate loadings during gasification. A saturation behavior of the kinetic parameter upon increasing the mineral loading was observed. The Langmuir-type modeling of this dependence further revealed that catalytically influenced devolatilization results in a char with higher oxidation reactivity, whereas for gasification, thermal annealing dominates. The systematically derived parameters provide a comprehensive description of catalytic effects, taking into account the type of mineral, the applied loading, the used atmosphere, and the fuel morphology. The derived activation energies can be used to include catalytic effects into combustion models.

Scalable Synthesis of Multi-Metal Electrocatalyst Powders and Electrodes and their Application for Oxygen Evolution and Water Splitting.

Multi-metal electrocatalysts provide nearly unlimited catalytic possibilities arising from synergistic element interactions. We propose a polymer/metal precursor spraying technique that can easily be adapted to produce a large variety of compositional different multi-metal catalyst materials. To demonstrate this, 11 catalysts were synthesized, characterized, and investigated for the oxygen evolution reaction (OER). Further investigation of the most active OER catalyst, namely CoNiFeMoCr, revealed a polycrystalline structure, and operando Raman measurements indicate that multiple active sites are participating in the reaction. Moreover, Ni foam-supported CoNiFeMoCr electrodes were developed and applied for water splitting in flow-through electrolysis cells with electrolyte gaps and in zero-gap membrane electrode assembly (MEA) configurations. The proposed alkaline MEA-type electrolyzers reached up to 3 A cm-2 , and 24 h measurements demonstrated no loss of current density of 1 A cm-2 .

Differentiating between Acidic and Basic Surface Hydroxyls on Metal Oxides by Fluoride Substitution: A Case Study on Blue TiO2 from Laser Defect Engineering.

Both oxygen vacancies and surface hydroxyls play a crucial role in catalysis. Yet, their relationship is not often explored. Herein, we prepare two series of TiO2 (rutile and P25) with increasing oxygen deficiency and Ti3+ concentration by pulsed laser defect engineering in liquid (PUDEL), and selectively quantify the acidic and basic surface OH by fluoride substitution. As indicated by EPR spectroscopy, the laser-generated Ti3+ exist near the surface of rutile, but appear to be deeper in the bulk for P25. Fluoride substitution shows that extra acidic bridging OH are selectively created on rutile, while the surface OH density remains constant for P25. These observations suggest near-surface Ti3+ are highly related to surface bridging OH, presumably the former increasing the electron density of the bridging oxygen to form more of the latter. We anticipate that fluoride substitution will enable better characterization of surface OH and its correlation with defects in metal oxides.

Defective Metal-Organic Frameworks with Tunable Porosity and Metal Active Sites for Significantly Improved Performance in Styrene Oxidation.

Metal active sites and sufficient porosity in metal-organic frameworks (MOFs) are crucial parameters determining the performances of catalysis, guest molecule adsorption, etc. Herein, through in situ introduction of Ru sites with different levels to Cu-BTC structure together with post-synthetic activation at 180 °C, a series of hierarchically porous CuRu-BTC (HP-CuRu-BTC) MOFs were obtained. Besides, selective thermal decomposition (STD) treatment was carried out at 240 °C to further tune the hierarchical pores and metal sites, yielding rare case of metal nanoparticles (NPs)@HP-CuRu-BTC composites. After full characterization by XRD, N2 physisorption, SEM, ICP and XPS, these HP-CuRu-BTC and NPs@HP-CuRu-BTC samples possess high surface area (682-1199 m2 g-1 ), hierarchical pores and highly distributed metal sites with reduced oxidation states (Cu+ and Ru2+ ), indicating regulation of both metal sites and hierarchical pores. The HP-CuRu-BTC and NPs@HP-CuRu-BTC were further employed as catalysts for the heterogeneous styrene oxidation reaction under mild condition. Compared to microporous Cu-BTC with unary metal component, HP-CuRu-BTC-3 and NPs@HP-CuRu-BTC-3 exhibited more than 2 times higher styrene conversion after 7 hours reaction under same condition.

Influence of the ZnCrAl Oxide Composition on the Formation of Hydrocarbons from Syngas.

The conversion of syngas into value-added hydrocarbons gains increasing attention due to its potential to produce sustainable platform chemicals from simple starting materials. Along this line, the "OX-ZEO" process that combines a methanol synthesis catalyst with a zeolite, capable of catalyzing the methanol-to-hydrocarbon reaction, was found to be a suitable alternative to the classical Fischer-Tropsch synthesis. Hitherto, understanding the mechanism of the OX-ZEO process and simultaneously optimizing the CO conversion and the selectivity toward a specific hydrocarbon remains challenging. Herein, we present a comparison of a variety of ZnCrAl oxides with different metal ratios combined with a H-ZSM-5 zeolite for the conversion of syngas to hydrocarbons. The effect of aluminum on the catalytic activity was investigated for ZnCrAl oxides with a Zn/Cr ratio of 4:1, 1:1, and 1:2. The product distribution and CO conversion were found to be strongly influenced by the Zn/Cr/Al ratio. Although a ratio of Zn/Cr of 1:2 was best to produce lower olefins and aromatics, with aromatic selectivities of up to 37%, catalysts with a 4:1 ratio revealed high paraffin selectivity up to 52%. Notably, a distinct effect of aluminum in the oxide lattice on the catalytic activity and product selectivity was observed, as a higher Al content leads to a lower CO conversion and a changed product spectrum. We provide additional understanding of the influence of different compositions of ZnCrAl oxides on their surface properties and the catalytic activity in the OX-ZEO process. Furthermore, the variation of the zeolite component supports the important role of the channel topology of the porous support material for the hydrocarbon production. In addition, variation of the gas hourly space velocity showed a correlation of contact time, CO conversion, and hydrocarbon selectivity. At a gas hourly space velocity of 4200 mL/gcat h, CO conversion as high as 44% along with a CO2 selectivity of 42% and a lower paraffin (C2 0-C4 0) selectivity of 41% was observed.

Interactions of water and short-chain alcohols with CoFe2O4(001) surfaces at low coverages.

Iron and cobalt-based oxides crystallizing in the spinel structure are efficient and affordable catalysts for the oxidation of organics, yet, the detailed understanding of their surface structure and reactivity is limited. To fill this gap, we have investigated the (001) surfaces of cobalt ferrite, CoFe2O4, with the A- and B-layer terminations using density functional theory (DFT/PBE0) and an embedded cluster model. We have considered the five-fold coordinated Co2+/3+ (Oh), two-fold coordinated Fe2+ (Td), and an oxygen vacancy, as active sites for the adsorption of water and short-chain alcohols: methanol, ethanol, and 2-propanol, in the low coverage regime. The adsorbates dissociate upon adsorption on the Fe sites whereas the adsorption is mainly molecular on Co. At oxygen vacancies, the adsorbates always dissociate, fill the vacancy and form (partially) hydroxylated surfaces. The computed vibrational spectra for the most stable configurations are compared with results from diffuse reflectance infrared Fourier transform spectroscopy.

Introducing Stacking Faults into Three-Dimensional Branched Nickel Nanoparticles for Improved Catalytic Activity.

Creating high surface area nanocatalysts that contain stacking faults is a promising strategy to improve catalytic activity. Stacking faults can tune the reactivity of the active sites, leading to improved catalytic performance. The formation of branched metal nanoparticles with control of the stacking fault density is synthetically challenging. In this work, we demonstrate that varying the branch width by altering the size of the seed that the branch grows off is an effective method to precisely tune the stacking fault density in branched Ni nanoparticles. A high density of stacking faults across the Ni branches was found to lower the energy barrier for Ni2+/Ni3+ oxidation and result in enhanced activity for electrocatalytic oxidation of 5-hydroxylmethylfurfural. These results show the ability to synthetically control the stacking fault density in branched nanoparticles as a basis for enhanced catalytic activity.

3D atomic-scale imaging of mixed Co-Fe spinel oxide nanoparticles during oxygen evolution reaction.

The three-dimensional (3D) distribution of individual atoms on the surface of catalyst nanoparticles plays a vital role in their activity and stability. Optimising the performance of electrocatalysts requires atomic-scale information, but it is difficult to obtain. Here, we use atom probe tomography to elucidate the 3D structure of 10 nm sized Co2FeO4 and CoFe2O4 nanoparticles during oxygen evolution reaction (OER). We reveal nanoscale spinodal decomposition in pristine Co2FeO4. The interfaces of Co-rich and Fe-rich nanodomains of Co2FeO4 become trapping sites for hydroxyl groups, contributing to a higher OER activity compared to that of CoFe2O4. However, the activity of Co2FeO4 drops considerably due to concurrent irreversible transformation towards CoIVO2 and pronounced Fe dissolution. In contrast, there is negligible elemental redistribution for CoFe2O4 after OER, except for surface structural transformation towards (FeIII, CoIII)2O3. Overall, our study provides a unique 3D compositional distribution of mixed Co-Fe spinel oxides, which gives atomic-scale insights into active sites and the deactivation of electrocatalysts during OER.

Thermicity of the Decomposition of Oxygen Functional Groups on Cellulose-Derived Chars.

The evolution of oxygen functional groups (OFGs) and the associated thermic effects upon heat treatment up to 800 °C were investigated experimentally as well as by theoretical calculations. A synthetic carbon with a carbonaceous structure close to that of natural chars, yet mineral-free, was derived from cellulose and oxidized by HNO3 vapor at different temperatures and for varied durations in order to generate char samples with different concentrations and distributions of OFGs. The functionalized samples were subjected to calorimetric temperature-programmed desorption measurements in correlation with an extensive effluent gas analysis, thereby focusing on the specific heat effects of individual OFG evolution. Interpretation of the experimental results was aided by density functional theory (DFT) calculations which allowed one to infer the thermal stability of different OFGs and the reaction energy associated with their evolution upon heating. Results showed that, with increasing temperature, H2O was released due to the loss of physisorbed water, the decomposition of clusters bound to carboxylic acids, and condensation reactions. The associated heat uptake amounted to about 100 kJ mol-1. Contrarily, the release of CO2, attributed to the decomposition of condensed acids, carboxylic acids, anhydrides, and lactones, resulted in a heat release of about 40 kJ mol-1. The most strongly pronounced thermic effects were detected for the release of CO, comprising highly exothermic effects due to the decomposition of condensed acids and carbonyls/quinones as well as endothermic effects attributed to anhydrides and phenols/ethers. Notably, anhydrides can be formed during the oxidative treatment as well as during heating by condensation of adjacent carboxylic acids. In the latter case, the two-step decomposition is overall highly exothermic, indicating the associated occurrence of pronounced carbon matrix rearrangements. DFT investigations suggest that these rearrangements not only affect the immediate OFG proximity but also involve several carbon sheets.

The Roles of Composition and Mesostructure of Cobalt-Based Spinel Catalysts in Oxygen Evolution Reactions.

By using the crystalline precursor decomposition approach and direct co-precipitation the composition and mesostructure of cobalt-based spinels can be controlled. A systematic substitution of cobalt with redox-active iron and redox-inactive magnesium and aluminum in a cobalt spinel with anisotropic particle morphology with a preferred 111 surface termination is presented, resulting in a substitution series including Co3 O4 , MgCo2 O4 , Co2 FeO4 , Co2 AlO4 and CoFe2 O4 . The role of redox pairs in the spinels is investigated in chemical water oxidation by using ceric ammonium nitrate (CAN test), electrochemical oxygen evolution reaction (OER) and H2 O2 decomposition. Studying the effect of dominant surface termination, isotropic Co3 O4 and CoFe2 O4 catalysts with more or less spherical particles are compared to their anisotropic analogues. For CAN-test and OER, Co3+ plays the major role for high activity. In H2 O2 decomposition, Co2+ reveals itself to be of major importance. Redox active cations in the structure enhance the catalytic activity in all reactions. A benefit of a predominant 111 surface termination depends on the cobalt oxidation state in the as-prepared catalysts and the investigated reaction.

A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols.

Selective oxidation of higher alcohols using heterogeneous catalysts is an important reaction in the synthesis of fine chemicals with added value. Though the process for primary alcohol oxidation is industrially established, there is still a lack of fundamental understanding considering the complexity of the catalysts and their dynamics under reaction conditions, especially when higher alcohols and liquid-phase reaction media are involved. Additionally, new materials should be developed offering higher activity, selectivity, and stability. This can be achieved by unraveling the structure-performance correlations of these catalysts under reaction conditions. In this regard, researchers are encouraged to develop more advanced characterization techniques to address the complex interplay between the solid surface, the dissolved reactants, and the solvent. In this mini-review, we report some of the most important approaches taken in the field and give a perspective on how to tackle the complex challenges for different approaches in alcohol oxidation while providing insight into the remaining challenges.

Liquid-Phase Cyclohexene Oxidation with O2 over Spray-Flame-Synthesized La1-x Srx CoO3 Perovskite Nanoparticles.

La1-x Srx CoO3 (x=0, 0.1, 0.2, 0.3, 0.4) nanoparticles were prepared by spray-flame synthesis and applied in the liquid-phase oxidation of cyclohexene with molecular O2 as oxidant under mild conditions. The catalysts were systematically characterized by state-of-the-art techniques. With increasing Sr content, the concentration of surface oxygen vacancy defects increases, which is beneficial for cyclohexene oxidation, but the surface concentration of less active Co2+ was also increased. However, Co2+ cations have a superior activity towards peroxide decomposition, which also plays an important role in cyclohexene oxidation. A Sr doping of 20 at. % was found to be the optimum in terms of activity and product selectivity. The catalyst also showed excellent reusability over three catalytic runs; this can be attributed to its highly stable particle size and morphology. Kinetic investigations revealed first-order reaction kinetics for temperatures between 60 and 100 °C and an apparent activation energy of 68 kJ mol-1 for cyclohexene oxidation. Moreover, the reaction was not affected by the applied O2 pressure in the range from 10 to 20 bar. In situ attenuated total reflection infrared spectroscopy was used to monitor the conversion of cyclohexene and the formation of reaction products including the key intermediate cyclohex-2-ene-1-hydroperoxide; spin trap electron paramagnetic resonance spectroscopy provided strong evidence for a radical reaction pathway by identifying the cyclohexenyl alkoxyl radical.

Formic Acid-Assisted Selective Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran over Bifunctional Pd Nanoparticles Supported on N-Doped Mesoporous Carbon.

Biomass-derived 5-hydroxymethylfurfural (HMF) is regarded as one of the most promising platform chemicals to produce 2,5-dimethylfuran (DMF) as a potential liquid transportation fuel. Pd nanoparticles supported on N-containing and N-free mesoporous carbon materials were prepared, characterized, and applied in the hydrogenolysis of HMF to DMF under mild reaction conditions. Quantitative conversion of HMF to DMF was achieved in the presence of formic acid (FA) and H2 over Pd/NMC within 2 h. The reaction mechanism, especially the multiple roles of FA, was explored through a detailed comparative study by varying hydrogen source, additive, and substrate as well as by applying in situ ATR-IR spectroscopy. The major role of FA is to shift the dominant reaction pathway from the hydrogenation of the aldehyde group to the hydrogenolysis of the hydroxymethyl group via the protonation by FA at the C-OH group, lowering the activation barrier of the C-O bond cleavage and thus significantly enhancing the reaction rate. XPS results and DFT calculations revealed that Pd2+ species interacting with pyridine-like N atoms significantly enhance the selective hydrogenolysis of the C-OH bond in the presence of FA due to their high ability for the activation of FA and the stabilization of H- .

Effect of Dipole Orientation in Mixed, Charge-Equilibrated Self-assembled Monolayers on Protein Adsorption and Marine Biofouling.

While zwitterionic interfaces are known for their excellent low-fouling properties, the underlying molecular principles are still under debate. In particular, the role of the zwitterion orientation at the interface has been discussed recently. For elucidation of the effect of this parameter, self-assembled monolayers (SAMs) on gold were prepared from stoichiometric mixtures of oppositely charged alkyl thiols bearing either a quaternary ammonium or a carboxylate moiety. The alkyl chain length of the cationic component (11-mercaptoundecyl)-N,N,N-trimethylammonium, which controls the distance of the positively charged end group from the substrate's surface, was kept constant. In contrast, the anionic component and, correspondingly, the distance of the negatively charged carboxylate groups from the surface was varied by changing the alkyl chain length in the thiol molecules from 7 (8-mercaptooctanoic acid) to 11 (12-mercaptododecanoic acid) to 15 (16-mercaptohexadecanoic acid). In this way, the charge neutrality of the coating was maintained, but the charged groups exposed at the interface to water were varied, and thus, the orientation of the dipoles in the SAMs was altered. In model biofouling studies, protein adsorption, diatom accumulation, and the settlement of zoospores were all affected by the altered charge distribution. This demonstrates the importance of the dipole orientation in mixed-charged SAMs for their inertness to nonspecific protein adsorption and the accumulation of marine organisms. Overall, biofouling was lowest when both the anionic and the cationic groups were placed at the same distance from the substrate's surface.

Anchoring of palladium nanoparticles on N-doped mesoporous carbon.

Pd nanoparticles deposited on nitrogen-doped mesoporous carbon are promising catalysts for highly selective and effective catalytic hydrogenation reactions. To design and utilize these novel catalysts, it is essential to understand the effect of N doping on the metal-support interactions. A combined experimental (X-ray photoelectron spectroscopy) and computational (density functional theory) approach is used to identify preferential adsorption sites and to give detailed explanations of the corresponding metal-support interactions. Pyridinic N atoms turned out to be the preferential adsorption sites for Pd nanoparticles on nitrogen-doped mesoporous carbon, interacting through their lone pairs (LPs) with the Pd atoms via N-LP - Pd dσ and N-LP - Pd s and Pd dπ - π* charge transfer, which leads to a change in the Pd oxidation state. Our results evidence the existence of bifunctional palladium nanoparticles containing Pd0 and Pd2+ centers.

Identifying the nature of the active sites in methanol synthesis over Cu/ZnO/Al2O3 catalysts.

The heterogeneously catalysed reaction of hydrogen with carbon monoxide and carbon dioxide (syngas) to methanol is nearly 100 years old, and the standard methanol catalyst Cu/ZnO/Al2O3 has been applied for more than 50 years. Still, the nature of the Zn species on the metallic Cu0 particles (interface sites) is heavily debated. Here, we show that these Zn species are not metallic, but have a positively charged nature under industrial methanol synthesis conditions. Our kinetic results are based on a self-built high-pressure pulse unit, which allows us to inject selective reversible poisons into the syngas feed passing through a fixed-bed reactor containing an industrial Cu/ZnO/Al2O3 catalyst under high-pressure conditions. This method allows us to perform surface-sensitive operando investigations as a function of the reaction conditions, demonstrating that the rate of methanol formation is only decreased in CO2-containing syngas mixtures when pulsing NH3 or methylamines as basic probe molecules.

In situ X-ray emission and high-resolution X-ray absorption spectroscopy applied to Ni-based bimetallic dry methane reforming catalysts.

The promoting effect of cobalt on the catalytic activity of a NiCoO Dry Methane Reforming (DMR) catalyst was studied by a combination of in situ Kß X-ray Emission Spectroscopy (XES) and Kß-detected High Energy Resolution Fluorescence Detected X-ray absorption spectroscopy (HERFD XAS). Following the calcination process, Ni XES and Kß-detected HERFD XAS data revealed that the NiO coordination in the NiCoO catalyst has a higher degree of symmetry and is different than that of pure NiO/γ-Al2O3. Following the reductive activation, it was found that the NiCoO/γ-Al2O3 catalyst required a relatively higher temperature compared to the monometallic NiO/γ-Al2O3 catalyst. This finding suggests that Co is hampering the reduction of Ni in the NiCoO catalyst by modulation of its electronic structure. It has also been previously shown that the addition of Co enhances the DMR activity. Further, the Kß XES spectrum of the partly reduced catalysts at 450 °C reveals that the Ni sites in the NiCoO catalyst are electronically different from the NiO catalyst. The in situ X-ray spectroscopic study demonstrates that reduced metallic Co and Ni are the primary species present after reduction and are preserved under DMR conditions. However, the NiCo catalyst appears to always be somewhat more oxidized than the Ni-only species, suggesting that the presence of cobalt modulates the Ni electronic structure. The electronic structural modulations resulting from the presence of Co may be the key to the increased activity of the NiCo catalyst relative to the Ni-only catalyst. This study emphasizes the potential of in situ X-ray spectroscopy experiments for probing the electronic structure of catalytic materials during activation and under operating conditions.