Enhanced stability of sub-nanometric iridium decorated graphitic carbon nitride for H2 production upon hydrous hydrazine decomposition.

Stabilizing metal nanoparticles is vital for large scale implementations of supported metal catalysts, particularly for a sustainable transition to clean energy, e.g., H2 production. In this work, iridium sub-nanometric particles were deposited on commercial graphite and on graphitic carbon nitride by a wet impregnation method to investigate the metal-support interaction during the hydrous hydrazine decomposition reaction. To establish a structure-activity relationship, samples were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. The catalytic performance of the synthesized materials was evaluated under mild reaction conditions, i.e. 323 K and ambient pressure. The results showed that graphitic carbon nitride (GCN) enhances the stability of Ir nanoparticles compared to graphite, while maintaining remarkable activity and selectivity. Simulation techniques including Genetic Algorithm geometry screening and electronic structure analyses were employed to provide a valuable atomic level understanding of the metal-support interactions. N anchoring sites of GCN were found to minimise the thermodynamic driving force of coalescence, thus improving the catalyst stability, as well as to lead charge redistributions in the cluster improving the resistance to poisoning by decomposition intermediates.

Computational Studies on Microreactors for the Decomposition of Formic Acid for Hydrogen Production Using Heterogeneous Catalysts.

Sustainable alternatives to conventional fuels have emerged recently, focusing on a hydrogen-based economy. The idea of using hydrogen (H2) as an energy carrier is very promising due to its zero-emission properties. The present study investigates the formic acid (FA) decomposition for H2 generation using a commercial 5 wt.% Pd/C catalyst. Three different 2D microreactor configurations (packed bed, single membrane, and double membrane) were studied using computational fluid dynamics (CFD). Parameters such as temperature, porosity, concentration, and flow rate of reactant were investigated. The packed bed configuration resulted in high conversions, but due to catalyst poisoning by carbon monoxide (CO), the catalytic activity decreased with time. For the single and double membrane microreactors, the same trends were observed, but the double membrane microreactor showed superior performance compared with the other configurations. Conversions higher than 80% were achieved, and even though deactivation decreased the conversion after 1 h of reaction, the selective removal of CO from the system with the use of membranes lead to an increase in the conversion afterwards. These results prove that the incorporation of membranes in the system for the separation of CO is improving the efficiency of the microreactor.

Selective decomposition of hydrazine over metal free carbonaceous materials.

Herein we report a combined experimental and computational investigation unravelling the hydrazine hydrate decomposition reaction on metal-free catalysts. The study focuses on commercial graphite and two different carbon nanofibers, pyrolytically stripped (CNF-PS) and high heat-treated (CNF-HHT), respectively, treated at 700 and 3000 °C to increase their intrinsic defects. Raman spectroscopy demonstrated a correlation between the initial catalytic activity and the intrinsic defectiveness of carbonaceous materials. CNF-PS with higher defectivity (ID/IG = 1.54) was found to be the best performing metal-free catalyst, showing a hydrazine conversion of 94% after 6 hours of reaction and a selectivity to H2 of 89%. In addition, to unveil the role of NaOH, CNF-PS was also tested in the absence of alkaline solution, showing a decrease in the reaction rate and selectivity to H2. Density functional theory (DFT) demonstrated that the single vacancies (SV) present on the graphitic layer are the only active sites promoting hydrazine decomposition, whereas other defects such as double vacancy (DV) and Stone-Wales (SW) defects are unable to adsorb hydrazine fragments. Two symmetrical and one asymmetrical dehydrogenation pathways were found, in addition to an incomplete decomposition pathway forming N2 and NH3. On the most stable hydrogen production pathway, the effect of the alkaline medium was elucidated through calculations concerning the diffusion and recombination of atomic hydrogen. Indeed, the presence of NaOH helps the extraction of H species without additional energetic barriers, as opposed to the calculations performed in a polarizable continuum medium. Considering the initial hydrazine dissociative adsorption, the first step of the dehydrogenation pathway is more favourable than the scission of the N-N bond, which leads to NH3 as the product. This first reaction step is crucial to define the reaction mechanisms and the computational results are in agreement with the experimental ones. Moreover, comparing two different hydrogen production pathways (with and without diffusion and recombination), we confirmed that the presence of sodium hydroxide in the experimental reaction environment can modify the energy gap between the two pathways, leading to an increased reaction rate and selectivity to H2.

Quantifying Morphology and Diffusion Properties of Mesoporous Carbon From High-Fidelity 3D Reconstructions.

A reliable quantitative analysis in electron tomography, which depends on the segmentation of the three-dimensional reconstruction, is challenging because of constraints during tilt-series acquisition (missing wedge) and reconstruction artifacts introduced by reconstruction algorithms such as the Simultaneous Iterative Reconstruction Technique (SIRT) and Discrete Algebraic Reconstruction Technique (DART). We have carefully evaluated the fidelity of segmented reconstructions analyzing a disordered mesoporous carbon used as support in catalysis. Using experimental scanning transmission electron microscopy (STEM) tomography data as well as realistic phantoms, we have quantitatively analyzed the effect on the morphological description as well as on diffusion properties (based on a random-walk particle-tracking simulation) to understand the role of porosity in catalysis. The morphological description of the pore structure can be obtained reliably both using SIRT and DART reconstructions even in the presence of a limited missing wedge. However, the measured pore volume is sensitive to the threshold settings, which are difficult to define globally for SIRT reconstructions. This leads to noticeable variations of the diffusion coefficients in the case of SIRT reconstructions, whereas DART reconstructions resulted in more reliable data. In addition, the anisotropy of the determined diffusion properties was evaluated, which was significant in the presence of a limited missing wedge for SIRT and strongly reduced for DART.

CNF-Functionalization as Versatile Tool for Tuning Activity in Cellulose-Derived Product Hydrogenation.

Carbon nanofibers (CNFs) have been functionalized by introducing O, N, and P containing groups in order to investigate the effect of support functionalization in Ru catalysed hydroxymethyl furfural (HMF) and levulinic acid (LA) hydrogenation. In the case of HMF, despite the fact that no effect on selectivity was observed (all the catalysts produced selectively gamma-valerolactone (GVL)), the functionalization strongly affected the activity of the reaction. O-containing and N-containing supports presented a higher activity compared to the bare support. On the contrary, in HMF hydrogenation, functionalization of the support did not have a beneficial effect on the activity of a Ru-catalysed reaction with respect to bare support and only CNFs-O behaved similarly to bare CNFs. In fact, when CNFs-N or CNFs-P were used as the supports, a lower activity was observed, as well as a change in selectivity in which the production of ethers (from the reaction with the solvent) greatly increased.

Ruthenium on Carbonaceous Materials for the Selective Hydrogenation of HMF.

We report the use of Ru catalysts supported in the activated carbon (AC) and carbon nanofibers (CNFs) for the selective production of liquid fuel dimethylfuran (DMF) and fuel additives alkoxymethyl furfurals (AMF). Parameters such as the reaction temperature and hydrogen pressure were firstly investigated in order to optimise the synthesis of the desired products. Simply by using a different support, the selectivity of the reaction drastically changed. DMF was produced with AC as support, while a high amount of AMF was produced when CNFs were employed. Moreover, the reusability of the catalysts was tested and deactivation phenomena were identified and properly addressed. Further studies need to be performed in order to optimise the stability of the catalysts.

Characterisation of gold catalysts.

Au-based catalysts have established a new important field of catalysis, revealing specific properties in terms of both high activity and selectivity for many reactions. However, the correlation between the morphology and the activity of the catalyst is not always clear although much effort has been addressed to this task. To some extent the problem relates to the complexity of the characterisation techniques that can be applied to Au catalyst and the broad range of ways in which they can be prepared. Indeed, in many reports only a few characterization techniques have been used to investigate the potential nature of the active sites. The aim of this review is to provide a critical description of the techniques that are most commonly used as well as the more advanced characterization techniques available for this task. The techniques that we discuss are (i) transmission electron microscopy methods, (ii) X-ray spectroscopy techniques, (iii) vibrational spectroscopy techniques and (iv) chemisorption methods. The description is coupled with developing an understanding of a number of preparation methods. In the final section the example of the supported AuPd alloy catalyst is discussed to show how the techniques can gain an understanding of an active oxidation catalyst.

Glycerol oxidation using gold-containing catalysts.

Glycerol is an important byproduct of biodiesel production, and it is produced in significant amounts by transesterification of triglycerides with methanol. Due to the highly functionalized nature of glycerol, it is an important biochemical that can be utilized as a platform chemical for the production of high-added-value products. At present, research groups in academia and industry are exploring potential direct processes for the synthesis of useful potential chemicals using catalytic processes. Over the last 10 years, there has been huge development of potential catalytic processes using glycerol as the platform chemical. One of the most common processes investigated so far is the catalytic oxidation of glycerol at mild conditions for the formation of valuable oxygenated compounds used in the chemical and pharmaceutical industry. The major challenges associated with the selective oxidation of glycerol are (i) the control of selectivity to the desired products, (ii) high activity and resistance to poisoning, and (iii) minimizing the usage of alkaline conditions. To address these challenges, the most common catalysts used for the oxidation of glycerol are based on supported metal nanoparticles. The first significant breakthrough was the successful utilization of supported gold nanoparticles for improving the selectivity to specific products, and the second was the utilization of supported bimetallic nanoparticles based on gold, palladium, and platinum for improving activity and controlling the selectivity to the desired products. Moreover, the utilization of base-free reaction conditions for the catalytic oxidation of glycerol has unlocked new pathways for the production of free-base products, which facilitates potential industrial application. The advantages of using gold-based catalysts are the improvement of the catalyst lifetime, stability, and reusability, which are key factors for potential commercialization. In this Account, we discuss the advantages of the using supported gold-based nanoparticles, preparation methods for achieving highly active gold-based catalysts, and parameters such as particle size, morphology of the bimetallic particle, and metal-support interactions, which can influence activity and selectivity to the desired products.

N-Modified Carbon-Based Materials: Nanoscience for Catalysis.

Carbon-based materials constitute a large family of materials characterized by some peculiarities such as resistance to both acidic and basic environments, flexibility of structure, and surface chemical groups. Moreover, they can be deeply modified by simple organic reactions (acid-base or redox) to acquire different properties. In particular, the introduction of N-containing groups, achieved by post-treatments or during preparation of the material, enhances the basic properties. Moreover, it has been revealed that the position and chemical nature of the N-containing groups is important in determining the interaction with metal nanoparticles, and thus, their reactivity. The modified activity was addressed to a different metal dispersion. Moreover, experiments on catalysts, showing the same metal dispersion, demonstrated that the best results were obtained when N was embedded into the carbon structure and not very close to the metal active site.

Au-Based Catalysts: Electrochemical Characterization for Structural Insights.

Au-based catalysts are widely used in important processes because of their peculiar characteristics. The catalyst performance depends strongly on the nature and structure of the metal nanoparticles, especially in the case of bimetallic catalysts where synergistic effects between the two metals can be occasionally seen. In this paper, it is shown that electrochemical characterisation (cyclovoltammetry CV and electrochemical impedance spectroscopy EIS) of AuPd systems can be used to determine the presence of an electronic interaction between the two metals, thus providing a strong support in the determination of the nature of the synergy between Au and Pd in the liquid phase oxidation of alcohols. However, it seems likely that the strong difference in the catalytic behavior between the single metals and the bimetallic system is connected not only to the redox behaviour, but also to the energetic balance between the different elementary steps of the reaction.

Gold colloids: from quasi-homogeneous to heterogeneous catalytic systems.

Ruby red colloids of gold have been used for thousands of years and in the past have attracted much attention due to their optical properties. Surface plasmon resonance (SPR) bands are responsible for gold colloid colors and typically appear for nanometer-sized gold nanoparticles (GNPs). These lie in the visible range and their position (and intensity) depends on the size, distribution of size, and shape of GNPs but also on their interaction with other materials (i.e., support). Scientists consider colloids as quasi-homogeneous systems, but because of their intrinsic thermodynamic instability, they need different capping agents providing sufficient stability. The strength and the nature of the interaction between the protective (or functionalizing) molecule and the GNP surface is of utmost importance. It can determine the catalytic properties of the nanoparticles, as they mainly interact with the active sites, thus interfering with reactant. Therefore, the protective layer should contribute to the colloid stability, but at the same time, it should not be irreversibly adsorbed on the active site of the GNP surface providing convenient accessibility to reactant. From a catalytic point of view, the milder the interaction is between the particle surface and the capping agent, the more the activity increases. Unfortunately, the reaction conditions often do not allow the required stability of GNPs, which constitutes a fundamental prerequisite for stable catalytic activity. Anchoring GNPs on suitable supports can circumvent the problem, and this technique is now considered a valuable alternative to classical methods to produce highly dispersed gold catalysts. In this Account, we describe the advantages in using this technique to produce gold heterogeneous catalysts of high metal dispersion on a large variety of supports with the possibility of tuning to a large extent the size and (even partially) the shape of GNPs. We also review our recent progress on the sol-immobilization technique. Specifically, we highlight how, depending on its nature, the protective agent not only mediates the activity of GNPs in alcohol oxidation process but also actively participates in the anchoring process and to the stability of GNPs depending on the support surface. We can also use the modification of the metal surface operated by the capping agent to prepare bimetallic species and influence the surface potential, which modifies the intrinsic activity of the GNP. In conclusion, this technique implies many contributions (sometimes not yet clarified factors) that are not simply concerning dimension and dispersion of GNPs or type of support. Chemists should make careful selection of the protective agent and reaction parameters depending on which support is used in which reaction.

AuRu/AC as an effective catalyst for hydrogenation reactions.

AuRu bimetallic catalysts have been prepared by sequential deposition of Au on Ru or vice versa obtaining different nanostructures: when Ru has been deposited on Au, a Au(core)-Ru(shell) has been observed, whereas the deposition of Au on Ru leads to a bimetallic phase with Ru enrichment on the surface. In the latter case, the unexpected Ru enrichment could be attributed to the weak adhesion of Ru on the carbon support, thus allowing Ru particles to diffuse on Au particles. Both structures result very active in catalysing the liquid phase hydrogenolysis of glycerol and levulinic acid but the activity, the selectivity and the stability depend on the structure of the bimetallic nanoparticles. Ru@Au/AC core-shell structure mostly behaved as the monometallic Ru, whereas the presence of bimetallic AuRu phase in Au@Ru/AC provides a great beneficial effect on both activity and stability.

Ultrasonic reactor set-ups and applications: A review.

Sonochemistry contributes to green science as it uses less hazardous solvents and methods to carry out a reaction. In this review, different reactor designs are discussed in detail providing the necessary knowledge for implementing various processes. The main characteristics of ultrasonic batch systems are their low cost and enhanced mixing; however, they still have immense drawbacks such as their scalability. Continuous flow reactors offer enhanced production yields as the limited cognition which governs the design of these sonoreactors, renders them unusable in industry. In addition, microstructured sonoreactors show improved heat and mass transfer phenomena due to their small size but suffer though from clogging. The optimisation of various conditions of regulations, such as temperature, frequency of ultrasound, intensity of irradiation, sonication time, pressure amplitude and reactor design, it is also discussed to maximise the production rates and yields of reactions taking place in sonoreactors. The optimisation of operating parameters and the selection of the reactor system must be considered to each application's requirements. A plethora of different applications that ultrasound waves can be implemented are in the biochemical and petrochemical engineering, the chemical synthesis of materials, the crystallisation of organic and inorganic substances, the wastewater treatment, the extraction processes and in medicine. Sonochemistry must overcome challenges that consider the scalability of processes and its embodiment into commercial applications, through extensive studies for understanding the designs and the development of computational tools to implement timesaving and efficient theoretical studies.

Modification of the terms of authorisation regarding the maximum inclusion level of a feed additive consisting of 4-hydroxy-2,5-dimethylfuran-3(2H)-one for cats and dogs (V. MANE FILS).

Following a request from the European Commission, EFSA was asked to deliver a scientific opinion on the proposed modification of the terms of the authorisation regarding the maximum inclusion level of a feed additive consisting of 4-hydroxy-2,5-dimethylfuran-3(2H)-one for cats and dogs. 4-Hydroxy-2,5-dimethylfuran-3(2H)-one is currently authorised for use as a sensory additive (functional group: flavouring compounds) for cats and dogs at a recommended maximum content of 5 mg/kg complete feed. The applicant is requesting a modification of the authorisation to increase the recommended maximum content of the additive up to 25 mg/kg complete feed for cats and dogs. Based on the toxicological data available, the FEEDAP Panel concludes that 4-hydroxy-2,5-dimethylfuran-3(2H)-one is safe for dogs at 25 mg/kg feed and for cats at 18 mg/kg feed. The additive is irritant to skin, eyes and to the respiratory tract and is a skin sensitiser. No further demonstration of efficacy is necessary.

Safety and efficacy of a feed additive consisting of vitamin B12 (cyanocobalamin) produced by fermentation with Ensifer adhaerens CGMCC 21299 for all animal species (NHU Europe GmbH).

Following a request from the European Commission, EFSA was asked to deliver a scientific opinion on the safety and efficacy of vitamin B12 (cyanocobalamin) produced by fermentation with a non-genetically modified strain of Ensifer adhaerens (CGMCC 21299), when used as a nutritional additive for all animal species. No viable cells or DNA of the production strain were detected in the additive. Therefore, cyanocobalamin produced by fermentation with E. adhaerens CGMCC 21299 does not raise safety concerns as regards to the production strain. The Panel on Additives and Products or Substances used in Animal Feed concluded that cyanocobalamin produced by fermentation with E. adhaerens CGMCC 21299 is considered safe for all animal species, for the consumers and the environment. Due to the presence of nickel, the additive is considered a skin and respiratory sensitiser. Inhalation and dermal exposure are considered a risk. Due to the lack of data, the Panel could not conclude on the potential of the additive to be an eye irritant. Cyanocobalamin produced by fermentation with E. adhaerens CGMCC 21299 is effective in meeting animal's nutritional requirements when administered via feed.

Safety and efficacy of a feed additive consisting of monensin sodium (Coxidin®) for chickens for fattening, chickens reared for laying, turkeys for fattening and turkeys reared for breeding (Huvepharma N.V.).

Following a request from the European Commission, EFSA was asked to deliver a scientific opinion on the safety and efficacy of monensin sodium (Coxidin®) as a coccidiostat for chickens for fattening, chickens reared for laying, turkeys for fattening and turkeys reared for breeding. The additive currently on the market complies with the existing conditions of authorisation. The FEEDAP Panel concluded that Coxidin® remains safe for turkeys for fattening (up to 16 weeks) and extends this conclusion to turkeys reared for breeding (up to 16 weeks). The Panel was not in the position to confirm that the current maximum authorised level of 125 mg monensin sodium/kg complete feed remains safe for chickens for fattening and chickens reared for laying. The use of monensin sodium from Coxidin® at the corresponding maximum authorised/proposed use levels in the target species is safe for the consumer. The existing maximum residue levels (MRLs) for poultry tissues ensure consumer safety. No withdrawal time is necessary. Both formulations of Coxidin® pose a risk by inhalation. The formulation with wheat bran as a carrier was neither irritant to the skin nor a skin sensitiser but it was irritant to the eyes. In the absence of data, no conclusions could be made on the potential of the formulation containing calcium carbonate to be irritant to skin and eyes and to be a skin sensitiser. The use of monensin sodium from Coxidin® in complete feed for the target species poses no risk for the terrestrial compartments and for sediment. No risk for groundwater is expected. For chickens for fattening the risk for aquatic compartment cannot be excluded, but no risks are expected for the other animal categories. There is no risk of secondary poisoning. Coxidin® is efficacious in controlling coccidiosis at a level of 100 mg/kg complete feed for chickens for fattening and at 60 mg/kg complete feed for turkeys for fattening. These conclusions are extended to chickens reared for laying and turkeys reared for breeding. The Panel noted that there are signs of development of resistance of Eimeria spp. to monensin sodium.

Safety and efficacy of a feed additive consisting of narasin (Monteban® G100) for chickens for fattening (Elanco GmbH).

Following a request from the European Commission, EFSA was asked to deliver a scientific opinion on the safety and efficacy of the coccidiostat narasin (Monteban® G100) for chickens for fattening. In a previous opinion, uncertainties remained on the identification and characterisation of the non-genetically modified production strain of the active substance narasin. The Panel could not conclude either on the safety of Monteban® G100 for chickens for fattening or on the efficacy of the additive at the minimum applied concentration. The FEEDAP Panel excluded risks for environment but the risk for sediment compartment could not be assessed. The applicant provided supplementary information to cover the data gaps and substituted the narasin production strain from Streptomyces spp. NRRL 8092 to Streptomyces spp. NRRL B-67771. The information submitted to taxonomically identify the production strain did not allow to assign it to any described microbial species. Based on the information provided, the Panel concluded that the use of Monteban® G100 did not raise safety concerns as regards the production strain for the target animal, consumer, user and environment. The Panel concluded that 70 mg narasin/kg complete feed was safe for chickens for fattening with a margin of safety of 1.4; narasin from Monteban® G100 was unlikely to increase shedding of Salmonella Enteritidis, Salmonella Typhimurium and Campylobacter jejuni. Narasin, when used in chickens for fattening at 70 mg/kg feed, was not expected to pose a risk to the aquatic compartment and to sediment, while a risk for the terrestrial compartment could not be excluded. No risk for groundwater was expected, nor for secondary poisoning via the terrestrial food chain, but the risk of secondary poisoning via the aquatic food chain could not be excluded. The Panel concluded that 60 mg narasin/kg feed was efficacious in controlling coccidiosis in chickens for fattening.

Safety and efficacy of a feed additive consisting of salinomycin sodium (Sacox®) for rabbits for fattening (Huvepharma N.V.).

Following a request from the European Commission, EFSA was asked to deliver a scientific opinion on the safety and efficacy of the coccidiostat salinomycin sodium (Sacox®) for rabbits for fattening. The EFSA Panel on Additives and Products or Substances used in Animal Feed (FEEDAP) concluded that the use of salinomycin sodium (SAL-Na) from Sacox® does not raise safety concerns for the target species, consumers, users and the environment with regard to the production strain. In the absence of adequate tolerance studies, the FEEDAP Panel could not conclude on the safety of SAL-Na from Sacox® for rabbits for fattening. The FEEDAP Panel concluded that the additive is safe for the consumer when it is used at the proposed maximum level of 25 mg SAL-Na/kg complete feed for rabbits and a withdrawal period of 1 day is respected. The following maximum residue limits (MRL) are proposed for the marker residue compound salinomycin (SAL): 0.2 and 0.03 mg SAL/kg for liver and kidney, respectively. The additive is not irritant to skin and eyes but should be considered a potential dermal and respiratory sensitiser. A risk for inhalation toxicity could not be excluded. The use of the SAL-Na from Sacox® in feed for rabbits for fattening up to the highest proposed level will not pose a risk for the terrestrial and aquatic compartment and ground water. The risk of secondary poisoning can be excluded for worm-eating birds and mammals, while it cannot be excluded for fish-eating birds and mammals. The FEEDAP Panel concludes that SAL-Na from Sacox® at the minimum concentration of 20 mg SAL-Na/kg complete feed has the potential to control coccidiosis in rabbits for fattening. Development of resistance to SAL-Na of field Eimeria spp. strains isolated from rabbits for fattening should be monitored.

Guidance for the assessment of detoxification processes in feed.

This statement provides scientific guidance on the information needed to support the risk assessment of the detoxification processes applied to products intended for animal feed in line with the acceptability criteria of the Commission Regulation (EU) 2015/786.

Disclosing the leaching behaviour of Pd@CMK3 catalysts in formic acid decomposition by electron tomography.

Supported nanocatalysts exhibit different performances in batch and fixed bed reactors for a wide range of liquid phase catalytic reactions due to differences in metal leaching. To investigate this leaching process and its influence on the catalytic performance, a quantitative 3D characterization of the particle size and the particle distribution is important to follow the structural evolution of the active metal catalysts supported on porous materials during the reaction. In this work, electron tomography has been applied to uncover leaching and redeposition of a Pd@CMK3 catalyst during formic acid decomposition in batch and fixed bed reactors. The 3D distribution of Pd NPs on the mesoporous carbon CMK3 has been determined by a quantitative tomographic analysis and the determined structural changes are correlated with the observed differences in activity and stability of formic acid decomposition using batch and fixed bed reactors.