Opportunities of MXenes in Heterogeneous Catalysis: V2C as Aerobic Oxidation Catalyst.

MXenes are two-dimensional nanomaterials having alternating sheets of one atom-thick early transition metal layer and one atom-thick carbide or nitride layer. The external surface contains termination groups, whose nature depends on the etching agent used in the preparation procedure from the MAX phase. The present concept proposes that, due to their composition, the metal-surface termination groups make MXenes particularly suited as heterogeneous catalysts for some reactions. This proposal comes from the consideration that early transition metal atoms bonded to hydroxyl and oxo groups are a general type of active sites in heterogeneous catalysis and that similar catalytic centers can also be present in the MXene structure. After having presented the concept, we have selected V2C Mxene as an example to illustrate its catalytic activity and to show how the catalytic performance varies when the surface groups are modified. As a test reaction, we selected the aerobic oxidation of indane to the corresponding indanol/indanone mixture using molecular oxygen as terminal oxidizing reagent. Two previously reported procedures to modify the surface groups, namely surface dehydroxylation by thermal treatment under diluted hydrogen flow and surface oxidation with ammonium persulfate to convert some surface groups into oxo groups were used, observing in both cases a decrease in the catalytic activity of V2C. Based on this, VIII/IV-OH are proposed as catalytic centers in this aerobic oxidation. Overall, the present concept shows the merits of MXenes in heterogeneous catalysis, based on their chemical composition and the surface functionality.

Selective Gas-Phase Hydrogenation of CO2 to Methanol Catalysed by Metal-Organic Frameworks.

Large scale production of green CH3 OH obtained from CO2 and green H2 is a highly wanted process due to the role of CH3 OH as H2 /energy carrier and for producing chemicals. Starting with a short summary of the advantages of metal-organic frameworks (MOFs) as catalysts in liquid-phase reactions, the present article highlights the opportunities that MOFs may offer also for some gas-phase reactions, particularly for the selective CO2 hydrogenation to CH3 OH. It is commented that there is a temperature compatibility window that combines the thermal stability of some MOFs with the temperature required in the CO2 hydrogenation to CH3 OH that frequently ranges from 250 to 300 °C. The existing literature in this area is briefly organized according to the role of MOF as providing the active sites or as support of active metal nanoparticles (NPs). Emphasis is made to show how the flexibility in design and synthesis of MOFs can be used to enhance the catalytic activity by adjusting the composition of the nodes and the structure of the linkers. The influence of structural defects and material crystallinity, as well as the role that should play theoretical calculations in models have also been highlighted.

Palladium Supported on Porous Chitosan-Graphene Oxide Aerogels as Highly Efficient Catalysts for Hydrogen Generation from Formate.

Adsorption of Pd(NH3)42+ in preformed chitosan-graphene oxide (CS-GO) beads and their subsequent reduction with NaBH4 afford well-dispersed, high dispersion (~21%) of uniformly sized Pd nanoparticles (~1.7 nm). The resulting Pd/CS-GO exhibits interesting catalytic activity for hydrogen generation by ammonium formate decomposition. The optimal GO proportion of 7 wt% allows reaching, at 60 °C, a turnover frequency above 2200 h-1-being outstanding among the highest values reported for this process to date. Interestingly, no formation of CO or CH4 was detected. The catalyst did not leach, although it underwent gradual deactivation, probably caused by the increase in the Pd average size that became over 3 nm after three uses. Our results are relevant in the context of efficient on-board hydrogen generation from liquid organic hydrogen carriers in transportation.

One-Step Pyrolysis Preparation of 1.1.1 Oriented Gold Nanoplatelets Supported on Graphene and Six Orders of Magnitude Enhancement of the Resulting Catalytic Activity.

Pyrolysis of chitosan films containing Au(3+) renders 1.1.1 oriented Au nanoplatelets (20 nm lateral size, 3-4 nm height) on a few layers of N-doped graphene (Au/fl-G), while the lateral sides were 0.0.1 oriented. Comparison of the catalytic activity of Au/fl-G films with powders of unoriented Au NPs supported on graphene showed that Au/fl-G films exhibit six orders of magnitude enhancement for three gold-catalyzed reactions, namely, Ullmann-like homocoupling, C-N cross coupling, and the oxidative coupling of benzene to benzoic acid. This enhancement is the result of the defined morphology, facet orientation of Au nanocrystals, and strong gold-graphene interaction.

p-n Heterojunction of doped graphene films obtained by pyrolysis of biomass precursors.

Nitrogen-doped graphene [(N)G] obtained by pyrolysis at 900 °C of nanometric chitosan films exhibits a Hall effect characteristic of n-type semiconductors. In contrast, boron-doped graphene [(B)G] obtained by pyrolysis of borate ester of alginate behaves as a p-type semiconductor based also on the Hall effect. A p-n heterojunction of (B)G-(N)G films is built by stepwise coating of a quartz plate using a mask. The heterojunction is created by the partial overlapping of the (B)G-(N)G films. Upon irradiation with a xenon lamp of aqueous solutions of H(2) PtCl(6) and MnCl(2) in contact with the heterojunction, preferential electron migration from (B)G to (N)G with preferential location of positive holes on (B)G is established by observation in scanning electron microscopy of the formation of Pt nanoparticles (NP) on (N)G and MnO(2) NP on (B)G. The benefits of the heterojunction with respect to the devices having one individual component as a consequence of the electron migration through the p-n heterojunction are illustrated by measuring the photocurrent in the (B)G-(N)G heterojunction (180% current enhancement with respect to the dark current) and compared it to the photocurrent of the individual (B)G (15% enhancement) and (N)G (55% enhancement) components.

Chitosan-templated synthesis of few-layers boron nitride and its unforeseen activity as a Fenton catalyst.

A novel procedure for the preparation of few-layers boron nitride (BN), either as films or as suspended BN platelets, is presented, based on the pyrolysis of chitosan, which serves both as a matrix to embed ammonium borate and as a template for BN synthesis. The resulting BN samples are characterized by XRD, Raman and X-ray photoelectron spectroscopy, and by TEM and AFM imaging. The samples exhibit deep UV emission, which is characteristic of high quality BN. This template synthesis and the easy exfoliation of BN platelets facilitate the use of BN as an extremely high-efficiency Fenton catalyst for the generation of highly aggressive hydroxyl radicals in water.

Graphenes as Efficient Metal-Free Fenton Catalysts.

Reduced graphene oxide exhibits high activity as Fenton catalyst with HO(.) radical generation efficiency over 82 % and turnover numbers of 4540 and 15023 for phenol degradation and H2 O2 consumption, respectively. These values compare favorably with those achieved with transition metals, showing the potential of carbocatalysts for the Fenton reaction.

Zeolites as catalysts in oil refining.

Oil is nowadays the main energy source and this prevalent position most probably will continue in the next decades. This situation is largely due to the degree of maturity that has been achieved in oil refining and petrochemistry as a consequence of the large effort in research and innovation. The remarkable efficiency of oil refining is largely based on the use of zeolites as catalysts. The use of zeolites as catalysts in refining and petrochemistry has been considered as one of the major accomplishments in the chemistry of the XXth century. In this tutorial review, the introductory part describes the main features of zeolites in connection with their use as solid acids. The main body of the review describes important refining processes in which zeolites are used including light naphtha isomerization, olefin alkylation, reforming, cracking and hydrocracking. The final section contains our view on future developments in the field such as the increase in the quality of the transportation fuels and the coprocessing of increasing percentage of biofuels together with oil streams. This review is intended to provide the rudiments of zeolite science applied to refining catalysis.

N-doped graphene derived from biomass as a visible-light photocatalyst for hydrogen generation from water/methanol mixtures.

There is much current interest in developing graphene-based materials as photocatalysts, particularly in the field of solar fuels and the photocatalytic generation of hydrogen. Graphene is a versatile material allowing different modification strategies to improve its activity. Thus, in the present manuscript we report that, in contrast to the lack of photocatalytic activity of undoped graphene, nitrogen doping introduces UV- and visible-light activity for hydrogen evolution; the efficiency of the material depends on the preparation conditions. The N-doped graphene is obtained by pyrolysis under an inert atmosphere of natural chitosan, which is considered a biomass waste, followed by ultrasound exfoliation, without the need of oxidation and reconstitution. The main parameter controlling the residual amount of nitrogen and the resulting photocatalytic activity is the pyrolysis temperature that produces an optimal material when the thermal treatment is carried out at 900 °C. Due to the fact that, in contrast to graphene oxide, N-doped graphene exhibits an almost "neutral" absorption spectrum, the material exhibits photocatalytic activity upon UV- (355 nm) and visible-light (532 nm) irradiation, and is able to generate hydrogen upon simulated sunlight illumination.

Copper nanoparticles supported on doped graphenes as catalyst for the dehydrogenative coupling of silanes and alcohols.

Copper nanoparticles (NPs) supported on a series of undoped and doped graphene materials (Gs) have been obtained by pyrolysis of alginate or chitosan biopolymers, modified or not with boric acid, containing Cu(2+) ions at 900 °C under inert atmosphere. The resulting Cu-G materials containing about 17 wt % Cu NPs (from 10 to 200 nm) exhibit high catalytic activity for the dehydrogenative coupling of silanes with alcohols. The optimal material consisting on Cu-(B)G is more efficient than Cu NPs on other carbon supports.

Nanometric Cu-ZnO Particles Supported on N-Doped Graphitic Carbon as Catalysts for the Selective CO2 Hydrogenation to Methanol.

The quest for efficient catalysts based on abundant elements that can promote the selective CO2 hydrogenation to green methanol still continues. Most of the reported catalysts are based on Cu/ZnO supported in inorganic oxides, with not much progress with respect to the benchmark Cu/ZnO/Al2O3 catalyst. The use of carbon supports for Cu/ZnO particles is much less explored in spite of the favorable strong metal support interaction that these doped carbons can establish. This manuscript reports the preparation of a series of Cu-ZnO@(N)C samples consisting of Cu/ZnO particles embedded within a N-doped graphitic carbon with a wide range of Cu/Zn atomic ratio. The preparation procedure relies on the transformation of chitosan, a biomass waste, into N-doped graphitic carbon by pyrolysis, which establishes a strong interaction with Cu nanoparticles (NPs) formed simultaneously by Cu2+ salt reduction during the graphitization. Zn2+ ions are subsequently added to the Cu-graphene material by impregnation. All the Cu/ZnO@(N)C samples promote methanol formation in the CO2 hydrogenation at temperatures from 200 to 300 °C, with the temperature increasing CO2 conversion and decreasing methanol selectivity. The best performing Cu-ZnO@(N)C sample achieves at 300 °C a CO2 conversion of 23% and a methanol selectivity of 21% that is among the highest reported, particularly for a carbon-based support. DFT calculations indicate the role of pyridinic N doping atoms stabilizing the Cu/ZnO NPs and supporting the formate pathway as the most likely reaction mechanism.

Climate-driven shifts in decapod larvae assemblages in a temperate estuary.

The study examines the complex impact of climatic patterns, driven by the North Atlantic Oscillation (NAO), on regional climate, hydrology, and sea surface temperatures. Focused on the period from 2003 to 2012, the research specifically investigates the influence of thermal variability on decapod larval communities. Monthly zooplanktonic sampling conducted at the Mondego Estuary, Portugal, entrance over a decade revealed the prevalence of Carcinus maenas, Diogenes pugilator, and Pachigrapsus marmoratus larvae. These assemblages displayed notable interannual and seasonal fluctuations, often corresponding with changes in sea surface temperatures. Significant system shifts around 2007, instigated by the large-scale NAO, led to subsequent modifications in sea surface temperature and decapod larvae communities' dynamics. Post-2007, there was an upward trajectory in both species' abundance and richness. Phenologically during the former period, the community exhibited two abundance peaks, with the earlier peak occurring sooner, attributed to heightened temperatures instead of the unique peak exhibited before 2007. The research further elucidated the occurrences of Marine Heatwaves (MHW) in the region, delving into their temporal progression influenced by the NAO. Although water temperature emerged as a crucial factor influencing decapod larvae communities annually and seasonally, the study did not observe discernible impacts of MHW events on these communities. These communities represent essential trophic links and are crucial for the survival success of adult decapods. Given the rapid pace of climate change and increasing temperatures, it is imperative to assess whether these environmental shifts, particularly in thermal conditions, affect these meroplanktonic communities.

Doped graphene as a metal-free carbocatalyst for the selective aerobic oxidation of benzylic hydrocarbons, cyclooctane and styrene.

Nitrogen (N)-, boron (B)-, and boron,nitrogen (B,N)-doped graphene (G) act as carbocatalysts, promoting the aerobic oxidation of the benzylic positions of aromatic hydrocarbons and cyclooctane to the corresponding alcohol/ketone mixture with more than 90 % selectivity. The most active material was the co-doped (B,N)G, which, in the absence of solvent and with a substrate/(B,N)G ratio of 200, achieved 50 % tetralin conversion in 24 h with a alcohol/ketone selectivity of 80 %. An FT-Raman spectroscopic study of a sample of (B,N)G heated at 100 °C in the presence of oxygen revealed new bands that disappeared upon evacuation and that have been attributed to hydroperoxide-like species formed on the G sheet based on the isotopic shift of the peak from 819 to 779 cm(-1) when (18)O2 was used as the oxidizing reagent. Furthermore, (B)G and (N)G exhibited high catalytic activity in the aerobic oxidation of styrene to benzaldehyde (BA) in 4 h. However, the product distribution changed over time and after 10 h a significant percentage of styrene oxide (SO) was observed under the same conditions. The use of doped G as catalyst appears to offer broad scope for the aerobic oxidation of benzylic compounds and styrene, for which low catalyst loading, mild reaction temperatures, and no additional solvents are required.

Preparation of graphene quantum dots from pyrolyzed alginate.

Pyrolysis at 900 °C under an inert atmosphere of alginate, a natural widely available biopolymer, renders a graphitic carbon that upon ablation by exposure to a pulsed 532 nm laser (7 ns, 50 mJ pulse(-1)) in acetonitrile, water, and other solvents leads to the formation of multilayer graphitic quantum dots. The dimensions and the number of layers of these graphitic nanoparticles decrease along the number of laser pulses from 100 to 10 nm average and from multiple layers to few layers graphene (1-1.5 nm thickness), respectively, leading to graphene quantum dots (GQDs). Accordingly, the emission intensity of these GQDs increases appearing at about 500 nm in the visible region along the reduction of the particle size. Transient absorption spectroscopy has allowed detection of a transient signal decaying in the microsecond time scale that has been attributed to the charge separation state.

Interannual variability in early life phenology is driven by climate and oceanic processes in two NE Atlantic flatfishes.

Early life phenology is a crucial factor for population dynamics in a climate change scenario. As such, understanding how the early life cycle of marine fishes is influenced by key oceanic and climate drivers is of chief importance for sustainable fisheries. This study documents interannual changes in early life phenology of two commercial flatfishes: European flounder (Platichthys flesus) and common sole (Solea solea) from 2010 to 2015 based on otolith microstructure. Using GAMs, we looked for correlations of the North Atlantic Oscillation (NAO), Eastern Atlantic pattern (EA), sea surface temperature (SST), chlorophyl a concentration (Chla) and upwelling (Ui) variation with the onset of hatch, metamorphosis, and benthic settlement day. We concluded that higher SST, more intensive upwelling, and EA were coincident with a later the onset of each stage, while increasing NAO induces an earlier onset of each stage. Although similar to S. solea, P. flesus showed a more complex interaction with the environmental drivers, most possibly because it is at its southern limit of its distribution. Our results highlight the complexity of the relationship between climate conditions and fish early life history, particularly those with complex life cycles that include migrations between coastal areas and estuaries.

Green, HF-Free Synthesis of MXene Quantum Dots and their Photocatalytic Activity for Hydrogen Evolution.

A general methodology to prepare MXene quantum dots (MxQDs) with yields over 20% by liquid-phase laser ablation of the MAX phase is reported. Mechanical and thermal shock by 532 nm laser pulses (7 ns fwhp, 50 mJ × pulse-1 , 1 Hz pulse frequency) produces MAX etching and exfoliation to form MXene QDs, avoiding the use of HF. The process can be followed by absorption and emission spectroscopy and by dynamic laser scattering and it appears to be general, being applied to Ti3 AlC2 , Ti2 AlC, Nb2 AlC, and V2 AlC MAX phases. Density functional theory calculations indicate that, depending on the surface terminal groups, the diminution of the MXene size to the nanometric scale makes it possible to control the band gap of the MXene. The photocatalytic activity of these MXene QDs for hydrogen evolution has been observed, reaching an H2 production for the most efficient Ti3 C2 QDs as high as 2.02 mmol × g-1 × h-1 .

Climate forcing on estuarine zooplanktonic production.

Estuaries are among the most valuable aquatic systems in the world and resolving how there are impacted by climate change is fundamental to their management under global change scenarios. In this study, a ten-year time series (2003-2013) of zooplankton in an estuarine area (Mondego estuary, Portugal) is used to determine the impact of climate variability on estuarine zooplanktonic secondary production. For that, a trend analysis of seasonal zooplankton production was applied and their link with large-scale, regional, and local environment was tested by Distance-based multivariate multiple regression (DistLM). The annual integrated production of zooplankton varied between 34.27 mg C m-3 (2003) and 179.804 mg C m-3 (2013). Results showed that estuarine and marine zooplanktonic production increased in the estuary, mostly during summer/autumn and spring/summer, respectively. Local and regional environmental forcing drove copepod production in the estuary, with large-scale regime shifts affecting both directly and indirectly.

Short and long-term temperature variations drive recruitment variability in marine and estuarine juvenile fishes.

Understanding the long-term effects of climatic factors on key species' recruitment is crucial to species management and conservation. Here, we analysed the recruitment variability of key species (Dicentrarchus labrax, Platichthys flesus, Solea solea, Pomatoschistus microps and Pomatoschistus minutus) in an estuary between 2003 and 2019, and related it with the prevailing local and large-scale environmental factors. Using a dynamic factor analysis (DFA), juvenile abundance data were grouped into three common trends linked to different habitat uses and life cycle characteristics, with significant effect of temperature-related variables on fish recruitment: Sea surface temperature and the Atlantic Multidecadal Oscillation. In 2010, a regime shift in the North Atlantic coincided with a shift in the common trends, particularly a decline in P. flesus and S. solea trend. This work highlights the thermophilic character of fish recruitment and the necessity to investigate key biological processes in the context of species-specific responses to climate change.

Tridimensional N, P-Codoped Carbon Sponges as Highly Selective Catalysts for Aerobic Oxidative Coupling of Benzylamine.

Two tridimensional N-doped porous carbon sponges (3DC-X) have been prepared by using cetyltrimethylammonium chloride (CTAC) and cetyltrimethylammonium bromide (CTAB) as soft templates and alginate to replicate the liquid crystals formed by CTA+ in water. Alginate is a filmogenic polysaccharide of natural origin having the ability to form nanometric defectless films around objects. Subsequent pyrolysis at 900 °C under an Ar flow of the resulting CTA+-polysaccharide assemblies result in 3DC-1 and 3DC-2, with the N percentages of 0.48 and 0.36 wt % for the materials resulting from CTAC and CTAB, respectively. Another four 3DC materials were obtained via pyrolysis of the adduct of phytic acid and chitosan, rendering an amorphous, N and P-codoped carbon sample (3DC-3 to 3DC-6). The six 3DC samples exhibit a large area (>650 m2 × g-1) and porosity, as determined by Ar adsorption. The catalytic activity of these materials in promoting the aerobic oxidation of benzylamine increases with the specific surface area and doping, being the largest for 3DC-4, which is able to achieve 73% benzylamine conversion to N-benzylidene benzylamine in solventless conditions at 70 °C in 5 h. Quenching studies and hot filtration tests indicate that 3DC-4 acts as a heterogeneous catalyst rather than an initiator, triggering the formation of hydroperoxyl and hydroxyl radicals as the main reactive oxygen species involved in the aerobic oxidation.

High C2-C4 selectivity in CO2 hydrogenation by particle size control of Co-Fe alloy nanoparticles wrapped on N-doped graphitic carbon.

A catalyst based on first-row Fe and Co with a record of 51% selectivity to C2-C4 hydrocarbons at 36% CO2 conversion is disclosed. The factors responsible for the C2+ selectivity are a narrow Co-Fe particle size distribution of about 10 nm and embedment in N-doped graphitic matrix. These hydrogenation catalysts convert CO2 into C2-C4 hydrocarbons, including ethane, propane, n-butane, ethylene and propylene together with methane, CO. Selectivity varies depending on the catalyst, CO2 conversion, and the operation conditions. Operating with an H2/CO2 ratio of 4 at 300°C and pressure on 5 bar, a remarkable combined 30% of ethylene and propylene at 34% CO2 conversion was achieved. The present results open the way to develop an economically attractive process for CO2 reduction leading to products of higher added value and longer life cycles with a substantial selectivity.