Continuous isomerisation of 2,5-dimethylfuran to 2,4-dimethylfuran over Ga-silicate.

2,4-dimethylfuran has a rare disubstitution pattern in the five-membered heterocyclic furan ring that is highly interesting chemically but challenging to access synthetically. We present a heterogeneously catalysed route to synthesise 2,4-dimethylfuran from commonly available 2,5-dimethylfuran using a zeolite packed-bed flow reactor. As supported by DFT calculations, the reaction occurs inside the zeolite channels, where the acid sites catalyse proton transfer followed by migration of a methyl group. The zeotype Ga-silicate (MFI type) appears superior to an aluminium-containing ZSM-5 by demonstrating higher selectivities and slower catalyst deactivation. This work provides new opportunities for the continuous valorisation of bio-feedstock molecules in the perspective of the emerging biorefinery era.

Going Beyond Silver in Ethylene Epoxidation with First-Principles Catalyst Screening.

Ethylene epoxidation is industrially and commercially one of the most important selective oxidations. Silver catalysts have been state-of-the-art for decades, their efficiency steadily improving with empirical discoveries of dopants and co-catalysts. Herein, we perform a computational screening of the metals in the periodic table, identify prospective superior catalysts and experimentally demonstrate that Ag/CuPb, Ag/CuCd and Ag/CuTl outperform the pure-Ag catalysts, while they still confer an easily scalable synthesis protocol. Furthermore, we show that to harness the potential of computationally-led discovery of catalysts fully, it is essential to include the relevant in situ conditions e.g., surface oxidation, parasitic side reactions and ethylene epoxide decomposition, as neglecting such effects leads to erroneous predictions. We combine ab initio calculations, scaling relations, and rigorous reactor microkinetic modelling, which goes beyond conventional simplified steady-state or rate-determining modelling on immutable catalyst surfaces. The modelling insights have enabled us to both synthesise novel catalysts and theoretically understand experimental findings, thus, bridging the gap between first-principles simulations and industrial applications. We show that the computational catalyst design can be easily extended to include larger reaction networks and other effects, such as surface oxidations. The feasibility was confirmed by experimental agreement.

Synthesis and Characterization of Catalytically Active Au Core─Pd Shell Nanoparticles Supported on Alumina.

A two-step seeded-growth method was refined to synthesize Au@Pd core@shell nanoparticles with thin Pd shells, which were then deposited onto alumina to obtain a supported Au@Pd/Al2O3 catalyst active for prototypical CO oxidation. By the strict control of temperature and Pd/Au molar ratio and the use of l-ascorbic acid for making both Au cores and Pd shells, a 1.5 nm Pd layer is formed around the Au core, as evidenced by transmission electron microscopy and energy-dispersive spectroscopy. The core@shell structure and the Pd shell remain intact upon deposition onto alumina and after being used for CO oxidation, as revealed by additional X-ray diffraction and X-ray photoemission spectroscopy before and after the reaction. The Pd shell surface was characterized with in situ infrared (IR) spectroscopy using CO as a chemical probe during CO adsorption-desorption. The IR bands for CO ad-species on the Pd shell suggest that the shell exposes mostly low-index surfaces, likely Pd(111) as the majority facet. Generally, the IR bands are blue-shifted as compared to conventional Pd/alumina catalysts, which may be due to the different support materials for Pd, Au versus Al2O3, and/or less strain of the Pd shell. Frequencies obtained from density functional calculations suggest the latter to be significant. Further, the catalytic CO oxidation ignition-extinction processes were followed by in situ IR, which shows the common CO poisoning and kinetic behavior associated with competitive adsorption of CO and O2 that is typically observed for noble metal catalysts.

Nanofluidic Trapping of Faceted Colloidal Nanocrystals for Parallel Single-Particle Catalysis.

Catalyst activity can depend distinctly on nanoparticle size and shape. Therefore, understanding the structure sensitivity of catalytic reactions is of fundamental and technical importance. Experiments with single-particle resolution, where ensemble-averaging is eliminated, are required to study it. Here, we implement the selective trapping of individual spherical, cubic, and octahedral colloidal Au nanocrystals in 100 parallel nanofluidic channels to determine their activity for fluorescein reduction by sodium borohydride using fluorescence microscopy. As the main result, we identify distinct structure sensitivity of the rate-limiting borohydride oxidation step originating from different edge site abundance on the three particle types, as confirmed by first-principles calculations. This advertises nanofluidic reactors for the study of structure-function correlations in catalysis and identifies nanoparticle shape as a key factor in borohydride-mediated catalytic reactions.

Light-Off in Plasmon-Mediated Photocatalysis.

In plasmon-mediated photocatalysis it is of critical importance to differentiate light-induced catalytic reaction rate enhancement channels, which include near-field effects, direct hot carrier injection, and photothermal catalyst heating. In particular, the discrimination of photothermal and hot electron channels is experimentally challenging, and their role is under keen debate. Here we demonstrate using the example of CO oxidation over nanofabricated neat Pd and Au50Pd50 alloy catalysts, how photothermal rate enhancement differs by up to 3 orders of magnitude for the same photon flux, and how this effect is controlled solely by the position of catalyst operation along the light-off curve measured in the dark. This highlights that small fluctuations in reactor temperature or temperature gradients across a sample may dramatically impact global and local photothermal rate enhancement, respectively, and thus control both the balance between different rate enhancement mechanisms and the way strategies to efficiently distinguish between them should be devised.

Hydrogen induced interface engineering in Fe2O3-TiO2 heterostructures for efficient charge separation for solar-driven water oxidation in photoelectrochemical cells.

Semiconductor heterostructure junctions are known to improve the water oxidation performance in photoelectrochemical (PEC) cells. Depending on the semiconductor materials involved, different kinds of junctions can appear, for instance, type II band alignment where the conduction and valence bands of the semiconductor materials are staggered with respect to each other. This band alignment allows for a charge separation of the photogenerated electron-hole pairs, where the holes will go from low-to-high valance band levels and vice versa for the electrons. For this reason, interface engineering has attracted intensive attention in recent years. In this work, a simplified model of the Fe2O3-TiO2 heterostructure was investigated via first-principles calculations. The results show that Fe2O3-TiO2 produces a type I band alignment in the heterojunction, which is detrimental to the water oxidation reaction. However, the results also show that interstitial hydrogens are energetically allowed in TiO2 and that they introduce states above the valance band, which can assist in the transfer of holes through the TiO2 layer. In response, well-defined planar Fe2O3-TiO2 heterostructures were manufactured, and measurements confirm the formation of a type I band alignment in the case of Fe2O3-TiO2, with very low photocurrent density as a result. However, once TiO2 was subjected to hydrogen treatment, there was a nine times higher photocurrent density at 1.50 V vs. the reversible hydrogen electrode under 1 sun illumination as compared to the original heterostructured photoanode. Via optical absorption, XPS analysis, and (photo)electrochemical measurements, it is clear that hydrogen treated TiO2 results in a type II band alignment in the Fe2O3-H:TiO2 heterostructure. This work is an example of how hydrogen doping in TiO2 can tailor the band alignment in TiO2-Fe2O3 heterostructures. As such, it provides valuable insights for the further development of similar material combinations.

To Every Rule There is an Exception: A Rational Extension of Loewenstein's Rule.

Loewenstein's rule, which states that Al-O-Al motifs are energetically unstable, is fundamental to the understanding and design of zeolites. Here, using a combination of electronic structure calculations and lattice models, we show under which circumstances this rule becomes invalid and how it can be rationally extended using the chabasite framework for demonstration.

Copper catalysis at operando conditions-bridging the gap between single nanoparticle probing and catalyst-bed-averaging.

In catalysis, nanoparticles enable chemical transformations and their structural and chemical fingerprints control activity. To develop understanding of such fingerprints, methods studying catalysts at realistic conditions have proven instrumental. Normally, these methods either probe the catalyst bed with low spatial resolution, thereby averaging out single particle characteristics, or probe an extremely small fraction only, thereby effectively ignoring most of the catalyst. Here, we bridge the gap between these two extremes by introducing highly multiplexed single particle plasmonic nanoimaging of model catalyst beds comprising 1000 nanoparticles, which are integrated in a nanoreactor platform that enables online mass spectroscopy activity measurements. Using the example of CO oxidation over Cu, we reveal how highly local spatial variations in catalyst state dynamics are responsible for contradicting information about catalyst active phase found in the literature, and identify that both surface and bulk oxidation state of a Cu nanoparticle catalyst dynamically mediate its activity.

Desorption products during linear heating of copper zeolites with pre-adsorbed methanol.

Desorption products from zeolites with medium (MFI) and small (CHA) pores and with and without ion-exchanged copper were studied during linear heating after the pre-adsorption of methanol using a chemical flow reactor with a gas phase Fourier transform infrared spectrometer. The methanol desorption profiles were deconvoluted and compared with those predicted from first-principles calculations. In situ diffuse reflectance infrared Fourier transform spectroscopy was used to study the samples during methanol desorption following a step-wise increase of the sample temperature. It is shown that well-dispersed copper species in the Cu-zeolite samples interact more strongly with methanol and its derivatives as compared to the bare zeolites, resulting in methanol desorption at higher temperatures. Moreover, the introduction of Cu leads to CO formation and desorption in larger amounts at lower temperatures compared to the bare zeolites. The formation and desorption of dimethyl ether (DME) from pre-adsorbed methanol takes place at different temperatures depending on both the influence of Cu and the zeolite topology. The Cu sites in zeolites lead to higher DME formation/desorption temperatures, while a small shift of DME desorption towards higher temperatures is observed for the CHA framework structure compared to the MFI framework structure.

Synergies of co-doping in ultra-thin hematite photoanodes for solar water oxidation: In and Ti as representative case.

Solar energy induced water splitting in photoelectrochemical (PEC) cells is one of the most sustainable ways of hydrogen production. The challenge is to develop corrosion resistant and chemically stable semiconductors that absorb sunlight in the visible region and, at the same time, have the band edges matching with the redox level of water. In this work, hematite (α-Fe2O3) thin films were prepared onto an indium-doped tin oxide (ITO; In:SnO2) substrate by e-beam evaporation of Fe, followed by air annealing at two different temperatures: 350 and 500 °C. The samples annealed at 500 °C show an in situ diffusion of indium from the ITO substrate to the surface of α-Fe2O3, where it acts as a dopant and enhances the photoelectrochemical properties of hematite. Structural, optical, chemical and photoelectrochemical analysis reveal that the diffusion of In at 500 °C enhances the optical absorption, increases the electrode-electrolyte contact area by changing the surface topology, improves the carrier concentration and shifts the flat band potential in the cathodic direction. Further enhancement in photocurrent density was observed by ex situ diffusion of Ti, deposited in the form of nanodisks, from the top surface to the bulk. The in situ In diffused α-Fe2O3 photoanode exhibits an improved photoelectrochemical performance, with a photocurrent density of 145 µA cm-2 at 1.23 VRHE, compared to 37 µA cm-2 for the photoanode prepared at 350 °C; it also decreases the photocurrent onset potential from 1.13 V to 1.09 V. However, the In/Ti co-doped sample exhibits an even higher photocurrent density of 290 µA cm-2 at 1.23 VRHE and the photocurrent onset potential decreases to 0.93 VRHE, which is attributed to the additional doping and to the surface becoming more favorable to charge separation.

Metal-polymer hybrid nanomaterials for plasmonic ultrafast hydrogen detection.

Hydrogen-air mixtures are highly flammable. Hydrogen sensors are therefore of paramount importance for timely leak detection during handling. However, existing solutions do not meet the stringent performance targets set by stakeholders, while deactivation due to poisoning, for example by carbon monoxide, is a widely unsolved problem. Here we present a plasmonic metal-polymer hybrid nanomaterial concept, where the polymer coating reduces the apparent activation energy for hydrogen transport into and out of the plasmonic nanoparticles, while deactivation resistance is provided via a tailored tandem polymer membrane. In concert with an optimized volume-to-surface ratio of the signal transducer uniquely offered by nanoparticles, this enables subsecond sensor response times. Simultaneously, hydrogen sorption hysteresis is suppressed, sensor limit of detection is enhanced, and sensor operation in demanding chemical environments is enabled, without signs of long-term deactivation. In a wider perspective, our work suggests strategies for next-generation optical gas sensors with functionalities optimized by hybrid material engineering.

Steps Control the Dissociation of CO2 on Cu(100).

CO2 reduction reactions, which provide one route to limit the emission of this greenhouse gas, are commonly performed over Cu-based catalysts. Here, we use ambient pressure X-ray photoelectron spectroscopy together with density functional theory to obtain an atomistic understanding of the dissociative adsorption of CO2 on Cu(100). We find that the process is dominated by the presence of steps, which promote both a lowering of the dissociation barrier and an efficient separation between adsorbed O and CO, reducing the probability for recombination. The identification of steps as sites for efficient CO2 dissociation provides an understanding that can be used in the design of future CO2 reduction catalysts.

Surface terminations of hematite (α-Fe2O3) exposed to oxygen, hydrogen, or water: dependence on the density functional theory methodology.

Hematite (α-Fe2O3) is the most stable and abundant iron oxide in nature, and is used in many important environmental and industrial technologies, such as waste-water treatment, gas sensors, and photoelectrocatalysis. A clear understanding of the structure, composition, and chemistry of the hematite surface is crucial for improving its function in these technologies. Here we employ density functional theory (DFT) together with the DFT+U approach using semi-local functionals, as well as hybrid functionals, to study the structure, stability, and electronic properties of the (0 0 0 1) surface exposed to oxygen, hydrogen, or water. The use of hybrid functionals allow for a description of strong correlation without the need for atom-specific empirical parameters (i.e. U). However, we find that PBE+U, and in part also PBE, give similar results as the hybrid functional HSE(12%) in terms of structure optimization. When it comes to stability, work function, as well as electronic structure, the results are sensitive to the choice of functionals, but we cannot judge which level of functional is most appropriate due to the lack of experimental observations.

The structure-function relationship for alumina supported platinum during the formation of ammonia from nitrogen oxide and hydrogen in the presence of oxygen.

We study the structure-function relationship of alumina supported platinum during the formation of ammonia from nitrogen oxide and dihydrogen by employing in situ X-ray absorption and Fourier transform infrared spectroscopy. Particular focus has been directed towards the effect of oxygen on the reaction as a model system for emerging technologies for passive selective catalytic reduction of nitrogen oxides. The suppressed formation of ammonia observed as the feed becomes net-oxidizing is accompanied by a considerable increase in the oxidation state of platinum as well as the formation of surface nitrates and the loss of NH-containing surface species. In the presence of (excess) oxygen, the ammonia formation is proposed to be limited by weak interaction between nitrogen oxide and the oxidized platinum surface. This leads to a slow dissociation rate of nitrogen oxide and thus low abundance of the atomic nitrogen surface species that can react with the adsorbed hydrogen species. In this case the consumption of hydrogen through the competing water formation reaction and decomposition/oxidation of ammonia are of less importance for the net ammonia formation.

Tailoring Charge Recombination in Photoelectrodes Using Oxide Nanostructures.

Optimizing semiconductor devices for solar energy conversion requires an explicit control of the recombination of photogenerated electron-hole pairs. Here we show how the recombination of charge carriers can be controlled in semiconductor thin films by surface patterning with oxide nanodisks. The control mechanism relies on the formation of dipole-like electric fields at the interface that, depending on the field direction, attract or repel minority carriers from underneath the disks. The charge recombination rate can be controlled through the choice of oxide material and the surface coverage of nanodisks. We provide proof-of-principle demonstration of this approach by patterning the surface of Fe2O3, one of the most studied semiconductors for light-driven water splitting, with TiO2 and Cu2O nanodisks. We expect this method to be generally applicable to a range of semiconductor-based solar energy conversion devices.

Schottky barrier formation and band bending revealed by first- principles calculations.

The formation of a Schottky barrier at the metal-semiconductor interface is widely utilised in semiconductor devices. With the emerging of novel Schottky barrier based nanoelectronics, a further microscopic understanding of this interface is in high demand. Here we provide an atomistic insight into potential barrier formation and band bending by ab initio simulations and model analysis of a prototype Schottky diode, i.e., niobium doped rutile titania in contact with gold (Au/Nb:TiO2). The local Schottky barrier height is found to vary between 0 and 1.26 eV depending on the position of the dopant. The band bending is caused by a dopant induced dipole field between the interface and the dopant site, whereas the pristine Au/TiO2 interface does not show any band bending. These findings open the possibility for atomic scale optimisation of the Schottky barrier and light harvesting in metal-semiconductor nanostructures.

Unravelling the dependence of hydrogen oxidation kinetics on the size of Pt nanoparticles by in operando nanoplasmonic temperature sensing.

We use a noninvasive nanoscale optical-temperature measurement method based on localized surface plasmon resonance to investigate the particle size-dependence of the hydrogen oxidation reaction kinetics on model supported Pt nanocatalysts at atmospheric pressure in operando. With decreasing average nanoparticle size from 11 down to 3 nm, the apparent reaction activation energy is found to increase from 0.5 up to 0.8 eV. This effect is attributed to an increase of the fraction of (100)-facet and edge and corner sites and their increasingly important role in the reaction with decreasing particle size.

The role of surface States in the oxygen evolution reaction on hematite.

Hematite (α-Fe2 O3 ) is an extensively investigated semiconductor for photoelectrochemical (PEC) water splitting. The nature and role of surface states on the oxygen evolution reaction (OER) remain however elusive. First-principles calculations were used to investigate surface states on hematite under photoelectrochemical conditions. The density of states for two relevant hematite terminations was calculated, and in both cases the presence and the role of surface states was rationalized. Calculations also predicted a Nerstian dependence on the OER onset potential on pH, which was to a very good extent confirmed by PEC measurements on hematite model photoanodes. Impedance spectroscopy characterization confirmed that the OER takes place via the same surface states irrespective of pH. These results provide a framework for a deeper understanding of the OER when it takes place via surface states.

Mechanism for limiting thickness of thin oxide films on aluminum.

A first-principles account of the observed limiting thickness of oxide films formed on aluminum during oxidizing conditions is presented. The results uncover enhanced bonding of oxygen to thin alumina films in contact with metallic aluminum that stems from charge transfer between a reconstructed oxide-metal interface and the adsorbed molecules. The first-principles results are compared with the traditional Cabrera-Mott (CM) model, which is a classical continuum model. Within the CM model, charged surface oxygen species and metal ions generate a (Mott) potential that drives oxidation. An apparent limiting thickness is observed as the oxidation rate decreases rapidly with film growth. The present results support experimental estimates of the Mott potential and film thicknesses. In contrast to the CM model, however, the calculations reveal a real limiting thickness that originates from a diminishing oxygen adsorption energy beyond a certain oxide film thickness.

Analysis of porphyrines as catalysts for electrochemical reduction of O2 and oxidation of H2O.

Bioinspired structures are promising as improved catalysts for various redox reactions. One example is metal hangman-porphyrines (MHP), which recently have been suggested for oxygen reduction/evolution reaction (ORR/OER). The unique properties of the MHPs are attributed to both the hangman scaffold and the C6F5 side groups. Herein, the OER/ORR over various transition metal MHPs is investigated by density functional theory calculations within an electrochemical framework. A comparison of the reaction landscape for MHP, metal porphyrine (MP) and metaltetrafluorophenyloporphyrines (MTFPP), allow for a disentanglement of the different roles of the hangman motif and the side groups. In agreement with experimental studies, it is found that Fe and Co are the best MHP metal centers to catalyze these reactions. We find that the addition of the three-dimensional moiety in the form of hangman scaffold does not break the apparently universal energy relation between *OH and *OOH intermediates. However, the hangman motif is found to stabilize the oxygen intermediate, whereas addition of C6F5 groups reduces the binding energy of all reaction intermediates. Our results indicate that the combination of these two effects allow new design possibilities for macromolecular systems with enhanced catalytic OER/ORR activity.