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.

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.

Structural Changes in Monolayer Cobalt Oxides under Ambient Pressure CO and O2 Studied by In Situ Grazing-Incidence X-ray Absorption Fine Structure Spectroscopy.

We have used grazing incidence X-ray absorption fine structure spectroscopy at the cobalt K-edge to characterize monolayer CoO films on Pt(111) under ambient pressure exposure to CO and O2, with the aim of identifying the Co phases present and their transformations under oxidizing and reducing conditions. X-ray absorption near edge structure (XANES) spectra show clear changes in the chemical state of Co, with the 2+ state predominant under CO exposure and the 3+ state predominant under O2-rich conditions. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis shows that the CoO bilayer characterized in ultrahigh vacuum is not formed under the conditions used in this study. Instead, the spectra acquired at low temperatures suggest formation of cobalt hydroxide and oxyhydroxide. At higher temperatures, the spectra indicate dewetting of the film and suggest formation of bulklike Co3O4 under oxidizing conditions. The experiments demonstrate the power of hard X-ray spectroscopy to probe the structures of well-defined oxide monolayers on metal single crystals under realistic catalytic conditions.

On-Line Composition Analysis of Complex Hydrocarbon Streams by Time-Resolved Fourier Transform Infrared Spectroscopy and Ion-Molecule Reaction Mass Spectrometry.

On-line composition analysis of complex hydrocarbon mixtures is highly desirable to determine the composition of process streams and to study chemical reactions in heterogeneous catalysis. Here, we show how the combination of time-resolved Fourier transform infrared spectroscopy and ion-molecule-reaction mass spectrometry (IMR-MS) can be used for compositional analysis of processed plant biomass streams. The method is based on the biomass-derived model compound 2,5-dimethylfuran and its potential catalytic conversion to valuable green aromatics, for example, benzene, toluene, and xylenes (BTX) over zeolite ß. Numerous conversion products can be determined and quantified simultaneously in a temporal resolution of 4 min-1 without separation of individual compounds. The realization of this method enables us to study activity, selectivity, and changes in composition under transient reaction conditions. For example, increasing isomerization of 2,5-dimethylfuran to 2,4-dimethylfuran, 2-methyl-2-cyclopenten-1-one, and 2-methyl-2-cyclopenten-1-one is observed as the catalyst is exposed to the reactant, while BTX and olefin formation is decreasing.

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.

Electronic Structure of the Hieber Anion [Fe(CO)3(NO)]- Revisited by X-ray Emission and Absorption Spectroscopy.

While the Hieber anion [Fe(CO)3(NO)]- has been reincarnated in the last years as an active catalyst in organic synthesis, there is still a debate about the oxidation state of the central Fe atom and the resulting charge of the NO ligand. To shed new light on this question and to understand the Fe-NO interaction in the Hieber anion, it is investigated in comparison to the formal 3d8 reference Fe(CO)5 and the formal 3d10 reference [Fe(CO)4]2- by the combination of valence-to-core X-ray emission spectroscopy (VtC-XES), X-ray absorption near-edge structure spectroscopy (XANES), and high-energy-resolution fluorescence-detected XANES. In order to extract information about the electronic structure, time-dependent density functional theory and ground-state density functional theory calculations are applied. This combination of experimental and computational methods reveals that the electron density at the Fe center of the Hieber resembles that of the isoelectronic [Fe(CO)4]2-. These observations challenge recent descriptions of the Hieber anion and reopen the debate about the experimentally and computationally determined Fe oxidation state and charge on the NO ligand.

Local anisotropy in single crystals of zeotypes with the MFI framework structure evidenced by polarised Raman spectroscopy.

Polarised Raman spectroscopy is used to characterise the local structure in single crystals of zeotypes, namely silicalite-1 and ZSM-5, which share the MFI framework structure. Attributes favourable for applying polarised Raman spectroscopy are the orthogonal axes of these single crystals and their size, i.e. 10 to 30 micrometers in all three directions. We show that the intensity of certain vibrational modes in silicalite-1 depends on the polarisation of the incident light, reflecting the anisotropic character of the molecular bonds contributing to these vibrations. Using these observations, and by estimating the depolarisation ratio (ρ) and the pseudo-order factor (f), we propose a more accurate assignment of the Raman active modes. More precisely, Raman intensities peaked at 294, 360, 383 and 472 cm-1 are attributed to bending modes in 10-, 6-, 5- and 4-membered rings, respectively. In the region of stretching modes, the vibration at 832 cm-1 is assigned to Si-O-Si bonds shared between 5-membered rings, which have an orientation parallel to the a-axis of the crystal. By virtue of having a strongly polarised character, the modes at 472 and 832 cm-1 can be used as orientational indicators. The proposed assignment is supported by the good agreement between experimental and simulated polar plots, where Raman intensities are plotted as a function of the polarisation angle of the incident light. Finally, upon partial substitution of Si atoms by Al, the crystalline structure is maintained and almost no spectroscopic changes are observed. The only significant difference is the increased width of most vibrational modes, which is consistent with the local lower symmetry. This is also seen in the angular dependence of selected vibrational modes that compared to the case of pure silicalite-1 appear less polarised. In the Raman spectrum of ZSM-5 a new feature at 974 cm-1 is observed, which we attribute to Al-OH stretching. In the high frequency range, the O-H stretching modes are observed which arise from the Si-O(H)-Al Brønsted acid sites. The intensity of the characteristic mode at 3611 cm-1 reveals an anisotropic character as well, which is in line with previous findings from solid state NMR that Al atoms distribute nonrandomly within the MFI framework structure.

Combining synchrotron light with laser technology in catalysis research.

High-energy surface X-ray diffraction (HESXRD) provides surface structural information with high temporal resolution, facilitating the understanding of the surface dynamics and structure of the active phase of catalytic surfaces. The surface structure detected during the reaction is sensitive to the composition of the gas phase close to the catalyst surface, and the catalytic activity of the sample itself may affect the surface structure, which in turn may complicate the assignment of the active phase. For this reason, planar laser-induced fluorescence (PLIF) and HESXRD have been combined during the oxidation of CO over a Pd(100) crystal. PLIF complements the structural studies with an instantaneous two-dimensional image of the CO2 gas phase in the vicinity of the active model catalyst. Here the combined HESXRD and PLIF operando measurements of CO oxidation over Pd(100) are presented, allowing for an improved assignment of the correlation between sample structure and the CO2 distribution above the sample surface with sub-second time resolution.

Catalytically Active Pd-Ag Alloy Nanoparticles Synthesized in Microemulsion Template.

This work investigates the possibility to form catalytically active bimetallic Pd-Ag nanoparticles synthesized in the water pools of a reversed microemulsion using methanol, a more environmental- and user-friendly reductant compared to hydrazine or sodium borohydride, which are commonly used for this type of synthesis. The nanoparticles were characterized with regards to crystallinity and size by X-ray diffraction and transmission electron microscopy. CO chemisorption and oxidation followed by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used for investigating the elemental composition of the surface and catalytic activity, respectively. Moreover, the structural composition of the bimetallic particles was determined by scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy. The particles were shown to be crystalline nanoalloys of around 5-12 nm. CO adsorption followed by in situ DRIFTS suggests that the particle surfaces are composed of the same Pd-Ag ratios as the entire particles, regardless of elemental ratio (i.e., no core-shell structures can be detected). This is also shown by numerical simulations using a Monte Carlo based model. Furthermore, CO oxidation confirms that the synthesized particles are catalytically active.

Methane adsorption and methanol desorption of copper modified boron silicate.

Boron silicate (BS) with a chabazite framework structure was synthesised using a direct route and rigorously characterized before it was ion-exchanged with copper to form Cu-BS. Employing in situ infrared spectroscopy, we show that Cu-BS is capable of oxidising methane to methoxy species and methanol interacts with the boron sites without deprotonation.

SO2 adsorption on silica supported iridium.

The interaction of SO2 with Ir/SiO2 was studied by simultaneous in situ diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry, exposing the sample to different SO2 concentrations ranging from 10 to 50 ppm in the temperature interval 200-400 °C. Evidences of adsorption of sulfur species in both absence and presence of oxygen are found. For a pre-reduced sample in the absence of oxygen, SO2 disproportionates such that the iridium surface is rapidly saturated with adsorbed S while minor amounts of formed SO3 may adsorb on SiO2. Adding oxygen to the feed leads to the oxidation of sulfide species that either (i) desorb as SO2 and/or SO3, (ii) remain at metal sites in the form of adsorbed SO2, or (iii) spillover to the oxide support and form sulfates (SO42-). Notably, significant formation of sulfates on silica is possible only in the presence of both SO2 and O2, suggesting that SO2 oxidation to SO3 is a necessary first step in the mechanism of formation of sulfates on silica. During the formation of sulfates, a concomitant removal/rearrangement of surface silanol groups is observed. Finally, the interaction of SO2 with Ir/SiO2 depends primarily on the temperature and type of gas components but only to a minor extent on the inlet SO2 concentration.

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.

Spatially and temporally resolved gas distributions around heterogeneous catalysts using infrared planar laser-induced fluorescence.

Visualizing and measuring the gas distribution in close proximity to a working catalyst is crucial for understanding how the catalytic activity depends on the structure of the catalyst. However, existing methods are not able to fully determine the gas distribution during a catalytic process. Here we report on how the distribution of a gas during a catalytic reaction can be imaged in situ with high spatial (400 µm) and temporal (15 µs) resolution using infrared planar laser-induced fluorescence. The technique is demonstrated by monitoring, in real-time, the distribution of carbon dioxide during catalytic oxidation of carbon monoxide above powder catalysts. Furthermore, we demonstrate the versatility and potential of the technique in catalysis research by providing a proof-of-principle demonstration of how the activity of several catalysts can be measured simultaneously, either in the same reactor chamber, or in parallel, in different reactor tubes.

An in situ sample environment reaction cell for spatially resolved X-ray absorption spectroscopy studies of powders and small structured reactors.

An easy-to-use sample environment reaction cell for X-ray based in situ studies of powders and small structured samples, e.g., powder, pellet, and monolith catalysts, is described. The design of the cell allows for flexible use of appropriate X-ray transparent windows, shielding the sample from ambient conditions, such that incident X-ray energies as low as 3 keV can be used. Thus, in situ X-ray absorption spectroscopy (XAS) measurements in either transmission or fluorescence mode are facilitated. Total gas flows up to about 500 mln/min can be fed while the sample temperature is accurately controlled (at least) in the range of 25-500 °C. The gas feed is composed by a versatile gas-mixing system and the effluent gas flow composition is monitored with mass spectrometry (MS). These systems are described briefly. Results from simultaneous XAS/MS measurements during oxidation of carbon monoxide over a 4% Pt/Al2O3 powder catalyst are used to illustrate the system performance in terms of transmission XAS. Also, 2.2% Pd/Al2O3 and 2% Ag - Al2O3 powder catalysts have been used to demonstrate X-ray absorption near-edge structure (XANES) spectroscopy in fluorescence mode. Further, a 2% Pt/Al2O3 monolith catalyst was used ex situ for transmission XANES. The reaction cell opens for facile studies of structure-function relationships for model as well as realistic catalysts both in the form of powders, small pellets, and coated or extruded monoliths at near realistic conditions. The applicability of the cell for X-ray diffraction measurements is discussed.

Mechanisms behind sulfur promoted oxidation of methane.

The promoting effect of SO2 on the activity for methane oxidation over platinum supported on silica, alumina and ceria has been studied using a flow-reactor, in situ infrared spectroscopy and in situ high-energy X-ray diffraction experiments under transient reaction conditions. The catalytic activity is clearly dependent on the support material and its interaction with the noble metal both in the absence and presence of sulfur. On platinum, the competitive reactant adsorption favors oxygen dissociation such that oxygen self-poisoning is observed for Pt/silica and Pt/alumina. Contrarily for Pt/ceria, no oxygen self-poisoning is observed, which seems to be due to additional reaction channels via sites on the platinum-ceria boundary and/or ceria surface considerably far from the Pt crystallites. Addition of sulfur dioxide generally leads to the formation of ad-SO(x) species on the supports with a concomitant removal and/or blockage/rearrangement of surface hydroxyl groups. Thereby, the methane oxidation is inhibited for Pt/silica, enhanced for Pt/alumina and temporarily enhanced followed by inhibition after long-term exposure to sulfur for Pt/ceria. The observations can be explained by competitive oxidation of SO2 and CH4 on Pt/silica, formation of new active sites at the noble metal-support interface promoting dissociative adsorption of methane on Pt/alumina, and in the case of Pt/ceria, formation of promoting interfacial surface sulfates followed by formation of deactivating bulk-like sulfate species. Furthermore, it can be excluded that reduction of detrimental high oxygen coverage and/or oxide formation on the platinum particles through SO2 oxidation is the main cause for the promotional effects observed.

Self-sustained kinetic oscillations in CO oxidation over silica-supported Pt.

Isothermal self-sustained kinetic oscillations in CO oxidation over silica-supported Pt at near-atmospheric pressure were studied by combined in situ Fourier transform infrared spectroscopy and mass spectrometry. The use of a specially designed reactor and careful choice of the physical properties of the catalyst and reaction conditions made it possible to eliminate diffusion limitations, to determine the maximum CO oxidation rate per Pt site in the purely kinetic regime and to clarify the mechanism of the oscillations. Specifically, our results indicate that during the high reactive periods the reaction mainly occurs on the oxide surface.