Search

The dynamic interactions between noble metal particles and reducible metal-oxide supports can depend on redox reactions with ambient gases. Transmission electron microscopy revealed that the strong metal-support interaction (SMSI)–induced encapsulation of platinum particles on titania observed under reducing conditions is lost once the system is exposed to a redox-reactive environment containing oxygen and hydrogen at a total pressure of ~1 bar. Destabilization of the metal–oxide interface and redox-mediated reconstructions of titania lead to particle dynamics and directed particle migration that depend on nanoparticle orientation. A static encapsulated SMSI state was reestablished when switching back to purely oxidizing conditions. This work highlights the difference between reactive and nonreactive states and demonstrates that manifestations of the metal-support interaction strongly depend on the chemical environment., Science, 376 (6596), ISSN:0036-8075, ISSN:1095-9203

The electron paramagnetic resonance of Ce3+ ions in ceria is a rather controversial issue. In this report we analyze the EPR line with g ∼1.97 in the absorption spectra of ceria powders, which has been assigned in many studies to Ce3+ ions. Such an assignment is not in agreement with the experimental data nor with the theory of spin-lattice relaxation of rare earth ions in solids. Our statement is supported by our experimental results based on EPR spectroscopy of nanocrystalline ceria, synthesized in various ways, as well as by magnetization measurements of as-prepared and vacuum-annealed ceria nanoparticles. The possible model of the paramagnetic center, which is assigned to the EPR line with g ∼1.97 in ceria, is an electron trapped near the surface Ce3+/Ce4+ redox pair.

Identification of active sites and phases in heterogeneous catalysts and the understanding of the reaction mechanism remain highly challenging. In most catalysts, the existence of a multitude of surface species, which are dynamic in relation to reaction conditions, presents a challenge of distinguishing those that are involved in the catalytic cycle from those which are spectators. The emergence of the field of single-site catalysts potentially eliminates these issues, although it can be argued that these systems remain dynamic and that multiple speciation, each a candidate for the active site, often remains a consideration. A perspective on how X-ray spectroscopy and characterization tools in general, can be used to correlate the number of active sites and the rate of their formation, in single-site and redox catalyst systems, is presented. The importance of observing proportionality between spectra features and the reaction rate, to differentiate between active sites and spectator species is discussed. Performing characterisation under catalyticly relevant conditions on structures that are demonstrably representative of actual catalysts is essential.

PtCu- and PdCu–mordenite allow for isothermal reaction at 200 °C for the stepwise methane to methanol conversion with comparably high yields. In contrast to traditional Cu-zeolites, these materials are more reactive under isothermal conditions than after high temperature activation., Chemical Science, 10 (1), ISSN:2041-6520, ISSN:2041-6539

The available knowledge of the electronically excited states of the copper dimer is limited. This is common for transition metals, as the high density of states hinders both experimental assignment and computation. In this work, two-color resonant four-wave mixing spectroscopy was applied to neutral Cu2 in the gas phase. The method yielded accurate positions of individual rovibrational lines in the I-X and J-X electronic systems. This revealed the term symbols for the I and J states as 1Πu (1u) and 1Σu+ (0u+), respectively. For the 63Cu2 isotopologue, accurate molecular constants were obtained. The characterization of the J state finally allowed decisive determination of its electron configuration. The J state is obtained from the ground state by promotion of a 3dπg electron into the weakly bonding 4pπu molecular orbital. From the data analysis, lifetimes of the I state (between 10 ps and 5 ns) and J state (66 ns) were inferred.

A detailed study on the synthesis of Pt3Co intermetallic nanoparticles supported on ceria via preferential chemical vapor deposition was conducted, leading to a fundamental understanding of the deposition process and the Co–Pt alloying. This understanding helps us to develop a facile and scalable method for preferential adding of a metal on the surface of another metal already supported on an oxide which facilitates the design of novel structured nanoparticles. The fluidized flow reactor eliminated the deposition profile and resulted in Pt3Co nanoparticles uniformly and homogeneously distributed on ceria. The kinetic study of cobalt deposition on the platinum surface, in accordance with DFT calculations, demonstrates that the platinum surface catalyzes the deposition reaction; while the ceria surface is inert in the preferential deposition temperature window between 150 °C and 180 °C. The obtained sample was characterized by in situ XRD, HAADF-STEM, FT-IR, and XAS methods. The results indicate the formation of uniform Pt3Co nanoparticles with an average size of 1.1 nm. This sample showed superior catalytic activity in preferential oxidation of CO with an almost twice higher CO conversion rate and CO2 selectivity compared to a classically synthesized sample with successive impregnation.

The potential of two-color resonant and degenerate four-wave mixing spectroscopy for investigations of the complex spectra of transition metal dimers is explored. Two-color resonant and degenerate four-wave mixing spectroscopy scans of the well-known A-X and B-X transitions in Cu2 are reported and compared with previous experimental data obtained from standard single-resonance techniques. The selectivity of the method is shown to enable the measurement of isotopologue pure spectra without the need for isotopically enriched metal targets. Specific subsets of the rovibronic structure are separated in a congested spectral region of overlapping transitions. The sensitivity of the method compares satisfactorily with linear spectroscopic methods such as laser-induced fluorescence and cavity ring-down. A new laser vaporization source for the production of transition metal dimers and clusters has been constructed. The new design aims for a high number density and maximum possible shot-to-shot stability. The possibilities of further applications of non-linear four-wave mixing spectroscopy to Cu2 and other transition metal dimers are discussed. Copyright © 2015 John Wiley & Sons, Ltd.

Metal organic frameworks (MOFs) have recently debuted as participants and solid supports in catalytic water splitting. Their porosity and structural versatility offer a tantalising consolidation of the components needed for solar light harvesting and water splitting. Herein, we describe a selection of relevant contemporary investigations that employ electrocatalysis, chemically introduced redox partners, and photo-catalysts to generate dioxygen and dihydrogen from water. The role of semiconducting MOFs in these systems is addressed, in tandem with band gap control by linker functionalisation and doping. Considered holistically, MOFs offer an impressive physical, spatial and chemical versatility with which to support and sustain water splitting reactions. Major challenges toward practical implementation do remain, but opportunities for development are evidently numerous.

The previously reported silica-grafted iridium(III) hydride complex [IrH(O–SBA-15)(POCOP)] (2), prepared by treating [IrH2(POCOP)] (1) (POCOP is 1,3-bis((di-tert-butylphosphino)oxy)benzene) with SBA-15 (mesoporous silica), hydrogenates liquid alkenes (1-decene, trans-5-decene, cyclohexene, styrene, and 4-phenyl-1-butene) at room temperature and under 1 atm H2. Internal alkenes react at a lower rate than the terminal ones. For the sake of comparison, the hydrogenation of the same substrates was studied with the homogeneous catalyst [IrH2(POCOP)] (1). The heterogeneous catalyst 2 hydrogenates 1-decene, cyclohexene, and 4-phenyl-1-butene faster than 1, whereas the opposite is true for styrene and trans-5-decene, which suggests that different active species are involved in the heterogeneous and homogeneous reactions. Catalysis by a truly heterogeneous species is supported by a series of “hot filtration tests”. NMR spectroscopic studies showed that 2 does not undergo degrafting upon longer exposure to alkenes compared to ethene under hydrogenation conditions.

The low energy electronic structure of the copper dimer has been re-investigated using non-linear four-wave mixing spectroscopy and high level ab initio calculations. In addition to the measurement of the previously reported A, B, and C electronic states, a new state denoted A' is identified with T

The low energy electronic structure of the copper dimer has been re-investigated using non-linear four-wave mixing spectroscopy and high level ab initio calculations. In addition to the measurement of the previously reported A, B, and C electronic states, a new state denoted A′ is identified with T0 = 20 100.4090(16) cm−1 (63Cu2). Rotational analysis of the A′–X (0,0) and (1,0) transitions leads to the assignment of A′ 1u. Ab initio calculations present the first theoretical description of the low energy states of the copper dimer in Hund’s case (c) and confirm the experimental assignment. The discovery of this new low energy excited state emphasizes that spin-orbit coupling is significant in states with d-hole electronic configurations and resolves a decades-long mystery in the initial assignment of the A state.

The potential of valence to core Al X-ray emission spectroscopy to determine aluminum distribution in ferrierite zeolites was investigated. The recorded emission spectra of four samples prepared with different structure directing agents exhibit slight variations in the position of the main emission peak and the intensity of its low energy shoulder. Theoretical calculations indicate that an increased intensity of the Kβx shoulder in the Al emission spectra can be linked to a predominant occupation of the T3 site by a single aluminum atom. This study thus suggests that valence to core X-ray emission spectroscopy can be applied to help determine the occupation of aluminum at crystallographic T-sites in zeolites.

The available knowledge of the electronically excited states of the copper dimer is limited. This is common for transition metals, as the high density of states hinders both experimental assignment and computation. In this work, two-color resonant four-wave mixing spectroscopy was applied to neutral Cu

Human papillomavirus (HPV)-induced oropharyngeal squamous cell carcinoma (OPSCC) remains increasing worldwide. We aimed to investigate if the HPV-prevalence of OPSCC in the Netherlands is rising as well, also in female patients. In addition, we evaluated the association between HPV-positive OPSCC and suspicious Pap results of the cervix in these female patients. Patients with OPSCC treated in the period 2000-2015 at the VU University Medical Center Amsterdam, were included (n = 926). The presence of an oncogenic HPV infection was determined by p16-immunostaining, followed by a high-risk HPV general primer 5+/6+ DNA PCR on the p16-immunopositive cases. A review of pathology reports in all female patients (n = 305) was undertaken to identify cytological signs of HPV-related (pre)cancer of the cervix. In total 281 of 926 (30.3%) OPSCCs were HPV-positive. Moreover, a significant increase in the prevalence of HPV-positive OPSCCs was observed from 14.0% in 2000 to 48.1% in 2015 (p 0.001). Among the female patients with an HPV-positive OPSSC (n = 70), the results of cervical smears were available in 56 of 70 patients (80.0%). Of the female patients with HPV-positive OPSCC, 9 of 56 (16.1%) patients had a vaginal cuff Papanicolaou (Pap) test ≥3b in their medical history compared to 7 of 168 (4.2%) in the HPV-negative group (p = 0.003). In conclusion, a continuous increase in the HPV-attributable fraction of OPSCC was demonstrated in the period 2000-2015 in the Amsterdam region. HPV-positive OPSCC has a significant association with a history of suspicious Pap results of the cervix in female patients.

We report the enhancement of infrared absorption of chemisorbed carbon monoxide on platinum in the gap of plasmonic nanoantennas. Our method is based on the self-assembled formation of platinum nanoislands on nanoscopic dipole antenna arrays manufactured via electron beam lithography. We employ systematic variations of the plasmonic antenna resonance to precisely couple to the molecular stretch vibration of carbon monoxide adsorbed on the platinum nanoislands. Ultimately, we reach more than 1500-fold infrared absorption enhancements, allowing for an ultrasensitive detection of a monolayer of chemisorbed carbon monoxide. The developed procedure can be adapted to other metal adsorbents and molecular species and could be utilized for coverage sensing in surface catalytic reactions.

Since its commercial introduction three-quarters of a century ago, fluid catalytic cracking has been one of the most important conversion processes in the petroleum industry. In this process, porous composites composed of zeolite and clay crack the heavy fractions in crude oil into transportation fuel and petrochemical feedstocks. Yet, over time the catalytic activity of these composite particles decreases. Here, we report on ptychographic tomography, diffraction, and fluorescence tomography, as well as electron microscopy measurements, which elucidate the structural changes that lead to catalyst deactivation. In combination, these measurements reveal zeolite amorphization and distinct structural changes on the particle exterior as the driving forces behind catalyst deactivation. Amorphization of zeolites, in particular, close to the particle exterior, results in a reduction of catalytic capacity. A concretion of the outermost particle layer into a dense amorphous silica–alumina shell further reduces the mass transport to the active sites within the composite., ISSN:2041-1723, Nature Communications, 8 (1)

Vibrational levels of the recently observed high-spin transition (1 Πu5-1 Πg5) of dicarbon [P. Bornhauser et al., J. Chem. Phys. 142, 094313 (2015)] are explored by applying non-linear double-resonant four-wave mixing and laser-induced fluorescence methods. The deperturbation of the d Πg3, υ = 8 and 1 Πg5, υ = 3 states results in accurate molecular constants for the υ = 3 "dark" quintet state. In addition, the spin-orbit interaction constant is determined and parameters for the upper Swan level d Πg3, υ = 8 are improved. The first excited vibrational state of 1 Πu5 is observed by performing perturbation-assisted intersystem crossing via "gateway" states in the d Πg3, υ=6∼1 Πg5,υ= 0 system. The rotationally resolved spectra yield 11 transitions to 1 Πu5, υ = 1 that include four spin-substates. Data reduction results in accurate molecular constants of this vibrational level in the shallow potential energy surface of this state. Finally, υ = 1 and 2 of the lower quintet state (1 Πg5) are measured by performing perturbation-assisted double-resonant excitation to the 1 Πu5, υ = 0 state and observing dispersed fluorescence. The obtained molecular constants are compared with high level ab initio computations at the multi-reference configuration interaction (MRCI) level of theory by using a large correlation consistent basis set or, alternatively, by applying the computationally less demanding method of explicitly correlated multi-reference configuration interaction (MRCI-F12). The spectroscopic accuracy of both methods is evaluated by comparison with the experimental findings.

Transient-pulse experiments in temporal-analysis-of-product (TAP) and conventional mode were performed to determine elementary reaction steps involved in the methane steam reforming reaction over Rh/Al2O3 and Rh/12 CeO2–Al2O3. TAP pulse experiments demonstrated that both catalysts dissociate methane at 773 K leading to hydrogen, carbon, and CHx adspecies. Conventional transient pulse experiments with alternating methane/argon and water pulses with a delay between them of 86, 73, and 5 s, respectively, showed that the reactivity of adspecies over the promoted and nonpromoted catalysts decrease with time. Ceria catalyzes the oxidation of carbon species by water.

Atomically dispersed rhodium catalysts supported on xSm2O3–yCeO2–Al2O3 supports were synthesized and applied in the methane steam reforming (MSR) reaction. Scanning transmission electron microscopy and MSR catalytic tests show that the dispersion of the metal, and thus the catalytic performance, depends on the support composition. The 12CeO2–Al2O3 support confers partial stability to rhodium, leading to the coexistence of atomically dispersed and nanosized rhodium particles during MSR at 773 K. A reaction mechanism with CH4 being activated on the atomically dispersed rhodium and CO being formed on the nanoparticles is proposed. A lower rate caused by the formation of carbon species is seen in the presence of atomically dispersed rhodium and in the absence of nanoparticles as CO formation is hindered.

The design and performance of a high-resolution transmission-type X-ray spectrometer for use in the 15-26 keV energy range at synchrotron light sources is reported. Monte Carlo X-ray-tracing simulations were performed to optimize the performance of the transmission-type spectrometer, based on the DuMond geometry, for use at the Super X-ray absorption beamline of the Swiss Light Source at the Paul Scherrer Institute. This spectrometer provides an instrumental energy resolution of 3.5 eV for X-ray emission lines around 16 keV and 12.5 eV for emission lines at 26 keV, which is comparable to the natural linewidths of the K and L X-ray transitions in the covered energy range. First experimental data are presented and compared with results of the Monte Carlo X-ray simulations.

Correction for ‘An adamantane-based COF: stability, adsorption capability, and behaviour as a catalyst and support for Pd and Au for the hydrogenation of nitrostyrene’ by M. M. Trandafir et al., Catal. Sci. Technol., 2016, 6, 8344–8354.

The structure of Ce3+, which is responsible for the low-temperature oxygen storage capacity of ceria, was determined by high-energy-resolution fluorescence detected X-ray absorption spectroscopy at the Ce L3 and L1 edges. Well-defined ceria nanoparticles (rods, truncated octahedra, and cubes) were synthesized hydrothermally and promoted by platinum nanoparticles. The electronic structure of Ce3+ does not depend significantly on the nature of the exposed crystallographic planes of CeO2 particles; it does, however, differ from the electronic structure of known stable compounds containing Ce3+ ions, such as CeAlO3, Ce(NO3)3·6H2O, and Ce2Zr2O7. Theoretical simulation of Ce L1 and L3 X-ray absorption spectra, quantitative analysis of the oxygen storage capacity, and X-ray diffraction data suggest that Ce3+ ions form both at the surface and in the near-surface layer. Surface and bulk Ce3+ ions are characterized by elongated Ce–O distances in the first coordination shell and almost the same Ce–Ce distances in th...

Hard X-ray RIXS permits monitoring the electronic state of materials under in situ conditions. Time resolved, or quick RIXS, allows following dynamic changes in the electronic structure, which enables the identification of short-lived reaction intermediates. Within, we report the development of in situ quick RIXS spectroscopy, using a wavelength dispersive spectrometer operated in the von Hamos geometry. The main operational characteristics and performance of this experimental setup are presented. Dynamic changes in electronic structure are reported for the first time with quick RIXS, demonstrated on the reduction of Au2O3. The quick RIXS methodology can be implemented at any spectroscopy hard X-ray beamline. Therefore we envisage this tool to be widely used in the study of materials dynamic electronic changes.

The cerium oxidation state in ceria doped Rh/Al2O3 catalysts during the methane steam reforming (MSR) reaction was determined by in situ X ray absorption spectroscopy and online mass spectrometry at 773 K. The catalysts were characterized by electron microscopy and X ray diffraction. The oxidation states of rhodium and cerium during MSR are related. A total of 25 of Ce4+ reduces to Ce3+ under a CH4/H 2O/He flow when the catalyst is active in MSR (773 K). A slight reoxidation occurs when the catalyst is sequentially exposed to steam diluted in helium at high temperatures showing that only a small fraction of CeO 2 x may be involved in the water activation step by reoxidation. The main role of ceria during reaction besides the activation of water is to stabilize the structure of alumina by forming CeAlO3 and to maintain the dispersion of rhodium. © 2013 American Chemical Society.

Oxygen storage materials such as ceria are used in many catalytic applications because they can reversibly bind and release oxygen. Tools are needed to observe and quantify this activity which involves a change in the cerium oxidation state and to understand the involvement of cerium in catalytic processes. To prove that cerium changes its oxidation state in the catalytic cycle the transient rates of Ce3+ formation and decay should be compared to the overall reaction rate. For such mechanistic studies the time resolution is essential as the quantification of the Ce3+ species should be faster than the reaction rate. However, it is challenging to follow the dynamic changes of the cerium oxidation state under reaction conditions, especially when the concentration of cerium atoms involved in the reaction cycle is low. In this paper, we evaluate the sensitivity of high-resolution X-ray emission-based methods for the in situ time-resolved quantification of small concentrations of Ce3+ in ceria-based materials. We demonstrate that resonant X-ray emission spectroscopy (RXES) at optimal excitation energy is more sensitive than high energy resolution off-resonant spectroscopy (HEROS) and non-resonant X-ray emission spectroscopy (non-resonant XES) and that it can track the reactivity of less than 0.3% of cerium atoms in a 1% Pt/CeO2 catalyst in a plug-flow reactor with sub-second time resolution. These results demonstrate that X-ray emission-based methods can be used as very sensitive tools and provide new insights into dynamic changes of the oxidation state in reducible oxides in a variety of applications.

The role of Sm 2 O 3 and CeO 2 promoters on the structural properties and catalytic behavior of Rh/ x Sm 2 O 3 – y CeO 2 –Al 2 O 3 catalysts during methane steam reforming (MSR) was investigated. Promoted catalysts showed higher reaction rates per surface Rh atom and improved stability compared to Rh/Al 2 O 3 . In situ X-ray absorption revealed that the structure of Rh particles in Rh/Al 2 O 3 changes drastically during MSR, while it was stable in the presence of Sm 2 O 3 –CeO 2 . Sintering of the active metal phase was the main cause of deactivation. STEM images showed stronger Rh agglomeration of the unpromoted catalyst with time on stream.

The gas phase hydrogenation of nitrobenzene over gold nanoparticles supported on titania and alumina was investigated by in situ infrared spectroscopy. Adsorption and reduction of nitrobenzene readily proceeded on both catalysts, and aniline remained adsorbed on the catalyst. On titania, decreasing amounts of nitrobenzene adsorb in each cycle. Adsorption was found to occur through hydrogen-bonding interactions with surface hydroxyl groups on both supports. In the presence of aniline, nitrobenzene adsorbs on other sites on titania, as it is not able to displace aniline from hydroxyl sites. On alumina, nitrobenzene displaces some aniline from the surface hydroxyl groups. Nitrobenzene and aniline thus compete, at least in part, for the same sites on alumina, but not on titania.

In X-ray computed tomography, the task of imaging only a local region of interest (ROI) inside a larger sample is very important. However, without a priori information, this ROI cannot be exactly reconstructed using only the image data limited to the ROI. We propose here an approach of region-of-interest tomography, which reconstructs a ROI within an object from projections of different fields of view acquired on a specific angular sampling scheme in the same tomographic experiment. We present a stable procedure that not only yields high-quality images of the ROI but keeps as well the quantitative contrast on the reconstructed images. In addition, we analyze the minimum number of projections required for ROI tomography from the point of view of the band region of the Radon transform, which confirms this number must be estimated based on the size of the entire object and not only on the size of the ROI.

In situ X-ray absorption spectroscopy identified the role of oxidized platinum species in generating a high-activity state in the oxidation of carbon monoxide over Pt/Al2O3, Pt/TiO2 and Pt/SiO2. Two activity regimes were identified: low-activity at high carbon monoxide concentration and low temperature, and high activity at low carbon monoxide concentration and high temperature. The change between the two regimes occurred suddenly; there was ignition when going from low to high activity, and extinction from high to low activity. Ignition only occurred when the concentration of oxygen was higher than that of carbon monoxide. The catalyst was metallic and covered with carbon monoxide at low activity and partially oxidic at high activity. Carbon monoxide poisoned the catalyst at low activity. The partially oxidized platinum catalysts showed high activity. The partially oxidized catalyst had a lower oxygen coordination number and shorter Pt–O bond length as expected for bulk PtO2, suggesting that a strongly defected platinum oxide was formed with a possibly square planar coordination. The catalyst structure showed a dynamic behavior and the amount of oxide depended on the concentration of reactants in the gas phase. Smaller particles of Pt/Al2O3 reached high activity at lower temperature than the larger particles. Pt/TiO2 reached high activity at the lowest temperature while Pt/SiO2 required the highest temperature. Parallel with the temperature of ignition, Pt/SiO2 was the most oxidized while Pt/TiO2 was the least oxidized.

The deposition of gold on silica tends to give large particles when using conventional techniques. We report the preparation of 0.8 ± 0.2 nm particles on a modified SBA-15 support. The method involves the functionalization of silica with amine groups and deposition of gold at basic pH. These catalysts are highly active and selective in the dehydrogenation of formic acid.

[EN] A bimodal (micro/mesoporous) COF was synthesized by coupling tetrakis-1,3,5,7-(4'-iodophenyl) adamantane with 4,4'-diethynylbiphenyl following a Sonogashira protocol. The COF preparation strategy led, however, to the incomplete recovery of the palladium catalyst and ICP-OES analysis indicated that around 0.1 wt% palladium remained inside the pores. Noteworthily, the remnant palladium catalyst is still accessible and can be valorised in additional catalytic reactions like the hydrogenation of nitrostyrene. Further deposition of 0.5 wt% active metals (like palladium or gold) enhanced the catalytic activity and tuned the catalyst selectivity with respect to analogous metal catalysts prepared using active carbon as a support. The resulting COF-supported metal NPs are stable and recyclable catalysts. Under normal conditions, this COF is also able to adsorb large amounts of weak electrophilic gases like carbon dioxide., This work was supported by a grant from the Romanian National Authority for Scientific Research, CNDI-UEFISCDI (project numbers PCCA-II-166/2012, PCCA-II-56/2014, and PN-II-ID-PCE-2011-3-0060). M. M. T. was supported by the strategic grant POSDRU/187/1.5/S/155559 Competitive Multidisciplinary Doctoral Research at European Level (CdocMD) cofinanced by the European Social Fund within the Sectorial Operational Program Human Resources Development 2007-2013. C. M. T. acknowledges funding from the PN2-128/2011 Project granted by the Romanian UEFISCDI Agency.

A series of well-defined catalysts based on platinum nanoparticles supported on amorphous silica–alumina with varying composition was prepared by incipient wetness impregnation. Quantitative structural characterization of alumina and aluminosilicate domains by 27Al (3Q) MAS NMR spectroscopy showed that the concentration of Bronsted acid sites determined by pyridine adsorption is related solely to the concentration of the aluminosilicate domain. However, only a very small fraction of the tetrahedrally coordinated aluminum led to the formation of Bronsted acid sites, because the negative charge on most of the tetrahedral Al–O was balanced by cationic aluminum oxide clusters. The preparation method led to small, uniform (0.6–0.8 nm) Pt particles on all aluminum-containing supports.

Au catalysts with different metallic particle sizes and supported on silica, alumina, titania, zirconia, ceria, and niobia were prepared, and the reduced catalysts were characterized by EXAFS spectroscopy. As the Au Au coordination number decreased, the interatomic bond length decreased. The Au Au bond length contraction appears to be independent of the support type. A correlation between the dispersion of Pt catalysts determined by hydrogen chemisorption and the EXAFS Pt Pt coordination number was established and used to determine the dispersion of fully reduced Au catalysts. In addition, the Au particle size was estimated using a literature correlation of the EXAFS coordination number. For particles larger than about 40 A, there was little change in the metallic bond length, whereas in catalysts with gold particles smaller than 30 A, the Au Au distance decreased with decreasing particle size, with a maximum contraction of about 0.15 A. Decreasing particle size also brought a decrease in the intensity of the white line of the XANES spectrum. Both the decrease in bond distance and white line intensity were consistent with an increase in the d-electron density of Au atoms in very small particles. Au particles smaller than about 30 A were also reactive to air, leading to oxidation of up to 15% of the atoms of the gold particles, depending on the size; larger particles were not oxidized. These oxidized Au atoms in small particles are suggested to be active for CO oxidation.

The structure of gold clusters of different sizes supported on various metal oxides (Al2O3, TiO2, SiO2) exposed to different CO oxidation conditions was investigated in situ using X-ray absorption spectroscopy at the Au L3 edge. In all catalysts, the only phase detected during catalysis was Au0. In the most active sample with small gold particles (Au/Al2O3), variation in the electronic structure of the gold clusters with changing reaction conditions was observed by XANES spectroscopy and ascribed to the adsorption of CO on the metallic gold clusters. FEFF8 calculations proved that the changes in the XANES signature of Au/Al2O3 can be explained by backdonation of d-electrons into the π ∗ orbitals of CO. For Au/Al2O3, the presence of Au O backscattering in the EXAFS suggested weak cluster–support interactions. For Au/SiO2 and Au/TiO2, the structure of the gold clusters remained unchanged throughout all experiments.

The Henry's equilibrium constant and enthalpy of n -hexane adsorption at low coverage have been determined at 423 K for zeolites of different structural types that have been subjected to different treatments. The Henry's constant and enthalpy of adsorption increased as the pore size decreased as MFI > MOR > BEA > FAU. Bronsted acid sites and non-framework Al produced through steaming gave smaller increases, and changes due to alkali or lanthanide ions had little effect. From the enthalpy of adsorption and the Henry's equilibrium constant, the entropy of adsorption was calculated. Despite the large differences in structure and composition of the zeolites, there is a linear correlation, or compensation relation, between the entropy and enthalpy of adsorption for all of the zeolites. Combined with previous work in which the same zeolites exhibited a linear kinetic compensation, or Constable relation, for the monomolecular cracking of n -hexane, these results support the proposal that the intrinsic acid strengths of active Bronsted acid sites in zeolites of different structures or silica-to-alumina ratios with Lewis acid sites from nonframework Al are similar.

This paper presents a catalytic and spectroscopic study of the dealumination of a commercial zeolite, Beta (Zeolyst). After dealumination by hydrochloric or oxalic acid and steaming (up to 973 K), increased activity and selectivity were found in the acylation of anisole with octanoic acid. The treatments led to the extraction of aluminum from the crystallographic T-sites in the framework structure, giving amorphous extraframework aluminum(–silicon)–oxide species in the case of steaming, while treatment with mineral acids or complexing agents removed framework and extraframework aluminum from the zeolite. 27 Al MQ MAS NMR revealed a changing of the aluminum coordinations and a shift from framework to extraframework species after steaming. Steaming removes aluminum from other crystallographic T-sites than acid leaching. Using nitrogen physisorption, no differences in the texture properties could be observed and XRD showed that the crystallinity was preserved, indicating high stability of the commercial zeolite Beta. The nature of the enhanced activity is suggested to result from higher accessibility of the active sites that are proposed to be associated with framework-connected aluminum atoms.

L2 and L3 X-ray absorption near edge spectra (XANES) on supported Pt particles, with and without chemisorbed hydrogen, are shown to reflect the type of hydrogen-binding site on the Pt surface. FEFF8 ab initio multiple scattering calculations are used to determine XANES spectral fingerprints for the atop vs threefold H binding sites on Pt. Comparison of the experimental XANES data with the theoretical fingerprints, and further theoretical results, show that the acid/base properties of the support have a profound influence on the hydrogen coverage, and therefore on the mode of hydrogen adsorption on the Pt surface. As the electron richness of the support oxygen atoms increases (i.e., with increasing alkalinity of the support), the H coverage increases and the hydrogen-binding site of the strongly adsorbed hydrogen changes from atop to threefold. This site change is primarily responsible for the observed changes in previously reported kinetic data, which show an increase in negative order (roughly from −1. 5t o−2.5) in hydrogen partial pressure for neopentane hydrogenolysis with increasing support alkalinity. This change in negative order directly reflects the greater number of vacant Pt sites that must be available to allow adsorption of the neopentane. A compensation relation is found in the kinetic data of Pt on different supports resulting directly from this change in hydrogen coverage. This implies that the experimentally determined kinetic parameters are apparent values. These apparent values are correlated to the intrinsic kinetic parameters via the thermodynamic properties of the sorption of the reactants, described by the Temkin equation. The TOF of neopentane hydrogenolysis over several catalysts, measured in previous work, decreases with the increasing alkalinity of the support. This can now be directly explained as the result of the change in hydrogen coverage using a Frumkin isotherm, implying that the neopentane adsorption becomes weaker with increased hydrogen coverage. These conclusions, that hydrogen drives the catalysis, are further supported by density functional calculations on small Pt 4 clusters, which show that the acid/base properties of the support have a much larger direct influence on Pt–H bonding than on Pt–CH n bonding.  2003 Elsevier Science (USA). All rights reserved.

Review of Scientific Instruments, 86 (12), ISSN:0034-6748, ISSN:1089-7623

Vibrational levels of the recently observed high-spin transition (1 Πu5−1 Πg5) of dicarbon [P. Bornhauser et al., J. Chem. Phys. 142, 094313 (2015)] are explored by applying non-linear double-resonant four-wave mixing and laser-induced fluorescence methods. The deperturbation of the d Πg3, υ = 8 and 1 Πg5, υ = 3 states results in accurate molecular constants for the υ = 3 “dark” quintet state. In addition, the spin-orbit interaction constant is determined and parameters for the upper Swan level d Πg3, υ = 8 are improved. The first excited vibrational state of 1 Πu5 is observed by performing perturbation-assisted intersystem crossing via “gateway” states in the d Πg3, υ=6∼1 Πg5,υ= 0 system. The rotationally resolved spectra yield 11 transitions to 1 Πu5, υ = 1 that include four spin-substates. Data reduction results in accurate molecular constants of this vibrational level in the shallow potential energy surface of this state. Finally, υ = 1 and 2 of the lower quintet state (1 Πg5) are measured by performi...

Phosphine metal–organic frameworks (P-MOFs) are crystalline porous coordination polymers that contain phosphorus functional groups within their pores. We present the use of X-ray absorption spectroscopy (XAS) at the P K-edge to determine the phosphine to phosphine oxide ratio in two P-MOFs with MIL-101 topology. The phosphorus oxidation state is of particular interest as it strongly influences the coordination affinity of these materials for transition metals. This method can determine the oxidation state of phosphorus even when the material contains paramagnetic nuclei, differently from NMR spectroscopy. We observed that phosphine in LSK-15 accounts for 72 ± 4% of the total phosphorus groups and that LSK-12 contains only phosphine oxide.

The influence of the support type and the metal precursor on the dispersion of rhodium after calcination and reduction was determined. The combination of electron microscopy and X-ray absorption analysis allowed the quantification of the amount of atomically dispersed rhodium in the samples. Higher amounts of atomically dispersed rhodium atoms are obtained when metal impregnation is performed with a rhodium acetate precursor in comparison to a rhodium chloride precursor over supports of the same composition. The stability of rhodium is improved with the addition of promoters; the co-presence of samaria and ceria in the support and metal impregnation with a rhodium acetate precursor leads to the highest amount of atomically dispersed rhodium remaining after reductive treatment at 773 K.

Uptake experiments under full catalytic conditions have been performed using a tapered element oscillating microbalance. The effect of acid leaching on the diffusivity and reaction of n -hexane in Pt/H–mordenite has been established from these experiments. For acid-leached Pt/H–mordenite the hydroisomerization activity is four times higher than for untreated Pt/H–mordenite. Acid leaching gives rise to an acceleration of the uptake of n -hexane under reaction conditions. Analysis of the data implies the faster uptake to be predominantly accounted for by a shorter intracrystalline diffusion path length, as diffusivities are in the same range for both catalysts. The shorter path length results from the mesoporous structure of H-mordenite generated by acid leaching. From the diffusivities, from the path lengths, and by using the Thiele model, it is inferred that the intrinsic rate constant of acid-leached Pt/H–mordenite is twice that of the untreated Pt/H–mordenite. The facilitated transport of reactants and products doubles the effectiveness factor, thus also contributing to the overall activity enhancement for n -hexane hydroisomerization after acid leaching.

This paper presents a catalytic, spectroscopic, calorimetric study of mildly steam-dealuminated mordenite (H-MOR). With increasing steam partial pressures at 673 K there is increasing dealumination, the loss of Brønsted acid sites and the presence of extra-framework Al (AlEF) likely in the zeolite pores. Additionally, the number of Lewis sites increases with increasing AlEF. As observed in previous studies, the catalytic activity for hydroisomerization of n-hexane and monomolecular cracking of n-butane and n-hexane initially increases and goes through a maximum with increasing steam severity. While the maximum increase in the rate of hexane cracking is about 5 times, the activation energies indicate that steaming does not significantly change the strength of the Brønsted acid sites. In the absence of AlEF, the enthalpy of alkane adsorption is constant at all surface coverages. However, in the presence of AlEF, the initial heat of alkane sorption increases by about 5–10 kJ/mol. After covering these adsorption sites, the heat of adsorption is identical to that on the unsteamed H-MOR until saturation coverage has been reached. The increased enthalpies of adsorption are suggested to occur on the AlEF, Lewis acid sites. Since the surface coverage of paraffins under typical reaction conditions is low, it is proposed that the enhanced rate is due to the increased initial heat of adsorption leading to a higher concentration of reactants in the steamed H-MOR zeolites.

X-Ray absorption near-edge structure (XANES) spectra at the Ti K-edge of TiO2 (rutile and anatase phases) have been analyzed. Theoretical analyses of the experimental data have been performed on the basis of both real-space full multiple scattering theory with muffin-tin approximation for potential shape and full-potential (beyond muffin-tin approximation) finite difference method. A full potential is required to satisfactorily reproduce experimental Ti K-edge XANES spectra in the TiO2.

A new apparatus for in situ x-ray absoprtion fine structure measurements in the medium energy range of 1000–3500 eV has been developed. Measurements can be performed in a gaseous environment (max. pressure 1 bar) at temperatures ranging from 80 to 750 K. Pre-treatments can be performed at 5 bar and 750 K in the same cell, after which XAFS measurements can be done without exposing the sample to ambient air. In a modular set-up several detector systems can be used: fluorescence detection using a gas proportional counter, a photodiode or a microstrip detector. All detectors are highly integrated into the cell, gaining solid angle for detection. Electron yield detection can be used simultaneously using conversion electron yield or total electron yield. The performance of the new apparatus is demonstrated by a study of the K edge of Al in Zeolite Beta. The Al content is as low as 2 wt%. It will be shown that octahedral framework Al is formed while adding gaseous water at room temperature after ammonia removal ...