Patricia J. Kooyman

Nanoscale chemical imaging of a working catalyst by scanning transmission X-ray microscopy

The modern chemical industry uses heterogeneous catalysts in almost every production process. They commonly consist of nanometre-size active components (typically metals or metal oxides) dispersed on a high-surface-area solid support, with performance depending on the catalysts’ nanometre-size features and on interactions involving the active components, the support and the reactant and product molecules. To gain insight into the mechanisms of heterogeneous catalysts, which could guide the design of improved or novel catalysts, it is thus necessary to have a detailed characterization of the physicochemical composition of heterogeneous catalysts in their working state at the nanometre scale. Scanning probe microscopy methods have been used to study inorganic catalyst phases at subnanometre resolution, but detailed chemical information of the materials in their working state is often difficult to obtain. By contrast, optical microspectroscopic approaches offer much flexibility for in situ chemical characterization; however, this comes at the expense of limited spatial resolution. A recent development promising high spatial resolution and chemical characterization capabilities is scanning transmission X-ray microscopy, which has been used in a proof-of-principle study to characterize a solid catalyst. Here we show that when adapting a nanoreactor specially designed for high-resolution electron microscopy, scanning transmission X-ray microscopy can be used at atmospheric pressure and up to 350 °C to monitor in situ phase changes in a complex iron-based Fisher–Tropsch catalyst and the nature and location of carbon species produced. We expect that our system, which is capable of operating up to 500 °C, will open new opportunities for nanometre-resolution imaging of a range of important chemical processes taking place on solids in gaseous or liquid environments.

Evolution of Fe species during the synthesis of over-exchanged Fe/ZSM5 obtained by chemical vapor deposition of FeCl3

The evolution of iron in over-exchanged Fe/ZSM5 prepared via chemical vapor deposition of FeCl3 was studied at each stage of the synthesis. Different characterization techniques (EXAFS, HR-XANES, 57 Fe Mossbauer spectroscopy, 27 Al NMR, EELS, HR-TEM, XRD, N2 physisorption, and FTIR spectroscopy) were applied in order to correlate the changes occurring in the local environment of the Fe atoms with migration and aggregation phenomena of iron at micro- and macroscopic scale. Mononuclear isolated Fe-species are formed upon FeCl3 sublimation, which are transformed into binuclear Fe-complexes during washing. During calcination, iron detached from the Bronsted sites migrates to the external surface of the zeolite, finally leading to significant agglomeration. Nevertheless, agglomeration of Fe can be strongly suppressed by adequately tuning the conditions of the calcination.  2002 Elsevier Science (USA). All rights reserved.

Hydroxylation of phenol with hydrogen peroxide on EUROTS-1 catalyst

Abstract The catalytic performance of a standard titanium molecular sieve catalyst, EUROTS-1, has been evaluated at four European universities. The hydroxylation of phenol into catechol and hydroquinone with aqueous H 2 O 2 solution was selected as catalytic test reaction. The use of standard EUROTS-1 catalyst permitted to establish a reproducible standard reaction procedure. The calcination conditions of the catalyst, the use of internal standards, and the nature and the amount of the solvent added to the reaction mixture were found to be major factors determining phenol conversion, product selectivity and hydrogen peroxide product yield.

Visualization of oscillatory behaviour of Pt nanoparticles catalysing CO oxidation

Many catalytic reactions under fixed conditions exhibit oscillatory behaviour. The oscillations are often attributed to dynamic changes in the catalyst surface. So far, however, such relationships were difficult to determine for catalysts consisting of supported nanoparticles. Here, we employ a nanoreactor to study the oscillatory CO oxidation catalysed by Pt nanoparticles using time-resolved high-resolution transmission electron microscopy, mass spectrometry and calorimetry. The observations reveal that periodic changes in the CO oxidation are synchronous with a periodic refacetting of the Pt nanoparticles. The oscillatory reaction is modelled using density functional theory and mass transport calculations, considering the CO adsorption energy and the oxidation rate as site-dependent. We find that to successfully explain the oscillations, the model must contain the phenomenon of refacetting. The nanoreactor approach can thus provide atomic-scale information that is specific to surface sites. This will improve the understanding of dynamic properties in catalysis and related fields.

MAS NMR, TPR, and TEM studies of the interaction of NiMo with alumina and silica–alumina supports

Abstract 1 H MAS NMR, 1 H spin-echo MAS NMR with Al irradiation, 29 Si MAS NMR, and 1 H→ 29 Si CP MAS NMR were used to investigate the deposition of Mo and Ni species on the surface of alumina and silica–alumina. Mo and Ni species first occupy the alumina sites and then the silica sites. The results of temperature-programmed reduction show that the weaker interaction between the Mo and Ni species and the silica–alumina support leads to better reducibility of the metal oxides on silica–alumina than on Al 2 O 3 . Mo and Ni species also interact with each other. Transmission electron microscopy proved that, after sulfidation, higher stacks of MoS 2 are formed on the silica–alumina support than on the Al 2 O 3 support. The higher stacking is responsible for the higher hydrodenitrogenation activity of the NiMoS catalysts supported on silica–alumina.

A MEMS Reactor for Atomic-Scale Microscopy of Nanomaterials Under Industrially Relevant Conditions

We present a microelectromechanical systems (MEMS) nanoreactor that enables high-resolution transmission electron microscopy (TEM) (HRTEM) of nanostructured materials with atomic-scale resolution during exposure to reactive gases at 1 atm of pressure. This pressure exceeds that of existing HRTEM systems by a factor of 100, thereby entering a pressure range that is relevant to industrial purposes. The nanoreactor integrates a shallow flow channel (35 ¿m high) with a microheater and with an array of electron transparent windows of silicon nitride. The windows are only 10 nm thick but are mechanically robust. The heater has the geometry of a microhotplate and is made of Pt embedded in a silicon nitride membrane. To interface the nanoreactor, a dedicated TEM specimen holder has been developed. The performance is demonstrated by the live formation of Cu nanoparticles in a catalyst for the production of methanol. At 120 kPa and for temperatures of up to 500°C , the formation of these nanoparticles can be observed clearly and with an exceptionally low thermal drift. HRTEM images of the nanoparticles show atomic lattice fringes with spacings down to 0.18 nm.

Salinity-dependent diatom biosilicification implies an important role of external ionic strength

The role of external ionic strength in diatom biosilica formation was assessed by monitoring the nanostructural changes in the biosilica of the two marine diatom species Thalassiosira punctigera and Thalassiosira weissflogii that was obtained from cultures grown at two distinct salinities. Using physicochemical methods, we found that at lower salinity the specific surface area, the fractal dimensions, and the size of mesopores present in the biosilica decreased. Diatom biosilica appears to be denser at the lower salinity that was applied. This phenomenon can be explained by assuming aggregation of smaller coalescing silica particles inside the silica deposition vesicle, which would be in line with principles in silica chemistry. Apparently, external ionic strength has an important effect on diatom biosilica formation, making it tempting to propose that uptake of silicic acid and other external ions may take place simultaneously. Uptake and transport of reactants in the proximity of the expanding silica deposition vesicle, by (macro)pinocytosis, are more likely than intracellular stabilization and transport of silica precursors at the high concentrations that are necessary for the formation of the siliceous frustule components.

Nanometric hollow spheres made of MSU-X-type mesoporous silica

Submicrometric hollow spheres with mesoporous walls were prepared by applying the synthesis of MSU-X type mesoporous silica at neutral pH. The application of ultrasound in the proper power range, to a mixture of nonionic polyoxyethylene surfactant and unreacted silicon alkoxide (tetraethylorthosilicate) gives a stable emulsion where the cavitation bubbles created by the ultrasound are trapped in the solution. Further hydrolysis and condensation of the silica by the addition of sodium fluoride freezes the structure and gives a powder that exhibits a single reflection in the X-ray diffraction pattern, characteristic of a 3D-wormhole ordered porous framework. Its main feature is a large hysteresis loop in the nitrogen adsorption/desorption isotherm. This hysteresis loop corresponds to the retention of condensed nitrogen within the voids of these particles that can play the role of tanks for volatile compounds.

High-temperature transformations of organised mesoporous alumina

The high-temperature behaviour of organised mesoporous alumina was investigated using X-ray powder diffraction, nitrogen adsorption isotherms and transmission electron microscopy. Based on the combination of these methods it has been shown that organised mesoporous alumina synthesised using long-chain carboxylic acids is transformed at temperatures between 600 and 1000 °C into a material which consists of a packing of δ-alumina particles.

Acid dealumination of ZSM-5

The acid dealumination of various samples of ZSM-5 is studied using mineral acids as the dealuminating agent. Apart from the synthesis method of the ZSM-5, the acid concentration, the temperature, and the duration of the acid treatment are varied. The results are evaluated using elemental analysis and 27 Al MAS n.m.r. A comparison with literature data is presented. It seems that the dealumination of MFI is not as straightforward as is often assumed.