Alfons Molenbroek - Academia.edu (original) (raw)

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Papers by Alfons Molenbroek

$Research paper thumbnail of Dehydrogenation kinetics of pure and nickel-doped magnesium hydride investigated by in situ time-resolved powder X-ray diffraction$

International Journal of Hydrogen Energy, 2006

Journal of Catalysis, 2000

Dynamic changes in the structure and catalytic activity of Cu/ZnO methanol synthesis catalysts ha... more Dynamic changes in the structure and catalytic activity of Cu/ZnO methanol synthesis catalysts have been investigated by a further developed in situ method, which combines X-ray diffraction (XRD), X-ray absorption fine structure spectroscopy (XAFS), and on-line catalytic measurements by mass spectrometry. The temperature-programmed reduction of copper was monitored both by in situ quick-EXAFS (QEXAFS) and on-line mass spectrometry. The results indicate that under typical mild reduction conditions very small copper particles (10-15Å) are formed. Upon change in the reduction potential of the methanol synthesis gas, reversible changes of the Cu-Cu coordination number are observed by EXAFS. These structural changes are accompanied by changes in the catalytic activity and the highest activities were observed after exposure to the most reducing conditions. In this state the catalyst exhibited low Cu-Cu coordination numbers. These results support the model that reversible changes in the wetting of ZnO by Cu may occur upon changes in the reaction conditions. The results also show that such dynamical changes in Cu morphology may influence the catalytic properties. All the conditions used in the above studies are less severe than those observed to result in bulk alloy formation. However, additional XAFS measurements at higher temperatures have been performed and EXAFS spectra have been simulated to address the possibility for Cu-Zn alloy formation as suggested by recent results in literature. Only under severe reduction conditions was significant alloying of copper and zinc observed in EXAFS in addition to the morphological changes. Such changes have not been seen in the Cu/SiO 2 system.

Synchrotron Radiation News, 2009

Heterogeneous catalysts are complex and dynamic multicomponent systems, usually consisting of sev... more Heterogeneous catalysts are complex and dynamic multicomponent systems, usually consisting of several elements and phases. The catalyst active sites are very sensitive to the environmental conditions (gas atmosphere, temperature, and pressure) and may undergo major changes during the course of the chemical reaction. A good understanding of the active catalytic species and their role in the catalytic reaction can only

J Phys Chem B, 1998

ABSTRACT

EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany, 2008

ABSTRACT

Most industrial high-temperature shift (HTS) catalysts consist of chromium-and iron-oxides. The s... more Most industrial high-temperature shift (HTS) catalysts consist of chromium-and iron-oxides. The stable iron-oxide phase under water-gas shift reaction conditions is magnetite, which actually is the active species [1]. Chromium is added as textural promoter to inhibit sintering of the iron-oxide, and hence preserve the surface area [2,3]. Cu is added as a promoter to enhance the catalyst activity [4]. In a HTS catalyst with 1%Cu, 6%Cr/Fe 3 O 4 it has been observed that, after 2000h of catalytic test, leaching the spent catalysts with ammonia solution is unable to remove all Cu. A small amount of Cu (0.36%) remains in the catalyst. Surprisingly, the activity of the catalyst is retained. The chemical state of the Cu promoter in the fresh and spent Cu-promoted Cr-stabilized iron-based water gas shift catalyst has been studied by in situ XAFS prior to reaction and under industrial HTS conditions (380˚C, 24bar, feed gas: 9.4 % CO, 6.25 % CO 2 , 1.25 % Ar, 45.6 % H 2 , 37.5% H 2 O). The in situ XAFS experiments have been performed at Hasylab(Germany) at beamline X1 and at MAXlab (Sweden) at beamline I811.

Introduction The awareness of soot abatement in the exhaust from diesel engines is currently incr... more Introduction The awareness of soot abatement in the exhaust from diesel engines is currently increasing due to new environmental legislations for exhaust specifications. An attractive approach for effective soot removal includes the introduction of filters on the diesel-driven vehicles and the functionalization of the filters for catalytic soot oxidation (1,2). Ceria-based materials are widely adopted for this purpose. It is generally accepted that the redox properties of ceria are crucial to the catalytic effect, but the detailed reaction mechanism and the location of the catalytic active sites are still being debated. For instance, it has been proposed that the reaction occurs at the soot-ceria interface (3) and that the reaction occurs through spill-over of oxygen from ceria to reaction centers located at the soot surfaces (2-4). Averaging techniques, such as temperature-programmed oxidation and thermo-gravimetric analysis, have mainly been used to study the soot oxidation reacti...