Keturakis C.,Lehigh University |
Gibson E.,Laboratoire Catalyse et Spectrochimie |
Vasic R.,Yeshiva University |
Tao F.,University of Notre Dame |
And 3 more authors.
AIChE 2012 - 2012 AIChE Annual Meeting, Conference Proceedings | Year: 2012
The bulk chromia-iron mixed oxide catalyst is the primary catalyst for the high temperature (310-450°C) water-gas shift (WGS) reaction for the production of hydrogen and carbon dioxide from steam and carbon monoxide. The commercial lifetime of pure magnetite catalysts is limited because of thermal sintering and chromium oxide addition, 8-12% Cr2O3, has been found to stabilize the surface area and extend the catalyst life to 2-5 years. Despite numerous characterization studies, the role of the chromia promoter is still not completely understood. The absence of fundamental in situ and operando spectroscopic studies of the bulk Cr2O3*Fe2O3 WGS shift catalyst during the WGS reaction in the catalysis literature has hindered the development of molecular level insights about the catalytic active sites, surface reaction intermediates and the reaction mechanism. In order to address the state of the iron oxide catalyst under reaction conditions, the role of the chromia promoter, and the nature of the catalytic active site, in situ Ambient Pressure-XPS (AP-XPS) and operando Raman, IR, and XAS spectroscopic studies under reaction conditions were undertaken in the present investigation. The operando Raman spectroscopy studies during the WGS confirm that crystalline Fe3O4 is the active bulk phase in Cr-doped samples and that crystalline Cr2O3 NPs are not present. The corresponding operando IR spectroscopy measurements revealed that dioxo surface (O=)2CrO2 species are also present under oxidizing conditions and become reduced during the WGS reaction. The IR measurements also revealed that no surface reaction intermediates are present, even when the reaction is performed at the WGS reaction temperature limit of ∼225°C. Operando XANES measurements of the Cr K-edge confirm the reduction of Cr+6 (dioxo species) to Cr+3 during steady-state WGS reaction conditions. The EXAFS Fe K-edge data confirm that the Cr-doped samples stabilize as Fe3O4 under steady-state WGS reaction conditions, while unpromoted Fe2O3 is active as an amorphous phase consisting mostly of a core of metallic Fe with an iron oxide shell consisting of an indistinguishable FeOx state (+2 or +3). Recent in situ AP-XPS data has revealed, for the first time, that some Cr+6 species are not reduced and remain on the catalyst surface during reaction conditions. The new fundamental insights are allowing for the establishment of molecular level models, based on direct observations during relevant WGS reaction conditions, for the WGS reaction mechanism and the nature of the catalytic active site for the high temperature chromia-iron WGS catalysts.
Aramburo L.R.,University Utrecht |
Karwacki L.,University Utrecht |
Cubillas P.,University of Manchester |
Asahina S.,JEOL Europe SAS Espace Claude Monet Allbe de Givemy |
And 15 more authors.
Chemistry - A European Journal | Year: 2011
A combination of atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), focused-ion-beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron-based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM-5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol-to-olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM-5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM-5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5-50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Ramirez J.,UNICAT |
Gutierrez-Alejandre A.,UNICAT |
Sanchez-Minero F.,National Polytechnic Institute of Mexico |
MacIas-Alcantara V.,UNICAT |
And 4 more authors.
Energy and Fuels | Year: 2012
HDS of 4,6-DMDBT over NiMoP/SBA-15 and NiMoP/(x)TiSBA-15 catalysts prepared using an heteropolyacid (H 3PMo 12O 40) and nickel citrate (C 12H 10Ni 3O 14) as Mo and Ni precursors was studied. To analyze the effect of calcination temperature on HDS activity, catalysts noncalcined and calcined at 773 K were prepared. The performance of the different catalysts during the hydrodesulfurization of 4,6-dimethyldibenzothiophene was compared with that of a reference catalyst prepared by impregnation with a solution containing ammonium heptamolybdate and nickel nitrate. Kinetic parameters for the HDS of 4,6-DMDBT were estimated using a simplified kinetic model. The catalysts were characterized by N 2 physisorption, X-ray diffraction, Raman, and IR of adsorbed CO at ∼100 K. The results show that for catalysts supported on pure SBA-15 the noncalcined catalyst prepared with H 3PMo 12O 40 (NiMoP(H-nc)/SBA-15) presents the highest number of active sites, the higher apparent reaction rate constant for the hydrogenation route, and therefore the best 4,6-DMDBT HDS activity. In contrast, for Ti-modified catalysts, NiMoP/(x)Ti-SBA-15, the highest HDS activity was found when 15% of TiO 2 was incorporated to SBA-15 and the catalyst was calcined at 773 K. This catalyst presented the highest TOF. © 2011 American Chemical Society.