10 research outputs found
Dynamics of CrO3âFe2O3 catalysts during the high-temperature water-gas shift reaction: molecular structures and reactivity
A series of supported CrO3/Fe2O3 catalysts were investigated for the high-temperature water-gas shift (WGS) and reverse-WGS reactions and extensively characterized using in situ and operando IR, Raman, and XAS spectroscopy during the high-temperature WGS/RWGS reactions. The in situ spectroscopy examinations reveal that the initial oxidized catalysts contain surface dioxo (Oâ)2Cr6+O2 species and a bulk Fe2O3 phase containing some Cr3+ substituted into the iron oxide bulk lattice. Operando spectroscopy studies during the high-temperature WGS/RWGS reactions show that the catalyst transforms during the reaction. The crystalline Fe2O3 bulk phase becomes Fe3O4 ,and surface dioxo (Oâ)2Cr6+O2 species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromiumâiron oxide catalyst surface is dominated by FeOx sites, but some minor reduced surface chromia sites are also retained. The Fe3â-xCrxO4 solid solution stabilizes the iron oxide phase from reducing to metallic Fe0 and imparts an enhanced surface area to the catalyst. Isotopic exchange studies with C16O2/H2 â C18O2/H2 isotopic switch directly show that the RWGS reaction proceeds via the redox mechanism and only O* sites from the surface region of the chromiumâiron oxide catalysts are involved in the RWGS reaction. The number of redox O* sites was quantitatively determined with the isotope exchange measurements under appropriate WGS conditions and demonstrated that previous methods have undercounted the number of sites by nearly 1 order of magnitude. The TOF values suggest that only the redox O* sites affiliated with iron oxide are catalytic active sites for WGS/RWGS, though a carbonate oxygen exchange mechanism was demonstrated to exist, and that chromia is only a textural promoter that increases the number of catalytic active sites without any chemical promotion effect
Anomalous reactivity of supported V2O5 nanoparticles for propane oxidative dehydrogenation: influence of the vanadium oxide precursor
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugĂ€nglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The oxidative dehydrogenation (ODH) of propane to propylene by supported vanadia catalysts has received much attention in recent years, but different reactivity trends have been reported for this catalytic reaction system. In the present investigation, the origin of these differing trends are investigated with synthesis of supported V/SiO2, V/TiO2, and V/Al2O3 catalysts prepared with three different vanadium oxide precursors (2-propanol/vanadyl triisopropoxide [VO(O-Pri)3] (VTI), oxalic acid/ammonium metavanadate [NH4VO3] (AMV), and toluene/vanadyl acetylacetonate [VO(C5H7O2)2] (VAA)) in order to elucidate the influence of the precursor on supported vanadia phase and propane ODH activity. In situ Raman spectroscopy revealed that the choice of vanadium precursor does not affect the dispersion of the supported vanadium oxide phase below 4 V nmâ2 (0.5 monolayer coverage), where only isolated and oligomeric surface VO4 species are present, and only the AMV precursor favors crystalline V2O5 nanoparticle (NP) formation below monolayer coverage (8 V nmâ2). The propane ODH specific reactivity trend demonstrated that there is no significant difference in TOF for the isolated and oligomeric surface VO4 sites. Surprisingly, V2O5 NPs in the âŒ1â2 nm range exhibit anomalously high propane ODH TOF values for the supported vanadia catalysts. This was found for all supported vanadium oxide catalysts examined. This comparative study with different V-precursors and synthesis methods and oxide supports finally resolves the debate in the catalysis literature about the dependence of TOF on the surface vanadium density that is related to the unusually high reactivity of small V2O5 NPs.DFG, SFB 546, Struktur, Dynamik und ReaktivitĂ€t von Ăbergangsmetalloxid-Aggregate
Anomalous reactivity of supported V2O5 nanoparticles for propane oxidative dehydrogenation: influence of the vanadium oxide precursor
CATL 2-Operando Raman and IR spectroscopy during the water-gas shift reaction over bulk Cr2O3*Fe2O3 mixed oxide catalysts
International audienc
New insights into the water-gas shift reaction over bulk Cr2O3*Fe2O3 mixed oxide catalysts: A combined operando Raman-IR-XAS-MS investigation
International audienc
New insights into the water-gas shift reaction over bulk Cr2O3*Fe2O3 mixed oxide catalysts: A combined operando Raman-IR-XAS-MS investigation
International audienc
New insights into the water-gas shift reaction over bulk Cr2O3*Fe2O3 mixed oxide catalysts: A combined operando Raman-IR-XAS-XPS investigation
International audienc
Surface ReOx Sites on Al2O3 and Their Molecular Structure Reactivity Relationships for Olefin Metathesis
International audienc
Surface ReO<sub><i>x</i></sub> Sites on Al<sub>2</sub>O<sub>3</sub> and Their Molecular StructureâReactivity Relationships for Olefin Metathesis
Supported
ReO<sub><i>x</i></sub>/Al<sub>2</sub>O<sub>3</sub> catalysts
were investigated for propylene metathesis as a
function of surface rhenia loading and extensively characterized with
in situ UVâvis, Raman, IR, XANES/EXAFS, and isotopic <sup>18</sup>Oâ<sup>16</sup>O exchange studies. The experimental studies
were complemented with DFT calculations using realistic models of
the alumina surface. The surface ReO<sub><i>x</i></sub> sites
were found to be isolated surface dioxo (Oî»)<sub>2</sub>ReO<sub>2</sub> species, which represent the most stable surface rhenia structures
on alumina as shown by DFT. Two distinct surface ReO<sub>4</sub> species,
however, were found to be present and only slightly differ in their
bridging ReâOâAl bond lengths brought about by anchoring
at different sites of the Al<sub>2</sub>O<sub>3</sub> support. The
deformed surface ReO<sub>4</sub>âI species preferentially anchor
at more basic Ό<sub>1</sub> Al<sub>IV</sub> and Ό<sub>1</sub> Al<sub>VI</sub> sites and are difficult to activate for propylene
metathesis. The surface ReO<sub>4</sub>âII species are formed
at more acidic Ό<sub>2</sub> Al<sub>VI</sub> and Ό<sub>3</sub> Al<sub>VI</sub> sites and are the catalytic active sites
for propylene metathesis. The surface ReO<sub>4</sub>âII sites
were readily activated by propylene while the deformed surface ReO<sub>4</sub>âI sites were almost not affected by propylene, with
only a few sites being activated. The steady-state propylene metathesis
reaction rates are much higher for the surface ReO<sub>4</sub>âII
sites than the deformed surface ReO<sub>4</sub>âI sites. The
formation of the less reactive deformed surface ReO<sub>4</sub>âI
species could be blocked by occupation of the Ό<sub>1</sub> Al<sub>IV</sub> sites with sacrificial surface TaO<sub><i>x</i></sub> species that resulted in catalysts exclusively containing
the more active surface ReO<sub>4</sub>âII sites on alumina.
This is <i>the first study</i> to demonstrate that the surface
ReO<sub>4</sub>âII sites are the precursors for the catalytic
active sites for propylene metathesis by supported ReO<sub>4</sub>/Al<sub>2</sub>O<sub>3</sub> catalysts and to molecularly design
olefin metathesis catalysts that exclusively contain isolated surface
ReO<sub>4</sub>âII sites
Dynamics of CrO<sub>3</sub>âFe<sub>2</sub>O<sub>3</sub> Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity
A series
of supported CrO<sub>3</sub>/Fe<sub>2</sub>O<sub>3</sub> catalysts
were investigated for the high-temperature water-gas shift
(WGS) and reverse-WGS reactions and extensively characterized using
in situ and operando IR, Raman, and XAS spectroscopy during the high-temperature
WGS/RWGS reactions. The in situ spectroscopy examinations reveal that
the initial oxidized catalysts contain surface dioxo (Oî»)<sub>2</sub>Cr<sup>6+</sup>O<sub>2</sub> species and a bulk Fe<sub>2</sub>O<sub>3</sub> phase containing some Cr<sup>3+</sup> substituted into
the iron oxide bulk lattice. Operando spectroscopy studies during
the high-temperature WGS/RWGS reactions show that the catalyst transforms
during the reaction. The crystalline Fe<sub>2</sub>O<sub>3</sub> bulk
phase becomes Fe<sub>3</sub>O<sub>4</sub> ,and surface dioxo (Oî»)<sub>2</sub>Cr<sup>6+</sup>O<sub>2</sub> species are reduced and mostly
dissolve into the iron oxide bulk lattice. Consequently, the chromiumâiron
oxide catalyst surface is dominated by FeO<sub><i>x</i></sub> sites, but some minor reduced surface chromia sites are also retained.
The Fe<sub>3â<i>â</i>x</sub>Cr<sub><i>x</i></sub>O<sub>4</sub> solid solution stabilizes the iron
oxide phase from reducing to metallic Fe<sup>0</sup> and imparts an
enhanced surface area to the catalyst. Isotopic exchange studies with
C<sup>16</sup>O<sub>2</sub>/H<sub>2</sub> â C<sup>18</sup>O<sub>2</sub>/H<sub>2</sub> isotopic switch directly show that the RWGS
reaction proceeds via the redox mechanism and only O* sites from the
surface region of the chromiumâiron oxide catalysts are involved
in the RWGS reaction. The number of redox O* sites was quantitatively
determined with the isotope exchange measurements under appropriate
WGS conditions and demonstrated that previous methods have undercounted
the number of sites by nearly 1 order of magnitude. The TOF values
suggest that only the redox O* sites affiliated with iron oxide are
catalytic active sites for WGS/RWGS, though a carbonate oxygen exchange
mechanism was demonstrated to exist, and that chromia is only a textural
promoter that increases the number of catalytic active sites without
any chemical promotion effect