Alexopoulos, Konstantinos

Balcaen, Veerle

Cristol, S

Heynderickx, Philippe

Marin, Guy

Poelman, Dirk

Poelman, Hilde

Reyniers, Marie-Françoise

English

Ghent University Academic Bibliography

Laboratory for Chemical TechnologyK. Alexopoulos, M.-F. Reyniers, H. Poelman, S. Cristol, V. Balcaen, P. M. Heynderickx, D. Poelman, G. B. MarinIn situ X-ray absorption study of the total oxidation of propane over CuO-CeO2 /Al2O3http://www.lct.ugent.be 16 WCOC, April 5-10, 2009Overview• Introduction• Experimental & analysis procedure• Results In situ XAS measurements Theoretical calculations • Conclusions2H2OC3H8CO2O2I0 IsamplexIntroduction3• Objective:obtain insight in the structure of the CuO-CeO2/Al2O3 catalyst under working (i.e. red-ox) steady state and transient conditions• Method:XANES at Cu K edge  •-------• Mass spectroscopyIntroductionStructure of the activephase (e.g. coordination, oxidation state)Conversion, selectivity3• Objective:obtain insight in the structure of the CuO-CeO2/Al2O3 catalyst under working (i.e. red-ox) steady state and transient conditions• Method:XANES at Cu K edge  •-------• Mass spectroscopyExperimental setupBEAMOvenCapillary made out of quartz (ID = 0.9 mm, OD = 1.0 mm) ID24 (in transmission)ESRFCat.quartz quartz10 mm~5 mg cat. (particle size: 75-100 μm)Fixed bed reactorGas flow4Experimental procedure• Pretreatment:1. Heating from RT to 423 K under He2. At 423 K under 10% O2/He for 1 h3. Heating under He from 423 K to first reaction temperature• Step response experiments performed, T= 573-723 K:1. Catalyst red-ox cycles: He | 10%O2/He  He | 2%C3H8/HeConstant flow rate = 1.5 10-5 mol/s2. Total oxidation reaction: He | (1%C3H8+ 5% or 10%O2)/HeSpace times = 38.3-127.7 kg s / mol Apart from the transient response, the catalytic system is allowed to reach its steady state in all experiments5Analysis procedure• MS: 0.1s/amu amu’s followed: 4, 12, 14, 18, 28, 29, 32, 41, 44 gas phase compositions: He (internal standard), CO, C3H8, O2, C3H6, CO2• XANES: 0.1s/spectrum Raw data in pixels (of the camera)  energy calibration by comparing a Cu foil measured in pixels (taken at ID24) to one measured in eV (taken at the energy scanning beamline BM26) Determination of edge position, background subtraction & normalization of calibrated raw data Solid phase compositions from spectral de-convolution: Standard spectra used: oxidized catalyst sample (Cu2+), reduced catalyst sample after treatment with propane (Cu0), Cu2O reference sample (Cu1+) Principal component analysis (PCA) to find number of phases and assess suitability of standards for describing a given set of spectra Linear combination fitting (LCF) to quantify the ratio of Cu2+/Cu1+/Cu0 6Analysis procedure• Theoretical XANES calculations:– FDMNES– Muffin-tin approximation– Models used: crystal structure of Cu, Cu2O, CuO– Cluster size of 8.3 Å– Convolution parameters obtained from the best fit with the references (Cu, Cu2O, CuO)– Variance:7Cu Cu2O CuOResults - In situ XAS measurements• XANES of the CuO-CeO2/Al2O3: while heating under He and after pretreatment No changes in the spectra8Results - In situ XAS measurements• XANES of the CuO-CeO2/Al2O3: He| 2%C3H8/He step-response experiment at 723 K9 Catalyst reduction: a two step process is foundTime:Results - In situ XAS measurements• Catalyst reduction at 723 K1. PCA: 3 components2. LCF & MS analysis: Mechanism of catalyst  reduction:Cu2+  Cu1+  Cu0 No CO2 when the catalyst is fully reduced The LCF analysis agrees with the MS results01020304050607080901000 250 500 750 1000time (sec)solid phase composition (%)012345678910gas phase composition (%)Cu(+2)Cu(+1)Cu(0)CO2C3H810Results - In situ XAS measurements• Catalyst reduction at 723 K1. PCA: 3 components2. LCF & MS analysis: Mechanism of catalyst  reduction:Cu2+  Cu1+  Cu0 No CO2 when the catalyst is fully reduced The LCF analysis agrees with the MS results01020304050607080901000 250 500 750 1000time (sec)solid phase composition (%)012345678910gas phase composition (%)Cu(+2)Cu(+1)Cu(0)CO2C3H810Results - In situ XAS measurements• XANES of the CuO-CeO2/Al2O3: He| 10%O2/He step-response experiment at 723 K11 Catalyst reduction is reversible with reoxidationTime:T (K) 573 623 673 723τred (s) 782.7 229.6 88.3 37.2τox (s) 5.7 1.0 0.5 0.5Results - In situ XAS measurements• Catalyst red/ox cycles: LCF analysisConversion of CuO-phase of CuO-CeO2/Al2O3 catalyst during reduction with 2%C3H8/HeConversion of Cu-phase of CuO-CeO2/Al2O3catalyst during re-oxidation with 10% O2/HetCuO e1XtCu e1X12 re-oxidation of catalyst occurs faster than its reduction at all T both processes speed up with Ty = -6.635x + 10.183R2 = 0.8661-2-10121.3 1.4 1.5 1.6 1.7 1.81000/T (1/K)ln(1 / τ ox )y = -8.3822x + 7.984R2 = 0.9997-7-6-5-4-3-21.3 1.4 1.5 1.6 1.7 1.81000/T (1/K)ln(1 / τ red )Results - In situ XAS measurements• Catalyst red/ox cycles: LCF analysis Activation energies for reduction & oxidation of CuO-CeO2/Al2O3Ea = 70.0 kJ/molEa = 55.2 kJ/mol Catalyst reduction requires a higher Ea than catalyst oxidation Therefore, during hydrocarbon oxidation over the metal oxide catalyst, a reaction that can occur via the following mechanism:the first step (i.e. catalyst reduction) will be rate-determiningM-O-M-O-MHCM- -M-O-MHCOO2O2ee13Results - In situ XAS measurements no changes in the spectra catalyst remains oxidized under total oxidation reaction conditions, in agreement with the rate determining step of the Mars-van-Krevelen reaction mechanism14• XANES of the CuO-CeO2/Al2O3: He| {1%C3H8-5%O2}/He step-response experiment at 723 KTime:Results - In situ XAS measurementsy = -8.4371x + 7.5399R2 = 0.9916-7.5-7-6.5-6-5.5-5-4.5-41.45 1.5 1.55 1.6 1.65 1.7 1.75 1.81000/T (1/K)ln(-rC3H8,o)• γ = yO2(0) / yC3H8(0) = 5• CO2: main product• CO: not observed• C3H6: very small traces (ca. 100 ppm) within experimental error• MS: Total oxidation reaction under steady state conditionsEa = 70.1 kJ/mol15Apparent activation energy from MS analysis agrees with the apparent activation energy from the LCF analysis for catalyst reduction (rate-determining step)00.20.40.60.811.21.48970 8980 8990 9000 9010 9020 9030Energy [eV]Normalised μ(E)Reduced CatalystTheoretical fitOxidized CatalystTheoretical fitResults - Structure of the active phaseCuO-CeO2/Al2O3 under steady-state reductive and oxidative conditionsStructure of active phase of catalyst modeled using the fcc Cu structure or the monoclinic CuO structure as input16 Good agreement between theory and experiment (variance < 1%)Results - Structure of the active phase1. Reduced catalyst Reduced catalyst same structure as metallic copper17Bond distance (Å) Crystal structureXANES fit valuesCu reference foil Reduced catalystCu-Cu 2.56 2.64 2.64Bond distance (Å) Crystal structureXANES fit valuesCuO reference sample Oxidized catalystCu-O 1.95 - 1.96 2.01 - 2.02 2.10 - 2.112. Oxidized catalyst Oxidized catalyst CuO-like structure with larger Cu-O distances compared to reference  Weaker Cu-O bonds Conclusions18• The copper phase of the catalyst remains oxidized during heating inHe, pretreatment and total oxidation reaction conditions• Based on the PCA and LCF results, three components are present duringreduction and re-oxidation cycles (i.e. Cu(+2), Cu(+1) and Cu(0)) and atwo-step mechanism is found• Theoretical spectra have been successfully fitted to experimentalXANES of the CuO-CeO2/Al2O3 catalyst: No differences between the structure of the reduced catalyst andmetallic copper The oxidized catalyst has a CuO-like structure with elongated Cu-Obonds, thus making removal of oxygen easierThank you for your attention19Questions ?

In situ X-ray absorption study of the total oxidation of propane over CuO-CeO2/Al2O3

https://biblio.ugent.be/publication/787805/file/813721

In situ X-ray absorption study of the total oxidation of propane over CuO-CeO2/Al2O3

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