Student : Alba Calvo GarcíaPromotor: Prof. Dr. M. F. Reyniers

Coach(es): Dr. Anilkumar Mettu, Dr. Hilde Poelman

Academic year: 2009-2010

Laboratory for Chemical Technology, Ghent University

Total oxidation over Cu based catalysts: Structure-activity relationship of metal oxides

1. Introduction: Justification, Objective, Analytical methods

2. Methodology2.1 Description of the process2.2 Experimental setup

3. Campaign November 2009 in Grenoble3.1 Synchrotron: source of X-rays3.2 Experiments

4. XANES analysis4.1 Processing of data: importing raw data, normalization4.2 Analysis of data : PCA, LCF, WL analysis

5. Results and discussion 5.1 Structure of the catalyst at ambient and pretreatment conditions 5.2 Temperature programmed reductions 5.3 Isothermal reductions

6. Conclusions

7. Future work

Overview

• Objective

Study the evolution of the active phase in CuO supported on CeO2/Al2O3 catalysts under different conditions

• Justification

Emission of VOCs: critical environmental problem → catalytic oxidation: effective technique for the elimination of VOCs

Propane: model VOC

• Analytical methods

Operando X-ray absorption spectroscopy in transmission mode at the Cu K edge and Ce L3 edge → structural information of the active phase of the catalyst during the reaction

On-line mass spectrometry → determinate the catalytic activity by measuring the concentration of the reactor effluent as a function of time

Introduction

Methodology: description of the processExperimental setup: in-situ/operando cell →

design resembles operation of a reactor under plug flow conditions

''Heart'' of the experimental cell: quartz capillary tube mounted horizontally in the setup (inner diameter 0.9mm, outer diameter 1mm) → undiluted ground catalyst (75-100µm) loaded between two glass wool plugs

Characterization of the catalyst: XAS → evaluation of unoccupied electronic states. Need for a source of X-rays

X-ray absorption coefficient (μ(E)) of a material is measured as a function of energy It = I0e − μx (Beer-Lambert‘s law)

Synchrotron radiation: used to provide a continuous source of photons → range 0.1–10 keV

Campaign November 2009, Grenoble: ESRF

All data were collected at the beamline 26 in the ESRF (European Synchrotron Radiation Facility) in Grenoble, France.

eMethodology: experimental setup

E xperimental setup in the beamline 26 (ESRF Grenoble, France)

Normal sequence of operation : Purging the setup in He, as it is an inert gas Calibrating the mass spectrometer with different gases Pre-treating the catalyst with oxygen 423K, 1h Purging the reactor with He Feeding the mixture and performing the experiment

Campaign November 2009, Grenoble: Experiments

ExperimentQuick C3H8-

TPRQuick H2-

TPR

Isothermal C3H8-

reduction

Isothermal H2-reduction

Total oxidation

Gas inlet (%) 5%C3H8/He 5%H2/He 5%C3H8/He 5%H2/He1%C3H8-

10%O2/He

T Cu K edge (ºC)

200 to 450ºC200 to 450ºC

300 to 450ºC per 50ºC

200 to 450ºC per 50ºC

300 to 400ºC per 50ºC

T Ce L3 edge (ºC)

100 to 400ºC -300 to 400ºC

per 50ºC200 to 400ºC

per 50ºC300 to 400ºC

per 50ºC

Pressure (atm)

1 1 1 1 1

Total flow (mol/s)

1.5 10-5 mol/s1.5 10-5

mol/s1.5 10-5

mol/s1.5 10-5 mol/s

0.5 - 1.5 10-5 mol/s

Experiments performed at the beam line 26:

Time resolution: fast scans → 1min/spectra; slow scans → 6, 15 or 20 min/spectra

2. Normalization → µ(E) = (µ (E) - µ0(E))/ µ0 (E0)

2.1. Value of E0 2.2. Pre-edge and post-edge 2.3. Normalized μ

(E)

XANES analysis: processing of dataAthena: interactive graphical utility for processing EXAFS data

1. Import raw data

3. Data analysis

XANES analysis: processing of dataData analysis

PCA (Sixpack): number of principal components to describe the evolution in a given set of spectra.

IND function number of components

Target transormation

Good selection of principal components

Acceptable SPOIL number (<6)

Good fitting of reference spectra

LCF analysis

XANES analysis: processing of dataData analysis

LCF (Athena): fits a linear combination of standard spectra to an unknown spectrum.

Different libraries of references:

pellets

Cu2+, Cu1+, Cu0 CuO, Cu2O, Cu capillaries

merged files

first spectrum, Cu2O reference,

last spectrum

pellets

Ce4+, Ce3+ CeO2, CeF3 capillaries

merged files

Molar fractions of Cu2+, Cu1+, Cu0 or Ce4+, Ce3+ vs T

XANES analysis: processing of dataData analysis

WL analysis: estimating the average oxidation state of Cu/Ce in each spectrum

Cu2+, Cu1+, Cu0 / Ce4+, Ce3+ references

EWL oxidation state

linear relationship : Ox. State = f(EWL)

EWL unknown spectra T unknown spectra

Oxidation state vs T

Reduction of the active phase causes a shift in the WL

Results and discussion: Initial structure of the catalystStructure of the catalyst at ambient and pretreatment conditions

Cu K XANES (references + catalyst) Ce L3 XANES (references + catalyst)

Cu2+ and Ce4+ species are present in the catalyst as prepared, the

active phase remains in its state during heating in He and after

pretreatment with oxygen

• weak pre-edge feature at 8978eV

• Shoulder feature at 8986eV

• White line at 8998eV

• Shoulder feature at 5721 eV

• White line 5731 eV

Results and discussion: Temperature programmed reductions

C3H8 TPR

Cu K XANES Ce L3 XANES

WL↓ and shifts to ↓energiesShoulder feature →Pre-edge peak ↓ 2 trends: Cu1+

and Cu0

Changes less appreciable in Ce L3 WL↑and shifts slightly to ↓energiesPronounced shoulder on the low energy side of the WL

References + catalyst after reduction

Results and discussion: Temperature programmed reductionsC3H8 TPR

Cu K XANES Ce L3 XANES

Final ox. state (400⁰C)= 3,8

Reduction 280-300 ⁰C100% Cu0 obtainedCu2O (300-450⁰C)

↓ max 380 ⁰C

Final ox. state (446⁰C) ≈ 0

↑T ↑reduction

Only 20,9% Ce3+ obtained

Results and discussion: Temperature programmed reductions

H2 TPR

Cu K XANES

Reduction with H2 → faster WL ↓ and shifts rapidly to ↓energiesShoulder feature →Pre-edge peak (Cu0)

CuO reference + catalyst at 226ºC

All spectra resemble Cu0 except for the first (226⁰)226⁰ → Catalyst is not totally oxidized

Cu K XANES

Results and discussion: Temperature programmed reductionsH2TPR

Cu K XANES

3 principal components → first one much more weight

EWL values are close to Cu0 except for the first one

Initial ox. state (226⁰C)= 1,2T ≥ 250⁰C → Ox. State ≈ 0

Results and discussion: Isothermal reductionsCu K XANES

T ≥ 250⁰C → Ox. State ≈ 0

T = 300⁰C → hardly reducedT > 350 ⁰C → Cu0

T = 350 ⁰C → features Cu2O

T ≥ 250⁰C → Spectra resemble the one of Cu0

Reduction with H2 starts at lower temperatures and occurs faster

Isothermal reduction with C3H8 Isothermal reduction with H2

Results and discussion: Isothermal reductionsCe L3 XANES

Isothermal reduction with C3H8 Isothermal reduction with H2

CeO2 is hardly reduced by C3H8, even at high T

Reduction ↑ by using H2 Bigger changes in the spectra

CeO2 and CeF3 references + catalyst at 400ºC

Pronounced shoulder on the low energy side of the WL

Results and discussion: Isothermal reductions

Isothermal reduction with C3H8 Isothermal reduction with H2

Ce L3 XANES

200ºC→Reduction starts (2 scans)

↑T isothermal reduction → ↑ final %CeF3

CeO2 is hardly reduced by C3H8

Final Ox. state (400⁰C)= 3,73

Reduction↑ by using H2 → 60,9% CeF3

Final Ox. state (400⁰C)= 3,04 (WL&LCF)

Conclusions• Study of X-ray data → evolution of the copper and cerium phase of the catalyst during the different experiments: starting temperature, activity, average oxidation state…

• At ambient conditions, heating in He and after pretreatment with oxygen, the catalyst is totally oxidized → Cu2+ and Ce4+ are the only species present

• Both Cu and Ce are active in the catalyst → differences: range of T, extent of reduction

• CuO reduction → 3 principal components; CeO2 reduction → 2 principal components • Reduction increases with T (CuO &CeO2 ) • T< 300ºC neither CuO nor CeO2 can be reduced by C3H8. Reduction with H2 starts at about 200ºC and occurs faster

• T ≥ 400ºC CuO is totally reduced by C3H8 or H2

• CeO2 is only slightly reduced by C3H8, even at high temperatures. Higher concentration of Ce3+ is obtained with H2

• A transient phase of Cu2O appears during the reduction of CuO : C3H8 (300 - 450ºC) ; H2 reduction : almost negligible (t resolution), appears at lower temperatures (200-250ºC ) → redox cycles occur via the following mechanism: CuO ↔ Cu2O ↔ Cu

Future work

- Synchronize temperature in XAS and MS measurements

- MS measurements should be performed on other gases apart from

the compounds involved in the reaction → detect leaks or capillary

breakage

- Experiments could be done in a Cu based catalyst and a Ce based

catalyst separately → how each element in the catalyst works

- XANES analysis in the Ce K edge could be performed → Better

resolution with the contribution of different coordination cells.

- XAS data could be examined in the EXAFS region as well →

structural information (distances between atoms, coordination

number..)

Thanks for your attention