Electron Transfer Effects in Ozone Decomposition on Supported Manganese Oxide
Rakesh Radhakrishnan†and S.Ted Oyama*,‡
En V ironmental Catalysis and Materials Laboratory,Departments of Chemical Engineering and Chemistry,
Virginia Polytechnic Institute and State Uni V ersity,Blacksburg,Virginia24061-0211
Jingguang G.Chen
Department of Materials Science and Engineering,Center for Catalytic Science and Technology,
Uni V ersity of Delaware,Newark,Delaware19716
K.Asakura
Catalysis Research Center,Hokkaido Uni V ersity,Sapporo,Hokkaido060-0811,Japan
Recei V ed:September13,2000;In Final Form:February21,2001
Manganese oxide catalysts supported on Al2O3,ZrO2,TiO2,and SiO2supports were used to study the effect
of support on ozone decomposition kinetics.In-situ laser Raman spectroscopy,temperature-programmed oxygen
desorption,surface area measurements,and extended and near-edge X-ray absorption fine structure(EXAFS
and NEXAFS)showed that the manganese oxide was highly dispersed on the surface of the supports.The
EXAFS spectra suggested that the manganese active centers on all of the surfaces were surrounded by five
oxygen atoms.These metal centers were found to be of a monomeric type for the Al2O3-supported catalyst
and multinuclear for the other supports.The NEXAFS spectra for the catalysts showed a chemical shift to
lower energy,and an intensity change in the L-edge features which followed the trend Al2O3>ZrO2>TiO2
>SiO
2
.The trends provided insights into the positive role of available empty d-states required in the reduction step of a redox reaction.The catalysts were tested for their ozone decomposition reactivity and reaction rates
were found to have a fractional order dependency(n<1)with ozone partial pressure.The apparent activation
energies for the reaction were found to be low(3-15kJ/mol).The support was found to influence the desorption
step(a reduction step)and this effect manifested itself in the preexponential factor of the rate constant for
desorption.Trends for this preexponential factor correlated with the NEXAFS trends and reflected the ease
of electron donation from the adsorbed species to the active center.
Introduction
Manganese oxide has been used as a catalyst for several chemical reactions including the decomposition of ozone,1 nitrous oxide,2-4and2-propanol;4,5the oxidation of methanol,6 ethanol,7benzene,8CO,4,5,9and propane;10and the reduction of nitric oxide11and nitrobenzene.12Oxides of manganese have been applied in air pollution control technology particularly in the abatement of contamination by volatile organic compounds (VOCs).13
Very few characterization and reactivity studies have been conducted on supported manganese oxides.Alumina-supported manganese oxide is the most frequently studied catalyst ap-pearing in the literature.5,14The popular techniques used to characterize the catalyst have been Raman spectroscopy,14,15 X-ray photoelectron spectrscopy(XPS),5,14X-ray diffraction (XRD),2,15magnetic susceptibility,2,16,17and electron spin resonance(ESR).17,18Limited work has been done on other supports.4,19,20
X-ray absorption spectroscopy(XAS)has emerged as a powerful technique for probing surface sites in catalysis. Extended X-ray absorption fine structure(EXAFS)21-23and near-edge X-ray absorption fine structure(NEXAFS)23-25 provide information about the local coordination and electronic properties of the catalyst active centers.The majority of X-ray absorption work on manganese oxides has been done on bulk materials.26-28The only supported manganese oxide catalyst studied using these techniques is MnO2/SiO2.19
This paper presents previously unreported EXAFS and NEXAFS measurements for Al2O3-,ZrO2-,TiO2-,and SiO2-supported manganese oxides.The catalysts were further char-acterized using in situ laser Raman spectroscopy and temperature-programmed desorption(TPD)of oxygen.The ozone decomposi-tion reactivity was studied to ascertain the effect of support on catalyst activity.The reactivity findings are explained through the NEXAFS results which provided insights into the electronic properties of the manganese active centers.
Experimental Section
Catalyst Preparation.Manganese oxide catalysts(3wt%) were prepared using aqueous solutions of manganese acetate (Mn(CH3COO)2•4H2O,Aldrich>99.99%).The supports used were Al2O3(Degussa,
Aluminum oxid C),ZrO2(Degussa,VP ZrO2),TiO2(Degussa,Titanoxid P25),and SiO2(Cabosil,L-90). These supports were impregnated with the precursor solution using the incipient wetness technique and the resulting samples
*Author to whom correspondence should be addressed.†Department of Chemical Engineering.
‡Departments of Chemical Engineering and Chemistry.4245
J.Phys.Chem.B2001,105,4245-4253
10.1021/jp003246z CCC:$20.00©2001American Chemical Society
Published on Web04/07/2001
were dried at393K for6h and calcined at773K for6h,both in air.
Raman Spectroscopy.Details about the experimental set up for the in-situ Raman spectroscopy system can be found elsewhere.29The system used an argon ion laser(514.5nm, Spex Lexel95)as a light source,a holographic notch filter (Kaiser,Super Notch Plus)for removing Rayleigh scattering, and a single-stage monochromator(Spex,500M)fitted with a CCD detector(Spex,Spectrum One)for spectral a
cquisition. The laser was operated at200mW and the detector slit width was set at100µm.The resolution of the Raman spectrometer was6cm-1.Catalyst samples were pretreated at773K for2h in oxygen flow prior to use.
TPD and Surface Area Studies.The number of active manganese sites on the surface of the catalyst were estimated using the temperature-programmed desorption of oxygen.The experiment was conducted in a flow apparatus equipped with a computer-interfaced mass spectrometer(Dycor/Ametek Model MA100).30The catalyst(0.9g)was loaded in a quartz reactor and pretreated in oxygen(Air products,Grade2.6)flow for2 h and cooled to233K using a2-propanol-liquid nitrogen gel mixture.Ozone generated from oxygen using an ozone generator (OREC,V5-0)was introduced into the reactor and the sample was exposed to a2mol%O3/O2mixture for2h.The temperature-programmed desorption was carried out in helium (Air products,Grade5.0)flow by heating the sample to1273
K at0.17K s-1while monitoring the m/e signals16(O),32 (O2),and48(O3).These measured values of the oxygen desorption(T<550K)were used to calculate the site densities (sites/g-catalyst)for turnover rate calculations.The desorption peak areas corresponding to the adsorbed species were calibrated in separate experiments using pulses of oxygen from a calibrated volume(39µmol).
The surface area measurements were carried out in a Micromeritics ASAP2000unit.The sample(0.5g)was loaded into a quartz reactor and degassed at473K in vacuum prior to all measurements.A five-point N2BET analysis was used to measure the specific surface area(S g)of each sample.The dispersion was calculated as the ratio of Mn sites involved in ozone chemisorption to the total Mn atoms in the sample. EXAFS Spectroscopy.The catalysts(0.13g for SiO2-and Al2O3-supported samples and0.065g for TiO2-supported samples)were pressed into self-supporting wafers and put into an in-situ cell with Kapton windows.They were then oxidized in oxygen flow at723K for1h and cooled to room temperature. The measurements were carried out in a transmission mode at the BL10B beam line of the Photon Factory,Institute for Material Science,(KEK-PF)with a2.5GeV ring energy and a 300-400mA ring current.The monochromator was a channel cut Si(311)crystal and the energy resolution was about1eV. The incident and transmitted X-rays were detected by N2-filled ionization chambers with lengths of14and28cm,respectively. The sample spectra were acquired twice with total accumulation time4-6s and a scan range of6186.1-7539.7eV(step)2-3 eV).The program FEFF631was used to elucidate the phase and amplitude functions used to fit the data.The fitting procedure used a three wave fitting function with a single Mn-O bond and two types of Mn-Mn bonds.These assignments agreed with crystallographic data for standard Mn compounds. NEXAFS Spectroscopy.The NEXAFS measurements for the catalysts were carried out at the NSLS(National Sy
nchrotron Light Source)using the Exxon U1A beamline.Details about the experimental setup can be found elsewhere.25The powder samples of the catalyst were oxidized at773K and stored in bottles.Prior to use,the catalysts were pressed into a sample holder cup with dimensions of1cm diameter and0.1cm depth and were not pretreated before measurements.The synchrotron radiation was monochromatized using a spherical grating monochromator(SGM).The energy range of the incident photons was set between630eV and680eV with an energy resolution∆E/E of approximately0.0025.
Kinetic Studies.The steady-state reaction rates for ozone decomposition were measured in a flow system as a function of ozone partial pressure(0.6kPa-3kPa)and temperature(293 K-340K).The total flow rate was670µmol s-1(1000cm3 min-1)and the pressure was atmospheric.An ozone generator (OREC,V5-0)was used for ozone production and the concen-trations were measured using a UV absorption-type ozone analyzer(IN-USA,AFX-H1).The samples(0.3g)were pressed into thin cylindrical wafers and mounted on a rotatable ceramic rod in a low-volume cell(V<25cm3).These samples were pretreated in flowing oxygen at773K for2h prior to running the steady-state kinetic measurements.The turnover rates were calculated on the basis of ozone conversions and the number of sites,estimated from oxygen TPD measurements,using the expression,rate)(conversion×flow rate×ozone concentra-tion)/number of sites.
Results
Raman Spectra.The MnO x/SiO2sample had a simple spectrum consisting of one band at664cm-1(Figure1a).The MnO x/TiO2catalyst showed two bands at510and632cm-1 (Figure1b).The MnO x/ZrO2had a complex spectrum with peaks at532,552,618,and638cm-1(Figure1c).Finally,the MnO x/ Al2O3sample also presented a simple spectrum(Figure1d)with a band at661cm-1.As will be discussed,the peaks in the SiO2-and Al2O3-supported samples correspond to dispersed manga-nese oxide species while those for the ZrO2and TiO2are due to the support.
TPD and Surface Area Results.Figures2-5show oxygen (m/e)32)TPD traces for the various Mn catalysts and supports. Figure1.In situ Raman spectra of supported manganese oxides in oxygen flow670µmol s-1(1000cm3min-1)at room temperature:(a) MnO x/SiO2;(b)MnO x/TiO2;(c)MnO x/ZrO2;(d)MnO x/Al2O3.Spectrum acquisitions conditions:laser power)200mW,resolution)6cm-1, exposure time)60s,n)60scans.
4246J.Phys.Chem.B,Vol.105,No.19,2001Radhakrishnan et
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Signals due to ozone(m/e)48)were not detected at any time
in the desorption experiments.
Figure2presents the oxygen TPD traces for the Al2O3support and the MnO x/Al2O3catalyst.For the Al
2O3support alone,no peaks were observed in the TPD traces taken for the cases with and without ozone adsorption(Figure2a,b).Peaks at830and 1095K were observed for the MnO x/Al2O3catalyst when the sample was not exposed to ozone(Figure2c).Oxygen peaks at 410,537,660,830,and1095K were recorded for the same sample after ozone adsorption(Figure2d).
Figure3shows similar TPD traces for the ZrO2support and the MnO x/ZrO2catalyst.The ZrO2support trace showed no peaks without prior ozone adsorption(Figure3a).The MnO x/ ZrO2catalyst TPD trace without ozone adsorption showed oxygen peaks at560,775,790,and1050K(Figure3c)while the trace with ozone adsorption showed oxygen peaks at370, 410,549,780,790,and1060K(Figure3d).After ozone adsorption,the trace from this support showed a mass32peak at532K(Figure3b).
Figure4shows the corresponding TPD traces for the TiO2 support and the MnO x/TiO2catalyst.The trace taken from the MnO x/TiO2catalyst without ozone adsorption showed oxygen peaks at770,836,and1005K(Figure4c)while the trace taken after ozone adsorption showed peaks at378,830,and995K (Figure4d).Again,no peaks were observed for TPD traces taken from the TiO2support for cases with and without ozone adsorption(Figure4a,b).
Figure  2.Oxygen(m/e)32)TPD trace for Al2O3-supported manganese oxide:(a)Al2O3without ozone adsorption;(b)Al2O3after ozone adsorption at233K;(c)MnO x/Al2O3without ozone adsorption;
(d)MnO x/Al2O3with ozone adsorption at233K.
Figure  3.Oxygen(m/e)32)TPD trace for ZrO2-supported manganese oxide:(a)ZrO2without ozone adsorption;(b)ZrO2after ozone adsorption at233K;(c)MnO x/ZrO2without ozone adsorption;
(d)MnO x/ZrO2with ozone adsorption at233K.Figure4.Oxygen(m/e)32)TPD trace for TiO2-supported manganese oxide:(a)TiO2without ozone adsorption;(b)TiO2after ozone adsorption at233K;(c)MnO x/TiO2without ozone adsorption;(d) MnO x/TiO2with ozone adsorption at233K.
Figure5.Oxygen(m/e)32)TPD trace for SiO2-supported manganese oxide:(a)SiO2without ozone adsorption;(b)SiO2after ozone adsorption at233K;(c)MnO x/SiO2without ozone adsorption;(d) MnO x/SiO2with ozone adsorption at233K.
Structure and O3Decomposition Activity of Mn Oxides J.Phys.Chem.B,Vol.105,No.19,2001
4247
Figure5shows the TPD traces for the SiO2support and the MnO x/SiO2catalyst.Oxygen peaks at755,865,and1225K were observed when the trace was taken without ozone adsorption on the MnO x/SiO2catalyst(Figure5c).After ozone adsorption,oxygen peaks at363,548,760,870,and1230K were identified(Figure5d).For the SiO2support,TPD traces revealed no peaks with and without ozone adsorption(Figure 5a,b).
Integration of the TPD peak areas corresponding to the desorption of adsorbed oxygen allowed the determination of
the active site density and corresponding dispersion values for the catalysts.These values have been summarized along with the corresponding surface area measurements in Table1.There was no direct correlation observed between the surface cover-ages and the percentage dispersion.The different manganese oxide coverages were accounted for in the TOR calculations on the basis of the estimated active site densities.
reactor4EXAFS Results.Figure6,shows the EXAFS spectrum for Mn3O4(Figure6a), -MnO2(Figure6b),MnO x/SiO2(Figure 6c),MnO x/TiO2(Figure6d),and MnO x/Al2O3(Figure6e).The EXAFS oscillations after ba
ckground subtraction in k-space are shown in Figure7.The dotted lines in the figure represent the fitting functions used to evaluate interatomic distances.The Fourier transforms of these lines are plotted in Figure8.The transforms for Mn3O4(Figure8a),γ-MnO2(Figure8b), -MnO2 (Figure8c),MnO x/SiO2(Figure8d),MnO x/TiO2(Figure8e), and MnO x/Al2O3(Figure8f)all have a broad feature at0.20 nm.The spectrum for the MnO x/ZrO2could not be obtained because of interference by the ZrO2support.The transforms for all the samples except the MnO x/Al2O3have features around 0.29and0.35nm.These results are discussed in detail later in the paper.
NEXAFS Results.Figure9presents the Mn L-edge NEX-AFS spectra for all of the catalysts compared to -MnO2.The peak at655eV for the MnO2represents the L II peak while the peaks at645and643eV represent the L III peaks.There is a shift in both the L II and L III edge features for the various supports which follows the trend Al2O3>ZrO2>TiO2>SiO2.The positions of the peaks are653.5and641.5eV,respectively, for the L II and L III features on the alumina-supported manganese oxide.
Steady-State Kinetics.The turnover rate for ozone decom-position for each catalyst was measured as a function of ozone partial pressure and temperature.The decomposition rates increased with increasing ozone partial pressure and temperature (Figure10-13).A fractional reaction order was obser
ved for all four catalysts and this order was found to be independent of temperature.Table2presents a summary of the turnover rates
Figure6.Mn K-edge EXAFS spectra:(a)Mn3O4;(b) -MnO2;(c) MnO x/SiO2;(d)MnO x/TiO2;(e)MnO x/Al2O3.
TABLE1:Active Site Densities and Surface Areas
catalyst S g/m2g-1t otal/µmol g-1
(O2/O3TPD)dispersion/%
MnO x/Al2O3924012 MnO x/ZrO24516347 MnO x/TiO247319 MnO x/SiO288134 Al2O396
ZrO250250.3 TiO252
SiO295Figure7.Mn K-edge EXAFS spectra in k3space:(a)Mn3O4;(b) -MnO2;(c)MnO x/SiO2;(d)MnO x/TiO2;(e)MnO x/Al2O3.
Figure8.Mn K-edge EXAFS spectra Fourier transforms:(a)Mn3O4;
(b)γ-MnO2;(c) -MnO2;(d)MnO x/SiO2;(e)MnO x/TiO2;(f)MnO x/ Al2O3.
4248J.Phys.Chem.B,Vol.105,No.19,2001Radhakrishnan et
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at two different ozone partial pressures and four different temperatures for the catalysts.
Discussion
Raman Spectra.The Raman spectra for the Al 2O 3-and SiO 2-supported catalysts had peaks at 661and 664cm -1,respectively (Figure 1).Since SiO 2has very weak spectral features 32at 604cm -1,804cm -1,and 974cm -1and Al 2O 3does not have any features in the region scanned,it can be concluded that the peaks observed were due to the manganese oxide species.Buciuman et al.15investigated manganese oxide species with Raman spectroscopy and found vibrational frequencies in the range of 659to 650cm -1for Mn 3O 4species.They also observed vibrational frequencies for R -Mn 2O 3at 650and 680cm -1and a frequency of 633cm -1for γ-Mn 2O 3.Wachs et al.,11in their studies on Al 2O 3-supported manganese oxide,have observed Raman peaks in the range of 637to 647cm -1for catalysts with
Figure 9.Mn L-edge NEXAFS spectra for catalysts compared to  -MnO 2.
Figure 10.Effect of temperature and ozone partial pressure on ozone decomposition rates over SiO 2-supported manganese oxide.Reactant flow )670µmol s -1.
Figure 11.Effect of temperature and ozone partial pressure on ozone decomposition rates over Al 2O 3-supported manganese oxide.Reactant flow )670µmol s -1
.
Figure
12.Effect of temperature and ozone partial pressure on ozone decomposition rates over TiO 2-supported manganese oxide.Reactant flow )670µmol s -1.
Figure 13.Effect of temperature and ozone partial pressure on ozone decomposition rates over ZrO 2-supported manganese oxide.Reactant flow )670µmol s -1.
TABLE 2:Summary of Turnover Rates
T /K ozone partial pressure/kPa MnO x /SiO 2(TOR/s -1)MnO x /Al 2O 3(TOR/s -1)MnO x /TiO 2(TOR/s -1)MnO x /ZrO 2
(TOR/s -1)293  1.5
0.210.160.0850.0313130.240.190.0990.0353280.310.180.1500.0373430.350.200.1900.047293  3.0
0.390.350.1500.0583130.470.380.2000.0593280.590.390.2600.066343
0.77
0.43
0.340
0.079
Structure and O 3Decomposition Activity of Mn Oxides J.Phys.Chem.B,Vol.105,No.19,20014249

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