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In Situ Characterisation of Heterogeneous Catalytic Reactions byRaman and IR Vibrational Spectroscopies on a single Instrument

- HORIBA Jobin Yvon S.A.S., 59650 Villeneuve dAscq, France- SensIR, Danbury, CT 06810, USA- Laboratoire de Catalyse de Lille, UMR 8010, Bt. C3, USTL, 59655 Villeneuve dAscq, France

Complementary IR and Ramanspectroscopies for the study of catalyticreactions

A catalyst is a chemical species able toincrease the speed of a thermodynamicallyallowed reaction. If many reactions are likely tooccur, a well selected catalyst may selectively

increase the speed of a given reaction at theexpense of the other reactions. Catalysts aretherefore of great interest to the industry asthey allow to improve yields and to better tunethe properties of the final product.

Vibrational spectroscopic characterisation cantake place all along the catalyst life from thecontrol of the synthesis of the precursor to thecontrol of the catalyst regeneration via theanalysis of the surface reaction mechanism. Tostudy catalytic reactivity, the strengths ofRaman include the higher spatial resolution

and the ability to probe the low wavenumberspectral range where most of the activephases (metal oxides or sulfides) have theircharacteristic lines. Thus Raman spectroscopyhas been applied in heterogeneous catalysisfor the characterization of bulk and supportedoxides. As a complementary technique, FTIRhas been more widely used for thecharacterisation of the surface acidity of thecatalysts through the adsorption of probemolecules (1).

The FTIR/Raman combination microprobe

system, using SameSpotTM

technology enablessimultaneous examination of one sample areaby both techniques. A single microscope issimultaneously coupled to a Ramanspectrograph and an infrared interferometer.While the spatial resolution is quite differentbecause of the diffraction limit imposed by thewavelength of the probing radiation, the laserand IR beams coincide at the sample, thusproviding information on essentially the samematerial area.

Taking advantage of the different informationavailable from the two techniques, this newtechnology enables the characterisation of on-going heterogeneous catalytic processes byusing a dedicated cell directly fitted to themicroscope stage. The ability to monitor thevery same reaction via both techniques in aquasi-simultaneous approach provides full and

complementary vibrational information aboutthe mechanisms involved at the surface of thecatalyst during a catalytic reaction.

This new technological improvement enablesus, as an example of application, to follow boththe modification of the molecular structure ofthe active phase by Raman spectroscopy witha 633nm laser excitation line and the nature ofthe different surface adsorbed reactionintermediates by IR spectroscopy during thereaction of NO decomposition at 473 K on analumina supported palladium.

Simultaneous IR and Raman study of acatalytic reaction: Experimental

1. In situ Raman and FTIR quasi-

simu l taneous spectroscop ic measurements

Raman and FTIR spectroscopicmeasurements were performed on a LabRAMIR spectrometer. This instrument is acombined dispersive Raman and FT-IRmicroscope. An all-reflecting objective(Cassegrain/Schwarzschild type) as shown in

Fig. 1 enables both Raman and FTIR spectrato be acquired. Indeed, this optical system hasbeen specifically designed and optimised toprovide an additional unique functionality to theLabRAM IR : the collection of Raman and IRspectra from the same spot without any needfor switching objectives. This greatly facilitatesthe operation and handling of the samples.

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After H2reduction at 573 K for 2 hours

The infrared data (Fig. 4B) showed that

formerly adsorbed water and carbonate

species were desorbed after the treatmentwhile the Raman features of PdO are nolonger observed (Fig. 3B). Such observationssuggest the cleanness of the catalyst surfaceand the reduction into metallic Pd.

As soon as the catalyst has been exposed toNO at 473 K,

the infrared spectrum (Fig. 4C) exhibitsbands at 1461 and 1561 cm

-1assigned to

chelating bidentate nitrito (-O2N)compounds (Fig. 5i).Simultaneously, the Raman spectrum (Fig.3C) exhibited a line at 1049 cm

-1attributed to

the presence of nitrate species (Fig. 5iv)adsorbed on alumina and two lines at 828 and1308 cm

-1assigned to nitro compounds (Fig.

5v).

After one hour exposure to NO,

the apparition ofa new set of infrared bandsat 1556 cm

-1, 1614 cm

-1and 1305 cm

-1(Fig.

4D) characterizes the formation of nitratospecies: bidentate nitrato (-O2NO) compounds(Fig. 5ii) and bridged nitrato compounds (Fig.5iii).

After cooling under NO to 298 K,

We could clearly notice changes in the IRspectrum (Fig. 4E) that can be attributed tothe formation ofnitro compounds (-NO2) (Fig.5v). In Raman spectroscopy, the linespreviously assigned to nitro species were nolonger observed in the spectrum (Fig. 3E)

whereas new lines can be attributed to nitratespecies.

Figure 3. In situ Raman spectra (633nmexcitation line) obtained on (A) a 1 wt. % Pd/-Al2O3 exposed to air at room temperature, (B)

after reduction under H2 at 573 K for 2h andafter 5x10

-3NO atm exposure at 473 K (C) for

30 mn, (D) for 1 hour, and (E) after coolingdown to room temperature under NO flow.

Figure 4. In situ IR spectra obtained on (A) a 1 wt.% Pd/-Al2O3 exposed to air at room temperature,(B) after reduction under H2 at 573 K for 2h and

after 5x10-3

NO atm exposure at 473 K (C) for 10mn, (D) for 1 hour, and (E) after cooling down to

room temperature under NO flow.

200 400 600 800 1000 1200 1400

Wavenumber / cm-1

Intensity/

a.u.

(A)

(B)

(C)

(D)

(E)

639

828

831

1050

1049

711

1308

125013501450155016501750185019502050Frequency / cm

-1

KubelkaMunk

(A)

(B)

(C)

(D)

(E)

1649

1509 1401

1461

1556

1561

1614 1

305

1328

1428

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