A contribution of in situ UV/Vis/NIR spectroscopy tocharacterize molybdenum oxide catalysts

S. Knobl,(b) D. Niemeyer,(b) J. Melsheimer,(b) R. Schlögl,(b) N. Abdullah,(a,b) S. B. Abd Hamid,(a)

(a) Combinatorial Technology and Catalysis Research Centre (COMBICAT) UNIVERSITI MALAYA 50603 Kuala Lumpur, Malaysia, combicat@um.edu.my

(b) Fritz-Haber Institut der MPG, Abteilung AC, 14195 Berlin, acsek@fhi-berlin.mpg.de

300 400 500 600 700 800

0

2

4

6

8

10

12

2,8 3,2 3,6 4,00

10

20

30

a

c

b

316

324

nm

310

Ku

bel

ka-M

un

k u

nit

s

wavelength (nm)

c

a

b

3.35

3.36

3.46

eV

d(

Kub

elka

-Mun

k)/d

(hv)

Photon energy (eV)

1000 1500 2000 2500

0,0

0,1

0,2

0,3

0,4

cb

a

1945

2150

2040

1440

1570

Ku

bel

ka-M

un

k u

nit

s

wavelength (nm)

Introduction 

Controlled precipitation (diffe- rent cursor concentration, temp- erature or nature of counter ca- tion) is used to obtain structural complexity without the need for chemical complexity [1, 2] (Po- ster 115).

The obtained materials (ortho- rhombic, hexagonal MoO3, su- pramolecular Mo36 and trimo- lybdate) undergo electronic changes in the presence of cer- tain gas atmospheres.

Analysis of such materials is carried out by in situ UV/Vis- NIR spectroscopy (this Poster).

References

[1] S.Knobl, G.A. Zenkovets, G.N. Kryukova, O. Ovsitser, D. Niemey-er, R. Schlögl und G. Mestl, J. Catal., 215, 2003, 177.[2] S. Knobl, O. Timpe, N.D. Orth-man, N. Abdullah, Q. Basher, R.E. Jentoft, F. Girgsdies, J. Kröhnert, J. B. Wagner, J. Melsheimer, D. Nie-meyer, S.B.A. Hamid and R. Schlögl, in preparation.[3] J. Melsheimer, M. Thiede, R. Ahmad, G. Tzolova-Müller and F.C. Jentoft, PCCP, 2003, 4366.

Molybdenum oxide catalysts prepared from 0.7 mol/L AHM with 1mol/L HNO3 at 30°C (a),

50°C (b) and 70°C (c) ;

ExperimentalSpectroscopic equipment

A commercial UV/Vis/NIR spectro-meter (Lambda 9, PerkinElmer)equipped with a BaSO4 coatedintegrating sphere was supplemen-ted with a new construction (spe-cially formed light conductor in ver-tical position) to measure in situdiffuse reflectance spectra of diffe-rent MoOx from room temperature(RT) to 749 K [3].

Spectroscopic measurements

The spectra were recorded both From 250 to 2500 nm (scan speed of 240 nmmin-1, slit 1 nm) and 250 to 800 nm (scan speed 60nmmin-1, slit 0.2 nm) with Spec-tralon (Labsphere) as a white Stan-dard in the reference position.Powder samples (ca. 0.6 g) werecharged in the home-made micro-reactor and fed with a flow of air, pure He or 21 % oxygen in helium.

Room temperature

spectroscopy

Summary (RT spectroscopy)The following average LMCT bands (nm) (I) and Eg’s (eV) (II) at RT are attributed to certainMoOx families:

(I) 322 (NH4+), 314 (K+); (II) 3.48 (NH4

+), 3.44 (K+) to supramolecular Mo36;

(I) 313 (NH4+), 319 (K+), 327 (Na+); (II) 3.35 (NH4

+); 3.30 (K+), 3.27 (Na+) to hexagonal Mo3;

(I) (296 (Li+); (II) 3.44 (Li+) to orthorhombic MoO3 and

(I) 284 (K+); (II) 3.77 (K+) to trimolybdate MoOx. From a blue shift of the LMCT band in the series supramolecular/hexagonal orthorhombic trimolybdate and a decreasing broadening of this band it may be concluded that the clustzersize decreases.

Summary (high temperature spectroscopy)

By increasing the temperature the band at 2150 nm begins to disappear at 553 K and the band at 1570 nm diminished around 633 K.

In Vis range a new band at 660/670 nm develops at 553 K which decreases in the presence of He and increases in the presence of O2 in He with increasing tempera- ture. This band is assigned to a d-d transition.

In He this band suffers a blue shift to about 570 nm at 673 K. The appearance of

the new Vis band is in line with TG/DSC data. At 553 K ammonia as counter ion decomposes to NOx and reduces the Mo matrix; ammonia is completely removed

from the sample at around 693 K (Poster 76).

structure

HNO3

(mol/L)conc.

(mol/)

sample

number

AHM

temp.

(OC)precursor

256

227

232

0.7 1

30

50

70

228 AHM 1.0 1 50

225 AHM 0.7 2 50

286

247

233

K2MoO40.28 5

30

50

70

243

246

253

K2MoO4 2 5

30

50

70

251

222

230

252

226

231

Li2MoO4

Na2MoO4

0.28

2 2

2

30

30

50

50

70

70

supramol.

hexag.

hexag.

supramol.

supramol.

supramol.

hexag.

hexag.

supramol.

supramol.

trimolybdate

orthorombic

orthorombic

orthorombic

hexag.

hexag.

hexag.

Spectra recorded from

room temperature up to

673 K under flow of Helium

1000 1500 2000 2500

0,00

0,07

0,14

1940

2030

2150

1570

1440

RT 553 K 473 K 673 K 513 K 723 K

Kub

elka

-Mun

k un

its

wavelength (nm)

1000 1500 2000 2500

0,0

0,2

0,4 RT 553 K 373 K 593 K 473 K

673 K

2150

2040

1940

1580

1444

Ku

bel

ka-M

un

k u

nit

s

wavelength (nm)

Spectra recorded from

room temperature up to

723 K under flow of 21 %

oxygen in Helium

Supramolecular Mo36

prepared from 0.7 mol/L AHM

with 1 mol/L HNO3 at 30°C

500 600 700 8000,0

0,2

0,4

0,6

673 K553 K

749 K513 K

660 nm

Kube

lka-

Mun

k un

its

wavelength (nm)

High temperature

spectroscopy

Spectra recorded from

room temperature up to

749 K under flow of 21 %

oxygen in Helium Supramolecular Mo36 prepared

from 0.7 mol/L AHM

with 1 mol/L HNO3 at 50°C

Spectra recorded from

room temperature up to

673 K under flow of Helium

500 600 700 8000,0

0,7

1,4

2,1

5

6

7

570

nm

593 K

553 K513 K

633 K

673 K

Kub

elka

-Mun

k un

its

670

nm

Kub

elka

-Mun

k un

its

wavelength (nm)

samplenumber group

NIR (nm)

hexag. MoO3 supram. Mo36 trimolybdate

1440

1570

1945

2040

2150

_

256, 227,232, 228, 225

-OH(H2O)ads

(H2O)ads

-OH(H2O)ads

-OH(H2O)ads

NH4+

1440

1570

1940

2030

2150

_

_

_

_

286, 247,233, 253,246, 243

1930

1790/1810

2000

2075

orthorhombic

1430 1435 1435-OH(H2O)ads

1940 1930-OH(H2O)ads

1790NO3-

2090 2095

2230 2250 2250

_

?

?

NO3-

_ _

hexag. MoO3group samplenumber

samplenumber

252, 226, 231

251, 222 230

970

1430

1790

1930

2280

1430

1820

1955

2280

-OH(H2O)ads

NO3-

-OH(H2O)ads

NO3-

?_

256

227

232

228

225

286

247

233

253

246

243

251

222

230

252

226

231

320

328

332

3.27

3.27

3.27

306

295

288 3.48

3.37

284

308

329 3.43

3.43

3.77

319

305

305 3.46

3.30

3.30

330

324

310

316

308

3.46

3.36

3.35

3.55

3.36

3.37

sample

number LMCT

(nm) Eg (eV)

Catalyst preparation

s. poster 115