Combined in-situ FTIR and on-line activity studies: applications to vanadia-titania DeNOx catalyst

77

Catiysk Today, 9 (1991) 77-82 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

IN-SITU FTIR AND ONLINE

COMBINED

APPLICATIONS

TO VANADIA-TITANIA

AClWlTY

STUDIES:

DeNO, CATALYST

Nan-Yu Topsoe and Henrik Topsee Haldor Topsoe Research Laboratories, Nymollevej 55, DK-2800 Lyngby, Denmark

ABSTRACT An approach for erforming combined in-situ FDR and on-line activi measurements has been described. l# e application of such studies to vanadia-titania DeXIOx catalysts is ‘ven. The trends in catalytic activity measured directly with the FTIR cell/reactor were f’ound to be in good agreement with separate activity measurements demonstrating that the present ex erimental a preach provides a direct link between the surface chemistry and the catalysis. +h e results sg ow that during the DeNOx reaction conditions, both Brensted and Lewis acid sites are present on the surface and they were seen to adsorb NHs strongly. It is seen that the surface V-OH and V=O groups are likely involved in the catalytic reactron.

INTRODUCI’ION The importance

of in-situ techniques cannot be overemphasized

in the field of cataly-

sis since most catalytic phenomena

and catalyst properties depend intimately on the state of

the catalyst. Thus, the information

obtained from non-in-situ

experiments may be drastically

different from that obtained under the relevant reaction conditions. In fact, a great fraction of the controversies in the past can be directly attributed to the lack of information under in-situ conditions and these controversies

example catalyst structure, nature of the active phases, promotion, deactivation interaction.

obtained

have lead to different views regarding for and support

The situation can in many cases be improved since various in-situ methods are

now available which in principle allow studies of catalysts to be carried out under real reaction conditions.

For example, we have in several instances constructed

both the catalytic activity and spectroscopic information Infrared information

spectroscopy is one of the interesting on catalyst materials.

where

techniques which may provide in-situ

(see e.g., Refs. l-5). In the present paper, a setup for

combined in-situ FDR and on-line activity measurements tages and limitations

equipments

can be recorded simultaneously.

is described. Some of the advan-

will be discussed and we will give an example to illustrate how the

combined in-situ FTIR and activity investigation

has provided information

directly relevant

to the catalysis of the titania supported vanadia DeNOx catalyst. The development

of effective DeNOx catalysts for the Selective Catalytic Reduction

or the so-called SCR process has received a great deal of attention the increasing environmental monly used catalysts o920-5861/91/$03.50

in recent years due to

demands of removing air pollutants. At present, the most com-

are based on vanadia-titania

system and numerous

01991 Elsevier Science Publishers B.V.

studies have

appeared in the literature reaction

mechanism

(Ref. 6) dealing with the physical characterization,

activity and

of these catalysts. However, to our knowledge, no combined

in-situ

on-line spectroscopic and activity studies on this catalyst system have been published. EXPERIMENTAL The in-situ experiments were carried out by combining basically three components:,(i) a feed system allowing the preparation talytic reactor connected

of the desired reactant mixtures, (ii) a FTIR cell/ca-

to the FTIR spectrometer,

this case is a mass spectrometer. This combination

and (iii) an analysis section which in

allows the simultaneous

the surface adsorbed species and the concentrations

of reactants

measurement

of

and products. The basic

features of the setup are illustrated in Figure 1.

MS

I

1

electronicaliy regulated gas feed system

I-

in-situ cell/reactor

Fig. 1. Schematic drawing of the experimental setup. &mole nreuaration; The vanadia-titania impregnating

sample studied which contained

TiOz in an oxalic acid solution of ammonium

ing and calcination

6 wt% VsOr was prepared metavanadate

by

followed by dry-

at 4OOOC.The titania support used was present as anatase and had a

surface area of 90 mr/g. Activitv measurements: The catalyst activity was measured on-line at one atmosphere mass spectrometer.

by a Balzers QMS420

The setup was designed in such a way to permit the mass spectrometer

to directly monitor both the reactant gas mixture by bypassing the in-situ ir ceil/reactor the reaction products exiting from the outlet of the cell. The mass spectrometer quantitatively calibration

analyzed using the fragmentation

patterns

determined

and

data were

experimentally

from

gases. The gas handling manifold as shown in Figure 1 has the capability which

allows mixing of the feed gas to give the different desired reactant compositions

for the

activity studies. The gas mixture used in the present study had the following composition: 340 ppm NO, 500 ppm NHs, 8% 02 and balance Ar. The total flow rate used under the study was 100 Nml/min. FTIR measurements: The ground catalyst sample of 170 mg was pressed into a self-supporting cm in diameter. This was mounted into a quartz in-situ ir cell/reactor

wafer of 2.86

with CaFz windows

where the catalyst was activated in the stream of the reaction feed gas at the chosen temperature. Changes occurring on the catalyst during the activation and reaction process were monitored directly at the given conditions without altering the position of the sample. The ir spectra were recorded on a Digilab FTS80 FTIR spectrometer with a MCT (Mercury-Cadmium-Telluride)

detector at a spectral resolution of 4 cm-?

RESULTS AND DISCUSSIONS In order to carry out combined

in-situ spectroscopic

and on-line

activity measure-

ments, the conditions in the in-situ cell should simulate those present in a catalytic reactor. Preferably, such in-situ cell should operate as a reactor at the same time to ensure that the spectroscopic and the catalytic information are obtained on the same sample under identical condition. However, this often poses great constraints on the design of the in-situ cell since it should meet the same stringent requirements reactors.

In particular,

channeling.

as those used in the design of laboratory

one should avoid the heat and mass transport

These requirements

limitations

and

can be difficult to satisfy in many spectroscopic measure-

ments. For example, in infrared spectroscopy, one is usually restricted by the choice of window materials and sample configurations.

Some of the limitations

ing for example with low conversion over the ir cell/reactor. a mass spectrometer

can be reduced by work-

The present setup coupled with

is ideal for this purpose. In spite of having a geometry quite different

from that encountered

in an ideal plug flow reactor, it was found that the present FI’IR

reactor-cell gave reasonable

activity results and moreover, the trends observed in the kinet-

ics were rather analogous to those observed in separate kinetic tests as shown below. Thus, it is clear from the results obtained that the present setup is ideal for establishing tive (or semi-quantitative)

qualita-

relationship between the surface chemistry and the catalysis. It is

however not the purpose in the present paper to give a detailed account of the mechanistic implication of the investigation

( this will be dealt with separately (Ref. 7)).

Some examples of the FHR results obtained during the course of the DeNOx reaction are shown in Figure 2. These are ir difference spectra (with the background spectrum of sample in Ar flow at the same temperature

subtracted)

measured on a 6% vanadia-titania

catalyst after different lengths of time in the flow of reaction gas mixture at 300°C (Fig. 2a-e). They, thus, reflect only the changes occurred on the surface due to the reaction. It can be seen that the major changes lie in the high frequency region between 2800 cm-1 and 3800

A0 b

LO

a

4000

3500

I

I

w

/L

I

3000 2500 2000 Wavenumbers

I

1500

Fig. 2. FTIR spectra of vanadia-titania catalyst during DeNOx reaction at 3OOOC.Figs. 2a-e are recorded after 30 sec., 3 min, 16 min., 30 min., and 20 hr on stream, respective1 . (The negative band at 2350 cm-1 is from small amount of CO1 in the background spectrum r

cm-1 consisting of the O-H and N-H stretching vibration

modes and the lower frequency

region between 1640 cm-1 and 1200 cm-1 due to the corresponding

deformation

modes. The

general intensity of these bands is seen to increase with reaction time. It is seen that the surface species are formed already after 30 seconds in the reaction gas (Fig. 2a) and the concentration

of these species approached

a steady state after 30 minutes (Fig. 2d). The

band at 1419 cm-1 is attributed to the asymmetric bending vibration of NH; (Ref. 8 ) whereas the bands at 1606 cm-l and 1237 cm-1 are due to the asymmetric and symmetric bending modes of coordinated

NH3 (Ref. 8), respectively. In the higher frequency region, broad

bands at 3019 cm-1 and 2808 cm-1 are due to the stretching vibration of NHf species whereas the N-H stretching band at 3364 cm-1 and 3334 cm-1 are assigned to the asymmetric and symmetric

stretching

vibrational

frequencies

of the coordinated

NH3, respectively.

other bands at 3256 cm-t and 3170 cm-l are assigned to the @IHs) resonance with the overtone of the asymmetric NH3 deformation.

The

split due to a Fermi

The appearance

of these

bands are accompanied by changes of the band at 3640 cm-t assigned to the O-H stretching vibration of the V-OH groups on the surface. Thus, these results indicate that not only are there Brensted and Lewis acid sites on the catalyst surface as already found in our earlier ir studies of NH3 adsorption on the same catalyst sample (Ref. 9) and other ir studies on simi-

81

lar systems (Refs. 10, 11). but these sites are covered with strongly adsorbed ammonia wader DeNOx reaction condition. The spectra also show an initial increase in the intensity of the band at 3640 cm-i due to V-OH (Ref. 8) and subsequent decrease in the intensity of this band and another band at 2040 cm-r due to V=.O (Ref. 8). Hence, the results indicate that V-OH and V =0 species are involved in the surface reaction. The MS analysis of the reactant and product gas stream during the reaction process could also follow the onset of reaction in that the NO and NH3 concentrations decreased accompanied by a simultaneous increase in N2 concentration.

4000

3500

2500 2000 3000 Wavenumbers

Fig. 3 FTIR spectra of the viva-ti~a at 3OO*C,and (c) difference spectrum

1500

catalyst mxder DeNOx reaction: (a) at 350°C, (b)

In order to study the effect of temperature on the DeNOx reaction, the reaction temperature was raised to 3WC. Figure 3 compares the background subtracted ir spectrum (i.e. only changes due to reaction are indicated) obtained after 1 hr reaction at 35flOC(Fig. 3a) and that after 1 hr reaction at 3OO’C(Fig. 3b). It is seen that the same absorption bands are present in the two cases but the band intensity of the spectrum at 35O*Cis lower than that at 3OOOC (Fig. 3~). Thus, the results indicate that the same ammonia adsorption species are present on the surface at the two temperatures but the steady state surface coverage is lower at the higher reaction temperature. The co~~~n~~ MS me~~ernen~ showed, however, a higher NO conversion at the higher reaction temperature. First order rate con. stants for NO conversion, k,,, were calculated from the MS data to be 56.5 and 60.9 Nlg-Q-t at 3OOOCand 350°C, respectively. This activity trend agrees both qualitatively and semi-quantitatively with laboratory plug flow reactor results where a higher DeNOx activity

82 was also found for the same catalyst at the higher reaction temperature. CONCLUSIONS The present study has demonstrated on-line FTIR and activity measurement vanadia-titania

the successful application

to the investigation

of a combined in-situ

of the DeNOx reaction over a

catalyst. The results show that it was possible to follow the catalyst reactivity

as well as the changes in surface adsorbed species during the catalysis under relevant reaction conditions. The Bronsted and Lewis acid sites found to be present from previous adsorption studies have been shown to be present under the reaction conditions

and these

sites are found to adsorb NH, strongly. The surface V-OH and V=O species are likely involved in the surface reaction. The activities measured by the present found to be in good qualitative

setup have been

agreement with separate results from a plug flow reactor.

Thus, it is shown that despite the fact that the ir cell/reactor

differs from a plug flow reac-

tor and the catalyst sample is in the form of a thin wafer, relevant information catalysis can be obtained by the present method. These type of information discussed separately, contribute

significantly to the development

concerning

can, as will be

of a “micro kinetic” reac-

tion mechanism (Ref. 7). ACKNOWLEDGMENTS Helpful discussions with B.S. Clausen, E.O. Tiimquist,

and J.A. Dumesic and experi-

mental assistance of K. Reiter are gratefully appreciated. REFERENCES 1.

Hair, M.L., ’ Infrared Spectroscopy in Surface Chemistry”, Marcel Dekker, New York

2.

fi%!r W C and Bell A. T J Catal 88 289 (1984) Winslow, P., z&d Bell, A. T., J: Catal., 86, i58 (1984) Kaul, D. J., and Wolf, E. E., J. Catal., 89,348 (1984) Tamaru, K., and Onishi, T., Appl. Spectrosc. Rev., 9(l), 133 (1975) Bosch, H., and Janssen, F., Catalysis Today, 2,369 (1988) Topsoe, N-Y., and Dumesic, J.A., to be published uir! 2,577 (1986) Busca, G., Topsoe, N-Y., La?m ubmnted for publication Inomata, M., Mori. K., Miyamoto, A., and Murakami, Y., J. Phys. Chem., 87, 761 1983) k ajadhyaksha, R. A., and Knozinger, H., Applied Catalysis, 51,81(1989)

: 5: 4: f lb. 11.