Jointly published by

Elsevier Science B.V., Amsterdam

and Akad@miai Kiad6, Budapest

React.Kinet. Catal.Lett.,

Vol. 56, No. i, 185-189

(1995) .

RKCL2670

HETEROGENEOUS PHOTOCATALYTIC REDUCTION OF NO IN THE PRESENCE OF

CONJUGATED POLYMERS

Zolt~n T6th, P@ter P@nzeli and Enik6 P6s~n

Institute of Physical Chemistry, Kossuth Lajos University,

H-4010 Debrecen, Hungary

Received February 6, 1995 Accepted May 22, 1995

The reaction of NO with NH 3 is photocatalytic in the

presence of ~oly(p-phenylene), polythiocyanogen and

Ti02, and leads to high N2/N20 ratios. The reduction

of NO in the presence of Cu-containing paracyanogen or

Cu is not a photocatalytic reaction, and yields

relatively low N2/N20 ratios.

INTRODUCTION

The catalytic reduction of nitrogen oxide (NO) is very

important from the standpoint of removal of air pollutants.

Different metals (Pt, Cu), metal ion-exchanged zeolites, and

metal-oxides (CuO, Cr203 , Fe203 ) are the most effective

catalysts for the heterogeneous reduction of NO. In this paper,

we present the results of our studies on the photoreduction of

NO in the presence of conjugated polymers. Among ~-bonded

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TOTH et al.: PHOTOCATALYTIC REDUCTION

conjugated polymers, polycyanogens (PC), polythiocyanogens

(PTC), and poly(p-phenylene)s (PPP) show remarkable

photocatalytic activity in the photoreduction of water [1-3],

carbon dioxide [4], carbonyl compounds [2], methyl-viologen

[3,5], and in the photoisomerization of olefins [2] in the

presence of sacrificial electron donors (e.g. EDTA, TEA, TEOA)

under UV (or VIS) light.

EXPERIMENTAL

NO gas was prepared from NaN02 and KI with H2SO 4 [6]. A

glass flask (120 cm 3, transparent for k > 305 nm light)

containing the solid photocatalyst, was filled with oxygen-free

NO, then i0 cm 3 of NH 3 solution was injected through a silicon

rubber septum. The reaction mixture was stirred at 30-40 ~

under unthermostated conditions. The N0-content of the gas

phase was determined spectrophotometrically as FeN02+ complex

(k = 450 nm), formed in the solution of Fe(NH4)2(S04)2 on the

action of 0.25 cm 3 of N0-containing sample. The relative

amount of the N 2 and N20 formed was determined mass-

spectrometrically (QGA-2, ATOMKI). A 150 W tungsten filament

spot lamp (PAR-38, TUNGSRAM) with 15 cm water-filter, and a 400

W quartz lamp (HgOK-400, TUNGSRAM, with intensive emission

lines at 310, 360 and 365 nm) served as light sources for VIS,

and UV irradiation, respectively. The preparation of the

conjugated polymers tested has been described earlier [3].

TiO 2 (Degussa P 25) a well-known photocatalyst (which has not

been investigated in NO photoreduction till now) was the

reference material. The sacrificial electron donors (NH 3-

solution, ascorbic acid, n-butyl-amine) were used without

further purification.

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TOTH et al.: PHOTOCATALYTIC REDUCTION

RESULTS AND DISCUSSION

Data in Table 1 show that poly(p-phenylene), PPP, iodine-

poor polythiocyanogen: (SCNI0.04) n, PTC, and Ti02 catalyze the

photoconversion of NO in the presence of NH 3. The rate of NO

photoconversion depends on the amount of the catalyst, the

energy and intensity of light, and the concentration of NH 3

solution (Fig. i) . (Note, that the photocatalytic reaction was

observed in the presence of n-butyl-amine or ascorbic acid,

too, but no reaction takes place in the absence of an electron

donor.)

Table 1

Conversion of NO after 2 h in the presence of different

photocatalysts(25 % NH3-solution, distance of light source: UV

52 cm, VIS 44 cm)

Catalyst Amount UV light VIS light in dark

PPP 5 mg ii %

i0 mg 34 % 29 % 2 %

20 mg 36 %

PTC 5 mg 24 %

i0 mg 29 % 19 %a 28 % 14 %b 5 %

20 mg 41%

PC-Cu i0 mg 53 % 59 % 60 %

Cu I0 mg 55 % 42 % 55 %

TiO 2 i0 mg 28 % 17 % 0 %

a b irradiation from a greater distance

We did not observe any photocatalytic activity of

paracyanogens, (CN)n, in the above reaction system. Mercury or

silver containing paracyanogens did not catalyze the NO

reduction, and the measured catalytic activity of the copper-

containig (87 wt.%) paracyanogen, PC-Cu, was the same as that

of the well-known Cu powder catalyst.

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TOTH et al.: PHOTOCATALYTIC REDUCTION

The difference between the two types of catalytic behavior

was explored not only by effect of irradiation, but by MS

investigating the relative amount of the two main reaction

products: N 2 and N20. The N2/N20 ratio was more than ten times

greater in the case of PPP, PTC and Ti02, (42, 2.2 and 1.9,

respectively) than in the case of PC-Cu (0.i) . These

observations can be explained by the well-known mechanism for

the heterogeneous catalytic dark reduction of NO with NH 3 [7].

The first steps are the associative adsorption of NO:

NO -+ N0ad s

and the dissociative adsorption of NH3:

NH 3 -+ NH2,ad s + Had s

The reaction of adsorbed NO with adsorbed NH 2 leads to the

formation of N2:

NOads + NH2, ads -+ N2, ads + H2Oads

and the reaction between adsorbed NO and adsorbed H yields

N20:

2NOad s + 2Had s -+ N2Oad s + H2Oad s

The photocatalysts may prefer the formation of N 2 to N20 by

the oxidation of adsorbed NH 3 with photogenerated holes:

NH3,ad s + h + -~ NH2,ad s + H+ad s

However, this speculation needs further experimental

evidence.

Acknowledgement. We are grateful to the Hungarian

Scientific Research Foundation for financial support (0TKA

Grant 1695/91. to Z.T6th), and to Dr. L~szl6 D6zsa for the MS

measurements.

188

TOTH et al.: PHOTOCATALYTIC REDUCTION

100 ~ m �9 ,u �9

I I k ~ ~ o - �9 •

-~ 80

N g O

Reect ion t i m e / h

Fig. i. Effect of ammonia concentration on the

photoreduction of NO catalyzed by PTC under UV

light

[] 25% ammonia A 20% ammonia

�9 12.5% ammonia �9 5% ammonia �9 0% ammonia

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TOTH et al.: PHOTOCATALYTIC REDUCTION

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