Journal of Sol-Gel Science and Technology 19, 775–778, 2000c© 2000 Kluwer Academic Publishers. Manufactured in The Netherlands.

Photocatalytic Reduction of NO with C2H6 on a Hollandite-Type Catalyst

JUN SUZUKI∗, KENJIRO FUJIMOTO†, TOSHIYUKI MORI, MAMORU WATANABEAND YOSHIO HASEGAWA∗∗

National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japanjsuzuki@nirim.go.jp

Abstract. The photocatalytic reduction of nitrogen monoxide (NO) with ethane on the hollandite type catalyst(K2Ga2Sn6O16 KGSO) was investigated. Using a closed-gas circulating system equipped with a Q-MASS detectorand in-situ diffuse reflectance FT-IR spectroscopy. The reactant gases of NO and13C2H6 decreased with theincreasing irradiation time. In contrast, the N2 yield increased proportionally to the conversion of13C2H6. Nitrogenoxides such as N2O did not reach their detectable levels. The NO adsorbed on KGSO was found to change to itsactivated species by UV irradiation. The oxidized products of C2H6 such as CH3CHO increased in proportion tothe reaction time. The present results strongly suggest that KGSO has remarkable photocatalytic activity for thereduction of NO with C2H6.

Keywords: hollandite-type compound, photocatalytic reduction, nitrogen monoxide, in-situ diffuse reflectanceFT-IR

1. Introduction

Nitrogen monoxide, a typical air pollutant, is dis-charged through the rexhaust gas of diesel engines andso on, and has been bringing about various environ-mental problems.

Selective catalytic reduction of NOx has been re-ported to effectively proceed on various ion-exchangedzeolites using C2H4, C3H6 or higher hydrocarbons asthe reductant [1–5]. Several reports have been pub-lished for research on the removal of NO by pho-tocatalysis [6–11]. Photocatalytic reduction seems aneffective and favorable way to decompose NOx speciesexhausted in the environment. For instance, some zeo-lites, such as Ti, V, Ag or Cu were added, have beenexamined from this viewpoint [9–11].

Hollandite-type compounds AxMyN8−yO16 (x≤ 2,y≤ 2) have a characteristic one-dimensional tunnel

∗To whom all correspondence should be addressed.†Joint-Graduate School Program between N.I.R.I.M and ScienceUniversity of Tokyo.∗∗Present address: Kaken Co. Ltd., Shikada 873-3, Asahi-mura,Kashima-gun, Ibaraki, 311-1416, Japan.

structure illustrated in Fig. 1. Recently, K2Ga2Sn6O16

( KGSO) among the family was found to promote theselective reduction of nitrogen monoxide with hydro-carbon when thermally activated [12].

In this study, using KGSO as a photocatalyst, its ac-tivity for the reduction of NO was examined in a closedgas circulating system, and the adsorbed species onKGSO in this reaction were analyzed by in-situ diffusereflectance FT-IR spectroscopy.

2. Experimental

The hollandite type catalyst (KGSO) was prepared bythe sol-gel process reported previously [13]. Dried gelwas crystallized at 973 K in the atmosphere, and theproducts were identified by powder XRD measure-ments using RAD-2B system (Rigaku Co. Ltd.). N2

adsorption and desorption isotherms were measured ona BELSORP28SA apparatus (Bel Jpn. Inc.). Specificsurface area and pore size distribution were estimatedaccording to the BET and the D-H method, respec-tively. UV-VIS spectra were collected on a U-4000spectrometer (Hitachi Co. Ltd.).

776 Suzuki et al.

Figure 1. Crystal structure of hollandite viewed along [001].

The photocatalytic reaction was carried out in aclosed gas-circulating system. In advance, the KGSOpowder was subjected to the pretreatment as follows; itwas heated at 873 K for 1 h in vacuo. Reactant gases ofNO (4000 ppm) and13C2H6 (2000 ppm) were broughtinto contact with 500 mg of KGSO powder in a quartzglass reactor. Gas components in this closed systemwere analyzed quantitatively by an on-line quadrupolemass spectrometer. The sample was irradiated throughthe reactor wall with a high pressure Hg lamp (400 W).The conversion and the yield rate were calculated fromthe observed concentration of reactants and products,respectively.

In-situ diffuse reflectance spectra were measured us-ing a FT-IR spectrometer equipped with a MCT detec-tor (FTS-45RD, Bio-Rad Jpn. Co. Ltd.). 120 mg ofthe catalyst powder was loaded into a small pan in thestainless steel airtight container with a KBr windowsettled in a sample room of the spectrometer. Afterpreheated at 873 K for 1 h in vacuo, KGSO was irradi-ated using an illuminator with a 400 W Xe lamp. NO(8000 ppm) and/or C2H6 (4000 ppm) were flowed intothe container at 10 mL/min.

3. Results and Discussion

KGSO fine powder was identified to be a single phaseby its XRD pattern. The BET and D-H analyses showedthat the powder had a specific surface area of 28 m2/gand was mesoporous. Its UV-VIS spectra exhibited theabsorption edge at about 340 nm, suggesting a band gapenergy of about 3.6 eV.

The photocatalysis on KGSO was examined on thebasis of the following model reaction (1).

2NO+ C2H6→ N2+ 2CO+ 3H2 (1)

Figure 2. Relationship between reaction time and photocatalyticactivity of KGSO in gas phase◦: NO,¤: 13C2H6,M: N2,♦: N2O,O: CO,+: CO2.

Figure 2 shows the relationship between photocat-alytic activities and reaction time in the closed-gas cir-culating system. The conversion of NO was much moremarked as compared to that of13C2H6, but this re-markable conversion even before UV irradiation wasattributed to a large adsorption capacity of KGSO forNO species. In contrast, the conversion of13C2H6 grad-ually increased with the increasing irradiation time.The yield of N2 rose nearly proportional to the conver-sion of 13C2H6. Any oxidized products derived from13C2H6 were below their detectable levels (<1 ppm) inthe present condition.

Figure 3 displays diffuse reflectance spectra of ad-sorbed NO on KGSO before and after UV irradiation.Prior to the irradiation, the strong adsorption band wasobserved at 1260 cm−1 after one hour adsorption ofNO followed by the exclusion of gaseous NO fromthe container. The similar spectra were observed for

Figure 3. FT-IR spectra of NO species adsorbed on KGSO beforeand after UV irradiation after the exclusion of gaseous NO with a Heflow.

Photocatalytic Reduction of NO with C2H6 on a Hollandite-Type Catalyst 777

NO and CO adsorbed on K-doped Fe/ZrO2 [14]. Thosewere assigned to the symmetric stretching vibration ofnitrogen oxide anion ([ONO]−) on the K+ ion, and theweak band at 1330 cm−1 was attributed to the asym-metric stretching vibration of the nitrogen oxide anion[14, 15]. According to that assignment, it is suggestedthat the present strong band is given by the NO adsorbedon K+ ions in a form of nitrogen oxide anion. Afterthe irradiation, the band at 1410 cm−1 progressivelygot larger and the band at 1260 cm−1 was decayed.In general, it is seemed that this band is assigned tothe asymmetric stretching vibration of nitro complex(−NO2) [16, 17].

In Fig. 4, the intensities of the two bands, or 1260and 1410 cm−1, were plotted against the reaction time.The band at 1260 cm−1 reached the maximum in inten-sity after about 3 h from the starting of NO adsorptionunder the dark condition, and under the irradiation, itsintensity was gradually weakened. In contrast, the bandat 1410 cm−1 became rapidly larger by the UV illumi-nation. Their intensity changes came to be depressedabout 6 h later after the irradiation.

The spectra in Fig. 5 were taken in a He stream con-taining NO and C2H6; taking account of the resultsin Fig. 4, the irradiation was started 3 h later after in-troduction of the sample gases. During the dark pe-riod, the bands at 1260 cm−1 and so on progressivelygrew up. When the irradiation started, the band at1410 cm−1 became clearer and the new band was ob-served at 2230 cm−1. The band at 2230 cm−1 had beenalso reported for the selective catalytic reduction of NOwith hydrocarbons on ion-exchanged zeolites [18–20].In these cases, the band was assigned to the asymmetric

Figure 4. Relationship between irradiation time and intensi-ties of each bands by in-situ IR spectra of KGSO◦: 1260 cm−1,

¤:1410 cm−1.

Figure 5. FT-IR spectra of adsorbed species on KGSO before/afterUV irradiation under flowing NO and C2H6 by in-situ FT-IR spectro-scopy.

stretching vibration of isocyanate (NCO), the asym-metric stretching vibration of cyanate (OCN), thestretching vibration of nitrile (CN); those specieswere regarded as the intermediates in the reductionprocess of NO with hydrocarbon. As can be seenin Fig. 5, the weak bands in the range of 1550 to1850 cm−1 became gradually larger after the irradi-ation and were confirmed to consist of two bands at1610 and 1720 cm−1 about 9 h later. Those bandscould be assigned to the stretching vibration of the car-bonyl groups (>CO) in aldehydes, carboxylic acidsand so on. Anpo and coworkers reported CH3CHO orCO2 as the products in the photocatalytic reduction ofNO with C3H6 [9]. Accordingly, it is considered thatthose bands were assigned to the stretching vibrationof C O in CH3CHO.

Therefore the speculated reduction process of NO inthe Eq. (1) was modified as the follows:

2NO+ C2H6→ N2+ CH3CHO+ H2O (2)

The NO adsorbed on KGSO was supposed to betransformed to the activated NO under UV irradiation.This activated species were expected to react with C2H6

and react in N2 and CH3CHO. The adsorbed speciessuch as isocyanate, cyanate or nitrile species were as-sumed to be produced by reacting adsorbed NO withC2H6.

4. Summary

In a closed-gas circulating system, the yield of N2

rose nearly proportional to the conversion of13C2H6.

778 Suzuki et al.

Nitrogen oxides such as N2O did not reach theirdetectable levels.

In-situ diffuse reflectance FT-IR spectra exhibitedthe strong absorption band due to NO adsorbed on K+

ions before UV irradiation. After the irradiation, theband due to the activated NO by UV light progres-sively got larger. The oxidized products of C2H6 suchas CH3CHO increased in proportion to the irradiationtime.

The present results strongly suggest that KGSO hasremarkable photocatalytic activity for the reduction ofNO with C2H6.

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