study of wo3-based sensing materials for nh3 and no detection

Xusheng Wanga,, Norio Miurab, Noboru Yamazoeba Department of Technical Physics, Xidian UniÕersity, Xi’an 710071, People’s Republic of China b Department of Materials Science and Technolo

Xusheng Wanga,), Norio Miurab, Noboru Yamazoeb

a

Department of Technical Physics, Xidian UniÕersity, Xi’an 710071, People’s Republic of China

b

Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu UniÕersity, Kasuga, Fukuoka 816, Japan

Received 30 July 1998; received in revised form 28 January 1999; accepted 11 May 1999

Abstract

Gas sensing materials of WO3 loaded with 1 wt.% metal oxides were prepared and applied for NH3 and NO detection The measurement of NH and NO sensing properties of the materials revealed that WO q 1 wt.% Mg, WO q 1 wt.% Zn, WO q 1 wt.%3 3 3 3

Mo and WO q 1 wt.% Re characterized good responses to NH3 3 and NO As a whole, these materials have low resistance, high sensitivity and fast response to NH3 and NO compared with pure WO The possibility of NH –NO equivalent point sensor was3 3 discussed It can be used for real time monitoring, and controlling the reduction of NO using NH q 2000 Elsevier Science S.A All3 rights reserved.

Keywords: NH and NO gas sensors; WO ; Metal oxides; NH –NO equivalent point sensor3 3 3

1 Introduction

The release of various chemical pollutants from

indus-tries, automobiles and homes into the atmosphere has been

causing global environmental issues Intense efforts are

being exerted on research and development of new

tech-nologies andror new chemical processes that can

elimi-nate or reduce air pollutants The typical example is the

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reduction of NO by some selective catalysts with NH3 1

But, the NO emitted varies with time and position, and

NH3 is also one of pollutants In the reduction of NO

using NH , it is very important to control the amount of3

NH to be used The monitoring of NO–NH3 3 reaction is

eagerly awaited in environmental pollution control Gas

sensors make it possible to detect air pollutants in real time

and can be used for feedback control systems of various

chemical processes Compactness and low cost will also be

advantageous, not only for application to mobile facilities

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but also for multi-site monitoring 2 WO -based semicon-3

ducting gas sensors have showed good responses, both to

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NH3 and NO 3,4 , and characterized the opposite

resis-tance changes to them, which makes it possible to detect

the NH3 or NO, and the surplus NO or NH3 after the

NO–NH reaction In this paper, the sensing materials of3

WO loaded with 1 wt.% metal oxides were prepared and3

characterized for NH and NO detection.3

)

Corresponding author Tel.: q86-29-8202560; fax: q86-29-5262281.

2 Experimental

Aqueous soakage method was used for preparing the sensing materials The WO3 powder was prepared by

pyrolyzing ammonium paratungstate NH4 10W O P12 41

5H O at 6008C for 2 h in air, and the doped materials2 were nitrate or acetate The sensing materials of WO3 loaded with 1 wt.% metal oxides were prepared by solving the doped original materials in de-ionized water at 60– 808C, adding the WO3 powder into the solution, stirring and heating until it dried, then calcining at 6008C for 2 h The sensor element was prepared by applying WO -based3 powder on an aluminum tube fitted with Pt wire elec-trodes, and calcined at 6008C for 1 h The NH3 and NO sensing properties of WO -based materials were measured3

Ž

in a conventional flow apparatus the gas flow rate was

fixed to 100 cm rmin in 3508C The sensitivities were

defined as S s R rRa g and S s R rR , respectively, forg a

NH3 and NO, where Ra and Rg were resistances of sensing element in air and in sample gas

3 Results and discussion

Fig 1 shows the sensitivity of WO loaded with 1 wt.%3

metal oxides except noble metals responding to 30-ppm

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NH3 and 40-ppm NO at 3508C for the sake of high sensitivity and fast response both to NH3 and NO, this 0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A All rights reserved.

PII: S 0 9 2 5 - 4 0 0 5 9 9 0 0 4 1 0 - 4

Fig 1 Sensitivity of WO and WO q 1 wt.% metal oxides to NH and NO at 3508C 3 3 3

working temperature is suitable For comparison, the

re-sult of pure WO was also shown From the rere-sult it can3

be seen that: the sensitivity and responding properties of

WO decayed with loading of alkali metals The respond-3

ing and recovery properties were obviously improved by

loading with alkali earth metals The materials of WO3

loaded with 1 wt.% Mg, Zn, Mo and Re showed good

responses both to NH and NO, and can be used for NH3 3

and NO gas sensors As a whole, these materials have low

resistance about 5 = 10 V, Mg-loaded material was

slightly higher , high sensitivity pure WO : S s 5.40 for3

30-ppm NH3 and S s 4.00 for 40-ppm NO; WO q 13

wt.% Mg, Zn, Mo and Re: S s 7.65, 5.06, 10.1, 9.46,

respectively, for 30 ppm NH , and S s 5.87, 8.33, 5.00,3

6.25, respectively, for 40 ppm NO , and fast response to

NH and NO compared with pure WO The materials of3 3

WO loaded with 1 wt.% Cr, La, Pr, Sm, Gd, Er, Tm and3

Yb showed good responses to NH , and hardly responded3

to NO, and the materials of WO loaded with 1 wt.% Ba,3

Te, and Pb showed good responses to NO and hardly

responded to NH These two kinds of materials can be3

used as NH and NO selective gas sensors, respectively.3

The characteristics of WO -based semiconducting mate-3

rials responding to NH3 and NO, i.e., the opposite

resis-tance changes to NH3 and NO, respectively, make it

possible to detect the surplus NO or NH3 after the NO–

NH3 reaction Fig 2 shows the transient responses of a

WO sensing element to NO mixed with NH From the3 3

measurement, it can be seen that the NH –NO equivalent3

point can be defined as the value of NH –NO, while the

sensitivity S s 1 in NO–NH3 mixture It is necessary for the existence of the NH –NO equivalent point that NH3 3 and NO must be reacted completely The NH –NO equiv-3 alent point sensor was designed as follows: the sensor is two-layer-structured, i.e., a catalyst layer was coated on the surface of the WO -based sensing element Two kinds3

of NO reduction catalysts of TiO q 10 wt.% CuO q 52 wt.% SiO and TiO q 8 wt.% V O q 1 wt.% WO q 52 2 2 5 3 wt.% SiO were used for the coating layers, and measured2 for various NO and NH concentrations Expedient results3 were obtained

Fig 2 Transient response of WO to NH mixed with NO.

designed NH –NO equivalent point sensor includes two3

parts: one is a sensing material and the other is a catalyst

This research gives a number of good candidates in

semi-conducting sensing materials for NH3 and NO detection

The NH rNO equivalent point sensor can be used for real3

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M de Boer, M.D Amiridis, J Catal 161 1996 211–221.

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w x 4 T Maekawa, J Tamaki, N Miura, N Yamazoe, Chem Lett 1992Ž . 639–643.