Ammonia Sensing Mechanism of Tungstated-Zirconia Thick Film

Extinction coefficient of NH4+ was determined by measuring IR band of NH4+ ...... R.K. Kotnala , Jyoti Shah , Mohan C. Mathpal , K.C. Verma , Sandeep ...
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12080

J. Phys. Chem. C 2007, 111, 12080-12085

Ammonia Sensing Mechanism of Tungstated-Zirconia Thick Film Sensor Atsushi Satsuma,*,† Ken-ichi Shimizu,† Koichi Kashiwagi,† Tadanori Endo,† Hiroyuki Nishiyama,‡ Shiro Kakimoto,‡ Satoshi Sugaya,‡ and Hitoshi Yokoi‡ Graduate School of Engineering, Nagoya UniVersity, Nagoya 464-0814, Japan, and R&D Center, NGK Spark Plug Co. Ltd., Komaki 485-8510, Japan ReceiVed: May 18, 2007; In Final Form: June 10, 2007

A sensor element of interdigital capacitor (IDC) electrodes covering with tungstated-zirconia thick film shows a good response and cross-sensitivity in gaseous ammonia sensing. The sensing mechanism of the tungstatedzirconia thick film has been investigated by means of in situ FT/IR and H/D isotope effect. In the absence of ammonia, the role of protonic conduction was clarified by the H/D isotope effect of water vapor on the conductivity. In the presence of gas phase ammonia, the conductivity of the tungstated-zirconia thick film significantly increased. The conductivity in the presence of ammonia showed unique temperature dependence, i.e., the conductivity at first increased with a decrease in temperature, showed the maximum at 633 K, and then decreased with further decrease in temperature. In situ FT/IR spectra revealed the presence of both ammonium ion adsorbed on Brønsted acid site and ammonia coordinated to Lewis acid site on the tungstated-zirconia surface. Temperature dependence and dynamic response of the conductivity were in good accordance with the surface concentration of ammonium ion adsorbed on Brønsted acid site. The contribution of ammonium ion to electric conductivity was indicated. The role of ammonium ion as a migrating species for ammonia sensing was verified from negligible H/D isotope effect of ammonia and mobility of ammonium ion.

1. Introduction There has been a worldwide effort to develop technologies for NOx removal from various sources because that causes photochemical smog formation and acid rain. Urea-SCR (selective catalytic reduction of NO by urea) is thought to be one of the promising technology for the removal of NOx emissions from large-scale diesel engine cars such as tracks and buses.1-7 In urea-SCR systems, NH3 as a reductant of NOx is supplied by on-site hydroxylation of urea ((NH2)2CO), and the selective reduction of NOx to N2 effectively proceeds over Fezeolite catalyst. For the effective and safety injection of urea into catalytic converter, a feedback system including measurement of leak ammonia concentration is desired. For the real diesel exhausts, a good response, cross-sensitivity to various interfering gases, and tolerance to high temperatures are required for ammonia sensors. So far, various types of ammonia sensor by using various sensing materials, such as proton conductor, semiconductor, and polymers, have been reported.8-17 Among them, the performance of zeolite thick film ammonia sensor is thought to meet these requirements for automobile use.8 Recently, we have reported that a sensor element of interdigital capacitor electrodes covering with thick film of tungstated-zirconia shows a good response and cross-sensitivity in ammonia sensing.18 From physicochemical characterizations of tungstated-zirconia, it was revealed that subnanometer-sized polytungstate clusters are uniformly dispersed on zirconia surface, and the presence of strong solid acid sites * To whom correspondence should be addressed. E-mail: satsuma@ apchem.nagoya-u.ac.jp. Tel: +81-52-789-4608. Fax: +81-52-789-3193. † Nagoya University. ‡ NGK Spark Plug Co. Ltd.

comparable to zeolite MFI was confirmed from a profile of NH3-temperature programmed desorption. The strong acid property is reasonable because tungstated-zirconia is usually applied as a solid acid catalyst such as skeletal isomerization of alkanes.19-21 The strong acid property may enable us to consider the ammonia sensing mechanism of tungstated-zirconia in analogy with proton-type zeolites. The contribution of protonic conduction on the sensing property is proposed on mordenite22 and MFI23-25 zeolites in a pellet and a thick film. A series of investigations by Franke and Simon et al. clarified the contribution of translational proton hopping in zeolites.23-25 They observed a significant increase of conductivity of MFI thick film under the presence of NH3 as compared to the NH3 free conditions, and pointed out adsorbed complex of NH3 as a migrating species.25 However, their discussion was not based on direct observation of the surface species. For improvement of the sensing performance of such solid acid utilized thick film sensors, a good understanding of the sensing mechanism and the roles of adsorbed species should be essential. Quantitative and dynamic analysis of adsorbed species may open up clearer understanding of the sensing mechanism. In the present study, to clarify the sensing mechanism of tungstated-zirconia thick film sensor, the surface species on tungstated-zirconia under working states have been analyzed by means of in situ FT/IR. First, the contribution of protonic conductivity is confirmed by dynamic analysis of surface adsorbed water and H/D isotope effect of H2O vapor. Second, the contribution of Brønsted and Lewis acid sites on tungstatedzirconia surface to the sensing property is identified. Finally, the role of NH4+ species as a migrating species under the presence of NH3 was demonstrated by the H/D isotope effect of NH3 and from temperature dependence of mobility of NH4+ ions.

10.1021/jp073833l CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007

Mechanism of Tungstated-Zirconia Thick Film Sensor

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2. Experimental Section For the preparation of tungstated-zirconia, hydrated zirconia was first prepared by hydrolysis of zirconium oxynitrate 2-hydrate (Kishida, 99%) in distilled water by gradual adding an aqueous NH4OH solution (1.0 mol dm-3), filtration of precipitate, washing with distilled water for three times, and dryness at 353 K for 24 h in air. Tungstated-zirconia was prepared by impregnation of the obtained hydrated zirconia with an aqueous solution of ammonium paratungstate (Mitsuwa) at pH 10 with an aqueous NH4OH solution.18,26 The suspension was evaporated in a rotary-evaporator. The solid thus obtained was dried and then calcined at 873 K for 3 h. The loading amount of WO3 is 25 wt %, BET surface area of tungstatedzirconia was 107 m2 g-1, and thus the surface W density is 6.5 W nm,-2 which is within the range of monolayer coverage. It should be noted that, because of the use of sintered ZrO2, the tungstated-zirconia used in the previous paper (WO3 loading amount of 10 wt %, BET surface area of 51.1 m2g-12) was different from that used in this study,18 but the surface W density of the previous sample (5.1 nm-2) was also within the range of monolayer coverage. A schematic electrode setup was reported previously.18 Interdigital electrodes were formed on the top of an alumina substrate in which a platinum heater and a platinum temperature sensor were integrated. The heater keeps the electrode at prescribed temperatures. The interdigital 15 lines of electrodes having 110 µm width, 90 µm interval, and 8 µm height on the top of an alumina substrate were covered with a tungstatedzirconia thick film of ca. 30 µm thickness as a sensing material by a screen-printing technique. The alumina substrate covered with the tungstated-zirconia thick film was calcined at 873 K for 3 h in air and assembled in a stainless case. Impedance measurements were carried out with an LCR meter (HIOKI 3532-80) with the frequency range of 4 Hz to 1 MHz. The electrodes were exposed in a conventional flow apparatus equipped with gas flow controllers and a heating facility. The electrodes were exposed to a flow gas containing various concentrations of NH3, 10% O2, 3% H2O, and N2 as a balance at a flow rate of 300 cm3 min-1. The gas mixture was preheated at 553 K and the electrode was heated at 473-773 K. The sensing experiment was performed with impedance measurement. The in situ FT/IR spectra were recorded using a JASCO FT/ IR-620 equipped with a quartz IR cell connected to a conventional flow reaction system, which was used in our previous studies.27,28 It should be noted that the FT/IR measurements were carried out not with the assembled electrode but with a selfsupporting wafer of the sensing material. The tungstated-zirconia powder was pressed into a 0.05 g wafer and mounted into the quartz IR cell with KBr windows. The spectra were measured by accumulating 20 scans at a resolution of 4 cm-1. A reference spectrum of the sensing material wafer was taken in flowing He, and was subtracted from each spectrum under the reaction conditions. Prior to each experiment, the wafer was heated in O2(10%)/N2(balance) at 773 K for 1 h, followed by cooling to measurement temperatures. A flow of various gas mixtures was then fed at a rate of 300 cm3 min-1. The composition of the gas mixture was the same as in the corresponding impedance measurements. The amount of adsorbed species was estimated from the integration of the IR bands on the basis of LambertBeer equation by using extinction coefficient in the same way as described earlier.28 Extinction coefficient of adsorbed water was determined by measuring IR band of adsorbed water on tungstated-zirconia followed by heating of the sample and

Figure 1. Impedance spectra of tungstated-zirconia thick film electrode in the absence (O) and presence of 3% water vapor (b) between 4 Hz and 1 MHz at various temperatures.

measurement of desorbed water by mass spectroscope. Extinction coefficient of NH4+ was determined by measuring IR band of NH4+ in prescribed amount of ammonium paratungstate diluted in KBr disk. 3. Results and Discussion 3.1. Protonic Conduction in Absence of Ammonia. First of all, the role of protonic conduction in the absence of ammonia is examined in this section. Figure 1 shows impedance spectra, so-called Cole-Cole plot, between 4 Hz and 1 MHz for the tungstated-zirconia thick film electrode in a flow of a gas mixture of O2(10%)/N2(balance) with or without 3% water vapor. The impedance spectra showed semicircles at higher frequencies and tails at lower frequencies. The profiles of impedance spectra indicate that the sample can be interpreted in terms of an RC parallel equivalent circuit.8,29,30 On the analogy of the zeolite thick film sensor,8,24 it is reasonable to consider that the former semicircle is assigned to the contribution of an RC parallel equivalent circuit of bulk electrolyte and/or grain boundaries, in which R is dependent on gas composition, and the latter tail part of the impedance plot at lower frequencies is dominated by the sample/electrode interface polarization.29 The resistance value of R of an RC parallel equivalent circuit at higher frequencies was defined from the cross section of the semicircle with the real axis.29,30 The conductivity σ (S cm-1) can be estimated from the following equation:

σ ) l/(R‚A)

(1)

where l (interval of interdigital electrode) ) 110 µm, A (effective area of electrode) ) 0.00285 cm2 in this case. As shown in the figure, the resistance value R at the intersection of the semicircle with the real axis at around 30 Hz varied with temperature and concentration of water vapor. The semicircle became smaller in the presence of water vapor, indicating the smaller resistance R, i.e., higher conductivity σ. At higher temperatures, the effect of water vapor reduced. Figure 2 shows in situ FT/IR spectra of tungstated-zirconia under a flow of 3% water vapor. A strong band was observed at 1615 cm-1 assignable to deformation vibration of adsorbed water molecule.31,32 This band decreased with an increase in temperature, indicating desorption of water molecule at higher temperatures. Negative bands were observed at 1015 and 2020 cm-1, which are assignable to ν(WdO) for 1015 cm-1 and 2ν(WdO) for 2020 cm-1, respectively.33,34 As we have reported previously, the supported tungsten oxide on tungstated-zirconia is present as polytungstate oxo-clusters sized around 0.6 nm.26 The negative bands of ν(WdO) and 2ν(WdO) indicate interaction of adsorbed water molecule with WdO oxo-species on polytungstate clusters. In the FT/IR spectra, positive bands were

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Figure 2. IR spectra of adsorbed water on tungstated-zirconia in the presence of 3% water vapor at various temperatures.

Figure 3. Time courses of the intensity of the relative conductivity (O, σ/σ0) and the surface concentration of adsorbed H2O at 673 K (2).

also observed at around 3500 and 990 cm-1. The former broad band is assignable to hydrogen-bonded O-H groups. Hattori et al. observed the band at near 990 cm-1 in IR spectra of tungstated-zirconia heated in the presence of hydrogen and assigned to acidic OH delocalized around WdO and Zr.34 The negative bands of WdO species and the positive bands of acidic OH groups indicate the conversion of WdO oxo-species on polytungstate clusters to W-OH species after the introduction of water vapor to tungstated-zirconia surface. The surface concentration of adsorbed water was estimated on the basis of the integrated band intensity of 1573 - 1642 cm-1 by using extinction coefficient of 4.12 × 10-18 cm-1 cm2 molecule-1. Figure 3 shows time-on-stream of the response of the relative conductivity and the surface concentration of adsorbed water at 673 K. After the introduction of water vapor, both the conductivity and the surface concentration of adsorbed water immediately increased. At the maximum of the adsorption, the surface concentration of H2O is 0.16 nm-2, which is far lower than the density of the W species of 6.5 nm-2. Under the present conditions, the adsorbed water molecules are highly isolated from each other. After water vapor purging, the band immediately decreased. The response of the adsorbed water is in good agreement with that of the relative conductivity. As shown in Figure 1 and Figure 3, the strong dependence of the conductivity on water vapor suggests the contribution of protonic conduction on tungstated-zirconia surface. To evaluate the conduction model, H/D isotope effect is examined and the conductivities in the presence of 3% of H2O or D2O vapor are plotted against reciprocal temperature in Figure 4. The introduction of water vapor results in the higher conductivity than that in the dry conditions. In all the temperatures examined, the conductivity was depressed by replacing H2O to D2O. Small temperature dependence of the conductivity below 600 K suggests water-cooperative proton conduction, which can be

Satsuma et al.

Figure 4. Conductivity of tungstated-zirconia in a flow of O2/N2 (0, base), with 3% H2O (O) and 3% D2O (b).

Figure 5. Impedance spectra of tungstated-zirconia thick film electrode in O2/N2 (O, base), with 3% H2O (b), with 150 ppm NH3 (4), and with both 150 ppm NH3 and 3% H2O (2) between 4 Hz and 1 MHz at 673 K.

observed in silica glasses.35 From the slope of the plot above 623 K, the activation energies in the presence of H2O and D2O are estimated as 48.4 and 63.1 kJ mol-1, respectively. The difference in the activation energy (14.7 kJ mol-1) is in good accordance with that in vibration energy of O-H (O-D) bond of water molecule (16 kJ mol-1). The presence of H/D isotope effect clearly indicates that the conduction on tungstatedzirconia includes the O-H bond fission in the rate-determining step. The H/D effect clearly indicates that the electric conduction of tungstated-zirconia is attributed to surface protonic conductivity in the absence of NH3, as well as the protonic conduction mechanism on zeolite.22-25,36 Very low concentration of water molecules (e.g., 0.16 nm-2 at 673 K) suggests that proton migrates by hopping from one hydroxyl site to another with O-H bond fission at higher temperatures. 3.2. Conductivity and Surface Adsorbed Species under Ammonia Containing Atmosphere. Figure 5 illustrates impedance spectra of the tungstated-zirconia thick film electrode at 673 K in the presence and the absence of ammonia and water vapor. The spectra showed basically similar profiles as in the absence of NH3; the spectra consist of semicircles at higher frequencies and tails at lower frequencies. The cross section of the semicircle or its extension with the real axis significantly decreased with the introduction of NH3. The figure indicates that the introduction of NH3 results in the decrease of R of an RC parallel equivalent circuit, i.e., the increase of the conductivity σ in the presence of NH3. The co-presence of NH3 and H2O results in the further decrease in R. It should be noted that the conductivity was hardly affected by the partial pressure of oxygen from 0% to 10% in the temperature range of 473-773 K, suggesting negligible contribution of semiconductive or anion conductive character of tungstated-zirconia.

Mechanism of Tungstated-Zirconia Thick Film Sensor

Figure 6. IR spectra of tungstated-zirconia in the presence of 150 ppm NH3 at various temperatures.

Figure 7. IR spectra of adsorbed species in the presence of (a) 3% H2O, (b) 150 ppm NH3, and (c) both 150 ppm NH3 and 3% H2O at 573 K.

Figure 6 shows in situ FT/IR spectra of adsorbed species on tungstated-zirconia under a flow of 150 ppm of NH3 at various temperatures. At 473 K, a band at 1420 cm-1 was observed, which is assignable to NH4+ adsorbed on Brønsted acid.37 Bands at 1600 and 1235 cm-1 assignable to coordinately held NH3 were also observed. Since the latter bands were remained even at 673 K, the bands are mainly due to coordinated NH3 strongly held on Lewis acid sites. The small shift of the band at 1235 cm-1 suggests some contribution of hydrogen-bonded NH3. The intensity of these bands gradually decreased with increasing temperature. In higher wavenumbers, a negative band of O-H stretching mode around 3600 cm-1 became significant at lower temperatures, while a broad band around 3000 cm-1 increased. The negative band of O-H stretching mode indicates the adsorption of NH4+ on surface acidic OH group, i.e., Brønsted acid site. The broad positive bands from 3400 to 2600 cm-1 can be assigned to N-H stretching mode of NH3 (3400-3100 cm-1) and the shifted O-H stretching due to the adsorption of NH4+. Xu et al. clarified that OH groups on tungstate oligomer performs as Brønsted acid sites having an acid strength similar to sulfated zirconia by means of solid-state NMR and DFT quantum chemical calculation.38 Based on their research, the strong adsorption of NH4+ on surface acidic OH group is reasonable. In Figure 7, the effect of atmospheres on the adsorbed species is compared at 573 K. In the co-presence of NH3 and H2O, the band at 1420 cm-1 assignable to NH4+ on Brønsted acid became more intense than in the absence of H2O. Comparing with the impedance spectra of the sample shown in Figure 5, the surface concentration of NH4+ is in accordance with the reduction of the resistance R, i.e., the increase in the conductivity. The intensity of the band at 1240 cm-1 assignable to NH3 on Lewis

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Figure 8. Response transient of tungstated-zirconia thick film electrode (O) and surface concentration of adsorbed NH4+ in a flow of 300 ppm of NH3 at 673 K (2).

Figure 9. Dependence of the electrical conductivity of tungstatedzirconia on surface concentration of adsorbed NH4+ at 573 K (b) and 673 K (O). Gaseous NH3 concentrations are 100, 300, 600, and 1200 ppm, respectively.

acid decreased in the co-presence of H2O, showing opposite trend in the conductivity. The results suggest contribution of NH4+ species to the conductivity. The response of the conductivity is compared with the surface concentration of adsorbed NH4+ in Figure 8. The surface concentration of adsorbed NH4+ was determined from the integrated band intensity of 1348-1496 cm-1 by using extinction coefficient of 2.38 × 10-17 cm-1 cm2 molecule-1. After the introduction of NH3 in the gas phase, the conductivity immediately increased. At the same time, the integrated band intensity of NH4+ also immediately increased and reached to the constant value. When gaseous NH3 was removed from the gas phase, both the conductivity and the surface concentration of NH4+ steeply decreased. In Figure 9, the conductivity and the surface concentration of NH4+ are compared when the concentration of gaseous NH3 and the operating temperature are varied. A good correlation was observed in these parameters. The agreement of the transient response (Figure 8) and the good correlation between the conductivity and the surface concentration of NH4+ (Figure 9) clearly demonstrates the contribution of adsorbed NH4+ to the conductivity. 3.3. Temperature Dependence of Conductivity and Adsorbed Species. Figure 10 shows logarithm of the conductivity as a function of reciprocal temperature. Unique temperature dependence was observed in the presence of NH3. At 773 K, the conductivity in the presence of NH3 was almost the same as that in the absence of NH3. In the presence of NH3, the conductivity at first increased with a decrease in temperature, showed the maximum at 633 K, and then decreased with further decrease in temperature. The figure also shows the H/D isotope effect of NH3 on the conductivity. The use of ND3 slightly reduced the conductivity above 633 K, while the H/D effect was not observed below 633 K.

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Figure 10. Conductivity of tungstated-zirconia in a flow of O2/N2(O), with 3% H2O(b), with 150 ppm NH3 (0), and with 150 ppm ND3 (9) as a function of reciprocal temperature.

Figure 11. Surface concentration of adsorbed NH4+ on tungstatedzirconia in a flow of O2/N2 with 150 ppm NH3 as a function of reciprocal temperature.

Satsuma et al. NH4+ in the range of 773-633 K is in accordance with the temperature dependence of the conductivity. At 773 K, the adsorption of NH4+ is negligible, and the conductivity was close to that in the absence of NH3. As temperature reduced, the surface concentration of NH4+ became higher, and the difference between the conductivity in the presence of NH3 and the base conductivity became larger. The accordance of the temperature dependence indicates that the surface concentration of NH4+ is the essential factor for the conductivity. Such unique temperature dependence in Figure 10 was also observed in zeolite thick film electrode, as reported by Franke et al.25,40 They explained such temperature dependence on the basis of the amount of adsorbed molecules, by comparing the adsorption-desorption behavior of zeolites reported in literatures. At lower temperatures below 393 K, the presence of excess adsorbed NH3 plays solvate molecules analogous to the liquid state and results in a Grottus-like proton transport. With increasing temperature, the surface species desorbs and the conduction model shifts to vehicle transport of protonated NH4+ and H3O+. In the present study, the quantitative effect of surface adsorbed species has been directly confirmed by means of in situ FT/IR measurement, and revealed the contribution of the surface NH4+ concentration to the conductivity. Since the adsorbed NH4+ is below 0.4 nm-2 and highly isolated above 473 K, the electric conduction on tungstated-zirconia can be rationalized by hopping mechanism of NH4+ as a migrating species in the examined temperature rage. Further discussion on the effect of surface concentration of NH4+ on the conductivity is carried out through estimation of mobility of NH4+ ions. Generally, the ionic conductivity is expressed by the product of the number of ions present and their respective mobility.41-45 Assuming that the surface NH4+ species is a migrating species on tungstated-zirconia, the conductivity can be written by the following equation:

σ ) FZµ[NH4+]

(2)

where σ, F, Z, µ, and [NH4+] denote the conductivity, Faraday constant, charge of migrating species, the mobility of migrating species, and the concentration of NH4+, respectively. Thus the mobility of NH4+ can be estimated as follows:

µ ) σ/FZ[NH4+]

Figure 12. H/D isotope effect on the mobility of ammonium ion in a flow of 150 ppm NH3 (0) and 150 ppm ND3 (9).

Since the results in the previous section demonstrated the contribution of surface adsorbed NH4+ species to the electric conductivity, quantitative effects of the surface NH4+ are examined in this section. The surface concentration of NH4+ on tungstated-zirconia is estimated from in situ FT/IR spectra and plotted as a function of reciprocal temperature in Figure 11. The concentration of NH4+ is negligible at 773 K and increased with a decrease in temperature. This profile is in accordance with NH3-TPD profile reported previously, i.e., the adsorbed NH3 monotonously desorbed as temperature increased and the desorption terminated at 773 K.18,39 It should be noted that the surface concentration of NH4+ is below 0.4 nm-2 even at 473 K; thus, the adsorbed NH4+ is highly isolated on the tungstated-zirconia surface. Compared with the Arrhenius plot of the conductivity, the increase in the surface concentration of

(3)

In order to clarify the adequacy of the above discussions, the mobility of NH4+ was estimated on the basis of the surface NH4+ concentration determined by in situ FT/IR. In Figure 12, logarithm of the mobility is plotted as a function of reciprocal temperature. A linear correlation was observed in the temperature rage of 500-630 K; i.e., Arrhenius type temperature dependence was observed in the mobility thus defined. The activation energy of the mobility in the presence of NH3 was 58.2 kJ mol-1. There is no difference in the mobility of NH3 and ND3 below 633 K, although a slight difference in the mobility was observed above 633 K. The result can be interpreted that the contribution of N-H fission of NH4+ to the rate-determining step of the electric conductivity is negligible in this temperature range, in other words, the contribution of NH4+ as a migrating unit. The slight difference of the mobility between NH3 and ND3 at higher temperatures (>633 K) suggests some contribution of the N-H bond fission, however, further investigation is necessary for the mechanistic discussion at higher temperatures because of the difficulty in quantification of the surface concentration of NH4+ at higher temperatures due to very low band intensity and low S/N ratio of in situ

Mechanism of Tungstated-Zirconia Thick Film Sensor FT/IR spectra. As the most important conclusion in Figure 12, it was clarified that the conductivity of tungstated-zirconia can be expressed by the product of the mobility and the surface concentration of NH4+. The hopping mechanism of NH4+ above 500 K was demonstrated in the present study. The surface NH4+ migrates on tungstated-zirconia surface with adsorbing surface acidic OH species which is generated by hydration of the WdO species on polytungstate clusters. Taking the surface concentration of adsorption sites into account, the conductivity may strongly depend on the surface concentration of polytungstate cluster and the conversion of WdO to W-OH. In the case of zeolite thick film electrode, Al content strongly affects the proton conductivity, as Franke et al. reported.24,46 They found the strong dependence of the activation energy on Al content and proposed classical hopping theory on the basis of the Debye-Hu¨ckel potential between Al sites, i.e., Brønsted acid sites. In the case of tungstated-zirconia, the similar effect of WO3 content is expected. The further study on the effect of WO3 content is now under investigation and will be reported in the near future. Conclusion The sensing mechanism of the interdigital capacitor electrodes covering with tungstated-zirconia thick film has been investigated through the quantitative analysis of surface adsorbed species by in situ FT/IR and the kinetic analysis by H/D isotope effect of H2O and NH3. It was confirmed that the electric conduction is basically attributed to protonic conduction on tungstated-zirconia surface. In the presence of gaseous NH3, the conductivity is strongly dependent on the surface concentration of NH4+ adsorbed on acidic OH species. The introduction of gas-phase NH3 results in adsorption of NH4+ on surface acidic OH species of tungstated-zirconia. The surface adsorbed NH4+ plays a role of charge carrier, and thus results in an increase of the electric conductivity. From Arrhenius type temperature dependence of the mobility of NH4+ and the absence of H/D isotope effect of gaseous NH3, it was proposed that the electric conductivity on tungstated-zirconia surface is achieved by NH4+ hopping in between the surface acidic OH species. Acknowledgment. The authors would like to appreciate to Prof. Takashi Hibino and Dr. Masahiro Nagao in Graduate School of Environmental Studies, Nagoya University, for his helpful discussions and suggestions. References and Notes (1) Burch, R. Catal. ReV. Sci. Eng. 2004, 46, 271. (2) Matsumoto, S. Catal. Today 2004, 90, 183. (3) Klingstedt, F.; Arve, K.; Era¨nen, K.; Murzin, D. Y. Acc. Chem. Res. 2006, 39, 273. (4) Koebel, M.; Elsener, M.; Kleemann, M. Catal. Today 2000, 59, 335. (5) Held, W.; Ko¨ning, A.; Richter, T.; Ruppe, L. SAE Paper 1990, 900496. (6) Shirahama, N.; Mochida, I.; Korai, Y.; Choi, K.-H.; Enjoji, T.; Shimohara, T.; Yasutake, S. Appl. Catal., B 2005, 57, 237.

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