D2O Exchange on SnO2 Materials in the Presence of CO

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HO/DO Exchange on SnO Materials in the Presence of CO: Operando Spectroscopic and Electric Resistance Measurements Roman G. Pavelko, Joong-Ki Choi, Atsushi Urakawa, Masayoshi Yuasa, Tetsuya Kida, and Kengo Shimanoe J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 30 Dec 2013 Downloaded from http://pubs.acs.org on December 30, 2013

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The Journal of Physical Chemistry

H2O/D2O Exchange on SnO2 Materials in the Presence of CO: Operando Spectroscopic and Electric Resistance Measurements Roman G. Pavelko†*, Joong-Ki Choi‡, Atsushi Urakawa§, Masayoshi Yuasa†, Tetsuya Kida†, Kengo Shimanoe† † Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, Kasugakoen 6-1, Kasuga, Japan

‡ Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kasugakoen 6-1, Kasuga,

Japan

§ Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, Tarragona, Spain

ABSTRACT: Water isotope exchange in the presence of CO on two undoped tin dioxides has been studied using modulation excitation diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and resistance measurements at 300 oC. Our results reveal that the material synthesized from tin tetrachloride (SnO2 Cl) manifests higher affinity to chemisorbed water than the one made from tin hydroxide acetate (SnO2 Ac). The latter was shown to exhibit a strong correlation between the evolution of surface OH groups (bridging type, involved in hydrogen bonding) and electric resistance upon increasing concentration of CO. Water desorption kinetics, being independent of CO concentration for both materials, was found to be slower for SnO2 Cl by ca. 30% with respect to SnO2 Ac. High affinity to water as well as low sensor signals to CO in humid air reported for SnO2 Cl were proposed to 1 ACS Paragon Plus Environment


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originate from traces of Cl ions (about 0.15 wt% for SnO2 Cl and 0.03 wt% for SnO2 Ac) and not microstructure, which has been confirmed to be similar for both materials. Two types of water adsorption and two CO sensing mechanisms are proposed for SnO2 Cl and SnO2 Ac on the basis of the results.

KEYWORDS: hydroxyl group, SnO2, DRIFTS, modulation, isotope exchange, water adsorption, adsorption kinetics, carbon monoxide, chlorine, gas sensor. INTRODUCTION Surface chemistry of metal oxides and its electronic aspects represent always an acute interest for great variety of applications, including catalysis, gas sensors, batteries, fuel cells and so on 1,2. Isotopic exchange, performed either under steady-state or transient (steady state isotopic transient kinetic analysis; SSITKA) conditions, is one of the few methods for surface characterization, providing intimate insights into reaction intermediates not hindered by spectators 3,4. The main feature of this method is, however, to study surface reactions without changing chemical potential of the reactive species, which cannot be achieved if the concentration of the reactants changes 5. Similar idea drove us to use the isotopic exchange to study electronic processes on the surface 6: in addition to steady-state chemical potential, the exchange induces no electronic effect, which permits to separate chemisorption and formation/consumption of free carriers, since these two phenomena often occur simultaneously 7. In spite of great importance for solid-state electrochemical devices, the isotopic exchange studies are rarely combined with electric measurements. Few examples are predominantly concerned to ionic conductivity in solids for fuel cell applications, e.g. O16/O18 and H2O/D2O

8-11

, H2/D2

12

13-15

. To our best knowledge first employment of isotope exchange to study

electronic effects has been reported by Chen and White 16. Authors used H2/D2 exchange to

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study strong metal-support interaction (SMSI) on titanium oxides doped with Pt. Comparing resistance measurements and exchange rates they proposed that hydrogen chemisorption depends on electronic transport between Pt and the support and is limited by small fraction of Pt sites in contact with TiO2. Recently we have combined H2/D2 exchange, monitored by DRIFTS, with resistance measurements to study correlations between exchange and resistance kinetics for SnO2 materials at 300 oC 6. This very combination permitted us to establish that catalytic formation of water on neat and Pd-doped SnO2 surfaces occurs through “fast” and “slow” hydrogen transfers. While resistance change of Pd-doped material was found to correlate only with formation of water and not with evolution of surface hydroxyls, resistance of the undoped SnO2 was found to have reverse trend. The results suggested us that different surface chemistry is involved in H2 sensing mechanism for both materials. Similar conclusion but for blank SnO2 materials, synthesized using different methods and precursors, can be derived from our another study of isotope exchanges monitored by operando DRIFTS 17. In this paper it was clearly shown that, depending on synthesis route, SnO2 materials manifest very different amounts and types of surface OH groups consumed or formed upon interaction with target gases and water. Consumption and redistribution of surface OH groups have been already reported many times for oxides exposed to N2 19,20

, CO

17,20-22

, C2H5OH 23, CO2 24, NOx

25

and NH3

26

18

, H2

in comparison to pure air. Together

with well-documented electronic effects occurring upon water chemisorption on semiconducting oxides

27

, a detailed investigation on water adsorption is vital not only for

electrochemical applications employing metal oxides but also for any catalytic reaction in the presence of water vapors. In this paper we report a study of water desorption from two undoped SnO2 materials in the presence of CO using H2O/D2O exchange and electric resistance measurements. Two



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materials, synthesized similarly from tin chloride and tin hydroxide acetate, have been chosen after evidencing that sample history and particularly precursor type affect greatly water adsorption on SnO2 materials and hence performance of gas sensors based thereof

17,28

. We

employ the modulation excitation approach, largely its synchronized data acquisition and averaging scheme, to study fast kinetics of D2O desorption, which has been shown to increase signal-to noise ratio of fast spectroscopic measurements

29,30

. On the basis of the obtained

results we infer the types of surface hydroxyls and water adsorption models for the two materials. Finally formation of donor states upon exposure to CO in the presence of water is discussed in the light of different surface reactivity found for the materials. EXPERIMENTAL SECTION Material Synthesis Two materials, compared in this study, were synthesized as follows. Tin(IV) hydroxide acetate and tin(IV) chloride (pentahydrate), dissolved in pure glacial acetic acid and deionized water respectively, were added dropwise to ammonia hydrocarbonate aqueous solution. The resulting precipitates of tin hydroxide were washed first with 0.6 M solution of NH4NO3 to remove Cl- ions and then with deionized water to remove other water-soluble impurities. The drain solutions were checked with AgNO3 to ensure the absence of chloride ions. For hydrothermal treatment 2.7±0.1 g of the washed precipitates was mixed with 200 ml of water. pH of the resulting colloid was adjusted to 10.5 through addition of NH4OH. The mixture was transferred to an autoclave and hydrothermally treated. The colloid was heated up to 200 oC in N2 atmosphere under continuous stirring. After 3 h at 200 oC the autoclave was allowed to cool down naturally. The obtained transparent sols of SnO2 particles were evaporated and dried at 150 oC. Hence, the two materials, being different only in their precursors, will be referred to as SnO2 Ac and SnO2 Cl, respectively.


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Since DRIFTS and resistance measurements required screen-printing and annealing of the materials, the resulting SnO2 powders were mixed with 1,2-propanediol and screen-printed onto alumina foil to perform the same heat treatment as for the operando study on sensors. The annealing protocol assumed heating in ambient air at 100, 200 and 300 oC with 3 h of dwell at each temperature, followed by annealing at 400 oC for 12 h and 580 oC for 3 h. Material Characterization The materials annealed as described above were characterized by means of TEM, XRD and N2 physisorption (BET analysis). TEM was performed using a FEI Tencai F20 microscope operating at 200 kV. X-ray diffraction patterns were recorded using RINT 2100 diffractometer (Rigaku Denki) operating at 60 kV with CuKα radiation. BET surface areas were calculated from N2 adsorption-desorption isotherms using BELSORP-mini II surface analyzer. Before the BET analysis, all the samples were degassed in vacuum (10 Pa) at 150 °C for 1 h. AgNO3 tests for Cl ions have been performed for SnO2 sols obtained after the hydrothermal treatment (HT) and for SnO2 powders obtained after evaporation of the sols at 150 oC. In the former case an excess of AgNO3 was added to the sol to cause precipitation of AgCl. In the latter case the powders (0.3-0.4 g) were ground and washed with hot deionized water in ultrasonic bath for 1 h four times until the Cl test of the drain solution was negative. The drain solutions were then collected and chloride ions were precipitated in a form of AgCl after an excess of AgNO3 has been added. The resulting precipitates have been quantitatively analyzed. DRIFTS and resistance measurements For DRIFTS and resistance measurements the materials were mixed with 1,2-propanediol and screen-printed on the front side of a planar Al2O3 substrate containing Au electrodes for read out of the resistance. The rear of the substrates contained Pt heater, which allowed us to



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control the temperature of the materials. After the deposition, the substrates were kept at room temperature for 12 h and annealed in ambient air step-wise at 100, 200, 300, 400 and 580 oC as described above. The annealed substrates were placed into the home-made environmental chamber for the operando measurements. The chamber, shown in Figure 1 a, was equipped with ZnSe window and electric contacts to read out resistance of the materials in the flow of desired atmosphere. The inner volume of the chamber amounted to 5.0±0.2 cm3 and the flow rate of 50 ccm was used throughout the experiments. The time constants of the set-up for gas exchange, estimated from the absorbance evolution of the CO2 band, were ca. 4 s at room temperature and ca. 3 s at the working sensor temperature. All measurements and preceding stabilization procedures to reach a steady-state resistance have been carried out at 300 oC. First the isotope exchange was performed in 0 ppm CO, and then the CO content in air was step-wise increased: 100, 150, 200, 300 and 500 ppm. The background gas contained 1.3 % H2O, which was replaced with 1.3 % D2O at each CO concentration. The materials were stabilized at least 12 h in the target atmosphere before each measurement.

Figure 1. Environmental chamber for operando DRIFTS and resistance measurements (a), and gas-mixing system to perform H2O/D2O exchange in the presence of CO (1 and 2 are the H2O and D2O bubblers, 3 is the pneumatically actuated 4-way valve).


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Figure 1 b shows the gas-mixing system used to prepare and dose the target gases. In our measurements the pneumatic 4-ways valve (3) was controlled via OPUS 7.0 software and thereby was synchronized with the DRIFT spectral acquisition. The valve permitted to switch between the H2O and D2O feeds within 10 ms and hence no contribution to the time constant of the cell from switching the flows was expected. DRIFTS measurements were performed on a Bruker Vertex 70v spectrometer (MCT detector) operating at a resolution of 4 cm-1 in the rapid scan time-resolved mode with 1 scan per spectrum and 0 s time delay between the spectra. The optical compartment of the spectrometer was evacuated, while sample compartment was purged with N2 during the measurements to minimize interference from the atmospheric gases. The modulation (i.e. periodic H2O/D2O concentration variation) experiment assumed acquisition of 30 spectra in atmosphere containing H2O, then switching to that containing D2O and another 30 spectra were recorded, resulting in 60 spectra per one H2O/D2O modulation period. The H2O/D2O modulation was repeated 20 times (loops) resulting in 1200 spectra acquired for each concentration of CO. The obtained single channel spectra for the last 15 loops (the first 5 loops were omitted to ensure the quasi-steady-state of the isotope exchange was reached) have been used to calculate the averaged spectra of one period of H2O/D2O exchange. Hence, each spectrum of the 60 averaged spectra is the mean of 15 spectra repeated consecutively at the same certain point in time of the H2O/D2O exchange experiment. The averaging procedure increases the signal-to-noise ratio for the rapid scan measurements

29

, as shown in Figures 2a and b. The averaging together with background

correction has been performed using MATLAB 7.11.



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Figure 2. Absorbance evolution at 2700 cm-1 upon one cycle of H2O/D2O exchange (a), the averaged absorbance at the same wavenumber (b). The absorbance spectra were calculated using the last spectrum in the modulation period as the reference. Our analysis will be divided into two parts: steady-state analysis and kinetic study (see Figure 2b). In the former we will use the last spectrum recorded in H2O before the switching to D2O. This spectrum reflects gas-solid equilibrium reached after D2O has been replaced with H2O at different backgrounds of CO. In the kinetic study we will use the first 30 spectra recorded in H2O to compare desorption rate of D2O at different backgrounds of CO. The obtained results will be compared with those of resistance measurement, performed along with the DRIFTS study. Resistance measurements in humid air as well as in the presence of CO in humid air have been recorded under the steady state, reached after ca. 12 h of stabilization in the respective background atmosphere. The results are given in the form of normalized resistance or sensor response, defined as resistance ratio: / , where is the resistance in humid air and is the resistance in humid air containing CO.


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RESULTS Material Characterization After drying at 150 oC SnO2 Ac and SnO2 Cl were found to crystalize in the Cassiterite structure with mean crystallite sizes of 4.4 and 3.2 nm, which is consistent with TEM measurements: 5±1 and 4±1 nm, respectively (not shown). BET analysis revealed that SnO2 Ac has higher pore volume than SnO2 Cl: 0.091 against 0.070 cm3/g. After the screen printing and annealing the pore volumes of the materials were found to be increased, still remaining slightly higher for SnO2 Ac: 0.160 against 0.110 cm3/g. Maximum of pore size distribution was found to be around 8 and 9 nm for SnO2 Ac and SnO2 Cl (insets in Figure 3a and b), respectively. Mean crystallite sizes were found to be rather close for both materials: 12.6 nm for SnO2 Ac and 13.5 nm SnO2 Cl, which is consistent with TEM: 16±5 and 17±6 nm respectively (Figures 3a and b). In summary, two materials are very similar after the annealing. The biggest difference concerns volume of the pores, which is about 40% higher for SnO2 Ac. Taking into account the similar pore, particle and crystallite sizes, the increase in pore volume suggests that the latter material consists of less agglomerated particles.

Figure 3. TEM images for SnO2 Ac (a) and SnO2 Cl (b), both annealed on Al foil using the same protocol as for sensors. Insets show the pore size distribution. 9 ACS Paragon Plus Environment


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Our results of Cl tests were negative for the precipitates before the HT and positive afterwards. The precipitate of AgCl, used as a marker for Cl ions, got dissolved gradually in SnO2 sols due to high concentration of ammonia, which prevented us from quantitative measurements of Cl concentration. However, once the SnO2 sols have been evaporated and the powders were dried at 120 oC contamination with Cl ions has been found ca. 0.03 wt% for SnO2 Ac and 0.15 wt% for SnO2 Cl. Energy dispersive X-ray analysis (EDX) showed no signal for Cl species on both surfaces. DRIFTS: steady state Figure 4 shows the last absorbance spectra in water vapor for the SnO2 materials taken before H2O has been replaced with D2O. The spectral features of H2O/D2O exchange agree well with the literature, manifesting sharp and broad bands at high and low frequencies

17,31

.

For SnO2 Ac and SnO2 Cl, the vibrations at 2740 and 2700 cm-1 are assigned to isolated terminal OD groups (ITOD) 32,33. Bands at lower frequencies – 2670 and 2597 cm-1 for SnO2 Ac and SnO2 Cl – are believed to be specific for isolated bridging ODs (IBOD)

32,33

. The

broad bands at 2527 and 2460 cm-1 have been reliably assigned in the literature as bridging OD groups involved in “hydrogen bonding” with neighboring OD groups (referred to as BBOD)

32-35

. The isotope ratio of 1.36±1, found for all spectral features, confirms the

stretching nature of the observed vibrations and indicates that the OD region reliably reflects the exchanged OH species. To avoid misinterpretation related to any artificial signals in the OH region (e.g. ice formation on the detector, atmospheric impact, undesirable H2O traces in the system, etc.) we will use only OD region in the following analysis. OD bands on SnO2 Cl are shifted towards lower frequencies in comparison with SnO2 Ac. The highest shift of ca. 67 cm-1 was observed for the BOD involved in the bonding (BBOD), while shift of ca. 40 cm-1 was found for terminal ODs. Another difference between the


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materials relates to the amount of ITOD groups, which was found to be about three times higher on SnO2 Cl.

Figure 4. DRIFT spectra of the isotopic exchange recorded under steady state for SnO2 Ac (a) and SnO2 Cl (b). Only OD region (highlighted) will be used in detailed evaluation. As can be seen from Figures 4a and b the amount of OD groups involved in the exchange decreases in the presence of CO for SnO2 Ac and remains fairly constant for SnO2 Cl. To estimate how the total amount of exchangeable OD groups depends on CO concentration in the background gas, first we calculate integral area of the bands between 2760 and 1840 cm-1 and then fit the obtained values with the first-order exponential function:

/

(1)

where A0 is the limiting band area in 100% CO, C is the total change of the band area for the given range of CO concentrations, [CO] is the concentration of CO in air and τ is the constant, which is inversely proportional to the rate of the exponential decay. The exponential function was empirically found to have lower fitting errors in comparison with Freundlichtype adsorption isotherms used to model gas-solid interactions on gas sensors (e.g. ⁄ , where m is the constant, and n ranges between 1 and 0

36-38

). Also Langmuir,

Langmuir-Freundlich and Temkin isotherms, widely used in the modeling of gas adsorption were found to have higher fitting errors than the function (1)39. 11 ACS Paragon Plus Environment


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Figure 5 shows evolution of the band integrals as a function of CO concentration and compares the exponential function with the Freundlich isotherm. The latter fails to fit the data obtained in pure air which was also the case for the rest of the above mentioned isotherms. Most probably the first-order exponent fits the obtained results better due to a complex nature of consumption of surface OD groups in the presence of CO, which cannot be taken into account by the Freundlich isotherm. The process clearly involves surface oxidation reactions, which differs from pure chemisorption (of e.g. water on SnO2) described well with the Langmuir-Freundlich isotherm

28

. Accordingly, surface concentration of OD groups can be

exponentially dependent on CO concentration in air and therefore only function (1) and decay constant (τ) will be used in the following analysis to compare the evolution of spectral features with that of resistance.

Figure 5. Evolution of band integrals between 2760 and 1840 cm-1 as a function of CO concentration in humid air. Full lines represents fitting with the exponential function (1), the dashed line corresponds to the Freundlich isotherm.

It was found that SnO2 Ac manifests a pronounced exponential decrease of surface OD groups with the decay constant (τ) of 234±72 ppm. On the SnO2 Cl surface the amount of the species remains almost unchanged in the presence of CO within an interval of ca. 10 a.u. of 12 ACS Paragon Plus Environment

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the band integral values. The band integrals used to calculate τ represent weighted average values of absorbance which mainly accounts for the dominant species. However, if we calculate τ for absorbance at each of the frequencies between 2760 and 1840 cm-1 τ will be around 230±70 ppm for BBODs and IBODs, while not for ITODs (see the Supporting Information). The bands of the latter were found to be slightly shifted upon exposure to CO, which is discussed in detail in the Supporting Information (Figures S-2a and b). The integral amount of surface ODs seems to be similar for both materials in pure humid air, while in 500 ppm CO the surface of SnO2 Ac contains OD species about 20% less than SnO2 Cl.

DRIFTS: kinetic study As evidenced from the steady state analysis all types of OD species on SnO2 Ac evolve at the same rate upon increase of CO concentration, while those on SnO2 Cl seem to be unchanged in the presence of CO. Reasons for such different behavior of the two surfaces might be related to the fact that OD/OH groups on SnO2 Ac are more basic and therefore more reactive. To compare the reactivity of OD groups on the two surfaces we performed kinetic study of D2O desorption. Analysis of the D2O desorption rates will also help us to differentiate between the fast and slow surface species as well as understand how their reactivity changes in the presence of CO. Desorption rate of D2O will be analyzed using 30 time-resolved spectra recorded in the course of H2O-containig feed with the reference spectrum taken again in D2O at the end of modulation period (see Figure 2b). Originally, the evolution of IR spectra upon periodic changes of the stimulus (in our case 20 loops of H2O/D2O exchange) assumed application of phase sensitive detection (PSD) and in-phase angle analysis

29,30

. Periodical oscillations of the time-resolved spectra occur at

certain frequency close to the frequency of the stimulus. Analysis of the phase lag between



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the different spectral features permits one to distinguish species oscillating at different phase angels thereby suggesting different processes involved in their formation 30. Here, instead of in-phase angle analysis we use the exponential function to fit the absorbance as a function of time as it is shown in Figure 6. The exponential decay function seems to be again appropriate since exchange of surface OD groups is assumed to be limited only by diffusion of H2O to the surface with deuterated hydroxyl groups

40,41

. The time constant – τ – gives similar to

phase lag information about the rate distribution, however in contrast to the phase lag it has higher time resolution, providing the lower fitting errors.

Figure 6. Representation of the fitting process for time-resolved averaged spectra in the ODregion. Figures 7a and b give distribution of the time constant over the OD region calculated from the time-resolved spectra recorded at different backgrounds of CO. For both materials τ seems to be weakly dependent on CO concentration, suggesting that D2O adsorption sites remain the same chemical activity in pure air as well as in the presence of CO. The mean D2O desorption rate seems to be slower by 25-45% on SnO2 Cl when compared with SnO2 Ac.


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Figure 7. Distribution of τ over the region between 2750 and 1900 cm-1 for SnO2 Ac (a) and SnO2 Cl (b) at different concentration of CO (upper part of the plots). Absorbance change upon H2O pulse in pure humid air (bottom part) is shown to confront the spectral features with their τ values. Terminal ODs groups are highlighted with yellow color. The fitting error for both materials does not exceed 0.1 (i.e. τ ±0.1). BOD and TOD groups can be easily defined based on the fact that the lowest τ values (the highest desorption rate) correspond to the frequency region between 2750 and 2630 cm-1 for both materials. In the case of SnO2 Ac, the highest τ (the lowest desorption rate) has been found for a region of 2400-2350 cm-1, which corresponds to BBOD groups. As for IBODs and ITODs, the species were found to desorb at the highest rate with τ of ca. 3±0.1 s, which is close to the time constant of the cell and therefore desorption of these species might be limited by the gas mixing. At frequencies between 2740 and 2670 cm-1 a sharp increase of the time constant is observed, suggesting a small fraction of isolated OD groups (at 2717 cm-1) manifest a desorption rate similar to that of BBODs. For SnO2 Cl the time constant was found to vary between 4.2 and 4.9 s with the highest values between 2470 and 2310 cm-1, corresponding to BBODs. Similarly to SnO2 Ac, the lowest values were found between 2750 and 2630 cm-1. However, in this case the band at 2700 cm-1, assigned to ITOD groups, corresponds well to the increased τ values in this



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region, which suggests that desorption of major part of ITOD groups depends on desorption rate of BODs. Resistance measurements Figure 8 a compares sensor responses to 0-500 ppm CO in humid air for SnO2 Ac and SnO2 Cl obtained in the environmental cell at 300 oC along with DRIFTS measurements. The experimental data were fitted with the exponential function (1) to compare the found trends with those of spectra evolution given in Figures 5 and S-1b. SnO2 Ac manifests sensor signals about two times higher than SnO2 Cl. The growth constant – τ – was found to be lower for the former material, suggesting its response curve is less linear and sensor resistance reaches saturation at lower concentrations of CO. Also for SnO2 Ac τ seems to be very close to the decay constant of band evolution (Figures 5 and S1b).

Figure 8. Sensor response (S=Ra/Rg) to CO in humid air with the reference in humid air (a); resistance evolution as a function of water vapor concentration and corresponding response to H2O with the reference in 1000 ppm H2O in air, i.e. R1000/Rg (b). In the absence of water vapor resistance of the materials is similar, which suggests a similar amount of donor states on both surfaces (Figure 8b). However, when concentration of water vapor increases from 10 to ca. 1000 ppm the resistance drops by 150 and 30 times for SnO2


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Cl and SnO2 Ac, respectively. High resistance change indicates that water chemisorption on SnO2 Cl creates about 5 times more donor states if compared to SnO2 Ac. On the other hand starting from ca. 1000 ppm resistance evolves similarly for both materials, which is reflected by close slopes of the response (dashed lines in Figure 8b). This means that when the two surfaces are slightly hydrolyzed the electronic effects of water chemisorption are similar for both. DISCUSSION Water adsorption According to kinetic study of D2O desorption (Figure 7), adsorption sites for water on the two SnO2 surfaces differ greatly. Namely, the desorption rate for ITODs on SnO2 Ac is the lowest among the surface species, while on SnO2 Cl desorption of ITODs is similar to BODs. Also on the latter surface the amount of ITOD groups is only ca. 20% lower than that of BBODs, while on SnO2 Ac the difference is 80% (compare νITOD at 2700 and 2740 cm-1 in Figures 4a and b). Similar concentration of terminal and bridging hydroxyls on SnO2 Cl means that adsorption sites for water represent a well-known combination of five-fold 34,42 coordinated in-plane Sn atom (i.e. Sn . The two sites give rise to ) and bridging oxygen

ITOD and IBOD (or BBOD) groups respectively in almost equivalent amounts, which also explains similar desorption rate for both surface species on SnO2 Cl. In contrast, on SnO2 Ac water seems to be adsorbed mainly on bridging oxygen vacancies (i.e. two Sn typically surrounded by bridging oxygens to accommodate hydrogen atom). This results in formation of mainly bridging species, as reported for defective oxide surfaces

1,34,43

. The predominant

type of water adsorption on both materials is shown in Figures 9a and b.



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Figure 9. Scale model for predominant type of water adsorption on (110) surfaces of SnO2 Cl (a) and SnO2 Ac (b).

Surface hydroxyl groups The surface OD groups, formed upon adsorption of D2O, manifest different vibrational frequencies for the materials in question. According to Bager’s rule, the lower the vibrational frequency, the longer the distance between the oxygen and hydrogen atoms

44

. Stretching

vibrations of BBOD and ITOD groups on SnO2 Cl are red-shifted by ca. 67 and 40 cm-1 in comparison with SnO2 Ac. The red-shift of the stretching vibrational frequency being directly related to the lower binding energy of the protons in surface OH groups can be caused by several factors 32,33,35,45 and two of them are likely: i- increase of bond order between tin and oxygen atom, constituting the OH group; ii- decrease of coordination number of tin atom bound to the hydroxyl group. A red-shift of up to 100 cm-1 should be expected if the surface oxygen atom increases its coordination number from 1 to 2 or new stronger bond evolves between metal and oxygen atoms, as was observed for Al2O3

33

and MoO3

45

. Influence of the factor (ii) seems to be

smaller since it concerns the atom which is not directly bound to hydrogen

33

. However,

different surrounding of the tin atom can still contribute to the observed band shifts in the materials, as shown in Figure 10.


Page 19 of 45

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Figure 10. Predominant types of bridging ODs on SnO2 Cl and SnO2 Ac, judging from their distinct stretching vibrations, adapted from 33.

Higher bond length of surface O-H groups, as well as higher bond order between tin and surface oxygen (see Figure 10) directly suggest that the binding energy for oxygen species is notably higher on SnO2 Cl. This explains well the slow kinetics of water desorption on this surface. As was shown in Figure 7 the time constants of D2O desorption were found to be higher for SnO2 Cl by ca. 30% in comparison to SnO2 Ac, indicating that the rate determining step in water desorption involves breakage of Sn-O and not H-O bond (otherwise the desorption kinetics would be faster on more acidic surface of SnO2 Cl). CO was found to have no effect on water desorption kinetics, meaning that concentration of surface oxygen species (incl. OHs) as well as interaction between them (incl. hydrogen bonding) has no effect on Sn-O bond strength. This assumption leads us to conclude that CO interacts only with surface species and does not reduce the bulk of the lattice at concentrations as high as 500 ppm in humid air. High binding energies of oxygen suggest that SnO2 Cl possesses high affinity to water. Together with low binding energy of protons in hydroxyls on its surface this explains the drastic change of resistance upon decrease of water concentration from ca. 1000 to 10 ppm in air (Figure 8 b). Interestingly, after the surfaces are exposed to ca. 1000 ppm H2O further



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Page 20 of 45

increase of humidity results in a similar resistance change (or response, see Figure 8 b) for both materials. This indicates that small amount of water easily compensates the large difference in binding energies and electronic effect of water adsorption becomes similar for the undoped SnO2 materials.

CO sensing mechanism In the light of the latter conclusions it becomes natural that SnO2 Ac manifests higher sensor signals to CO in respect to SnO2 Cl. Low sensor signal of the latter corresponds well with the absence of the OD band evolution at various CO concentrations in humid air. Most probably the IR absorbance decreases in the OD region for SnO2 Cl as well, but the change is below the detection limit. In the case of SnO2 Ac the correlation between IR absorption decay (234±72 ppm, Figure 5) and sensor response growth (200±34 ppm, Figure 8a) both as a function of CO concentration suggests that BOH are involved in the formation of surface donor states. Two mechanisms seem to be reasonable to explain the consumption of bridging hydroxyls. First assumes direct interaction of the target gas with surface OHs, which is accepted for various oxidation mechanisms on noble metals and oxide supported catalysts

6,46-51

. This

interaction apparently occurs through formation of hydrocarboxyl radical, leading to formation of formic acid or CO2 51-54 as shown in Figure 11a. In the latter case hydrogen atom has been proposed to recombine with the neighboring OH group and desorb as water molecule

54-57

. Another mechanism, shown in Figure 11b, can be speculated assuming a

competition between water and CO molecules for the same surface oxygen species 17,20.


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Figure 11. Interaction between CO and SnO2 surface: direct consumption of surface BOHs (a) and competitive reaction of CO and H2O with BOs (b). Most probably both mechanisms should be considered functional for SnO2 surfaces. Indeed, taking into account higher acidity of surface hydroxyls on SnO2 Cl, which means shorter residence time and lower diffusion barrier of the proton within the row of bridging oxygens

43,58-60

, it seems reasonable to suppose that for this surface the mechanism

accounting for competitive reaction between CO, H2O for surface oxygen (Figure 11b) is more feasible. Alternatively, surface of SnO2 Ac having highly reactive and basic hydroxyls with higher residence time of the proton on the bridging oxygens, seems to undergo a direct consumption of surface hydroxyls according to Figure 11a.

The effect of the precursor The found difference between the materials apparently stems from different precursors used to synthesize tin hydroxide for the HT, since two materials were found to be very similar in terms of pore distribution, crystallite and particle sizes. The total growth rate of SnO2 crystals during the HT was found to be 2.3±0.3 nm/h for SnO2 Cl and 5±1 nm/h for SnO2 Ac. As known, crystal growth of oxide materials in water media occurs via condensation reactions between hydroxyl groups

61,62

. However, if tin hydroxide contains

residual chlorine, its substitution with OH groups can be rate limiting. Also hydrolytic



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Page 22 of 45

activity of the neighboring to chlorine OH groups can be drastically different from those without the halide. Results of Briois et al. help us to contrast better the possible effect of chloride and organic ligands on SnO2 growth

63

. The authors compared chemistry of SnO2 sol-gel route using tin

tetrachloride and acetylacetone-modified tin chlorides. They found that condensation rate of various acetylacetone-modified tin(IV) chlorides is controlled by substitution of Cl ions. This results in similar particle growth kinetics for both precursors containing only Cl ligands and with Cl ligands partially substituted by acetylacetonates. Thus, it appears likely that growth rate of SnO2 is retarded by Cl ions which are released in solution as the condensation progresses. This assumption is supported by Fujihara et al., who used method similar to us to synthesize SnO2 sol

64

. The authors reported that Cl ions were not found by AgNO3 test

before the HT, however after the treatment formation of AgCl was observed in the supernatant water. We confirm the same for both mother solutions tested with AgNO3 after the HT. For dry SnO2 powders contamination with Cl ions has been found about 0.03 wt% and 0.15 wt% for SnO2 Ac and SnO2 Cl, respectively. Despite positive Cl tests, EDX showed no signal for Cl species on both surfaces, which is again in agreement with Fujihara et al, who used XPS and found no contaminations for all their samples 64. The Cl ions are known to be stable on oxide surfaces up to 700 oC 65. Presence of Cl ions in SnO2 Cl sample is hypothesized to be the main reason for the observed difference in surface activity and sensing properties between the materials in question.

CONCLUSION Traces of Cl in SnO2 materials with concentration of around 0.15 wt% seem to induce dramatic changes in water adsorption kinetics and CO sensing mechanism. The material


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synthesized from Cl-free precursor manifests more basic surface hydroxyls with higher desorption rate of water. Electronic effect of CO chemisorption quantitatively correlates with consumption of bridging hydroxyls on the latter surface, while no such correlation was found for the material synthesized from tin chloride. For SnO2 Ac a direct consumption of bridging hydroxyls is proposed as the main reaction path to produce surface donor states. On SnO2 Cl a competitive reaction between CO and H2O with surface oxygen species is believed to be the predominant type of sensing mechanism. For both materials CO induces no changes in water desorption kinetics. On SnO2 Ac adsorption of CO perturbs slightly the isolated terminal OH groups, which were found however to have a low impact on formation of donor states.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]; address: Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, Kasuga-Koen 6-1, Kasugashi, Fukuoka 816-8580, Japan; telephone: +81-(0)-92-583-7876, fax:+81-(0)-92-583-7538.

ACKNOWLEDGEMENT RGP gratefully acknowledges a scholarship from the Japan Society for the Promotion of Science and financial support from JSPS grant-in-aid No 23·01343. RGP and J-KC are thankful to all PhD students of Dr. Urakawa, as well as to Prof. E. Llobet and Dr. R. Calavia for miscellaneous support in the experimental implementation of the operando measurements.



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ASSOCIATED CONTENT Supporting Information Text and four figures related to the calculation of the decay constant (τ) of absorbance at each frequency between 2760 and 1840 cm-1 for steady state condition. This material is available free of charge via the Internet at http://pubs.acs.org

ABBREVIATIONS BOD, bridging OD group; BBOD, bonded bridging OD group; BO, bridging oxygens; TOD, terminal hydroxyl; ITOD, isolated terminal OD group; BOH, bridging OH group.

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Figure 1. Environmental chamber for operando DRIFTS and resistance measurements (a), and gas-mixing system to perform H2O/D2O exchange in the presence of CO (1 and 2 are the H2O and D2O bubblers, 3 is the pneumatically actuated 4-way valve). 63x23mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2. Absorbance evolution at 2700 cm-1 upon one cycle of H2O/D2O exchange (a), the averaged absorbance at the same wavenumber (b). 65x24mm (300 x 300 DPI)



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Figure 3. TEM images for SnO2 Ac (a) and SnO2 Cl (b), both annealed on Al foil using the same protocol as for sensors. Insets show the pore size distribution. 165x80mm (300 x 300 DPI)


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DRIFT spectra of the isotopic exchange recorded under steady state for SnO2 Ac (a) and SnO2 Cl (b). Only OD region (highlighted) will be used in detailed evaluation. 65x25mm (300 x 300 DPI)



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Figure 5. Evolution of band integrals between 2760 and 1840 cm-1 as a function of CO concentration in humid air. Full lines represents fitting with the exponential function (1), the dashed line corresponds to the Freundlich isotherm. 65x52mm (300 x 300 DPI)


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Representation of the fitting process for time-resolved averaged spectra in the OD-region. 54x36mm (300 x 300 DPI)



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Distribution of τ over the region between 2750 and 1900 cm-1 for SnO2 Ac (a) and SnO2 Cl (b) at different concentration of CO (upper part of the plots). Absorbance change upon H2O pulse in pure humid air (bottom part) is shown to confront the spectral features with their τ values. Terminal ODs groups are highlighted with yellow color. The fitting error for both materials does not exceed 0.1 (i.e. τ ±0.1). 65x24mm (300 x 300 DPI)


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Figure 8. Sensor response (S=Ra/Rg) to CO in humid air with the reference in humid air (a); resistance evolution as a function of water vapor concentration and corresponding response to H2O with the reference in 1000 ppm H2O in air, i.e. R1000/Rg (b). 65x24mm (300 x 300 DPI)



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Scale model for predominant type of water adsorption on (110) surfaces of SnO2 Cl (a) and SnO2 Ac (b). 46x12mm (300 x 300 DPI)


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Predominant types of bridging ODs on SnO2 Cl and SnO2 Ac, judging from their distinct stretching vibrations, adapted from 22b. 54x19mm (300 x 300 DPI)



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Interaction between CO and SnO2 surface: direct consumption of surface BOHs (a) and competitive reaction of CO and H2O with BOs (b). 46x12mm (300 x 300 DPI)


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TOC 86x67mm (300 x 300 DPI)


D2O Exchange on SnO2 Materials in the Presence of CO

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