Active Sites on Nanocrystalline Tin Dioxide Surface: Effect of

Aug 29, 2014 - (22) The analytic EPR spectra were recorded at 110 K. Besides, EPR spectra registration at 5 K under liquid helium cooling and on heati...
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Active Sites on Nanocrystalline Tin Dioxide Surface: Effect of Palladium and Ruthenium Oxides Clusters Artem V. Marikutsa, Marina N. Rumyantseva,* Elizaveta A. Konstantinova, Tatyana B. Shatalova, and Alexander M. Gaskov Department of Chemistry, Moscow State University, Vorobyevy Gory 1-3, Moscow 119991, Russia S Supporting Information *

ABSTRACT: Active sites of nanocrystalline tin dioxide materials with variable particle size, surface area, and catalytic modifiers were studied. Effect of palladium and ruthenium oxides clusters on the activity and concentration of tin dioxide surface centers was evaluated by temperature-programmed desorption techniques using probe molecules, FTIR spectroscopy, EPR, and thermogravimetric methods. The surface site concentration decrease was observed with an increase of SnO2 particle size and BET area decrease. The active sites of SnO2 were found to be selectively promoted by the additives. Accumulation of surface OH groups including hydroxyl spin centers and Broensted acid sites was characteristic for SnO2/PdOx nanocomposites as a result of water chemisorption enhancement due to proposed electronic clusters−support interaction. Ruthenium oxide was shown to increase the concentration of chemisorbed oxygen species via oxygen spillover route.

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INTRODUCTION Nanocrystalline tin dioxide is one of the most utilized materials for semiconductor gas sensors, as it combines appropriate structural, adsorptive, and electrophysical properties.1,2 The advantages of tin dioxide based chemical sensors include sensitivity to low concentrations (about threshold limit value) of toxic air pollutants such as CO,3,4 NOx,4,5 H2S,6 and NH3,7 stability in air, and low cost.2 Semiconductor sensors operation in its essence is a transformation of chemical information on ambient gas (usually, air) composition into an electric signal measured in dc mode as a resistance (conductance) response. Yamazoe et al. distinguish the main stages of gas sensing process: reception and transduction.8,10 Reception includes reversible redox chemical interactions of gas molecules with sensor surface. Consequent charge density redistribution between the semiconductor surface and bulk gives rise to electric transduction phenomena. To enhance the sensitivity, nanosized materials with increased surface-to-volume ratio are of use. The materials with particle size less than 10 nm and surface area higher than 100 m2/g are of particular interest.9 From the transduction point of view, decreasing the particle size in the nanometer range increases the impact of surface phenomena to conductance. The electrophysical concepts of sensor signal dependence on particle size were thoroughly studied by Yamazoe et al.,8,10,11 Barsan et al.,12 Trakhtenberg et al.,13 and Zaretskiy et al.14 Finding an optimal nanoparticle size corresponding to a high surface sensitivity combination with measurable bulk conductivity was an encouraging challenge in these studies. Recently, the role of pore size affecting diffusion in a sensing layer was found critical, as it determined opposite trends in the dependence of sensitivity to reducing gases on SnO2 particle size.15 However, the implication of increased © 2014 American Chemical Society

active surface area of sensor nanomaterials for the gas reception processes which are primarily chemical phenomena has hardly been considered. The main limitation of semiconductor metal oxide sensors is the lack of selectivity, especially when the analytes belong to the same redox gas group, as do most toxic gases. For example, CO, NH3, H2S, and volatile organics are reductive gases; nitrogen dioxide, ozone, and halogens are oxidative ones. Redox interactions with metal oxide surface being nonselective, particular molecules from these groups contribute indistinguishably to the integral sensor response. An effective tool to increase sensor materials selectivity is chemical modification by noble metal additives.2−5 It is based on an assumption that gas reception is determined by active sites on the sensor surface. Once introduced, the modifiers create specific catalytic sites which selectively promote the interaction with particular reductive or oxidative gases. Recently, the possibility to improve selectivity to volatile organics depending on the molecule structure was demonstrated through the modification of the active sites on tin dioxide surface.16 An active site is a local (of an atomic or molecular size range order) surface region possessing definite chemical properties.17 According to the bulk and surface structure of tin dioxide, its intrinsic surface sites include undercoordinated tin cations, surface anions O2− and oxygen vacancies, and chemisorbed molecular water and OH groups that constitute the hydrated surface species.1,17 Once introduced, catalytic additives form extrinsic sites, usually treated as clusters attached to the SnO2 surface.2 Received: July 18, 2014 Revised: August 29, 2014 Published: August 29, 2014 21541

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Table 1. Materials Composition and Microstructure Characteristics particle size, nm sample

Tanneal (SnO2), °C

SnO2-300 SnO2-300/PdOx SnO2-300/RuOy SnO2-500 SnO2-500/PdOx SnO2-500/RuOy SnO2-700 SnO2-700/PdOx SnO2-700/RuOy SnO2-1000 SnO2-1000/PdOx SnO2-1000/RuOy

300 300 300 500 500 500 700 700 700 1000 1000 1000

crystal phasea,b cassiterite cassiterite cassiterite cassiterite cassiterite cassiterite cassiterite cassiterite cassiterite cassiterite cassiterite cassiterite

SnO2a,c

(SnO2) (SnO2) (SnO2) (SnO2) (SnO2) (SnO2) (SnO2) (SnO2) (SnO2) (SnO2) (SnO2) (SnO2)

3−6 3−6 3−6 10−15 10−15 10−15 20−30 20−30 20−30 35−50 35−50 35−50

modifierd 1−3 2−5

2−5 3−8

BET area, m2/g 95−100 90−95 90−95 20−25 20−25 20−25 7−10 7−10 7−10 2−5 2−5 2−5

a According to XRD.19,25,27 bAccording to electron diffraction.18,19,27 cEvaluated by TEM.18,19,27 dEstimated by HRTEM, STEM, and EDX mapping.26

and ground. After this it was divided into four parts, which were annealed at different temperatures of 300, 500, 700, and 1000 °C for 24 h. The modification by 1 wt % of PdOx or RuOy was performed via the obtained tin dioxide impregnation by the noble metal acetylacetonate precursors.25,27 In a typical procedure, nanocrystalline SnO2 powder was stirred with ethanol solution of Pd(acac)2 or Ru(acac)3 under heating to 100 °C to evaporate the solvent. The as-obtained deposits were annealed to decompose the acetylacetonate precursors: SnO2/ Pd(acac)2 at 225 °C and SnO2/Ru(acac)3 at 265 °C for 24 h. The materials characterization by X-ray diffraction (XRD), surface area measurements via nitrogen adsorption (BET), high-resolution transmission electron microscopy (HRTEM), electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), X-ray near-edge absorption spectroscopy (XANES), and extended X-ray absorption fine structure (EXAFS) was discussed previously.18,19,25−27 The sample composition and microstructure parameters are summarized in Table 1. The active sites of the materials were investigated by different techniques according to their origin and chemical properties. Oxidative sites were examined by temperature-programmed reduction with hydrogen (TPR-H2) using a Chemisorb 2750 (Micromeritics) instrument. The powders (∼50 mg) were tested without pretreatment by heating to 800 °C at a rate of 10 °C/min under 10 vol % H2 in argon gas flow (10 mL/min). Acid sites were tested by temperature-programmed desorption of ammonia (TPD-NH3) using the Chemisorb 2750 instrument. Before measuring, the granulated powders (∼100 mg) were pretreated under helium at 200 °C, cooled under synthetic air to room temperature, and saturated with 5 vol % NH3 in N2. The TPD patterns were registered during the heating of samples to 800 °C at a rate of 8 °C/min under He flow (30 mL/min). The concentrations of acid sites were calculated from desorbed ammonia volume supposing single NH3 molecule per site occupation. Surface spin centers were studied by EPR spectroscopy; the instrumentation and procedure details can be found elsewhere.22 The analytic EPR spectra were recorded at 110 K. Besides, EPR spectra registration at 5 K under liquid helium cooling and on heating to 200−450 °C was also performed. Hydrated surface species were characterized by FTIR using a PerkinElmer Spectrum One spectrometer. FTIR spectra were recorded in transmission mode in the 4000−1000 cm−1 wavenumber region with

In recent studies we observed that nanocrystalline SnO2 modification by palladium and ruthenium oxides yielded nanocomposites with specific gas sensitivity to CO and NH3, respectively.18,19 Catalytic impact of RuO2 clusters to deep ammonia oxidation yielding nitrogen oxides was proposed as a reason for drastic NH3-sensitivity increase at raised temperature (150−200 °C).19 In fact, ruthenium- and palladium-based systems are efficient selective heterogeneous catalysts of the reactions of NH3 and CO oxidation, respectively, their specificity being due to relevant metal−gas and metal−oxygen bond energies.20,21 However, in the case of SnO2/PdOx interaction with CO the observed sensor characteristics, e.g., alteration of the sensor signal temperature profile, maximum response shift to near-room-temperature range (25−50 °C), and promoting effect of humidity, could not be addressed solely to the catalytic action of PdOx clusters.18 Previously Barsan et al. had established Pd-induced CO + OH groups reaction taking place during the sensing process; however, the role of additive was unclear.22−24 We detected this interaction proceeding at temperatures as low as room temperature on SnO2/PdOx surface, concomitant with direct CO oxidation by palladium oxide.18,25 Yet a relation between these features and their impact to CO sensing process were not defined. Recently nanocrystalline SnO2 oxygen exchange was found to be accelerated by RuOy clusters that mirrored the modifier effect on surface oxygen species.26 Thus, an understanding of surface phenomena and especially gas sensitization provided by chemical modification cannot be advanced with a neglect of the interference between extrinsic (modifier) and intrinsic active sites on tin dioxide surface. The present paper is to summarize qualitative and quantitative effects of microstructural parameters and catalytic additives on the active sites of nanocrystalline tin dioxide modified by palladium and ruthenium oxides.

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MATERIALS AND METHODS Nanocrystalline SnO2, SnO2/PdOx, and SnO2/RuOy materials were used for the study. Tin dioxide samples with variable mean particle size and BET surface area were obtained by aqueous hydrolysis route.18 SnCl4·5H2O (>99% pure, SigmaAldrich) was dissolved in deionized water. After that, 1 M ammonia solution was dropwise added until pH 6 was reached. The precipitate of SnO2·nH2O was washed several times with deionized water to remove chloride ions, dried in air at 50 °C, 21542

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Table 2. Active Sites Concentration on the Surface of Blank and Modified Nanocrystalline Tin Dioxide active site concentration, μmol/m2 spin center,c ×10−6

acid siteb sample SnO2-300 SnO2-300/PdOx SnO2-300/RuOy SnO2-500 SnO2-500/PdOx SnO2-500/RuOy SnO2-700 SnO2-700/PdOx SnO2-700/RuOy SnO2-1000 SnO2-1000/PdOx SnO2-1000/RuOy a

modifier 0.93 ± 0.05, including 70 atom % PdO (amorphous), 25 atom % Pd0, 5 atom % Pd3+ 0.84 ± 0.05, including 80 atom % RuO2 (struct), 20 atom % Ru3+

hydrated speciesd

oxidativea (O2,chem)

Brønsted

Lewis

OH·

O2−

H2Osurf

OHsurf

11.8 ± 0.4 13.6 ± 0.5

0.6 ± 0.2 0.9 ± 0.2

2.4 ± 0.5 3.2 ± 0.4

2.6 ± 0.3 5.2 ± 0.5

14 ± 2 9±1

3.0−3.5 4.1−4.7

8.5−8.8 14.0−14.5

14.9 ± 0.5

0.6 ± 0.2

2.1 ± 0.2

2.1 ± 0.3

17 ± 3

3.5−4.0

12.0−12.4

7.6 ± 0.6 8.8 ± 0.7 12.2 ± 0.5

0.5 0.8 0.5 0.5 0.9 0.6 0.6 0.9 0.6

± ± ± ± ± ± ± ± ±

0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.4 0.3

1.5 2.1 1.3 1.2 1.3 1.3 0.7 1.1 0.8

± ± ± ± ± ± ± ± ±

0.4 0.3 0.2 0.3 0.2 0.1 0.4 0.5 0.4

2.5 ± 0.7 total

0.3 ± 0.1 total

From TPR-H2. bFrom TPD-NH3. cEvaluated by EPR. dEvaluated by TG-DSC.

resolution of 4 cm−1. Powders (∼5 mg) were ground with 100 mg of KBr and pressed into pellets. The hydrated species concentration was evaluated by TG−DSC analysis with the use of a NETZSCH STA 409 PC instrument. During the measurements about 20 mg of a powder was heated to 800 °C at a rate of 10 °C/min under argon flux (10 mL/min).

dioxide is well studied by either theoretical or experimental methods.29 It is considered to result in several possible surface species, the predominant type of which depends on temperature:12,29 physisorbed O2,phys (T < −70 °C), molecular chemisorbed O2,chem and ionosorbed O2− (T < 160−200 °C), atomic ionosorbed O− (200 < T < 450 °C), and O2− (T > 450 °C). In TPR-H2 experiments surface oxidative species provide hydrogen consumption at lower temperature range (Figure 1), the high-temperature peaks at 450−650 °C being due to tin dioxide bulk reduction.27 Taking into account that hydroxyl groups were also reported to possibly oxidize H2,30 the processes of surface species reduction at T = 100−230 °C could be summarized by the following schemes:

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RESULTS AND DISCUSSION The experimentally determined active sites on the surface of nanocrystalline tin dioxide based materials were divided into the following groups according to their origin and chemical activity. (a) Modifiers. Palladium and ruthenium oxides formed extrinsic surface sites on tin dioxide surface. By HRTEM, STEM, and EDX mapping, the additives were shown to segregate in the form of clusters attached to SnO2 nanoparticles.19,26,27 HRTEM and STEM micrographs with EDX maps for SnO2-700/PdOx, SnO2-300/PdOx, and SnO2-300/ RuOy can be found in Supporting Information. From XANES, EXAFS, XPS, and EPR studies18,19,25,26 the additives were found in mixed-valence oxidized states as indicated in Table 2. The PdO x composition corresponds to predominantly amorphous PdO mixed with Pd0 and a small fraction of Pd3+. Though the localization of different oxidation states could not be determined, an assumption was made that the clusters had complex structure SnO2−Pd3+/Pd0/PdO, Pd3+ being stabilized in the interface with tin dioxide, while Pd0 and PdO supposedly constituted outer layers of the clusters.25 Predominantly structured RuO2 was found in SnO2/RuOy; however, up to 20 atom % of the additive was in the Ru3+ state.19 It was assumed to result from an incomplete oxidation of the Ru(acac)3 precursor during the synthesis of the nanocomposites. EPR data showed that Ru3+ was mostly a surface species readily oxidized after heating or gas exposure; however, about 1 atom % of Ru3+ was supposedly stabilized in SnO2 subsurface lattice.19,25 Supporting this assumption is the proximity of Sn4+ (0.69 Å) and Ru3+ (0.68 Å) six-coordinated cations radii.28 (b) Oxidative Sites. Oxidative surface sites were attributed to chemisorbed oxygen species. Oxygen adsorption on tin

O2,chem + 2H 2 → 2H 2O −

O2 + 2H 2 → 2H 2O + e −

O + H 2 → H 2O + e

−

2OHsurf + H 2 → 2H 2O

(1) −

(2) (3) (4)

Because of the lack of selectivity, these species could not be distinguished by TPR-H2. Yet the oxidative sites concentration was calculated in an account of molecular chemisorbed species O2,chem (Table 2). First, the temperature of surface species reduction corresponds to molecular adsorbed oxygen predominance,12,29 while the total amount of oxidative sites exceeds by 6 orders the concentration of ionosorbed O2− estimated by EPR (Table 2). Second, the entire hydroxyl concentration was estimated by TG-DSC to a value close to the O2,chem concentration obtained from TPR-H2 (about 10 μmol/m2). However, in the reaction with hydrogen the stoichometric ratio of OHsurf/H2 = 2:1 (eq 4) is 4 times higher than that of O2,chem/ H2 = 1:2 (eq 1). Hence, in an account of solely hydroxyl reduction the TPR-H2 estimated OHsurf concentration would be 4 times higher than that of O2,chem, i.e., higher than 40 μmol/ m2. It means that even if all the hydroxyls participated in reaction 4, their actual concentration independently measured by TG-DSC is about 4 times lower than the total amount of oxidative sites obtained from TPR-H2. The oxidative species concentration decreased with tin dioxide particle size so that on 21543

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ΔHdes/ΔSdes of probe molecules desorption is a characteristic of acid sites strength. Weak (Brønsted) acid sites bring about low-temperature (T < 200 °C) desorption peaks on the TPDNH3 spectra.32 These sites are recognized as surface OH groups having an acidic character, usually ascribed to rooted (or bridging) hydroxyls.30 At temperature T > 200 °C ammonia desorbs from Lewis acid sites, i.e., coordinately unsaturated tin cations (Sn4+cus). In some studies medium (T = 200−400 °C) and strong Lewis sites (T > 400 °C) are distinguished in this temperature region.32,33 The former was assigned to coordinately unsaturated (cus) Sn4+cus cations, the latter to Sn2+cus on a reduced surface adsorbing NH3 stronger because of covalent binding.33 In our case the pretreatment of samples included heating in air so that such attribution was excluded. Wide dispersion of TPD curves in high-temperature range (Figure 2)

Figure 1. TPR-H2 profiles of blank (1), PdOx- (2), and RuOy-modified (3) samples based on nanocrystalline SnO2 annealed at different temperatures: 300 °C (a), 500 °C (b), 700 °C (c), and 1000 °C (d).

the materials with BET area lower than 20 m2/g they could not be quantified. Nanocrystalline SnO2 modification resulted in an increase of oxygen species concentration, the effect being larger for SnO2/ RuOy (Table 2). It could be explained from oxygen isotopic exchange studies by the facilitation of O2 spillover on the noble metal oxide clusters.27 The higher promoting effect of RuOy clusters was addressed to higher oxygen adsorption energy and Ru4+/Ru3+ interplay.26 Importantly, spillover mechanism implies O2 dissociation on the catalytic clusters yielding atomic oxygen,26,27 the species regarded as much more reactive than molecular ones.31 In the case of SnO2/RuOy the spillovermediated oxygen exchange was observed to take place already at 200 °C, while for it to proceed on blank and PdOx-modified materials, heating to T > 310 °C was necessary.26 (c) Acidic Sites. Acid sites play a key role in the interaction with basic molecules like NH3. Ammonia serves as probe molecule for these species examination by TPD-NH3. The thermodynamic condition of spontaneous desorption is ΔG = ΔHdes − T ΔSdes < 0

Figure 2. TPD-NH3 profiles of blank (1), PdOx- (2), and RuOymodified (3) samples based on nanocrystalline SnO2 annealed at different temperatures: 300 °C (a), 500 °C (b), 700 °C (c), and 1000 °C (d).

(5)

might arise from local coordination distortions of Sn4+cus on the surface of nanocrystalline tin dioxide. Furthermore, these desorption curves shift to the region characteristic of medium Lewis sites (200−400 °C) as SnO2 particle size increases (Figure 2). This could reflect an increasing number of Sn4+cus sites with minimum coordination vacancies and lower acidity in comparison with highly distorted sites anticipated on the

where desorption enthalpy ΔHdes is positive (desorption is an endothermic process) and entropy ΔSdes > 0 due to gas evolvement. The desorption enthalpy depends on bond strength with the adsorbate. Since entropy value depends on gas molecules quantity (taken as one NH3 molecule per site) rather than on the type of adsorption site, the temperature T ≥ 21544

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surface of small nanocrystals. The total concentration of Lewis acid sites decreased as tin dioxide particle size increased and BET surface area decreased. The effect of additives on strong acid sites could not be estimated unambiguously. On the one hand, the highertemperature desorption peaks emergence at T ≤ 350 °C implying only medium Lewis sites to exist on the nanocomposites surface might reflect the Sn4+cus coordination saturation due to the modifiers-induced increase of chemisorbed oxygen concentration (Table 2). However, a side process of NH3 oxidation catalyzed by the additives is not excluded, which avoids Lewis sites quantification on the noble metal oxide modified SnO2 surface. However, a systematic increase of Brønsted sites concentration on the SnO2/PdOx nanocomposites, in contrast to blank and RuOy-modified samples, was observed (Table 2). No significant effect of the difference of tin dioxide particle size and BET area on the concentration of weak acid sites on its surface was observed. (d) Surface Spin Centers. In the EPR spectra paramagnetic surface sites determine a complex signal at H = 3350−3360 G, its integrated intensity decreasing with tin dioxide BET area (Figure 3). Surface O2− and OH· species

Figure 4. ERP spectrum of SnO2-300 sample registered at 5 K.

EPR signal width (ΔH) is the reciprocal of the excited spin lifetime (Δτ) according to the uncertainty principle:36 ΔE Δτ = gμB ΔH Δτ ≥ ℏ

(6)

where ΔE is the excitation energy, g is the g-factor, μB is the Bohr magneton, and ℏ is the Planck constant. The reason for the excited spin lifetime shortening upon heating is the induction of spin−lattice relaxation. High susceptibility to temperature-induced phonon vibrations suggests that VO• is bulk rather than surface spin centers. The PdOx- and RuOy-modified tin dioxide materials possessed the same surface paramagnetic sites except oxygen vacancies and paramagnetic species of modifiers (Pd3+ and Ru3+, respectively).25 No VO• centers were detected in the nanocomposites, supposedly for the reason of electronaccepting action of the additives (see discussion below) facilitating full ionization of the oxygen vacancies. Moreover, the modifier species (Pd3+ and Ru3+vol) assumed to partly incorporate in the SnO2 lattice could serve as acceptor bulk defects compensating the donor-like oxygen vacancies. In addition, the concentration of Pd3+ in SnO2/PdOx (∼4 × 10−6 mol/g) and Ru3+vol in SnO2/RuOy (∼8 × 10−7 mol/g) exceeds that of VO• in tin dioxide (∼3 × 10−8 mol/g). Concerning the intrinsic surface spin sites of SnO2, the introduction of RuOy had no detectable impact on their concentration.25 But on the surface of SnO2/PdOx a twice increased concentration of OH· and a 1.5-fold decreased amount of O2− were determined (Table 2). The promotion of hydroxyl radicals formation could be due to PdOx-enhanced surface hydration at the expense of ionosorbed oxygen depletion:

Figure 3. Experimental EPR spectra of nanocrystalline tin dioxide samples: SnO2-1000 (1), SnO2-700 (2), and SnO2-300 (3) registered at 110 K. Simulated spectra of O2− (4) and OH· (5) fitted to the experimental SnO2-300 spectrum.

were recognized corresponding to this signal.25,34 Their concentrations were discerned via experimental spectra simulation as a superposition of spin centers signals (Figure 3). Yet it was performed only for ultradispersive samples based on the SnO2-300 matrix. On the rougher samples integral O2− + OH· amount could only be estimated because of the low spin centers amount (Table 2). The total spin centers concentration decreased as tin dioxide particle size increased and BET surface area decreased. Recently we have shown that the concentration of hydroxyl radicals conformed to humidity variations because of the proposed equilibrium with chemisorbed water.34 Besides, registering EPR spectra at liquid helium temperature, a lowintense and broadened (ΔH ≈ 20 G) signal with g-factor 1.9812 was detected (Figure 4). According to the literature,35 it was assigned to single-charged oxygen vacancies VO•. The concentration of VO• sites of about 2 × 1016 g−1 (versus ∼1015 g−1 for the sum of O2− and OH· amount) was obtained for unmodified SnO2-300 (Table 2). In the spectra measured at higher temperatures (e.g., at 110 K, Figure 3) this signal results from an extensive broadening which, provided with a constant spin centers concentration, leads to the amplitude decrease. An

O2− + 2H 2Osurf = OH− + 3OH·

(7)

Registering the EPR spectra at room temperature and on heating to 200−450 °C revealed that both O2− and OH· species exist on the surface of blank and modified tin dioxide, implying their possible participation in gas sensing, as the SnO2-based sensors are typically operated at these temperatures.3−6 However, these active sites concentration cannot be calculated because of extreme signal widening, nor would in situ EPR studies be of use to define their role in the interaction with gas molecules. (e) Hydrated Species. A variety of hydrated species was deduced from FTIR data (Figures 5 and 6) to exist on the surface of nanocrystalline SnO2. For the sake of simplicity these species could be grouped in molecular (denoted as H2Osurf) and dissociated (OHsurf) ones. The bending H−O−H vibrations band at 1620 cm−1 is characteristic of the former. The broad band at 3650−2000 cm−1 of stretching O−H 21545

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Figure 5. FTIR spectra of SnO2-300 sample pretreated under variable relative humidity (RH): 1% RH (1), 20% RH (2), and 35% RH (3).

Figure 7. Thermograms (a) and DSC curves (b) of SnO2-300 sample pretreated under variable relative humidity (RH): 1% RH (1), 20% RH (2), and 35% RH (3).

Figure 6. FTIR spectra of nanocrystalline blank and modified tin dioxide samples: SnO2-300 (1), SnO2-300/PdOx (2), and SnO2-300/ RuOy (3).

at 200 °C < T < 600 °C was due to desorption of surface OH groups bound to the surface by stronger covalent bonds for which cleavage required heating to such high temperatures. This assignment agrees with previously reported thermodesorption data.37 Low-temperature shifting of the mass-loss regions (Figure 7a) and of the endothermic DSC bends (Figure 7b) is notable with the humidity increase. It can be accounted for by a decrease of hydrated species binding energy as a result of the surface hydration during competitive H2O chemisorption. Calculating the hydrated species concentrations from the mass loss values confirmed the predominance of OHsurf on the surface of materials (Table 2). It can be noted that surface hydroxyls include but are not restricted to the discussed above weak acid sites and paramagnetic OH· species. From the total OHsurf concentration of ∼10 mmol/m2 less than 10% of the sites perform Brønsted acidity, while the OH· percentage is as low as 10−4 %. Tin dioxide modification by PdOx and RuOy additives resulted in such a hydrated layer modulation that rooted hydroxyls OH···OH associated by hydrogen bonds were the predominant surface hydrated species, as follows from a comparison of FTIR spectra (Figure 6). The lower vibrational wavenumber (3400−3200 cm−1) of OH···OH compared to isolated OH species (3640−3500 cm−1) mirrors the O−H bond loosening and strengthening of that of M−OH that is typical for rooted hydroxyls.30 The reason for increased impact of OH···OH to the hydrated species of modified samples might be a larger concentration of rooted OH groups and/or their higher acidic nature, which for SnO2/PdOx agrees with TPDNH3 data. Noteworthy, with the FTIR patterns of modified samples looking similar, the OHsurf and H2Osurf absorption

vibrations is contributed by either hydrated species. The following adsorbates could be distinguished from this spectral region: terminal OH groups (3640−3600 cm−1),23 isolated rooted OH groups (3600−3500 cm−1),23,24 associates of Hbound rooted hydroxyls OH···OH and chemisorbed H2O (3400−3200 cm−1),23,24,30 protonated molecular species H3O+ (3300−3150, 2650−2470, and 1700−1670 cm−1),24 and H5O2+ (3000−2850, 2250−2200, and 1700−1660 cm−1).24 The stretching band intensity and broadness indicate, in a complex, the content of the hydrated species; however surface OH groups are the prevailing ones on the surface of nanocrystalline SnO2 (Figure 5). Because of the wide variety of hydrated surface species and a strong interference of corresponding IR signals, their particular concentrations could not be estimated. Yet it was done for the average amounts of molecular and dissociated aqueous adsorbates by means of thermogravimetric analysis. Figure 7 illustrates the TG-DSC results for SnO2-300 sample pretreated under variable humidity. Two mass loss regions were distinguished on the thermograms, the amplitudes of both of them increasing with humidity (Figure 7a). Despite the low intensity of the calorimetric signals, two bends of the DSC curves could be noted at ∼100 and 450−550 °C, the temperatures corresponding to intense mass loss on both the stages (Figure 7b). Emergence of the bends on DSC curves, in which general rising was due to sample heating, corresponds to some heat consumption, presumably as a result of endothermic desorption processes. Thus, the mass loss at T < 200 °C was attributed to molecular water desorption (Figure 7a), while that 21546

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bands of SnO2/PdOx were more intense (Figure 6). In line with this, thermogravimetric study revealed tin dioxide surface enrichment by both molecular and dissociated hydrated species due to modification by PdOx and RuOy, other things being equal (Figure 8). The quantitative data derived from TG-DSC indicate increasing surface concentrations of H2Osurf and OHsurf: SnO2 < SnO2/RuOy < SnO2/PdOx (Table 2).

Figure 9. Schematic band diagrams of cluster−support interface in SnO2/PdOx (a) and SnO2/RuOy (b) nanocomposites. E(PdO/Pd) and E(RuO2/Ru3+) are the absolute electrode potentials of PdO/Pd and RuO2/Ru3+ RedOx couples. EF(PdO) and EF(RuO2) are the work functions of PdO and RuO2. EC, EF, ED, and EV are the conduction band, Fermi level, donor level, and valence band energy, respectively, of SnO2.

(Table 2). The literature value of the crystalline RuO2 work function40 is about (E0 − EF) = 5.0−5.1 eV. Taking into account the possible RuO2/Ru3+ RedOx couple formation, its electrode potential (E) corresponding to the process

Figure 8. Thermograms of nanocrystalline blank and modified tin dioxide samples: SnO2-300 (1), SnO2-300/PdOx (2), and SnO2-300/ RuOy (3).

RuO2 + 4H+ + e− = Ru 3 + + 2H 2O

The reason for modifiers influence on the hydrated surface species could be an enhancement of atmospheric H2O chemisorption on tin dioxide provided by the catalytic clusters. The superior hydrating effect of PdOx is evidenced by not only the accumulation of molecular and dissociated aqueous forms but also the higher concentrations of such reactive hydrated species as Brønsted acidic sites, OH· spin centers (Table 2), and hydrogen-bound OH···OH associates (Figure 6). Taking into account that Pd2+ and Pd3+ cations have larger radii and lower charges than Sn4+ and Ru4+, the stimulation of H2O chemisorption on the surface of SnO2/PdOx materials is not of the origin of Lewis acid−base interactions. The hydration effect of modifiers was assumed to arise from cluster−support electronic effects. The PdOx clusters consist mainly of PdO and Pd0 (Table 2). The work function values of PdO and metallic Pd were estimated to be 6.0 and 4.8 eV, respectively.38 Moreover, these species could form a redox couple in the PdOx clusters. To account for the electronic interaction, the redox potential of PdO/Pd couple in the clusters should be compared with the work function value of support. For this purpose, the electrode potential should be taken in the absolute scale, i.e., related to vacuum level. In this context, it corresponds to the energy needed to bring an electron from the PdO/Pd electrode to the vacuum and can be compared with the work function of SnO2, the latter being an energy of electron transfer from tin dioxide bulk (from the Fermi level) to the vacuum. The absolute electrode potential of PdO/Pd couple could be estimated3,39 to be E0 − E(PdO/Pd) ≈ 5.5 eV. With tin dioxide having a lower work function (E0 − EF = 4.8 eV),39 a depletion layer in its subsurface layer contacting with PdOx clusters could arise (Figure 9a). We suppose that the electron deficiency of tin dioxide near the interface area promotes the chemisorption of donor-like H2O molecules, as well as their further dissociation which is regarded to produce free charge carriers.19 In this respect the lower hydrating effect in SnO2/RuOy should be due to less electron-acceptor action of RuOy clusters. These clusters comprise primarily structured RuO2 with a fraction of Ru3+

(8)

was evaluated using the literature value of standard potential (E0)41 and the Nernst equation (at pH 7): E = E0 − 0.059 × 4 × pH = 1.99 − 0.059 × 4 × 7 = 0.34 V

(9)

which corresponds to ∼4.8 eV below vacuum level, i.e., similar to the work function of SnO2. The schematic band diagram of SnO2−RuOy interface is illustrated by Figure 9b. Thus, modification by catalytic additives of palladium and ruthenium oxides results in promotion of different active sites on nanocrystalline tin dioxide surface. Both the modifiers, and to a larger extent RuOy, increase the concentration of chemisorbed oxygen which is a predominant oxidative species type on the SnO2 surface. In contrast to this, PdOx renders a hydration effect increasing the amount of hydrated species on tin dioxide surface, including reactive hydroxyl species. The influence of such a specific interference between catalytic modifiers and tin dioxide active sites on the materials interaction with gases and sensing performance is a matter of forthcoming publications.

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CONCLUSIONS Different types of active sites were detected and quantified on the surface of nanocrystalline tin dioxide. Oxidative centers were deduced to be represented mainly by chemisorbed oxygen species. Lewis (tin cations) and Brønsted (acidic OH groups) acid sites were distinguished by means of TPD-NH3. Paramagnetic surface centers of ionosorbed oxygen O2− and OH· were determined by EPR to constitute about 10−4% of total chemisorbed oxygen and surface hydroxyls amounts, respectively. A variety of hydrated surface species were observed by FTIR: chemisorbed H2O with its protonated forms and surface OH groups, comprising mostly hydrogenbond associated hydroxyls besides Brønsted acid sites and OH· radicals. The active sites concentrations fall with an increase of SnO2 mean particle size and a decrease of the materials BET 21547

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surface area. Tin dioxide surface modification by palladium and ruthenium oxides was found to result in catalytic sites formation comprising mixed-valence oxidized noble metal clusters. A specific promoting effect of modifiers on the intrinsic active centers of SnO2 was established. The modifiers facilitated an increase of oxidative sites concentration, presumably via oxygen spillover, the effect being larger for SnO2/RuOy samples. Palladium oxide demonstrated a prominent surface hydration activity, providing the increase of general hydrated species concentration and, in particular, of reactive hydroxyls, such as Broensted acidic OH sites, OH· spin centers, and hydrogen-bound OH···OH associates. Electronic cluster−support interaction was proposed as a factor controlling the modifier-stimulated water chemisorption on the SnO2/PdOx surface.

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ASSOCIATED CONTENT

S Supporting Information *

HRTEM and STEM micrographs of SnO2-700/PdOx sample and STEM micrographs with the insets of EDX maps for SnO2300/PdOx and SnO2-300/RuOy samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +7 495 939 5471. Fax: +7 495 939 0998. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Russian Science Foundation (Grant 14-19-00120) for financial support. REFERENCES

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