Role of PdOx and RuOy Clusters in Oxygen ... - ACS Publications

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Role of PdOx and RuOy Clusters in Oxygen Exchange between Nanocrystalline Tin Dioxide and the Gas Phase Artem V. Marikutsa,† Marina N. Rumyantseva,*,† Dmitry D. Frolov,† Igor V. Morozov,† Alexander I. Boltalin,† Anna A. Fedorova,† Ilya A. Petukhov,† Lada V. Yashina,† Elizaveta A. Konstantinova,‡ Ekaterina M. Sadovskaya,§ Artem M. Abakumov,∥ Yan V. Zubavichus,⊥ and Alexander M. Gaskov† †

Department of Chemistry, Lomonosov Moscow State University, Moscow, 119234, Russia Department of Physics, Lomonosov Moscow State University, Moscow, 119234, Russia § Boreskov Institute of Catalysis, RAS, Novosibirsk, 630090, Russia ∥ EMAT, University of Antwerp, Antwerp, B-2020, Belgium ⊥ NRC “Kurchatov Institute”, Moscow, 123182, Russia ‡

S Supporting Information *

ABSTRACT: The effect of palladium- and ruthenium-based clusters on nanocrystalline tin dioxide interaction with oxygen was studied by temperature-programmed oxygen isotopic exchange with mass-spectrometry detection. The modification of aqueous sol−gel prepared SnO2 by palladium and, to a larger extent, by ruthenium, increases surface oxygen concentration on the materials. The revealed effects on oxygen exchangelowering the threshold temperature, separation of surface oxygen contribution to the process, increase of heteroexchange rate and oxygen diffusion coefficient, decrease of activation energies of exchange and diffusionwere more intensive for Ru-modified SnO2 than in the case of SnO2/Pd. The superior promoting activity of ruthenium on tin dioxide interaction with oxygen was interpreted by favoring the dissociative O2 adsorption and increasing the oxygen mobility, taking into account the structure and chemical composition of the modifier clusters. the surface of modified sensor material,1,9 modulation of SnO2 electric properties.3,5,6,10 Since oxygen is a key participant in the processes resulting in sensor signal formation, the comparative study of blank and modified tin dioxide interaction with oxygen would contribute to the understanding of the role of modifiers in promoting the interaction with gases utilized in gas sensing. The interaction of SnO2 with oxygen has been extensively studied by various experimental (TPD, EPR, IR spectroscopy,12 XPS,13 electric,14,15 and work function measurements15) and theoretical (DFT) methods.13,16 It was shown that O2 may chemisorb on SnO2 surface: (i) in ionic molecular (O2−, O22‑) or dissociated atomic (O−, O2‑) states;12,13 (ii) in the form of molecular dipoles.15 The type of predominant chemisorbed oxygen depends on temperature, oxygen concentration, structure and oxidation state of SnO2 surface,12−15 and so on. The role of lattice oxygen has not been well determined because it is usually supposed not to participate in the chemical

1. INTRODUCTION Tin dioxide is an n-type semiconductor extensively used as the gas sensitive material in chemoresistive sensors. The surface modification of tin dioxide by Pd or Ru additives has been widely exploited to improve the sensors characteristics, like selectivity, sensitivity, and optimal operating temperature for the detection of reductive gases, such as CO,1−4 H2,4,5 hydrocarbons,2,6,7 NH3,8,9 and so on. The sensor performance of nanocrystalline SnO 2 is commonly accepted to be determined by oxygen chemisorption and chemical reactions between the chemisorbed or lattice oxygen and target gas molecules.10 Such processes are accompanied by the changes in free electron concentration, surface potential modulation, and electric conductivity change. To account for the role of additives (like Pd or Ru) in improving tin dioxide sensor performance with reductive gases numerous hypotheses have been proposed, regarding either physical or chemical effects, which are sometimes contradicting or lacking experimental confirmation: specific gas adsorption on the active surface sites formed by a modifier,1,2,5 spillover of gas molecules to the surface of sensitive layer,4,11 catalysis of target gas oxidation on © 2013 American Chemical Society

Received: August 29, 2013 Revised: October 2, 2013 Published: October 4, 2013 23858

dx.doi.org/10.1021/jp408646k | J. Phys. Chem. C 2013, 117, 23858−23867

The Journal of Physical Chemistry C

Article

Synchrotron Radiation Center (NRC “Kurchatov Institute”, Moscow, Russia).25 The spectra were collected in the fluorescence yield mode using a Si(220) channel-cut monochromator and a Si avalanche photodiode (FMB Oxford) as a detector. Spectra of reference compounds, including Ru and Pd foils, RuO2, Ru(acac)3, PdO and Pd(acac)2 were measured in the transmission mode using two ionization chambers filled with appropriate Ar−Xe mixtures. The standard data processing was performed using IFFEFFIT software suite.26 The extracted normalized EXAFS oscillations were fitted using theoretical photoelectron phase and amplitude functions calculated by FEFF6.5 included into IFEFFIT over the k-range 2.0−15.0 Å−1 and the R-range 1.3−3.6 Å for SnO2/Ru and the k-range 2.0− 12.0 Å−1 and the R-range 1.1−2.2 Å for SnO2/Pd. Quantity of surface oxygen species was estimated by H2temperature programmed reduction (TPR-H2) using the Chemisorb 2750 (Micromeritics) instrument. The powders were tested without a pretreatment. The gas used was 10 vol % H2 in argon, gas flow rate 10 mL/min; heating from room temperature to 1073 K with the rate 10 K/min. Electron paramagnetic resonance (EPR) spectra were recorded at 110 K by the standard Bruker EPR spectrometer ELEXSYS-580 (X-band, sensitivity is ∼1010 spin/G). The values of g-factors and spin center concentrations were calculated using standard samples Mn2+ and CuCl2·2H2O, respectively. 2.3. Mass-Spectrometric Study of Oxygen Isotopic Exchange. The oxygen exchange experiments were performed in a dynamic temperature-programmed isotopic exchange (TPIE) mode. The measurements were carried out in a flow reactor heated with a constant rate, through which the test gas containing Ar, 16O2, 16O18O and 18O2 was passed. The detailed description of experimental setup and conditions is given in ref 23. Ion currents at mass numbers 32 (16O2), 34 (16O18O) and 36 (18O2) were recorded as raw data along with the sample temperature. The molecular oxygen fractions were calculated by the following equations:

interaction with gases, yet oxygen vacancies surface and bulk diffusion was recently revealed to affect the electrical properties drift of SnO2 sensors.17 By the way, the effect of noble metal additives on SnO2 interaction with oxygen has not been established firmly. Palladium nanoparticles on SnO2 nanowires were observed to increase the electric response to O2 gas5 that was attributed to enhanced dissociative oxygen adsorption at Pd sites. On the contrary, by means of DFT simulations it was shown that Pd and Pt on reduced SnO2(110) surface should increase the activation energy for oxygen dissociation,18 while insignificant changes of O2 adsorption energy were predicted. Other DFT studies agree that Ru sites if created on the SnO2 surfaces greatly facilitate oxygen adsorption and dissociation.19,20 Oxygen isotopic exchange is a powerful technique for the evaluation of oxygen mobility in oxidation catalysts21,22 or oxygen conductive materials (YSZ, CeO2−ZrO2). The present paper continues and extends our previous work23 on temperature-programmed oxygen isotopic exchange (TPIE) study of nanocrystalline SnO2. The main objective of this work is to highlight the influence of modifiers on SnO2 interaction with oxygen depending on the composition and structure of noble metals clusters.

2. MATERIALS AND METHODS 2.1. Synthesis of Nanocrystalline SnO 2 and Its Modification by Pd and Ru. Nanocrystalline SnO2 was synthesized via aqueous deposition of α-stannic acid followed by washing and drying. The dried SnO2*nH2O powder was annealed at 573 K for 24 h. The modification of SnO2 was performed by impregnation with ethanol solutions of Pd(acac)2 or Ru(acac)3.24 The quantity of Pd and Ru elements was 1 wt %. The impregnated and dried SnO2/Pd(acac)2 and SnO2/ Ru(acac)3 powders were annealed at 498 and 538 K, respectively, for 24 h to decompose the precursors. 2.2. Materials Characterization. Phase composition was examined by X-ray powder diffraction (XRD) with the DRON3M instrument (wavelength λ = 1.54059 Å (Cu Kα 1 radiation)). The crystallite size (dXRD) of SnO2 was calculated from the broadening of the (110) XRD peak using Scherer equation. The specific surface area was measured by nitrogen adsorption using the BET model with the Chemisorb 2750 instrument (Micromeritics). The microstructure was characterized by high resolution transmission electron microscopy (HRTEM), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray (EDX) analysis with the Tecnai G2 transmission electron microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed using Axis Ultra DLD (Kratos, UK) spectrometer equipped with a monochromatic Al Kα X-ray source. Charge neutralization was applied, providing the C 1s peak position of 285.0 eV. XP-spectra were fitted by the Gaussian−Lorentzian convolution functions with simultaneous optimization of the background parameters. XP-spectra of Pd(acac)2 and Ru(acac)3 references were recorded for the evaluation of modifiers chemical states. Synchrotron radiation-based X-ray absorption spectroscopy (XAS) was applied in order to determine the chemical state of ruthenium and palladium species in modified nanocrystalline tin oxide samples. The Ru K-edge (22.117 keV) and Pd K-edge (24.350 keV) XANES/EXAFS spectra were measured at the Structural Materials Science beamline of the Kurchatov

f32 =

I32 I32 + I34 + I36

(1)

f34 =

I34 I32 + I34 + I36

(2)

f36 =

I36 I32 + I34 + I36

(3)

where I32, I34, and I36 are the ion currents at mass numbers 32, 34, and 36. The atomic fraction of 18O in the test gas was calculated from molecular oxygen fractions using the equation α=

f34 2

+ f36

(4)

The blank SnO2 was examined after pretreatment in 16O2 using an equilibrated test gas mixture as described in ref 23. The modified SnO2/Ru and SnO2/Pd samples were tested without any pretreatment to prevent morphologic transformations. The modified samples were tested with a nonequilibrium gas mixture containing 40% of 18O2, 15% of 16 18 O O, and 45% of 16O2 from the total oxygen content. 23859



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3. RESULTS AND DISCUSSION

size of 5−10 nm in all the samples. Electron diffraction patterns (Figure 1a, inset) confirmed SnO2 as the only crystalline phase. Palladium tends to form clusters with the size of 1− 5 nm attached to the surface of tin dioxide (Figure 2a) when it was

3.1. Materials Composition and Microstructure. According to XRD data, the SnO2, SnO2/Pd, and SnO2/Ru samples consist of cassiterite (SnO2) phase with average crystallite size (dXRD) calculated using Scherer equation of about 2−5 nm. Their BET surface areas were estimated to about 90−95 m2/g. The modification of tin dioxide had no observable effect on the crystallite size and surface area. Low magnification TEM (Figure 1a) and HRTEM images (Figure 1b) revealed agglomerated irregularly shaped particles with the

Figure 2. (a) High-resolution micrograph of Pd-based clusters when deposited on coarse crystalline tin dioxide (particle size more than 50 nm). Inset shows HAADF-STEM image of the Pd cluster along with the EDX signal profiles for the Sn-L and Pd-L lines. (b) HAADFSTEM image of the nanocrystalline SnO2/Ru sample; inset: EDX map of the selected area (blue − Sn; yellow − Ru).

deposited on SnO2 with rather large crystallites (about 50 nm; SnO2 annealed at 973 K). However, if nanocrystalline SnO2 (5−10 nm; annealed at 573 K) was modified by palladium, no clear sign of Pd segregation was observed. An analogous situation was encountered in the case of nanocrystalline SnO2/ Ru. Although the amounts of about 1 wt % of each modifier were detected by EDX, the visualization of either Pd or Ru clustering in the corresponding nanocomposites by HRTEM or HAADF-STEM was hindered for the following probable

Figure 1. (a) A representative low magnification TEM image of the materials (on an example of SnO2/Ru); inset: electron diffraction pattern with the diffraction rings indexed according to cassiterite structure. (b) A representative high-resolution micrograph of the materials (on the example of SnO2/Ru). The TEM and HRTEM images of the other samples are similar. 23860



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Figure 3. (a) Ru K-edge XANES spectra of nanocrystalline SnO2/Ru and reference substances (Ru foil, RuO2, and Ru(acac)3). (b) Pd K-edge XANES spectra of nanocrystalline SnO2/Pd and reference substances (Pd foil, PdO, and Pd(acac)2).

inspection of the experimental spectra (see Supporting Information), however, indicates that the sample can actually store an appreciable fraction of up to 20% Ru3+ presumably due to incomplete decomposition of Ru(acac)3 precursor or, to some extent, the oxygen deficiency in RuO2. The latter suggestion is supported by a slight elongation of Ru−O and Ru···Ru interatomic distances in the SnO2/Ru sample given by EXAFS fit as compared to bulk RuO2.30 Pd K-edge XANES spectra for SnO2/Pd and reference compounds are shown in Figure 3b. It is evident that the mainedge position in the spectrum is shifted by ca. 7 eV with respect to the spectrum of Pd foil but is very similar to that in PdO or Pd(acac)2. This means that the oxidation state of palladium in the sample is +2 to a high level of confidence. Moreover, the XANES line shape of SnO2/Pd is very similar to that of PdO rather than to the spectrum of Pd(acac)2 reference. It indicates that palladium in the investigated SnO2/Pd sample is likely to represent PdO predominant species, which is in accordance to XPS data. Nevertheless, the EXAFS spectrum of SnO2/Pd shown in Figure 5a is substantially different from that of PdO. Indeed, no Pd···Pd contributions at 2.5−3.5 Å were observed therein, which implies a strongly distorted amorphous-like short-range order. An adequate fitting of the experimental EXAFS spectrum is achieved with a single Pd−O contribution (Figure 5b) at ∼2.00 Å and coordination number 4. 3.2. Surface Oxygen Species Content. The oxygen surface species in the samples were investigated by TPR-H2, XPS, and EPR methods. The TPR curves of blank and modified SnO2 are compared in Figure 6. Two general H2-consumption lines were observed: the less intense one at lower temperature (370−570 K) and the major one at higher temperature (720− 920 K). The low-temperature peaks were assigned23,27 to the reduction of surface species (O−, O2−, OH groups) according to the reactions:

reasons: (i) more homogeneous modifier distribution over small tin dioxide nanoparticles (annealed at 573 K), unlikely for Pd-modified coarse crystalline SnO2 where small clusters of Pd contrasted on the background of large SnO2 grains. (ii) Close atomic numbers of Ru, Pd, and Sn producing weak contrast. However, EDX mapping of nanocrystalline SnO2/Ru (Figure 2b, inset) revealed ruthenium segregations with a size of about 2−5 nm on the background of agglomerated SnO2 nanoparticles. This observation is in line with the reported elsewhere tendency of Ru to segregate on tin dioxide surface confirmed by means of DFT modeling.20 Palladium clusters demonstrate mixed Pd 3+ /Pd 2+ /Pd 0 valence state with Pd2+ to Pd0 atomic content about 2:1 (XPS data27) and a small fraction of Pd3+ (EPR data24); the latter is supposed to occur on a tin dioxide−palladium cluster interface. Ruthenium presents in the SnO2/Ru sample in the mixed Ru3+/Ru4+ state where the amount of Ru3+ is about of 20 at. % of the total Ru content (XPS and EPR data24,28). In the present study, XAS was implemented to validate the ruthenium and palladium speciation. Figure 3a compares Ru K-edge XANES spectrum of SnO2/ Ru with those of reference compounds, i.e., Ru foil and Ru(acac)3 and RuO2. The main-edge position in SnO2/Ru is shifted slightly toward higher photon energies with respect to Ru(acac)3 and the general near-edge structure is very similar to RuO2 reference both measured by us and reported in literature,29 which implies that Ru atoms are most probably present in the samples in the +4 oxidation state. Fourier transform of Ru K-edge EXAFS spectrum for SnO2/Ru (Figure 4a) is dominated by the Ru−O first coordination shell contribution but additionally reveals distinct peaks at rather long distances of 2.5−3.5 Å (without phase correction), indicative of a rather ordered structure. The observed EXAFS pattern can be fairly well simulated by the rutile-type RuO2 local-structure model (Figure 4b) taking into account the closest Ru−O (1.93 and 2.03 Å) and Ru···Ru (3.14 and 3.57 Å) contributions. Importantly, no direct Ru−Ru bonds as in Ru metal were found. Furthermore, the SnO2 local structure model gives a poorer fit. Hence, ruthenium in the modified SnO2 is deduced to comprise structured RuO2 nanoclusters. A closer 23861

O−2(surf) + 2H 2(g) = 2H 2O(g) + e−

(5)

− O(surf) + H 2(g) = H 2O(g) + e−

(6)

2OH(surf) + H 2(g) = 2H 2O(g)

(7)



Article

Figure 4. (a) Fourier transforms (FTs) of Ru K-edge EXAFS spectra for nanocrystalline SnO2/Ru and reference substances (Ru foil, RuO2, and Ru(acac)3). (b) Results of quantitative analysis of EXAFS spectra for nanocrystalline SnO2/Ru: experimental (solid lines) and best-fit theoretical curves (circles).

Figure 5. (a) FTs of Pd K-edge EXAFS spectra for nanocrystalline SnO2/Pd and reference substances (Pd foil, PdO, and Pd(acac)2). (b) Results of quantitative analysis of EXAFS spectra for nanocrystalline SnO2/Pd: experimental (solid lines) and best-fit theoretical curves (circles).

The high-temperature H2 consumption was due to SnO2 bulk reduction:23,27 SnO2(s) + 2H 2(g) = Sn(1) + 2H 2O(g)

(8)

Tin dioxide modification by Pd and Ru shifted the H2consumption lines to lower temperature. The peak of surface species reduction on pristine tin dioxide was centered at 500 K, while those of Pd- and Ru-modified samples were at 420 K. Bulk SnO2 reduction peak was shifted from 800 K (unmodified SnO2) to 765 K in the case of SnO2/Pd and to 755 K for SnO2/Ru sample. This could be evidence of either or both the factors: (i) higher surface and bulk oxygen mobility in modified tin dioxide, and (ii) some catalytic activity of PdOx and RuOy clusters in such reduction−oxidation processes. The overall surface oxygen species concentrations calculated from low-temperature H 2-consumption peaks areas are summarized in Table 1. The estimation was performed for diatomic oxygen species, which are known to dominate on the surface of SnO2 at relatively low temperatures.12 The latter are denoted further as O2− for simplicity; however, not only ionic, but also neutral O2 chemisorbed species are implied.

Figure 6. H2-TPR spectra of nanocrystalline SnO2, SnO2/Pd, and SnO2/Ru samples.

Under this assumption, the TPR stoichiometry determination, which is necessary for evaluating the chemisorbed oxygen concentration, simplifies to that of eq 5. Yet, the actual 23862



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Although the difference between the samples is not striking, the trend of [Osurf]:[Osum] values agrees with that of surface species concentrations suggested by TPR-H2. It supported the conclusion about increasing surface oxygen content in the row: SnO2 < SnO2/Pd < SnO2/Ru. Rather high Osurf fractions (up to about 40 at. %) might result from small nanocrystals size (high surface-to-volume ratio) and large surface area available for adsorption. Total O:Sn atomic ratios (Table 1) are less than the stoichometric value of 2.0; however, the modified SnO2 samples retained more oxygen (O:Sn ratio of 1.90−1.96) than the unmodified one (O:Sn = 1.80). Noteworthy, tin was found to occur solely in the Sn4+ state in the materials (XP-spectra not shown) despite oxygen deficiency. The EPR signal of nanocrystalline SnO2 consists of several lines attributed to O2− and OH·radicals.24 The modification of tin dioxide by Pd results in 2-fold increase of OH·concentration and 1.5-fold decrease of O2− quantity. The introduction of ruthenium has no significant effect on the concentration of oxygen and hydroxyl radicals (Table 1). 3.3. Oxygen Exchange on SnO2, SnO2/Pd, and SnO2/ Ru. The molecular oxygen fractions of 16O2, 18O16O, and 18O2 calculated by eqs 1−3 for the nanocrystalline blank and modified tin dioxide are depicted in Figure 8 in the dependence on temperature. The starting temperatures of oxygen exchange were recognized by the molecular isotopic fraction deviations from the initial plateaus. The detectable exchange processes started from 700 K in pristine SnO2, while the modification by palladium and ruthenium reduced the temperature to 590 and 480 K, respectively. Such an essential temperature decrease evidence a promotive effect of the modifiers on oxygen exchange in nanocrystalline tin dioxide, the effect being stronger in the case of SnO2/Ru. The atomic 18O fractions (α) calculated via the eq 4 and molecular 18O16O fractions ( f 34) for the tested samples are compared versus temperature in Figure 9. The data treatment and oxygen exchange modeling was exercised as in our previous publication,23 which can be addressed for more calculation details. The decrease of atomic 18O fractions with the process initiation suggested heteroexchange, i.e., the interaction between gaseous O2 molecules and the oxygen of oxides constituent, as the predominant route of oxygen exchange in the samples. The heteroexchange mechanism implies the exchange of O atoms between O2 molecules from gas phase and constituent oxygen of the solid, e.g., via the processes expressed in the scheme 9−10. Integrating the α(T) curves, the amounts of substituted oxygen were calculated. They were about 60 at. % from total oxygen amount in SnO2, 54 at. % of oxygen in SnO2/Pd, and 46 at. % in the case of SnO2/Ru. The α(T) and f 34(T) data are of particular analytical importance, because from their fitting by theoretical dependencies the mechanism type and kinetic parameters of exchange can be deduced. The quantitative model is based on a combination of mass balance equations formulating the isotopic exchange on the surface and oxygen diffusion into the bulk of materials. The kinetic characteristics, such as apparent exchange rate, diffusion coefficient, activation energies of oxygen exchange, and diffusion, are included in this model as parameters in the differential equations; the variables are α, f 34, and T. The simulated α(T) and f 34(T) curves for each sample in comparison with the experimental ones are plotted in Figure 9. The experimental data for oxygen exchange on SnO2 were successively simulated only in the frame of so-called simple heteroexchange model (Figure 9a). This mechanism was

Table 1. Surface Oxygen Species Concentrations TPR sample SnO2 SnO2/ Pd SnO2/ Ru

XPS

EPR

[O2]a

[Obulk]/ [Sn]

[Osurf]/ [OSum]b

[Osum]/ [Sn]c

[O2−]d

[OH·]e

11.8 ± 0.4 13.6 ± 0.5

2.12 2.06

0.35 0.36

1.80 1.96

13.9 8.7

2.6 5.2

14.9 ± 0.5

2.16

0.38

1.90

14.0

2.6

a

Surface concentration of conditional O2 species (see text) evaluated by H2-TPR, 10−6 mol/m2. bFraction of surface oxygen estimated by XPS. cTotal oxygen-to-tin atomic ratio estimated by XPS. dConcentration of paramagnetic centers determined by EPR,24 10−12 mol/m2 . e Concentration of paramagnetic centers determined by EPR,24 10−12 mol/m2 .

surface species composition is more complex and includes various aqueous (hydroxyls) and oxygen (ionic and neutral) derivatives with different stoichiometry in H2-assisted reduction. The representative values of surface oxygen concentrations (Table 1) were found to increase in the following order SnO2 < SnO2/Pd < SnO2/Ru. Since the difference between blank and modified tin dioxide of about 2−3 × 10−6 moles of O2− per 1 m2 of samples surface exceeds the molar concentrations of either Pd or Ru (0.9−1 × 10−6 moles of element per 1 m2 of samples surface), the increase of lowtemperature H2 consumption is not only due to the reduction of PdOx (0 < x < 1) or RuOy (1.5 < y < 2). Consequently, the modification of SnO2 by palladium and to a larger extent by ruthenium increased the materials surface affinity to chemisorbed oxygen-related species. The XP-spectra of the investigated samples in the O 1s binding energy region are shown in Figure 7. In each case, the

Figure 7. XP-spectra in O 1s binding energy region of SnO2 (a), SnO2/Pd (b), and SnO2/Ru (c) samples.

experimental curves could be reliably fitted by at least two peaks: the first one centered at 530.8 eV originated from bulk O2− (denoted as Obulk) anions (database value 530.9 eV); the second one at 531.7−531.9 eV was attributed to strongly bound surface species (Osurf), such as O2−, O−, OH, etc.13,27 The XPS-evaluated Osurf fractions in the summary constituent oxygen (Osum = Osurf + Obulk) are compared in Table 1. 23863



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Figure 8. Temperature dependences of molar 16O2, 16O18O, and 18O2 fractions in the outlet gas flow during TPIE measurements for SnO2 (a), SnO2/Pd (b), and SnO2/Ru (c).

Figure 9. Temperature dependences of 18O isotopic fractions (α: filled triangles − experimental values; line − simulation) and 18O16O molecular fractions ( f 34: hollow squares − experimental values; line − simulation) for SnO2 (a), SnO2/Pd (b), and SnO2/Ru (c). Dotted lines correspond to the exchange of surface oxygen on the samples.

established as the predominant route of oxygen exchange in pristine tin dioxide. Simple heteroexchange usually proceeds via the Iley−Rideal mechanism and consists in direct gas phase oxygen molecule interaction with an oxygen atom of the oxide according to the following scheme:19,28 18

O2(g) + 16O(s) ↔ 16O18O(g) + 18O(s)

16 18

O O(g) + 16O(s) ↔ 16O2(g) + 18O(s)

could be appropriately fitted (Figure 9b,c) only if an additional exchange mechanism, namely, multiple heteroexchange, was taken into consideration. Multiple heteroexchange includes preliminary O2 molecule dissociation followed by the exchange of adsorbed oxygen atoms and lattice oxide anions. This mechanism can be formalized as a diatomic exchange between oxygen gas molecule and two oxygen atoms from the material surface:

(9) (10)

The kinetic parameters evaluated for the reference temperature (T = 573 K) and used in the simulation of simple heteroexchange on SnO2 are presented in Table 2. Unlike blank SnO2, in the cases of modified materials (SnO2/ Pd and SnO2/Ru), the experimental α(T) and f 34(T) data

18

O2(g) + 216O(s) ↔ 16O2(g) + 218O(s)

23864

(11)



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the first step in the processes scheme may be taken as a limiting one. Then the kinetics of this combination of processes can be formalized analogously to that of a multiple heteroexchange stage (eqs 11−13). Thus, the emergence of the multiple heteroexchange mechanism and the activation of oxygen exchange with modified tin dioxide were attributed to fast migration of dissociated oxygen from PdOx or RuOy clusters to the surface of SnO2 support and back. It should be noted that spillover effect has been commonly declared for metallic nanoparticles and supported clusters.21,31 However, in the present study, the spillover-like activity was observed for oxidized PdOx and RuOy clusters on nanocrystalline tin dioxide. The spillover effect of the modifiers also accounts for the increment of oxygen diffusion coefficients (Table 2), since the reported parameters characterize the diffusion into the samples structure as a whole. In this context, the promotion of surface oxygen diffusion, which is the essence of spillover, would contribute to the intensification of oxygen migration into the nanocrystalline samples through the grains interfaces,21 as well as the diffusion of atomic oxygen into the nanocrystals bulk. These are the phenomena that were assumed to be reflected by the increment of apparent diffusion coefficients (Table 2) estimated from the experimental results. Atomic oxygen bulk diffusion into the oxide grains could be due to the oxygen vacancies surface-to-bulk diffusion in SnO2, which was shown by Lopez et al. to take place under oxygen-containing atmosphere,17 like in our experimental conditions. Low-temperature peaks could be deconvoluted from the α(T) curves of blank and Pd-modified tin dioxide (Figure 9a,b, dotted lines). In the case of SnO2, the calculated amount of substituted oxygen corresponding to this peak was close to the content of structural oxygen anions in the SnO2(110) surface layer, taking into account the specific surface area. The position of this recognized peak (750−850 K) close to the exchange starting temperature means that corresponding oxygen species are more readily exchangeable than the others (lattice oxygen anions in the bulk). The higher activity of the former determines that these oxygen species initiate heteroexchange at the appropriate temperature. Thus, the contribution of materials surface oxygen to the isotopic exchange could be distinguished. In the case of SnO2/Pd, the exchange of surface oxygen resulted in a low-temperature shoulder on the α(T) curve (Figure 9b, dotted line). It was shifted to lower temperature (600 K) than the surface oxygen peak of SnO2, which agreed with the noticed decrease of heteroexchange starting temperature. For the SnO2/Ru sample, the initial peak attributed to the exchange of surface oxygen was temperatureresolved from the rest part of α(T) curve (Figure 9c). It was even more shifted to lower temperature (530 K), implying that, in this case, the surface oxygen species had been significantly more mobile than in the blank and Pd-modified samples. Separation of the impact of surface oxygen species from that of bulk O atoms to entire exchange is consistent with the largest decrease of exchange starting temperature noticed for SnO2/ Ru. It suggests one extra feature of strong promoting effect of Ru-modifier on tin dioxide oxygen exchange: an essential activation of surface oxygen species. Having established that the role of PdOx and RuOy clusters in facilitating the oxygen exchange on nanocrystalline SnO2 consists in the initiation of spillover-based heteroexchange mechanism (eq 13), the difference between the two modifiers activities needs to be interpreted. In a study of 16O2/18O2 homoexchange on various metal-doped oxides,33 the authors

Table 2. Estimated Kinetic Parameters of Oxygen Exchange sample parametera

SnO2 −2

−1

R1 (T = 300 °C), m s R2 (T = 300 °C), m−2 s−1 D (T = 300 °C), m2 s−1 E(R1), kJ/mol E(R2), kJ/mol E(D), kJ/mol

1.3 × 10 none 6.1 × 10−24 130 none 80 12

SnO2/Pd

SnO2/Ru

2.6 × 10 5.7 × 1014 1.9 × 10−22 130 110 80

2.6 × 1012 3.6 × 1017 3 × 10−21 130 130 60

12

R1 − rate of simple heteroexchange; R2 − rate of multiple heteroexchange; D − oxygen diffusion coefficient; E − corresponding activation energy. a

18

O2(g) + 216O(s) ↔ 16O18O(g) + 16O(s) + 18O(s)

16 18

O O(g) + 216O(s) ↔ 16O2(g) + 16O(s) + 18O(s)

(12) (13)

Thus, oxygen exchange on SnO2/Pd and SnO2/Ru was deduced to proceed jointly via two parallel routes: (i) the simple heteroexchange, like in unmodified tin dioxide; (ii) the multiple heteroexchange brought about by the modifiers. The kinetic parameters used for the modeling of oxygen exchange in modified tin dioxide samples are compared in Table 2. The multiple heteroexchange is characterized by 2 orders of magnitude higher exchange rate R2 and lower activation energy E(R2) than those of simple heteroexchange (R1 and E(R1), respectively). The influence of modifiers also provided the increased rates of simple heteroexchange and oxygen diffusion coefficients in comparison with those of blank SnO2. As can be seen from the data in Table 2 and comparing with the abovediscussed starting temperatures of the processes, the samples activity in oxygen heteroexchange increased in the order: SnO2 < SnO2/Pd < SnO2/Ru. Noteworthy, this observation correlates with the order of increment of oxygen surface species concentrations on the samples revealed by TPR-H2 and XPS. Noble metal clusters supported on an oxide are known to possess spillover effect facilitating oxygen exchange with the gas phase.21,22,31 In the present study the spillover effect is proposed for the role of PdOx and more prominently of RuOy clusters supported on SnO2 in the promotion of oxygen exchange. For the spillover-assisted oxygen exchange the following mechanism was suggested:21 18

O2(g) ↔ 218O(NMOx)

(dissociation)

18

(spillover)

O(NMOx) ↔ 18O(surf)

18

O(surf) + 16O(bulk) ↔ 16O(surf) + 18O(bulk)

16

O(surf) ↔ 16O(NMOx)

18

(spillover)

O(NMOx) + 16O(NMOx) ↔ 18O16O(g)

216O(NMOx) ↔ 16O2(g)

(exchange)

(recombination)

(recombination)

Here NMOx designates the noble metal oxide cluster (PdOx or RuOy), while surf and bulk implies the surface and bulk of tin dioxide. Supposing the rate of molecular oxygen dissociation on the active sites to be lower than that of atomic oxygen spillover due to high O−O binding energy (493 kJ/mol32 for gaseous O2), 23865



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oxygen exchangelowering the threshold temperature, separation of surface oxygen contribution to the process, increase of heteroexchange rate and oxygen diffusion coefficient, decrease of activation energies of exchange and diffusionwere more intensive for Ru-modified SnO2 than in the case of SnO2/Pd. The superior promoting activity of ruthenium on tin dioxide interaction with oxygen was interpreted by favoring the dissociative O2 adsorption and increasing the oxygen mobility, taking into account the structure and chemical composition of the modifiers clusters.

attributed higher activity and oxygen storage capacity of ceriasupported Ru than those of CeO2/Pd to a difference in oxygen desorption energy. Indeed, elemental Ru is known to possess higher oxygen affinity due to the vacancies in the valence electron levels (basic configuration 4d75s1), and forms a stable oxide RuO2.34 In its turn, Pd is more inert because of stable 4d105s0 configuration, and its oxide PdO is readily decomposed at raised temperature.34 This difference could also be confirmed by the trend of noble metal oxides formation energies:35 ΔfH = −150/−200 kJ/mol for RuO2 and ΔfH = −50/−100 kJ/mol for PdO (for SnO2 it is −580 kJ/mol). As for the SnO2supported metals, by means of DFT simulations, Pd clusters were shown to afford a minor influence on the adsorption energy of O2 on a reduced SnO2(110) surface;18 however, Pd was stated to inhibit the dissociation of adsorbed O2 due to a calculated increase of transition barrier. On the contrary, other DFT studies conclude that Ru-doping of SnO2 single crystal surfaces significantly improves their O2 adsorption properties: Ru-sites possess higher oxygen adsorption energy than Snsites,20 a fact that is more prominent for coordinatively unsaturated Ru atoms.19 In the presently investigated SnO2/Pd and SnO2/Ru materials, the situation is complicated by the different mixed chemical states of the additives. The palladiumbased clusters containing the detected amounts of Pd3+, Pd2+, and Pd0 were assumed to have a complex structure with the metallic core under PdO shell, Pd3+ being at the interface between SnO2 grains and PdO/Pd clusters or diffused into the subsurface layer of SnO2.24 Under this condition, the spillover of oxygen molecules could proceed on the external (PdO-like) surface. The stronger promoting effect of ruthenium on tin dioxide interaction with oxygen could be explained as follows. The additive in SnO2/Ru was shown to form the RuO2structured clusters with the predominance of Ru4+ and an admixture of Ru3+ cations (XAS, XPS, EPR data). RuO2 is a rutile-type structure, which is characterized by oxygen deficiency. The formation of oxygen vacancies in RuO2 is favored by the ability of ruthenium to exist in the Ru3+ oxidation state. Having electron donor properties, these oxygen vacancies were found to considerably increase the energy of O2 adsorption on the defective RuO2 surface.36 To conclude, the superior effect of RuOy clusters in promoting oxygen exchange on tin dioxide is believed to consist in (i) the stimulation of oxygen adsorption (and dissociation) via the formation of surface active sites, such as anion vacancies, undercoordinated Ru4+, and/or Ru3+ cations and (ii) the acceleration of surface species spillover due to enhanced oxygen mobility provided by the Ru4+/Ru3+ interplay in the RuO2(Ru3+) clusters.

■

ASSOCIATED CONTENT

* Supporting Information S

XANES spectrum of SnO2/Ru simulation by linear combination of 0.8 mol % RuO2 and 0.2 mol.% Ru(acac)3; EXAFS spectrum of SnO2/Ru simulations by 0.8 mol.% RuO2 and 0.2 mol % Ru(acac)3 via k3χ(k) and k2χ(k) weighting schemes. This information is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +7-495-9395471. Fax: +7-495-9390998. E-mail address: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Russian Ministry of Science and Education (state contract No. 14.B37.21.0087). EPR measurements have been effectuated using the facilities of the Collective Use Center at the Moscow State University supported by Federal Contract 16.552.11.7081.

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REFERENCES

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