Platinum Supported on Ta2O5 as a Stable SO3 Decomposition

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Pt Supported on TaO as a Stable SO Decomposition Catalyst for Solar Thermochemical Water Splitting Cycles Alam S.M. Nur, Takayuki Matsukawa, Eri Funada, Satoshi Hinokuma, and Masato Machida ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00195 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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ACS Applied Energy Materials

Pt Supported on Ta2O5 as a Stable SO3 Decomposition Catalyst for Solar Thermochemical Water Splitting Cycles Alam S.M. Nur, Takayuki Matsukawa, Eri Funada, Satoshi Hinokuma, and Masato Machida*

†

Department of Applied Chemistry and Biochemistry,

Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan Corresponding author: Prof. Masato Machida Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan Tel/Fax: +81-96-342-3651 E-mail: [email protected]

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ABSTRACT Pt supported on Ta2O5 was found to be a very active and stable catalyst for SO3 decomposition, which is a key reaction in solar thermochemical water-splitting processes. During continuous reaction testing at 600 °C for 1,800 h, the Pt/Ta2O5 catalyst showed no noticeable deactivation (activity loss ≤ 1.5% per 1,000 h). This observed stability is superior to that of the Pt catalyst supported on anatase TiO2 developed in our previous study, and to those of Pt catalysts supported on other SO3-resistant metal oxides Nb2O5 and WO3. The higher stability of Pt/Ta2O5 is due to the abundance of metallic Pt (Pt0), which favors the dissociative adsorption of SO3 and the smooth desorption of the products (SO2 and O2). This feature is in accordance with a lower activation energy and a less negative partial order with respect to O2. Pt sintering under the harsh reaction environment was also suppressed to a significant extent compared to that observed with the use of other support materials. Although a small fraction of the Pt particles were observed to have grown to more than several tens of nanometers in size, nanoparticles smaller than 5 nm were largely preserved and were found to play a key role in stable SO3 decomposition.

KEYWORDS Thermochemical water splitting, Sulfur-iodine process, SO3 Decomposition, Platinum catalyst, Support material, Tantalum oxide

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1. INTRODUCTION Solar thermochemical water-splitting cycles using the sulfur-iodine process are expected to emerge as being a key for large-scale and cost-effective production of H2. Using concentrated solar radiation as a heat source, endothermic H2 and O2 evolution reactions are driven indirectly as follows:1-5 H2SO4 → H2O + SO2 + 1/2O2

(1)

2HI → H2 + I2

(2)

SO2 + I2 + 2H2O → H2SO4 + 2HI

(3)

Reaction 1 involves the decomposition of H2SO4 into SO3 and H2O, followed by the decomposition of SO3 into SO2 and O2. Although the first step proceeds in the gas phase above 400 °C, the second step requires much higher temperatures (≥ 800 °C), and that is why the conventional sulfur-iodine process was designed to be combined with nuclear heat sources. Even for solar heat, several research groups have been studying SO3 decomposition at a higher temperature (≥800 °C), which can be generated by solar-tower and solar-dish collectors.6, 7 Although higher temperatures tend to favor greater efficiency, the practical operation temperature should be restricted below ≤650 °C due to material limitations of thermochemical reactors and heat fluid. In order for reaction 1 to proceed with the use of a heated fluid supplied by solar-trough collectors (~600 °C), active and stable SO3 decomposition catalysts that work at much more moderate temperatures are necessary.7-9 It should also be noted that lower temperature leads to degrade the SO3 conversion and thermal efficiency of the cycle; the equilibrium conversion of SO3 to SO2 is below 40% at 600 °C, and thus an equilibrium-shift reactor is indispensable. Catalyticmembrane reactors are potential candidates for such advanced reactors,10 and they enable O2 to be separated from a catalyst bed so that the forward SO3 decomposition reaction can be favored and the reaction efficiency improved. Although supported-Pt catalysts are considered to be the most promising for this temperature range,2, 11, 12

most previous studies have focused on how they work at high temperatures (≥ 800 °C),8, 13-28 where

the catalysts start to deactivate. Therefore, in order to design efficient catalysts for solar thermochemical

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water splitting cycles, optimizing the Pt-support material is important for ensuring the activity and stability of the catalyst. γ-Al2O3 is the most widely used Pt support in industry. However, its instability in the presence of SO3 leads to the formation of Al2(SO4)3 overlayers at ~600 °C that block access to the active Pt sites.25 TiO2,2, 16-18 SiO2,2, 29 and SiC22-24, 27, 28 have been reported as SO3-resistant support materials in the literature. Among these candidates, TiO2 has been studied extensively as an SO3resistant support, but most previous studies have focused only on the rutile TiO2 phase,16-18 which is thermodynamically stable at high reaction temperatures of ~800 °C. In our previous study,29 we found that Pt/anatase is much more active and stable than Pt/rutile at 600 °C, and its steady-state SO3 conversion efficiency is 10-fold higher than that of Pt/rutile because Pt is mainly present in the active metallic state on anatase TiO2, whereas less-active Pt oxides (PtO2 and PtO) are dominant on rutile TiO2. This stark contrast is rationalized by the fact that metallic Pt favors the dissociative adsorption of SO3 and the smooth desorption of the products (SO2/O2).19 During continuous catalytic testing at 600 °C for 1,000 h, Pt/anatase exhibited activity loss of approximately 4% (of the initial activity). To further reduce Pt activity loss under harsh reaction conditions, the present study is extended to support materials other than conventional TiO2 that are stable against SO3. Herein, we report for the first time that a Pt catalyst supported on Ta2O5 is much more stable than conventional Pt catalysts for the SO3 decomposition reaction at 600 °C. Among possible SO3-resistant metal oxides (TiO2, Nb2O5, Ta2O5, and WO3), which were studied comparatively as support materials for Pt catalysts, the long-term stability of Pt/Ta2O5 for the SO3 decomposition reaction at 600 °C was demonstrated over a total of 1,800 h. The catalysts before and after stability testing were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), transmission electron microscopy (TEM), temperature-programmed desorption (TPD) and other techniques. The origin of the high stability of Pt/Ta2O5 is discussed based on the results of these analyses in terms of the Pt oxidation state and metal-support interactions.

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2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. Supported Pt catalysts (1 wt% loading) were prepared by a wet impregnation method that used an aqueous solution of Pt(NO2)2(NH3)2 (Tanaka Precious Metals) and oxide powder supports (Ta2O5, Nb2O5, TiO2, and WO3). The anatase TiO2 was supplied by the Catalysis Society of Japan (JRC-TIO-8). The other oxides were purchased from Wako Pure Chemical Industries and used without further treatment. As-impregnated samples were calcined at 500 °C for 3 h in air and aged under the catalytic reaction conditions (14 vol% SO3, 18 vol% H2O, and N2 balance) at 600 °C for 2 h prior to catalytic testing. XRD analysis was performed using monochromated Cu Kα radiation (30 kV, 20 mA; Rigaku Multiflex). The chemical composition was analyzed using X-ray fluorescence spectroscopy (XRF, EDXL300, Rigaku). XPS was performed using monochromated Al Kα radiation (12 keV, K-Alpha; Thermo Fisher Scientific). The binding energy (EB) was charge-referenced to C 1s at 285 eV. TEM micrographs were acquired using an FEI TECNAI F20 operating at 200 kV. Brunauer-Emmett-Teller surface areas (SBET) were calculated using N2 adsorption isotherms obtained at −196 °C (Belsorp, Microtrac-bel Japan). The Pt metal dispersion was determined by pulsed CO chemisorption at 50 °C (Belcat, Microtrac-bel Japan) after the catalyst was reduced using H2 at 400 °C. For Pt/TiO2, however, the reduction temperature was 200 °C in order to avoid the strong metal support interaction (SMSI) effects.30, 31 The metal dispersion (DPt) was calculated from the molar ratio of chemisorbed CO to loaded Pt by assuming that the chemisorption stoichiometry of Pt:CO was 1:1. Temperature-programmed desorption of oxygen (O2-TPD) was measured in a flow reactor (Belcat-A, Microtrac-Bel Japan) to analyze the thermal dissociation behavior of the Pt oxide. Prior to the measurements, the sample was treated under a flow of 20 vol% O2/He at 500 °C for 1 h. After cooling, the sample was heated from

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ambient temperature to 800 °C at a constant rate of 10 °C min−1 in a flow of He. The gas leaving the sample was analyzed by an online quadrupole mass spectrometer (Belmass, Microtrac-Bel Japan). Pt LIII-edge XANES spectra were obtained at the BL9A station of the Photon Factory (PF), High Energy Accelerator Research Organization (KEK), and at the BL01B1 station of SPring-8, Japan Synchrotron Radiation Research Institute (JASRI). The spectra were recorded at ambient temperature in transmission mode using an ionization chamber filled with N2 for the incident beam, another chamber filled with 50% N2 + 50% Ar for the transmitted beam, and a Si(111) double-crystal monochromator. The catalysts and reference samples (PtO2 and Pt) were mixed with boron nitride (BN) powder to achieve an appropriate absorbance at the edge energy. The XAFS data were processed using the IFEFFIT software package (Athena and Artemis).

2.2. Catalytic Reactions. The catalytic reactions were carried out in a flow reactor (Supporting Information, Figure S1). Sulfuric acid (95%) was pumped and vaporized at 450 °C in a flow of N2 to thermally decompose into SO3 and H2O. The resulting gas mixture containing 14 vol% SO3, 18 vol% H2O, and N2 balance was then supplied to a catalyst bed. Although no such a carrier gas (N2) is used in the practical applications, the dilute SO3 is useful for monitoring the catalytic activity in terms of the concentration of O2 produced by SO3 decomposition (reaction 1). A granular catalyst (10–20 mesh) was affixed into a quartz tube (inner diameter = 8 mm) plugged with quartz wool at either end of the catalyst bed. The catalyst was diluted with dense SiC grains (1.1–1.4 mm in size; Shin-Etsu Chemical). These grains have good heat conductivity, which means that heat transfer to the catalysts could be improved and the plug-flow criteria satisfied. The SiC grains were confirmed to be inactive for SO3 decomposition at temperatures below 750 °C. The gas effluent from the catalyst bed was then bubbled into an aqueous solution of NaOH (to remove SO3) and dried using a dry ice-ethanol trap. The O2 concentration was measured using a magneto-pneumatic oxygen analyzer (Horiba MPA3000) and a gas chromatographer equipped

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with a thermal conductivity detector (He carrier, MS-5A column, Shimadzu GC8A). The steady-state conversion of SO3 to SO2 was calculated using the concentration of O2 in the gas effluent. The obtained value was consistent with the SO2 concentration in the effluent gas, which was determined using iodimetric titration. Kinetic analyses were performed at a weight hourly space velocity (WHSV) of 22–880 g-H2SO4 g−1 h−1. The gas concentrations and reaction temperatures were varied so that the steady-state conversions would not exceed 20%. The temperature dependence of the SO3 decomposition rates yielded linear relationships between the logarithmic reaction rates and the reciprocal absolute temperatures (1/T), and these were used to calculate the apparent activation energies (Ea) of the SO3 decompositions. The effects of the SO3 and O2 partial pressures on the SO3 decompositions were examined at around 600 °C. Initially, the SO3 partial pressure (pSO3) was varied from 2 to 16 kPa without O2 being supplied in the gas feed. Then the O2 partial pressure (pO2) in the gas feed was varied from 0.2 to 0.9 kPa while pSO3 was maintained at 14 kPa. The partial orders for SO3 (m) and O2 (n) were calculated using the empirical rate equation that gives the rate as k pSO

3

m

pO n. A catalyst-stability test was performed in a similar 2

manner. At a constant temperature of 600 °C, the reaction mixture was supplied at WHSV = 11 gH2SO4 g−1 h−1, and the deactivation behavior was recorded in terms of SO3 conversion as a function of time-on-stream.

3. RESULTS AND DISCUSSION 3.1. SO3 Decomposition Activity and Kinetic Analysis. Four metal oxides (TiO2, Nb2O5, Ta2O5, and WO3), which are not expected to form sulfates under a SO3 atmosphere, were used as support materials for Pt. Because rutile TiO2 stabilizes inactive Pt oxide at 600 °C, as reported previously,29, 32 anatase TiO2 was used in the present study. Table 1 lists the values of SBET, DPt, and the kinetic parameters for the as-prepared Pt catalysts. Regardless of their different

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SBET and DPt values, these catalysts show similar SO3 decomposition activities. For instance, when the catalytic reaction is carried out at T = 600 °C and WHSV = 11 g-H2SO4 g-cat−1 h−1, the observed steadystate SO3 conversions are comparable and more than 90% of the equilibrium value equal to 38%. The kinetic analysis for SO3 decomposition over these catalysts was performed at a greater WHSV (22–880 g-H2SO4 g−1 h−1) in the temperature range 500–650 °C to ensure that the steady-state conversions did not exceed 20% (see Figures S2–S4 in the Supplementary data). As shown in Table 1, Pt/Ta2O5 exhibits partial orders with respect to SO3 and O2 of m = 0.43 and n = −0.03, respectively, whereas the other catalysts show a more negative dependence on O2. The negative order with respect to O2 implies that the oxygen release after the dissociation of SO3 into SO2 and O is a key step in determining the overall reaction rate. SO3 decomposition is therefore inhibited by the presence of oxygen, which is considered to be strongly bound onto the surface of Pt. The least negative value of n for Pt/Ta2O5 should be beneficial, in particular, for high conversion efficiencies. Pt/Ta2O5 also exhibits the smallest value of Ea (78 kJ mol−1) among the tested catalysts. As reported in our previous paper,29 SO3 decomposition activity is strongly affected by the Pt oxidation state. Metallic Pt (Pt0) favors the dissociative adsorption of SO3 and the fast removal of the products (SO2 and O2) from the surface, whereas Pt oxides (Pt2+ or Pt4+) are much less-active. Figure 1 shows the O2 desorption profiles of the as-prepared Pt catalysts that contain Pt in the form of oxides after calcination in air. O2 desorption from Pt/Ta2O5 begins at the lowest temperature of approximately 400 °C, and is completed at ≤ 500 °C. The cumulative amount of O2 desorption nearly equals the stoichiometric decomposition from PtO2 to Pt. Conversely, O2 desorption from the other catalysts occurs at higher temperatures than that from Pt/Ta2O5. This indicates that Pt oxides are less stabilized on the Ta2O5 support than on the other supports, and thus metallic Pt species are favored. This feature is closely associated with the less negative partial order of the SO3 decomposition rate with respect to O2 that is shown in Table 1. The SO3 decomposition reaction steps over Pt can be described as follows:

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SO3s → SO2s + Os; subscript s means adsorbed species

(4)

SO2s → SO2

(5)

Os + Os → O2

(6)

The activity and deactivation of the catalysts can be determined by the rate of the dissociation of adsorbed SO3 species (4) and the removal rate of the decomposition products from the surface of the metal nanoparticles (5 and 6). Rashkeev et al. investigated the mechanism of SO3 decomposition over supported Pt catalysts using density-functional-theory (DFT)-based first-principle calculations.19 They found that the rate for reaction 4 is always faster than the rates for reactions 5 and 6, suggesting that Pt is more efficient when its metallic state is favored over its oxidized states. Metallic Pt surfaces should also be more efficient for the detachment of O atoms from SO3 species (reaction 4) compared to that from oxidized Pt surfaces. Ramos-Fernandez et al. studied the interaction between Pt metal and a Ta2O5 support by means of FTIR spectroscopy of CO chemisorption,33 and found that CO linearly adsorbed on Pt on Ta2O5 showed a lower wavenumber as compared to those found for CO adsorbed on Pt on other oxide supports. Therefore, the electronic density of Pt is considered to be increased via interaction with Ta2O5, which affects the dissociation ability of the S−O bond. The present O2-TPD results clearly demonstrate that Pt loaded on Ta2O5 should be most efficient in removing these decomposition products. These features are beneficial for catalytic activity and stability under a strongly oxidizing atmosphere containing SO3 and O2.

3.2. Stability of Supported Pt Catalysts. The long-term stability of Pt catalysts under harsh reaction environments is very important for practical applications. The catalyst-stability tests were performed at T = 600 °C and WHSV = 11 g-H2SO4 g-cat−1 h−1. Figure 2 plots the relative activity, which is normalized by the average conversion during the first 50 h, as a function of time-on-stream. Because the initial SO3 conversions of these catalysts are similar

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(more than 90% of the equilibrium conversion equal to 38%), the relative activities of the different catalysts can be compared directly. Clearly, the extent of deactivation is strongly dependent on the support material and decreases in the sequence WO3 > Nb2O5 > TiO2 > Ta2O5. Among the materials tested, Pt/Ta2O5 shows no noticeable deactivation over 1,800 h of continuous use, and the overall deactivation after 1,000 h is only 1.5% of the initial SO3 conversion. This is lower than the activity loss of Pt/TiO2 (approximately 4% per 1,000 h). The deactivation observed for Pt/Ta2O5 is less than 3% after 1,800 h of the reaction. The deactivation kinetics of SO3 decomposition over Pt/Ta2O5 was studied using a simple power-law equation, which is given by Equation 7.34-39 This equation assumes that the concentration of the active sites is a time-dependent power function of the remaining active sites, and the rate of deactivation is independent of the chemical species involved. This is reasonable considering the absence of significant vaporization loss of Pt during the stability test, as described below, ୢ௔

−‫ݎ‬ௗ = − = −݇ௗ ܽௗ ୢ௧

(7)

where rd and kd are the rate of deactivation and rate constant for deactivation, respectively, and d is the order of deactivation. The relative catalyst activity (a) is expressed as the rate of reaction over the deactivated catalyst with time (rt) divided by that over the fresh catalyst (r0). When the Equation 7 was applied to the Pt/TiO2(anatase) catalyst in our previous study, the observed activity data as a function of the time-on-stream showed good fit with the case of d = 2.29 Therefore, the same analysis protocol was applied to the present Pt/Ta2O5 system to estimate the deactivation after a longer duration. The following equations can be obtained by integrating Equation 7 (d = 2) with initial limits of t = 0 and a = 1: (1− a)/a = kd t

(8)

Figure 3 plots the simulated deactivation curves with different kd values together with the observed activity data for Pt/Ta2O5, suggesting that kd would be approximately 1.5 × 10−5 h−1 and the estimated

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deactivation after 1 year (~8,000 h) is nearly 10%. This is superior to the estimated deactivation for Pt/TiO2 (> ~25%) in our previous study.29 To the best of our knowledge, Pt/Ta2O5 has not been studied for SO3 decomposition and is among the most stable catalyst materials under the present reaction conditions at around 600 °C.

3.3. Characterization of Tested Catalysts. The spent Pt/Ta2O5 catalyst after stability testing for 1,800 h was characterized to elucidate the reasons for its high resistance against deactivation. Spent catalysts were taken from both the upstream and downstream parts of the catalyst bed, denoted as “up” and “down,” respectively. The analyzed Pt loadings of the spent catalysts were 0.95 wt% (up) and 0.98 wt% (down). The slight decrease, especially in the upstream part, indicates a possible loss of Pt during the reaction. Significant Pt volatilization loss was not observed obviously for all the tested catalysts shown in Figure 2. The SBET value decreased from 5 to 3 m2 g−1 (both up and down). The DPt value of the spent catalyst could not be determined by the pulsed CO chemisorption technique, probably because of weak CO chemisorption onto the spent Pt/Ta2O5. The particle size of Pt/Ta2O5 before and after stability testing was therefore obtained by TEM (Figure 4). Because of the high dark contrast due to large particles of Ta2O5, the nanoparticles of Pt could only be observed near the surface region, as indicated by the arrows. Highly dispersed Pt particles smaller than 2 nm are dominant in the fresh catalysts. Although slightly larger Pt nanoparticles of 3–5 nm are mainly observed in the spent catalysts, local agglomeration leads to more significant growth of Pt particles larger than 20 nm, as shown in the bottom two pictures in Figure 4. In contrast to the uniform Pt particle size of the fresh catalyst, the Pt particle size of the spent catalyst was heterogeneous. Significant differences are not observed between the upstream and downstream parts of the catalyst bed. Figure 5 shows the XRD patterns of Pt/Ta2O5 before and after stability testing for 1,800 h. The asprepared catalyst shows diffraction peaks due to Ta2O5, but no diffraction peaks ascribable to Pt because of the low crystallinity of Pt oxides. After the stability test, the diffraction peaks for Ta2O5 remain

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unchanged. The only change observed in the spent catalyst is the appearance of a very weak peak for Pt metal crystallites. Although, in our previous study, a similar phenomenon was observed for Pt supported on anatase TiO2 after catalytic stability testing under the same conditions for 1,000 h,29 the Pt peak for the present Pt/Ta2O5 catalyst is much less intense. Because the same trend was confirmed even when compared to Pt/Nb2O5 and Pt/WO3, which were used for the catalyst-stability testing in a much shorter period of time (500 h and 115 h, respectively) (Supporting Information, Figure S5), the Pt/Ta2O5 catalyst showed the highest sintering resistance. The size of the Pt crystallites in the spent Pt/Ta2O5 was estimated to be close to 30 nm by X-ray line-broadening analysis made using the Scherrer equation. These results suggest a bimodal distribution of Pt particle sizes after stability testing, with sizes ranging from several nm to over 30 nm. Pt nanoparticles that have survived on the surface of the Ta2O5 are considered to play a key role in the stable catalytic performance shown in Figure 2. The surface Pt oxidation state for Pt/Ta2O5 was analyzed using Pt 4f XPS (Figure 6). The fresh catalyst shows the three different peak sets for Pt 4f7/2 and Pt 4f5/2, which were assigned according to data from the literature.40 Peaks for Pt 4f7/2 (EB ~72.5 eV) and Pt 4f5/2 (EB ~76.5 eV) were assigned to Pt2+ and the peaks for Pt 4f7/2 (~75 and 75.5 eV) and Pt 4f5/2 (~77.5 and 78 eV) were assigned to Pt4+. In addition, peaks at the lowest EB values (71 and 74.5 eV) were assigned to metallic Pt0. As shown in Figure 6(b and c), the catalyst after the stability test exhibits metallic Pt as the dominant species, whereas oxidized Pt species are weaker or absent. Because XPS analyzes only near the surface, the Pt oxidation state for the whole catalyst was studied by Pt LIII-edge XANES (Figure 7). As-prepared Pt/Ta2O5 presents a spectrum similar to that of reference PtO2, while the spectrum is transformed into a shape similar to that for a Pt metal foil after stability testing. Therefore, not only the surface but also the particle bulk is stabilized in the form of the active metallic state under the SO3 decomposition atmosphere at 600 °C. These results suggest that the formation and preservation of metallic Pt nanoparticles is the main reason for the higher catalytic activity and long-term stability of Pt/Ta2O5. Although some Pt shows significant particle growth, the extent of such local agglomeration is much

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lower than that for other catalysts, such as the Pt/TiO2 from our previous study (Supporting Information, Figure S5).29 To prevent this local agglomeration and to further enhance long-term stability, the preparation of porous-structured and/or surface-textured Ta2O5 to enhance metal-support bonding is our next challenge, and will be reported in due course.

4. CONCLUSION The present study demonstrated for the first time that Pt/Ta2O5 exhibits a much greater stability in catalytic SO3 decomposition than that of Pt supported on other SO3-resistant support materials. During continuous reaction testing at 600 °C for 1,800 h, the catalyst showed no noticeable deactivation (activity loss ≤ 1.5% per 1,000 h). Thus, the observed stability is superior to that of Pt/anatase, which was developed in our previous study, and to those of catalysts supported on other SO3-resistant metal oxides Nb2O5 and WO3. This superior stability is closely associated with metallic Pt nanoparticles stabilized on Ta2O5, which are highly efficient for dissociation of SO3 and the smooth removal of the decomposition products (SO2/O2). Thus, Pt/Ta2O5 is a promising candidate for SO3 decomposition in solar thermochemical water splitting for CO2-free H2 production.

ASSOCIATED CONTENT Supporting Information Arrhenius plots, partial pressure dependences for SO3 decomposition, and XRD patterns of spent catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

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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 interests.

ACKNOWLEDGMENTS This work was supported by the Council for Science, Technology, and Innovation (CSTI), Crossministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST) and JSPS KAKENHI (Grant Number 16H02418). The XANES experiments were performed at BL9A with the approval of PF, KEK (Proposal No. 2016G527), and at the BL01B1 of SPring-8 with the approval of JASRI (Proposal No. 2017A1050).

AUTHOR INFORMATION E-mail: [email protected] Tel/Fax: +81-96-342-3651

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Table 1 Surface areas, metal dispersions, and kinetic parameters of SO3 decomposition over the supported catalysts Catalyst

Partial order c

SBET

DPt

/ m2 g−1

/%

m

n

/kJ mol−1

Pt/TiO2

47

9a

0.65 d

−0.08 d

94 f

Pt/Nb2O5

7

7b

0.30 d

−0.09 e

93 f

Pt/Ta2O5

5

24 b

0.43 d

−0.03 e

78 g

Pt/WO3

15

21 b

0.31 d

−0.25 e

83 g

Ea

a

Determined by pulsed CO chemisorption after H2 reduction at 200 °C.

b

Determined by pulsed CO chemisorption after H2 reduction at 400 °C.

c

Rate = k pSO3m pO2n. d Measured at 600 °C. e Measured at 570 °C.

f

Measured at 500-570 °C. g Measured at 540~650 °C.

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FIGURES

Pt/TiO2 Pt/Nb2O5 Pt/Ta2O5 Pt/WO3

O2 desorption rate / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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300

400

500

600

700

Temperature / °C Figure 1. O2-TPD profiles for Pt catalysts supported on SO3-resistant metal oxides measured in a stream of He. Heating rate: 10 °C min−1.

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Ta2O5

100

TiO2 Relative activity / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90

WO3

Nb2O5

80 70 60 50 0

500

1000

1500

2000

Time-on-stream / h Figure 2. Results of catalyst-stability tests at 600 °C for Pt catalysts supported on SO3-resistant metal oxides. WHSV = 11 g-H2SO4 g-cat−1 h−1.

17



105

Relative activity / %

100

a b c

95 90

d

85 80 75 70 0

500

1000

1500

2000

105 100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

95

a

90 85

b

80

c

75

d

70 0

2000

4000

6000

8000

Time-on-stream / h Figure 3. Deactivation rate analysis for Pt/Ta2O5 at 600 °C. The dotted lines are drawn using the kinetic expression with the order of deactivation, d = 2, and the deactivation rate constants, kd = (a) 1.5 × 10−5, (b) 3.0 × 10−5, (c) 4.1 × 10−5, and (d) 6.2 × 10−5 h−1.

18


Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(e)

(d)

Pt Ta2O5 Ta2O5

Pt

20 nm

Pt

20 nm

Figure 4. TEM images for Pt/Ta2O5: (a) as prepared and (b-e) after 1,800 h of catalyst-stability testing conducted for SO3 decomposition at 600 °C. Photographs (b,d) and (c,e) were taken from the downstream and upstream parts of the catalyst bed, respectively. Pt particles are indicated by arrows.

19



Page 20 of 28

Ta2O5 Pt (c)

(b)

(a)

0

10

20

30 40 2θ / degree

50

60

70

Figure 5. XRD patterns for Pt/Ta2O5: (a) as prepared and (b and c) after 1,800 h of catalyst-stability testing conducted for SO3 decomposition at 600 °C. The top two patterns (b) and (c) were taken from the downstream and upstream parts of the catalyst bed, respectively.

20


Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pt0

(c)

Pt2+

(b)

Pt4+ Pt4+

(a)

81 80 79 78 77 76 75 74 73 72 71 70 69

Binding energy / eV Figure 6. Pt 4f XPS for Pt/Ta2O5: (a) as prepared and (b and c) after 1,800 h of catalyst-stability testing conducted for SO3 decomposition at 600 °C. The top two spectra, (b) and (c), were taken from the downstream and upstream parts of the catalyst bed, respectively.

21



6

5 Normalized absorption / a. u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4 (a)

3 (b)

2 Pt foil

1 PtO2

0 11550

11560

11570

11580

Energy / eV

Figure 7. Pt LIII XANES for Pt/Ta2O5: (a) as prepared and (b) after 1,800 h of catalyst-stability testing conducted for SO3 decomposition at 600 °C.

22


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Ta2O5

100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TiO2

90

WO3

80

Nb2O5

SO3 SO2+1/2O2

70 60

Pt

50

SO3-resistant metal oxides

40 0

500

1000

1500

2000

Time-on-stream / h

28


Page 28 of 28

Platinum Supported on Ta2O5 as a Stable SO3 Decomposition

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