Pd@HyWO3–x Nanowires Efficiently Catalyze the CO2

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Pd@HyWO3−x Nanowires Efficiently Catalyze the CO2 Heterogeneous Reduction Reaction with a Pronounced Light Effect Young Feng Li,† Navid Soheilnia,† Mark Greiner,‡ Ulrich Ulmer,† Thomas Wood,† Abdinoor A. Jelle,§ Yuchan Dong,† Annabelle Po Yin Wong,† Jia Jia,§ and Geoffrey A. Ozin*,† †

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada Fritz-Haber-Institut der Max-Planck-Gesselschaft, Faradayweg 4-6, 14195 Berlin, Germany § Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario M5S 3E4, Canada ‡

S Supporting Information *

ABSTRACT: The design of photocatalysts able to reduce CO2 to valueadded chemicals and fuels could enable a closed carbon circular economy. A common theme running through the design of photocatalysts for CO2 reduction is the utilization of semiconductor materials with high-energy conduction bands able to generate highly reducing electrons. Far less explored in this respect are low-energy conduction band materials such as WO3. Specifically, we focus attention on the use of Pd nanocrystal decorated WO3 nanowires as a heretofore-unexplored photocatalyst for the hydrogenation of CO2. Powder X-ray diffraction, thermogravimetric analysis, ultraviolet−visiblenear infrared, and in situ X-ray photoelectron spectroscopy analytical techniques elucidate the hydrogen tungsten bronze, HyWO3−x, as the catalytically active species formed via the H2 spillover effect by Pd. The existence in HyWO3−x of Brønsted acid hydroxyls OH, W(V) sites, and oxygen vacancies (VO) facilitate CO2 capture and reduction reactions. Under solar irradiation, CO2 reduction attains CO production rates as high as 3.0 mmol gcat−1 hr−1 with a selectivity exceeding 99%. A combination of reaction kinetic studies and in situ diffuse reflectance infrared Fourier transform spectroscopy measurements provide a valuable insight into thermochemical compared to photochemical surface reaction pathways, considered responsible for the hydrogenation of CO2 by Pd@HyWO3−x. KEYWORDS: tungsten trioxide, CO2 hydrogenation, hydrogen tungsten bronze, heterogeneous photocatalysis, hydrogen spillover

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INTRODUCTION

generated electrons of high enough energy to enable CO2 reduction.11−14 WO3 has an electronic band gap in the visible spectral range (∼2.4 eV) and a relatively positive valence band edge that can facilitate water oxidation.15−19 Even though its conduction band edge is rather positive, WO3 could potentially catalyze hydrogenation and deoxygenation reactions of CO2. These attributes relate to the ability of WO3 to form hydrogen bronzes, which contain Brønsted protons, excess electrons, and oxygen vacancies (VO) in its lattice.20−22 The introduction of a hydrogen dissociation catalyst such as Pd onto the surface of WO3 enhances hydrogen bronze formation via the hydrogen spillover reaction.23−27 Protons and electrons are inserted throughout the bulk of the WO3 lattice and become stabilized as Brønsted acidic OH groups and W(V) sites, respectively,28,29 functioning as active sites for the

The conversion of CO2 into value-added chemicals and fuels is gaining interest for its potential benefits to reduce greenhouse gas global warming and provide the chemical industry with a sustainable C1 feedstock.1,2 The high thermodynamic and kinetic stability of CO2 continues to challenge efforts aimed at its utilization. A promising approach for CO2 reduction at industrially relevant scales involves heterogeneous catalytic hydrogenation. Thermochemical conversions commonly involve an irreducible metal oxide support, exemplified by Al2O3, ZrO2, and SiO2 integrated with a transition-metal cocatalyst, typically Ni, Pt, Pd, and Ru.3−7 These thermochemical processes often involve surface bicarbonate intermediates formed at the interface between the metal and support via reaction of CO2 with surface hydroxides. These surface bicarbonates are subsequently reduced by dissociated H2 on the metal at the interface between metal and the support.8−10 The photochemical hydrogenation of CO2 is of a more recent vintage than these thermochemical studies. Photocatalysts for CO2 reduction are commonly metal oxide semiconductors designed to provide conduction band energies and photo© XXXX American Chemical Society

Special Issue: Artificial Photosynthesis: Harnessing Materials and Interfaces for Sustainable Fuels Received: March 27, 2018 Accepted: May 23, 2018

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DOI: 10.1021/acsami.8b04982 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) SEM and HRTEM (inset) images of WO3 nanowires, (b) STEM and HRTEM (inset) images of Pd@WO3. Inset scale bars equal 1 nm. XPS spectra of Pd@WO3 under 0.3 mbar H2 at 200 °C: (c) Pd 3d and (d) W 4f.

Figure 2. (a) UV−vis−NIR spectra of Pd@WO3 after heating in air at 200 °C for 12 h, after exposure to H2 for 12 h, and after heating under a flow of 5% H2/Ar at 200 °C for 3 h (photograph in inset), normalized to the absorption peak at 250 nm. (b) TGA plot of Pd@WO3. Dashed gray lines indicate gases switching from N2 to air to H2..

bicarbonates in stark contrast to the photochemical generation of formates, carbonates, and carboxylates.

reduction of CO2. In addition, oxygen vacancies formed in nonstoichiometric WO3−x enable the light-assisted stoichiometric extraction of O atoms from CO2 to form CO.30 The integration of Pd, a hydrogen spillover catalyst on WO3, inspires the possibility that Pd@HyWO3−x could contain highly active sites for CO2 hydrogenation on the entire HyWO3−x surface rather than being limited to the vicinity of Pd. In this study, WO3 nanowires loaded with Pd nanocrystals provide a synthetic pathway to Pd@HyWO3−x. This material is investigated as a photocatalyst for the light-assisted hydrogenation of CO2. The presence of Brønsted acidic OH groups, W(V) sites, and VO in the material is defined by UV−vis-NIR spectroscopy, in situ X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) experiments. Catalyst testing of Pd@HyWO3−x for CO2 hydrogenation revealed high selectivity toward CO formation with a decrease in activation energy from 54 to 31 kJ/mol under solar illumination. Kinetic studies of the apparent reaction orders of CO2 and H2 for CO formation indicate a shift from a CO2 dependent reaction in the dark to one dependent on H2 under illumination. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments revealed the thermochemical production of surface

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RESULTS AND DISCUSSION Characterization. SEM and HRTEM confirmed the morphology of WO3 (Figure 1a) as nanowires around 50 nm in diameter and several micrometers in length. An observed dspacing of 0.38 nm along the length of the nanowires indicate a (010) growth direction, also suggested by the major peak on the powder X-ray diffraction (PXRD) pattern (Figure S5) at 23.4° corresponding to the (010) planes. After photodeposition of 1 wt % Pd on WO3 (Pd@WO3), scanning transmission electron microscopy (STEM) and high-resolution transmission electron microscopy (HRTEM) (Figure 1b) revealed dark spots on the nanowires, attributed to the Pd nanocrystals roughly 10 nm in diameter with an observed lattice parameter of 0.24 nm corresponding to the Pd (111) planes. The Brunauer−Emmett−Teller (BET) surface area was also measured to be 77 m2 g−1. The state of Pd and W centers under catalytically relevant conditions was investigated by in situ XPS at 200 °C under 0.3 mbar H2. The Pd 3d region (Figure 1c) contained two asymmetric doublets centered at 335 and 341 eV and 337 and 344 eV. The lower binding energy pair B

DOI: 10.1021/acsami.8b04982 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces corresponds to the Pd0 3d5/2 and 3d3/2 peaks, respectively, whereas the higher binding energy doublet suggests oxidized Pd species likely residing at the Pd surface and/or the Pd−WO3 interface. The W 4f region (Figure 1d) contains two overlapping W 4f5/2 and 4f7/2 doublets centered at 36.0 and 38.1 eV and 34.5 and 36.7 eV. The higher binding energy pair corresponds to the W6+ centers, whereas the low binding energy pair indicate the presence of W5+ centers from HyWO3 and/or VO formation.31,32 Significant population of the W 5d orbitals and bronze formation is thus likely to occur at catalytically relevant conditions. Hydrogenation of Pd@WO3. Introduction of H2 was observed to slightly lower the bandgap of WO3 from 3.3 to 3.2 eV (Figure S7) and significantly increase the optical absorbance of Pd@WO3 in the visible and NIR region relative to the 300 nm peak (Figure 2a) and less so for WO3 (Figure S9). This broadband absorption is attributed to the intra- and interband transitions within Pd along with the polaron band of HyWO3−x. This optical transition from dark yellow to a dark blue also suggests the possibility toward utilizing the entire solar spectrum for photothermally enhanced (photo)catalysis rather than a small fraction for bandgap excitation. The photothermal capabilities under a H2 /CO 2 environment are further demonstrated in Figure S8, whereby films of Pd@HyWO3−x are heated to 200 °C when irradiated by a 120 W Xe light source. The distinction between whether VO and/or OH correlate with the formation of W5+ centers upon hydrogenation was investigated by TGA. As described in Figure 2b, introduction of H2 at low temperature (50 °C) resulted in rapid weight gain of Pd@WO3 compared to a slower increase for WO3. After 1 h, the weight increase of Pd@WO3 stabilized to 1.2% compared to 0.6% for WO3 and 0.5% for 1 wt % Pd@Al2O3 (Figure S2). The much larger weight increase of Pd@WO3 is indicative of H2 dissociation by Pd and its insertion into WO3 as HxWO3, producing W5+ centers and OH groups. Upon heating, Pd@ WO3 lost significantly more weight than WO3 and Pd@Al2O3, dropping below the initial weight at 130 °C, indicative of VO formation rather than solely H2 desorption. The confirmed capacity for OH groups and VO formation under H2 is thus promising toward the absorption and subsequent catalytic reduction of CO2. CO2 Hydrogenation by Pd@WO3. A comparison of the catalytic activity of Pd@WO3 to WO3 and Pd@Al2O3 toward CO2 hydrogenation is shown in Figure 3. In a batch reactor

illuminated by a 300 W Xe arc lamp, WO3, Pd@WO3, and Pd@ Al2O3 were observed to selectively produce 13CO when 13CO2 was used, ruling out carbon contamination with selectivity over 13 CH4 exceeding 99% (Figure S12). Pd@WO3 and WO3 produced CO at 1.3 and 0.3 mmol g−1 hr−1, respectively, in the illuminated batch reactions, significantly higher than Pd@ Al2O3. Under flow conditions (Figures 3 and 4a and b), Pd@

Figure 4. (a) Temperature dependence of the CO production rate on Pd/WO3 in the dark (black) and under illumination (red). (b) The corresponding Arrhenius plot with activation energies noted. (c and d) CO production rate dependence on partial pressure of CO2 and H2 at different temperatures and illumination.

WO3 was also observed as the most active for CO production with a decrease in the apparent activation energy from 54 to 31 kJ/mol upon illumination by a 120 W Xe lamp. A slight photoenhancement was also observed for WO3, whereas Pd@ Al2O3 had no significant light effect. The greater CO production of Pd@WO3 over Pd@Al2O3 is attributed to the redox activity of WO3 and the mobility of dissociatively absorbed hydrogen throughout the bulk of WO3 rather than being limited to the Pd−WO3 interface. Additionally, the capability of the WO3-based catalysts to utilize light more effectively compared to Al2O3 is likely attributed to the smaller bandgap of WO3 facilitating the generation of excitons to further facilitate the reduction of CO2. The wavelength dependence of illumination on CO formation (Figure S7) also suggests a photoexcitation process responsible for the photoenhancement of WO3 whereby illumination by 300−480 nm light resulted in the largest enhancement. An external and internal quantum yield of 0.0056 and 0.051%, respectively, for CO production was determined for the Pd@WO3 sample in the batch reactor. A comparison with previously studied light driven reverse water gas shift catalysts is included in Table S3. Partial Pressure Dependence of CO2 Hydrogenation by Pd@WO3. Apart from the increased CO production, illumination was also observed to affect the apparent reaction orders of CO2 (α) and H2 (β) (Figures 4c and d). In a thermal process, α and β are observed to increase from 0.72 to 0.94 and 0.10 to 0.25, respectively, as the temperature increased from 473 to 523 K. This is similar to a study on Pd@Al2O3 which reported α and β values around 0.4−0.6 and 0.2, respectively,9 suggesting the thermally driven process is highly dependent on CO2 partial pressure, whereas H2 dissociation occurs faster and

Figure 3. CO production rates over various catalysts with (red) and without (black) illumination at 250 °C and in a batch reactor with 13 CO2 illuminated by a 300 W Xe arc lamp (blue). *WO3 was heated at 300 °C in the flow experiments. C

DOI: 10.1021/acsami.8b04982 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) In situ DRIFTS measurements of Pd@WO3 under a flow of H2:CO2 at 200 °C in the dark after 2 h (black) and under illumination for 2 h (red). (b) Possible reaction pathway of CO formation via interaction of CO2 with VO and/or hydroxide groups.

not rate limiting. With illumination by a 120 W Xe arc lamp, α decreased to 0.29, and β increased to 0.62, suggesting a photochemical pathway distinct from the thermal process where the rate-determining step depends on H2 more than CO2. Although the penetration depth of light limited the accuracy of the kinetics study, the observation of a distinct photochemical effect remains relevant. In Situ DRIFTS. In situ DRIFTS experiments in the dark and light (Figure 5a) were performed to probe the species on the active catalyst surface, providing insight into the surface chemistry responsible for CO2 hydrogenation. Prior to DRIFTS measurements, Pd@WO3 was initially heated under a flow of He/H2 at 200 °C to form the defect laden Pd@HyWO3−x and to remove any surface contaminants. Upon introduction of CO2, peaks appeared within the region 2000−2100 and 1900− 1950 cm−1. This indicates the generation of monodentate and bidentate carbonyl species, respectively, on the surface both in the dark and light.33,34 Peaks centered at 1693 and 1404 cm−1 along with a weak feature at 1200 cm−1 appeared in the dark, indicating the accumulation of bicarbonate species on the surface likely from the interaction of CO2 with surface hydroxide. Upon illumination, it is observed that formate (1388 and 1558 cm−1), carboxylate (1245 and 1670 cm−1), and carbonate (1278, 1593, and 1765 cm−1) species begin accumulating while the bicarbonate peaks disappear, suggesting the acceleration of bicarbonate reduction.10,11,35−39 On the basis of the operando DRIFTS observations, two pathways are proposed, as illustrated in Figure 5b. On the left, a Langmuir−Hinshelwood pathway describes the formation of surface bicarbonates via the interaction of CO2 with surface hydroxides followed by hydrogenation to formate and then CO via dissociated H atoms on Pd10 (Pd−WO3 interface) or on the surface of the HyWO3−x bronze. On the right, a Mars−van

Krevlen pathway is described, whereby surface CO2 species react with a VO to form a carboxylate intermediate before dissociating as CO. The potential of these reactions to occur away from the Pd−WO3 interface presents an interesting route to improved catalyst utilization. In accordance with the observed reaction orders α and β in the dark and light, it is likely that surface bicarbonate species react sluggishly in the dark, leading to a higher dependence on CO2 (α > β). Under illumination, the increase in β over α indicates a H2 dependent step becoming rate limiting, which may suggest the increased reactivity of surface bicarbonate species on the illuminated catalyst surface. In the Mars−van Krevlen pathway, this likely corresponds to an increased conversion rate of bicarbonate to carboxylate on the surface, resulting in the shift in the rate-limiting step toward the regeneration of VO by H2. Additionally, the disappearance of bicarbonate and accumulation of formate suggests the promotion of bicarbonate hydrogenation to formate in the Langmuir−Hinshelwood pathway under illumination, also leading to a lower β.

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CONCLUSION Pd nanocrystals nucleated and grown on WO3 nanowires catalyze the reduction of CO2 by H2 to CO with an impressively high conversion rate and selectivity. TGA, XPS, and UV−vis−NIR experiments revealed the formation of HyWO3−x under reaction conditions arising from Pd promoted H2 dissociation and spillover and the ability of WO3 to store protons and electrons within its lattice as a hydrogen tungsten bronze. Under illumination, CO production was observed to significantly increase along with a decrease in the apparent activation energy from 54 to 31 kJ/mol. Concomitantly, the observed rate law changed from r = k[CO2]0.7[H2]0.1 in the D

DOI: 10.1021/acsami.8b04982 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces dark to r = k[CO2]0.3[H2]0.6 under illumination. Additional insight into this change stems from in situ DRIFTS experiments, where distinct pathways for the photochemical and thermochemical CO2 reduction reaction are observed.

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Thermo Scientific Kα with an Al Kα X-ray source. SEM and STEM was done using a Hitachi S-5200 HRSEM. HRTEM was done using a Hitachi CFE-TEM HF3300. In Situ DRIFTS. A Thermo Scientific iS50 FTIR spectrometer equipped with a Harrick Scientific Praying Mantis reaction chamber was used for the DRIFTS studies. The sample used for DRIFTS studies was diluted by KBr due to the high IR absorbance of HyWO3−x. It was determined that 5 wt % Pd@WO3 to KBr resulted in the best signal-to-noise ratio, although the spectra still had poor signal-to-noise ratio and peak resolution. Powder was placed into the sample cup without any packing, and gases were flowed through the powder from top to bottom at a total flow rate of 20 sccm. Prior to measurement, the sample was heated under a flow of 1:1 H2:He at 200 °C for 2 h, and a background measurement was done. The gas composition was then changed to 1:1 and H2:CO2, as spectra were collected. A 120 W Xe arc lamp was also used to illuminate the sample through a quartz window.

EXPERIMENTAL SECTION

Materials. Tungsten(VI) chloride (WCl6 > 99.9%), sodium tetrachloropalladate (Na2PdCl4, 99.95%), 1 wt % palladium on alumina (Pd@Al2O3), and Carbon-13C dioxide (13CO2, 99%) were purchased from Sigma-Aldrich and used without further purifications. Anhydrous ethanol was purchased from Commercial Alcohols and used as received. Synthesis of WO3 Nanowires. WO3 nanowires were prepared by a modified literature report of the alcoholysis of WCl6 in ethanol.30 WCl6 (150 mg) was dissolved in 50 mL of anhydrous ethanol, transferred into a 100 mL Teflon lined steel autoclave, and heated at 180 °C for 24 h. The resulting blue W18O49 was retrieved by centrifugation, washed with ethanol and DI water, dried under vacuum, and oxidized in air at 200 °C overnight. Synthesis of Pd@WO3 Nanowires. Pd@WO3 was prepared by photodeposition of Pd onto WO3. In general, 50 mg of WO3 was dispersed into 75 mL of ethanol by sonication followed by illumination from a 120 W Xe arc lamp while stirring under inert atmosphere. Na2PdCl4 (1.6 mg, 98%, Aldrich) dissolved in 75 mL of ethanol was then added to the flask at a rate of 1 mL/min. After all of the solution was added, the reaction was kept stirring under illumination for an additional 30 min. The Pd@WO3 nanocrystal−nanowire composite was then collected by centrifugation, washed with water and ethanol, and dried under vacuum and then air at 200 °C. CO2 Hydrogenation. Flow experiments were done in a plug flow capillary reactor which consisted of the catalyst packed into a quartz tube and immobilized with quartz wool on both ends. Nine milligrams of Pd@WO3, 16 mg of Pd@Al2O3, and 14 mg of WO3 were used in this study. The quartz tube had an inner diameter of 2 mm and was positioned on a groove carved out of a copper block to accommodate the tube. An OMEGA temperature controller was attached to a heating cartridge inserted into the copper block along with a thermocouple inserted into the quartz tube in contact with the catalyst bed for control of the catalyst temperature. A 120 W Xe arc lamp was used to illuminate the catalyst plug at a measured intensity of 2 W cm−2. CO2, H2, and He were flowed through in various ratios with a combined flow of 4 sccm controlled by Alicat Scientific digital flow controllers. Cutoff filters by Newport were used to control the illumination wavelengths. The amounts of CO and CH4 produced were determined using gas chromatography−mass spectrometry (GCMS, 7890B and 5977A, He carrier, Agilent). Reactions carried out in a batch reactor used 1 mg of sample deposited on borosilicate filters placed into a stainless-steel reactor equipped with a quartz window and pressurized to 27 psi of 1:1 H2:13CO2. A 300 W Xe arc lamp illuminated the catalyst with a measured intensity of 1.36 W cm−2 for 2 h followed by 13CO and 13 CH4 measurement by a flame ionization detector (FID) in a SRI8610 GC and an Agilent 5975C MSD. Thermogravimetric Analysis. TGA experiments were done on a Discovery Series TGA from TA Instruments. A constant gas flow of 10 sccm was used at all times. An initial bake under N2 at 150 °C was done to remove any adsorbed species followed by an oxidation step under air at 250 °C for 3 h. The samples were then cooled to 50 °C, 5% H2/Ar was introduced, and the system was allowed to equilibrate for 1 h. Lastly, a temperature ramp to 300 °C at 1 °C/min was applied under the 5% H2/Ar flow. Characterization. PXRD patterns were collected on a Bruker D2Phaser X-ray diffractometer using Cu Kα radiation at 30 kV. Diffuse reflectance spectra were collected using a Lambda 1050 UV/vis/NIR PerkinElmer spectrometer with an integrating sphere. In situ XPS experiments were performed at the ISISS beamline of BESSY II/HZB (Berlin, Germany) at 0.3 mbar of H2 heated to 200 °C. The Pd 3d and W 4f regions were studied with a photon energy of 560 and 202 eV, respectively. XPS of as-synthesized Pd@WO3 was also performed on a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04982. SEM and STEM of Pd@Al2O3 (Figure S1); photograph and TGA of Pd@Al2 O3 under H2 (Figure S2); photograph of as prepared and oxidized WO3 (Figure S3); electron microscopy of WO3 and Pd@WO3 before and after heating under H2 at 200 °C (Figure S4); PXRD (Figure S5); XPS (Figure S6); bandgap determination (Figure S7); photothermal measurements (Figure S8); UV−vis−NIR of WO3 and Pd@Al2O3 (Figure S9); CO production data (Table S1); stability tests (Figure S10); Pd loading dependence (Figure S11); CO and CH4 selectivity comparison (Figure S12); DRIFTS peak assignments (Table S2); comparison with literature (Table S3). (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mark Greiner: 0000-0002-4363-7189 Ulrich Ulmer: 0000-0003-1692-2694 Annabelle Po Yin Wong: 0000-0002-5341-1036 Geoffrey A. Ozin: 0000-0002-6315-0925 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.A.O. is a Government of Canada Research Chair in Materials Chemistry and Nanochemistry. Financial support for this work was provided by the Ontario Ministry of Research Innovation (MRI); Ministry of Economic Development, Employment and Infrastructure (MEDI); Ministry of the Environment and Climate Change (MOECC); Connaught Innovation Fund; Connaught Global Challenge Fund; and the Natural Sciences and Engineering Research Council of Canada (NSERC).

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DOI: 10.1021/acsami.8b04982 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX