Photo-thermal coupling factor achieving CO2 reduction based on

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Photo-thermal coupling factor achieving CO2 reduction based on palladium-nanoparticle-loaded TiO2 Chenyu Xu, Wenhui Huang, Zheng Li, Bowen Deng, Yanwei Zhang, Mingjiang Ni, and Kefa Cen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00272 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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Photo-thermal coupling factor achieving CO2 reduction based on palladium-nanoparticle-loaded TiO2 Chenyu Xu, Wenhui Huang, Zheng Li, Bowen Deng, Yanwei Zhang*, Mingjiang Ni*, Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China

* Tel.: +86-571-87952040, E-mail address: [email protected] (Y. Zhang); Tel.: +86-571-87951369

[email protected] (M. Ni)

Abstract Solar fuels have attracted great interest as an alternative use for solar energy. However, the challenges are high temperatures and low solar utilization for thermochemical and photochemical conversion methods, respectively. To lower the temperature in thermochemistry and increase solar energy utilization, a photo-thermochemical cycle (PTC) has been reported for carbon dioxide (CO2) reduction and improved by palladium-nanoparticle-loaded TiO2 (PNT). A maximum and stable carbon monoxide (CO) production of 11.05 μmol/h/g is demonstrated using 1.0PNT, which is 8.27 x the CO produced by P25 in the PTC. The PNT can enhance light utilization by a redshift photoresponse range and visible light absorbance of localized surface plasmon resonances (LSPRs). Photo-induced electron and hole pairs (EHPs) could be more readily separated. More available charge carriers would induce more photo-induced vacancies in the photoreaction, which serve a key role in the PTC. Additionally, Pd can promote CO2 absorbance to form Pd-CO2- and Pd-CO2--VO on the defective surface in the thermal reaction. Finally, CO production can be enhanced by a photo-thermal coupling factor, and a reaction mechanism is proposed for the complete cycle based on both theoretical calculations and experiments. Keywords: CO2 reduction; solar fuel; LSPR; photo-thermochemical; palladium.

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1. Introduction Modern society based on fossil fuel combustion leads to lots of environmental issues. Meanwhile, carbon dioxide (CO2) levels are increased, which is a significant contributor to global warming.1,

2

However, as a carbon source, CO2, which is

abundant in the atmosphere, is also a cheap feedstock for carbon fuels. To produce suitable carbon fuels, a variety of CO2 conversion methods have been widely studied.3, 4

Solar energy attracts much attention with its clean, abundance and sustainability.

However, the nature of sunlight makes it unsteady and hard to be stored. Combining solar energy with CO2 reduction, the production of solar fuels is a promising yet challenging approach to meet future social need of both energy and environment.5 Based on solar thermal application, the two-step thermochemical cycle utilizing heat to dissociate metal oxides (MxOy), could avoid direct CO2 splitting.6-8 With the effect of that heat energy converted into chemical energy, the carbon fuels could be produced by massive natural resources, solar energy and CO2, as following steps: 2 MxOy → 2 MxOy-1 + O2 (g) (thermochemistry, > 1273 K)

(1)

MxOy-1 + CO2 (g) → MxOy + CO (g) (thermochemistry, > 773 K)

(2)

This method is a promising and attractive system because of its simplicity, facile recyclability, and high effectiveness and utilization of energy. Lots of researchers are attracted to the method and make efforts for improving cycle efficiency and reducing the MxOy splitting temperature by using multifarious metal-oxide redox pairs (primarily Zn/ZnO, Ce2O3/CeO2, FeO/Fe3O4, and SnO/SnO2).7, 9 Recently, a novel photo-thermochemical cycle (PTC) has been reported by Zhang 2


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et al. for the reduction of H2O and CO2.10-13 A photochemical reaction is introduced into the two-step thermochemical cycle to replace MxOy splitting stage with high temperature. Cycle temperature achieving an impressive decrease is reduced to between 373 and 873 K. Photoinduced oxygen vacancies (VOs) are produced on the surface of the metal oxide under ultraviolet-visible (UV-Vis) light in the first photochemical step. In the second thermochemical step, CO2 is reduced by the photoinduced VOs below 873 K. Finally, CO2 could be overall dissociated into carbon monoxide (CO) and dioxygen (O2). Comparing to original photocatalysis and thermal catalysis used solar energy, UV-Vis light of sunshine could be utilized to overcome high energy barrier of O2 generation reaction at room temperature in PTC. Then visible-IR could provide needed temperature for thermal reaction. The PTC gets the utmost out of solar light, thereby sharing accountability with different expert parts. Meanwhile, O2 and fuels could be generated separately, which solved the other difficult issue. Yoon et al. and Wang et al. also reported PTC water splitting and CO2 reduction.14, 15 According to the previous PTC mechanism, metal ions and oxygen ions attract photoinduced electrons and holes, respectively, under UV-Vis light. Then O2 and an extrinsic surface state on the metal oxide could be formed in an inert atmosphere, thus, forming.13 The photogenerated electron-hole pairs (EHPs) act as “fuel” in whole cycle, which are “burned” to produce VOs. Then, H2O and CO2 are reduced by VOs on the defective surface under heating. Clearly, both photo and thermal factors should be considered in PTC. The requirements of PTC material include a wider photoresponse 3


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range, a lower EHPs recombination rate, a lower VO formation energy and a suitable thermal catalysis for CO2 and H2O reduction. Loading TiO2 with noble-metal nanoparticles (Pd, Pt, Au, Ag, etc.) has been widely researched to overcome the shortcomings of TiO2 because the noble-metal particles reduce the rate of EHP recombination and lead to efficient charge separation.16, 17 Noble-metal nanoparticles are also promising for harvesting the energy of visible-light photons for chemical reactions because of their localized surface plasmon resonance (LSPR) properties.18-20 LSPR could generate higher energy electrons than electrons produced by original photocatalysis. And the surpassing electrons could overcome higher energy barriers to process difficult reaction by concentrating the diffuse solar flux.21, 22 In this paper, quantum Pd nanoparticles in the form of quantum dots were produced to achieve LSPR in the visible region.23, 24 Additionally, CO2 has been activated on the Pd nanoparticle surface in an abundance of research.25, 26 The activation, which could promote CO2 reduction in the thermal reaction, was investigated in this work. Both photo and thermal factors were proved to enhance CO2 reduction based on palladium quantum dots. A maximum stable CO production of 11.05 μmol/h/g was achieved using 1.0 palladium-nanoparticles-loaded TiO2 (PNT), which was 8.27 x the amount of CO produced by P25. Both multiple experiments and theoretical calculations have been carried out to deeply investigate the mechanism and key factors in the PTC method.

2. Results and discussion 4


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2.1. Photo-thermochemical performance To examine the influences of every step’s time, temperature and catalyst stability on the overall PTC, a series of experiments were conducted for PNT, as shown in Figure 1. Eight cycles were implemented independently for certain illumination times (from 0 min to 70 min, in 10 min increments) in a He atmosphere and under heating at in a CO2 atmosphere 773 K for 1 h, as shown in Figure 1a. A substantial CO production increase was showed from 0 min to 50 min, whereas CO production only slightly changed from 50 min to 70 min. These results indicated that 50 min was the optimal photochemical time for 1.0PNT. According to previous research, the VO concentration increased with increasing irradiation time and that the total amount of VOs was maximized at approximately 50 min for 1.0PNT.14, 15 Eight independent cycles were carried out to study the influence of the thermochemical time in Figure 1b. The 1.0PNT samples were irradiated for 50 min and heated for 0 min, 10 min, 20 min, 30 min, 40 min, 50 min, 60 min and 70 min (in this order) at 773 K. These results indicated that 50 min was the optimal thermochemical time for 1.0PNT. The maximum output of CO was achieved at 50 min, which was due to the increased number of VOs in 1.0PNT requiring more time for consumption. In the figure 1c, the CO yield increased with thermochemical temperature in the second thermal step over seven independent cycles. The 1.0PNT samples were illuminated for 50 min and heated for 50 min. Little CO was generated at 323 K, 373 K and 473 K with PNT, whereas the CO yield dramatically increased from 473 K to 773 K. The CO yield significantly decreased at 873 K. The thermal crystal-transition of TiO2 (from anatase 5


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to rutile) also began around this temperature, which represents an additional uncontrollable factor. Therefore, the experimental temperature was limited to 873 K to avoid high temperatures. As shown in Figure 1d, according to the above experiments with single variable, five successive cycles were carried out with the PNT samples illuminated for 50 min in the photochemical step. In the thermochemical step, the PNT samples were heated for 50 min at 773 K. The CO yield was stable for 0.1PNT, 0.5PNT and 1.0PNT. However, the CO yield decreased each consecutive cycle for 1.5PNT, and after five cycles, the production of CO dropped below the detection limit. In previous research, the amount of produced CO was approximately 2.23 μmol/g for each cycle using P25. Additionally, the average amounts of generated CO were 5.95, 13.96 and 18.42 μmol/g (2.67 x, 6.27 x and 8.27 x greater than the amount of CO generated using P25) for each cycle using 0.1PNT, 0.5PNT and 1.0PNT, respectively.

2.2. Crystal structure and morphology The field-emission scanning electron microscope (FESEM) images of the original and cycled 1.0PNT were used to study the PTC (Figure 2a and 2b). The PNT powder was composed of microparticles and constructed by a combination of nanoparticles with diameters between 20 nm and 40 nm. Additionally, the images of the original and cycled samples showed no evident change. The compositional analysis of 1.0PNT was performed by energy-dispersive X-ray spectroscopy (EDXS), as shown in Figure 2c. The EDXS mapping images of 1.0PNT suggested that Pd was homogeneously dispersed on the nanoscale level in Figure 2d and 2e.27 Figure 2f and 2g shows the 6


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presence of Ti and Pd, respectively, in the 1.0PNT sample, in good agreement with the results of the EDXS analysis. Studying the size of Pd nanoparticles (PNs) is important because Pd size would improve the reduction reaction efficiency.26 Detailed crystal structure and composition of the PNT samples have been obtained by using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD), and these results are shown in Figure 2h-k and Figure 3, respectively. In Figure 2h-k, many PNs were observed on the nano-TiO2 particles in all PNT samples. Moreover, the various PNT samples showed interesting and distinct differences. The PNs of 0.1PNT were the smallest amongst all samples, with diameters between 1.0 and 1.8 nm, and these particles were too small to confirm their shapes, as shown in Figure 2h. For 0.5PNT, the PNs grew with increased diameters between 1.9 and 2.2 nm; the lattice spacings of the two crystalline polymorphs were 0.351 and 0.220 nm, corresponding to the (101) crystal facet of anatase and the (111) crystal facet of metallic Pd, respectively (see Figure 2i).27 In 1.0PNT, the diameters of the particles grew to between 2.1 to 3.5 nm; however, the lengths of the sides were between 1 to 1.5 nm. Additionally, the (110) crystal facet of rutile was found (Figure 2j). The PNs of 1.5PNT grew to be substantially larger than those of 0.1PNT. Intact “hexagonal PNs”, with diameters of approximately 5.06 nm, were found, and the (111) crystal facet of metallic Pd was also observed (Figure 2k). The XRD patterns of all PNT samples were obtained before and after (signed as original samples and cycled samples, respectively) the PTC tests at 773 K in Figure 3a. Both anatase and rutile phases were observed, where the crystal facets were denoted 7


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by a black “A” and a red “R” respectively.10 There were few differences between the XRD patterns of the original and cycled samples for crystalline phases of TiO2. Except for cycled 1.5PNT, no Pd material or other oxide peaks were shown in the XRD results, indicating that the Pd atoms have a high dispersity without significant changes amongst the original samples, which agrees with the HRTEM and EDXS mapping results. Additionally, after the PTC experiments, the (111) and (200) crystal facets of Pd appeared in the XRD pattern of 1.5PNT,28, 29 indicating that the PNs may grow large enough by agglomeration for detection after the PTC, which is in good agreement with HRTEM results (see Figure S1d). The agglomeration of Pd led to a continuous increase in the PN size, which might have caused metallic Pd particles to be a centre of electron-hole recombination. This agglomeration may explain the lower CO production using 1.5PQTD. However, according to the XRD (Figure 3a) and HRTEM (Figure S1) results, the degree of agglomeration was significantly less on 0.1PNT, 0.5PNT and 1.0PNT, which may result from the better dispersities and smaller crystal sizes of the PNs in these PNT samples than the PNs in 1.5PNT. The effect of the thermal temperature on the crystal structure was evaluated using XRD analysis (shown in Figure 3b). The XRD showed no clear change for 1.0PNT when the temperature was lower than 773 K. However, the mass ratio between anatase and rutile decreased when 1.0PNT was cycled at 873 K. A crystal transition between anatase and rutile occurs at 873 K. The (111) and (200) crystal facets of metallic Pd were observed, further supporting that Pd agglomerated at 873 K, which is in accordance with the HRTEM results in Figure S3 and may cause the decreased CO 8


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production at 873 K.

2.3. EHP separation and optical absorption properties Photoinduced EHPs act as “fuel” for the photochemical step at a superficial level.14 A low rate of EHP recombination could produce more “fuel” for the reaction. To study the separation of photoinduced EHPs, the PL spectra of the PNT samples and P25 were examined in the region from 350 to 630 nm, and the results are shown in Figure 4a. The PNT samples displayed weaker PL intensities than bare P25, which suggests that the undesirable recombination of EHPs was effectively suppressed through the introduction of the PNs.30 Additionally, a clear trend was observed in which a higher Pd mass ratio resulted in a weaker PL intensity in the original samples. This trend may be due to more available EHPs producing more Ti3+ and VOs, as indicated by previous research.14, 15 Additionally, this reasoning may contribute to the higher CO production as the Pd mass ratio increased from 0.1PNT to 0.5PNT to 1.0PNT. The optical absorption properties of P25 and the PNT samples and the optical band gap of the materials were examined by UV-Vis diffuse-reflectance spectroscopy (DRS), as shown in Figure 5b. All samples exhibited intense optical absorption ranging from 200 to 380 nm, indicating that the materials were photoactivated under UV illumination.31 Clearly, the PNT samples increased absorption, and the absorption edge was extended compared with that of P25.32 Moreover, the band gap energies (Eg values) of P25 and the PNT samples were calculated using the following empirical 9


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equation: (αhν)2 = A2(hν - Eg)n

(3)

In equation (5), the absorption coefficient, Planck constant, light frequency, proportionality constant and band gap energy are represented by α, h, ν, A, and Eg, respectively. n is equal to 1 for a direct transition for TiO2.33 As shown in the inset of Figure 4b, the Eg values of 0.1PNT, 0.5PNT, 1.0PNT and 1.5PNT were approximately 3.22 eV, 3.18 eV, 3.12 eV and 3.08 eV, which were smaller than that of P25 (Eg = 3.23 eV). These results indicated that the absorption intensity in the visible region increased from the PNs, which suggests that more EHPs can be produced, enhancing the photochemical reaction in the PTC. It should be noted that noble metal loading on the surface of semiconductors would not change the band gap of samples theoretically. However, there were some chemical interactions between noble metal and TiO2, which would impact TiO2 surface structure. For example, chemical bonds (e.g. Pd-O-Ti) may be formed by loading noble metal on TiO2. So Pd2+ could be detected in the XPS results. Changes of surface state may influence structure and expand light absorption ability of materials.34 Moreover, evident peaks were observed at approximately 715 nm for the PNT samples (see Figure 4b and S3). Upon increasing the Pd loading, the intensity of the peak (λmax ≈ 710 nm) increased, except for 1.5PNT. This peak was attributed to the LSPR of the PNs on the surface of TiO2, which overlapped with the absorption range of PNT in the visible region and may have an influence on the PTC reactions.34 Control experiments (see Figure S5) showed that P25 could only be affected by UV 10


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light. Additionally, both UV and visible light could contribute to CO production using PNT, which confirms the improvement of PTC by LSPR. LSPR may be a key factor for the visible-light response of 1.0PNT, which could be excited by visible light (Figure 5a). As shown in Figure 5b, then generated hot electrons could transport from the high LSPR state of the PNs to the low conduction band (CB) state of TiO2 in the PNT samples. Finally, electrons could react with Ti4+ and induce more available VOs on the surface of TiO2 for CO2 reduction. For the reason that hot electrons had higher energy than normal excited electrons, more VOs with high formation energy could only be produced by hot electrons. On the other hand, the visible light energy could not be utilized in the previous work. Hot electrons generated by LSPR could make use of visible part energy, which would improve energy utilization from light source in the PTC. Additionally, hot electrons overstepped Schottky barrier and transferred from Pd to TiO2. The process reduced the recombination rate and increase availability of electrons in the photochemical step. To better understand the amplified electric field and charge transport caused by LSPR, three-dimensional finite-difference time-domain (3D-FDTD) simulations were performed to calculate the spatial electric-field distribution of a PNT as a function of the incident wavelength of light, as shown in Figure 6. The electric field of the PNT was stronger than that for TiO2, and the interface between the PN and TiO2 was “hot” at an excitation wavelength of 200 nm (Figure 6a); this indicated that electrons transfer from TiO2 to the PNs, which was designated the normal condition.27 When the excitation wavelength increased to 400 nm, near the inflection point of the UV-Vis 11


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DRS spectrum, the electric field of the PN interior was as strong as that of TiO2 (Figure 6b). At the edge of the PN, the electric field was very strong. Although the transfer ability became weaker, the electrons still transferred from TiO2 to the PN. However, in Figure 6c, the “hot spot” almost extended outside the PN at an excitation wavelength of 600 nm. As shown in Figure 6d, at an excitation wavelength of 715 nm, a very strong electric-field distribution existed on the TiO2 near the edge of the PN, and a comparable electric field did not exist on the PN; this indicated that most electrons transferred from the PN to TiO2, which may correspond to the LSPR conditions mentioned above.35 Moreover, the electric-field vector is shown in Figure 6f-6j, clearly indicating that the direction of the electric field started to change at an excitation wavelength of 600 nm (see Figure 6h). The direction of the electric field was fully reversed at an excitation wavelength of 715 nm (see Figure 6i), which may be caused by the LSPR.36 The variation of the electric field is intuitively shown in Movie S1. Above calculation results indicate that hot electrons could be generated on the PNs by LSPR and transferred from PNs to TiO2 based on the irradiation around 715 nm. Hot electrons possessed higher energy compared with normal excited electrons generated by P25, which led to much more utilization of light energy. These hot electrons could reduce recombination rate of EHPs and produce more difficult VOs on the surface. Especially, the VOs at the edge of PNs and TiO2 could benefit from LSPR. Due to increased VOs productions, more CO2 would be reduced to produce more CO. The simulation results are in good agreement with the experiments and characterization results. 12


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2.4 Charge-carrier transport and surface-state analysis To study the PTC mechanism, X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) were employed to detect the surface-state changes and the carrier-transport properties for three different states by using PNT. In Figure 7, States A and B represent the 1.0PNT before and after illumination by UV-Vis light in a He atmosphere, respectively. State C was obtained from the 1.0PNT sample subjected to a PTC cycle at 773 K. The results of the Pd 3d XPS peak-differentiation-imitating analysis are shown in Figure 7a. The peaks centred at 335.5 and 336.6 eV were assigned to Pd0 3d5/2 and Pd2+ 3d5/2, respectively. The peaks centred at 340.8 and 342.2 eV were assigned to Pd0 3d3/2 and Pd2+ 3d3/2, respectively.37, 38 Clearly, Pd0 and Pd2+ were present during the whole cycle. The presence of Pd2+ may be caused by the imbalanced charge between metallic Pd and Ti ions in the oxide TiO2. This could lead to formation of the Pd-O-Ti bond.39 The decrease in Pd2+ may be induced by VO formation at the Pd-O-Ti bond, going from State A to B. Additionally, the recovery of Pd2+ may be induced by Pd-O-Ti bond formation, going from State B to C. As shown in Figure 7b, the binding energy of Ti 2p3/2 reduced from 458.6 to 458.2 eV when going from State A to B and then returned to 458.8 eV at State C,13, 40 and the shifts were beyond the experimental errors of ±0.2 eV. This phenomenon can be due to Ti3+ generated by photochemical reaction under UV-Vis illumination. Additionally, the appearance of Ti3+ was also found at g = 2.003 after UV-Vis irradiation by ESR spectra (Figure S6).13 Furthermore, there was hardly any Ti3+ before irradiation and after the cycle. The O 1s spectra are shown in Figure 13


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7c. The XPS analysis shows asymmetric O 1s spectra that can be divided into three peaks. The binding energy values at 530.1 eV, 531.8 eV, and 532.4 eV were assigned to oxygen associated with O species in the lattice (OL), near the vacancies or defects (OV) and chemisorbed or dissociated (OC) species.39, 41 The results indicated that OV increased after irradiation and decreased after the thermal reaction. As mentioned above, photoexcited electrons may lead to VOs and O2 production on the TiO2 surface. The Ti-O bond may break, and VOs could be produced in the photo-step and consumed in the thermal reaction under a CO2 atmosphere in the PTC (Figure 7c). To uncover the underlying reasons for the enhancement of thermal CO2 reduction on the defect surface, in situ time-resolved diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) was utilized to characterize the adsorption modes in the second thermal step. After irradiation in the in situ reaction chamber filled with a He atmosphere at room temperature, the samples were heated to 473 K with a CO2/He-mixture flow during in situ DRIFTS measurements. As shown in Figure 8a, exposure of the irradiated P25 to CO2 induced the formation of CO adsorbed on the Ti atom (CO-Ti at 2205 cm-1), carboxylate ((CO2-)as at 1648 cm-1), bidentate carbonate ((b-CO32-)as at 1630 cm-1 and (b-CO32-)s at 1375 cm-1) and monodentate carbonate ((m-CO32-)as at 1548 cm-1 and (m-CO32-)s at 1455 and 1422 cm-1).5, 42 The CO-TiO2 and (CO2-)as peak increased from 2 to 50 min and remained unchanged from 50 to 60 min (Figure 8a). This indicated that CO2 was adsorbed on the VO of TiO2 and formed (CO2-)as by accepting e- from Ti3+ (Figure 8b). Then, CO2- on the VO could be heated to CO in the thermal reaction. However, CO-Ti and CO2- peaks were very weak and 14


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unchanged after 50 min. The CO2- group was a key intermediate but difficult to form on the TiO2 surface, which may have led to a dynamic obstruction. CO2 could be adsorbed on the VO and form CO2-, but the step proceeds slowly. Additionally, the CO2- formation step hinders CO2 reduction in the thermal reaction. As shown in Figure 8c, exposure of the irradiated 1.0PNT to CO2 induced the formation of CO adsorbed on the Pd atom (CO-Pd from 2150 to 2050 cm-1), carboxylate ( (CO2-)as at 1672 cm-1 and Pd-CO2--VO at 1625 cm-1), bidentate carbonate ( (b-CO32-)s at 1281 cm-1) and monodentate carbonate ( (m-CO32-)as at 1566 cm-1 and (m-CO32-)s at 1392 cm-1).42-44 The CO-Pd peak increased from 2 to 8 min, decreased from 10 to 45 min and then disappeared. In contrast, an evident (CO2-)as peak at 1672 cm-1 decreased from 2 to 10 min and increased from 15 to 60 min. Interestingly, a peak at 1625 cm-1 increased from 2 to 8 min, decreased from 10 to 45 min and then disappeared, which was assigned to CO2- adsorbed on both Pd and VO (Pd-CO2--VO) (Figure 8c). As shown in Figure 8d, Pd could adsorb CO2 and easily form CO2- on the surface after the photoreaction. Electrons could transfer from electron-rich Pd to the CO2 molecule to form CO2-, and VO could be filled with an O atom of CO2-. Then, the Pd-CO2--VO would be formed on the surface and heated to CO in the thermal reaction. Finally, CO was desorbed from the 1.0PNT surface. The Pd-CO2--VO existed before all VOs were used up. It may be a key intermediate group between CO2- (adsorbed on Pd) and CO. It should be pointed out that the CO2- peak rose again after 15 min. Furthermore, this indicated that Pd could continue adsorbing CO2 after the consumption of VOs, but CO could not be produced without VO. Two contrast 15


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experiments have been taken to ensure the conclusion in Figure S7. And details about Pd-CO2--VO peak have been discussed in the supporting information.

2.5. Density functional theory (DFT) calculations To study the activation of Pd for photo-absorbance, density of states (DOS) and charge density difference were calculated. For the pure surface Ti36O72 model (Figure S8a), the valence band (VB) consisted of Ti 3d and O 2p states, and the CB primarily consisted of Ti 3d states. The DOS of spin-up and spin-down was symmetric, indicating that there was no magnetic moment in Figure 9a. After loading a Pd atom onto the surface (Figure S8b), new states appeared on the Pd-loaded surface, as shown in Figure 9b. The Pd atom was an electron capture centre, which caused these two states in the DOS and the shift of the Femi level.45 Both spin-up and spin-down interstitial hybrid states in the DOS served as shallow acceptor energy levels near the VB top of the Pd-loaded surface, which could lead to expansion of the light absorption range.45 The lifetimes of the photoinduced EHPs could be prolonged by these impurity states, as well as generating more available carries to improve the reaction efficiency.45, 46 The electron capture centre could be demonstrated by the charge density difference. As shown in Figure S9, the charge density difference was obtained by subtracting the charge density of one isolated loaded Pd and anatase (101) surface from that of anatase (101) with the Pd atom adsorbed. The charge tended to localize on the Pd atom near the O atoms, which was beneficial for CO2 activation through effective binding with CO2- intermediate, consistent with the DOS analysis 16


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and IR results. To dive deep into the VOs formation and CO2 reduction process on the PNT surface, the whole reaction process was calculated. The VOs formation was recognized as the first stage of PTC. There were four different configurations of O sites (O2C, O3C1, O3C2 and O3C3) on the pure TiO2 surface. The selected VO sites on the pure surface (see Figure S10a) were marked with yellow (Figure S10b-S10e). As shown in Figure S11a, a Pd4 cluster placed on the top of three O2C sites of TiO2 was constructed. Four different O2C sites (O2C-p1, O2C-p2, O2C-p3 and O2C-p4, see Figure S11b-S11e) and three different O3C sites (O3C-p1, O3C-p2 and O3C-p3, see Figure S11f-S11h) were selected to calculate the VOs formation energy near the Pd4 cluster. As shown in Table S1, the VOs of O2C and O2C-p1 were the smallest among those on the surface of pure TiO2 and Pd4-TiO2, respectively, which means that two VOs were created most easily on these sites. On the basis of the in situ time-resolved DRIFTS results, the thermochemical CO2 reduction process was calculated on the defective surface of TiO2 and Pd4-TiO2 with the above two representative VOs. The adsorption, reaction and desorption energy results are shown in Table S2. Additionally, the whole reaction processes of pure and Pd4-loaded TiO2 are shown in Figure 10. The results of these calculations indicated that the VO formation energy of Pd4-TiO2 was smaller than pure TiO2, which means VO could be easier generated on the surface with Pd4. It might be caused that Pd4 cluster could attract the O atoms on the surface of TiO2. The attraction led to longer Ti-O bonds, which were easier to be broken.45 For example, according to DFT results, two Ti-O bonds connected to O2C-p1 were 1.845Å and 1.873 Å before 17


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loading Pd4, respectively. After Pd4 was loaded on the surface, these two Ti-O bonds have been added to 1.890 Å and 1.937 Å, respectively. The VO sites on the Pd4-TiO2 surface had better adsorption power than did the VO sites on the pure TiO2 surface. Electrons were transferred from Pd to CO2 to form CO2-, when CO2 was adsorbed on the Pd. The key intermediate group, CO2-, was readily formed on the Pd4-TiO2 surface in good agreement with the DRIFTS results. According to the DRIFTS results, not all of CO2- could be converted to CO. Especially, Pd-CO2- at the edge of Pd and TiO2 may be adsorbed by VO between Ti and Pd atoms (Pd-VO-Ti), and then Pd-CO2--VO could generate, as Pd-CO2--VO (2.50 eV) has a lower energy than Pd-CO2- (2.65 eV). The calculation results well explained the experimental results in the Figure 8c and 8d. Moreover, CO formation could occur more readily on the Pd4-TiO2 surface, which may cause the presence of CO-Pd peak immediately (Figure 8c). The DFT results also showed that CO adsorption energy was bigger on the Pd. It may indicate that CO was difficult to be desorbed on the PNT. To consider DRIFTS results, CO increased quickly in 10 minutes and was desorbed in 45 minutes based on PNT. But CO generated slowly by using pure P25. For the reason that CO2 was reduced in an airtight chamber under high temperature, CO had enough time and temperature to be desorbed. CO2 activation may play a more important kinetic factor of CO generation. To accelerate CO2 adsorption and reduction under high temperature, Pd could promote CO production effectively and CO desorption could be carried out fast to avoid catalyst poisoning. As a consequence of the calculations, PNT was proved to promote both photo and thermal reactions and acted as an effective photo-thermal 18


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coupling factor for CO reduction in this work.

3. Conclusions and outlook To enhance the performance of the PTC for CO2 reduction, PNT samples were applied for better optical properties and thermo-catalytic action caused by PNs. The experimental results indicated that PNT improved the production of CO in the PTC. A maximum and stable production of CO of 11.05 μmol/h/g was produced using 1.0PNT, which was 8.27 x the CO produced using P25 in the PTC. The PNs were stable after cycling at 773 K for 0.1PNT, 0.5PNT and 1.0PNT. PNT samples led to the absorption of more visible light and promoted the separation of EHPs resulting from the Schottky barrier and LSPR, which increased Ti3+ and VOs. CO2-, which is a key intermediate group in CO2 reduction, was formed easily via electron-rich Pd. Moreover, the Pd-CO2--VO may be generated before CO production, when CO2- reacted with VO. Additionally, the PNs showed good CO generation performance on the TiO2 surface. Finally, an effective photo-thermal coupling factor using PNT samples was proved to improve CO reduction in this work by both experiments and calculations. Additional studies of the PTC should be carried out to study the mechanism and enhance solar fuel production. In spite of some limitations currently, the PTC is a prospective approach, particularly if efforts are made to enhance both optical and thermal steps by utilized materials.

4. Experimental Section 19


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4.1. Fabrication of PNT thin films Herein, four different mass ratios of Pd to Ti were chosen to produce the PNT samples: 0.1 wt% PNT (0.1PNT), 0.5 wt% PNT (0.5PNT), 1.0 wt% PNT (1.0PNT) and 1.5 wt% PNT (1.5PNT). All PNT samples were prepared using TiO2 powder (Degussa-P25), who composed of 80% anatase and 20% rutile with average particle size of 30 nm, as the support and PdCl2 as the Pd source. Using 1.0PNT as an example, 0.99 g of P25 was mixed with 50 mL of deionized water as a solvent under stirring at 343 K. Then, 3 g of citric acid was added to the suspension to create a reducing environment. After the solute was homogeneously dispersed, 0.833 mL of a PdCl2 solution (0.113 M) was added dropwise to the suspension under vigorous stirring. A NaOH aqueous solution was utilized to adjust the pH value of the suspension to 10. The resulting suspension was stored at 343 K for 5 h, and then, crystals were collected by vacuum filtration using a funnel and were washed with a few millilitres of deionized water. The produced samples were dried in a vacuum oven at 333 K. The produced PNT powder (50 mg) was added to deionized water (5 mL) in a tube. After ultrasonic dispersion for 10 min, the suspension was input to a round quartz saucer with diameter of 5 cm at 383 K and stored for 3 h. Finally, the PNT film formed on the quartz saucer. Different ratios (0.1 wt%, 0.5 wt%, 1.0 wt% and 1.5wt %) of PNT in the thin films were produced by a similar procedure.

4.2．Photo-thermochemical cycling experiments An elaborate experimental platform was designed in this study in Figure S12. A 20


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stainless-steel reactor (approximately 200 mL) has been utilized to place the samples under a quartz cover. The thin-film sample (approximately 50 mg) covered on a quartz saucer (approximately 25 cm2), which was positioned on the reactor floor. The PNT films were preheated in situ at the thermo-step temperature for a sufficient time under a pure CO2 atmosphere to remove volatile compounds (e.g., residual moisture and carbon residue) and avoid misleading results in the reactor at the pretreatment stage. After thorough preheated process, misleading CO or other volatile compounds were undetectable and excluded by GC. Every cycle of PTC was taken as follow procedure. Helium gas (He) was input to the reactor (100 mL/min) in the first stage of the PTC. The He gas could prevent reoxidation of the thin-film samples in a protective atmosphere. A mercury lamp (500 W, > 254 nm) was used to illuminate the thin-film samples for a set time at room temperature to generate VOs on the sample surface in the He atmosphere. After the photochemical reaction, the light was turned off. And CO2 gas was input into the system for 10 min through the other gas circuit, forming a CO2 atmosphere in the reactor. An airtight chamber was formed after closing gas valves. Then CO2 and VOs were heated for a set time. A gas chromatograph (Agilent 7820A) with a thermal conductivity detector (TCD) was utilized to detect the gaseous products. After that, a whole cycle was finished and various measurements were conducted over set cycles to confirm the augmented CO production and to study the role of PNT under various conditions of PTC experiments.

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4.3. Analysis and characterization methods The structural properties of the obtained samples were characterized by FESEM, HRTEM, EDXS and XRD. The FESEM images were recorded by using an scanning electron microscope (FEISIRION-100). A FEI Tecnai G2 F20 S-TWIN instrument has been employed to record the HRTEM and EDXS micrographs by using an acceleration voltage of 200 kV. The XRD patterns were obtained by using a Cu-Kα radiation source (Kα = 1.540598 Å) operated at 40 kV and 100 mA. The diffraction patterns were obtained over a 2θ range of 10° to 85° at intervals of 0.02° and with a time step of 0.3 s. PL and UV-Vis DRS are conducted to investigate the optical properties. PL patterns were obtained using an Edinburgh Instruments FLS 920 with a 325-nm excitation source at room temperature, and the UV-Vis DRS spectra were acquired with a UV-Vis spectrophotometer (200-800 nm, SHIMADZU UV-3600, Japan). To analyse the surface-state changes and charge-carrier transport on the surface of PNT in the PTC, XPS, ESR and in situ DRIFTS were employed. An ESCALAB 250Xielectron spectrometer produced by Thermo Fisher Scientific has been equipped to record the XPS data. In the preparation of test sample, adventitious carbon was used as reference to determine all binding energies with C 1s peak (284.6 eV). ESR results were obtained by using 1.0PNT sample powders in a quartz tube on a Bruker A300 at 77 K. A Nicolet 6700 spectrometer was utilized to obtain IR spectra, which were equipped with a liquid nitrogen cooled HgCdTe (MCT) detector and displayed in absorbance units with a resolution of 4 cm−1, using 64 scans. The in situ DRIFTS studies were studied by using a reaction chamber produced by Harrick 22


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Scientific in a Praying Mantis DRIFTS accessory.

4.4. Theory calculations and simulations First-principles calculations using DFT have already been used to study the DOS and charge density difference of pure and Pd-loaded anatase (101) models, implemented in the Vienna ab initio simulation package (VASP 4.6). Here, we used the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06), which mixes general gradient approximation (GGA) exchange with screened Hartree–Fock (HF) exchange. The correlation part was defined by Perdew, Burke, and Ernzerhof (PBE), whereas a range-separation approach was used for the exchange part. A mixing of 25% of exact HF and 75% of PBE exchange was utilized at short range, while the standard PBE exchange was remained at long range. The range separation parameter was fixed at 0.2 Å. GGA functional would underestimate band gap commonly. It could be corrected with the replacement of HSE06 formalism. According to the XRD results, TiO2 anatase (101) was the most of the exposed crystal face and customarily chosen as the representative face in this work. A slab of Ti36O72 with a surface area of 10.89 × 7.55 Å2 was selected, and the slab thickness was three layers (see Figure S13). There were four types of oxygen: a bridging two-fold coordinated oxygen atom (O2C), three types of three-fold coordinated oxygen atoms (O13C, O23C, and O33C), and two types of titanium atoms: five/six-fold coordinated titanium atoms (Ti5C/Ti6C), as shown in Figure S13b and S13c. To economize valuable computing power, one Pd atom was utilized to calculate the DOS by the screened hybrid functional HSE06. The 23


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configuration with the Pd atom between two Ti5C atoms was stable after optimization (Figure S8b). Additionally, the configuration was utilized to calculate the DOS and charge density of Pd-loaded anatase (101). As shown in Figure S11a, a Pd4 cluster placed on the top of three O2C sites of TiO2 was constructed to process continued calculations. The VO formation energy was calculated by the following equation: Evf = Eds + 1/2EO2 - Eps

(4)

In the equation (4), Evf represents the VO formation energy; Eds and Eps represent the total energy of the defective and pure surface, respectively; and a single oxygen molecule’s total energy was represented by EO2, which could be calculated in a 10 × 10 × 10 vacuum box. To study the LSPR caused by PNT, the optical absorption was simulated by a FDTD method (Lumerical Solutions, v.8.11.387). A model of PNT was determined based on the HRTEM photographs of 1.0PNT and the metallic Pd crystal shape, in which a PN was constructed as a regular icosahedron (diameter was approximately 3 nm) located on the TiO2 surface, as shown in Figure S14. The size of the PNT model matched the average values. A plane wave with a broad wavelength range from 200 nm to 800 nm was used as the normal incidence. Additionally, the polarization direction of the excitation light was parallel to the x-axis. The optical constants of bulk TiO2 and Pd were adopted from the values proposed by Lumerical Solutions.19 The boundary of the model was divided into meshes with sizes of 0.05 nm.

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Supporting Information HRTEM images (Figures S1, S3), supplementary CO production (Figures S2, S5), irradiation spectrum of light source (Figure S4), ESR spectra (Figure S6), in situ DRIFTS for contrast experiments (Figure S7), computational details (structures, numerical values, etc.) (Figure S8, S10, S11 and S13; Table S1, S2), charge density difference calculation (Figure S9), FDTD model (Figure S14)

Acknowledgements This work was financially supported by the Innovative Research Groups of the National Natural Science Foundation of China (51621005), the Zhejiang Provincial Natural Science Foundation (LR18E060001) and the Fundamental Research Funds for the Central Universities (2017FZA4014). The authors gratefully acknowledge this support.

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Figure 1. CO production (by mass) of several PTCs: (a) the 1.0PNT samples were illuminated in a He atmosphere for 0, 10, 20, 30, 40, 50, 60 and 70 min and heated at 773 K for 1 h in a CO2 atmosphere; (b) the 1.0PNT samples irradiated for 50 min in a He atmosphere and heated for 0, 10, 20, 30, 40, 50, 60 and 70 min at 773K in a CO2 atmosphere; (c) the 1.0PNT samples illuminated for 50 min in a He atmosphere and heated at different temperatures for 50 min in a CO2 atmosphere; (d) five successive PTCs with all samples illuminated for 50 min in a He atmosphere and heated at 773 K for 50 min in a CO2 atmosphere.

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Figure 2. FESEM images of (a) original and (b) cycled 1.0PNT. (c) EDXS pattern of 1.0PNT. TEM image of (d) 1.0PNT and the corresponding EDXS mappings of 1.0PNT in the region shown in panel a, indicating the distributions of (e) both Ti and Pd, (f) Ti, and (g) Pd. HRTEM images of original (h) 0.1PNT, (i) 0.5PNT, (j) 1.0PNT and (k) 1.5PNT.

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Figure 3. XRD images: (a) full patterns of all original and cycled PNT samples at 773 K. (b) All cycled 1.0PNT samples at 373, 473, 573, 673, 773 and 873 K; inset: magnified region of the metallic Pd peak. Each crystal facet is denoted by a black “A’’(anatase) or a red “R” (rutile). 35


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Figure 4. (a) PL patterns of P25 and the PNT samples; (b) UV-Vis DRS spectra of the original P25 and the PNT samples. Inset: determination of the optical band gap.

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Figure 5. Processes involved in the (a) LSPR excitation and (b) hot electron reaction. CB and VB represent the conduction and valence bands of the semiconductor, respectively. EF refers to the Fermi energy level.

Figure 6. Spatial distribution of the LSPR-induced enhancement of the electric field intensity from the FDTD simulations for PNT at excitation wavelengths of (a) 200, (b) 400, (c) 600, (d) 715 and (e) 800 nm. The electric-field vector at excitation wavelengths of (f) 200, (g) 400, (h) 600, (i) 715 and (j) 800 nm. Here, E denotes the vector of the electric field. 37


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Figure 7. (a) XPS peak-differentiation-imitating analysis of the Pd 3d peaks for State A (before UV-Vis irradiation), State B (after UV-Vis irradiation) and State C (cycled) of 1.0PNT. XPS patterns of (b) Ti 2p and (c) O 1s at the various states in the PTC of 1.0PNT.

Figure 8. In situ time-resolved DRIFTS analysis of (a) a P25 sample and (b) a possible pathway of CO2 reduction on the defect surface of P25. In situ DRIFTS analysis of (c) the 1.0PNT sample and (d) a possible pathway of CO2 reduction on the defect surface of PNT in the thermal reaction of PTC. The “as” and “s” subscripts mean asymmetric vibration and 38


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symmetric vibration, respectively.

Figure 9. Density of states (DOS) for (a) pure anatase (101) and (b) Pd-loading anatase (101).

Figure 10. The whole reaction process of pure and Pd4-loaded TiO2.

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Photo-thermal coupling factor achieving CO2 reduction based on

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