- Email: [email protected]
Pd-TiO2 Schottky heterojunction catalyst boost the electrocatalytic hydrodechlorination reaction
Journal Pre-proofs Pd-TiO2 Schottky Heterojunction Catalyst Boost the Electrocatalytic Hydrodechlorination Reaction Kaifeng Wang, Song Shu, Min Chen, Junxi Li, Kai Zhou, Jian Pan, Xialing Wang, Xiangjun Li, Jianping Sheng, Fan Dong, Guangming Jiang PII: DOI: Reference:
S1385-8947(19)32076-5 https://doi.org/10.1016/j.cej.2019.122673 CEJ 122673
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
27 June 2019 18 August 2019 30 August 2019
Please cite this article as: K. Wang, S. Shu, M. Chen, J. Li, K. Zhou, J. Pan, X. Wang, X. Li, J. Sheng, F. Dong, G. Jiang, Pd-TiO2 Schottky Heterojunction Catalyst Boost the Electrocatalytic Hydrodechlorination Reaction, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.122673
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Pd-TiO2 Schottky Heterojunction Catalyst Boost the Electrocatalytic Hydrodechlorination Reaction Kaifeng Wanga,#, Song Shua,#, Min Chena, Junxi Lia, Kai Zhouc, Jian Panb, Xialing Wanga, Xiangjun Lia, Jianping Shenga, Fan Donga, Guangming Jianga*
a Engineering
Research Center for Waste Oil Recovery Technology and Equipment, Ministry of
Education, Chongqing Technology and Business University, Chongqing 400067, China
b
Environmental Technology Innovation Center of Jiande, Hangzhou 311607, China
c
Analytical and Testing Center, Chongqing University, Chongqing 401331, China #
These authors contribute equally to this work.
Corresponding
author. Tel.: +86-23-62768316; Fax: +86-23-62768317.
E-mail address: [email protected] (G.M. Jiang)
1
Abstract Electrocatalytic hydrodechlorination (EHDC) provides one promising solution to mitigate challenges from the water pollutions by persistent chlorinated organics. Palladium (Pd) has shown appealing properties as EHDC catalysts, while the low earth abundance has urged us to raise its activity and minimize usage. Herein, one novel semiconductor-metal Pd-TiO2 Schottky heterojunction catalyst was developed, which notably outperformed the conventional Pd-C catalyst in mass activity, dechlorination degree and energy selectivity towards EHDC of 2,4-dichlorophenol (2,4-DCP) in water. A combined experimental and DFT calculation study on mechanism revealed that the primary product phenol had a considerable negative effect on EHDC by competing the active sites with 2,4-DCP. The enhanced performance of Pd-TiO2 originated from the Schottky heterojunction-induced metal-support interactions, which optimized the Pd electronic structure and balanced the 2,4-DCP adsorption and phenol desorption on Pd. This study highlights one novel strategy to boost the metal performance in electrocatalytic hydrogenation reactions. Keywords: Electrocatalysis; Chlorinated organics; Persistent organic pollutant; Wastewater; Hydrodechlorination
2
1. Introduction The massive use of chlorinated phenols (CPs) in agriculture, pharmaceuticals petroleum and polymer industries have raised growing public concerns due to their variable mobility in environment, highly carcinogenic to aquatic environments and human health, and strong resistance to natural degradation [1-2]. The C-Cl bond is the origin of their toxicity and persistence, and the CPs could be, therefore, detoxified by replacing the chloride with hydrogen atom [3-4]. Following this principle, various hydrodechlorination technologies have been developed, including the H2/Fe0-driven chemical
hydrodechlorination
[5-6],
photocatalytic
hydrodechlorination
and
electrocatalytic hydrodechlorination (EHDC) [4]. In all these technologies, amounts of atomic hydrogen (H*) were produced, which served as the reductive agents for the replacement reaction. In recent years, the EHDC, which can continuously produce H* at cathode via electrolysis of water, has aroused intensive research interests, due to its high efficiency, low apparatus cost and little risk of secondary pollution (use of clean renewable electricity as electron donator, and no yield of toxic intermediates) [7-9]. The noble metal palladium (Pd) is usually adopted as the EHDC catalyst due to its high efficiency in producing and retaining H* on surface [10]. Nevertheless, Pd is one precious metal, and is also pointed out to be not active enough when performed alone (even in a nanoscale), hampering the scale development of the EHDC technology [11]. It is, therefore, highly desired to develop the strategies that can efficiently improve the performance of Pd and minimize its consumption. The first strategy that comes to the mind is to engineer the Pd into a 3D porous architecture [12], or support 3
it on a 3D porous support (such as the Ni and Cu foam) [13, 14]. The formed hierarchical structure can not only maximize the exposure of active sites, but also facilitate the mass diffusion among catalyst surfaces. However, this method does not really raise the intrinsic activity of Pd, leading to the limited improvements in its performance. The more efficient one is to tune the electronic structure of Pd by alloying it with a transition metal M (such as Ni and Cu) [4, 15] or forming a heterojunction with another material (such as the carbon-based materials [16], oxides [17], nitrides [18,19] and phosphides [20]). It is claimed that the formation of PdM alloy enabled to modulate the d band center of Pd by manipulating its surface strain in particle, and optimize its binding strengths with reactants and intermediate products for an enhanced reaction. This strategy has been made full use of in improving the Pd activity in many reactions, such as the oxygen reduction reaction [21] and hydrogen evolution reaction [22]. However, the synthesis of PdM alloy NPs with desired surface strain and electronic structure usually involves time-consuming and harsh synthetic conditions. The heterojunction catalyst utilizes the strong metal-support interactions, by which the metal reactivity is modified along with the redistribution of the spatial charges between the support and metal [23-25]. Recently, researchers have developed one novel Schottky heterojunction electrocatalyst with the metal loading on an n-type semiconductor [26-29]. In this Schottky heterojunction, the position of their Fermi levels manipulates the direction of electron transfer, while their differences determine the number of the electrons transferred [30, 31]. In principle, the electrons 4
tend to transfer from one phase with a more negative Fermi level to the other, and the transfer does not cease till their Fermi levels get balanced. With the predicted electron transfer direction and number, as well as the diverse semiconductor library, the Schottky heterojunction catalysts should make big differences in the field of heterogeneous catalysis. It is well known that a higher electron density on Pd can promote its performance in reduction reactions, such as the hydrogen evolution reaction and some hydrogenation reactions [32-34]. To enrich the electrons on Pd for an enhanced EHDC reaction, the n-type semiconductor TiO2 was thus selected to form the Pd-TiO2 Schottky heterojunction catalyst, in which the TiO2 with a more negative Fermi level of -0.192 V vs. Normal hydrogen electrode (NHE) [35] exported some electrons to Pd. With the electron-rich Pd NPs, an improved EHDC efficiency could be anticipated on the Pd-TiO2 catalyst. To verify this speculation, the Pd-TiO2 Schottky heterojunction catalyst was prepared, and then characterized by XRD, TEM, PL and XPS to see whether the Schottky heterojunction was formed, and how the electrons were transferred. The EHDC efficiency, reaction pathway, dechlorination degree and kinetics on Pd-TiO2 were assessed and compared to those on Pd-C. Finally, the involved mechanism on how the Schottky heterojunction modulated the EHDC performance was explored by a combined experimental and DFT calculation study.
2. Experimental 2.1. Materials Titanium dioxide (TiO2, 99.9%, anatase type, Aladdin Industrial Co., Ltd) and 5
carbon black (C, Vulcan XC-72R, Cabot Co., Ltd) were adopted as two supports. Prior to use, both of them were treated by concentrated HCl, washed with deionized water and vacuum dried at room temperature. The Na2[PdCl4] (> 98%) and Nafion® solution (0.5% in ethanol) were provided by Sigma-Aldrich Co., Ltd. NaBH4, 2,4-DCP, NaOH, p-chlorophenol (p-CP), o-chlorophenol (o-CP), phenol (P) and chromatography-grade methanol were purchased from Sinopharm Group Chemical Reagent Co., Ltd. 2.2. Catalyst preparation The Pd-TiO2 Schottky heterojunction catalyst was prepared by a method that was developed from the published work [36]. Typically, 16 mg of TiO2 were dispersed in 40 mL of aqueous solution that contains 16 mg of Na2[PdCl4]. The dispersion was then kept intense magnetically stirring for 1.0 h after its pH was adjusted to 10 by a NaOH aqueous solution (1.0 M). Next, 10 mL of NaBH4 solution (1.0 M) was dropwise added into the above dispersion, the color of which gradually turned from white to grey. After 1.0 h of reaction, the grey precipitate was collected via filtration, washed with water/ethanol for three times, and vacuum dried at 60 °C for further use. For comparison, the Pd-C catalyst was prepared, under the identical procedure but replacing the TiO2 with carbon black. The working electrode was made of one 3 cm × 2 cm × 280 μm Toray 090 carbon paper with the catalysts evenly pasted on its two sides. Specifically, 10 mg of the catalyst and 5 mg of carbon black powders were dispersed by intense sonication in 3.8 mL of the ethanol that contains 25 μL of Nafion solution. The resultant catalyst ink 6
was then drop-casted onto carbon paper under an infrared heat lamp. With the slow evaporation of solvent, a uniform film of the catalyst could be formed on the surface of carbon paper substrate. 2.3. Characterization The solid phases in catalysts were analyzed on an X-ray diffractometer (XRD) diffractometer with Cu Kα radiation. (Model D/max RA, Rigaku Co., Japan). The particle morphology and its crystal lattice fringe were examined on a transmission electron microscopy (TEM, JEOL, JEM-2010 instrument, Japan). The Pd loading on catalysts were analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Tianrui 2060t, China). The Pd electronic structure was investigated using X-ray photoelectron spectroscopy (XPS) with a Thermos ESCALAB 250 spectrometer and an Al Kα radiation source (Thermo Fisher Scientific, UK). The C1s peak at 284.6 eV was used for calibration. The Photoluminescence feature was recorded on a Fluorescence spectrophotometer (PL, F-7000, Hitachi, Japan). The linear sweep voltammetry (LSV), and the CO and H* stripping voltammetry were conducted on an Autolab potentiostat (Metrohm Instrument Company, Autolab 302, Swiss). The concentrations of chlorophenols (CPs) and phenol (P) were determined by the high-performance liquid chromatography (HPLC, Shimadzu 2010-AT) with an ODS-SP separation column (150 × 4.6 mm) and a UV detector. A mixture of methanol, water and phosphoric acid (volume ratio: 60/40/0.4) was adopted as the mobile phase (1.0 mL min-1). The concentration of chlorine ion (Cl-) was measured by ion chromatography (IC, Dionex ICS 1100, Thermo Fisher Scientific, UK). All 7
samples before the analysis were filtered by 0.45 μm cellulose membrane. 2.4. Electrocatalytic Hydrodechlorination of 2,4-DCP in Water All the EHDC experiments were controlled by a CHI760 workstation (Chenhua Instruments Company, China), and performed in an H-shaped cell at 25 °C. The whole cell was separated into one cathode and anode chamber (each one is 150 mL), and a cation-exchange membrane was used to prevent the Cl- transfer between chambers. A three-electrode system was employed for the study, with the catalyst-loaded carbon paper, Ag/AgCl (3.0 M KCl, 0.201 V vs Standard hydrogen electrode, 25 °C) and Pt foil as the working, reference and counter electrode, respectively. For each test, both the cathode and anode chambers were feed with 100 mL of 50 mM Na2SO4 solution, while the cathode chamber was further charged with 1 mL of 2,4-DCP stocking solution (5 g of 2,4-DCP in 1 L of methanol). During tests, the solution in cathode chamber was magnetically stirred under a N2 flow, while that in anode chamber were bubbling with N2 to remove the generated oxygen on anode. Aliquots (0.5 mL) were taken from the system at certain intervals for analysis. The intrinsic activity of the catalyst was evaluated by its TOF (min-1) at the end of reaction:
TOF
(C0 -Ct ) V M 2,4-DCP nPd D t
(2)
Where C0 and Ct correspond to the concentration of 2,4-DCP (mg L-1) at the reaction time of 0 and t; t is the reaction time (min); M2,4-DCP is the molar mass of 2,4-DCP (163 g mol-1); V is the volume of liquid in reactor (0.1 L); nPd denotes the Pd loading on electrode (mmol); D is the proportion of Pd atom in one NP dispersed on the 8
surface, and can be roughly estimated by:
D
6 CPd M Pd 109 100% d NA
(3)
Where CPd is the specific atomic concentration on Pd NP surface (1.27 × 1019 atoms m-2); MPd is the molar weight of Pd (106.42 g mol-1); NA is the Avogadro’s number (6.02 × 1023 atoms mol-1); ρ is the density of Pd (1.202 × 107 g m-3); d (nm) is the mean particle size, which was obtained from TEM image. 2.5. DFT calculation Surfaces were molded as periodically repeating slabs separated with a vacuum spacing greater than 15 Å. TiO2 (101) surface was 3 ×3 slab containing 36 Ti and 72 O atoms in a supercell. The graphene structure was molded as a 2D slab with 2 atom layers, 106 atoms. For both structures, half of bottom atomic layers in the slab were frozen to bulk position, and top layers were fully relaxed in the simulations. The Pd4 (111) cluster was put on the supporter surface as an active species because of its relatively stable three-dimensional configuration in both free and supported situation. Nonspin-polarized DFT-D2 calculations in this study were performed using the Vienna
Ab-Initio
Simulation
Package
(VASP5.4.1)
program
with
the
projector-augmented wave (PAW) pseudopotentials and the generalized gradient approximation
(GGA)
of
Perdew-Burke-Ernzerhof
(PBE)
to
treat
the
exchange-correlation potential of electrons [37,38]. The cutoff energies for plane waves were chosen to be 450 eV. A Morkhost-Pack mesh of k-points 3 × 3 × 1 was used to sample the two-dimensional Brillouin zone for geometry optimization. The partial occupancies close to the Fermi level were evaluated with the Gaussian 9
smearing width of 0.1 eV. The solvation program VASPsol in which the solvent was treated as a continuum dielectric was used to handle the effects of solvent [39]. In geometry optimizations, all the structures were relaxed up to the residual atomic forces smaller than 0.03 eV/Å, and the total energy was converged to 10−4 eV. The adsorption energy (Eads) was calculated as: Eads = Etot – (Esub + Emol)
(3)
Where Etot, Esub and Emol depict the total energy of the adsorption complex, the substrate and the isolated adsorbed molecules, respectively.
3. Results and discussion 3.1. Electrode preparation and characterization The Pd-TiO2 Schottky-heterojunction catalyst was synthesized by a facile one-pot wet-chemical reduction method. TEM image in Fig. 1a shows that the black Pd NPs are well dispersed on the grey TiO2 surface, and have spherical shape with about 5.0 nm size. The representative HRTEM image in Fig. 1b shows a d-spacing of 0.22 nm for Pd and 0.36 nm for TiO2, matching well with the (111) plane of Pd phase and the (101) plane of anatase TiO2 phase. XRD analyses on both the TiO2 and Pd-TiO2 reveal the pure anatase phase of TiO2 in both samples, and the metallic Pd phase in Pd-TiO2 sample by the (111) and (200) diffraction peaks at 2θ = 40.12° and 46.36°. Here Pd-C heterojunction catalyst was prepared as a reference sample. The XRD pattern in Fig. 1c confirms the formation of metallic Pd phase, while the TEM image in Fig. 1d shows that the 5.2 nm Pd NPs are uniformly deposited over the carbon layers. The ICP-AES analyses determined the Pd loading to be close at 0.241 ± 10
0.012 and 0.239 ± 0.016 mgPd mg-1catalyst on Pd-C and Pd-TiO2 catalysts. Their electrochemical active specific surface areas (ECSA), estimated from CO-stripping experiments, are also very close to be 19.5 and 22.7 m2 gPd-1 (See Fig. S1). The Pd-TiO2 and Pd-C, with the close Pd loading and active site number, offer the ideal platform to study the Schottky heterojunction effect on the EHDC performance.
Fig. 1.
In principle, the formation of Schottky heterojunction in Pd-TiO2 will drive parts of electrons transferred from TiO2 to Pd at the interface under dark conditions (See a schematic diagram in Fig. S2), while promote the separation of hot charge carriers in TiO2 under light illumination [40]. Accordingly, to verify the successful formation of Schottky heterojunction in Pd-TiO2, comparative study on the electronic states of Pd in Pd-C and Pd-TiO2, and the charge carrier separation efficiency in TiO2 and the Pd-loaded TiO2 were conducted by using the XPS and PL techniques. Fig. 2a presents the core-level Pd 3d XPS spectra of Pd-TiO2 and Pd-C, both of which show two pairs of doublet peaks in a similar profile. The predominant peaks at about 335 and 340 eV correspond to the 3d5/2 and 3d3/2 spin-orbitals of the metallic Pd, while the shoulder peaks at around 337 and 342 eV resemble the 3d5/2 and 3d3/2 levels of Pd2+ species in the form of PdO. Notably, the Pd0 3d5/2 peak of Pd-TiO2 locates at a binding energy of 335.25 eV, smaller than that of the standard metallic Pd (335.60 eV), while both the Pd 3d3/2 and 3d5/2 peaks of Pd-TiO2 shows an obvious negative shift in comparison to those of Pd-C. Further area integration of the Pd2+ (SPd2+) and Pd0 (SPd0) peak reveal 11
that the SPd2+/SPd0 in Pd-TiO2 is only 0.12, lower than that of 0.32 in Pd-C. The XPS results demonstrate that the TiO2-supported Pd indeed owns more electrons in comparison to the carbon-supported Pd, and are more tolerant to the oxidation under atmosphere. The PL spectra of TiO2 and Pd-TiO2 in Fig. 2b clearly point to the positive effect of heterojunction construction on reducing the intensity of omitted light, which is one key signal of the improved hot carrier separation in Pd-TiO2. Based on the displayed electron transfer direction from TiO2 to Pd, and the improved charger carrier separation under light illumination, we were ascertained that the Schottky heterojunction catalyst Pd-TiO2 had been successfully prepared.
Fig. 2.
3.2. EHDC Performance To probe into the Schottky heterojunction effect on EHDC performance, the TiO2, Pd-TiO2 and Pd-C were used as the cathode catalysts, respectively, to detoxify the 2,4-DCP in aqueous solutions. The 2,4-DCP removal efficiency, reaction pathway, EHDC degree and reaction kinetics on these three catalysts were then examined. Fig. 3a presents the C/C0-reaction time profiles under one fixed potential of -0.85 V (vs. Ag/AgCl, the same below). It clearly shows that the TiO2 is inert, while the Pd-C and Pd-TiO2 are both active to remove 2,4-DCP. Fig. 3b summarizes the TOFs of Pd-TiO2 and Pd-C under the potentials of -0.65, -0.75, -0.85 and -0.95 V, where the Pd-TiO2 is indicated to be more active by its larger TOF values than those of Pd-C. 12
Considering the inertness of TiO2 in EHDC, the enhanced performance of Pd-TiO2 should arise from the Schottky heterojunction effect. To find what the removed 2,4-DCP has been converted to on our catalysts, the species evolved during the reaction were analyzed and quantified by a combined technique of HPLC and chloride ion (Cl-) analyzer. The results reveal that in both the Pd-TiO2 and Pd-C systems, only 2,4-DCP, o-CP, P and Cl- are detected, and no other species (such as cyclohexanol) emerge even at an extended reaction time (See Fig. S3), which suggest that the EHDC is terminated at P, and no further hydrogenation reaction occurs. Among the detected species, the concentrations of P and o-CP increase rapidly with the consumption of 2,4-DCP (Fig. 3c), but the mass balance of carbon keeps relatively steady, further demonstrating a fairly good reaction selectivity towards EHDC, as well as a leading reaction pathway of 2,4-DCP→o-CP + Cl- →P + Cl-. Similar reaction pathways were also reported by some researchers, and the cleavage superiority of p-C-Cl was usually explained in terms of the steric hindrance effect [42, 43]. The EHDC degree, described by the molar ratio of the chlorine-free P in product, is one critical descriptor of the EHDC completeness and safety. Fig. 3d plots the molar ratio of o-CP and P in product as a function of reaction time. It is found that on Pd-C, the o-CP keeps a relatively high constitution of around 30% in product during the 180-min reaction, suggestive of an inefficient and incomplete EHDC process. The number, however, declines to below 5% on Pd-TiO2 at the end of reaction, which indicates that the Schottky heterojunction contributes to a more complete EHDC, in comparison to the Pd-C catalyst. 13
Fig. 3.
The durability of the Pd-TiO2 catalyst was evaluated by repeating the EHDC reaction on one Pd-TiO2 electrode for five times. The nearly undeteriorated efficiency (Fig. 4a) and the little change in the CV profile after the five tests (Fig. 4b) indicated that the presented Pd-TiO2 catalysts were relatively durable. The Pd2+ concentration in the solution was also tracked at the end of each run of reaction. No Pd element was detected, indicating no Pd was leached during the EHDC. The high durability of Pd-TiO2 could be ascribed to the strong metal-support interactions in the constructed Schottky heterojunction structure [41]. Moreover, the EHDC was usually conducted at a very negative potential (-0.85 V) and in an oxygen-free and neutral solution, both of which prevented the Pd against the oxidative corrosion.
Fig. 4.
3.3. Mechanism To probe into the origin of Schottky heterojunction effect, we used the classic Langmuir-Hinshelwood (L-H) model at the first stage to describe the EHDC kinetics on Pd-C and Pd-TiO2 (See the details in Supplementary Materials). As illustrated in Fig. 5a, a pretty good linear plottings of 1/r0 vs. 1/C0 are obtained for both the 14
Pd-TiO2 (R2 = 0.98) and Pd-C (R2 = 0.98) systems, which reflect that the EHDC reaction is rate-controlled by the surface reaction of 2,4-DCP with H*. Regarding the critical role of H*, the H* generation kinetics on Pd-TiO2 and Pd-C were firstly studied by LSV method. A more positive onset potential for the conversion reaction of H+ to H* as well as the large current density in the H* generation region are observed on the LSV curve of Pd-C, which reveals that the Pd-C is more powerful in producing H* (Fig. S5). Such an efficiency order of Pd-C > Pd-TiO2, however, rightly reverses their EHDC performance order (Pd-TiO2 > Pd-C). Fig. 5b plots the current efficiency of Pd-C and Pd-TiO2 versus the reaction time, which is intended to show how many of the generated H* are consumed by EHDC (the rest is evolved into molecular H2). Though the current efficiency of Pd-TiO2 nearly doubles that of Pd-C, both of their absolute values are relatively low, leaving only 25.8% for Pd-TiO2 and 18% for Pd-C at the end of reaction. On basis of the lower H* utilization rate and the inconsistency between the order of H* generation efficiency and EHDC performance for Pd-C and Pd-TiO2, we inferred that the effective adsorption and activation of 2,4-DCP on catalyst was the more important step of EHDC reaction.
Fig. 5.
On basis of the above discussions, we speculated that the promoting effect of Schottky heterojunction on EHDC might lie at its ability in intensifying the adsorption and activation of 2,4-DCP on the Pd surface. Here the influences of mass transfer
15
limitations of 2,4-DCP in solution and its adsorption by support were not taken into consideration as the EHDC on Pd-C and Pd-TiO2 were conducted under identical conditions (e.g. intensive stirring) and the support itself had no power to provide H* and activate 2,4-DCP [44, 45]. To verify this speculation, we adopted the DFT calculations to embody the adsorption energy (Eads) of 2,4-DCP and the C-Cl activation degree (characterized by the C-Cl bond length) on Pd-C and Pd-TiO2. To be simple, four Pd atoms in a tetrahedra shape supported on a leading exposed TiO2 (101) plane and C (002) plane were employed to represent the heterojunction structure [46]. The detailed structure optimization process and the optimal cross-section configurations of Pd-TiO2 and Pd-C are shown in Fig. S6 and Fig. 6a-b. In the optimized structure, the Pd NP, indeed, has much stronger interactions with TiO2 support, evidenced by the transfer of nearly 0.81 electrons from TiO2 to Pd (See Fig. S7 and Table S1). In comparison, the positive Bader charge of Pd means that 0.49 of the electrons are transferred from Pd to C. 2,4-DCP molecule prefers to be adsorbed with its ring plane parallel to one Pd plane at the support-metal interface. The Eads calculation results demonstrate that 2,4-DCP is more preferably adsorbed on Pd-C with a more negative value of -1.23 eV (vs. -1.01 eV on Pd-TiO2). Accompanying the strong adsorption, the o- and p-C-Cl bond lengths of 2,4-DCP on Pd-C are 1.768 and 1.831 Å, much longer than those of 1.754 and 1.751 on Pd-TiO2, demonstrating that the Pd-C interface is more powerful in activating the C-Cl bonds. These results are, however, unexpected as the effective adsorption and activation of the pollutant on Pd-C inversely yield an inferior EHDC performance. 16
In one heterogeneous catalysis, besides the intrinsic adsorption and C-Cl bond activation, the adsorption of 2,4-DCP on Pd is interfered by the desorption kinetics of the intermediate product (i.e. P and Cl-) from Pd [47-49]. Generally, an effective desorption of the product will accelerate the refreshment of the active sites for a new round of reactions, which is thus essential to maintain an efficient and continuous catalytic reaction. In this case, we further studied the adsorption behaviors of P and Cl- on Pd-C and Pd-TiO2 by DFT calculations. The results in Fig. 6c-d and Fig. S8 show that the P is adsorbed on catalyst in a very close configuration to 2,4-DCP, and its binding strength with Pd is comparable to that of 2,4-DCP but much stronger than that of Cl-, which indicate that the P is one very potential species that may compete the active sites with 2,4-DCP on the catalyst. On the other hand, the maximum Eads of P on Pd-TiO2 is only -0.82 eV, much more positive than that of -1.19 eV on Pd-C, evidencing that the P desorption on the Pd-TiO2 interface is much easier.
Fig. 6.
Up to here, we realized that the P (not Cl-) should exert a considerable negative effect on EHDC by its strong adsorption on Pd, which might lower the possibility or raise the difficulty for 2,4-DCP to reach the active sites, and form an extra energy barrier for EHDC reaction. The Pd-TiO2 had a weakened binding strength with P, which facilitated the P desorption and intensified the 2,4-DCP adsorption over Pd, leading to an enhanced EHDC performance. Here, we attributed the improved 17
performance of Pd-TiO2 in adsorbing 2,4-DCP and desorbing P to the Schottky heterojunction effect, which functioned to modify the electronic structure of Pd and optimize its interactions with 2,4-DCP and P. 3.4. Improved phenol-tolerance on the Schottky heterojunction catalyst To verify the negative effect of P and its alleviation on the Pd-TiO2 Schottky heterojunction catalyst, we firstly compared the EHDC performances of Pd-C and Pd-TiO2 electrodes before and after being pre-immersed in a P solution, respectively. The obtained TOF results in Fig. 7a clearly show that the electrodes pre-immersed in the P solutions display significant decays in their EHDC activities. Furthermore, the decay extent is enlarged when a more concentrated P solution is used. By these observations, we are ascertained that the P indeed poses an adverse effect on EHDC by occupying the active sites. Fig. 7a also shows that with the P concentration increasing from 0 to 10 and to 30 mg L-1, the TOFs of Pd-TiO2 declines by 3.7% and 1.0%, much smaller than those of Pd-C (35.7% and 27.0%). The smaller TOF decay on Pd-TiO2 provides the solid evidence of the alleviated negative effect of P on Schottky heterojunction catalyst.
Fig. 7.
The alleviated negative effect of P on the Schottky heterojunction catalyst was further verified by an H* stripping method. In this method, the H* was generated, and adsorbed/absorbed over the catalyst at -0.7 V for 60 s (at this condition the H* would 18
not evolve into H2), which was then oxidized by a positive scanning. The oxidation current can reflect the transferred electron number during oxidation, and be used to quantify the generated H*. Assuming that the P-occupied Pd sites are deactivated in H* generation, the generated H* number difference on the electrode before and after being pre-immersed in P solutions can be used to characterize the P tolerance of the catalysts. Fig. 7b presents the H* stripping profiles. It shows that the H* yield declines only by 10.31% on the P-treated Pd-TiO2 electrode, but by a larger extent of 38.65% on Pd-C under the identical condition. These H* stripping results substantially support the viewpoint that the Pd-TiO2 Schottky heterojunction catalyst has a less tendency to adsorb the P molecules, which enables to promote the P desorption and the 2,4-DCP adsorption on Pd, and ease the negative effects from P.
4. Conclusions In summary, this work reports one novel semiconductor-metal Pd-TiO2 Schottky heterojunction catalyst for EHDC of CPs in water. This catalyst is proved to be superior to the conventional Pd-C catalyst in the aspects of removal efficiency, dechlorination degree and current efficiency. The superiority originates from the strong Schottky heterojunction-induced metal-support interactions between Pd and TiO2, by which the Pd NPs gather electrons from TiO2. The Pd NPs with the higher electron density have a less tendency to adsorb the P, which promotes the adsorption of 2,4-DCP over the catalyst. Overall, our work successfully utilizes the schottky heterojunction effect for optimization of the EHDC performance, and the presented strategy can also be extended to other hydrogenation reaction systems. 19
Acknowledgements The present work is financially supported by National Natural Science Foundation of China (51878105), Venture & Innovation Support Program for Chongqing Overseas Returnees (cx2017066), the Program for the Top Young Talents of Chongqing, Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJQN201800829), Research Startup Foundation of Chongqing Technology and Business University (2016-56-01 and 2016-56-02).
References [1] M.Á. Arellano-González, I. González, A.-C. Texier, Mineralization of 2-chlorophenol by sequential electrochemical reductive dechlorination and biological processes, J. Hazard. Mater. 314 (2016) 181-187. [2] X. Song, Q. Shi, H. Wang, S. Liu, C. Tai, Z. Bian, Preparation of Pd-Fe/graphene catalysts by photocatalytic reduction with enhanced electrochemical oxidation-reduction properties for chlorophenols, Appl. Catal. B: Environ. 203 (2017) 442-451. [3] W. Xie, S. Yuan, X. Mao, W. Hu, P. Liao, M. Tong, A. Alshawabkeh, Electrocatalytic activity of Pd-loaded Ti/TiO2 nanotubes cathode for TCE reduction in groundwater, Water Res. 47 (2013) 3573-3582. [4] R. Liu, H. Chen, L. Fang, C. Xu, Z. He, Y. Lai, H. Zhao, D. Bekana, J. Liu, Au@Pd bimetallic nanocatalyst for carbon-halogen bond cleavage: An old story with new insight into how the activity of Pd is influenced by Au, Environ. Sci. Technol. 52 (2018) 4244-4255. [5] F. He, Z. Li, S. Shi, W. Xu, H. Sheng, Y. Gu, Y. Jiang, B. Xi, Dechlorination of excess trichloroethene by bimetallic and sulfidated nanoscale zero-valent iron, Environ. Sci. Technol. 52 (2018) 8627-8637. [6] Y. Mu, F. Jia, Z. Ai, L. Zhang, Iron oxide shell mediated environmental remediation properties of nano zero-valent iron, Environ. Sci.: Nano 4 (2017) 27-45. [7] P. Shao, J. Tian, X. Duan, Y. Yang, W. Shi, X. Luo, F. Cui, S. Luo, S. Wang, Cobalt silicate 20
hydroxide nanosheets in hierarchical hollow architecture with maximized cobalt active site for catalytic oxidation, Chem. Eng. J. 359 (2019) 79-87. [8] P. Shao, J. Tian, F. Yang, X. Duan, S. Gao, W. Shi, X. Luo, F. Cui, S. Luo, S. Wang, Identification and regulation of active sites on nanodiamonds: establishing a highly efficient catalytic system for oxidation of organic contaminants, Adv. Funct. Mater. 28 (2018) 1705295. [9] X. Mao, A. Ciblak, K. Baek, M. Amiri, R. Loch-Caruso, A.N. Alshawabkeh, Optimization of electrochemical dechlorination of trichloroethylene in reducing electrolytes, Water Res. 46 (2012) 1847-1857. [10] B.P. Chaplin, M. Reinhard, W.F. Schneider, C. Schuth, J.R. Shapley, T.J. Strathmann, C.J. Werth, Critical review of Pd-based catalytic treatment of priority contaminants in water, Environ. Sci. Technol. 46 (2012) 3655-3670. [11] Z. Sun, X. Wei, Y. Han, S. Tong, X. Hu, Complete dechlorination of 2,4-dichlorophenol in aqueous solution on palladium/polymeric pyrrole-cetyl trimethyl ammonium bromide/foam-nickel composite electrode, J. Hazard. Mater. 244-245 (2013) 287-294. [12] C. Jiang, H. Yu, Y. Lu, S. Zhu, Z. Geng, M. Huo, X. Wang, Preparation of spike-like palladium nanoparticle electrode and its dechlorination properties, Thin Solid Films 664 (2018) 27-32. [13] L. Yang, Z. Chen, D. Cui, X. Luo, B. Liang, L. Yang, T. Liu, A. Wang, S. Luo, Ultrafine palladium nanoparticles supported on 3D self-supported Ni foam for cathodic dechlorination of florfenicol, Chem. Eng. J. 359 (2019) 894-901. [14] Y. Wu, L. Gan, S. Zhang, B. Jiang, H. Song, W. Li, Y. Pan, A. Li, Enhanced electrocatalytic dechlorination of para-chloronitrobenzene based on Ni/Pd foam electrode, Chem. Eng. J. 316 (2017) 146-153. [15] H. Xu, H. Xu, Z. Chen, X. Ran, J. Fan, W. Luo, Z. Bian, W.X. Zhang, J. Yang, Bimetallic PdCu nanocrystals immobilized by nitrogen-containing ordered mesoporous carbon for electrocatalytic denitrification, ACS Appl. Mater. Interfaces 11 (2019) 3861-3868. [16] Ding, Y.; Sun, X. L.; Wang, Z. L.; Sun, S. H., Misfit dislocations in multimetallic core-shelled nanoparticles, Appl. Phys. Lett. 100 (2012) 111603. [17] Z. Lou, J. Zhou, M. Sun, J. Xu, K. Yang, D. Lv, Y. Zhao, X. Xu, MnO2 enhances electrocatalytic hydrodechlorination by Pd/Ni foam electrodes and reduces Pd needs, Chem. Eng. 21
J. 352 (2018) 549-557. [18] W. Fu, S. Shu, J. Li, X. Shi, X. Lv, Y. Huang, F. Dong, G. Jiang, Identifying the rate-determining step of the electrocatalytic hydrodechlorination reaction on palladium nanoparticles, Nanoscale (2019), 10.1039/C9NR04634H. [19] S. Yang, D.Y. Chung, Y.-J. Tak, J. Kim, H. Han, J.-S. Yu, A. Soon, Y.-E. Sung, H. Lee, Electronic structure modification of platinum on titanium nitride resulting in enhanced catalytic activity and durability for oxygen reduction and formic acid oxidation, Appl. Catal. B: Environ. 174-175 (2015) 35-42. [20] W. Guo, S. Niu, W. Shi, B. Zhang, W. Yu, Y. Xie, X. Ji, Y. Wu, D. Su, L. Shao, Pd–P nanoalloys supported on a porous carbon frame as an efficient catalyst for benzyl alcohol oxidation, Catal. Sci. Technol. 8 (2018) 2333-2339. [21] Y. Holade, R.G. da Silva, K. Servat, T.W. Napporn, C. Canaff, A.R. de Andrade, K.B. Kokoh, Facile synthesis of highly active and durable PdM/C (M = Fe, Mn) nanocatalysts for the oxygen reduction reaction in an alkaline medium, J. Mater. Chem. A 4 (2016) 8337-8349. [22] H.Y. Chen, M.X. Jin, L. Zhang, A.J. Wang, J. Yuan, Q.L. Zhang, J.J. Feng, One-pot aqueous synthesis of two-dimensional porous bimetallic PtPd alloyed nanosheets as highly active and durable electrocatalyst for boosting oxygen reduction and hydrogen evolution, J. Colloid Interf. Sci. 543 (2019) 1-8. [23] Z. Lou, J. Zhou, M. Sun, J. Xu, K. Yang, D. Lv, Y. Zhao, X. Xu, MnO2 enhances electrocatalytic hydrodechlorination by Pd/Ni foam electrodes and reduces Pd needs, Chem. Eng. J. 352 (2018) 549-557. [24] Z. Lou, J. Xu, J. Zhou, K. Yang, Z. Chao, Y. Li, Y. Liu, L. Lou, X. Xu, Insight into atomic H* generation, H2 evolution, and cathode potential of MnO2 induced Pd/Ni foam cathode for electrocatalytic hydrodechlorination, Chem. Eng. J.374 (2019) 211-220. [25] Z. Lou, Y. Li, J. Zhou, K. Yang, Y. Liu, S.A. Baig, X. Xu, TiC doped palladium/nickel foam cathode for electrocatalytic hydrodechlorination of 2,4-DCBA: Enhanced electrical conductivity and reactive activity, J. Hazard. Mater. 362 (2019) 148-159. [26] J. Hou, Y. Sun, Y. Wu, S. Cao, L. Sun, Promoting active sites in core-shell nanowire array as Mott-Schottky electrocatalysts for efficient and stable overall water splitting, Adv. Funct. Mater. 28 (2018) 1704447. 22
[27] H. Liu, X. Liu, W. Yang, M. Shen, S. Geng, C. Yu, B. Shen, Y. Yu, Photocatalytic dehydrogenation of formic acid promoted by a superior PdAg@g-C3N4 Mott–Schottky heterojunction, J. Mater. Chem. A 7 (2019) 2022-2026. [28] Y.Y. Cai, X.H. Li, Y.N. Zhang, X. Wei, K.X. Wang, J.S. Chen, Highly efficient dehydrogenation of formic acid over a palladium-nanoparticle-based Mott-Schottky photocatalyst, Angew. Chem. Int. Ed. 52 (2013) 11822-11825. [29] Z. Zhuang, Y. Li, Z. Li, F. Lv, Z. Lang, K. Zhao, L. Zhou, L. Moskaleva, S. Guo, L. Mai, MoB/g-C3N4 interface materials as a Schottky catalyst to boost hydrogen evolution, Angew. Chem. Int. Ed. 57 (2018) 496-500. [30] X.H. Li, M. Antonietti, Metal nanoparticles at mesoporous N-doped carbons and carbon nitrides: functional Mott-Schottky heterojunctions for catalysis, Chem. Soc. Rev. 42 (2013) 6593-6604. [31] Y. Liu, J. Guo, E. Zhu, L. Liao, S.J. Lee, M. Ding, I. Shakir, V. Gambin, Y. Huang, X. Duan, Approaching the Schottky-Mott limit in van der Waals metal-semiconductor junctions, Nature 557 (2018) 696-700. [32] H. Wang, Z. Lu, S. Xu, D. Kong, J.J. Cha, G. Zheng, P.C. Hsu, K. Yan, D. Bradshaw, F.B. Prinz, Y. Cui, Electrochemical tuning of vertically aligned MoS2 nanofilms and its application in improving hydrogen evolution reaction, Proc. Natl. Acad. Sci. USA 110 (2013) 19701-19706. [33] J. Xie, J. Zhang, S. Li, F. Grote, X. Zhang, H. Zhang, R. Wang, Y. Lei, B. Pan, Y. Xie, Controllable disorder engineering in oxygen-incorporated MoS2 ultrathin nanosheets for efficient hydrogen evolution, J. Am. Chem. Soc. 135 (2013) 17881-17888. [34] C. Wan, Y.N. Regmi, B.M. Leonard, Multiple phases of molybdenum carbide as electrocatalysts for the hydrogen evolution reaction, Angew. Chem. Int. Ed. 53 (2014) 6407-6410. [35] M. Jakob, H. Levanon, Charge distribution between UV-Irradiated TiO2 and gold nanoparticles: Determination of shift in the Fermi level. Nano Lett. 3 (2003) 353-358. [36] X. Xu, J. Luo, L. Li, D. Zhang, Y. Wang, G. Li, Unprecedented catalytic performance in amine syntheses via Pd/g-C3N4 catalyst-assisted transfer hydrogenation, Green Chem. 20 (2018) 2038-2046. [37] S. Grimme, Semiempirical GGA ‐ type density functional constructed with a long ‐ range 23
dispersion correction, J. Comput. Chem. 27 (2006) 1787-1799. [38] B. Hammer, L. B. Hansen, J. K. Nørskov, Improved adsorption energetics within density-functional theory using revised Perdew-Burke-Ernzerhof functionals, Phys. Rev. B 59 (1999) 7413. [39] M. Fishman, H.L. Zhuang, K. Mathew, W. Dirschka, R.G. Hennig, Accuracy of exchange-correlation functionals and effect of solvation on the surface energy of copper, Phys. Rev. B 87 (2013). [40] J. Fu, J. Yu, C. Jiang, B. Cheng, g-C3N4-Based Heterostructured Photocatalysts, Adv. Energ. Mater. 8 (2018) 1701503. [41] G. Yuan, M.A. Keane, Liquid phase hydrodechlorination of chlorophenols over Pd/C and Pd/Al2O3: a consideration of HCl/catalyst interactions and solution pH effects, Appl. Catal. B Environ. 52 (2004) 301-314. [42] G. Yuan, M.A. Keane, Aqueous-Phase Hydrodechlorination of 2,4-Dichlorophenol over Pd/Al2O3: Reaction under Controlled pH, Ind. Eng. Chem. Res. 46 (2007) 705–715. [43] S. Shu, W. Fu, P. Wang, W. Cen, Y. Chu, F. Wei, X. Zhang, F. Dong, G. Jiang, Electrocatalytic hydrodechlorination of 2,4-dichlorophenol over palladium nanoparticles: The critical role of hydroxyl group deprotonation, Appl. Catal. A-Gen. 583 (2019) 117146. [44] Y. Fang,
K.N. Heck, P.J. J. Alvarez, M.S. Wong, Kinetics analysis of Palladium/Gold
nanoparticles as colloidal hydrodechlorination catalysts, ACS Catal. 1 (2011) 128-138. [45] M. Munoz, M. Kaspereit, B.J.M. Etzold, Deducing kinetic constants for the hydrodechlorination of 4-chlorophenol using high adsorption capacity catalysts, Chem. Eng. J. 285 (2016) 228-235. [46] M. Munoz, G.R. Zhang, B.J.M. Etzold, Exploring the role of the catalytic support sorption capacity on the hydrodechlorination kinetics by the use of carbide-derived carbons, Appl. Catal. B: Environ. 203 (2017) 591-598. [47] J.N. Sandoval, D. Ortiz, M. Munoz, Z.M. Pedro, J.A. Casas, On the deactivation and regeneration
of
Pd/Al2O3
catalyst
for
aqueousphase
hydrodechlorination
of
diluted
chlorpromazine solution, Catal. Today (2019), 10.1016/j.cattod.2019.06.028. [48] G. Yuan, M.A. Keane, Catalyst deactivation during the liquid phase hydrodechlorination of 2,4-dichlorophenol over supported Pd: influence of the support, Catal. Today 88 (2003) 27-36. 24
[49] E. Diaz, A.F. Mohedano, Jose A.CasasJuan J.Rodriguez, Analysis of the deactivation of Pd, Pt and Rh on activated carbon catalysts in the hydrodechlorination of the MCPA herbicide, Appl. Catal. B-Environ. 181 (2016) 429–435.
Figure captions Fig. 1. Representative (a) TEM and (b) HRTEM images of the Pd-TiO2 composite; (c) XRD patterns of Pd-TiO2, Pd-C and TiO2; (d) One typical TEM image of the Pd-C composite. Fig. 2. (a) Comparison in the Pd 3d XPS spectra of Pd-TiO2 and Pd-C. (b) PL patterns of Pd-TiO2 and TiO2 samples under the UV-light illumination (leading wavelength of 300 nm). Fig. 3. (a) Reaction time-dependent 2,4-DCP concentration with the Pd-TiO2, Pd-C and TiO2 electrodes at a fixed potential of -0.85 V in N2-saturated Na2SO4 aqueous solution (initial 2,4-DCP concentration: 50 mg L-1); (b) TOF of Pd-TiO2 and Pd-C under the potentials of -0.65, -0.75, -0.85 and -0.95 V, respectively. (c) The concentration evolution of the 2,4-DCP and the intermediate products during EHDC of 2,4-DCP on the Pd-TiO2 and Pd-C electrodes; (d) Reaction time-dependent o-CP concentration in the dechlorination products. Fig. 4. (a) Durability test for Pd-TiO2 catalyst in EHDC reaction; (b) CV profiles of the fresh Pd-TiO2 and the Pd-TiO2 after the use for five times. (Conditions: 0.1 M HClO4 aqueous solution; scanning rate: 10 mV s-1, electrode area: 2 cm × 3 cm). Fig. 5. (a) Langmuir-Hinshelwood (L-H) representations and (b) the reaction time-dependent current efficiency of the EHDC reaction over Pd-TiO2 and Pd-C. Fig. 6. Side view of the optimized adsorption configurations evaluated for 2,4-DCP on (a) Pd-TiO2, (b) Pd-C, and P on (c) Pd-TiO2 and (d) Pd-C. Fig. 7. (a) TOFs of the Pd-TiO2 and Pd-C electrodes before and after being pre-immersed in P solutions; (b). The H* stripping profiles of Pd-TiO2 and Pd-C electrodes.
25
Fig. 1.
26
Fig. 2.
27
Fig. 3.
28
Fig. 4.
29
Fig. 5.
30
Fig. 6.
31
Fig. 7.
Graphical Abstract
Highlights A novel TiO2-Pd Schottky heterojunction catalyst was developed for EHDC reaction TiO2-Pd outperforms the C-Pd in EHDC efficiency, degree and energy selectivity Schottky heterojunction-induced metal-support interaction is the origin of activity 32
Phenol exerts a considerable negative effect on EHDC by competing the active sites The formation of Schottky heterojunction eases the negative effect of phenol
33
S1385-8947(19)32076-5 https://doi.org/10.1016/j.cej.2019.122673 CEJ 122673
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
27 June 2019 18 August 2019 30 August 2019
Please cite this article as: K. Wang, S. Shu, M. Chen, J. Li, K. Zhou, J. Pan, X. Wang, X. Li, J. Sheng, F. Dong, G. Jiang, Pd-TiO2 Schottky Heterojunction Catalyst Boost the Electrocatalytic Hydrodechlorination Reaction, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.122673
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Pd-TiO2 Schottky Heterojunction Catalyst Boost the Electrocatalytic Hydrodechlorination Reaction Kaifeng Wanga,#, Song Shua,#, Min Chena, Junxi Lia, Kai Zhouc, Jian Panb, Xialing Wanga, Xiangjun Lia, Jianping Shenga, Fan Donga, Guangming Jianga*
a Engineering
Research Center for Waste Oil Recovery Technology and Equipment, Ministry of
Education, Chongqing Technology and Business University, Chongqing 400067, China
b
Environmental Technology Innovation Center of Jiande, Hangzhou 311607, China
c
Analytical and Testing Center, Chongqing University, Chongqing 401331, China #
These authors contribute equally to this work.
Corresponding
author. Tel.: +86-23-62768316; Fax: +86-23-62768317.
E-mail address: [email protected] (G.M. Jiang)
1
Abstract Electrocatalytic hydrodechlorination (EHDC) provides one promising solution to mitigate challenges from the water pollutions by persistent chlorinated organics. Palladium (Pd) has shown appealing properties as EHDC catalysts, while the low earth abundance has urged us to raise its activity and minimize usage. Herein, one novel semiconductor-metal Pd-TiO2 Schottky heterojunction catalyst was developed, which notably outperformed the conventional Pd-C catalyst in mass activity, dechlorination degree and energy selectivity towards EHDC of 2,4-dichlorophenol (2,4-DCP) in water. A combined experimental and DFT calculation study on mechanism revealed that the primary product phenol had a considerable negative effect on EHDC by competing the active sites with 2,4-DCP. The enhanced performance of Pd-TiO2 originated from the Schottky heterojunction-induced metal-support interactions, which optimized the Pd electronic structure and balanced the 2,4-DCP adsorption and phenol desorption on Pd. This study highlights one novel strategy to boost the metal performance in electrocatalytic hydrogenation reactions. Keywords: Electrocatalysis; Chlorinated organics; Persistent organic pollutant; Wastewater; Hydrodechlorination
2
1. Introduction The massive use of chlorinated phenols (CPs) in agriculture, pharmaceuticals petroleum and polymer industries have raised growing public concerns due to their variable mobility in environment, highly carcinogenic to aquatic environments and human health, and strong resistance to natural degradation [1-2]. The C-Cl bond is the origin of their toxicity and persistence, and the CPs could be, therefore, detoxified by replacing the chloride with hydrogen atom [3-4]. Following this principle, various hydrodechlorination technologies have been developed, including the H2/Fe0-driven chemical
hydrodechlorination
[5-6],
photocatalytic
hydrodechlorination
and
electrocatalytic hydrodechlorination (EHDC) [4]. In all these technologies, amounts of atomic hydrogen (H*) were produced, which served as the reductive agents for the replacement reaction. In recent years, the EHDC, which can continuously produce H* at cathode via electrolysis of water, has aroused intensive research interests, due to its high efficiency, low apparatus cost and little risk of secondary pollution (use of clean renewable electricity as electron donator, and no yield of toxic intermediates) [7-9]. The noble metal palladium (Pd) is usually adopted as the EHDC catalyst due to its high efficiency in producing and retaining H* on surface [10]. Nevertheless, Pd is one precious metal, and is also pointed out to be not active enough when performed alone (even in a nanoscale), hampering the scale development of the EHDC technology [11]. It is, therefore, highly desired to develop the strategies that can efficiently improve the performance of Pd and minimize its consumption. The first strategy that comes to the mind is to engineer the Pd into a 3D porous architecture [12], or support 3
it on a 3D porous support (such as the Ni and Cu foam) [13, 14]. The formed hierarchical structure can not only maximize the exposure of active sites, but also facilitate the mass diffusion among catalyst surfaces. However, this method does not really raise the intrinsic activity of Pd, leading to the limited improvements in its performance. The more efficient one is to tune the electronic structure of Pd by alloying it with a transition metal M (such as Ni and Cu) [4, 15] or forming a heterojunction with another material (such as the carbon-based materials [16], oxides [17], nitrides [18,19] and phosphides [20]). It is claimed that the formation of PdM alloy enabled to modulate the d band center of Pd by manipulating its surface strain in particle, and optimize its binding strengths with reactants and intermediate products for an enhanced reaction. This strategy has been made full use of in improving the Pd activity in many reactions, such as the oxygen reduction reaction [21] and hydrogen evolution reaction [22]. However, the synthesis of PdM alloy NPs with desired surface strain and electronic structure usually involves time-consuming and harsh synthetic conditions. The heterojunction catalyst utilizes the strong metal-support interactions, by which the metal reactivity is modified along with the redistribution of the spatial charges between the support and metal [23-25]. Recently, researchers have developed one novel Schottky heterojunction electrocatalyst with the metal loading on an n-type semiconductor [26-29]. In this Schottky heterojunction, the position of their Fermi levels manipulates the direction of electron transfer, while their differences determine the number of the electrons transferred [30, 31]. In principle, the electrons 4
tend to transfer from one phase with a more negative Fermi level to the other, and the transfer does not cease till their Fermi levels get balanced. With the predicted electron transfer direction and number, as well as the diverse semiconductor library, the Schottky heterojunction catalysts should make big differences in the field of heterogeneous catalysis. It is well known that a higher electron density on Pd can promote its performance in reduction reactions, such as the hydrogen evolution reaction and some hydrogenation reactions [32-34]. To enrich the electrons on Pd for an enhanced EHDC reaction, the n-type semiconductor TiO2 was thus selected to form the Pd-TiO2 Schottky heterojunction catalyst, in which the TiO2 with a more negative Fermi level of -0.192 V vs. Normal hydrogen electrode (NHE) [35] exported some electrons to Pd. With the electron-rich Pd NPs, an improved EHDC efficiency could be anticipated on the Pd-TiO2 catalyst. To verify this speculation, the Pd-TiO2 Schottky heterojunction catalyst was prepared, and then characterized by XRD, TEM, PL and XPS to see whether the Schottky heterojunction was formed, and how the electrons were transferred. The EHDC efficiency, reaction pathway, dechlorination degree and kinetics on Pd-TiO2 were assessed and compared to those on Pd-C. Finally, the involved mechanism on how the Schottky heterojunction modulated the EHDC performance was explored by a combined experimental and DFT calculation study.
2. Experimental 2.1. Materials Titanium dioxide (TiO2, 99.9%, anatase type, Aladdin Industrial Co., Ltd) and 5
carbon black (C, Vulcan XC-72R, Cabot Co., Ltd) were adopted as two supports. Prior to use, both of them were treated by concentrated HCl, washed with deionized water and vacuum dried at room temperature. The Na2[PdCl4] (> 98%) and Nafion® solution (0.5% in ethanol) were provided by Sigma-Aldrich Co., Ltd. NaBH4, 2,4-DCP, NaOH, p-chlorophenol (p-CP), o-chlorophenol (o-CP), phenol (P) and chromatography-grade methanol were purchased from Sinopharm Group Chemical Reagent Co., Ltd. 2.2. Catalyst preparation The Pd-TiO2 Schottky heterojunction catalyst was prepared by a method that was developed from the published work [36]. Typically, 16 mg of TiO2 were dispersed in 40 mL of aqueous solution that contains 16 mg of Na2[PdCl4]. The dispersion was then kept intense magnetically stirring for 1.0 h after its pH was adjusted to 10 by a NaOH aqueous solution (1.0 M). Next, 10 mL of NaBH4 solution (1.0 M) was dropwise added into the above dispersion, the color of which gradually turned from white to grey. After 1.0 h of reaction, the grey precipitate was collected via filtration, washed with water/ethanol for three times, and vacuum dried at 60 °C for further use. For comparison, the Pd-C catalyst was prepared, under the identical procedure but replacing the TiO2 with carbon black. The working electrode was made of one 3 cm × 2 cm × 280 μm Toray 090 carbon paper with the catalysts evenly pasted on its two sides. Specifically, 10 mg of the catalyst and 5 mg of carbon black powders were dispersed by intense sonication in 3.8 mL of the ethanol that contains 25 μL of Nafion solution. The resultant catalyst ink 6
was then drop-casted onto carbon paper under an infrared heat lamp. With the slow evaporation of solvent, a uniform film of the catalyst could be formed on the surface of carbon paper substrate. 2.3. Characterization The solid phases in catalysts were analyzed on an X-ray diffractometer (XRD) diffractometer with Cu Kα radiation. (Model D/max RA, Rigaku Co., Japan). The particle morphology and its crystal lattice fringe were examined on a transmission electron microscopy (TEM, JEOL, JEM-2010 instrument, Japan). The Pd loading on catalysts were analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Tianrui 2060t, China). The Pd electronic structure was investigated using X-ray photoelectron spectroscopy (XPS) with a Thermos ESCALAB 250 spectrometer and an Al Kα radiation source (Thermo Fisher Scientific, UK). The C1s peak at 284.6 eV was used for calibration. The Photoluminescence feature was recorded on a Fluorescence spectrophotometer (PL, F-7000, Hitachi, Japan). The linear sweep voltammetry (LSV), and the CO and H* stripping voltammetry were conducted on an Autolab potentiostat (Metrohm Instrument Company, Autolab 302, Swiss). The concentrations of chlorophenols (CPs) and phenol (P) were determined by the high-performance liquid chromatography (HPLC, Shimadzu 2010-AT) with an ODS-SP separation column (150 × 4.6 mm) and a UV detector. A mixture of methanol, water and phosphoric acid (volume ratio: 60/40/0.4) was adopted as the mobile phase (1.0 mL min-1). The concentration of chlorine ion (Cl-) was measured by ion chromatography (IC, Dionex ICS 1100, Thermo Fisher Scientific, UK). All 7
samples before the analysis were filtered by 0.45 μm cellulose membrane. 2.4. Electrocatalytic Hydrodechlorination of 2,4-DCP in Water All the EHDC experiments were controlled by a CHI760 workstation (Chenhua Instruments Company, China), and performed in an H-shaped cell at 25 °C. The whole cell was separated into one cathode and anode chamber (each one is 150 mL), and a cation-exchange membrane was used to prevent the Cl- transfer between chambers. A three-electrode system was employed for the study, with the catalyst-loaded carbon paper, Ag/AgCl (3.0 M KCl, 0.201 V vs Standard hydrogen electrode, 25 °C) and Pt foil as the working, reference and counter electrode, respectively. For each test, both the cathode and anode chambers were feed with 100 mL of 50 mM Na2SO4 solution, while the cathode chamber was further charged with 1 mL of 2,4-DCP stocking solution (5 g of 2,4-DCP in 1 L of methanol). During tests, the solution in cathode chamber was magnetically stirred under a N2 flow, while that in anode chamber were bubbling with N2 to remove the generated oxygen on anode. Aliquots (0.5 mL) were taken from the system at certain intervals for analysis. The intrinsic activity of the catalyst was evaluated by its TOF (min-1) at the end of reaction:
TOF
(C0 -Ct ) V M 2,4-DCP nPd D t
(2)
Where C0 and Ct correspond to the concentration of 2,4-DCP (mg L-1) at the reaction time of 0 and t; t is the reaction time (min); M2,4-DCP is the molar mass of 2,4-DCP (163 g mol-1); V is the volume of liquid in reactor (0.1 L); nPd denotes the Pd loading on electrode (mmol); D is the proportion of Pd atom in one NP dispersed on the 8
surface, and can be roughly estimated by:
D
6 CPd M Pd 109 100% d NA
(3)
Where CPd is the specific atomic concentration on Pd NP surface (1.27 × 1019 atoms m-2); MPd is the molar weight of Pd (106.42 g mol-1); NA is the Avogadro’s number (6.02 × 1023 atoms mol-1); ρ is the density of Pd (1.202 × 107 g m-3); d (nm) is the mean particle size, which was obtained from TEM image. 2.5. DFT calculation Surfaces were molded as periodically repeating slabs separated with a vacuum spacing greater than 15 Å. TiO2 (101) surface was 3 ×3 slab containing 36 Ti and 72 O atoms in a supercell. The graphene structure was molded as a 2D slab with 2 atom layers, 106 atoms. For both structures, half of bottom atomic layers in the slab were frozen to bulk position, and top layers were fully relaxed in the simulations. The Pd4 (111) cluster was put on the supporter surface as an active species because of its relatively stable three-dimensional configuration in both free and supported situation. Nonspin-polarized DFT-D2 calculations in this study were performed using the Vienna
Ab-Initio
Simulation
Package
(VASP5.4.1)
program
with
the
projector-augmented wave (PAW) pseudopotentials and the generalized gradient approximation
(GGA)
of
Perdew-Burke-Ernzerhof
(PBE)
to
treat
the
exchange-correlation potential of electrons [37,38]. The cutoff energies for plane waves were chosen to be 450 eV. A Morkhost-Pack mesh of k-points 3 × 3 × 1 was used to sample the two-dimensional Brillouin zone for geometry optimization. The partial occupancies close to the Fermi level were evaluated with the Gaussian 9
smearing width of 0.1 eV. The solvation program VASPsol in which the solvent was treated as a continuum dielectric was used to handle the effects of solvent [39]. In geometry optimizations, all the structures were relaxed up to the residual atomic forces smaller than 0.03 eV/Å, and the total energy was converged to 10−4 eV. The adsorption energy (Eads) was calculated as: Eads = Etot – (Esub + Emol)
(3)
Where Etot, Esub and Emol depict the total energy of the adsorption complex, the substrate and the isolated adsorbed molecules, respectively.
3. Results and discussion 3.1. Electrode preparation and characterization The Pd-TiO2 Schottky-heterojunction catalyst was synthesized by a facile one-pot wet-chemical reduction method. TEM image in Fig. 1a shows that the black Pd NPs are well dispersed on the grey TiO2 surface, and have spherical shape with about 5.0 nm size. The representative HRTEM image in Fig. 1b shows a d-spacing of 0.22 nm for Pd and 0.36 nm for TiO2, matching well with the (111) plane of Pd phase and the (101) plane of anatase TiO2 phase. XRD analyses on both the TiO2 and Pd-TiO2 reveal the pure anatase phase of TiO2 in both samples, and the metallic Pd phase in Pd-TiO2 sample by the (111) and (200) diffraction peaks at 2θ = 40.12° and 46.36°. Here Pd-C heterojunction catalyst was prepared as a reference sample. The XRD pattern in Fig. 1c confirms the formation of metallic Pd phase, while the TEM image in Fig. 1d shows that the 5.2 nm Pd NPs are uniformly deposited over the carbon layers. The ICP-AES analyses determined the Pd loading to be close at 0.241 ± 10
0.012 and 0.239 ± 0.016 mgPd mg-1catalyst on Pd-C and Pd-TiO2 catalysts. Their electrochemical active specific surface areas (ECSA), estimated from CO-stripping experiments, are also very close to be 19.5 and 22.7 m2 gPd-1 (See Fig. S1). The Pd-TiO2 and Pd-C, with the close Pd loading and active site number, offer the ideal platform to study the Schottky heterojunction effect on the EHDC performance.
Fig. 1.
In principle, the formation of Schottky heterojunction in Pd-TiO2 will drive parts of electrons transferred from TiO2 to Pd at the interface under dark conditions (See a schematic diagram in Fig. S2), while promote the separation of hot charge carriers in TiO2 under light illumination [40]. Accordingly, to verify the successful formation of Schottky heterojunction in Pd-TiO2, comparative study on the electronic states of Pd in Pd-C and Pd-TiO2, and the charge carrier separation efficiency in TiO2 and the Pd-loaded TiO2 were conducted by using the XPS and PL techniques. Fig. 2a presents the core-level Pd 3d XPS spectra of Pd-TiO2 and Pd-C, both of which show two pairs of doublet peaks in a similar profile. The predominant peaks at about 335 and 340 eV correspond to the 3d5/2 and 3d3/2 spin-orbitals of the metallic Pd, while the shoulder peaks at around 337 and 342 eV resemble the 3d5/2 and 3d3/2 levels of Pd2+ species in the form of PdO. Notably, the Pd0 3d5/2 peak of Pd-TiO2 locates at a binding energy of 335.25 eV, smaller than that of the standard metallic Pd (335.60 eV), while both the Pd 3d3/2 and 3d5/2 peaks of Pd-TiO2 shows an obvious negative shift in comparison to those of Pd-C. Further area integration of the Pd2+ (SPd2+) and Pd0 (SPd0) peak reveal 11
that the SPd2+/SPd0 in Pd-TiO2 is only 0.12, lower than that of 0.32 in Pd-C. The XPS results demonstrate that the TiO2-supported Pd indeed owns more electrons in comparison to the carbon-supported Pd, and are more tolerant to the oxidation under atmosphere. The PL spectra of TiO2 and Pd-TiO2 in Fig. 2b clearly point to the positive effect of heterojunction construction on reducing the intensity of omitted light, which is one key signal of the improved hot carrier separation in Pd-TiO2. Based on the displayed electron transfer direction from TiO2 to Pd, and the improved charger carrier separation under light illumination, we were ascertained that the Schottky heterojunction catalyst Pd-TiO2 had been successfully prepared.
Fig. 2.
3.2. EHDC Performance To probe into the Schottky heterojunction effect on EHDC performance, the TiO2, Pd-TiO2 and Pd-C were used as the cathode catalysts, respectively, to detoxify the 2,4-DCP in aqueous solutions. The 2,4-DCP removal efficiency, reaction pathway, EHDC degree and reaction kinetics on these three catalysts were then examined. Fig. 3a presents the C/C0-reaction time profiles under one fixed potential of -0.85 V (vs. Ag/AgCl, the same below). It clearly shows that the TiO2 is inert, while the Pd-C and Pd-TiO2 are both active to remove 2,4-DCP. Fig. 3b summarizes the TOFs of Pd-TiO2 and Pd-C under the potentials of -0.65, -0.75, -0.85 and -0.95 V, where the Pd-TiO2 is indicated to be more active by its larger TOF values than those of Pd-C. 12
Considering the inertness of TiO2 in EHDC, the enhanced performance of Pd-TiO2 should arise from the Schottky heterojunction effect. To find what the removed 2,4-DCP has been converted to on our catalysts, the species evolved during the reaction were analyzed and quantified by a combined technique of HPLC and chloride ion (Cl-) analyzer. The results reveal that in both the Pd-TiO2 and Pd-C systems, only 2,4-DCP, o-CP, P and Cl- are detected, and no other species (such as cyclohexanol) emerge even at an extended reaction time (See Fig. S3), which suggest that the EHDC is terminated at P, and no further hydrogenation reaction occurs. Among the detected species, the concentrations of P and o-CP increase rapidly with the consumption of 2,4-DCP (Fig. 3c), but the mass balance of carbon keeps relatively steady, further demonstrating a fairly good reaction selectivity towards EHDC, as well as a leading reaction pathway of 2,4-DCP→o-CP + Cl- →P + Cl-. Similar reaction pathways were also reported by some researchers, and the cleavage superiority of p-C-Cl was usually explained in terms of the steric hindrance effect [42, 43]. The EHDC degree, described by the molar ratio of the chlorine-free P in product, is one critical descriptor of the EHDC completeness and safety. Fig. 3d plots the molar ratio of o-CP and P in product as a function of reaction time. It is found that on Pd-C, the o-CP keeps a relatively high constitution of around 30% in product during the 180-min reaction, suggestive of an inefficient and incomplete EHDC process. The number, however, declines to below 5% on Pd-TiO2 at the end of reaction, which indicates that the Schottky heterojunction contributes to a more complete EHDC, in comparison to the Pd-C catalyst. 13
Fig. 3.
The durability of the Pd-TiO2 catalyst was evaluated by repeating the EHDC reaction on one Pd-TiO2 electrode for five times. The nearly undeteriorated efficiency (Fig. 4a) and the little change in the CV profile after the five tests (Fig. 4b) indicated that the presented Pd-TiO2 catalysts were relatively durable. The Pd2+ concentration in the solution was also tracked at the end of each run of reaction. No Pd element was detected, indicating no Pd was leached during the EHDC. The high durability of Pd-TiO2 could be ascribed to the strong metal-support interactions in the constructed Schottky heterojunction structure [41]. Moreover, the EHDC was usually conducted at a very negative potential (-0.85 V) and in an oxygen-free and neutral solution, both of which prevented the Pd against the oxidative corrosion.
Fig. 4.
3.3. Mechanism To probe into the origin of Schottky heterojunction effect, we used the classic Langmuir-Hinshelwood (L-H) model at the first stage to describe the EHDC kinetics on Pd-C and Pd-TiO2 (See the details in Supplementary Materials). As illustrated in Fig. 5a, a pretty good linear plottings of 1/r0 vs. 1/C0 are obtained for both the 14
Pd-TiO2 (R2 = 0.98) and Pd-C (R2 = 0.98) systems, which reflect that the EHDC reaction is rate-controlled by the surface reaction of 2,4-DCP with H*. Regarding the critical role of H*, the H* generation kinetics on Pd-TiO2 and Pd-C were firstly studied by LSV method. A more positive onset potential for the conversion reaction of H+ to H* as well as the large current density in the H* generation region are observed on the LSV curve of Pd-C, which reveals that the Pd-C is more powerful in producing H* (Fig. S5). Such an efficiency order of Pd-C > Pd-TiO2, however, rightly reverses their EHDC performance order (Pd-TiO2 > Pd-C). Fig. 5b plots the current efficiency of Pd-C and Pd-TiO2 versus the reaction time, which is intended to show how many of the generated H* are consumed by EHDC (the rest is evolved into molecular H2). Though the current efficiency of Pd-TiO2 nearly doubles that of Pd-C, both of their absolute values are relatively low, leaving only 25.8% for Pd-TiO2 and 18% for Pd-C at the end of reaction. On basis of the lower H* utilization rate and the inconsistency between the order of H* generation efficiency and EHDC performance for Pd-C and Pd-TiO2, we inferred that the effective adsorption and activation of 2,4-DCP on catalyst was the more important step of EHDC reaction.
Fig. 5.
On basis of the above discussions, we speculated that the promoting effect of Schottky heterojunction on EHDC might lie at its ability in intensifying the adsorption and activation of 2,4-DCP on the Pd surface. Here the influences of mass transfer
15
limitations of 2,4-DCP in solution and its adsorption by support were not taken into consideration as the EHDC on Pd-C and Pd-TiO2 were conducted under identical conditions (e.g. intensive stirring) and the support itself had no power to provide H* and activate 2,4-DCP [44, 45]. To verify this speculation, we adopted the DFT calculations to embody the adsorption energy (Eads) of 2,4-DCP and the C-Cl activation degree (characterized by the C-Cl bond length) on Pd-C and Pd-TiO2. To be simple, four Pd atoms in a tetrahedra shape supported on a leading exposed TiO2 (101) plane and C (002) plane were employed to represent the heterojunction structure [46]. The detailed structure optimization process and the optimal cross-section configurations of Pd-TiO2 and Pd-C are shown in Fig. S6 and Fig. 6a-b. In the optimized structure, the Pd NP, indeed, has much stronger interactions with TiO2 support, evidenced by the transfer of nearly 0.81 electrons from TiO2 to Pd (See Fig. S7 and Table S1). In comparison, the positive Bader charge of Pd means that 0.49 of the electrons are transferred from Pd to C. 2,4-DCP molecule prefers to be adsorbed with its ring plane parallel to one Pd plane at the support-metal interface. The Eads calculation results demonstrate that 2,4-DCP is more preferably adsorbed on Pd-C with a more negative value of -1.23 eV (vs. -1.01 eV on Pd-TiO2). Accompanying the strong adsorption, the o- and p-C-Cl bond lengths of 2,4-DCP on Pd-C are 1.768 and 1.831 Å, much longer than those of 1.754 and 1.751 on Pd-TiO2, demonstrating that the Pd-C interface is more powerful in activating the C-Cl bonds. These results are, however, unexpected as the effective adsorption and activation of the pollutant on Pd-C inversely yield an inferior EHDC performance. 16
In one heterogeneous catalysis, besides the intrinsic adsorption and C-Cl bond activation, the adsorption of 2,4-DCP on Pd is interfered by the desorption kinetics of the intermediate product (i.e. P and Cl-) from Pd [47-49]. Generally, an effective desorption of the product will accelerate the refreshment of the active sites for a new round of reactions, which is thus essential to maintain an efficient and continuous catalytic reaction. In this case, we further studied the adsorption behaviors of P and Cl- on Pd-C and Pd-TiO2 by DFT calculations. The results in Fig. 6c-d and Fig. S8 show that the P is adsorbed on catalyst in a very close configuration to 2,4-DCP, and its binding strength with Pd is comparable to that of 2,4-DCP but much stronger than that of Cl-, which indicate that the P is one very potential species that may compete the active sites with 2,4-DCP on the catalyst. On the other hand, the maximum Eads of P on Pd-TiO2 is only -0.82 eV, much more positive than that of -1.19 eV on Pd-C, evidencing that the P desorption on the Pd-TiO2 interface is much easier.
Fig. 6.
Up to here, we realized that the P (not Cl-) should exert a considerable negative effect on EHDC by its strong adsorption on Pd, which might lower the possibility or raise the difficulty for 2,4-DCP to reach the active sites, and form an extra energy barrier for EHDC reaction. The Pd-TiO2 had a weakened binding strength with P, which facilitated the P desorption and intensified the 2,4-DCP adsorption over Pd, leading to an enhanced EHDC performance. Here, we attributed the improved 17
performance of Pd-TiO2 in adsorbing 2,4-DCP and desorbing P to the Schottky heterojunction effect, which functioned to modify the electronic structure of Pd and optimize its interactions with 2,4-DCP and P. 3.4. Improved phenol-tolerance on the Schottky heterojunction catalyst To verify the negative effect of P and its alleviation on the Pd-TiO2 Schottky heterojunction catalyst, we firstly compared the EHDC performances of Pd-C and Pd-TiO2 electrodes before and after being pre-immersed in a P solution, respectively. The obtained TOF results in Fig. 7a clearly show that the electrodes pre-immersed in the P solutions display significant decays in their EHDC activities. Furthermore, the decay extent is enlarged when a more concentrated P solution is used. By these observations, we are ascertained that the P indeed poses an adverse effect on EHDC by occupying the active sites. Fig. 7a also shows that with the P concentration increasing from 0 to 10 and to 30 mg L-1, the TOFs of Pd-TiO2 declines by 3.7% and 1.0%, much smaller than those of Pd-C (35.7% and 27.0%). The smaller TOF decay on Pd-TiO2 provides the solid evidence of the alleviated negative effect of P on Schottky heterojunction catalyst.
Fig. 7.
The alleviated negative effect of P on the Schottky heterojunction catalyst was further verified by an H* stripping method. In this method, the H* was generated, and adsorbed/absorbed over the catalyst at -0.7 V for 60 s (at this condition the H* would 18
not evolve into H2), which was then oxidized by a positive scanning. The oxidation current can reflect the transferred electron number during oxidation, and be used to quantify the generated H*. Assuming that the P-occupied Pd sites are deactivated in H* generation, the generated H* number difference on the electrode before and after being pre-immersed in P solutions can be used to characterize the P tolerance of the catalysts. Fig. 7b presents the H* stripping profiles. It shows that the H* yield declines only by 10.31% on the P-treated Pd-TiO2 electrode, but by a larger extent of 38.65% on Pd-C under the identical condition. These H* stripping results substantially support the viewpoint that the Pd-TiO2 Schottky heterojunction catalyst has a less tendency to adsorb the P molecules, which enables to promote the P desorption and the 2,4-DCP adsorption on Pd, and ease the negative effects from P.
4. Conclusions In summary, this work reports one novel semiconductor-metal Pd-TiO2 Schottky heterojunction catalyst for EHDC of CPs in water. This catalyst is proved to be superior to the conventional Pd-C catalyst in the aspects of removal efficiency, dechlorination degree and current efficiency. The superiority originates from the strong Schottky heterojunction-induced metal-support interactions between Pd and TiO2, by which the Pd NPs gather electrons from TiO2. The Pd NPs with the higher electron density have a less tendency to adsorb the P, which promotes the adsorption of 2,4-DCP over the catalyst. Overall, our work successfully utilizes the schottky heterojunction effect for optimization of the EHDC performance, and the presented strategy can also be extended to other hydrogenation reaction systems. 19
Acknowledgements The present work is financially supported by National Natural Science Foundation of China (51878105), Venture & Innovation Support Program for Chongqing Overseas Returnees (cx2017066), the Program for the Top Young Talents of Chongqing, Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJQN201800829), Research Startup Foundation of Chongqing Technology and Business University (2016-56-01 and 2016-56-02).
References [1] M.Á. Arellano-González, I. González, A.-C. Texier, Mineralization of 2-chlorophenol by sequential electrochemical reductive dechlorination and biological processes, J. Hazard. Mater. 314 (2016) 181-187. [2] X. Song, Q. Shi, H. Wang, S. Liu, C. Tai, Z. Bian, Preparation of Pd-Fe/graphene catalysts by photocatalytic reduction with enhanced electrochemical oxidation-reduction properties for chlorophenols, Appl. Catal. B: Environ. 203 (2017) 442-451. [3] W. Xie, S. Yuan, X. Mao, W. Hu, P. Liao, M. Tong, A. Alshawabkeh, Electrocatalytic activity of Pd-loaded Ti/TiO2 nanotubes cathode for TCE reduction in groundwater, Water Res. 47 (2013) 3573-3582. [4] R. Liu, H. Chen, L. Fang, C. Xu, Z. He, Y. Lai, H. Zhao, D. Bekana, J. Liu, Au@Pd bimetallic nanocatalyst for carbon-halogen bond cleavage: An old story with new insight into how the activity of Pd is influenced by Au, Environ. Sci. Technol. 52 (2018) 4244-4255. [5] F. He, Z. Li, S. Shi, W. Xu, H. Sheng, Y. Gu, Y. Jiang, B. Xi, Dechlorination of excess trichloroethene by bimetallic and sulfidated nanoscale zero-valent iron, Environ. Sci. Technol. 52 (2018) 8627-8637. [6] Y. Mu, F. Jia, Z. Ai, L. Zhang, Iron oxide shell mediated environmental remediation properties of nano zero-valent iron, Environ. Sci.: Nano 4 (2017) 27-45. [7] P. Shao, J. Tian, X. Duan, Y. Yang, W. Shi, X. Luo, F. Cui, S. Luo, S. Wang, Cobalt silicate 20
hydroxide nanosheets in hierarchical hollow architecture with maximized cobalt active site for catalytic oxidation, Chem. Eng. J. 359 (2019) 79-87. [8] P. Shao, J. Tian, F. Yang, X. Duan, S. Gao, W. Shi, X. Luo, F. Cui, S. Luo, S. Wang, Identification and regulation of active sites on nanodiamonds: establishing a highly efficient catalytic system for oxidation of organic contaminants, Adv. Funct. Mater. 28 (2018) 1705295. [9] X. Mao, A. Ciblak, K. Baek, M. Amiri, R. Loch-Caruso, A.N. Alshawabkeh, Optimization of electrochemical dechlorination of trichloroethylene in reducing electrolytes, Water Res. 46 (2012) 1847-1857. [10] B.P. Chaplin, M. Reinhard, W.F. Schneider, C. Schuth, J.R. Shapley, T.J. Strathmann, C.J. Werth, Critical review of Pd-based catalytic treatment of priority contaminants in water, Environ. Sci. Technol. 46 (2012) 3655-3670. [11] Z. Sun, X. Wei, Y. Han, S. Tong, X. Hu, Complete dechlorination of 2,4-dichlorophenol in aqueous solution on palladium/polymeric pyrrole-cetyl trimethyl ammonium bromide/foam-nickel composite electrode, J. Hazard. Mater. 244-245 (2013) 287-294. [12] C. Jiang, H. Yu, Y. Lu, S. Zhu, Z. Geng, M. Huo, X. Wang, Preparation of spike-like palladium nanoparticle electrode and its dechlorination properties, Thin Solid Films 664 (2018) 27-32. [13] L. Yang, Z. Chen, D. Cui, X. Luo, B. Liang, L. Yang, T. Liu, A. Wang, S. Luo, Ultrafine palladium nanoparticles supported on 3D self-supported Ni foam for cathodic dechlorination of florfenicol, Chem. Eng. J. 359 (2019) 894-901. [14] Y. Wu, L. Gan, S. Zhang, B. Jiang, H. Song, W. Li, Y. Pan, A. Li, Enhanced electrocatalytic dechlorination of para-chloronitrobenzene based on Ni/Pd foam electrode, Chem. Eng. J. 316 (2017) 146-153. [15] H. Xu, H. Xu, Z. Chen, X. Ran, J. Fan, W. Luo, Z. Bian, W.X. Zhang, J. Yang, Bimetallic PdCu nanocrystals immobilized by nitrogen-containing ordered mesoporous carbon for electrocatalytic denitrification, ACS Appl. Mater. Interfaces 11 (2019) 3861-3868. [16] Ding, Y.; Sun, X. L.; Wang, Z. L.; Sun, S. H., Misfit dislocations in multimetallic core-shelled nanoparticles, Appl. Phys. Lett. 100 (2012) 111603. [17] Z. Lou, J. Zhou, M. Sun, J. Xu, K. Yang, D. Lv, Y. Zhao, X. Xu, MnO2 enhances electrocatalytic hydrodechlorination by Pd/Ni foam electrodes and reduces Pd needs, Chem. Eng. 21
J. 352 (2018) 549-557. [18] W. Fu, S. Shu, J. Li, X. Shi, X. Lv, Y. Huang, F. Dong, G. Jiang, Identifying the rate-determining step of the electrocatalytic hydrodechlorination reaction on palladium nanoparticles, Nanoscale (2019), 10.1039/C9NR04634H. [19] S. Yang, D.Y. Chung, Y.-J. Tak, J. Kim, H. Han, J.-S. Yu, A. Soon, Y.-E. Sung, H. Lee, Electronic structure modification of platinum on titanium nitride resulting in enhanced catalytic activity and durability for oxygen reduction and formic acid oxidation, Appl. Catal. B: Environ. 174-175 (2015) 35-42. [20] W. Guo, S. Niu, W. Shi, B. Zhang, W. Yu, Y. Xie, X. Ji, Y. Wu, D. Su, L. Shao, Pd–P nanoalloys supported on a porous carbon frame as an efficient catalyst for benzyl alcohol oxidation, Catal. Sci. Technol. 8 (2018) 2333-2339. [21] Y. Holade, R.G. da Silva, K. Servat, T.W. Napporn, C. Canaff, A.R. de Andrade, K.B. Kokoh, Facile synthesis of highly active and durable PdM/C (M = Fe, Mn) nanocatalysts for the oxygen reduction reaction in an alkaline medium, J. Mater. Chem. A 4 (2016) 8337-8349. [22] H.Y. Chen, M.X. Jin, L. Zhang, A.J. Wang, J. Yuan, Q.L. Zhang, J.J. Feng, One-pot aqueous synthesis of two-dimensional porous bimetallic PtPd alloyed nanosheets as highly active and durable electrocatalyst for boosting oxygen reduction and hydrogen evolution, J. Colloid Interf. Sci. 543 (2019) 1-8. [23] Z. Lou, J. Zhou, M. Sun, J. Xu, K. Yang, D. Lv, Y. Zhao, X. Xu, MnO2 enhances electrocatalytic hydrodechlorination by Pd/Ni foam electrodes and reduces Pd needs, Chem. Eng. J. 352 (2018) 549-557. [24] Z. Lou, J. Xu, J. Zhou, K. Yang, Z. Chao, Y. Li, Y. Liu, L. Lou, X. Xu, Insight into atomic H* generation, H2 evolution, and cathode potential of MnO2 induced Pd/Ni foam cathode for electrocatalytic hydrodechlorination, Chem. Eng. J.374 (2019) 211-220. [25] Z. Lou, Y. Li, J. Zhou, K. Yang, Y. Liu, S.A. Baig, X. Xu, TiC doped palladium/nickel foam cathode for electrocatalytic hydrodechlorination of 2,4-DCBA: Enhanced electrical conductivity and reactive activity, J. Hazard. Mater. 362 (2019) 148-159. [26] J. Hou, Y. Sun, Y. Wu, S. Cao, L. Sun, Promoting active sites in core-shell nanowire array as Mott-Schottky electrocatalysts for efficient and stable overall water splitting, Adv. Funct. Mater. 28 (2018) 1704447. 22
[27] H. Liu, X. Liu, W. Yang, M. Shen, S. Geng, C. Yu, B. Shen, Y. Yu, Photocatalytic dehydrogenation of formic acid promoted by a superior PdAg@g-C3N4 Mott–Schottky heterojunction, J. Mater. Chem. A 7 (2019) 2022-2026. [28] Y.Y. Cai, X.H. Li, Y.N. Zhang, X. Wei, K.X. Wang, J.S. Chen, Highly efficient dehydrogenation of formic acid over a palladium-nanoparticle-based Mott-Schottky photocatalyst, Angew. Chem. Int. Ed. 52 (2013) 11822-11825. [29] Z. Zhuang, Y. Li, Z. Li, F. Lv, Z. Lang, K. Zhao, L. Zhou, L. Moskaleva, S. Guo, L. Mai, MoB/g-C3N4 interface materials as a Schottky catalyst to boost hydrogen evolution, Angew. Chem. Int. Ed. 57 (2018) 496-500. [30] X.H. Li, M. Antonietti, Metal nanoparticles at mesoporous N-doped carbons and carbon nitrides: functional Mott-Schottky heterojunctions for catalysis, Chem. Soc. Rev. 42 (2013) 6593-6604. [31] Y. Liu, J. Guo, E. Zhu, L. Liao, S.J. Lee, M. Ding, I. Shakir, V. Gambin, Y. Huang, X. Duan, Approaching the Schottky-Mott limit in van der Waals metal-semiconductor junctions, Nature 557 (2018) 696-700. [32] H. Wang, Z. Lu, S. Xu, D. Kong, J.J. Cha, G. Zheng, P.C. Hsu, K. Yan, D. Bradshaw, F.B. Prinz, Y. Cui, Electrochemical tuning of vertically aligned MoS2 nanofilms and its application in improving hydrogen evolution reaction, Proc. Natl. Acad. Sci. USA 110 (2013) 19701-19706. [33] J. Xie, J. Zhang, S. Li, F. Grote, X. Zhang, H. Zhang, R. Wang, Y. Lei, B. Pan, Y. Xie, Controllable disorder engineering in oxygen-incorporated MoS2 ultrathin nanosheets for efficient hydrogen evolution, J. Am. Chem. Soc. 135 (2013) 17881-17888. [34] C. Wan, Y.N. Regmi, B.M. Leonard, Multiple phases of molybdenum carbide as electrocatalysts for the hydrogen evolution reaction, Angew. Chem. Int. Ed. 53 (2014) 6407-6410. [35] M. Jakob, H. Levanon, Charge distribution between UV-Irradiated TiO2 and gold nanoparticles: Determination of shift in the Fermi level. Nano Lett. 3 (2003) 353-358. [36] X. Xu, J. Luo, L. Li, D. Zhang, Y. Wang, G. Li, Unprecedented catalytic performance in amine syntheses via Pd/g-C3N4 catalyst-assisted transfer hydrogenation, Green Chem. 20 (2018) 2038-2046. [37] S. Grimme, Semiempirical GGA ‐ type density functional constructed with a long ‐ range 23
dispersion correction, J. Comput. Chem. 27 (2006) 1787-1799. [38] B. Hammer, L. B. Hansen, J. K. Nørskov, Improved adsorption energetics within density-functional theory using revised Perdew-Burke-Ernzerhof functionals, Phys. Rev. B 59 (1999) 7413. [39] M. Fishman, H.L. Zhuang, K. Mathew, W. Dirschka, R.G. Hennig, Accuracy of exchange-correlation functionals and effect of solvation on the surface energy of copper, Phys. Rev. B 87 (2013). [40] J. Fu, J. Yu, C. Jiang, B. Cheng, g-C3N4-Based Heterostructured Photocatalysts, Adv. Energ. Mater. 8 (2018) 1701503. [41] G. Yuan, M.A. Keane, Liquid phase hydrodechlorination of chlorophenols over Pd/C and Pd/Al2O3: a consideration of HCl/catalyst interactions and solution pH effects, Appl. Catal. B Environ. 52 (2004) 301-314. [42] G. Yuan, M.A. Keane, Aqueous-Phase Hydrodechlorination of 2,4-Dichlorophenol over Pd/Al2O3: Reaction under Controlled pH, Ind. Eng. Chem. Res. 46 (2007) 705–715. [43] S. Shu, W. Fu, P. Wang, W. Cen, Y. Chu, F. Wei, X. Zhang, F. Dong, G. Jiang, Electrocatalytic hydrodechlorination of 2,4-dichlorophenol over palladium nanoparticles: The critical role of hydroxyl group deprotonation, Appl. Catal. A-Gen. 583 (2019) 117146. [44] Y. Fang,
K.N. Heck, P.J. J. Alvarez, M.S. Wong, Kinetics analysis of Palladium/Gold
nanoparticles as colloidal hydrodechlorination catalysts, ACS Catal. 1 (2011) 128-138. [45] M. Munoz, M. Kaspereit, B.J.M. Etzold, Deducing kinetic constants for the hydrodechlorination of 4-chlorophenol using high adsorption capacity catalysts, Chem. Eng. J. 285 (2016) 228-235. [46] M. Munoz, G.R. Zhang, B.J.M. Etzold, Exploring the role of the catalytic support sorption capacity on the hydrodechlorination kinetics by the use of carbide-derived carbons, Appl. Catal. B: Environ. 203 (2017) 591-598. [47] J.N. Sandoval, D. Ortiz, M. Munoz, Z.M. Pedro, J.A. Casas, On the deactivation and regeneration
of
Pd/Al2O3
catalyst
for
aqueousphase
hydrodechlorination
of
diluted
chlorpromazine solution, Catal. Today (2019), 10.1016/j.cattod.2019.06.028. [48] G. Yuan, M.A. Keane, Catalyst deactivation during the liquid phase hydrodechlorination of 2,4-dichlorophenol over supported Pd: influence of the support, Catal. Today 88 (2003) 27-36. 24
[49] E. Diaz, A.F. Mohedano, Jose A.CasasJuan J.Rodriguez, Analysis of the deactivation of Pd, Pt and Rh on activated carbon catalysts in the hydrodechlorination of the MCPA herbicide, Appl. Catal. B-Environ. 181 (2016) 429–435.
Figure captions Fig. 1. Representative (a) TEM and (b) HRTEM images of the Pd-TiO2 composite; (c) XRD patterns of Pd-TiO2, Pd-C and TiO2; (d) One typical TEM image of the Pd-C composite. Fig. 2. (a) Comparison in the Pd 3d XPS spectra of Pd-TiO2 and Pd-C. (b) PL patterns of Pd-TiO2 and TiO2 samples under the UV-light illumination (leading wavelength of 300 nm). Fig. 3. (a) Reaction time-dependent 2,4-DCP concentration with the Pd-TiO2, Pd-C and TiO2 electrodes at a fixed potential of -0.85 V in N2-saturated Na2SO4 aqueous solution (initial 2,4-DCP concentration: 50 mg L-1); (b) TOF of Pd-TiO2 and Pd-C under the potentials of -0.65, -0.75, -0.85 and -0.95 V, respectively. (c) The concentration evolution of the 2,4-DCP and the intermediate products during EHDC of 2,4-DCP on the Pd-TiO2 and Pd-C electrodes; (d) Reaction time-dependent o-CP concentration in the dechlorination products. Fig. 4. (a) Durability test for Pd-TiO2 catalyst in EHDC reaction; (b) CV profiles of the fresh Pd-TiO2 and the Pd-TiO2 after the use for five times. (Conditions: 0.1 M HClO4 aqueous solution; scanning rate: 10 mV s-1, electrode area: 2 cm × 3 cm). Fig. 5. (a) Langmuir-Hinshelwood (L-H) representations and (b) the reaction time-dependent current efficiency of the EHDC reaction over Pd-TiO2 and Pd-C. Fig. 6. Side view of the optimized adsorption configurations evaluated for 2,4-DCP on (a) Pd-TiO2, (b) Pd-C, and P on (c) Pd-TiO2 and (d) Pd-C. Fig. 7. (a) TOFs of the Pd-TiO2 and Pd-C electrodes before and after being pre-immersed in P solutions; (b). The H* stripping profiles of Pd-TiO2 and Pd-C electrodes.
25
Fig. 1.
26
Fig. 2.
27
Fig. 3.
28
Fig. 4.
29
Fig. 5.
30
Fig. 6.
31
Fig. 7.
Graphical Abstract
Highlights A novel TiO2-Pd Schottky heterojunction catalyst was developed for EHDC reaction TiO2-Pd outperforms the C-Pd in EHDC efficiency, degree and energy selectivity Schottky heterojunction-induced metal-support interaction is the origin of activity 32
Phenol exerts a considerable negative effect on EHDC by competing the active sites The formation of Schottky heterojunction eases the negative effect of phenol
33