Palladium hydride with high-index facets for enhanced methanol oxidation

Xiaoyun Guo a, Zheng Hu a, Jianxin Lv a, Jianqiang Qu *a and Shi Hu *ab
aDepartment of Chemistry, School of Science, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072, China. E-mail: jqqu@tju.edu.cn; rychushi@gmail.com
bInstitute of Energy, Hefei Comprehensive National Science Center, 230031, China

Received 19th May 2021 , Accepted 17th June 2021

First published on 22nd June 2021


Abstract

Nobel metal catalysts with high-index facets feature a high density of steps and kink sites, which bring about high activity but could be unstable during the electrocatalytic process. Doping with interstitial hydrogen atoms is a unique and effective way to regulate the electronic structure of the host materials. The formation of hydride also helps to stabilize the active sites on the surface of catalysts. Herein, we demonstrate the conformal doping of H atoms into the Pd nanostructure with preferential exposure of {730} facets, forming concave nanocubes of palladium hydride. Compared to the palladium counterparts, the palladium hydride catalysts show enhanced activity and stability in electrocatalytic methanol oxidation, and the structural differences between the Pd and PdH catalysts are revealed by XRD and X-ray photoelectron spectroscopy. Our work presents a powerful strategy for designing durable catalysts with high performance by combining high-index facet with interstitial atom doping.


Introduction

Highly active and durable electrocatalysts for the methanol oxidation reaction (MOR) are vital to the development and application of alkaline direct methanol fuel cells (ADMFCs). Noble metals like Pt are insuperable in many situations due to their unique activity and excellent stability; however, ADMFCs still suffer from the low MOR kinetics, even in the latest platinum (Pt)-based anode electrocatalysts.1–6 Besides, the scarcity of Pt restricts its wide usage in the application scenarios with the increasing demand of performance. Over the past decade, much effort has been devoted to non-Pt nanomaterials with relatively high abundance on earth for high MOR activity in an alkaline medium through a variety of modification methods.7–9

Palladium nanocrystals have attracted great interest in recent years owing to their unique properties and applications in catalysis. The catalytic activity of Pd nanocrystals strongly depends on their size and shape.10 In the past few decades, controlled synthesis of Pd nanocrystals with various shapes has been demonstrated. Compared with the Pd nanoparticles enclosed by low-index facets such as {111}, {100}, and {110},11 Pd nanocrystals with high-index facets have shown enhanced catalytic activities (arising from high densities of atomic steps and kinks).12,13 However, the activity could be compromised by the poor stability of these sites during the reaction.

Besides morphology modification, compositional tuning via interstitial doping of light atoms such as H,14,15 B,16–18 and C19 provides an effective strategy to tune the electronic structure of the nanomaterials. With the introduction of small atoms into the host lattice, the crystal lattice of the host will be expanded, and charge transfer will occur between the host and the guest atoms. Both effects will influence the catalytic activity and selectivity by adjusting the electronic structure. Hydrogen, the smallest atom, can be easily incorporated into the lattice of the host metals like Pd, forming metal hydrides. Palladium hydride has attracted much attention for its wide application prospects, such as hydrogen sensing,20,21 hydrogen purification22 and catalysis.14,15,23–25 However, palladium hydride obtained by traditional methods such as hydrogen pressurization, electrochemical scanning, and NaBH4 treatment was unstable,26–28 as hydrogen atoms leach out over hours even at room temperature, which becomes a major limiting factor for their application. Recently, Huang et al. proposed a solution for the stabilization of palladium hydride colloids with controlled morphology and size via hydrothermal treatment of palladium nanostructures in dimethylformamide (DMF). This made it possible to study the catalytic performance of various nanostructures of palladium hydride.23 While palladium nanostructures with low-index facets of {100}, {111} and {110} have been demonstrated by this protocol, palladium hydride with high-index facet has never been demonstrated.

In this work, we successfully incorporated hydrogen atoms into Pd concave nanocubes bounded by high-index {730} facets without significantly changing the morphology, and explored the role of H in tuning the catalyst structure and electrocatalytic activity in methanol oxidation. The introduction of hydrogen not only expands the crystal lattice of palladium but also causes the transfer of electrons between Pd and H, which changes the electronic structure of Pd. Furthermore, the formation of palladium hydride enhanced the structure stability, and the catalysts exhibit enhanced MOR performance in both activity and durability as compared to their Pd counterparts.

Experimental procedure

Chemicals and materials

Palladium chloride (PdCl2), potassium hydroxide (KOH), and L-ascorbic acid (AA) were purchased from Shanghai Aladdin Co. Ltd; sodium chloride (NaCl), poly (vinyl pyrrolidone) (PVP, Mw ≈ 40[thin space (1/6-em)]000 and Mw ≈ 58[thin space (1/6-em)]000), and potassium bromide (KBr; 99%) were purchased from Shanghai Macklin Biochemical Co., Ltd; N,N-dimethylformamide (DMF), ethanol, and acetone were purchased from Tianjin Yuanli Chemical Co., Ltd; Ar (99.999%) was purchased from Tianjin Huanyu High-Tech Gas Co. Ltd. Deionized (DI) water with a resistivity of 18.2 MΩ cm was used in all the experiments. Nafion (5.0 wt%) solution was purchased from Sigma-Aldrich. All the chemicals were used as received without further purification.

Synthesis of materials

Synthesis of Pd nanocubes. The Pd nanocubes were synthesized using a modified method by Younan Xia et al.29 Typically, L-ascorbic acid (60 mg), KBr (600 mg), and PVP (Mw ≈ 40[thin space (1/6-em)]000) (80 mg) were dissolved in deionized water (8.0 mL) in a 25 mL round bottom flask, which was preheated at 80 °C, under stirring at least for 10 min to obtain a homogeneous solution. Subsequently, 3 mL of an aqueous solution containing PdCl2 (34.4 mg) and NaCl (22.6 mg) was added to the vial. The sealed vial was kept in an oil bath at 80 °C under stirring for 3 h. The final product was collected by centrifugation and further purified by water/ethanol mixture, and dispersed in 10 mL water to make a seed solution.
Synthesis of Pd concave nanocubes. The Pd concave nanocubes were synthesized using a modified method by Younan Xia et al.30 In a typical synthesis, 7.7 mL of an aqueous solution containing PVP (Mw ≈ 58[thin space (1/6-em)]000) (105 mg), AA (60 mg) and KBr (300 mg), and 0.3 mL of the Pd seeds were mixed in a glass vial. The mixture was heated to 60 °C under stirring. Then, 3.0 mL of an aqueous solution containing PdCl2 (8.8 mg) and NaCl (5.8 mg) was added to the mixture by pipette under stirring. The synthesis was allowed to proceed at 60 °C for 3 h. The resulting products were collected by centrifugation (10[thin space (1/6-em)]800 rpm, 10 min) and washed with water three times to remove excess PVP.
Synthesis of β-palladium hydride (PdH0.43) concave nanocubes. Typically, the as-synthesized Pd nanocubes above were dispersed in 40 mL of DMF with the assistance of ultrasonication for 10 min. The obtained homogeneous solution was transferred to a Teflon-lined stainless-steel autoclave and then heated to 160 °C for 2 h before it was cooled to room temperature. The resulting products were collected by centrifugation (10[thin space (1/6-em)]800 rpm, 10 min) and washed with water three times.
Preparation of carbon-supported nanoparticles. The carbon powder (Vulcan XC-72R) and catalysts were dispersed in ethanol. The ethanol solutions of the catalyst nanoparticles and the carbon powder were sonicated for 2 h. Then the carbon supported nanoparticles were collected by centrifugation. The final products were obtained with a Pd loading of ca. 20%.

Material characterization

The morphology images of the products were taken on a JEOL-1230 transmission electron microscope (TEM) operated at 100 kV. High-resolution transmission electron microscopy (HR-TEM) was carried out on a Talos F200X transmission electron microscope (Thermo Fisher Scientific). Powder X-ray diffraction (XRD) analysis was carried out on a D8-Focus diffractometer (Bruker AXS) using Cu Kα radiation (λ = 1.5418 Å) with a current of 40 mA and a voltage of 40 kV. X-ray photoelectron spectroscopy (XPS) data were collected using an ESCALAB-250Xi spectrometer (Thermo Fisher Scientific) with Al Kα radiation as the X-ray source for excitation. The binding energy was calibrated against the carbon 1 s line.

Electrochemical characterization

Electrochemical measurements were performed using an electrochemical workstation (CHI660E, and CS-150H) with a standard three-electrode system at room temperature. A Hg/HgO electrode and a platinum foil electrode (1 cm × 1 cm) were used as a reference and counter electrode, respectively. The catalyst ink was prepared by mixing the carbon-supported catalyst with 980 μL of ethanol and 20 μL of Nafion (5.0 wt%) to form a suspension under ultrasonication. 8 μL of the catalyst ink of all the powder samples was drop-casted onto a glassy carbon (GC) electrode and dried at room temperature to obtain the working electrode (a catalyst-modified glassy carbon electrode with a diameter of 4 mm). The total loading weights of metals were kept constant at around 64 μg cm−2 for the Pd and PdH catalysts. All potentials in this paper were measured against the Hg/HgO reference electrode and converted to the pH-independent RHE reference scale by E (vs. RHE) = E (vs. HgO) + 0.098 + 0.059 × pH.

For every measurement, the fresh 1 M KOH electrolyte filled in a clean electrochemical cell was deoxygenated with a steady stream of ultrahigh-purity Ar for at least 20 min. The working electrode was first electrochemically cleaned via potential cycling between 0.05 V and 0.9 V versus the RHE for 20 cycles at 200 mV s−1 until a stable cyclic voltammogram was recorded.

The CO stripping measurements were carried out in a 1 M KOH solution by cycling between 0.05 V and 1.2 V versus RHE at 10 mV s−1 for the electrochemical surface area determination.

Results and discussion

The design of the catalyst is based on the strong hydrogen-absorption characteristics of palladium, which will induce a phase transition to palladium hydride PdHx with an expanded lattice constant. The synthesis of Pd concave nanocubes bounded by high-index {730} facets follows a modified seed-mediated method proposed by Younan Xia et al.,30 using Pd nanocubes as the seeds29 (Fig. S1). The products were collected and redispersed in ethanol for the following characterization studies (Fig. 1).
image file: d1dt01625c-f1.tif
Fig. 1 TEM and HRTEM images of (a and b) Pd concave nanocubes, and (c and d) β-PdH concave nanocubes respectively. (e) XRD analysis of Pd and β-PdH concave nanocubes.

As shown by the TEM image (Fig. 1a and S2), the cubic nanocrystal exhibited a darker contrast in the center than at the edges, confirming the formation of a concave cubic structure. The TEM images in Fig. S2 show the Pd concave nanocubes with high purity (>96%). Fig. 1b shows the high-resolution TEM (HRTEM) image of an individual Pd concave nanocube viewed along the 〈100〉 zone axis, as confirmed by the corresponding Fourier transform (FT) pattern (Fig. 1b). By measuring the tilt angles of edges at the surface (Fig. 1b), these concave surfaces could be indexed as the {730} planes, consistent with the previously reported results.30

The concave nanocubes of Pd were then subjected to hydrogen doping. To avoid the possible loss of high-index facets during hydrogen doping, the doping method of Huang et al. is modified by applying a 2-hour hydrothermal treatment in DMF.23Fig. 1c shows the TEM image of the nanocubes after the treatment. The concave cubic morphology was well retained after the H doping process. Fig. 1d shows the HRTEM image of an individual PdH concave nanocube with a lattice parameter of around 1.99 Å, larger than that of palladium nanocubes (1.95 Å).

The powder X-ray diffraction (XRD) (Fig. 1e) analysis further proved the successful incorporation of H into Pd. The (111) diffraction peak shifted to a smaller angle due to the expansion of the crystal lattice, consistent with the TEM observation. The peak position in the XRD pattern (2θ = 39.1°) suggests a H/Pd ratio of 0.43 in the β-palladium hydride, based on the relationship between lattice parameter and composition in the palladium–hydrogen system.

Besides lattice expansion, the interstitial hydrogen doping also induces charge transfer from Pd to H due to their difference in electronegativity. In order to investigate the electronic structure variation between Pd and PdH0.43, X-ray photoelectron spectroscopy (XPS) spectra of the two samples were collected to investigate the variation of valence states with all peaks calibrated against C 1s (Fig. 2). As can be seen from Fig. 2, the Pd 3d5/2 and Pd 3d3/2 peaks in both concave nanocubes are deconvoluted into two components. In Pd nanocubes, Pd0 represents metallic Pd in the bulk, and PdII is attributed to the oxidized species on the surface. The deconvoluted main peak of Pd 3d shifted from 334.85 eV in Pd to 335.43 eV in PdH0.43 and PdII becomes the dominant species in PdH0.43 nanocubes due to the presence of interstitial hydrogen atoms. Besides, the relative contribution of surface oxide species was lower in PdH0.43, which could be attributed to the formation of air-stable palladium hydride. Pd atoms in PdH0.43 are partially ionized by the interstitial hydrogen atoms and are less easily oxidized at the surface. The band structures of Pd and PdH0.43 were characterized by the valence-band spectrum and are shown in Fig. S3. PdH0.43 showed a narrower bandwidth of 2.13 eV as compared to 2.59 eV of Pd, with a difference similar to the previously reported value.23


image file: d1dt01625c-f2.tif
Fig. 2 Pd 3d XPS spectra of Pd concave nanocubes and β-PdH concave nanocubes.

The electrochemical behavior of Pd and PdH0.43 concave nanocubes was studied in a typical three-electrode system by loading the concave nanocube catalysts onto the conductive support of Vulcan XC-72 (denoted Pd/C and PdH0.43/C, respectively) before depositing the slurry on a glassy carbon electrode. The surface features of the catalysts were assessed by cyclic voltammetry (CV) in an Ar-saturated 0.1 M HClO4 solution. As shown in Fig. S4, the underpotential adsorption and desorption features of hydrogen in PdH0.43 were not as strong as those of Pd (0–0.4 V) and the peak position was slightly shifted in the potential range of 0 V to 0.4 V, indicating different atomic configurations on the surface of Pd and PdH0.43. The H stripping test also proved the difference in the electronic structure between Pd and PdH (Fig. 3a and b). Pd has a strong electrochemical hydrogen storage capacity at a low potential and the absorbed hydrogen can be further released by applying a more positive voltage. As shown in Fig. 3a, the much higher H-UPD desorption current in the red curve of the linear scanning voltammogram corresponds to the electrochemically stored hydrogen during the low-potential holding stage (0.025 V vs. RHE for 60 s).27 The current drops and almost coincides with the anodic section of the CV during the second scan (blue curve in Fig. 3a), as the stored interstitial hydrogen is fully released during the first scan. On the contrary, PdH0.43 showed negligible hydrogen storage and release through the low-potential holding and anodic scan (Fig. 3b), even with a prolonged potential-holding time (Fig. S5).


image file: d1dt01625c-f3.tif
Fig. 3 Cyclic voltammogram and electrochemical hydrogen stripping of (a) Pd and (b) PdH concave nanocubes in Ar-saturated 0.1 M HClO4 electrolyte (scan rate = 50 mV s−1) after holding the potential at 0.025 V vs. RHE for 60 s. (c and e) Cyclic voltammograms of Pd and PdH concave nanocubes in an Ar-saturated 1 M methanol + 1 M KOH electrolyte (scan rate = 50 mV s−1). (d) CO stripping of Pd and PdH concave nanocubes in 1 M KOH electrolyte (scan rate = 10 mV s−1). (f) Histograms of mass peak activities and ECSA peak activities for methanol oxidation.

The MOR activity of the two catalysts was measured in an Ar-saturated electrolyte containing 1 M KOH and 1 M CH3OH and analyzed by CV, which includes mass activity (Fig. 3c) and specific activity (Fig. 3e). CO stripping was collected to calculate the electrochemical active surface area (ECSA) in 1 M KOH electrolyte at a scan rate of 10 mV s−1 (Fig. 3d and S6). Both the catalysts show two oxidation peaks, where the anodic peaks correspond to the oxidation of CH3OH, while the cathodic peaks are associated with further oxidation of freshly formed intermediates that are generated during the anodic scan. The MOR mass activity and MOR specific activity at PdH0.43 are 922.9 A g−1 and 58.2 mA cm−2, which are 1.5-fold and 1.9-fold those of Pd (613.2 A g−1 and 29.2 mA cm−2), respectively (Fig. 3f). With the retained high-index facets after hydrogen doping, the PdH concave nanocubes exhibited superior activity when compared with the commercial Pt/C and some of the recently reported Pd-based electrocatalysts, as shown in Fig. S6 and Table S1.

The electrocatalytic stability of electrocatalyst is also a key criterion for evaluating their MOR performances. Continuous CV tests are carried out to investigate the durability of Pd (Fig. 4a) and PdH0.43 (Fig. 4b) for MOR. After 200 cycles, PdH0.43 shows an undecayed MOR current density of 55.9 A m−2, while the anodic peak current of Pd shows a limited increase to 31.9 A m−2.


image file: d1dt01625c-f4.tif
Fig. 4 Continuous ECSA normalized CV curves of (a) Pd and (b) PdH concave nanocubes in Ar-saturated 1 M methanol + 1 M KOH electrolyte at 50 mV s−1. Normalized chronoamperometric curves of (c) Pd and (d) PdH concave nanocubes, recorded at an applied potential of 0.824 V vs. RHE for the MOR.

The catalytic durability of the catalysts was also evaluated via chronoamperometry (CA), as shown in Fig. 4c and d. The CA curves of Pd and PdH0.43 catalysts were measured in 1 M KOH + 1 M CH3OH. The current was collected by applying a polarization potential of 0.824 V (vs. RHE) to these catalysts for 8000 s. Compared with Pd, the MOR on PdH0.43 concave nanocubes exhibits a higher oxidation current during the entire 8000 s test. The current density at the end of the CA test of PdH0.43 concave nanocubes shows a mild decrease trend, the PdH0.43 maintains a current density of 34.36 A m−2, while Pd only maintains a current density of 0.58 A m−2, indicating the high durability of PdH0.43 concave nanocubes for the MOR. The enhanced electrochemical durability could be attributed to the structural stability of palladium hydride as compared to Pd.

During the MOR, a poisonous intermediate of CO is generated, which significantly decreases the electrocatalytic activity of the electrocatalysts due to its strong adsorption at the Pd surface. Thus, the oxidation behaviors of COads at Pd and PdH0.43 concave nanocubes were investigated by electrochemical CO stripping (Fig. 3d). During CO stripping test, the CO stripping peak showed up at a lower voltage (0.76 eV) on PdH0.43 compared to Pd (0.78 eV) indicating weaker CO adsorption on PdH0.43, possibly resulting from the larger lattice parameter and the different valence band structures of PdH0.43. Therefore, the enhanced catalytic efficiency in the MOR could be ascribed to the weaker CO binding on the PdH0.43 concave nanocubes.7,31,32

Conclusions

In summary, we fabricated PdH0.43 with high-index {730} facets from Pd concave nanocubes by doping of interstitial hydrogen atoms. The PdH0.43 concave nanocubes exhibit a different composition and electronic structure from their Pd counterparts, as revealed by XRD and XPS. The incorporation of H not only expanded the lattice of Pd, but also caused the transfer of electrons from Pd to H, both of which modified the electronic structure of Pd in PdH0.43 and enhanced the catalytic activity in the MOR. The formation of hydride also strengthens the structure stability and electrochemical durability in the MOR. This work reveals the key role of interstitial atoms like H in fine-tuning the structure for enhanced electrocatalytic performance.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Tianjin, China under No. 18JCYBJC20600 and Institute of Energy, Hefei Comprehensive National Science Center (No. 19KZS207).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d1dt01625c

This journal is © The Royal Society of Chemistry 2021