Regular Article
Interface and defect engineer of titanium dioxide supported palladium or platinum for tuning the activity and selectivity of electrocatalytic nitrogen reduction reaction

https://doi.org/10.1016/j.jcis.2019.05.105Get rights and content

Abstract

Electrocatalytic N2 reduction reaction (NRR) under ambient condition is considered as an alternative and environmental-friendly technique to substitute the conventional process of Haber-Bosch for NH3 production. However, there are still hurdles for researchers to control the balance between N2 activation and competitive hydrogen evolution reaction (HER) to obtain high selectivity of NRR. Herein, we synthesized Pt/TiO2 and Pd/TiO2 hybrids by using laser ablation in liquid (LAL) technology combined with hydrothermal treatment and compared their activity and selectivity of N2 reduction. The results concluded that Pt/TiO2 exhibited a higher NH3 yield rate whereas Pd/TiO2 achieved a better FE for artificial N2 fixation, confirming that enhanced activity surely needs more electrons and protons to participate in the reaction, but the limited protons and electrons furnishing could restrain HER activity and improve selectivity of NRR. Comparing with Pt/TiO2, Pd/TiO2 hybrids could serve as a superior catalyst for keeping a balance relationship between HER and NRR to realize excellent selectivity and high yield rate simultaneously in an alkaline solution. Overall, this work will provide a significant practice to rational design electrocatalysts for NRR at ambient conditions and Pd-based materials might open an electrocatalyst paradigm to solve the global energy and ecological crisis.

Introduction

Ammonia (NH3) as a promising energy carrier not only attribute to agriculture, plastic and textile industries, but also dedicate to stable hydrogen storage [1], [2]. For more than a centenary, Haber-Bosch process is a mainstay for NH3 production by using Fe-based catalysts at stringent technical requirements (Temperature: ∼700 K Pressure: ∼100 atm). This harsh reaction condition consumes huge quantities of fossil fuels and generates excessive CO2 emission [3], [4], [5]. Therefore, it is particularly urgent to find sustainable and environmental alternatives. Electrocatalytic N2 reduction reaction (NRR) is supposed to an attractive approach for NH3 production due to its mild reaction condition and abundant material source [6], [7], [8]. However, the conversion is difficult in breaking Ntriple bondN bond by strong bond energy (∼940 kJ mol−1), low polarizability and absent dipole moment of inert N2 [9], [10]. Furthermore, the side reaction of hydrogen evolution reaction (HER) is another huge challenge for enhancing the selectivity of NRR. This is not just because of their similar redox potential (0 V vs. Normal hydrogen electrode (NHE) for HER and 0.092 V vs. NHE for NRR), but that the attaching HER only requires two electrons to produce one H2 molecule, while the NRR needs six-electron charges transfer for one N2 molecule [11], [12]. Thus, it is crucial to control and balance the relationship between NRR and HER for obtaining high activity and selectivity of N2 reduction simultaneously. And so far, no effective electrocatalysts has been discovered that can produce high-yield NH3 in surrounding environments. Hence, exploring appropriate material systems is of great significance on understanding the reaction kinetics process of NRR.

Noble metal materials can absorb intermediate reactants easily and moderately due to their incomplete d-orbitals electrons, which means that the metallic materials are conducive to the formation of active sites in catalytic reactivity [13], [14], [15], [16]. Much attention has been focused on metallic materials for electrocatalytic N2 reduction recently, such as Au, Ag, Ru and Pd [17], [18], [19], [20]. However, single noble metals usually tend to reunion and agglomeration during the process of electrocatalytic reaction. Metal oxides as supports are commonly implemented to build heterogeneous materials, that could improve the dispersion of metals, modify the electronic structure, and subsequently enhance the stability and selectivity of the resultant catalysts [21], [22], [23], [24], [25], [26], [27]. Yan’s group synthesized a high dispersed Au-loaded TiO2 catalyst and reported that the formed chemical bond of Au-O-Ti could improve the stability of Au, accelerate electrons transfer, and accordingly achieve an overall performance of NRR gain [28]. Nevertheless, it is worth reminding that an excellent NRR response catalyst is inevitably accompanied by an outstanding HER performance due to the more easily activated protons rather than Ntriple bondN bonds. Therefore, a catalyst with high performance of NRR should possess an excellent activity for NH3 production even though accompanying with the serious H2 generation [29]. But even so as above mentioned, almost no researchers have strived for developing efficient N2 reduction catalysts with the conditions of strong H2 evolution. In this sense, our efforts were devoted to exploring and developing the N2/NH3 conversation process through comparing metal-oxide (TiO2) catalysts supported by different metal materials of Pt and Pd, which had been demonstrated as superior electrocatalysts for HER and NRR, respectively [5], [16].

TiOx colloids produced by laser ablation in liquid (LAL) usually possess negatively charged surface with obvious oxygen vacancies, as presented in previous works [30], indicating that the positively charged ions could be easily transferred to the surface of TiOx and further ripening and crystallization after hydrothermal treatment. Moreover, TiO2 as one of metal oxides has the merits of remarkable stability and beneficial interactions [31], [32], [33], [34], [35], [36], [37]. Thus, in this work, TiO2 was selected as the supporter, Pt and Pd respectively loaded on the surface of TiO2 to form Pt/TiO2 and Pd/TiO2 by using LAL technique combined with hydrothermal treatment without using any additional chemical reducing agent. Subsequently, the compared N2 reduction activity and selectivity of Pt/TiO2 and Pd/TiO2 were investigated in the same conditions, and of particular concern was that Pt/TiO2 had the lower Faradaic efficiency (FE) but the higher N2 reduction performance, whereas Pd/TiO2 possessed the higher FE but the relatively lower N2 yield rate. This result indicated that a large number of protons and electrons can lead to high HER and seriously limit NRR selectivity, and comparing to Pt/TiO2, Pd/TiO2 can enable abundant protons and electrons to participate in the NRR process to realize high selectivity and achieve a considerable NH3 yield rate simultaneously. Therefore, Pd-based heterogeneous could serve as glorious electrocatalysts of NRR to balance the reaction between H2 evolution and N2 activation and simultaneously boost the NRR selectivity and NH3 yield rate. This work would provide guidance for reasonably designing heterogeneous catalysts to balance the relationship between HER and NRR for the N2 chemical absorption improving and N2 transformation performance enhancing in ambient conditions.

Section snippets

Materials

Titanium (Ti) target (99.99%, diameter × thickness: 15 mm × 3 mm) was purchased from Zhongnuo Co. Ltd. Chloropalladite acid (H2PdCl4), sodium hydroxide (NaOH), ammonia chloride (NH3Cl), Chloroplatinic acid (H2PtCl6), Nafion solution, hydrogen peroxide (H2O2), hydrochloric acid (HCl), sodium nitroferricyanide (C5FeN6Na2O), sulfuric acid (H2SO4), sodium salicylate (C7H5NaO3), sodium citrate (Na3C6H5O7·2H2O), sodium hypochlorite (NaClO), and para-(dimethylamino) benzaldehyde were purchased from

Results and discussion

The phase structure and relative crystallinity of the as-synthesized products were confirmed by XRD analysis as shown in Fig. 2a, it clearly displays that the diffraction peaks of hydrothermal-treated TiOx colloids could be mostly indexed to the anatase TiO2, the strong diffraction angles were found at 25.33°, 37.79°, 48.05°, 53.90°, 55.08° and 62.74°according to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1) and (2 0 4) crystal planes of anatase TiO2 (JCPDS NO. 01-071-1166). Few TiO2 of other

Conclusions

In summary, Pd/TiO2 and Pt/TiO2 hybrids fabricated by LAL technology combined with hydrothermal treatment were systematically investigated for tuning the activity and selectivity of NRR under ambient conditions. Pt/TiO2 exhibited a higher NH3 yield rate whereas Pd/TiO2 achieved a better FE for artificial N2 fixation, confirming that enhanced activity surely needs more electrons and protons to participate in the reaction, but the limited protons and electrons furnishing could restrain HER

Acknowledgements

This work was supported by the Instrument Developing Project of the Chinese Academy of Sciences (grant no. YZ201627), the National Natural Science Foundation of China (NSFC nos. 51801205, 11674321, 51572103, 51571186, and 11504375), and the Distinguished Young Scholar of Anhui Province (grant no. 1808085J14). C. L. also thanks the support from the Chinese Academy of Sciences/State Administration of Foreign Experts Affairs (CAS/SAFEA) International Partner-ship Program for Creative Research

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