Conversion of confined metal@ZIF-8 structures to intermetallic nanoparticles supported on nitrogen-doped carbon for electrocatalysis

Nano Research, Mar 2018

We report a facile strategy to synthesize intermetallic nanoparticle (iNP) electrocatalysts via one-pot pyrolysis of a zeolitic imidazolate framework, ZIF-8, encapsulating precious metal nanoparticles (NPs). ZIF-8 serves not only as precursor for N-doped carbon (NC), but also as Zn source for the formation of intermetallic or alloy NPs with the encapsulated metals. The resulting sub-4 nm PtZn iNPs embedded in NC exhibit high sintering resistance up to 1,000 °C. Importantly, the present methodology allows fine-tuning of both composition (e.g., PdZn and RhZn iNPs, as well as AuZn and RuZn alloy NPs) and size (2.4, 3.7, and 5.4 nm PtZn) of the as-formed bimetallic NPs. To the best of our knowledge, this is the first report of a metal-organic framework (MOF) with multiple functionalities, such as secondary metal source, carbon precursor, and size-regulating reagent, which promote the formation of iNPs. This work opens a new avenue for the synthesis of highly uniform and stable iNPs. Open image in new window

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Conversion of confined metal@ZIF-8 structures to intermetallic nanoparticles supported on nitrogen-doped carbon for electrocatalysis

Nano Research Conversion of confined metal@ZIF-8 structures to intermetallic nanoparticles supported on nitrogen-doped carbon for electrocatalysis Zhiyuan Qi 1 2 3 Yuchen Pei 1 2 3 Tian Wei Goh 1 2 3 Zhaoyi Wang 0 2 3 Xinle Li 1 2 3 Mary Lowe 2 3 Raghu V. Maligal-Ganesh 1 2 3 Wenyu Huang 1 2 3 0 Department of Chemistry, Beijing Normal University , Beijing 100875 , China 1 Ames Laboratory, U.S. Department of Energy, Ames , Iowa 50011 , USA 2 Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018 3 Department of Chemistry, Iowa State University , Ames, Iowa 50011 , USA 1 Introduction Pt-based alloys have been intensively studied as potential electrocatalysts in polymer electrolyte membrane fuel cells (PEMFCs) for decades [ 1–4 ]. The secondary metals in the alloys can improve the fuel cell performance by reducing Pt usage, tailoring the electronic properties of surface sites, and controlling the binding strength of the adsorbed molecules [5]. Many Pt alloy catalysts are more active than Pt in the oxygen reduction and methanol oxidation reactions (ORR and MOR, respectively) [ 1–3, 6 ]. However, a significant challenge for many alloys is represented by the loss of activity due to leaching of metals via oxidative dissolution under the electrochemical reaction conditions [7]. The leaching of metals also inevitably leads to surface reconstruction [ 8 ]. Improving the structural and compositional homogeneity of Pt alloys is an essential requirement to enhance their catalytic performance. Intermetallic compounds are special alloys with ordered structure and well-defined stoichiometry [ 9 ], which makes them an attractive alternative to random alloys in terms of activity, stability, and mechanisms [ 10–16 ]. However, one of the challenges for the synthesis of intermetallic nanoparticles (iNPs) is the high-temperature sintering required for the formation of intermetallic phases [ 9 ], which results in large particles and insufficient utilization of Pt in the catalysts. The encapsulation of nanoparticles (NPs) in inorganic shells (i.e., silica, titania, and zirconia) is an effective approach to enhance their thermal stability [ 17, 18 ]. By using mesoporous silica (mSiO2) as the encapsulation shell, our group obtained small and uniform PtZn iNPs (3.2 ± 0.4 nm) on multi-walled carbon nanotubes (MWNTs) with enhanced electrocatalytic properties [16]. The same encapsulation strategy has also been used for synthesizing other Pt-based alloys and iNPs [ 19, 20 ]. In order to use the iNPs in electrocatalysis, an etching process with hazardous chemicals (e.g., HF and NaOH) is needed to remove the poorly conductive mSiO2 shell. Carbon encapsulation, on the other hand, can be used to prevent aggregation of NPs and provide a highly conductive matrix for electrocatalysis. Metal-organic frameworks (MOFs), an emerging class of porous crystalline materials, are widely used in the synthesis of metal NPs with controlled size [ 21, 22 ]. Because of their high functional tunability and uniform cavities, two main approaches, namely “ship in a bottle” [ 23, 24 ] and “bottle around ship” [ 25 ], are used to confine the growth of the NPs [ 26 ]. However, most MOFs are only thermally stable between 250 and 500 °C [ 27 ], and thus cannot be directly used as the matrix for the synthesis of iNPs, which requires high-temperature annealing. Recently, nanostructures derived from pyrolysis of MOFs have attracted increasing attention [ 28–31 ]: Relevant examples include (heteroatom-doped) porous carbons, metal alloys/metal oxides, and their hybrid composites [ 32–37 ]. These MOF-derived carbon materials can be applied as highly efficient electrocatalysts or catalyst supports [ 38, 39 ]. Since the typical pyrolysis temperatures (600– 1,000 °C) are suitable for the formation of intermetallic compounds, we envision that the simultaneous formation of iNPs and porous carbon could be achieved by one-pot pyrolysis of MOF-encapsulated metal NPs. This general methodology could allow the synthesis of a broad range of iNPs supported on porous carbon. Herein, we report its application to the synthesis of uniform PtZn iNPs encapsulated within N-doped porous carbon (denoted as Pt-Zn@NC), starting from Pt NPs encapsulated in ZIF-8 (Pt@ZIF-8). The size of the PtZn iNPs can be easily tuned by altering the original size of the Pt NPs. To the best of our knowledge, the monodisperse PtZn iNPs (2.4 ± 0.4 nm) prepared in this study are the smallest iNPs synthesized to date. Remarkably, the small iNPs in Pt-Zn@NC exhibited high resistance to aggregation, up to 1,000 °C. The present facile methodology was also extended to the synthesis of PdZn/RhZn iNPs and AuZn/RuZn alloyed NPs. This study represents the first attempt to use MOF-metal NPs composites as precursors for the synthesis of intermetallic compounds. 2 Experimental We synthesized NC-encapsulated M-Zn iNPs (denoted as M-Zn@NC (M = Pt, Rh, Ru, (...truncated)


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Zhiyuan Qi, Yuchen Pei, Tian Wei Goh, Zhaoyi Wang, Xinle Li, Mary Lowe, Raghu V. Maligal-Ganesh, Wenyu Huang. Conversion of confined metal@ZIF-8 structures to intermetallic nanoparticles supported on nitrogen-doped carbon for electrocatalysis, Nano Research, 2018, pp. 3469-3479, Volume 11, Issue 6, DOI: 10.1007/s12274-018-2016-x