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)