Topics in Current Chemistry (2019) 377:4
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reactions, during which they must avoid the aggregation of single metal atoms
and dissolution. For example, Bulushev et al. synthesized single Pt-group metal
(Ru, Pb, and Pt) atoms on N-doped carbon nanofibers via incipient wetness
impregnation [17]. Experimental and theoretical simulation results revealed that
these metal atoms are coordinated by pyridinic N, leading to an ionic/electrondeficient state of these single metal atoms, thereby preventing their aggregation.
Choi et al. reported a sulfur-doped zeolite-templated carbon stabilizing high
concentration of single Pt atoms (5 wt%) [21]. Interestingly, unlike conventional
Pt-based materials that catalyze ORR with four-electron transfer, this catalyst
selectively produces H 2 O 2 with a two-electron pathway. This work suggests that
the coordination environment of single Pt atoms plays a significant role in electrocatalysis and affects the reaction mechanisms.
In addition, this strategy can also be used to prepare carbon-based singleatom non-noble-metal catalysts. In nature, an iron porphyrin core structure in
cytochrome c oxidase is recognized as the active site for catalyzing the ORR
[38, 39]. To mimic the natural system, transition metal macrocyclic molecules
supported on carbon materials have been designed and applied in electrocatalysis [40–42]. For example, Cao et al. reported iron phthalocyanine with an axial
ligand anchored on single-walled carbon nanotubes (CNTs) [43], while Hijazi
et al. prepared a covalent cobalt porphyrin network supported on CNT surfaces
[44]. Wang et al. synthesized cobalt ions coordinated with N-, O-, S-doped graphene [45], and Ding et al. reported single cobalt ions bridged by ionic liquid
polymer on CNT surfaces [46]. These carbon-based single-atom metal catalysts
obtained through wet-chemical routes possess a relatively high concentration of
single metal atoms, but they usually suffer from poor long-term stability.
2.3 High‑Temperature Pyrolysis
High-temperature pyrolysis of precursors is the most widely adopted method
for preparing carbon-based single-atom metal catalysts. Metal ions adsorbed on
conducting polymer, metal–organic frameworks (MOFs), and transition metal
macrocyclic molecules are the most common precursors [47–51]. During the
high-temperature treatment (generally above 700 °C), isolated metal atoms can
be directly doped into the carbon matrix or strongly coordinated by heteroatomdoped carbon, which gives isolated metal atoms high stability. Importantly, if
the temperature is too high, coordination sites can be easily broken, leading to
inactive nanosized metal species rather than enrichment of isolated metal atoms.
To date, a series of carbon-based single-atom metal (Pt, Ru, Fe, Co, Ni, Cu,
and Zn) catalysts have been prepared using high-temperature pyrolysis [52–57].
More importantly, this method involves low-cost raw materials and apparatuses,
and the final catalysts exhibit a robust structure and high performance. However, there is still a challenge in increasing the metal loading, as metal atoms
are easily converted to nanoclusters and nanoparticles during high-temperature
treatment.
Reprinted from the journal
132
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reactions, during which they must avoid the aggregation of single metal atoms
and dissolution. For example, Bulushev et al. synthesized single Pt-group metal
(Ru, Pb, and Pt) atoms on N-doped carbon nanofibers via incipient wetness
impregnation [17]. Experimental and theoretical simulation results revealed that
these metal atoms are coordinated by pyridinic N, leading to an ionic/electrondeficient state of these single metal atoms, thereby preventing their aggregation.
Choi et al. reported a sulfur-doped zeolite-templated carbon stabilizing high
concentration of single Pt atoms (5 wt%) [21]. Interestingly, unlike conventional
Pt-based materials that catalyze ORR with four-electron transfer, this catalyst
selectively produces H 2 O 2 with a two-electron pathway. This work suggests that
the coordination environment of single Pt atoms plays a significant role in electrocatalysis and affects the reaction mechanisms.
In addition, this strategy can also be used to prepare carbon-based singleatom non-noble-metal catalysts. In nature, an iron porphyrin core structure in
cytochrome c oxidase is recognized as the active site for catalyzing the ORR
[38, 39]. To mimic the natural system, transition metal macrocyclic molecules
supported on carbon materials have been designed and applied in electrocatalysis [40–42]. For example, Cao et al. reported iron phthalocyanine with an axial
ligand anchored on single-walled carbon nanotubes (CNTs) [43], while Hijazi
et al. prepared a covalent cobalt porphyrin network supported on CNT surfaces
[44]. Wang et al. synthesized cobalt ions coordinated with N-, O-, S-doped graphene [45], and Ding et al. reported single cobalt ions bridged by ionic liquid
polymer on CNT surfaces [46]. These carbon-based single-atom metal catalysts
obtained through wet-chemical routes possess a relatively high concentration of
single metal atoms, but they usually suffer from poor long-term stability.
2.3 High‑Temperature Pyrolysis
High-temperature pyrolysis of precursors is the most widely adopted method
for preparing carbon-based single-atom metal catalysts. Metal ions adsorbed on
conducting polymer, metal–organic frameworks (MOFs), and transition metal
macrocyclic molecules are the most common precursors [47–51]. During the
high-temperature treatment (generally above 700 °C), isolated metal atoms can
be directly doped into the carbon matrix or strongly coordinated by heteroatomdoped carbon, which gives isolated metal atoms high stability. Importantly, if
the temperature is too high, coordination sites can be easily broken, leading to
inactive nanosized metal species rather than enrichment of isolated metal atoms.
To date, a series of carbon-based single-atom metal (Pt, Ru, Fe, Co, Ni, Cu,
and Zn) catalysts have been prepared using high-temperature pyrolysis [52–57].
More importantly, this method involves low-cost raw materials and apparatuses,
and the final catalysts exhibit a robust structure and high performance. However, there is still a challenge in increasing the metal loading, as metal atoms
are easily converted to nanoclusters and nanoparticles during high-temperature
treatment.
Reprinted from the journal
132