(127c) Atomically Dispersed Rhodium on Self-Assembled Phosphotungsticacid: Structural Features and Catalytic CO Oxidation Properties

Single-atom catalyst (SAC), which refers to a class of catalysts
where bare metal atoms are atomically dispersed or anchored on the support, has
recently emerged as a new frontier in heterogeneous catalysis.^[1] A key
challenge encountered is to stabilize isolated metals on a support without
compromising catalytic activity. As the surface energy of single metal atoms is
higher than corresponding metal clusters and nanoparticles, the atoms are
highly mobile and tend to form aggregates during synthetic procedures.^[2] Keeping the metal loading at a very
low level to minimize agglomerations is a common practise to tackle the
problem. Another strategy is to anchor single atoms on a support with stronger
metal-support interaction.^[3] Nevertheless, it remains
difficult to effectively stabilize isolated atoms at increased metal loading,
which would hinder the applications of SACs. Inspired by the classic
coordination chemistry that four-coordinate metal complexes in square-planar or
tetrahedron geometry are intrinsically more stable, we propose constructing or
introducing four-coordinating anchoring sites with suitable geometry onto a support
to prepare high loading, stable SACs.^[4] In this work, phosphotungstic acid (PTA, H₃PW₁₂O₄₀) was selected to examine the feasibility of the
approach.^[5]

PTA has a classical Keggin structure with one P atom in the center
and caged by 12 octahedral WO₆-units linked together by 24 bridging
oxygen atoms (O_br). There are another 12 corner oxygen atoms (O_c),
each of which is double-bonded with an addenda W atom. With 36 oxygen atoms
exposed PTA furnishes a range of coordination sites, including the single
corner site, the bridge site (e.g., the O_c-O_br-bridge
site), the three-fold hollow site (3-H_O_c and 3-H_O_br),
and the four-fold hollow site (4-H). Thus, it represents an ideal platform to
differentiate the ability of various sites to stabilize SACs. Previously, there are studies on the combination of metal complexes
with heteroply acids. To
our knowledge, there has been no report on active SACs on PTA,
with the accurate location, electronic state and catalytic mechanism of single
metal atoms being determined.

Figure
1. The configuration of the atomically dispersed Rh catalyst and its catalytic
application.

Herein, atomically dispersed Rh catalysts supported on PTA with Rh
loading up to 0.9 wt% were prepared through a self-assembly method. As shown in
Figure 1, Rh stays exclusively as single atom species coordinated to six oxygen
atoms, plausibly located at the 4-fold hollow site on one PTA molecule together
with a chemically adsorbed O₂ molecule. The catalyst is active in CO
oxidation affording turnover frequencies between 0.2 and 1.7 s⁻¹ from 165 to 195 ¡ãC, with an apparent activation energy of 127
kJ/mol. The catalyst is highly stable, well maintaining the Keggin structure of
PTA as well as the single-atom identity of Rh after three catalytic cycles (50
to 400 ¡ãC). CO activation is mainly achieved on Rh via the formation of
dicarbonyl species, while oxygen activation and transfer mainly occur through
PTA. The proposed catalytic cycle consists of an alternation of Rh(CO)₂³⁺
species and Rh(CO)₂¹⁺ species on PTA, during which CO
transforms into CO₂ with oxygen activation being rate-determining.

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