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Chen et al. Microstructures 2023;3:2023025 https://dx.doi.org/10.20517/microstructures.2023.12 Page 7 of 31

environment (e.g., humid atmosphere), resulting in encapsulation failure, metal agglomeration, and a
dramatic decrease in stability. To optimize the stability of the TiO overlayers, Liu et al. reported that
2
Au/TiO catalysts modified with melamine and annealed at 600 °C in N atmosphere and further treatment
2
2
at 600 °C in air atmosphere to form an amorphous and permeable TiO encapsulation layer on Pt NPs,
x
which is extremely stabilized against re-oxidize in air, in stark contrast to the retreat of the TiO
x
encapsulation layer by later oxidation treatment in previous SMSI (as shown in Figure 3F) . Subsequently,
[52]
they expanded this strategy to TiO -supported Pd and Rh NPs, a promising way for designing supported
2
platinum group metal-based catalysts with high activity and stability .
[53]
Adsorbate-mediated SMSI (A-SMSI)
Notably, the aforementioned examples of SMSI and O-SMSI typically rely on high-temperature thermal
treatment (≥ 500 °C) while inevitably decreasing the catalytic performance due to blockage of metal active
sites. Matsubu et al. reported an adsorbate-mediated SMSI (A-SMSI) encapsulated strategy that forms with
the treatment of TiO - and Nb O -supported Rh NPs in 20CO :2H atmosphere at a relatively low
2
2
2
2
5
temperature (150-300 °C) . In situ spectroscopy and microscopy demonstrated that the thickness of the A-
[54]
SMSI overlayer is almost twice that of the SMSI overlayer, thanks to the adsorbates HCO species strongly
x
bounded on the support to induct the formation of oxygen-vacancy, prompting the migration of HCO -
x
functionalized support onto the metal to form an extremely stabilized encapsulated state against re-
oxidation by the air atmosphere (As shown in Figure 4A, line a-b-c). Later, the A-SMSI strategy was labeled
as “low-temperature induction” and “encapsulation stability” with its extended application to Cu/CeO
[55]
2
and Ru/MoO . Nowadays, the induction of A-SMSI overlayer is not restricted to CO -H atmospheres
[56]
2
3
2
only. For example, Li et al. successfully constructed A-SMSI on a commercial Cu/ZnO/Al O catalyst under
3
2
H /H O/CH OH/N mixture atmosphere at 300 °C (as shown in Figure 4B) and found that the ZnO species
3
2
x
2
2
migrated onto the surface of metallic Cu NPs to constitute a stable encapsulated structure, and the proper
0
[57]
degree of encapsulation could be achieved by adjusting the exposure time . Combined with DFT
calculations, the improved methanol steam reforming reaction activity of Cu/ZnO/Al O catalysts was
2
3
attributed to the increased number of the ZnO -Cu interfacial sites (as shown in Figure 4C).
x
Wet-chemistry SMSI (wcSMSI)
Besides the A-SMSI strategy enables encapsulation at relatively low temperatures, the wcSMSI method is
another effective strategy for achieving SMSI in an aqueous solution at room temperature, which not only
avoids the conventional high-temperature redox condition causing pre-sintered metal NPs but also
effectively stabilizes the metal NPs against re-oxidation. As an example, an Au/TiO -wcSMSI catalyst
2
synthesis was reported by Zhang et al. that the average diameter of the TiO overlay was 2 nm, and the
2
supported Au NPs were completely encapsulated . In addition, they provide a proof-of-concept design to
[58]
reveal the mechanisms of enhancing the catalytic activity of Au cluster on inert SiO support covered by
2
55
TiO overlayers (Au @Ti/SiO ) through DFT calculations (as shown in Figure 4D), thereby extending the
x
2
55
wet-chemistry method to inert oxide supported field. Recently, Hao et al. synthesized a Pt@TiO /TiO
x
2
catalyst via the wcSMSI strategy, which consists of Pt NPs decorated by amorphous TiO overlayers and
x
exhibits extremely active and stable in C H and C H combustion compared with the conventional
6
3
3
8
0
4+ [59]
3+
supported Pt/TiO catalyst owing to the electronic interaction between Pt and TiO (Pt /Pt ↔Ti /Ti ) .
x+
2
x
Since the wcSMSI process protects the severe sintering of the NPs from the high temperature, the stability of
catalysts is also substantially ensured.
Reaction-induced SMSI (R-SMSI)
In particular, the abovementioned types of SMSI constructions predominantly rely on the redox oxide
supports, such as TiO , ZnO, Nb O , etc. However, the relatively redox-inert supports (e.g., Mg- and Al-
5
2
2
based oxide supports) are unsuitable for the construction of SMSI as their surface activation is challenging.