In recent years, yolk–shell structured materials with a void space
between the core and the shell have been intensively studied. Many
methods such as selective etching, bottom-up or soft templating,
Ostwald ripening, galvanic replacement, etc. have been developed to
produce yolk–shell structured materials. The void space within
the shell provides a unique confined space for the yolk material in
confined catalysis, drug release, lithium-ion batteries, etc.The
yolk–shell structured material can be regarded as a nanoreactor for
catalytic reactions. Several reports have shown that yolk–shell
structured material with noble metal inside the shell had high
catalytic activities in several important reactions, including
reduction of nitrophenol, oxidation of aerobic alcohol, Suzuki
reaction, etc. Acid–base bi-functional catalysts with acid and
base sites residing in either the shell or the core were also
reported.However, most related studies were focused on either
the shell or the yolk, while the role of the void space between was
largely neglected. In addition, the activity of a nanoreactor with a
metal oxide core was less explored, and metal oxides are an
important class of catalysts that need to be explored. And in most
studies, porous shells had irregular pore structures, as a mesoporous shell is more difficult to be produced in yolk–shell materials.
However, for catalysis, a mesoporous shell is preferred because of
the advantages of the mesoporous pores in diffusion.