Catalysis inside a box
Résumé
Supported nanoparticles are usually formed after post-synthesis modifications such as ion-exchange or wetness impregnation. Such classical chemical and physical methods for the control the size of supported metallic nanoparticles are generally not compatible with harsh conditions of applications (high temperature, presence of steam). Sintering of nanoparticles usually occurs, leading to particle growth and eventually to a degradation of physical properties.
Tough encapsulation can provide a strict control of the nanoparticle size as well as a limitation of aggregation
at high temperature; encapsulated nanoparticles are often hardly accessible due to diffusion limitations
of reactants in sub-nanometric micropores. Recent developments in zeolite synthesis have offered a
new class of materials with hierarchical structures or unusual morphologies in which the mean diffusion
path is considerably reduced. Those zeolites with inter-crystalline and/or intra-crystalline mesopore
systems are particularly adapted for supporting nanoparticles both in microporous cavities of the framework
and in mesoporous channels. More sophisticated architectures such as yolk/core–shell materials, in which
nanoparticles are protected against poisoning or sintering by a thin zeolite shell, are particularly attractive. In
contrast to nanometric intrinsic cavities of zeolite frameworks, a large internal void is available for chemical
reactions and such catalysts can be considered as nanoreactors in which the reaction is essentially governed
by the permeability of the shell.
We will describe a generic process for synthesizing nanoalloys of controlled size (2-10 nm) and composition encapsulated in zeolite nanoboxes. The originality of the synthesis design is that the nanoboxes act both as nanoreactors in which the bimetallic particles are formed and as protective ultra-microporous shells, which prevent sintering by coalescence even under harsh conditions. Moreover, the present process allows controlling the size and compositions of NPs which are factors that ultimately determine the catalytic properties. A series of mono- (Au, Pd, Pt) or bi- (AuAg, PdAg, PtAg and PdPt) metallic nanoparticles individually encapsulated in hollow silicalite-1 single crystals have been prepared and characterized.
In the talk, main aspects of heterogeneous catalysis will be revisited, namely: activity, selectivity, deactivation and diffusion limitation.