(350e) Stabilization By Atomic Layer Deposition of Copper Catalysts for Liquid Phase Reactions

Precious metal catalysts are used extensively in the
petrochemical industry and are anticipated to have a significant role in future
biorefineries.  While it would be desirable to replace precious metal catalysts
(e.g., platinum) with base metals that are more abundant (e.g., copper), these
base metal catalysts are susceptible to sintering and leaching under
liquid-phase reaction conditions.  Previously, we have reported a method to stabilize
copper nanoparticles on g-Al₂O₃ by atomic
layer deposition (ALD) of an alumina overcoat for the liquid phase
hydrogenation of biomass-derived furfural.  This overcoat provides resistance
against sintering and leaching of the nanoparticles without significantly
affecting the reaction kinetics. In order to broaden the impact of this result,
a more fundamental understanding of the phenomena that lead to the pore opening
of the overcoat and the stabilizing interaction between the overcoat and metal
nanoparticle surface is required.

In order to achieve such a mechanistic
understanding, traditional surface science and reaction kinetics studies are
combined with in situ and operando measurements.  For example, in
situ and operando XAS measurements demonstrate that the copper
nanoparticles simultaneously undergo sintering and leaching during reaction,
but ALD overcoated catalysts remain stable during reaction.  Additionally, hydrogen
chemisorption, Fourier transform infrared spectroscopy, and scanning tunneling
microscopy are used to demonstrate that a selective interaction between the
overcoat and the least coordinated copper surface atoms causes the increased
stability of the ALD overcoated catalysts.  This fundamental understanding of
the interfacial interaction is crucial to extending this result to other metals
and overcoat materials.

            An
understanding of the pore opening process in the overcoat also has the
potential to broaden the impact of the new overcoating technique by providing a
degree of control over the final pore size.  Solid state ²⁷Al magic
angle spinning nuclear magnetic resonance demonstrates significant
restructuring in the overcoat during heat treatment.  Powder X-ray diffraction
also shows that the overcoat has restructured after heat treatment, and in
situ small angle X-ray scattering (SAXS) shows the evolution of the pores
during the heat treatment process.  Using SAXS, we are also able to probe the
effect of different heat treatment parameters (time, temperature, ramp rate) on
the resulting pore size, which gives us the potential to further refine the
selectivity of the catalyst via size exclusion.