Integration of SiO2 aerogels in microchips and their role as catalyst supports for H₂ combustion devices

Silica aerogels are porous materials with high specific surface area, open
porosity, and low thermal conductivity. These unique properties make them
promising candidates for applications such as catalyst supports and thermal
insulation. Although silica aerogels have already been incorporated in
microsystems; their high shrinkage and fragile behaviour have limited their use to
particles, thin films and droplets. This work investigates (I) a process for realising
shrinkage-free silica aerogel monoliths inside microfluidic channels and (II) their
application as catalytic supports in hydrogen combustion microdevices.
The silica aerogels were realised using a sol-gel method with a two-step
catalysis. Tetraethyl orthosilicate was used as the silica precursor. To overcome
the shrinking and improve the mechanical strength of the aerogels, CO2
supercritical drying and mechanical additives (polyethylene glycol and carbon
nanotubes) were applied. Critical process parameters observed were the filling
of the microchannels during gelation to avoid shrinkage inside the channel; one week aging time; and twenty exchange cycles during drying to completely
remove the ethanol from the pores and avoid pore collapse. The resulting
aerogels were successfully integrated in the microchannels without shrinkage.
The various compositions of aerogels exhibited high specific surface areas, in a
range of 374 to 551 m2/g and mesoporosity. The aerogels were also successfully
reinforced by polyethylene glycol and carbon nanotubes (2 to 8 wt.%). The
reinforced aerogel monoliths showed an increase of the compressive strength
up to three times higher than pure silica aerogels.
To explore catalytic applications, the aerogels were functionalized with
platinum and ruthenium nanoparticles with concentrations of 2 to 10 wt.%. For this
study, a new chip design was used, featuring a polyimide membrane-based open
cavity with platinum thermal structures, enabling both the nanoparticle
integration and in-situ reaction characterization. Nanoparticles were dispersed in
ethanol and infiltrated into the aerogel after drying, ensuring deep penetration
and uniform distribution. The catalytic system was characterised using SEM,
STEM, EXD, XRD and H2 chemisorption. Additionally, the catalytic combustion was
monitored directly in the chip by measuring the resistance change of the
thermistor. The final nanoparticle integrated aerogel system could initiate
hydrogen combustion in the chip at different loading of Platinum (2 to 10 wt%)
and gas composition (0.5 – 2 % vol H2/air). The catalytic reaction also proceeded
independently of pre-heating of the system, where a variation of 40oC was
measured.
Overall, this work establishes a scalable methodology for integrating silica
aerogels into microfluidic channels without shrinkage and demonstrates their
viability as catalytic supports for platinum nanoparticles in hydrogen combustion
devices.