AmericanChemicalSociety.com

The development of more efficient and stable catalysts has been an increasingly important goal for chemists and materials scientists for both economic and environmental merits.  The significant progress in the synthesis of colloidal metal nanoparticles allows for the design of catalysts with superior performance by taking advantage of nanoparticles’ high surface-to-volume ratio and their shape-dependent surface structure.  However, because of their high surface energies, nanoparticles tend to coagulate and/or change shape when taking part in catalytic reactions and eventually lose their initial activity and selectivity. 

With the support from ACS PRF grant, we have been actively exploring methods to assemble catalytic nanoparticles in appropriate host materials to form composite nanostructures that can improve the stability, recyclability, and catalytic selectivity of nanocatalysts.  In an earlier work, we studied the introduction of superparamagnetic components to the catalyst support to significantly improve the recycling efficiency.  Magnetically responsive hierarchical assemblies of silica colloids have been developed as recoverable supports of nanocatalysts for liquid phase reactions.  Such unique structures possess both high separation efficiency and relatively high surface area.   

To greatly improve the stability of catalyst particles against coagulation, we have further developed a general “encapsulation and etching” strategy for fabricating nanocatalyst systems where catalyst nanoparticles are protected within porous oxide shells.  In this strategy, catalyst particles are first stabilized by encapsulating in a layer of oxides such as SiO₂ and TiO₂.  Then, a unique “surface-protected etching” process is performed to conveniently convert dense oxide coatings into porous shells so that chemical species can reach the core catalysts to participate in reactions while the shells still act as physical barriers preventing the aggregation of the catalyst particles.  We have demonstrated that the permeation rate of chemical species through the shells can be controlled by varying the extent of etching.  To further increase the loading of catalyst particles, we have also developed a core-satellite nanocomposite catalyst system by first immobilizing a monolayer of metal nanocatalysts on the surface of silica supports, then coating over the composite particles with another layer of silica to fix the position of metal nanoparticles, and finally exposing the catalyst particles to outside chemical species by creating mesopores in the outer shells through the “surface-protected etching”.  In addition to the “surface-protected etching” route, we have also discovered a spontaneous dissolution-regrowth process that allows convenient conversion of solid silica spheres into hollow particles with porous shells. 

Last year, our efforts have been made mainly on studying the stabilization effect of the porous shells to the nanoparticle catalysts in gas phase reaction at high temperatures. By collaborating with Prof. Francisco Zaera in my department, we used carbon monoxide (CO) adsorption to test the accessibility of the Pt nanoparticles in these catalysts.  We have also carried out tests to study the stability of the mesoporous-shell-protected catalysts, and found out that the platinum nanoparticle catalysts gain significant stability against thermal sintering.  With the protection from a thin mesoporous silica layer, the Pt nanoparticles did not show evident sintering at temperatures as high as 875 K.  We have also started research on the study of the catalytic activity and stability of the proposed nanoreactors in gas phase model reactions, such as the preferential (or selective) oxidation of carbon monoxide to carbon dioxide and hydrogenation/isomerization of 2-butene.  Preliminary results have been reported in journals such as Adv. Funct. Mater. and PCCP.  We are currently continuing the studies along this direction.  

Our work along this direction has been reported in journals such as Angew. Chem. and Nano Lett.  Last year I was invited to write a review article in Nano Today to summarize the synthesis part of this work, and a feature article in Adv. Funct. Mater. to highlight the unique stabilization effect of the mesoporous shells in heterogeneous catalysis.

Funding from ACS PRF grant has helped me to obtain important preliminary results for seeking further supports from major funding agencies.  A project on the magnetically tunable photonic crystal nanostructures have been recommended for funding by NSF for the CAREER Award.  I was also chosen to receive the 2009 Cottrell Scholar Award from the Research Corporation for Science Advancement, DuPont Young Professor Grant, and 3M Nontenured Faculty Grant.  Last year, I have also launched a new project on the development of efficient photocatalysts for water splitting using visible light.  The new photocatalysts was designed on the basis of above nanostructured composite catalysts.  Our preliminary results obtained through partial support from ACS PRF grant, have resulted in a join grant from DOE (with Prof. Francisco Zaera and Prof. Chris Bardeen at my department).

The ACS PRF grant also helped me to support graduate and undergraduate students in their research trainings.  I have been committed to expanding research-based learning at UCR and in the wider community by engaging undergraduate students in my research program.  As a result of their significant contributions, several undergraduates have already obtained interesting results and deservingly been coauthors on publications.  With partial support from ACS PRF, in the past two years I have also been able to recruit four high school students into my group to carry out supervised research.  The students were encouraged to demonstrate their results in their schools, thus increasing awareness and promoting excitement about science education and careers.  Two of them have won medals in the San Bernardino County Science Fair Competition and in the California State Science Fair Competition.