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Particle ALD Catalysis

Metal, and metal oxide supported catalysts are important in a variety of applications ranging from Fisher-Tropsch synthesis, methanation, water splitting and photocatalysis etc. Sol-gel and incipient wetness methods are the traditional ways to prepare the supported catalyst, but these techniques offer little control of surface structure and distribution of catalyst materials on a support surface. Catalyst materials prepared by Atomic Layer Deposition (ALD) offer significant advantages in terms of the precision control over the size of nanoclusters, thin film thickness and the ability to deposit a homogeneous distribution of active metals over high surface area supports.

Recently, catalyst materials such as iron, cobalt, nickel, ruthenium, and platinum have been successfully deposited on particles via ALD in our group. Metal oxides typically deposit in the form of conformal films whereas noble metals deposit as islands or nanoclusters as seen in the images below. Below-left shows a TEM image of Fe2O3 continuous thin films coated on ZrO2 nanopowder. Metal oxide thin films can be reduced under hydrogen environment to metal particles, as seen below-center. The bright spots in the below-right image are the Pt nanoparticles deposited via ALD, which are homogenously distributed on the particulate TiO2 substrate. The nanoparticle size of the metal can be controlled by the coating temperature and cycles which gives the opportunity to optimize the catalyst particle size, and improve the dispersion and efficiency of the catalyst.  The ALD process tends to deposit these catalyst materials at rates of ~ 0.15 Å/cycle.

 

LEFT) TEM image of Fe2O3 coated on ZrO2 nanopowder; CENTER) TEM image of Fe particles produced via the reduction of Fe2O3 ALD films; RIGHT) STEM image of Pt coated on TiO2 nanopowder.

 

Stoichiometric thin film catalyst, such as CoxFe(1-x)O2  for the water splitting process was successfully developed by alternating CoO ALD and Fe2O3 ALD. The compositional parameter (x) and thin film thickness are easy to be controlled by ALD cycles which are tough for the traditional methods. By using very thin films catalyst with optimized stoichiometry where the diffusion limitations are reduced, the kinetics and conversions of water splitting will be greatly enhanced.

 

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© 2012. Team Weimer


University of Colorado at Boulder University of Colorado at Boulder