Materials for Catalysis – Bridging the materials and pressure gap to understand catalysts

This research is focused on understanding how the properties of a support material influence a supported metal nanoparticles geometric structure, particle stability, and catalytic activity. Currently there are two main thrusts of this project.

Thrust 1 is focused on understanding how the support material used to support a metal nanoparticle changes during the solution phase synthesis of a catalytic nanoparticle and how these changes influence the properties of a supported metal nanoparticle.

Thrust 2 is focused on growing catalyst materials on non-traditional support materials, such as carbides like SiC, and comparing the properties of these materials to the same catalysts grown on the corresponding (and more common) metal oxide, SiO2. This approach enables us to understand the role the oxide anion plays in mediating the properties of a supported metal catalyst.

Understanding metal-support interactions has been a constant goal of catalysis research. There are two general approaches that have been taken to understand this problem; model ultra high vacuum (UHV) and classic solution phase synthesis of supported metal catalysts. UHV studies focus on growing by evaporation well defined and clean catalytic nanoparticles (i.e. free of typical impurities like carbon) on single crystal support materials. Traditional solution phase approaches generally involve growing nanoparticles on high surface area metal oxide supports via precipitation routes from solutions.

There are advantages and disadvantages to both approaches. The benefits UHV studies include the ability to prepare chemically pure catalysts clusters, usually through evaporation without impurities however supports are typically in a non-equilibrium condition (highly reduced, no water, no atmospheric interactions, vacuum) making it difficult to extrapolate UHV results to real systems. Solution grown catalysts are more realistic, but consequently have a wide range of sizes and chemical impurities from the solution phase synthesis techniques.

	Fig. 1. Rotating cup agitates powders during deposition coating every surface with catalytic nanoparticles.

Our Approach. In order to bridge the pressure gap and materials gap described above our approach to catalyst research involves using vapor deposition to deposit supported metal nanoparticles on realistic, commercially available, high surface area support materials, Figure 1. There are several advantages to using this approach over traditional preparation techniques.

• Homogeneous catalysts can be grown on the same high surface area support material used in traditional solution phase synthesis programs, Figure 2.
• The catalysts that are grown are chemically very clean since the starting material is a high purity metal target (similar to UHV studies).
• Vapor deposition is completely scalable. It is already used to make the reflective coatings on CD’s, windows, and potato chip bags.
• Now catalysts can be grown easily in a one-step process on virtually any support material expanding the variety of support materials which can be investigated and reduces the number of experimental variables which need to be controlled.
• We can purposely dose the support material with impurities before depositing the catalyst clusters and study the effect of impurities.

	Fig. 2. Z-contrast STEM image of gold nanopariticles on γ-Al2O3

We have the ability to grow virtually any material onto any support material. Presently we are focused on investigating support precious metal catalysts such as gold, platinum, silver, and palladium. Most of our research so far has used gold as a probe metal because: [1] the catalytic activity of gold nanoparticles is highly dependent on the support material, [2] gold has a very low melting point (1337 K) which leads to catalyst clusters that are inherently unstable compared to the higher melting platinum (2041 K) allowing us to study how changes in the surface chemistry of a support influence nanoparticle stability, and [3] gold is extremely heavy making it easy to characterize via electron microscopy.

	Fig. 3. Image of argon plasma used to create metal nanoparticles.

Recent Highlight – Understanding the origin of catalytic activity of Au on TiO2: Using the vapor deposition process we were able to determine that surface hydroxyls, introduced during the solution phase synthesis of supported gold clusters, were essential for stabilizing the gold clusters against coarsening at ambient conditions. In addition to increasing the metal cluster stabilities, we found that the activity of the gold clusters increased dramatically from 0.008 mol CO/mol Au•sec to 0.28 mol CO/mol Au•sec with the pH dependent introduction of surface hydroxyls to the TiO2 surface. [J. Phys. Chem. C 2009, 113, pg 269-281]

Recent Highlight – Stabilizing gold nanoparticles by the choice of support material: We recently showed that gold nanoparticles grown on high surface area silica are remarkably stable against coarsening at 500oC in air for a month compared to the more commonly studied Au/TiO2 catalysts. This temperature is above the Tammann temperature where particle coarsening is expected to be severe. This result was not expected based on classic UHV studies. [J. Catal. 2009. 262, pg 92-101]

The financial assistance for this research is provided by DOE-OS and is gratefully acknowledged.

The principal technical contact for this project is Dr. Gabriel M. Veith,
tel. (865) 574-0027, e-mail veithgm@ornl.gov.