(771f) Improving Thermal Stability of Supported Metal Catalysts Via Phosphonic Acid Self-Assembled Monolayers

Improving Thermal Stability of Supported Metal Catalysts
via Phosphonic Acid Self-Assembled Monolayers

Alexander H. Jenkins¹, Charles
B. Musgrave¹, and J. Will Medlin¹*

¹University of Colorado, Boulder, CO 80303 (USA)

*will.medlin@colorado.edu

Transition metal catalysts supported on metal-oxides
are ubiquitous in heterogeneous catalysis, with the precious metal catalyst
market size alone exceeding $17 billion.¹ For many applications metal
nanoparticle (NP) size plays an important role in the activity and selectivity
of catalyzed reactions. For instance, low-temperature CO oxidation on supported
Au is thought to require Au NPs in the 2-3 nm range.² Even for
applications that do not rely on a specific particle size, smaller NPs are
generally favored due to their high ratio of surface metal atoms which can
participate in the reaction. However, throughout a typical catalyst’s lifetime,
metal particles are constantly growing due to Ostwald ripening, where adatom
diffusion across the support surface leads to the growth of large metal
clusters at the expense of smaller ones. At high operating temperatures this
diffusion becomes facile and catalysts can rapidly deactivate due to NP growth.
This problem of thermal instability is especially prevalent in weakly binding
metals, such as Au, and highly disperse catalysts, such as single-atom
catalysts. The development of a universal method to enhance thermal stability and
longevity across a wide range of supported catalysts would not only be
advantageous economically and environmentally, but also lead to more freedom in
catalyst design and implementation.

We have developed one such method for improving the
thermal stability of supported metal catalysts via alkyl-phosphonic acid (PA)
self-assembled monolayers (SAMs). It has been previously shown that these PA
SAMs selectively bind to metal-oxide support materials such as TiO₂
and Al₂O₃, while leaving metal nanoparticles completely
undecorated.³ Thus, when applied to a supported metal catalyst, PA
SAMs can act to increase the barrier for adatom surface diffusion while leaving
the bulk metal sites unaffected. However, PA SAMs have also been shown to
affect the activity and selectivity of catalytic sites at the metal-support
interface based on the reaction mechanism and functionalization of the PA tail
group. In this project, we demonstrated the use of PA SAMs to improve the
thermal stability of supported Au catalysts (Figure 1) as well as single-atom
Rh catalysts. Transmission electron microscopy (TEM) images of thermally
treated samples showed little change in PA coated catalysts but significant increases
in NP size for uncoated ones. We also showed the effects of these monolayers on
a variety of relevant reactions. For Au/TiO₂, we found that PA SAMs
deactivated the catalyst for CO oxidation, but improved activity for acetylene
hydrogenation. We also observed a reduction in poisoning via coke formation on
the PA coated catalysts.

(1)      Occams Business Research and Consulting. (2018, April)
Global Precious Metal Catalysts Market - Technologies, Market share and
Industry Forecast to 2024. Retrieved from Research & Markets.

(2)       N. Lopez, T. V. W. Janssens, B. S. Clausen,
Y. Xu, M. Mavrikakis, and T. Bligaard, “On the origin of the catalytic activity
of gold nanoparticles for low-temperature CO oxidation,” vol. 223, pp. 232–235,
2004.

(3)       J. Zhang et al., “Control of
interfacial acid-metal catalysis with organic monolayers,” Nat. Catal.,
vol. 1, no. 2, pp. 148–155, 2018.