Technical ApproachOpportunity and SignificanceConventional supported catalysts, in which precious metals interact with highsurface area metal oxides, have been widely used in industrial applications dueto their ability to enhance catalytic performance for a wide array of reactions.Supported catalysts have been traditionally preferred because they can beprepared via simple synthesis techniques, typically involving precious metaldeposition onto support materials.

Catalytic Performance

Dr. Eranda Nikolla Chemical Engineering

We thankfully acknowledge the financial support of the National Science Foundation (DMREF-1436193), the Young Scholars Summer Research Program

at the University of Colorado, Boulder and Wayne State University.

Technical Objectives & Methods

Here, we study two design methods for synthesizing 3-dimensional active sites:I. An inverted catalytic system with controlled metal@metal oxide core-shell

structure (Pd@TiO2) that enhances metal – metal oxide interactionsII. A catalyst with single precious metal atoms supported on functionalized

metal oxide (Pt1/TiO2) surface modified with phosphonic acid self-assembled monolayers that maximizes atom efficiency by minimizingnanoparticle size.

References

Laura Ivette Paz Herrera

Design of 3-Dimensional Active Sites for Selective Catalysis

Selective HDO of benzyl alcohol towards toluene:

Temperature - 190°C, gas flow rates - 140 ml/min (H2=20%, benzyl alcohol=0.03%), conversion=6.1%

Conventional catalysts, however, fail to provide thermal nanoparticle stability,leading to:

Precious metals, are rare, expensive, and non-renewable, inorganic materials.For this reason, development of methods to minimize the use of preciousmetals, by maximizing atom efficiency, is key.To tackle this matter, efforts were made to efficiently design and modify the3-dimensional environment of the catalytic surface in order to increaseavailability of active sites. This was accomplished by tailoring the synthesis ofthe catalysts via controlled studies of reaction parameters and conditions.

Formation of large precious metal nanoparticles

Decrease in the overall exposed surface area of

the precious metal

Poor catalytic activity

The main purpose of this study is to minimize the use of precious metals in supported catalysts by:

✓ Maximizing atom efficiency✓ Enhancing metal – metal oxide interfacial sites✓ Increasing specific activity✓ Improving metal atom/nanoparticle stability

Synthesizing single atoms onto metal oxide material, surface decorated with aphosphonic acid self-assembled monolayer

❶ Phosphonic acid self-assembled monolayer deposition on TiO2 support

❷ Removal of organic ligands (tails)

❸ Platinum single-atom deposition followed by calcination

Sol-Gel Synthesis Method:

Tuning Pd@TiO2 shell structure at room temperature:

❶ Pd nanoparticles are pre-synthesized prior to coating of the TiO2 Shell

❷ CTAB is used as hydrophilic ligand to modify the metal nanoparticle surfaces

❸ Metal alkoxide precursors are used to synthesize metal oxide-shell usingalcohols as solvents

❹ De-ionized water is used as an initiator to start the sol-gel shell reaction

✓ Increased density of metal-metal oxide interface sitesthat completely surround the metal particles

✓ Improve chemical and thermal stability of small metalnanoparticles through encapsulation with metal oxides

✓ Porous shell provides spatially regulated delivery ofreactants to the interfacial sites

=

20 nm

I. Inverted Core-Shell Structure: Pd@TiO2

II. Single Metal Atoms on Functionalized Metal Oxide Support: Pt1/TiO2

❸

✓ Single precious metal atoms maximize catalyst efficiency by exposing 100%of active sites on the catalytic surface

✓ Phosphonic acid self-assembled monolayers provide stability and preventformation of large precious metal nanoparticles

❶ ❷ ❸

❶ ❷

Figure 2. Process followed to synthesize systems of Pd@TiO2

Figure 3. Pd@TiO2 core-shellstructures observed under TEM

Figure 4. Process used to synthesize single Pt atoms on phosphonic acid SAMs functionalized

TiO2 support

Figure 1. Structure of a conventional catalytst composed of a precious metal nanoparticle supported on a metal oxide

✓ Angew. Chem. Int. Ed. 2017, 56 (23), 6594-65980✓ J. Am. Chem. Soc. 139 (40), 14150-14165✓ Phys. Rev. E 1993, 48 (5), 3692-3704

Pathway leading todesired product:

toluene

Temperature – 350 °C, gas flow rates – 50 ml/min (CO: 1%, O2: 20%. He: 79%)

Probe reaction: Hydrodeoxygenation (HDO)

R2O + 2 H2 → H2O + 2 RHAccessible conformations of aromatics

are restricted and HDO selectivity is increased

Probe reaction: CO Oxidation2 CO + O2 → 2 CO2

✓ Exposed surface area of precious metal is increased and catalytic

activity is improved

Figure 5. Comparison of a) selectivity and b) turnover frequency number of supported vs. encapsulated systems

a)

Figure 6. a) Characterization and b) catalytic testing of traditional vs. functionalized supported systems

I. Inverted Core-Shell Structure: Pd@TiO2

II. Single Metal Atoms on Functionalized Metal Oxide Support: Pt1/TiO2

b)