(52f) Reversible Transformation from Pt Single Atoms to Sub-Nanometer Particles for Low Temperature CO Oxidation

Reversible
Transformation from Pt Single Atoms to Sub-Nanometer Particles for Low Temperature
CO Oxidation

Xavier Isidro Pereira Hernández¹⁺, Andrew
DeLaRiva²⁺, Haifeng Xiong², Eric J. Peterson²,
Deepak Kunwar², Berlin Sudduth¹, Yong Wang^1,3,*
and Abhaya K. Datye^2,*

¹ Voiland
School of Chemical Engineering and Bioengineering, Washington State University,
Pullman, WA 99164.

² Department of Chemical and Biological Engineering and Center for
Micro-Engineered Materials, University of New Mexico, Albuquerque, New Mexico
87131, USA.

³
Institute
for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA
99354.

*Corresponding
authors: Prof. A. K. Datye (datye@unm.edu)
and Prof. Y. Wang (yong.wang@pnnl.gov)

⁺ These
authors contributed equally to this work

Abstract:
While single atom catalysts can provide high catalytic activity and
selectivity, application in industrial catalysts demands long term performance
and the ability to regenerate the catalyst. We have investigated factors that
lead to improved catalytic reactivity of Pt for low temperature CO oxidation. 
Single atom Pt/CeO₂ becomes active for CO oxidation under lean
condition only at elevated temperatures, because CO is strongly bound to ionic Pt
sites.  Activating the catalyst with a reduction treatment, even under mild
conditions, leads to onset of CO oxidation activity even at room temperature. This
high activity state involves the transformation of mononuclear Pt species to
sub-nanometer sized Pt particles.  Under oxidizing conditions, the Pt can be
restored to its stable, single atom state.  The key to facile regeneration is
the ability to create mobile Pt species and suitable trapping sites on the
support, making this a prototypical catalyst system for industrial application
of single atom catalysis.

1.     Introduction

One of the main
objectives during the synthesis of heterogeneous catalysts is to have the active
transition metal in an atomically dispersed form, in order to maximize atom
efficiency.  This can be achieved either through trapping the atomic species [1] or
by limiting the transition metals weight loading [2],
[3]. 
However, in most cases, increases in temperature under reaction conditions leads
to sintering and loss of this dispersed form.  Therefore, heterogeneous
catalysts are based on catalyst supports that aid in the reaction and/or
provide stability to the transition metal. Ceria is just one of the numerous
supports that are used for Platinum Group Metals (PGM).  Ceria is a commonly
used component in automotive exhaust catalysts, often used as an additive for
oxygen storage and in lean NOx traps.  The unique ability of Ceria to change
from Ce⁺⁴ to Ce⁺³ and the facile synthesis methods that
can produce various morphologies make it ideal as a support for Pt. We show
here a ceria-supported Pt single-atom catalyst with great stability even at
temperatures as high as 800°C, as previously reported [4],
and its activation (and structural changes) to achieve high activity for CO
oxidation (low CO oxidation light-off temperatures) while maintaining its
stability. Furthermore, we show that the catalyst can be restored to its
initial state by high-temperature treatment under oxidative conditions,
granting it flexibility to operate under the desired conditions. We propose
that the Pt/Ceria catalyst be considered as a prototypical system that can
toggle between an atomically dispersed form and self-assemble into a high activity
state, providing the design elements for a thermally stable catalyst for
industrial applications.

2.    
Experimental

The catalysts were synthesized via Dry Impregnation (DI) [5]. The
activity measurements were performed using a stainless-steel reactor tube and
monitored by a micro Gas Chromatograph (GC). The catalyst was characterized by
means of High-Angle Annular Dark-Field Scanning Transmission Electron
Microscopy (HAADF-STEM), Diffuse Reflectance Infrared Fourier Transform
Spectroscopy (DRIFTS), X-Ray Photoelectron Spectroscopy (XPS), Raman
Spectroscopy and X-Ray Powdered Diffraction (XRD).

3.     Results and
Discussion

The
activity measurements for the As Synthesized catalyst (prior to activation) and
the activated catalysts are observed on Figure 1. The activation of the
catalyst is performed at 275°C under CO flow. It is seen that when the Pt/CeO₂
catalyst is activated, the light-off temperature decreases drastically
(approximately 200°C).

Figure 1. CO oxidation reaction on a 1wt.%Pt/CeO₂ catalyst,
As Synthesized and Activated.

DRIFTS
measurements were performed on the catalyst As Synthesized, Activated and
Regenerated and are observed on Figure 2. The catalyst as synthesized shows a very
well defined peak at 2095 cm^-1, assigned to CO adsorbed on ionic Pt.
Other authors have reported the adsorption of CO on ionic Pt at similarly high
wavenumbers [6], [7]. After being
activated, the presence of two peaks at 2086 cm^-1 and 2073 cm^-1
is observed. These two peaks are assigned to CO adsorbed linearly on Pt
nanoparticles (NPs). At the same time, the Activated catalyst shows a peak at
2110 cm^-1, assigned to CO adsorbed on ionic Pt. The reason for the
blueshift between the As Synthesized and Activated catalysts is associated to
the presence of CO dipole-dipole interactions in the latter one, due to the
existence of Pt NPs near the Pt single atoms. After performing a regeneration
at 800°C under O₂ flow, the catalyst shows the same behavior as the
As Synthesized catalyst, suggesting it can be restored to the initial state.

Figure 2. CO oxidation reaction on a 1wt.%Pt/CeO₂
catalyst, As Synthesized, Activated and Regenerated.

To
further study the state of the catalyst, XPS measurements were performed and
can be observed on Figure 3. The Ce 3d region shows that changes on the support
start occurring during activation at temperatures as low as 125°C, in which Ce⁴⁺
is reduced to Ce³⁺. Very little difference is observed between
activation at 125°C and 275°C. Once regenerated, all the Ce³⁺ is
converted to Ce⁴⁺, although, this behavior is expected since CeO₂
can be oxidized relative easily. Analyzing the Pt 4f region, it can be observed
that unlike on CeO₂, activation at 125°C only reduces partially the
Pt²⁺ species, which are present in the catalyst As Synthesized.
Activating the catalyst at 275°C shows a more well defined peak at 71.2 eV,
associated to Pt⁰. Once again, it is seen that the regeneration step
restores the Pt species to their original state.

Figure 3. XPS spectra of the 1wt.% Pt/CeO2 catalyst. Left: Ce3d region. Right: Pt4f region. As Synthesized (Black), Activated at 125°C
(Red), Activated at 275°C (Blue), Regenerated at 600°C (Green).

To confirm the results obtained by the
previous characterization techniques, HAADF-STEM images on the Activated and
Regenerated catalyst are presented on Figure 4. The Activated Catalyst (left)
shows the presence of Pt species of 1 nm or smaller size (bright spots). These
sub-nanometer particles are not observed anymore after regeneration (right) due
to the redispersion of the Pt atoms.

Figure 4. HAADF-STEM images of the 1wt.% Pt/CeO₂ catalyst.
Left: Activated catalyst. Right: Regenerated catalyst.

Conclusions

We presented a pathway to
synthesize a Pt single-atom catalyst that is stable at temperatures as high as
800°C. This catalyst can be activated by a reduction treatment, leading to high
activity for CO oxidation at low temperatures. Furthermore, the catalyst shows the
capability to be restored to its initial state by regenerating it under an
oxidation treatment at high temperatures, granting it flexibility to operate
under the desired conditions in an industrial environment.

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