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CH04.04.09

Operando XAS-XRD Reveals Structural Dynamics in CoPt Nanoparticles under Dry Reforming of Methane Conditions

When and Where

Apr 24, 2024
10:45am - 11:00am
Room 443, Level 4, Summit

Presenter(s)

Co-Author(s)

David Niedbalka1,Marcel Janak1,Diana Piankova1,Paula Abdala1,Christoph Müller1

ETH Zurich1

Abstract

David Niedbalka1,Marcel Janak1,Diana Piankova1,Paula Abdala1,Christoph Müller1

ETH Zurich1
Gaining insight into how the geometric and electronic structure of (mono)metallic nanoparticles is modified through the addition of a second metal (bimetallic nanoparticles) and how such structural changes affect in turn their catalytic properties1 is crucial for the rational advancement of catalysts. Further, as the structure of a catalyst is often dynamic2 ex-situ characterization methods may be insufficient to describe the active phases of a catalyst. Therefore, operando studies are key to correlate a catalyst’s structure to its performance while relying at the same time on well-defined model systems.

The dry reforming of methane (DRM) is a reaction that converts CH4 and CO2 into a synthesis gas at 600-1000 °C and is typically catalyzed by transition metals such as Ni, Co, or Pt. However, such monometallic catalysts often suffer from deactivation due to particle growth, carbon deposition, and/or oxidation.

In this study, we investigate SiO2-supported, bimetallic CoPt nanoparticles and their monometallic counterparts (CoPt/SiO2, Co/SiO2, and Pt/SiO2) for the DRM. Prior to the catalytic DRM tests, all catalysts were activated in-situ in a H2/N2 mixture (1-2 h). The bimetallic CoPt/SiO2 catalyst shows superior activity and stability under DRM conditions (800 °C and 1 bar, CH4:CO2 ratio = 1, space velocity = 30.000 ml g-1 h-1) in comparison to its monometallic counterparts. Specifically, while Pt/SiO2 showed the lowest CH4 conversion (10%), Co/SiO2 underwent deactivation, resulting in a decrease in CH4 conversion from 30% to 15% within 120 min. Conversely, CoPt/SiO2 showed a stable performance of 35% CH4 conversion over 6 h.

To elucidate the structure of the active phase in CoPt/SiO2 under DRM conditions, and to probe structural dynamics, we conducted operando experiments using combined synchrotron X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). Co K-edge and Pt L3-edge XAS analysis shows that in CoPt/SiO2 during in-situ H2 activation Co and Pt become both fully reduced to their metallic states. We further observe differences in the Co K-edge and Pt L3-edge XAS features of CoPt/SiO2 when compared to Co/SiO2 or Pt/SiO2, possibly due to a charge transfer between the metals, orbital hybridization in the CoPt alloy and/or a change in the local structure of Pt and Co.3 Rietveld analysis of the acquired XRD data after in-situ activation at 800 °C indicates the formation of two types of CoPt alloys: an ordered (intermetallic) CoPt (52%) and a random CoPt alloy (48%). Interestingly, upon switching to DRM conditions (800 °C, CH4:CO2 ratio = 1) XRD analysis revealed an instantaneous transformation of the intermetallic phase into a random alloy. This phase transition was also evidenced in the Pt L3-edge XAS data but did not change the electronic structure/oxidation state of Co (i.e., Co K-edge remained invariant prior to and during DRM). In contrast, under DRM conditions, Co/SiO2 underwent a partial oxidation showcasing the stabilization of the metallic state of Co through its alloying with Pt. This stabilization, combined with changes in the electronic/local structure and site isolation that very likely suppresses coking on Pt, contribute to the superior activity of CoPt/SiO2 compared to Co/SiO2 and Pt/SiO2.

We also observe a phase transition from a random to intermetallic alloy during the cooling down to room temperature of the reacted catalyst, underscoring the significance of operando characterization in capturing dynamic changes and identifying the catalytically active phase.

(1) Nakaya, Y.; Furukawa, S. Chem. Rev. 2023, 123 (9), 5859-5947.
(2) Chavez, S.; Werghi, B.; Sanroman Gutierrez, K. M.; Chen, R.; Lall, S.; Cargnello, M. J. Phys. Chem. B 2023, 127 (5), 2127-2146.
(3) Lee, Y. S.; Rhee, J. Y.; Whang, C. N.; Lee, Y. P. Phys. Rev. B 2003, 68 (23), 235111.

Keywords

operando | phase transformation

Symposium Organizers

Yuzi Liu, Argonne National Laboratory
Michelle Mejía, Dow Chemical Co
Yang Yang, Brookhaven National Laboratory
Xingchen Ye, Indiana University

Session Chairs

Tao Sun
Tao Zhou

In this Session