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

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 <i>ex-situ</i> characterization methods may be insufficient to describe the active phases of a catalyst. Therefore, <i>operando</i> studies are key to correlate a catalyst’s structure to its performance while relying at the same time on well-defined model systems.<br/><br/>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.<br/><br/>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 <i>in-situ</i> 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.<br/><br/>To elucidate the structure of the active phase in CoPt/SiO₂ under DRM conditions, and to probe structural dynamics, we conducted <i>operando </i>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/SiO₂during <i>in-situ</i> 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 <i>in-situ</i> 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.<br/><br/>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 <i>operando</i> characterization in capturing dynamic changes and identifying the catalytically active phase.<br/><br/>(1) Nakaya, Y.; Furukawa, S. <i>Chem. Rev. </i><b>2023</b>, <i>123</i> (9), 5859-5947.<br/>(2) Chavez, S.; Werghi, B.; Sanroman Gutierrez, K. M.; Chen, R.; Lall, S.; Cargnello, M. <i>J. Phys. Chem. B </i><b>2023</b>, <i>127</i> (5), 2127-2146.<br/>(3) Lee, Y. S.; Rhee, J. Y.; Whang, C. N.; Lee, Y. P. <i>Phys. Rev. B </i><b>2003</b>, <i>68</i> (23), 235111.