A major facet of transitioning from fossil fuels to green and renewable energy solutions involves the removal, capture and storage of carbon dioxide (CO₂) from the environment. One method is by CO₂ hydrogenation, which requires a catalyst to spur the reaction, frequently including metal-oxide catalysts in which metal-support interactions (MSIs) play an important role. 

Researchers from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, Stony Brook University, DOE’s Argonne National Laboratory and several other institutions used a suite of in situ techniques to study the behavior and structural and chemical properties of a Cu@TiO_x core@shell catalyst under CO₂ hydrogenation. Their work was published in ACS Catalysis.

In a core@shell structure, one type of active system (the core) is encapsulated by a shell of a different material to enhance catalytic performance. These experiments focused on an inverse oxide/metal catalyst configuration using a copper nanowire core with a titanium oxide (titania) shell. Such catalysts have been shown to offer improved stability and activity over the conventional metal/oxide arrangement.

Through the use of an entire range of in situ characterization techniques – including time-resolved experiments with X-ray absorption spectroscopy (XAS), ambient pressure X-ray photoelectron spectroscopy (AP-XPS), environmental transmission electron microscopy (E-TEM), and X-ray diffraction at the 17-BM-B beamline of the Advanced Photon Source, a DOE Office of Science user facility at Argonne – the investigators sought to achieve a comprehensive understanding of the structure and behavior of the Cu@TiO_x catalyst under CO₂ activation and hydrogenation, a functional picture that cannot be obtained with typical steady state studies.

The dynamic characteristics of this catalyst system became immediately evident even during the standard pretreatment used for CO₂ hydrogenation, when the H₂ pretreatment at temperatures of above 250 degrees Celsius resulted in cracking of the titania shell and migration of Cu particles from the core to the top of the oxide shell. This, along with other configuration changes, was caused by metal-support interactions. The migrating Cu particles are about 20-40 nm in diameter and are speckled with clusters of TiO_x and Cu-Ti-O_x. With this altered structure, the system displayed highly dynamic yet wholly reversible catalytic characteristics that were dependent on temperature and chemical environment.

In situ tracking during CO₂ exposure at room temperature showed that this configuration changes again, with the TiO_x and Cu-Ti-O_x aggregates becoming a layered structure atop the Cu surfaces. Under higher temperatures above 180 degrees Celsius and exposure to H₂, this oxide layer vanished, and the only detected reaction product was CO, indicating partial decomposition of CO₂ on the surface. The in-situ observations indicate oxidation of Cu and TiO_x at room temperatures by CO₂ with reduction of CuO_x and Cu-Ti-O_x at higher temperatures by H₂. 

It was clear that neither CO₂ nor H₂ behaved as passive reactants and that both had significant, yet reversible, effects on the composition and morphology of the Cu@TiO_x catalyst. These properties moved toward a dynamic equilibrium when under simultaneous oxidation and reduction in a CO₂ and H₂ mixture. The work is a new and valuable window into the very active processes involved with a core@shell catalyst under normal operational conditions. Such insights may provide important clues for the design and fabrication of new and more efficient catalytic materials for dealing with the trapping and conversion of CO₂. – Mark Wolverton

See: K. Deng^1,2, X. Chen³, J. Moncada², K.L. Salvatore¹, N. Rui², W. Xu⁴, S. Xiang¹, N. Marinkovic⁵, A.I. Frenkel^2,1, G. Zhou³, S.S. Wong¹, J.A. Rodriguez^1,2, “Observing chemical and morphological changes in a Cu@TiO_x core@shell catalyst: Impact of reversible metal-oxide interactions on CO₂ activation hydrogenation," ACS Catal. 2024, 14, 15, 11832-11844

Author affiliations: ¹Stony Brook University; ²Brookhaven National Laboratory; ³State University of New York; ⁴Argonne National Laboratory; ⁵Columbia University

The work done at the Chemistry Division of BNL was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and Catalysis Science Program under contract No. DE-SC0012704. J.A.R. and J.M. received funding from a DOE Office of Science Distinguished Scientist Fellow Award. X.C. and G.Z. acknowledge the support by the National Science Foundation (NSF) under DMR 1905422. A.I.F. acknowledges support of XAFS data analysis by the U.S. DOE, Office of Science, Office of Basic Energy Sciences grant no. DE-SC0022199. The Cu@TiO_x core–shell nanowires, that inspired the current work, were initially produced in SSW’s laboratory, supported by the U.S. National Science Foundation under Grant No. CHE-1807640. This research used resources of beamlines 7-BM (QAS), 28-ID-2 (XPD), and 23-ID-2 (IOS) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. The QAS beamline operations were supported in part by the Synchrotron Catalysis Consortium (U.S. DOE, Office of Basic Energy Sciences, Grant No. DE-SC0012335). This research used resources of beamline 17-BM at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The morphology of the catalyst was studied at the Electron Microscopy Facility (FEI Titan 80-300 and FEI Talos 200×) of the Center for Functional Nanomaterials (CFN), which is a U.S. Department of Energy Office of Science User Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. The authors are grateful to D. Zakharov for his help with some of the E-TEM and EELS studies.

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