Unveiling non-thermal catalytic origin of direct current-promoted catalysis for energy-efficient transformation of greenhouse gases to valuable chemicals

Catalytic dry reforming of methane (DRM) is a key reaction for the sustainable utilization of major greenhouse gases, CO₂ and CH₄. However, conventional DRM often suffers from severe catalyst deactivation due to high temperature requirements. Applying direct current (DC) to catalyst materials has emerged as a promising strategy to overcome these limitations, yet the underlying DC-enhanced catalytic mechanism remains elusive. Here, we unveil the non-thermal catalytic origin of DC-applied DRM over Pd/CeO₂ through multimodal operando analyses, providing a microscopic physicochemical framework for the rational design of next-generation DRM catalysts beyond the limitations of conventional thermal catalysis.

Catalytic dry reforming of methane (DRM) offers a promising strategy for the sustainable utilization of greenhouse gases, CH₄ and CO₂. However, its practical application has long been hampered by severe catalyst deactivation under harsh thermal conditions, typically above 800 °C. In recent years, catalysis under direct current (DC) has emerged as a groundbreaking non-thermal approach that enables DRM to proceed at significantly lower temperatures (~200 °C) with enhanced catalytic activity and stability. Despite this progress, the underlying microscopic physicochemical origin of the DC-applied catalytic enhancement remains elusive. Competing hypotheses, including electric field effects, current-induced charge-driven mechanism, surface protonics, and Joule heating, have been proposed, yet no comprehensive framework has been established. This ambiguity continues to impede the rational design and optimization of DC-applied catalytic systems.

In this work, the research group led by Toshiki Sugimoto (Associate Professor at Institute for Molecular Science) and Yasushi Sekine (Professor at Waseda University) establish a definitive charge-driven mechanism underlying the non-thermal catalytic enhancement observed in DC-applied DRM, focusing on Pd/CeO₂ as a model catalyst. By combining multimodal operando analyses, including infrared (IR) thermal imaging, spatially resolved mid-IR and visible-to-near IR spectroscopies integrated with real-time mass spectrometry, and ex-situ soft X-ray absorption spectroscopy, the research group directly identified key reaction intermediates in non-thermal DRM that emerge exclusively in current-carrying domains under DC application (Figure 1).　Notably, the DRM activity exhibits a positive correlation with the applied current and electrical conductivity, but a negative correlation with the applied voltage, indicating that the catalytic enhancement is governed by charge injection rather than electric field strength and Joule heating. Crucially, the research group succeeded in detecting both electrons and holes generated in CeO₂ during DC application and revealed a quantitative correlation between the density of injected charges and the DRM activity. Furthermore, soft X-ray spectroscopic analysis uncovers an unconventional mechanism of hole generation in the nominally n-type CeO₂ semiconductor; local lattice strain induced by partial Ce⁴⁺ reduction promotes electron transfer from O 2p to Ce 4f orbitals via ligand-to-metal charge transfer, thereby enabling the redox reaction cycles essential for DRM. These findings overturn the conventional belief that hole-driven oxidation is unlikely in n-type semiconductors, and instead highlight a cooperative mechanism between trapped electrons and strain-induced holes as the microscopic origin of non-thermal catalysis under DC application.

Thus, this study not only establishes a noble physicochemical framework for understanding non-thermal catalytic enhancement in DC-applied systems but also provides guiding principles for the rational design of coke-resistant, low-temperature catalytic DRM systems for the sustainable chemical transformation of greenhouse gases. In addition, the discovery of an unconventional hole-generation pathway through a nontrivial interplay between electrons and holes in a nominally n-type semiconductor under DC bias expands the conventional understanding of charge carrier dynamics beyond the classical models in semiconductor physics and provides a new conceptual foundation in the interdisciplinary field of heterogeneous catalysis and solid-state science.

Information of the paper：

Authors: Harunobu Tedzuka, Hikaru Saito, Nobuki Matsumoto, Masanari Nagasaka, Hiromasa Sato, Yasushi Sekine, and Toshiki Sugimoto
Journal Name: The Journal of Physical Chemistry Letters
Journal Title: Nonthermal Catalytic Origin of DC-Enhanced Dry Reforming of Methane Unveiled by Multimodal Operando Analyses
DOI: 10.1021/acs.jpclett.5c03159
 

Financial Supports

JSPS KAKENHI

Grant-in-Aid for Early-Career Scientists, JP25K18117
Grant-in-Aid for Transformative Research Areas (A), JP 24H02205
Grant-in-Aid for Scientific Research (A), JP 24H00487
Grant-in-Aid for Specially Promoted Research, JP 23H05404

Fund for the Promotion of Joint International Research (International Leading Research), JP 23K20034

JST CREST, Japan, (Grant No. JPMJCR22L2)

JST ACT-X, Japan, (Grant No. JPMJAX24D7)

BL3U of UVSOR Synchrotron Facility, Institute for Molecular Science (IMS program 24IMS6009, 23IMS6811, and 23IMS6615)

Demonstration Project of Innovative Catalyst Technology for Decarbonization through Regional Resource Recycling, the Ministry of the Environment, Japan.