Controlled Reconstruction of Cu-Ag Binary Catalysts for Steering C₁/C₂ Chemicals Pathway in CO₂ Electroreduction

Abstract

전기화학적 이산화탄소 환원 반응 (CO₂RR)은 이산화탄소를 부가가치 화합물로 전환할 수 있는 유망한 기술로 주목받고 있다. 단일 금속 구리 (Cu)는 C-C 결합 형성을 가능하게 하는 독특한 촉매 특성을 지니지만, 반응 중간체 안정화의 한계로 인해 선택성이 낮고 산물 분포 제어가 어렵다는 문제가 있다. 이를 극복하기 위해 은 (Ag)을 도입한 Cu-Ag 이종금속 촉매가 제안되었으며, Ag는 CO 생성을 촉진하고 Cu는 이를 추가 환원하여 다탄소 화합물을 생성하는 역할을 수행한다. 그러나 최근 Cu-Ag 시스템에서는 동일한 촉매와 반응 조건에서도 메탄 (CH₄) 또는 에틸렌 (C₂H₄) 및 에탄올 (C₂H₅OH)과 같은 C2 화합물이 우세하게 생성되는 선택성 양면성 현상이 보고되고 있으며, 그 근본적인 기작은 아직 명확히 규명되지 않았다.

본 논문에서는 Cu-Ag 촉매에서 CO₂RR 동안 발생하는 표면 재건현상 (reconstruction)이 C₁/C₂ 선택성을 지배하는 핵심 인자임을 규명하였다. Arc Plasma Deposition (APD)을 이용하여 Ag 증착량을 정밀하게 제어함으로써 서로 다른 계면 밀도와 초기 구조를 갖는 Cu-Ag 촉매를 제작하였고, 이를 통해 Ag 로딩에 따른 선택성 변화를 체계적으로 분석하였다.

정적 조건에서 CO₂RR을 수행한 경우, Cu 용출 및 재증착에 의해 표면이 Cu-rich 상태로 재구성되며 CH₄가 우세하게 생성되는 C₁ 지배적 시스템이 형성되었다. 특히 Ag 로딩이 높은 500 shot 촉매에서는 Cu 도메인이 더욱 분절되어 C-C 결합이 억제되고 protonation 경로가 우세해짐에 따라 CH₄ 선택성이 더욱 크게 증가하였다. 반면, 반응 전 펄스 전위(pulse potential)를 인가하여 비평형 재건현상을 유도한 경우, 동일한 Cu-Ag 촉매에서도 선택성이 C₂ 중심으로 전환되었다. Cu-Ag 15 shot 촉매에서는 결함이 풍부한 Cu-rich ad-particle이 형성되어 CO 중간체의 안정화 및 C-C 결합이 촉진되었으며, Cu-Ag 500 shot 촉매에서는 Cu-rich ad-particle과 Ag aggregate가 공존하는 독특한 표면 구조가 형성되었다. 이 과정에서 Cu/Ag 계면에서의 전자밀도 증가가 XPS 결합에너지 이동으로 확인되었으며, 이는 C-C 결합 반응의 활성화를 더욱 촉진하였다.

XRD, SEM, HR-TEM, STEM-EDS, XPS 및 in-situ XAS 분석을 통해, 이러한 선택성 변화는 새로운 벌크 상 형성이 아닌 표면 및 근표면 영역에서의 재건현상에 의해 유도됨을 확인하였다. 본 연구는 Cu-Ag 촉매에서 C1/C2 선택성을 결정하는 핵심 인자가 조성 자체가 아니라 반응 중 형성되는 재건된 활성 구조임을 명확히 제시한다. 결론적으로, APD 기반 계면 설계와 펄스 전위에 의한 재건 제어를 통해 Cu-Ag 촉매의 C₁/C₂ 선택성을 정밀하게 조절할 수 있음을 입증하였으며, 본 연구는 Cu-Ag 시스템에서의 선택성 양면성 문제에 대한 통합적인 기작 모델을 제시함과 동시에 재건 제어형 CO₂RR 촉매 설계를 위한 새로운 전략을 제공한다.

핵심어: 전기화학적 이산화탄소 환원 반응, 표면 재건현상, Cu-Ag 이종금속 촉매, 펄스 전위, 선택성 양면성

|Electrochemical carbon dioxide reduction reaction (CO₂RR) has emerged as a promising strategy for converting CO₂ into value added fuels and chemicals, contributing to sustainable carbon utilization and carbon neutral energy systems. Among various metal catalysts, monometallic Cu is unique in its ability to catalyze C-C coupling reactions, enabling the formation of multi-carbon products. However, Cu-based catalysts often suffer from limited selectivity control and broad product distributions, primarily due to their intrinsic tendency toward competing protonation pathways and dynamic surface reconstruction during reaction. To overcome these challenges, bimetallic Cu-Ag catalysts have been widely explored, leveraging the complementary roles of Ag for *CO generation and Cu for subsequent C-C coupling. Despite extensive efforts, Cu-Ag systems frequently exhibit a puzzling selectivity bifurcation, producing predominantly either methane (CH₄) or C₂ products such as ethylene (C₂H₄) and ethanol (C₂H₅OH) under similar electrochemical conditions. This inconsistent selectivity behavior has hindered rational catalyst design, and the fundamental mechanistic origin of selectivity bifurcation in Cu-Ag catalysts remains unresolved.

In this work, we systematically investigate the role of surface reconstruction as the key factor governing C₁ and C₂ selectivity in Cu-Ag catalysts during CO₂RR. Cu-Ag catalysts with precisely controlled Ag loadings were fabricated using arc plasma deposition (APD), enabling fine tuning of Cu/Ag interfacial density. This synthesis strategy allowed direct comparison of catalysts with distinct initial Ag distributions but similar overall morphology, providing an ideal platform to isolate reconstruction driven effects from compositional variables. Under static CO₂RR conditions, both Cu-Ag 15 shot and 500 shot catalysts underwent pronounced surface reconstruction through Cu dissolution and redeposition, resulting in Cu-rich surface states. These reconstructed surfaces exhibited CH₄-selective behavior, with higher Ag loading leading to more pronounced CH₄ formation. This trend is attributed to enhanced fragmentation of Cu domains by Ag incorporation, which suppresses contiguous Cu ensembles required for C-C coupling and instead favors protonation dominated reaction pathways.

In contrast, pulse activated CO₂RR induced a fundamentally different reconstruction pathway that redirected the reaction toward C₂ formation. Pulse activation generated Cu-rich ad-particle structures with high defect density in both Cu-Ag 15, 500 shot catalysts and promoted Ag aggregation near the surface, leading to expanded and spatially coupled Cu/Ag interfacial regions in Cu-Ag 500 shot catalyst. Comprehensive structural and compositional analyses using XRD, SEM, in-situ XAS, HR-TEM, STEM-EDS and XPS depth profiling revealed that pulse activation drives non-equilibrium surface and near surface reconstruction without inducing bulk phase transformation. Notably, XPS binding energy downshifts observed exclusively in pulse activated Cu-Ag 500 shot catalysts indicate the formation of a unique interfacial electronic structure characterized by enhanced local electron density at the Cu/Ag interfaces. These electronic modifications are closely correlated with the emergence of Ag-rich aggregates adjacent to defect-rich Cu ad-particle domains.

As a result of these reconstruction-driven structural and electronic changes, pulse-activated Cu-Ag catalysts exhibited strong suppression of CH₄ formation and a dramatic enhancement of C₂H₄ and C₂H₅OH selectivity. The selectivity transition is attributed to the synergistic combination of stabilized *CO-derived intermediates on defect-rich Cu ad-particle domains and pulse induced electronic reconstruction that collectively lower the kinetic barrier for C-C coupling. Importantly, the extent of C₂ enhancement depends on the initial Ag loading, with Cu-Ag 500 shot catalyst showing the most pronounced selectivity shift due to the formation of extensive Cu/Ag interfacial contact and electronically modified active sites.

Overall, this study demonstrates that surface reconstruction is the decisive factor controlling C₁ and C₂ selectivity in Cu-Ag catalysts, rather than the intrinsic composition of the as-prepared catalyst. By linking static and pulse induced reconstruction pathways to distinct CH₄ and C₂ selective states, this work provides a unified mechanistic framework for understanding selectivity bifurcation in Cu-Ag systems. The integration of APD-based interface engineering with pulse-controlled reconstruction offers a general and scalable strategy for designing reconstruction driven-CO₂RR catalysts with tunable selectivity toward multi-carbon products and provides broader insights into dynamic catalyst behaviors under electrochemical operating conditions.

Keywords: CO₂RR, Pulse potential, Cu-Ag catalyst, Surface reconstruction, Selectivity bifurcation