| 摘要: | 近年來，環境污染和能源需求的日益增長已成為全球共同面臨的挑戰。為了因應這個挑戰，減少碳排放是必要的，而二氧化碳還原反應(CO2 reduction reaction, CO2RR)則是可行的方法之一。CO2RR利用由再生能源產生的電能將二氧化碳轉化為一氧化碳(CO)與甲酸(HCOOH)等高價值產物，這種反應被認為能夠實現真正的碳中和。然而，電化學CO2RR觸媒面臨的挑戰包含:與析氫反應(hydrogen evolution reaction, HER)的競爭，CO2RR活性低，產物選擇性不佳等。為了解決上述問題，目前科學家們已經開發了許多二元觸媒。銅(Cu)與錫(Sn)雙元觸媒具有良好的協同效應而被廣泛用作CO2RR觸媒，但為了更好的效能，雙元觸媒的相與組成仍需要進一步探討與優化。
本研究製備了相同原子比(Cu/Sn=99/1)的Cu-SnO2/C與CuO-SnO2/C，X射線光電子能譜(X-ray photoelectron spectroscopy)的分析結果證實了Cu與Sn原子之間具電子轉移效應。在不同電壓下，Cu-SnO2/C 與 CuO-SnO2/C比Cu/C，CuO/C及SnO2/C 有更高的CO法拉第效率，顯示Cu / CuO與SnO2之間的協同效應能有效抑制HER反應，並提高CO選擇性。另外，CuO-SnO2/C展現最佳的CO選擇性及穩定性，在-0.8 V (vs. RHE)下，CO法拉第效率達到95 %，並在10小時後仍維持90%，相比Cu-SnO2/C僅有80%且維持5小時，證實CuO-SnO2之間的介面比起Cu-SnO2介面有更強的協同效應，對於改善CO選擇性及穩定性有最佳的效果。
另外，Cu與Sn原子比例與CO2RR表現的關係也被進一步探討，99CuO-SnO2/C達到95%的CO法拉第效率，隨著Sn含量上升，13CuO-SnO2/C (Cu/Sn=93/7) 的CO法拉第效率僅剩46 %，另外發現有40%的HCOOH法拉第效率。在原位X光吸收光譜(in-situ X-ray absorption spectroscopy)分析中發現，99CuO-SnO2/C具有最佳的CO選擇性的原因源於其Sn-Cu鍵結，當添加少量的Sn，Sn容易與Cu結合並產生大量Sn-Cu鍵結；隨著Sn含量上升，Sn原子間容易團聚並形成Sn-Sn鍵結，使得產物選擇性從CO轉向HCOOH。另外，SnO2修飾CuO觸媒預防SnO2被還原成Sn，維持CO2-CO轉換的活性位點。
為了確認 CuO跟SnO2之間的協同效應，製備了CuO 與 SnO2物理混合 (Cu/Sn=99/1)樣品，其只有73% CO法拉第效率並只能維持5小時，證明物理混合只產生有限的協同效應，且確認大量的CuO-SnO2介面結合是達到最佳CO2RR效能的關鍵。
本研究證明了CuO和SnO2對CO2-CO轉化的協同效應，並探討了原子比對選擇性的影響，為應用於CO2RR的觸媒設計提供了新的見解。
;As the increasing demand for energy and the rising environmental pollution become global challenges, the electrochemical carbon dioxide reduction reaction (CO2RR) has emerged as a potential solution for reducing carbon emissions. CO2RR uses electric energy from sustainable energy sources to convert CO2 into high-value products, such as carbon monoxide (CO) and formic acid (HCOOH), offering a promising path towards true carbon neutrality. However, CO2RR faces several hurdles, including competition with the hydrogen evolution reaction, low CO2RR activity, and poor product selectivity. To address these issues, scientists have developed many binary catalysts, including copper (Cu) and tin (Sn) bimetallic catalysts, which exhibit strong synergistic effects in CO2RR. However, to improve the performance, the phases and compositions on binary catalysts needs further exploration and optimization.
In this study, Cu-SnO2/C and CuO-SnO2/C with the same atomic ratio of Cu/Sn = 99/1 were prepared. X-ray photoelectron spectroscopy analysis confirmed the electron transfer effect between Cu and Sn atoms. Under different voltages, Cu-SnO2/C and CuO-SnO2/C showed higher CO faradaic efficiency (FECO) than Cu/C, CuO/C, and SnO2/C, indicating that the synergistic effect between Cu/CuO and SnO2 could effectively suppress the HER reaction and enhance CO selectivity. Additionally, CuO-SnO2/C exhibited the best CO selectivity and stability, with a FECO of 95% at -0.8 V (vs. RHE) and maintaining 90% after 10 hours, compared to 80% for Cu-SnO2/C within 5 hours. These results indicate that the CuO-SnO2 interface demonstrates a superior synergistic effect compared to Cu-SnO2 interface, leading to the best improvement of CO selectivity and stability.
Furthermore, the relationship between the Cu/Sn atomic ratio and CO2RR performance was further explored. 99CuO-SnO2/C achieved a FECO of 95%, while the FECO of 13CuO-SnO2/C (Cu/Sn=93/7) only reached 46% and showed a FEHCOOH of 40%. In-situ X-ray absorption spectroscopy analysis revealed that the best CO selectivity of 99CuO-SnO2/C was due to the Sn-Cu bonding. When a small amount of Sn was added, numerous Sn-Cu bonds were formed. As the Sn content increased, Sn tended to aggregate and form Sn-Sn bonds, causing a shift in selectivity from CO to HCOOH. In addition, SnO2-modified CuO catalysts could prevent SnO2 from being reduced to Sn and maintain the activity site for CO2-CO conversion.
To confirm the synergistic effect between CuO and SnO2, physically mixed samples of CuO and SnO2 with Cu/Sn of 99/1 were prepared, which only achieved a FECO of 73% and maintained activity for 5 hours. This result highlights that physical mixing only produces limited synergistic effects, and a large number of CuO-SnO2 interfaces are crucial for achieving optimal CO2-CO conversion.
This study demonstrates the synergistic effect of CuO and SnO2 on CO2-CO conversion and investigates the impact of the atomic ratio on selectivity, providing new insights for catalyst design in CO2RR applications. |
