由於大氣中二氧化碳濃度的增加導致全球暖化，明顯地改變氣候和自然生態的平衡。因此，透過二氧化碳的電化學還原反應(carbon dioxide electrochemical reduction reaction, CO2RR) 生產高附加價值的化學品被認為是實現碳循環的途徑之一。然而，電化學CO2RR面臨的挑戰在於其與析氫反應(hydrogen evolution reaction, HER)的競爭和產物選擇性不佳。因此，設計和製備具有低過電位、高法拉第效率和高選擇性的觸媒是CO2RR技術發展的關鍵考量因素。
本研究分為兩個部分。在第一部分，製備了In/Sn原子比為 3/1 和 1/1 的In-Sn二元觸媒。In-Sn奈米顆粒的結構為In、In(OH)3和少量InSn4，而表面有些許的In(OH)x和SnOx。表面In(OH)3不僅有效抑制HER以提高CO2RR活性，並與CO2相互作用形成In-CO3-物種並還原為甲酸。因此，InxSny奈米顆粒展現出優異的性能。在-1.0 V下，甲酸法拉第效率為92.6 %，且實現了10小時的出色穩定性。在第二部分，設計了一系列Ag-Sn二元奈米顆粒應用於CO2RR。觸媒結構主要由Ag、Ag3Sn和SnO2組成，並且有少量 Ag 氧化物和大量 SnOx在觸媒表面。表面的氧化錫，為形成甲酸的主要活性位點。根據電化學結果，Ag觸媒的一氧化碳法拉第效率為94.9 %，也不產出任何甲酸產物，而SnO2 的甲酸法拉第效率為71.0 %。有趣的是，Ag4Sn展現最佳的甲酸選擇性，在-1.0 V下，具有最高的甲酸法拉第效率為90.9 %和分電流密度為-4.64 mA cm-2。此外，在CO2RR穩定性測試中，可以保持優異的甲酸法拉第效率長達10小時，證明其對電化學CO2RR形成甲酸的潛力。基於上述結果，產物選擇性的轉變、HCOOH選擇性、活性和穩定性的優化源於Ag-Sn中金屬和氧化物在電催化中的協同作用。本研究對於通過優化Sn基奈米顆粒的組成和結構以提高CO2RR選擇性的方法提供了指引方針。

The increasing concentration of CO2 in the atmosphere leads to global warming, which greatly changes the climate and the ecological balance of nature. Therefore, electrochemical CO2 reduction reaction (CO2RR) to produce highly value-added products is regarded as a prospective path toward carbon cycling. However, the challenges in electrochemical CO2RR are its competition to hydrogen evolution reaction (HER) and unsatisfied products selectivity. Therefore, the design and preparation of highly effective catalysts with low overpotential, high faradaic efficiency (FE), and high selectivity is key consideration for the development of CO2RR technology.
This research is divided into two parts. In the first part, the binary In-Sn catalysts with In/Sn atomic ratios of 3/1 and 1/1 have been prepared. In -Sn nanoparticles (NPs) are composed of In, In(OH)3 and few InSn4 with some surface In hydroxides and Sn oxides. The catalyst with surface In hydroxide not only effectively suppresses the HER to increase the CO2RR activity but also interacts with CO2 to form In-CO3- species which then is reduced to formate. Therefore, the InxSny NPs show outstanding performance with a HCOOH FE of 92.6 % and excellent durability for 10 h at -1.0 V.
In the second part, a series of Ag-Sn binary NPs was designed for CO2RR. Structural characterizations reveal that the existence of Ag, Ag3Sn and SnO2 with few surface Ag oxides and many surface SnOx. The surface SnOx is the major active site in the formation of formate. According to the electrochemical results, Ag catalyst exhibits high CO FE of 94.9 %, without any formate products while SnO2 shows the formate FE of 71.0 %. It is interesting to find that Ag4Sn particularly shows excellent catalytic CO2RR to formate, with maximum formate FE of 90.9 % and partial current density of -4.64 mA cm-2 at the overpotential of -1.0 V (vs.RHE). Moreover, the catalytic activity of Ag4Sn remains reasonably stable over a 10 h period of electrolysis, demonstrating its potential for electrochemical CO2RR to formate. Based on the above results, the transformation of product selectivity, the optimization of the HCOOH selectivity, activity and stability stem from the synergistic effect between metals and oxides in Ag-Sn during electrocatalysis. This research provides guidelines for the improvement of CO2RR selectivity by optimization of compositions and structures of Sn-based NPs.

1. N. S. Spinner, J. A. Vega, W. Mustain, Recent progress in the electrochemical conversion and utilization of CO2. Catal. Sci. Technol. 2012, 2, 19-28.
2. Y. I. Hori, Electrochemical CO2 reduction on metal electrodes, in Modern aspects of electrochemistry. Modern aspects of electrochemistry 2008, Springer. 89-189.
3. D. T. Whipple, P. J. Kenis, Prospects of CO2 utilization via direct heterogeneous electrochemical reduction. J. Phys. Chem. Lett. 2010, 1, 3451-3458.
4. T. K. Todorova, M. W. Schreiber, and M. Fontecave, Mechanistic understanding of CO2 reduction reaction (CO2RR) toward multicarbon products by heterogeneous copper-based catalysts. ACS Catal. 2019, 10, 1754-1768.
5. H. Xiao, W. A. Goddard, T. Cheng, Y. Liu, Cu metal embedded in oxidized matrix catalyst to promote CO2 activation and CO dimerization for electrochemical reduction of CO2. Proc. Natl. Acad. Sci. 2017, 114, 6685-6688.
6. A. Liu, M. Gao, X. Ren, F. Meng, Y. Yang, L. Gao, Q. Yang, T. Ma, Current progress in electrocatalytic carbon dioxide reduction to fuels on heterogeneous catalysts. J. Mater. Chem. A 2020, 8, 3541-3562.
7. W. Zhang, Y. Hu, L. Ma, G. Zhu, Y. Wang, X. Xue, R. Chen, S. Yang, Z. Jin, Progress and perspective of electrocatalytic CO2 reduction for renewable carbonaceous fuels and chemicals. Adv. Sci. 2018. 5, 1700275.
8. D. M. Fernandes, A. F. Peixoto, and C. Freire, Nitrogen-doped metal-free carbon catalysts for (electro) chemical CO2 conversion and valorisation. Dalton Trans. 2019, 48, 13508-13528.
9. S. Zhao, S. Li, T. Guo, S. Zhang, J. Wang, Y. Wu, Y. Chen, Advances in Sn-based catalysts for electrochemical CO2 reduction. Nano-Micro Lett. 2019, 11, 1-19.
10. R. Kortlever, J. Shen, K. J. P. Schouten, F. Calle-Vallejo, and M. T. Koper, Catalysts and reaction pathways for the electrochemical reduction of carbon dioxide. J. Phys. Chem. Lett. 2015, 6, 4073-4082.
11. Peterson, Andrew A.; Norskov, Jens K., Activity descriptors for CO2 electroreduction to methane on transition-metal catalysts. J. Phys. Chem. Lett. 2012, 3, 251-258.
12. J. H. Zhou, Y. W. Zhang, Metal-based heterogeneous electrocatalysts for reduction of carbon dioxide and nitrogen: mechanisms, recent advances and perspective. React. Chem. Eng. 2018, 3, 591-625.
13. J. W. Vickers, D. Alfonso, and D. R. Kauffman, Electrochemical carbon dioxide reduction at nanostructured gold, copper, and alloy materials. Energy Technol. 2017, 5, 775-795.
14. A. Vasileff, C. Xu, Y. Jiao, Y. Zheng, S. Z. Qiao, Surface and interface engineering in copper-based bimetallic materials for selective CO2 electroreduction. Chem. 2018, 4, 1809-1831.
15. Y. B. Huang, Q. Wang, J. Liang, X. Wang, R. Cao, Soluble metal-nanoparticle-decorated porous coordination polymers for the homogenization of heterogeneous catalysis. J. Am. Chem. Soc. 2016, 138, 10104-10107.
16. J. Zhang, M. Qiao, Y. Li, Q. Shao, X. Huang, Highly active and selective electrocatalytic CO2 conversion enabled by Core/Shell Ag/(Amorphous-Sn (IV)) nanostructures with tunable shell thickness. ACS Appl. Mater. Interfaces 2019. 11, 39722-39727.
17. E. E. Benson, C. P. Kubiak, A. J. Sathrum, J. M. Smieja, Electrocatalytic and homogeneous approaches to conversion of CO2 to liquid fuels. Chem. Soc. Rev. 2009. 38, 89-99.
18. Y. Hori, H. Wakebe, T. Tsukamoto, O. Koga, Electrocatalytic process of CO selectivity in electrochemical reduction of CO2 at metal electrodes in aqueous media. Electrochim. Acta 1994, 39, 1833-1839.
19. Z. Sun, T. Ma, H. Tao, Q. Fan, B. Han, Fundamentals and challenges of electrochemical CO2 reduction using two-dimensional materials. Chem 2017, 3, 560-587.
20. J. T. Feaster, C. Shi, E.R. Cave, T. Hatsukade, D. N. Abram, K. P. Kuhl, C. Hahn, J. K. Norskov, T. F. Jaramillo, Understanding selectivity for the electrochemical reduction of carbon dioxide to formic acid and carbon monoxide on metal electrodes. Chem. 2017, 7, 4822-4827.
21. J. Wu, Y. Huang, W. Ye, Y. Li, CO2 reduction: from the electrochemical to photochemical approach. Sci. Adv. 2017. 4, 1700194.
22. D. H. Won, C. H. Choi, J. Chung, M. W. Chung, E. H. Kim, and S. I. Woo, Rational design of a hierarchical tin dendrite electrode for efficient electrochemical reduction of CO2. ChemSusChem 2015, 8, 3092-3098.
23. Z. M. Detweiler, J. L. White, S. L. Bernasek, A. B. Bocarsly, Anodized indium metal electrodes for enhanced carbon dioxide reduction in aqueous electrolyte. Langmuir 2014, 30, 7593-7600.
24. H. Zhang, Y. Ma, F. Quan, J. Huang, F. Jia, L. Zhang, Selective electro-reduction of CO2 to formate on nanostructured Bi from reduction of BiOCl nanosheets. Electrochem. Commun. 2014, 46, 63-66.
25. B. Innocent, D. Pasquier, F. Ropital, F. Hahn, J. M. Léger, and K. Kokoh, FTIR spectroscopy study of the reduction of carbon dioxide on lead electrode in aqueous medium. Appl. Catal. B 2010, 94, 219-224.
26. F. Cai, D. Gao, R. Si, Y. Ye, T. He, S. Miao, G. Wang, X. Bao, Effect of metal deposition sequence in carbon-supported Pd–Pt catalysts on activity towards CO2 electroreduction to formate. Electrochem. Commun. 2017, 76, 1-5.
27. J. Wu, F. G. Risalvato, S. Ma, and X. D. Zhou, Electrochemical reduction of carbon dioxide III. The role of oxide layer thickness on the performance of Sn electrode in a full electrochemical cell. J. Mater. Chem. A 2014, 2, 1647-1651.
28. B. Kumar, V. Atla, J. P. Brian, S. Kumari, T. Q. Nguyen, M. Sunkara, J. M. Spurgeon, Reduced SnO2 porous nanowires with a high density of grain boundaries as catalysts for efficient electrochemical CO2‐into‐HCOOH conversion. Angew. Chem. Int. Ed. 2017, 56, 3645-3649.
29. J. Wang, S. Ning, M. Luo, D. Xiang, W. Chen, X. Kang, Z. Jiang, S. Chen, In-Sn alloy core-shell nanoparticles: In-doped SnOx shell enables high stability and activity towards selective formate production from electrochemical reduction of CO2. Appl. Catal. B 2021, 288, 119979.
30. W. Luc, C. Collins, S. Wang, H. Xin, K. He, Y. Kang, F. Jiao, Ag–Sn bimetallic catalyst with a core–shell structure for CO2 reduction. J. Am. Chem. Soc. 2017, 139, 1885-1893.
31. S. Zhang, P. Kang, T. J. Meyer, Nanostructured tin catalysts for selective electrochemical reduction of carbon dioxide to formate. J. Am. Chem. Soc. 2014, 136, 1734-1737.
32. Q. Shao, P. Wang, X. Huang, Opportunities and challenges of interface engineering in bimetallic nanostructure for enhanced electrocatalysis. Adv. Funct. Mater. 2019, 29, 1806419.
33. J. He, K. E. Dettelbach, D. A. Salvatore, T. Li, C. P. Berlinguette, High‐throughput synthesis of mixed‐metal electrocatalysts for CO2 reduction. Angew. Chem. Int. Ed. 2017, 129, 6164-6168.
34. S. Rasul, D. H. Anjum, A. Jedidi, Y. Minenkov, L. Cavallo, K. Takanabe, A highly selective copper–indium bimetallic electrocatalyst for the electrochemical reduction of aqueous CO2 to CO. Angew. Chem. Int. Ed. 2015, 127, 2174-2178.
35. G. O. Larrazábal, A. J. Martín, S. Mitchell, R. Hauert, J. Pérez-Ramírez, Synergistic effects in silver–indium electrocatalysts for carbon dioxide reduction. J. Catal. 2016, 266-277.
36. J. Li, M. Zhu, and Y. F. Han, Recent Advances in Electrochemical CO2 Reduction on Indium‐Based Catalysts. ChemCatChem 2021, 13, 514-531.
37. S. Gao, Y. Lin, X. Jiao, Y. Sun, Q. Luo, W. Zhang, D. Li, J. Yang, Y. Xie, Partially oxidized atomic cobalt layers for carbon dioxide electroreduction to liquid fuel. Nature 2016. 529, 68-71.
38. S. Huo, Z. Weng, Z. Wu, Y. Zhong, Y. Wu, J. Fang, H. Wang, Coupled metal/oxide catalysts with tunable product selectivity for electrocatalytic CO2 reduction. ACS Appl. Mater. Interfaces 2017, 9, 28519-28526.
39. Q. Li, J. Fu, W. Zhu, Z. Chen, B. Shen, L. Wu, Z. Xi, T. L. Wang, J. J. Zhu, Tuning Sn-catalysis for electrochemical reduction of CO2 to CO via the core/shell Cu/SnO2 structure. J. Am. Chem. Soc. 2017, 139, 4290-4293.
40. D. Gao, Y. Zhang, Z. Zhou, F. Cai, X. Zhao, W. Huang, Y. Li, J. Zhu, P. Liu, F. Yang, Enhancing CO2 electroreduction with the metal–oxide interface. J. Am. Chem. Soc. 2017, 139, 5652-5655.
41. G. n. O. Larrazábal, A. J. Martín, S. Mitchell, R. Hauert, J. Pérez-Ramírez, Enhanced reduction of CO2 to CO over Cu–In electrocatalysts: catalyst evolution is the key. ACS Catal. 2016, 6, 6265-6274.
42. M. Schreier, F. Héroguel, L. Steier, S. Ahmad, J. S. Luterbacher, M. T. Mayer, J. Luo, M. Grätzel, Solar conversion of CO2 to CO using Earth-abundant electrocatalysts prepared by atomic layer modification of CuO. Nature Energy 2017, 2, 1-9.
43. Z. Cai, Y. Wu, Z. Wu, L. Yin, Z. Weng, Y. Zhong, W. Xu, X. Sun, H. Wang, Unlocking Bifunctional Electrocatalytic Activity for CO2 Reduction Reaction by Win-Win Metal–Oxide Cooperation. ACS Energy Lett. 2018, 3, 2816-2822.
44. Q. Lai, N. Yang, G. Yuan, Highly efficient In–Sn alloy catalysts for electrochemical reduction of CO2 to formate. Electrochem. Commun. 2017, 83, 24-27.
45. I. Berregi, G. Del Campo, R. Caracena, and J. Miranda, Quantitative determination of formic acid in apple juices by 1H NMR spectrometry. Talanta 2007, 72, 1049-1053.
46. J. L. White, A. Bocarsly, Enhanced carbon dioxide reduction activity on indium-based nanoparticles. J. Electrochem. Soc. 2016, 163, H410.
47. Z. Bitar, A. Fecant, E. Trela-Baudot, S. Chardon-Noblat, D. Pasquier, Electrocatalytic reduction of carbon dioxide on indium coated gas diffusion electrodes—Comparison with indium foil. Appl. Catal. B 2016, 189, 172-180.
48. J. E. Pander III, M. F. Baruch, A. Bocarsly, Probing the mechanism of aqueous CO2 reduction on post-transition-metal electrodes using ATR-IR spectroelectrochemistry. ACS Catal. 2016, 6, 7824-7833.
49. B. Zha, C. Li, J. Li, Efficient electrochemical reduction of CO2 into formate and acetate in polyoxometalate catholyte with indium catalyst. J. Catal. 2020, 382, 69-76.
50. W. J. Dong, C. J. Yoo, J. L. Lee, Monolithic nanoporous In–Sn alloy for electrochemical reduction of carbon dioxide. ACS Appl. Mater. Interfaces 2017, 9, 43575-43582.
51. Y. Chen, M. W. Kanan, Tin oxide dependence of the CO2 reduction efficiency on tin electrodes and enhanced activity for tin/tin oxide thin-film catalysts. J. Am. Chem. Soc. 2012, 134, 1986-1989.
52. F. Li, L. Chen, G. P. Knowles, D. R. MacFarlane, J. Zhang, Hierarchical mesoporous SnO2 nanosheets on carbon cloth: a robust and flexible electrocatalyst for CO2 reduction with high efficiency and selectivity. Angew. Chem. Int. Ed. 2017, 129, 520-524.
53. G. Liu, Z. Li, J. Shi, K. Sun, Y. Ji, Z. Wang, Y. Qiu, Y. Liu, Z. Wang, P. Hu, Black reduced porous SnO2 nanosheets for CO2 electroreduction with high formate selectivity and low overpotential. Appl. Catal. B 2020, 260, 118134.
54. H. Liang, H. Xi, S. Liu, X. Zhang, H. Liu, Modulation of oxygen vacancy in tungsten oxide nanosheets for Vis-NIR light-enhanced electrocatalytic hydrogen production and anticancer photothermal therapy. Nanoscale 2019, 11. 18183-18190.
55. M. F. Baruch, J. E. Pander III, J. L. White, A. B. Bocarsly, Mechanistic insights into the reduction of CO2 on tin electrodes using in situ ATR-IR spectroscopy. ACS Catal. 2015, 5, 3148-3156.
56. R. Zhang, W. Lv, L. Lei, Role of the oxide layer on Sn electrode in electrochemical reduction of CO2 to formate. Appl. Surf. Sci. 2015, 356, 24-29.
57. B. A. Manning, S. R. Kanel, E. Guzman, S. W. Brittle, I. E. Pavel, Oxidative dissolution of silver nanoparticles by synthetic manganese dioxide investigated by synchrotron X-ray absorption spectroscopy. J. Nanopart. Res. 2019, 21, 1-14.
58. T. Vidaković, Kinetics of methanol electrooxidation on PtRu catalysts in a membrane electrode assembly. 2005, Otto-von-Guericke-Universität Magdeburg.
59. H. A. Gasteiger, N. Marković, P. N. Ross Jr, E. Cairns, Electro-oxidation of small organic molecules on well-characterized Pt-Ru alloys. Electrochim. Acta 1994, 39, 1825-1832.
60. X. Wang, W. Xiao, J. Zhang, Z. Wang, X. Jin, Nanoporous Ag-Sn derived from codeposited AgCl-SnO2 for the electrocatalytic reduction of CO2 with high formate selectivity. Electrochem. Commun. 2019, 102, 52-56.
61. Y. W. Choi, F. Scholten, I. Sinev, B. Roldan Cuenya, Enhanced Stability and CO/Formate Selectivity of Plasma-Treated SnOx/AgOx Catalysts during CO2 Electroreduction. J. Am. Chem. Soc. 2019, 141, 5261-5266.