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Catalysis Selects Its Own Favorite Facets
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Catalysis Selects Its Own Favorite Facets Ruixuan Qin1 and Nanfeng Zheng1,* Catalytic performance of metal nanocrystals in many reactions is determined by their exposed facets. In a recent issue of Joule, Cui and colleagues report a novel self-selective strategy that utilizes CO2 electroreduction itself to smartly pick up the best catalytic facets for producing high-performance CO2 reduction catalysts.
The exposed surface of metal catalysts plays a crucial role in determining their overall (electro)catalytic performance, such as activity and selectivity, creating the so-called ‘‘facet effect.’’ The facet effect has been recently well-demonstrated in the metal-catalyzed electroreduction reaction of CO2 (CO2RR). For example, a small portion of Cu high-index stepped facets is demonstrated as the main contributor to the overall CO2RR activity.1 Studies based on both crystalline and polycrystalline Cu catalysts also reveal that different facets of Cu vary the product selectivity. Although Cu(111) produces more CH4 than Cu(100), stepped facets exhibit an activity one order higher than that of Cu(111) toward the C-C coupled products.2 The optimized facets for CO2RR vary with metal compositions and experimental conditions as well.3 For instance, the high-index facets of Bi, such as (012) and (104), exhibit high performance for the selective production of formate.4 Both single-crystal and thin-film studies revealed that Ag(110) having the larger density of undercoordinated sites exhibited higher activity than Ag(111) and Ag(100) in CO2 reduction to CO.5,6 Most of heterogeneous metal catalysts used nowadays are prepared by the conventional methods such as impregnation, co-precipitation and deposition-precipitation, electrodeposition,
etc. These methods usually yield metal nanoparticles of Wulff shapes enclosed by low-energy facets.7 To better utilize the facet effect, researchers have made tremendous efforts in the past two decades to develop wetchemical methods for preparing metal nanoparticles of specific facets. Binding agents (e.g., surfactants and small coordinating molecules) are often used to selectively reduce the surface energy of certain facets and thus induce the formation of well-defined exposure surfaces, deviating the produced nanoparticles from the Wulff construction.8,9 Then a typical protocol to identify the best facet for the desired catalytic reactions is to compare catalytic performances of pre-made metal nanocrystals bound by different facets. In a recent issue of Joule, Cui and colleagues demonstrated a smart strategy to self-select the best facets for CO2RR.10 In the developed strategy, CO2 was introduced in the course of the electroreductive deposition of Pb and Cu catalysts. The CO2 reduction intermediates generated in situ readily served as effective binding agents to induce the formation of high-performance catalytic facets for CO2RR. As shown in Figure 1, when the electrodeposition of Pb on three-dimensional
carbon fiber paper was carried out under Ar and CO2 atmosphere, irregular and uniform octahedral Pb particles enclosed by eight Pb(111) facets were obtained, respectively. Regardless of the Pb precursor, electrolyte, and electrodeposition conditions, octahedral Pb particles were always obtained when CO2 is introduced, suggesting the importance of CO2 in the uniform formation of Pb(111). Pb is one of the best candidates for the production of formate because of the stronger binding of O sites than C sites on Pb. However, different Pb facets still exhibit different catalytic activity.3 The formate partial current density (jformate) of the Pb catalyst obtained with the presence of CO2 (SELF-CAT-Pb) reached 22 mA/cm2 at 1.2 V versus reversible hydrogen electrode (RHE). This number was more than twice of that of the normal Pb catalyst obtained in the absence of CO2 although both catalysts had the similar electrochemical active surface area (ESCA). Moreover, SELFCAT-Pb exhibited an enhanced formate Faradaic efficiency (FE) of 90.5%, much higher than 69% over the normal Pb catalyst. An excellent stability of SELFCAT-Pb was also observed during the 6-hour test at 1.2 V versus RHE. In the study, the authors applied DFT calculations to illustrate why Pb(111) was self-selected by CO2RR and performed the best in the reaction. Although HCOOH is typically produced through the intermediate of COOH or OCHO, their DFT calculations illustrated that OCHO is thermodynamically favored over different Pb
1State
Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and National & Local Joint Engineering Research Center for Preparation Technology of Nanomaterials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2019.07.011
Chem 5, 1924–1937, August 8, 2019 ª 2019 Elsevier Inc. 1935
CO2 atmosphere. By mapping the surface energies of the three facets with 0%–25% coverage of OCHO under different applied potentials, the Wulff shape evolution was evaluated. The analysis revealed that the formation of Pb(111) was indeed favored by the presence of CO2, especially under applied potential lower than 0.5 V versus RHE. The self-selective strategy was also successfully applied to produce highperformance Cu catalysts for CO2RR. Compared with the normal Cu catalyst, a larger portion of Cu steps is observed when the electrodeposition of Cu was performed under CO2 atmosphere. In Cu-catalyzed CO2RR, CO is a thermodynamically favored intermediate.2 The CO intermediate binds more strongly on Cu step site,1 thus leading to the formation of abundant Cu step sites in the presence of CO2. The resultant SELFCAT-Cu catalyst also exhibited an enhanced performance over the normal Cu catalyst obtained in the absence of CO2, although both of them had a similar ECSA. SELF-CAT-Cu exhibited FEs of 54.5% toward formate at 0.7 V versus RHE and 42.2% toward C2+ products at 1.0 V versus RHE. In comparison, FEs of the normal Cu catalyst toward formate and C2+ products were only 27% and 20.9%, respectively, under the same conditions. Figure 1. Influence of CO2 on the Morphology and CO2RR Performance of Pb- and Cu-Based Catalysts Fabricated by Electrodeposition (A) Schematic illustration of the electroreductive formation of Pb nanoparticles of different shapes and exposed surfaces under Ar and CO 2 atmospheres. (B and C) SEM images of normal Pb and SELF-CAT-Pb. Scale bar, 1 mm; insert: 200 nm. (D) Comparison of formate partial current density using geometric surface area of lead-based catalysts under various potentials. (E and F) SEM images of normal Cu and SELF-CAT-Cu. Scale bar, 500 nm; insert, 100 nm. (G) Comparison of FEs of reduction products for copper-based catalysts under various potentials. Figures adapted from Cui and colleagues. 10
facets such as (111), (100), and (211). But OCHO binds more strongly on (111) than on (100) or (211). The limiting potentials over Pb(111), (100), and (211) were estimated to be 0.42, 0.86, and 0.78 V versus
1936 Chem 5, 1924–1937, August 8, 2019
RHE, respectively. The strongest binding of OCHO on Pb(111) not only makes Pb(111) serve as the most effective facet for CO2RR but also thermodynamically selects Pb(111) during the electrodeposition of Pb under
In summary, the work has nicely demonstrated a novel self-selective strategy to utilize the electrocatalytic CO2 reduction itself for controlling the formation of exposed surfaces of metal catalysts during their electrodeposition. The selective strong binding of the CO2 reduction intermediates on specific metal facets was proposed as the main driving force for the selective process. This work is expected to stimulate more research effort on the development of new catalyst preparation methodologies by introducing reactants or intermediates of a targeted catalytic reaction, which should not be limited to
electrocatalysis, to facilitate the formation of desired catalytic facets. 1. Liu, X., Xiao, J., Peng, H., Hong, X., Chan, K., and Nørskov, J.K. (2017). Understanding trends in electrochemical carbon dioxide reduction rates. Nat. Commun. 8, 15438.
electrochemical CO2 reduction. Chem. Rev. 119, 6631–6669. 4. Koh, J.H., Won, D.H., Eom, T., Kim, N.-K., Jung, K.D., Kim, H., Hwang, Y.J., and Min, B.K. (2017). Facile CO2 electro-reduction to formate via oxygen bidentate intermediate stabilized by high-index planes of Bi dendrite catalyst. ACS Catal. 7, 5071.
2. Nitopi, S., Bertheussen, E., Scott, S.B., Liu, X., Engstfeld, A.K., Horch, S., Seger, B., Stephens, I.E.L., Chan, K., Hahn, C., et al. (2019). Progress and perspectives of electrochemical CO2 reduction on copper in aqueous electrolyte. Chem. Rev. 119, 7610– 7672.
5. Clark, E.L., Ringe, S., Tang, M., Walton, A., Hahn, C., Jaramillo, T.F., Chan, K., and Bell, A.T. (2019). Influence of atomic surface structure on the activity of Ag for the electrochemical reduction of CO2 to CO. ACS Catal. 9, 4006–4016.
3. Xu, S., and Carter, E.A. (2019). Theoretical insights into heterogeneous (Photo)
6. Hoshi, N., Kato, M., and Hori, Y. (1997). Electrochemical reduction of CO2 on single crystal electrodes of silver Ag(111), Ag(100),
and Ag(110). J. Electroanal. Chem. 440, 283–286. 7. Bell, A.T. (2003). The impact of nanoscience on heterogeneous catalysis. Science 299, 1688–1691. 8. Liu, P., Qin, R., Fu, G., and Zheng, N. (2017). Surface coordination chemistry of metal nanomaterials. J. Am. Chem. Soc. 139, 2122– 2131. 9. Tao, A.R., Habas, S., and Yang, P. (2008). Shape control of colloidal metal nanocrystals. Small 4, 310–325. 10. Wang, H., Liang, Z., Tang, M., Chen, G., Li, Y., Chen, W., Lin, D., Zhang, Z., Zhou, G., Li, J., Lu, Z., et al. (2019). Self-selective catalyst synthesis for CO2 reduction. Joule 3, https:// doi.org/10.1016/j.joule.2019.05.023.
Chem 5, 1924–1937, August 8, 2019 1937
Catalysis Selects Its Own Favorite Facets Ruixuan Qin1 and Nanfeng Zheng1,* Catalytic performance of metal nanocrystals in many reactions is determined by their exposed facets. In a recent issue of Joule, Cui and colleagues report a novel self-selective strategy that utilizes CO2 electroreduction itself to smartly pick up the best catalytic facets for producing high-performance CO2 reduction catalysts.
The exposed surface of metal catalysts plays a crucial role in determining their overall (electro)catalytic performance, such as activity and selectivity, creating the so-called ‘‘facet effect.’’ The facet effect has been recently well-demonstrated in the metal-catalyzed electroreduction reaction of CO2 (CO2RR). For example, a small portion of Cu high-index stepped facets is demonstrated as the main contributor to the overall CO2RR activity.1 Studies based on both crystalline and polycrystalline Cu catalysts also reveal that different facets of Cu vary the product selectivity. Although Cu(111) produces more CH4 than Cu(100), stepped facets exhibit an activity one order higher than that of Cu(111) toward the C-C coupled products.2 The optimized facets for CO2RR vary with metal compositions and experimental conditions as well.3 For instance, the high-index facets of Bi, such as (012) and (104), exhibit high performance for the selective production of formate.4 Both single-crystal and thin-film studies revealed that Ag(110) having the larger density of undercoordinated sites exhibited higher activity than Ag(111) and Ag(100) in CO2 reduction to CO.5,6 Most of heterogeneous metal catalysts used nowadays are prepared by the conventional methods such as impregnation, co-precipitation and deposition-precipitation, electrodeposition,
etc. These methods usually yield metal nanoparticles of Wulff shapes enclosed by low-energy facets.7 To better utilize the facet effect, researchers have made tremendous efforts in the past two decades to develop wetchemical methods for preparing metal nanoparticles of specific facets. Binding agents (e.g., surfactants and small coordinating molecules) are often used to selectively reduce the surface energy of certain facets and thus induce the formation of well-defined exposure surfaces, deviating the produced nanoparticles from the Wulff construction.8,9 Then a typical protocol to identify the best facet for the desired catalytic reactions is to compare catalytic performances of pre-made metal nanocrystals bound by different facets. In a recent issue of Joule, Cui and colleagues demonstrated a smart strategy to self-select the best facets for CO2RR.10 In the developed strategy, CO2 was introduced in the course of the electroreductive deposition of Pb and Cu catalysts. The CO2 reduction intermediates generated in situ readily served as effective binding agents to induce the formation of high-performance catalytic facets for CO2RR. As shown in Figure 1, when the electrodeposition of Pb on three-dimensional
carbon fiber paper was carried out under Ar and CO2 atmosphere, irregular and uniform octahedral Pb particles enclosed by eight Pb(111) facets were obtained, respectively. Regardless of the Pb precursor, electrolyte, and electrodeposition conditions, octahedral Pb particles were always obtained when CO2 is introduced, suggesting the importance of CO2 in the uniform formation of Pb(111). Pb is one of the best candidates for the production of formate because of the stronger binding of O sites than C sites on Pb. However, different Pb facets still exhibit different catalytic activity.3 The formate partial current density (jformate) of the Pb catalyst obtained with the presence of CO2 (SELF-CAT-Pb) reached 22 mA/cm2 at 1.2 V versus reversible hydrogen electrode (RHE). This number was more than twice of that of the normal Pb catalyst obtained in the absence of CO2 although both catalysts had the similar electrochemical active surface area (ESCA). Moreover, SELFCAT-Pb exhibited an enhanced formate Faradaic efficiency (FE) of 90.5%, much higher than 69% over the normal Pb catalyst. An excellent stability of SELFCAT-Pb was also observed during the 6-hour test at 1.2 V versus RHE. In the study, the authors applied DFT calculations to illustrate why Pb(111) was self-selected by CO2RR and performed the best in the reaction. Although HCOOH is typically produced through the intermediate of COOH or OCHO, their DFT calculations illustrated that OCHO is thermodynamically favored over different Pb
1State
Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and National & Local Joint Engineering Research Center for Preparation Technology of Nanomaterials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2019.07.011
Chem 5, 1924–1937, August 8, 2019 ª 2019 Elsevier Inc. 1935
CO2 atmosphere. By mapping the surface energies of the three facets with 0%–25% coverage of OCHO under different applied potentials, the Wulff shape evolution was evaluated. The analysis revealed that the formation of Pb(111) was indeed favored by the presence of CO2, especially under applied potential lower than 0.5 V versus RHE. The self-selective strategy was also successfully applied to produce highperformance Cu catalysts for CO2RR. Compared with the normal Cu catalyst, a larger portion of Cu steps is observed when the electrodeposition of Cu was performed under CO2 atmosphere. In Cu-catalyzed CO2RR, CO is a thermodynamically favored intermediate.2 The CO intermediate binds more strongly on Cu step site,1 thus leading to the formation of abundant Cu step sites in the presence of CO2. The resultant SELFCAT-Cu catalyst also exhibited an enhanced performance over the normal Cu catalyst obtained in the absence of CO2, although both of them had a similar ECSA. SELF-CAT-Cu exhibited FEs of 54.5% toward formate at 0.7 V versus RHE and 42.2% toward C2+ products at 1.0 V versus RHE. In comparison, FEs of the normal Cu catalyst toward formate and C2+ products were only 27% and 20.9%, respectively, under the same conditions. Figure 1. Influence of CO2 on the Morphology and CO2RR Performance of Pb- and Cu-Based Catalysts Fabricated by Electrodeposition (A) Schematic illustration of the electroreductive formation of Pb nanoparticles of different shapes and exposed surfaces under Ar and CO 2 atmospheres. (B and C) SEM images of normal Pb and SELF-CAT-Pb. Scale bar, 1 mm; insert: 200 nm. (D) Comparison of formate partial current density using geometric surface area of lead-based catalysts under various potentials. (E and F) SEM images of normal Cu and SELF-CAT-Cu. Scale bar, 500 nm; insert, 100 nm. (G) Comparison of FEs of reduction products for copper-based catalysts under various potentials. Figures adapted from Cui and colleagues. 10
facets such as (111), (100), and (211). But OCHO binds more strongly on (111) than on (100) or (211). The limiting potentials over Pb(111), (100), and (211) were estimated to be 0.42, 0.86, and 0.78 V versus
1936 Chem 5, 1924–1937, August 8, 2019
RHE, respectively. The strongest binding of OCHO on Pb(111) not only makes Pb(111) serve as the most effective facet for CO2RR but also thermodynamically selects Pb(111) during the electrodeposition of Pb under
In summary, the work has nicely demonstrated a novel self-selective strategy to utilize the electrocatalytic CO2 reduction itself for controlling the formation of exposed surfaces of metal catalysts during their electrodeposition. The selective strong binding of the CO2 reduction intermediates on specific metal facets was proposed as the main driving force for the selective process. This work is expected to stimulate more research effort on the development of new catalyst preparation methodologies by introducing reactants or intermediates of a targeted catalytic reaction, which should not be limited to
electrocatalysis, to facilitate the formation of desired catalytic facets. 1. Liu, X., Xiao, J., Peng, H., Hong, X., Chan, K., and Nørskov, J.K. (2017). Understanding trends in electrochemical carbon dioxide reduction rates. Nat. Commun. 8, 15438.
electrochemical CO2 reduction. Chem. Rev. 119, 6631–6669. 4. Koh, J.H., Won, D.H., Eom, T., Kim, N.-K., Jung, K.D., Kim, H., Hwang, Y.J., and Min, B.K. (2017). Facile CO2 electro-reduction to formate via oxygen bidentate intermediate stabilized by high-index planes of Bi dendrite catalyst. ACS Catal. 7, 5071.
2. Nitopi, S., Bertheussen, E., Scott, S.B., Liu, X., Engstfeld, A.K., Horch, S., Seger, B., Stephens, I.E.L., Chan, K., Hahn, C., et al. (2019). Progress and perspectives of electrochemical CO2 reduction on copper in aqueous electrolyte. Chem. Rev. 119, 7610– 7672.
5. Clark, E.L., Ringe, S., Tang, M., Walton, A., Hahn, C., Jaramillo, T.F., Chan, K., and Bell, A.T. (2019). Influence of atomic surface structure on the activity of Ag for the electrochemical reduction of CO2 to CO. ACS Catal. 9, 4006–4016.
3. Xu, S., and Carter, E.A. (2019). Theoretical insights into heterogeneous (Photo)
6. Hoshi, N., Kato, M., and Hori, Y. (1997). Electrochemical reduction of CO2 on single crystal electrodes of silver Ag(111), Ag(100),
and Ag(110). J. Electroanal. Chem. 440, 283–286. 7. Bell, A.T. (2003). The impact of nanoscience on heterogeneous catalysis. Science 299, 1688–1691. 8. Liu, P., Qin, R., Fu, G., and Zheng, N. (2017). Surface coordination chemistry of metal nanomaterials. J. Am. Chem. Soc. 139, 2122– 2131. 9. Tao, A.R., Habas, S., and Yang, P. (2008). Shape control of colloidal metal nanocrystals. Small 4, 310–325. 10. Wang, H., Liang, Z., Tang, M., Chen, G., Li, Y., Chen, W., Lin, D., Zhang, Z., Zhou, G., Li, J., Lu, Z., et al. (2019). Self-selective catalyst synthesis for CO2 reduction. Joule 3, https:// doi.org/10.1016/j.joule.2019.05.023.
Chem 5, 1924–1937, August 8, 2019 1937