Porous Metal Nanocrystal Catalysts: Can Crystalline Porosity Enable Catalytic Selectivity?

Abstract

Open AccessCCS ChemistryMINI REVIEW6 Jun 2022Porous Metal Nanocrystal Catalysts: Can Crystalline Porosity Enable Catalytic Selectivity? Xiaowen Min, Hao Lv, Yusuke Yamauchi and Ben Liu Xiaowen Min Key Laboratory of Green Chemistry and Technology of the Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064 , Hao Lv Key Laboratory of Green Chemistry and Technology of the Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064 , Yusuke Yamauchi Australian Institute for Bioengineering and Nanotechnology (AIBN), School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072 JST-ERATO Yamauchi Materials Space-Tectonics Project, International Research Centre for Materials Nanoarchitechtonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044 and Ben Liu *Corresponding author: E-mail Address: [email protected] Key Laboratory of Green Chemistry and Technology of the Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064 https://doi.org/10.31635/ccschem.022.202201892 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Catalytic selectivity is a central issue in the efficiency of catalytic processes. A large number of strategies have been developed to improve the catalytic selectivity of metal catalysts at the atomic and molecular levels, for instance, alloying secondary elements, fabricating metal-support interactions, and introducing surface ligands. Recently, macro/mesoscopic pores and cavities have been demonstrated as an alternative route to promote catalytic selectivity of metal nanocrystal catalysts. The promotion effects of continuous crystalline porosity include (1) more catalytically active sites that accelerate the favorable catalytic routes to targeted products, (2) confined spaces that increase the retention time of key intermediates and remarkably promote the selective catalysis toward desired products, and (3) an optimized electronic structure and coordination environment of active metal sites that tailor the reaction trends of selective catalysis toward desired products. In this minireview, we summarize recent advances in porosity-enabled catalytic selectivity of metal nanocrystal catalysts with focused discussions of CO2 reduction electrocatalysis and selective hydrogenation reactions. The mechanisms that allow for the continuous porosity that enables the catalytic selectivity of metal nanocrystal catalysts are discussed in detail. We end this minireview by proposing current challenges and offering future opportunities in this research field. Download figure Download PowerPoint Introduction Nanostructured metal nanocrystals have attracted dramatically increased attention in nanoscience and nanotechnology. A library of metal nanostructures with precisely controlled morphology, size, and composition is playing a key role in the design of novel functional nanomaterials for their wide application. Porous metal nanocrystals with a continuous crystalline framework are a new kind of nanostructured material. They have been widely studied as novel catalysts for their applications in catalysis and electro/photocatalysis.1–7 Not only do the abundant and highly penetrated pores/cavities tremendously enlarge catalytically active sites, but the continuous crystalline frameworks also facilitate electron transfer and stabilize the metal catalysts by inhibiting the Ostwald ripening process, creating the structural add-in synergies to exceptionally enhance catalytic activity and stability of porous metal nanocrystal catalysts (Figure 1).8–16 For instance, mesoporous palladium (Pd) nanocrystals displayed remarkably enhanced activity and stability in the hydrogenation of styrene, whose activity was 3.1- and 2.1-fold higher than the activity of commercial Pd nanoparticles and Pd black, respectively.17 Similarly, multimetallic PdAgCu nanoparticles with abundant pores and interior hollow cavities exhibited superior ethanol electrooxidation performance, which was 1.58–6.18 times higher than that of its counterpart catalysts.18 Furthermore, the continuous crystalline porosity of plasmonic Au/Ag nanocrystals produced multiple "hot spots" around the pores and thus enabled significant coupling of localized surface plasmon resonance, resulting in enhanced performance in light scattering and surface-enhanced Raman scattering.14,19,20 Figure 1 | Schematic illustrations of physiochemical properties and catalytic performances (activity, stability, and selectivity) of porous metal nanocrystal catalysts. Download figure Download PowerPoint Along with high activity and stability, catalytic selectivity is another essential issue in promoting the performance of catalytic reactions. An ideal catalyst is indeed not only highly active and stable for catalysis but also extremely selective toward targeted products. Generally, to promote selective catalysis, metal catalysts need to be further modified to optimize the surface coordination structures of metal active sites at atomic and/or molecular levels. These strategies include the formation of metal alloys by introducing main and/or transition group elements (even including nonmetals and metalloids), the fabrication of metal-support interaction by loading on functional supports, the modification of functional organic ligands, and so on.21–28 In parallel, continuous porosity of metal nanocrystal catalysts optimizes the surface electronic structure of metal sites and endows a confined space for key intermediates in mesoscopic levels, thus performing well in selective catalysis (Figure 1). One of the pioneering studies reported by Lu et al.29 demonstrated that porous Ag nanocrystal catalysts were highly selective for promoting the electrocatalytic CO2 reduction reaction (CO2RR) while inhibiting its competitive hydrogen evolution reaction (HER). Very recently, our group found that continuous porosity weakened the chemisorption capacity of alkenes within porous palladium-phosphorous (PdP) nanocrystal catalysts and selectively promoted the semihydrogenation of alkynes to industrially important alkenes.30 Overall, the innovation in tuning catalytic selectivity of metal nanocrystal catalysts by porosity engineering is still in its infancy, and there are many issues to be understood further and many to be discovered. In this minireview, we focus on the crystalline porosity-enabled catalytic selectivity of metal catalysts with continuous crystalline frameworks. Both macroporous/mesoporous metal nanocrystals and hollow metal nanocrystals are referenced. We start our discussions with electrocatalytic CO2RR and demonstrate how the continuous crystalline porosity (pores and cavities) selectively promotes CO2RR (to CO and even to the value-added C2+ products) and inhibits their competitive HER. We highlight the fact that continuous porosity not only tunes the surface electronic structures of porous/cavity metal nanocrystal catalysts for energetically changing reaction trends but also provides the confined pore/cavity spaces for the stabilization of key intermediates, which synergistically promote CO2RR electrocatalysis toward desired CO (over Ag, Au, etc.) and C2+ products (over Cu/Cu2O catalysts). Selective hydrogenation reactions are then discussed as another example to reveal the importance of continuous crystalline porosity in promoting catalytic selectivity of metal nanocrystal catalysts. In the end, we describe the challenges and future opportunities in this research field. However, supported metal nanocrystals within porous frameworks (e.g., zeolites and metal–organic frameworks and their derivatives) and their enabled catalytic selectivity have not been covered in this minireview. Readers are encouraged to refer to the relevant excellent reviews for more detail.31–34 Crystalline Porosity-Enabled Selectivity in CO2RR Electrocatalysis CO2RR versus HER Electrocatalytic reduction of CO2 can not only obtain the value-added chemicals and/or fuels (CO and other hydrocarbons (C2+) products) but also be an important step to close the environmental carbon cycle.35 Generally, electrocatalytic CO2RR happens in aqueous electrolytes (for example: CO2 + 2 H+ + 2 e− → CO + H2O). Competitively, the reduction of protons to H2 (known as the HER, 2 H+ + 2 e−→ H2) is kinetically more favorable, thus possibly inhibiting the selectivity of CO2RR electrocatalysis. Theoretically, developing new strategies that can promote the CO2RR and/or deactivate the HER is of crucial importance for practical applications of CO2RR electrocatalysis. Among various strategies, engineering nanoscale porosity of metal nanocrystal catalysts at macro/mesoscopic levels has recently been confirmed as an effective route to catalyze CO2RR electrocatalysis. Herein, we start the discussion of how continuous crystalline porosity enables catalytic selectivity of metal nanocrystal catalysts from controlling the competitive reactions of CO2RR and HER. In a pioneering work, Lu et al.29 found that nanoporous Ag (np-Ag) nanocrystal catalysts were highly efficient in promoting CO2RR electrocatalysis. The np-Ag catalysts with the framework thickness of 50–200 nm and the pore size of 100 nanometers were first prepared through a classic alloying-dealloying method (Figure 2a). The authors compared the surface areas and the active sites of np-Ag with nonporous Ag polycrystalline and confirmed that crystalline porosity had two important effects in CO2RR electrocatalysis (Figure 2b). On the one hand, the penetrated nanopores tremendously enlarged the surface area of active Ag sites, 150 times higher than nonporous Ag polycrystalline. On the other hand, the highly curved surface of np-Ag produced the crystalline steps and exposed the high-density active sites that remarkably activated the catalysts more than 20 times over its nonporous counterpart. When investigating the catalyst in CO2-saturated 0.5 M KHCO3, the authors found that np-Ag achieved a stable current of 18 mA cm−2 and a superior CO Faradaic efficiency of approximately 92% at the potential of −0.6 V (vs the reversible hydrogen electrode) (Figure 2c). The CO Faradaic efficiency was still as high as 90% (9.0 mA cm−2) at −0.5 V and 79% (3.3 mA cm−2) at −0.4 V, respectively. In comparison, nonporous Ag favorably catalyzed the HER; its CO Faradaic efficiency and current density were only 1.1% and 0.47 mA cm−2, respectively. The authors further performed the Tafel analysis (Figure 2d). A very low Tafel plot of 58 mV dec−1 in the low overpotentials suggested a faster electrocatalytic kinetics of np-Ag for CO2RR. Meanwhile, the sharp decrease in Tafel plots at the high overpotentials was seen. The authors deduced that crystalline porosity facilitated the formation of CO2− intermediates (CO2 + e− → CO2−) and accelerated the reaction of CO2− into CO (CO2− + 2H+ + e− → CO + H2O). Although this work was conceptual, it definitely highlighted the structural synergies of crystalline porosity in promoting CO2RR electrocatalysis of metal catalysts. Figure 2 | (a) TEM image and (b) corresponding schematic illustration of nanoporous silver (np-Ag) catalyst. (c) Electrocatalytic CO2RR performance of np-Ag and nonporous Ag collected in CO2-saturated 0.5 M KHCO3 at −0.6 V. (d) Tafel analysis of np-Ag and nonporous Ag. (e) SEM image of porous Au film. (f) Electrocatalytic CO2RR performances of porous Au films with different thicknesses. (a–d) Adapted with permission from ref 29. Copyright 2014 Springer Nature. (e and f) Adapted with permission from ref 36. Copyright 2015 American Chemical Society. Download figure Download PowerPoint Surendranath's group36 gave more evidence on how crystalline porosity facilitated the CO2RR and suppressed the HER in electrocatalysis. By using porous gold (Au) films with different film thicknesses (200-nm pores and 70-nm framework thickness) as the catalysts (Figure 2e), they investigated their CO2RR performance in CO2-saturated 0.1 M KHCO3. Obviously, porous Au film with the thinnest thickness of 0.5 μm displayed the lowest CO Faradaic efficiencies for CO2RR under all potentials. In sharp contrast, the CO Faradaic efficiencies were higher over the films with thicker thicknesses (Figure 2f). The opposite tendency was also observed for the HER, which was exemplified by the fact that, in comparison to the thinnest counterpart, the thickest film exhibited a 10-fold decrease in HER. These results indicated the promotion of CO2RR and the inhibition of HER enabled by the crystalline porosity engineering. The authors further carried out electrochemical rotating studies and found that the HER performance was remarkably limited by the diffusion of reactants while the CO2RR had a minor effect on the diffusion rates. Therefore, the authors ascribed the higher CO2RR selectivity to the confinement effect of crystalline porosity that kinetically limited the transport of reactants for the HER but preserved them well for the CO2RR. Subsequently, the same group demonstrated, when porous Ag films were the catalysts, that not only did activity for the HER decrease 10-fold, but a threefold increase in activity for the CO2RR (to CO) was also achieved, indicating the structural synergies of crystalline porosity in promoting CO2RR electrocatalysis.37 Several other groups have also reported similar phenomena and enhancement mechanisms to promote CO2RR electrocatalysis toward CO and even other C1 products.38–42 In comparison to the larger pores (>50 nm), mesoporous metals with the smaller pore sizes of <10 nm have big differences in promoting CO2RR electrocatalysis. Recent work reported by Du's group43 demonstrated that nanosized crystalline mesoporosity can provide a confined pincer effect for changing the chemisorption behaviors of CO2 and H2O and remarkably enhance the electrocatalytic selectivity of CO2RR. Mesoporous Zn (P-Zn) nanoparticles with an average pore size of 3.5 nm were chosen as a model catalyst for evaluating CO2RR performance (Figure 3a). By means of various characterizations, including aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM), in situ spectroscopy, and density functional theory (DFT) analysis, P-Zn nanoparticles composed of randomly distributed pores and highly concave surfaces, in which the mesopores consisted of a corner Zn atom and two adjacent planar Zn atoms (Figure 3b). When being used as the catalyst for CO2RR electrocatalysis, P-Zn nanoparticles achieved a superior CO Faradaic efficiency of 98.1% at −0.95 V. As controls, the highest Faradaic efficiencies of less porous Zn (L-Zn) and nonporous Zn (N-Zn) were 92.5% and 83% (both at −1.15 V), respectively (Figure 3c). The CO yield of P-Zn nanoparticles reached 350.2 mmol mg−1 h−1 at −0.85 V, which was 2.82 and 8.91 times higher than L-Zn (124.2 mmol mg−1 h−1) and N-Zn (39.3 mmol mg−1 h−1), respectively. Differently, at the same potentials, Zn foil solely produced H2. The results confirmed the importance of nanosized crystalline mesoporosity in promoting CO2RR electrocatalysis. In-situ Zn K-edge extended X-ray absorption fine structure spectra showed that the P-Zn nanoparticles with highly concave surfaces exposed a number of low coordinated atoms. The coordination number of P-Zn nanoparticles was the lowest at 4.2. In contrast, the coordination numbers of L-Zn and N-Zn were 4.9 and 5.4, respectively. The low coordinated atoms remarkably changed the chemisorption capacity of CO2 and H2O. In-situ Fourier transform infrared spectroscopy further exhibited that P-Zn favorably adsorbed CO2 than H2, while L-Zn and N-Zn had the stronger adsorption capacity for H2O as compared to CO2. The results were strongly consistent with the low coordinated atoms of catalysts. DFT calculations confirmed that, in comparison to its counterparts, more low coordinated Zn atoms of P-Zn produced a nanoconfined pincer effect that remarkably facilitated the capture of CO2/H2O, decreased the distances of active intermediates, and lowered the energy barriers of CO2RR (Figure 3d). The structural synergies of nanosized mesopores over P-Zn thus selectively reduced CO2 to CO (but inhibited the HER) through a confined pincer mechanism. Figure 3 | (a) HAADF-STEM image and (b) corresponding schematic illustration of porous Zn (P-Zn) catalyst. (c) CO Faradaic efficiencies of catalysts (P-Zn, L-Zn, N-Zn, and Zn foil) at different potentials. (d) DFT-calculated reaction trends of catalysts for CO2RR electrocatalysis. Adapted with permission from ref 43. Copyright 2020 Wiley-VCH. Download figure Download PowerPoint CO2RR to C2+ products In comparison to producing CO, the deeper CO2RR electrocatalysis toward value-added hydrocarbons (C2+) products is more desirable due to its broad potential in various industrial applications. Current catalysts for CO2RR electrocatalysis toward C2+ products are being developed on Cu-based materials.44 Recently, it has been demonstrated that, compared with the tuning of surface structures and compositions, crystalline porosity engineering is also highly effective for electrochemical conversion of CO2 to C2+ products.45 In 2016, Yang et al.46 demonstrated that both the width and depth of porous Cu electrodes remarkably affected CO2RR electrocatalysis toward different products. They first prepared three porous Cu electrode catalysts with different pores, including 30 nm width/40 nm depth, 30 nm width/70 nm depth, and 300 nm width/40 nm depth, by an easy sputtering method (Figure 4a). When carrying out CO2RR electrocatalysis in CO2-saturated 0.1 M KHCO3, remarkable differences in product selectivity were achieved (Figure 4b). The authors found that nonporous Cu produced mainly C1 products (48% of Faradaic efficiency of CO and CH4) while their Faradaic efficiency over porous Cu with the largest pores (300 nm width/40 nm depth) decreased 18%. Meanwhile, more HCOOH (32%) was produced over the porous Cu catalyst. In sharp contrast, porous Cu with smaller widths displayed a decrease of C1 products and, more importantly, a remarkable increase of C2 products. Among them, porous Cu with 30 nm width/40 nm depth produced 38% of C2H4 while the film with 30 nm width/70 nm depth produced 38% of C2H6. The authors further studied their electrochemical kinetics (Figure 4c). The twofold higher CO2-specific activities were achieved over porous Cu catalysts with smaller pore width (30 nm) than larger ones (300 nm), which coincided with the results from the H2-specific activities. The authors deduced that crystalline porosity affected the surface proton concentrations and accelerated the C–C coupling reaction, thereby enlarging their product selectivity toward C2 products. Meanwhile, the deeper pores (70 nm) provided a longer retention time for intermediate species, facilitating the deeper electrochemical reduction of CO2 from C2H4 to C2H6. Meanwhile, Lv et al.48 found that the crystalline porosity of the Cu electrode not only accelerated the transport of products (especially gas products) that pumped out of the electrode-electrolyte interface but also increased surface pH (by the generation of hydroxide ions) on the Cu electrode that inhibited the HER, synergistically reducing CO2 to the value-added C2+ products at the high currents. Figure 4 | (a) SEM images of porous Cu electrodes with different widths and depths. (b) CO2RR selectivity over the catalysts in CO2-saturated 0.1 M KHCO3 at −0.96 V. (c) Electrocatalytic specific activities of catalysts for reducing CO2. (d) SEM images of hierarchical porous Cu5Zn8 catalyst. (e) Faradaic efficiencies over hierarchical porous CuxZny catalysts in CO2-saturated 0.1 M KHCO3 at −0.8 V. (a–c) Adapted with permission from ref 46. Copyright 2017 Wiley-VCH. (d and e) Adapted with permission from ref 47. Copyright 2020 Elsevier. Download figure Download PowerPoint Recently, our group found that the hierarchical porous structures of Cu/Zn catalysts selectively electroreduced CO2 toward liquid C2+ products.47 The hierarchical porous films on the carbon paper electrode with the macropores of 320 nm and the mesopores of 20 nm were prepared by the co-template method, where 350 nm of poly(methyl methacrylate) nanoparticles and 23 nm of silica-containing polymer (poly(ethyleneoxide) -block-poly(3-(trimethoxysilyl)propymethacrylate) were used for the macropore-forming and mesopore-forming templates, respectively. Scanning electron microscopy (SEM) images showed that the macropores were interconnected and nearly hexagonal close-packed while the mesopores were uniformly dispersed within the macroporous frameworks (Figure 4d). In the hierarchical pores, the macropores kinetically accelerated the transports of reactants and products out of films while the mesopores confined the key intermediates for the deeper reduction, both of which thus synergistically promoted CO2RR electrocatalysis toward liquid C2+ products. Especially when the hierarchical porous film was Cu5Zn8, the superior Faradaic efficiency of CH3CH2OH and CH3COOH (58%) was reached at CO2-saturated 0.1 M KHCO3 at −0.8 V (Figure 4e). Comparatively, the hierarchical films with fewer layers produced more H2 and C1 products. The results also confirmed that the nanosized mesopores of porous Cu-based catalysts, compared with the larger macropores, could increase the retention time of intermediates and thus facilitate the deeper CO2RR electrocatalysis toward value-added C2+ products. In 2019, O'Mara et al.49 reported an active site-separated nanoparticle catalyst composed of an Ag core and porous Cu shells ([email protected]) for the cascade electrocatalysis of multiple CO2RRs toward C2+ products. In comparison to CO2, CO was more active and was easily reduced into C2+ products. This work proposed that the Ag core first reduced CO2 into CO and then stored spillover CO within porous Cu frameworks. Meanwhile, the high concentration of the CO intermediate confined in exterior crystalline pores drove their subsequent C–C coupling into C2 and even C3 products on the porous Cu catalysts. The cascade CO2RR electrocatalysis over [email protected] produced five main products: CO, C2H4, C2H5OH, C2H5CHO, and C3H7OH at −0.6 and −0.65 V. By contrast, only trace C3H7OH was produced over porous Cu nanoparticles (with no Ag core). This concept was very interesting and extended crystalline porosity engineering to cascade CO2RR electrocatalysis toward C3 products. Crystalline pores of Cu-based catalysts also stabilized the oxidated Cu (Cu+) which was more active for deeply catalyzing CO2 toward C2+ products.50–52 Generally, active Cu+ (e.g., Cu2O) is easily reduced to metallic Cu during CO2RR electrocatalysis, degrading the formation of C2+ products via the C–C coupling. Yang et al.53 recently reported that crystalline porosity provided a confinement space for the intermediates and also protected the oxidated Cu+, producing the structural synergies for deeply electroreducing CO2 toward C2+ fuels. The authors first synthesized porous Cu2O nanoparticles composed of 5–50 nm pores through a facile method (Figure Meanwhile, and Cu2O were also prepared for The method of that nanosized mesopores facilitated the confinement effect of intermediates and the C–C coupling toward C2+ products (Figure In comparison, the intermediates easily from the and the retention time of carbon intermediates for subsequent C–C coupling. Electrocatalytic CO2RR results confirmed that porous Cu2O produced a Faradaic efficiency of C2+ products, including C2H4 CH3COOH and at V. The reached the highest of at V (Figure Competitively, more C1 products were produced over and and their were in the of 0.5 and Raman spectra of catalysts were to their structural (Figure Porous Cu2O catalysts well the Raman being performed even for CO2RR electrocatalysis at V for 20 indicating oxidated Cu+ was protected by confined crystalline In contrast, its counterpart catalysts and Cu2O) into metallic Cu within 2 In to the results from the authors that crystalline porosity of Cu-based catalysts not only the active Cu+ around the porous surfaces but also the confinement environment for the key intermediates, which synergistically facilitated the conversion (Figure Figure | (a) image and (b) concentration and of C2+ on porous Cu2O catalyst in (c) product selectivity of CO2RR electrocatalysis over and Cu2O catalysts collected in the M (d) Raman spectra of porous Cu2O catalyst at V collected in M (e) Schematic illustration of CO2RR to C2+ products over porous Cu2O catalyst. Adapted with permission from Copyright 2020 American Chemical Society. Download figure Download PowerPoint Not only the porous structures but also the structures the spaces that increase the retention time for the key intermediates and promote CO2RR electrocatalysis toward C2+ products. In et demonstrated that Cu nanoparticles a similar to porous to the retention time of C2 and thus promote C3 The authors first prepared Cu2O nanoparticles with an average size of nm and then them with to structures on the electrochemical reduction with CO, Cu nanoparticles were Meanwhile, the sizes of on the Cu nanoparticles were further through controlling the times during the a hierarchical structure was to be an ideal to catalytic performance of Cu The that active CO the chemisorption on the surface of Cu and selectively reduced into C2 the C2 were in the interior Cu which as the active key intermediates for further reduction with CO to the C3 products. The DFT calculations demonstrated that both electrocatalytic and kinetics facilitated the selective coupling of C2 with CO into C3 product in Cu The authors further demonstrated that Cu nanoparticles with larger produced a higher C3 product of with a remarkable Faradaic efficiency of at V. Meanwhile, the high Faradaic efficiencies of C2 products, including and were also Furthermore, when compared with its counterpart catalysts with smaller and the C3 product was also the highest over the Cu with larger The on with the from the calculations for all of the catalysts. Overall, results confirmed the importance of crystalline cavities and in synergistically promoting the deeper electrochemical reduction toward value-added C3 products. A similar work has also been reported by Liu et that the porous Cu the intermediates and the C–C coupling toward C2+ products. A very recent work reported by Liu et further how the numbers of Cu/Cu2O nanoparticles selectively promoted the CO2RR electrocatalysis toward C2+ products. The authors first prepared Cu2O nanoparticles and then synthesized Cu2O nanoparticles by means of the Ostwald ripening The Cu2O nanoparticles with 1 2 and 3 shells were on the times of the Ostwald ripening A transmission electron microscopy image confirmed the formation of Cu2O nanoparticles (Figure When the in comparison to the with fewer nanoparticles

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