Tunability and Scalability of Single-Atom Catalysts Based on Carbon

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Tunability and Scalability of Single-Atom Catalysts Based on Carbon Nitride Zupeng Chen, Sharon Mitchell, Frank Krumeich, Roland Hauert, Sergii Yakunin, Maksym V. Kovalenko, and Javier Pérez-Ramírez ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06148 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 2, 2019

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ACS Sustainable Chemistry & Engineering

Tunability and Scalability of Single-Atom Catalysts Based on Carbon Nitride Zupeng Chen†, Sharon Mitchell†*, Frank Krumeich‡, Roland Hauert§, Sergii Yakunin‡§, Maksym V. Kovalenko‡§ and Javier Pérez-Ramírez†* †Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1, 8093 Zürich, Switzerland ‡Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1, 8093 Zürich, Switzerland §Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland *Corresponding authors. Sharon Mitchell: [email protected] Javier Pérez-Ramírez: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Carbon nitrides are promising hosts for single-atom catalysts (SACs) based on small amounts of precious metals dispersed as isolated atoms, presenting tantalizing opportunities to reduce the cost and for higher efficiency as compared to traditional nanoparticle-based formulations. Heteroatom doping represents a straightforward method to tailor the (opto-)electronic properties of carbon nitrides and could, therefore, extend the tunability of SACs. This paper compares the impact of modifying graphitic carbon nitride with phosphorus, boron, sulfur, and fluorine on the interaction with palladium. As an aliovalent dopant, phosphorus is found to appreciably increase the electron density of carbon nitride, thereby lowering the oxidation state of the metal. The stability of single atoms depends on the dopant (D) content, with nanoparticle formation observed at higher concentrations (e.g. molar D:Pd ratio >1), which is linked to a weaker metal-host interaction. Evaluation in the three-phase semi-hydrogenation of 2-methyl-3-butyn-2-ol, an important building block in fine-chemical manufacturing, evidences an enhanced reaction rate (up to 5.4 times) upon doping with phosphorus that is governed by the P/Pd molar ratio. The selectivity to the desired product approaches 100%, outperforming the commercial Lindlar-type Pd-Pb/CaCO3 catalyst (78%). Looking toward the future implementation, scalability aspects of SACs based on carbon nitrides, such as the choice of precursor, synthesis conditions, and the trade-off between the host surface area and yield, are addressed. Extrapolation of the superior catalytic properties and robust stability are confirmed in a continuous-flow reactor. These findings identify key steps in the design of single-atom catalysts based on carbon nitride for large-scale application.

KEYWORDS: Single-atom heterogeneous catalysis, Carbon nitride, Heteroatom doping, Scaleup, Exfoliation, Metal-carrier interaction, Palladium, Alkyne semi-hydrogenation


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INTRODUCTION Catalysis is currently undergoing a revolution driven by the need of the chemical industry to adapt to diverse renewable feedstocks and to reduce the impact on the environment. In doing so, it faces important hurdles such as the scarcity of platinum group metals, which are presently indispensable in the synthesis of many commodity chemicals, and the development of more energy-efficient processes.1-3 Single-atom heterogeneous catalysts (SACs) containing metals finely dispersed as isolated sites are widely targeted as a strategy to tackle this.4-6 In the design of these materials, the choice of the host is crucial since it determines the stability, the coordination environment, and the related electronic properties of the metal center.7 Carbon nitrides (C3N4) offer unique potential for the stabilization of single atoms due to the presence of abundant multidentate coordination sites within the macroheterocycles intrinsic to their structure.8-12 The high chemical and structural diversity of C3N4 presents a wide space for tailoring the properties of SACs. In the liquid-phase selective hydrogenation of alkynes, where SACs have been demonstrated as sustainable alternatives to the commercial Lindlar-like (PdPb/CaCO3) catalysts, varying the morphology, carbon to nitrogen ratio, and lattice structure have all been shown to influence the activity.10,13-14 However, compared to the decades of research that have been devoted to the design of ligands in metal complexes used as homogeneous catalysts,15-16 the impact of varying the chemical composition of C3N4 to develop SACs with tunable properties has not been addressed. Elemental doping that deliberately introduces substitutional impurities (e.g. P, S, B, F, O, C, or I), into the framework of C3N4, has demonstrated to be a practical strategy to tune the electronic and optical properties, and thus improve the performance of metal-free materials in photo(electro)catalytic applications including water splitting, CO2 reduction, and C-H bond oxidation, and bioimaging.8,17-22


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To date, most of the developed strategies for the synthesis of SACs have been confined to the milligram or gram scale and are cost prohibitive and impractical for production in larger quantities. Carbon nitride is widely regarded as scalable and has recently been commercialized. Considering the versatility of C3N4 as a host for SACs, it is thus highly desired to translate the findings into industrial applications. However, the scale-up of SACs brings distinct challenges, which may require revisions of the unit operations or chemical precursors, and the process must be economic (high yield, low cost) and sustainable (green chemistry). In this work, we study the impact of heteroatom doping (i.e., P, B, S, and F) in a high-surface-area exfoliated form of graphitic carbon nitride (ECN) on the stabilization and associated catalytic performance of palladium single atoms. Detailed analysis of the properties of P-doped ECN confirms the gradual incorporation of the heteroatom, while photoluminescence (PL) analysis (steady-state and time-resolved) suggests that the phosphorous incorporation is accompanied by overall decrease of PL emission while part that corresponds to graphitic material decreases faster than exfoliated one. The effect of heteroatom doping on the stabilization and properties of palladium is studied by varying the dopant content. To gain insights into the impact of P doping of C3N4 on the catalytic performance of Pd species, the obtained catalysts are evaluated in three-phase semi-hydrogenation of 2-methyl-3-butyn-2-ol, an important building block in the synthesis of vitamin E. Looking beyond laboratory research, scalability aspects of SACs based on graphitic carbon nitride are subsequently addressed, confirming the amenability to large-scale applications.


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EXPERIMENTAL SECTION Catalyst preparation. Graphitic carbon nitride (BCN) in bulk form was prepared by calcining dicyandiamide (10 g, Sigma Aldrich) at 823 K (2.3 K min−1) for 4 h in static air. Exfoliation of BCN (2 g) was typically undertaken by thermal treatment at 773 K for 5 h (5 K min−1) in static air, leading to ECN. The heteroatom (i.e., P, B, S, and F) doped ECN samples were prepared by grinding ECN (1 g) with the desired amount of NaH2PO2 (0.05, 0.1, 0.2, 0.5, 0.75, or 1 g), B2O3 (0.01 g), S powder (1 g), or NH4F (0.25 g), and the mixture was heated at 773 K (P and F doping) or 813 K (B and S doping) for 1 h in N2 atmosphere (2 K min−1). Afterwards, the resulting products were washed in distilled water at room temperature under stirring for 24 h, and finally collected by filtration, washed extensively with distilled water and ethanol successively, and dried at 338 K overnight in an electro-thermostatic blast oven. To study the economy of the most widely employed precursors, various BCN hosts were prepared by calcining cyanamide, urea, guanidinium chloride, or melamine (10 g, Sigma-Aldrich) at 823 K (2.3 K min−1) for 4 h in static air, and the exfoliated form of ECN was obtained by thermal etching of the resulting BCN at 773 K for 1-8 h (5 K min−1). Metal introduction. The different carbon nitride materials (0.5 g) were dispersed in distilled water (20 cm3) and sonicated for 1 h at room temperature. Afterwards, an aqueous solution of Pd(NH3)4(NO3)2 containing 5 wt.% Pd (0.05 cm3) was added and the resulting mixture was stirred overnight to enable complete adsorption. In the case of the microwave-assisted deposition, the resulting solution was treated in a microwave reactor (CEM Discover SP), with a cyclic program of 15 s irradiation and 3 min cooling in 20 repetitions (100 W). In the case of the wet impregnation, the resulting solution was further stirred for 6 h at 333 K. The desired products were collected by


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filtration, washed extensively with distilled water and ethanol successively, and dried at 338 K overnight in an electro-thermostatic blast oven. Catalytic tests. The three-phase hydrogenation reactions were conducted in a microwave reactor (CEM Discover SP) for 20 min at 323 K under stirring (0.4 M 2-methyl-3-butyn-2-ol or nitrobenzene in toluene (1.5 cm3), catalyst (15 mg)), with an initial hydrogen pressure of 3 bar. The reaction mixture was then filtered and the liquid products were collected in a vial. Catalytic tests under flow conditions were conducted in a micro-reactor (H-Cube ProTM, ThalesNano) at T = 323 K, P = 3 bar, FL(alkyne + toluene) = 1 cm3 min–1, FG(H2) = 36 cm3 min–1. Hydrogen generated in situ by electrolysis and the feed solution flow concurrently through a cylindrical cartridge (I.D. = 3.5 mm) filled with a mixture of the catalyst (0.1 g) and silicon carbide (0.2 g, particle size = 0.2-0.4 mm). The liquid product was collected every 30 min and analyzed by gas chromatography using an HP-5 capillary column and flame ionization detector (HP-6890). The conversion was calculated by dividing the concentration of the transformed substrate by the initial concentration. The selectivity was determined by dividing the concentration of the particular product to the concentration of reacted substrate. Characterization. The crystalline order of the samples was assessed by X-ray diffraction (XRD) in an X’Pert PRO MPD diffractometer (PANalytical) using Bragg-Brentano geometry and Ni-filtered Cu K ( = 0.1541 nm) radiation. Diffraction patterns were acquired from 5-70° 2 (0.05° step size; 1.5 s per step). The structure of P-doped ECN was studied by

31P

nuclear

magnetic resonance spectroscopy using a Bruker AVANCE III HD NMR spectrometer (16.4 T magnetic field; corresponding to a 1H Larmor frequency of 700.13 MHz). The CP/MAS (cross-polarization/magic angle spinning) spectra were acquired under high-power decoupling (SPINAL-64) at a contact time of 2 ms and a recycle delay of 1 s. A total of 4096 and 40960 scans


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were acquired for ECN-0.25P and ECN-2.01P, respectively. The electronic properties and composition of surface species was investigated by X-ray photoelectron spectroscopy (XPS) with a Quantum 2000 spectrometer (Physical Electronics Instruments) using monochromatic Al K radiation (46.95 eV electron analyzer pass energy; 45° emission angle; 5×10−8 Pa residual pressure). The collected spectra were referenced to the ternary carbon peak of C3N4 at C1s = 288.3 eV. The inelastic background (Shirley type) was subtracted prior to peak fitting. The average oxidation state of palladium ( OS Pd ) was determined from the relative peak areas of the different palladium species in the Pd3d XPS spectra. The host composition was confirmed by elemental analysis using a LECO CHN-900 combustion furnace. The Pd content was measured by inductively coupled plasma-optical emission spectrometry (ICP-OES) with a Horiba Ultra 2 instrument featuring photomultiplier tube detection. Prior to analysis the solids were dissolved under sonication in a piranha solution. The porous properties of pre-evacuated samples (423 K, 10 h) were studied by argon sorption in a Micrometrics 3Flex instrument (77 K). The specific surface area was estimated from the Brunauer-Emmett-Teller (BET) method. Conventional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) of the samples was conducted using a Talos F200X instrument (200 kV) and. Aberration-corrected (AC-)STEM was conducted on a HD2700CS (Hitachi) microscope with a cold field emitter operated at 200 kV and probe corrector (CEOS) resulting in a resolution of approximately 1 Å.23-24 This setup collects incoherently scattered electrons which avoids any Bragg diffraction contrast and results in pure atomic number (Z) contrast.25 Thereby, a high contrast between the palladium species and the host material was achieved. The HD2700CS was operated in the ultra-high resolution mode (UHR) and images (1020 × 1024 pixels) were acquired with pixel dwell times of 15 μs, which lead to sufficient signal-to-noise ratios for palladium single atom visibility. The


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samples for microscopy studies were obtained by dispersing the powders in ethanol and putting some droplets onto copper or nickel TEM grids coated with a lacey carbon foil. Photoluminescence (PL) spectra were acquired by an optical fiber-coupled CCD spectrometer (LR1, ASEQ Instruments) while 355 nm laser light (frequency-tripled, picosecond Nd:YAG laser, Duetto from Time-Bandwidth) was used for the excitation. For the rejection of excitation source scattered light from collected PL emission was made by a long-pass optical filter with an edge at 400 nm. Time-resolved photoluminescence (TR-PL) measurements were done using a time-correlated single photon counting (TCSPC) self-made setup, equipped with an SPC-130-EM counting module (Becker & Hickl GmbH) and an IDQ-ID-100-20-ULN avalanche photodiode (Quantique) with the same excitation laser frequency-tripled picosecond laser. The average radiative lifetimes were determined as a squared sum of products of squared lifetimes and corresponding amplitudes normalized by squared sum of products of lifetimes and amplitudes. For experiments with selection of a spectral range (Figure S1), the PL emission was cut out by combination of short and long-pass optical filters.

RESULTS AND DISCUSSION Tunability of the metal-carrier interaction. Alongside nitrogen-based ligands, phosphorus ligands are perhaps the most widely utilized in transition metal chemistry and have played a strong role in the advancement of homogeneous catalysts in both academic and industrial fields.15-16,26 For this reason, we initially focused on this dopant. Phosphorous-doped ECN samples were obtained by thermal treatment of ECN with varied amount of P-containing source (NaH2PO2). The carriers were coded as ECN-xP, where x denotes the amount of introduced phosphorous (Table S1). The successful incorporation of P into the framework of C3N4 was confirmed by 31P solid-state


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cross-polarization/magic angle spinning nuclear magnetic resonance (CP/MAS NMR) spectra, showing well-resolved peaks ranging from -30 to 10 ppm (Figure 1a). The peak at 1.2 ppm is ascribed to terminal P=O sites (P1) that form during the post-treatment step, while the signals at -8.6 and -20.2 ppm are attributed to phosphorus located in corner (P2) and bay (P3) sites, respectively.19,27 The chemical environment of phosphorous was further verified by P2p X-ray photoelectron spectroscopy (XPS, Figure 1b). The main peak centered at 133.6 eV corresponds to P-N or P=N bonding, while the shoulder at 134.9 eV can be attributed to P=O bond.19,27 The influence of P incorporation on the intrinsic electronic properties was studied by PL spectroscopy. The emission intensity of steady-state PL (Figure 1c) quenches significantly upon increasing P amount due to the opening of additional non-radiative relaxation channel and can be associated with improved separation and transfer of charges in the modified samples. Furthermore, the time-resolved PL analysis (Figure 1d,e) reveals a decreased average exciton lifetime in the P-doped ECN, suggesting that the relaxation of a fraction of charges occurs more readily via non-radiative paths, which might due to the charge or energy transfer to new localized states that were formed during the doping process.28-31 Note that the emission bands at 400-450 nm decay faster than that above 500 nm (Figure S1). The two emission peaks at 439 and 460 nm in all samples can be ascribed to the exfoliated and graphitic form of C3N4, respectively,32 and the latter decays faster with increasing P content. In parallel, a slight increase in the intensity of N1s signal around 400.9 eV is observed which is attributed to the presence of surface -NHx groups, pointing towards the formation of defects at the surface (Figure 1f and S2). Interestingly, the P-doping process did not disturb the periodic structure and physical properties of ECN (Table S1), but even slightly enhanced the crystallinity, as revealed by the corresponding X-ray diffraction (XRD) patterns (Figure S2).


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Palladium was introduced on P-doped ECN via microwave-assisted deposition strategy targeting a content of 0.5 wt.%. Evaluation by (aberration-corrected) high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) verifies the atomic dispersion of Pd over the P-doped hosts with heteroatom contents below 0.39 wt.% (Figure 2a-c), while nanoparticles are observed in the samples with P contents above 0.67 wt.% (Figure 2d-f). Attempts to isolate Pd atoms over ECN-2.01P by reducing the amount of metal (targeting 0.22 and 0.09 wt.%) were unsuccessful (Figure S3), which could be due to fast charge transfer between Pd species and the host. Further insight into the metal-carrier interaction was accessed by Pd3d5/2 XPS analysis (Figure S4), showing two main peaks at 338.3 and 336.3 eV ascribed to Pd4+ and Pd2+ species based on the formal assignments, respectively. The metallic signature at 334.9 eV was observed only in ECN-2.01P-Pd. Furthermore, the average Pd oxidation state decreases from 3.10 to 2.48 in the case of pristine ECN-Pd and ECN-2.01P-Pd, respectively (Figure 3a). It is known that phosphorous contains one more valence electron compared to carbon, thus by replacing the C in the structure of C3N4, four electrons covalently bond to the neighboring N atoms adopting a planar structure, and the remaining one electron delocalizes into the conjugated heptazine ring.19,27 The formation of the joint electronic system between the positively charged P and electron rich C3N4 framework is expected to favor reductive catalysis reactions and the localization of charges also increases the conductivity of the material.33 These results confirm the expected strong interaction of P-doped ECN with supported Pd species, in line with the microscopy analysis. The absence of palladium reflections in the XRD patterns in all cases corroborates the high dispersion of the metal (Figure S5). To study the impact of heteroatom doping on the catalytic performance, the obtained catalysts were evaluated in the semi-hydrogenation of 2-methyl-3-butyn-2-ol, and benchmarked with


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Lindlar catalyst (PdPb/CaCO3), the dominant industrial catalyst in liquid-phase hydrogenations despite drawbacks of poor metal utilization (5%) and toxicity.34-35 Over a fivefold increase in the reaction rate towards 2-methyl-3-buten-2-ol (from 0.22 × 103 to 1.18 × 103 mol-ene molPd-1 h-1 in ECN-Pd and ECN-0.25P-Pd, respectively) is achieved upon increasing the P/Pd ratio to 0.72 (Figure 3b). Above this value, the rate gradually decreases to 0.71 × 103 mol-ene molPd-1 h-1 for ECN-0.67P-Pd, possibly reflecting the significant disruption of the framework. Though slightly decreased, the selectivity towards the alkenol approaches 100% for all the investigated samples. The same behavior was observed in nitrobenzene hydrogenation, reaching the optimized catalytic performance towards the formation of aniline at a P content of 0.25 wt.% (Figure S6). In comparison, the selectivity to the desired product over the Lindlar catalyst is only 78% due to the over-hydrogenation to 2-methyl-3-butan-2-ol (22% selectivity).13 To verify the versatility of heteroatom doping, some representative atoms (i.e., B, S, and F) were also incorporated into C3N4 framework with similar doing level (ca. 0.2 wt.%) as the best performing ECN-0.25P-Pd (Figure 4). The absence of Pd nanoparticles by AC-STEM analysis suggests that the metal is also atomically dispersed on these samples. However, no obvious improvement was observed in 2-methyl-3-butyn-2-ol semi-hydrogenation (Figure S7). Scalability of SACs based on carbon nitride. Although SACs based on carbon nitride are highly tunable, several aspects in their synthesis need to be addressed to enable their practical application (Figure 5). In the synthesis of the host, the first major decision is the choice of precursor, which influences both the price and the efficiency of the thermal polymerization. While previous studies have compared the effectiveness of different precursors based on the properties of the resulting carbon nitrides, little information is available about the yields.36-38 To gain insight into the comparative viability, bulk carbon nitrides were prepared from the five most widely


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employed reagents (cyanamide, dicyanamide, urea, guanidinium chloride, and melamine) by calcination at 823 K for 4 h in static air. Characterization of the resulting materials confirms the similar chemical composition and crystalline structure (Table 1, Figure S8). The surface areas were also generally low (< 20 m2 g-1), except in the case of urea which resulted in a carbon nitride of 108 m2 g-1. In terms of yield, the highest value was obtained with melamine (54%), while urea was the least efficient at 2%. The latter value could be slightly improved (5%) by decreasing the polymerization temperature to 753 K for 2 h, but the surface area also dropped substantially (34 m2 g-1). Considering the observed yields and the catalogue prices of the precursors, the estimated cost of producing 1 kg of BCN ranges between 61 USD when using melamine to 31396 USD for cyanamide. The latter price is similar to that of the commercially available BCN (14586 and 158100 USD, respectively). Given the lower price from melamine, this was selected as the precursor for the scalable synthesis of carbon nitride. High surface area hosts are desirable for the preparation of SACs to lower the areal densities of isolated centers for a given metal content. The porosity of carbon nitrides can be increased by two main approaches: direct synthesis via templating or post-synthetically by exfoliation.39-42 Templating routes face significant hurdles to application at large scale since they typically employ silica-based materials that require the use of toxic and highly corrosive hydrofluoric acid to remove.43-45 On the other hand, exfoliation routes such as the thermal oxidation exfoliation of BCN in air, are more convenient and controllable. However, the yields may be very low (e.g. 2%).46-47 To gain insight into the scalability of ECN, the trade-off between the host porosity and the yield was studied by varying the etching time (Figure 6a). The specific surface area increases significantly from 9 to 214 m2 g−1 after thermal treatment of BCN at 773 K for 8 h. However, the yield decreases to 7%, suggesting a layer-by-layer oxidation etching process.47 As can be seen in


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representative TEM images (Figure 6b), the dense and irregularly shaped laminar BCN gradually evolves into rumpled nanosheets, and some in-plane meso- and macropores are observed in the case of 8 h exfoliated ECN. Based on these results, ECN-3h was selected as the host for preparation of SACs giving a yield of 41% and surface area of 148 m2 g-1. Palladium was first introduced into this host by microwave-assisted deposition, and the resulting SAC matches the rate of 2-methyl-3-buten-2-ol formation (0.28 × 103 mol-ene molPd-1 h-1) of the standard ECN-Pd (0.22 × 103 mol-ene molPd-1 h-1), and longer exfoliation time (5 or 8 h) only slightly boost the performance (0.29 × 103 or 0.36 × 103 mol-ene molPd-1 h-1, respectively) (Figure 7a). However, this strategy is limited by the size of microwave reactor and thus is impractical for mass production of SACs. Therefore, Pd atoms were further introduced by conventional wet impregnation at 333 K for 6 h. Despite resulting in inferior performance (0.11 × 103 mol-ene molPd-1 h-1), this approach can be easily scaled up under green synthesis conditions. The stability of the resulting ECN-3h-Pd-imp was evaluated in a continuous configuration. The constant reaction rate of 2-methyl-3-buten-2-ol formation (Figure 7b) excludes the effects of deactivation caused by leaching or aggregation of palladium. Analysis of the used catalyst evidences the preserved metal dispersion (Figure 7c). To prepare heteroatom-doped carbon nitrides two possible routes can be envisaged: the direct synthesis of a heteroatom-doped BCN or the post-synthetic modification of ECN-3h. The former approach appears attractive since it reduces the overall number of unit operations. However, comparison of the efficiency of exfoliation of a P-doped BCN reveals that the yield decreases to 7% after 3 h thermal treatment and the surface area drops to 37 m2 g−1.


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CONCLUSION The impact of heteroatom doping in the framework of carbon nitride on metal stabilization and the catalytic efficiency of Pd atoms in alkyne semi-hydrogenation reactions has [have?] been demonstrated. The substitution of corner or bay carbon atoms in the heptazine ring by phosphorus was confirmed by NMR and XPS, and the photoluminescence analysis revealed the formation of new localized states during the doping process. Palladium single atoms can be stabilized with P contents < 0.39 wt.%, while nanoparticles were formed on the hosts with more P. An optimized catalytic performance in semi-hydrogenation reactions was observed with 0.25 wt.% P, leading to a reaction rate over 5 times higher than the pristine C3N4. However, further introduction of P is detrimental due to more significant disruption of the framework. Key criteria for the practical application of Pd single-atom catalysts based on carbon nitride were identified for the first time (e.g. choice of precursor, synthesis condition, thermal exfoliation time, and metal deposition method), showing competitive performance and robust stability in semi-hydrogenation reactions. The findings provide insights into the large-scale synthesis and industrial application of single-atom catalysts based on graphitic carbon nitride.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Table with the characterization data, N1s XPS spectra, XRD, and additional time-resolved PL analysis of the P-doped ECN hosts and associated metal-containing catalysts, STEM images of ECN-2.01P with 0.09 and 0.22%Pd, the result of nitrobenzene hydrogenation, additional catalytic tests, and the XRD spectra of the BCN hosts prepared from the most widely employed precursors (PDF).


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGMENTS We thank ETH Zurich and the Swiss National Science Foundation (Grant no. 200021-169679) for financial support, ScopeM at ETH Zurich for access to its facilities, E. Vorobyeva for some microscopy analysis, Dr. R. Verel for NMR measurements.

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Figure 1. (a)

31P

CP/MAS NMR spectra, (b) P2p XPS spectra, (c) steady-state, and (d)

time-resolved PL spectra of P-doped ECN. In (a), different P species and their associated resonances are labeled (atom color codes: gray, C; purple, N; orange, P; red, O). The legend in (d) applies to (a-c). Dependence of (e) the exciton lifetime (determined by time-resolved PL) and (f) the relative content of -NHx defects of the total N content (determined by the N1s XPS, Figure S2) as a function of P content.


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Figure 2. AC-STEM images of (a) ECN-Pd, (b) ECN-0.25P-Pd, (c) ECN-0.39P-Pd, (d) ECN-0.67P-Pd, (e) ECN-1.06P-Pd, and (f) ECN-2.01P-Pd. Insets in (a-c) evidence the atomic dispersion of Pd. Isolated Pd atoms and nanoparticles are identified by red and yellow circles, respectively.


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Figure 3. (a) Average oxidation state of palladium ( OS Pd ) as a function of the P content. Inset, the illustration of the effect of P doping at corner site upon the electronic properties of C3N4 structure. The parentheses indicate the sites that will further polymerize. Color codes: gray, C; purple, N; orange, P. (b) Reaction rate (in 103 mol-ene molPd-1 h-1) and selectivity to 2-methyl-3buten-2-ol in the hydrogenation of 2-methyl-3-butyn-2-ol over selected catalysts. Reaction conditions:

Wcat = 15 mg,

VL (2-methyl-3-butyn-2-ol + toluene) = 1.5 cm3,

T = 323 K,

and

PG (H2) = 3 bar.


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Figure 4. (a) B1s, (b) S2p, (c) F1s XPS spectra and schematic structures of the ECN-0.17B, ECN-0.24S, and ECN-0.26F hosts, respectively, and the (d) XRD, (e) Pd3d XPS spectra, and (f) AC-STEM images of the corresponding Pd-containing samples. Insets in (f) evidence the atomic dispersion of Pd and the isolated Pd atoms are identified by red circles.


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Figure 5. Steps (blue boxes) in the preparation of Pd-SACs based on ECN indicating the key samples derived (green boxes) and key considerations for the scalability. Heteroatom doping (orange boxes) is optional and can be approached during the thermal polymerization or as a post-synthetic modification of ECN.


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Page 28 of 31

Figure 6. (a) Trade-off between the surface area and yield with the exfoliation time of BCN prepared from melamine. (b) TEM images of the corresponding BCN, and ECN after 3 h and 8 h thermal exfoliation.


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Figure 7. (a) Reaction rate (in 103 mol-ene molPd-1 h-1) and selectivity to 2-methyl-3-buten-2-ol in the hydrogenation of 2-methyl-3-butyn-2-ol over Pd SACs based on carbon nitrides of different surface area. The Pd single atoms were incorporated via two approaches, i.e., microwave-assisted deposition (shaded light blue) and wet impregnation (shaded light yellow), respectively. (b) Stability test of the Pd SAC on ECN-3h (by wet impregnation) in semi-hydrogenation of 2-methyl-3-butyn-2-ol in flow, and (c) the corresponding AC-STEM images of the fresh and used catalysts.


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Table 1. Characterization data of the BCN hosts prepared from the most widely employed precursors. Yield SBETc) [%] [m2 g−1]

Costd) [USD kg-1]

Samplea)

Precursor

Formulab)

BCN-1

cyanamide

C3N4.55H2.04O0.28

4

16

31396

BCN-2

urea

C3N4.54H2.01O0.33

2

108

705

BCN-3e)

urea

C3N4.59H2.22O0.33

5

34

285

BCN-4

guanidinium chloride

C3N4.52H1.72O0.16

20

19

408

BCN-5

dicyandiamide

C3N4.53H1.77O0.17

21

6

155

BCN-6f)

dicyandiamide

C3N4.53H1.65O0.09

19

6

171

BCN-7

melamine

C3N4.54H1.66O0.09

54

9

61

BCN-8f)

melamine

C3N4.54H1.59O0.06

54

8

61

BCN-9g)

-

C3N4.47H1.70O0.16

-

17

14586

BCN-10h)

-

C3N4.55H1.81O0.18

-

17

158100

a)Prepared

method.

at 823 K for 4 h (ramp, 2.3 K min−1) in static air. b)Elemental analysis. c)BET

d)Calculated

from the yield and the price of the precursor (Sigma-Aldrich,

https://www.sigmaaldrich.com/switzerland-schweiz.html). (ramp, 5 K min−1). f)Prepared under N2 atmosphere. h)Purchased

e)Prepared

g)Purchased

at 753 K for 2 h

from Carbodeon Ltd.

from American Elements Ltd.


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TOC Graphic

Synopsis. The tunability and scalability of palladium single-atom catalysts based on tailored carbon nitrides is assessed for the sustainable semi-hydrogenation of functionalized alkynes.


31

Tunability and Scalability of Single-Atom Catalysts Based on Carbon

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