Open AccessCCS ChemistryRESEARCH ARTICLE5 Sep 2022Incorporating Sulfur Atoms into Palladium Catalysts by Reactive Metal–Support Interaction for Selective Hydrogenation Zhen-Yu Wu†, Hang Nan†, Shan-Cheng Shen, Ming-Xi Chen, Hai-Wei Liang, Chuan-Qi Huang, Tao Yao, Sheng-Qi Chu, Wei-Xue Li and Shu-Hong Yu Zhen-Yu Wu† Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Institute of Energy, Hefei Comprehensive National Science Center, Department of Chemistry, Institute of Biomimetic Materials and Chemistry, Anhui Engineering Laboratory of Biomimetic Materials, University of Science and Technology of China, Hefei 230026 †Z.-Y. Wu and H. Nan contributed equally to this work.Google Scholar More articles by this author , Hang Nan† Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Institute of Energy, Hefei Comprehensive National Science Center, Department of Chemistry, Institute of Biomimetic Materials and Chemistry, Anhui Engineering Laboratory of Biomimetic Materials, University of Science and Technology of China, Hefei 230026 †Z.-Y. Wu and H. Nan contributed equally to this work.Google Scholar More articles by this author , Shan-Cheng Shen Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Institute of Energy, Hefei Comprehensive National Science Center, Department of Chemistry, Institute of Biomimetic Materials and Chemistry, Anhui Engineering Laboratory of Biomimetic Materials, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Ming-Xi Chen Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Institute of Energy, Hefei Comprehensive National Science Center, Department of Chemistry, Institute of Biomimetic Materials and Chemistry, Anhui Engineering Laboratory of Biomimetic Materials, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Hai-Wei Liang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Institute of Energy, Hefei Comprehensive National Science Center, Department of Chemistry, Institute of Biomimetic Materials and Chemistry, Anhui Engineering Laboratory of Biomimetic Materials, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Chuan-Qi Huang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Hangzhou Institute of Advanced Studies, Zhejiang Normal University, Hangzhou, Zhejiang 311231 Google Scholar More articles by this author , Tao Yao National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author , Sheng-Qi Chu Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 Google Scholar More articles by this author , Wei-Xue Li Department of Chemical Physics, School of Chemistry and Materials Science, iCHeM, CAS Center for Excellence in Nanoscience, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author and Shu-Hong Yu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, CAS Center for Excellence in Nanoscience, Institute of Energy, Hefei Comprehensive National Science Center, Department of Chemistry, Institute of Biomimetic Materials and Chemistry, Anhui Engineering Laboratory of Biomimetic Materials, University of Science and Technology of China, Hefei 230026 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101428 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Developing highly active and selective catalysts for the hydrogenation of nitroarenes, an environmentally benign process to produce industrially important aniline intermediates, is highly desirable but very challenging. Pd catalysts are generally recognized as active but nonselective catalysts for this important reaction. Here, we report an effective strategy to greatly improve the selectivity of Pd catalysts based on the reactive metal–support interaction, by which the sulfur atoms doped in the carbon supports are extracted out and react with Pd at 500 °C to form Pd4S nanoparticle catalysts. The Pd4S catalysts that are formed display high selectivity of >98% at complete conversion to diverse nitroarenes (with more than 20 examples) under mild conditions. Density functional theory calculations reveal that the incorporation of S atoms into Pd breaks up the Pd ensembles, which results in the preferential adsorption of nitro groups on the Pd4S surfaces and thus selective hydrogenation of substituted nitroarenes to corresponding anilines. Download figure Download PowerPoint Introduction Hydrogenation reactions have attracted continuing interest due to their great significance in the chemical industry and scientific research.1–7 A typical but important example is the hydrogenation of substituted nitroarenes, which produces substituted anilines as key intermediates for the manufacture of pharmaceuticals, agrochemicals, dyes, and functional polymers.8–14 A big challenge for this reaction is the selective reduction of the nitro group when more than one reducible group (e.g., –Cl, –C=C, –C=O, –OH) is present in the nitroarenes, as conventional supported noble metal catalysts (e.g., Pd or Pt) cannot discriminate different functional groups for selective hydrogenation.8,12,13,15–20 Developing new hydrogenation catalysts with high intrinsic chemoselectivity is a straightforward and effective strategy to tackle this tough problem.8,21–27 Some important progress has been achieved recently by employing Au-based, Ag-based, and pyrolyzed Fe(or Co)/nitrogen/carbon catalysts for selective reduction of the nitro group of substituted nitroarenes.8,21,25,28–30 Nevertheless, these catalysts always suffer from low activity and thus often require harsh reaction conditions, such as high temperature (≥120 °C), high H2 pressure (≥5.0 MPa), and long reaction time (≥12 h).8,21,25,28,31 Another effective strategy is improving chemoselectivity of conventional hydrogenation catalysts (mainly Pt-group catalysts), which possess poor catalytic selectivity but high intrinsic activity.13,15,20,31–35 A traditional approach is the modification of active metal species with inorganic or organic modifiers. However, those modifiers not only decrease the activity via coverage of the active sites, but also raise environmental problems.20,34,36 Other ways of improving selectivity of Pt-group catalysts mainly include size control of active components (even at the single-atomic level),11,12,18,31 formation of alloys or intermetallic compounds,17,32,37–40 and regulation by the pore structure of supports.33,35,41 Particularly, construction of intermetallic compounds, featuring atomically ordered and thermodynamically stable structures with defined stoichiometry, can change the adsorption/desorption properties of the relevant reaction species, which makes it possible to catalyze the nitroarene hydrogenation reaction in the desired direction.32,37,40 Furthermore, well-defined atomic arrangements provide an excellent platform to study the catalytic mechanism, enabling better performance optimization and catalyst design.32,37 Inspired by these promising works, we propose that incorporating nonmetallic elements (i.e., sulfur) into active Pd metal lattices to form compounds with well-defined structures is a feasible way to create selective nitroarene hydrogenation catalysts that will work well under mild conditions. Herein, we report a novel nitroarene hydrogenation catalyst, that is, crystalline Pd4S nanoparticles supported on S-doped carbon (Pd4S/SC), which is prepared through the reactive metal–support interaction (RMSI). RMSI, a chemical reaction that generally occurs on the metal–oxide support interface under high temperatures, was recently employed to prepare bimetallic structures.42–44 However, RMSI has never before been explored in the carbon-supported metal catalysts. Here, we demonstrate an example of carbon support-based RMSI, in which the sulfur atoms doped in the carbon supports are extracted out at 500 °C and incorporated into Pd to form crystalline Pd4S nanoparticle catalysts. In contrast to conventional Pd/C catalysts with poor selectivity, the prepared Pd4S catalyst gives high selectivity of >98% at complete conversion for structurally diverse nitroarenes (with more than 20 examples) under mild conditions. Density functional theory (DFT) calculations disclose that the geometric effect induced by the S incorporating into the Pd lattice results in the preferential adsorption of the nitro group on the Pd4S surfaces and thus selective hydrogenation of nitroarenes to corresponding anilines. Experimental Methods Chemicals and materials SiO2 fumed powder and commercial Pd/C catalyst were purchased from Sigma-Aldrich (St. Louis, MO). Commercial Pt/C catalyst was obtained from Alfa Aesar (Ward Hill, MA). Other chemicals were commercially procured from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and used as received without further purification. Synthesis of SC supports and a-PdxS/SC-b catalysts In a typical experiment for synthesizing SC, 2.0 g of 2,2′-bithiophene, 1.0 g of Co(NO3)2·6H2O, and 2.0 g of SiO2 fumed powder (7 nm; Sigma-Aldrich S5130) were first added into 120 mL of tetrahydrofuran and stirred for ca. 2 h. Then, the solvent was removed by rotary evaporation. The obtained dried powder was subsequently carbonized under flowing N2 for 2 h at 800 °C. The carbonized product was then leached in 2.0 M NaOH for 3 days and 0.5 M H2SO4 at 90 °C for 8 h to remove SiO2 templates and metallic species, and finally afforded SC. An inductively coupled plasma-atomic emission spectrometer (ICP-AES) test showed that the residual Co content in SC was very low (0.24 wt %). To synthesize a-PdxS/SC-b catalysts, PdCl2 was first dissolved in HCl aqueous solution to generate a H2PdCl4 solution with a concentration of 2.815 mg mL−1. Then, a certain amount of H2PdCl4 aqueous solution and 60 mg of SC was added into 30 mL deionized water. After stirring for 6 h and sonication for another 0.5 h, the water was removed by rotary evaporation. The obtained powder was treated by thermal reduction at 300–700 °C under flowing 5% H2/Ar for 2 h to afford final a-PdxS/SC-b catalysts, where a and b are the weight ratios of Pd to SC and reduction temperatures, respectively. Catalyst characterizations Transmission electron microscopy (TEM) images were taken with a Hitachi H7700 transmission electron microscope with a charge-coupled device (CCD) imaging system and an accelerating voltage of 100 kV. High-angle annular dark-field scanning TEM (HAADF-STEM) and high-resolution TEM (HRTEM) images were acquired by using a JEM-ARM 200F atomic resolution analytical microscope (JEOL, Tokyo, Japan) operating at an accelerating voltage of 200 kV. X-ray diffraction (XRD) data were collected on a Philips X’Pert PRO SUPER X-ray diffractometer (Almelo, The Netherlands) equipped with graphite monochromatic Cu Kα radiation (λ = 1.54056 Å). The energy-dispersive spectroscopic (EDS) line-scan and EDS elemental mapping was carried out on a Talos F200X transmission electron microscope (FEI, Hillsboro, OR) at an accelerating voltage of 200 kV equipped with an energy dispersive detector. N2 sorption analysis was recorded on an ASAP 2020 accelerated surface area and porosimetry instrument (Micromeritics, Norcross, GA), equipped with automated surface area, at 77 K using Barrett–Emmett–Teller (BET) calculations for the surface area. X-ray photoelectron spectroscopy (XPS) was performed on an X-ray photoelectron spectrometer (ESCALab MKII, Thermo Scientific, Waltham, MA) with an excitation source of Mg Kα radiation (1253.6 eV). The X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of Pd K-edge were measured at BL14W1 beam line of Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China). The XANES tests of S K-edge were carried out at the 4B7A beamline of Beijing Synchrotron Radiation Facility (BSRF, Beijing, China). The XANES tests of S L-edge were performed at the BL11U beamline of National Synchrotron Radiation Laboratory (NSRL, Hefei, China). The S XANES spectra in the figures have been normalized to the background before and after the main features. The gas chromatograph (GC) instrument was equipped with a Restek-5 capillary column (5% diphenyl and 95% dimethylsiloxane, 0.32 mm diameter, 60 m length), and a flame-ionization detector (FID). GC (Shimadzu GC-2014, Nakagyo-ku, Kyoto, Japan) was used to calculate the conversion and selectivity of the catalytic products. Pd K-edge XAFS analysis The obtained EXAFS data were processed according to the standard procedures utilizing the ATHENA module in the IFEFFIT software packages. The EXAFS spectra were obtained by subtracting the postedge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, the χ(k) data were Fourier-transformed (FT) to real (R) space using a hanning windows (dK = 1.0 Å−1) to separate the EXAFS contributions from different coordination shells. Least-squares curve parameter fitting was carried out using the ARTEMIS module of the IFEFFIT software packages to obtain the quantitative structural parameters around central atoms. Catalytic reaction experiments The hydrogenation of 4-chloronitrobenzene was carried out in a stainless steel autoclave equipped with a pressure control system and a magnetic stirrer (1200 rpm). In a typical hydrogenation experiment, 1.0 mmol of 4-chloronitrobenzene and 0.094 mol % Pd of catalysts were mixed with 1 mL of ethyl acetate in a reaction glass vial. The reaction vial was then placed into a 100 mL steel Parr autoclave. The autoclave was purged three times with H2 and then charged with 0.6 MPa of H2 before the reaction mixture was stirred for 1 h at 80 °C. After the reaction was completed, the autoclave was placed into a cool water bath to stop the reaction. Then, the H2 was released slowly, and 60 μL of o-xylene was added to the system as the internal standard. Next, 10 mL of ethyl acetate was added to dilute the reaction solution. The products were analyzed by GC. In the recyclability test, all reactions were carried out for 0.75 h according to the procedures above. After each run, the used catalyst was separated by vacuum filtration and washed with deionized water and ethanol. Then the obtained sample was dried at 80 °C overnight before being tested in the next run. For the gram-scale hydrogenation of the 4-chloronitrobenzene test, the reaction was conducted by using 25 mmol of 4-chloronitrobenzene, 0.0188 mol % Pd of catalysts, and 15 mL of ethyl acetate as solvent under 1.0 MPa of H2 at 100 °C. The hydrogenation experiment procedure remained the same. After the reaction was performed for 1 h, the autoclave was placed into a cool water bath to stop the reaction. Then the remaining hydrogen gas was discharged, and 0.1 mL liquid mixture was extracted to analyze the product by GC. Afterward, the autoclave was charged with pure H2 again to perform the next run. In the experiment of hydrogenation of substituted nitroarenes, the substrates and products were determined by GC or GC-mass spectrometry (GC-MS). The conversion and selectivity were calculated by GC analysis, which were determined by GC peak areas. DFT calculation method DFT calculations were performed using the Vienna ab initio Simulation Package (VASP) based on the projected augmented wave (PAW) method.45–48 The nonlocal correlation functional of van der Waals density functional (vdW-DF) scheme proposed by Dion et al.49 was used to account for vdW interaction. The exchange effect of the electron was described with the optB86b exchange functional.50 This combined optB86b-vdW functional was verified to have a good description of the vdW interaction between molecules and metal surfaces.50,51 Optimized lattice constants of 3.941 Å for Pd and a = 5.18 Å, c = 5.65 Å for Pd4S were used. For Pd(111), Pd(100), and Pd(110) surfaces, slabs of 4, 4, and 5 layers, with supercells of (5 × 5), (4 × 4), and (3 × 4) size were used, respectively. For Pd4S(110), Pd4S(100), Pd4S(001), Pd4S(102), and Pd4S(112) surfaces, (2 × 2) super cells of about 9 Å atomic layer thickness were used, respectively. Slabs were separated with 20 Å of vacuum. The upper two layers of each slab were relaxed. A plane wave basis set with energy cutoff of 400 eV and (2 × 2) k-points were used to ensure the convergence of total ground-state energy. All geometrical optimization was performed until the force acting on relaxing atoms was less than 0.02 eV Å−1. Results and Discussion Synthesis and characterization of the catalysts The first step of the synthesis is to prepare the SC support by carbonization of molecular precursors with SiO2 nanoparticles as hard templates (see Experimental Methods for details).52,53 The SC support possessed a high specific surface area of 1164 m2 g−1, a large pore volume of 3.24 cm3 g−1, and a particularly high S content of 13.2 wt % ( Supporting Information Figures S1 and S2a and Table S1). The Pd4S/SC catalyst was then prepared by the wet impregnation of H2PdCl4 (5 wt % Pd) and subsequent H2 reduction at 500 °C. The S-enriched carbon supports enabled the reaction of Pd with S atoms to form Pd4S nanoparticles based on the RMSI (Figure 1a). As far as we know, it is the first example of synthesizing metal sulfide catalysts by RMSI on the metal–carbon interface. Figure 1 | Synthesis and characterization of the catalyst. (a) Schematic illustration for the preparation processes of Pd4S/SC catalyst based on RMSI. (b) Low-magnification HAADF-STEM image of the Pd4S/SC catalyst. Inset shows the particle size distribution of Pd4S. (c) High-resolution HAADF-STEM of a Pd4S nanoparticle. (d) EDS elemental mapping images of the Pd4S/SC catalyst. (e) Line-scan EDS elemental distribution curves of an individual Pd4S nanoparticle. (f) XRD pattern of the Pd4S/SC catalyst. (g) Crystal structure model of Pd4S. Download figure Download PowerPoint We first made HAADF-STEM observations to characterize the microstructure of the prepared catalysts. HAADF-STEM images and the corresponding particle size distribution showed that the nanoparticles with an average diameter of 5.8 ± 1.8 nm were homogeneously distributed on the SC supports (Figure 1b, the inset in Figure 1b, and Supporting Information Figure S3). The high-resolution HAADF-STEM image of an individual nanoparticle revealed well-defined lattice fringes with spacings of 0.243 and 0.220 nm, which were consistent with (102) and (112) planes of Pd4S (Figure 1c). Additionally, HRTEM also showed two sets of lattice fringes from Pd4S ( Supporting Information Figure S4). EDS elemental mapping images indicated that the S element was distributed over the whole carbon matrix, while line-scan curves confirmed that both of Pd and S elements were concentrated in individual particles (Figures 1d and 1e). Subsequently, we performed XRD to identify crystalline phase compositions of the prepared catalyst. The XRD pattern displayed main diffraction peaks at 35.2°, 36.6°, 39.5°, 40.8°, 42.8°, 48.2°, and 51.8° (Figure 1f), which corresponded well to the (200), (102), (210), (112), (211), (202), and (212) planes, respectively, of tetragonal Pd4S (JCPDS 73-1387). The space group of Pd4S is P-421c with eight palladimn atoms in general positions and two sulfur atoms in positions (0, 0, 0) and (1/2, 1/2, 1/2) (Figure 1g).54 Overall, the above characterizations unambiguously proved the formation of highly crystalline Pd4S nanoparticles on the SC supports based on RSMI. We then explored the effect of reduction temperature and Pd contents on RSMI for the catalyst preparation. All of the catalysts are referred to as a-PdxS/SC-b, where a and b are the weight ratio of Pd to SC and reduction temperatures, respectively. XRD analyses clearly indicated the vital role of reduction temperature on RSMI for the catalyst synthesis ( Supporting Information Figure S5). Unlike the highly crystalline Pd4S phase of 5%-PdxS/SC-500 (i.e., aforementioned Pd4S/SC), no diffraction peaks were found for the sample prepared at 300 °C ( Supporting Information Figure S5a), indicating its amorphous structure or the ultrasmall size of the nanoparticles. For the 700 °C sample (5%-PdxS/SC-700), we observed the coexistence of diffraction peaks corresponding to Pd4S and metallic Pd ( Supporting Information Figure S5b), suggesting the partial decomposition of Pd4S to form metallic Pd phase at 700 °C. Along with the gradual evolution of Pd structures depending on temperatures, the particle size sharply increased from 0.90 ± 0.24 nm for 5%-PdxS/SC-300 to 14.1 ± 6.2 nm for 5%-PdxS/SC-700 when the H2-reduction temperature rose from 300 to 700 °C ( Supporting Information Figures S6 and S7). When we fixed the H2-reduction temperature at 500 °C and changed the Pd contents in the range of 2∼10 wt %, similar nanoparticle size distributions were found ( Supporting Information Figures S8 and S9). However, we also observed the emergence of the metallic Pd phase for the high-Pd-content (10 wt %) sample at 500 °C ( Supporting Information Figure S10), implying the deficiency of S atoms in the SC supports to form Pd4S for a high Pd content. Additionally, N2 adsorption–desorption tests revealed that all of the a-PdxS/SC-b catalysts possessed high specific surface areas of ca. 1000 m2 g−1 and large pore volumes of >3.5 cm3 g−1 ( Supporting Information Figures S2b–S2f and Table S1). Electronic properties of the catalysts We further conducted XAFS measurements to reveal the effects of reduction temperatures on the electronic and local structures of the catalysts based on RSMI. The Pd K-edge XANES profiles in Figure 2a showed that the near-edge feature of 5%-PdxS/SC-300 was extremely close to that of PdS, while those of 5%-PdxS/SC-500 and 5%-PdxS/SC-700 were quite similar to that of Pd foil, indicating that the valence states of Pd in 5%-PdxS/SC-300, 5%-PdxS/SC-500, and 5%-PdxS/SC-700 were close to Pd2+, Pd0, and Pd0, respectively. The white line, feature A, and feature B peaks increased or decreased gradually with the reduction temperature, demonstrating that the valence states of the Pd element in 5%-PdxS/SC-b decreased accordingly. Figure 2b shows the FT k2-weight EXAFS curves of 5%-PdxS/SC-b as well as those of reference PdS and Pd foil. 5%-PdxS/SC-300 exhibited a prominent peak at ca. 1.8 Å, agreeing well with Pd–S coordination of the PdS reference, and no other obvious peak for Pd–Pd was observed. When the reduction temperature was increased to 500 and 700 °C, a new peak located at ca. 2.6 Å pointing to Pd–Pd coordination was found. Of note, the intensity of Pd–Pd coordination gradually increased with the reduction temperature, accompanying the gradually weakened Pd–S scattering from 5%-PdxS/SC-300 to 5%-PdxS/SC-700. Figure 2 | Electronic properties of the catalysts. (a) XANES spectra at the Pd K-edge of the 5%-PdxS/SC-b catalysts, PdS, and Pd foil. The inset is the magnified image of pre-edge XANES spectra. (b) FT at the Pd K-edge of 5%-PdxS/SC-b catalysts, PdS, and Pd foil. (c) WT of 5%-PdxS/SC-b catalysts, PdS, and Pd foil. (d) Pd XPS spectra of 5%-PdxS/SC-b catalysts. Download figure Download PowerPoint Wavelet transform (WT) analysis was further conducted to probe the atomic dispersion of Pd in 5%-PdxS/SC-b (Figure 2c). The WT contour plots of PdS showed only one intensity maximum at 1.8 Å attributed to Pd–S coordination, while Pd foil displayed only one intensity maximum at 2.6 Å from Pd–Pd coordination. Clearly, the intensity of Pd–S coordination in WT contour plots of 5%-PdxS/SC-b catalysts decreased gradually when the reduction temperature increased from 300 to 700 °C, accompanying the breaking of Pd–S bonds and the increase of Pd–Pd coordination. Furthermore, we carried out EXAFS fitting to obtain the quantitative chemical configuration of the Pd atoms in the 5%-PdxS/SC-b catalysts ( Supporting Information Figure S11 and Table S2). For 5%-PdxS/SC-300, there was only one coordination configuration (i.e., Pd–S) at ca. 2.31 Å with a coordination number of ca. 3.9, very close to the PdS reference with an average Pd–S bond length of 2.32 Å and a coordination number of about 3.6. As expected, 5%-PdxS/SC-500 and 5%-PdxS/SC-700 possessed two coordination configurations, that is, Pd–S and Pd–Pd. The coordination numbers of Pd–S and Pd–Pd were 2.1 and 5.5 in 5%-PdxS/SC-500, and 1.7 and 6.5 in 5%-PdxS/SC-700, respectively. Pd EXAFS results in k-space for 5%-PdxS/SC-b catalysts, PdS and Pd foil are provided in Supporting Information Figure S12. We also performed XPS tests to further characterize the electronic properties of 5%-PdxS/SC-b catalysts (Figure 2d). The Pd 3d XPS spectrum of 5%-PdxS/SC-300 consisted of asymmetric Pd 3d5/2 and 3d3/2 peaks centered at 337.2 and 342.5 eV, respectively, which are the characteristics of Pd2+.55,56 The Pd 3d5/2 and Pd 3d3/2 peaks sharply dropped for 5%-PdxS/SC-500 and 5%-PdxS/SC-700, and two distinct peaks at 335.6 and 340.9 eV emerged, which were indicative of Pd0.55,56 The ratios of Pd0/Pd2+ were 0, 3.06, and 3.75 for 5%-PdxS/SC-300, 5%-PdxS/SC-500, and 5%-PdxS/SC-700, respectively. In the S K-edge, L-edge XANES and XPS spectra, we only identified the C–S–C bond from the SC support and did not find the Pd–S bond from PdxS ( Supporting Information Figures S13 and S14), probably due to the relatively low content of Pd–S bonds compared with that of C–S–C bonds in 5%-PdxS/SC-b catalysts. Given the above HADDF-STEM, XRD, XAFS, and XPS analyses, we proposed the evolution process of Pd species on the SC supports with the reduction temperature based on RMSI. At the low reduction temperature of 300 °C, the Pd precursors react with S atoms that are extracted out from SC supports to form loosely assembled PdxS nanoclusters with exclusive Pd–S coordination configuration. At 500 °C, along with the slight aggregation/ripening, the Pd and S atoms in the particles will rearrange into well-defined crystalline Pd4S structures. Further raising the temperature to 700 °C will cause some S atoms to leave the Pd4S lattice, that is, the partial decomposition of Pd4S into metallic Pd. Catalytic performance in hydrogenation of 4-chloronitrobenzene We chose hydrogenation of 4-chloronitrobenzene as a model reaction to investigate the catalytic performance of a-PdxS/SC-b catalysts, in which the selective hydrogenation of the nitro group to produce 4-chloroaniline is desired (Figure 3a).35,40 The catalytic reactions were performed under a mild condition (T = 80 °C, P (H2) = 0.6 MPa) for all the a-PdxS/SC-b catalysts. Commercial Pd/C and Pt/C were also investigated under the identical conditions for comparison. As expected, the commercial Pd/C and Pt/C catalysts showed the high conversion of 100% after 1 h reaction but poor 4-chloroaniline selectivity of 56.2% and 74.5%, respectively (Figure 3b), as dechlorination occurred on both catalysts to form the aniline as side product. The SC support alone did not show any catalytic activity toward the 4-chloronitrobenzene hydrogenation. Remarkably, the 5%-PdxS/SC-500 catalyst exhibited a superior 4-chloroaniline selectivity of >99.9%, along with the complete conversion with 1 h reaction (Figure 3b). Additionally, we conducted the hydrogenation reaction under harsh reaction conditions, including longer reaction time and higher reaction temperature, to further demonstrate the outstanding selectivity of the 5%-PdxS/SC-500 catalyst. As shown in Supporting Information Figure S15, 5%-PdxS/SC-500 still retained excellent selectivity of >99.9% when the reaction time
Search from 250 M+ research papersSearch from 250 M+ research papers
Feb 28, 2024
Haifeng Zhang + 5
Open Access