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The design and development of high performance catalysts has received considerable attention in selective hydrogenation reactions but remains a major challenge. Here we report a monatomic RuNi alloy (SAA) in which individual Ru atoms are immobilized on the surface of Ni nanoparticles via Ru-Ni coordination, which is accompanied by an electron transfer from subsurface Ni to Ru. To our knowledge, the best catalyst 0.4% RuNi SAA simultaneously showed higher activity (TOF value: 4293 h–1) and chemoselectivity for the selective hydrogenation of 4-nitrostyrene to 4-aminostyrene (yield: >99%), the highest level by compared with known heterogeneous catalysts. In situ experiments and theoretical calculations show that the Ru-Ni interface sites, as internal active sites, promote preferential breaking of NO bonds with a lower energy barrier of 0.28 eV. In addition, synergistic Ru-Ni catalysis favors the formation of intermediates (C8H7NO* and C8H7NOH*) and accelerates the rate-determining step (hydrogenation of C8H7NOH*).
Functionalized aromatic amines, important building blocks of fine chemicals, have important industrial applications in the production of pharmaceuticals, agrochemicals, pigments and polymers1,2,3. The catalytic hydrogenation of readily available nitroaromatic compounds over heterogeneous catalysts has attracted considerable attention as an environmentally friendly and recyclable method for the synthesis of amines with added value4,5,6,7. However, the chemoselective reduction of -NO2 groups while retaining other reducible groups such as alkenes, alkynes, halogens, or ketones is a highly desirable but rather challenging task8,9,10,11. Therefore, rational use of heterogeneous catalysts for the specific reduction of -NO2 groups without affecting other reducible bonds is highly desirable12,13,14. Many noble-metal-free catalysts have been investigated to catalyze the hydrogenation of nitroarenes, but the harsh reaction conditions prevent their wide application15,16. Although noble metal catalysts (such as Ru17, Pt18, 19, 20 or Pd21, 22, 23) are active under mild reaction conditions, they typically suffer from high cost, suboptimal selectivity, and low atom utilization. Thus, obtaining highly active and chemoselective catalysts by rational design and fine tuning of the fine structure remains a major challenge24,25,26.
Monatomic Alloy (SAA) catalysts have maximum noble metal efficiency, special geometric and electronic structure, provide unique active sites, and provide outstanding catalytic performance by breaking the characteristic linear scaling behavior27,28,29,30,31. Doped single atoms and host metal atoms in SAA can serve as dual active sites, facilitating the activation of multiple substrates or allowing different elementary reaction steps to occur at different sites32,33,34. In addition, heterometallic associations between isolated impurity metal atoms and host metals can lead to idiosyncratic synergistic effects, although the understanding of such synergistic effects between two sets of metal sites at the atomic level remains controversial35,36,37,38. For the hydrogenation of functionalized nitroarenes, the electronic and geometric structures of active sites must be designed in such a way as to accelerate the activation of exclusively nitro groups. As a rule, electron-deficient nitro groups are predominantly adsorbed on the nucleophilic regions of the catalyst surface, while in the subsequent hydrogenation pathway, cooperative catalysis of neighboring active sites will play an important role in controlling reactivity and chemoselectivity4,25. This prompted us to explore SAA catalysts as a promising candidate for improving the catalytic efficiency of chemoselective hydrogenation of nitroaromatic compounds, as well as further elucidating the relationship between active site structure and atomic scale catalytic performance.
Here, catalysts based on monatomic RuNi alloys were prepared based on a two-stage synthetic approach, including the structural-topological transformation of a layered double hydroxide (LDH) followed by electro-displacement treatment. RuNi SAA exhibits exceptional catalytic efficiency (>99% yield) for the chemoselective hydrogenation of 4-nitrostyrene to 4-aminostyrene with a turnover frequency (TOF) of up to ~4300 mol-mol Ru-1 h-1, which is the highest level among heterogeneous catalysts registered under similar reaction conditions. Electron microscopy and spectroscopic characterization showed that isolated Ru atoms are dispersed on the surface of Ni nanoparticles (~8 nm), forming a stable Ru-Ni coordination, resulting in negative Ru sites (Ruδ-) due to electron transfer from subsurface Ni to Ru. In situ FT-IR, XAFS studies and density functional theory (DFT) calculations confirmed that sites at the Ru-Ni interface as internal active sites facilitate nitro. Activated adsorption (0.46 eV) differs from that of the monometallic nickel catalyst. (0.74 eV). In addition, hydrogen dissociation occurs in neighboring Ni positions, followed by hydrogenation of intermediates (C8H7NO* and C8H7NOH*) in Ruδ positions. The synergistic effect of support doping in the RuNi SAA catalyst results in outstanding nitroarenes hydrogenation activity and selectivity, which can be extended to other rare noble metal catalysts used in structure sensitive reactions.
Based on the transition of the structural topology of layered double hydroxide (LDH) precursors, we prepared monometallic Ni deposited on amorphous Al2O3 substrates. After that, a set of RuNi/Al2O3 bimetallic samples with different Ru content (0.1–2 wt %) was accurately synthesized by electrodisplacement to deposit Ru atoms on the surface of Ni nanoparticles (NPs) (Fig. 1a). Inductively coupled plasma atomic emission spectrometry (ICP-AES) measurements clearly gave the elemental composition of Ru and Ni in these samples (Supplementary Table 1), which is close to the theoretical feedstock loading. The SEM images (Supplementary Figure 1) and BET results (Supplementary Figures 2–9 and Supplementary Table 1) clearly show that the morphological structure and specific surface area of ​​the RuNi/Al2O3 samples do not undergo obvious changes during electrochemical treatment. – the process of moving. The X-ray pattern (Fig. 1b) shows a series of characteristic reflections at 2θ 44.3°, 51.6°, and 76.1°, indicating phases (111), (200), and (220) of typical Ni (JCPDS 004–0850). Notably, the RuNi samples do not show reflections of metallic or oxidized Ru, indicating a high dispersion of Ru varieties. Transmission electron microscopy (TEM) measurements of monometallic Ni and RuNi samples (Fig. 1c1–c8) show that nickel nanoparticles are well dispersed and immobilized on an amorphous Al2O3 support with similar particle sizes (7.7–8.3 nm). HRTEM images (Figs. 1d1–d8) show a uniform lattice period of about 0.203 nm in the Ni and RuNi samples, corresponding to the Ni(111) planes, however, the lattice edges of the Ru particles are absent. This indicates that Ru atoms are highly dispersed on the sample surface and do not affect the Ni lattice period. Meanwhile, 2 wt% Ru/Al2O3 was synthesized by the deposition-deposition method as a control, in which Ru clusters were uniformly distributed on the surface of the Al2O3 substrate (Supplementary Figs. 10-12).
a Scheme of the synthesis route for RuNi/Al2O3 samples, b X-ray diffraction patterns of Ni/Al2O3 and various RuNi/Al2O3 samples. c1−c8 TEM and d1−d8 HRTEM grating images with respective particle size distributions of monometallic Ni, 0.1 wt%, 0.2 wt%, 0.4 wt%, 0.6 wt%, 0, 8% wt., 1 wt. Striped image. % and 2 wt.% RuNi. “au” means arbitrary units.
The catalytic activity of RuNi samples was studied by chemoselective hydrogenation of 4-nitrostyrene (4-NS) to 4-aminostyrene (4-AS). The 4-NS conversion on pure Al2O3 substrate was only 0.6% after 3 hours (Supplementary Table 2), indicating little catalytic effect of Al2O3. As shown in fig. 2a, the original nickel catalyst exhibited extremely low catalytic activity with a 4-NS conversion of 7.1% after 3 hours, while 100% conversion could be achieved in the presence of the monometallic Ru catalyst under the same conditions. All RuNi catalysts showed a significantly increased hydrogenation activity (conversion: ~100%, 3 h) compared to the monometallic samples, and the reaction rate was positively correlated with Ru content. This means that Ru particles play a decisive role in the hydrogenation process. Interestingly, the product selectivity (Fig. 2b) varies greatly depending on the catalyst. For the less active pure nickel catalyst, the main product was 4-nitroethylbenzene (4-NE) (selectivity: 83.6%) and the selectivity of 4-AC was 11.3%. In the case of monometallic Ru, the C=C bond in 4-NS is more susceptible to hydrogenation than -NO2, leading to the formation of 4-nitroethylbenzene (4-NE) or 4-aminoethylbenzene (4-AE); the selectivity of 4-AC was only 15.7%. Surprisingly, RuNi catalysts with a relatively low Ru content (0.1–0.4 wt%) showed excellent selectivity (>99%) to 4-aminostyrene (4-AS), indicating that it is NO2 and not vinyl, is uniquely chemoselective. When the content of Ru exceeded 0.6 wt.%, the selectivity of 4-AS decreased sharply with increasing loading of Ru, while the selectivity of 4-AE increased instead. For the catalyst containing 2 wt% RuNi, both the nitro and vinyl groups were highly hydrogenated with a high selectivity to 4-AE of 98%. To study the effect of Ru dispersion state on the catalytic reaction, 0.4 wt% Ru/Al2O3 samples were prepared (Supplementary Figures 10, 13 and 14) in which Ru particles were mostly dispersed as individual atoms followed by a few Ru clusters. (quasi-atomic Ru). The catalytic performance (Supplementary Table 2) shows that 0.4 wt% Ru/Al2O3 improves the 4-AS selectivity (67.5%) compared to the 2 wt% Ru/Al2O3 sample, but the activity is quite low (conversion: 12.9). %; 3 hours). Based on the total number of metal sites on the surface determined by CO pulsed chemisorption measurements, the turnover frequency (TOFmetal) of the RuNi catalyst was obtained at low 4-NS conversion (Supplementary Fig. 15), which showed a trend first to increase and then to decrease with increasing increase in Ru loading (Supplementary Fig. 16). This suggests that not all surface metal sites act as native active sites for RuNi catalysts. In addition, the TOF of the RuNi catalyst was calculated from Ru sites to further reveal its intrinsic catalytic activity (Fig. 2c). As the content of Ru increases from 0.1 wt. % to 0.4 wt. % RuNi catalysts showed almost constant TOF values ​​(4271–4293 h–1), which indicates the localization of Ru particles in atomic dispersion (possibly with the formation of RuNi SAA). ) and serves as the main active site. However, with a further increase in the loading of Ru (within 0.6–2 wt %), the TOF value decreases significantly, which indicates a change in the intrinsic structure of the active center (from atomic dispersion to Ru nanoclusters). In addition, to our knowledge, the TOF of the 0.4 wt% RuNi (SAA) catalyst is at the highest level among metal catalysts previously reported under similar reaction conditions (Supplementary Table 3), further demonstrating that monoatomic RuNi alloys provide excellent catalytic properties. spectacle. Supplementary Figure 17 shows the catalytic performance of a 0.4 wt% RuNi (SAA) catalyst at various pressures and temperatures of H2, where H2 pressure of 1 MPa and reaction temperature of 60 °C were used as optimal reaction parameters. sample containing RuNi 0.4 wt. % (Fig. 2d), and no significant decrease in activity and yield was observed over five consecutive cycles. X-ray and TEM images of the 0.4 wt% RuNi catalyst used after 5 cycles (Supplementary Figures 18 and 19) showed no significant changes in the crystal structure, indicating a high stability of the selective hydrogenation reaction. In addition, the 0.4 wt% RuNi (SAA) catalyst also provides excellent yields of amines for the chemoselective hydrogenation of other nitroaromatic compounds containing halogens, aldehydes, and hydroxyl groups (Supplementary Table 4), demonstrating its good applicability.
a Catalytic conversion and b distribution of 4-nitrostyrene hydrogenation products in the presence of monometallic Ni, Ru, and RuNi catalysts with different Ru content (0.1–2 wt %), c in the catalytic dynamic range, Turnover frequency (TOF) on RuNi catalysts c depending on Ru per mole. d Test for the possibility of reuse of 0.4 wt.% RuNi catalyst for five consecutive catalytic cycles. ln (C0/C) is based on the reaction time of the hydrogenation of e-nitrobenzene and f-styrene with a mixture of nitrobenzene and styrene (1:1). Reaction conditions: 1 mmol reagent, 8 ml solvent (ethanol), 0.02 g catalyst, 1 MPa H2, 60°C, 3 hours. Error bars are defined as the standard deviation of three replicates.
To further investigate the significant chemoselective difference, hydrogenation of a mixture of styrene and nitrobenzene (1:1) was also carried out in the presence of monometallic catalysts Ni, Ru, 0.4 wt% RuNi, and 2 wt% RuNi, respectively (Supplementary Fig. 20). Although the chemoselectivity of the reactions hydrogenation of functional groups is consistent, indeed there are some differences in the selectivity of intramolecular and intermolecular hydrogenation due to molecular allosteric effects. As shown in fig. 2e,f, the curve ln(C0/C) versus reaction time gives a straight line from the origin, indicating that both nitrobenzene and styrene are pseudo-first order reactions. Monometallic nickel catalysts showed extremely low hydrogenation rate constants for both p-nitrobenzene (0.03 h-1) and styrene (0.05 h-1). Notably, a preferable styrene hydrogenation activity (rate constant: 0.89 h-1) was achieved on the Ru monometallic catalyst, which is much higher than the nitrobenzene hydrogenation activity (rate constant: 0.18 h-1). In the case of a catalyst containing RuNi(SAA) 0.4 wt. % nitrobenzene hydrogenation is dynamically more favorable than styrene hydrogenation (rate constant: 1.90 h-1 vs. 0.04 h-1), indicating a preference for the -NO2 group. over C hydrogenation = bond C. For a catalyst with 2 wt. % RuNi, the rate constant of hydrogenation of nitrobenzene (1.65 h-1) decreased compared to 0.4 wt. % RuNi (but still higher than that of the mono-metal catalyst), while the hydrogenation rate of styrene increased dramatically (rate constant: 0.68). h−1). This also indicates that with a synergistic effect between Ni and Ru, the catalytic activity and chemoselectivity towards -NO2 groups are significantly increased compared to RuNi SAA.
To visually determine the dispersion states of Ru and Ni compounds, an imaging method using high-angle ring dark scanning electron microscopy with aberration correction (AC-HAADF-STEM) and element mapping by energy dispersive spectroscopy (EDS) were performed. The EMF elemental map of the sample with 0.4 wt% RuNi content (Fig. 3a, b) shows that Ru is highly uniformly dispersed on the nickel nanoparticles, but not on the Al2O3 substrate, the corresponding AC-HAADF-STEM image (Fig. 3c) shows, It can be seen that the surface of Ni NPs contains many bright spots of the atomic size of Ru atoms (marked by blue arrows), while neither clusters nor Ru nanoparticles are observed. Fig. 3d), demonstrating the formation of monatomic RuNi alloys. For a sample containing RuNi 0.6 wt. % (Fig. 3e), single Ru atoms and a small amount of bulk Ru particles were observed on Ni NPs, which indicates a small aggregation of Ru atoms due to an increased load. In the case of a sample with 2 wt% RuNi content, many large Ru clusters on Ni NPs were found in the HAADF-STEM image (Fig. 3f) and EDS elemental mapping (Supplementary Fig. 21), indicating a large accumulation of Ru.
a HAADF-STEM image, b corresponding EDS mapping image, c high resolution AC-HAADF-STEM image, d magnified STEM image and corresponding intensity distribution of the 0.4 wt% RuNi sample. (e, f) AC–HAADF–STEM images of samples containing 0.6 wt. % RuNi and 2 wt. % RuNi, respectively.
Compared to Ni/Al2O3 and Ru/Al2O3 samples, DRIFTS spectra of CO adsorption in situ were performed (Fig. 4a) to further study the structural details of samples containing 0.4 wt. %, 0.6 wt. % and 2 wt. % RuNi. CO adsorption on a Ru/Al2O3 sample gives a main peak at 2060 cm-1 and another broad peak at 1849 cm-1 attributed to linear CO adsorption on Ru and bridging on two neighboring Ru atoms, respectively CO39,40. For the monometallic Ni sample, a strong peak is observed only at 2057 cm–1, which is attributed to linear CO41,42 in the nickel region. For the RuNi sample, in addition to the main peak at 2056 cm-1, there is a distinct shoulder centered at ~2030 cm-1. The Gaussian peak fitting method was used to reasonably deconvolve the distribution of RuNi samples in the 2000-2100 cm-1 range and the distribution of CO in the Ni (2056 cm-1) region and the Ru (2031-2039 cm) region. Two peaks were linearly adsorbed – 1) (Fig. 4b). Interestingly, from the Ru/Al2O3 samples (2060 cm–1) to the RuNi samples (2031–2039 cm–1), the linearly related CO peak in the Ru region undergoes a significant redshift and increases with increasing Ru content. This indicates an increased electronegativity of the Ru particles in the RuNi sample, which is the result of electron transfer from Ni to Ru, increasing the d-π electron feedback from Ru to the antibonding CO 2π* orbital. In addition, for a sample containing 0.4 mass% RuNi, no bridging adsorption peak was observed, indicating that the Ru particles exist as isolated Ni atoms (SAA). In the case of samples with 0.6 wt. % RuNi and 2 wt. % RuNi, the presence of bridging CO confirms the existence of Ru multimers or clusters, which is in good agreement with the AC-HAADF-STEM results.
a In situ CO-DRIFTS spectra of Ni/Al2O3, Ru/Al2O3 and 0.4 wt.%, 0.6 wt.%, 2 wt.% RuNi samples with helium gas flow in the range 2100–1500 cm-1 for 20 min. b Scaled and Gaussian-fitted spectra of the RuNi/Al2O3 sample with fixed peak positions and FWHM. c In situ Ru K-edge XANES spectra and d EXAFS Fourier transform spectra of various samples. K2-weighted wavelet transform of XAFS K-edge Ru signals based on the Morlet wavelet for e Ru samples from e Ru foil, f 0.4 wt% RuNi and g RuO2. “au” means arbitrary units.
Normalized in situ X-ray absorption structure X-ray absorption structure (XANES) spectra were performed to study the electronic and geometric structures of RuNi samples with Ru foil and RuO2 samples. As shown in fig. 4c, as the Ru loading decreases, the intensity of the white line gradually decreases from the Ru/Al2O3 samples to the RuNi samples. Meanwhile, the intensity of the white line of the XANES spectrum at the K-edge of Ni shows a slight increase from the original Ni sample to the RuNi sample (Supplementary Fig. 22). This indicates a change in the electron density and coordination environment of the Ru compounds. As shown in the X-ray photoelectron spectroscopy (XPS) spectra (Supplementary Fig. 23), the Ru0 peak of the RuNi sample shifted to a lower binding energy and the Ni0 peak shifted to a higher binding energy compared to monometallic Ru and Ni. , which additionally demonstrates electron transfer from Ni atoms to Ru atoms in RuNi SAA. The Bader charge analysis of the RuNi SAA(111) surface shows that the isolated Ru atoms carry negative charges (Ruδ-) transferred from the subsurface Ni atoms (Supplementary Fig. 24), which is consistent with the in situ DRIFTS and XPS results. To study the detailed coordination structure of Ru (Fig. 4d), we performed extended X-ray absorption fine-grained spectroscopy (EXAFS) in the Fourier transform. Sample containing RuNi 0.4 wt. % has a sharp peak at ~2.1 Å, located in the region between the Ru-O (1.5 Å) and Ru-Ru (2.4 Å) shells, which can be attributed to the Ru-Ni coordination44, 45. Data fitting results EXAFS (Supplementary Table 5 and Supplementary Figures 25–28) show that the Ru-Ni pathway has a coordination number (CN) of 5.4, while there is no Ru-Ru and Ru-O coordination at 0.4 wt. % RuNi sample. This confirms that the main Ru atoms are atomically dispersed and surrounded by Ni, forming a monoatomic alloy. It should be noted that the peak intensity (~2.4 Å) of Ru-Ru coordination appears in a sample of 0.6 wt. % RuNi and is enhanced in the sample by 2 wt. % RuNi. In particular, EXAFS curve fitting showed that the Ru-Ru coordination numbers increased significantly from 0 (0.4 wt.% RuNi) to 2.2 (0.6 wt.% RuNi) and further increased to 6.7 (2 wt.% .% RuNi), respectively, indicating that as the Ru load increases, the Ru atoms gradually aggregate. The K2-weighted wavelet transform (WT) of Ru K-edge XAFS signals was further used to study the coordination environment of Ru species. As shown in fig. 4e, Ru foil lobes at 2.3 Å, 9.7 Å-1 refer to the Ru-Ru contribution. In a sample containing RuNi 0.4 wt. % (Fig. 4f) there are no lobes at k = 9.7 Å-1 and 5.3 Å-1, except for the central bond of Ru with Ru atoms and O atoms (Fig. 4g); Ru-Ni are observed at 2.1 Å, 7.1 Å-1, which proves the formation of SAA. In addition, the EXAFS spectra at the K-edge of Ni for different samples showed no significant differences (Supplementary Fig. 29), indicating that the coordination structure of Ni is less influenced by surface Ru atoms. In short, the results of the AC-HAADF-STEM, in situ CO-DRIFTS, and in situ XAFS experiments confirmed the successful preparation of RuNi SAA catalysts and the evolution of Ru particles on Ni NPs from single atoms to Ru multimers by increasing the Ru load. In addition, the HAADF-STEM images (Supplementary Fig. 30) and EXAFS spectra (Supplementary Fig. 31) of the RuNi SAA catalysts used showed that the dispersion state and coordination structure of the Ru atoms did not change significantly after 5 cycles, proving that the stable RuNi SAA catalyst .
H2-TPD measurements were performed to study the dissociative adsorption of hydrogen on various catalysts and the results showed that all of these catalysts have a strong H2 dissociation capacity with a desorption peak at ~100 °C (Supplementary Fig. 32). The results of quantitative analysis (Supplementary Fig. 33) did not show a clear linear correlation between reactivity and the amount of hydrogen desorption. In addition, we performed experiments with D2 isotopes and obtained a kinetic isotope effect (KIE) value of 1.31 (TOFH/TOFD) (Supplementary Fig. 34), suggesting that the activation and dissociation of H2 are important but not rate-limiting steps. DFT calculations were performed to further investigate the adsorption and dissociation behavior of hydrogen on RuNi SAA versus metallic Ni alone (Supplementary Fig. 35). For RuNi SAA samples, H2 molecules preferentially chemisorb over single Ru atoms with an adsorption energy of -0.76 eV. Subsequently, hydrogen dissociates into two active H atoms on the hollow sites of Ru-Ni RuNi SAA, overcoming the energy barrier of 0.02 eV. In addition to the Ru sites, H2 molecules can also be chemisorbed on the upper sites of the Ni atoms adjacent to Ru (adsorption energy: -0.38 eV) and then dissociated into two Hs at the Ru-Ni and Ni-Ni hollow sites. Atomic barrier 0.06 eV. On the contrary, the energy barriers for the adsorption and dissociation of H2 molecules on the Ni(111) surface are -0.40 eV and 0.09 eV, respectively. The extremely low energy barrier and insignificant differences indicate that H2 easily dissociates on the surface of Ni and RuNi surfactants (Ni-site or Ru-site), which is not a key factor affecting its catalytic activity.
Activated adsorption of certain functional groups is critical for the selective hydrogenation of substrates. Therefore, we performed DFT calculations to investigate possible configurations of 4-NS adsorption and active sites on the RuNi SAA(111) surface, and the optimization results are shown in Supplementary Fig. 36. Seemingly parallel configuration (Fig. 5a and Supplementary Fig. 36e), in which N atoms are located in Ru-Ni hollow sites and two O atoms are bonded to the Ru-Ni interface shows the lowest adsorption energy level (-3.14 eV). This suggests a thermodynamically more favorable adsorption regime compared to vertical and other parallel configurations (Supplementary Fig. 36a–d). In addition, after the adsorption of 4-HC on RuNi SAA(111), the length of the N-O1 (L(N-O1)) bond in the nitro group increased to 1.330 Å (Fig. 5a), which is much longer than the length of the gaseous 4- NS (1.244 Å) (Supplementary Fig. 37), even exceeding L (N-O1) (1.315 Å) on Ni (111). This indicates that the activated adsorption of N–O1 bonds on the surface of RuNi PAA is significantly enhanced compared to the initial Ni(111).
a Adsorption configurations of 4-HC on Ni(111) and RuNi SAA(111) (Eads) surfaces (side and top views). Ru – violet, Ni – green, C – orange, O – red, N – blue, H – white. b In situ FT-IR spectra of gaseous and chemisorbed 4-HC on monometallic surfactants Ni, Ru, RuNi (0.4 wt. %) and 2 wt. % RuNi, respectively. c Normalized in situ XANES and d-phase-corrected Fourier EXAFS at the Ru K-edge of 0.4 wt % RuNi PAA during 4-NS adsorption (RuNi SAA–4NS) and hydrogenation steps (RuNi SAA–4NS–H2) .Transformation spectra ;…e Projection density of states (PDOS) of the initial surface of RuNi SAA(111), N-O1 in gaseous 4-NS and adsorbed 4-NS on RuNi SAA(111). “au” means arbitrary units.
To further test the adsorption behavior of 4-NS, in situ FT-IR measurements were performed on Ni monometallic, Ru monometallic, 0.4 wt% RuNi (SAA), and 2 wt% RuNi catalysts (Fig. 5b). The FT-IR spectrum of gaseous 4-NS exhibited three characteristic peaks at 1603, 1528, and 1356 cm–1, which were assigned to ν(C=C), νas(NO2), and νs(NO2)46,47,48. In the presence of monometallic Ni, redshifts of all three bands are observed: v(C=C) (1595 cm–1), νas(NO2) (1520 cm–1), and νs(NO2) (1351 cm–1). , which indicates the chemisorption of C=C and -NO2 groups on the Ni surface (most likely, in the configuration of parallel adsorption). For a sample of monometallic Ru, redshifts of these three bands (1591, 1514, and 1348 cm–1, respectively) relative to monometallic Ni were found, which indicates a slightly enhanced adsorption of nitro groups and С=С bonds on Ru. In the case of 0.4 wt. % RuNi (SAA), the ν(C=C) band is centered at 1596 cm–1, which is very close to the monometallic Ni band (1595 cm–1), indicating that the vinyl groups tend to adsorb Ni on the RuNi SAA sites. In addition, in contrast to the monometallic catalyst, the relative intensity of the νs(NO2) band (1347 cm-1) is much weaker than the νas(NO2) band (1512 cm-1) on 0.4 wt.% RuNi ( SAA) , which has been associated with cleavage of the NO bond to -NO2 to form a nitroso intermediate according to previous studies49,50. A similar phenomenon was also observed in the sample with a RuNi content of 2 wt.%. The above results confirm that the synergistic effect of the bimetallic centers in PAA RuNi promotes the polarization and dissociation of nitro groups, which is in good agreement with the optimal adsorption configuration obtained by DFT calculations.
In situ XAFS spectroscopy was carried out to study the dynamic evolution of the electronic structure and coordination state of RuNi SAA during 4-NS adsorption and catalytic reaction. As can be seen from the K-edge XANES spectrum of Ru (Fig. 5c), after adsorption of 4-HC, 0.4 wt. % RuNi PAA, the absorption edge is significantly shifted towards higher energies, which is accompanied by an increase in the intensity of the white line, which indicates that Ru species Partial oxidation occurs due to electron transfer from Ru to 4-NS. In addition, the phase-corrected Fourier transform EXAFS spectrum of adsorbed 4-NS RuNi SAA (Fig. 5d) shows a clear enhancement of signals at ~1.7 Å and ~3.2 Å, which is associated with the formation of Ru-O coordination. The XANES and EXAFS spectra of 0.4 wt% RuNi SAA returned to their original state after a 30 minute injection of hydrogen gas. These phenomena indicate that nitro groups are adsorbed on Ru sites via Ru-O bonds based on electronic interactions. As for the XAFS spectra of the Ni-K edge in situ (Supplementary Fig. 38), no obvious changes were observed, which may be due to the effect of dilution of Ni atoms in the bulk phase on surface Ni particles. The predicted density of states (PDOS) of RuNi SAA (Fig. 5e) shows that the unoccupied state of the nitro group above the Femi level broadens and moves below the Femi level in the adsorbed state, which additionally indicates that electrons from the d-state of RuNi SAA transition to the unoccupied state in −NO2. The charge density difference (Supplementary Fig. 39) and the Bader charge analysis (Supplementary Fig. 40) show that the integrated electron density of 4-NS accumulates after its adsorption on the surface of RuNi SAA (111). In addition, the -NO2 charge density was significantly increased compared to the vinyl group in 4-NS due to electron transfer at the Ru-Ni interface, indicating specific activation of the NO bond in the nitro group.
In situ FT-IR was performed to monitor the catalytic process of the 4-NS hydrogenation reaction on catalyst samples (Fig. 6). For the initial nickel catalyst (Fig. 6a), only a slight decrease in the density of the nitro (1520 and 1351 cm-1) and C=C (1595 cm-1) bands was observed when passing H2 for 12 min, which indicates that − Activation NO2 and C=C are rather weak. In the presence of monometallic Ru (Fig. 6b), the ν(C=C) band (at 1591 cm–1) rapidly narrows within 0–12 min, while the νs(NO2) and νas(NO2) bands are strongly reduced. Slow This indicates preferential activation of the vinyl group for hydrogenation, leading to the formation of 4-nitroethylbenzene (4-NE). In the case of 0.4 wt. % RuNi (SAA) (Fig. 6c), the νs(NO2) band (1347 cm–1) rapidly disappears with the influx of hydrogen, accompanied by a gradual decay of ν(N=O ) ; a new band centered at 1629 cm-1 was also observed, attributed to bending vibrations of NH. In addition, the band for ν(C=C) (1596 cm–1) shows only a rather slight decrease after 12 min. This dynamic change confirms the polarization and hydrogenation of -NO2 to -NH2 by 0.4 wt% RuNi (SAA) based on the unique chemoselectivity towards 4-aminostyrene. For a sample of 2 wt. % RuNi (Fig. 6d), in addition to the appearance of a new band at 1628 cm–1 attributed to δ(NH), the ν(C=C) band mainly decreases and disappears with increasing band of the nitro group (1514 and 1348 cm–1). This indicates that C=C and -NO2 are effectively activated due to the presence of Ru-Ru and Ru-Ni interfacial centers, respectively, which corresponds to the formation of 4-NE and 4-AE on 2 wt.% RuNi catalyst.
In situ FT-IR spectra of 4-NS hydrogenation in the presence of monometallic Ni, b monometallic Ru, c 0.4 wt% RuNi SAA, and d 2 wt% RuNi in H2 flow at 1700–1240 cm– Range 1 was recorded as the reaction gas after 0, 3, 6, 9 and 12 minutes, respectively. “au” means arbitrary units. Potential energy distributions and corresponding optimized structures for C=C hydrogenation and NO scission into 4-NS on e Ni(111) and f RuNi SAA(111) surfaces. Ru – violet, Ni – green, C – orange, O – red, N – blue, H – white. “ads”, “IS”, “TS”, and “FS” represent the adsorption state, the initial state, the transition state, and the final state, respectively.
Potential pathways for 4-NS transformation to Ni(111) and RuNi SAA(111), including C=C hydrogenation and NO bond cleavage, were investigated by DFT calculations to further elucidate the critical role of 4-NS. Sections of the Ru-Ni interface for the production of 4-AS targets. For the Ni(111) surface (Fig. 6e), the energy barriers for NO scission and hydrogenation of vinyl groups in the first stage are 0.74 and 0.72 eV, respectively, which indicates that the chemoselective hydrogenation of nitro groups in 4-HC is unfavorable. for monometallic nickel surfaces. On the contrary, the energy barrier for NO dissociation is only 0.46 eV higher than that of RuNi SAA (111), which is much lower than that of C=C bond hydrogenation (0.76 eV) (Fig. 6f). This unambiguously confirms that the Ru–Ni interfacial centers effectively lower the energy barrier for NO scission in nitro groups, leading to a thermodynamically preferable reduction of nitro groups compared to C=C groups on the RuNi surfactant surface, which agrees with the experimental results.
The reaction mechanism and calculated energy curves of 4-NS hydrogenation on RuNi SAA were investigated based on DFT calculations (Fig. 7), and the detailed adsorption configuration of the main steps is shown in Supplementary Fig. 41. To optimize the calculation program, energy-producing barriers for water molecules were excluded from the calculations. plate models9,17. As shown in fig. 7, the 4-NS molecules are first absorbed in parallel on the RuNi surfactant, and two O atoms in the nitro group are bound to the Ru-Ni interfacial centers (S0; step I). Subsequently, the NO bond attached to the Ru site is broken, which is accompanied by the formation of a nitroso intermediate (C8H7NO*) at the Ru-Ni interface site and O* at the empty Ni site (S0 → S1 via TS1; energy barrier: 0.46 eV, second step ). O* radicals are hydrogenated by active H atoms to form H2O molecules with an exotherm of 0.99 eV (S1 → S2). Energy barriers for the hydrogenation of the C8H7NO* intermediate (Supplementary Figures 42 and 43) indicate that reactive H atoms from hollow Ru-Ni sites preferentially attack O atoms over N atoms, resulting in C8H7NOH* (S2 → S4; energy barrier TS2: 0.84 eV, step III). The N atoms in C8H7NOH* were then hydrogenated to form C8H7NHOH* after crossing the 1.03 eV barrier (S4→S6; step IV), which is the defining step of the entire reaction. Next, the N–OH bond in C8H7NHOH* was broken at the Ru–Ni interface (S6 → S7; energy barrier: 0.59 eV; stage V), after which OH* was hydrogenated to HO (S7 → S8; exotherm: 0.31 eV) After that, the N atoms of the Ru-Ni hollow sites in C8H7NH* were additionally hydrogenated to form C8H7NH2* (4-AS) with an energy barrier of 0.69 eV (S8 → S10; step VI). Finally, 4-AS and HO molecules were desorbed from the RuNi-PAA surface, and the catalyst returned to its original state (step VII). This unique interfacial structure between single Ru atoms and Ni substrates, accompanied by the synergistic effect of host doping in RuNi SAA, results in the outstanding activity and chemoselectivity of 4-NS hydrogenation.
Rice. 4. Schematic diagram of the mechanism of the hydrogenation reaction of NS to 4-AS on the RuNi PAA surface. Ru – violet, Ni – green, C – orange, O – red, N – blue, H – white. The inset shows the distribution of the potential energy of 4-NS hydrogenation on the RuNi SAA(111) surface, calculated on the basis of DFT. “S0″ represents the initial state, and “S1-S10″ represents a series of adsorption states. “TS” stands for transition state. The numbers in brackets represent the energy barriers of the main steps, and the remaining numbers represent the adsorption energies of the corresponding intermediates.
Thus, RuNi SAA catalysts were obtained using electrosubstitution reactions between RuCl3 and Ni NPs obtained from LDH precursors. Compared with previously reported monometallic Ru, Ni and other heterogeneous catalysts, the resulting RuNi SAA showed superior catalytic efficiency for 4-NS chemoselective hydrogenation (4-AS yield: >99%; TOF value: 4293 h-1). The combined characterization including AC-HAADF-STEM, in situ CO-DRIFTS, and XAFS confirmed that Ru atoms were immobilized on Ni NPs at the one-atom level via Ru-Ni bonds, which was accompanied by electron transfer from Ni to Ru. In situ XAFS, FT-IR experiments, and DFT calculations showed that the Ru-Ni interface site serves as an internal active site for preferential activation of the NO bond in the nitro group; synergism between Ru and neighboring Ni sites facilitates intermediate activation and hydrogenation, thereby greatly improving catalytic efficiency. This work provides insight into the relationship between bifunctional active sites and the catalytic behavior of SAA at the atomic level, paving the way for the rational design of other two-way catalysts with desired selectivity.
The analytical reagents used in the experiment were purchased from Sigma Aldrich: Al2(SO4)3 18H2O, sodium tartrate, CO(NH2)2, NH4NO3, Ni(NO3)2 6H2O, RuCl3, ethanol, 4-nitrostyrene (4- NS), 4-aminostyrene, 4-nitroethylbenzene, 4-aminoethylbenzene and nitrostyrene. Purified water was used in all experiments.
Hierarchical NiAl LDHs were synthesized as precursors by in situ growth. First, urea (3.36 g), Al2(SO4)3·18H2O (9.33 g) and sodium tartrate (0.32 g) were dissolved in deionized water (140 ml). The resulting solution was transferred to a Teflon-coated autoclave and heated to 170°C for 3 h. The resulting precipitate was washed with distilled water and thoroughly dried, after which it was calcined at 500°C (2°C min–1; 4 h) to obtain amorphous Al2O3 . Then Al2O3 (0.2 g), Ni(NO3)2 6H2O (5.8 g) and NH4NO3 (9.6 g) were dispersed in purified water (200 ml) and the pH was adjusted to ~6.5 by adding 1 mol l -1 ammonia water. . The suspension was transferred into a flask and kept at 90°C for 48 h to obtain NiAl-LDH. Then NiAl-LDH powder (0.3 g) was reduced in a stream of H2/N2 (10/90, v/v; 35 ml min–1) at 500°C for 4 h (heating rate: 2°C min -1). Preparation of samples of monometallic nickel (Ni/Al2O3) deposited on amorphous Al2O3. The deposited bimetallic samples of RuNi were synthesized by the electrodisplacement method. Typically, a fresh sample of Ni/Al2O3 (0.2 g) was dispersed in 30 ml of pure water, then a solution of RuCl3 (0.07 mmol l-1) was added slowly and stirred vigorously for 60 minutes under the protection of an N2 atmosphere. The resulting precipitate was centrifuged, washed with pure water, and dried in a vacuum oven at 50°C for 24 h, obtaining a sample containing 0.1% RuNi. Before the catalytic evaluation, the freshly synthesized samples were preliminarily reduced in an H2/N2 flow (10/90, v/v) at 300°C (heating rate: 2°C min–1) for 1 h, and then heated in N2 Cool to room temperature. For reference: samples with Ru/Al2O3 content of 0.4% and 2% by mass, with actual Ru content of 0.36% by mass and 2.3% by mass, were prepared by precipitation by precipitation and heated at 300 °C (consumption of H2/ N2 : 10/90, v/v, heating rate: 2 °C min–1) for 3 hours.
X-ray diffraction (XRD) experiments were carried out on a Bruker DAVINCI D8 ADVANCE diffractometer with a Cu Kα radiation source (40 kV and 40 mA). A Shimadzu ICPS-7500 Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES) was used to determine the actual abundance of elements in various samples. Scanning electron microscopy (SEM) images were imaged using a Zeiss Supra 55 electron microscope. N2 adsorption-desorption experiments were performed on a Micromeritics ASAP 2020 device and specific surface area was calculated using the Brunauer-Emmett-Teller (BET) multipoint method. Transmission electron microscopy (TEM) characteristics were performed on a JEOL JEM-2010 high-resolution transmission electron microscope. High Angle Aberration Corrected Scanning Transmission Electron Microscope Dark Field (AC-HAADF) – STEM with FEI Titan Cube Themis G2 300 with spherical aberration corrector and Energy Dispersive X-ray Spectroscopy (EDS) system and JEOL JEM-ARM200F instrument) and EDS mapping measurements . Fine structure X-ray absorption spectroscopy (XAFS) in situ K-edge of Ru and Ni K-edge was measured on channels 1W1B and 1W2B of the Beijing Synchrotron Radiation Facility (BSRF) of the Institute of High Energy Physics (IHEP), China. Academy of Sciences (KAN). Pulsed CO chemisorption and temperature-programmed hydrogen desorption (H2-TPD) experiments were performed on a Micromeritics Autochem II 2920 instrument using a thermal conductivity detector (TCD). The in situ DRIFTS and FT-IR experiments were carried out on a Bruker TENSOR II infrared spectrometer equipped with a modified in situ reaction cell and a highly sensitive MCT detector. Detailed characterization methods are described in the Supplementary Information.
First, the substrate (4-NS, 1 mmol), solvent (ethanol, 8 ml) and catalyst (0.02 g) were carefully added to a 25 ml stainless steel autoclave. The reactor was then completely purged with 2.0 MPa (>99.999%) hydrogen 5 times, and then pressurized and sealed to 1.0 MPa with H2. The reaction was carried out at 60°C at a constant stirring speed of 700 rpm. After the reaction, the resulting products were identified by GC-MS and quantitatively analyzed using a Shimadzu GC-2014C gas chromatography system equipped with a GSBP-INOWAX capillary column (30 m×0.25 mm×0.25 mm) and a FID detector. The 4-nitrostyrene conversion and product selectivity were determined as follows:
Turnover frequency (TOF) values ​​were calculated as mol 4-NS converted per mol metal sites per hour (mol4-NS mol-1 h-1) based on low 4-NS conversion (~15%). As for the number of Ru nodes, Ru-Ni interface nodes and the total number of surface metal atoms. For the recyclability test, the catalyst was collected by centrifugation after the reaction, washed three times with ethanol, and then re-introduced into the autoclave for the next catalytic cycle.
All density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP 5.4.1). The Generalized Gradient Approximation (GGA) PBE function is used to describe electron exchange and correlation conditions. The Projector Augmented Wave (PAW) method is used to describe the interaction between atomic nuclei and electrons. The Grimm DFT-D3 method describes the effect of van der Waals interactions between the substrate and the interface. Calculation of Energy Barriers by Climbing Elastic Bands with Image Boost (CI-NEB) and Dimer Methods. A frequency analysis of the oscillations was carried out, confirming the presence of only one imaginary frequency in each transition state (Supplementary Figures 44–51). More detailed calculations are described in the additional information.
The main data that supports the plots in this article is provided in the source data files. Other data relevant to this study are available from the respective authors upon reasonable request. This article provides the original data.
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