Highly efficient subnanometer Ru-based catalyst for ammonia synthesis via an associative mechanism

Yanliang Zhou,Qianjin Sai,Zhenni Tan,Congying Wang,Xiuyun Wang,Bingyu Lin,Jun Ni,Jianxin Lin,Lilong Jiang

National Engineering Research Center of Chemical Fertilizer Catalyst,Fuzhou University,Fuzhou 350002,China

Keywords:Ammonia synthesis Sustainability Subnanometer Ru clusters Associative route Catalysis

ABSTRACTThe industrial manufacture of ammonia (NH3) using Fe-based catalyst works under rigorous conditions.For the goal of carbon-neutrality,it is highly desired to develop advanced catalyst for NH3 synthesis at mild conditions to reduce energy consumption and CO2 emissions.However,the main challenge of NH3 synthesis at mild conditions lies in the dissociation of steady N≡N triple bond.In this work,we report the design of subnanometer Ru clusters(0.8 nm)anchored on the hollow N-doped carbon spheres catalyst (Ru-SNCs),which effectively promotes the NH3 synthesis at mild conditions via an associative route.The NH3 synthesis rate over Ru-SNCs(0.49%(mass)Ru)reaches up to 11.7 mmol NH3.(g cat)-1.h-1 at 400°C and 3 MPa,which is superior to that of 8.3 mmol NH3.(g cat)-1.h-1 over Ru nanoparticle catalyst(1.20% (mass) Ru).Various characterizations show that the N2H4 species are the main intermediates for NH3 synthesis on Ru-SNCs catalyst.It demonstrates that Ru-SNCs catalyst can follow an associative route for N2 activation,which circumvents the direct dissociation of N2 and results in highly efficient NH3 synthesis at mild conditions.

1.Introduction

Ammonia (NH3) is one of the most critical ingredients for the manufacture of chemical fertilizer relating to the agricultural and food production[1,2].Besides,NH3has been regarded as a promising clean energy carrier because of the high energy density(3 kW.h.kg-1) and the convenience of storage and transportation[3].The traditional NH3synthesis comes from the Haber–Bosch(HB) process with fossil fuels as feedstock,which consumes about 2% of the global energy supply and accounts for～1.5% of world’s CO2emission [2,4].To realize the magnificent blueprint of carbon-neutrality,it is highly desired to develop NH3synthesis routes that are both environmentally friendly and energy-saving[5].In recent years,with the booming of renewable sources,the technological process of ‘‘renewable energy power →electrolytic H2production →NH3synthesis →NH3utilization”has aroused great attention due to the carbon-free process [6].Nevertheless,the pressure of H2production from pressurized water electrolysis systems is usually in the range of 1.6–3.2 MPa,the NH3synthesis based on the traditional Fe-based catalysts at this condition cannot realize desired performance [7,8].Therefore,it is of significance to exploit advanced catalysts that can efficiently work at mild conditions (e.g.,≤400 °C and 3 MPa) for NH3synthesis to avoid expensive pressure ramping of H2from the electrolysis system.

Over the traditional Fe-or Ru-based catalysts,N2is firstly dissociated at a catalyst surface,and then the adsorbed N atom is hydrogenated stepwise to form NH3,which is well known as dissociative mechanism [9,10].Therein,the activation of N2is regarded as the rate-determining step(RDS)due to the highly steady triple bond of N2(945 kJ.mol-1).Besides,the dissociation barrier of N2on a catalyst is negatively correlated with the desorption energies of NHxspecies (i.e.BEP relation),resulting in the difficulty to realize efficient NH3synthesis [11,12].Recently,it has been reported that the activation of N2via an associative route is favorable in some special catalysts such as Ba-Ru-Li/AC[13],RhCo3cluster[14],Co3-Mo3N[15],and so on,which can bypass the direct N2dissociation.Therein,the activation barrier of N2decreases obviously due to the partial hydrogenation of N2to form N2Hxspecies and it no longer obeys the BEP relation.For instance,the dissociation energy of the N-N bond from*N2H4intermediates is 297 kJ.mol-1,which is less than one-third of that of N≡N triple bond [16].Hence,the development of efficient catalysts that follow associative mechanism rather than dissociative mechanism is an effective strategy to circumvent the direct dissociation of N2and realize desired NH3synthesis activity at mild conditions.

Previous theoretical calculations have shown that the most active sites over Ru metal for N2dissociation are ensembles of five Ru atoms(the so-called B5sites),which expose a three-fold hollow site and a bridge site close together to assure N2molecular not bonding in the same Ru atom [17,18].The quantity of B5sites would be maximized when the size of Ru particle is around 2 nm,while it would decrease obviously with the decrease of Ru particle size below 2 nm [17].Thus,the Ru particle size below 2 nm is believed to possess low reaction activity and do not raise much attention for NH3synthesis.Nevertheless,recently,it has been reported that the Li promoted Ba-Ru/AC can block the Ru B5sites and make the N2hydrogenation to NH3with efficiency via an associative route [13].In addition,DFT calculations show that the small clusters such as Fe3or RhCo3clusters preferentially undergo N2hydrogenation via an associative route for NH3synthesis with a low reaction barrier [12,14].Hence,it is reasonable to speculate that reaction mechanism of NH3synthesis over subnanometer Ru clusters may be different from that on Ru nanoparticles.It is highly desirable to develop efficient subnanometer Rubased catalysts and uncover the reaction mechanim for NH3synthesis at mild conditions.

In this work,we synthesized the N-doped carbon spheres supported subnanometer Ru clusters (Ru-SNCs) catalyst for NH3synthesis at mild conditions.It is found that the Ru-SNCs catalyst exhibits a higher NH3synthesis rate compared with the Ru nanoparticle catalyst.Various characterizations show that N2H4species are the main intermediates in NH3synthesis over Ru-SNCs catalyst.It demonstrates that the Ru-SNCs catalyst follows an associative route to realize efficient NH3synthesis,which is different from the traditional Ru-based catalyst via a dissociative route.This work discovers the associative route over subnanometer Ru clusters catalyst and provides an important implication to develop efficient catalysts for NH3synthesis at mild conditions.

2.Experimental

2.1.Catalyst preparation

2.1.1.Synthesis of hollow N-doped carbon spheres (HNCSs) support

The preparation method of HNCSs support has been reported previously [19].In detail,the 25% NH3solution (5 ml),ethanol(120 ml),and deionized water (40 ml) were mixed together.After that,a series of poloxamer(0.6 g),tetraethyl orthosilicate(5.6 ml),resorcinol(0.8 g),and formaldehyde(1.12 ml)was added stepwise into the solution and stirred for 1.5 h.Then,melamine(0.63 g)and formaldehyde (0.84 ml) were added and continually stirred at 100 °C for 24 h.The resulting suspension was further filtrated and washed with deionized water to obtain the solid product.It was then calcined at 700 °C under Ar atmosphere for 3 h.Finally,the sample was immersed in 40% HF solution and etched for several hours to remove SiO2substrate.After filtration and washing,the sample was dried at 80 °C overnight to obtain the HNCSs support.

2.1.2.Synthesis of subnanometer Ru-based and nanoparticle Ru-based catalysts

Typically,a quantity of ruthenium acetylacetone (Ru(acac)3)was dissolved in isopropyl alcohol (30 ml),and the resulting solution was stirred evenly at 0 °C.Then,the HNCSs support (100 mg)was slowly added into the solution and further stirred for 1 h.After filtration and washing by deionized water,the product was dried at 80°C for one night.Finally,the sample was pyrolyzed by 10%H2/Ar atmosphere (60 ml.min-1) at 400 °C for 4 h.Through varying the content of Ru(acac)3in these samples (Ru loading ≤0.5% (mass)),a series of HNCSs supported subnanometer Ru cluster catalysts was synthesized.In comparison,the synthetic procedure of Ru nanoparticle catalyst was similar to that of Ru cluster catalysts except the Ru content was controlled at 1.2% (mass) and the stirring process was conducted at room temperature.Besides,the typical Ru-based catalysts containing Ru/MgO and Ru/AC (activated carbon) were prepared by impregnation for comparison.

2.2.NH3 synthesis activity test

The catalytic activities of the as-synthesized catalysts in NH3synthesis were measured in a pressure fixed bed reactor.In detail,0.3 g of a catalyst (1.1–1.7 mm) was diluted with appropriate amount of quartz powder(1.1–1.7 mm)and loaded into the stainless steel reactor tube.Before measurement,the catalysts were reduced by feed gas (25%N2–75%H2) at 400 °C for 3 h.Then,NH3synthesis activities of catalysts were measured at 400 °C and 3 MPa with a weight hourly space velocity (WHSV) of 60 000 ml.g-1.h-1.Besides,NH3synthesis rates of catalysts were evaluated under different reaction pressures (0.2–5 MPa) and temperatures(250–400°C).The outlet NH3was dissolved in 0.02 mol.L-1H2SO4solution,which was then analyzed by ion chromatography.To evaluate the intrinsic activity,the turnover frequency based on the total Ru metal (TOFRutotal) was calculated via the following equation:

where,r is the NH3synthesis rate(mol.(g cat)-1.s-1),nMrepresents the molar amount of Ru (mol.(g cat)-1) over catalysts determined by ICP-AES analysis.

2.3.Catalyst characterization

Powder X-ray diffraction (XRD) patterns were acquired on an X’Pert Pro diffractometer (PANalytical) using Cu Kα radiation source.Nitrogen physical adsorption experiment was conducted on an ASAP 2020 apparatus.The specific surface areas of samples were calculated by BET (Brunauer-Emmett-Teller) equation.The Ru loadings of the catalysts were acquired by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis.Raman spectroscopy was measured on a LabRam confocal microprobe Raman instrument equipped with an Ar laser.

Catalyst morphologies were detected on a Hitachi Model S-4800 scanning electron microscopy (SEM).Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM),and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of catalysts were obtained on a JEM-2010 instrument.The energydispersive X-ray (EDX) spectroscopy analysis was acquired on a JEM-2010 instrument.

UV–vis absorption spectroscopy (UV–vis) was measured on a Perkin Elmer Lambda spectrometer.In the exit of the NH3synthesis reactor,we used a flask trap including para-(dimethylamino)benzaldehyde and sulfur acid solution to capture the products during NH3synthesis reaction for 15 min at different temperatures.After that,the collected solution was measured by UV–vis to detect the intermediates in NH3synthesis.

In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) under the atmosphere of 25%N2–75%D2was conducted on a Nicolet Nexus FT-IR spectrometer in the range of 1000–3000 cm-1.Before measurements,the catalyst was reduced by 10% H2/Ar with a flow of 30 ml min-1at 400 °C for 2 h.Then it was cooled down to the given temperature to obtain a background spectrum.Then,the sample was treated by a 25%N2–75%D2mixture at different temperatures.Finally,the spectrum of catalyst was collected over time.Notably,the sample spectrum needed to deduct the corresponding background spectrum at different temperatures.

3.Results and Discussion

3.1.Synthesis and characterization of subnanometer Ru-based catalysts

The schematic for the synthesis of hollow N-doped carbon spheres(HNCSs)supported Ru clusters(Ru-SNCs)or nanoparticles(Ru-NPs)is shown in Fig.1.In detail,the particle sizes of Ru metal are regulated by the variations of Ru loading and stirring temperature.For the preparation of subnanometer Ru clusters,the mixture of HNCSs,isopropyl alcohol,and Ru(acac)3was stirred evenly at zero degree,and the Ru loading was controlled below 0.5%(mass).After the pyrolysis of precursors under H2atmosphere at 400 °C,the HNCSs supported subnanometer Ru clusters (xRu-SNCs) catalysts were obtained,where the x presents Ru content over these samples.Inductively coupled plasma atomic emission spectroscopy(ICP-AES)shows that the Ru contents over these catalysts are 0.13% (mass),0.30% (mass),and 0.49% (mass) (Table 1),and thus the samples are herein donated as 0.1Ru-SNCs,0.3Ru-SNCs and 0.5Ru-SNCs,respectively.By contrast,through increasing Ru loading and altering the stirring at room temperature,the HNCSs supported Ru nanoparticles catalyst (1.2Ru-NPs) was synthesized for comparison.

Fig.1.Schematic of the preparation of Ru-SNCs and Ru-NPs catalysts.

The morphological features of the synthesized catalysts were measured by scanning electron microscopy (SEM).As displayed in Fig.2,the uniform microspheres with hollow structure can be clearly observed over these catalysts.Statistical analysis for ca.100 microspheres shows that the average diameter of carbon spheres is in the range of 160–180 nm(Fig.2),which demonstrates that the Ru loading has a small impact on the morphology of HNCSs support.High-resolution transmission electron microscopy(HRTEM) images with different image magnifications do not exhibit any sights of Ru nanoparticles over 0.1Ru-SNCs(Fig.3(a)–3(b)),0.3Ru-SNCs(Fig.3(c)–3(d)),and 0.5Ru-SNCs(Fig.3(e)–3(f)),implying the highly dispersed Ru species as tiny clusters or single atoms over these samples.In comparison,distinct Ru nanoparticles are detected over 1.2Ru-NPs catalyst (Fig.3(g)).The lattice spacing over 1.2Ru-NPs catalyst is around 0.23 nm in the inset of Fig.3(g),which is attributed to(1 0 0)crystal planes of Ru metal.Moreover,the average size of Ru nanoparticles over 1.2Ru-NPs catalyst is near 4.4 nm (Fig.3(h)).

Fig.2.SEM images and the corresponding carbon spheres diameter distribution of (a)-(b).0.1Ru-SNCs,(c)-(d).0.3Ru-SNCs,(e)-(f).0.5Ru-SNCs,and (g)-(h).1.2Ru-NPs samples.

Fig.3.HRTEM images of the (a)-(b) 0.1Ru-SNCs,(c)-(d) 0.3Ru-SNCs,(e)-(f) 0.5Ru-SNCs,(g) 1.2Ru-NC and (h) the corresponding size distribution of Ru nanoparticle.

In order to get insight into the state of subnanometer Ru species,high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)technique was conducted.As shown in Fig.4(a),the tiny Ru clusters can be clearly witnessed over 0.5Ru-SNCs catalyst.More images of 0.5Ru-SNCs in different areas and magnifications have been added in the Fig.S1 (Supplementary Material).Statistical analysis for ca.100 particles shows that the average size of Ru clusters is near 0.8 nm (Fig.4(b)).Furthermore,energy-dispersive X-ray spectroscopy (EDX) was conducted to detect the distribution of elements.As displayed in Fig.4(c),the homogeneous dispersion of C and N elements,as well as tiny Ru clusters can be found over 0.5Ru-SNCs sample.These results demonstrate that the HNCSs supported subnanometer Ru clusters catalysts were successfully synthesized.

Fig.4.(a)HAADF-STEM image and(b)the size distribution of Ru clusters over 0.5Ru-SNCs catalyst;(c)EDX mapping of Ru,N,and C elements over the 0.5Ru-SNCs catalyst.

XRD patterns were measured to study the crystal structures of the as-prepared catalysts.Fig.5(a) shows that all catalysts do not present the diffraction peaks of Ru species,indicating the low loading and tiny clusters of Ru species as confirmed by HRTEM.Meanwhile,there are the detections of broad diffraction peaks at around 23.4° and 43.8°,which are assigned to the(0 0 2)crystal planes of graphite carbon and the (1 0 0) planes of disordered carbon,respectively [19,20].The graphitic feature of these catalysts was further confirmed by Raman detections.Raman spectra (Fig.5(b))of all catalysts exhibit the absorption bands at 1334 and 1597 cm-1,which is attributed to the D and G bands of defective carbon and sp2-bonded graphitic carbon,respectively[21].Besides,the ID/IGvalues of these samples are in the range of 0.83–0.90(Table 1),confirming that the Ru loading has little effect on carbon defective configuration.Nitrogen physical adsorption was carried out to measure the specific surface area of these catalysts,and the parameters were summarized at Table 1.The nitrogen adsorption/desorption isotherms of these catalysts are shown in Fig.S2,which are belong to the type IV isothermal curve with H3hysteresis loop,demonstrating the existence of mesoporous structure.The specific surface areas and pore diameters of these samples are similar in the range of 479–559 m2.g-1and 9.9–11.7 nm,respectively.These characterizations demonstrate that the as-prepared catalysts possess the similar structural properties except for the differences in the loading and particle size of Ru metal.

Table 1 Element composition and textural properties of the as-prepared catalysts

Fig.5.(a) X-ray diffraction patterns and (b) Raman spectra of 0.1Ru-SNCs,0.3Ru-SNCs,0.5Ru-SNCs,and 1.2Ru-NPs catalysts.

3.2.Catalyst activity and stability

In order to explore the effect of Ru particle size on NH3synthesis,the reaction activities of the as-synthesized catalysts were conducted under a feed gas of 25%N2–75%H2and a WHSV of 60 000 ml.(g cat)-1.h-1.As depicted in Fig.6(a),the NH3synthesis rate over the subnanometer Ru catalysts increases obviously along with higher loading of Ru metal at 400 °C and 3 MPa.Notably,the NH3synthesis rate over 0.5Ru-SNCs can reach up to 11.7 mmol NH3.(g cat)-1.h-1,which is～6.5 times and～3.1 times that of 0.1Ru-SNCs(1.8 mmol NH3.(g cat)-1.h-1) and 0.3Ru-SNCs (3.8 mmol NH3.(g cat)-1.h-1),respectively.Nevertheless,the NH3synthesis rate over 1.2Ru-NPs catalyst is only 8.3 mmol NH3.(g cat)-1.h-1even with high Ru loading,which is lower than that over 0.5Ru-SNCs.To demonstrate the superior performance of 0.5Ru-SNCs,typical Rubased catalysts including Ru/MgO and Ru/AC with similar Ru content (0.50%(mass)) were also prepared and evaluated for comparison.Under the same reaction conditions,the NH3synthesis rate over 0.5Ru-SNCs catalyst is～3.7 and～13.0 times that of Ru/MgO(3.2 mmol NH3.(g cat)-1.h-1) and Ru/AC (0.9 mmol NH3.(g cat)-1-.h-1) as shown in Fig.6(a),respectively.For intrinsic comparison,turnover frequency of Ru metal (TOFRutotal) was calculated on the basis of total Ru atoms in the catalysts.As shown in Fig.6(b),0.5Ru-SNCs catalyst possesses the superior TOFRutotalvalue of 0.0670 s-1,which is 3.5-folds that of 0.0193 s-1over 1.2Ru-NPs,and also outperforms most of Ru-based catalysts ever reported in Table S1 [22–24].

Fig.6.(a) NH3 synthesis rate over the catalysts at 400 ℃and 3 MPa.(b) TOFRutotal of the catalysts at 400 ℃and 3 MPa.(c) NH3 synthesis rate over 0.5Ru-SNCs at different pressures and 400 ℃.(d) The stability test over 0.5Ru-SNCs at 400 ℃and 3 MPa.

Furthermore,the effect of reaction pressure on the catalytic activity of 0.5Ru-SNCs was investigated.As shown in Fig.6(c),NH3synthesis rate over 0.5Ru-SNCs is positively associated with the increase of pressure,demonstrating the likely absence of hydrogen poisoning over subnanometer Ru clusters [25].For instance,NH3synthesis rate increases from 3.3 to 20.1 mmol NH3-.(g cat)-1.h-1along with the increase of reaction pressure from 0.2 to 5 MPa at 400°C.Typically,0.5Ru-SNCs has a higher NH3synthesis rate of 6.5 mmol NH3.(g cat)-1.h-1in comparison with 1.2Ru-NPs (3.0 mmol NH3.(g cat)-1.h-1at 400 °C and 1MPa (Table S1).Besides,the NH3synthesis rates of 0.5Ru-SNCs and 1.2Ru-NPs from 250 to 400 °C were measured.Fig.S3 shows that there is a 12.3 times rise of NH3synthesis rate from 0.95 to 11.7 mmol NH3.(g cat)-1.h-1at 3 MPa along with the increased reaction temperature from 250 to 400°C.Moreover,the reaction activity of 0.5 Ru-NPs is higher than that of 1.2Ru-NPs at different temperatures.It is known that the stability of catalyst is a crucial factor for NH3synthesis.The thermal stability investigation of the 0.5Ru-SNCs catalyst was performed at 400 °C and 3 MPa.As displayed in Fig.6(d),the NH3synthesis rate remains constant value of 11.7 (mmol NH3).(g cat)-1.h-1in a run of 50 h test.In order to illustrate the variation of Ru clusters,the used 0.5Ru-SNCs catalyst was measured by HRTEM characterization.As shown in Fig.S4,after careful examination of different HRTEM images,almost no obviously Ru nanoparticles are observed.Furthermore,HAADF-STEM measurement and statistical analysis(Fig.S5) show that the average particle size of Ru over 0.5Ru-SNCs only slightly increases from 0.8 nm to 0.9 nm after NH3synthesis.The results of activity test and structure characterization indicate the excellent activity and stability of subnanometer 0.5Ru-SNCs for NH3synthesis under mild conditions.

3.3.Reaction mechanism over subnanometer Ru catalysts

The reaction mechanisms of NH3synthesis mainly contain dissociative and associative routes,where adsorbed N2directly dissociates or hydrogenates step by step into NNHx(x=1–6) to form NH3[10,12].For traditionnal Ru nanoparticle catalysts,N2molecular prefers to directly dissociate over Ru B5step sites via a dissociative route.Nevertheless,the number of B5step sites would decrease obviously when the Ru nanopartice size is below 2 nm,especially in subnanometer scale [9,17].Therefore,the reaction mechanism of NH3synthesis over subnanometer Ru clusters or Ru single-atoms catalysts may be different from the dissociative route over Ru nanoparticle.For instance,atomically dispersed Ru-based catalysts were reported to activate N2molecular through an associative route [26,27].Moreover,the activation of N2via an associative route presents a low barrier due to the circumvention of direct dissociation of N2as proved by experimental and theoretical analysis[26,28].Accordingly,the apparent activation energies(Ea) over 0.5Ru-SNCs and 1.2Ru-NPs were measured.As shown in Fig.7,the Eafor NH3synthesis on 1.2Ru-NPs derived from the Arrhenius plots is 99 kJ.mol-1,which is similar to the Ru NPs(98 kJ.mol-1) and Cs-Ru/MgO (106 kJ.mol-1) catalysts that obey the dissociation route [29].By contrast,there is significant decrease of Ea,which is 64 kJ.mol-1over 0.5Ru-SNCs.The low Eavalue suggests that the activation of N2over 0.5Ru-SNCs is facile,which may be resulted from a different reaction route.

Fig.7.Arrhenius plots over the 0.5Ru-SNCs and 1.2Ru-NPs catalysts.

In order to demonstrate the reaction mechanism over the subnanometer 0.5Ru-SNCs cayalyst,UV–vis absorption spectroscopy(UV–vis) was conducted to detect the intermediate species in NH3synthesis.A long time of 15 min was take to capture the products during NH3synthesis at different temperatures via a flask trap containing para-(dimethylamino) benzaldehyde and sulfur acid solution,and the collected solution was measured via UV–vis.As displayed in Fig.8(a),the absorption band at ca.468 nm can be observed in the products of 0.5Ru-SNCs catalyst at 250 °C,which is ascribed to the N2H4species[30–32].Notably,the band intensity of the N2H4species decreases along with the increase of reaction temperature,demonstrating the facile conversion of N2H4intermediates species at higher temperature [32].In comparison with 0.5Ru-SNCs,almost no signal of N2H4species can be observed over 1.2Ru-NPs at different temperatures (Fig.8(b)).There results demonstrate that the existence of associative route over 0.5Ru-SNCs,which is different from the 1.2Ru-NPs catalyst with a dissociative route.

Fig.8.UV–vis absorbance spectra of the products over (a) 0.5Ru-SNCs and (b) 1.2Ru-NPs catalysts at different temperatures during NH3 synthesis.

Fig.9.In situ DRIFTS investigation of 0.5Ru-SNCs after the treatment of 25%N2–75%D2 at different temperatures.

In situ DRIFTS experiment under the atmosphere of 25%N2–75%D2was further performed to identify the surface intermediate species after N2molecules activation.As shown in Fig.9,0.5Ru-SNCs shows the absorption band at 2321 cm-1,which is ascribed to the N-D stretching vibration of*N2Dxintermediate [33,34].Meanwhile,the intensity of this band decreases along with the elevated temperature,which is in line with the results of UV–vis,suggesting the facile conversion of*N2Dxspecies at higher temperature.The results clearly demonstrate that N2H4is the main intermediate species over 0.5Ru-SNCs during NH3synthesis.Taking the results of UV–vis,in situ DRIFTs and Eavalues into consideration,it is deduced that the subnanometer 0.5Ru-SNCs catalyst can follow an associative route for N2activation,which bypasses the direct N2dissociation and results in superior NH3synthesis performance at mild conditions.

4.Conclusions

In summary,we have developed a simple approach for the preparation of subnanometer Ru clusters loaded on HNCSs catalyst via regulating Ru loading and stirring temperature.The average size of Ru subnanometer clusters over 0.5Ru-SNCs is around 0.8 nm.The 0.5Ru-SNCs catalyst exhibits a superior NH3synthesis rate of 11.7 mmol NH3.(g cat)-1.h-1at 400°C and 3 MPa,which is superior to that of 8.3 mmol NH3.(g cat)-1.h-1over 1.2Ru-NPs catalyst.Moreover,the TOFRutotalvalue over 0.5Ru-SNCs is 3.5 times that of 1.2Ru-NPs catalyst.Various characterizations reveal that N2preferentially undergo the hydrogenation to N2Hxintermediates via an associative mechanism,which bypasses the bottleneck of N2dissociation over traditional Ru-based catalysts.Therefore,the developed Ru-SNCs catalyst presents superior NH3synthesis performance at mild conditions.This study provides an important implication to develop efficient subnanometer Ru-based catalysts for NH3synthesis via an associative route at mild conditions.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are grateful to the Key Research &Development Program of National Natural Science Foundation of China(22038002) and the National Natural Science Foundation of China(21972019,22108037) for financial support of this work.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.01.015.