The highly selective catalytic hydrogenation of CO2 to CO over transition metal nitrides

Yichao Wu,Zhiwei Xie,Xiaofeng Gao,Xian Zhou,Yangzhi Xu,Shurui Fan,Siyu Yao,Xiaonian Li,Lili Lin,*

1 Institute of Industrial Catalysis,State Key Laboratory of Green Chemistry Synthesis Technology,College of Chemical Engineering,Zhejiang University of Technology,Hangzhou 310014,China

2 Key Laboratory of Biomass Chemical Engineering of Ministry of Education,College of Chemical and Biological Engineering,Zhejiang University,Hangzhou 310027,China

Keywords:Carbon dioxide Chemical reaction Catalysis Reverse water gas shift (RWGS) reaction Transition metal nitride In-situ X-ray diffraction characterization

ABSTRACT Three transition metal-like facet centered cubic structured transition metal nitrides,γ-Mo2N,β-W2N and δ-NbN,are synthesized and applied in the reaction of CO2 hydrogenation to CO.Among the three nitride catalysts,the γ-Mo2N exhibits superior activity to target product CO,which is 4.6 and 76 times higher than the other two counterparts of β-W2N and δ-NbN at 600°C,respectively.Additionally,γ-Mo2N exhibits excellent stability on both cyclic heating–cooling and high space velocity steady state operation.The deactivation degree of cyclic heating–cooling evaluation after 5 cycles and long-term stability performance at 773 and 873 K in 50 h are all less than 10%.In-situ XRD and kinetic studies suggest that the γ-Mo2N itself is able to activate both of the reactants CO2 and H2.Below 400 °C,the reaction mainly occurs at the surface of γ-Mo2N catalyst.CO2 and H2 competitively adsorbe on the surface of catalyst and CO2 is the relatively stronger surface adsorbate.At a higher temperature,the interstitial vacancies of the γ-Mo2N can be reversibly filled with the oxygen from CO2 dissociation.Both of the surface and bulk phase sites of γ-Mo2N participate in the high temperature CO2 hydrogenation pathway.

1.Introduction

The increasing concentration of CO2,which causes global warming and ocean acidification,is one of the major challenges of the sustainable development of human society [1,2].The fixation and catalytic transformation of CO2are the major routes to convert CO2into useful resources and mitigate the negative effect of CO2emission[3–5].However,there are only limited cases in the industry that directly use CO2as a substrate,such as urea synthesis [6]and polycarbonate synthesis [7].The hydrogenation of CO2into CO or synthesis gas,known as the reverse water gas shift (RWGS)reaction,is a promising and flexible route for CO2conversion,as the products could be used to convert into methanol and other valuable hydrocarbons through well-developed C1processes [8–10].Detailed energy balance estimation also suggests that the two-step conversion of CO2to methanol via RWGS+syngas methanol synthesis route is more favorable and energy efficient[11].As a result,developing an inexpensive and highly active RWGS catalyst with considerable selectivity and stability at high temperature is one of the indisputably important topics for CO2conversion.

In the previous studies,the copper based inexpensive catalysts have been reported highly selective and efficient for the RWGS reaction [12–14].In comparison,the other 3d transition metal[15,16] and noble metal centers [17,18] tend to convert part of the feed CO2into undesirable C1byproduct,methane.Despite of the good catalytic activity and selectivity,the Cu based catalysts suffer significantly from the deactivation at high temperature[19].Due to its low Tammann temperature and redox properties,highly dispersed Cu tend to sinter in the presence of steam [20],which makes the Cu based catalysts not suitable for the high temperature RWGS reaction.Recently,the early transition metal carbide (TMC) and TMCs supported metal catalysts have been extensively studied and reported to be another group of highperformance catalysts,exhibiting superior activity at different space velocity [21,22].For example,the hexagonal β-Mo2C has been reported to be more active than Cu based catalysts [20].The transition metal nitrides (TMN) usually share similar geometric and electronic structures with the TMCs and thus exhibit similar noble metal like behaviors with the carbide catalysts [23].Meanwhile,it has also been reported that the TMNs are more stable than the carbides in the oxidative atmosphere,which makes them potential alternatives of the TMCs in the RWGS reaction[24,25].To the best of our knowledge,although the catalytic performances of transition metal carbides for RWGS have been studied extensively [26,27],there are limited number of studies on the transition metal nitrides,especially little attention has been paid to the intrinsic structure changes during reaction condition.Therefore,a systematic study on the catalytic properties of TMNs in the CO2hydrogenation to CO is important to evaluate their applicability as the RWGS catalysts.

Herein,we have synthesized a serial of face centered cubic(FCC) structured transition metal nitrides (γ-Mo2N,β-W2N and δ-NbN) for the high temperature RWGS reaction.By comparing the catalytic performances of the molybdenum,tungsten and niobium nitrides,we discovered that all the three nitrides showed high selectivity to CO,but the molybdenum nitride exhibited the highest activity to CO.The conversion rate of CO2is times higher than the other two counterparts.Additionally,the stability of the γ-Mo2N isre excellent that the deactivation rates are less than 10%in both cyclic and steady state tests,suggesting the γ-Mo2N is a promising catalyst for RWGS reaction.The in-situ XRD and kinetic studies have demonstrated that the bulk structure of the molybdenum nitride is stable under the high temperature condition,and the oxygen from CO2dissociation on the catalyst surface is able to be removed by the hydrogen.At low temperature,the reaction mainly occurs at the surface.While at over 400 °C,the bulk phase vacancies will also involve in the reaction based on the lattice expansion of in-situ XRD,which enhances the dissociation of CO2for higher activity.

2.Materials and Methods

2.1.The preparation of transition metal nitrides

The transition metal nitrides were synthesized following literature reported procedures [24,28,29].MoO3,WO3and Nb2O5were purchased from Alfa Aesar and grinded into fine powder without other treatment.Typically,0.8 g precursor was loaded in a tubular fixed bed reactor with the inner diameter of 8 mm.The sample was first in the flow of Ar for 20 min to remove the remained air in the reactor,and then heated to 700°C at a heating rate of 5°C.min-1in the flow of NH3(100 ml.min-1).After heated to the target temperature,the sample was kept at 700 °C for another 2 hrs.After cooling down to room temperature,the catalyst was passivated in the flow of pure CO2(20 ml.min-1) for 6 hours.

2.2.Catalyst characterization

2.2.1.X-ray diffraction (XRD)

X-ray diffraction(XRD)patterns were recorded with a PANalytical X’Pert PRO powder diffractometer using Cu Kα radiation(λ=0.1541 nm).The working voltage was 40 kV and the working current was 40 mA.The patterns were collected with a 2θ range from 10° to 80° at a step of 1°/min.

2.2.2.Transmission electron microscopy (TEM)

The TEM characterizations were conducted using Tecnai F30 electron microscope operating at 300 kV.All samples were dispersed in ethanol by ultrasonication.The suspension was then deposited onto a copper grid with carbon film and dried at 60 °C for measurements.

2.3.Catalytic performance evaluation of the RWGS reaction

The performance evaluation for RWGS reaction was carried out in a fixed-bed flow stainless steel reactor.10 mg catalyst was diluted with 10 mg quartz sand,and then packed into the quartz tubular reactor.Prior to the catalytic performance evaluation,the catalyst was reduced in a stream of 25% H2/N2(10 ml.min-1H2:30 ml.min-1N2) at 590 °C for 1 h under atmospheric pressure.Then,the catalyst was cooled down to 350 °C in the flow of inert gas.At 350°C,the reaction atmosphere was switch to the reaction gas of CO2,H2and N2mixture at a ratio of 1:3:1(10 ml.min-1CO2:30 ml.min-1H2:10 ml.min-1N2).The reaction was performed under atmospheric pressure.The catalytic activity test was performed at 350,400,450,500,550 and 600 °C respectively,and the temperature was held at each step for 2 h.The reactants and products in the exhaust gas flow from the reactors were analyzed by an online gas chromatographer (8860 GC System,Agilent)equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID) using inlet standard method.The CO2conversion and CO selectivity were calculated based on the GC results using the following equations.

The stability test of the γ-Mo2N catalyst was performed under heating–cooling cyclic condition and steady state condition respectively.In the cyclic test,10 mg catalyst was mixed with 10 mg quartz sand and activated using the same procedure as the activity evaluation test.The reaction condition is the same with the performance evaluation.The RWGS reaction was performed at 350,400,450,500,550 and 600°C,and each step for 2 h.After one cycle,the catalyst was cool to 350°C to start the next cycle.The same procedure was carried out for commercial CuZnOAl2O3catalyst for the cyclic stability test.The steady state of γ-Mo2N catalyst was performed at 500 and 600 °C respectively.The space velocity was increased to 720000 ml.g-1.h-1(24 ml.min-1CO2,72 ml.min-1H2and 24 ml.min-1N2).The test was lasted for 50 h.

2.4.In-situ XRD characterization

The in-situ XRD experiment was performed at 17 BM beamline of Advanced Photon Source (APS),Argonne National Laboratory.The wavenumber of the incident X-ray is 0.015425 nm.The images of the powder diffraction were collected by a planar CCD detector.The images were integrated to obtain the diffraction pattern.All patterns were treated by the GSAS-II package[30].The Clausen Cell[31]was used in the in-situ XRD characterization.About 2 mg catalyst powder was loaded in the quartz capillary.First,the catalyst was reduced in a stream of 25%H2/N2(2 ml.min-1H2:6 ml.min-1N2) at 590 °C for 1 h.Then,the catalyst was cooled down to room temperature in the flow of Ar.Then,the reaction atmosphere was switch to the reaction gas of CO2and H2mixture at a ratio of 1:3(2 ml.min-1CO2:6 ml.min-1H2).The reaction was performed under atmospheric pressure.The temperatures of 350 °C,400 °C,450 °C,500 °C,550 °C and 600 °C are each held for 15 min,respectively.

2.5.Kinetic studies of the molybdenum nitride catalyst for RWGS reaction

The kinetic study was performed using the fixed-bed reactor.The conversion of CO2was controlled below 5% at all the tested condition.The H2partial pressure was set at 7.8,15.5,31.1 and 38.8 kPa,and the CO2partial pressure was tuned from 3.1 to 61.5 kPa at each constant p(H2).The CO2conversion rate was determined using the same method with the activity test.The apparent reaction order of the CO2is determined using simple order model.

3.Results and Discussion

3.1.The XRD characterization of as synthesized transition metal nitride catalysts

The FCC structured transition metal nitrides,γ-Mo2N,β-W2N and δ-NbN were synthesized via temperature program nitrification method using ammonia as the nitrogen source [32].The XRD profiles of the as-synthesized transition metal nitrides were shown in Fig.1.All the catalysts exhibit typical facet centered cubic crystal structure [33].The major diffraction peaks in the 2θ range from 20° to 80° can be attributed to the lattice index of (111),(200),(220),(311) and (222) respectively.Judging from the relatively positions of the diffraction peaks,it can be seen that the niobium nitride appears at a relatively lower angle,indicating the δ-NbN has the largest lattice parameter (0.248 nm),while the tungsten nitride has the smallest size of crystal unit cell (0.242 nm).The 2θ angle of the diffraction peaks of δ-NbN sample is slightly higher than the standard PDF card of δ-NbN,which can be explained by the relatively lower bulk phase N/Nb ratio than the stoichiometric value.

3.2.TEM characterization of the TMN catalysts

The morphology of the TMN catalysts was observed using TEM images(Fig.2).All of the three catalysts exhibit porous appearance.Each of the nitride catalysts can be regarded as the construction of numbers of small irregular shaped nitride nanoparticles.According to the previous studies,a huge lattice expansion occurs during the phase transformation from metal oxides to transition metal nitrides during the temperature program nitrification process[34].Due to the strong tension,cracks form in the crystallite of the TMN precursor,generating the porous appearance.Based on the electron diffraction fringe,the interplane distances of the γ-Mo2N,δ-NbN and β-W2N were determined as 0.251,0.251 and 0.245 nm respectively,which are close to the plane distance of(111)surface.The slightly larger interplane distance than the data obtained from powder diffraction profiles may suggested that the nitrification degree of the surface is larger than the bulk phase.

3.3.The evaluation of catalytic performances of TMN catalysts for RWGS reaction

The catalytic activity,selectivity and stability of three transition metal nitride catalysts were evaluated using a fixed bed flow reactor(Fig.3).The quantification was performed by online GC system using N2as the inlet standard.The dependence of CO2hydrogen activity with temperature was first evaluated by the stepwise reaction in the range from 350°C to 600°C(Fig.4(a)).Although all the tested catalysts showed very high selectivity in the CO2hydrogenation reaction with merely CH4forming during the evaluation,the activities for CO2hydrogenation showed huge difference from each other.As shown in Fig.3(a),the niobium nitride is almost inert for the RWGS reaction.Even when the temperature reached 600°C,the overall CO2conversion is only 0.6%.The β-W2N showed moderate CO2hydrogenation activity.The CO2convresion at 600 °C is about 14%.In comparison,the CO2conversion of γ-Mo2N reaches 45.9% at the same temperature.What’s more,the low temperature CO2conversion of γ-Mo2N is also remarkable.The space time yield of the γ-Mo2N catalyst in the RWGS reaction at 350 °C is 21.9 mol CO.(g cat)-1.h-1,nearly comparable to the mass activity of β-W2N at 500 °C.The mass activity of γ-Mo2N at 600 °C is as high as 123 mol CO.(g cat)-1.h-1,while the mass activities of β-W2N and δ-NbN are only 35.2 and 1.6 mol CO.(g cat)-1.h-1(Fig.3(b)).Therefore,it can be concluded that the γ-Mo2N can be regarded as a promising RWGS catalyst among nitrides based on reaction activity and selectivity.The improvement of the synthesis method of γ-Mo2N catalyst to enhance its surface area will be an effective method to promote the CO2hydrogenation activity.

Fig.1.The XRD patterns of transition metal nitride catalysts of γ-Mo2N,β-W2N and δ-NbN.

Fig.2.The TEM and HR-TEM images of transition metal nitrides.(a)–(c) The TEM images and (d)–(f) the HR-TEM images of γ-Mo2N,β-W2N and δ-NbN catalysts.

Furthermore,the stability of the γ-Mo2N was evaluated using two methods.In the first test,the stability was evaluated in five successive heating–cooling cycles from 350 °C to 600 °C.The CO2conversion at each temperature was measured(Fig.3(c)-(d)).From the first to the fifth cycle,the CO2conversion reduced from 45.9 to 42.8%,which equals to a deactivation of 6.7%,suggesting the molybdenum nitride catalyst can well maintain its activity under start-up shut-down cycles(Fig.3(c)).On the contrary,the CO2conversion of the commercial Cu-ZnO-Al2O3catalyst dropped from 49.5% to 18.1% (Fig.3(d)).The catalyst deactivated completely at the low temperature region.Therefore,the robustness of γ-Mo2N is superior over Cu based catalyst facing unstable operation condition.The stability under steady state was further evaluated under a much harsher condition (Fig.4).In the experiment,the space velocity was further increased to 720,000 ml.(g cat)-1.h-1.In a 50 hrs stability test at 500 °C and 600 °C.The deactivation rate is only 6% and 9% respectively,demonstrating the excellent stability of γ-Mo2N catalyst for the RWGS reaction.

Fig.4.Steady state stability evaluation of the γ-Mo2N catalyst.

3.4.The in-situ XRD

The in-situ XRD experiment was performed to understand the structure variation of molybdenum nitride catalyst under working condition (Fig.5).The procedure of in-situ XRD experiment was shown in Fig.5(a).The XRD profile of the fresh catalyst,the passivated one,showed typical features of the FCC Mo2N and the lattice parameter of the sample is 0.2424 nm,smaller than the lattice of the activated γ-Mo2N of 0.2396 nm.Based on the structure of the γ-Mo2N,it is known that at least half of the interstitial sites is vacant.The oxygen species introduced from the passivation procedure may occupy these sites and induce the expansion of the lattice parameter[35,36].As a result,after the reductive activation of molybdenum nitride at high temperature,the oxygen atoms doped in the lattice are removed and the lattice shrinks to a smaller value.After activated by the H2/N2(1:3) atmosphere,the γ-Mo2N was exposed to the RWGS gas feed.And then,the catalyst underwent stepwise heating to each reaction temperature of 350 °C,400 °C,500 °C,600 °C and kept 10 min at each step (Fig.5(b)).With the increased reaction temperature,the diffraction peaks gradually moved to the lower angle (Fig.5(c)),corresponding to the lattice parameter expanded to 0.2405 nm after the temperature reached 350 °C (Fig.5(d)).This phenomenon can be attributed to the thermal induced lattice expansion.During the RWGS reaction from 350°C to 400°C,the lattice parameter remained constant,suggesting there is no incorporation of the oxygen into the bulk phase of molybdenum nitride.During the heating process to 500 °C,an unexpected non-thermal lattice expansion has been observed(Fig.5(d)).The position of all diffraction peaks shifted significantly to lower angles (5.73°–5.64° for the γ-Mo2N (111) diffraction,see Fig.5(c)).Even when the temperature reached steady state at 500 °C and 600 °C,the lattice of molybdenum nitride remained expanding.As there is no other feature appeared in the in-situ XRD profiles,it can be concluded that the active phase of the molybdenum nitride is stable under RWGS reaction condition,which effectively explains the excellent stability of γ-Mo2N catalysts.The non-thermal lattice expansion is attributed to the chemical reaction induced doping of the oxygen atoms from CO2dissociation into the interstitial site of the nitride lattice.Comparing the reductive activation and the RWGS reaction,it is reasonable that the oxygen dopants can be reduced by the hydrogen existed in the treatment atmosphere.Therefore,for the RWGS reaction above 400°C,not only the surface,the bulk phase vacant interstitial sites also involve in the RWGS reaction,which can be described using the classic Mars van Krevelen model [37,38].Below 400 °C,the CO2hydrogenation is probably mainly catalyzed by the surface active sites.The oxygen species from CO2dissociation may not be able to migrate into the bulk interstitial sites.

3.5.The kinetic studies of the γ-Mo2N catalyst in the RWGS reaction at 350 °C

After understanding the CO2activation and hydrogenation behavior at high temperature (above 400 °C),we further explored the reaction behavior of CO2and H2in the low temperature reaction using kinetic method.The apparent kinetic order of CO2is determined at low CO2conversion condition at different constant hydrogen partial pressures,in order to understand the low temperature reaction mechanism of the molybdenum nitride catalyst.As shown in the Fig.6(a),under each specific H2partial pressure,the reaction rate of RWGS reaction increased with the raised CO2partial pressure.As the p(CO2) reached sufficiently high,the improvement of the reaction rate became less significant.The relationship of the p(CO2)-activity showed typical Langmuir-Hinshelwood characteristics,which suggests that the CO2is a relatively stronger surface adsorbate,compared with H2.Indeed,comparing the activity at the same p(CO2),it can be seen that the reaction rate increased significantly along with the rising H2pressure,especially at high p(CO2),indicating the activation of H2is the major factor that inhibits the further improvement of the reaction rate.The calculation of the apparent reaction order of the CO2(Fig.6(b))showed that the at low CO2partial pressure,the reaction order of CO2is larger than the high p(CO2) region,which also demonstrates the existence of high concentration of CO2has negative effect on the activation of H2.Meanwhile,with the increasing of p(H2) from 7.8 to 38.8 kPa,the CO2apparent order increased from 0.1 to 0.33.This result suggests that enhancing the H2adsorption will weaken the adsorption of CO2.Similar trend has also been observed from the calculated H2order (Fig.6(c)),where the order of H2increased from 0.39 to 0.59 with the increasing p(CO2) (3.1–30.8 kPa).In other word,CO2and H2are probably competitively adsorbed on the molybdenum nitride surface.In other word,the surface active sites of γ-Mo2N can activate both CO2and H2.CO2is the relatively stronger adsorbing species than H2.How to effectively enhance the activation ability of H2is the critical factor to enhance its RWGS activity at low temperature.Adding transition metal as the promoter is probably an effective solution to meet this target [24].

Fig.5.The in-situ XRD characterization of γ-Mo2N under RWGS atmosphere in the stepwise reaction condition.(a)the experimental detail of the in-situ XRD experiment;(b)the in-situ XRD profiles of stepwise to each reaction temperature of 350 °C,400 °C,500 °C,600 °C;(c) the contour map of the in-situ XRD profiles,the movement of the diffraction peaks were marked in the figure;(d) The variation of the lattice parameter of fresh γ-Mo2N,and γ-Mo2N under reduction and RWGS reaction condition,the magenta and red line-symbol curve is the lattice parameter and the grey dash line is the temperature profile of the in-situ XRD experiment.

Fig.6.The kinetic study of the γ-Mo2N catalyst in the RWGS reaction.(a)The dependence of the intrinsic activity of γ-Mo2N with the partial pressure of CO2;The calculation of the apparent kinetic order of (b) CO2 and (c) H2 using simple order model.

4.Conclusions

In summary,by comparing the catalytic performance of FCC structured γ-Mo2N,β-W2N and δ-NbN,we have discovered that the γ-Mo2N exhibits superior activity over other TMN counterparts.The mass specific activity at 600 °C reaches 123 mol CO.(g cat)-1.h-1,five times higher than the second active β-W2N.Moreover,the extraordinary stability of γ-Mo2N under the heating–cooling cyclic and steady-state operation condition makes it a potential alternative catalyst of the traditional metal-based catalyst for RWGS reaction.In-situ XRD and kinetic studies reveal that the CO2hydrogenation reaction mainly occurs over the surface at the low temperature region and the vacant interstitial sites in the bulk phase will involve in the reaction at above 400 °C.Both CO2and H2can be activated on the γ-Mo2N,and the H2activation is the major factor determining the reaction rate.Enlarging the mass specific surface area and introducing transition metal centers as H2dissociation center are probably the effective ways to further enhance the RWGS reaction performance of the γ-Mo2N catalysts.

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

This work was financially supported by the National Natural Science Foundation of China(22002140),Zhejiang Provincial Natural Science Foundation of China(LR21B030001 and LR22b030003),Young Elite Scientist Sponsorship Program by CAST (No.2019QNRC001).Use of the Advanced Photon Source (beamlines 17-BM,for in-situ XRD characterization) was supported by the U.S.DOE under contract no.DE-AC02-06CH11357.