原位生成非晶纳米钴高效催化氨硼烷的醇解制氢

陈 浩 俞哲健 徐丹丹 李 洋 王明明 夏良敏 罗书平

（浙江工业大学绿色化学合成技术国家重点实验室培育基地，杭州 310014）

0 Introduction

In order to solve the problems of over consuming fossil fuels and the accompanied environmental pollution of our earth,there is an urgent need to search for efficient,clean,renewable and rich energy resources[1-4].Hydrogen is considered to be the ideal candidate of clean energy due to it is easily obtained and burning cleanly[5-9].However,the search for safe and efficient hydrogen storage materials is a key challenging issue in the development of the“hydrogen economy”[10-11].The ammonia borane complex （AB）is considered as a promising candidate for on-board hydrogen applications because of its high hydrogen density （19.6%（w/w））and high stability[12-14].

Generally,there are two primary ways to release hydrogen from AB,catalytic thermolysis and catalytic hydrolysis （include methanolysis）respectively.The former usually requires high temperature and the rate of dehydrogenation of AB is low,while the latter can release 3.0 equivalents of hydrogen in the presence of a suitable catalyst.Moreover,the comparison between catalytic hydrolysis and catalytic methanolysis of AB is whether ammonia is liberated.The catalytic methanolysis of AB release hydrogen gas without ammonia evolution and yield the recyclable methanolysis by-product ammonium tetramethaoxyborate[15-17].

Due to the advantages of nano-catalysis,the current research has been concentrated on the development of some metal nanoparticles possessing high activity and reusability in the hydrolytic or methanolic dehydrogenation of AB.Rh[18],Pd[19],Co-Ni[20],Co-GO[21],Co-Cu[22]and Co-Ag[23]nanoparticles exhibited excellent catalytic activity. Although the active metal nanoparticles-catalyzed hydrolysis or methanolysis of AB possessed high hydrogen production efficiency,sometimes the preparation was cumbersome.Although the homogeneous catalysts for the methanolysis of AB was generally higher efficient than the heterogeneous catalysts,people tended to nano-catalysis.Few of the studies of homogeneous catalysis for the methanolysis or hydrolysis of ammonia borane were done.However,many hydrogen transfer reactions of ammonium borane as an additive have been investigated[24-25].

Herein,a series of pincer Co complexes have been designed and synthesized in our previous work[26].Then using them as catalysts,the methanolysis of ammonia borane was investigated in homogeneous system.Later,the in-situ formed amorphous Co nanoparticles was discovered and studied for the methanolysis of ammonia borane in heterogeneous system.Importantly,the Co-NPs possessed highest activity and reusability in the methanolic dehydrogenation of AB.Proposed catalytic mechanism was put forward based on the amorphous Co-NPs were characterized by transmission electron microscopy（TEM）,X-ray diffraction （XRD）,Fourier transform infrared spectra （FT-IR）and X-ray photoelectron spectroscopy （XPS）.What was more,we also studied the kinetics of methanolysis of AB.Especially,the activation energy of the methanolysis of ammonia borane towards amorphous Co nanoparticles was calculated to be 20.00 kJ·mol-1,which was close to that of some noble metal-based catalysts[27-28].

1 Experimental

1.1 Chemicals

A series of Co-complexes were synthesized from our previous work[26]. The chemicals used in the experiment were purchased from Energy Chemical.Methanol was an analytical level and was used after heavy steaming with non-aqueous anaerobic treatment.All pieces of glassware were washed several times with aqua regia and ordinary distilled water.Unless otherwise noted,all manipulations were carried out under an inert atmosphere.

1.2 In-situ preparation for amorphous Co nanoparticles and its characters

To a 100 mL round bottom flask was added CoCl2（0.390 g,3 mmol）and NH3BH3（0.930 g,30 mmol）in the glovebox.MeOH （10 mL,250 mmol）was added dropwisely and slowly to avoid a strong hydrogen production reaction.The mixture was stirred at room temperature until no gas was generated.After the completion of the reaction,the Co NPs was obtained in vacuo.Other metal nanoparticles were prepared in the same way.The morphology and microstructure of the samples were characterized by TEM on a FEI TECBAI G2 F30 instrument operated at 300 kV.The XRD patterns were obtained with a PANalytical X,Pert PRO diffractometer with Cu Kα1radiation in the 2θscan range from 10°to 80°at room temperature（40 kV and 100 mA）.The FT-IR spectra were obtained with a nicolet 670 FT-IR spectrometer using a KBr pellet technique.The XPS was performed with a Thermo Fisher（Escalab 250Xi）using Al Kα X-rays as an excitation source （1 486.8 eV）.

1.3 Evaluation of the catalytic activity

To assess the catalytic activity of the amorphousCo nanoparticles for methanolysis of AB, the classicwater-displacement method involving determining therate of hydrogen generation was performed. Theexperimental apparatus used in this study wasidentical to that reported by Matthias Beller[29].

Fig.1 TEM images （a,b）and particle size histogram （c）of Co NPs （washed by the degassed water）

2 Results and discussion

Initially,the Co NPs was characterized with TEM,XRD,FTIR and XPS.Firstly,the TEM images of sample was proved to be the Co NPs （Fig.1（a,b））.The morphology of the sample could be clearly observed.The particles were mainly spatially discrete globular particles with sizes on the nanoscale（approximately 4 nm in diameter）（Fig.1c）.Because of its strong magnetism,the Co NPs prepared tended to aggregate to some extent.Secondly,according to the standard card （PDF No.73-0365）,the XRD patterns of the Co NPs （Fig.S1a）was ammonium chloride.The result showed that there was the formation of ammonium ion in the process of catalytic reaction.After washed by the degassed water,as could be seen in Fig.S1b,there might be amorphous Co NPs.Then,FTIR was used to detect surface chemical information of the sample （Fig.S2）.The Co NPs had two broad absorption bands in the infrared spectrum.The broad absorption band centered at 3 400 cm-1could be assigned to the vibration of the surface residual NH and OH groups[30].The framework bands in the range 500～1 300 cm-1was Co NPs,which was similar to the characteristic absorption band of nano alumina[31].Finally,XPS was used to further investigate the localized valence orbitals of the transition metal composite （Fig.S3）.As could be seen in Fig.S3b,the peak of Co composite could be deconvoluted into three peaks at binding energy 782.1,780.1 and 778.5 eV.The first two peaks were related to Co2+and Co3+,and the last peak could be assigned to the metallic Co[32].This suggestion that Co was formed by the reduction of CoⅡwith AB during the methanolysis process possibly.There were also three peaks located at 529.3,530.8 and 532.4 eV in the O1s spectrum（Fig.S3c）which were attributed to the cobalt oxide and other substances.

Next,the methanolysis dehydogenation of AB catalyzed by homogeneous and heterogeneous cobalt catalysts was researched.Cobalt complexes 1～4 were chosed because they displayed high catalytic activity in the reduction reaction by AB.Co NPs and other metal NPs were easy prepared by the metal reduction by AB in-situ method.As was shown in Table 1,the Co NPs and complex 1 displayed higher performance for hydrogen production.The generated hydrogen was measured by a drainage method,using a burette to observe the volume of hydrogen.The volume of hydrogen produced versus time was plotted in Fig.2.We have proceeded the twice experiments with the methanolysis of AB catalyzed by Co NPs and complex 1 respectively.Due to the similar rate and amount of hydrogen evolution,it showed good reproducibility.As the reaction gone,the structure of the complex 1 might be destroyed by the AB which leaded to the decrease of hydrogen production rate.However,in-situ formed amorphous Co NPs exhibited high activity all the time and its turnover frequency （TOF）was calculated to be 515 molH2·molmetal-1·h-1during the first hydrogen release process （Fig.2）.Compared with some noble metal-based catalysts[27-28,33],the catalytic performance of Co NPs was poor.However,the value of TOFwas much higher than those of Cu-based catalysts,which could only reach up to 19 molH2·molmetal-1·h-1[34].Moreover,Sun et al.[35]and Filiz et al.[36]demonstrated that metallic Co was very active towards the methanolysis of AB.

Table 1 Rate of hydrogen production by catalysts

Fig.2 Twice hydrogen production of ammonia borane（2 mmol）by complex 1 （1.2 mg）and Co NPs（1.2 mg）in methanol（4 mL）

Subsequently,the stability of Co NPs catalyzing AB was measured by adding another equivalent（2 mmol）of AB into the mixture after the previous cycle（Fig.3）.The 10th cycles were tested.The turnover number for hydrogen production （TON）could reach 6 000 in the 10th process.It was worth pointing out that the rate of hydrogen generation by Co NPs was highest in the first cycle and then decreased very slowly in other cycles,which identified with the catalytic activity of Co NPs.The decrease of catalytic activity was mainly attributed to the Co containing active species adsorbed on the surface of the trimethylborate and a slight aggregation of the Co NPs during the reaction[37].The Co NPs dispersed in solution or supported on suitable solid materials with large surface area could catalyze AB more quickly,suggesting its good stability.Additionally,the surface electronic properties and geometrical structure of the nanoparticles determine the catalytic activity,selectivity and stability[38].

Fig.3 Recycling of Co NPs （1.2 mg）in methanol（4 mL）,with addition of aqueous AB （2 mmol）to system in each cycle （the entire system remained 298 K）

Fig.4 （a）Influence of catalyst amount on hydrogen generation rate;（b）Fitting plot of hydrogen generation rate （R H2）vs catalyst amount （w cat）

Furthermore, the kinetics studies for the methanolysis reaction of AB have been explored.More specifically,the amounts of catalyst,the concentrations of AB and the temperatures were experimented on the methanolysis of AB,respectively.The effect of the amount of catalyst on the hydrogen generat rate of the catalytic reaction was first studied,where the concentration of AB was maintained at 2 mmol at 298 K and the catalyst amounts were divided into 1.0,1.2,1.5 and 2.0 mg （Fig.4）.The initial rate of hydrogen generation was determined from the initial nearly linear portion of each plot.As was seen in Fig.4a,the rate of hydrogen production was improved with an increased amount of the catalyst in a certain range,because a larger dosage of the catalysts could provide more active sites for catalytic reaction.In addition,the number of catalysts and the rate of hydrogen production were linearly correlated （Fig.4b）.The results showed that the catalytic methanolysis reaction of AB was first-order with respect to the catalyst amount,which was consistent with a relevant report byÖzkar et al.[16].

The influence of AB concentration on the hydrogen evolution rate was also evaluated （Fig.5）.The hydrogen generation rate barely changed with the different amounts of AB （0.5,1.0,1.5 and 2.0 mmol）,using Co NPs catalyst （1.2 mg）at 298 K.The studies suggested that the methanolysis reaction of the AB complex was zero-order with respect to the AB amounts.Thus we could infer that the methanolysis reaction of the AB complex on the catalyst surface was a rate-limiting step[39].

In order to get activation energy （Ea）for the methanolysis reaction,the effect of various temperatures （288,298,303 and 313 K）was also discussed（Fig.6a）.According to Arrhenius plot and response characteristics indicating that the reaction was quasi zero-order with respect to the AB concentration[40],the Eawas calculated to be 20.00 kJ·mol-1.Although the H2evolution rate for the amorphous Co nanoparticles was lower than that of the Pt-based catalysts,the activation energy for amorphous Co nanoparticles was close to that of some Pt-based catalysts.

Finally,the product was probed after the reaction was completed.In the previous work,ammonium tetramethoxyborate was the only product in methanolysis reaction of AB[15-17].The B-containing product was monitored by the11B NMR spectrum in the process of reaction （Fig.7）.When the reaction was over or the reaction was going half an hour,the11B NMR （18.69）revealed the formation of trimethylborate,which disagreed with the ammonium tetramethoxy-borate（8.7）.

Fig.5 （a）Effect of amounts of AB on the hydrogen generation rate;（b）Fitting plot of the hydrogen generation rate （R H2）vs concentration of AB （C AB）in 4 mL methanol

Fig.6 （a）Plot of time vs volume of hydrogen generated from methanolysis of AB （2 mmol）catalyzed by Co NPs （1.2 mg）at different temperatures;（b）Corresponding Arrhenius plot

Fortunately,the plausible mechanism of the AB methanolysis reaction was predicted （Fig.8）.Cobalt dichlorotetrakis （methanol）-coordination compound,formed by cobalt chloride in methanol solution,was reduced to metallic Co by AB and then reunited together.In the process of first step,there might be the formation of ammonium ion,which was consistent with the XRD of Co NPs.As pointed out by Xu et al.[41],AB interacted with the surface of metal to form an active complex,which might be the rate-determining step.The active complex was then attacked by a CH3OH molecule,which readily leaded to dissociation of the BN bonds to produce the trimethylborate along with the release of H2and NH3[26].

Fig.7 11B NMR of（a）trimethylborate,（b）product when the reaction was going half an hour and（c）product when the reaction was complete

Fig.8 Plausible mechanism of the methanolysis of AB catalyzed by the Co NPs

3 Conclusions

In summary,we found that in-situ formed amorphous Co nanoparticles exhibited notable activity towards hydrogen generation from the methanolysis of AB at room temperature.The possible mechanism of the AB methanolysis reaction was the surface catalysis of metals.Furthermore,we have investigated the kinetics of the methanolysis reaction of AB in detail.The activation energy was determined to 20.00 kJ·mol-1.Their tunable catalytic properties shown here indicated that Co composite had great potential in developing AB as a hydrogen storage material for fuel cell applications.The results provided new insight into the enhanced performance of the Co hybrids and might help for the design of advanced catalysts.

Supporting information isavailable at http://www.wjhxxb.cn