2017, Vol. 31
Ammonia synthesis reaction plays a key role in the development of heterogeneous catalysis and related industries. Magnetite based catalyst with Fe3O4 as precursor has been investigated for more than 100 years and no further significant improvement are expected[1-2]. However, in 1986, we discovered a novel wüstite-based catalyst with Fe1-xO as the precursor of the catalyst[3]. This catalyst shows high activity, high reducibility and has been applied to various ammonia plants. As reported, it presents completive perfor-mance with Ru/C catalysts[4].
Recently, most work has been focused on the development of non-Fe catalysts, such as Ru/C[5-7] or new carrier materials for Ru catalysts[8-10]. In addition, cobalt promoted with other metals[11-13] was suggested to be the potential catalyst for ammonia synthesis. Relatively little work centers around the improvement on the iron based catalysts. As we all know, the cost of iron catalysts is much lower than other catalysts. Therefore, we suggest that modification and enhancement of the existing iron catalysts are worth noting.
In iron based catalysts, promoters play major role in the performance of catalysts[14]. Actually, wüstite precursor is favourable for the dispersion of Al2O3 and MgO promoters[15]. Although almost all the elements in the periodical table have been explored, still there are reports on new promoters. In addition to the well consolidated promoters, such as oxide of potassium, magnesium and aluminium, lithium, rare earth gangue and niobium oxide were found to be potential promoters for iron catalysts[16-19]. Interestingly, photoprompted hot electrons were found to be effective for ammonia synthesis even under atmospheric pressure[20]. It was confirmed that plasmonic metal nanoparticles can not only generate highly reactive electrons and holes but also induce resonant surface excitations through plasmonic decay, which can catalyze dissociation and redox reactions under mild conditions[21].
It is well accepted that ammonia synthesis reaction is structure sensitive over iron and ruthenium catalysts[22-24]. The crystallite size and shapes have significant effect on the activity of catalyst. With the increase of crystallite size, rate of ammonia decomposition over Fe surface decreases dramatically[25-26]. Size-depen-dent transformation of α-Fe into γ-Fe4N was also observed[27]. In addition, as iron based catalysts are usually prepared by fusing process, the surface is relatively smooth and the surface area is low. Hence, improvement of surface roughness is helpful for the activity of catalyst[28].
Surprisingly, high-energy ball-milling is a potential route for the preparation of iron catalysts for ammonia synthesis[29]. By ball-milling of either oxide components or commercial iron catalyst, the activity increased slightly. We suggest that nano and micro iron and iron oxides produced during ball-milling modifies the surface of catalyst and contribute to the activity.
In the present study, we propose a simple and efficient way to modify the commercial pre-reduced wüstite based catalyst with nano and micro Fe particles. The preparation of nano and micro Fe particles and modification of pre-reduced wüstite based catalyst was fulfilled via the in-situ solid-state reaction at room temperature.
1 Materials and Methods 1.1 Preparation of Fe2(C2O4)3·5H2OFe(NO3)3·9H2O (from Guangdong Guanghua Sci-Tech Co. Ltd.) and H2C2O4·2H2O (from Shanghai Lingfeng Chemical Reagent Co. Ltd.) were analytical grade reagents and used as received. According to the molar ratio of Fe(NO3)3·9H2O and oxalic acid=2:3, desired weight of Iron(Ⅲ) nitrate nonahydrate (Fe(NO3)3·9H2O) and oxalic acid (H2C2O4·2H2O) were prepared. Then, Fe(NO3)3·9H2O and H2C2O4·2H2O were milled into fine powder, respectively. And then fine Fe(NO3)3·9H2O powder was well mixed with fine oxalic acid powder in an agate mortar. During the milling, pungent gases were emitted. Subsequently, the homogeneous mixture was transferred into a beaker, and then dried at 110 ℃ for 5 h. Finally, the product was collected for structure and property characterization.
1.2 Preparation of catalystThe detailed process is illustrated in Fig. 1. According to the Fe(NO3)3·9H2O / H2C2O4·2H2O feed mole ratio and Fe loading, desired weight of prereduced wüstite based catalyst (0.4~0.8 mm)was well mixed with fine Fe(NO3)3·9H2O powder in an agate mortar. Then, desired amounts of fine oxalic acid powder were introduced into the mortar and milled for 30 min. During the milling, pungent gases were emitted. Finally, the sample was dried at 110 ℃ for 5 h. Prior to the activity tests, the modified catalysts were subject to calcination at 400~450 ℃ or in-situ calcination and reduction at the same temperature.
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Figure 1 Schematic illustration of modification of pre-reduced wüstite based catalyst with nano and micro Fe particles via solid-state reaction |
To investigate the morphology of the catalysts, SEM performed on a Hitachi S-4700 (Ⅱ) electron microscope (Tokyo, Japan) with an accelerating voltage of 15 kV was used. The samples were gold coated by cathodic sputtering.
The crystal phases of the catalysts were analyzed by powder X-ray diffraction (XRD), using a PANaly-tical X'Pert Pro diffractometer (Cu Kα radiation, λ = 0.154 056 nm, 2θ=10°~80°).
Thermogravimetric analysis in argon(Ar-TG) was conducted on NETZSCH-STA449C. Weight about 10 mg catalyst (particle size 0.154~0.050 mm) and heat it to 700 ℃ at a heating rate of 10 ℃/min under argon with the flow rate of 30 mL/min.
Temperature programmed reduction(TPR) profiles of samples (weight about 30 mg catalyst) were generated on a PX200 catalyst characterization instrument. H2/Ar mixture (5%H2 -95%Ar) was used as the redu-cing gas and heated at a heating rate of 10 ℃/min from 30 to 850 ℃ with the flow rate of 30 mL/min.
1.4 Activity test for ammonia synthesisCatalytic performance evaluation was carried out in a continuous flow fixed-bed reactor(id=14 mm). The catalyst bed was loaded in the isothermal zone of the reactor, and both the ends of the catalyst bed were filled with quartz with the size range of 1.0~2.0 mm to prevent the ammonia synthesis gas from channeling. The feed gas was derived from the decomposition of ammonia with deep removal of H2O, CO, CO2 and residual NH3 over Pd, 13 X, and 5 A molecular sieves and compressed by a compressor. Before catalyst acti-vity test, the catalysts were reduced by the mixture of N2 and H2 (H2:N2= 3:1) according to the following temperature program:heating to 400 ℃ and standing for 4h, then changing to 425 ℃ and standing for 6 h, then changing to 450 ℃ and standing for 10 h, and then changing to 475 ℃ and standing for 4 h, at the pressure of 5.0 MPa and space velocity of 30 000 h-1. Then, the activity was tested at the reaction temperature of 450, 425 and 400 ℃, GHSV of 30 000 h-1 and pressure of 15 MPa. In addition, to investigate the stability of the catalyst for ammonia synthesis, the catalyst was over heating at 500 ℃ for 15 h.
2 Results and discussion 2.1 Characterization of catalystsThe solid-state reaction of Fe(NO3)3·9H2O with oxalic acid can be expressed as the following equation (R1). According to the reaction, the pungent gas emission during milling should be HNO3 and N2O5. This reaction is confirmed by the differential scanning calorimetry (DSC) experiments as demonstrated in Fig. 2.
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Figure 2 DSC (differential scanning calorimetry) curves of (a) Fe(NO)3·9H2O, (b) H2C2O4·2H2O and (c) Fe2(C2O4)3·5H2O |
As shown in DSC curves, clear endothermal peaks of dehydration and decomposition of Fe(NO3)3·9H2O at temperatures between 50~200 ℃ were detected, while H2C2O4·2H2O dehydrates and decomposes at 120 and 205 ℃, respectively. These results are consistent with the reports in literature[30-31]. However, with the molar ratio of 3:2, once H2C2O4·2H2O and Fe(NO3)3·9H2O are well mixed, their decomposition behaviors differ from the pure components significantly. The DSC curve of the mixture is almost identical to that of Fe2(C2O4)3·5H2O indicating the complete reaction of R1[32]. Unlike gas-phase reactions, solid-state is not limited by thermodynamics, resulting in complete formation of Fe2(C2O4)3·5H2O[33].
| $ \begin{array}{l} {\rm{2Fe(N}}{{\rm{O}}_{\rm{3}}}{{\rm{)}}_{\rm{3}}}{\rm{\cdot9}}{{\rm{H}}_{\rm{2}}}{\rm{O + 3}}{{\rm{H}}_{\rm{2}}}{{\rm{C}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}{\rm{\cdot2}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to \\ {\rm{F}}{{\rm{e}}_{\rm{2}}}{{\rm{(}}{{\rm{C}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}{\rm{)}}_{\rm{3}}}{\rm{\cdot5}}{{\rm{H}}_{\rm{2}}}{\rm{O + 6HN}}{{\rm{O}}_{\rm{3}}}{\rm{ + 19}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \end{array} $ | (R1) |
The morphologies of product of Fe2(C2O4)3·5H2O and wüstite based catalysts are displayed in Fig. 3. Prior to the solid-state reaction, the pre-reduced wüstite-based catalyst exhibits smooth and clean surface (Fig. 3a). Following the solid-state reaction of Fe(NO)3·9H2O with H2C2O4·2H2O in the presence of pre-reduced wüstite-based catalyst, it presents rough surface (Fig. 3b). Clearly, following solid-state reaction, the surface of pre-reduced wüstite based catalyst is fully covered by Fe2(C2O4)3·5H2O which is the major product of solid-state reaction. Moreover, the morphology of Fe2(C2O4)3·5H2O in presence of pre-reduced wüstite based catalyst differs from that of pure Fe2(C2O4)3·5H2O sample. Prior to the calcination, both morphologies of Fe2(C2O4)3·5H2O reveal assembling of sheet structure (Fig. 3b and 3d). Follo-wing calcination at 450 ℃, the surface of pre-reduced wüstite based catalyst is covered by cross-linked array of nanobelt (Fig. 3c). With the same treatment, pure Fe2(C2O4)3·5H2O decomposes into fine particles after calcination at 400 ℃ (Fig. 3e) or 450 ℃(Fig. 3f). This difference attributes to the interaction between the pre-reduced wüstite-based catalyst and Fe2(C2O4)3·5H2O particles.
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Figure 3 SEM images of pre-reduced wüstite based catalysts before and after solid-state reaction and SEM images of Fe2(C2O4)3·5H2O, product of Fe2(C2O4)3·5H2O calcinated at different temperature. (a) pristine pre-reduced wüstite catalyst, (b) pre-reduced wüstite catalyst after solid-state reaction, (c) calcination of sample (b) at 450 ℃ for 3 h, (d) Fe2(C2O4)3·5H2O, (e)product of Fe2(C2O4)3·5H2O calcinated at 400 ℃ and (f) product of Fe2(C2O4)3·5H2O calcinated at 450 ℃. |
The XRD result of the solid-state reaction of Fe(NO)3·9H2O with H2C2O4·2H2O in the absence of pre-reduced wüstite based catalyst confirms the formation of pure Fe2(C2O4)3·5H2O (Fig. 4a). No impurity was detected by XRD pattern. In addition, weak diffraction peaks imply small crystal size of Fe2(C2O4)3·5H2O prepared by solid-state reaction.
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Figure 4 XRD patterns of Fe2(C2O4)3·5H2O (a) and pre-reduced wüstite based catalyst before and after modification (b) |
The interaction is reinforced by the results of XRD experiments. As shown in Fig. 4b, XRD pattern of pristine pre-reduced wüstite based catalyst shows most α-Fe formed which is the active phase for ammonia synthesis. The presence of Fe2O3 is resulted from the surface oxidation during sample preparation for XRD experiment. The pre-reduced wüstite based catalyst was modified with Fe2(C2O4)3·5H2O (5% based on the Fe content in Fe2(C2O4)3·5H2O) shows much weaker diffraction peaks of α-Fe. Unlike XRD pattern of pure Fe2(C2O4)3·5H2O in Fig. 4a, no diffraction peaks of Fe2(C2O4)3·5H2O are observed, indicating that Fe2(C2O4)3·5H2O interacts with the surface of pre-reduced wüstite based catalyst and disperses on the catalyst surface uniformly during solid-state reaction.
It is consistent with the results of TG as demonstrated in Fig. 5 and Fig. 6. The decomposition process of Fe2(C2O4)3·5H2O can be divided into three stages (R2 to R5).
| $ {\rm{F}}{{\rm{e}}_{\rm{2}}}{{\rm{(}}{{\rm{C}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}{\rm{)}}_{\rm{3}}} \to {\rm{2Fe}}{{\rm{C}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}{\rm{ + 2C}}{{\rm{O}}_{\rm{2}}} $ | (R2) |
| $ {\rm{3F}}{{\rm{e}}_{\rm{2}}}{{\rm{(}}{{\rm{C}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}{\rm{)}}_{\rm{3}}} \to {\rm{2F}}{{\rm{e}}_{\rm{3}}}{{\rm{O}}_{\rm{4}}}{\rm{ + 10C}}{{\rm{O}}_{\rm{2}}}{\rm{ + 8CO}} $ | (R3) |
| $ {\rm{Fe}}{{\rm{C}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}} \to {\rm{F}}{{\rm{e}}_{1 - }}_x{\rm{O + CO + C}}{{\rm{O}}_{\rm{2}}} $ | (R4) |
| $ 4{\rm{F}}{{\rm{e}}_{1 - }}_x{\rm{O}} \to {\rm{F}}{{\rm{e}}_{\rm{3}}}{{\rm{O}}_{\rm{4}}} + \alpha - {\rm{Fe}} $ | (R5) |
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Figure 5 TG and DTG experiments of Fe2(C2O4)3·5H2O prepared via solid-state reaction Fe(NO)3·9H2O with H2C2O4·2H2O |
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Figure 6 TG and DTG experiments of wüstite based catalyst doped with Fe2(C2O4)3·5H2O. (a) TG, (b) DTG |
As illustrated in Fig. 5, three peaks of DTG curve at 172, 260 and 448 ℃ correspond to reactions R2 and R3, reaction R4 and reaction R5, respectively. According to the DSC curve, endothermal and exothermal peaks around 170 ℃ can be assigned to reactions R2 and R3, respectively. Two exothermal peaks at 260 and 448 ℃ can be assigned to reaction of R4 and R5, respectively. Moreover, the much weaker peak at 448 ℃ suggest that only very small amounts of iron oxide further decomposes into Fe3O4 and Fe. In addition, the total weight loss of reactions R2-R5 (59%, 32% for reaction R2 and R3, 27% for reactions R4 and R5) agrees well with the calculated value (58.96%).
As shown in Fig. 6a, with the modification of 5% of Fe particles (based on the Fe content in Fe2(C2O4)3·5H2O), the weight loss of modified wüstite-based catalyst is about 6.41%, which is much lower than that of calculated value (8.6%). We suggest that part of Fe2(C2O4)3·5H2O dehydrates and decomposes during the drying process as the wet paste has to be dried after solid-state reaction. In addition, as displayed in Fig. 6b, decomposition of Fe2(C2O4)3·5H2O on pre-reduced wüstite based catalyst at temperatures of 175, 225, 309 and 578 ℃. Compared with pure Fe2(C2O4)3·5H2O (in the absence of iron catalyst, Fig. 5), reaction temperatures of R2 and R3 split into two temperatures, from 172 to 175 and 225 ℃. Especially, decomposition temperature of FeC2O4 (R4) increases from 260 to 309 ℃, suggesting the strong interaction between FeC2O4 and pre-reduced wüstite based catalyst. The formation of new peak at 225 ℃ was caused by the weak interaction between Fe2(C2O4)3·5H2O and the surface of pre-reduced wüstite based catalyst. As a result, reaction temperature of R3 almost keeps unchanged after solid-state reaction in the presence of pre-reduced wüstite based catalyst. However, due to the interaction between FeC2O4 and pre-reduced wüstite based catalyst, reaction temperature of R2 is elevated significantly. The shift of decomposition temperature of Fe1-xO (R5) indicates the even stronger interaction with the surface of pre-reduced wüstite based catalyst. Clearly, these nano and micro particles derived from the decomposition of Fe2(C2O4)3·5H2O are significantly stabilized by this interaction. We suggest that it also plays a fundamental role in the performance of modified catalyst.
The H2-TPR profiles shown in Fig. 7 represent the reduction behavior of pre-reduced wüstite based cata lyst before and after modification. According to the results of H2-TPR, reduction peaks of pre-reduced wüstite based catalyst before and after modification center around 519 and 537 ℃, respectively. Clearly, with modification, the reduction temperature of pre-reduced wüstite based catalyst increased by 18 ℃. In addition, for pre-reduced wüstite based catalyst, the starting reduction temperature is about 400 ℃ and the end temperature is about 600 ℃, while the corresponding temperatures for modified pre-reduced wüstite based catalyst are 270 and 700 ℃, respectively. The starting reduction temperature is 130 ℃ lower than pre-reduced wüstite based catalyst, which was attribute to the reduction of surface or more dispersed Fe2(C2O4)3·5H2O over the catalysts. The end reduction temperature is 100 ℃ higher than pre-reduced wüstite based catalyst, which can be attribute to the strong interaction between modified Fe particles and pre-reduced wüstite based catalyst. In summary, the results of H2-TPR indicate the formation of strong interaction between modified Fe particles and pre-reduced wüstite based catalyst, which was well agreed with TG and DTG (Fig. 6).
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Figure 7 H2-TPR patterns of pre-reduced wüstite based catalyst before and after modification |
The activities of pristine and modified pre-reduced wüstite based catalyst for ammonia synthesis were evaluated at temperatures of 400, 425 and 450 ℃, reaction pressure of 15 MPa and space velocity of 30 000 h-1. The results are summarized in Fig. 8(a).
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Figure 8 Activity of pre-reduced wüstite based catalyst for ammonia synthesis at 15 MPa and space velocity of 30 000 h-1 (a) before over-heating and (b) after over-heating at 500 ℃ for 15 h. |
With modification, the activity of pre-reduced wüstite based catalyst is enhanced significantly. It increases with Fe loading (neat Fe based on the Fe content in Fe2(C2O4)3·5H2O) until the maximum acti-vity is achieved at Fe loading of 5%. When loadings are higher than 5%, the activity of modified catalyst drops with loading. With the Fe loading of 5%, ammonia concentrations at the outlet of the reactor are improved from 12.4% to 15.6% at 450 ℃, 11.0% to 14.8% at 425 ℃ and 9.4% to 13% at 400 ℃, respectively. As iron catalyst for ammonia synthesis is a well consolidated catalyst with a development history of more than 100 years, no further significant improvement is expected[4]. Therefore, the present work is a surprising progress on iron-based catalyst for ammonia synthesis. Via simple and one-step modification (solid-state reaction), the activity of pre-reduced wüstite based catalyst is enhanced by 25% to 38%. It is a significant progress for iron based catalyst for ammonia synthesis. With the Fe loading higher than 5%, we suggest that the surface of pre-reduced wüstite based catalyst is heavily covered by the particles. As we all know, in the absence of promoters, Fe is rather inactive for ammonia synthesis[34]. So with excessive amounts of nano and micro Fe particles, most doped Fe can not contact with promoters, while with small amounts of Fe doping, these particles are dispersed on the surface of pre-reduced wüstite based catalyst and interact with promoters located on the surface.
As demonstrated in Fig. 8(b), modified wüstite based catalysts exhibit high stability for ammonia synthesis. After over heating at 500 ℃ for 15 h, identical activity was detected. Clearly, as mentioned previously, the strong interaction between nano and micro iron particles stabilizes the particles and prevent them from sintering.
3 ConclusionsVia simple and one-step modification (in-situ solid-state reaction of Fe(NO3)3·9H2O with H2C2O4·2H2O in the presence of pre-reduced wüstite based catalyst at room temperature), the activity of pre-reduced wüstite based catalyst is improved significantly. The so-lid-state reaction of Fe(NO3)3·9H2O with H2C2O4·2H2O at room temperature yields Fe2(C2O4)3·5H2O completely and disperses on the surface of pre-reduced wüstite based catalyst. Following calcination or reduction, formed Fe1-xO or Fe particles interact with the surface of pre-reduced wüstite based catalyst strongly, and thus stabilizes the nano and micro particles during reaction. As a result, high stability of catalyst is achieved. We propose it is a potential route for the improvement of ammonia synthesis catalyst and may be applied to other systems.
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