Highly Dispersed Co Nanoparticles Prepared by an Improved Method for Plasma-Driven NH₃ Decomposition to Produce H₂

Abstract

Previous studies reveal that combining non-thermal plasma with cheap metal catalysts achieved a significant synergy of enhancing performance of NH₃ decomposition, and this synergy strongly depended on the properties of the catalyst used. In this study, techniques of vacuum-freeze drying and plasma calcination were employed to improve the conventional preparation method of catalyst, aiming to enhance the activity of plasma-catalytic NH₃ decomposition. Compared with the activity of the catalyst prepared by a conventional method, the conversion of NH₃ significantly increased by 47% when Co/fumed SiO₂ was prepared by the improved method, and the energy efficiency of H₂ production increased from 2.3 to 5.7 mol(kW·h)⁻¹ as well. So far, the highest energy efficiency of H₂ formation of 15.9 mol(kW·h)⁻¹ was achieved on improved prepared Co/fumed SiO₂ with 98.0% ammonia conversion at the optimal conditions. The improved preparation method enables cobalt species to be highly dispersed on fumed SiO₂ support, which creates more active sites. Besides, interaction of Co with fumed SiO₂ and acidity of the catalyst were strengthened according to results of H₂-TPR and NH₃-probe experiments, respectively. These results demonstrate that employing vacuum-freeze drying and plasma calcination during catalyst preparation is an effective approach to manipulate the properties of catalyst, and enables the catalyst to display high activity towards plasma-catalytic NH₃ decomposition to produce H₂.

1. Introduction

NH₃ decomposition has been considered to be an attractive route to supply CO_x-free H₂ for proton exchange membrane fuel cell (PEMFC) vehicles [1,2,3]. Until now, the noble metal Ru, due to its high turnover frequency (TOF), is still the most active component for NH₃ decomposition, and the formation rate of H₂ reached as high as 4.0 mol/(h·g_cat)⁻¹ using K-Ru/MgO-CNTs catalyst with complete conversion of ammonia at 450 °C, but the scarcity and high price of Ru limits its use on a large scale [4,5,6]. Whereas, cheap metal catalysts show low activity towards NH₃ decomposition due to the strong adsorption of N atoms onto the surface of cheap metal catalysts [1,7,8,9,10]. As far as we know, the highest formation rate of H₂ was 2.0 mol/(h·g_cat)⁻¹ using CeO₂-doped Ni/Al₂O₃ catalyst with 98.3% ammonia conversion at 550 °C [7]. Recently, the combination of non-thermal plasma with cheap metal catalyst displayed a powerful ability in enhancing NH₃ decomposition [11,12,13]; 99.9% conversion of NH₃ was achieved in combination mode, but only 7.4% and 7.8% was obtained for Fe-based catalyst alone and plasma alone, respectively, which experienced an unexpected strong synergy between plasma and catalyst [11], and this synergy strongly depended on the properties of catalyst [12,14].

The preparation approach of catalyst could directly affect the properties of the catalyst, such as crystal size, shape, composition, acidity, and basicity [15,16,17]. Normally, catalyst preparation was operated in a thermodynamic equilibrium state of gas, liquid, and solid state, but faces the limitation of thermodynamic equilibrium. For example, the calcination temperature for supported metal catalysts is usually over 500 °C, and high temperature operation causes aggregation of metal particles, but low temperature operation results in incomplete decomposition of catalyst precursors. Besides, it is difficult to achieve a high dispersion of catalyst with a high metal loading above 20 wt % in a thermodynamic equilibrium state.

Non-thermal plasma is the fourth state of matter and characterized by non-equilibrium character. Typically, the overall gas temperature in a field of non-thermal plasma can be as low as room temperature, while the generated free electrons are highly energetic with a typical electron temperature of 1–10 eV, which can collide with carrier gas to produce chemically reactive species such as radicals, excited species, and ions [18]. Such a characteristic of plasma enables some thermodynamically unfavorable chemical reactions to proceed at moderate conditions, especially for inert molecule conversion, such as CO₂, CH₄, and N₂ [19,20,21,22,23,24].

Similarly, the non-equilibrium character of non-thermal plasma also benefits catalyst preparation to achieve controllable morphology and chemical property by controlling the reaction rate of nucleation and crystal growth in a non-equilibrium environment. Different from the conventional thermal process, the catalyst preparation with plasma is not based on the thermal effect, but on the inelastic collision of those energetic species (free electrons, radicals, excited species and ions) with catalyst precursors to accomplish the purpose of calcination or treatment. Catalyst preparation with plasma has attracted increasing interest since the 1990s [25,26,27,28,29,30,31,32,33,34], and a variety of plasmas, such as glow discharge, radio frequency discharge, microwave discharge, and dielectric barrier discharge, were employed for calcination and reduction of supported catalyst, which can make metal highly dispersed on a support with a narrow distribution of particle size, manipulate metal–support interaction, and shorten the time of catalyst preparation due to high reaction rates in the plasma process. Besides, with regard to the characteristic of low temperature, plasma removal of template was well developed for synthesis of microporous and mesoporous materials, instead of thermal removal that could destroy the porous structure of the materials [35]. Bogaerts and coworkers found that plasma could be formed inside pores of material with pore size above 200 μm at 20 kV by two-dimensional fluid modeling, and the possibility of discharge forming inside pores and discharge behavior strongly depended on pore size and applied voltage [36]; this observation helps to understand the process of plasma removal of template. Very recently, Wang, et al. and Di, et al. summarized the advances in preparation of catalyst with plasmas, and the mechanism of preparation was discussed as well [29,30]. Although low temperature operation of non-thermal plasma enables catalyst preparation in a more efficient and more controllable way, normally, low temperature operation leads to incomplete decomposition or removal of precursors, along with poor growth of materials with residues.

In this study, a novel combination of vacuum-freeze drying technique with atmospheric pressure dielectric barrier discharge (DBD) calcination technique was proposed for the preparation of supported Co catalyst with a high Co loading of 30 wt %. To calcine catalyst completely, DBD reactor was placed in a furnace to prevent heat dissipation of electric heat from discharge, so as to keep plasma calcination at a temperature of about 400 °C by tuning energy input of power supply (note that the furnace here is not used to heat the reactor, but used for electric heat preservation). Compared to Co/fumed SiO₂ with conventional preparation method, the improved preparation method enabled the conversion of plasma-catalytic NH₃ to increase by 47%, and greatly enhanced the formation rate of H₂. Besides, the reaction performance can be further improved through increasing specific energy input.

2. Results and Discussion

2.1. Characterization

The physicochemical properties of as-prepared Co catalysts were examined using various characterization techniques, including X-ray diffraction (XRD), X-ray fluorescence (XRF), transmission electron microcopy (TEM), H₂ temperature-programmed reduction (H₂-TPR), and NH₃ temperature-programmed desorption (NH₃-TPD). In this study, the fumed SiO₂ used as a support for Co catalyst was an amorphous material with a Brunauer–Emmett–Teller (BET) surface area of 297.8 m²·g⁻¹. The theoretical Co loading was designed to be 30 wt %, but the actual Co loading through XRF analysis was 27.7 wt % and 27.4 wt % for the improved prepared catalyst and the conventional prepared catalyst, respectively (see Tables S1 and S2 in Supporting Information). Figure 1 shows the XRD patterns of as-prepared fumed SiO₂-supported Co catalysts using conventional and improved preparation methods, respectively. Besides, pure fumed SiO₂ was analyzed as a reference in Figure 1 (a). Clearly, the same diffraction peaks were observed at 2θ of 31.1, 36.7, 44.6, 59.2, and 65.2 as shown in Figure 1 (b) and (c), which matched well with the characteristic structure of Co₃O₄ (JCPDS file No: 43-1003), and those diffraction peaks represented the (220), (311), (400), (511), and (440) planes of Co₃O₄, respectively [37,38]. Namely, the difference in preparation approach did not influence the phase structure of Co catalysts, and they both finally existed in the form of Co₃O₄ over fumed SiO₂ support. However, by contrast, the intensity of diffraction peaks of Co catalyst prepared with improved method was weaker than that with conventional preparation method, suggesting that the average particle size of the former is smaller than that of the latter according to the Debye–Scherrer formula [39]; this observation is also supported by the results of TEM as follows.

TEM images of as-prepared Co catalyst supported on fumed SiO₂ using different approaches were shown in Figure 2. Clearly, a very poor dispersion of Co catalyst was observed on fumed SiO₂ using the conventional preparation method, and the particle size of Co was much larger than 5 nm; some particle sizes were around 50 nm, as shown in Figure 2a,b. However, the use of combining vacuum-freeze drying and plasma calcination techniques in the process of catalyst preparation enabled the Co particles to disperse highly and homogeneously onto the fumed SiO₂ support, and the average Co particle size was less than 5 nm, mostly around 2–3 nm in Figure 2c,d. Actually, it is difficult to obtain such smaller nanoparticles with a high metal loading of about 27 wt % using the conventional preparation method.

Using NH₃ as probe molecule, the influence of preparation approach on the chemical properties of catalyst was evaluated through NH₃-TPD, as displayed in Figure 3. Clearly, two major desorption peaks were observed, one at the low temperatures of 150–220 °C corresponded to the weak adsorption of NH₃ on the catalyst, and the other at the high temperatures of 220–350 °C was attributed to the strong adsorption of NH₃. It is worth noting that the desorption amount of NH₃ over Co catalyst prepared with the improved method was much higher than that with the conventional preparation method, revealing that the improved method leads to an increase in the number of active sites for NH₃ adsorption; this finding can be ascribed to the high dispersion of Co nanoparticles, as evidenced by the results of TEM in Figure 2. In addition, the desorption temperature of adsorbed NH₃ on the catalyst with improved preparation method shifted towards higher temperature, reflecting that the binding ability of NH₃ with the catalyst was stronger than that with the catalyst prepared using conventional preparation method. This inferred that the acidity of catalyst was strengthened by the improved preparation method as well and, more importantly, the increase in active site number and acid strength both facilitated the adsorption of NH₃ on the catalyst, finally promoting the dissociation of NH₃ on the catalyst.

H₂-TPR technique was used to evaluate the reduction behavior of Co₃O₄/fumed SiO₂ prepared with different methods, and the resulting profiles are displayed in Figure 4. Clearly, the reduction of Co₃O₄ on fumed SiO₂ support occurred in the temperature range of 275–550 °C. Two groups of reduction peaks were observed, i.e., the low temperature reduction peaks (α) consisted of α₁ and α₂ in the range of 275–400 °C, and the high temperature reduction peaks (β) with a consecutive-broad peak consisted of β₁ and β₂ in the range of 370–550 °C. More importantly, by contrast, the reduction temperature of catalyst with improved preparation method shifted towards higher temperature, representing that the improved method strengthened the interaction of Co with fumed SiO₂ support. This difference in metal–support interaction can be explained by the difference in particle sizes of Co catalyst prepared by different methods (Figure 2). Actually, the reduction process of as-prepared catalyst was very complicated, since these peaks obtained were heavily overlapped. Therefore, the analysis of each peak area using peak fit function (Gaussian) of Origin software was employed to understand the H₂-TPR profiles obtained (see Figure S1 in Supporting Information), the area ratio of β₁/β₂ was found to be 1/3, which is quantitatively consistent with the theoretical value (1/3) of area ratio of Co₃O₄ reduction peaks [40,41]. This indicates that β₁ and β₂ corresponded to the two-step reduction of Co³⁺ → Co²⁺ → Co⁰ of Co₃O₄, as do α₁ and α₂ based on 5/16 (≈ 1/3) area ratio of α₁/α₂. Besides, the result of XRD in Figure 1 also supported the assignment of α and β to Co₃O₄. According to the reduction temperature of Co₃O₄, the low temperature reduction peaks (α₁ and α₂) could be due to the reduction of bulk Co₃O₄, whereas the high temperature reduction peaks (β₁ and β₂) were attributed to the reduction of Co₃O₄ that interacted with fumed SiO₂ [42,43].

Interestingly, the above results reveal that the application of vacuum-freeze drying and plasma calcination techniques in the preparation process of catalyst not only results in highly dispersed metal nanoparticles along with the increase of active site number, but also strengthens the acidity of catalyst and the metal–support interaction. Thus, it is feasible and crucial to manipulate the properties of catalysts through exploiting novel preparation techniques.

2.2. Performance of Prepared Catalyst in Plasma-Catalytic NH₃ Decomposition

Our previous studies showed that Co-based catalyst exhibited the best activity towards NH₃ decomposition to H₂ in the presence of DBD plasma [12]. Here, the influence of catalyst preparation method on the performance of plasma-catalytic NH₃ decomposition was investigated, as shown in Figure 5. Compared to the conventional preparation method, Co/fumed SiO₂ catalyst prepared with the improved method greatly promoted the reaction performance, and the conversion of NH₃ increased from 25.8 to 72.7% at the reaction temperature of 400 °C in Figure 5a, increased by a factor of almost 3 and, correspondingly, the energy efficiency of H₂ formation increased from 2.3 to 5.7 mol(kW·h)⁻¹ in Figure 5b. In addition, changing the reaction temperature from 300 °C to 450 °C through increasing DBD energy input resulted in a significant increase of NH₃ conversion by 80.8% (from 16.1 to 96.9%) in the case of catalyst prepared by the improved method whereas, at the same conditions, the NH₃ conversion only increased by 47.3% (from 4% to 51.3%) over catalyst using the conventional preparation method. Note that the reaction temperature required for complete conversion of NH₃ in the case of using improved preparation method shifted towards lower temperature, at least 50 °C lower in comparison with that using conventional preparation method in Figure 5a.

Combining the results of characterizations in Figure 1 to 4, the improved preparation method did not affect the phase composition of catalyst (Figure 1), but significantly increased the dispersion of catalyst with a narrow particle size of 2–3 nm (Figure 2), which actually creates much more active sites for NH₃ decomposition, enhancing the specific reactivity of catalyst, and this is also directly evidenced by the result of NH₃-probe experiments presented in Figure 3. Notably, the adsorption amount of NH₃ over the catalyst with improved preparation method is much larger than that with conventional preparation method (Figure 3), this directly points to the fact that enhancing the adsorption step of NH₃ decomposition is one of the reasons for the high activity of catalyst with improved preparation method. Recently, CoPt/TiO₂ with Co particle size of ~1 nm displayed a much higher Fischer–Tropsch reaction rate, which was also found to be due to increasing the amount of active site caused by using plasma-assisted preparation [44]. More importantly, in this study, the improved preparation method increased the acid strength of catalyst as well (Figure 3), as demonstrated by the increase in adsorption strength of NH₃ over catalyst, which can promote the dissociation step of NH₃; this is another crucial reason that explains the high activity of catalyst with the improved preparation method. Besides, the improved preparation method strengthened the interaction of Co with fumed SiO₂ (Figure 4), indicating the difference in electronic structure of catalyst with different preparation methods, and this could influence the activity of catalyst as well.

In addition, using Co/fumed SiO₂ catalyst prepared by the improved method, the influence of the combining mode of plasma and catalyst was investigated on the performance of plasma-catalytic NH₃ decomposition, as shown in Scheme 1 and Figure 6. About 3 g Co/fumed SiO₂ was packed in the reactor with a packing volume of about 3.1 mL, and the combining mode of plasma and catalyst changed through changing discharge volume “V”, but the packed catalyst was fixed. Namely, changing “V” from 3.3 to 0.4 mL enabled the catalyst to be partly packed in the field of plasma, as shown in Scheme 1.

In Figure 6a, interestingly, the conversion of ammonia was greatly enhanced with discharge volume decrease, and partly packing catalyst into the discharge area was found to be better than that of full-packing mode. Among the cases studied, the discharge volume with 0.4 mL showed the best activity towards NH₃ decomposition, in this case, the reaction temperature with 98.0% NH₃ conversion was only 380 °C, which was 140 °C lower than that in the case of catalyst alone. At the reaction temperature of 380 °C, the conversion of NH₃ over Co/fumed SiO₂ is only 6.2% without plasma whereas, at the same conditions, the use of DBD plasma significantly enhanced the reaction performance, and the conversion of NH₃ increased by a factor of 16 (from 6.1% to 98.0%) with decreasing discharge volume from 3.3 to 0.4 mL. Correspondingly, the energy efficiency of H₂ formation increased from 11.9 to 15.9 mol(kW·h)⁻¹; this is the highest H₂ formation rate obtained in ammonia decomposition so far, as shown in Figure 6b. In addition, Figure 6c displayed that the specific energy input (SEI) significantly increased with decreasing discharge volume, which might be the reason for the high performance shown in Figure 6a,b. To exclude the effect of heat caused by SEI increasing on the reaction performance, the reaction temperatures with different discharge volumes were all controlled at around 350 °C by adjusting energy input, then the relationship of ammonia conversion and SEI was presented in Figure 6d. Clearly, the conversion of ammonia increased with SEI increasing, demonstrating that the high performance resulting from high SEI was not due to heating of the catalyst. Furthermore, our previous studies revealed that increasing energy input of discharge can significantly facilitate the desorption of the strong-adsorbed N from catalyst surface (rate-limiting step in ammonia decomposition) [11], thus, the nature of the contribution of high SEI was to accelerate the rate-limiting step of ammonia decomposition.

3. Materials and Methods

3.1. DBD Plasma-Catalytic Reactor

NH₃ decomposition for H₂ generation was carried out in a DBD reactor with a catalyst bed in the discharge area at atmospheric pressure (Scheme 2). The DBD reactor was a typical cylindrical reactor using a stainless-steel rod (2 mm o.d.) as a high-voltage electrode placed along the axis of a quartz tube (10 mm o.d. × 8 mm i.d.) which was used as a discharge dielectric. An aluminum foil sheet tightly covered the outside of the quartz cylinder and served as a ground electrode. A 3 mm of discharge gap was used, and catalyst was fully packed in the discharge area unless otherwise noted. The DBD reactor was connected to an AC high voltage power supply with a peak voltage of up to 30 kV and a variable frequency of 5–20 kHz. In this study, the discharge frequency was fixed at 12 kHz, and NH₃ with a purity of 99.999% was fed into the DBD reactor at a total flow rate of 40 ml/min. The products of NH₃ decomposition were analyzed on-line using a gas chromatograph (Shimadzu GC-2014) equipped with a thermal conductivity detector (TCD). The input power driving the reaction was determined from the product of the apparent voltage and current of AC power supply, and the discharge power was measured using a four-channel digital oscilloscope (Tektronix DPO 3012, high-voltage probe Tektronix P6015A, Tektronix Tech. Corp., Beaverton, OR, USA, current probe Pearson 6585, Pearson Electronics, Inc., San Jose, CA, USA).

To evaluate the reaction performance of plasma-catalytic NH₃ decomposition to produce H₂, the conversion of NH₃ was calculated using Equation (1). The energy efficiency of H₂ formation (mol(kW∙h)⁻¹), defined as the number of moles of H₂ produced per kilowatt hour, was calculated using Equation (2). The specific energy input (SEI), defined as the energy input per discharge volume, was calculated using Equation (3).

$${\mathrm{X}}_{{\mathrm{NH}}_3}(\%)=\frac{{\mathrm{moles}\mathrm{of}\mathrm{NH}}_3\mathrm{converted}}{{\mathrm{moles}\mathrm{of}\mathrm{initial}\mathrm{NH}}_3}\times 100$$

(1)

$${\mathrm{E}}_{{\mathrm{H}}_2}=\frac{3·{\mathrm{X}}_{{\mathrm{NH}}_3}·{\mathrm{F}}_{{\mathrm{NH}}_3}\times 60}{2\times 1000\times 22.4\times \mathrm{P}}$$

(2)

• $E_{H_2}$: Energy efficiency of H₂ formation, mol(kW∙h)⁻¹
• $X_{N{H}_3}$: Conversion of NH₃ , %
• $F_{N{H}_3}$: NH₃ flow rate, $\mathrm{mL}·{\min }^{-1}$
• P: Plasma power, kW

$$\mathrm{SEI}=\frac{\mathrm{energy}\mathrm{input}\left(\mathrm{W}\right)}{\mathrm{discharge}\mathrm{volume}\left(\mathrm{mL}\right)}$$

(3)

3.2. Catalyst Preparation

Cobalt nitrate was provided by the Tianjin Kermel Chemical Reagent Co., Ltd (Tianjin, China). Fumed SiO₂ was purchased from the Dalian Luming Nanometer Material Co., Ltd (Dalian, China). Catalysts were synthesized either using the conventional preparation method and improved preparation method. Incipient wetness impregnation was used in this study. Briefly, cobalt nitrate (the theoretical metal loading was 30 wt %) was dissolved in deionized water. The support of fumed SiO₂ was calcined, in advance, at 400 °C for 5 h to remove impurities, such as H₂O, before impregnation, and then the pretreated support was added to the cobalt nitrate solution and stirred until it was thoroughly mixed. For “conventional preparation method”, the resulting mixture was kept at room temperature for 3 h and dried in air overnight at 110 °C. The dried sample was finally calcined in air at 540 °C for 5 h. Different from the conventional preparation method, for the “improved preparation method”, the resulting mixture was kept at room temperature for 3 h, followed by vacuum-freeze drying overnight at −50 °C before dried in air at 120 °C for 5 h, then the dried sample was calcined in a He-DBD plasma environment at 400 °C for 3 h to obtain the as-prepared catalyst. In addition, all the as-prepared catalysts were treated in NH₃-DBD plasma at 400 °C for 0.3–1.0 h to reduce catalysts before evaluating their activity in NH₃ decomposition.

3.3. Catalyst Characterization

X-ray diffraction (XRD) patterns of as-prepared catalysts were recorded using a Rigaku D-Max 2400 X ray diffractometer with Cu K_α radiation. Transmission electron microcopy (TEM) was used to characterize metal particles formed on the support surface (FEI Tecnai G2 F30 microscope, point resolution 0.2 nm, operated at 300 kV, Utrecht, Netherlands).

The reduction behavior of as-prepared catalyst was evaluated by H₂ temperature-programmed reduction (H₂-TPR) using a Chemisorption instrument (ChemBET 3000, Quantachrome, Boynton Beach, FL, USA). The sample (100 mg) was pretreated at 500 °C for 1 h under He flow (20 mL/min), and then cooled to 50 °C. The pretreated sample was exposed to a H₂/He mixture (10 vol% H₂) and was heated from 150 to 800 °C at a constant heating rate of 14 °C/min to get a H₂-TPR profile. The acid–base properties of the as-prepared catalyst were tested by NH₃ temperature-programmed desorption (NH₃-TPD) using the same Chemisorption instrument with operating H₂-TPR. The sample (140 mg) was pretreated at 500 °C for 1 h under He flow (20 mL∙min⁻¹), and then cooled to 150 °C. The pretreated sample was saturated with NH₃ for 30 min, and then purged with He flow for 1 h at 150 °C. The TPD profile was recorded while the sample was heated from 150 to 600 °C at a constant heating rate of 14 °C∙min⁻¹ under He flow.

The specific surface area (S_g) of fumed SiO₂ support was tested by N₂ physisorption at −196 °C (Micrometrics ASAP 2020, Norcross, GA, USA). Prior to the N₂ physisorption measurement, fumed SiO₂ was degassed at 350 °C for 3 h, and S_g was calculated using the Brunauer−Emmett−Teller (BET) equation.

The metal loading of fumed SiO₂ supported catalyst with different preparation methods was determined using X-ray fluorescence (XRF, SRS-3400, Bruker, Germany).

4. Conclusions

CO_x-free H₂ generation from plasma-catalytic NH₃ decomposition has been significantly promoted over Co/fumed SiO₂ catalyst prepared with an improved preparation method, which featured the use of vacuum-freeze drying and DBD plasma calcination techniques during catalyst preparation. Compared with the activity of the catalyst prepared by the conventional preparation method, the conversion of NH₃ increased by 47% on Co/fumed SiO₂ catalyst prepared by improved method and, correspondingly, the energy efficiency of H₂ production increased from 2.3 to 5.7 mol(kW·h)⁻¹. The enhanced activity was mainly attributed to the high dispersion of Co particles on fumed SiO₂ with a narrow particle size distribution (2–3 nm), which brought more active sites, stronger acidity, and a strong metal–support interaction. In addition, the reaction performance was significantly improved with the increase of specific energy input. At 380 °C, the highest energy efficiency of H₂ formation achieved, so far, was 15.9 mol(kW·h)⁻¹ over improved prepared Co/fumed SiO₂ catalyst with 98.0% ammonia conversion at the optimal conditions.