Plasma driven ammonia synthesis over silica: The role of the catalyst morphology and porous structure

doi:10.21203/rs.3.rs-3108484/v1

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Plasma driven ammonia synthesis over silica: The role of the catalyst morphology and porous structure

https://doi.org/10.21203/rs.3.rs-3108484/v1

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Non-thermal plasma (NTP) has opened novel routes for ammonia production, providing a powerful and high-performance rivalling the conventional Haber-Bosch process. Yet, the enhancement of ammonia yield over NTP driven catalytic processes is still challenging. In this work, ammonia synthesis performance is tailored by the morphology and textural properties including surface area and pore size volume of various silica catalysts. Experimental results indicated the effects of different catalyst surface areas, pore sizes, and morphology on the surface discharge. Spherical macroporous silica displays the highest ammonia production rate of 0.14 mmol min^-1 m^-2 at a power of 15W. The spherical macroporous silica offers a more uniform discharge, enhancing the reaction of unreacted N₂ and H₂ and allowing their diffusion and absorption inside the pores to form ammonia. The findings from this work provide insights for tailoring catalyst porous structure and morphology for ammonia production powered by non-thermal plasma.

Non-thermal plasma

ammonia synthesis

mesoporous silica

macroporous silica

plasma catalysis.

Ammonia is an important building block for fertilizers and many other chemicals production [1]. Moreover, ammonia has recently been used as a sustainable fuel for mobile and remote applications [2]. Ammonia offers a lower cost per unit of stored energy. As an example, 182 days of ammonia storage incurred only $0.54/kg-H₂ compared to almost $15/kg-H₂ (pure hydrogen) storage [3]. Ammonia also has a high volumetric energy density (7.1–2.9 MJ/L), added to widespread production, handling, and distribution capacity with better commercial viability [4]. Therefore, long-sustained ammonia production can be foreseen [5]. The application of plasma technology for ammonia production has been considered a promising alternative to replace the traditional Haber-Bosch (H-B) process, which is often operated at harsh conditions such as high temperature (650–750 K) and pressure (50–200 bar) [6, 7]. Non-thermal plasma (NTP) driven ammonia synthesis can be operated at atmospheric pressure and low temperature (< 300 ^oC).[8] In NTP, electrons are highly energetic with electron temperature in the range 10⁴-10⁵ K (1–10 eV), while the bulk gas temperature remains as low as room temperature [9–11]. Unlike thermal catalysis routes requiring high temperatures to dissociate the highly stable N≡N triple bond and the use of noble catalysts such as Ru or Rh [12]. Plasma activation can eliminate equilibrium barriers exhibited by thermal systems and secure nitrogen conversion with hydrogen to ammonia at milder bulk conditions [13, 14]. Previous studies have also indicated that the combination between plasma and catalyst can deliver vibrationally excited N₂ effectively [15]. More importantly, plasma-based reaction systems offer great flexibility, safety, and straightforward switch ON/OFF, and do not require complex heat/chilling systems [16, 17]. Such advantages place NTP as a high-potential technology for decentralized small-scale ammonia synthesis powered by intermittent renewables, which increases the overall economic capability of the plasma process.

Nonetheless, NTP-mediated ammonia production is currently dealing with two major challenges: (i) the complexity of plasma-catalyst interactions; and (ii) plasma-induced reverse reactions (ammonia decomposition) that take place during ammonia formation reactions concomitantly [18–20]. The first obstacle leads to the lack of solid understanding of the plasma-catalyst synergism, which is crucial for further optimization and scaling-up of the ammonia production process driven by NTP. The second challenge, on the other hand, diminishes ammonia product yield significantly. Researchers have indicated that the decomposition of ammonia freshly generated in the plasma reaction regime takes place by electron impact dissociation during the micro-discharges [21–24]. Moreover, studies also reveal that the presence of a catalyst with a certain pore size and morphology can limit the in situ decomposition of ammonia. For instance, our recent work confirmed that the octahedral shape of CC3 crystal [25] or the pore size of mesoporous SBA-15 [2] can offer shielding effects reducing ammonia decomposition and improving ammonia production rates. In terms of catalyst selection, solid oxides are considered more robust and affordable relative to their metal-based counterparts [26]. Furthermore, solid oxides possess flexible textural, compositional, and morphological properties that can be tailored for various catalytic reactions. Silica is among such solid oxides and has recently emerged as a potential candidate for plasma-assisted ammonia synthesis. Silica possesses low electronegativity (about 3.38), and low electrical resistivity that can lead to more stable and uniform plasma discharges [2]. Our recent DFT calculations [15] and experimental results [25] have revealed that a desirable catalyst is the one that can delay the recombination of adsorbed H* into H₂ enabling it to instead bind to adsorbed N₂^activated to form NH*. Also, it is necessary to prevent the in situ decomposition of ammonia due to the plasma exposure. This latest can be achieved with the use of catalysts with high surface areas and large pore size which can promote mass transfer of guest-product species. In fact, it has been indicated that active plasma species can occur inside catalyst pores[27]; nonetheless, the mechanism underlying the plasma behaviour when employing porous materials i.e., silica with different surface areas and pore size ranges remains unclear. Further, due to such complex interactions between the plasma discharge and the catalyst, rational design of porous catalysts needs to be attempted thoughtfully.

Herein, this work reports NTP driven ammonia synthesis over silica catalysts with different morphologies such as sphere and gyroids with various pore regimes, including mesoporous, and macroporous. The influence of the morphology and surface area of silica catalysts on plasma driven ammonia production and in situ decomposition are addressed thoughtfully. Moreover, the mechanism of plasma discharge both on the silica surface and pores is discussed with support from the adsorption-desorption experiments and in situ diagnosis. The findings from this work provide significant insights into tailoring catalysts that can limit the recombination of intermediates to form initial reactants and the decomposition of ammonia product in the plasma regime.

Catalyst preparation

Macroporous gyroid-shaped silica. Macroporous gyroid silica was prepared via wet chemical method. The precussor solution consisted of tetrapropylammonium bromide (TPAB), NH₄OH, Tetraethyl orthosilicate (TEOS), and deionized water was mixded and stirred for 4h at room temperature. Another solution for spherical templates was prepared at the same time. In this template solution, 4g polystyrene (PS) beads and 20g toluene was stirred for 2h at room temperature. These two solutions were then mixed and stirred at 60 ^oC for 24 h. The white solid obtained was calcined at 600 ^oC with a heating rate of 1 ^oC.min^− 1 for 4 h in air.

Macroporous spherical-shaped silica. Macroporous spherical-shaped silica was prepared by the hydrothermal method. The first solution consisted of 5g TPAB and 20g of deionized water and was stired for 1h at 40 ^oC temperature. 10 mL of NH₄OH were added to this solution and it was kept stirring for 1h. Then, 4.9g TEOS was added to the mixture and the solution was stirred for 1 h at 40 ^oC temperature. A second solution consisted of 4g of polystyrene (PS) beads and 20g of toluene was stirred at room temperature for 1h. These two solutions were mixed and stirred at 40 ^oC for 24 h. The mixture was then transferred to a Teflon lined hydrothermal autoclave to age at 80 ^oC for 24 hours. The white solid collected from the autoclave was rinsed three times with ethanol and water, respectively prior to calcination at 600 ^oC with a heating rate of 1 ^oC.min^− 1 for 6h in air.

Mesoporous spherical-shaped silica. To prepare mesoporous silica particles, cetyltrimethylammonium bromide (CTAB) were added to the solution consisted of N, N-dimethylformamide (DMF), NaOH, and deionized water. The solution was stirred for 30 min before adding TEOS and kept stirring at 40 ^oC for 24 h. The precursor solution was aged in a Teflon autoclave at 80 ^oC for 12 hrs. The white solid obtained was calcined at 500 ^oC with a heating rate of 1 ^oC.min^− 1 for 4 h in air.

Mesoporous gyroid-shaped shape. Mesoporous silica with SBA-15-gyroid morphology was synthesized via the sol-gel method, modified from elsewhere [28]. The process was initiated by pre-hydrolyzation of TEOS in ethanol solution by an acid-catalysis to obtain an oligomeric silica solution, which was added dropwise to a solution of P123 triblock copolymer and electrolyte in water and ethanol magnetically stirred at room temperature for 4 hours. The final molar composition of the mixture was 1.0 TEOS:0.03 P123:1.0 MgSO₄: 65 H₂O:0.005 HCl:40 ethanol. The mother gel was aged for 12 hours at 30^oC to maintain the interconnection of the sol-gel network. The solid products were obtained after being dried at 50^oC overnight and centrifuged three times with deionized water at 5000 RPM for 10 minutes to remove electrolytes, further dried at 80^oC overnight. The solid products obtained were calcined to completely remove the block copolymer species in an oven at 500°C at a 1°C/min ramp for 12 h in air.

Catalyst characterization

Powder X-Ray Diffraction patterns were collected using a X’Pert PRO MPD X-ray Diffraction system operated at 30 kV and 25 mA with a Cu Kα1 radiation (λ = 1.54 Å). The morphology of the catalysts was inspected by scanning electron microscopy (SEM) JEOL JSM-7401F, with an acceleration voltage of 10 kV. We also performed Transmission Electron Microscopy (TEM) Analysis. The catalyst samples were also examined for their morphology using a FEI Talos F200X microscope operating at 200 kV. To prepare the TEM specimens, the catalyst and reference samples were dispersed as fine powders on standard holey carbon-on-copper 300 mesh TEM grids. Scanning Transmission Electron Microscopy (STEM) images were captured from these specimens employing a high annular angle dark-field (HAADF) detector and a sub-nanometer electron probe. The utilization of STEM-HAADF imaging offered a twofold advantage. Firstly, it allowed for strong mass contrast, facilitating effective visualization of the pore structure. Secondly, the use of STEM imaging significantly minimized the overall electron-beam exposure of the samples compared to imaging in regular TEM mode. Consequently, the adverse effects such as radiation damage on beam-sensitive samples were notably mitigated. The prepared catalysts were degassed at 120°C under a vacuum for 12 hours. Nitrogen physisorption isotherms were conducted at 77 K (ASAP 2020 Plus, Micrometric) to determine Brunauer-Emmett-Teller (BET) surface area, pore volume, and pore size for fresh catalysts.

Catalytic performance tests

Silicas with different porosities were tested in an in-house-built atmospheric DBD reactor [13, 29, 30]. For the catalytic tests in this reactor, N₂ and H₂ cylinders were connected to the reactor using mass flow controllers (MFC). The reactions were carried out at 25 sccm of total flow rate with various nitrogen and hydrogen feed ratios (1:1, 1:2, 1:3) (N₂:H₂). The plasma power varied from 2.5–15 Watts with 25 ± 0.5 kHz frequency. A wire from Midwest Tungsten Service was employed as an inner electrode. The outer electrode was made of tinned copper mesh acting as the ground electrode. The electrical characterization was carried out by measuring the applied voltage to the reactor by employing a high-voltage probe (Tektronix P6015A). The charge was calculated using the voltage measurement across a capacitor. The two probes were connected to an oscilloscope (Tektronix TDS2014C). The capacitor was connected to the reactor in series with the ground electrode. The high-voltage power supply was connected to the reactor using a litz wire and alligator clips. The inner electrodes were placed at the centre of the quartz tube with an ID of 4 mm and an OD of 6.35 mm. The gases collected from the reactor exit were sent to an online gas chromatograph to determine the ammonia synthesis rate. The quantification was performed using an Agilent 8860A GC with an HP-PLOT U column (30 m × 0.320 mm × 10 µm) and hydrogen as the carrier gas. The light emitted from the discharge was led through an optical system. The measurements were recorded using a dual-channel UV-VIS-NIR spectrophotometer in scope mode (Avantes Inc., USB2000 Series). The spectral range was from 200–1100 nm, using a line grating of 600 lines/mm and a resolution of 0.4 nm. A bifurcated fibre optic cable with 400 µm was employed. For accuracy, integration time was set at 5 seconds with 100 averaging.

Catalyst characterization

Figure 1 presents the morphology of all the prepared silica catalysts, including mesoporous spheres and gyroids, and macroporous spherical and gyroid silica. Figure 1(a) also displays the presence of well-defined mesoporous spherical silica while Fig. 1(b) displays well-defined macroporous silica with larger spheres. To gain a better understanding of the silica morphology, STEM and HRTEM characterizations were also performed. The high angle dark-field STEM images (Fig. 1(c)&(d)) of the meso-and macro-porous silica confirm the well-defined spheres. The mesoporous spherical-shaped silica presents inhomogeneous particle size distribution while the macroporous spherical-shaped silica own a more uniform particle size. The mesoporous spherical-shaped silica shows particle size ranging from 100 nm to 800 nm while the macroporous spherical silica size was larger, about 2µm. The HRTEM image (Fig. 1(e)) of the mesoporous spherical-shaped silica shows a pore size of around 20 nm; nonetheless, the presence of the microporous pores was observed. In the meanwhile, the pore size of macroporous spherical-shaped silica displays a pore size of around 50–80 nm (Fig. 1(f)). Nonetheless, such pore sizes are not uniform throughout the macroporous silica spheres. We, therefore, propose that macroporous silica comprise a mixture of micro-, meso-, and macropores. Similar observations have been reported by other researchers [31], who observed a boosted role of silica with hollow micro–meso–macroporous structure on the adsorption of Cr⁵⁺. The gyroid morphology of meso-and macro-porous silicas was observed from their respective SEM images (Fig. 1(g)&(h)). The gyroid silica with microporous pore-regime owns a larger size. Such difference in catalyst’ morphology is crucial for chemical reaction performance as it determines the electrical properties of the plasma and the associated molecular reaction rate, thereby, altering the plasma-catalytic synergism [11]. The next section will verify the effects of these morphological differences on the catalytic performance.

N₂ adsorption-desorption characterization was performed for all the prepared catalysts and the result are shown in Fig. 2. The N₂ adsorption-desorption curve of the mesoporous silicas of both mesoporous gyroid- and spherical-shaped display type IV affirming the presence of mesoporous structure. In contrast, the N₂ adsorption-desorption curve of the macroporous silicas including spherical- and gyroid-shaped morphology displayed type VI isotherms verifying the presence of macroporous structure, but with a mixture of micro-and meso-porous pore regime [17]. Moreover, the H₂ pore shape of mesoporous silicas shows bottleneck constrictions in their structure. While the H₄ pore type shape verifies the presence of slit pores in the macroporous silicas, which further confirms the mixtre of micro-, meso-, and macro-porous structures.

The specific surface of all prepared catalysts are shown in Table 1. It is worth noting that the macroporous spherical-shaped silica owns a surface area with 634 m²/g, which is significantly smaller than the macroporous gyroid-shaped silica (928 m²/g). Similar trend was observed from mesoporous silicas, where gyroid-shaped (870 m²/g) possesses larger surface area than spherical-shaped counterpart (521 m²/g). Pore size and pore volume of both marco-and meso-porous silicas also displays similar trend, where gyroid-shaped morphologies own larger pore size. Noticably, the mesoporous gyroid-shaped silica displayed largest pore size of 6.44 nm. Such irregular trends in the pore sizes are due to the co-existence of micro-, meso-, and macro-porous structures in the macroporous silicas and microporous structure in the mesoporous silicas. The N₂ adsorption analysis only delivered the average values. Similar observations have been reported by other researchers [31–33].

Table 1

Textural properties for all prepared catalysts.
Catalyst	Morphology		Surface Area	Pore Size	Pore Volume
Catalyst	Morphology		m²/g	nm	cm³/g
Macroporous silica		Sphere	634	3.42	0.54
Macroporous silica		Gyroids	928	4.20	1.00
Mesoporous silica		Sphere	521	3.92	0.51
Mesoporous silica		Gyroids	870	6.44	1.39

Electrical properties and in situ OES diagnostic. It is important to characterize the electrical properties of all prepared silica catalysts in the plasma environment to provide better understanding of the influence of the morphology on the discharge, and hence, the impact on ammonia production rate. Figure 3(a) shows the voltage-charge curve with a well-defined Lissajous loop for all the prepared silica catalysts under the plasma regime. The area of this loop represents the energy dissipated by the discharge period. The electrical power was calculated by multiplying this value with the waveform frequency (AC coupling).

P_electrical=${f}_{AC}\oint \text{v}\left(\text{t}\right)\text{d}\text{Q}$ (1)

It can be seen that the largest dissipated energy was observed from the presence of the macroporous silica with both spherical and gyroids shapes. In fact, the spherical structure is proven to offer a more uniform discharge than other particle forms. The voltage-time graph (Fig. 3(b)) exhibits discharged pk-to-pk voltage of 8.97 kV with pulsed width of 43.8µS. From the voltage and current curves, the time-averaged electrical power consumed by the plasma actuator was computed from:

P_avg=$\oint \text{v}\left(\text{t}\right)\text{*}\text{q}\left(\text{t}\right)\text{*}\text{d}\left(\text{t}\right)= \frac{\text{f}}{2{\pi }}\text{*}\text{S}$ (2)

The OES spectrum collected during plasma reactions is shown in Fig. 3(c). All species detected from the OES spectrum were consistent with our previous studies. The intensity of all detected peaks was stronger with the presence of the macroporous silica in the packed-bed plasma reactor. In this work, we also tested plasma catalytic pulsing (Plasma On/Off) to gain insights on the adsorption and desorption behaviour of ammonia with various silica catalysts. The results show that the morphology and porous structure influence the desorption of ammonia. It can be seen that when plasma was Off macroporous spherical-shaped silica owns a better ammonia desorption rate relative to other silica catalysts. This observation reveals that more ammonia was captured inside macropores of spherical silicas, and verifies our assumption on the possible formation of ammonia taking inside of the pores.

Catalytic performance. The catalytic activity for plasma driven ammonia production is shown in Fig. 4. The influence of the catalyst’ morphology and porous structure on the plasma driven ammonia production is thoughfully addressed in this context. The macroporous spherical-shaped silica displays the highest ammonia production rate of 0.14 µmol min^-1 m^-2 at a power of 15W followed by the macroporous silica with gyroid morphology of 0.08 µmol min^-1 m^-2. The reason for the better performance of the macroporous spherical-shaped silica catalyst is attributed to the penetration of plasma active species into the macropores to promote unreacted N₂ and H₂ diffusing inside the pores to form ammonia. However, it is also worth noting that in situ decomposition of ammonia products can lead to the reduction in ammonia production rate [34, 35], taking place concomitantly. Macropores can offer a better reaction pathway to form more ammonia but also promote the diffusion of plasma active species to decompose ammonia freshly formed. It has been proved experimentally that plasma discharge can have an important effect within the lowest limit of pore size of 50 nm at a very short distance and time [36]. From the performance results, the penetration of plasma active species into macropores mostly promoted the reaction between unreacted N₂ and H₂ to form ammonia. As depicted in Fig. 1, gyroid hollows are larger than spheres, and the in situ decomposition, in this case, would be more enhanced. Conversely, mesoporous silicas were not able to accelerate the reaction of unreacted N₂ and H₂ inside mesopores [1, 15, 24, 30]. The H₂-rich environment, for example, N₂:H₂ of 2 and 3, Fig. 4(b)&(c), slightly promotes the ammonia production observed from all prepared catalysts Fig. 4(d).

Figure 5 displays a comparison of ammonia yield obtained with various silica catalysts in this work and with other materials reported in the literature at similar reaction conditions. An ammonia energy yield of around 7.5 g-NH₃ kWh^− 1 could be achieved over the macroporous silicas, verifying the roles of pore and morphology role in ammonia energy yield. Such findings are good demonstrations of tailoring morphology and porosity of either catalyst or support to achieve better ammonia energy yield.

In this work, plasma driven ammonia production over silica catalysts with various morphology including spherical and gyroid, and different porous structure such as meso-, and macroporous were investigated. The influence of silica morphology and porosity on plasma powered ammonia production was investigated thoughtfully. The ammonia production rate was favored in the following order: macroporous spherical-shaped > macroporous gyroid-shaped silica > mesoporous spherical-shaped silica > mesoporous gyroid-shaped silica. The larger the pore size offers better plasma active species diffusion to boost the formation of ammonia. Spherical-shaped silicas was favourable for obtaining higher ammonia production rate given their better uniform plasma discharge and limitations of in situ ammonia decomposition from the plasma regime relative to gyroid morphology. Whether a commercial non-thermal plasma process for direct ammonia production will be available soon as did the non-thermal plasma-assisted ozone production, is progressively contingent on the progress of achieved fundamental understandings of catalyst and operational optimization. The findings from this work provide important steps for development of earth-abundant catalysts by tailoring their morphology and porous structure to achieve better plasma ammonia production yield.

Author Contributions

Hoang M.Nguyen: Performed material characterization, Formal Analysis, Methodology, Reviewing and Editing; FNU Gorky: Performed ammonia synthesis experiments; Jacek B. Jasinski: TEM characterization of silicas; Maria L. Carreon: Investigation, Supervision, Funding acquisition, Project administration, Reviewing and Editing.

Funding Maria L. Carreon acknowledges the Department of Energy Office of Science DE-SC0023261, NSF-CBET award No. 2203166, and NSF-CAREER award No. 2235247.

Ethical Approval

Not Applicable.

Supplementary Information The online version contains supplementary material available at (website link)

Data Availability A reasonable request to the corresponding authors may permit the data used to support the results of this study.

Conflict of interest I declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

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No competing interests reported.

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Plasma driven ammonia synthesis over silica: The role of the catalyst morphology and porous structure

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Catalyst characterization

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