Fundamental studies of ruthenium species supported on boron nitride nanotubes: metal loading and pretreatment effects on CO oxidation

Abstract

Multiwalled boron nitride nanotube (BNNT), as a catalyst support, has become one of the promising materials due to its high oxidation resistance and thermal stability. In this work, ruthenium (Ru) supported on BNNT catalysts with different metal loadings and treatment conditions was investigated for CO oxidation as a model reaction. To understand the physicochemical properties of the prepared samples, a suite of techniques, including FTIR, UV-Raman, SEM, TEM, and XPS, was utilized. The results showed that the RuO_x species were located on both the interior and the exterior surfaces of the BNNT, and an increase in metal loading led to increased active sites. 1 wt% RuO_x/BNNT (oxidized) exhibited better catalytic activity than 1 wt% Ru/BNNT (reduced), indicating that treatment conditions significantly affect the catalytic properties. Reaction conditions, such as GHSV and the O₂/CO ratio, were varied to further investigate the external mass transfer limitations and reaction mechanism of the 1 wt% RuO_x/BNNT catalyst. The peculiar tubular morphology of the BNNT resulted in negligible external mass transfer limitation, and the catalyst might primarily follow the Eley–Rideal (ER) mechanism over the Langmuir–Hinshelwood (LH) mechanism.

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1. Introduction

Boron nitride nanotube (BNNT) has attracted considerable attention as a promising catalyst support due to its high oxidation resistance and superior thermal stability.^1–3 The BNNT is generally oxidized after 800 °C, allowing it to be effective for high-temperature applications.^3,4 High thermal stability can minimize the chronic problems of catalyst sintering and deactivation caused by support collapse. In addition, these properties are one of the important keys for hindering the nanoparticle sintering caused by particle migration and coalescence and Ostwald ripening mechanisms.^5,6 Recently, numerous theoretical studies reported that the BNNT can be a good candidate as a catalyst support due to its unique properties.^7–12 Q. Chen et al. designed Ru_B@BNNT (doped a Ru atom into the B vacancy in the BNNT) using the density functional theory (DFT) method and reported that the catalyst stability is superior to that of Ru@hexagonal boron nitride nanosheet (h-BNNS).¹³ The authors claimed that Ru_B@BNNT exhibits strong hybridization at the Fermi level, leading to high structural stability. R. Chen et al. investigated the feasibility of arsenic(V) adsorption from water solutions over Fe₃O₄/BNNT and concluded that the BNNT is a promising support material due to its high oxidation resistance and physical stability.¹⁴ Our previous research also demonstrated that platinum group metals (PGMs) supported on functionalized BNNT (e.g., Pd/f-BNNT, Pt/f-BNNT, and Rh/f-BNNT) catalysts exhibited outstanding catalytic activity and stability for NO reduction by CO oxidation due to the synergetic effect of PGMs and BNNTs.^15,16

Supported Ru catalysts have shown high catalytic activity in diverse fields, such as ammonia decomposition,^17,18 water splitting,¹⁹ CO₂ methanation,²⁰ CO oxidation,²¹etc. In the past decades, researchers have tried to reduce the Ru content in the catalysts due to its finite resources, while achieving similar or better catalytic performance. It has been acknowledged that the oxidation state of Ru species can significantly affect the catalytic performance. K. Xu et al. investigated the oxidation state effects of Ru species for CO oxidation using in situ diffuse reflectance infrared Fourier-transform spectroscopy (in situ DRIFTS).²² The authors demonstrated that Ruⁿ⁺ (4 ≤ n ≤ 6) are active species for the CO oxidation reaction, while Ru⁰ are inactive species. J. Li et al. changed the oxidation state of Ru species supported on CeO₂ nanorod by varying oxidizing and reducing synthesis conditions.²³ The authors found that the catalytic stability and performance on CO oxidation of the 5Ru/CeO₂ NR-r with Ruⁿ⁺ (4 ≤ n ≤ 6) species is better than that of the 5Ru/CeO₂ NR-o with Ru⁶⁺ species. W. Li et al. reported that the catalytic performance on the CO oxidation of Ru/graphene aerogels was decreased after reduction treatment.²⁴ The authors concluded that the reduction treatment step (e.g., Ru⁴⁺ to Ru⁰) made the Ru species inactive on graphene aerogel supports, which led to deactivation. These results indicate that the oxidation state of Ru species plays a key role in the catalytic performance for the CO oxidation reaction.

Based on the published papers, the BNNT as catalyst support has a potential to improve the catalytic activity and stability. However, most existing research was conducted as theoretical studies due to BNNT's supply shortages. Herein, we report an empirical study of RuO_x/BNNT catalysts with different Ru loadings and pretreatment steps (e.g., oxidizing and reducing). The physical properties and morphology of the synthesized catalysts were characterized by spectroscopic and microscopic techniques. CO oxidation as a model chemical reaction over the synthesized catalysts was studied to understand the effect of Ru loading and oxidation state on the catalytic performance.

2. Experimental section

2.1. Catalyst synthesis

Boron nitride nanotube (BNNT, purity >90 wt%, NAiEEL Technology) and ruthenium(III) acetylacetonate (Ru(C₅H₇O₂)₃, Sigma-Aldrich) were used as a support material and a surface species, respectively. The materials were used without further purification. For the synthesis of RuO_x/BNNT catalysts, the following three steps were applied: (1) predetermined amounts of Ru precursor and BNNT were mixed using a mortar and pestle for 20 min. (2) The Ru precursor was evaporated and interacted with the BNNT surface in an N₂ flow (UHP grade, total flow rate of 20 mL min⁻¹, 10 °C min⁻¹) at 170 °C for 2 h and cooled to room temperature. (3) The sample was calcined in air (dry air, 20% oxygen and 80% nitrogen, total flow rate of 20 mL min⁻¹, 10 °C min⁻¹) at 400 °C for 4 h. The oxidized samples were denoted as x wt% RuO_x/BNNT (x = 0.25, 0.5, and 1). In the case of 1 wt% Ru/BNNT catalyst, 1 wt% RuO_x/BNNT was reduced in a H₂ flow (10% H₂ balance with N₂, total flow rate of 60 mL min⁻¹, 10 °C min⁻¹) at 300 °C for 5 h. All samples were sieved (500 μm, Fieldmaster) to obtain uniform particle sizes.

2.2. Characterization of catalysts

To understand the molecular structures and bonding vibration of the prepared samples, Fourier-transform infrared (FTIR, Nicolet iS50, Thermo Scientific) and UV-Raman spectroscopy (325 nm, Renishaw inVia confocal Raman microscope) were used. UV-Raman spectra were collected in the range of 1000–1800 cm⁻¹, and the acquisition time and the accumulation of the final spectrum were 10 s and 30 scans, respectively. X-ray photoelectron spectroscopy (XPS) was used to investigate the oxidation state of the synthesized samples and obtained using monochromatic Al-Kα radiation (hν = 1486.6 eV). The surface morphology of the catalysts was characterized by scanning electron microscopy (SEM, EmCrafts CUBE II). Transmission electron microscopy (TEM, JEM 2100F, JEOL)/EDS was performed to study the Ru species on the BNNT surface. Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) measurements were conducted to determine the specific surface area (SSA) and pore size distribution (PSD), respectively, of the bulk BNNT using a Quantachrome NOVAtouch® instrument. The tests were performed at −196 °C with N₂ (UHP grade) as adsorption–desorption gas. Prior to measurement, the samples were degassed at 300 °C for 4 h under vacuum to remove moisture and volatiles.

2.3. Catalytic activity tests

The catalytic performance on CO oxidation was evaluated in a fixed bed quartz reactor (OD 9.6 mm, ID 7 mm) connected with a mass flow controller (SLA5800 Series, Brooks Instrument). 40 mg of catalyst was loaded in the middle of the reactor and held in place by quartz wool on each side. The sample was pretreated in a He flow (30 mL min⁻¹) at 400 °C for 30 min and then cooled to room temperature. For the catalytic CO oxidation experiment, the composition of the gas mixtures was 2 mL min⁻¹ CO, 20 mL min⁻¹ O₂, and 28 ml min⁻¹ He (total flow rate 50 mL min⁻¹ and GHSV 75 000 mL g_catalyst⁻¹ h). The reaction temperature was increased from room temperature to 400 °C at a ramping rate of 1 °C min⁻¹. The composition of the product gas was analyzed using an online gas chromatograph (TRACE™ 1300 GC, Thermo Scientific) equipped with a thermal conductivity detector (TCD) and a capillary column (Carboxen 1010 PLOT). The CO conversion was calculated using the following equation:
where [CO]_inlet and [CO]_outlet represent the influent and effluent CO concentrations at a certain temperature, respectively.

3. Results and discussion

3.1. FTIR and UV-Raman analysis

To understand the molecular structure of the samples, FTIR and UV-Raman spectroscopy were performed. For comparison purposes, BNNT and Ru(acac)₃ samples' spectra were also obtained. Fig. 1(a) shows the color changes of the metal precursor, BNNT, and prepared samples (e.g. Ru(acac)₃ (red), BNNT (white), 1 wt% Ru(acac)₃/BNNT (pink), 1 wt% Ru(acac)_y/BNNT (light pink), and 1 wt% RuO_x/BNNT (grey)). As shown in Fig. 1(b), the spectra of BNNT, 1 wt% Ru(acac)₃/BNNT (after premixing step), 1 wt% Ru(acac)_y/BNNT (after evaporation step), and 1 wt% RuO_x/BNNT (after calcination step) showed the same peaks at 762 and 1330 cm⁻¹, which corresponded to the B–N bending and B–N stretching modes, respectively.^15,16 Compared to the BNNT, 1 wt% Ru(acac)₃/BNNT displayed a distinct peak at 452 cm⁻¹, which is assigned to an acetylacetonate vibration. Other Ru(acac)₃ related peaks between 500 and 1600 cm⁻¹, however, were not observed in the 1 wt% Ru(acac)₃/BNNT spectra due to the overlap by the strong B–N bending and stretching peaks. In the case of 1 wt% Ru(acac)_y/BNNT and 1 wt% RuO_x/BNNT, the peak at 452 cm⁻¹ disappeared, indicating that the metal ligands were eliminated. However, the sample colors of 1 wt% Ru(acac)_y/BNNT (light pink) and 1 wt% RuO_x/BNNT (grey) were different (Fig. 1(a)). This result indicates that the sample after the evaporation step contains a small quantity of acetylacetonate residues and it was fully eliminated after the calcination step. As shown in Fig. 1(c), the UV-Raman spectra of the BNNT and three supported Ru samples (e.g., premixed, evaporated, and calcined) showed a peak at 1368 cm⁻¹, which corresponds to the E_2g vibration mode of the BNNT.²⁵ Since the E_2g peak was not shifted, even after the calcination step, it is expected that the BNNT has structural stability.

Fig. 1 (a) A digital photo, (b) FTIR spectra, and (c) UV-Raman spectra of (i) Ru(acac)₃, (ii) as-received BNNT, (iii) 1 wt% Ru(acac)₃/BNNT after premixing step, (iv) 1 wt% Ru(acac)_y/BNNT after evaporation step, and (v) 1 wt% RuO_x/BNNT after calcination step.

3.2. SEM and TEM analysis

The surface morphology of the 0.25, 0.5, and 1 wt% RuO_x/BNNT and 1 wt% Ru/BNNT was investigated using SEM. All samples showed a 3–5 μm tube length as shown in Fig. 2. This is in agreement with the bulk BNNT length studied in our previous research,¹⁵ indicating that the catalyst synthesis processes did not affect the tube length and structure.

Fig. 2 SEM images of (a) 0.25 wt% RuO_x/BNNT, (b) 0.5 wt% RuO_x/BNNT, (c) 1 wt% RuO_x/BNNT, and (d) 1 wt% Ru/BNNT.

Fig. 3 shows the TEM results of 0.25, 0.5, and 1 wt% RuO_x/BNNT and 1 wt% Ru/BNNT. The inner diameter of the BNNT was 10–35 nm and the outer diameter was 50–70 nm, which was consistent with the pore size of the nanotube structure measured by BET (Fig. S1†). According to TEM images in low magnification (Fig. 3(a)–(d)), the presence of RuO_x species increased with increasing metal loading percentage, suggesting a corresponding augmentation of active sites on the BNNT surface. As shown in Fig. 3(e)–(h), Ru nanoparticles were observed on the outer wall surface of the BNNT. The particle sizes were 10–20 nm and some clusters were observed. It is important to note that the nanoparticles were also consistently observed in the inner wall surface of the BNNT across all samples (Fig. 3(i)–(l)).Furthermore, these particles were found throughout the nanotubes, regardless of their position, whether at the edges or in the middle. Similar results have been reported by D. Ugarte et al.²⁶ The authors synthesized carbon nanotubes (CNTs) filled with silver particles, which was possible with tubes having an inner diameter of ≥4 nm. The authors claimed that wide nanotube cavities (or through pores) are preferred for filling since narrow cavities are greatly affected by van der Waals repulsion forces than wide cavities, inhibiting metal penetration into the tube cavities. X. Pan et al. reported that catalyst stability could be improved by locating metal nanoparticles inside CNTs due to the confinement effect.²⁷ The curvature of the CNT induces a shift in π-electron density from the interior surface to the external surface, causing the internal metal nanoparticles to donate more electrons to the electron-deficient internal surface. This results in stronger interactions between the internal metal and internal surface than the external metal and external surface, resulting in increased catalyst stability. Z. Peralta-Inga et al. computed the electrostatic potential on the inner and outer surfaces of BNNTs and reported that the potentials on the former are more positive than on the latter (i.e., the curvature of the BNNT induces the π-electron density to be shifted towards the external surface than the interior surface).²⁸ Thus, metal nanoparticles inside the internal surface of the BNNT would also be strongly confined. In the present work, the wide nanotube cavities of BNNTs (inner diameater: 10–35 nm) would allow Ru nanoparticles to be formed inside the inner surface of the BNNT, leading to the confinement effect.

Fig. 3 TEM images of 0.25, 0.5, and 1 wt% RuO_x/BNNT and 1 wt% Ru/BNNT: (a–d) low magnification, (e–h) Ru species on the outer wall of the BNNT, and (i–l) Ru species on the inner wall of the BNNT. Dotted circles: RuO_x or Ru particles.

To further explore the shape and location of RuO_x particles inside the BNNT, pristine BNNT and 1 wt% RuO_x/BNNT were investigated. Fig. 4(a) shows the TEM results of pristine BNNT, where regular dark spots and an independent BNNT (red arrow) inside another BNNT were observed. Based on the previous study by A. Celik-Aktas et al., these dark spots indicate high crystallinity, which is characteristic of BNNTs synthesized with a double-helix structure.²⁹ It is worthwhile to note that RuO_x particle sizes (e.g., 15.1 nm and 32.5 nm) and shapes (e.g., oval and circle) were varied with different inner diameters of the BNNT (Fig. 4(b)). Individual BNNTs can disturb the migration of partial Ru precursors during catalyst synthesis, leading to the growth of particles in locations with different inner diameters. If the RuO_x species had grown on the outer wall surface of the BNNT, the particle sizes would not match the inner wall line (Fig. 3(g)).

Fig. 4 TEM images of (a) BNNT and (b) 1 wt% RuO_x/BNNT.

3.3. XPS analysis

In order to analyze the oxidation state of Ru species in the 1 wt% RuO_x/BNNT and 1 wt% Ru/BNNT catalysts, XPS characterization was performed. Fig. 5 demonstrates the XPS spectra of the Ru 3d energy regions. The variation of peak intensity was observed, especially at ≤283.5 eV. Three different Ru 3d_5/2 peaks at around 280.8, 281.6, and 283.1 eV can be assigned to Ru⁴⁺, Ruⁿ⁺ (4 ≤ n ≤ 6), and Ru⁶⁺, respectively.^30,31 S. López-Rodríguez et al. studied the oxidation state of Ru of the series of RuO_x/CeO₂ catalysts by combining TPR, XRD, and XPS.³² Although the authors did not fully distinguish the oxidation state of the Ru, the authors assigned the Ru3d_5/2 regions containing Ru⁴⁺ and cationic species of Ru (or Ruⁿ⁺) in the CeO₂ lattice. K. Qadir et al. reported in situ XPS for Ru nanoparticles under oxidation and reduction conditions.³³ The authors provided the reversibility of Ru between Ru⁰ (279.8 eV) and Ru⁴⁺ (280.7 eV). It should be noted that the analysis of Ru3d_3/2 regions at >284 eV was more complicated since the C 1s peak (∼285.0 eV) is overlapped with Ru peaks. Based on a literature review, it is concluded that the 1 wt% RuO_x/BNNT (oxidation treatment) sample contains Ruⁿ⁺ species predominantly, while the 1 wt% Ru/BNNT (reduction treatment) sample contains Ru⁴⁺ species predominantly. The XPS results provide that the oxidation state of Ru species supported on BNNT depends on the treatment conditions, which may influence the catalytic properties.

Fig. 5 XPS spectra of Ru 3d for 1 wt% RuO_x/BNNT and 1 wt% Ru/BNNT. The Shirley-type background was applied.

3.4. Catalytic CO oxidation performance

Fig. 6(a) and Table 1 show the catalytic performance of the series of RuO_x/BNNT samples as well as the bulk BNNT. CO oxidation would occur simultaneously at the active sites on the exterior and interior surface since the BNNT provides enough space to penetrate the carbon monoxide (kinetic diameter: 3.76 Å) and oxygen molecules (kinetic diameter: 3.46 Å).³⁴ The temperature at 50% CO conversion (T₅₀) was decreased with increasing RuO_x loadings: 1 wt% RuO_x/BNNT (208 °C) < 0.5 wt% RuO_x/BNNT (216 °C) < 0.25 wt% RuO_x/BNNT (240 °C) at GHSV 75 000 mL g_catalyst⁻¹ h. It is worth noting that the bulk BNNT did not show any catalytic activity, indicating that RuO_x species on the BNNT surface are active sites for CO oxidation. This agrees with the previous report by I. Rossetti et al.³⁵ The authors investigated the effect of Ru loading on ammonia synthesis and claimed that increasing the Ru content from 1.9 wt% to 3.8 wt% led to an increase in active sites. The reported results for CO oxidation using supported Ru catalysts are summarized in Table 2.

Fig. 6 CO conversion as a function of reaction temperature. (a) Ru loading effect, (b) treatment condition effect, (c) GHSV effect using 1 wt% RuO_x/BNNT, and (d) CO-to-O₂ ratio effect using 1 wt% RuO_x/BNNT. Reaction conditions: (a and b) GHSV = 75 000 (mL g_catalyst⁻¹ h), CO : O₂ ratio = 1 : 10, (c) CO : O₂ ratio = 1 : 10, (d) GHSV = 75 000 (mL g_catalyst⁻¹ h).

Table 1 The CO conversion results of RuO_x/BNNT catalysts with different Ru loadings, treatment steps, and experimental conditions

Catalysts	CO : O2 gas ratio	GHSV (mL gcat^−1 h^−1)	T 10 (°C)	T 50 (°C)	T 90 (°C)
Loading and treatment condition effects
Bulk BNNT	1 : 10	75 000	—	—	—
0.25 wt% RuOx/BNNT	1 : 10	75 000	224	240	300
0.5 wt% RuOx/BNNT	1 : 10	75 000	200	216	253
1 wt% RuOx/BNNT	1 : 10	75 000	194	208	218
1 wt% Ru/BNNT	1 : 10	75 000	230	246	262

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GHSV effect
1 wt% RuOx/BNNT	1 : 10	37 500	191	200	210
1 wt% RuOx/BNNT	1 : 10	75 000	194	208	218
1 wt% RuOx/BNNT	1 : 10	150 000	194	208	215

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CO : O2 ratio effect
1 wt% RuOx/BNNT	1 : 10	75 000	194	208	218
1 wt% RuOx/BNNT	1 : 5	75 000	192	205	214
1 wt% RuOx/BNNT	1 : 2.5	75 000	209	231	241
1 wt% RuOx/BNNT	1 : 1	75 000	226	244	260

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Table 2 Catalytic performance comparison with previously reported catalysts for CO oxidation

Catalysts	Ru loading (wt%)	Treatment step	CO conc. (vol%)	CO : O2 gas ratio	GHSV (mL gcat^−1 h^−1)	T 10 (°C)	T 50 (°C)	T 90 (°C)	Ref.
Ru/CeO2NR-r	5.0	Reduction	1	1 : 20	36 000	25	50	102	23
Ru/Al2O3	2.0	—	1.564	1 : 2	25 200	120	134	180	36
Ru/C12A7:e^−	2.0	—	1.564	1 : 2	25 200	87	125	134	36
Ru(1.5)CeO2	1.5	Reduction	1	1 : 0.5	Contact time W/FCO = 7.4 gcat h molCO^−1	50	90	140	37
Ru NWs/TiO2	1.2	—	1	1 : 1	12 600	84	126	143	38
fcc-Ru NPs/γ-Al2O3	1.0	Reduction	1	1 : 1	20 000	115	141	160	39
Ru/TiO2	1.0	Oxidation	3500 ppm	—	60 000	78	120	128	40
Ru/CeO2NR-r	1.0	Reduction	1	1 : 20	36 000	47	78	105	41
Ru/SiO2-r	1.0	Reduction	1	1 : 20	36 000	236	297	370	41
RuOx/BNNT	1.0	Oxidation	4	1 : 10	150 000	194	208	215	This work

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The effect of oxidizing and reducing treatment was investigated (Fig. 6(b)). The T₁₀, T₅₀, and T₉₀ of 1 wt% RuO_x/BNNT were 194, 208, and 218 °C and those of 1 wt% Ru/BNNT were 230, 246, and 262 °C, respectively. This result provides that the catalytic property for the CO oxidation is directly related to the oxidation state of Ru. XPS results (Fig. 5) show that 1 wt% RuO_x/BNNT has mainly Ruⁿ⁺ species, while 1 wt% Ru/BNNT has primarily Ru⁴⁺ species. J. Li et al. investigated the effect of metal oxidation state using Ru supported on CeO₂ and demonstrated that the Ruⁿ⁺-rich surface is more favorable for the catalytic CO oxidation than the Ru⁴⁺- or Ru⁶⁺-rich surface.²³ K. Xu et al. studied the Ru active species using DRIFTS and reported that the CO adsorbed on Ruⁿ⁺ species is more rapidly converted to CO₂ gases than that on Ru⁰ species at the same temperature.²² Thus, the predominant Ruⁿ⁺ species in 1 wt% RuO_x/BNNT would improve the catalytic performance as active species.

The effect of the mass transfer limitation on the CO oxidation of 1 wt% RuO_x/BNNT was investigated by changing the GHSV conditions from 37 500 to 150 000 mL g_catalyst⁻¹ h. As shown in Fig. 6(c), the CO catalytic performance was very similar (or slightly changed) with increasing GHSV conditions, indicating that 1 wt% RuO_x/BNNT is not affected by the external mass transfer limitation. This result originates from the peculiar tubular morphology of nanotubes, specifically the absence of micropores. This is further confirmed in Fig. S1† showing the BET and PSD data. In the case of the conventional porous supported catalysts, a low external-to-internal surface area ratio is expected. Consequently, it leads to difficulties in diffusion and accessibility of reactants to active sites. In addition, the products hinder the diffusion of the reactants by escaping to the same path of the blind pores, thereby contributing to the formation of stagnant film.⁴² On the other hand, the nanotube structure contains cavities (or through pores) and a high external-to-internal surface area ratio, facilitating easy diffusion and accessibility of reactants to active sites. Moreover, the products do not impede the diffusion of reactants by escaping through the other exit, thereby resulting in the formation of negligible stagnant film. The SSA from the BET analysis was calculated to be ∼48.50 m² g⁻¹, and the isotherm (Fig. S1(a)†) confirms a meso-macroporous configuration. The PSD and cumulative pore volume data (Fig. S1(b)†) reveal a mesoporous range of ∼10–30 nm with minor N₂ adsorption, while the majority of adsorption occurred on the external surface of the BNNT. These data support the hypothesis of negligible mass transfer limitation over RuO_x/BNNT catalysts during the CO oxidation reaction. CNT-based catalysts have consistently exhibited similar results, although experimental demonstration is still debatable.^43–45

The Eley–Rideal (ER) and the Langmuir–Hinshelwood (LH) mechanisms have been proposed to understand the CO oxidation reaction over the supported metal oxide catalysts.^46–48Fig. 7 shows the proposed ER and RH mechanisms. The ER mechanism occurs in the interaction between a CO gas molecule and an adsorbed O atom on the active site. Initially, O₂ is adsorbed and dissociates into two O atoms (O₂(g) + 2* ↔ 2O*) on the active site. Then, a CO molecule approaches the active site and reacts with the adsorbed O atom to form a CO₂ molecule (CO(g) + O* ↔ CO₂(g) + *). In the case of the LH mechanism, the reaction occurs through the interaction between an adsorbed CO molecule and an adsorbed O atom on the active site. Both CO and O₂ molecules are respectively adsorbed on the active site (O₂(g) + 2* ↔ 2O* and CO(g) + * ↔ CO*). Then an adsorbed CO molecule reacts with a vicinal adsorbed O atom to form a CO₂ molecule (CO* + O* ↔ CO₂(g) + 2*). The reaction mechanism of the CO oxidation over 1 wt% RuO_x/BNNT was investigated by varying the CO-to-O₂ ratio from 1 : 1 to 1 : 10. As shown in Fig. 6(d), the CO conversion was improved with increasing O₂ ratio from 1 : 1 to 1 : 5 ratio: T₅₀ = 205 °C (1 : 5) < 231 °C (1 : 2.5) < 244 °C (1 : 1). Further increasing the O₂/CO ratio, however, did not affect the catalytic activity. C. H. F. Peden et al. studied the CO oxidation mechanism on the Ru(001) surface using DRIFTS with varied O₂/CO ratios.⁴⁹ Since the chemisorbed CO peak was gradually decreased with increasing oxygen pressure, the authors claimed that the Ru(001) surface follows the ER mechanism under oxidizing conditions over the LH mechanism. It was also reported that the CO₂ formation rate increases with the oxygen pressure up to 2.5 torr, beyond which it remains constant regardless of oxygen pressure. C. Huang et al. investigated the CO oxidation of Ru/hBN catalyst using the periodic DFT method.⁵⁰ The authors reported that the O₂ molecules will be primarily adsorbed on the Ru atoms over CO molecules since the adsorption energy of O₂ (−2.43 eV) is lower than that of CO (−1.95 eV). Based on the literature review and obtained activity results, it is concluded that the 1 wt% RuO_x/BNNT catalyst is favorable for the ER mechanism over the LH mechanism (Fig. 7).

Fig. 7 Schematic illustration of the proposed reaction mechanism for CO oxidation on 1 wt% RuO_x/BNNT catalyst.

4. Conclusions

In this study, RuO_x/BNNT catalysts with different Ru loadings and pretreatment steps were synthesized and applied to CO oxidation as a model reaction. TEM results confirmed that the Ru nanoparticles with size 10–20 nm were introduced to both the internal and the external surfaces of the BNNT. The CO conversion results improved with increasing Ru loading and the oxidized catalyst exhibited higher activity than the reduced sample. XPS revealed that Ruⁿ⁺ species shifted to Ru⁴⁺ species after reduction treatment, suggesting that Ruⁿ⁺ plays a critical role in enhancing the catalytic activity. The 1 wt% RuO_x/BNNT catalyst was further investigated by changing the reaction conditions (e.g., GHSV and O₂/CO ratio). The GHSV conditions did not affect the activity, suggesting no external mass transfer limitation on the catalyst. The absence of micropores in the nanotube made a high ratio of external to internal surface area compared to the conventional porous materials, resulting in easy accessibility of reactants to metal active sites without external diffusion limitation. In addition, both open ends of the nanotube can minimize the diffusion competition between the reactants and the products, contributing to the formation of negligible stagnant film. The catalyst will be favorable to the ER mechanism over the LH mechanism, as the catalytic performance was increased with increasing CO-to-O₂ ratio from 1 : 1 to 1 : 5.

Data availability

The data that support the findings of the research are available on request from the corresponding author.

Author contributions

J. Choi: methodology, formal analysis, writing – original draft, writing – review & editing. A. Pophali: formal analysis, writing – review & editing. B. Kim: formal analysis. K. Yoon: formal analysis. H. Song: formal analysis. S. Shim: formal analysis. J. Kim: formal analysis. T. Kim: supervision, writing – review & editing, project administration, funding acquisition. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors would like to thank the Advanced Energy Research and Technology Center (AERTC) for the use of the facilities at Stony Brook University. T. Kim acknowledges funding support from the National Science Foundation (NSF-CBET-2050824). This work is supported by R&D Grants from the Ministry of Trade, Industry and Energy (ATC+ Project Grant No. 20017989) of the Republic of Korea. The authors would also like to thank Prof. Benjamin Hsiao and Dr. Rasel Das for providing BET analyzer.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cy00945b

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