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Article

Adjacent Reaction Sites of Atomic Mn2O3 and Oxygen Vacancies Facilitate CO2 Activation for Enhanced CH4 Production on TiO2-Supported Nickel-Hydroxide Nanoparticles

by
Praveen Kumar Saravanan
1,†,
Dinesh Bhalothia
2,†,
Amisha Beniwal
2,
Cheng-Hung Tsai
1,
Pin-Yu Liu
1,
Tsan-Yao Chen
2,
Hong-Ming Ku
3,* and
Po-Chun Chen
1,*
1
Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei 10608, Taiwan
2
Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan
3
Chemical Engineering Practice School, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2024, 14(7), 410; https://doi.org/10.3390/catal14070410
Submission received: 11 June 2024 / Revised: 23 June 2024 / Accepted: 24 June 2024 / Published: 28 June 2024
(This article belongs to the Special Issue Catalysis on Stable Molecules (CO2, CO, CH4, N2, NH3) Activation and Their Transformation, 2nd Edition)
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Abstract

:
The catalytic conversion of carbon dioxide (CO2) to methane (CH4) through the “Sabatier reaction”, also known as CO2 methanation, presents a promising avenue for establishing a closed carbon loop. However, the competitive reverse water gas shift (RWGS) reaction severely limits CH4 production at lower temperatures; therefore, developing highly efficient and selective catalysts for CO2 methanation is imperative. In this regard, we have developed a novel nanocatalyst comprising atomic scale Mn2O3 species decorated in the defect sites of TiO2-supported Ni-hydroxide nanoparticles with abundant oxygen vacancies (hereafter denoted as NiMn-1). The as-prepared NiMn-1 catalyst initiates the CO2 methanation at a temperature of 523 K and delivers an optimal CH4 production yield of 21,312 mmol g−1 h−1 with a CH4 selectivity as high as ~92% at 573 K, which is 45% higher as compared to its monometallic counterpart Ni-TiO2 (14,741 mmol g−1 h−1). Physical investigations combined with gas chromatography analysis corroborate that the exceptional activity and selectivity of the NiMn-1 catalyst stem from the synergistic cooperation between adjacent active sites on its surface. Specifically, the high density of oxygen vacancies in Ni-hydroxide and adjacent Mn2O3 domains facilitate CO2 activation, while the metallic Ni domains trigger H2 splitting. We envision that the obtained results pave the way for the design of highly active and selective catalysts for CO2 methanation.

1. Introduction

The escalating levels of carbon dioxide (CO2) emissions resulting from anthropogenic activities have underscored the urgent need for effective CO2 utilization methods. To this end, the heterogeneous catalytic transformation of CO2 to methane (CH4), known as CO2 methanation, presents a promising avenue for mitigating CO2 concentrations and advancing the development of green fuels [1,2]. In this process, hydrogen (H2) (generated from water electrolysis using renewable electricity) reacts with CO2 to produce CH4. Unlike H2, CH4 offers advantages in terms of storage, transportation, and application in energy-intensive industries [3]. However, due to its exothermic nature, CO2 methanation is thermodynamically limited at high temperatures [4]. Therefore, the development of highly efficient heterogeneous catalysts capable of achieving high CH4 yields at low temperatures is imperative.
Over the years, extensive research has been conducted to understand the intricate mechanisms and key intermediates involved in CO2 methanation to design the powerful heterogeneous catalysts that can deliver a high CH4 production yield with improved selectivity at low temperatures. Generally, two primary CO2 methanation pathways (i.e., associative and dissociative) have gained widespread acceptance [5]. The associative pathways entail the sequential addition of hydrogen (*H) atoms to adsorbed carbon dioxide (*CO2) molecules, resulting in the formation of CH4. In this pathway, there is a direct involvement of CO2 without its prior dissociation into carbon monoxide (CO) and oxygen (O) species. Based on the adsorption center (C-center or O-center) of CO2 molecules on the catalyst’s surface, the association pathway can further follow two different routes (i.e., formate and carboxyl). In the formate pathway, the *CO2 molecules are generally adsorbed via the O-center to form bidentate formates (HCOO*) as the intermediate products. These formats undergo further hydrogenation to produce CH4 [6]. Another associative pathway is the carboxyl pathway. In this case, the *CO2 molecules are first converted to the carboxyl (*COOH) intermediates and finally produce CH4 via sequential hydrogenation [7]. For the dissociation pathway, instead of direct hydrogenation of *CO2 molecules, they dissociate into *CO and *O intermediates first. Obtained *CO intermediates undergo further hydrogenation to form carbon species (*C) and later CH4, whereas the *O species react with *H atoms to produce water (H2O) [8]. Based on the aforementioned reaction pathways, it can be concluded that the primary factor contributing to the CO2 methanation reaction is the selection of a catalyst. To achieve higher CH4 production with the desired selectivity, the catalyst should possess two adjacent reaction sites for CO2 adsorption/activation and H2 splitting. Thus far, the catalysts comprising metal oxide-supported nanoparticles are at the forefront, where the metal oxide supports are mainly involved in the CO2 activation step while the active metals (nanoparticles) favor the H2 splitting [9]. Among various support materials, titanium dioxide (TiO2) has attracted considerable attention for its ability to improve CO2 methanation processes [10,11]. TiO2 supports offer advantages such as a high oxygen storage capacity, the improved dispersion of metal nanoparticles (NPs), and the promotion of CO2 adsorption [12,13]. Moreover, surface oxygen vacancies (OV) in support materials have been implicated in triggering CO2 activation, further enhancing catalytic performance [14,15]. In terms of active metals, precious metals, especially Ru, and Rh-based catalysts are widely acknowledged for their exceptional performance in CO2 methanation [16,17]. Nevertheless, their high cost and limited scalability pose significant barriers to their widespread industrial adoption. As a result, there is increasing interest in investigating alternative catalysts that offer higher activity levels at lower material costs. In response, nickel (Ni)-based catalysts have emerged as a potential contender to noble metal-based catalysts due to their high CO2 conversion efficiency and relatively cheap price as compared to other transition metals [18,19,20]. Despite potential advantages, the strong adsorption energy of carbonaceous species on the Ni surface causes passivation and subsequent degradation (i.e., coke effect) of Ni-based catalysts during CO2 methanation [21]. Reducing the adsorption energy of carbonaceous species can mitigate this passivation effect on Ni atoms, thereby enhancing the chemical stability of Ni-based catalysts in CO2 methanation. In addition, the high onset temperature, lower CH4 production yields, and lower selectivity towards desired products are key issues limiting the practical and scalable liability of Ni-based catalysts. Various strategies have been explored to address the aforementioned technical bottlenecks in the development of an ideal design configuration for a CO2 methanation catalytic system. These design strategies include modifying particle size or shape (configuration effect) [22], introducing dopants (heteroatomic intermix) [23], employing mixed oxide supports (co-catalysis effect) [24], and inducing lattice strain (formation of core–shell or cluster-in-cluster structures) [25]. However, regardless of the strategy adopted, achieving a uniform surface composition is often inevitable. This limitation restricts the selectivity of intermediates and significantly suppresses the CO2 methanation efficiency. To address these bottlenecks simultaneously, configuring the catalyst surface with heteroatomic domains unevenly distributed in a local region within the diffusion length of intermediate species proves to be an effective strategy.
In this study, we have developed a novel design configuration of a highly selective and efficient heterogeneous catalyst for CO2 methanation, which consists of atomic-scale Mn2O3 species decorated at the defect sites of TiO2-supported Ni-hydroxide nanoparticles enriched with abundant oxygen vacancies (hereafter denoted as NiMn-1). This NiMn-1 catalyst exhibits remarkable performance in CO2 methanation, initiating the reaction at 523 K and achieving an optimal CH4 production yield of 21,312 mmol g−1 h−1 with a CH4 selectivity of ~92% at 573 K. This represents a 45% increase compared to its monometallic counterpart, Ni-TiO2 (14,741 mmol g−1 h−1). Through comprehensive physical investigations combined with gas chromatography analysis, we have confirmed that the outstanding activity and selectivity of the NiMn-1 catalyst arise from the synergistic cooperation between adjacent active sites on its surface. Specifically, the high density of oxygen vacancies in the Ni-hydroxide and adjacent Mn2O3 domains facilitate CO2 activation, while the metallic Ni domains trigger H2 splitting. The results are anticipated to offer valuable insights for developing improved catalysts for the Sabatier reaction.

2. Results and Discussion

2.1. Physical Structure Inspections

High-resolution transmission electron microscopy (HRTEM) was employed to investigate the surface morphology and crystal structure of the as-synthesized Ni-TiO2 and NiMn catalysts and control samples. For clarification, the HRTEM image of bare TiO2 (i.e., P25) is first discussed. As shown in Figure S1a, the TiO2 NPs exhibit a rocky surface with a high density of terraced (defect) regions and a d-spacing value of 0.342 nm. It is frequently reported in the literature that these terraced regions serve as nucleation centers for the subsequent crystal growth of Ni (hydroxide) NPs on the TiO2 surface [26,27]. Furthermore, as shown in Figure 1a, the Ni(OH)x NPs are grown spherically with a typical diameter range of 3–4 nm. On top of that, it is interesting to observe that the d-spacing value of TiO2 is increased from 0.342 nm to 0.348 as compared to bare TiO2 after the growth of Ni(OH)x NPs on the surface of TiO2. Considering the bigger ionic radius of Ni2+ (0.69 Å) as compared to Ti4+ (0.605 Å), such an increased d-spacing value of Ni-TiO2 can be attributed to the substitution of Ti4+ sites via Ni2+ in the TiO2 lattice and consistently confirmed by the Rietveld refinement in the subsequent section [28]. Conversely, the Mn-TiO2 (Figure S1b) shows a significantly suppressed d-spacing value of 0.312 nm, which is obvious due to the substitution of Ti4+ sites via Mn3+ with a smaller ionic radius of 0.58 Å as compared to Ti4+ [29]. More interestingly, the NiMn-1 catalyst shows a similar d-spacing value to that of Ni-TiO2 (Figure 1b). Considering the significant substitution of Ti4+ sites via Mn3+ in the Mn-TiO2, this similar d-spacing value, along with nearly the same configuration (shape and size), confirms that Mn2O3 species are accommodated in the defect sites of Ni(OH)x NPs (denoted by the orange square in the zoom-in region of Figure 1b) instead of the TiO2 surface in the NiMn-1 catalyst. Moreover, the d-spacing value of 0.244 nm (corresponding to the (10–14) plane of Ni(OH)x (materials project id: mp-626794)) of Ni domains (yellow region in Figure 1b) indicates that Ni is present in the form of Ni(OH)x. Furthermore, increasing the Mn content to 3 wt.% leads to the formation of sub-nanometer clusters of Mn2O3 on the surface of Ni(OH)x. As shown in Figure 1c, the Mn2O3 clusters exhibiting lower Z contrast in the HRTEM image are found on the TiO2-supported Ni(OH)x surface, and they could be determined as Mn due to its lower Z number (Z = 25) in the bimetallic system. In addition, the increased d-spacing value of 0.377 as compared to bare TiO2 and Ni-TiO2 can be attributed to the severe surface oxidation and higher extent of Ti4+ substitution via Ni2+.
Furthermore, the X-ray diffraction (XRD) patterns were analyzed to obtain more insights into the crystal structure. Figure 2a shows the XRD pattern of the NiMn catalyst and control samples. Accordingly, the TiO2 support comprises both anatase and rutile phases, with the anatase phase the dominating phase [30]. Notably, the unchanged peak profiles (position and intensity) of TiO2 for all the catalysts under investigation suggest that Ni and Mn NPs are grown on the surface of TiO2 without disturbing the inner crystal structure. In addition, the presence of a suppressed peak centered at 44.48° (corresponding to the Ni(OH)x (1 0 -17) facet (mp-626794)) confirms that Ni is present in the form of Ni(OH)x in Ni-TiO2 and NiMn catalysts. Furthermore, the absence of diffraction peaks corresponding to the Mn indicates that Mn2O3 species are present in the amorphous phase. In addition, Figure S2 shows the XRD pattern of the NiMn catalyst and control samples after the reaction. Further, the Rietveld refinement (Figure 2b–e) was employed to obtain the cell volumes to cross-reference the former HRTEM result, and the obtained results are summarized in Table S1. For instance, the substitution of Ti4+ via Mn3+ in the anatase phase of TiO2 is confirmed by the observed decrease in the unit cell volume (V) and lattice parameters (a and c) in the Mn-TiO2 (due to the small ionic radius of Mn3+ (0.58 Å) as compared to Ti4+ (0.605 Å)) as compared to Ni-TiO2. On the other hand, due to the larger ionic radius of Ni2+ (0.69 Å), the higher unit cell volume (V) of Ni-TiO2 is observed. Although these results differ from the unchanged peak profiles of TiO2 observed in various samples, this discrepancy can be attributed to the limitation of such substitution affecting only the surface regions rather than the inner lattices.
To gain deeper insights into the local atomic and electronic structure of the Ni and Mn atoms in the NiMn catalyst, X-ray absorption spectroscopy was conducted at the Ni and Mn K-edges. In Figure 3a, the X-ray absorption near-edge spectroscopy (XANES) spectra of NiMn catalysts are compared with standard samples (Ni foil and Ni(OH)x and the control sample (Ni-TiO2)). Notably, the similarity of the XANES profile of Ni-TiO2 and NiMn catalysts to that of Ni(OH)x confirms that Ni is present in the form of Ni(OH)x in these samples [31]. Consistent with the XANES peak profiles, the similar peak position (peak O) in the first derivative curve (Figure 3b) indicates the similarity of the oxidation state of Ni-TiO2 and NiMn catalysts to that of Ni(OH)x. In addition, the NiMn-3 exhibits the highest whiteline intensity (HA), suggesting the highest amount of surface chemisorbed oxygen on its surface, which can be attributed to the presence of sub-nanometer Mn-oxide clusters on its surface [32]. Furthermore, Figure 3c presents the Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) spectra of these samples at the Ni K-edge, with corresponding structural parameters summarized in Table 1, and the overlay fitting curves are provided in Figure S3. As shown in Figure 3c, the peaks B and C correspond to the Ni-O bond pairs in the first and second coordination shells, while the peak D refers to the Ni-Ni bond pair. As listed in Table 1, with the same amount of Ni-loading and the presence of additional Mn-oxide, the NiMn catalysts exhibit smaller coordination numbers (CN)s for Ni-O bond pairs along with the suppressed peak B in the first coordination shell as compared to Ni-TiO2. Such a scenario suggests the presence of abundant oxygen vacancies in the NiMn catalysts [33,34]. Moreover, the similar EXAFS peak profiles and wavelet transform (WT) patterns (Figure 3d) of Ni-TiO2 and NiMn catalysts as that of Ni(OH)x again confirm the Ni(OH)x phase of Ni in these samples.
Figure 3e depicts the XANES spectra of NiMn catalysts and standard samples (Mn-foil, Mn2O3, and MnO2) at the Mn K-edge. The position of the inflection point (IP) refers to the oxidation state of target atoms [35]. Accordingly, the NiMn catalysts exhibit a similar position of inflection points and peak profiles in the first derivative curve (Figure 3f) to that of Mn2O3, confirming that Mn is present in the form of Mn2O3 in NiMn catalysts. Figure 3g presents the FT-EXAFS spectra of the NiMn catalysts and standard samples at the Mn K-edge. Correspondingly, fitting curves are depicted in Figure S4, and the model-simulated structural parameters are summarized in Table 1. Consistent with the inflection point’s position in the XANES spectra, the nearly similar peak position and WT profiles (Figure 3h) of the NiMn catalysts and Mn2O3 in the EXAFS spectra again confirm the Mn2O3 state of Mn in the NiMn catalyst.
X-ray photoelectron spectroscopy (XPS) (Figure 4) has been utilized to elucidate the oxidation state and binding energies of the constituting elements in the NiMn catalyst. Figure 4a and Figure 4d, respectively, compare the XPS spectra of NiMn-1 and NiMn-3 catalysts at Ni-2p orbitals. In a Ni-2p spectrum, the doublet peaks at 852.20 eV and 871 eV, respectively, corresponding to the photoelectron emission from Ni-2p3/2 and Ni-2p1/2 orbitals, while the satellite peaks at 860 eV and 878 eV are contributions from the oxide species. These emission peaks from Ni-2p3/2 and Ni-2p1/2 orbitals are further deconvoluted to reveal the different oxidation states of Ni atoms in NiMn catalysts, and the obtained results are summarized in Table S2. The obtained results confirm that Ni atoms are present in the form of Ni(OH)x in these samples. More importantly, compared to the Ni-TiO2 (Figure S5 and Table S3), the higher binding energies of Ni2+ and Ni(OH)x confirm the severe electron relocation from Ni domains. Considering the lower electronegativity of Mn as compared to Ni (i.e., ENi > EMn), such electron transfer can be attributed to the oxygen vacancies, as they have a higher EN as compared to Ni [36]. Figure 4b and Figure 4e, respectively, compare the XPS spectra of NiMn-1 and NiMn-3 catalysts at Mn-2p orbitals, where the deconvolution results suggest that Mn3+ and Mn3+ are dominating oxidation states. More importantly, compared to NiMn-3, the NiMn-1 catalyst exhibits significantly lower binding energy. Such a scenario can be attributed to the severe electron relocation from Mn to Ni domains in the defect sites. Finally, the XPS spectra at O-1s orbitals (Figure 4c,f and Figure S5) confirm the presence of oxygen vacancies in NiMn and Ni-TiO2 samples [37].

2.2. CO2 Conversion Performance

The CO2 conversion performances of the NiMn catalysts, as well as the control samples (Ni-TiO2 and Mn-TiO2), were assessed over a temperature range from room temperature (RT) to 573 K. This evaluation was conducted using a flow reactor system operating under atmospheric pressure, with a continuous flow of H2/CO2 (3/1) atmosphere. Figure 5a,b show the CO and CH4 production yields for the NiMn catalysts and control samples. Accordingly, compared to CH4, the significantly higher CO production yield for Mn-TiO2 indicates that the reverse water gas shift (RWGS) reaction dominates over CO2 methanation at the 573 K temperature. Such a scenario can be attributed to the absence of active sites for H2 splitting and suggests that Mn-oxide reaction sites promote CO2 activation. It is frequently reported in the literature that the metal oxides are favorable reaction sites for CO2 activation. However, they are inert for H2 splitting [38]. On the other hand, Ni-TiO2 exhibits a higher CO production yield up to a temperature of 523 K, confirming the domination of the RWGS reaction over CO2 methanation. However, raising the temperature from 523 K to 573 K leads to a dramatically enhanced CH4 production yield of 14,741 mmol g−1 h−1 with a CH4 selectivity of 83.6% (Figure 5c), which is an 8-fold improvement compared to the CH4 production yield at a 523 K temperature (1850 mmol g−1 h−1). These results together confirm that, although the CO2 activation reaction sites (i.e., oxygen vacancies) exist at the lower temperature range, the active sites favoring H2 splitting are however generated at high temperatures. Previously published studies reported that oxygen vacancies actively promote CO2 dissociation, while the Ni-hydro (oxide) NPs are generally reduced into metallic Ni at a temperature of 573 K in the presence of H2. Therefore, it can be concluded that the oxygen vacancies in the Ni(OH)x and adjacent metallic Ni domains synergistically facilitate the CO2 dissociation and H2 splitting, respectively, during CO2 methanation on the surface of Ni-TiO2. Furthermore, the NiMn-1 catalyst exhibits a 45% enhancement in CH4 production with a yield of 21,312 mmol g−1 h−1 with a CH4 selectivity of 91.9% (Figure 5d) as compared to Ni-TiO2, which is obvious due to the presence of additional reaction sites (i.e., Mn2O3) for CO2 activation. At any rate, the suppression in the CH4 production yield and the CH4 selectivity (Figure 5e) for NiMn-3 catalysts can be attributed to the surface coverage of oxygen vacancy sites via sub-nanometer Mn2O3 clusters and surface oxide formation. For comparison, the CH4 production yield of NiMn-1 NC has been compared with the previous literature, as shown in Table S4.

3. Experimental Section

3.1. Preparation of Bimetallic Catalysts

As shown in Scheme 1, the synthesis of TiO2 (P25; 80% anatase and 20% rutile, Degussa)-supported NiMn binary catalysts involved a series of steps, including chemisorption, galvanic replacement, and the wet chemical reduction of metal ions with precise control over reaction sequence and time. Initially, 2 g of TiO2 (3 wt.% solution in D.I. water, equivalent to 60 mg of TiO2) was dispersed in 0.51 g of an aqueous solution containing 0.1 M of nickel (II) chloride hexahydrate (NiCl2·6H2O, Showa Chemical Co., Ltd., Minato-ku, Tokyo, Japan) and stirred at 400 rpm for 4 h (first step). This resulted in adsorption of Ni2+ ions on the TiO2 surface (Ni2+_ads-TiO2) containing 0.51 mmoles (3 mg) of Ni2+ ions with a weight ratio of Ni/TiO2 = 5 wt.%. Subsequently (2nd step), 5 mL of a water solution containing 0.03 g of sodium borohydride (NaBH4; 99%, Sigma-Aldrich Co., St. Louis, MO, USA) was added to the Ni2+_ads-TiO2 solution and stirred at 400 rpm for 10 s. This led to the formation of metastable Ni metal nanoparticles (Ni-TiO2), which later transformed into Ni(OH)x due to the presence of NaBH4. In the 3rd step, 0.11 g (1 wt.% of the TiO2) of Mn precursor solution containing 0.011 mmoles of Mn metal ions (0.1 M of Manganese (II) chloride tetrahydrate (MnCl2·4H2O), Showa Chemical Co., Ltd.) was added into the Ni(OH)x-TiO2 solution. Here, Mn2+ ions were reduced by the excess NaBH4 added in the 2nd step. Herein it is worth noticing that, with a quick reaction time (10 s), ultra-low metal loading of Mn, and the absence of a galvanic replacement reaction (because the electronegativity of Ni is higher than Mn), the Mn atoms are mainly accommodated in the defect sites (formed due to rapid nucleation in the presence of strong reducing agent) of Ni(OH)x. The resulting products were washed with acetone, IPA, and D.I. water, centrifuged, and dried at 70 °C. In the remainder of this article, the NiMn catalyst with 1 wt.% of Mn is denoted as NiMn-1. Similarly, the NiMn catalyst with 3 wt.% Mn is also prepared for comparison and denoted as NiMn-3. The control samples (Ni-TiO2 and Mn-TiO2) were synthesized similarly by immersing Ni (Mn) precursor solution in TiO2 followed by adding NaBH4.

3.2. Physical Characterizations

To discern the disparities in physical attributes between the control sample and the NiMn catalysts, different Mn-loading, microscopy, and X-ray spectroscopy outcomes were employed for analysis. High-resolution transmission electron microscopy (HRTEM) images were captured at the electron microscopy center of the National Taipei University of Technology in Taiwan. This allowed for the visualization of the crystal structure and surface morphology of the materials in their as-prepared states. X-ray diffraction (XRD) spectra were analyzed using a D8 instrument (operating at 40 kV and 40 mA with Cu-Kα radiation) at the National Taipei University of Technology (NTUT), Taiwan. This facilitated the examination of the crystallographic characteristics. For the determination of the oxidation states and surface compositions of the nanoparticles, X-ray photoelectron spectroscopy (XPS) was employed at beamline BL-24A of the Taiwan Light Source (TLS) located at the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The quantification of surface compositions was achieved by integrating each peak. After eliminating the initial peak with a Shirley-type background, the experimental curve was fitted using a combination of Lorentzian and Gaussian lines. X-ray absorption spectroscopy (XAS) of exploratory samples was conducted at beamline BL-17C1 of the NSRRC, Taiwan. The obtained spectra were normalized using ATHENA 3 software. In the EXAFS (extended X-ray absorption fine structure) analysis, the pre-edge and post-edge backgrounds from the XAS spectra were excluded. The normalized spectra were transformed from energy to k-space and subsequently weighted by k3 to facilitate the differentiation of backscattering contributions from distinct coordination shells.

3.3. CO2 Conversion Performance Analysis

The catalytic performances of NiMn catalysts and control samples were evaluated with a previously reported protocol [39]. More detailed discussion has been given in Supplementary S1.

4. Conclusions

In this study, we have developed a novel nanocatalyst denoted as NiMn-1, comprising atomic-scale Mn2O3 species decorated in the defect sites of TiO2-supported Ni-hydroxide nanoparticles, with abundant oxygen vacancies. The as-prepared material demonstrated remarkable performance, initiating CO2 methanation at a temperature of 523 K and yielding an optimal CH4 production of 21,312 mmol g−1 h−1 with a CH4 selectivity of ~92% at 573 K. This represents a 45% enhancement compared to its monometallic counterpart, Ni-TiO2. Physical investigations, coupled with gas chromatography analysis, confirm that the exceptional activity and selectivity of the NiMn-1 catalyst arise from the synergistic cooperation between adjacent active sites on its surface. Specifically, the high density of oxygen vacancies in the Ni-hydroxide and adjacent Mn2O3 domains facilitate CO2 activation, while the metallic Ni domains trigger H2 splitting. These findings lay the groundwork for the design of highly active and selective catalysts tailored for CO2 methanation, offering promising prospects for advancing sustainable energy conversion technologies.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal14070410/s1, 1. Investigation of Gaseous Products; Figure S1: HRTEM images of control samples ((a) P25 and (b) Mn-TiO2); Table S1: refined lattice parameters and figures of merit from the Rietveld refinement of Mn-TiO2, Ni-TiO2, NiMn-1, and NiMn-3 NCs using XRD data; Figure S2: X-ray diffraction pattern of the NiMn catalysts and control samples (Mn-TiO2, Ni-TiO2) after the reaction; Figure S3: model analysis fitting curves compared with experimental FT-EXAFS spectra at the Ni K-edge of (a) Ni-TiO2, (b) NiMn-1, and (c) NiMn-3 catalysts; Figure S4: model analysis fitting curves compared with experimental FT-EXAFS spectra at the Mn K-edge of (a) Mn2O3, (b) NiMn-1, and (c) NiMn-3 catalysts; Figure S5: X-ray photoelectron spectroscopy of experimental NC of Ni 2p and O1s orbitals of Ni TiO2 NC; Table S2: XPS determined the elemental chemical states of experimental samples; Table S3: XPS determined the binding energies of experimental samples; Table S4: the catalytic performance of various catalysts for CO2 methanation [23,30,31,32,33,34,35,36,37,38,39,40,41].

Author Contributions

Conceptualization, P.-C.C. and T.-Y.C.; methodology, P.K.S.; software, P.K.S.; validation, D.B., P.K.S. and P.-C.C.; formal analysis, P.K.S. and H.-M.K.; investigation, P.-C.C. and T.-Y.C.; resources, P.-C.C.; data curation, P.K.S., D.B., A.B., C.-H.T. and P.-Y.L.; writing—original draft preparation, P.K.S.; writing—review and editing, D.B. and P.K.S.; visualization, D.B.; supervision, P.-C.C.; project administration, P.-C.C.; funding acquisition, P.-C.C. and H.-M.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science and Technology Council, Taiwan (NSTC 109-2923-E-007-005-, NSTC 109-3116-F-007-001-, NSTC 109-2112-M-007-030-MY3 and NSTC 110-2221-E-027-022-MY3).

Data Availability Statement

The data presented in this study are available on request from the corresponding authors.

Acknowledgments

The authors express their gratitude to the staff of the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan, for help in various synchrotron-based measurements. P.-C. Chen acknowledges the funding support from the National Science and Technology Council, Taiwan, (MOST 110-2221-E-027-022-MY3) and the National Taipei University of Technology—King Mongkut’s University of Technology Thonburi Joint Research Program (NTUT-KMUTT-110-01). In addition, the authors acknowledge the Precision Research and Analysis Centre at NTUT.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 14 00410 g001
Figure 1. The HRTEM images of (a) Ni−TiO2, (b) NiMn−1, and (c) NiMn−3 catalysts. The Forward Fourier Transformation (FFT) patterns of the selected area in the HRTEM area are shown in the bottom left insets. The d-spacing values are calculated by using Inverse Fourier-transformed (IFT) images and their corresponding line histograms (insets).
Figure 1. The HRTEM images of (a) Ni−TiO2, (b) NiMn−1, and (c) NiMn−3 catalysts. The Forward Fourier Transformation (FFT) patterns of the selected area in the HRTEM area are shown in the bottom left insets. The d-spacing values are calculated by using Inverse Fourier-transformed (IFT) images and their corresponding line histograms (insets).
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Figure 2. (a) X-ray diffraction pattern of the NiMn catalysts and control samples (Mn−TiO2, Ni−TiO2). The Rietveld refinement fitting profiles utilizing XRD data of (b) Mn−TiO2, (c) Ni−TiO2, (d) NiMn−1, and (e) NiMn-3 catalysts. Black spheres represent the experimental data; the solid red lines correspond to the fitted data; the solid blue line illustrates the disparity between the observed and fitted data.
Figure 2. (a) X-ray diffraction pattern of the NiMn catalysts and control samples (Mn−TiO2, Ni−TiO2). The Rietveld refinement fitting profiles utilizing XRD data of (b) Mn−TiO2, (c) Ni−TiO2, (d) NiMn−1, and (e) NiMn-3 catalysts. Black spheres represent the experimental data; the solid red lines correspond to the fitted data; the solid blue line illustrates the disparity between the observed and fitted data.
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Figure 3. X-ray absorption spectroscopy of NiMn catalysts compared with reference samples. (a) XANES, (b) 1st derivative, (c) FT−EXAFS spectra, and (d) WT patterns of NiMn catalysts and reference samples at Ni K−edge. (e) XANES, (f) 1st derivative, (g) FT−EXAFS spectra, and (h) WT patterns of NiMn catalysts and reference samples at Mn K−edge.
Figure 3. X-ray absorption spectroscopy of NiMn catalysts compared with reference samples. (a) XANES, (b) 1st derivative, (c) FT−EXAFS spectra, and (d) WT patterns of NiMn catalysts and reference samples at Ni K−edge. (e) XANES, (f) 1st derivative, (g) FT−EXAFS spectra, and (h) WT patterns of NiMn catalysts and reference samples at Mn K−edge.
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Figure 4. Comparative X-ray photoelectron spectroscopy of NiMn-1 and NiMn-3 catalyst at (a–d) Ni-2p, (b–e) Mn-2p, and (c–f) O-1s orbitals.
Figure 4. Comparative X-ray photoelectron spectroscopy of NiMn-1 and NiMn-3 catalyst at (a–d) Ni-2p, (b–e) Mn-2p, and (c–f) O-1s orbitals.
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Figure 5. Gas chromatography (GC)−determined (a) CO and (b) CH4 production yields of NiMn−1, and NiMn−3 catalysts compared with control samples (Ni−TiO2 and Mn−TiO2). The CO and CH4 selectivities of (c) Ni−TiO2, (d) NiMn−1, and (e) NiMn−3 at temperature of 573 K.
Figure 5. Gas chromatography (GC)−determined (a) CO and (b) CH4 production yields of NiMn−1, and NiMn−3 catalysts compared with control samples (Ni−TiO2 and Mn−TiO2). The CO and CH4 selectivities of (c) Ni−TiO2, (d) NiMn−1, and (e) NiMn−3 at temperature of 573 K.
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Scheme 1. The schematic representation of sample growth for the NiMn catalysts.
Scheme 1. The schematic representation of sample growth for the NiMn catalysts.
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Table 1. Quantitative results of X-ray absorption spectroscopy model analysis at Ni and Mn K-edges of experimental samples.
Table 1. Quantitative results of X-ray absorption spectroscopy model analysis at Ni and Mn K-edges of experimental samples.
SampleNi K-EdgeMn K-Edge
Bond PairCNRR-FactorBond PairCNRR-Factor
Ni-TiO2Ni-O5.692.041
Ni-Ni3.713.1180.00233N/AN/AN/AN/A
Ni-MnN/AN/A
NiMn-1Ni-O5.282.038 Mn-O4.201.951
Ni-Ni2.643.1140.00905Mn-Mn5.943.1840.0201
Ni-Mn1.203.089 Mn-Ni3.983.410
NiMn-3Ni-O5.642.040 Mn-O3.801.921
Ni-Ni4.753.0880.00344Mn-Mn5.993.2050.0198
Ni-Mn1.272.948 Mn-Ni3.233.425
Note: The sigma^2 for Ni K-edge is 0.0057, and the sigma^2 for Mn K-edge is 0.0090. CN (Coordination number), R (Radial distance).
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Saravanan, P.K.; Bhalothia, D.; Beniwal, A.; Tsai, C.-H.; Liu, P.-Y.; Chen, T.-Y.; Ku, H.-M.; Chen, P.-C. Adjacent Reaction Sites of Atomic Mn2O3 and Oxygen Vacancies Facilitate CO2 Activation for Enhanced CH4 Production on TiO2-Supported Nickel-Hydroxide Nanoparticles. Catalysts 2024, 14, 410. https://doi.org/10.3390/catal14070410

AMA Style

Saravanan PK, Bhalothia D, Beniwal A, Tsai C-H, Liu P-Y, Chen T-Y, Ku H-M, Chen P-C. Adjacent Reaction Sites of Atomic Mn2O3 and Oxygen Vacancies Facilitate CO2 Activation for Enhanced CH4 Production on TiO2-Supported Nickel-Hydroxide Nanoparticles. Catalysts. 2024; 14(7):410. https://doi.org/10.3390/catal14070410

Chicago/Turabian Style

Saravanan, Praveen Kumar, Dinesh Bhalothia, Amisha Beniwal, Cheng-Hung Tsai, Pin-Yu Liu, Tsan-Yao Chen, Hong-Ming Ku, and Po-Chun Chen. 2024. "Adjacent Reaction Sites of Atomic Mn2O3 and Oxygen Vacancies Facilitate CO2 Activation for Enhanced CH4 Production on TiO2-Supported Nickel-Hydroxide Nanoparticles" Catalysts 14, no. 7: 410. https://doi.org/10.3390/catal14070410

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