Pd-Decorated 2D MXene (2D Ti₃C₂Ti_x) as a High-Performance Electrocatalyst for Reduction of Carbon Dioxide into Fuels toward Climate Change Mitigation

Abstract

Palladium nanoparticles (Pd NPs) have attracted considerable attention recently for their excellent catalytic properties in various catalysis reactions. However, Pd NPs have some drawbacks, including their high cost, susceptibility to deactivation, and the possibility of poisoning by intermediate products. Herein, Pd nanoparticles with an average diameter of 6.5 nm were successfully incorporated on electronically transparent 2D MXene (Ti₃C₂Ti_x) nanosheets (Pd-MXene) by microwave irradiation. Considering the synergetic effects of ultra-fine Pd NPs, together with the intrinsic properties of 2D MXene, the obtained Pd-MXene showed a specific surface area of 97.5 m²g⁻¹ and multiple pore channels that enabled excellent electrocatalytic activity for the reduction of CO₂. Further, the 2D Pd-MXene hybrid nanocatalyst enables selective electroreduction of CO₂ into selective production of CH₃OH in ambient conditions by multiple electron transfer. A detailed explanation of the CO₂RR mechanism is presented, and the faradic efficiency (FE) of CH₃OH is tuned by varying the cell potential. Recyclability studies were conducted to demonstrate the practical application of CO₂ reduction into selective production of CH₃OH. In this study, metal and MXene interfaces were created to achieve a highly selective electroreduction of CO₂ into fuels and other value-added chemical products.

1. Introduction

Catalysis plays a dominant role in environmental protection, energy storage and conversion, and biofuel production. High-performance catalysis requires the development of advanced and optimal catalysts [1]. A great deal of attention has been paid to them, especially in metal catalysis. Especially, noble metal nanoparticles (NPs) have gained attention due to their unique size, shape, and stability, making them useful in several research fields, including catalysis and energy and the environment [2,3]. Studies have shown that noble metals, such as Pd-based nanoparticles, possess excellent catalytic properties [4]. Palladium-based metallic nanoparticles have been used in electrochemical oxidations and reductions, bio/chemical sensors, hydrogen storage, hydrogenation of bio-oil compounds, carbon–carbon coupling reactions, etc. [2,5,6]. The size and distribution of Pd NPs determine their catalytic activity. Smaller Pd NPs can aggregate with each other due to their excessive surface free energy, resulting in an unfavorable agglomeration which causes a noticeable diminution in the catalytic activity. To solve this issue, a significant effort has been invested in Pd NP incorporation on various two dimensional materials (2D), such as graphene, metal oxide nanosheets, polymer supports, and metal carbides [2]. Undoubtedly, the Pd-decorated graphene shows excellent catalytic activity with high stability in various applications including electrochemical redox reactions, chemical reforming, and fuel conversion from flue gases [7,8,9]. Additionally, graphene provides a solid substrate for Pd NPs, which prevents the agglomeration of the Pd NPs, and thus maintains the catalyst active sites for various catalysis applications [8]. The graphene-supported Pd nanohybrid catalyst is suitable for converting flue gases, such as CO₂, into fuels and value-added chemicals [10]. In recent decades, CO₂ reduction into liquid fuels has been the most actively studied research area using various Pd metal based nanostructured catalysts.

In recent years, nH*-Pd10-graphene-based electrodes have shown excellent CO₂ reduction reaction (CO₂RR) into formate intermediate (HCOO*), which is crucial in the production of formic acid and alcohols [10]. In addition, Pd/three-dimensional graphene (Pd/3D-RGO) has demonstrated exceptional CO₂RR to produce a formate. Palladium NPs combined with 3D graphene form a special structure that converts CO₂ into formate [6]. Furthermore, a single-atom palladium (Pd) catalyst on nitrogen-doped carbon catalyzes CO₂ into CO twice as fast as its counterpart based on nanoparticles of Pd [11]. To optimize CO₂RR performance, the stoichiometry between Pd and graphene or carbon can be modulated to optimize synergistic and cooperative interactions between the two materials. In contrast, the Pd-Graphene system is mainly used to convert CO₂ into carbon monoxide (CO), and formate intermediates rather than producing hydrocarbons and alcohols that can be used for high-value-added products. In order to increase CO₂ adsorption as well as selectively produce alcohols and hydrocarbons from CO₂ reduction, a highly desirable new class of 2D materials is required. Based on the recent reports, two-dimensional (2D) transition metal carbides/nitrides (MXenes) are highly utilized for their CO₂ adsorption and their conversion into fuels and useful chemicals [12,13]. The metal carbides (M₂C) with M-C-M stoichiometry (M indicates transition metals) form two-dimensional layered structures in carbon atoms sandwiched between the hexagonal metal atoms [14]. The high density of active metal atoms at the surface makes them more efficient electrocatalysts than metal oxides and pure metals. Moreover, MXene (M₃C₂Tx) converts CO₂ into CO, CH₃OH, CH₂O, and HCOOH through bicarbonate species [15]. In order to attain the appropriate current density and reaction rate, these catalysts are required to get high potentials with an excessive energy, limiting their utilization as chemical or fuel catalysts.

Based on this investigation, two main problems have been identified with the use of Pd-based electrocatalysts and MXene in CO₂RR: (i) Pd and its nanocomposites mainly contribute to the reduction of CO₂ into CO, and format intermediates, and (2) MXene converts CO₂ into alcohols, but has lower current densities and reaction rates, resulting in less CO₂RR performance. Considering the above demerits, hybrid nanostructures of Pd-MXene may hold great promise for converting CO₂ into alcohols and fuels. The combination of the Pd NPs with MXene nanohybrids may possess higher CO₂ affinity with high current densities and reaction rates while converting CO₂ into fuels and alcohols [16,17]. Especially, Pd based catalysts possessed mainly conversion of CO₂ into CO intermediates which further protonates into the formation of CH₃OH, CH₃CH₂OH, etc., over MXene surfaces. The preparation of metallic Pd nanoparticles on MXene surfaces is very challenging as the Pd particles must be small in size with homogeneous distributions toward reduction of CO₂ into selective production of alcohols and fuels [12].

In order to overcome these challenges, smaller-sized Pd NPs were successfully incorporated onto MXene surfaces, which were further used as excellent electrocatalysts for CO₂ conversion into CH₃OH under ambient conditions. This study involved exfoliating multilayered MXene in a DMSO-aqueous solution under sonication into a few layers, then fabricating Pd-decorated MXene (Pd-MXene) using microwave irradiation. Various analytical techniques confirmed the successful incorporation of Pd nanoparticles into MXene. Further, different operating conditions were optimized in order to reduce CO₂ into CH₃OH. A detailed overview of the CO₂RR mechanism and the tuning of the faradic efficiency of CH₃OH is presented. The role of Pd and MXene in the hybrid catalyst is discussed in detail. The recyclability of Pd-MXene has been demonstrated for practical CO₂ reduction to CH₃OH.

2. Results

2.1. Physicochemical Properties of the Electrocatalysts

An X-ray diffraction analysis was performed to study the crystal structure of the as-prepared Pd nanoparticles, 2D MXene nanosheets, and Pd-MXene nanocomposites, as demonstrated in Figure 1a–c. The XRD pattern of Pd NPs in Figure 1a shows the three strong, broadened, and high intense diffraction peaks of (111), (200), (220) attributed at 2θ = 40.3, 46.7, 68.2°, respectively, associated with a face-centered cubic (fcc) Pd phase (JCPDS no. 05-0681) [18]. The Pd NPs had a crystallite size of approximately 7.5 nm as calculated by the Scherrer formula, indicating the size and crystallinity of the Pd particles were controlled by the microwave process and NaBH₄ used as the reducing agent. Furthermore, Figure 1b shows the XRD pattern of the pristine 2D MXene powder. The well-defined peaks obtained at 8.9°, 18.2°, 27.7°, and 61.5° can be allocated to the diffractions of (002), (004), (006), and (110) planes of MXene [19,20]. The XRD pattern of Pd-MXene catalysts shown in Figure 1c indicates that Pd-MXene nanocomposites are dual crystalline in nature, with Pd and functionalized MXene nanosheets as their primary phases. In the diffraction pattern, the broadened and high intense diffraction peaks at 40.2°, 46.5°, and 68.1° that Pd exhibits are associated with the (111), (200), (220) crystal planes, respectively, which correspond to fcc Pd (JCPDS no. 05-0681), while the well-defined peaks of (002), (008), (103), (104), (107), and (110) at 2θ = 7.5°, 27.1°, 34.1°, 36.2°, 45.2° and 61.4°, respectively, can be assigned to the diffraction of the functionalized hexagonal 2D MXene nanosheets [21]. A non-appearance of the (006) plane and the appearance of (103), (104), and (107) indicate that MXene nanosheets were functionalized and exfoliated with DMSO. MXene nanosheets exhibited a shift in peak positions due to functionalization and composite effects. The DMSO forms hydroxyl- and oxygen- functional groups on the surfaces of the MXene (Ti₃C₂Ti_x) nanosheets. A plane of (111) of Pd in the nanocomposites was used to estimate the crystallite size of the Pd NPs, which was estimated to be 5.8 nm by the Scherrer formula, which indicates that MXene nanosheets reduce the size of the Pd NPs. Particularly, MXene nanosheets reduce the aggregation of Pd metallic nuclei, leading to smaller particles and more uniform distributions.

To study the morphology and structural characteristics of MXene powder, exfoliated MXene nanosheets, as-prepared Pd NPs, and Pd-MXene nanocomposites, SEM, TEM, and SAED analysis were used. Figure 2a,b shows SEM micrographs of multilayered MXene that reveal the nanosheets of MXene are stacked together with 10 to 15 nanosheets. DMSO with an aqueous solution was used to exfoliate multilayered MXene nanosheets into a few layered MXene sheets using a sonication method, and their TEM image is presented in Figure 2c. The TEM image shows transparent monolayers and a few layers of MXene, indicating that DMSO with water is an effective solvent for exfoliating multilayered MXene nanosheets. Particularly, the DMSO generates OH functionalization between the multilayered MXene which enhances the distance between the nanosheets, allowing the individual sheets to be easily separated. The SEM image of Pd NPs in Figure 2d shows that the particles are uniformly sized without any agglomeration. The inset Figure 2d depicts the TEM image of Pd NPs, demonstrating spherical-like particles and diameters within the range of 6 to 7 nm, reliable with the XRD results. This investigation reveals that the microwave irradiation enables the synthesis of uniform sphere-like Pd NPs under controlled conditions. Furthermore, microwave irradiation is effectively involved in the synthesis of Pd-MXene nanohybrids. SEM micrographs of Figure 2e show that the Pd NPs were successfully incorporated into the exfoliated MXene nanosheets. Furthermore, TEM micrographs with different magnification revealed that spherical-like Pd NPs with a diameter of 5 to 6.5 nm were decorated on the surfaces of MXene nanosheets, as shown in Figure 2f,g. Furthermore, Figure 2h shows the magnified HR-TEM image of the Pd NP and its lattice interplanar distance is estimated to be 0.22 nm in the crystal plane (111) of the fcc Pd NPs [22].

As shown in Figure 2i, HR-TEM images of hexagonal MXene nanosheets show their lattice fringes with d-spacing of 0.27 nm, which is in good agreement with previous reports [21]. Moreover, in the SAED pattern shown in Figure 2j, the Pd catalytic sites exhibit the lattice planes of (111), (200), and (220), which is more consistent with the XRD results. Furthermore, the MXene nanosheets in hybrid nanocomposites have a hexagonal structure which is confirmed by the SAED pattern in Figure 2k [21]. The results of this investigation indicate that fcc Pd NPs are successfully incorporated onto MXene nanosheets, which are highly desirable catalysts for electrocatalysis. A surface property and a chemical state of Pd-MXene nanocatalysts were examined using XPS. As shown in Figure 3a, the XPS survey spectra clearly reveals the presence of C, Pd, O, Ti 2p, and Ti 2S in the Pd-MXene nanocatalysts, indicating that the Pd-MXene has been prepared by microwave irradiation. In Figure 3b, the deconvoluted Pd 3d XPS spectrum of the Pd-MXene nanocatalysts denotes that Pd3d_5/2 and Pd3d_3/2 double peaks located at 336.69 and 342.13 eV correspond to metallic Pd^o, whereas a small peak at 338.5 eV corresponds to Pd-O, indicating NaBH₄ is capable of completely reducing Pd²⁺ ions into metallic Pd^o under microwave irradiation [22]. The percentage of Pd^o was determined to be approximately 86.4%, which indicates that Pd mostly exists in metallic form in the Pd-MXene hybrid nanostructures [18].

Figure 3c shows the deconvoluted Ti 2p XPS spectrum of MXene in the hybrid nanocatalysts. The Ti 2p core level exhibits two doublets of Ti 2p_3/2 and Ti 2p_1/2 at 455.0 and 461.7 eV, respectively, which are separated by 5.7 eV. The peak at 453.8 eV corresponds to the Ti-C in the MXene [23]. The 454.8 eV (Ti 2p_3/2) and 461.5 eV (Ti 2p_1/2) peaks are ascribed to Ti²⁺, while the 457.2 eV (Ti 2p_3/2) and 462.7 eV (Ti 2p_1/2) peaks are attributed to Ti³⁺, indicating the various chemical stats of the Ti in the MXene [20]. Furthermore, Ti⁴⁺ exhibits two noticeable peaks at 457.5 eV (Ti 2p_3/2) and 463.2 eV (Ti 2p_1/2) indicating the sample contains mixed valence states of the Ti phases. Further, the peaks at 458.8 eV (Ti 2p_3/2) and 465.1 eV (Ti 2p_1/2) are associated with the formation of Ti–O on the surface, which was formed by the oxidation of MXene (TiO₂) [23]. The deconvoluted spectrum of C1 in Figure 3d indicates the presence of three major peaks at 283.3, 284.5, and 286.5 eV, corresponding to C-Ti-Tx, C-C, and C-O bonds, respectively. In addition, Figure 3e shows the O 1s spectrum with four components located at 530.9, 531.3, and 533.2 eV corresponding to Ti-O, C-Ti-(OH)_x, Ti-H₂O, and C-OH, respectively, indicating that DMSO primarily introduces OH ions on the surfaces of MXene nanosheets [19]. Additionally, hydroxyls (OH) and oxygen-functional groups enhance the hydrophilicity of Pd-MXene and allow for higher ionic conductivity during electrocatalysis. In addition, N₂ adsorption-desorption isotherms were analyzed to examine the textural properties of Pd-MXene nanocomposites (Figure 3f). The relative pressure (P/P_o) range between 0.5 and 1.0 was obtained in a typical type-IV isothermal loop and an H₃-type hysteresis loop. The Pd-MXene showed a specific surface area (SSA) of 97.5 m²g⁻¹, which is higher than the recently reported MXene-based nanocatalysts [19,24,25]. The inset Figure 3f shows the Barrett–Joyner–Halenda (BJH) pore size and pore volume distribution of the Pd-MXene nanocatalysts. The Pd-MXene nanocatalysts exhibited pore sizes and total pore volumes between 8–20 nm, and 0.289 cm³g⁻¹, respectively, suggesting a superiority over the various MXene-based nanocomposites [25,26]. The higher SSA and pore channels in Pd-MXene nanocatalysts could enhance their electrocatalytic activities, which could improve CO₂ adsorption and electro redox behavior for reducing CO₂ into fuels.

2.2. CO₂RR Performance

The electrocatalytic properties of the Pd NPs, MXene, and Pd-MXene were carried out at room temperature in a gas-tight 3-electrode cell. Figure 4a shows the LSV curves of the Pd NPs, MXene, and Pd-MXene electrodes at 10 mV s⁻¹ as the scan rate in 1.0 M KHCO₃ as the electrolyte solution. A Pd NP exhibits an overpotential of −229 mV, while MXene exhibits an overpotential of −502 mV, indicating that Pd is a more active metallic electrocatalyst than MXene. However, a hybrid Pd-MXene electrode exhibited similar electrocatalytic behavior to Pd NPs at an overpotential of −398mV, suggesting that metallic Pd and MXene nanosheets are combined to perform superior electrocatalytic activity than MXene nanosheets. Electrocatalytic performance has been enhanced dramatically due to the large surface area of 2D MXene nanocomposites and the high conductivity of the metallic Pd phase. A CO₂-saturated KHCO₃ electrolyte solution was used as the electrolyte solution for LSV experiments over Pd nanoparticles, MXene, and Pd-MXene nanocomposites at room temperature. As shown in Figure 4b, the Pd-MXene electrode exhibited a lower onset potential (E_o) of −154 mV than the Pd (E_o = −278 mV) and MXene (E_o = −365 mV) electrodes. A lower E_o is caused by the CO₂ reduction process occurring on the electrode surfaces. Specifically, the Pd-MXene exhibits synergistic electrochemical effects, as CO₂ is largely accommodated on the surface of the MXene nanosheets, while the metallic sites of Pd are primarily involved in the reduction of CO₂, which enhances the CO₂RR process as compared to pure Pd and MXene-based electrodes. Moreover, Pd-MXene also displayed a higher difference in current density (j = −4.1 mV cm⁻²) than previously reported Cu/Ti₃C₂T_x (−0.12 mA cm²), Ti₃C₂T_x/g-C₃N₄ (−2.9 mA cm²), and Ti₃C₂T_x/Bi₂WO₆ nanosheets (−3.3 mA cm²), and was also better than most MXenes reported in the literature [27,28,29].

Figure 5a shows the FE (%) of the product distribution of CO₂RR for Pd, MXene, and Pd-MXene nanocomposites. As a result, Pd electrodes have estimated FEs of CH₃OH, CH₃CH₂OH, HCOOH, CO, and H₂ to be 11.8, 10.01, 20.08, 26.4, and 31.8%, respectively, suggesting the Pd plays a key role in producing H₂ via HER, as well as the Pd’s metallic catalytic sites promoting a higher rate of electrochemical reduction of CO₂ into CO and HCOOH. Particularly, metallic Pd ions adsorb more H⁺ ions in the cathodic chamber, which are then further converted into H₂ with the addition of e⁻. Furthermore, Pd metallic sites stimulate protonation of 2e⁻/2H⁺, leading to higher FEs of CO and HCOOH. Additionally, the MXene electrode enhanced the FEs of CH₃OH (22.3%) and CH₃CH₂OH (15.1%), while slight decreases were observed in H₂ (24.5%), CO (21%), and COOH (17.1%). The results indicate that MXene is capable of promoting CH₃OH and CH₃CH₂OH production via 6e⁻/6H⁺ and 12e⁻/12H⁺ processes during CO₂RR and reducing HER activity simultaneously. On the other hand, a Pd-MXene electrode exhibits a much higher FE of CH₃OH (43.5%), while CH₃CH₂OH, HCOOH, CO, and H₂ exhibit lower FEs of 12.3, 13.4, 16.2 and 14.8%, respectively. Specifically, the Pd-MXene promotes the 6e⁻/6H⁺ process which enhances the formation of CH₃OH. Especially, the exfoliated 2D Ti₃C₂T_x nanosheets exhibit abundant adsorption sites, defects, and high electron density, which improve CO₂ adsorption, while metallic electrocatalytic sites of Pd provide larger adsorption sites for H⁺ ions, which enable the reduction of CO₂ into selective formation of CH₃OH. The reactions of CO₂RR products take place as follows [3,30,31]:

$$Anode:4h^{+}+2H_{2}O \to O_2+4H^{+}$$

(1)

$$2H^{+}+2e^{-}+C{O}_2 \to CO+H_{2}O$$

(2)

$$2H^{+}+2e^{-}+C{O}_2 \to HCOOH$$

(3)

$$6H^{+}+6e^{-}+C{O}_2 \to C{H}_{3}OH+H_{2}O$$

(4)

$$12H^{+}+12e^{-}+2C{O}_2 \to C{H}_{3}C{H}_{2}OH+3H_{2}O$$

(5)

$$HER activity: 2H^{+}+2e^{-} \to H_2$$

(6)

The cathodic potential of −0.5 V vs. RHE effectively reduces CO₂ to selectively generate CH₃OH via 6e⁻/6H⁺ process over and Pd-MXene nanocomposites. Additionally, the applied potential plays an imperative role in improving the selectivity of the CH₃OH using the optimized Pd-MXene electrode. The applied potential was varied from −0.2 to −0.8 V vs. RHE and 1.0 M KHCO₃ saturated with CO₂. Figure 5b shows distributions of the CO₂RR products at various potentials for the Pd-MXene electrode. As the applied potential was increased from −0.2 to −0.5 V compared to RHE, the FE of CH₃OH increased, while other products such as CO, CH₃CH₂OH, and HCOOH decreased. The maximum FE of 68% was obtained for CH₃OH at −0.5 V vs. RHE over the Pd-MXene electrode, indicating that the 6e⁻/6H⁺ process dominates the other protonation/reduction processes including the HER, 2e⁻/2H⁺, and 12e⁻/12H⁺ processes [31,32]. The FE of CH₃OH decreased when the applied voltage was raised from −0.6 to −0.8 V vs. RHE, while the FEs of CO and H₂ increased. As a result of the higher potential, CO and H₂ are formed over the Pd-MXene electrode surfaces. CO₂RR to CH₃OH is favored more than CO and H₂ production on the Pd-MXene electrode, especially at low overpotential, whereas CO and H₂ production dominate at higher potentials. Additionally, a stability test was conducted on Pd-MXene electrode in CO₂-saturated 1.0 M KHCO₃ as an electrolyte solution. Over a period of 10 consecutive cycles, the FE of CH₃OH remained stable for the first five cycles, then decreased from 68% to 66% for the next five cycles, as shown in Figure 5c. The results designate that the Pd-MXene electrode is stable during long-term CO₂ reduction. A chemically stable Pd and oxygen-functionalized MXene nanosheet showed higher electrochemical stability for CO₂RR. A reaction pathway for reducing CO₂ to CH₃OH over Pd-MXene is illustrated in Figure 5d. At the initial stage of the CO₂RR, the electrolyte increases CO₂ concentration, which then adsorbs on the surfaces of the 2D geometrical structure of MXene in the Pd-MXene nanocomposites. During the second step, H⁺ ions are generated via the electrolysis of water from the anodic reaction, which migrates to the cathodic chamber where they interact with Pd metal sites and form metal-hydride (Pd-H⁺) [33]. Additionally, CO₂ molecules were activated with electrons to form CO₂^•−, which could adsorb on MXene surfaces and accelerate the formation of adsorbed -CO species [30,32]. A study published elsewhere indicated that these CO intermediates promoted methanol production. Moreover, the Ti-active sites in the MXene were capable of accommodating CO intermediates, which were further protonated to form adsorbed -CHO, which were then reduced to -CH₂O, -CH₃O and finally reduced to CH₃OH via the 6e^-/6H⁺ process [34]. The Pd-MXene electrode had higher CO₂RR and stability than any other electrode, making it a superior electrode for producing CH₃OH under optimal conditions. Accordingly, Pd-MXene electrodes are highly competitive with other electrode materials that have been recently studied in terms of their ability to reduce CO₂ into selectively produced CH₃OH, which includes the following materials: Cu_2−xSe(y) nanocatalysts, Ru polypyridyl carbene catalyst on N-doped porous carbon, transition metals supported on g-C₃N₄, transition metal oxides, boron phosphide nanoparticles, hierarchical Pd/SnO₂ nanosheets, Cu/Ti₃C₂Tx, metal-free B₂S sheet, defects-rich Ni₃S₂-CNFs nanoheterostructures, etc., [27,30,35,36,37,38,39,40,41]. The Pd-MXene was shown to be an effective candidate for reducing flue gases into fuels in order to control climate change.

3. Materials and Methods

3.1. Materials

Methanol (CH₃OH, 99.9%), lithium fluoride (LiF), Palladium (II) chloride (PdCl₂, 99.9%), ethanol (CH₃CH₂OH, 99%), formic acid (HCOOH), potassium hydroxide (KOH), sodium sulfate (Na₂SO₄), sulfuric acid (H₂SO₄, 95–98%), hydrochloric acid (HCl, 37%), carbon black, and Nafion solution (5% w/w) were purchased from Sigma Aldrich, Merck.

3.2. Synthesis of Pd-MXene Nanocomposites

HF etching has been described elsewhere as a promising method for preparing 2D MXene nanosheets from Ti₃AlC₂ MAX phase. Typically, 10 g of MAX phase powder is dispersed in 200 mL of aqueous water (H₂O) and HF (48%) solution under magnetic stirring for 300 rpm. In a fume hood, the complete solution is heated to 40 to 45 °C for two days while being stirred magnetically. In order to settle the MXene suspension, 700 mL of distilled water (DIW) is added to the reacted solution and left for 24 h. The MXene suspension is then re-dispersed into 100 mL water and mixed with 5% HCl and 95% 2-propanol, and centrifuged 10 times at 5000× g rpm for 5 min. The MXene is then dried at 70 °C for 12 h in a vacuum oven. In order to synthesize Pd-MXene, MXene nanopowder is dispersed in 30 mL of DMSO submersed in a water bath sonicator and mixed for 12 h at 40 °C to form Ti₃C₂T_x suspension. As a result, 0.5 g of PdCl₂ salt (0.5 g) is dispersed in Ti₃C₂T_x-water suspension (0.5 g) under a magnetic stirring condition for 30 min. Subsequently, 10 mL of NaBH₄ (0.1 M) is added to the obtained-above Pd²⁺-MXene (Ti₃C₂T_x) solution in a microwave oven (1100 W and 2.45 GHz; 60 s) to form Pd^o-MXene nanocomposites, as previously described [12]. Additionally, the unreacted impurities and salts are removed by washing the sample several times with DIW and ethanol. Finally, the as-prepared samples are dried in a vacuum oven at 80 °C for 12 h prior to further physico-chemical characterizations.

3.3. Characterization

An HR-TEM (Titan TEM 300 kV) with SAED and a SEM (JEOL JSM-7610F FEG-SEM) is used to determine the morphology and composition of MXene, Pd NPs, and Pd-MXene nanocomposites. The crystalline structure and surface area of the as-prepared nanocomposites are determined using X-ray diffraction (XRD, PANalytical Empyrean), and Quantachrome Autosorb 06 surface area analyzers, respectively. The chemical states and elemental information of the Pd-MXene nanocomposites are determined by X-ray photoelectron spectroscopy (XPS) (PHI5000 Version Probe III).

3.4. CO₂RR Experiments

A Biologic VMP-300 is used to conduct the CO₂RR experiments in a gas-tight cell separated by a membrane, and CO₂ gas flow is controlled by a gas flow meter. As anode, Pt sheet is used, and saturated calomel electrode (SCE) is used as reference electrode, which is further converted to RHE using E(vs RHE) = E(vs SCE) + 0.244 + 0.059pH. To prepare the working electrode, 5 wt.% of Nafion solution containing 95 wt.% as-prepared electrocatalyst is sonicated for 30 min to prepare electrode ink. An electrode ink solution is drop-cast on a carbon sheet and further dried at 70 °C for 4 h in a vacuum oven to obtain a working electrode. Prior to the CO₂RR experiments, leaner sweep voltammetry measurements (LSV) with potential windows of 0 to −1.2 V at 10 mVs⁻¹ as the scan rate in 1 M KOH electrolyte solution are conducted to assess electrode electrochemical activity. The LSV of various Pd, MXene, and Pd-MXene electrodes is performed with and without CO₂-saturated KOH as the electrolyte. In the CO₂RR experiments, liquid and gas samples are collected and the quantities of the converted products are determined using CMS-QP2010 Ultra Shimadzu with Flame Ionization Detector (FID) and microGC 3000 (Inficon) with Thermal Conductivity Detector (TCD). The following equation can be used to calculate the faradic efficiency of each product,

$$FE(\%)=\frac{z.n.F}{q}100$$

(7)

where q is the total charge applied to the CO₂RR process, F is the Faraday constant (F = 96,485 C·mol⁻¹), n is the number of moles produced, and z is the theoretical number of e− exchanged to form the desired product. A comparison of different applied potentials versus FE of the products (−0.4 to −1.2 V vs. RHE) is also conducted. The recycling study is conducted on a Pd-MXene electrode at −0.7 V vs. RHE for 10 consecutive cycles.

4. Conclusions

In summary, Pd nanoparticles with a smaller diameter were grown on electronically transparent 2D MXene nanosheets using microwave irradiation. In the TEM and SEM images, the Pd NPs displayed a spherical-like morphology with a diameter of 6 nm and were decorated onto MXene nanosheets. XPS analysis revealed that most metallic Pd nanoparticles formed on hexagonal MXene nanosheets. BET and BJH studies showed that the Pd-MXene nanohybrids had a specific surface area of 97.5 m²g⁻¹, a pore size of 8–20 nm, and a pore volume of 0.0289 cm³g⁻¹. The LSV studies revealed that the obtained Pd-MXene exhibits better electrocatalytic activity than pure Pd NPs and MXene nanosheets in the presence of CO₂-saturated 1.0 MKHCO₃ as the electrolyte solution. A Pd-MXene electrode was fabricated for the reduction of CO₂ into CH₃OH under ambient conditions. Pd-MXene electrode possessed a higher FE of 67.8% for CH₃OH at −0.5 V compared to RHE in CO₂-saturated electrolyte solution, demonstrating that Pd and 2D geometrical structure of MXene enhance CO₂ reduction reaction and CH₃OH selectivity. Furthermore, the Pd-MXene catalyst was tested over ten consecutive recycling runs to demonstrate its practical applications. The results of this study will open new pathways for the deployment of metallic nanoparticle-incorporated MXene for the reduction of CO₂ into highly selective production of fuels and chemicals under ambient conditions, thereby achieving carbon neutrality.