MXENE NANOSHEET INK FOR PALLADIUM RECOVERY AND ITS MANUFACTURING METHOD, PALLADIUM RECOVERY METHOD USING MXENE NANOSHEET INK, AND ELECTROCHEMICAL CATALYST USING RECOVERED PALLADIUM AND ITS MANUFACTURING METHOD

Abstract

The present invention relates to a MXene nanosheet ink for palladium recovery, a method of manufacturing the same, a method of recovering palladium using a MXene nanosheet ink, an electrochemical catalyst using recovered palladium, and a method of manufacturing the same that are capable of significantly improving the recovery efficiency of palladium ions in water.

Claims

1. A MXene nanosheet ink for palladium recovery, which is a solution in which MXene nanosheets of a chemical formula below are dispersed, wherein a zeta potential () of the MXene nanosheets is 0 mV or higher in an acidic solution with a pH of 7 or below. ##STR00005## (where M is a transition metal, X is one of carbon or nitrogen or a combination thereof, T is a surface functional group and is at least one of O, F, OH, or Cl, and n is an integer from 1 to 4).

2. The MXene nanosheet ink of claim 1, wherein, in an acidic solution of pH 4 to 5, the zeta potential () of the MXene nanosheets exhibits 20 mV or higher.

3. The MXene nanosheet ink of claim 1, wherein, in an acidic solution of pH of 4 to 5, the surface functional groups present on a surface of the MXene nanosheets are protonated and saturated by hydrogen ions (H.sup.+).

4. The MXene nanosheet ink of claim 1, wherein, in an acidic solution of pH of 4 or lower, the surface functional groups present on a surface of the MXene nanosheets are protonated and saturated by hydrogen ions (H.sup.+), and the zeta potential () remains constant upon pH decrease.

5. The MXene nanosheet ink of claim 1, wherein the chemical formula is Ti.sub.3C.sub.2T.sub.x.

6. The MXene nanosheet ink of claim 1, wherein the MXene nanosheet ink has a dispersion concentration of 2 g/L or less.

7. The MXene nanosheet ink of claim 1, wherein a maximum adsorption capacity for Pd.sup.2+ is 800 mg/g or more.

8. The MXene nanosheet ink of claim 1, wherein a maximum adsorption capacity for Pd.sup.2+ is 1000 mg/g or more.

9. The MXene nanosheet ink of claim 1, wherein a maximum adsorption capacity for Pd.sup.2+ is 1900 mg/g or more.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] FIG. 1 is a process schematic view for describing a method of manufacturing MXene nanosheets according to an embodiment of the present invention.

[0049] FIG. 2 is an actual photograph of Ti.sub.3C.sub.2T.sub.x nanosheet ink prepared according to Experimental Example 1.

[0050] FIG. 3 illustrates the results of measuring the specific surface areas of MAX (Ti.sub.3AlC.sub.2) powder, Ti.sub.3C.sub.2T.sub.x flakes, and Ti.sub.3C.sub.2T.sub.x nanosheets.

[0051] FIG. 4 illustrates the results of measuring the hydrodynamic radius (H.sub.R) of Ti.sub.3C.sub.2T.sub.x nanosheets according to HF concentration.

[0052] FIG. 5 illustrates the experimental results of the zeta potential () of Ti.sub.3C.sub.2T.sub.x nanosheets according to HF concentration.

[0053] FIG. 6 illustrates the experimental results of the adsorption capacity (ge) according to the amount of Ti.sub.3C.sub.2T.sub.x nanosheets introduced.

[0054] FIG. 7 illustrates the experimental results of the adsorption capacity (ge) according to pH changes.

[0055] FIG. 8A illustrates the experimental results of the recovery rates of Ti.sub.3C.sub.2T.sub.x nanosheets and carbon black (CB) according to the initial concentration (Ci) of palladium.

[0056] FIG. 8B illustrates the experimental results of changes in the hydrodynamic radius (H.sub.R) of Ti.sub.3C.sub.2T.sub.x nanosheets and carbon black (CB) according to reaction time.

[0057] FIG. 9 illustrates the isothermal adsorption curve of Ti.sub.3C.sub.2T.sub.x nanosheet ink for Pd.sup.2+.

[0058] FIG. 10 is a reference view summarizing the maximum adsorption capacity (qm) and adsorption equilibrium time for each Pd adsorbent.

[0059] FIG. 11 illustrates the experimental results of the selectivity of Ti.sub.3C.sub.2T.sub.x nanosheets according to HF concentration.

[0060] FIG. 12 illustrates the actual photographs, SEM images, and SEM-EDS analysis results for a spent Pd catalyst.

[0061] FIG. 13 illustrates the experimental results of the Pd.sup.2+ recovery rate from waste liquid including the actual used spent Pd catalyst.

[0062] FIG. 14 is a FESEM image of Pd@Ti.sub.3C.sub.2T.sub.x.

[0063] FIGS. 15A, 15B, 15C, 15D, 15E, 15F and 15G illustrate the analysis results for describing the mechanism involved in palladium recovery by Ti.sub.3C.sub.2T.sub.x nanosheets.

[0064] FIG. 16 illustrates the experimental results of changes in palladium recovery rates according to the number of adsorption-desorption cycles.

[0065] FIG. 17 is an optical microscope image of palladium desorbed from Pd@Ti.sub.3C.sub.2T.sub.x nanosheets.

[0066] FIG. 18 illustrates the experimental results of the current density and overpotential measured for a Pd/NM catalyst and Pd/CB catalyst prepared in Experimental Example 8.

[0067] FIG. 19 illustrates the experimental results of the HER overpotential and mass-specific activity of noble metals according to the Pd weight of the Pd/NM catalyst.

[0068] FIG. 20 illustrates the HER Tafel slope and electrochemical resistance analysis graph of the Pd/NM catalyst.

[0069] FIG. 21 illustrates the electrochemical resistance analysis results of the Pd/NM catalyst.

[0070] FIG. 22 illustrates the constant current experimental results of the Pd/NM catalyst.

[0071] FIG. 23 illustrates the experimental results comparing the HER characteristics of Pd/NM catalysts recovered and prepared from ultrapure water and waste liquid environments, respectively, using linear sweep voltammetry (LSV).

DETAILED DESCRIPTION OF THE INVENTION

[0072] The present invention provides a technology that can dramatically improve the recovery efficiency of palladium ions in water using MXene nanosheet ink.

[0073] In the present invention, MXene nanosheet ink refers to an aqueous solution in which MXene nanosheets are dispersed. By introducing the MXene nanosheet ink into an acidic solution including palladium ions, the palladium ions are adsorbed onto the MXene nanosheets, allowing the recovery of palladium ions.

[0074] In the present invention, the mechanism involved in the recovery of palladium ions may be largely divided into three stages (first Mechanism to third Mechanism).

[0075] The first mechanism involves the electrostatic attraction between protonated MXene nanosheets and palladium-chlorine complexes, causing the palladium-chlorine complexes to adsorb onto the surface of the protonated MXene nanosheets. The second mechanism is the reduction of the palladium ions into palladium nanoparticles through a redox reaction between the adsorbed palladium ions and the MXene nanosheets. The third mechanism involves the aggregation and precipitation of MXene nanosheets due to the electrostatic attraction between the anionic palladium nanoparticles and the protonated MXene nanosheets.

[0076] In the first mechanism, for electrostatic attraction to occur, the MXene nanosheets need to carry a positive charge, and the palladium ions need to exhibit anionic characteristics.

[0077] The surface of MXene nanosheets is provided with surface functional groups, such as F, O, and OH, which are formed during the MXene nanosheet manufacturing process. These F, O, and OH functional groups are hydrophilic functional groups that are easily protonated by hydrogen ions (H.sup.+) in an acidic solution environment. As the hydrophilic functional groups (F, O, OH) on the surface of the MXene nanosheets become protonated, the surface of the MXene nanosheets acquires a positive charge. The method of manufacturing MXene nanosheets will be described below in detail.

[0078] Meanwhile, palladium ions (Pd.sup.2+) in water exist in a cationic state, but in a strongly acidic environment, they combine with Cl.sup., F.sup., NO.sub.3.sup.2, and similar anions in the acidic solution, acquiring anionic characteristics. For example, when Pd.sup.2+ combines with Cl.sup., it forms [PdCl.sub.4].sup.2. In the present invention, a substance in which palladium ions combine with anions in an acidic solution is referred to as a palladium-anion complex, and [PdCl.sub.4].sup.2 is an example of a palladium-anion complex.

[0079] As described above, in an acidic solution, the surface of the MXene nanosheets becomes protonated and acquires a positive charge, while the palladium ions form palladium-anion complexes with anionic characteristics. As a result, the palladium-anion complexes are adsorbed onto the surface of the protonated MXene nanosheets. Through this mechanism, it becomes possible to adsorb palladium ions onto the MXene nanosheets.

[0080] In addition, in an acidic solution environment where palladium ions (Pd.sup.2+) coexist with other metal ions, such as Mg.sup.2+, Cu.sup.2+, Ni.sup.2+, Ca.sup.2+, K.sup.+, and Na.sup.+, only the palladium ions (Pd.sup.2+) combine with anions to form palladium-anion complexes, while the other metal ions do not combine with anions and remain in their cationic states. Due to this characteristic, electrostatic attraction occurs only between the palladium-anion complexes and the MXene nanosheets, enabling the selective adsorption of palladium ions through this mechanism.

[0081] From the above, it can be understood that the adsorption of palladium ions onto MXene nanosheets is determined by whether the surface of the MXene nanosheets carries a positive charge. The presence of a positive charge on the surface of MXene nanosheets may also be expressed by the zeta potential () of the MXene nanosheet surface. That is, when the zeta potential () of the MXene nanosheet surface is greater than 0, the palladium-anion complexes are adsorbed onto the surface of the MXene nanosheets due to electrostatic attraction. Conversely, when the zeta potential () is less than 0, no electrostatic attraction occurs between the MXene nanosheet surface and the palladium-anion complexes.

[0082] The zeta potential () of the MXene nanosheet surface is influenced by the pH and the surface functional groups of the MXene nanosheets. As described above, in an acidic solution environment, the surface functional groups of MXene nanosheets are protonated by hydrogen ions (H.sup.+). The pH and surface functional groups of the MXene nanosheets are interrelated, thereby determining the zeta potential () of the MXene nanosheet surface.

[0083] The higher the zeta potential () of the MXene nanosheet surface, the better the recovery rate and selectivity of the palladium-anion complexes. The zeta potential () of the MXene nanosheet surface is determined by the degree of protonation on the MXene nanosheet surface. The degree of protonation is directly related to the bonding extent between surface functional groups and hydrogen ions (H.sup.+). As the pH decreases, the concentration of hydrogen ions (H.sup.+) increases, which inevitably causes the zeta potential () of the MXene nanosheet surface to increase. That is, the best method to enhance the recovery rate and selectivity of palladium-anion complexes is to lower the pH as much as possible.

[0084] However, in practical environments for recovering palladium ions, such as semiconductor waste liquid including palladium ions, the pH ranges from 2 to 5. This indicates that a relatively high zeta potential () needs to be maintained even at higher pH levels, such as around pH levels of 4 to 5.

[0085] To maintain a high value of zeta potential () on the MXene nanosheet surface even at pH levels of 4 to 5, the surface functional group density of the MXene nanosheets needs to be optimized.

[0086] The zeta potential () increases as the MXene nanosheet surface becomes protonated through the bonding of surface functional groups with hydrogen ions (H.sup.+). Since the concentration of hydrogen ions (H.sup.+) increases as the pH decreases, the MXene nanosheet surface needs to be provided with sufficient surface functional groups to bond with hydrogen ions (H.sup.+).

[0087] However, when the density of surface functional groups is either too high or too low, it adversely affects the recovery rate and selectivity of palladium ions.

[0088] When the density of surface functional groups is too high, many unbound surface functional groups remain without bonding to hydrogen ions (H.sup.+). This indicates a low degree of protonation on the MXene nanosheet surface, and the low degree of protonation leads to a low level of zeta potential () on the MXene nanosheet surface. In this case, even if the pH is further lowered and the concentration of hydrogen ions (H.sup.+) increases, unbound surface functional groups may still remain. Due to these unbound residual surface functional groups, the increase in zeta potential () becomes inherently limited. Therefore, when the density of surface functional groups is too high, both the recovery rate and selectivity of palladium ions deteriorate.

[0089] Additionally, when the density of surface functional groups is too low, the surface functional groups of MXene nanosheets may theoretically bond with all available hydrogen ions (H.sup.+), even at higher pH levels where the hydrogen ion concentration is relatively low, reaching a protonation saturation state. By analogy, because the amount of surface functional groups is too small, even a small amount of hydrogen ions (H.sup.+) present may result in a state where all the surface functional groups on the MXene nanosheets becoming protonated. As such, since the amount of surface functional groups available to bond with hydrogen ions (H.sup.+) is limited, the maximum value of the zeta potential () is inevitably constrained. Furthermore, even if the pH is further lowered and the concentration of hydrogen ions (H.sup.+) increases, the zeta potential () remains nearly unchanged because the protonation of the surface functional groups is already in a saturated state. For example, when the highest zeta potential () is achieved at pH 2, even if the pH is lowered to 1, the zeta potential () remains at the same level as it was at pH 2. Therefore, when the density of surface functional groups is too low, the selectivity for palladium ions may not be poor, but the recovery rate of palladium ions inevitably decreases.

[0090] However, when the density of surface functional groups is too low, as referenced in the experimental examples described below, the insufficient surface functional group density hinders the smooth exfoliation of MXene nanosheets. This results in larger MXene nanosheet sizes, which degrade the adsorption characteristics for palladium ions. Theoretically, when the density of surface functional groups is too low, protonation saturation needs to occur at relatively high pH levels, such as pH 4 to 5, as described above. However, due to the significantly larger size of the MXene nanosheets, the adsorption characteristics for palladium ions deteriorate, and actual protonation saturation occurs at lower pH levels (pH 1 to 2).

[0091] In light of these points, the surface functional groups of MXene nanosheets need to be controlled to an optimal density. That is, an appropriate amount of surface functional groups need to be present on the surface of the MXene nanosheets. When an appropriate amount of surface functional groups is present, the maximum zeta potential () appears at a relatively higher pH. That is, protonation saturation occurs at a relatively higher pH, resulting in the maximum zeta potential (). Even if the pH is further lowered, the corresponding maximum zeta potential () is maintained.

[0092] Additionally, when an appropriate amount of surface functional groups is present on the surface of the MXene nanosheets, the MXene nanosheets are formed at a small size. This avoids the issue of reduced adsorption performance caused by the significantly large MXene nanosheets, in cases where the surface functional group density is too low, as described above.

[0093] As such, when an appropriate amount of surface functional groups is present, the maximum zeta potential () is achieved even at relatively high pH levels, such as pH 4 to 5. This indicates that a high recovery rate and selectivity for palladium ions may be secured across a wide range of pH conditions.

[0094] Therefore, in the present invention, optimizing the surface functional group density of MXene nanosheets means ensuring that the zeta potential () of the MXene nanosheet surface reaches its maximum value or maintains a zeta potential above a certain value (e.g., 20 or higher) at relatively high pH levels, such as pH 4 to 5. Detailed measures to achieve these zeta potential characteristics may include the use of process conditions.

[0095] As an embodiment, the surface functional group density may be optimized by controlling the concentration of HF in the HF solution. The surface functional groups (F, O, OH) of MXene nanosheets are generated during the etching process of the precursor of MXene nanosheets, MAX, using a strong acid, such as an HF solution. The higher the HF concentration in the HF solution, the higher the density of the surface functional groups. Therefore, the surface functional group density may be optimized by controlling the concentration of HF in the HF solution.

[0096] In addition to controlling the HF concentration in the HF solution, adjusting the process temperature during MAX etching may also be considered. Further, the optimization of surface functional group density may be achieved by controlling other process conditions as well.

[0097] The important point is to control the surface functional group density so that the zeta potential () of the MXene nanosheet surface reaches its maximum value or maintains a value of a certain value or more at relatively high pH levels (e.g., pH 4 to 5). In this case, the surface functional group density may be achieved by controlling various process conditions, such as the HF concentration in the HF solution and the process temperature during the etching of MAX.

[0098] With reference to the experimental examples described below, when palladium ion recovery was performed using MXene nanosheets with optimized surface functional group density, it was shown that palladium ions were recovered at 100%, while in an environment where other metal ions coexisted, the recovery rate of other metal ions was as low as 0.9% (Cu.sup.2+). The maximum adsorption capacity (q.sub.m) was shown as 1983.3 mg/g.

[0099] Comparing the maximum adsorption capacity of 1983.3 mg/g in the present invention with known technologies, it can be seen that it is more than 10 times superior to the maximum adsorption capacity of 184.56 mg/g reported in Non-Patent Document 1, which used MXene flakes. Furthermore, it is over twice as effective compared to the previously known best-performing polymer-based adsorbent (poly-Cys-g-PDA@GPUF, refer to Non-Patent Document 4) with a maximum adsorption capacity of 785 mg/g.

[0100] The above describes the first mechanism.

[0101] Through the first mechanism, it can be seen that palladium ions may be adsorbed onto MXene nanosheets, as well as, by optimizing the surface functional group density of the MXene nanosheets, a high recovery rate and selectivity for palladium ions may be achieved even at relatively high pH levels (pH 4 to 5).

[0102] Next, the second mechanism and third mechanism will be described.

[0103] The second mechanism involves the reduction of the adsorbed palladium ions into palladium nanoparticles through a redox reaction between the palladium ions and the MXene nanosheets. In the second mechanism, the palladium ions refer to Pd.sup.2+, which forms the palladium-anion complex. That is, in the second mechanism, Pd.sup.2+ is reduced to palladium nanoparticles (Pd NPs) through a redox reaction between Pd.sup.2+ and the MXene nanosheets.

[0104] The reduction of palladium ions (Pd.sup.2+) to palladium nanoparticles indicates that electrons () are donated to the palladium ions (Pd.sup.2+). The occurrence of electron donation implies that an oxidation reaction takes place on the MXene nanosheets. That is, through the redox reaction between Pd.sup.2+ and the MXene nanosheets, Pd.sup.2+ is reduced to palladium nanoparticles.

[0105] With reference to the experimental results described below, it was analyzed that the CTiOH of the MXene nanosheets is oxidized to CTiO and/or TiO by the adsorbed Pd.sup.2+, and Pd.sup.2+ is reduced to palladium nanoparticles by electrons () donated from the OH groups on the surface of the MXene nanosheets. Additionally, due to this redox reaction, the peaks corresponding to TiC, Ti(II), and Ti(III) decrease, while the peak corresponding to Ti(IV) increases. Furthermore, it was analyzed that-F, one of the surface functional groups on the MXene nanosheets, polarizes adjacent oxygen atoms, increasing the overall polarity of the MXene nanosheets, thereby serving to facilitate the reduction reaction of Pd.sup.2+.

[0106] As such, the redox reaction between Pd.sup.2+ and the MXene nanosheets enables the reduction of Pd.sup.2+ to palladium nanoparticles.

[0107] The reduction of Pd.sup.2+ to palladium nanoparticles through the second mechanism is a critical mechanism both for the practical recovery of palladium and for the utilization of the recovered palladium as an electrochemical catalyst. In addition, the occurrence of the second mechanism enables the third mechanism. That is, when the reduction of Pd.sup.2+ to palladium nanoparticles through the second mechanism does not occur, the third mechanism will not take place.

[0108] As described above in the Background Art, nanoscale adsorbents have the issue of being difficult to recover due to their small size. In contrast, in the present invention, the recovery of palladium-adsorbed MXene nanosheets becomes significantly easier due to the second and third mechanisms.

[0109] The third mechanism, as described above, involves the aggregation and precipitation of MXene nanosheets due to the electrostatic attraction between the anionic palladium nanoparticles and the protonated MXene nanosheets.

[0110] Once the reduction of Pd.sup.2+ to palladium nanoparticles proceeds through the second mechanism, the third mechanism immediately follows, inducing the aggregation of the MXene nanosheets.

[0111] The palladium nanoparticles generated through the second mechanism inherently exhibit anionic characteristics. As a result, electrostatic attraction occurs between the anionic palladium nanoparticles and the protonated MXene nanosheets. Due to the electrostatic attraction between the palladium nanoparticles and the protonated MXene nanosheets, adjacent MXene nanosheets aggregate with each other, and the MXene nanosheet agglomerates precipitate within the solution.

[0112] As described above, through the second and third mechanisms, Pd.sup.2+ is reduced to form palladium nanoparticles. The electrostatic attraction between the anionic palladium nanoparticles and the protonated MXene nanosheets induces the aggregation and precipitation of the MXene nanosheets. Consequently, the palladium-adsorbed MXene nanosheets may be recovered very easily.

[0113] The reason that the aggregation and precipitation of MXene nanosheets may occur and be accelerated lies in the reduction of Pd.sup.2+ to palladium nanoparticles through the second mechanism, as described above. With reference to the experimental examples described below, it was confirmed that approximately 82.1% of the palladium content in the aggregated and precipitated material consists of palladium metal. In other words, this means that 82.1% of the adsorbed palladium ions are reduced to palladium nanoparticles.

[0114] In addition, the palladium-adsorbed MXene nanosheets may be recycled by desorbing the palladium from the MXene nanosheets through a regeneration process and then being manufactured back into MXene nanosheet ink, or the palladium-adsorbed MXene nanosheets themselves may be utilized as electrochemical catalysts, such as hydrogen evolution reaction (HER) catalysts.

[0115] In summary, the three mechanisms, i.e., first mechanism to third mechanism, involved in the recovery of palladium ions from water using MXene nanosheet ink according to the present invention have been described. Through these first to third mechanisms, the recovery rate and selectivity of palladium ions can be improved. Additionally, the palladium-adsorbed MXene nanosheets can be easily recovered, regenerated and utilized as electrochemical catalysts.

MXene Nanosheet Ink and Method of Manufacturing the Same

[0116] A method of manufacturing MXene nanosheet ink according to an embodiment of the present invention will be described as follows. FIG. 1 is a process schematic view for describing a method of manufacturing MXene nanosheets according to an embodiment of the present invention.

[0117] First, MAX, represented by Chemical Formula 1, is reacted with a strong acid solution to prepare MXene flakes, represented by Chemical Formula 2.

##STR00003## [0118] (where M is a transition metal, A is a group A element of the periodic table, X is carbon and/or nitrogen, and n is an integer from 1 to 4).

##STR00004## [0119] (where M is a transition metal, X is carbon and/or nitrogen, T is at least one selected from O, F, OH, and Cl, and n is an integer from 1 to 4).

[0120] As an embodiment, M.sub.n+1AX.sub.n may be any one of Ti.sub.3AlC.sub.2, Ti.sub.2CdC, Sc.sub.2InC, Ti.sub.2AlC, Ti.sub.2GaC, Ti.sub.2InC, Ti.sub.2TlC, V.sub.2AIC, V.sub.2GaC, Cr.sub.2GaC, Ti.sub.2AlN, Ti.sub.2GaN, Ti.sub.2InN, V.sub.2GaN, Cr.sub.2GaN, Ti.sub.2GeC, Ti.sub.2SnC, Ti.sub.2PbC, V.sub.2GeC, Cr.sub.2AlC, Cr.sub.2GeC, V.sub.2PC, V.sub.2AsC, Ti.sub.2SC, Zr.sub.2InC, Zr.sub.2TlC, Nb.sub.2AlC, Nb.sub.2GaC, Nb.sub.2InC, Mo.sub.2GaC, Zr.sub.2InN, Zr.sub.2TIN, Zr.sub.2SnC, Zr.sub.2PbC, Nb.sub.2SnC, Nb.sub.2PC, Nb.sub.2AsC, Zr.sub.2SC, Nb.sub.2SC, Hf.sub.2InC, Hf.sub.2TlC, Ta.sub.2AlC, Ta.sub.2GaC, Hf.sub.2SnC, Hf.sub.2PbC, Hf.sub.2SnN, Hf.sub.2SC, V.sub.3AlC.sub.2, Ti.sub.3SiC.sub.2, Ti.sub.3GeC.sub.2, Ti.sub.3SnC.sub.2, Ta.sub.3AlC.sub.2, Ti.sub.4AlN.sub.3, V.sub.4AlC.sub.3, Ti.sub.4GaC.sub.3, Ti.sub.4SiC.sub.3, Ti.sub.4GeC.sub.3, Nb.sub.4AlC.sub.3, or Ta.sub.4AlC.sub.3, or a combination thereof.

[0121] The strong acid solution is a diluted solution of hydrofluoric acid (HF) or hydrochloric acid (HCl).

[0122] The HF concentration or HCl concentration in the strong acid solution is not particularly limited but may be set to 5 to 45%. Additionally, the reaction temperature for M.sub.n+1AX.sub.n and the strong acid solution is not particularly limited but may be set to 15 to 55 C.

[0123] By reacting a MAX (M.sub.n+1AX.sub.n) with a strong acid solution, the A component of M.sub.n+1AX.sub.n is etched and removed, thereby forming surface functional groups such as O, F, OH, and Cl, and manufacturing MXene flakes of Chemical Formula 2. MXene flakes form a two-dimensional layered structure composed of multiple MXene nanosheets stacked together.

[0124] Next, the MXene flakes are introduced into an exfoliation solution to exfoliate the MXene flakes into MXene nanosheets. The exfoliation solution is a solution in which an intercalant is dissolved. The intercalant is inserted into the layered structure of the MXene flakes, causing the MXene flakes to exfoliate into nanosheet form. The substance of intercalant is not particularly limited, and in an example, tetramethylammonium hydroxide (TMAOH) may be used.

[0125] Once the exfoliation of MXene flakes into MXene nanosheets is complete, the residual intercalant is removed using ultrapure water or similar.

[0126] Then, by dispersing the MXene nanosheets in water, the preparation of MXene nanosheet ink is completed. The surface functional groups formed on the surface of MXene nanosheets are easily dispersed in water due to their hydrophilic characteristics. Ultrasonication may be applied to achieve uniform dispersion.

[0127] The MXene nanosheets dispersed in the MXene nanosheet ink achieve protonation saturation of their surface functional groups under acidic conditions with a pH of 4 to 5. This results in the MXene nanosheets having either the maximum zeta potential () or a zeta potential value of a certain value or more, in one embodiment, a zeta potential () of 20 or more.

[0128] To achieve these zeta potential characteristics, the surface functional group density of the MXene nanosheets may be adjusted. The surface functional group density of the MXene nanosheets may be adjusted by controlling the process conditions during the reaction between MAX and the strong acid solution. As an embodiment, the MXene nanosheets with the aforementioned zeta potential characteristics may be manufactured by controlling factors such as the concentration of the strong acid solution and the reaction temperature between MAX and the strong acid solution.

Palladium Recovery Method Using MXene Nanosheet Ink

[0129] A method of recovering palladium using MXene nanosheet ink according to an embodiment of the present invention will be described as follows.

[0130] The MXene nanosheet ink prepared according to an embodiment of the present invention is introduced into an acidic solution including palladium ions (Pd.sup.2+). The acidic solution may also include ions such as Mg.sup.2+, Cu.sup.2+, Ni.sup.2+, Ca.sup.2+, K.sup.+, and Na.sup.+ in addition to palladium ions.

[0131] As the MXene nanosheet ink is introduced into the acidic solution, palladium ions are adsorbed onto the surface of the MXene nanosheets. The palladium-ion-adsorbed MXene nanosheets then aggregate with each other and precipitate. In this case, to ensure the dispersion stability of the MXene nanosheets, it is preferable for the concentration of MXene nanosheets in the MXene nanosheet ink to be 2 g/L or less.

[0132] The adsorption of palladium ions onto the surface of the MXene nanosheets, as well as the aggregation and precipitation of the MXene nanosheets, are explained by the previously described first to third mechanisms.

[0133] That is, through the first mechanism, the palladium-chlorine complexes are adsorbed onto the surface of the protonated MXene nanosheets due to the electrostatic attraction between the protonated MXene nanosheets and the palladium-chlorine complexes. Additionally, through the second mechanism, the adsorbed palladium ions are reduced to palladium nanoparticles by a redox reaction between the palladium ions and the MXene nanosheets. Further, finally, through the third mechanism, the MXene nanosheets aggregate and precipitate due to the electrostatic attraction between the anionic palladium nanoparticles and the protonated MXene nanosheets.

Electrochemical Catalyst Using Recovered Palladium and Method of Manufacturing the Same

[0134] An electrochemical catalyst using the recovered palladium according to an embodiment of the present invention and a method of manufacturing the same are described as follows.

[0135] As described earlier in the method of recovering palladium using MXene nanosheet Ink, palladium ions are adsorbed onto the surface of MXene nanosheets by introducing MXene nanosheet ink into a solution including palladium ions. The reduction of palladium ions to palladium nanoparticles induces the aggregation and precipitation of the MXene nanosheets. The precipitate at this stage may be referred to as a palladium nanoparticle-immobilized MXene nanosheet aggregate.

[0136] The palladium nanoparticle-immobilized MXene nanosheet aggregate may be used in two ways. One approach is the desorption of palladium from MXene nanosheets through the regeneration process described above, which is then recycled into MXene nanosheet ink, and the other approach is the use as an electrochemical catalyst.

[0137] The electrochemical catalyst serves to facilitate electrochemical reactions when a power supply is applied, and may be used in a variety of electrochemical reactions. For example, the electrochemical catalysts are used in electrochemical hydrogen evolution reactions (HER).

[0138] In electrochemical reactions, electrochemical catalysts, which require the application of a power supply, typically have a structure where the catalytic material is coated onto a conductive electrode.

[0139] For such applications as an electrochemical catalyst, the electrochemical catalyst according to the present invention has a structure in which a palladium nanoparticle-immobilized MXene nanosheet aggregate is coated onto the surface of a conductive electrode.

[0140] Additionally, the following process may be used to manufacture the electrochemical catalyst according to the present invention.

[0141] First, a palladium nanoparticle-immobilized MXene nanosheet aggregate is mixed with an ion-conductive binder solution to prepare a catalyst ink. The ion-conductive binder solution is a solution in which an ion-conductive binder is dissolved. The ion-conductive binder physically binds the palladium nanoparticle-immobilized MXene nanosheet aggregates while also serving to mediate ion conduction between the palladium nanoparticles. The ion-conductive binders such as perfluorosulfonic acid-based ionomers may be used. Examples include commercially available products such as Nafion and Aquivion.

[0142] Next, the catalyst ink is applied to the surface of the conductive electrode to form a coating, completing the manufacture of the electrochemical catalyst. The coating of the catalyst ink may be performed using methods such as drop casting, as an example. Additionally, the conductive electrode is not limited to its composition. For example, the conductive electrode may be composed of carbon-based conductive materials such as pyrolytic graphite, carbon foam, carbon paper, or glassy carbon, taking into account properties such as allowing current to flow to the catalyst during the electrochemical reaction, supplying electrons for electron exchange with molecules, corrosion resistance, and the redox reaction potential window.

[0143] In summary, the MXene nanosheet ink for palladium recovery and method of manufacturing the same, the method of recovering palladium using MXene nanosheet ink, and the electrochemical catalyst using the recovered palladium along with the method of manufacturing the same have been described according to an embodiment of the present invention. Hereinafter, a more detailed description of the present invention will be provided through experimental examples.

Experimental Example 1: Manufacture of Mxene Nanosheet Ink

[0144] A mixture of TiC powder, Al powder, and Ti powder in a molar ratio of 2:1:1 was ball milled under a nitrogen atmosphere at 25 C. The resulting mixture was then heated at 1450 C. for 2 hours to synthesize MAX (Ti.sub.3AlC.sub.2) powder.

[0145] 1 g of Ti.sub.3AlC.sub.2 powder was added to 20 mL of HF solutions with concentrations of 5%, 15%, 25%, 35%, and 45%, respectively. The mixtures were stirred at 25 C. for 24 hours to prepare Ti.sub.3C.sub.2T.sub.x flakes. Subsequently, each Ti.sub.3C.sub.2T.sub.x flake was exfoliated using TMAOH (tetramethylammonium hydroxide) to prepare Ti.sub.3C.sub.2T.sub.x nanosheets. Specifically, Ti.sub.3C.sub.2T.sub.x flakes were introduced to a solution prepared by dissolving 10 mL of TMAOH in 50 mL of ultrapure water. The mixture was stirred at 25 C. for 48 hours. Afterward, TMAOH was removed using ultrapure water, obtaining Ti.sub.3C.sub.2T.sub.x nanosheets.

[0146] Finally, Ti.sub.3C.sub.2T.sub.x nanosheets were introduced into ultrapure water and subjected to ultrasonication to manufacture Ti.sub.3C.sub.2T.sub.x nanosheet ink, in which the Ti.sub.3C.sub.2T.sub.x nanosheets were dispersed. FIG. 2 is an actual photograph of the Ti.sub.3C.sub.2T.sub.x nanosheet ink prepared according to Experimental Example 1.

Experimental Example 2: Properties of MXene Nanosheet

[0147] The specific surface area, hydrodynamic radius (H.sub.R), and zeta potential () characteristics of the Ti.sub.3C.sub.2T.sub.x nanosheets manufactured according to Experimental Example 1 were analyzed.

[0148] The specific surface areas of MAX (Ti.sub.3AlC.sub.2) powder, Ti.sub.3C.sub.2T.sub.x flakes, and Ti.sub.3C.sub.2T.sub.x nanosheets were measured (see FIG. 3). The specific surface area of Ti.sub.3C.sub.2T.sub.x flakes (approximately 5 m.sup.2/g) showed a slight increase compared to Ti.sub.3AlC.sub.2 (approximately 3 m.sup.2/g). In contrast, the specific surface area of Ti.sub.3C.sub.2T.sub.x nanosheets was measured at approximately 25 m.sup.2/g, representing about a fivefold increase compared to Ti.sub.3C.sub.2T.sub.x flakes.

[0149] The hydrodynamic radius (H.sub.R) of Ti.sub.3C.sub.2T.sub.x nanosheets was measured based on the HF concentration (see FIG. 4). The results showed that the hydrodynamic radius (H.sub.R) was largest when a 5% HF solution was applied. This result is attributed to the insufficient HF concentration, which led to a too low surface functional group density on the MXene nanosheets, preventing proper exfoliation of the MXene nanosheets. In contrast, when 15% and 25% HF solutions were applied, the hydrodynamic radius (H.sub.R) of Ti.sub.3C.sub.2T.sub.x nanosheets was found to be the smallest, similar to each other, indicating that the exfoliation of Ti.sub.3C.sub.2T.sub.x nanosheets was carried out effectively.

[0150] Meanwhile, as the HF concentration increased beyond 25%, the hydrodynamic radius (H.sub.R) of Ti.sub.3C.sub.2T.sub.x nanosheets exhibited an exponential increase. This trend may be explained by the Ti.sub.3C.sub.2T.sub.x lattice defects. As the HF concentration increases, the reaction with MAX becomes more vigorous, leading to the formation of defective vacancies in the Ti.sub.3C.sub.2T.sub.x lattice. These defects induce the aggregation of carbon atoms, leading to the formation of an amorphous carbon structure, while also causing the oxidation of Ti atoms into TiO.sub.2. As a result, the surface functional group density and specific surface area characteristics are degraded. It can be seen that when 35% and 45% HF solutions were applied, the lattice oxidation resulted in the binding energy peak at 458.9 eV corresponding to Ti(IV) with a high area ratio. In contrast, for Ti.sub.3C.sub.2T.sub.x nanosheets with a 15% HF solution applied, the Ti(IV) peak exhibited a very small area, indicating that lattice oxidation was minimized.

[0151] The hydrodynamic radius (H.sub.R) of Ti.sub.3C.sub.2T.sub.x nanosheets also affects the palladium adsorption characteristics of Ti.sub.3C.sub.2T.sub.x nanosheets.

[0152] The zeta potential () of Ti.sub.3C.sub.2T.sub.x nanosheets was measured based on the HF concentration.

[0153] With reference to FIG. 5, in an alkaline environment, all Ti.sub.3C.sub.2T.sub.x nanosheets exhibited a negative charge regardless of the HF concentration. As the pH decreased, the potential value tended to increase due to the increased protonation of surface functional groups. For Ti.sub.3C.sub.2T.sub.x nanosheets with a 15% HF solution applied, the zeta potential () was the highest at approximately 35 mV at pH 4, and this value remained as the pH was lowered to 1.

[0154] This is attributed to differences in surface functional group density. Ti.sub.3C.sub.2T.sub.x nanosheets with a 15% HF solution applied have a lower surface functional group density compared to Ti.sub.3C.sub.2 T.sub.x nanosheets with 35% and 45% HF solutions applied. Therefore, for Ti.sub.3C.sub.2T.sub.x nanosheets with 35% and 45% HF solutions applied, a higher concentration of hydrogen ions (H.sup.+) and consequently a lower pH are required to increase the zeta potential (9), compared to Ti.sub.3C.sub.2T.sub.x nanosheets with a 15% HF solution applied. For example, when the pH decreases from 10 to 8, the zeta potential (C) of Ti.sub.3C.sub.2T.sub.x nanosheets with a 15% HF solution applied shows a significant change. In contrast, the zeta potential () of Ti.sub.3C.sub.2T.sub.x nanosheets with 35% and 45% HF solutions applied remains almost unchanged, indicating that the increased hydrogen ion (H.sup.+) concentration due to the pH reduction has little effect on the zeta potential () increase.

[0155] In conclusion, Ti.sub.3C.sub.2T.sub.x nanosheets with a 15% HF solution applied maintained the maximum zeta potential () of approximately 35 mV in the pH range of 1 to 4. This indicates that the surface functional groups of the Ti.sub.3C.sub.2T.sub.x nanosheets were saturated with hydrogen ions (H.sup.+) at pH 4. Meanwhile, Ti.sub.3C.sub.2T.sub.x nanosheets with 5% and 45% HF solutions applied exhibited relatively lower zeta potential () compared to samples with other HF concentrations. This is likely due to incomplete exfoliation of the Ti.sub.3C.sub.2T.sub.x nanosheets, lattice defects or the like.

Experimental Example 3: Palladium Recovery Characteristics of MXene Nanosheet

[0156] Ti.sub.3C.sub.2T.sub.x nanosheet ink was introduced into a palladium solution with a concentration of 500 mg/L at pH 1 to 3, and the mixture was stirred at 200 rpm. After stirring, the precipitate was filtered using a filtration membrane, and the Pd concentration was measured using ICP-OES. The concentration of Ti.sub.3C.sub.2T.sub.x nanosheets in the Ti.sub.3C.sub.2T.sub.x nanosheet ink was differently set to 0.1, 0.2, and 0.3 g/L.

[0157] The adsorption capacity at equilibrium (ge) of Ti.sub.3C.sub.2T.sub.x nanosheets was measured as 1937.3 mg/g at a Ti.sub.3C.sub.2T.sub.x nanosheet concentration of 0.1 g/L and 1932.3 mg/g at a Ti.sub.3C.sub.2T.sub.x nanosheet concentration of 0.2 g/L (see FIG. 6). When the concentration of Ti.sub.3C.sub.2T.sub.x nanosheets was 0.3 g/L, the adsorption capacity (ge) decreased significantly. This is attributed to the saturation of adsorption sites. As the pH increased from 1 to 3, the adsorption capacity of Ti.sub.3C.sub.2T.sub.x nanosheets for Pd.sup.2+ showed a decreasing trend (see FIG. 7).

[0158] For comparison, a palladium recovery experiment was conducted using carbon black (CB). Carbon black (CB) is considered one of the promising materials for Pd catalyst spacers due to its low cost, large specific surface area, and excellent electrical conductivity.

[0159] Ti.sub.3C.sub.2T.sub.x nanosheets demonstrated a 100% palladium recovery rate (Re %) for initial palladium concentrations (Ci) ranging from 0.1 to 10 mg/L. In contrast, commercial carbon black (CB) showed a maximum recovery rate (Re %) of 65.6% for an initial palladium concentration (Ci) of 10 mg/L (see FIG. 8A). Additionally, for carbon black (CB), the recovery rate (Re %) tended to decrease as the initial palladium concentration (Ci) decreased. These results are attributed to differences in the electrostatic attraction acting on the palladium-anion complexes, which are supported by the zeta potential () of Ti.sub.3C.sub.2T.sub.x nanosheets and carbon black (CB). This is because the zeta potential () of Ti.sub.3C.sub.2T.sub.x nanosheets is approximately 36.8 mV, whereas the zeta potential () of carbon black (CB) is approximately 3.1 mV.

[0160] Additionally, an analysis of the precipitation behavior of Ti.sub.3C.sub.2T.sub.x nanosheets and carbon black (CB) (see FIG. 8B) showed that the hydrodynamic radius (H.sub.R) of carbon black (CB) did not significantly increase after Pd adsorption. This is due to its reliance on electrostatic attraction that are not effective for precipitation. In contrast, for Ti.sub.3C.sub.2T.sub.x nanosheets, the hydrodynamic radius (H.sub.R) increased rapidly and stabilized within 60 minutes, forming micrometer-sized precipitates (approximately 9.5 m) that settled in the solution.

Experimental Example 4: Maximum Adsorption Capacity of MXene Nanosheet

[0161] The isothermal adsorption curve of Ti.sub.3C.sub.2T.sub.x nanosheet ink for Pd.sup.2+ was obtained, and it was confirmed that the data fit the Freundlich and Redlich-Peterson models (R.sup.20.97) better than the Langmuir model (R.sup.20.88) (see FIG. 9). These results indicate that Pd.sup.2+ is adsorbed onto Ti.sub.3C.sub.2T.sub.x nanosheets through pseudo-multilayer adsorption behavior.

[0162] The maximum adsorption capacity (q.sub.m) of Ti.sub.3C.sub.2T.sub.x nanosheets for Pd.sup.2+ was calculated to be 1983.3 mg/g. Additionally, the adsorption capacity of Ti.sub.3C.sub.2T.sub.x nanosheets reached equilibrium within 60 minutes.

[0163] The maximum adsorption capacity (q.sub.m) of Ti.sub.3C.sub.2T.sub.x nanosheets (1983.3 mg/g) and the adsorption equilibrium time (60 minutes or less) are significantly superior to the publicly-known Pd adsorbents. With reference to Table 1 below (maximum adsorption capacity (q.sub.m) and adsorption equilibrium time for each Pd adsorbent) and FIG. 10, the result is more than twice as superior compared to the polymer-based adsorbent (poly-Cys-g-PDA@GPUF, refer to Non-Patent Document 4), which is known for having the best palladium recovery properties with a maximum adsorption capacity of 785 mg/g.

[0164] The significantly superior maximum adsorption capacity (q.sub.m) and adsorption equilibrium time characteristics of Ti.sub.3C.sub.2T.sub.x nanosheets according to the present invention are attributed to the fact that Ti.sub.3C.sub.2T.sub.x nanosheets possess not only adsorption properties but also reductive properties. That is, the excellent maximum adsorption capacity (q.sub.m) and adsorption equilibrium time characteristics are exhibited due to the ability of adsorbing palladium-anion complexes through electrostatic attraction and reducing Pd.sup.2+ to Pd nanoparticles through a redox reaction between Pd.sup.2+ and the Ti.sub.3C.sub.2T.sub.x nanosheets. This is a clear distinction from conventional Pd adsorbents, which rely solely on their adsorption properties.

TABLE-US-00001 TABLE 1 Equilibrium Adsorbent q time type Adsorbent (mg g.sup.1) (min) Ref. Metal-based P -F O S O M 196 1440 [1] Adsorbents TiO.sub.2NP 12 30 [2] MOF -based Si-TpAL 48 300 [3] Adsorbents MNP-O3 3 480 [4] AHPP-MOF 284 420 [5] Fe O nanoparticle 11 180 [6] TMS 10 00 [7] (IV)-based MOF 120 25 [8] MIL-101(Cr)-NH 278 720 [9] MOF-AFH 193 180 [10] Ti T nanosheet ink 1983.3 60 This work Polymer-based PEPEI 509 1080 [11] adsorbents CD18 6E 1 180 [12] Biomass-based poly-Cys-g-PDA@ PUF 785 30 [13] adsorbents GCCR 120 120 [14] DMPAPER 224 300 [15] AMPT 2 4 120 [16] C8H2C 340 120 [17] PEIAB 136 1440 [18] TCM 42 150 [19] Carbon-based PAH-CNT 187 1 0 [20] adsorbents O 81 120 [21] MC 64 1080 [22] O-TOABr 93 30 [23] B-N-WSBP biochar 134 180 [24] indicates data missing or illegible when filed

REFERENCE

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Highly efficient and acid-resistant metal-organic frameworks of MIL-101 (Cr)NH2 for Pd (II) and Pt (IV) recovery from acidic solutions: Adsorption experiments, spectroscopic analyses, and theoretical computations. J. Hazard. Mater. 387, 121689 (2020). [0175] 10. Tang, J., Chen, Y., Wang, S., Kong, D. & Zhang, L. Highly efficient metal-organic frameworks adsorbent for Pd(II) and Au(III) recovery from solutions: Experiment and mechanism. Environ. Res. 210, 112870 (2022). [0176] 11. Bratskaya, S., Privar, Y., Ustinov, A., Azarova, Y. & Pestov, A. Recovery of Au (III), Pt (IV), and Pd (II) using pyridylethyl-containing polymers: Chitosan derivatives vs synthetic polymers. Ind. Eng. Chem. Res. 55, 10377-10385 (2016). [0177] 12. Grad, O., Ciopec, M., Negrea, A., Duteanu, N., Vlase, G., Negrea, P., Dumitrescu, C., Vlase, T. & Vod-, R. Precious metals recovery from aqueous solutions using a new adsorbent material. Sci. Rep. 11, 2016 (2011). [0178] 13. 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Experimental Example 5: Selectivity of MXene Nanosheet for Palladium Ions

[0190] In actual waste liquid including palladium, various metal ions (Mg.sup.2+, Cu.sup.2+, Ni.sup.2+, Ca.sup.2+, K.sup.+, Na.sup.+) are present, making the selective recovery of palladium critically important.

[0191] A solution was prepared in which each of Mg.sup.2+, Cu.sup.2+, Ni.sup.2+, Ca.sup.2+, K.sup.+, and Na.sup.+ had an initial concentration (Ci) of 100 mg/L, and Pd.sup.2+ had an initial concentration (Ci) of 1 mg/L. Ti.sub.3C.sub.2T.sub.x nanosheet ink (0.2 g/L) was then introduced into the solution. The pH of the solution was not adjusted and was measured to be 3.9. In addition, for comparison, Ti.sub.3C.sub.2T.sub.x nanosheets with 5%, 15%, 25%, 35%, and 45% HF solutions applied were each prepared as inks and introduced into the solution.

[0192] With reference to FIG. 11, when Ti.sub.3C.sub.2T.sub.x nanosheet ink with a 15% HF solution applied was introduced, it exhibited a palladium recovery rate of approximately 100%. Simultaneously, the recovery rates of other metal ions were analyzed to be less than 1.0%. This result demonstrates the exceptional recovery rate and selectivity for palladium ions.

[0193] When Ti.sub.3C.sub.2T.sub.x nanosheet ink with a 5% HF solution applied was introduced, it exhibited high recovery (89.9%) and selectivity. However, the recovery rate and selectivity were slightly lower compared to those achieved with the ink with a 15% HF solution applied. This result is attributed to the fact that the zeta potential () of Ti.sub.3C.sub.2T.sub.x nanosheets with a 15% HF solution applied is 35.0 mV, whereas that of Ti.sub.3C.sub.2T.sub.x nanosheets with a 5% HF solution applied is only 22.3 mV. As described above, this is due to the lower surface functional group density and incomplete exfoliation of Ti.sub.3C.sub.2T.sub.x nanosheets.

[0194] When HF solutions with a concentration of 25% or higher were applied, the surface of the Ti.sub.3C.sub.2T.sub.x nanosheets exhibited a negative charge or a low zeta potential () even under pH 3.9 conditions (35% HF solution: 13.1 mV, 25% HF solution: 3.8 mV, 45% HF solution: 8.4 mV). These zeta potential () characteristics result in low affinity for palladium-anion complexes and electrostatic attraction with coexisting metal ions. Therefore, the selectivity for palladium ions inevitably decreases. Furthermore, as previously described, lattice defects are present when 25%, 35%, and 45% HF solutions are applied.

[0195] As an additional experiment, a palladium ion recovery experiment of Ti.sub.3C.sub.2T.sub.x nanosheets was conducted on an actually used spent Pd catalyst. Pd catalysts are typically configured to be in the form of being coated on an alumina (-Al.sub.2O.sub.3) support structure (see FIG. 12). The total organic carbon (TOC) concentration in the waste liquid including the actually used spent Pd catalyst was 1.42 mg/L, the Pd.sup.2+ concentration was 10.80 mg/L, and the Al.sup.3+ concentration was 4.18 mg/L.

[0196] Ti.sub.3C.sub.2T.sub.x nanosheet ink was introduced into the waste liquid including the actually used spent Pd catalyst, resulting in the recovery of Pd.sup.2+ at 100% without the recovery of TOC or Al.sup.3+ (see FIG. 13). For comparison, carbon black (CB) was introduced as a Pd.sup.2+ adsorbent into the waste liquid including the actually used spent Pd catalyst. The results showed a low Pd.sup.2+ recovery rate, along with a high recovery rate of TOC and Al.sup.3+. These experimental results demonstrate that the Ti.sub.3C.sub.2T.sub.x nanosheet ink of the present invention is the optimal Pd.sup.2+ adsorbent for recovering palladium from actually used spent Pd catalysts because of exceptional recovery rate and selectivity, while being cost-effective to manufacture.

Experimental Example 6: Palladium Recovery Mechanism of MXene Nanosheet

[0197] As described earlier, the first to third mechanisms play a role in the palladium recovery using MXene nanosheets in the present invention. These first to third mechanisms are supported by the experimental results and analyses of Experimental Example 6.

[0198] To elucidate the palladium recovery mechanism of Ti.sub.3C.sub.2T.sub.x nanosheet ink under acidic conditions, various analyses were conducted on the palladium-adsorbed Ti.sub.3C.sub.2T.sub.x nanosheets (Pd@Ti.sub.3C.sub.2T.sub.x).

[0199] FIG. 14 illustrates the FESEM image of Pd@Ti.sub.3C.sub.2T.sub.x, revealing that a crystalline structure is uniformly covering the surface of the Ti.sub.3C.sub.2T.sub.x nanosheets. Additionally, with reference to the HRTEM image of Pd@Ti.sub.3C.sub.2T.sub.x in FIG. 15A, it can be seen that Pd.sup.2+ adsorbed on the surface of Ti.sub.3C.sub.2T.sub.x nanosheets has been reduced to palladium nanoparticles (Pd Nps). This result is consistent with the XRD analysis results (see FIG. 15B). Furthermore, the HRXPS spectrum of Pd@Ti.sub.3C.sub.2T.sub.x for Pd 3d shows that the Pd@Ti.sub.3C.sub.2T.sub.x precipitate exhibits two separate Pd 3d peaks corresponding to the Pd.sup.2+ and reduced Pd metal states with a high fraction of 82.1% (see FIG. 15C and Table 2). This indicates that most of the Pd.sup.2+ adsorbed onto the Ti.sub.3C.sub.2T.sub.x nanosheets through electrostatic attraction is converted into crystalline palladium nanoparticles. Additionally, these results contrast with those of carbon black (CB), which only exhibited peaks corresponding to Pd.sup.2+.

TABLE-US-00002 TABLE 2 Fitted Binding peak energy area Peak (eV) (%) Assignment Pd 3d 335.6/341.0 82.1 Pd(0) 337.3/343.1 17.9 Pd(III)

[0200] After Pd recovery, the HRXPS spectrum analysis of Ti.sub.3C.sub.2T.sub.x nanosheets for O 1s indicates that the surface functional group OH donates electrons to reduce Pd.sup.2+ adsorbed on the Ti.sub.3C.sub.2T.sub.x nanosheets (see FIG. 15D). Additionally, CTiOH is oxidized to CTiO and/or TiO by Pd.sup.2+. After Pd recovery, it can be seen that the HRXPS Ti 2p spectrum of Ti.sub.3C.sub.2T.sub.x nanosheets shows a significant reduction in peaks related to TiC, Ti(II), and Ti(III), while the peak corresponding to Ti(IV) increases due to redox reactions (see FIG. 15E).

[0201] Meanwhile, the HRXPS spectrum of Ti.sub.3C.sub.2T.sub.x nanosheets for F 1s indicates that the surface functional group F primarily exists in the form of TiF (see FIG. 15F). Due to its very high electronegativity, F may not directly contribute to Pd recovery. However, fluorine (F) atoms adjacent to oxygen atoms polarize the oxygen atoms, increasing the overall polarity of the Ti.sub.3C.sub.2T.sub.x nanosheets. This induces the active reduction of Pd.sup.2+. As a result, after Pd recovery, the peaks corresponding to TiF and CF at 684.3 eV and 685.7 eV, respectively, shifted to higher binding energies of 684.7 eV (+0.4 eV) and 685.9 eV (+0.2 eV). From the above, it can be inferred that the electron transfer from fluorine to oxygen induces an active redox reaction between Pd.sup.2+ and Ti.sub.3C.sub.2T.sub.x nanosheets.

[0202] In summary, under acidic conditions, the palladium-anion complex ([PdCl.sub.4].sup.2) is adsorbed onto positively charged Ti.sub.3C.sub.2T.sub.x nanosheets through electrostatic attraction (see the left schematic view in FIG. 15G). The Pd.sup.2+ adsorbed on Ti.sub.3C.sub.2T.sub.x nanosheets is easily reduced to palladium nanoparticles (Pd NPs) because the reduction potential of Pd.sup.2+ is greater than that of Ti.sub.3C.sub.2T.sub.x nanosheets (see the middle schematic view in FIG. 15G). As such, adsorption and reduction occur across multiple Ti.sub.3C.sub.2T.sub.x nanosheets, leading to the formation of Pd@Ti.sub.3C.sub.2T.sub.x precipitates (see the right schematic view in FIG. 15G). This facilitates easy recovery of the palladium.

Experimental Example 7: Regeneration of Palladium-Adsorbed MXene Nanosheet

[0203] It was confirmed that palladium can be desorbed from Pd@Ti.sub.3C.sub.2T.sub.x nanosheets, allowing the Ti.sub.3C.sub.2T.sub.x nanosheet ink to be reused. Thiourea, an environmentally friendly, cost-effective eluent with excellent kinetics and selectivity, was applied to elute the adsorbed palladium.

[0204] By mixing 0.3 M thiourea with 0.5 M HCl and introducing Pd@Ti.sub.3C.sub.2T.sub.x nanosheets, it was confirmed that approximately 99.7% of the palladium was desorbed (see Table 3). The palladium-desorbed Ti.sub.3C.sub.2T.sub.x nanosheet ink was used in palladium adsorption experiments, followed by repeated desorption of palladium. It was confirmed that the palladium recovery rate gradually decreased as the adsorption-desorption cycles were repeated (see FIG. 16). After repeating the adsorption-desorption cycle 10 times, the palladium recovery rate decreased by approximately 13%, indicating a reduction of approximately 1.3% in palladium recovery per cycle. In contrast, for carbon black (CB), the palladium recovery rate decreased by approximately 4.4% per adsorption-desorption cycle.

TABLE-US-00003 TABLE 3 Desorption Eluting Molarity efficiency agent (M) (%) Acidic thiourea 0.1 93.2 (in 0.5M HCl) 0.2 96.9 0.3 99.1 0.4 99.7

[0205] Meanwhile, the palladium desorbed from Pd@Ti.sub.3C.sub.2T.sub.x nanosheets could be ultimately recovered by simply evaporating the solution. The palladium purity was found to be 99.9% (see FIG. 17).

Experimental Example 8: Electrocatalytic Properties of Pd@ Ti.SUB.3.C.SUB.2.Ty for Hydrogen Evolution Reaction

[0206] The palladium solution concentration during palladium recovery with MXene nanosheets described in Experimental Example 3 was adjusted from 10 to 500 mg/L at pH 1 to 3. This resulted in the formation of Pd/NM (nanosheet MXene) complexes with Pd weight ratios of 5 to 62 wt % adjusted. The properties of these complexes as electrochemical catalysts for hydrogen evolution reaction (HER) were then analyzed.

[0207] To analyze the catalyst properties, a process was performed to prepare catalyst ink followed by coating onto an electrode structure. The catalyst ink was prepared by mixing 10 mg of Pd/NM, 20 L of Nafion solution, and 1 mL of an ultrapure water and ethanol mixed solution. The prepared catalyst ink was coated onto a glassy carbon-based electrode using the drop casting method, manufacturing the Pd/NM catalyst working electrode.

[0208] In a 0.1 M HClO.sub.4 electrolyte, a three-electrode system was configured using the Pd/NM catalyst working electrode, a glassy carbon counter electrode, and an Ag/AgCl reference electrode. The magnitude of current density and overpotential were measured using linear sweep voltammetry (LSV). The results confirmed that the HER catalytic efficiency of Pd/NM was superior to that of the Pd complex adsorbed on carbon black (Pd/CB) (see FIG. 18).

[0209] The HER overpotential and noble metal mass-specific activity of the manufactured Pd/NM catalysts were compared based on the Pd weight (see FIG. 19). The mass activity (jm), which represents noble metal mass-specific activity, was calculated by dividing the current density at 0.065 V vs. RHE by the Pd weight. It was confirmed that as the Pd weight ratio increased, the overpotential decreased, as well as the HER mass activity (jm) improved.

[0210] Additionally, HER Tafel slopes and electrochemical impedance analysis graphs for the manufactured Pd/NM catalysts were measured. It was confirmed that as the Pd weight ratio increased, the Tafel slope values decreased, indicating a trend of a faster HER reaction rate. The Pd/NM catalyst with a Pd weight ratio of 57 wt % (57-Pd/NM) exhibited the lowest value of 59 mV/dec (see FIG. 20). Electrochemical impedance analysis results indicated that as the Pd weight ratio increased, the amount of reduced Pd nanoparticles between Ti.sub.3C.sub.2T.sub.x nanosheets also increased, providing a larger active area, resulting in lower charge transfer resistance (Rct) (see FIG. 21).

[0211] To measure the durability of the Pd/NM catalyst, constant current tests were performed for 72 hours at a constant current density of 10 mA/cm.sup.2. The Pd/NM catalyst with a Pd weight ratio of 57 wt % (57-Pd/NM) maintained a stable voltage without significant voltage increase over 72 hours. These results, together with TEM images of Ti.sub.3C.sub.2T.sub.x nanosheets and reduced Pd nanoparticles remaining stable after 72 h constant current test, indicate the stability of the Pd/NM catalyst in HER reaction (see FIG. 22).

[0212] To evaluate and compare the HER characteristics of the manufactured Pd/NM catalyst and Ti.sub.3C.sub.2T.sub.x nanosheets that recovered Pd from actual wastewater, the HER characteristics of Ti.sub.3C.sub.2T.sub.x nanosheets with Pd respectively recovered from a palladium solution with the same Pd concentration (500 ppm) and from the waste liquid including the spent Pd catalyst used in Experimental Example 5, were compared using linear sweep voltammetry (LSV) (see FIG. 23). Due to the selective recovery performance of MXene, which adsorbs only Pd ions, it was confirmed that when manufactured into a catalyst, both solutions exhibited equivalent hydrogen evolution reaction characteristics.