Palladium (0) nanoparticles distributed on lanthanum (III) oxide as an effective catalyst for the methanolysis of hydrazine-borane to produce hydrogen

Pd (0) nanoparticles (NPs) distributed on lanthanum (III) oxide were ex situ generated from the reduction of Pd2+ ions using NaBH4 as reducing agent. The Pd/La2O3 displayed good catalytic activity in H2(g) releasing from the hydrazine-borane (HB) methanolysis reaction and it was identified by advanced techniques. Pd/La2O3 was found to be an active catalyst procuring three equiv. H2(g) per mole of HB. The results from TEM images represent the formation of Pd (0) NPs with an average particle size of 1.94 ± 0.1 nm on the surface of La2O3. Moreover, Pd/La2O3 with various Pd loadings were prepared and tested as catalyst in the methanolysis reaction to find the optimum metal loading on La2O3 support. The highest H2 formation rate was achieved with 3.0 wt% Pd. Pd/La2O3 catalyst exhibited a turnover frequency (TOF) value of 24.4 mol H2 mol Pd−1 min−1 in the reaction conditions. Additionally, the effect of different catalyst concentrations and temperatures on the reaction kinetics for the methanolysis of HB catalyzed by Pd/La2O3.


Palladium (0) nanoparticles distributed on lanthanum (III) oxide as an effective catalyst for the methanolysis of hydrazine-borane to produce hydrogen
Hydrogen can be stored in various forms, including high-pressure gaseous, liquid, metal, or chemical hydrides.Storing hydrogen as a gas in high-pressure vessels or as a liquid at cryogenic temperatures poses serious environmental, safety, and cost issues.These challenges have spurred increased research and applications related to materials capable of storing hydrogen in the solid phase.Materials such as carbon nanotubes [22][23][24], metal hydrides and amine boranes [25][26], nanomaterials [27], and organometallic structures [28] are among those explored.Among these materials, amineboranes and their derivatives have come to the forefront due to their stable structures, high hydrogen content per unit volume, being economical and environmentally friendly and have been the subject of many studies [29][30][31][32][33][34][35].Within these derivative compounds, hydrazine-borane (N 2 H 4 BH 3 , HB) has captured researchers' attention as a promising boron-based compound, boasting high hydrogen content (15.4 wt%) with four protic (N-H) hydrogens compared to three hydrogen (B-H) hydrides [36][37][38][39][40][41][42][43][44].Research on HB has revealed its advantages, such as being solid at room temperature, water solubility, environmental nontoxicity, and stability against spontaneous hydrolysis, similar to other amine-boranes.Additionally, it has been specifically identified to possess a high gravimetric hydrogen density.Hydrogen stored in HB can be released through pyrolysis [45], hydrolysis [46] (1), and methanolysis [47] (2) reactions.However, the most promising approach for hydrogen production from HB involves complete dehydrogenation, wherein the BH 3 group undergoes hydrolysis, selectively separating N 2 H 4 into N 2 and H 2 (3).Nevertheless, to maximize the efficiency of HB as a hydrogen storage material, undesired decomposition of N 2 H 4 into NH 3 and N 2 (4) should be avoided.This undesired catalytic decomposition not only reduces the produced amount of H 2 but also generates NH 3 , which can have a toxic effect on fuel cells.Studies have shown that these reaction pathways largely depend on catalyst design and reaction conditions.In this context, using a basic catalyst support and operating in a highly alkaline solution environment selectively advances the catalytic reaction in the desired direction [48][49].
Two main problems encountered during the hydrolysis of amine-borane (AB) are that the hydrogen stored in the NH 3 group within the structure cannot be completely liberated under hydrolysis conditions, and NH 3 can have a poisoning effect on the catalyst at high substrate concentrations.On the other hand, these problems encountered during the hydrolysis of AB are not experienced in the hydrogen production from HB.In addition to considering the mechanistic properties of the catalytic reaction, it is possible to find out the hydrogens in the N 2 H 4 group in the structure of HB in the specifically conducted hydrogen generation processes.In these selectively carried out reactions, dehydrogenation of 1 mole of N 2 H 4 BH 3 yields H 2 and N 2 in a mole ratio of 5:1.Due to the aforementioned advantages, studies aiming to produce hydrogen from HB through both hydrolysis and methanolysis have gained attention.The hydrolysis reaction can be conducted under relatively milder conditions, yielding satisfactory hydrogen evolution efficiency.However, there is an issue related to the inherent instability of HB against self-hydrolysis.In the alcoholysis reaction of HB, it has been observed that HB is more stable in methanol and does not undergo self-methanolysis.This suggests that effective hydrogen evolution from HB can be achieved in the presence of a suitable catalyst [50][51][52].
Research aimed at developing hydrogen production processes from HB via methanolysis has highlighted the indispensable need for suitable nanocatalysts to achieve fast, efficient, and controllable reaction environments.Nanocatalysts possess distinct physical and chemical properties compared to metal ingots, making them of great interest in recent years [53][54][55][56][57][58][59][60].Metal nanocatalysts can show high efficiency and selectivity in catalytic reactions where metal ingots are not active.The main reason for the high catalytic efficiency they show in the reactions is the increase in the number of catalytically effective atoms on their surfaces despite the reduction in particle sizes [61][62][63][64].
Despite the positive properties mentioned above, metal nanoparticles (NPs) are kinetically unstable against the formation of metal nuggets and aggregation due to their high surface energies.Such agglomerations that will occur in the catalyst system can shorten the catalyst's lifespan and reduce reaction efficiency.To prevent this undesirable situation, in other words, to keep the nanocatalysts colloidally stable in the solution environment, suitable stabilizing groups have been used.Carbonbased materials, metal oxides, metal-organic lattice structures, polymers, and zeolites are among these groups [65][66][67][68].
The key parameters in HB dehydrogenation include the presence of a selective and active catalyst capable of decomposing both the BH 3 and N 2 H 4 groups within the HB structure, as well as the development of support systems to prevent catalyst aggregation.The objective is to create efficient catalyst systems by combining rhodium, ruthenium, iridium, nickel, platinum, palladium with metals such as cobalt and copper, along with distinctive support supplies [69][70][71][72][73][74][75][76][77][78][79].
Rare earth elements (REE) are widely recognized for their unique chemical structures and exceptional catalytic, magnetic, and electronic properties, making them valuable in various industries and biotechnological applications.Lanthanum oxide (La 2 O 3 ), among REE oxides, finds applications in sensors, electronic and luminescent devices, fuel cells, magnetic data storage, antimicrobial agents, catalysis, automobiles, water treatment, phosphate removal, and biomedical research [80][81][82][83].With a bandgap ranging from 4.3 eV to 5.8 eV, La 2 O 3 is a semiconductor material with the largest bandgap among REE oxides.It crystallizes in a hexagonal system structure with the P3m1 space group and has a low photon energy of about 400 cm -1 , making it an excellent host matrix when doped with metals or other metal oxides [84][85][86].
In our study, we prepared La 2 O 3 -supported palladium nanoparticles (Pd NPs)-Pd/La 2 O 3 -for the methanolysis-based dehydrogenation of HB, and assessed the efficiency of the catalyst under mild reaction conditions.Characterization using powder X-ray diffraction (P-XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), SEMelemental mapping, transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and inductively coupled plasma optical emission spectroscopy (ICP-OES) confirmed that Pd (0) NPs were well spread on the surface of La 2 O 3 .Pd/ La 2 O 3 exhibited good catalytic activity in the methanolysis of HB, with an initial turnover frequency (TOF) of 24.4 mol H 2 mol Pd -1 min -1 at 25 ± 0.1 °C, marking it as the second heterogeneous catalytic system ever reported for HB methanolysis.Furthermore, stability and durability experiments revealed that the Pd/La 2 O 3 catalyst maintained its catalytic efficiency and activity to a great extent even after the 5th cycle, highlighting its high reusability and stability as a heterogeneous catalyst for HB methanolysis.

Synthesis of La 2 O 3 supported Pd (0) NPs
To synthesize the Pd/La 2 O 3 catalyst, we first prepared a 5-mL solution containing 3% by weight Pd metal (11.22 mg, 42.1 µmol) using Pd(NO 3 ) 2 •2H 2 O salt.To this solution prepared using deionized water, 150 mg of La 2 O 3 was added and the mixture was stirred at 750 rpm for 3 h.After stirring, an excess amount of NaBH 4 was added dropwise to reduce Pd(II) to Pd(0), (with an estimated metal/NaBH 4 molar ratio of 15).Once the bubbling completely ceased, indicating complete reduction of Pd(II) to Pd(0), the resulting product was filtered and washed with deionized water (3 × 20 mL) for purification.Finally, the Pd/La 2 O 3 catalyst obtained from these processes was dried under appropriate conditions (Scheme).

Catalytic activity in the HB methanolysis
The main method used to assess the catalytic efficiency of Pd/La 2 O 3 in the methanolysis of HB is to measure the gas formation rate.For this purpose, it was attempted to determine the catalytic efficiency by volumetrically measuring gas formation rate in different parameters.A graduated glass cylinder, resembling a gas burette, was used to determine the volume of H 2 gas produced during HB methanolysis.This involved periodically monitoring the displacement of water in the graduated cylinder due to the released H 2 gas [87].Temperature regulation was achieved by circulating water in the system, where a 50 mL single-necked reaction balloon was positioned on a magnetic stirrer.Gas volume released was determined by measuring the volumetric displacement of water in the graduated cylinder, placed on a jacketed Schlenk, designed for temperature regulation.The necessary graphs were generated accordingly.Following the experimental setup, 100 mg (25.8 µmol, 4.69 mM) of Pd/La 2 O 3 catalyst was placed on the Schlenk apparatus set at 298 K, and then 4 mL of dried methanol was added.The mouth of the jacketed Schlenk, whose temperature was controlled by means of a circulator, was sealed to prevent gas escape, and the system was stirred for about 15 min until it reached equilibrium.Subsequently, 46 mg of HB dissolved in 1 mL of dried methanol was injected into the experimental setup, maintaining a fixed stirring speed of 750 rpm.Finally, the volume (mL) of gas released due to HB methanolysis was recorded against time (min), and the corresponding graphs were plotted.
Scheme.Schematic presentation of the synthesis of Pd/La 2 O 3 catalyst.
The methanol used in the experimental studies was dried through the following procedure: Methanol was dried by heating on iodine-activated magnesium with a magnesium loading of 0.5-5.0g/L [88].Small pieces of magnesium metal were introduced into methanol and refluxed for 2-3 h under a nitrogen atmosphere.The dry methanol was then cooled to room temperature and stored in an inert environment.

Determination of most effective Pd loading for Pd/La 2 O 3
Various Pd-loaded samples (1.0%-4.0%by weight) were prepared to determine the most effective Pd loading for Pd/La 2 O 3 .These samples, each weighing 100 mg, were tested for H 2 gas release from the methanolysis of HB (46 mg, 200 mM HB) at 298 K.The highest H 2 formation rate was achieved for the catalyst sample containing 3.0 wt% Pd.Subsequently, the catalyst sample with 3.0 wt% Pd loading was used for further catalytic reactions.

Durability experiments
To assess the catalytic reusability and durability of the Pd/La 2 O 3 catalyst in the methanolysis of HB at room temperature, a specific amount of the catalyst was used to prepare a 5-mL solution (100 mg, 25.8 µmol, 4.69 mM).This solution, along with 200 mM HB, was employed in a series of experiments.After each experiment, once the targeted reaction was complete, an equal amount of HB was reintroduced to the reaction medium.The durability data for the Pd/La 2 O 3 catalyst in the methanolysis of HB was recorded as a percentage.

Results and discussion
P-XRD, XPS, TEM, HRTEM, SEM, and ICP-OES analyses were conducted on the Pd/La 2 O 3 catalyst to assess its structural and morphological properties.Initially, ICP-OES analysis was utilized to determine the distribution of Pd metal content on La 2 O 3 , revealing a Pd content of 2.7 ± 0.1 wt%.It is important to note that the theoretical metal content of the Pd/La 2 O 3 catalyst was designed to contain 3.0 wt% Pd.
SEM analysis was performed to examine the distribution of Pd NPs on the La 2 O 3 surface, which served as a support, and to characterize the surface morphology (Figures 1a-1c).To achieve the desired image quality, the sample of the Pd/La 2 O 3 catalyst was coated with gold for approximately 100 s.SEM images and corresponding elemental mapping of Pd/La 2 O 3 confirmed the presence of La, O, and Pd elements and the uniform dispersion of Pd NPs within the Pd/La 2 O 3 catalyst.On the other hand, Pd exhibited a homogeneous distribution similar to other elements but with a lower density.This is also associated with the contribution of Pd (0) NPs at lower concentrations compared to other elements.As confirmed by the SEM elemental mapping in the obtained SEM images, it is noticed that Pd NPs were extremely uniformly distributed on La 2 O 3 surface (Figures 1d-1f).
XPS analysis was employed to identify the elements present in the Pd/La 2 O 3 catalyst and elucidate its surface characteristics.The survey analysis of the Pd/La 2 O 3 catalyst in Figure 3a revealed the presence of the Pd element along with the framework elements of La 2 O 3 .High-resolution spectrum of Pd 3d bands displayed two prominent peaks at 334.4 and 339.9 eV corresponding to metallic Pd 3d 5/2 and Pd 3d 3/2 , respectively [32,64,91].This indicates reduction of Pd 2+ ions used as a precursor and the existence of metallic Pd in the Pd (0) form within the catalyst sample.Additionally, small peaks observed at 342.4 eV and 336.2 eV were associated with Pd-O 3d 3/2 and Pd-O 3d 5/2 bonds, respectively [92].
TEM images of the Pd/La 2 O 3 catalyst with 3.0 wt% Pd loading are presented in Figure 4.These images reveal that Pd (0) NPs are uniformly distributed on the surface of La 2 O 3 at various magnifications.A particle size histogram was generated by counting more than 100 nontouching particles, determining the mean diameter of Pd (0) NPs as 1.94 ± 0.1 nm.The TEM-EDX spectrum of the Pd/La 2 O 3 catalyst (2.7 ± 0.1 wt% Pd) in Figure 5c confirms the presence of La, O, and Pd as the only elements, consistent with XPS analysis in Figure 3a.Additionally, the HRTEM image of the Pd/La 2 O 3 catalyst indicates a d-spacing of 0.21 nm corresponding to the Pd (111) lattice plane (Figure 4d) [32,64,91].
Before conducting catalytic activity tests and detailed kinetic studies, we conducted two significant preliminary investigations.Firstly, the catalytic nature of La 2 O 3 and Pd/La 2 O 3 materials in the target catalytic reaction was compared under the same conditions to determine the net reactivity of Pd (0) nanoparticles in the methanolysis of HB.At the end of the    experiment, it was determined that the metal-free La 2 O 3 was not catalytically active in this catalytic conversion (Figure S1).Secondly, Pd-loaded La 2 O 3 samples containing varying percentages of Pd were prepared, and their catalytic activities were tested in the methanolysis of HB. Figure 5 illustrates the H 2 evolution graph from HB methanolysis catalyzed by Pd/La 2 O 3 with different Pd loading ranging from 1.0% to 4.0 wt% Pd.Upon addition of the HB substrate, H 2 production commenced without an observed induction time and continued until the gas equivalent reached 3.0 in the catalytic methanolysis reaction.The sample with 3.0 wt% Pd loading was established as the most active catalyst for H 2 generation from HB methanolysis.The H 2 generation rate (TOF) of the catalyst was calculated as 24.4 mol H 2 mol Pd -1 min -1 for Pd/La 2 O 3 with 3.0 wt% Pd.Higher palladium loadings resulted in a decrease in the surface area of the supporting material and failure to reach active sites of the catalyst.Therefore, Pd/La 2 O 3 samples with a 3.0 wt% Pd ratio were selected as the optimum ratio for all other experiments.
Figure 6 illustrates the graphs of H 2 gas volume (mL) versus H 2 gas time (min) released when Pd/La 2 O 3 is utilized as a catalyst in the methanolysis of 200 mM HB at 298 K. Based on the reaction data employing catalysts at varying concentrations, the Pd/La 2 O 3 catalyst demonstrated outstanding catalytic activity across all reactions.Upon examining the graphs, it is evident that the hydrogen formation processes start without any induction period at all concentrations and continue until all HB in the solution is converted into products, indicating an almost linear methanolysis process of HB.Drawing a graph of Pd concentration versus the rate of hydrogen formation on a logarithmic scale reveals a correlation (see inset of Figure 6), showing that the HB methanolysis process using the Pd/La 2 O 3 catalyst yields a straight line with a slope of approximately 2.0 concerning the Pd/La 2 O 3 catalyst concentration.
A significant outcome of the study is the achievement of complete conversion of HB methanolysis within 5 min at 298 K using 2.7 ± 0.1 wt% Pd.The TOF (turnover frequency) value for the Pd/La 2 O 3 catalyst in HB methanolysis was calculated as 24.4 mol H 2 mol Pd -1 min -1 at 298 K, representing an important result among catalytic systems employed in HB methanolysis, as summarized in Table .Additionally, the Pd/La 2 O 3 catalyst represents the second heterogeneous catalytic system used in HB methanolysis.
To assess the effect of HB concentration on the formation rate of H 2 gas released during catalytic methanolysis, catalytic studies were conducted using various HB concentrations at a constant catalyst concentration ([Pd] = 4.69 mM) and temperature (298 K).The volume of H 2 gas released in the methanolysis reactions initiated with different HB concentrations was measured at regular intervals, and a volume (mL)/time (min) graph was generated, as shown in Figure 7. Upon examination of the graph, the effect of HB concentration on the reaction's initial rate appears to be rather limited.The graph of the initial concentration of HB versus the hydrogen production rate yields a line with a slope of ≈0.8 (inset of Figure 7), indicating that HB methanolysis catalyzed by Pd/La 2 O 3 proceeds kinetically by half-order with HB concentration.
So far, the reaction mechanism of HB methanolysis has not been studied extensively, similar to the AB methanolysis reaction, which has been rarely explored in the literature [95][96][97].In this context, it is estimated that the reaction mechanism of HB methanolysis in the presence of the Pd/La 2 O 3 catalyst can be elucidated as follows, drawing parallels with AB:   Table .Comparing thermodynamic parameters and activities of catalysts used in the formation of H 2 as a result of HB methanolysis.A similar process repeats until the other two H 2 molecules are produced (Figure 8) [96][97].
In catalytic methanolysis reactions with an HB concentration of 200 mM and a Pd/La 2 O 3 catalyst amount of 100 mg (4.69 mM), the effect of temperature changes (288, 298, 308, and 318 K) on the H 2 formation rate was measured at regular intervals.The volume/time graph obtained is shown in Figure 9. Upon examination of the obtained graph, it is observed that catalytic methanolysis is completed rapidly, even at temperatures below room temperature, and the H 2 formation rate changes in direct proportion to the temperature.For the methanolysis of HB catalyzed by the Pd/La 2 O 3 catalyst, activation parameters such as E a , ΔH # , and ΔS # were calculated from temperature-dependent graphs drawn using Arrhenius and Eyring kinetic equations (Figures S2 and S3).The slope of the linear part of each hydrogen generation versus time plot at different temperatures indicates the reaction rate constant kobs of the methanolysis reaction.While the Arrhenius curve (Figure 9, inset) was used for the activation energy (E a ), which was found to be 53.66 kJ/mol (Figure S2), the Eyring curve was used to calculate the activation enthalpy (ΔH # = 51 kJ/mol) and activation entropy (ΔS # = 48.5 J/mol×K) values (Figure S3).This E a value is the lowest calculated Ea for HB methanolysis compared to the catalytic systems summarized in Table.After detailed kinetic studies, the recyclability performance of the Pd/La 2 O 3 catalyst in hydrogen production from HB methanolysis was investigated (Figure 10).The study aimed to determine the catalytic recycling durability of the prepared Pd/La 2 O 3 catalyst, when all of the HB was converted to the product, more HB was added to the solution medium and the catalyst efficiency in repeated catalytic reactions was determined.At the end of the methanolysis reactions continued for up to 5 cycles, it was determined that the Pd/La 2 O 3 catalyst exhibited extremely high stability and durability, preserving 81% of the initial yield and achieving approximately 99% conversion even in the 5th cycle.Some reduction in catalytic activity can be attributed to the closure of the catalyst's active surfaces by methanolysis reaction products or to the increase in particle size or aggregation of Pd (0) NPs.This is also evident in the TEM image of the product isolated as a result of the 5th catalytic cycle, with clusters circled in Figure 10b.

Conclusions
The prepared Pd/La 2 O 3 catalyst yielded significant results in the hydrogen production from HB methanolysis: (i) Using an easy and reproducible impregnation-reduction technique, we synthesized Pd(0) NPs supported on La 2 O 3 ; (ii) To elucidate the structural and morphological characteristics of Pd/La 2 O 3 catalyst, we employed SEM, SEM-Elemental Mapping, XRD, XPS, ICP-OES, TEM, HRTEM and TEM-EDX techniques; (iii) Compared with the initial turnover frequency of the second heterogeneous system used for the HB methanolysis, the Pd/La 2 O 3 catalyst exhibits a catalytic efficiency of 24.4 min -1 at 298 K; (iv) The properties of Pd/La 2 O 3 , characterized by high recyclability and excellent resistance to aggregation, have been verified.The catalyst demonstrates remarkable stability during catalytic recycles, retaining 81% of the initial yield and exceeding 99% in the 5th recycling; (v) It has been demonstrated that the methanolysis of HB in the presence of the Pd/La 2 O 3 catalyst follows second and ≈first-order kinetics with respect to Pd and HB concentrations, respectively.This conclusion is drawn from detailed kinetic studies that are dependent on Pd/La 2 O 3 catalyst and HB concentrations.For the HB methanolysis catalyzed by the Pd/La 2 O 3 catalyst, temperature-dependent kinetic studies were conducted, leading to anticipated values for the activation energy (E a ), enthalpy of activation (ΔH # ), and entropy of activation (ΔS # ) parameters (E a = 53.66kJ/mol, ΔH # = 51 kJ/ mol, ΔS # = 48.5 J/mol×K).Notably, the calculated E a value is the lowest reported for HB methanolysis compared to other catalytic systems in the literature.

Figure 1 .
Figure 1.SEM image at different scales (a-c), along with corresponding elemental mapping La (d), O (e), and Pd (f) on the surface of the Pd/La 2 O 3 catalyst.

Figure 3 .
Figure 3. Survey scan (a) and Pd 3d core level XPS spectra of Pd/La 2 O 3 catalyst.
(i) adsorption and activation of methanol on Pd NPs on La 2 O 3 , facilitating the absorption of HB molecules; (ii) scission of O-H bond in CH 3 OH to produce H* and *OCH 3 ; (iii) attack of OCH 3 on the B atom in HB, leading to the formation of

Figure 6 .
Figure 6.Graphs of the released gas volume vs. time for 200 mM HB methanolysis in different concentrations of Pd/La 2 O 3 catalyst ([Pd] = 3.52, 4.69, 5.87 and 7.04 mM) at 298 K. Inset: Plot of rate of the hydrogen generation vs. palladium concentration (in logaritmic scale).

Figure 7 .
Figure 7. Graphs of released gas volume vs. time for HB methanolysis in different concentrations of HB ([HB] = 100, 150, 200, and 250 mM) at 298 K. Inset: Plot of rate of hydrogen generation vs. concentration of HB (in logaritmic scale).

N 2 H 4 BH 2 -
OCH 3 and another H; (iv) combination of two H to form one H 2 molecule, which then exits the catalyst surface.

Figure 9 .
Figure 9. Graphs of the gas volume vs. time for the methanolysis of HB starting with 200 mM HB plus 100.0 mg Pd/La 2 O 3 catalyst (4.69 mM) at different temperatures (288, 298, 308, and 318 K).

Figure 10 .
Figure 10.(a) Time plots of catalytic methanolysis of HB by up to 5 recycle for the Pd/La 2 O 3 catalyst at 298 K and, (b) TEM image of Pd/ La 2 O 3 catalyst obtained after the 5th catalytic recycle.

Figure S1 .
Figure S1.Graphs of released volume of gas vs. time for 200 mM HB methanolysis with La 2 O 3 and Pd/La 2 O 3 catalyst at 298 K.