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Article

Catalytic Role of Nickel in Hydrogen Storage and Release Using Dibenzyltoluene as a Liquid Organic Hydrogen Carrier

by
Jesús Rodríguez Ruiz
,
Nuria García-Mancha
,
Roberto Campana
and
Carlos Tardío
*
Centro Nacional Del Hidrógeno, Prolongación Fernando El Santo, s/n, 13500 Puertollano, Spain
*
Author to whom correspondence should be addressed.
Energies 2025, 18(16), 4429; https://doi.org/10.3390/en18164429
Submission received: 14 July 2025 / Revised: 5 August 2025 / Accepted: 12 August 2025 / Published: 20 August 2025
(This article belongs to the Special Issue Advanced Energy Storage Technologies)
Browse Figures
Versions Notes

Abstract

Liquid Organic Hydrogen Carriers (LOHCs) represent a promising technology for the safe storage and transport of hydrogen. Its technical development largely depends on the catalysts used in the hydrogenation and dehydrogenation processes. Typically, noble metal-based monometallic catalysts are employed, although they present limitations in terms of cost and availability. This study uses the DBT system to explore the potential of nickel (Ni) as a catalytic alternative. In dehydrogenation, its role as an additive in low-loaded Pt-based catalysts (0.25 wt%) was evaluated, showing a significant increase in activity, with dehydrogenation levels exceeding 95%, compared to 82% obtained with monometallic Pt catalysts. This improvement is attributed to the formation of Pt-Ni alloys. On the other hand, although the bimetallic systems were not effective in hydrogenation, a commercial Ni/Al2O3-SiO2 catalyst was tested, achieving hydrogenation degrees of 80% in just 40 min, after pressure and catalyst loading optimization. These results position Ni as a key component in LOHC catalysis, either as an effective additive in Pt-based systems or as an active metal itself, due to its excellent performance and low cost. This paves the way for economically viable and efficient catalytic solutions for hydrogen storage applications, bridging the gap between performance and practicality.

1. Introduction

Currently, our society is immersed in an energy and environmental crisis as a consequence of the use of fossil fuels. The ongoing emissions of CO2 and other greenhouse gases have led to global warming and a drastic climate change [1,2]. In this context, an energy transition toward more sustainable energy production methods, such as wind or solar energy, is essential [3]. However, the intermittent and fluctuating nature of the renewable energy sources [4], as well as their uneven distribution across different regions of the planet, necessitates the development of efficient and effective storage and transport systems to ensure the utilization and distribution of the clean energy generated [5].
Hydrogen has emerged as a promising energy carrier, capable of storing clean energy generated from renewable energies and facilitating its transport and use in different industrial and mobility sectors [6,7,8]. However, the use of hydrogen presents some critical issues, especially regarding safety and efficient storage. Current technologies, such as compressed hydrogen, cryogenic hydrogen, or solid materials, require high energy costs and involve risks associated with gas handling [9,10].
In this context, Liquid Organic Hydrogen Carriers (LOHCs) have emerged as an attractive hydrogen storage method. LOHCs are compounds capable of absorbing and releasing hydrogen through reversible chemical reactions of hydrogenation and dehydrogenation [11,12,13]. This technique offers high gravimetric hydrogen storage capacity, compatibility with the existing fossil fuels infrastructures, and a low production cost [14].
Several aromatic compounds have been described as LOHCs, such as N-ethylcarbazole [15], toluene [16], benzyltoluene [17], and dibenzyltoluene [18]. Among them, dibenzytoluene (DBT) has received great attention because of its high thermal and chemical stability, low toxicity, and high hydrogen storage density (~6.2 wt%) [19,20,21].
Noble metal-based catalysts are required for C–H bond formation and cleavage in the hydrogenation and dehydrogenation reactions, respectively. Pt is the most employed in dehydrogenation reactions, due to its strong capacity to active C–H bonds [22,23,24], following by Pd [25,26]. In contrast, hydrogenation reactions offer more diversity in terms of metals, such as Rh, Ru in addition to Pt and Pd [5,27,28,29]. However, in all cases, the high cost and low availability are significant limitations that must be addressed.
Different alternatives have been proposed to reduce catalyst costs and improve the catalytic activity. For example, in the dehydrogenation reaction, the addition of secondary metals, such as Ni, Mg, Zn, Co, Mo, or Mn, has reported a positive impact on the process, enhancing the catalytic performance and productivity [30,31,32,33]. Conversely, non-noble metal-based catalysts have been tested in hydrogenation reactions, showing similar catalytic activities to those of noble metal-based catalyst [34,35]. In this regard, nickel (Ni) has attracted growing attention due to its versatility as a catalytic metal. Thus, it has been employed in steam methane reforming, hydrogen production from ammonia, alkaline electrolysis and hydrogenation reactions (including LOHC) [32,36,37,38,39].
Hence, in this work, we investigated the potential role of Ni in the development of new catalysts for dehydrogenation and hydrogenation of the DBT system. The results showed that Ni emerges as a key element in advancing LOHC catalysis, offering the potential to enable more efficient and economically viable catalytic systems.

2. Materials and Methods

2.1. Materials

H18-DBT and H0-DBT were supplied by Solutia Europe SPRL, Ghent, Belgium. Al2O3 support, Ni(NO3)2·6H2O (99%) and 66 wt% Ni/Al2O3-SiO2 were purchased from Thermo-Fisher Scientific Int, Madrid, Spain. H2PtCl6·6H2O (37.5 wt% of Pt) was bought from Merck KGaA, Darmstadt, Germany. All gases were supplied by Air Products and Chemicals.

2.2. Experimental Setup

The experimental setup employed for the dehydrogenation reactions was described in our previous work [22]. All dehydrogenation reactions were performed with 20 mL of H18-DBT and 6.10 g of catalyst at 290 °C for 2.5 h.
Hydrogenation experiments were carried out in a 600 mL stainless-steel Parr batch autoclave (Type 4568, Parr Instrument Company, Allentown, PA, USA), coupled with a Parr high-pressure gas burette filled with H2 to record gas consumption during the reaction (controlled by a pressure transmitter). Stirring and reaction temperature were controlled by a Parr Controller (Type 4848). The mixture was heated with an electric heating jacket.
After loading with H0-DBT (100 mL) and catalyst, the reactor was purged with N2, heated, and pressurized (to the pressure and temperature established for each catalyst). The reaction started when the stirrer speed was increased to 950 rpm. This stirring rate was selected to ensure complete homogenization of the reaction mixture and to minimize mass transfer limitations, based on our previous studies. During the experiment, the reaction pressure was maintained constant by continuously dosing hydrogen from the burette.
The degree of dehydrogenation (DoD) was determined using Equation (1), where gH2,released represents the amount of hydrogen released during the reaction, and gH2,max is the total mass of hydrogen in the H18-DBT initially placed in the reactor. Similarly, the catalytic activity was evaluated by calculating the productivity (P) (Equation (2)), where gH2 is the mass of hydrogen released, gPt is the mass of active metal (Equation (3)), and t is the reaction time.
D o D % = g H 2 , r e l e a s e d g H 2 , m a x · 100
P = g H 2 g P t · t
g P t = T o t a l   c a t a l y s t   m a s s   g ·   0.25 100
In the hydrogenation reaction, the total H2 consumption was determined from the pressure drop observed in the gas burette during the reaction. The degree of hydrogenation (DoH) was calculated using Equation (4), where nmax corresponds to the maximum amount of hydrogen storable in the amount of H0-DBT placed into the reactor.
D o H   ( % ) = Δ P · V R · T n m a x · 100

2.3. Catalyst Preparation

Monometallic Pt/Al2O3 and bimetallic Pt-Nix/Al2O3 catalysts, with a fixed Pt loading (0.25 wt %) and variable Ni loading (x = 0.1, 0.25, and 0.5 wt %), were synthesized via wetness impregnation. H2PtCl6·6H2O and Ni(NO3)2·6H2O were employed as metallic precursors for Pt and Ni, respectively. The required quantities of each precursor were calculated to prepare 10 g of catalyst. Thus, 0.066 g of H2PtCl6·6H2O was used to achieve 0.25 wt % Pt loadings. For Ni loadings of 0.1, 0.25 and 0.5 wt %, 0.049 g, 0.124 g, and 0.247 g of Ni(NO3)2·6H2O were used, respectively.
The precursor(s) was/were dissolved in 30 mL of deionized water. Then, Al2O3 support was added to the solution and stirred for 30 min. No pH adjustment was applied. Then, the mixture was dried in a vacuum oven at 110 °C for 15 h. Next, the sample was calcined in a muffle furnace for 3 h at the same calcination temperatures as those employed by Alconada et al. [40]. After completing the impregnation process, the catalysts were reduced before being tested in the corresponding reaction using a stream of H2 at 400 °C for 45 min.

2.4. Characterization

Textural properties were derived from the N2 adsorption–desorption isotherms at −196 °C using a Micromeritics 3Flex Instrument. Prior to analyses, samples were degassed at 350 °C for 3 h.
Metal loading was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) using an Agilent 5100 Instrument (Agilent Technologies Inc., Santa Clara, CA, USA). Samples were previously digested in an acid solution (mixture of HCl, 35%, and HNO3) using an Ethos Up Milestone microwave microwave (Milestone Srl, Sorisole, BG, Italy).
H2-TPR analyses were carried out using a Micromeritics 3Flex Instrument equipped with a Thermal Conductivity Detector (TCD) (Micromeritics Instrument Corp., Norcross, GA, USA). Calcined samples were pre-treated in situ (100 °C, 30 min) with Ar to eliminate impurities. Afterward, the samples were cooled down to 50 °C and again heated to 950 °C (10 K/min) with 10% v/v H2/Ar gas flow.
The same instrument was employed to perform H2-pulse chemisorption analysis. The samples were reduced under H2 stream at 400 °C for 1 h, then purged with Ar at the same temperature for 1 h, and finally cooled down to 35 °C. Small pulses of H2 (10% H2/Ar v/v) were sent to the reactor with intervals between each pulse of 1.5 min.
The gas phase was analyzed using an Agilent Gas Chromatography (490 Micro-GC QUAD) coupled to a low-dead-volume micro thermal conductivity detector. The capillary analytical column Poraplot Q 10 m was used at 40 °C. Hydrogen was used as the carrier gas. The GC injection temperature was 50 °C, with an injection time of 200 ms.

3. Results and Discussion

3.1. Influence of the Impregnation Order on Catalytic Activity

Given the presence of both metals in the Pt-Ni catalysts, we initially aimed to assess whether the sequence of Pt and Ni impregnation on the support has a significant impact on the catalytic behaviour of the system. Therefore, as a representative bimetallic system, the Pt-Ni0.25/Al2O3 catalyst was synthesized using the three possible impregnation routes:
  • Simultaneous impregnation—(denoted Pt-Nisimul); the corresponding amounts of Pt and Ni precursors were dissolved together in a single aqueous solution, into which the support was added. The resulting mixture was then dried, and the material was calcined (450 °C).
  • Sequential impregnation—Pt followed by Ni (denoted Pt-Niseq); initially, only the Pt precursor was dissolved and impregnated onto the support. After calcination (450 °C), the material was then introduced into another aqueous solution of the Ni precursor, followed by a second drying and calcination step (500 °C).
  • Sequential impregnation—Ni followed by Pt (denoted Ni-Ptseq); this procedure is analogous to the previous one, with the order of metal impregnation reversed.
The study of the synthesized catalysts by ICP-OES revealed that the Ni and Pt loadings were consistent with the desired values, allowing us to conclude that any differences in catalytic performance among the samples are not due to variations in metal loading (Table 1).
The catalysts were tested in the dehydrogenation of H18-DBT (Figure 1). The catalytic performance observed is strongly influenced by the sequence of metal impregnation. The catalyst synthesized by the Ni-Ptseq method exhibited the highest catalytic activity, reaching a dehydrogenation degree of 96.5% in 130 min. The Pt-Niseq method also allowed to obtain a catalyst with good catalytic performance, achieving a dehydrogenation degree of 91% in 150 min. In both cases, the dehydrogenation curves tend to level off during the final stages of the reaction, indicating the completion of the dehydrogenation process without reaching full dehydrogenation, possibly due to catalytic deactivation phenomena. This effect was not observed with the catalyst synthesized using the Pt-NiSimul method, as the dehydrogenation curve continues to rise beyond 150 min of reaction time, reaching the maximum dehydrogenation level (86%) at 185 min of reaction.
To explain these results, the catalysts obtained by each of the impregnation methods were studied by H2-pulse chemisorption (Table 1). The amount of chemisorbed H2 and the metal surface area follow the order: Ni-Ptseq > Pt-Niseq > Pt-Nisimul. In contrast, metallic particle size follows the inverse order. These results support the observed trend in catalytic activity as a function of the impregnation method, since these parameters are directly related to the number of active metal sites; therefore, higher chemisorbed H2 values correspond to greater catalytic activity.
Moreover, this trend aligns with previous studies highlighting the critical role of impregnation order in bimetallic systems [41]. Shu et al. demonstrated that impregnating Ni before Pt on γ-Al2O3 results in stronger Pt–Ni interactions, leading to improved hydrogenation activity [42]. Similarly, Zhang et al. [43] and Naicker et al. [44] observed higher metal dispersion and greater active site accessibility in sequentially impregnated catalysts. Betti et al. [45] further support this view, showing that the order of metal addition alters the surface acidity and metal distribution, which are key factors governing catalytic behaviour.
The improved catalytic performance observed for the Ni–Ptseq configuration, in comparison to the Pt–Niseq and Pt–Nisimul strategies, highlights the relevance of metal deposition order in bimetallic catalyst preparation. While this effect is consistently observed, its mechanistic origin remains unclear. Several factors may contribute, such as differences in alloying extent, metal dispersion, or the possible formation of interfacial sites; however, a definitive understanding requires more advanced characterization and kinetic studies.

3.2. Catalyst Characterization

The monometallic Pt/Al2O3 and bimetallic Pt–Nix/Al2O3 catalysts, synthesized using the Ni–Ptseq impregnation method, were characterized. ICP-OES analysis confirmed that the metal content in each catalyst was close to the nominal value, validating the effectiveness of the established impregnation method (Table 2).
The N2-adsorption–desorption isotherms showed, in all cases, a Type IV shape and a typical H1-type hysteresis loop, confirming the mesoporous structure of the catalysts (Figure S1). Upon incorporation of Ni into the Pt/Al2O3 system, a gradual decrease in surface area and pore volume was observed with increasing Ni loading. Specifically, the surface area decreased from 265.35 m2/g for Pt/Al2O3 to 250.14 m2/g for Pt-Ni0.5/Al2O3. A similar trend was observed in the pore volume, which decreased slightly from 0.842 to 0.809 cm3/g. The average pore diameter also exhibited a marginal decline, from 12.94 nm to 12.57 nm. Overall, the results suggest that the incorporation of Ni modifies the porous structure of the catalyst in a concentration-dependent manner. While the observed reductions in textural properties are relatively modest, they may influence metal dispersion and reactant accessibility (Table 2).
H2-TPR profiles of the catalysts reveal distinct reduction features associated with the different metal species present and their interactions with the support and each other (Figure 2). The Pt/Al2O3 catalyst presented two peaks: The first, centred around 225 °C, is assigned to the reduction in oxidized Pt species (PtOx) highly dispersed on the alumina surface. The second peak, appearing around 360 °C, is attributed to PtOx species more strongly bound to the alumina support, possibly forming surface complexes or interacting with Lewis acid sites of alumina [46].
Upon the introduction of Ni into the catalyst, the first peak remains unchanged. However, the second peak shifts progressively to higher temperatures, moving up to ~416 °C at 0.5 wt% Ni. This shift is attributed to the formation of Pt–Ni alloys, which are known to be more thermally stable and more difficult to reduce than monometallic Pt oxides. The alloying effect modifies the electronic environment of both Pt and Ni, increasing the reduction temperature as the Ni content increases and the degree of alloying becomes more significant [47].
A third peak, only observed in the bimetallic Pt–Ni catalysts and located between 782 °C and 893 °C, was associated with the reduction in NiO species strongly interacting with the alumina support, likely in the form of highly dispersed or embedded Ni2+ ions. As the Ni content increases, this peak becomes more intense and shifts to lower temperatures, indicating that Ni species become less stabilized by the support, possibly due to particle agglomeration or saturation of high-energy anchoring sites [40,48]. In addition, the presence of Pt may facilitate the reduction in NiO via a hydrogen spillover mechanism. In this process, H2 molecules dissociate on Pt sites, and atomic hydrogen migrates to adjacent NiO regions, enhancing their reducibility [49,50].
H2-TPR results were complemented by H2-pulse chemisorption analysis. As can be seen in Table 3, a progressive decrease in metal dispersion is observed with increasing Ni content, from 77.24% in the monometallic Pt/Al2O3 catalyst to 52.78% in Pt-Ni0.5/Al2O3. This trend could be attributed to the formation of Pt–Ni bimetallic particles, in which Ni0 is incorporated into the metallic structure, modifying the morphology and reducing the surface fraction of exposed Pt sites. This hypothesis requires further validation through XPS analysis to confirm the presence of Ni0. In addition, the particle size increases from 1.46 nm to 2.14 nm, consistent with enhanced sintering or particle growth, likely promoted by the presence of metallic Ni.
Moreover, the amount of chemisorbed H2 per gram of catalyst and per mole of Pt also decreased with higher Ni loading. This reduction indicates a lower number of metallic sites accessible for hydrogen adsorption, which can be attributed to a decrease in the metallic area exposed to the reaction, as well as to a weakening of the interaction between H2 and the active centres, as a consequence of the Pt–Ni alloy. Overall, these results confirm the formation of Pt–Ni alloys in the bimetallic catalysts, consistent with observations from H2-TPR analysis.

3.3. Catalytic Activity in the Dehydrogenation of H18-DBT

Monometallic Pt and bimetallic Pt–Ni catalysts were tested in dehydrogenation reactions. As shown in Figure 3a, the monometallic catalyst achieved a DoD of 82% after 150 min of reaction. This catalytic behaviour is similar to that of a commercial Pt/Al2O3 catalyst with 0.5 wt% Pt previously reported by our group [22], although only half that Pt content is employed in this case. Therefore, this new Pt catalyst offers a clear economic advantage by delivering comparable results at a lower cost. This result was notably improved by the bimetallic catalysts, which exhibited values ranging from 92% to 96%, depending on the Ni loading.
Interestingly, increasing the Ni content did not lead to a significant enhancement in dehydrogenation performance or productivity, as the maximum dehydrogenation levels for all bimetallic formulations were reached within similar reaction times (120 min approximately). These findings are also reflected in the productivity values, which remained relatively stable regardless of the Ni loading, suggesting that beyond a certain threshold, the contribution of additional Ni does not proportionally translate into improved catalytic efficiency.
Likewise, it is striking that in the first minutes of the reaction, both the monometallic catalyst and the bimetallic catalysts report similar productivity, which explains why the dehydrogenation curves appear practically overlapped at the beginning of the reaction, until after 30 min of reaction, the activity of the monometallic catalyst begins to decline (Figure 3b).
The experimental results appear to contradict the measured values of active metallic area, as the dehydrogenation activity improves despite a decrease in this parameter with increasing Ni content in the bimetallic Pt-Nix/Al2O3 catalysts (Table 3). This behaviour suggests that catalytic performance is not governed solely by the exposed metal surface area, but also by significant electronic and structural factors.
In particular, the enhanced activity observed at higher Ni loadings can be attributed to the formation of Pt–Ni alloys, which induce electronic modifications in the Pt sites. Consequently, the Pt d-band centre shifts downward, thereby weakening the interaction between dissociated hydrogen species (H*) and the Pt surface, and promoting their desorption—an essential step in the dehydrogenation process [51,52].
Additionally, H* generated on active metal can migrate via a hydrogen spillover mechanism to NiO species, possibly through interaction with -OH groups on the support, where it is stabilized and released, thereby preventing site blockage on Pt and enhancing reaction turnover [30].
Gas chromatography was employed to assess the purity of the hydrogen stream generated in each reaction (Table S1). As shown, the presence of Ni in the catalytic system does not significantly affect the purity of the released gas, which remains comparable to that obtained with the monometallic catalyst and exceeds 99.99% in all cases.

3.4. Catalytic Activity in the Hydrogenation of H0-DBT

Given the excellent catalytic activity exhibited by the catalysts, particularly the bimetallic ones, in dehydrogenation reactions, we proceeded to evaluate their performance in the hydrogenation of H0-DBT. The experiments were carried out using 2.10 g of catalyst, at a pressure of 30 bar and a temperature of 290 °C. As shown in Figure 4, the monometallic catalyst achieves a DoH of 93% after 150 min of reaction. However, the bimetallic catalysts displayed an undesired effect: the presence of Ni in the catalyst led to a significant decrease in hydrogenation efficiency compared to the monometallic catalyst, with similar values observed among them, around 70%. Notably, increasing the Ni loading led to a further decline in the reaction rate, as reflected by the initial slopes of the reaction profiles.
Once again, the differences in catalytic activity observed between the monometallic and bimetallic catalysts can be attributed to electronic and structural modifications induced by the presence of Ni. From an electronic perspective, alloying Pt with Ni shifts the Pt d-band centre downward, which weakens the metal’s ability to activate and dissociate H2 and reduces the adsorption strength of H0-DBT on the metal surface, since H0-DBT adsorption involves π–d interactions between the aromatic rings and the d orbitals of Pt [53,54]. This leads to diminished catalytic activity at the Pt sites, subsequently reducing the overall reaction rate.
In addition, part of the Ni in the catalyst is present as NiO, a species that is catalytically inactive for the hydrogenation step. However, NiO may partially block or cover Pt active sites, thereby decreasing the number of accessible catalytic centres. This could explain why increasing the Ni loading leads to a further decline in hydrogenation rate.
In light of these results, one possible route for improving catalyst performance in hydrogenation reactions could involve achieving the complete reduction in Ni species to metallic Ni0, ensuring that it contributes as an additional active phase alongside Pt. To accomplish this, it would be necessary to increase the reduction temperature [55]. However, it is well known that higher reduction temperatures can promote sintering of Pt particles, leading to a loss of dispersion and an increase in particle size, decreasing the active metallic area [56].
This trade-off underscores the inherent challenge in developing a dual-function catalyst—defined as a system capable of efficiently facilitating both dehydrogenation and hydrogenation—due to the distinct mechanistic demands of each reaction.

3.5. Noble Metal-Free Catalyst Alternative for Hydrogenation of H0-DBT

Although the monometallic catalyst demonstrated excellent activity in the hydrogenation of H0-DBT, the underwhelming performance of the bimetallic systems prompted us to explore an alternative. Our continued goal of cost reduction naturally steered us toward developing systems free of noble metals. In this context, Ni-based catalytic systems have attracted considerable interest due to their promising behaviour in hydrogenation reactions involving LOHCs [57,58,59]. Specifically, their application in the hydrogenation of H0-DBT has also been explored, with reports indicating high conversions and reaction efficiencies [60,61].
In supported catalysts, the nature of the support plays a key role not only in dispersing the active metal but also in enhancing reaction mechanisms. Al2O3 is a typical support for Ni-based catalysts, with a predominance of Lewis acid sites, which favours the adsorption of electron-rich aromatic molecules. However, it lacks significant Brønsted acidity. A successful strategy to introduce Brønsted acid sites, which facilitate H2 spillover by promoting the migration of dissociated hydrogen from the metal, is to incorporate SiO2 into the alumina matrix or surface. In that way, the formation of Si–O–Si and Si–O–Al bonds can eliminate vacancies and reduce the creation of low-reducing NiAl2O4 spinel structures. The incorporation of mixed Al2O3–SiO2 supports in Ni-based systems boosts catalytic activity by providing both Brønsted and Lewis acid sites, while simultaneously enhancing nickel reducibility and catalyst stability [34,61].
In this context, we employed the commercially available 66 wt % Ni/Al2O3–SiO2 catalyst for H0-DBT hydrogenation. Initially, the reaction conditions used to test the catalyst activity were practically the same as those reported by Ding [61] in a previous study: 150 °C, 65 bar, and 10 g of catalyst, corresponding to a mass ratio of 10 wt % relative to the reactant. Unlike the referenced study, our work did not employ any solvent, aiming to assess the reaction’s feasibility under solvent-free conditions, a common approach in Pt-based hydrogenation processes. This strategy promotes greater environmental sustainability and economic viability in industrial applications.
As shown in Figure 5a, 95% DoH was achieved after 40 min of reaction. This demonstrates that the Ni-based catalyst exhibited a catalytic activity markedly superior to that previously reported for Pt-based catalysts, further supporting the suitability of Ni as a non-noble metal alternative in hydrogenation reactions. Motivated by this outstanding performance, we proceeded to evaluate the effect of reducing the catalyst loading, specifically to 5 wt % and 1 wt %.
Surprisingly, the reduction in catalyst loading to 5 wt % did not compromise the performance observed previously; on the contrary, it led to an improvement, with a comparable hydrogenation degree being reached after only 30 min of reaction. The improved performance observed could be attributed to better dispersion of the catalyst particles, diminished diffusion limitations, or changes in the reaction kinetics due to the altered catalyst concentration. Nonetheless, these explanations remain speculative and would require further investigation to be confirmed. In contrast, the system with 1 wt % exhibits a slow and linear increase in DoH, reaching only ~25% after 40 min, indicating a strong dependence of reaction kinetics on catalyst availability.
To further optimize the H0-DBT hydrogenation, the effect of pressure was investigated while maintaining a constant mass ratio of 5 wt% (Figure 5b). The results show that high pressures enhance both the hydrogenation rate and the final DoH). However, even when the pressure was reduced from 65 bar to 35 bar (nearly a 50% decrease), the DoH remained relatively high, achieving ~80% after 40 min. This indicates that, although higher pressures improve reaction kinetics, the system maintains considerable activity even at lower hydrogen pressures. These findings suggest that efficient hydrogenation can still be achieved under milder conditions, offering potential benefits in lowering operational expenses and enhancing safety, without significantly sacrificing performance.
The results obtained pave the way for the development and synthesis of novel Ni-based catalysts, capitalizing on nickel’s favourable economic and catalytic properties. Future studies will focus on exploring key variables such as the nature of the support and the potential incorporation of other, preferably non-noble, metals to further enhance performance and sustainability.

4. Conclusions

In this work, we developed and tested a Pt/Al2O3 monometallic catalyst and a series of Pt-Ni/Al2O3 bimetallic catalysts for H2 transport and storage using LOHC technology.
The introduction of Ni into the Pt-based catalyst significantly enhanced its performance in the dehydrogenation reaction, reaching hydrogen release levels of up to 95%. This improvement is attributed to two synergistic effects: (i) the formation of a Pt–Ni alloy, which facilitates the desorption of hydrogen atoms from the catalyst surface, and (ii) the presence of Ni oxides, which promote hydrogen spillover, further enhancing dehydrogenation efficiency.
Conversely, during hydrogenation reactions, the Pt-Ni bimetallic catalysts showed lower activity compared to the excellent performance reported by the monometallic Pt catalyst. This behaviour is likely due to a weakening of the H2 adsorption strength on Pt sites caused by alloying with Ni, which alters the electronic structure and reduces the ability of Pt to activate molecular hydrogen.
In parallel, a Ni-based catalyst supported on alumina–silica was also evaluated and demonstrated excellent performance in the hydrogenation of H0-DBT, achieving conversion levels of approximately 80%, under lower Ni loading and significantly milder conditions than those typically required for Ni-based systems.
These results underscore the key role that Ni can play both as a promoter in noble metal-based catalytic systems and as a promising alternative to these critical metals. Owing to its cost-effectiveness and high catalytic efficiency, Ni stands out as a valuable component in the design of next-generation catalysts. Its application can significantly contribute to the development of more sustainable and economically viable LOHC technologies, fully aligned with the goals of energy transition and low-carbon energy storage.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en18164429/s1. Figure S1: N2 adsorption–desorption isotherms of the synthesized catalysts; Table S1: H2 purity during dehydrogenation reactions for each synthesized catalyst.

Author Contributions

Conceptualization, C.T., J.R.R. and R.C.; methodology, C.T. and N.G.-M.; validation, C.T. and N.G.-M.; formal analysis, C.T., J.R.R. and R.C.; investigation, C.T.; resources, R.C.; data curation, C.T.; writing—original draft preparation, C.T.; writing—review and editing, J.R.R. and R.C.; supervision, J.R.R. and R.C.; project administration, J.R.R.; funding acquisition, R.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was co-funded by Center for Industrial Technological Development (CDTI) and European funds from the Recovery and Resilience Mechanism, in the call for 2021 of the MISIONES Program, within the framework of the State Program to Catalyze Innovation and Business Leadership, of the State Plan for Scientific, Technical and Innovation Research 2021–2023, with the support of the Ministry of Science and Innovation, under project GreenH2Pipes (Generation of hydrogen (H2) by electrolysis, its injection into the natural gas network and its transport from liquid carriers) with file number MIG-20211014 and Complementary Renewable Energy and Hydrogen Plan of the Recovery, Transformation and Resilience Plan funded by the European Union—Next Generation-EU (grant number C17.I01.P01).

Data Availability Statement

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

Acknowledgments

The authors would like to express their sincere gratitude to Cristina Esteban for her supervision, continuous support, and oversight throughout the project, and to Exolum Solution S.L. for its essential contribution to both funding and implementing the research project. The authors also thank Cristina Alonso and Cristina Ballester for their valuable help in reviewing the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Influence of metal impregnation order on the performance of Pt–Ni0.25/Al2O3 catalysts in the dehydrogenation of H18-DBT. Reaction conditions: T = 290 °C; P = 1 bar; Catalyst mass = 6.10 g.
Figure 1. Influence of metal impregnation order on the performance of Pt–Ni0.25/Al2O3 catalysts in the dehydrogenation of H18-DBT. Reaction conditions: T = 290 °C; P = 1 bar; Catalyst mass = 6.10 g.
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Figure 2. H2-TPR profiles of Pt–Al2O3 and Pt-Nix/Al2O3 catalysts.
Figure 2. H2-TPR profiles of Pt–Al2O3 and Pt-Nix/Al2O3 catalysts.
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Figure 3. (a) Evolution of DoD with the different synthesized catalysts. Reaction conditions: T = 290 °C; P = 1 bar; Catalyst mass = 6.10 g. (b) Initial productivity values. Error bars indicate the standard deviation obtained from three replicate measurements (n = 3).
Figure 3. (a) Evolution of DoD with the different synthesized catalysts. Reaction conditions: T = 290 °C; P = 1 bar; Catalyst mass = 6.10 g. (b) Initial productivity values. Error bars indicate the standard deviation obtained from three replicate measurements (n = 3).
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Figure 4. Evolution of DoH with the different synthesized catalysts. Reaction conditions: T = 290 °C; P = 30 bar; Catalyst mass = 2.10 g.
Figure 4. Evolution of DoH with the different synthesized catalysts. Reaction conditions: T = 290 °C; P = 30 bar; Catalyst mass = 2.10 g.
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Figure 5. Conversion rate of H0-DBT hydrogenation over Ni/Al2O3–SiO2 catalyst (a) at varying catalyst loadings (relative to H0-DBT mass). Reaction conditions: T = 150 °C; P = 65 bar and (b) at different pressures. Reaction conditions: T = 150 °C; Catalyst mass ratio of 5 wt % (relative to H0-DBT).
Figure 5. Conversion rate of H0-DBT hydrogenation over Ni/Al2O3–SiO2 catalyst (a) at varying catalyst loadings (relative to H0-DBT mass). Reaction conditions: T = 150 °C; P = 65 bar and (b) at different pressures. Reaction conditions: T = 150 °C; Catalyst mass ratio of 5 wt % (relative to H0-DBT).
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Table 1. Metal content and H2-pulse chemisorption results.
Table 1. Metal content and H2-pulse chemisorption results.
Impregnation MethodMetal Content (Pt/Ni) (wt %) aMetallic Area (m2/gPt) bParticle Metal Size (nm) bH2 Chemisorption (µmol/µmolPt) b
Pt-Nisimul0.23/0.251162.010.24
Pt-Niseq0.23/0.201371.730.28
Ni-Ptseq0.26/0.271551.510.32
a Determined by ICP-OES. b Determined by H2-pulse chemisorption.
Table 2. Metal content, BET surface area, pore volume, and average pore size of the catalysts prepared.
Table 2. Metal content, BET surface area, pore volume, and average pore size of the catalysts prepared.
CatalystMetal Content (Pt/Ni) (wt %) aBET Surface Area (m2/g) bPore Volume (cm3/g)Pore Size (nm)
Pt/Al2O30.22/-2650.84212.94
Pt-Ni0.1/Al2O30.25/0.132630.83312.88
Pt-Ni0.25/Al2O30.27/0.292570.80812.87
Pt-Ni0.5/Al2O30.23/0.472500.80912.57
a Determined by ICP-OES. b Determined by N2 adsorption–desorption isotherm measurement.
Table 3. H2-pulse chemisorption results of the synthesized catalysts.
Table 3. H2-pulse chemisorption results of the synthesized catalysts.
CatalystDispersion (%)Metallic AreaParticle Size (nm)H2 Chemisorbed
(m2/gsample)(m2/gPt)(µmol/gcat)(µmol/molPt)
Pt/Al2O377.240.48190.761.464.950.38
Pt-Ni0.1/Al2O373.080.45180.471.544.680.37
Pt-Ni0.25/Al2O362.630.39154.681.804.020.31
Pt-Ni0.5/Al2O352.780.32130.652.143.380.26
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Ruiz, J.R.; García-Mancha, N.; Campana, R.; Tardío, C. Catalytic Role of Nickel in Hydrogen Storage and Release Using Dibenzyltoluene as a Liquid Organic Hydrogen Carrier. Energies 2025, 18, 4429. https://doi.org/10.3390/en18164429

AMA Style

Ruiz JR, García-Mancha N, Campana R, Tardío C. Catalytic Role of Nickel in Hydrogen Storage and Release Using Dibenzyltoluene as a Liquid Organic Hydrogen Carrier. Energies. 2025; 18(16):4429. https://doi.org/10.3390/en18164429

Chicago/Turabian Style

Ruiz, Jesús Rodríguez, Nuria García-Mancha, Roberto Campana, and Carlos Tardío. 2025. "Catalytic Role of Nickel in Hydrogen Storage and Release Using Dibenzyltoluene as a Liquid Organic Hydrogen Carrier" Energies 18, no. 16: 4429. https://doi.org/10.3390/en18164429

APA Style

Ruiz, J. R., García-Mancha, N., Campana, R., & Tardío, C. (2025). Catalytic Role of Nickel in Hydrogen Storage and Release Using Dibenzyltoluene as a Liquid Organic Hydrogen Carrier. Energies, 18(16), 4429. https://doi.org/10.3390/en18164429

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