Precious metal catalysts are widely used in the field of heterogeneous catalysis in general and the dehydrogenation process for liquid organic hydrogen carrier (LOHC) in particular. However, their catalytic activity and selectivity are difficult to improve simultaneously due to the characteristics of transition metals. This study developed a Pt2.5Cu0.1/Al2O3-H2 catalyst that can break the negative correlation between catalytic activity and selectivity, improving its common performance and reducing costs. This method achieved highly dispersed nanoparticles (NPs) and co-localization to form the unique PtCux alloy with reduced Pt electron density by anchoring very little Cu doped Pt based alumina catalyst. And it also suppressed strong metal support interaction (SMSI), as confirmed by characterization results such as XPS and HRTEM. These made it exhibit excellent bimetallic synergistic catalytic dehydrogenation activity and selectivity in perhydromonobenzyltoluene (12H-MBT), compared to Pt2.6/Al2O3-H2. The reaction energy barrier of 12H-MBT dehydrogenation was measured to be relative low (~ 94 kJ/mol), and the rate determined step of the whole catalytic dehydrogenation was also identified to be 4H-MBT → MBT.
Article
Alumina supported highly dispersed platinum-copper nanocatalyst with good dehydrogenation performance for perhydromonobenzyltoluene as a hydrogen carrier
https://doi.org/10.21203/rs.3.rs-5279341/v1
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Al2O3 supported Pt-Cu catalyst
dehydrogenation
reaction kinetics and mechanism
perhydromonobenzyltoluene
synergistic catalysis
With the increasing emission requirements of countries around the world and the global energy crisis, it is urgent to explore renewable and environmentally friendly clean energy. Hydrogen energy is believed to have significant contribution in the future energy system1,2. Safe and effective storage is one of the most critical factors restricting the development of efficient hydrogen utilization. Various hydrogen storage technologies have been widely studied, such as high-pressure gaseous hydrogen storage, low-temperature liquid hydrogen storage and solid material hydrogen storage technology. However, they still have their own problem in the long distance and large scale hydrogen transportation3, and the way based on liquid organic hydrogen carrier (LOHC) can be a hopeful key to this problem. With LOHC-based method, reversible catalytic hydrogenation/dehydrogenation reactions of oil-like compounds can be carried out under mild conditions,4 endowing the advantages high hydrogen storage capacity, good safety, and compatibility with existing fuel infrastructure5. At present, various LOHCs have been extensively studied, such as cyclohexane, methylcyclohexane (MCH), and dibenzyltoluene (DBT), etc. These have relatively complete hydrogenation preparation technologies and have basically achieved industrialization. However, the important influencing factors are still the slow hydrogen evolution kinetics rate, high dehydrogenation temperature, and low selectivity6–8. Monobenzyltoluene (MBT), having lower viscosity than DBT, has huge potential in hydrogen storage application. In addition to the large theoretical hydrogen storage capacity (6.17wt%), it also has the characteristics of suitable dehydrogenation temperature (below 563K), low volatility, safety and non-toxicity9.
The key issue to improve the competitiveness of MBT-based hydrogen storage technique is to solve the problems of high dehydrogenation temperature, side reactions, poor stability. So far, precious metal catalysts such as Pt, Pd, and Ru have been confirmed to be effective in the dehydrogenation process of common aromatic compounds10–12. Especially Pt-based catalysts have good potential in improving the dehydrogenation performance of 12H-MBT. For example, Kwak et al.13 investigated the dehydrogenation performance of different hydrogen carriers (MCH, 12H-MBT, and 18H-DBT) on a 0.5 wt%-Pt/γ-Al2O3 catalyst at 280–340 ℃. It was found that the dehydrogenation rate of 12H-MBT at 270 ℃ was only about 52%, while the cost would greatly increase if high loaded precious metals were used. To solve the above problems, appropriate approaches can be adding a second metal component, changing the support and the preparation process. In terms of support replacement, the Wasserscheid research group14 compared the performance of Pt catalysts using C, Al2O3, and SiO2 as supports in 12H-MBT and 18H-DBT systems. They found that the dehydrogenation rate of Pt/C (5 wt%) for the former was 87% at 270 ℃. However, for 12H-MBT, there are relatively few other catalyst improvement methods available for reference, which can be learned from other hydrogen carriers. For example, Guo et al.15 improved the anti-coking ability of Cu-Pt alloy formed by doping Cu in Pt/S-1, resulting in a conversion rate of 92.26% for MCH dehydrogenation. Wang et al.16 prepared bimetallic PdCu/r-GO catalysts loaded with reduced graphene oxide in different ratios. The Pd1.2Cu/rGO catalyst achieved a 100% selectivity for the final dehydrogenation product of N-ethylcarbazole at 453 K. The amount of Pd was reduced by more than 60% compared to typical commercial and else-where reported catalysts. Shi et al.17 also investigated the effect of Pt/Al2O3 with surface hydroxyl groups and surface oxygen vacancies obtained by plasma treatment on the reversible hydrogenation and dehydrogenation reactivity of 18H-DBT. Corma et al.18 compared the effect of Pt/NaY zeolite catalyst samples with different metal dispersions obtained on the dehydrogenation efficiency of MCH. The purpose was to explore the relationship between catalyst structure and catalytic activity. So far, the kinetics and mechanisms of cyclic hydrogenation and dehydrogenation of common aromatic compounds such as MCH, 12H-NEC, and 18H-DBT have also been relatively well deliberated19–21. For 12H-MBT, hydrogenation technology is quite mature, and catalytic hydrogenation mechanism is also rather well understood22–23. However, apart from relatively few explorations on catalytic performance, there is also a lack of detailed research on the mechanism of catalytic dehydrogenation. This is a challenge in hydrogen storage technology for this system-dehydrogenation process.
Aiming at overcoming these problems, this study employed a preparation strategy of anchoring and confinement on low loading Pt based alumina catalysts modified with a small amount of Cu. It achieved nanoscale highly dispersed metal particles, formed unique PtCux alloy and suppressed strong metal support interaction (SMSI)24. This strategy involved further calcination and fixation of Pt-Cu precursor. For 12H-MBT, this catalyst had excellent bimetallic synergistic effect on dehydrogenation activity and product selectivity, which was superior to the previously reported precious metal catalyst15. The structure of the target catalyst was systematically characterized by tools like high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectronic spectroscopy (XPS). The reaction mechanism, in particular the reaction determining step of the 12H-MBT dehydrogenation are discussed based on the characterization and kinetic results.
Catalyst characterization of basic physiochemical properties
As shown in Fig. 1, the purpose of carrying out a small amount of Cu doping modification on Pt/Al2O3 catalyst was to achieve the low-cost catalyst with high dispersion and small particle size. It could have the bimetallic synergistic effect and weakened SMSI to some extent. Extensive analytical methods were conducted in order to examine the differences between Pt2.5Cu0.1/Al2O3-H2 catalyst with alloy synergistic effect, pure Pt and other Pt-Cu bimetallic catalysts. It further investigated the substantive reasons for the synergistic effect between Pt and Cu. The characterization results of the basic physical and chemical properties of the prepared catalyst were included in Figure S1 and Tables S1~S2 of the first part of Supplementary Information. The characterization results closely related to the bimetallic synergistic effect are shown below.
Characterization on the metal component properties
XPS spectroscopy was used to analyze the chemical state changes of Pt-Cu catalysts and pure Pt catalysts with different preparation methods. The XPS spectra of Pt 4f and Cu 2p (Fig. 2a-b) showed that Pt mainly existed in a metallic state on catalyst. The asymmetric peak orbital energy spectra with binding energies ranging from 70.0 to 70.3 eV and 74.1 to 74.9 eV correspond to Pt 4f7/2 and Pt 4f5/2, separately. The Pt 4f5/2 binding energy (74.8 eV) of Pt atoms in Pt2.5Cu0.1/Al2O3-H2 catalyst was slightly higher than that of Pt atoms in Pt2.6/Al2O3-H2 catalyst (74.6 eV)25. And This was also higher than that of Pt atoms in Pt2.5Cu0.1/Al2O3-MR catalyst (74.7 eV). These indicated that the addition of Cu contributed to partial electron transfer of Pt and Pt atoms carried partial positive charges (Ptd+). In addition, compared with Pt and Cu catalysts prepared by formaldehyde reduction, the number of electrons transferred from Pt atoms to Cu increased slightly and the electron density of Pt decreased. The electronegativity of Cu atoms was larger, making it easier to synergistically catalyze hydrogen absorption. Due to the low load of Cu in both catalysts, the weak signal peaks at the binding energies of~931.6eV-933.7eV and~952.4eV-953.8eV were attributed to the Cu 2p3/2 and Cu 2p1/2 peaks, respectively26. At this point, the Cu 2p3/2 binding energy (931.6 eV) of Cu atoms in Pt2.5Cu0.1/Al2O3-H2 was lower than that of Cu 2p3/2 in Pt2.5Cu0.1/Al2O3-MR catalyst (933.7 eV). This further confirmed that some electrons in Pt atoms in Pt-Cu catalyst had transferred to Cu atoms and proved the existence of PtCux alloy. Moreover, the Pt 4f5/2 binding energy (74.7 eV) of Pt atoms in Pt2.5Cu0.1/Al2O3-MR was slightly higher than that in Pt2.6/Al2O3-MR catalyst (74.5 eV). It indicated that the addition of Cu generated partial electron transfer between Pt atoms and Cu atoms as well. However, there was no significant electron transfer between Pt atoms and Cu atoms due to the absence of hydrogen reduction after calcination and fixation. It is worth mentioning that the Al 2p binding energy of Al atoms in Pt2.5Cu0.1/Al2O3-H2 was also higher than that in Pt2.6/Al2O3-H2, and the signal of the characteristic peak was stronger at this time. It was revealed that the addition of Cu formed the alloy structure, which weakened SMSI as well.
The adsorption characteristics and electronic structure of Pt and Pt-Cu surfaces were investigated adopting CO-DRIFT in-situ spectroscopy at room temperature, as shown in Fig. 2c-d. In Pt2.5Cu0.1/Al2O3-H2 and Pt2.6/Al2O3-H2 samples, sharp peaks centered around~2000-2100 cm-1 could be observed. These corresponded to CO molecular linear adsorption on the Pt (111) surface with a coordination number of 927,28. The small peak extending to~2125 cm-1 appeared in Pt2.6/Al2O3-H2 sample except for the narrow peak centered at 2067-2075 cm-1 with Pt2.5Cu0.1/Al2O3-H2 sample. This phenomenon was due to the linear adsorption of CO molecules on the low coordination edge and angle Pt sites, and the relatively low adsorption strength of CO on this catalyst. The above behavior indicated that, on the one hand, Cu doped Pt catalysts increased the surface Pt exposure rate. It resulted in more linear adsorption of CO molecules on the Pt (111) surface (combined with CO chemisorption results). On the other hand, many studies have shown that inhibiting the adsorption of small molecules (such as CO and H2) was the typical behavior in the presence of SMSI29. Meanwhile, the active center of Pt doped with Cu suppressed the interaction between the metal and the support (combined with XPS results). In addition, the peak centered at 1830-1850 cm-1 appeared, proving that the adsorption of CO in Pt2.5Cu0.1/Al2O3-H2 and Pt2.6/Al2O3-H2 samples was both low adsorption. It meant that CO molecules were adsorbed between three Pt atoms, and the adsorption strength of the former was lower. These behaviors explicated that both were relatively small particle sizes, and the former had a larger particle size, making CO molecule adsorption more difficult30,31.
In order to observe the interaction between Pt and Cu particle sizes in sample Pt2.5Cu0.1/Al2O3-H2, HRTEM was exploited as shown in Fig. 3a. Among all the samples, the exposed surfaces of Al2O3 were mainly (110) surfaces. These illustrated that the supports of the catalysts were mainly γ-Al2O3. However, the exposed surfaces of Pt and Cu clusters were mainly (111) surfaces, which was related to the low loading of Cu, so there was little exposure of crystal planes. Meanwhile, it could be distinctly observed that numerous Pt nanoparticles (NPs) were directly loaded onto the γ-Al2O3 surface32,33. Compared with Pt2.5Cu0.1/Al2O3-MR, it could be seen that Pt2.5Cu0.1/Al2O3-H2 had clearer alloy metal lattice stripes and higher dispersion of Pt and Cu particle sizes. This explained that the reduction of Pt and Cu was more stable at this time, and SMSI was also weaker. It was further certificated that the synergistic catalytic effect of the catalyst was the combined result of interactions between the co-coordination of Pt and Cu and the formation of alloy (Fig. 3b). It was confirmed in DFT calculations and the HRTEM results further assisted the conclusions of XRD, BET, SEM, XPS, and CO-DRIFT (see the XRD, BET and SEM results in Fig.S1-A, Fig.S1-B and Fig.S1-E in Supplementary Information).
The unique electron-transfer alloy structure of Pt-Cu in Pt2.5Cu0.1/Al2O3-H2 might be the essential reason for synergistic effect. It could have the high dehydrogenation activity and strong structural retention ability compared to the Pt2.6/Al2O3-H2 catalyst. On the other hand, the formation of Pt-Cu unique electron transfer alloy structure was related to the high dispersion and nanoscale particles. This obtained by hydrogen reduction after Pt and Cu metal components are calcined and fixed. It determined that its dehydrogenation activity far exceeds that of the catalyst obtained by liquid-phase reduction. In other words, adjusting the electron distribution between Pt and Cu with different proportions of bimetallic precursors without significantly changing the size of Pt NPs could achieve the highly dispersed and alloyed state. Furthermore, further calcination of the precursor on the support also further anchors and limits the metal distribution, reducing SMSI.
Dehydrogenation performance of the Pt-Cu catalysts and the kinetic study
As shown in Fig. 4a-c, when the Pt load was below 2.5%, the addition of Cu had the reverse effect. When the loading was greater than 2.5% and Pt: Cu=10, the synergistic effect was observed. And the catalytic activity of Pt-Cu catalysts with higher loading was significantly higher than that of catalysts with lower loading, requiring less reaction time. This was related to the number of active components. In Fig. 4d, it could be clearly seen that the appropriate addition of Cu in a short period of time significantly improved the dehydrogenation catalytic performance of Pt. It further illustrated the synergistic catalytic effect between Pt (Pt≥2.5wt%) and Cu at this time. The best catalytic activity of catalyst A was selected for reactant analysis in Figure 4e, and the effect of reduction method on dehydrogenation performance was compared. Finally, it was confirmed that Pt2.5Cu0.1/Al2O3-H2 catalyst could achieve 100% dehydrogenation, and the degree of complete dehydrogenation was closely related to the cracking of reactants. And the above catalyst exhibited outstanding catalytic activity for 3 hours under the above reaction conditions. The reasons for the occurrence of the above were that, on the one hand, the addition of Cu might have the “group effect” similar to that of additives (Re or Sn) in Pt reforming catalysts34,35 for lower loading Pt. It would actually have a greater impact on the catalytic performance of Pt components. Right at this moment, the electronic and geometric effects of Cu were extremely significant, which would actually weaken the catalytic dehydrogenation activity of the active component Pt. On the other hand, Cu doping to form alloys and other surface dilution methods could disrupt the “cluster effect” for higher loading Pt, especially for Pt2.5Cu0.1/Al2O3 catalyst. It corresponded to the results of XRD and BET (see Fig.S1 in the Supplementary Information).
And the competitive adsorption between overflow hydrogen and products at the interface between Pt active sites and Pt active sites on Pt-Cu catalysts might promote the desorption of dehydrogenation products of 12H-MBT. Therefore, this could improve dehydrogenation activity. Especially for the H2-TPD, TEM, XRD, and in-situ infrared spectra of 12H-MBT adsorption reaction on Pt2.5Cu0.1/Al2O3 catalyst, this could also be demonstrated. Keane et al. also believed that overflow hydrogen had an important promoting effect on the catalytic performance of Pt like materials36. Fig. 4d further indicated that the 12H-MBT dehydrogenation activity was highest on the Pt2.5Cu0.1/Al2O3-H2 catalyst. At a reaction time of 3 hours, the dehydrogenation rate was close to 100% and the complete dehydrogenation rate was also relatively high. This was higher than Pt2.6/Al2O3-H2 and much higher than the Pt2.5Cu0.1/Al2O3-MR catalyst. Moreover, compared with reported literature, the reaction time was greatly shortened15. In addition, the dehydrogenation rate of Pt2.5Cu0.1/Al2O3-MR catalyst was higher than that of Pt2.6/Al2O3-MR catalyst within 3 hours. This was the reason why large particle metals were more conducive for substrate reaction in catalyzing the 12H-MBT dehydrogenation reaction. It was remarkable that the metal particle size of Pt2.5Cu0.1/Al2O3-H2 catalyst was larger than that of Pt2.6/Al2O3 and Pt2.6/Al2O3-MR, and the Pt grains doped with Cu were highly dispersed on the support. In the meanwhile, Pt coexists with Cu and forms alloy interactions, while PtCux alloy further synergistically catalyzes dehydrogenation by coordinating with C negative ions. Besides, the unique electronic structure of Pt-Cu alloy promoted the transfer of electrons from Pt to Cu and reduced the electron density of Pt. Thus it suppressed excessive dehydrogenation and hydrogenation in the 12H-MBT cycle.
Fig. 5 showed the 12H-MBT’s dehydrogenation activity data of Pt-Cu/Al2O3 and Pt/Al2O3 catalysts with different preparation methods and loading amounts at different temperatures. Compared with Pt/Al2O3-H2 catalysts with the same total loading, Pt-Cu/Al2O3 catalysts with high Pt loading (≥ 2.5wt%) exhibited higher 12H-MBT conversion rate, 0H-MBT yield, and TOF value. Among them, the data of Pt2.5Cu0.1/Al2O3-H2 catalyst and Pt2.6/Al2O3-H2 catalyst illuminated that the former catalyst had the higher CO adsorption capacity than the latter catalyst. This elaborated the higher number of surface hydrogen adsorption active sites, which was the intrinsic reason for its high intrinsic activity (TOF value). In addition, the intrinsic activity of Pt2.5Cu0.1/Al2O3-H2 catalyst (70.27min-1) was higher than that of other catalysts. This was due to the high dispersion of the metal surface and the increase in the number of active sites caused by the presence of Pt-Cu alloys. Without doubt, the reaction temperature also had the certain influence on it. The results showed that the higher the temperature, the higher the intrinsic activity due to the presence of in-situ hydrogen. The intrinsic activity of this catalyst was not significantly different at 250 ℃ and 260 ℃. It guided that there was still further improvement space in the subsequent dehydrogenation temperature reduction for the 12H-MBT system. Surprisingly, pure Pt catalysts also exhibited excellent intrinsic activity at low loading levels, providing another approach for subsequent catalyst research.
Corresponding kinetic analysis was conducted for the sake of further unveiling the mechanism of the excellent catalytic dehydrogenation performance of Pt2.5Cu0.1/Al2O3-H2 catalyst on 12H-MBT. As shown in Fig. 6a, the dehydrogenation reaction rate of 12H-MBT increased with the increase of temperature. And the substrate dehydrogenation rate was the fastest and the reaction degree was the most complete at 260 ℃. When the temperature dropped to 220 ℃, the final dehydrogenation amount at 390 minutes was about one-third of the final dehydrogenation amount at 260 ℃. This indicated that the reaction was endothermic and the increase in temperature was more conducive to the substrate reaction. Furthermore, the viscosity of the reaction substrate decreased with the increase of temperature. This was beneficial to better dispersion of the catalyst in the reaction system and sufficiently contact with the 12H-MBT reaction substrate. It facilitated the mass transfer of hydrogen gas in the reaction phase as well, making it easier for hydrogen gas to overflow from the reaction system. Thus, this promoted the occurrence of dehydrogenation reaction and accelerating the reaction. What's more, it could be observed from the graph that the dehydrogenation amount at 250 ℃ after 210 minutes of the reaction was roughly the same as that at 260 ℃.
These facts showed that the Pt2.5Cu0.1/Al2O3-H2 catalyst can effectively reduce the dehydrogenation temperature of the 12H-MBT system, implicating that the catalyst could effectively reduce the reaction energy barrier. More importantly, the amount of dehydrogenation increased linearly with reaction time at the early stages of the dehydrogenation reaction. It could be concluded that the dehydrogenation reaction rate was independent of the 12H-MBT concentration, i.e. this reaction could be regarded as an apparent zeroth-order one over the 12H-MBT concentration. The five straight lines could be linearly fitted from the initial reaction data of the five curves in Fig. 6a. The slope of the straight line corresponded to the apparent reaction rate constant k of the substrate dehydrogenation reaction at different temperatures corresponding to the fitting results shown in Fig. 6b-f. It could be inferred that the reaction rate tends to increase exponentially as the reaction temperature increases by 10 ℃.
By the calculation based on the data in Fig. 7a, the apparent activation energy (Ea) of the dehydrogenation reaction of 12H-MBT was 93.8 kJ/mol, which was relatively small and indicated easy occurrence of the reaction. This was in line with the fitting results of the previous reaction kinetics model as well. On this Ea value also confirmed that the catalytic reaction rate was mainly controlled by the catalytic conversion step instead of diffusion process. Fig. 7b-f provided the concentration curves of intermediates during the dehydrogenation process on the 12H-MBT of the Pt2.5Cu0.1/Al2O3-H2 catalyst at different temperatures. This further comprehended the possible dehydrogenation mechanism with excellent catalytic activity and product formation mechanism.10H-MBT, 6H-MBT, 4H-MBT, and MBT could be detected taking advantage of gas chromatography and gas chromatography-mass spectrometry existed among the analyzed intermediates. It could be apparently noticed from the graph that the concentration of substrate 12H-MBT in the reaction at 220 ℃ and 230 ℃ showed the stable decrease. However, the concentration of 12H-MBT at 240 ℃, 250 ℃, and 260 ℃ showed the cliff like decrease within 210 minutes.
The complete dehydrogenation product, MBT, was nearly unobservable at low temperatures of 220 ~230 ℃. The MBT concentration increased with reaction time as the reaction temperature increased to 240 ℃, and the concentration significantly increased with time at 260 ℃. This illustrated that the conditions required for the generation of 12H-MBT were more stringent. In terms of the 10H-MBT intermediate, with the progress of reaction time, the reaction basically generated subsequent products at different reaction temperatures. And only a small part could be detected in the prophase reaction, indicating that the macroscopic first step dehydrogenation of the reaction was easy to occur. As for the 6H-MBT intermediate, its concentration showed the trend of first increasing and then decreasing with time at different temperature ranges. The reaction was completed at the lowest temperature of 220 ℃ and the highest temperature of 260 ℃, describing that its macroscopic second step dehydrogenation was also relatively simple. Observing the changes in the intermediate 4H-MBT, it could be learnt about that its concentration slowly increased with time at 220 ℃, and was basically undetectable at other temperatures. This indicated that it was unable to continue dehydrogenation at low temperatures combined with the concentration changes of MBT. In the matter of other high temperatures, the final dehydrogenation product wasn't basically detected. These collectively indicated that the last dehydrogenation step from 4H-MBT to MBT for this continuous dehydrogenation reaction was the most stringent reaction condition. And the reaction was the slowest in the entire continuous dehydrogenation process as well, which could be further determined that it was the rate determining step of the reaction.
Combined with the above experimental results, there was still reduction room for further exploration of the catalyst’s effect on the dehydrogenation temperature of the system. The basic current detection methods could only detect intermediates in the dehydrogenation process of 12H-MBT, but their isomeric structures couldn't be detected. In summary, the dehydrogenation process of 12H-MBT under the action of Pt2.5Cu0.1/Al2O3-H2 catalyst was as follows: 12H-MBT→ 10H-MBT→ 6H-MBT→ 4H-MBT→ MBT. Theoretically, the dehydrogenation process mainly focused on one ring, and the starting position of dehydrogenation started from the most stable C3+. Then the position of the dehydrogenation double bond was located at the methylene group connected to the methyl group, and finally formed a large π bond. This process could refer to other polycyclic aromatic hydrocarbon dehydrogenation processes37,38. In the complete dehydrogenation process of 12H-MBT, the reaction energy barrier was mainly concentrated in the R9-R12 step (counted by the number of H atoms removed). And the following three most likely reaction paths were given and Path 1 had the lowest overall structural energy among them, and the rate-determining step existed in the release of the last two molecular H2 for a 12H-MBT o, i.e., from 4H-MBT to MBT, as expressed by:
Mechanism of 12H-MBT dehydrogenation on Pt2.5Cu0.1/Al2O3-H2 catalyst
DRIFT was adopted to study the surface adsorption and reaction process of the Pt2.5Cu0.1/Al2O3- H2 catalyst using 12H-MBT as the probe in order to better understand the structure-activity relationship of this catalyst. The in-situ infrared spectra of the reaction substrate molecules adsorbed on the Pt2.5Cu0.1/Al2O3- H2 catalyst at different temperatures were compared. And the changes in the adsorption active species of 12H-MBT during dehydrogenation could determine the reaction mechanism of 12H-MBT dehydrogenation on Pt2.5Cu0.1/Al2O3-H2 catalyst. In Fig. 8, the characteristic peaks of the adsorbed species were highlighted by subtracting the background from the in-situ adsorption at room temperature on Pt2.5Cu0.1/Al2O3-H2 catalyst in N2 gas. There was the certain change in the infrared spectrum when the carrier gas switched from N2 to the 12H-MBT mixture and adsorption was saturated at 25 ℃. After further heating, the weak characteristic peaks at 2853 cm-1, 2928 cm-1, and 2974 cm-1 (Fig. 6a) belonged to the C-H2 symmetric stretching mode, C-H2 asymmetric stretching mode, and C-H3 asymmetric stretching mode, respectively. With the increase of temperature, the C-H2 asymmetric stretching mode disappeared and the intensity of other vibration mode characteristic peaks decreased. This explained that increasing temperature aroused the desorption or conversion of 12H-MBT adsorbed species. It further confirmed that the catalyst was equipped with preeminent catalytic dehydrogenation activity for this reaction and the desorption energy was not too high at this time.
As the reaction temperature increased, it was observed that the peak at 2140 cm-1 belonged to the combined frequency peak of C-H bending vibration and C=C symmetric stretching vibration in the low frequency range. A significant blue shift occurred, indicating that the coordination adsorption and activation of 12HMBT on Pt and PtCux active sites were the initial steps of 12H-MBT dehydrogenation. The charge transfer and activation of H-H bonds might also have an impact on the subsequent reaction process, accompanied by further dehydrogenation39,40. In the low frequency range, the negative stretching vibration characteristic peak attributed to C=C appeared around 1620 cm-1 on the spectrum from 220 ℃ to 260 ℃ and the intensity didn't significantly decrease. This proved that the substrate underwent dehydrogenation reaction at this location and the dehydrogenation process was relatively complex. It was found on the sample that at 1453 cm-1, 1421 cm-1, 1385 cm-1, and 973 cm-1 as well (Fig. 6b), the former exhibited C=C symmetric stretching vibration on the hexagonal ring. However, the latter three exhibited C-H bending vibration on the hexagonal ring. These characteristic peak results attested that the adsorption mode of 12H-MBT on the prepared samples was consistent with the previously reported Pt adsorption mode, both lying flat at the sample interface41,42.
All of the above suggested that the adsorption capacity of 12H-MBT on Pt2.5Cu0.1/Al2O3-H2 was relatively powerful, which wouldn’t significantly inhibit the dehydrogenation reaction of 12H-MBT. Moreover, the initial step of 12H-MBT dehydrogenation was by no means the rate determining step. And the subsequent consecutive elementary reactions accompanied by the continuous dehydrogenation of the hexagonal ring to form the benzene ring derivatives were the real reaction energy barriers to be overcome. This was consistent with the experimental results.
In this study, the Pt2.5Cu0.1/Al2O3-H2 catalyst with alloy anchored by calcination exhibited excellent bimetallic synergistic catalytic activity for the 12H-MBT dehydrogenation. It was accompanied by the first complete reaction mechanism proposed to break the negative correlation between catalytic activity and selectivity. The comprehensive results including XPS and HRTEM indicated that the unique electron transfer alloy structure of Pt-Cu promoted the transfer of electrons from Pt to Cu. And it resulted in electron migration between the metal and the support to weaken SMSI, reducing the adsorption of aromatic products, and inducing appropriate H2 adsorption. Highly dispersed Pt components in nanoparticles were related to the addition of Cu and the anchoring of metal components through calcination. This reduced the aggregation of Pt components, which facilitated the synergistic catalysis of bimetallic components. The unique electronic structure and spatial configuration of the synergistic catalytic mechanism between Pt and Cu lead to a dehydrogenation process where the rate-determining step was the conversion from 4H-MBT to MBT.
Chemicals
Monobenzyltoluene was purchased from Hubei Xinkang Pharmaceutical Chemical Co., Ltd and used as a hydrogenation reactant. H2PtCl6·6H2O, imidazolidinyl urea, methanal, ethylene glycol, sodium hydrate and dichloromethane were purchased from Sinopharm Group Chemical Reagent Co., Ltd. Cu(NO3)2·3H2O was purchased from Shanghai Maclin Biochemical Technology Co., Ltd. Aluminium oxide and Ru/Al2O3 were purchased from Shaanxi Ruike New Material Co., Ltd. All atmospheres used are of ultra-pure grade (>99.99%).
Catalyst preparation
(1) Equal-volume impregnation method with hydrogen reduction post-treatment
Alumina calcined in the muffle furnace at 500 °C for 4 h was used as the catalytic support. And 0.265g H2PtCl6·6H2O and 0.015g Cu(NO3)2·3H2O were dissolved in 3.6 mL deionized water to form the precursor solution of the target product. Then the precursor solution was added to the 4g roasted alumina support and subjected to vacuum immersion for 12 hours. The above sample aged for 1 hour was dried at 120 ℃ for 2 hours, and then further calcined at 500 ℃ for 4 hours. Finally, the sample was heated to 500℃ in H2 (40 ml/min) and reduced for 4 h. The obtained catalyst was denoted as Pt2.5Cu0.1/Al2O3-H2 (the subscript numbers represent weight percentage, wt%) catalyst. Similarly, the metal ratio transformed could obtain different loads of Pt-Cu catalysts, which could be named accordingly.
(2) Equal volume impregnation with pre-coordination and methanal reduction (MR)
The precursor solution of Pt2.0Cu0.1/Al2O3-H2 catalyst during the preparation process was sonicated for 1 hour, then appropriate imidazolidinylurea (with the same molar amount of metal) was added. After drying the above samples, add the following solution and reduce them at 75 ℃ for 4 hours. The solution treatment process was as follows: add 60ml formaldehyde to a solution mixed with 40ml ethylene glycol and 40ml deionized water, and adjust the pH value to about 11 by adding 0.1mol/L NaOH. Finally, Pt2.5Cu0.1-Al2O3-MR catalyst was obtained, and similarly, other catalysts with different metal ratios could also be prepared.
Catalyst characterization
The sample composition and crystal equality information were obtained by the D8 Advance XRD X-ray powder diffractometer. And the X-ray source was tested under the conditions of utilizing Cu target (40kV, 40mA, λ = 0.1541 nm), ranging from 4.5° to 90°, step size of 0.02° and operating power of 2.2 kW.
The specific surface area, pore volume, and maximum pore size of the sample catalysts were analyzed using the Micromeritics ASAP 2460 BET automatic adsorption instrument. It was tested with high-purity nitrogen in a liquid nitrogen ultra-low temperature environment.
High resolution transmission electron microscopy (HRTEM) imaging analysis was performed on the samples in virtue of the JEOL JEM-F200 electron microscope in Japan.
The morphology of samples could be analyzed by SEM-EDX on the German ZEISS GeminiSEM 300 ultra-high resolution field emission scanning electron microscope. Energy spectrum mapping tests could be operated with the use of the energy spectrometer (Smartedx) to observe element distributions and contents.
The X-ray photoelectron spectroscopy (XPS) test required the use of the American Thermo Scientific K-Alpha X-ray source. After taking the appropriate amount of sample and pressing it onto the sample disk, the sample was placed into the sample chamber of the aforementioned instrument. When the pressure in the sample chamber was less than 2.0×10-7mbar, the sample tested at the spot size of 400 μm, working voltage of 12 kV and filament current of 6 mA. Eventually, the peaks in the spectrum obtained from the above tests were located based on the external reference value of 284.9 eV of the C1s peak and rectified for charge effects.
H2-TPD testing on the samples was manipulated under the action of the American Micromeritics AutoChem II 2920 chemical adsorption instrument. Firstly, 0.1g sample was fixed in a quartz tube, preprocessed in the H2 (50ml min-1) atmosphere at 300 ℃ for 2 h to dislodge the passivation behavior on the surface of the metal particles. And it was blown for 1 hour with He (50ml min-1) and cooled at 50 ℃. Secondly, 10% H2/Ar mixture (50ml min-1) was introduced for 1 h until saturation, and the Ar airflow (50ml min-1) was switched to blow for 1 h to remove weak H2 physical adsorption on the surface. Finally, the gas was desorbed at a heating rate of 10 ℃/min to 600 ℃ in the Ar atmosphere, and the desorbed gas was detected utilizing TCD. The CO chemical adsorption capacity was measured by CO pulse on the HP chemical adsorption instrument. Firstly, 0.2g sample was placed in a U-tube reactor, reduced at 300 ℃ in H2/Ar flow (50 ml min-1) for 2 h and cooled to room temperature. Then it was pulsed multiple times by CO (50ml min-1) in the Ar airflow and the chemical adsorption signal of CO was detected by a TCD detector until there was no chemical adsorption signal of CO. The CO chemical adsorption capacity was the difference between the total amount of CO pulses and the total amount of CO efflux, which could determine the dispersion of Pt particles on the support.
The CO Fourier diffuse reflectance infrared spectroscopy (CO-DRIFT) was measured utilizing the Thermo iS10 spectrometer. The testing process was as follows: Firstly, the sample was beneficiated for 2h in the H2 flow (50 ml min-1) at 300 ℃ before CO adsorption. The background spectrum of the sample could be collected when the sample was cooled to room temperature in N2 flow (50 ml min-1). Subsequently, the CO flow rate of 50ml min-1 was added to the spectral cell to allow the sample to adsorb for 30 minutes and reach saturation. Then the sample was cleaned continuously with a flow rate of 50ml min-1 of N2 until the infrared signal of the sample stops changing and collected CO-DRIFT spectra. Furthermore, the 12H-MBT DRIFT test was conducted on the Bruker INVENIO-S. infrared spectrometer in Germany. Firstly, the sample was placed in an in-situ cell and pretreated in an H2 flow (50 ml min-1) at 300 ℃ for 2 hours to eliminate partial oxidation behavior on the surface of the metal particles. Then, the background spectrum of the sample could be gathered after cooling the sample in N2 flow (50 ml min-1) to room temperature. Spectra were collected after saturating the N2 mixture (60ml min-1) containing 12H-MBT at room temperature. And then the sample was heated at the rate of 5 ℃ min-1 to the given temperature (220, 230, 240, 250, 260 ℃) for 180 seconds before sequentially collecting the corresponding spectral signals. All spectra require background subtraction to highlight the reaction adsorption spectral signal of 12H-MBT.
12H-MBT’s dehydrogenation test and its reaction kinetics
(1) Preparation of 12H-MBT
12H-MBT was prepared through complete hydrogenation of MBT. 30.0g NEC as the reactant and 3.0g of 5wt% Ru-Al2O3 catalyst were added and reacted with pure H2 for 2 hours in an autoclave equipped with a mechanic stirrer. The reaction conditions were 413 K, 5 Mpa, and 600 rpm. MBT could be completely hydrogenated to afford the product of 12H-NEC (≥ 99%) under reaction conditions reaction conditions described above43. And elemental analysis of the reaction product was used to further confirm that successive preparation of 12H-NEC (C:H = 6.4:1).
(2) Dehydrogenation performance of 12H-MBT
Similarly, 20.0g as-prepared 12H-MBT and 2.0g PtyCuz/Al2O3-H2/MR catalysts (y and z denoted the mass loading of Pt and Cu in percentage, respectively) were added to an auto clave equipped with a condenser and a H2 gas outlet connected by a flow meter. And the above dehydrogenation reaction was carried out at 260 °C, atmospheric pressure, and a stirring rate of 600 rpm. Similarly, while maintaining the above reaction conditions unchanged, 100.0g as-prepared 12H-MBT and 10.0g of Pt2.5Cu0.1-Al2O3-H2 catalyst were reacted on different temperatures (220,230, 240, 250,260 ℃) as the kinetic experiments. The composition of dehydrogenation products was determined by a gas chromatograph (Agilent 7820A) and a gas chromatography-mass spectrometer (PE CLARUS 680+SQ8). The release of hydrogen was recorded by an electronic display flow recorder, which could be referred to in the experimental equipment documentation and related calculation formulas.
The calculation method of some interested quantities are described as follows. The dehydrogenation performance was represented as the degree of dehydrogenation (DoD), which is related to degree of hydrogenation (DOH), as shown in (1) and (2), respectively. These were defined as the ratio of the amount of hydrogen stored in LOHC to the maximum potential hydrogen storage capacity of LOHC44. Moreover, the selectivity of the full dehydrogenation product i.e., MBT, could be expressed as the total degree of dehydrogenation (TDoD). The activation energy of the apparent dehydrogenation reaction of the system was received by the Arrhenius formula, which was shown in equation (3). In equation (3), k represents the reaction rate constant, R is the molar gas constant, R has a value of 8.314 J∙mol-1∙K-1, T is the absolute temperature of the reaction and A is the pre factor45. To deserve to be mentioned, the intrinsic activity of the catalyst could be reflected by the value of turnover frequency (TOF). It was more consistent with the initial stage of the reaction because the instantaneous reaction rate was approximately equal to the average reaction rate46. TOF could be expressed by formula (4), where r·AC was the rate constant for 12H-MBT consumption per minute of per gram catalyst (mol· min−1 gcat-1 ) , ntotal was molar mass of Pt and D is the the active site dispersion of per gram catalyst (gcat-1)47
$$\:DoH\:=\:\frac{{n}_{{H}_{2,}max}－{n}_{{H}_{2,}released}}{{n}_{{H}_{2,}max}}$$
1
$$\:DoD\:=1-DOH\:\:\:\:\:\:\:\:\:\:$$
2
$$\:Ink=\:\frac{\:{－E}_{a}}{RT}＋InA\:\:\:\:\:\:$$
3
$$\:TOF=\:\frac{r·A{C}_{·}}{{n}_{total}·D}\:\:\:\:\:\:\:\:\:\:\:\:$$
4
Author Contributions
Qiuyue Ding: investigation, data collection, reaction test, writing-original draft preparation, Yixuan Zhang: catalytic test, reaction setup construction, discussion. Huijie Wei: reaction setup construction. Qing Li: catalyst preparation, characterization, paper revision. Yanyan Xi: catalyst characterization, discussion. Songqing Hu: research funding provision, writing and organization. Xufeng Lin: idea development, research funding provision, writing and organization.
Declaration of Competing Interest
The authors declare no competing financial interest.
Data availability
The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files. Should any raw data files be needed in another format they are available from the corresponding author (X. Lin, via [email protected]) upon reasonable request.
Appendix A. Supplementary data
Supplementary data Supplementary data to this article can be found online at https://doi.xx.
Acknowledgements
Supports from the National Natural Science Foundation of China (21576291) and from the Fundamental Research Funds for the Central Universities (23CX03007A) is gratefully acknowledged.
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No competing interests reported.