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.