Dehydrogenation Mechanism of Liquid Organic Hydrogen Carriers: Dodecahydro‐N‐ethylcarbazole on Pd(111)

Abstract

Dodecahydro-N-ethylcarbazole (H12-NEC) has been proposed as a potential liquid organic hydrogen carrier (LOHC) for chemical energy storage, as it combines both favourable physicochemical and thermodynamic properties. The design of optimised dehydrogenation catalysts for LOHC technology requires a detailed understanding of the reaction pathways and the microkinetics. Here, we investigate the dehydrogenation mechanism of H12-NEC on Pd(111) by using a surface-science approach under ultrahigh vacuum conditions. By combining infrared reflection-absorption spectroscopy, density functional theory calculations and X-ray photoelectron spectroscopy, surface intermediates and their stability are identified. We show that H12-NEC adsorbs molecularly up to 173 K. Above this temperature (223 K), activation of C-H bonds is observed within the five-membered ring. Rapid dehydrogenation occurs to octahydro-N-ethylcarbazole (H8-NEC), which is identified as a stable surface intermediate at 223 K. Above 273 K, further dehydrogenation of H8-NEC proceeds within the six-membered rings. Starting from clean Pd(111), C-N bond scission, an undesired side reaction, is observed above 350 K. By complementing surface spectroscopy, we present a temperature-programmed molecular beam experiment, which permits direct observation of dehydrogenation products in the gas phase during continuous dosing of the LOHC. We identify H8-NEC as the main product desorbing from Pd(111). The onset temperature for H8-NEC desorption is 330 K, the maximum reaction rate is reached around 550 K. The fact that preferential desorption of H8-NEC is observed even above the temperature threshold for H8-NEC dehydrogenation on the clean surface is attributed to the presence of surface dehydrogenation and decomposition products during continuous reactant exposure.

Alkyl chain length-dependent surface reaction of dodecahydro-N-alkylcarbazoles on Pt model catalysts

“…In fact, this reaction is observed to occur both on industrial catalysts (<2% dealkylation after heating NEC for 72 h at 543 K in contact with a commercial Pt on AlOx catalyst 14 ) and on model catalytic surfaces. Dealkylation of H 12 -NEC on a Pt(111) single crystal is observed starting at 390 K, 10, 15 while on Pd(111) the onset of dealkylation is found already at 350 K. 11 The same reaction was found to take place on supported Pt and Pd model systems, where the activation barrier of C-N bond scission is reduced as compared to single crystals. 12 The aim of this work is to study modified LOHCs based on the carbazole system.…”

“…Figure 4(c) does not show any multilayer contribution, since the amount of H 12 -NBC deposited on the surface was significantly lower. Since we know from previous studies and from the experiments on Pt(111) that the multilayer desorbs up to 225 K, 11,15 we can exclude any multilayer contribution above this temperature. Therefore, we chose the spectra recorded after annealing to 300 K to reference the data to the amount of adsorbate chemisorbed on the Pt particle surfaces.…”

“…From these observations, we conclude that the behavior of H 12 -NPC and H 12 -NBC during adsorption is similar to the one of H 12 -NEC that was already addressed in great detail in our previous studies. [10][11][12][13]15 Next, we discuss the reactions occurring during heating of the adsorbed layers. To eliminate any contributions of the multilayer signal, which are difficult to separate from peaks of the chemisorbed species, for these experiments adsorption was performed at ∼250 K (except for the C 1s experiment for H 12 -NPC, where the molecule was adsorbed at 140 K and then heated to 250 K).…”

“…Concerning the dehydrogenation reaction, one specifically relevant aspect is the avoidance of unwanted side reactions during unloading, such as dealkylation of the N-alkyl substituent that would lead to an unfavorable change in the liquidus range of the LOHC system. In this context, we extensively studied H 12 -NEC regarding its dehydrogenation mechanism on various model catalysts including Pt(111), 10 Pd(111), 11 and on both Al 2 O 3 -supported Pt 12 and Pd 13 nanoparticles in ultrahigh vacuum (UHV) using high-resolution X-ray photoelectron spectroscopy (HR-XPS), infrared reflection-absorption spectroscopy (IRAS), and temperature-programmed molecular beam methods (TPMB). The N 1s spectrum of the nitrogen atom of NEC and H 12 -NEC is very suitable to monitor the C-N bond scission in HR-XPS.…”

“…On the single crystal surface, we obtain a detailed quantitative analysis of the reaction due to the highly defined nature of the substrate. 10,11,15 On the other hand, the investigation of supported model catalysts provides a more realistic picture of reactions taking place on the complex surface of an industrial heterogeneous catalyst. Note that for the hydrogenlean compounds, the abbreviations NEC, NPC, and NBC will be used (equivalent to H 0 -NEC, H 0 -NPC, H 0 -NBC, respectively).…”