Dehydrogenation of Dodecahydro‐N‐ethylcarbazole on Pd/Al₂O₃ Model Catalysts

Abstract

To elucidate the dehydrogenation mechanism of dodecahydro-N-ethylcarbazole (H(12)-NEC) on supported Pd catalysts, we have performed a model study under ultra high vacuum (UHV) conditions. H(12)-NEC and its final dehydrogenation product, N-ethylcarbazole (NEC), were deposited by physical vapor deposition (PVD) at temperatures between 120 K and 520 K onto a supported model catalyst, which consisted of Pd nanoparticles grown on a well-ordered alumina film on NiAl(110). Adsorption and thermally induced surface reactions were followed by infrared reflection absorption spectroscopy (IRAS) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) in combination with density functional theory (DFT) calculations. It was shown that, at 120 K, H(12)-NEC adsorbs molecularly both on the Al(2)O(3)/NiAl(110) support and on the Pd particles. Initial activation of the molecule occurs through C-H bond scission at the 8a- and 9a-positions of the carbazole skeleton at temperatures above 170 K. Dehydrogenation successively proceeds with increasing temperature. Around 350 K, breakage of one C-N bond occurs accompanied by further dehydrogenation of the carbon skeleton. The decomposition intermediates reside on the surface up to 500 K. At higher temperatures, further decay to small fragments and atomic species is observed. These species block most of the absorption sites on the Pd particles, but can be oxidatively removed by heating in oxygen at 600 K, fully restoring the original adsorption properties of the model catalyst.

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

“…The high binding energy signal (H 12 -NXC adsorbed on Al 2 O 3 ) shifts towards higher values by ∼0.5 eV, while it decreases and vanishes at approximately 400 K. Previous studies suggest that the adsorbate is mobile in this temperature regime and migrates from the support towards the metal deposits. Similar observations were made on Pd/Al 2 O 3 both for H 12 -NEC 13 and other adsorbates. 27 The coexistence of various non-equivalent adsorption sites on highly dispersed Pt aggregates as compared to Pt(111) causes a general broadening of all peaks.…”

“…Interestingly, the stepwise increase of the dealkylation with temperature is not observed on Pt(111) and also not on Pd(111), 11 while previous studies addressing the reaction of H 12 -NEC on supported Pd 13 and Pt 12 model catalysts clearly indicate a similar trend. Consequently, we assign the first step at ∼350 K to dealkylation at highly reactive low-coordinated defect sites on the metal particles.…”

“…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).…”

Thermodynamic Evaluation of Potential Organic Hydrogen Carriers

“…[13,14] In 2005 Pez et al proposed N-ethylcarbazole as a carrier material to ease the dehydrogenation, [15,16] and different groups have continued to research this carrier material. [17][18][19][20] Alternative carrier materials, such as azaborines, are being investigated by other groups. [21] For the example of toluene the (de)hydrogenation is shown in Equation (1).…”

Energy storage in residential and commercial buildings via Liquid Organic Hydrogen Carriers (LOHC)

“…1. Extensive studies on this matter have been performed and published, amongst others by F. Sotoodeh et al [18][19][20][21] Detailed studies about the lifetime and reusability of catalysts do not exist yet. For dehydrogenation Ahluwalia et al assume a catalyst productivity of 500 000 kg LOHC per one kg of catalyst and catalyst costs of 150 Euro per kg.…”