Naphthalene on Ni(111): Experimental and Theoretical Insights into Adsorption, Dehydrogenation, and Carbon Passivation

Abstract

An attractive solution to mitigate tars and also to decompose lighter hydrocarbons in biomass gasification is secondary catalytic reforming, converting hydrocarbons to useful permanent gases. Albeit that it has been in use for a long time in fossil feedstock catalytic steam reforming, understanding of the catalytic processes is still limited. Naphthalene is typically present in the biomass gasification gas and to further understand the elementary steps of naphthalene transformation, we investigated the temperature dependent naphthalene adsorption, dehydrogenation and passivation on Ni(111). TPD (temperature-programmed desorption) and STM (scanning tunneling microscopy) in ultrahigh vacuum environment from 110 K up to 780 K, combined with DFT (density functional theory) were used in the study. Room temperature adsorption results in a flat naphthalene monolayer. DFT favors the dibridge[7] geometry but the potential energy surface is rather smooth and other adsorption geometries may coexist. DFT also reveals a pronounced dearomatization and charge transfer from the adsorbed molecule into the nickel surface. Dehydrogenation occurs in two steps, with two desorption peaks at approximately 450 and 600 K. The first step is due to partial dehydrogenation generating active hydrocarbon species that at higher temperatures migrates over the surface forming graphene. The graphene formation is accompanied by desorption of hydrogen in the high temperature TPD peak. The formation of graphene effectively passivates the surface both for hydrogen adsorption and naphthalene dissociation. In conclusion, the obtained results on the model naphthalene and Ni(111) system, provides insight into elementary steps of naphthalene adsorption, dehydrogenation, and carbon passivation, which may serve as a good starting point for rational design, development and optimization of the Ni catalyst surface, as well as process conditions, for the aromatic hydrocarbon reforming process.

Investigation of the surface species during temperature dependent dehydrogenation of naphthalene on Ni(111)

“…In both cases, the only desorption process observed is the desorption of molecular hydrogen accompanied by irreversible structural changes in the naphthalene overlayer. This is consistent with the findings of Yazdi et al 7 where scanning tunneling microscopy (STM) and TPD were used to study naphthalene on Ni(111). H 2 desorption coincides with the disappearance of ordered naphthalene patches in the temperature range of 380-600 K with the first desorption peak at 450 K. At temperatures above 580 K, carbon chain and network growth is reported, and it is assumed that a graphenelike layer is formed.…”

“…Santarossa et al 14 used density functional theory (DFT) calculations to show that a flat lying di-bridge-7 is the optimal adsorption configuration of naphthalene on Pt, Pd, and Rh. Similar findings were recently reported by Kolsbjerg et al 24 for naphthalene on Pt(111) and by Yazdi et al 7 for naphthalene on Ni(111) where the di-bridge-7 is the most favored configuration. Additionally, DFT calculations by Yazdi et al show that the adsorption of naphthalene is accompanied by a pronounced de-aromatization, similar to that seen for benzene on Ni(100) and Cu(110).…”

“…In a previous structural investigation of naphthalene on Ni(111), 7 we have shown that naphthalene molecules adsorb flat on smooth terraces, preferring a geometry of aromatic rings positioned over Ni-bridge sites. Temperature programmed desorption (TPD) showed that desorption of H 2 occurs in two steps, where the second step coincides with the conversion of carbon atoms to a graphenelike structure, causing rapid passivation of the catalyst.…”

“…45 A 5 L dose at 110 K results in a chemisorbed monolayer and a physisorbed multilayer of naphthalene. 7 In the spectrum recorded after dosing at 110 K, one sharp resonance is observed at 3057 cm −1 , corresponding to multilayer naphthalene, in addition to the nonresonant background. The observed resonant frequency is in the middle of the aromatic C-H stretching region and matches the frequencies found by Jakob and Menzel 46 and Lehwald et al 47 for multilayer benzene on Ru(001) and Ni(111), respectively.…”

“…Temperature programmed desorption (TPD) showed that desorption of H 2 occurs in two steps, where the second step coincides with the conversion of carbon atoms to a graphenelike structure, causing rapid passivation of the catalyst. 7 In the present study, we aim to elucidate the mechanism of naphthalene dehydrogenation and graphene formation on the Ni(111) surface by investigating the surface species during the temperature dependent dehydrogenation.…”