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Naphthalene on Ni(111): experimental and theoretical insights into adsorption, dehydrogenation and carbon passivation
KTH, School of Engineering Sciences (SCI), Applied Physics, Material Physics, MF.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology. KTH.ORCID iD: 0000-0002-5395-599X
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

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 in use for long time in fossil feedstock catalytic steam reforming, the 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 ultra-high 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 di-bridge[7] geometry but the potential energy surface is rather smooth. 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 K 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.

Keywords [en]
Ni(111), surface chemistry, naphthalene, graphene, carbon passivation, dehydrogenation, DFT
National Category
Other Chemical Engineering Other Chemistry Topics Other Physics Topics
Research subject
Chemical Engineering; Chemistry; Physics
Identifiers
URN: urn:nbn:se:kth:diva-213362OAI: oai:DiVA.org:kth-213362DiVA, id: diva2:1136889
Funder
Swedish Energy AgencySwedish Research Council
Note

QC 20170830

Available from: 2017-08-29 Created: 2017-08-29 Last updated: 2017-08-30Bibliographically approved
In thesis
1. Catalytic Conversion of Undesired Organic Compounds to Syngas in Biomass Gasification and Pyrolysis Applications
Open this publication in new window or tab >>Catalytic Conversion of Undesired Organic Compounds to Syngas in Biomass Gasification and Pyrolysis Applications
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Reliable energy supply is a major concern and crucial for development of the global society. To address the dependency on fossil fuel and the negative effects of this reliance on climate, there is a need for a transition to cleaner sources. An attractive solution for replacing fossil-based products is renewable substitutes produced from biomass. Gasification and pyrolysis are two promising thermochemical conversion technologies, facing challenges before large-scale commercialization becomes viable. In case of biomass gasification, tar is often and undesired by-product. An attractive option to convert tar into syngas is nickel-based catalytic steam reforming (SR). For biomass pyrolysis, catalytic SR is in early stages of investigation as a feasible option for bio-crude conversion to syngas.

The focus of the thesis is partly dedicated to describe research aimed at increasing the knowledge around tar reforming mechanisms and effect of biomass-derived impurities on Ni-based tar reforming catalyst downstream of gasifiers. The work focuses on better understanding of gas-phase alkali interaction with Ni-based catalyst surface under realistic conditions. A methodology was successfully developed to enable controlled investigation of the combined sulfur (S) and potassium (K) interaction with the catalyst. The most striking result was that K appears to lower the sulfur coverage and increases methane and tar reforming activity. Additionally, the results obtained in the atomistic investigations are discussed in terms of naphthalene adsorption, dehydrogenation and carbon passivation of nickel.

Furthermore, the thesis describes research performed on pyrolysis gas pre-conditioning at a small-industrial scale, using an iron-based catalyst. Findings showed that Fe-based materials are potential candidates for application in a pyrolysis gas pre-conditioning step before further treatment or use, and a way for generating a hydrogen-enriched gas without the need for bio-crude condensation.

Abstract [sv]

Tillförlitlig energiförsörjning är en stor utmaning och avgörande för utvecklingen av det globala samhället. För att ta möta beroendet av fossil råvara och de negativa effekter som detta beroende medför för klimatet finns ett stort behov av en övergång till renare energiråvaror. En attraktiv lösning är att ersätta nuvarande fossil råvara med produkter från biomassa. Förgasning och pyrolys är två lovande teknologier för termokemisk omvandling av biomassa. Kommersialisering av dessa teknologier är inte helt problemfritt. I fallet förgasning så behöver, bl.a. oönskade tyngre kolväten (tjära) hanteras innan den producerade orenade produktgasen kan användas i syntesgastillämpningar. Ett effektivt alternativ för detta är gaskonditionering vid höga temperaturer, baserade på katalytisk ångreformering med en nickelkatalysator. Katalytisk ångreformering är en möjlig teknik för omvandling av bioråvara, producerad från pyrolys av biomassa, till syntesgas.

Avhandlingen fokuserar delvis på att beskriva den forskning som utförts för att öka kunskapen kring mekanismer för tjärreformering och effekterna av föroreningar från biomassan på en nickelkatalysator nedströms förgasare. Arbetet bidrar till en bättre förståelse av hur alkali i form av kalium (K) i gasfasen upptas, jämviktas och växelverkar med ytan hos nickelkatalysatorn under fullt realistiska förhållanden. Inledningsvis utvecklades en metod för att möjliggöra kontrollerade studier av den kombinerade effekten av S och K, vilken inkluderar exakt dosering av alkali till en produktgas, eliminering av transienter i katalysatoraktiviteten samt katalysatorkarakterisering. Det mest lovande resultatet är att K både sänker ytans svavelinnehåll och ökar aktiviteten för omvandlingen av metan och tjära. För att ytterligare fördjupa kunskaperna i mekanismerna för tjärnedbrytning utfördes experimentella och teoretiska ytstudier på en enkristallnickelyta med naftalen som modellförening. Resultat avseende naftalenadsorption, dehydrogenering av naftalen och kolpassivering av nickelytan diskuteras.

Därutöver så beskriver avhandlingen den forskning som utförts inom förkonditionering av pyrolysgas med en järnkatalysator för varsam deoxygenering av biooljan och vätgasproduktion. Detta utfördes vid en småskalig industriell anläggning. De experimentella studierna visar att den undersökta järnkatalysatorn resulterar i en vätgasberikad gas och att den är en potentiell kandidat för tillämpning i ett förkonditioneringssteg.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. p. 126
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2017:36
Keywords
tar reforming, biomass gasification, Ni-based catalyst, potassium, sulfur, pyrolysis gas, bio-crude conditioning, gas conditioning, Fe-based catalyst, tjärreformering, biomassaförgasning, Ni-baserad katalysator, kalium, svavel, pyrolysgas, konditionering bio-råolja, gaskonditionering, Fe-baserad katalysator
National Category
Chemical Process Engineering Other Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-213368 (URN)978-91-7729-509-9 (ISBN)
Public defence
2017-09-29, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
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Supervisors
Note

QC 20170830

Available from: 2017-08-30 Created: 2017-08-30 Last updated: 2017-09-07Bibliographically approved

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Ghadami Yazdi, MiladH. Moud, PouyaEngvall, KlasGöthelid, Mats

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