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DFT study of the water gas shift reaction on Ni(111), Ni(100) and Ni(110) surfaces
University of Borås, Faculty of Textiles, Engineering and Business.
University of Borås, Faculty of Textiles, Engineering and Business.
University of Borås, Faculty of Textiles, Engineering and Business.
2015 (English)In: Surface Science, ISSN 0039-6028, Vol. 644, 53-63 p.Article in journal (Refereed) Published
Abstract [en]

Density functional theory (DFT) calculations were used to study the water gas shift (WGS) reaction on Ni(111), Ni(100) and Ni(110) surfaces. The adsorption energy for ten species involved in the reaction together with activation barriers and reaction energies for the nine most important elementary steps were determined using the same model and DFT methods. The results reveal that these energies are sensitive to the surface structure. In spite of this, the WGS reaction occurs mainly via the direct (also referred to as redox) pathway with the CO + O → CO2 reaction as the rate determining step on all three surfaces. The activation barrier obtained for this rate limiting step decreases in the order Ni(110) > Ni(111) > Ni(100). Therefore, if O species are present on the surfaces then the WGS reaction is fastest on the Ni(100) surface. However, the barrier for desorption of H2O (which is the source of the O species) is lower than its dissociation reaction on the Ni(111) and Ni(100) surfaces, but not on the Ni(110) surface. Hence, at low H2O(g) pressures, the direct pathway on the Ni(110) surface will dominate and will be the rate limiting step. The calculations also show that the reason that the WGS reaction does not primarily occur via the formate pathway is that this species is a stable intermediate on all surfaces. The reactions studied here support the Brønsted–Evans–Polanyi (BEP) principles with an R2 value of 0.99.

Place, publisher, year, edition, pages
2015. Vol. 644, 53-63 p.
Keyword [en]
Water gas shift reaction; DFT; Nickel; Ni(111); Ni(100); Ni(110)
National Category
Engineering and Technology
Research subject
Resource Recovery
Identifiers
URN: urn:nbn:se:hb:diva-3287DOI: 10.1016/j.susc.2015.09.014OAI: oai:DiVA.org:hb-3287DiVA: diva2:871882
Available from: 2015-11-17 Created: 2015-11-17 Last updated: 2015-12-18
In thesis
1. Computational studies of nickel catalysed reactions relevant for hydrocarbon gasification
Open this publication in new window or tab >>Computational studies of nickel catalysed reactions relevant for hydrocarbon gasification
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Sustainable energy sources are of great importance, and will become even more important in the future. Gasification of biomass is an important process for utilization of biomass, as a renewable energy carrier, to produce fuels and chemicals. Density functional theory (DFT) calculations were used to investigate i) the effect of co-adsorption of water and CO on the Ni(111) catalysed water splitting reaction, ii) water adsorption and dissociation on Ni(111), Ni(100) and Ni(110) surfaces, as well as iii) formyl oxidation and dissociation, iv) hydrocarbon combustion and synthesis, and v) the water gas shift (WGS) reaction on these surfaces.

The results show that the structures of an adsorbed water molecule and its splitting transition state are significantly changed by co-adsorption of a CO molecule on the Ni(111) surface. This leads to less exothermic reaction energy and larger activation barrier in the presence of CO which means that far fewer water molecules will dissociate in the presence of CO.

For the adsorption and dissociation of water on different Ni surfaces, the binding energies for H2O and OH decrease in the order Ni(110) > Ni(100) > Ni(111), and the binding energies for O and H atoms decrease in the order Ni(100) > Ni(111) > Ni(110). In total, the complete water dissociation reaction rate decreases in the order Ni(110) > Ni(100) > Ni(111).

The reaction rates for both formyl dissociation to CH + O and to CO + H decrease in the order Ni(110) > Ni(111) > Ni(100). However, the dissociation to CO + H is kinetically favoured. The oxidation of formyl has the lowest activation energy on the Ni(111) surface.

For combustion and synthesis of hydrocarbons, the Ni(110) surface shows a better catalytic activity for hydrocarbon combustion compared to the other surfaces. Calculations show that Ni is a better catalyst for the combustion reaction compared to the hydrocarbon synthesis, where the reaction rate constants are small.

It was found that the WGS reaction occurs mainly via the direct pathway with the CO + O → CO2 reaction as the rate limiting step on all three surfaces. The activation barrier obtained for this rate limiting step decreases in the order Ni(110) > Ni(111) > Ni(100). Thus, the WGS reaction is fastest on the Ni(100) surface if O species are present on the surfaces. However, the barrier for desorption of water (as the source of the O species) is lower than its dissociation reaction on the Ni(111) and Ni(100) surfaces, but not on the Ni(110) surface. Therefore the direct pathway on the Ni(110) surface will dominate and will be the rate limiting step at low H2O(g) pressures. The calculations also reveal that the WGS reaction does not primarily occur via the formate pathway, since this species is a stable intermediate on all surfaces.

All reactions studied in this work support the Brønsted-Evans-Polanyi (BEP) principles.

Place, publisher, year, edition, pages
Borås: Högskolan i Borås, 2015. 56 p.
Series
Skrifter från Högskolan i Borås, ISSN 0280-381X ; 60
Keyword
DFT, H2O, CO, adsorption, dissociation, formyl, hydrocarbon combustion, hydrocarbon synthesis, water gas shift, gasification, Ni(111), Ni(110), Ni(100)
National Category
Engineering and Technology
Research subject
Resource Recovery
Identifiers
urn:nbn:se:hb:diva-323 (URN)978-91-87525-67-4 (ISBN)978-91-87525-68-1 (ISBN)
Public defence
2015-09-29, E310, University of Borås, Allégatan 1, Borås, 10:00 (English)
Available from: 2015-09-03 Created: 2015-06-29 Last updated: 2015-12-18Bibliographically approved

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