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The Effect of Carbon Monoxide Co-Adsorption on Ni-Catalysed Water Dissociation
University of Borås, School of Engineering.
University of Borås, School of Engineering.
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2013 (English)In: International Journal of Molecular Sciences, ISSN 1422-0067, E-ISSN 1422-0067, Vol. 14, no 12, 23301-23314 p.Article in journal (Refereed)
Abstract [en]

The effect of carbon monoxide (CO) co-adsorption on the dissociation of water on the Ni(111) surface has been studied using density functional theory. The structures of the adsorbed water molecule and of the transition state are changed by the presence of the CO molecule. The water O–H bond that is closest to the CO is lengthened compared to the structure in the absence of the CO, and the breaking O–H bond in the transition state structure has a larger imaginary frequency in the presence of CO. In addition, the distances between the Ni surface and H2O reactant and OH and H products decrease in the presence of the CO. The changes in structures and vibrational frequencies lead to a reaction energy that is 0.17 eV less exothermic in the presence of the CO, and an activation barrier that is 0.12 eV larger in the presence of the CO. At 463 K the water dissociation rate constant is an order of magnitude smaller in the presence of the CO. This reveals that far fewer water molecules will dissociate in the presence of CO under reaction conditions that are typical for the water-gas-shift reaction.

Place, publisher, year, edition, pages
M D P I AG , 2013. Vol. 14, no 12, 23301-23314 p.
Keyword [en]
water adsorption, water dissociation, nickel, water gas shift reaction, CO, H2O, DFT
Keyword [sv]
Energi och material
Research subject
Resource Recovery
Identifiers
URN: urn:nbn:se:hb:diva-1719DOI: 10.3390/ijms141223301ISI: 000330219800008PubMedID: 24287907Local ID: 2320/13136OAI: oai:DiVA.org:hb-1719DiVA: diva2:869788
Available from: 2015-11-13 Created: 2015-11-13 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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