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Computational studies of nickel catalysed reactions relevant for hydrocarbon gasification
University of Borås, Faculty of Textiles, Engineering and Business.
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 [en]
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: urn:nbn:se:hb:diva-323ISBN: 978-91-87525-67-4 (print)ISBN: 978-91-87525-68-1 (print)OAI: oai:DiVA.org:hb-323DiVA: diva2:827655
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
List of papers
1. DFT study of the adsorption and dissociation of water on Ni(111), Ni(110) and Ni(100) surfaces
Open this publication in new window or tab >>DFT study of the adsorption and dissociation of water on Ni(111), Ni(110) and Ni(100) surfaces
2014 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 627, 1-10 p.Article in journal (Refereed) Published
Abstract [en]

Water adsorption and dissociation on catalytic metal surfaces play a key role in a variety of industrial processes, and a detailed understanding of this process and how it is effected by the surface structure will assist in developing improved catalysts. Hence, a comparative study of the adsorption and dissociation of water on Ni(111), Ni(110) and Ni(100) surfaces, which is often used as catalyst, has been performed using density functional theory. The results show that the adsorption energies and dissociation rates depend on the surface structure. The adsorption energies for H2O and OH decrease in the order Ni(110) > Ni(100) > Ni(111), and for the O and H atoms the adsorption energies decrease in the order Ni(100) > Ni(111) > Ni(110). In addition, the splitting of water to OH and H has lower activation energies over less packed Ni(110) and Ni(100) surfaces compared to the highly packed Ni(111) surface. The subsequent splitting of the OH to O and H also has the lowest activation energy on the Ni(110) surface. At 463 K, which is typical for industrial processes that include the water gas shift reaction, the H2O splitting is approximately 6000 and 10 times faster on the Ni(110) surface compared to the Ni(111) and Ni(100) surfaces, respectively, and OH splitting is 200 and 3000 times faster, respectively. The complete water dissociation reaction rate decreases in the order Ni(110) > Ni(100) > Ni(111).

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Adsorption, Dissociation, Nickel, Water, DFT, Resource Recovery, Resursåtervinning
National Category
Chemical Engineering
Research subject
Resource Recovery
Identifiers
urn:nbn:se:hb:diva-1867 (URN)10.1016/j.susc.2014.04.006 (DOI)000338621500001 ()2320/13744 (Local ID)2320/13744 (Archive number)2320/13744 (OAI)
Available from: 2015-11-13 Created: 2015-11-13 Last updated: 2017-11-03Bibliographically approved
2. The Effect of Carbon Monoxide Co-Adsorption on Ni-Catalysed Water Dissociation
Open this publication in new window or tab >>The Effect of Carbon Monoxide Co-Adsorption on Ni-Catalysed Water Dissociation
Show others...
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) Published
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
Keyword
water adsorption, water dissociation, nickel, water gas shift reaction, CO, H2O, DFT, Energi och material
National Category
Chemical Sciences Physical Chemistry Theoretical Chemistry
Research subject
Resource Recovery
Identifiers
urn:nbn:se:hb:diva-1719 (URN)10.3390/ijms141223301 (DOI)000330219800008 ()24287907 (PubMedID)2320/13136 (Local ID)2320/13136 (Archive number)2320/13136 (OAI)
Available from: 2015-11-13 Created: 2015-11-13 Last updated: 2017-12-01Bibliographically approved
3. A density functional theory study of hydrocarbon combustion and synthesis on Ni surfaces
Open this publication in new window or tab >>A density functional theory study of hydrocarbon combustion and synthesis on Ni surfaces
2015 (English)In: Journal of Molecular Modeling, ISSN 1610-2940, Vol. 21, no 3Article in journal (Refereed) Published
Abstract [en]

Combustion and synthesis of hydrocarbons may occur directly (CH → C + H and CO → C + O) or via a formyl (CHO) intermediate. Density functional theory (DFT) calculations were performed to calculate the activation and reaction energies of these reactions on Ni(111), Ni(110), and Ni(100) surfaces. The results show that the energies are sensitive to the surface structure. The dissociation barrier for methylidyne (CH → C + H: catalytic hydrocarbon combustion) is lower than that for its oxidation reaction (CH + O → CHO) on the Ni(110) and Ni(100) surfaces. However the oxidation barrier is lower than that for dissociation on the Ni(111) surface. The dissociation barrier for methylidyne dissociation decreases in the order Ni(111) > Ni(100) > Ni(110). The barrier of formyl dissociation to CO and H is almost the same on the Ni(111) and Ni(110) surfaces and is lower compared to the Ni(100) surface. The energy barrier for carbon monoxide dissociation (CO → C + O: catalytic hydrocarbon synthesis) is higher than that of for its hydrogenation reaction (CO + H → CHO) on all three surfaces. This means that the hydrogenation to CHO is favored on these nickel surfaces. The energy barrier for both reactions decreases in the order Ni(111) > Ni(100) > Ni(110). The barrier for formyl dissociation to CH + O decreases in the order Ni(100) > Ni(111) > Ni(110). Based on these DFT calculations, the Ni(110) surface shows a better catalytic activity for hydrocarbon combustion compared to the other surfaces, and Ni is a better catalyst for the combustion reaction than for hydrocarbon synthesis, where the reaction rate constants are small. The reactions studied here support the BEP principles with R2 values equal to 0.85 for C-H bond breaking/forming and 0.72 for C-O bond breaking /forming reactions.

Keyword
DFT, Hydrocarbon combustion, Hydrocarbon synthesis, Nickel
National Category
Engineering and Technology
Identifiers
urn:nbn:se:hb:diva-264 (URN)
Available from: 2015-06-25 Created: 2015-06-25 Last updated: 2015-12-18
4. DFT study of the water gas shift reaction on Ni(111), Ni(100) and Ni(110) surfaces
Open this publication in new window or tab >>DFT study of the water gas shift reaction on Ni(111), Ni(100) and Ni(110) surfaces
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.

Keyword
Water gas shift reaction; DFT; Nickel; Ni(111); Ni(100); Ni(110)
National Category
Engineering and Technology
Research subject
Resource Recovery
Identifiers
urn:nbn:se:hb:diva-3287 (URN)10.1016/j.susc.2015.09.014 (DOI)
Available from: 2015-11-17 Created: 2015-11-17 Last updated: 2015-12-18

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