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First-Principles Study on the Mechanism of Photoselective Catalytic Reduction of NO by NH3 on Anatase TiO2(101) Surface
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0001-6994-9802
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0003-0007-0394
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 12, 6359-6364 p.Article in journal (Refereed) Published
Abstract [en]

A promising method for NO abatement is photoselective reduction with a proper semiconductor, such as TiO2. Here we report a systematic theoretical study on NO abatement through an adsorbed NH3 molecule on the anatase TiO2(101) surface. The reaction mechanism proposed by experiments has been verified. The key process, namely, the oxidation of the adsorbed NH3 molecule by photogenerated hole, has been investigated by two different methods: one is to use the triplet state to mimic the real excited state and the other is to inject a hole to the slab by the adsorption of center dot OH radical. Both methods give almost the same result, and the oxidation of the NH3 molecule is found to be a concerted proton coupled charge transfer process. The center dot NH2 radical, resulting from the oxidation of NH3, can be attacked by a NO molecule from the gas phase to form a NH2NO complex spontaneously. The decomposition of this complex to N-2 and H2O is the rate limiting step of the overall reaction. This multistep decomposition process consists of the following sequences: the H atom transfers to the O atom in the molecule first to form HNNOH that further decomposes to N-2 and OH groups, and the latter group recombines to produce the H2O molecule.

Place, publisher, year, edition, pages
2014. Vol. 118, no 12, 6359-6364 p.
Keyword [en]
Free radical reactions, Free radicals, Molecules, Oxidation, Surface reactions, Titanium dioxide, Catalytic reduction, Charge transfer process, Decomposition process, First-principles study, Photogenerated holes, Rate-limiting steps, Reaction mechanism, Theoretical study
National Category
Physical Chemistry
URN: urn:nbn:se:kth:diva-144942DOI: 10.1021/jp501427kISI: 000333578300043ScopusID: 2-s2.0-84897389524OAI: diva2:715500

QC 20140505

Available from: 2014-05-05 Created: 2014-05-05 Last updated: 2014-05-22Bibliographically approved
In thesis
1. Theoretical Studies on the Molecular Mechanisms of Photo-Catalytic Reactions on TiO2 Surfaces
Open this publication in new window or tab >>Theoretical Studies on the Molecular Mechanisms of Photo-Catalytic Reactions on TiO2 Surfaces
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Photocatalysis is a promising technology that can effectively convert the solar energyinto sustainable green energy. However, theoretical studies on the molecular mechanisms of photocatalytic reactions are rare. This thesis is devoted to investigate several typical photocatalytic reactions on the surfaces of the most popular photocatalysis TiO2 with density functional theory. We start our study with the characterization of both the free and trapped hole on the surface generated by the light. The oxidation of physisorbed H2O molecule by the hole trapped at bridge oxygen on rutile TiO2(110) surface has been studied. The hole is found to transferto the molecule via the anti-bonding orbital as a result of the hybridization between the hole orbital and the HOMO of the molecule. The energy and symmetry mismatching between the trapped hole orbital and the HOMO of the molecule explains why the trapped hole cannot directly transfer to the chemisorbed H2O molecule. On the other hand, we have found that the chemisorbed H2O moleculecan be more efficiently oxidized by the free hole with a lower barrier and higher reaction energy compared to the oxidation by the trapped hole. In this reaction, the free hole is transferred to the chemisorbed H2O after the dissociation. This is different from the oxidation of chemisorbed H2O on anatase TiO2(101) surface by free hole, in which the hole is transferred concertedly with the dissociation of themolecule.

    In order to understand the hole scavenger ability of organic molecules, the oxidation of three small organic molecules (CH3OH, HCOOH and HCOH) onanatase TiO2(101) surface has been systematically investigated. The concerted hole and proton transfer is found for all these molecules. The calculations suggestthat both kinetic and thermodynamic effects need to be considered to correctly describe the hole transfer process. The order of hole scavenging power is found tofollow: HCOH > HCOOH > CH3OH > H2O, which agrees well with experiments.

    Photo-selective catalytic reduction of the NO by NH3 and the photooxidationof CO by O2 are closely related to the environment application. Both reactionsinvolve the formation and/or breaking of non R–H bonds. The mechanism for the photoreduction of NO proposed by experiment has been verified by our calculations.The role of the hole is to oxidize the adsorbed NH3 into ·NH2 radical, which canform a NH2NO complex with a gaseous NO molecule easily. The photooxidation of CO by O2 is the first multi-step photoreaction we ever studied. By combining thepotential energy surfaces at the ground and excited state we have found that thehole and electron both take part in the reaction. A molecular mechanism which is in consistent with various experiments is proposed.

    These studies show that density functional theory is a powerful tool for studying the photocatalytic reaction. Apparently, more work needs to be done in orderto improve the performance of the existing materials and to design new ones thatcan take advantage of the solar light more efficiently

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. xii, 70 p.
TRITA-BIO-Report, ISSN 1654-2312 ; 2014:10
TiO2, First-principles, photocatalysis
National Category
Theoretical Chemistry
Research subject
urn:nbn:se:kth:diva-145146 (URN)978-91-7595-176-8 (ISBN)
Public defence
2014-06-10, FA32, Roslagstullsbacken 21, Stockholm, 10:00 (English)

QC 20140522

Available from: 2014-05-22 Created: 2014-05-12 Last updated: 2014-05-22Bibliographically approved

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