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Theoretical Studies on the Molecular Mechanisms of Photo-Catalytic Reactions on TiO2 Surfaces
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0001-6994-9802
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.
Series
TRITA-BIO-Report, ISSN 1654-2312 ; 2014:10
Keyword [en]
TiO2, First-principles, photocatalysis
National Category
Theoretical Chemistry
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-145146ISBN: 978-91-7595-176-8 (print)OAI: oai:DiVA.org:kth-145146DiVA: diva2:717138
Public defence
2014-06-10, FA32, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20140522

Available from: 2014-05-22 Created: 2014-05-12 Last updated: 2014-05-22Bibliographically approved
List of papers
1. Location of Trapped Hole on Rutile-TiO2(110) Surface and Its Role in Water Oxidation
Open this publication in new window or tab >>Location of Trapped Hole on Rutile-TiO2(110) Surface and Its Role in Water Oxidation
2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 14, 7863-7866 p.Article in journal (Refereed) Published
Abstract [en]

The trapped hole and its nature on rutile TiO2(110) surface has been fully examined by first-principles GGA+U method with U(p) ranged from 3.0 to 7.0 eV. The bridge oxygen is found to be the most stable hole trapping site, and it is of a p-type orbital perpendicular to the bridge oxygen row. How the hole reacts with a H2O molecule above the surface is investigated with constrained minimization method. The highest occupied molecular orbital of approaching H2O is found to hybridize with the hole orbital and to form bonding and antibonding orbitals. An electron is seen to be transferred from H2O to the bridge oxygen mediated by the formed bonding state. The electron transfer is accompanied by H2O dissociation concertedly, which results in a hydroxyl radical adsorbed on the surface sharing the hole orbital with an in-plane oxygen atom. The reaction pathway is also estimated.

National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-94042 (URN)10.1021/jp300753f (DOI)000302591300031 ()2-s2.0-84859743415 (Scopus ID)
Note
QC 20120508Available from: 2012-05-08 Created: 2012-05-07 Last updated: 2017-12-07Bibliographically approved
2. GGA plus U Study on the Mechanism of Photodecomposition of Water Adsorbed on Rutile TiO2(110) Surface: Free vs Trapped Hole
Open this publication in new window or tab >>GGA plus U Study on the Mechanism of Photodecomposition of Water Adsorbed on Rutile TiO2(110) Surface: Free vs Trapped Hole
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 2, 1027-1034 p.Article in journal (Refereed) Published
Abstract [en]

The initial step of O-2 evolution reaction on a TiO2 surface is a long-standing puzzle. A recent scanning tunneling microscopy experiment showed that the H2O molecule adsorbed on rutile TiO2(110) surface could decompose under ultraviolet illumination (Tan, S. J.; et al. J. Am. Chem. Soc., 2012, 134, 9978). The underlying reaction mechanism is now examined by our GGA+U study, in which the oxidation of the H2O molecule by both free and trapped holes has been carefully investigated. It is found that the transfer of the hole trapped at the bridge oxygen to the molecule is hindered by the mismatch between the energy and spatial symmetry of the trapped hole orbital and the highest occupied molecule orbital of H2O. The entire oxidation reaction has a high energy barrier and is barely exothermic. In contrast, the oxidation of the molecule by the free hole is energetically more favorable. The free hole is transferred to the H2O molecule via the in-plane oxygen atom when the molecule stays in the transient dissociation state. This mechanism may also be applicable to the photooxidation of other R OH type molecules adsorbed on the rutile TiO2(110) surface.

Keyword
Transient Absorption-Spectroscopy, Initio Molecular-Dynamics, Total-Energy Calculations, Wave Basis-Set, Photocatalytic Dissociation, TIO2 Nanoparticles, Photooxidation, Adsorption, Methanol, Oxidation
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-141965 (URN)10.1021/jp409605y (DOI)000330417100033 ()2-s2.0-84892756656 (Scopus ID)
Note

QC 20140227

Available from: 2014-02-27 Created: 2014-02-27 Last updated: 2017-12-05Bibliographically approved
3. A Comparative Theoretical Study of Proton-Coupled Hole Transfer for H2O and Small Organic Molecules (CH3OH, HCOOH, H2CO) on the Anatase TiO2(101) Surface
Open this publication in new window or tab >>A Comparative Theoretical Study of Proton-Coupled Hole Transfer for H2O and Small Organic Molecules (CH3OH, HCOOH, H2CO) on the Anatase TiO2(101) Surface
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 37, 21457-21462 p.Article in journal (Refereed) Published
Abstract [en]

The high oxidation power of the photogenerated hole in TiO2 has made it useful in many applications. It is of fundamental importance to understand how the hole transfers from the catalysis to adsorbates. We have performed a comparative study on the mechanism for the first proton-coupled hole transfer process in water, methanol, formic acid, and formaldehyde on the anatase TiO2(101) surface. Our results show that this process for all the molecules is concerted rather than sequential. Both the kinetic and thermodynamic effects need to be taken into account. The hole scavenging power for the four molecules under investigation is found to follow the order formaldehyde > formic acid > methanol > water, which agrees well with various experiments.

Keyword
Transient Absorption-Spectroscopy, Total-Energy Calculations, Wave Basis-Set, Tio2 Anatase, Formic-Acid, Photocatalytic Dissociation, Water Oxidation, Charge-Transfer, Trapped Holes, Kinetic-Model
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-145187 (URN)10.1021/jp505854t (DOI)000342118500017 ()2-s2.0-84949116189 (Scopus ID)
Note

QC 20141021. Updated from submitted to published.

Available from: 2014-05-14 Created: 2014-05-14 Last updated: 2017-12-05Bibliographically approved
4. First-Principles Study on the Mechanism of Photoselective Catalytic Reduction of NO by NH3 on Anatase TiO2(101) Surface
Open this publication in new window or tab >>First-Principles Study on the Mechanism of Photoselective Catalytic Reduction of NO by NH3 on Anatase TiO2(101) Surface
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.

Keyword
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
Identifiers
urn:nbn:se:kth:diva-144942 (URN)10.1021/jp501427k (DOI)000333578300043 ()2-s2.0-84897389524 (Scopus ID)
Note

QC 20140505

Available from: 2014-05-05 Created: 2014-05-05 Last updated: 2017-12-05Bibliographically approved
5. First principles study of O2 adsorption on reduced rutile TiO2-(110) surface under UV illumination and its role on CO oxidation
Open this publication in new window or tab >>First principles study of O2 adsorption on reduced rutile TiO2-(110) surface under UV illumination and its role on CO oxidation
2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 2, 956-961 p.Article in journal (Refereed) Published
Abstract [en]

Oxidation of CO by O2 on the reduced rutile TiO2(110) surface under UV illumination has been explored by first-principles simulations. It is found that, at the ground state, the O2 molecule prefers to be adsorbed at the oxygen vacancy horizontally; whereas under photo excitation, it can capture a hole as it transforms itself into a near-perpendicular geometry. Such a photoexcited O2 can be effectively connected to the CO molecule to form a O-O-CO complex, which can then convert to CO2 by overcoming a small barrier. This mechanism can be applied to both low and high O2 coverage and is consistent with the off-normal desorption behavior of the CO2 observed in recent experiments.

Keyword
CO molecule, Co oxidation, Desorption behavior, First-principles simulations, First-principles study, Oxidation of CO, Photo-excitations, Rutile TiO, UV illuminations
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-118195 (URN)10.1021/jp310443p (DOI)000313932800025 ()2-s2.0-84872725612 (Scopus ID)
Note

QC 20130215

Available from: 2013-02-15 Created: 2013-02-13 Last updated: 2017-12-06Bibliographically approved

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