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Coadsorption properties of CO2 and H2O on TiO2 rutile (110): A dispersion-corrected DFT study
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10.1063/1.4739088
/content/aip/journal/jcp/137/7/10.1063/1.4739088
http://aip.metastore.ingenta.com/content/aip/journal/jcp/137/7/10.1063/1.4739088

Figures

Image of FIG. 1.
FIG. 1.

(a) STM image (6 × 6 nm2, 1.5 V, 5 pA) after dosing CO2 on a clean, reduced rutile (110) at T = 50 K. A bridging hydroxyl (OHb) feature is observed in addition to oxygen vacancies (VO). (b) STM image (9 × 9 nm2, 1.5 V, 10 pA) after dosing water on a clean, reduced rutile (110) at T = 50 K. Water molecules are found to adsorb on the Ti rows. A lattice grid with the intersections of vertical and horizontal lines corresponding to the Ti(5f) atoms is superimposed on the STM images. This enables one to see that the centers of the water features are located on top of Ti(5f) atoms. (c) STM image (10 × 10 nm2, 1.5 V, 10 pA) after dosing CO2 on the water/rutile (110) surface at T = 50 K. No clustering between the adsorbed water and CO2 is observed. (d) Apparent height profiles along the three species indicated in (c). The apparent heights were measured with respect to the surface Ti(5f) plane in the STM image.

Image of FIG. 2.
FIG. 2.

(a) Front (uppermost row) and top (bottom row) views of the main adsorption configurations of CO2 on the oxidized (non-defective) [configurations C(1)-C(5)] and defective [configuration C(6)] rutile (110) surfaces: C(1) monodentate adsorption state at Ti(5f) in a tilted configuration; C(2) bidentate adsorption along the Ti(5f) row in a lying-down configuration; C(3) adsorption in a bent configuration with formation of a C–Ob bond; C(4) adsorption parallel to the surface, perpendicular to the bridging oxygen rows, and located in-between the bridging oxygen atoms; C(5) configuration similar to C(4) but located on-top a bridging oxygen atom. This configuration is a second-order saddle point; C(6) adsorption at a bridging oxygen defect in a tilted configuration. (b) Minimum energy pathway for interconversion reaction of a CO2 molecule from the tilted, linear C(1) to the bent C(3) configuration. Calculations have been done using a (4×2) supercell with four and five O–Ti2O2–O trilayers. The energy barriers (Eb) and reaction energies (ΔEr) are indicated in the inset table. For increased clarity, the oxygen atoms of CO2 are indicated in pink, and the oxygen atoms of the rutile slab are indicated in red.

Image of FIG. 3.
FIG. 3.

(a) Front (uppermost row) and top (bottom row) views of the main adsorption configurations of H2O on the oxidized [configurations W(1)-W(5)] and defective [configuration W(6)-W(7)] rutile (110) surfaces: W(1) and W(2) laying down and vertical adsorption configurations at 1/8 ML coverage; W(3)-W(5) adsorption at 1 ML coverage in a configuration with similar and alternating orientations of the molecules, respectively. The arrangements for W(4) and W(5) differ through orientation of the water molecules on successive Ti rows; W(6) molecular adsorption at a VO defect site; W(7) dissociated state at a VO site with formation of two OHb species. Panels (b) and (c) show the minimum energy pathways for dissociation of a H2O molecule at 1/8ML and at 1 ML, respectively. The labeling for the terminal OHt and bridging OHb species is also shown in the inset corresponding to the last image in panel (b). The energy barriers and reaction energies obtained from (4×2) slab calculations with four and five trilayers are indicated in the inset tables.

Image of FIG. 4.
FIG. 4.

Front (uppermost row) and top (bottom row) views of the main coadsorption configurations of CO2 and H2O species on (a) oxidized [configurations M(1)-M(7)] and (b) defective [configurations MD(1)-MD(4)] rutile (110) surfaces. The views for the front configurations of M(2), M(3), and M(7) are taken along the direction and along the [001] direction for all the other configurations.

Image of FIG. 5.
FIG. 5.

Front (uppermost row) and top (bottom row) views of the main coadsorption configurations of CO2 with a terminal OHt [configurations HT(1)-HT(7)] and a bridging OHb [configurations HB(1)-HB(4)] on the oxidized rutile (110) surface. For the defective surface only the coadsorption configurations of CO2 with an OHt species [HTD(1)-HTD(3)] are indicated. The views for the front configurations of HT(2), HT(3), and HB(4) are taken along direction and along [001] for all the other configurations.

Image of FIG. 6.
FIG. 6.

Relative stabilities of the formic acid and formate species on the (a) oxidized and (b) defective surfaces, respectively, and of the bicarbonate species on the (c) oxidized and (d) defective surfaces, respectively. For each diagram the reference states used are as follows: (a) C(1) and 2OHb(7) separated by large distances, (b) C(6) and 2OHb(7) adsorbed on different rows as indicated by the first inset figure in panel (b), (c) C(1) and W(1) configurations separated at large distances, and (d) C(6) and W(1) separated at larger distances. The relative energy of each configuration with respect to the chosen reference energy is also indicated. In each panel, the index of the indicated configurations corresponds to the relative stability order starting with the least stable configuration. The formic acid and formate species are indicated with the acronym FA and the bicarbonate species with the acronym BC.

Image of FIG. 7.
FIG. 7.

Minimum energy pathways for reactions of a CO2 molecule with coadsorbed H2O on the oxidized (110) surface: (a) reaction initiated from configuration M(5) in which CO2 is located above and perpendicular to the Ob row with formation of a bicarbonate and an OHt species; (b) reaction initiated from configuration M(6) in which two H2O molecules simultaneously interact with a CO2 molecule with formation of a carbonic acid molecule and two OHt species; (c) reaction initiated from M(4) where proton transfer from H2O to the bent CO2 leads to formation of a bicarbonate and OHt species; (d) reaction initiated from M(2) leading to formation of a bicarbonate species.

Image of FIG. 8.
FIG. 8.

Minimum energy reaction pathways for reaction of a CO2 molecule with three to five H2O molecule on the rutile (110) surface. (a) Reaction proceeds with an initial dissociation of the H2O molecule bonded to the surface, followed by a proton transfer between H2O molecules, and ultimately to the CO2 molecule with formation of a bicarbonate species; (b) formation of a carbonic acid molecule following a proton transfer between two H2O molecules and reaction of the resulting OH and H entities with the CO2 molecule; (c) and (d) reactions of a CO2 molecule adsorbed at a defect site to give carbonic acid. The transfer of a proton between H2O molecules leads to formation of OH and H fragments which then interact with CO2 to form carbonic acid.

Image of FIG. 9.
FIG. 9.

Minimum energy reaction pathway for BC(2) → BC(3) interconversion (a) direct and (b) mediated by a H2O molecule. In the latter case, the proton of the BC(2) species is transferred first to a H2O molecule resulting in a hydronium ion followed by a transfer back to the carbonate at a different atomic site.

Image of FIG. 10.
FIG. 10.

Minimum energy pathways for reaction of a CO2 molecule with coadsorbed OH species: (a) reaction initiated from the HT(6) configuration corresponding to CO2 + 2OHt and leading to formation of a carbonate species; (b) reaction initiated from HB(4) configuration of CO2 + OHb and leading to formation of a bicarbonate species; (c) and (d) nucleophilic attack of OHt with insertion into the CO2 bond and bicarbonate formation on the oxidized and defective surfaces, respectively.

Tables

Generic image for table
Table I.

Adsorption energies, representative geometrical parameters, vibrational frequencies, and variation of the total Bader charge of CO2 and H2O molecules adsorbed at different sites on oxidized and defective rutile (110) surfaces in the (4×2) supercell.

Generic image for table
Table II.

Adsorption energies, representative geometrical parameters, vibrational frequencies, and variation of the total Bader charge of a CO2 molecule coadsorbed with a H2O molecule on the oxidized [M(1)-M(7)] and defective [MD(1)-MD(4)] rutile (110) surfaces.

Generic image for table
Table III.

Adsorption energies, representative geometrical parameters, vibrational frequencies, and variation of the total Bader charge of a CO2 molecule coadsorbed with a terminal OHt on the oxidized [HT(1-7)] and defective [HTD(1-3)] rutile (110) surfaces and with a bridging OHb [HB(1-4)] on the oxidized (110) surface.

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/content/aip/journal/jcp/137/7/10.1063/1.4739088
2012-08-16
2014-04-25
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Coadsorption properties of CO2 and H2O on TiO2 rutile (110): A dispersion-corrected DFT study
http://aip.metastore.ingenta.com/content/aip/journal/jcp/137/7/10.1063/1.4739088
10.1063/1.4739088
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