Methods section is more concise with elimination of some...

April 25, 2017

Dr. Rainer Glaser, Professor of Chemistry

Editor, Inorganic Chemistry

Department of Chemistry, University of Missouri-Columbia

Columbia, MO 65211

RE: REVISED

Carbon Capture, Utilization, and Storage. A New Method for Carbon Dioxide

Photoreduction Catalyzed by Oxygen Deficient TiO2 Surfaces By: Melissa Hopkins, Parker Smith, and Ethan Zars

Dear Dr. Glaser,

Thank you for your comments on April 20 and the inclusion of the peer reviews on our work. We

used their suggestions as guidance for some necessary revisions. Based on reviewer

recommendation, we have made the following changes:

Major Revisons

[M.1] Methods section is more concise with elimination of some characterization

descriptions.

[M.2] Discussion is made clearer by reworking of sentences and increased discussion of

Lewis acid and base theory.

[M.3] Top part of Figure 1 was corrected to match the figure legend.

[M.4] Scheme 2 is corrected with labeling for bidentated carbonates.

Melissa Hopkins

Email: [email protected]

Parker Smith

Email: [email protected]

Ethan Zars Email: [email protected]

Department of Chemistry

University of Missouri-Columbia

105 Chemistry Building

601 S. College Avenue

Columbia, MO 65211

USA

mailto:[email protected]:[email protected]:[email protected]

2

Response to Reviewer 1

[1.1] See M.3 and M.4.


[2.1] See M.2

[2.2] Addition of a few sentences describing how the mechanism of reduced

intermediates to the products is unknown.

[2.3] Binding structures for the different doped surfaces are not given in the cited

references.

[2.4] See M.3 and M.4.


[3.1] See M.1.

[3.2] Table 1 gives rates of reduction and mass efficiencies for the discussed catalysts.

[3.3] We don’t find that the addition of color would help convey the message more

clearly.

Sincerely,

Melissa, Parker, and Ethan

1


Photoreduction Catalyzed by Oxygen Deficient TiO2 Surfaces.

Melissa Hopkins, Parker Smith, and Ethan Zars

Department of Chemistry, University of Missouri-Columbia, Columbia, MO, USA 65201

Email: [email protected]; [email protected];

[email protected]


2

Abstract

The photocatalytic reduction of carbon dioxide, a known greenhouse gas, utilizes

properties of semiconducting materials in the formation of products such as carbon

monoxide and methane. Titanium dioxide (TiO2) surfaces are the most commonly

reported materials used due to the effective band gap of molecular orbitals and absorption

in the near-visible region of the electromagnetic spectrum, allowing for activation via

sunlight irradiation. TiO2 has three polymorphs of anatase, rutile, and brookite which

have varying properties with regard to the formation of oxygen vacancies that increase

efficacy. Additionally, TiO2 surfaces can become more effective catalysts through the

use of dopants such as Ce, Cu and Pd lattices, and Pt, though cost efficiencies factor into

the feasibility of such compounds for commercial usage.

3

Introduction

One of the most pressing issues today is global climate change, primarily caused

by anthropogenic greenhouse gases.1 One anthropogenic greenhouse gas that is of

particular concern is carbon dioxide (CO2) because of its role in global climate change.2

It is, however, possible to remove and utilize CO2 from the atmosphere by various

capture, utilization, and storage methods.3 The main challenge that needs to be overcome

in CO2 utilization is the reduction of this highly stable, but also completely oxidized

molecule, to products such as carbon monoxide (CO) and methane (CH4) which can be

more easily utilized as chemical feedstock or fuels.

Many sources of energy are available for the reduction of CO2, but the process

can only contribute to a decrease in global CO2 levels if the energy source is renewable

and does not emit CO2. Solar radiation is a particularly attractive energy source because

it is abundant, renewable, and powerful enough to effectively reduce CO2 to CO and CH4

among other products.4 Many methods for photocatalytic reduction of CO2 have been

reported and most involve various forms of TiO2 surfaces in the three main phases: rutile,

brookite, and anatase. TiO2 surfaces are effective reducers of CO2 because they form

semiconductors with a HOMO/LUMO bandgap large enough reduce CO2 and almost

small enough to absorb in the visible region of the electromagnetic spectrum.4 For the

industrial use of TiO2 as a photocatalyst in CO2 reduction, the cost of the catalyst must

remain low, but the reduction of CO2 must still be effective. Luckily, anatase-phase TiO2

can effectively reduce CO2, and this activity is enhanced by partial hydration of the

anatase TiO2 surface.5 Furthermore, creating oxygen vacancies in brookite-phase TiO2

allows for the photocatalytic reduction of CO2 to CO and CH4 products. 6

4

In this paper we will compare the cost7 efficiencies of brookite-phase TiO2

photocatalysts with oxygen deficiencies, TiO2 photocatalysts doped with Ce,8 Pd and

Cu,9 and Pt10 (Scheme 1) and we find that the Pd and Cu doped TiO2 photocatalysts are

the most effective photocatalysts by mass, but Pd is extremely expensive. When the cost

efficiencies are compared, the brookite-phase TiO2 with oxygen vacancies proves to be

the most cost efficient.

Scheme 1. Representation of the Four TiO2 Surfaces Tested.

Materials and Methods

Preparation of TiO2 Photocatalyst

TiO2 crystals were synthesized by hydrolysis and hydrothermal methods to form

their three phases: rutile, anatase, and brookite. The rutile phase was synthesized by

combining TiCl4 with ethanol and water. The solution was stirred and then kept in an

oven for 24 hours. The white precipitate was collected, washed with water, and dried in

an oven. Anatase was synthesized by combining aqueous Ti(NH3Lac)2·(OH)2 with urea

and then diluting the solution in water. This solution was then sealed in an autoclave and

placed in an oven at 160 ºC for 24 hours. Next the precipitate was collected, washed with

water, and dried overnight in an oven at 60 ºC. Brookite was prepared following the

same method as rutile except using a much higher concentration of urea. All three

powders were calcined at 400 ºC. To prepare oxygen-deficient TiO2 crystals the as-

prepared TiO2 crystals were treated with helium gas at an elevated temperature.

5

Characterization of TiO2 Crystals

X-ray Diffraction (XRD), Raman, Transmission electron spectroscopy (TEM),

UV-Vis, and DRIFTS-IR spectroscopy were used to characterize the TiO2 surfaces.

XRD (Figure S1) is a useful analytical technique for the characterization and

identification of crystals because it produces data that is unique to certain materials and

those data are available for comparison in a database.11

Raman spectroscopy (Figure S1) was also used to characterize and identify TiO2

polymorphs. The spacing between Raman absorbance peaks will always be the rotational

constant multiplied by four (except in the case of symmetric rotors) and therefore the

bond length or reduced mass of the bonding atoms can be determined. Raman

spectroscopy is used to identify TiO2 crystal phases because Raman spectroscopy only

detects vibrational or rotational modes that result in a change of polarizability of the

bond. The different phases of TiO2 will have a unique set of Raman active modes

(brookite has 36 active modes, anatase has 5, and rutile has 3) that can be used to identify

and characterize the crystal structures.

Transmission electron microscopy (TEM) is a further technique that is used to

identify and characterize TiO2 (Figure S2) and is especially useful because it allows the

user to visualize the crystal structure. TEM allows for finer resolution of small objects

than traditional visible light microscopy because the size of objects that can be discerned

is limited by the wavelength of the radiation interacting with them. The wavelength

associated with electrons transmitted through a sample is smaller than the wavelengths of

visible light.

While the above techniques allow for the characterization and identification of the

gross crystal structure, they are not sufficient for detecting oxygen vacancies or other

6

defects in the crystals. UV-Vis spectroscopy was used to detect oxygen vacancies in the

TiO2 polymorphs (Figure S3). In TiO2 crystals with oxygen vacancies there will be an

increased absorption in lower frequency radiation (as compared to TiO2 crystals without

oxygen vacancies) because these oxygen vacancies have compressed and inconsistent

electron energy level gaps.

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is used to

characterize the roles of oxygen vacancies (VO) and Ti3+ sites in He-treated and

unpretreated surfaces (Figure S4). DRIFTS studies are similar to IR spectral studies, but

more specific detail can be observed in the peaks due to greater sensitivity. Light is

passed through a non-absorbing material, such as KBr, then interacts with the sample by

reflecting within the crystal structure. The result is a linear relationship between spectral

intensity and sample concentration thus giving more specificity than a regular IR spectra.

Performance Evaluation by DRIFTS-IR Spectroscopy and Gas Chromatography

DRIFTS-IR spectroscopy can be used to track changes in amounts of certain

molecules by observing intensity changes of the characteristic peaks over time.

Following Beer’s Law, an increase in absorbance at certain characteristic wavenumbers

corresponds to an increased concentration of molecules with that characteristic absorption

frequency. In order to measure the effectiveness of a catalyst system the disappearance

of the H2O absorption peak at 1640 cm-1 and the appearance of the HCO3- peak at 1420

cm-1 over time need to be considered. Differences in the absorbances at these

wavenumbers will be directly proportional to the intermediates and reactants they

represent and will be related to CO and CH4 production which is exemplified in Figure 1.

CO2 gas was passed through a water bubbler to become loaded with H2O vapor

and then allowed to pass over the photocatalyst under a 150 W lamp. For each test 100

7

mg of TiO2 was used. Identity and concentration determination of the product gases was

performed using gas chromatography using both a thermal conductivity detector and a

flame ionization detector. The gas chromatograms were compared to standard eluents to

identify and quantify the products. The production of CO and CH4 product gases was

calculated over a 6 hour photolumination period. These gas chromatography data yield

CO and CH4 production in units of µmol·g-1 (µmol product per gram of catalyst). In

order to compare the cost efficiency of different catalyst systems the product yield needs

to be scaled to one hour of reaction time and then the price of the catalyst system ($·kg-1)

is divided by the product yield per hour. The cost12 efficiencies and performance data of

TiO2 photocatalysts with and without oxygen deficiencies, TiO2 photocatalysts doped

with Ce (0.28 mol %),8 Pd and Cu (11.5 mol % and 2.27 mol %, respectively),9 and Pt (1

wt %)10 are shown in Table 1.

8

Table 1. Cost and Mass Efficiencies of Four Selected Photocatalyst Systems.

Catalyst Co-Catalyst Co-catalyst

abundance

Yield of CH4

per hour

(μmolg-1h-1)

Price of

reagents per

kg catalyst

($·kg-1)

Price of

reagents per

kg catalyst

per yield of

CH4 ($·g·h·

μmol-1·kg-1)

TiO2 - - 0.57 57.00 100

TiO2 Ce 0.28 mol % 0.89 338.43 380.26

TiO2 Pd and Cu

11.5 mol %

Pd

2.27 mol %

Cu

10.00 22,847.46 2,284.75

TiO2 Pt 1 wt % 2.65 1,978.47 746.59

9

Figure 1. DRIFTS-IR spectra of CO2 and H2O interaction with (a, top) unpretreated

anatase (TiA(UP)) and (b, top) brookite (TiB(UP)), (a, bottom) He-treated anatase

(TiA(He)) and (b, bottom) brookite (TiB(UP)) irradiated by radiation in the UV and

visible regions as a function of time. The light was turned on after the samples sat in the

dark for 15 min.

10

Results

All the XRD (Figure S1) intensity peaks corresponded well to the database11

values. Raman spectra (Figure S1) also show the characteristic Raman modes of

anatase, rutile, and brookite. Brookite has 36 possible Raman active modes and in the

spectrum many peaks are visible. Rutile shows one small Raman absorbance peak at 234

cm-1 and two larger peaks at 446.5 and 608.5 cm-1 corresponding to the three possible

Raman active modes of rutile. Anatase shows Raman absorbance peaks at 142.4, 192.5,

393.6, 514.2, and 638.1 cm-1 corresponding to the five Raman modes of anatase. TEM

also was used to characterize the TiO2 crystals. Only the pretreated samples were

analyzed by TEM because the XRD and Raman data showed no significant changes

following helium treatment. The TEM images (Figure S2) for the anatase crystals show

an interplanar spacing of approximately 0.350 nm and irregularly shaped hexagons,

circles, or rectangles with an average size of about 9 nm. A smaller interplanar spacing

of 0.322 nm and short nanoellipses and elliptical rods are observed for the rutile phase

crystals. The brookite crystals exhibit randomly dispersed nanorods with a length of

approximately 100 nm and diameter of 20 nm. The interplanar spacing of brookite is

0.544 nm and the lattice spacing is observed to be 0.344 nm. All of these TEM

characterizations correspond well with previous crystallographic descriptions of anatase,

rutile, and brookite TiO2 crystals.

Additionally, there was no discernible difference in the XRD and Raman before

and after helium treatment meaning the total crystal structure was not significantly altered

by helium treatment. The helium treatment was an effective way to remove oxygen from

the TiO2 surface and these oxygen deficient defects were detected by UV-Vis (Figure

S3) and DRIFTS-IR spectroscopy (Figure S4). For the brookite phase the absorbance

before 400 nm is noticeably reduced after helium treatment and a broad, gradual decline

11

between 400 and 550 nm can be seen. These changes are also observed in the anatase-

phase UV-Vis spectrum and indicate the formation of an oxygen hole or vacancy which

can efficiently capture visible light. The near identical UV-Vis spectrum for the rutile-

phase before and after helium treatment indicates the absence of oxygen vacancies in

rutile-phase TiO2.

The oxygen vacancies detected in the UV-Vis spectrum allow for the addition of

water to the TiO2 structure. When water replaces oxygen in this structure it creates

anomalies in the surface structure that allow for greater binding and reduction of CO2.5,6

This is because bending of the normally linear CO2 molecule decreases the

HOMO/LUMO energy gap and allows for an easier addition of an electron into the

antibonding orbital of CO2. The possible binding forms of CO2 to TiO2 surfaces are

given in Scheme 2. Simple coordination of CO2 to the TiO2 surface includes two forms

each of linear coordination and non-linear coordination. An oxygen atom can be

incorporated into the CO2 to form two forms each of monodentated and bidentated

carbonates. When water is added to the TiO2 surface two forms each of monodentated

and bidentated bicarbonates result. In all these cases the electropositive carbon atom

coordinates or bonds with an electronegative oxygen atoms and the electronegative

oxygen atoms in CO2 coordinate or bind with the electropositive Ti atoms in the TiO2

surface. Water addition to the TiO2 surface breaks up the surface and allows for more

stable coordination, especially of the monodentated bicarbonate species. All of the

bicarbonate and carbonate species shown in Scheme 2 correspond to the reduction

products CO2-, HCO3-, and bidentate CO3- (b-CO3-) observed in the DRIFTS-IR spectra.

The CO and CH4 detected in the gas chromatograms used to quantify the performance of

the TiO2 surface are products of further reactions of CO2-, HCO3-, and bidentate CO3- (b-

CO3-) by mechanisms that are yet to be elucidated.

12

Scheme 2. Possible surface structures of CO2 on TiO2 surfaces. CO2 can either be a

simple aggregate, a carbonate (C), or a bicarbonate (B). The reactions to form C

and B are shown.

13

Figure 1 shows DRIFTS-IR spectra of TiA(UP) and TiB(UP) with photo-illumination

and in the dark. Peaks for reduced carbon species and H2O were nearly identical in both

scenarios indicating that no new carbon species were formed. However, a DRIFTS-IR

spectrum of TiA(He) in the dark, shows a rapid formation of three reduced carbon

species in the first 5 minutes: CO2-, HCO3-, and b-CO3-. Over time, the intensity of the

H2O band increases, the CO2-band disappears, and there is a gradual decrease in HCO3-

and b-CO3-. This suggests that H2O can replace the adsorbed b-CO3- as it occupies the

oxygen vacancies, possibly due to the higher binding energy of TiO2 with H2O compared

to CO2.

The DRIFTS-IR spectrum of TiA(He) in photo-illumination is shown in Figure 1

and the intensities of the HCO3- and b-CO3- peaks increase with prolonged irradiation

time and the H2O peak decreases. This contrast to the dark spectra is evidence of

electrons trapped in defect sites transferring to the adsorbed CO2. There is also an

appearance of CO2- around 1675 and 1249 cm-1 that occurs by photo-induced activation

and reduction rather than re-adsorption since there was no evidence of CO2 dehydration,

the other possible cause of the CO2- presence. The increase in intensity for the reduced

carbon species, CO2-, HCO3-, and b-CO3- suggests that these three are the primary

intermediates for CO2 photoreduction on anatase surfaces and that H2O disappears as it

fulfills its role as an electron donor and fills the holes left behind in the TiO2 surface.

The DRIFTS-IR spectra of TiB(He) in the dark and shows similar peaks to

TiA(He) in the dark with the formation of the three carbon containing species. There is a

remarkable difference in the spectra with photo-illumination, however, with TiB(He)

showing two new bands at 1716 and 1379 cm-1 attributed to the formation of formic acid

(HCOOH) rather than the bands at 1675 and 1249 cm-1 for the formation CO2-. This

difference suggests that the primary intermediates for the brookite surfaces are CO2-,

14

HCO3-, and HCOOH. All these intermediates are subsequently reduced to CO and CH4

which are the detected and measured reduction products.

Discussion

It is apparent that an electron donor is necessary for the photocatalytic reduction

of CO2 to CO and CH4. The oxygen already present on TiO2 surfaces can act as an

effective electron donor but the oxygen atom in water is more effective.5,6 The

intermediates detected by DRIFTS-IR suggest the incorporation of water into CO2 as the

final reduction products are formed. In fact, almost any deformation from a uniform

TiO2 surface will increase CO2 reduction because the oxygen atoms will be made more

reactive. This increase in reactivity could be because oxygen vacancies have increased

absorption of light in the visible region and allow for the addition of water into the

vacancy. When the oxygen vacancies are formed the adjacent Ti atoms are reduced to

Ti3+, a less stable oxidation state than Ti4+ because of the full octet of Ti4+, which likely

also contributes to the increased photoreduction activity of oxygen-poor TiO2 surfaces.

The mechanism of CO2 addition to hydrated TiO2 surfaces is shown in Scheme 3 and

shows that water addition to TiO2 creates two different active sites for CO2 reduction and

bridging bicarbonates result.

It is also important for a dopant to not be too electronegative so the electron

donation can occur. For this reason fluorine is not used as a dopant. Nitrogen would also

have the appropriate electronegativity properties to be used as a dopant and nitrogen

containing amine-doped TiO2 surfaces are employed in photocatalytic CO2 reduction.13

In this way the electron donor can be thought of as a Lewis base. Normally a hard Lewis

base would be ideal for the reduction of the hard Lewis acid CO2.

15

Scheme 3. Addition of water to TiO2 surface followed by adsorption of CO2 and

formation of bicarbonates.

Oxygen, even from water, as well as other smaller main group elements are not

the best electron donors due to their small size and lack of d-orbitals. The most effective

dopants by mass are larger transition metals and lanthanides which, although formally

considered Lewis acids, will act as soft Lewis bases when reducing CO2. Palladium is

already known to be an effective donor into π* antibonding orbital of molecular

oxygen,15,16 and a similar activity can be expected in CO2 reduction. Additionally,

transition metals are stable in a variety of oxidation states with Pd making especially

frequent use of the Pd0/Pd2+ redox couple. Ce, while much larger than oxygen, is also not

a particularly effective dopant because its valence electrons will occupy s-orbitals and

will not be able to donate into the π* antibonding orbital of CO2 as effectively as valence

electrons in d-orbitals would. It seems that a soft Lewis acid, such as Pd and Pt in a low

oxidation state, can act as a hard Lewis base even more effectively than formal hard

Lewis bases. Large transition metals are the ideal dopants but are very expensive. For

16

this reason molecules of main group elements, including water, are the most cost efficient

(Table 1) dopants for TiO2 surfaces.

Other potentially useful dopants would include heavier Group 3 elements such as

Ga and In which will form stable +1 ions because of the resulting fully filled d-shell.

Phosphorus and sulfur compounds would also be interesting to study for potential

application as electron donors in TiO2 surfaces because they are inexpensive and not too

electronegative to disfavor electron donation. Lighter transition metals that oxidize to

form empty or half-filled d-shells could also be promising. Some of these metals could

include the Fe and Co groups or the Ti and Sc groups. One problem that needs to be

considered with lighter transition metals is that the energy difference between principle

quantum numbers will still be relatively high.

Conclusion

Brookite-phase TiO2 surfaces were synthesized and characterized by XRD,

Raman spectroscopy, TEM, UV-Vis spectroscopy, and DRIFTS-IR spectroscopy. It was

found that brookite-phase TiO2 was made a more effective photocatalyst for CO2

reduction when oxygen vacancies were introduced. This is likely due to a combination of

factors such as increased visible light absorption, replacement of oxygen atoms with

more reactive water and hydroxyl deformations, and formation of more readily reducing

Ti3+ ions.

Catalytic performances and cost efficiencies of oxygen-poor brookite-phase TiO2

were compared to Ce, Pd and Cu, and Pt-doped TiO2 surfaces and the oxygen-poor

brookite-phase TiO2 had the lowest catalytic performance but the highest cost efficiency.

The Pd and Cu-doped TiO2 had the highest catalytic performance but the high cost of this

material makes it unsuitable for industrial use. Further research into compounds of main

17

group elements and lighter transition metals is necessary to find dopants that are both low

in cost and high in catalytic performance.

Supplementary Material Available

In the appendix a more detailed description of the synthesis of the anatase, rutile,

and brookite phases of TiO2 is given along with a more detailed description of the helium

treatment process that is used to create oxygen deficiencies in the TiO2 surfaces. Spectral

data characterizing the TiO2 surfaces are also available in the appendix with XRD,

Raman, TEM, UV-Vis, and IR characterizations reported.

(1) United States Environmental Protection Agency: Climate Change

https://www.epa.gov/climatechange/climate-change-basic-information (accessed April

10, 2017)

(2) United States Environmental Protection Agency: Greenhouse Gas Emissions

https://www.epa.gov/ghgemissions/overview-greenhouse-gases (accessed April 10, 2017)

(3) International Energy Agency: Industrial Applications of CCS

https://www.iea.org/topics/ccs/ccssubtopics/industrialapplicationsofccs/ (accessed April

10, 2017)

(4) Li, K.; Peng, B.; Peng, T. Recent Advances in Heterogeneous Photocatalytic CO2

Conversion to Solar Fuels. ACS Catal. 2016, 6, 7485-7527.

(5) Mino, L.; Spoto, G.; Ferrari, A. M. CO2 Capture by TiO2 Anatase Surfaces: A

Combined DFT and FTIR Study. J. Phys. Chem. C 2014, 118, 25016-25026.

References

https://www.epa.gov/climatechange/climate-change-basic-informationhttps://www.epa.gov/ghgemissions/overview-greenhouse-gaseshttps://www.iea.org/topics/ccs/ccssubtopics/industrialapplicationsofccs/

18

(6) Liu, L.; Zhao, H.; Andino, J.; and Li, Y. Photocatalytic CO2 Reduction with H2O on

TiO2 Nanocrystals: Comparison of Anatase, Rutile, and Brookite Polymorphs and

Exploration of Surface Chemistry. ACS Catal. 2012, 2, 1817-1828.

(7) Sigma Aldrich http://www.sigmaaldrich.com/united-states.html (accessed April 4,

2017)

(8) Matĕjová, L.; Kočí, K.; Reli, M.; Čapek, L.; Hospodková, A.; Peikertová, P.; Matĕj,

Z.; Obalová, L.; Wach, A.; Kuśtrowski, P.; Kotarba, A. Preparation, characterization and

photocatalytic properties of cerium doped TiO2: On the effect of Ce loading on the

photocatalytic reduction of carbon dioxide. Appl. Catal. B 2014, 152-153, 172-183.

(9) Long, R.; Li, Y.: Liu, Y.; Chen, S.; Zheng, X.; Gao, C.; He, C.; Chen, N.; Qi, Z.;

Song, L.; Jiang, J.; Zhu, J.; and Xiong, Y. Isolation of Cu Atoms in Pd Lattice: Forming

Highly Selective Sites for Photocatalytic Conversion of CO2 to CH4. J. Am. Chem. Soc.

2017, 139, 4486-4492.

(10) Mao, J.; Ye, L.; Li, K.; Zhang, X.; Liu, J.; Peng, T.; and Zan, L. Pt-Loading

Reverses the Photocatalytic Activity Order of Anatase TiO2 {001} and {010} Facets for

Photoreduction of CO2 to CH4. Appl. Catal. B 2014, 144, 855-862.

(11) Hudson Institute of Mineralogy

http://webmineral.com/MySQL/xray.php?lambda=&lambda1=&ed1=&ed2=&ed3=&min

max=10&chem=TiO2&submit=Submit#.WOUc9jvyuUk (accessed April 5, 2017)

(12) Sigma Aldrich http://www.sigmaaldrich.com/united-states.html (accessed April 4,

2017)

http://www.sigmaaldrich.com/united-states.htmlhttp://webmineral.com/MySQL/xray.php?lambda=&lambda1=&ed1=&ed2=&ed3=&minmax=10&chem=TiO2&submit=Submit#.WOUc9jvyuUkhttp://webmineral.com/MySQL/xray.php?lambda=&lambda1=&ed1=&ed2=&ed3=&minmax=10&chem=TiO2&submit=Submit#.WOUc9jvyuUkhttp://www.sigmaaldrich.com/united-states.html

19

(13) Liao, Y.; Cao, S.-W.; Yuan, Y.; Gu, Q.; Zhang, Z.; Xue, C. Efficient CO2 Capture

and Photoreduction by Amine Functionalized TiO2. Chem. Eur. J. 2014, 20, 10220-

10222.

(15) Landis, C. R.; Morales, C. M.; Stahl, S. S. Insights into the Spin-Forbidden

Reaction between L2Pd0 and Molecular Oxygen. J. Am. Chem. Soc. 2004, 126, 16302-

16303.

(16) Popp, B. V.; Wendlandt, J. E.; Landis, C. R.; Stahl, S. S. Reaction of Molecular

Oxygen with an NHC-Coordinated Pd0 Complex: Computational Insights and

Experimental Implications. Angew. Chem. Int. Ed. 2007, 46, 601-604.

S1

Supporting Information


Photoreduction Catalyzed by Water-Doped TiO2 Surfaces.

Melissa Hopkins, Parker Smith, and Ethan Zars

Department of Chemistry, University of Missouri-Columbia, Columbia, MO, USA 65201

Email: [email protected]; [email protected];

[email protected]


S2

Table of Contents

Details of TiO2 anatase, rutile, and brookite syntheses S3

Figure S1 XRD and Raman spectra of TiO2 surfaces S4

Figure S2 TEM images of TiO2 surfaces S5

Figure S3 UV-Vis spectra of TiO2 surfaces S6

Figure S4 DRIFTS-IR spectra of the –OH groups of TiO2 surfaces S7

Bibliography S8

S3

The TiO2 nanocrystals are prepared in the three polymorphs: rutile, anatase, and

brookite. The rutile crystals are synthesized by adding a 2 molar ratio of titanium

tetrachloride dropwise to a 20 molar ratio of ethanol, which forms a transparent yellow

solid. The solid is then added to distilled water with a 280 molar ratio. Once combined,

the solution is stirred for thirty minutes, dried in a 50 ⁰C oven for 24 hours, and

centrifuged to collect the white precipitate. Anatase crystals are synthesized using 10 mL

of titanium bis(ammonium lactate) dihydroxide aqueous solution, desired amount of 0.1

M urea, and distilled water to reach a final volume of 100 mL. The solution is placed in a

sealed Teflon-lined autoclave and set in a 160 ⁰C oven for 24 hours. Upon cooling in air,

a precipitate is formed and then separated by centrifugation. The collected solid is

washed with distilled water until a pH of 7 is achieved then dried overnight in an oven set

at 60 ⁰C. Brookite is synthesized in a similar manner to anatase, though 7 M urea is used

in place of the 0.1 M solution. All three solids are calcined at 400 ⁰C for three hours with

a 2 ⁰C/min heating rate.

In addition to these non-pretreated materials, helium treated samples are prepared

to create an oxygen deficiency in the structures. This is accomplished by providing a flow

of He at 120 mL/min inside of diffuse reflectance infrared Fourier transform

spectroscopy (DRIFT) reaction cell or inside of a photo reactor for 1.5 hours at 220 ⁰C.

The pretreated samples are denoted as Ti(He) and the non-pretreated samples as Ti(UP).

In sum, the three polymorphs, rutile, anatase, and brookite, with treated and untreated

samples are denoted as TiR(He), TiR(UP), TiA(He), TiA(UP), TiB(He), and TiB(UP)

respectively.

S4

Figure S1. (a) XRD profiles of unpretreated brookite (TiB(UP)), rutile (TiR(UP)), and

anatase (TiA(UP)) phase TiO2. (b) XRD profiles of helium-treated brookite (TiB(He)),

rutile (TiR(He)), and anatase (TiA(He)) phase TiO2. (c) Raman absorbance spectra of

unpretreated brookite (TiB(UP)), rutile (TiR(UP)), and anatase (TiA(UP)) phase TiO2.

S5

Figure S2. TEM images for (a, b) helium treated anatase (TiA(He), (c, d) helium treated

rutile (TiR(He)), and (e, f) helium treated brookite (TiB(He)) TiO2 crystals.

S6

Figure S3. UV-Vis spectra of unpretreated (TiR(UP), TiB(UP), TiA(UP)) and He-

treated (TiR(He), TiB(He), TiA(He) rutile, brookite, and anatase phase TiO2. The inset

picture shows the color differences of the three TiO2 polymophs after He treatment.

S7

Figure S4. DRIFTS-IR spectra of OH groups in the region 3000−3800 cm−1 for (a)

unpretreated anatase (TiA(UP)), brookite (TiB(UP)), and rutile (TiR(UP)) and (b) He-

treated anatase (TiA(He)), brookite (TiB(He)), and rutile (TiR(He)) phase TiO2 crystals.

S8

Bibliography

Liu, L.; Zhao, H.; Andino, J.; and Li, Y. Photocatalytic CO2 Reduction with H2O on

TiO2 Nanocrystals: Comparison of Anatase, Rutile, and Brookite Polymorphs and

Exploration of Surface Chemistry. ACS Catal. 2012, 2, 1817-1828.

Long, R.; Li, Y.: Liu, Y.; Chen, S.; Zheng, X.; Gao, C.; He, C.; Chen, N.; Qi, Z.; Song,

L.; Jiang, J.; Zhu, J.; and Xiong, Y. Isolation of Cu Atoms in Pd Lattice: Forming

Highly Selective Sites for Photocatalytic Conversion of CO2 to CH4. J. Am. Chem. Soc.

2017, 139, 4486-4492.

Mao, J.; Ye, L.; Li, K.; Zhang, X.; Liu, J.; Peng, T.; and Zan, L. Pt-Loading Reverses the

Photocatalytic Activity Order of Anatase TiO2 {001} and {010} Facets for

Photoreduction of CO2 to CH4. Appl. Catal. B 2014, 144, 855-862.

Matĕjová, L.; Kočí, K.; Reli, M.; Čapek, L.; Hospodková, A.; Peikertová, P.; Matĕj, Z.;

Obalová, L.; Wach, A.; Kuśtrowski, P.; Kotarba, A. Preparation, characterization and

photocatalytic properties of cerium doped TiO2: On the effect of Ce loading on the

photocatalytic reduction of carbon dioxide. Appl. Catal., B 2014, 152-153, 172-183.

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