Mechanistic Study of CO2

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article 2

2

2

Mechanistic Study of CO Photoreduction with HO on Cu/TiO Nanocomposites by In Situ X-ray Absorption and Infrared Spectroscopies Lianjun Liu, Cunyu Zhao, Jeffrey T Miller, and Ying Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10835 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 18, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Mechanistic Study of CO2 Photoreduction with H2O on Cu/TiO2 Nanocomposites by In Situ X-ray Absorption and Infrared Spectroscopies Lianjun Liua, Cunyu Zhaoa, Jeffrey T. Millerb,c, Ying Lia,d* a

University of Wisconsin-Milwaukee, Mechanical Engineering Department, Milwaukee, WI

53211, USA b

Argonne National Laboratory, Chemical Science and Engineering Division, Argonne, IL

60439, USA c

Purdue University, School of Chemical Engineering, West Lafayette, IN 47907, USA

d

Texas A&M University, Department of Mechanical Engineering, College Station, TX 77843,

USA

*Corresponding Author: Ying Li, Ph. D., Associate Professor, Texas A&M University Email: [email protected]; Tel: +1 979-862-4465; Fax: +1 979-845-3081

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 33

ABSTRACT Cu/TiO2 composites are extensively studied for photocatalytic reduction of CO2 with H2O, but the roles of Cu species (Cu2+, Cu+, or Cu0) is not well understood and the photocatalyst deactivation mechanism is seldom addressed. In this work, we have employed in situ techniques, i.e., X-ray absorption spectroscopy (XAS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), to explore the surface chemistry of Cu/TiO2 composites under CO2 photoreduction environment. We found that the air-calcined Cu/TiO2 (Cu/Ti(air)) surface was dominated by isolated Cu2+ sites, while the one post-treated with H2 at 200 °C (Cu/Ti(H2)) was rich in Cu+ and oxygen vacancy (VO). Cu/Ti(H2) showed more than 50% higher activity than Cu/Ti(air) for CO2 photoreduction to CO, mainly resulting from the synergy of Cu+, OH groups and VO that could scavenge holes to enhance electron transfer, provide CO2 adsorption sites, and facilitate the activation and conversion of the adsorbed CO2 (HCO3− and CO2−). Meanwhile, the consumption of OH groups and Cu+ active sites by holes may result in the deactivation of Cu/Ti(H2). Moreover, in situ XAS results directly demonstrated that (1) the photo-induced oxidation of Cu+ to Cu2+ changed the surrounding environments of Cu by increasing the coordination number; (2) thermal treatment by H2 could not fully recover the OH and Cu+ sites to their original states; and (3) adding hole scavengers (e.g., methanol) maintained or even increased the more active Cu+ species from the photoreduction of Cu2+, thus leading to a higher and more stable CO2 reduction activity. Findings in this work and the application of in situ XAS technique will help develop a more efficient photocatalyst for CO2 photoreduction and advance the understanding of the reaction mechanism and surface chemistry.


2

Page 3 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Copper-deposited titanium dioxide (Cu/TiO2) has been extensively studied for the photocatalytic reduction of CO2 with H2O,1-9 an artificial photosynthesis process that holds potential to supply a renewable energy and a sustainable environment. 10-12 The Cu species in the Cu/TiO2 composites serve as a co-catalyst providing reaction sites and promoting the separation and transport of photo-exited charge carriers (electron-hole pairs), largely depending on the chemical valence of Cu. Highly dispersed surface Cu+ species are often reported to be the most active compared to Cu2+ and Cu0, 3, 9, 13-14 while other researchers suggested Cu2+ or Cu0 as the most active species.1, 15-16

Our previous work tailored the Cu valence in Cu-deposited P25 TiO2 and revealed the roles

of Cu species in enhancing CO2 conversion efficiency 7. We found that (1) as-prepared (or aircalcined) Cu/TiO2 was dominated by Cu2+, (2) thermal treatment of the as-prepared sample in He and in H2 at 200 °C resulted in the transition of Cu2+-rich to Cu+-rich and mixed Cu+/Cu0, respectively; (3) the photocatalytic activity was in the order of H2-treated > He-treated > unpretreated. However, the change of Cu valence during the photocatalytic process and how that could affect CO2 photoreduction is still not well understood. On the other hand, it has been challenging to accurately measure the surface Cu valence during the reaction, and consequently, ex situ characterization before and after the reaction is commonly performed. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), including X-ray absorption near edge structure (XANES) and extended Xray absorption fine structure (EXAFS), are representative characterization techniques.14, 17-18 For example, Tseng et al. combined XPS and EXAFS to identify the chemical states and location of Cu in 2wt%Cu/TiO2 before and after H2 reduction (at 300 oC).14, 18 They found that Cu(I) species were easily aggregated and reduced to Cu(0) by H2 treatment, which led to poor performance in


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 33

CO2 photoreduction. However, ex situ measurements have intrinsic uncertainties and may not represent the state of the active catalyst because the samples will be inevitably exposed to air that may change the surface property. This limitation demands advanced in situ techniques to monitor the Cu valence and coordination structure and accurately correlate with the catalytic activity. To the best of our knowledge, no research has been done to identify the chemical states of Cu species during CO2 photoreduction process by in situ XANES and EXAFS. In addition, many studies have been done to approach the pathways of charge transfer and CO2 activation/conversion over TiO2-based photocatalysts,4, 9, 12, 19-22 but very limited studies have been reported to address the photocatalyst deactivation mechanism, a phenomenon generally observed in CO2 photoreduction (especially at the gas-solid interface operated in a continuous flow mode), including our previous studies

3, 7

and other literature reports for

Cu/TiO2-based materials.4-5, 23 Possible reasons for deactivation are saturation of the adsorption sites on the TiO2 surface with intermediates products, diminishment of the adsorption capability, and/or photo-oxidation of products back into CO2 by O2 produced in the reaction, but the exact mechanism behind the deactivation is still not clear and the method to regenerate the catalyst and recover the activity is not explored in the literature. In this work, we have integrated in situ XANES/EXAFS and DRIFTS to further advance the mechanistic understanding of the CO2 photoreduction on Cu/TiO2 by monitoring the surface intermediates, Cu valence and coordination structure under the reaction environment. We have measured the photocatalytic activities of Cu/TiO2 and correlated them with the material properties. By comparing different regeneration methods and controlling the reaction conditions, we have revealed the reasons for catalyst deactivation and found the method to regenerate the catalyst.


4

Page 5 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2. Experimental 2.1. Synthesis of Cu/TiO2 catalysts The Cu/TiO2 nanocomposites were prepared by hydrothermal synthesis of TiO2 nanocrystals first and subsequently loading Cu species via complex-precipitation reaction. Briefly, to prepare anatase TiO2, 10 mL Titanium bis (ammonium lactate) dihydroxide (TALH) aqueous solution (50 wt%) and a desired amount of 0.1 M urea (0.6g) were mixed followed by the addition of distilled water to reach a final volume of 100 ml. The resulting solution was transferred into a Teflon-lined autoclave (150 ml), which was sealed and placed in an electric oven held at 160 °C for 24 h. Then, the autoclave was naturally cooled in air. The precipitates were separated by centrifugation, washed with distilled water until pH 7, and dried overnight at 60 °C in an oven. Our previous work found that for either Cu/TiO2(air) (non-pretreated) or Cu/TiO2(H2), 1 at.% Cu loading led to 40%-50% higher activity of CO2 photoreduction than 5 at.% Cu loading,7 This is because too high a loading of Cu species may cause particle aggregation and serve as charge recombination centers. In this work, we hence only focus on the study of 1%Cu/TiO2 sample. To prepare Cu/TiO2, in a beaker A, a desired amount of TiO2 (0.5 g) was dispersed in 50 ml H2O (Solution A). A certain amount of ammonia was added dropwise till pH 9-10, where the solution is negatively charged. In a beaker B, a desired amount of Cu(NO3)2 (Cu/(Cu+Ti) = 1 at. %) was dissolved in 50 ml H2O (Solution B). Ammonia was added to solution B till a dark blue color Cu(NH3)42+ was formed (positively charged). Subsequently, solution B was slowly transferred into solution A under vigorously stirring. After 3 h, the precipitate was collected, washed by distilled water till pH 7, and dried at 80 oC overnight. The composite was finally calcined at 400 oC for 2 h in air to obtain CuO/TiO2. To obtain reduced Cu/TiO2, CuO/TiO2 was


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 33

in situ pretreated by H2 at 200 oC for 1 h prior to catalytic reaction. The samples without and with H2 treatment were denoted as Cu/Ti(air) and Cu/Ti(H2), respectively. 2.2. Photocatalytic reduction of CO2 The photocatalytic conversion of CO2 with H2O vapor was conducted in a home-made quartz tube reactor operating in a continuous flow mode.24 For each test, 40 mg catalyst was used and evenly dispersed onto a glass-fiber filter that was placed at the center of a photoreactor. Two 250 W infrared (IR) lamps were used to heat up the reactor to a designated temperature of 150 oC. CO2 (99.999%, Praxair) (at a flow rate of 120 sccm) then continuously passed through a water bubbler to bring a CO2+H2O gas mixture to purge the reactor for 2 h. After that, the flow rate was reduced to 4.0 sccm for reaction. A 100 W mercury vapor lamp was used as the photoexcitation source, and the light intensity is about 10 mW/cm2 in the UV range (< 390 nm). To obtain Cu/Ti(H2), right before reaction, the Cu/Ti(air) was in situ pretreated by H2 200 oC for 1 h to reduce the Cu species. The same procedure was conducted on Cu/Ti(air) and Cu/Ti(H2) for CO2 photoreduction. The gaseous products in the reactor effluent were continuously analyzed by a gas chromatograph (GC, Agilent 7890A) equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID). Samples were taken at a 15 min interval. 2.3. In situ DRIFTS for CO2 adsorption/conversion All IR spectra were recorded on a Nicolet 6700 spectrometer (Thermo Electron) equipped with a liquid nitrogen cooled HgCdTe (MCT) detector.25 The spectra were displayed in absorbance units, and acquired with a resolution of 4 cm–1, using 32 scans. The DRIFTS studies were performed in a Praying Mantis DRIFTS accessory and a reaction chamber (Harrick Scientific, HVC-DRP). The reaction cell is equipped with a heater and a temperature controller, as well as a sample cup in the center. The dome of the DRIFTS cell has two KBr windows allowing IR


6

Page 7 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


transmission and a third (quartz) window allowing transmission of irradiation introduced through a liquid light guide (Newport) that connects to a 450 W Xe lamp (Oriel). Prior to the CO2 adsorption, the Cu/TiO2 was in situ treated by H2 for 1 h at 200 °C to obtain Cu/Ti(H2). The temperature was then lowered to 150 oC, and the background spectrum in the presence of the sample was collected. Next, FTIR spectra were recorded as a function of time to investigate the dynamics of the reactants (e.g., CO2 and H2O) adsorption in the dark and desorption/conversion under UV-vis irradiation. The same procedure has been done on Cu/Ti(air) but without H2 treatment. 2.4. In situ XANES/EXAFS for CO2 photoreduction In situ XANES and EXAFS of Cu K-edge was carried out at beam-line 10D at the Advanced Photon Source (APS) at Argonne National Laboratory, IL. Cu K-edge fluorescence XANES spectra were measured from 200 eV below the edge to 700 eV above the edge using a passivated implanted planar silicon detector. During the experiments, a Cu foil was used for energy calibration. XANES/EXAFS spectra were treated employing the Winxas 3.2 package. Figure S1 shows the schematic diagram of the in situ XAS setup for CO2 photoreduction. For the Cu/Ti(air), the sample was coated on a glass slide (1 cm × 1 cm) as a thin film by a doctor blade method. The thin film was loaded on a stainless steel sample holder that can be placed inside a homemade tube reactor. On the one side of the reactor, a circular hole was made and taped with a Teflon film (DuPont Inc.), which allows UV light and fluorescence X-ray transmission. At the two ends of the reactor, the valves were sealed with a Kapton film to allow X-ray penetration. Prior to reaction, CO2 + H2O vapor was introduced into the reactor for 2 h. After that, the reactor was sealed in a closed system, and then transferred to the beam-line. An IR lamp was used to heat the reactor to the target temperature (150 oC), and a mercury vapor UV lamp was used as


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 33

light source. The XANES/EXAFS spectra were collected in the dark and under UV irradiation. For Cu/Ti(H2), the same procedure has been conducted for in situ XANES/EXAFS. The only difference is that prior to adsorption, Cu/Ti(air) was subjected to H2 treatment at 200 oC for 1 h to get Cu/Ti(H2).

3. Results and discussion 3.1. In situ XANES/EXAFS analysis of Cu species To understand the structural difference and track the changes of Cu species on Cu/Ti(air) and Cu/Ti(H2) during the reaction of photocatalytic CO2 reduction, the catalysts were subjected to in situ XANES and EXAFS analyses. Figure 1 shows Cu K-edge in situ XANES spectra for Cu/Ti(air) and Cu/Ti(H2) under different reaction conditions, as well as the XANES spectra for Cu(I) (Cu2O) and Cu(II) (Cu(AcAc)2) references. In Figure 1a, the reference Cu(I) compound has a typical whiteline (WL) at about 8.983 keV, the intensity of which is dependent on the type of ligands. At the Cu(I) WL energy there is little intensity of Cu(II) compounds. The reference Cu(II) compound has a small pre-edge (PE) feature at 8.979 keV. Typically for compounds with both Cu(I) and Cu(II) the very small Cu(II) pre-edge is overlapped with the Cu(I) edge and often not observed. As shown in Figure 1b, Cu/Ti(air) in the dark at 50 °C demonstrates a very weak Cu(I) WL peak, directly confirming that Cu/Ti(air) surface is covered with nearly all Cu2+ with an insignificant number of Cu+ (as proved by EXAFS analysis described later in this paper and listed in Table 1). When processing the reaction with CO2 and H2O under UV irradiation for 60 min (still at 50 °C), the feature of Cu(I) WL remained almost the same. Subsequent heating the reaction system to 150 oC under UV irradiation for another 60 min induced only a very small decrease of Cu(I) WL.


8

Page 9 of 33

(b) 1.4

(a) Cu(AcAc)2

1.0

Cu(I) WL

0.8

0.6

0.4

Cu(II) PE

0.2

0.0 8.96

8.97

8.98

o

Dark 50 C o UV 60 min 50 C o UV 120 min 150 C

1.2

Cu2O

Normalized Absorption


1.2

1.0

Fresh Cu/Ti(air) (CO2 +H2O)

0.8

0.6

0.4

Cu(I) WL

0.2

8.99

9.00

0.0 8.96

9.01

8.97

(c) 1.4

Fresh Cu/Ti(H2)

0.8

9.00

9.01

9.00

9.01

o


1.0

8.99

(d)1.4

o

Dark 50 C o UV 60 min 50 C o UV 80 min 150 C o UV 120 min 150 C

1.2

8.98

Photon Energy [keV]

Photon Energy [keV]


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(CO2 +H2O) 0.6

0.4

Dark 50 C o UV 20 min 50 C o UV 40 min 150 C o UV 60 min 150 C

1.2

1.0

0.8

Used Cu/Ti(H2) (CO2 + 0.6%CH3OH/H2O)

0.6

0.4

Cu(I) WL

Cu(I) WL

0.2

0.2

0.0 8.96

8.97

8.98

8.99

9.00

9.01

0.0 8.96

8.97

Photon Energy [keV]

8.98

8.99

Photon Energy [keV]

Figure 1. In situ XANES spectra for (a) Cu2O and Cu(AcAc)2 references, (b) fresh Cu/Ti(air) in the presence of CO2 + H2O vapor, (c) fresh Cu/Ti(H2) in the presence of CO2 +H2O vapor, and (d) used Cu/Ti(H2) in the presence of CO2 + 0.6%CH3OH/H2O vapor Figure 1c shows in situ XANES spectra of Cu K-edge for Cu/Ti(H2) in the presence of CO2+H2O vapor. One can see that the Cu(I) WL shoulder is near 8.983 keV in the dark at 50 °C, and the fraction of Cu+/Cu2+ is about 60/40 (see Table 1). Upon UV irradiation for 40 min at 50 o

C, there were a small decrease of Cu(I) WL and large changes in the XANES near 8.986 keV

due to the changing ratio of Cu+/Cu2+ at the overlapping XANES edges. However, no further


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 33

changes were observed when increasing the irradiation time to 60 min. A subsequent increase of the temperature to 150 oC under UV irradiation for 80 min led to further decrease of Cu+ and increase of Cu2+. After 120 min UV there were no further changes. This is an important finding that directly proves Cu+ species reacted with holes during the photocatalytic reaction and were oxidized to Cu2+ (Cu+ + h+ → Cu2+). The loss of Cu+ during reaction suggests that Cu+ species are the active sites, and the significant oxidation of Cu+ to Cu2+ that occurs during the reaction may cause the deactivation of Cu/Ti(H2). Since Cu+ species were sacrificed during the CO2 photoreduction, we conducted in situ XANES on the used Cu/Ti(H2) in the presence of 0.6%CH3OH/H2O to further study the stability of Cu+. As shown in Figure 1d, unlike the trend with H2O vapor only (Figure 1c), the WL peak intensity of Cu+ gradually increased while that of Cu2+ decreased with UV irradiation time and higher temperature. Obviously, adding hole scavengers into the CO2 photoreduction system not only stabilizes Cu+ species but also induce the formation of more Cu+ sites possibly through photoreduction of Cu2+.


10

Page 11 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Structure parameters, Cu-O bond distance (R), and coordination number (CN) of Cu/Ti(H2) and Cu/Ti(air) before and during the photocatalytic reaction process. σ is the DebyeWaller factor Sample Cu2O

Reference

2

Cu(AcAc)2

Reference

4

1.92

Dark, 50°C

4.0

1.94

4.0

1.0

1.0 Cu(II)

UV 60 min, 50°C

3.8

1.94

4.0

1.1

1.0 Cu(II)

UV 120 min, 150°C

3.9

1.94

4.0

1.2

1.0 Cu(II)

Dark, 50°C

2.8

1.92

4.0

0.3

0.60 Cu(I) + 0.40 Cu(II)

UV 60 min, 50°C

3.4

1.93

4.0

0.5

0.30 Cu(I) + 0.70 Cu(II)

UV120 min, 150°C

3.6

1.93

4.0

0.8

0.20 Cu(I) + 0.80 Cu(II)

Dark, 50°C

3.7

1.93

4.0

0.8

0.15 Cu(I) + 0.85 Cu(II)

UV 20 min, 50°C

3.5

1.93

4.0

0.7

0.25 Cu(I) + 0.75 Cu(II)

UV 60 min, 150°C

3.2

1.92

4.0

0.2

0.40 Cu(I) + 0.60 Cu(II)

(CO2 + H2O)

Fresh Cu/Ti(H2) (CO2 + H2O)

Used Cu/Ti(H2) (CO2+0.6%CH3OH /H2O)

NCu-O

σ∆2 (× 103)

R (Å) 1.85

Fresh Cu/Ti(air)

Treatment

Eo, (eV)

Est. Fraction Cu(I)-Cu(II) (±0.1) Cu(I) ref Cu(II) ref

To study the coordination geometry of the Cu atoms and their bond distances during the reaction, we fit in situ EXAFS data at the Cu K-edge. The EXAFS data were Fourier transformed to the r-space, and the data range taken for the transformation was 2.6-11.4 Å-1 in the k-space. Structural parameters of Cu species were obtained using experimental phase and amplitude functions determine for the Cu(AcAc)2 reference (4 Cu-O at 1.92 Å) for data in r-space within the interval of 1.0-1.9 Å. The structural parameters, i.e., coordination number (CN), bonddistance (R), Debye-Waller factor (σ2), and inner potential (Eo) for correction are summarized in Table 1. Here we only focus on the Cu-O shells of Cu/Ti(air) and Cu/Ti(H2), since the higher shells are small leading to larger uncertainties in the fits. As shown in Figure 2, for both


11


Cu/Ti(air) and Cu/Ti(H2), the Fourier transformed EXAFS oscillations show one main sharp peak at 1.45 Å (phase uncorrected distance), corresponding to the Cu-O bond in the isolated CuO species. Cu/Ti(air) and Cu/Ti(H2) also show similar Cu-O bond distance (1.93 Å) regardless of UV irradiation, reaction time, reaction temperature, and treatment conditions. The small higher shell peaks suggest that the Cu species are present as single site ions on the TiO2 support rather than Cu oxide clusters.

o

Cu/Ti(air) dark 50 C o Cu/Ti(air) UV 120 min 150 C o Cu/Ti(H2) dark 50 C

FT Magnitude (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 33

0.0

o

Cu/Ti(H2) UV 120 min 150 C

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

R (Å) 2

Figure 2. The Fourier transformed k χ(k) EXAFS of Cu/Ti(air) and Cu/Ti(H2) catalysts before and after photocatalytic reduction of CO2 (∆k = 2.5 – 11.0 Å-1)

Another important finding in Table 1 is that the CN of Cu on Cu/Ti(air) and Cu/Ti(H2) change with the reaction conditions. Generally, Cu+ has a CN of 2 Cu-O bonds while Cu2+ has a CN of 4. Hence, we can estimate the fraction of Cu ions from the Cu-O coordination number, i.e., the fraction of Cu+ = (4 – CN)/2. It is clearly seen from Table 1 that before, during and after


12

Page 13 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reaction, Cu/Ti(air) always has a CN of 4.0, same as the Cu2+ reference, thus, is nearly 100% Cu2+. Cu2+ species on the Cu/Ti(air) showed little change during reaction. On the other hand, H2 treatment resulted in the significant decrease of CN from 4 to 2.8 over Cu/Ti(H2), corresponding to a faction of 60%Cu+/40%Cu2+. Subsequent UV irradiation (no matter without and with heating) induced the increase of CN gradually to 3.6 with increasing reaction time and temperature. At the end, only 20%Cu+ was left. Table 1 also shows that the CN of Cu and the fraction of Cu+/Cu2+ on the used Cu/Ti(H2) (CN = 3.7, 15%Cu+/85%Cu2+) has changed under UV irradiation when CO2 was bubbled through a CH3OH/H2O solution, i.e., the CN of Cu on the used Cu/Ti(H2) deceases to 3.2 and the amount of Cu+ increases from 15% to 40%. It should be noted that it is more difficult to recover the population of Cu+ by thermal H2 treatment than by adding hole scavengers like CH3OH. The in situ XANES/EXAFS data on materials characterization are used to correlate with the photocatalytic performance described in the next section. 3.2.

Photocatalytic performance of Cu/TiO2

Photocatalytic conversion of CO2 on TiO2, Cu/Ti(air) and Cu/Ti(H2) was conducted at 150 oC (an optimum temperature found in our previous work),26 and CO was found to be the major product. Since the CO production rates of TiO2 and Cu/Ti(air) at low temperature (50 oC, without heating) were less than 1 µmol g-1 h-1, more than 5 times lower than that at 150 oC, we reported the activity data at 150 oC only in this paper. To exclude the thermal-induced catalytic effect and the possibility of surface organic contaminants that may induce CO production, background tests at 150 oC were conducted by introducing He + H2O vapor to the reactor in the presence of the Cu/TiO2 catalysts. No matter in the dark or under UV-vis irradiation, the amount of CO produced was not detectable. This confirms that the produced CO was indeed derived


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 33

from photocatalytic conversion of CO2 instead of carbonaceous species. FITR characterization results further confirm that there are negligible carbonaceous contaminants on the catalyst surface, since no obvious peaks for carbonates, bicarbonate or hydrocarbon appear on either Cu/Ti(H2) or Cu/Ti(air) (see Figure S2). Figure 3 compares the CO production rate by TiO2, Cu/Ti(air) and Cu/Ti(H2). TiO2 had a maximum CO production rate of 5.5 µmol g-1 h-1 at the initial 0.5 h, but after that the rate decreased to ~1.2 µmol g-1 h-1 at 5 h. Compared to bare TiO2, Cu/Ti(air) and Cu/Ti(H2) showed an enhanced activity, perhaps since the addition of Cu reduces the charge recombination rate.1-2, 6 More importantly, the Cu/Ti(H2) demonstrated about 50% higher activity than the Cu/Ti(air) for CO production. In agreement with our previous work and the literature reports,7,

27-28

the

treatment of Cu/TiO2 with H2 often led to the formation of abundant Cu+ species and oxygen vacancy (VO) due to the removal of surface oxygen species. As a result, the enhancement is likely because Cu/Ti(H2) surface was dominated with Cu+ and VO, while Cu/Ti(air) surface was rich in Cu2+ species alone. The in situ XANES/EXAFS result demonstrated that Cu2+ was not the active site and Cu2+ is stable during the CO2 photoreduction with H2O vapor process. In addition, VO may react with CO2 to produce CO, which may make additional enhancement on the CO production on Cu/Ti(H2). To investigate such effect of VO due to hydrogen treatment in this work, we measured the CO production by Cu/Ti(H2) in the dark and under photo-illumination sequentially (Figure S3). A very small amount of CO was indeed detected even in the dark period upon introducing CO2 over Cu/Ti(H2), suggesting CO2 may react with VO to produce CO. Through this non-catalytic process, CO production reached a maximum of ~1.0 µmol g-1 h-1 and decreased to zero after 1 h. By contrast, immediately upon photo-illumination, a much larger amount of CO (4.5 µmol g-1 h-1) was produced and reached its


14

Page 15 of 33

maximum of 7.5 µmol g-1 h-1 after 1.5 h. Even after 7 h, the CO product still retained at 3.5 µmol g-1 h-1. This result demonstrates that (1) CO was produced mainly through a photocatalytic process, and (2) the main roles of Vo and Cu+ are acting as active sites to enhance CO2 adsorption/activation and scavenging holes to promote charge separation. Another important finding in Figure 3 is that for all catalysts, the CO production rate decayed after reaching the peak value. For example, Cu/Ti(air) showed the maximum rate of 5.7 µmol g-1 h-1 at 0.5 h and dramatically reduced its activity to 1.5 µmol g-1 h-1 within 7 h (by 74%). The activity of Cu/Ti(H2) also decreased from the maximum (7.5 µmol g-1 h-1) to 3.5 µmol/g/h at 7 h (by 53%). The results clearly indicate that the catalysts deactivate during the reaction. 8 7

-1

-1

CO production rate (µ mol g h )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 5

Cu/Ti(H2)

4 3

Cu/Ti(air)

2 1

TiO2

0 0

1

2

3

4

5

6

7

Irradiation time (h) Figure 3. The rate of CO production from CO2 photoreduction with H2O vapor over TiO2, Cu/Ti(air) and Cu/Ti(H2) under UV-vis irradiation.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

In addition to CO production, CH4 was detected to be a minor product from CO2 reduction. However, the amount of CH4 produced was much lower than that of CO. As shown in Figure S4, the CH4 production rate for Cu/Ti(H2) could only reach a maximum of ~ 0.4 µmol/g/h (at 1.5 h), more than one-magnitude lower than that of CO production (~7.5 µmol/g/h at 1.5 h). The CH4 production for Cu/Ti(air) was even much lower than that of Cu/Ti(H2) and approached the detection limit of the GC (thus considered negligible). Hence, we believe the analysis of CO production only (and neglecting the minimal CH4 production) is sufficient and more reliable to reflect the catalyst activity and stability. Besides the measurement of reductive product like CO, the measurement of oxidative product such as O2 was investigated using Cu/Ti(H2) and Cu/Ti(air). It is noted that there was always background O2 and N2 (200 – 400 ppm) inside the photoreactor, even though we purged the reactor with a CO2–H2O mixture at a high flow rate for a few hours. Hence, to better indicate the O2 production we calculated the volumetric ratio of O2/N2 in the effluent gas before, during, and after the photoreaction, as also suggested in our previous work

25, 29

and those from other

groups.30-31 Before the photoreaction using Cu/Ti(H2) as the catalyst, the O2/N2 ratio reached a steady state of 0.40. Immediately upon photoillumination, the O2/N2 ratio dramatically decreased to 0.15 in the first 30 min but thereafter increased gradually, indicating the O2 level ratio is controlled by the kinetics of O2 generation and consumption. O2 can be produced from H2O oxidation by photo-generated holes (H2O + 2h+ → 2H+ + 1/2O2). Meanwhile, it can be consumed by reacting with photoexcited electrons (O2 + e– → O2–) or backward reactions (e.g., CO + O2 → CO2). Oxygen may also participate in the oxidation of Cu+ to Cu2+ and filling in the surface oxygen vacancies. The initial drop of O2 suggests that the consumption of O2 outweighed its generation at the beginning. By comparison, when using Cu/Ti(air) as the catalyst, the O2/N2


16

Page 17 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ratio dropped from 0.25 to 0.20 in the first 30 min and then gradually increased. The much smaller rate of O2/N2 ratio decrease using Cu/Ti(air) suggests that O2 consumption was less prominent because of the fewer number of Cu+ and oxygen vacancies on Cu/Ti(air). After the initial surface effects that consumed more background O2, the O2 generation process began to overweigh the concurrent O2 consumption process, and thus resulting in the net increase of the O2/N2 ratio with time. When the O2 generation/consumption processes reached equilibrium after 6 h, the O2/N2 ratio was finally stabilized. When the light was turned off after photoreaction, the O2/N2 ratio decreased, again verifying that O2 was produced during the photocatalytic CO2 reduction process.

3.3. Exploration of catalyst deactivation mechanism To better understand the catalyst’s mechanism of deactivation, several regeneration experiments were conducted sequentially after the 1st cycle of CO2 photoreduction using the same sample of Cu/Ti(H2). In the 2nd cycle the used Cu/Ti(H2) was calcined in air at 400 oC and treated again by H2 at 200 oC. In the 3rd cycle, the catalyst was calcined in air treated with 1 ml 28%H2O2, followed by re-reduction by H2. In the 4th cycle, the catalyst was reduced in H2 and the reaction included a mixture of CH3OH/H2O vapor (CH3OH/H2O = 0.6 vol.%/2.2 vol.%) along with CO2. The results for each regeneration procedure are shown in Figure 4. In the 2nd cycle regeneration process, calcination at 400 oC in air was conducted to eliminate the reaction intermediates or surface carbonates, and the H2 reduction at 200 oC was supposed to restore the Cu+ and VO species. However, the CO production rate in the 2nd cycle was much lower than in the 1st cycle, and continued to decrease with increasing reaction time.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

This result suggests that surface carbonates are not likely a key factor in the deactivation of the catalysts. It is possible that deactivation could result from (1) the consumption of surface OH groups, (2) the limited availability of electrons, and (3) the change of Cu+ species (valence and coordination structure). To confirm these ideas, the deactivated catalyst was treated with 1 ml H2O2 on the used Cu/Ti(H2) (i.e., the 3rd cycle), since photo-irradiation can easily induce the dissociation of H2O2 into OH fragments. Subsequently, the catalyst was re-reduced by H2 to restore the Cu+ sites and decompose any remaining H2O2. As shown in Figure 4 (the 3rd cycle), the initial CO production rate was enhanced from 2.0 µmol g-1 h-1 to 5.5 µmol g-1 h-1. Because of the limited number of OH species produced by the regeneration treatment, the catalyst activity decreased after 1.5 h. This result clearly demonstrates the important role of OH groups that are thought to act as hole scavengers, thus enabling electrons to reduce CO2. On the other hand, it also shows the OH groups cannot be fully regenerated in the presence of continuous water vapor, likely because water oxidation by holes is a rate limiting step. Hence, the loss of OH groups could be one important reason for catalyst deactivation. To confirm the role of hole scavengers, we continuously introduced 0.6 vol.% CH3OH vapor (hole scavenger) into the reaction (i.e., the 4th cycle). Unlike the previous regeneration procedures, the CO production rate in the 4th cycle was very stable and about twice the conversion as that at the end of the 3rd cycle. In a separate experiment, we introduced 0.6%CH3OH/H2O over the fresh Cu/Ti(air) and Cu/Ti(H2) in a continuous flow reaction mode, and the results are shown in Figure S5. The CO production rate reached a steady state of 6.0 and 9.5 µmol g-1 h-1 for Cu/Ti(air) and Cu/Ti(H2), respectively, in contrast to the declining rate observed without hole scavengers (Figure 2). The results indicate the importance of hole scavenger (e.g., CH3OH) and surface OH groups in CO2 photoreduction.


18

Page 19 of 33

It is worth mentioning that the majority of the produced CO came from CO2, not CH3OH, because comparison experiments conducted in our previous work with 0.6%CH3OH/H2O alone (without CO2) showed a minimal amount of CO production, while in the presence of CO2, the CO production rate linearly increased with CO2 partial pressure and it was five times higher at 97% CO2 than that without CO2. 32-33 8

1st

2nd Cu/Ti(H2)

7

-1

3rd

Regeneration conditions:

4th



o

o

1) 400 C air 1 h o 2) in situ 200 C H2 1h

-1

CO production rate (µ mol g h )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1) 400 C air 1 h,

o

1) in situ 200 C H2 1h

2) dip 1 ml 28% H2O2

2) continuously buble 0.6%CH3OH/H2O vapor

o

3) in situ 200 C H2 1h

6 5 4 3 2 1 0 0

2

4

6

8

10

12

14

16

18

20

Irradiation time (h)

Figure 4. The CO production rate over Cu/Ti(H2) during four different cycles. All the tests were conducted continuously on the same sample.

To further demonstrate the consumption of OH groups during the CO2 photoreduction reaction, we used in situ DRITFS to track the changes of OH before and after reaction and after thermal regeneration by H2. The corresponding IR spectra are shown in Figure 5. Before reaction Cu/Ti(H2) shows a sharp Ti4+-OH peak at 3670 cm-1 and a shoulder at 3722 cm-1 for Ti3+-OH.7 But after reaction, the Ti3+-OH shoulder completely disappeared, and the Ti4+-OH was also weakened and overlapped with a broad band (3600 cm-1-2600 cm-1) of adsorbed H2O. This result clearly demonstrates that the OH groups bonded with Ti3+ sites are very active and easily consumed because of the hole trapping effect, while the Ti4+-OH sites are only covered with the


19


adsorbed H2O. Thermal regeneration by H2 induced the re-appearance of Ti4+-OH with a very weak shoulder for Ti3+-OH. The incomplete regeneration of the Ti3+-OH groups is probably the reason for the inability to fully recover in initial activity with only thermal treatment of the sample by H2 (see Figure 4, the 2nd cycle). 0.1

Cu/Ti(H2) after reaction

4+

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

Ti -OH 3670

3+

Ti -OH 3722

Cu/Ti(H2) before reaction

Cu/Ti(H2) after H2 regeneration

3800

3600

3400

3200

3000

2800

2600

-1

Wavenumber (cm )

Figure 5. IR spectra identifying the OH groups on Cu/Ti(H2) before and after reaction and after thermal regeneration. Comparison of the data in Figure 4 also revealed that in the presence of 0.6%CH3OH vapor, the steady-state rate of CO production by the used Cu/Ti(H2) in the 4th cycle (5.0 µmol g-1 h-1) was lower than the maximum in the 1st cycle when only water vapor was present (7.5 µmol g-1 h-1). However, when fresh Cu/Ti(H2) was used in the presence of 0.6%CH3OH, the rate of CO production was much higher and more stable (9.5 µmol g-1 h-1, Figure S5). These results indicate that in addition to the effect of surface OH groups and hole scavengers, the chemical states of Cu species may affect the activity and stability as well. To approach the evolution of Cu species


20

Page 21 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


before and after reaction and after regeneration, in situ DRIFTS of CO adsorption on the Cu/Ti(H2) was obtained. Briefly, Cu/Ti(air) was reduced in situ by H2 at 200 oC in the DRIFTS cell to get Cu/Ti(H2). Once the temperature was cooled to 15 oC from 200 oC in helium (He), 5%CO/He was then introduced onto the Cu/Ti(H2) for 30 min. After CO2 photoreduction process and regeneration (calcination at 400 oC in air first and treatment at 200 oC by H2 again), the same procedure on CO adsorption was conducted on the same sample. The spectra were collected as a function of time for each treatment. For comparison, CO adsorption on the Cu/Ti(air) was also obtained. As shown in Figure 6, Cu/Ti(air) displays a very weak peak for Cu+-(CO)2 at 2109 cm1 34-35

,

but this peak is much stronger on the Cu/Ti(H2). This result suggested that Cu/Ti(H2) had

dominating Cu+ species while the Cu/Ti(air) surface was rich in Cu2+ with negligible Cu+, in good agreement with aforementioned in situ XANES results. The peak intensity of Cu+-(CO)2 on Cu/Ti(H2) remarkably decreased after reaction, suggesting some Cu+ species were oxidized back to Cu2+ possibly by photo-excited holes. Thus, the hole scavenging effect of Cu+ could also contribute to the higher activity of Cu/Ti(H2). In addition, a new weak shoulder appeared at 2126 cm-1 representing linear Cu+-CO, indicating that the coordination structure of Cu+ species had changed after reaction. Another interesting finding in Figure 6 was that thermal regeneration of the used Cu/Ti(H2) induced not only an increase of the peak intensity for Cu+-(CO)2 but also a shift from 2109 cm-1 to 2114 cm-1. This result indicated that more Cu+ species were produced after regeneration by H2, but the coordination environment of the regenerated Cu+ may be different from the original,35 which could also explain why the activity of the regenerated Cu/Ti(H2) (Figure 4, the 2nd cycle) was not recovered to the initial state. On the other hand, as evidenced by in situ XANES/EXAFS results, addition of a reducing agent like CH3OH is


21


capable of converting less active Cu2+ to more active Cu+, thus recovering the activity and maintaining the conversion stability (Figure 4, the 4th cycle).

0.05

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

Cu/Ti(H2) after H2 regeneration

+

Cu -(CO)2 2114

Cu/Ti(H2) before reaction 2109

Cu/Ti(H2) after reaction +

Cu -CO 2126 4+

Cu/Ti(air)

Ti -CO 2175 2161 2192

2250

2200

2150

2100 -1

2050

Wavenumber (cm ) Figure 6. In situ DRIFTS spectra of the adsorption of CO (as a probing molecule) on Cu/Ti(air) and Cu/Ti(H2) before and after reaction and after thermal regeneration by H2. 3.4. In situ DRIFTS for CO2 adsorption and conversion To verify the favorable activation and conversion of CO2 on the reduced Cu/TiO2, in situ DRIFTS for CO2 photoreduction was conducted on Cu/Ti(air) and Cu/Ti(H2). The in situ DRIFTS analysis was carried out in two sequential steps. First, CO2 adsorption on the sample surface was studied by introducing a CO2/H2O mixture to the IR cell at 150 oC for 60 min in the dark until the intensities of adsorption peaks reached saturation or remained unchanged (Step 1). Subsequently, the cell was kept in a closed system and the UV-vis light was turned on for 180 min to investigate the photocatalytic conversion of reaction intermediates and the desorption of


22

Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


products (Step 2). The in situ DRIFTS spectra recorded in each of the two steps are shown in Figure 7. As shown in Figure 7a, exposure of the Cu/Ti(air) to CO2+H2O induced the formation of CO2− (at 1664 cm−1), bidentate carbonate (b-CO32−, at 1621 and 1376 cm−1), monodentate carbonate (m-CO32−, at 1565 and 1518 cm−1), and bicarbonate (HCO3−, at 1424 and 1222 cm−1).7, 20

In general, the formation of CO2− and HCO3− requires the presence of metal ionic sites (e.g.,

Ti3+) and OH groups, respectively.20, 36 The surface Ti3+ and OH groups play an important role as CO2 adsorption sites, which is critical in CO2 photoreduction. Subsequent photo-irradiation for 120 min results in a decrease of HCO3− and CO2− by 28%-30% (calculated by 1 − PUV-vis120/PDark, where PUV-vis120 and PDark refer to the peak areas of HCO3− and CO2− in spectrum 4 and in spectrum 1, respectively, in Figure 7), while the b-CO32− and m-CO32− remain unchanged regardless of the illumination time. This result indicates that HCO3− and CO2− are likely more active intermediates and more easily converted than b-CO32− and m-CO32−. In Figure 7b, exposure of the Cu/Ti(H2) to CO2+H2O also led to the formation of HCO3−, CO2−, b-CO32− , and m-CO32−. Photo-irradiation for 120 min caused the reduction of HCO3− and CO2− by 70%-85%, over 1.5 times more than that on the Cu/Ti(air), indicating that the HCO3− and CO2− species are more easily activated and dissociated on the Cu/Ti(H2).


23


2-

(a)

m-CO3

0.02

1565

-

HCO3

2-

Absorbance (a.u.)

1424 b-CO3 1376 (4) UV-vis 120 min 1518 (3) UV-vis 80 min (2) UV-vis 40 min (1) Dark 2-

b-CO3

1621 -

CO2

1664

-

HCO3 1222

1900

1800

1700

1600

1500

1400

1300

1200

1100

-1

Wavenumber (cm )

(b)

1561

0.025

2-

m-CO3 1516

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

2-

HCO3 b-CO3

1424 1378

(4) UV-vis 120 min (3) UV-vis 80 min

2-

b-CO3

(2) UV-vis 40 min

1627 1666 -

(1) Dark

CO2

-

HCO3 1222

1900

1800

1700

1600

1500

1400

-1

1300

1200

1100

Wavenumber (cm )

Figure 7. In situ DRIFTS spectra for CO2+H2O vapor adsorption on (a) Cu/Ti(air) and (b) Cu/Ti(H2) in the dark and under UV-vis irradiation


24

Page 25 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


By analyzing all the results from photocatalytic activities, XANES/EXAFS and DRFITS, mechanistic schemes are illustrated in Figure 8 to compare the different active species and reaction pathways during the photocatalytic reaction process, on TiO2, Cu/Ti(air) and Cu/Ti(H2), respectively. On bare TiO2, the rate of CO2 photoreduction by electrons is limited by the rate of hole scavenging through water oxidation. In the presence of Cu2+ species on Cu/Ti(air), Cu2+ can be reduced by photoelectron to produce Cu+ (reduction potential of Cu2+/Cu+ = 0.16 V vs SHE), which in turn can be oxidized back by holes to Cu2+. This redox reaction to some extent promotes the rate limiting hole scavenging effect. Although Cu2+ competes with CO2 for electrons, the overall photocatalytic reaction is enhanced (through enhanced hole scavenging) compared with bare TiO2. The population of Cu2+ remains almost the same as indicated from the XAS results, because all produced Cu+ species are oxidized back to Cu2+. For the case of Cu/Ti(H2), the initial high population of Cu+ species significantly promotes hole scavenging, which further promotes the overall photocatalytic reaction. The population of Cu+ species decreases over time and converts to Cu2+, indicating the oxidation reaction dominates the Cu2+/Cu+ redox reaction. This is one reason the catalyst deactivates overtime. In addition, the Ti3+, Vo, and surface OH groups on Cu/Ti(H2) also provide additional CO2 adsorption sites, enhance the charge separation and facilitate the conversion of intermediates (CO2− and HCO3−) to CO. However, Ti3+, Vo and OH groups are consumed during the photocatalytic reaction, which is another reason for catalyst deactivation. In summary, although the active sites on Cu/Ti(H2) including highly dispersed Cu+, Ti3+, Vo, and surface OH can all promote CO2 photoreduction, they are not easily regenerated as demonstrated from the regeneration experiments (Figure 4). The most effective way of regeneration is the addition of a small amount of methanol vapor, which not only scavenges


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

holes to enhance charge transfer but also induces photo-reduction of Cu2+ into Cu+ to provide additional active sites, thus leading to a higher and more stable activity.

(a)

hv TiO2  → ecb− (TiO2 ) + hvb+ (TiO2 )

CO2 + 2H + + 2e− → CO + H 2O 2 H 2O + 4h + → 4 H + + O2 (Hole scavenging is rate limiting)

(b)

(c)



Cu 2+ + e − → Cu +

Cu + + h + → Cu 2 +

Cu + + h + → Cu 2 +

Cu 2+ + e − → Cu +

CO2 + 2 H + + 2e− → CO + H 2O

CO2 + 2H + + 2e− → CO + H 2O

2 H 2O + 4h + → 4 H + + O2

2 H 2O + 4h + → 4 H + + O2

(The presence of surface Cu2+ induces Cu2+/Cu+ redox reactions [0.16V vs SHE] that helps scavenge holes)

(Ti3+ − −Ti3+ ) + CO2 → (Ti 4+ − O 2− − Ti 4+ ) + CO (Rich Cu+ on the surface further promotes hole scavenging; meanwhile, Ti3+ and oxygen vacancy help CO2 reduction )

Figure 8. Schematic illustration of the reaction pathways of CO2 photoreduction and water oxidation over (a) TiO2, (b) Cu/Ti(air), and (c) Cu/Ti(H2) photocatalysts.

4. Conclusion This work is the first to combine in situ XANES/EXAFS and DRIFTS to study CO2 photoreduction with H2O and investigate the change of coordination structure and chemical state of Cu species on Cu/TiO2 as well as the photocatalyst deactivation mechanism. The in situ XANES/EXAFS results directly demonstrate that the oxidized sample (Cu/Ti(air)) and the reduced sample (Cu/Ti(H2)) surface were dominated by monomeric Cu2+ and Cu+, respectively. Cu/Ti(H2) exhibited 50% higher activity of CO production than Cu/Ti(air), mainly benefit from the synergy of OH groups (bonded with Ti3+), Cu+ and oxygen vacancies that can provide CO2


26

Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


adsorption sites, act as hole scavengers, and facilitate the activation of adsorbed CO2. The in situ DRIFTS and XANES/EXAFS results also suggest that the deactivation of Cu/Ti(H2) is caused by the sacrifice of Ti3+-OH and Cu+ sites (being oxidized back to Cu2+) by holes and the irreversible changes in coordination environment of Cu by increasing its coordination number. Only thermal regeneration by H2 cannot recover the Ti3+-OH and Cu+ sites back to the original states. On the other hand, introducing hole scavengers (e.g., methanol) onto the Cu/TiO2 is a feasible way to improve the photocatalytic stability, because charger transfer is enhanced and more Cu+ sites are generated from the photoreduction of Cu2+ by photo-excited electrons. This work has provided useful insights in the CO2 photoreduction mechanism and suggested ways toward a more efficient and stable photocatalytic system for photocatalytic CO2 conversion to fuels. Especially, the application of in situ XANES/EXAFS has provided an innovative way to monitor the oxidation state, active sites and coordination environment of photocatalysts during the photocatalytic process. Supporting Information Supplementary materials for this paper is available, including the schematic of the in situ synchrotron XAS set-up for CO2 photoreduction, in situ FTIR characterization, CO production over Cu/Ti(H2) in the dark, CH4 production over Cu/Ti(H2) under light irradiation, and CO2 photoreduction in the presence of hole scavengers. Acknowledgements This work is supported by National Science Foundation (NSF) Early Faculty CAREER Award (CBET-1538404). Partial funding for JTM was provided by Chemical Sciences, Geosciences and Biosciences Division, U.S. Department of Energy, under contract DE-AC0-06CH11357. Use of the Advanced Photon Source is supported by the U.S. Department of Energy, Office of Science,


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

and Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. References 1.

Ohta, K.; Ueda, Y.; Nakaguchi, S.; Mizuno, T., Photochemical Reduction of CO2 Using

Cu-loaded Silicate Rock Powder Suspended in Water. Canadian J. Chem.-Revue Canadienne De Chimie 1998, 76, 228-233. 2.

Yang, H.-C.; Lin, H.-Y.; Chien, Y.-S.; Wu, J. C.-S.; Wu, H.-H., Mesoporous TiO2/SBA-

15, and Cu/TiO2/SBA-15 Composite Photocatalysts for Photoreduction of CO2 to Methanol. Catal. Lett. 2009, 131, 381-387. 3.

Li, Y.; Wang, W.-N.; Zhan, Z.; Woo, M.-H.; Wu, C.-Y.; Biswas, P., Photocatalytic

Reduction of CO2 with H2O on Mesoporous Silica Supported Cu/TiO2 Catalysts. Appl. Catal. BEnviron. 2010, 100, 386-392. 4.

Srinivas, B.; Shubhamangala, B.; Lalitha, K.; Reddy, P. A. K.; Kumari, V. D.;

Subrahmanyam, M.; De, B. R., Photocatalytic Reduction of CO2 over Cu-TiO2/Molecular Sieve 5A Composite. Photochem. Photobio. 2011, 87, 995-1001. 5.

Wang, W.-N.; Park, J.; Biswas, P., Rapid Synthesis of Nanostructured Cu-TiO2-SiO2

Composites for CO2 photoreduction by Evaporation Driven Self-assembly. Catal. Sci. Tech. 2011, 1, 593-600. 6.

Liu, D.; Fernandez, Y.; Ola, O.; Mackintosh, S.; Maroto-Valer, M.; Parlett, C. M. A.;

Lee, A. F.; Wu, J. C. S., On the Impact of Cu Dispersion on CO2 Photoreduction over Cu/TiO2. Catal. Commun. 2012, 25, 78-82. 7.

Liu, L.; Gao, F.; Zhao, H.; Li, Y., Tailoring Cu Valence and Oxygen Vacancy in Cu/TiO2

Catalysts for Enhanced CO2 Photoreduction Efficiency. Appl. Catal. B-Environ. 2013, 134, 349358. 8.

Seriani, N.; Pinilla, C.; Crespo, Y., Presence of Gap States at Cu/TiO2 Anatase Surfaces:

Consequences for the Photocatalytic Activity. J. Phys.Chem. C 2015, 119, 6696-6702. 9.

Zhu, S.; Liang, S.; Tong, Y.; An, X.; Long, J.; Fu, X.; Wang, X., Photocatalytic

Reduction of CO2 with H2O to CH4 on Cu(I) Dupported TiO2 Nanosheets with Defective {001} facets. Phys. Chem. Chem.Phys. 2015, 17, 9761-9770.


28

Page 29 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10.

Ovcharov, M. L.; Shvalagin, V. V.; Shcherban, N. D.; Nazarkovskii, M. A.; Granchak, V.

M., Photocatalytic Reduction of CO2 Using Titanium Dioxide and Metal-Semiconductor Nanostructures Made from Titanium Dioxide. Theo. Exper. Chem. 2013, 49, 172-177. 11.

Marszewski, M.; Cao, S.; Yu, J.; Jaroniec, M., Semiconductor-based Photocatalytic CO2

Conversion. Mater. Horizons 2015, 2, 261-278. 12.

Liu, L.; Li, Y., Understanding the Reaction Mechanism of Photocatalytic Reduction of

CO2 with H2O on TiO2-Based Photocatalysts: A Review. Aerosol Air Qual. Res. 2014, 14, 453469. 13.

Roy, S. C.; Varghese, O. K.; Paulose, M.; Grimes, C. A., Toward Solar Fuels:

Photocatalytic Conversion of Carbon Dioxide to Hydrocarbons. ACS Nano 2010, 4, 1259-1278. 14.

Tseng, I. H.; Chang, W. C.; Wu, J. C. S., Photoreduction of CO2 Using Sol-gel Derived

Titania and Titania-supported Copper Catalysts. Appl. Catal. B-Environ. 2002, 37, 37-48. 15.

Yin, G.; Nishikawa, M.; Nosaka, Y.; Srinivasan, N.; Atarashi, D.; Sakai, E.; Miyauchi,

M., Photocatalytic Carbon Dioxide Reduction by Copper Oxide Nanocluster-Grafted Niobate Nanosheets. ACS Nano 2015, 9, 2111-2119. 16.

Slamet; Nasution, H. W.; Purnama, E.; Kosela, S.; Gunlazuardi, J., Photocatalytic

Reduction of CO2 on Copper-doped Titania Catalysts Prepared by Improved-Impregnation Method. Catal. Commun. 2005, 6, 313-319. 17.

Tseng, I. H.; Wu, J. C. S.; Chou, H. Y., Effects of Sol-gel Procedures on the

Photocatalysis of Cu/TiO2 in CO2 Photoreduction. J. Catal. 2004, 221, 432-440. 18.

Tseng, I. H.; Wu, J. C. S., Chemical States of Metal-loaded Titania in the Photoreduction

of CO2. Catal. Today 2004, 97, 113-119. 19.

Indrakanti, V. P.; Kubicki, J. D.; Schobert, H. H., Photoinduced Activation of CO2 on Ti-

based Heterogeneous Catalysts: Current state, Chemical physics-based Insights and Outlook. Energy Environ. Sci. 2009, 2, 745-758. 20.

Yang, C.-C.; Yu, Y.-H.; van der Linden, B.; Wu, J. C. S.; Mul, G., Artificial

Photosynthesis over Crystalline TiO2-Based Catalysts: Fact or Fiction? J. Am. Chem.Soc. 2010, 132, 8398-8406. 21.

Kou, Y.; Nabetani, Y.; Masui, D.; Shimoda, T.; Takagi, S.; Tachibana, H.; Inoue, H.,

Direct Detection of Key Reaction Intermediates in Photochemical CO2 Reduction Sensitized by a Rhenium Bipyridine Complex. J. Am.Chem. Soc. 2014, 136, 6021-6030.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

22.

Page 30 of 33

Tu, W.; Zhou, Y.; Zou, Z., Versatile Graphene-Promoting Photocatalytic Performance of

Semiconductors: Basic Principles, Synthesis, Solar Energy Conversion, and Environmental Applications. Adv. Func. Mater. 2013, 23, 4996-5008. 23.

Sasirekha, N.; Basha, S. J. S.; Shanthi, K., Photocatalytic Performance of Ru Doped

Anatase mounted on Silica for Reduction of Carbon Dioxide. Appl. Catal. B-Environ. 2006, 62, 169-180. 24.

Zhao, C.; Liu, L.; Rao, G.; Zhao, H.; Wang, L.; Xu, J.; Li, Y., Synthesis of Novel MgAl

Layered Double Oxide Grafted TiO2 Cuboids and Their Photocatalytic Activity on CO2 Reduction with Water Vapor. Catal. Sci. Tech. 2015, 5, 3288-3295. 25.

Liu, L.; Zhao, H.; Andino, J. M.; 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. 26.

Liu, L.; Zhao, C.; Zhao, H.; Pitts, D.; Li, Y., Porous Microspheres of MgO-Patched TiO2

for CO2 Photoreduction with H2O Vapor: Temperature-Dependent Activity and Stability. Chem. Commun. 2013, 49, 3664-3666. 27.

Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S., Increasing Solar Absorption for Photocatalysis

with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746-750. 28.

Xiong, L.-B.; Li, J.-L.; Yang, B.; Yu, Y., Ti3+ in the Surface of Titanium Dioxide:

Generation, Properties and Photocatalytic Application. J. Nanomater. 2012. 2012, ID:831524 29.

Zhao, H.; Liu, L.; Andino, J. M.; Li, Y., Bicrystalline TiO2 with Controllable Anatase-

brookite Phase Content for Enhanced CO2 Photoreduction to Fuels. J. Mater. Chem. A 2013, 1, 8209-8216. 30.

Wang, W.-N.; An, W.-J.; Ramalingam, B.; Mukherjee, S.; Niedzwiedzki, D. M.;

Gangopadhyay, S.; Biswas, P., Size and Structure Matter: Enhanced CO2 Photoreduction Efficiency by Size-Resolved Ultrafine Pt Nanoparticles on TiO2 Single Crystals. J. Am. Chem. Soc. 2012, 134, 11276-11281. 31.

Yaghoubi, H.; Li, Z.; Chen, Y.; Ngo, H. T.; Bhethanabotla, V. R.; Joseph, B.; Ma, S.;

Schlaf, R.; Takshi, A., Toward a Visible Light-Driven Photocatalyst: The Effect of MidgapStates-Induced Energy Gap of Undoped TiO2 Nanoparticles. ACS Catal. 2015, 5, 327-335.


30

Page 31 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


32.

Liu, L.J., Pitts, D. T., Zhao, H.L., Zhao, C.Y., Li,Y., Silver-incorporated Bicrystalline

(Anatase/brookite) TiO2 Microspheres for CO2 Photoreduction with Water in the Presence of Methanol. Appl. Catal. A: Gen. 2013, 467, 474-482. 33.

Zhao, C.Y., A. K., Zhao, H.L., Zhang, Q.Y., Li,Y., Ultrasonic Spray Pyrolysis Synthesis

of Ag/TiO2 Nanocomposite Photocatalysts for Simultaneous H2 Production and CO2 Reduction. Inter. J. Hydrogen Energy 2012, 37, 9967-9976. 34.

Jin, H.; You, R.; Zhou, S.; Ma, K.; Meng, M.; Zheng, L.; Zhang, J.; Hu, T., In-situ

DRIFTS and XANES Identification of Copper Species in the Ternary Composite Oxide Catalysts CuMnCeO During CO Preferential Oxidation. Inter. J. Hydrogen Energy 2015, 40, 3919-3931. 35.

Coloma, F.; Bachiller-Baeza, B.; Rochester, C. H.; Anderson, J. A., Infrared Study of

Competitive Crotonaldehyde and CO Adsorption on Cu/TiO2. Phys. Chem. Chem. Phys.2001, 3, 4817-4825. 36.

Su, W.; Zhang, J.; Feng, Z.; Chen, T.; Ying, P.; Li, C., Surface Phases of TiO2

Nanoparticles Studied by UV Raman Spectroscopy and FT-IR Spectroscopy. J. Phys. Chem. C 2008, 112, 7710-7716.


31


Page 32 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

32

Page 33 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48



Mechanistic Study of CO2

Recommend Documents