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## Research Paper

Applied Science and Convergence Technology 2020; 29(2): 36-39

Published online March 30, 2020

https://doi.org/10.5757/ASCT.2020.29.2.036

## Photocatalytic CO2 Reduction and Thermal CO Oxidation to CO2 over Cu/Ni-loaded TiO2 Photo and Thermal Catalysts

Hee Jung Yoona, Ju Hyun Yangb, and Youngku Sohna,b,*

aDepartment Chemistry, Yeungnam University, Gyeongsan 38541, Republic of Korea
bDepartment of Chemistry, Chungnam National University, Daejeon 34134, Republic of Korea

Correspondence to:youngkusohn@cnu.ac.kr

Received: March 17, 2020; Accepted: April 2, 2020

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.

### Abstract

Bimetallic cocatalysts have played an important role in increasing the catalytic activities of various metal oxide supports. Herein, Cu and Ni cocatalysts were loaded onto TiO2 nanoparticles and tested for both thermal CO oxidation and photocatalytic CO2 reduction activities. It was observed that the thermal CO oxidation onset started to appear at approximately 200 °C. Photocatalytic CO2 reduction products were commonly found to be CO, CH4, and CH3OH, with total yields of 14.7, 3.2, and 6.1 μmol after 13 h of UVC irradiation, respectively. The 5 mol% Cu/Ni-loaded sample showed the highest thermal CO oxidation activity, whereas the 1 mol% Cu/Ni-loaded sample showed the highest CO2 reduction activity. The demonstration experiments for CO oxidation and CO2 reduction could provide valuable information for developing efficient catalysts for sustainable energy and environmental solutions.

Keywords: Cocatalyst, TiO2, CO oxidation, CO2 reduction, Photocatalyst

### 1. Introduction

Titanium dioxide (TiO2) has been widely used as a model catalyst material in various areas such as photo and thermal catalytic applications for energy and environment [1-6]. To satisfy increasing demand, considerable effort has been devoted to the designing of new catalysts with higher catalytic performance [1,7-9]. Among various strategies, guest transition metal doping/loading of a host metal oxide has been extensively employed [1,7,10-13]. For recycling energy production using CO2 reduction, Ola and Maroto-Vale loaded Cr, V, and Co onto honeycomb monolithic TiO2 structures, tested vapor-phase CO2 photoreduction, and observed CH4 and acetaldehyde production rates of 4.87 and 11.13 μmol g−1 h−1, respectively, for 0.5 wt% V–TiO2 monoliths [9]. Low et al. prepared a TiO2/Ti3C2 composite and tested photocatalytic CO2 reduction to achieve a CH4 production rate of 0.22 μmol h−1 under a 300 W Xe arc lamp. Xu et al. reported 1D/2D TiO2/MoS2 hybrid nanostructures prepared using the hydrothermal transformation method. They reported that CH4 and CH3OH yields reached up to 2.86 and 2.55 μmol g−1 h−1, respectively, for photocatalytic CO2 reduction over 10 % MoS2/TiO2 [12]. Bare TiO2 fibers were reported to show only CH3OH as a reduction product with a yield of 0.72 μmol g−1 h−1. Jung et al. reported that when mesoporous TiO2 and few-layered MoS2 were decorated on graphene, the CO production yield by CO2 photocatalytic reduction reached 93.2 μmol g−1 h−1, which is 14.5 times that of bare TiO2 [13]. Pham and Lee used Cu/V co-doped TiO2 on polyurethane as a CO2 photoreduction catalyst and reported CH4 and CO production rates of 933 and 588 μmol g−1 h−1 under visible-light irradiation (0.05 W/cm2) [8].

For thermal CO oxidation tests [7,14,15], Fang et al. prepared CuO/TiO2 catalysts using coprecipitation and impregnation methods and reported a CO oxidation onset at approximately 50 °C [14]. The high catalytic activity was attributed to the interfacial bond formation of hybrid Ti–O–Cu and the consequent stabilization of Cu+ species. DeSario et al. prepared low-valent (0, 1+) Cu nanoparticles (2–3 nm) on TiO2 aerogels and reported CO oxidation onset below 100 °C with a high conversion rate, which was primarily determined by the Cu0/1+: Cu2+ ratio [16].

To acquire new information on the relationship between thermal CO oxidation and photocatalytic CO2 reduction, we introduced Cu/Ni-bimetallic catalysts loaded onto TiO2 nanoparticles (NPs) and performed both CO oxidation and CO2 reduction experiments. The present thermal CO oxidation and CO2 reduction demonstration tests could be very useful methodologies for developing Ti oxide-based thermal and photocatalysts.

### 2. Experimental details

Anatase titanium (IV) oxide nanopowder (99.7 %, < 25 nm particle size, Sigma-Aldrich) was used as received as a support material. For the preparation of the Cu/Ni-loaded TiO2 NPs, appropriate amounts of Ni2+ (nickel (II) nitrate hexahydrate, 98 %, Samchun, Korea) and Cu2+ (copper (II) nitrate trihydrate, 99 %, Daejung, Korea) solutions were added to a TiO2 dispersed solution. The mixed solution was fully dried before characterization. A Hitachi S-4800 scanning electron microscope (SEM) was used to examine the morphology of the prepared samples (1, 2, and 5 mol% Cu/Ni-loaded TiO2 NPs). A Hitachi H-7600 transmission electron microscope (TEM) at 100 kV was also used to examine the morphology. The crystal phases of the powder samples were examined using a PANalytical X’Pert Pro MPD X-ray diffractometer (XRD) under Cu Kα radiation (40 kV and 40 mA). Using a Thermo-VG Scientific K-alpha+ spectrometer with a monochromatic Al Ka X-ray source and a hemispherical energy analyzer, the surface chemical states were examined by conducting a survey of Cu 2p, Ni 2p, Ti 2p, and O 1s X-ray photoelectron spectra. UV-visible absorption and Raman spectra of the powder samples were obtained using a SCINCO NeoSys-2000 UV–Vis spectrophotometer and a Bruker Senterra Raman spectrometer (a laser excitation wavelength of 532 nm), respectively. Thermal CO oxidation experiments in a flow reactor were conducted using two different detectors of a CO2 sensor and an SRS RGA200 quadrupole mass spectrometer. A powder sample of 20 mg was mounted in a U-type quartz tube. The flow rate of CO (1.0 %)/O2 (2.5 %)/N2 was fixed at 40 mL/min. A heating rate of 20 K/min was fixed for the detection of CO2 (CO oxidation product) using a mass spectrometer. To use the CO2 sensor, the sample temperature was increased to the desired temperature and stabilized for 30 min before measuring the CO2 concentration using the CO2 sensor. Photocatalytic CO2 reduction experiments were performed in a stainless-steel reactor with a quartz window on top. The powder sample was pasted on a glass slide (3 cm × 3 cm) and placed in the reactor with 40 μL of deionized water. Afterwards, the reactor was flushed and filled with pure CO2 (99.999 %) gas. The reactor was then placed under four 15 W UVC (200–280 nm) lamps for 12 h. The CO2 reduction gas products (e.g., CO, CH4, and CH3OH) were analyzed using a YL 6500 gas chromatograph (Young In Chromass Co., Ltd.) equipped with a flame ionization detector and a thermal conductivity detector. A gas volume of 0.5 mL was injected into two different columns of 40/60 Carboxen-1000 and HP-PlotQ-PT for GC analysis.

### 3. Results and discussion

Figure 1 shows the XRD patterns of bare TiO2 NPs and 5 mol% Cu/Ni-loaded TiO2 NPs. For the bare TiO2 powder sample, all the XRD peaks were attributed to the tetragonal crystal phase (JCPDS 1-084-1285). A dominant peak at 2θ = 25.3° was assigned to the (101) plane. The (004), (200), (105), (211), (204), and (220) planes were assigned to the corresponding peaks [17]. For the 5 mol% Cu/Ni-loaded TiO2 powder sample, new (but weak) XRD peaks clearly appeared at 2θ = 37.3° and 43.3°. These peaks were attributed to the (111) and (200) planes of the cubic crystal phase (Fm-3m) of NiO (JCPDS 1-078-0429) or NixCu1-xO (JCPDS 1-078-0645). The 2θ peak at approximately 43° was also possibly due to the (111) plane of metallic Cu. This is discussed below.

Figure 1. XRD patterns of bare TiO2 NPs and 5 mol% Cu/Ni-loaded TiO2 NPs.

Figure 2 shows the SEM and TEM images of bare TiO2 NPs and 5 mol% Cu/Ni-loaded TiO2 NPs. The SEM image of the bare TiO2 powder shows large cluster aggregates of small particles. The SEM image of the Cu/Ni-loaded (5 mol%) TiO2 sample appeared to be bigger cluster aggregates of particles of various sizes. TEM images show particle size distribution of approximately 20 nm.

Figure 2. SEM (top) and TEM (bottom) images of the bare TiO2 and Cu/Ni-loaded (5 mol%) TiO2 samples.

Figure 3 displays the UV-visible absorption and Raman spectra of the bare TiO2 and Cu/Ni-loaded TiO2 powder samples. The absorption edge appeared at approximately 390 nm, corresponding to a band gap of 3.2 eV for TiO2 [17]. Upon introducing Cu and Ni, the absorption edge increased to a longer wavelength in the visible region. The absorption in the visible region increased with increasing amounts of Cu and Ni. Four major Raman active peaks were found at 144, 394, 514, and 637 cm−1, and became weaker and broader as the number of metal ions increased. The former two peaks (144 and 394 cm−1) were attributed to the Eg and B1g O–Ti–O bending modes, respectively, and shifted to higher wavenumbers with increasing guest metal ions. The latter two were attributed to A1g and Eg Ti–O stretching modes, respectively, and shifted to lower wavenumbers with increasing metal ions [10,15]. This indicates that the bending and stretching modes are oppositely affected by the loaded metal ions.

Figure 3. Reflectance UV-Vis absorption (left) and Raman spectra (right) of bare TiO2 and Cu/Ni-loaded (1,2, and 5 mol%) TiO2 samples.

Figure 4 displays CO oxidation profiles along with the reaction temperature obtained using two different methods: 1) CO2 gas sensor detection and 2) temperature-programmed reaction mass spectrometry [7,11,18]. For the CO2 gas sensor detection method, the sample temperature was increased to the desired temperature and stabilized for 30 min. Subsequently, the CO2 concentration reading on the sensor was recorded. For the first run of 1 and 2 mol% Cu/Ni-loaded TiO2 samples, the CO oxidation started to appear at approximately 300 °C. The 5 mol% loaded sample showed CO oxidation onset just above 200 °C. For the second run, all the samples showed CO oxidation onset at approximately 200 °C. However, the slope of the CO2 concentration change was different and showed the order of 2 mol% << 1 mol% < 5 mol%. The concentration for the 5 mol% sample exhibited the sharpest changes at different temperatures. For bare TiO2 NPs, no CO oxidation activity was observed below 500 °C, which is in good agreement with the literature [14]. For the CO oxidation profiles obtained using temperature-programmed reaction spectrometry, the sample heating rate was fixed at 20 K/min. For the first run, CO oxidation onsets were observed at 240, 300, and 340 °C for 5, 2, and 1 mol% samples, respectively. The 5 mol% sample showed the highest CO oxidation activity. For the second run, the differences in the onset temperatures became smaller. CO oxidation onsets were observed at 200, 240, and 250 °C for 5, 2, and 1 mol% samples, respectively. The CO oxidation activity increased after the first run, possibly due to an increase in crystallinity and metal alloy formation [7]. Based on the CO oxidation profiles using two different methods, it was concluded that the heating rate and stabilization were important factors affecting the catalytic activity.

Figure 4. 1st (left panel) and 2nd run (right panel) CO oxidation reaction profiles measured using a sensor (top) and a mass spectrometry (bottom).

Photocatalytic CO2 reduction experiments (Fig. 5) were performed, and three different reduction products of carbon monoxide (CO), methane (CH4), and methanol (CH3OH) were obtained. The total CO production yields after UVC irradiation for 13 h were observed to be 0.4, 14.7, 10.2, and 9.1 μmol for bare TiO2 NPs, CuNi (1 mol%)/TiO2, CuNi (2 mol%)/TiO2, and CuNi (5 mol%)/TiO2, respectively. Compared with that of bare TiO2 NPs, the CO yield saw an increase by a factor of 35. The total CH4 production yields for 13 h were observed to be 0.6, 3.2, 2.0, and 1.9 μmol for bare TiO2 NPs, CuNi (1 mol%)/TiO2, CuNi (2 mol%)/TiO2, and CuNi (5 mol%)/TiO2, respectively. The total CH3OH production yields were observed to be 1.0, 6.0, 4.9, and 5.1 μ mol for bare TiO2 NPs, CuNi (1 mol%)/TiO2, CuNi (2 mol%)/TiO2, and CuNi (5 mol%)/TiO2, respectively. The total reduction yields showed the order of CH4 < CH3OH < CO. In addition, the photocatalytic CO2 reduction activity showed the order of bare TiO2 NPs << CuNi (5 mol%)/TiO2 < CuNi (2 mol%)/TiO2 < CuNi (1 mol%)/TiO2. Overall, the CuNi (1 mol%)/TiO2 sample showed the highest CO2 reduction activity and the CO, CH4, and CH3OH production rates were observed to be 1.13, 0.25, and 0.46 μmol h-1, respectively. These values were higher than those reported in the literature [19]. For the mechanism of CO2 reduction, photogenerated electrons (e) and holes (h+) are created, which participate in the formation of H+, consequently forming CO, CH4, and CH3OH, as described below.

Figure 5. Total CO2 reduction (CO, CH4, and CH3OH) yields (μmol) for of bare TiO2 and Cu/Ni-loaded (1,2, and 5 mol%) TiO2 samples.
$Cu / Ni - TiO 2 + h ν → Cu / Ni - TiO 2 ( VB , h + ) + Cu / Ni - TiO 2 ( CB , e − ) .$ $H 2 O + h + → • OH + H +$ $H + + e − → 1 / 2 H 2$ $CO 2 + 2 H + + 2 e − → CO + H 2 O$ $CO 2 + 8 H + + 8 e − → CH 4 + 2 H 2 O$ $CO 2 + 6 H + + 6 e − → CH 3 OH + H 2 O$

A survey XPS scan (not shown here) was performed to examine the surface chemical species. Two strong Ti 2p and O 1s peaks were observed because of the host TiO2 matrix. Weaker Cu 2p, Ni 2p, and C 1s peaks were observed and were attributed to guest metal (Cu and Ni) ions and surface impurities, respectively. Figure 6 displays high-resolution Cu 2p, Ni 2p, Ti 2p, and O 1s XPS spectra. For the Cu 2p XPS peaks, the dominant Cu 2p3/2 and Cu 2p1/2 peaks were found at binding energies (BEs) of 932.7 and 952.4 eV, with a spin-orbit splitting energy of 19.7 eV. These peaks can be assigned to metallic Cu and/or Cu2O [20]. A negligible satellite peak at approximately 942.0 eV indicated that Cu was not in the +2 oxidation state [15]. To distinguish Cu from Cu2O, Cu LMM Auger lines were examined in the XPS survey. Two broad Cu LMM Auger peaks were observed at BEs of 570 (minor) and 565 eV (major). This indicates that Cu is mainly metallic with a minor Cu2O state. For Ni 2p XPS peaks, two Ni 2p1/2 and Ni 2p3/2 peaks were observed at BEs of 856.1 and 873.8 eV, respectively, with a spin-orbit splitting energy of 17.7 eV. These peaks could be attributed to the surface NiO and/or Ni(OH)2 species. Broad and strong satellite peaks at 862 eV indicate the +2 oxidation state of Ni. For the Ti 2p XPS peaks, two Ti 2p3/2 and Ti 2p1/2 XPS peaks were observed at 459.0 and 464.6 eV, with a spin-orbit splitting of 5.6 eV. This is attributed to the 4+ oxidation state of TiO2 [12,15]. Two O 1s XPS peaks were observed at 530.2 and 532.0 eV, mainly due to lattice oxygen (O2-) of TiO2 and other surface oxygen species (e.g., OH and H2O) species, respectively [12,15].

Figure 6. Cu 2p, Ni 2p, Ti 2p, and O 1s XPS spectra for 5 mol% CuNi-loaded TiO2 sample.

### 4. Conclusions

Cu and Ni co-loaded TiO2 NPs were prepared and tested for both thermal CO oxidation and photocatalytic CO2 reduction performance. The CuNi (1 mol%)/TiO2 sample showed the highest CO2 reduction activity, and the total CO, CH4, and CH3OH production yields for UVC irradiation after 13 h were 14.7, 3.2, and 6.1 μmol, respectively. Bare TiO2 NPs exhibited CO2 reduction yields below 1 μ mol. For thermal CO oxidation, the CuNi (5 mol%)/TiO2 sample showed the highest CO oxidation activity with a CO oxidation onset at 200 °C. This demonstration test could provide guidance for developing thermal CO oxidation and photocatalytic CO2 reduction catalysts.

### Acknowledgements

This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (2016R1D1A3B04930123).

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