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Catalysis Today

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Photocatalytic reduction of CO2 to hydrocarbons by using photodeposited Ptnanoparticles on carbon-doped titania

Minoo Tasbihia,⁎, Michael Schwarzea, Miroslava Edelmannováb, Camillo Spöric, Peter Strasserc,Reinhard Schomäckera

a Department of Chemistry, Technical University of Berlin, Straße des 17. Juni 124, 10623-Berlin, Germanyb Institute of Environmental Technology, VŠB-Technical University of Ostrava, 17. Listopadu 15, Ostrava, Poruba 708 33, Czech Republicc The Electrochemical Catalysis, Energy and Materials Science Laboratory, Department of Chemistry, Technical University of Berlin, Straße des 17. Juni 124, 10623 Berlin,Germany

A R T I C L E I N F O

Keywords:PhotocatalysisPhotodepositionPt/C-TiO2

CO2 reductionMethane

A B S T R A C T

Photocatalytic reduction of CO2 with H2O was performed in a top-irradiation stainless-steel photoreactor withPt/C-TiO2 as the photocatalyst. Pt/C-TiO2 photocatalysts with different amount of Pt (0.5–3.0 wt.%) weresynthesized by the photodeposition method and were characterized in detail by X-ray powder diffraction (XRD),nitrogen physisorption measurement (BET), UV–vis diffuse reflectance spectroscopy, inductively coupled plasmaoptical emission spectrometry (ICP-OES), scanning electron microscopy (SEM), transmission electron micro-scopy (TEM), and photoelectrochemical measurements. Results revealed the photocatalytic reduction of CO2

increased by loading Pt on the surface of C-TiO2. The main reaction product was methane (CH4), however,hydrogen (H2) and carbon monoxide (CO) were also detected. The highest yields of CH4, H2, and CO wereachieved in the presence Pt/C-TiO2 with a nominal loading of 0.88 wt.%, resulting from the efficient interfacialtransfer of photogenerated electrons from C-TiO2 to Pt as it is evidenced from photoelectochemical measure-ments.

1. Introduction

One main environmental challenge in our time is to avoid or reducethe impacts of global warming. In recent years, consumption of fossilfuels generated greenhouse gasses (GHG) especially carbon dioxide(CO2), regarded as the main cause of the worldwide global warming[1]. It is vital to reduce the accumulation of CO2 in the atmosphere.Therefore, researchers make special efforts to convert CO2 into moreuseful compounds. Photocatalytic reduction of CO2 with H2O is con-sidered as a promising method to simultaneously reduce the level ofCO2 emission and produce renewable and sustainable fuels [1,2].However, the development of efficient photocatalysts for CO2 conver-sion still remains in the developing phase [3]. In the photocatalyticreduction of CO2 three main reactions (a–c) have been proposed andvalidated. The main products are methane (CH4) and carbon monoxide(CO) [4]. H2 is a product from photocatalytic water splitting which isusually competing with CO2 reduction:

CO2 + 8H+ + 8e− → CH4 + 2H2O (a)

CO2 + 2H+ + 2e− → CO + H2O (b)

2 H2O → 2H2 + O2 (c)

Titania (TiO2), as the most explored semiconductor, has been highlyinvestigated for photocatalytic applications due to its outstandingchemical and thermal stability. However, titania has two main draw-backs: low photocatalytic activity and limited utilization of solar en-ergy, resulting from the fast recombination rate of photoinduced elec-tron-hole pairs and high band gap energy. Therefore it is required toimprove the electron-hole separation efficiency and light utilizationability of titania [5,6]. Several methods have been used to improve thephotocatalytic efficiency of TiO2 materials by developing the structuralmodification of titania with metal [2,7–9] or metal-free strategies[10–13]. Among them, surface modification of titania is widely appliedfor inhibiting the recombination of photogenerated electron-hole pairson TiO2 [14–17]. Different metal nanoparticles including Pt, Au, Ag,and Pd have been loaded on TiO2 and proved to be effective for en-hancing its photocatalytic activity and in fact, Pt indicates as the mosteffective co-catalyst to utilize the photogenerated electrons for the CO2

reduction to methane [18–21]. Pt is the most studied co-catalyst whichis widely employed in various systems including TiO2, titanates and

https://doi.org/10.1016/j.cattod.2018.10.011Received 11 July 2018; Received in revised form 7 September 2018; Accepted 8 October 2018

⁎ Corresponding author.E-mail address: minoo.tasbihi@tu-berlin.de (M. Tasbihi).

Catalysis Today 328 (2019) 8–14

Available online 15 October 20180920-5861/ © 2018 Elsevier B.V. All rights reserved.

T

carbon nitrides [22]. An optimal loading of the co-catalyst at which thehighest photocatalytic activity is achieved is always searched for [23].In addition, different parameters such as loading, elemental composi-tion, particle size, dispersion, structure, morphology can influence theCO2 reduction. Furthermore, the size of Pt nanoparticles is also playinga critical role in the photocatalytic reduction of CO2 as shown by Donget al. [24]. In principle doping with non-metals creates heteroatomicsurface structure and modify the physic-chemical properties and ac-tivity of TiO2. Generally, dopant metals act as a sink and trap theelectron generated by the semiconductor under excited state [4].

In this research, Pt nanoparticles supported by photodeposition oncarbon-doped titania (Pt/C-TiO2, 0.5–3.0 wt.% Pt) was used as thephotocatalyst and investigated towards the photocatalytic reduction ofCO2. C-TiO2 was used as the source of titania because of its higherphotocatalytic activity in comparison to pure titania [25–27]. The aimof this work is focused on the effect of Pt on the physicochemicalproperties of C-TiO2 and its photocatalytic activity towards the photo-catalytic reduction of CO2

2. Experimental

2.1. Co-photocatalyst preparation

The desired amount of H2PtCl6.6H2O solution (8 wt.% in H2O) toobtain catalyst loadings of 0.5–3.0 wt% was dissolved under stirringinto 180ml of distilled water. The solution was bubbled with N2 gas for15min to remove the dissolved oxygen. Thereafter, 2 g of C-TiO2

powder was added under vigorous stirring. The suspension was irra-diated with a 300W Xenon lamp (L.O.T. Oriel Quantum Design)equipped with a cut-off filter (λ > 395 nm), for 2 h. After 2 h, 20 mlmethanol was subsequently added to the suspension and irradiationwas continued for further 2 h. The suspension was then centrifugated(Biofuge stratus, Heraeus, 8500 rpm, 15min) to collect the catalyst,which was washed with distilled water and acetone, and finally dried at80 °C for 24 h under a reduced pressure of about 80mbar.

2.2. Characterization

The crystalline phases of synthesized photocatalyst powders wereexamined by XRD. The XRD patterns were obtained on a Bruker D8Advance using Cu Kα radiation in the range between 10 and 80° with astep size of 0.034°. The average crystallite size was estimated by theScherrer equation. The Brunauer–Emmett–Teller (BET) surface area,pore volume and pore size of the powder specimen were calculatedfrom the nitrogen adsorption–desorption isotherms at 77 K, using theMicromeritics-Gemini chemisorption system. The Pt loading was mea-sured by inductively coupled plasma optical emission spectrometry(ICP-OES, Varian Inc., USA). To dissolve the platinum, a microwave

method (Discover SPeD, CEM, USA) was used. Calibration of the setupwas done with a standard platinum solution (concentration of1000mg L−1, Sigma-Aldrich). The diffuse reflectance UV/Vis absorp-tion spectra were measured using a UV–vis spectrophotometerequipped with an integrating sphere (LAMBDA 650 UV/Vis with150mm integrating sphere, Perkin Elmer, USA). Indirect band-gapenergies were determined by plotting the Kubelka–Munk transforma-tion of the original diffuse reflectance spectra vs. photon energy (Tauc’splot): F(R) = (1-R∞)2/2R∞, where R∞ is the measured reflectance (R∞

= Rsample/Rstandard).Morphology of as-prepared photocatalysts was obtained by trans-

mittance electron microscopy (TEM) (TECNAI G220, FEI company, USAoperated at 200 kV, with LaB6 electron emitter) and scanning electronmicroscopy (SEM) (Hitachi SEM type SU8030 microscope operated atan acceleration voltage of 10 kV and a probe current of 15 pA).

Photoelectrochemical measurements were carried out using a pho-toelectric spectrometer coupled with potentiostat and 150W Xe lamp(Instytut Fotonowy, Poland). The photocurrent responses were re-corded using a three-electrode setup. A platinum wire and saturatedAg/AgCl were used as the counter and reference electrodes, respec-tively. The working electrode was prepared by depositing the photo-catalyst powder onto an indium-tin oxide (ITO) foil and 0.1M KNO3

solution was used as an electrolyte. The photocurrent spectra were re-corded within the range of 280–470 nm with the step of 10 nm in thepotential range of -0.2 to 1.0 V, step 0.1 V. Before the measurement, thecell with electrolyte was purged by argon to ensure an oxygen freeenvironment. The argon purge was also kept constant during the wholemeasurement.

2.3. Photocatalytic reaction test

The photocatalytic reduction of CO2 proceeded in a homemadebatch stainless-steel reactor (volume 357ml) with a quartz window. Asthe light source, an 8W Hg UV lamp (peak intensity at 254 nm wave-length; Ultra-Violet Products Inc.) was used, which was placed on top ofthe quartz window of the reactor (Fig. 1).

The reactor was filled with 0.1 g of the investigated photocatalystand 100ml of a 0.2 M NaOH solution. Before the start of the photo-catalytic reaction, the reactor was tightly closed and purged with CO2.The pressure sensor (Greisinger, GMSD 3.5 BAE) was placed at the topof the reactor to control the experiment.

The gaseous samples were analyzed every two hours for a total re-action time of 14 h. The samples were taken with a gastight syringe(Hamilton Co., Reno, USA) and analyzed in a gas chromatograph(Shimadzu Tracera GC-2010Plus) equipped with barrier discharge de-tector (BID) for its composition. Each experiment was repeated in orderto ensure the reproducibility of the experimental data. The blank testswere also performed.

Fig. 1. The photoreactor system for photocatalytic reduction of CO2.

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3. Results and discussion

3.1. Physic-chemical structure of co-photocatalyst

The XRD patterns of C-TiO2 and Pt/C-TiO2 photocatalysts are shownin Fig. 2. The crystalline phase of TiO2 and Pt/TiO2 are composed ofanatase (JCPDS No. 21-1272) as indicated by the peaks emerging with(101), (004), (200), (211), (204), (220), and (215). In detail, C-TiO2

contains anatase crystalline structure which by loading Pt nanoparticlesremains accordingly. This result is in agreement with the literature[6,17]. No peaks for Pt compounds were detected for catalysts withloadings of 0.5 till 1.5 wt.% Pt (Fig. 2), however as it is shown in Fig. 2,by loading 3.0 wt.% Pt, there is a small shoulder around 40° which is Pt(01-087-0647). We assume that in case of low Pt loading due to almostsimilar ionic radii of Ti4+ and Pt4+, Pt ions are replacing some Ti ionsin the crystal lattice points [28,29]. It is also possible that small dis-persed Pt, maybe as a single atom species, is present on TiO2 surfacewhich cannot be observed through XRD.

The calculated crystallite size, BET surface area, Pt loading, andband gap energy of investigate d photocatalysts are listed in Table 1.

In detail, the crystallite size of the anatase titania varies from 7.67to 9.40 nm. The crystallite size of C-TiO2 enhances by increasing the Ptamount as it is 8.14 nm, 8.32 nm, 9.31 nm and 9.40 nm in case of0.5 wt.% Pt, 1.0 wt.% Pt, 1.5 wt.% Pt and 3.0 wt.% Pt loading, respec-tively [8,30].

The specific surface area of photocatalysts varied from 215m2/g to238m2/g. C-TiO2 exhibited a specific surface area of 232m2/g whilethe specific surface area increases to 238m2/g with increasing amountof Pt to 1.0 wt.%. The specific surface area then decreases to 215 m2/gfor 3.0 wt.% Pt loading. This behavior could be caused by the growth inparticle size and may be caused by the deposition of Pt NPs within thepores of C-TiO2 on the surface [6,31].

The corresponding values of band-gap energies were determined byusing the Tauc plot. The band-gap energy of all catalysts is about3.3 eV. As expected, no significant changes are observed upon deposi-tion of Pt [30].

The actual amount of Pt was measured by ICP –OES. As it can beseen, the Pt amount by photodeposition is very close to the nominalamount for 0.5–3.0 wt%. A loading efficiency of about 90% was ob-tained by photodeposition as a result of the higher BET surface area ofthe catalysts [8]. In contrast, lower BET surface usually results in lowerloadings at similar conditions [32].

The morphology and structure of TiO2 and 1.0 wt.% Pt/TiO2 areshown in Fig. 3 (TEM images) and Fig. 4 (SEM images) which clearlyshow the different morphology and structure of C-TiO2 and Pt/C-TiO2

photocatalysts. It is essential to mention that the 1.0 wt.% Pt/TiO2

photocatalyst was chosen as it shows the highest activity for photo-catalytic CO2 reduction.

From Fig. 3, the lattice fringes of C-TiO2 nanoparticles are clear andconfirm the crystal form of anatase for C-TiO2. No Pt nanoparticles canbe seen in the TEM images which can be due to the integration of ul-trafine Pt nanoparticles inside the titania structure which is in ac-cordance with the N2 sorption results and XRD results (Fig. 2 andTable 1).

As shown in Fig. 4, the morphology of C-TiO2 did not change byloading Pt nanoparticles, as it was expected. In contrast to TEM, whereno Pt particles were found, some bigger Pt nanoparticles (agglomerates)on the surface of C-TiO2 can be detected (white points which aremarked with a red circle) because in SEM a bigger fraction of thesample is imaged. It is also obvious that Pt nanoparticle are nothomogenously dispersed on the titania powders indeed part of the Ptnanoparticle are presented on the surface and part are integrated insidethe titania structure.

To get a better insight into the utilization of photons during irra-diation, C-TiO2 and Pt/C-TiO2 catalysts were investigated by photo-lectrochemical measurements that confirmed the generation of chargecarriers after irradiation. Fig. 5 shows the dependence of generatedphotocurrent on wavelength and it is obvious that the number of chargecarriers is significantly lowered when platinum loading is too high.Lower loadings of platinum slightly increase the generation of chargecarriers compared to pure C-TiO2 which means that loading lowamount of Pt inhibits the recombination rate of photogenerated elec-tron-hole pairs (in case of 0.5 wt.% Pt/C-TiO2 and 1.0 wt.% Pt/C-TiO2)(Fig. 5).

3.2. Photocatalytic reduction of CO2

After characterization of the catalysts, their photocatalytic activitytowards the photocatalytic reduction of CO2 was studied. Figs. 6 and 7show the yields of products as a function of irradiation obtained for atime interval from 0 to 14 h. The main product is methane, however,carbon monoxide is also detected in the lower amount. Hydrogen is alsoformed as a product coming from the photocatalytic splitting of water.The product yields (μmol/gcat.) were detected in this order: a) hydrogen(H2), b) methane (CH4) and c) carbon monoxide (CO).

As it is shown, for pure C-TiO2, generation of H2, methane, and COoccurred only after an irradiation time of 8 h. In the presence of Pt asco-catalyst, the reaction is significantly accelerated. In detail, by in-creasing the amount of Pt from 0.46 wt.% (0.5 wt.% Pt/C-TiO2) to0.88 wt.% (1.0 wt.% Pt/C-TiO2), the yields of CH4, CO and H2 increasewhile the production rate of CH4 intensively increases (Figs. 6 and 7).

Fig. 2. XRD patterns of investigated photocatalysts (A and Pt indicate the peaksof the anatase phase of titania and platinum, respectively).

Table 1The characterization of investigated photocatalysts.

Photocatalyst Crystallite size(nm)

Surface area(SBET)(m2 g−1)

Pt contentfromICP-OES(wt.%)

Indirectband-gapenergy(eV)

C-TiO2 7.67 232 0 3.260.5 wt.% Pt/C-

TiO2

8.14 233 0.46 3.26

1.0 wt.% Pt/C-TiO2

8.32 238 0.88 3.27

1.5 wt.% Pt/C-TiO2

9.31 226 1.35 3.27

3.0 wt.% Pt/C-TiO2

9.40 215 2.62 3.25

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This behavior could be due to the enhances the enrichment of electrondensity on the catalysts surface or on the other hand, as it is written inTable 1, loading of 0.88 wt.% Pt (1.0 wt.% Pt/C-TiO2) causes increasingthe crystallite size and the specific surface area to 8.32 nm and 238m2 g−1, respectively. By increasing more Pt loading to 1.35 wt.%(1.5 wt.% Pt/C-TiO2) the rate of CH4 production decreases while therate of CO and H2 slightly decrease (Figs. 6 and 7). A similar trend is

observed by loading 2.26 wt.% Pt (3.0 wt.% Pt/C-TiO2), as by addingmore amount Pt the rate of all reaction products decrease (Figs. 5 and6). These results are in agreement with our previous work [8] and theresults observed by Xie [33].

The highest yields of all products were achieved in the presence of1 wt. % Pt/C-TiO2. Among the Pt/C-TiO2 samples, the order of yield ofCH4 production as the desired product is 1.0 wt.%<1.5 wt.%<0.5 wt.

Fig. 3. TEM images of C-TiO2 and 1.0 wt.% Pt/C-TiO2.

Fig. 4. SEM images of C-TiO2 and 1.0 wt.% Pt/C-TiO2 photocatalysts.

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%<3wt.%. The amount of CH4 is always higher than the amount ofCO and Pt can work as an electron trap to provide more electrons forthe CO2 reduction which is useful for CH4 production [6,17,34]. As it isshown in Fig. 6, the amount of CO and H2 are decreasing by time whilethe formation of methane increases with irradiation time in the pre-sence of Pt/C-TiO2 photocatalysts.

In general, photocatalytic reduction of gaseous CO2 depends on twofactors: the capability of the photocatalyst to adsorb CO2 and the effi-ciency of the transfer of the excited electrons. Thus the surface char-acteristics of the photocatalyst, such as surface charge and surface area,are very important parameters influencing the photocatalytic CO2 re-duction activity [16]. C-TiO2 photocatalysts were used as titania sourcebecause M. Janus [35] investigated the adsorption of CO2 on bare TiO2

(anatase structure obtained from POLICE) and C, N-TiO2 which wascalcined for 1 h at different temperatures (T=100 to 600 °C). The re-sults show that the C, N-TiO2 shows higher CO2 absorption in com-parison to bare and commercial TiO2 (for example Degussa P-25). Also,a higher surface area is beneficial for CO2 adsorption and the 1.0 wt.%Pt/C-TiO2 photocatalyst can supply more adsorption site for CO2 mo-lecules so that the concentration of localized CO2 on the TiO2 surface ishigher and the photocatalytic reduction of CO2 is accelerated [14,36]. Itis important to mention that high surface area photocatalysts with highporosity show higher photocatalytic activity [37–39]. In addition, theZeta potential of C-TiO2 is more negative [40] compared to commercialP25 [41], which also is beneficial for CO2 adsorption. To provide moreelectrons for CO2 reduction, catalysts with a higher photocurrent re-sponse might be beneficial because of better utilization of the photons.As shown in Fig. 5, the photocurrent densities of 0.5 wt.% Pt/C-TiO2

and 1.0 wt.% Pt/C-TiO2 are higher than C-TiO2 showing that their ef-ficiency for electron-hole pair separation is higher.

It is worth to mention that the photocatalytic reduction of CO2 is avery complex reaction with very low yields and the development ofefficient photocatalysts for CO2 conversion under solar irradiation stillremains in the developing phase. In fact, photocatalytic reduction ofCO2 is always with the parallel water decomposition reaction whichcompeting with CO2 reduction. The redox potentials for CO2 reductionis close to that of H2O to H2 and activation of H2O is generally beingmuch easier than CO2, it makes it so that the photocatalysts tends toreduce H2O to H2. Therefore, a suitable catalyst and reaction condition(such as irradiation source, CO2 concentration, amount of H2O, reactorgeometry, light position and …) can considered as major challenges in

photocatalytic CO2 reduction. In addition, the economically and en-vironmentally-friendly reduction of CO2 to value added chemicals ishighly desired which is possible if renewable energy such as solar en-ergy is used as an energy source.

4. Conclusion

Pt/C-TiO2 photocatalysts with different Pt amount (0.5–3.0 wt.%)

Fig. 5. The dependence of photocurrent on wavelength. Measured at 1 V ex-ternal potential.

Fig. 6. Time dependence of yields of H2, CO and CH4, after 14 h of irradiationin the presence of investigated photocatalysts (amount of photocatalyst = 0.1g).

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were synthesized by the photodeposition method and tested towardsthe photocatalytic reduction of CO2. The crystallite size of C-TiO2 in-creases by Pt loading while the surface area decreases. The actualamount of Pt was measured by ICP-OES and is only slightly lower thanthe nominal value, which can be attributed to the higher surface area ofC-TiO2. CH4, CO, and H2 were detected as the main reaction products.The photocatalytic activity of C-TiO2 increases by loading Pt, wherebythe amount of Pt is crucial for the performance of the photocatalyst. Thehighest yields for all products were achieved for an actual Pt loading of0.88 wt.%, where utilization of photo-induced electrons via charge se-paration is optimized.

Acknowledgment

This project was funded by the Federal Ministry of Education andResearch of Germany under the" CO2Plus funding measure - Use of CO2

to broaden the raw material basis "under the grant number 033RC003and by the International Postdoc Initiative (IPODI) of the EuropeanUnion and also supported of the project LO1208 “TEWEP” fromMinistry of Education, Youth and Sports of Czech Republic.

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