Journal of Catalysis 370 (2019) 88–96

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Journal of Catalysis

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The synergic degradation mechanism and photothermocatalyticmineralization of typical VOCs over PtCu/CeO2 ordered porous catalystsunder simulated solar irradiation

https://doi.org/10.1016/j.jcat.2018.12.0030021-9517/� 2018 Elsevier Inc. All rights reserved.

⇑ Corresponding author.E-mail address: antc99@gdut.edu.cn (T. An).

Jiejing Kong, Guiying Li, Meicheng Wen, Jiangyao Chen, Hongli Liu, Taicheng An ⇑Guangzhou Key Laboratory of Environmental Catalysis and Pollution Control, Guangdong Key Laboratory of Environmental Catalysis and Health Risk Control, School ofEnvironmental Science and Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou 510006, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 November 2018Revised 4 December 2018Accepted 5 December 2018

Keywords:PhotothermocatalysisVOCs mineralizationPtCu alloyCoke resistanceSynergic mechanism

The major challenges facing catalysts for mineralization of volatile organic compounds (VOCs) includepoor photocatalytic efficiency, high cost of thermocatalytic oxidation, and low stability. Here, a highlyactive and stable PtCu/CeO2 ordered porous nanostructure catalyst was synthesized and efficientlyapplied in the instant mineralization of paraffinic VOCs under photothermocatalytic conditions. Opticaland morphological characterization shows that low loadings of PtCu alloy clusters on CeO2 surfaces leadto enhanced light harvesting, improved charge separation, and increased capacity for reactive oxygengeneration and then promote a dynamic balance of Cu1+,2+/Cu0 with high photothermocatalytic stability.Reaction kinetics calculations and carbon deposits investigation reveal that photothermocatalytic syner-gism mainly comes from the acceleration of the Mars–van Krevelen redox cycle by photocatalysis and theenhancement of coke resistance by thermocatalysis. The strategy here would provide insight into thedesign of highly effective catalysts, as well as in-depth understanding of the synergic mechanism of pho-tothermocatalytic mineralization of refractory VOCs.

� 2018 Elsevier Inc. All rights reserved.

1. Introduction

Straight-chain paraffinic volatile organic compounds (VOCs),such as n-pentane emitted from refinery and e-waste dismantlingprocesses, are always hard to mineralization completely by a singlecatalysis technique, resulting in serious atmosphere pollution [1,2].Current pursuit of more efficient and economical resolution foratmospheric issues has greatly promoted both the developmentof catalysts and the enhancement of catalytic reaction processes[3–5]. Though photocatalysis offers a viable opportunity fordegrading low-concentration VOCs under mild conditions, mostphotocatalysts suffer from low quantum yield [6,7]. For example,only 15% of n-pentane can be mineralized over P25 after 365-nmUV illumination for 60 min [8]. Moreover, carbon deposits areprone to occur during photocatalytic processes, and coke accumu-lation would poison the active site of the catalyst, leading to lowphotocatalytic efficiency [9]. Thermocatalytic oxidation is anothereffective method for deep mineralization of VOCs, even changingcoke into CO2 [10,11]. However, this traditional thermocatalytictreatment alone also has some significant drawbacks, in that sup-

ported noble metal catalysts (more than 0.5 wt% Pt loading) andhigh temperature (more than 500 �C) are always required for effec-tive operation in paraffinic VOCs mineralization, resulting in highenergy costs [5,12,13].

Much research has suggested that combining thermocatalysiswith photocatalysis to maximize synergistic effects could over-come their limitations [14–18]. Compared with traditional TiO2

catalysts, nanostructured CeO2 shows more efficiency in pho-tothermocatalytic removal of VOCs, owing to its unique opticalproperties, remarkable Ce4+/Ce3+ redox properties, high oxygenrelease/storage capacity, and favorable thermostability [16]. Con-sidering that the catalytic reaction is a typical surface process, suf-ficient contact with VOCs and easy mass transfer facilitation can berealized over the ordered porous structures and improve the cat-alytic performance [19]. In addition, supported precious metals,especially Pt, have been well established as stabilizers of photoex-cited electrons for suppressing charge recombination during pho-tocatalytic processes [20] and highly active catalysts foractivating both adsorbed oxygen and surface lattice oxygen todirectly oxidize VOCs [21]. However, the coarsening of the noblemetal particles and the sintering of the support, leading to the lossof active sites and irreversible deactivation, is an importantconcern for their application [10]. In comparison with single

J. Kong et al. / Journal of Catalysis 370 (2019) 88–96 89

noble-metal-supported catalysts, alloy nanostructures have unpar-alleled advantages: (1) Significant promotion of the generationand/or separation of photogenerated charges can be expecteddue to the formation of novel energy structure in alloy structure[22]; (2) alloying results in the decrease of metal nanoparticle sizeand the increase of surface atoms of metal, which improves theactivity and cost effectiveness of catalysts [23]; (3) alloying couldnot only minimize carbon deposits [24] but also help in the regen-eration of noble metal particles by providing nucleation sites onthe support surface [25]. Zhang et al. [23] reported that muchenhanced catalytic efficiency of benzyl alcohol oxidation took placeon PtCu/TiO2 in comparison with Pt/TiO2, and Cu was not catalyt-ically active in the dark. They claimed that PtCu alloy could notonly stabilize Pt particles to be small, but also induce partially pos-itive charging of Pt atoms, and thus expedite catalytic oxidation onPt sites. However, the stability of Cu in PtCu/TiO2, particularlyunder oxidizing conditions, is a serious concern for long-term cat-alytic applications due to its sensitivity to the oxidation.

The synergetic effect of photothermocatalysis over noble metal/semiconductor oxides generally suggests that both the spillover Osadatoms from noble metal nanoparticles and the lattice oxygen insemiconductor oxides first oxidize VOCs, and then the reactiveoxygen species (such as �OH and �O2

�) generated by photocatalysisaccelerate the reoxidation of the reduced catalyst [26]. Mostreports follow this photothermocatalytic mechanism in whichphotocatalysis facilitates the thermocatalytic process, but littleconcern has been shown for how thermocatalysis promotes thephotocatalytic process. As mentioned above, coke formationalways occurs during the photocatalytic oxidation process, whilethermocatalysis facilitates the regeneration of the catalyst byin situ coke elimination [11,27]. In this context, photothermocat-alytic reactions could benefit enhanced coke resistance, thoughcarbon deposition in photothermocatalytic degradation of VOCshas rarely been reported up to now.

Here, to attain full utilization of solar energy for the economicaland efficient catalytic treatment of straight-chain paraffinic VOCs,a PtCu/CeO2 hybrid catalyst, with tiny PtCu alloy clusters(�2.2 nm) highly dispersed on CeO2 ordered porous structures,was developed and applied to photothermocatalytic mineraliza-tion of n-pentane under simulated solar irradiation. The dynamicbalance of Cu1+,2+/Cu0, together with its stabilization of Pt nanopar-ticles in PtCu/CeO2, was demonstrated by TEM and XPS. In compar-ison with CeO2, PtCu/CeO2 exhibits enhanced light-harvestingefficiency, improved charge separation, and increased capacityfor the generation of reactive oxygen species, as well as high activ-ity and stability for the mineralization of n-pentane. Reactionkinetics calculations and carbon deposit investigation have alsobeen performed to understand the synergic effect of photothermo-catalytic mineralization of VOCs.

2. Experimental

2.1. Preparation of catalysts

Three-dimensionally ordered macroporous (3DOM) CeO2 wasprepared by a colloidal crystal template method. Equimolaramounts (2 mmol) of citric acid and Ce(NO3)3�6H2O were dissolvedin a 10 mL mixture of ethylene glycol and methanol (volume ratio1:1). This precursor solution was contacted with polymethylmethacrylate (PMMA) microspheres with a diameter of 300 nmfor 12 h. After impregnation, excess precursor solution wasremoved using vacuum filtration. The precipitate was dried over-night at 50 �C in a vacuum oven and then calcined under Ar at130 �C for 1 h, followed by linearly increasing the temperatureto 600 �C at a rate of 1 �C min�1. After a dwell time of 5 h, the

atmosphere was changed to air, and the sample was kept at600 �C for another 3 h to remove PMMA colloidal crystal templatesto form 3DOM CeO2.

PtCu/CeO2 was prepared using the deposition–precipitationmethod. 3DOM CeO2 (0.2 g) was dispersed evenly in 40 mL aque-ous solution comprising H2PtCl6 and Cu(CH3COO)2, and the sus-pension was vigorously stirred magnetically for 30 min to reachadsorption equilibrium. Subsequently, the pH of the precursorsolution was adjusted to 8 with NaOH aqueous solution. There-after, the suspension was thermostatically held at 80 �C for 4 hunder vigorous magnetic stirring. The whole reaction was carriedout in the dark. The resultant product was filtered and washedfor several times, dried overnight at 50 �C in a vacuum oven, andthen annealed at 400 �C for 2 h at 5 �C min�1 under H2 flow. Thefinal concentration of Pt and Cu in the samples was confirmedusing inductively coupled plasma mass spectrometry (ICP-MS,ICAP RQ); Pt content was approximated to 0.08 wt% and Pt/Cumolar ratio was close to 1:2. For comparison, Pt/CeO2 and Cu/CeO2 were prepared by loading Pt and Cu, respectively, on theCeO2 surface with the same fabrication process as above.

2.2. Catalyst characterization

The morphologies and crystal analysis were obtained by trans-mission electron microscopy (TEM) and high-resolution TEM(HRTEM) measurements over an FEI Talos-F200S instrument atan accelerating voltage of 200 kV. High-angle annular dark-field(HAADF) imaging in the scanning transmission electron micro-scopy (STEM) mode was performed on the same electron micro-scope with the sample tilted about a single axis using a Fischioneultranarrow gap tomography holder. X-ray diffraction (XRD) wasperformed to characterize the crystal structures and compositionsof the catalysts using a D-MAX diffractometer (CuKa radiation).The Brunauer–Emmett–Teller (BET) surface area, pore volume,and pore size distribution of the samples were measured with aMicromeritics ASAP 2020 Plus HD88 instrument using adsorptionof N2 at 77 K. Prior to adsorption analyses, the catalysts weredegassed in flowing N2 at 300 �C for 3 h. X-ray photoelectron spec-troscopy (XPS) was performed using an ESCALAB 250Xi spectrom-eter (Thermo Fisher Scientific, AlKa, hm = 1486.6 eV) under highvacuum of �2 � 10�7 Pa.

Temperature-programmed reduction (TPR) analysis was con-ducted on a TP-5078 Autochem absorption analyzer, with approx-imately 50 mg of the sample loaded into a quartz tube. The samplewas irradiated through the small windows on both sides of the fur-nace; the simulated solar irradiation came from a 300W Xe lamp(k = 300–800 nm, optical power density 15 mW cm�2). The TPRprofile was recorded from 30 to 800 �C at a heating rate of5 �C min�1 under 5% H2/N2 with a flow rate of 30 mL min�1 andthe H2 uptake was measured by a TCD detector.

Ultraviolet–visible (UV–vis) diffuse reflectance spectra (DRS)were recorded on a UV–vis spectrophotometer (UV3600 Plus)equipped with an integrating sphere attachment within a rangeof 200–900 nm and with BaSO4 as the reflectance standard. Photo-luminescence (PL) spectra were characterized by a steady state andtransient state fluorescence spectrometer (FS5) with a Xe lamp(excitation wavelength 380 nm) as light source. Photocurrentsspectra were obtained on an electrochemical analyzer (CHI660E)under visible light illumination from a Xe lamp (100 mW cm�2)in a standard three-electrode system, composed of the as-prepared samples of ca. 1.0 cm2 active area (working electrodes),Pt wire (counter electrode), and Ag/AgCl (saturated KCl) (referenceelectrode). The electrolyte was N2-saturated 0.5 mol L�1 Na2SO4.An electron paramagnetic resonance (EPR) spectrometer(EMXPlus-10/12, Bruker) was used for the measurement of theEPR signals of photoinduced radicals, which were spin-trapped

90 J. Kong et al. / Journal of Catalysis 370 (2019) 88–96

by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The EPR signals ofthe DMPO-trapped radicals (DMPO��OH and DMPO��O2

�) wererecorded at ambient temperature. All freshly prepared suspensions(DMPO: 10 lL; solvent: 3 mL water (for DMPO��OH, react with�O2

�) or methanol (for DMPO��O2�, act as hole scavenger); mCatal.:

3 mg) were mixed directly and then transferred into a cylindricalquartz cell (length 100 mm, diameter 2 mm). A 300 W Xe lamp(k = 300–800 nm) was used in situ as a photoexcitation lightsource. After irradiation for 600 s, the signals of DMPO��OH andDMPO��O2

� were measured on an EPR spectrometer.

2.3. Activity test

Photothermocatalytic reactions were conducted in a tube (i.d.= 6 mm) fixed-bed reactor at atmospheric pressure, and the cata-lyst was irradiated through two small windows on both sides ofthe furnace. The simulated solar irradiation came from a 300WXe lamp (k = 300–800 nm, optical power density 15 mW cm�2).Approximately 0.1 g of catalyst was packed into the reactor. n-Pentane vapor was generated by passing N2 at a certain flow ratethrough the n-pentane solution (�99%). Then the vapor was mixedwith dry air. The final n-pentane concentration of �50 ppm wasfixed in the feeding gas by controlling the solution temperatureand N2 flow rate through the saturator. A gas hourly space velocity(GHSV) value of 30,000 mL h�1 g�1 was tested. The n-pentane andCO2 concentrations were respectively measured using GC-9800(FID) and GC9800 (FID, equipped with a Ni-catalyst-based metha-nizer) gas chromatographs. Three to five concentration data weremeasured at each temperature, with around a 15 min interval.The concentration datum adopted showed less than 10% differencefrom other measured data at each temperature. The n-pentanemineralization was assessed by the equation

Fig. 1. (a, b) SEM and TEM images of CeO2; (c–e) PCu particle size distributions and (HREDX elemental mappings.

Mineralization % ¼CO2½ �out

5

C5H12½ �in� 100%; ð1Þ

where C5H12½ �in is the inlet molar quantity of vaporous n-pentane,

CO2½ �out is the outlet molar quantity of CO2, andCO2½ �out

5 means the cor-responding converted n-pentane molar quantity.

3. Results and discussion

3.1. Structural properties

The XRD patterns in Fig. S1 in the Supporting Information exhi-bit all the characteristic peaks of both CeO2 and PtCu/CeO2, agree-ing well with the cubic CeO2 phase structure (JCPDS No. 34-0394).However, the diffraction peaks of platinum and copper are notfound in PtCu/CeO2, and the reason may be attributed to its lowcontent. Fig. 1 exhibits the morphology and elemental distributionmappings of the catalysts. SEM and TEM images show the orderedtexture of CeO2 with macropores (120 nm) and uniformly sizedwalls (Fig. 1a and b) interconnected with smaller windows. Ultra-small particles with a narrow size distribution are highly dispersedon the CeO2 surface, as shown in the TEM (Fig. 1c and d). HRTEMimages in Fig. 1e exhibits well-crystallized clusters with a latticespacing of about 1.89 Å, just between the values of Cu (2 0 0)(1.80 Å) and Pt (2 0 0) (1.96 Å), indicating the formation of a stablePtCu alloy structure. This result is identical with those reported inprevious literature [23]. The average diameter of PtCu clusters isaround 2.2 nm, and the reduction of obtained PtCu/CeO2 did notalter the ordered pore structure. Elemental distribution mappingsof Ce, Pt, and Cu are shown in Fig. 1f, where Ce, Cu, and a smallamount of Pt are present in all the samples, further demonstratingthe homogeneous distribution of PtCu nanoparticles on CeO2. The

)TEM images of PtCu/CeO2; (f) HAADF images in STEM mode and the corresponding

J. Kong et al. / Journal of Catalysis 370 (2019) 88–96 91

BET surface area, average pore size, and pore volume of CeO2 andPtCu/CeO2 are listed in Table S1. A slight increase in specific surfacearea, with decreases in both the average pore size and the pore vol-ume of PtCu/CeO2, is related to the deposition of ultrasmall PtCuparticles in the existing large pores of CeO2.

3.2. Catalytic performance

The effect of PtCu alloy clusters on the photocatalytic perfor-mance for mineralization of n-pentane is investigated in Fig. 2a.No obvious n-pentane degradation was found over PtCu/CeO2

without light irradiation. Although the activity of both catalystsfor photocatalytic degradation of n-pentane is poor, PtCu/CeO2

exhibits relatively high mineralization efficiencies for n-pentane,approximately 3 times higher than for CeO2 under simulated solarillumination (k = 300–800 nm, optical power density15 mW cm�2)within 2 h. Figs. 2b and S2 show the photothermocatalytic miner-alization activity of n-pentane as a function of temperature overthe catalysts. The photo/thermomineralization of n-pentane wasnegligible without the catalyst. In contrast with CeO2, Cu/CeO2

exhibited no enhanced activity in the dark, indicating that the Cucomponent is not catalytically active for the thermocatalytic min-eralization of n-pentane (Fig. S2). However, PtCu/CeO2 exhibitedmuch more improved photothermocatalytic activity (Fig. S2b), inwhich nearly complete mineralization (>95%) temperature of n-pentane for PtCu/CeO2 and Pt/CeO2 is obtained as 400 and450 �C, respectively, indicating that alloying Pt with Cu couldgreatly promote the photothermocatalytic process with lowerenergy costs. As we know, the Tamman temperature for Cu is406 �C, and Cu would sinter at higher temperatures (>406 �C).However, in the temperature range 450–600 �C, there is no obviousdifference in photothermocatalytic performance between PtCu/CeO2 and Pt/CeO2 (Fig. S2b), indicating that Cu has a negligibleeffect on photothermocatalysis at such high temperatures; there-fore no decrease of catalytic activity was observed over PtCu/CeO2 even when Cu was already sintered. Moreover, the completemineralization temperature of photothermocatalytic n-pentaneover PtCu/CeO2 is 200 �C lower than that in thermocatalytic treat-ment alone (Fig. 2b), demonstrating a synergistic effect betweenphotocatalysis and thermocatalysis over PtCu/CeO2.

To obtain further intrinsic insight into the performance differ-ence between PtCu/CeO2 and CeO2 for n-pentane oxidation, theMears criterion [28] and the Weisz–Prater criterion [29] were cal-culated and evaluated at T100, T50, and T10 (or the temperature atwhich corresponding n-pentane mineralization was achieved) toestimate the influence of external and internal diffusion effectson the reaction rates in this study (see the Supporting Informationfor details). The calculated values at T100, T50, and T10 all show thatNM < 0.15 and NW-P < 0.3 for the catalysts (Table S2), indicating thatboth the external and internal diffusion effects on catalytic perfor-mance can be negligible [30]. Since n-pentane-lean conditions(50 ppm in simulated air), a thin catalyst bed, and well-distributed catalyst size (177–250 lm) were used, the influenceof heat transfer can also be overlooked. After the mass and heattransfer effects were excluded, Ea values could be evaluated foreach sample from the slope of the Arrhenius plots (see the Sup-porting Information for details), as shown in Fig. 2c. The estimatedEa value was around 20.1 kJ mol�1 for PtCu/CeO2 under simulatedsolar irradiation, which was observably lower than those of PtCu/CeO2 in the dark (28.6 kJ mol�1), CeO2 under simulated solar irradi-ation (54.8 kJ mol�1), and CeO2 in the dark (65.2 kJ mol�1)(Table S2), indicating that the existence of a photothermocatalyticsynergetic effect of PtCu/CeO2 can greatly decrease the activationenergy of n-pentane oxidation by the lattice oxygen in CeO2.Fig. S3 shows the turnover frequency (TOF, mineralizedn-pentane molecules per exposed active Pt site per time) of

PtCu/CeO2 for n-pentane mineralization less than 10%. The resultsreveal that PtCu/CeO2 under simulated solar irradiation has higherTOF values (1.7 � 10�3 s�1 at 200 �C) than in the darkness(0.8 � 10�3 s�1 at 200 �C), indicating that photoactivation couldremarkably promote the catalytic oxidation of n-pentane.

To further explore the applicability of the as-developed catalyst,the photothermocatalytic performance of PtCu/CeO2 at variousinlet concentrations of n-pentane (10, 50, 100 ppm) was alsochecked (Fig. 2d). Obviously, PtCu/CeO2 performs well even undera high n-pentane concentration of 100 ppm, with an n-pentanemineralization of 93% at 400 �C under simulated solar irradiation.The stability of PtCu/CeO2 was tested over thermocatalytic andphotothermocatalytic mineralization of n-pentane with a totalreaction time of 141 h in Fig. 2e. At the first 10 h for simulated solarirradiation at 400 �C, 95% n-pentane mineralization was obtained,which to 80% when the light was turned off. After a dwell timeof 121 h, the light was turned on again and n-pentane mineraliza-tion returned to 95%, indicating high photothermocatalytic perfor-mance stability. Moreover, there is no obvious coarsening oragglomeration of PtCu clusters on the surface of PtCu/CeO2 afterthe stability test, further demonstrating the excellent stability ofPtCu/CeO2 (Fig. S4). All these results reveal that the photocatalyticprocess under simulated solar irradiation can greatly lower thetemperature required for thermocatalytic mineralization of n-pentane and promote thermocatalysis, without deactivation ofPtCu/CeO2 catalysts, showing advantages over singlephotocatalysis.

3.3. Optical properties

For the purpose of understanding the photoassisted activityenhancement of PtCu/CeO2, its optical properties including lightresponse, charge separation, and oxidative radical generation areevaluated and illustrated in Fig. 3. Compared with CeO2, PtCu/CeO2 shows more apparent absorption in visible and near-infrared wave bands (Fig. 3a), accompanied by a dramatic colorchange from yellow to gray. This is probably due to the surface dis-order layer with oxygen vacancies created after hydrogen treat-ment of the obtained PtCu/CeO2 [31,32]. The estimated band gap(Eg) value of both catalysts is approximately 2.9 eV (Fig. 3a inset).The valence band edges obtained from valence band spectra ofCeO2 and PtCu/CeO2 are estimated to be ca. 2.5 and 2.3 eV, respec-tively, implying that PtCu loadings and hydrogen treatmentslightly changed the valence band structure of PtCu/CeO2

(Fig. 3b). In combination with the estimated Eg values, their con-duction band values are estimated to be ca. �0.4 for CeO2 and�0.6 eV for PtCu/CeO2, respectively. Charge recombination andseparation were analyzed using photoluminescence spectroscopyand photoelectrochemical methods, as exhibited in Fig. 3c and d.PtCu/CeO2 shows weaker PL intensity and stronger photocurrentthan CeO2, indicating that PtCu/CeO2 has lower charge recombina-tion but higher charge separation than that of CeO2 alone. Further-more, electrochemical impedance spectroscopy (EIS) was alsoperformed to study the apparent charge transfer resistance at theinterface of the catalysts. As shown in Fig. S5, the radius forPtCu/CeO2 is smaller than that for CeO2, both in the dark and undersimulated solar irradiation, implying a higher charge transfer effi-ciency for PtCu/CeO2. It is well known that the surface reactionprocess depends strongly on oxidative radicals’ generation, asoxidative radicals with powerful oxidizing ability can directlydegrade VOCs [33,34], and an indirect DMPO-assisted EPR tech-nique is used to evaluate the generation of �OH and �O2

� radicalsover the samples under simulated solar irradiation (Fig. 3e and f).After being illuminated for 10 min, both catalysts show weak char-acteristic quartet signals of DMPO–�OH with a characteristic inten-sity ratio of 1:2:2:1, indicating that the concentration of �OH from

Fig. 2. (a) Photocatalytic mineralization of 50 ppm n-pentane on PtCu/CeO2 and CeO2 at 30 �C under simulated solar illumination (k = 300–800 nm); (b, c) initial activity andArrhenius plots of PtCu/CeO2 and CeO2 catalysts in the dark or under light irradiation; (d) effect of n-pentane inlet concentration on the performance of PtCu/CeO2; (e) long-term stability test for PtCu/CeO2 at GHSV = 30,000 mL h�1 g�1 and T = 400 �C.

92 J. Kong et al. / Journal of Catalysis 370 (2019) 88–96

photoinduction is very low, as the holes (h+) in their valence bandswithout adequate capacity could hardly oxidize H2O to generate�OH (2.72 eV at pH 7) [7]. In addition, DMPO–�O2

� EPR signals underthe same conditions are only observed in PtCu/CeO2 rather than inCeO2. Considering that the appearance of DMPO–�O2

� is mainly dueto surface reaction between oxygen and photoexcited electrons, wecan infer that the photoexcited electrons can be stabilized on PtCuclusters and then activate oxygen. Moreover, PtCu loadings inducea significant negative shift of the conduction band to enhance thereductive potential, and then supply a strong potential foradsorbed oxygen reduction [35,36].

3.4. H2 temperature-programmed reduction

As the reduction behavior of the catalysts is very important forcatalytic oxidation of VOCs, the reducibility of the metallic ionfrom high to low valence and the nature of taking up oxygen weremeasured by H2 TPR analysis [37]. H2 TPR profiles of CeO2 andPtCu/CeO2 catalysts are shown in Fig. 4. It can be observed that adistinct reduction peak centered at 679 �C is found for CeO2 inthe dark, which is due to the reduction of lattice oxygen and theinner Ce4+, and a lower-temperature reduction peak was probably

caused by the reduction of Ce4+ to Ce3+ ions in the outermost lay-ers, as well as the reduction of the surface adsorption oxygen spe-cies of CeO2. Interestingly, when the catalysts were exposed undersimulated sunlight (k = 300–800 nm) or after the loading of PtCualloy clusters, the reduction peaks shifted to the low-temperature range, indicating that the mobility of surface latticeoxygen species is generally improved by photoexcitation and PtCuloadings [38]. As the catalytic oxidation process could be facilitatedby the high mobility of surface lattice oxygen, PtCu/CeO2 underphotothermocatalytic conditions gives better catalytic perfor-mance for n-pentane mineralization.

3.5. XPS and EPR characterization

To gain insight into the stable photothermocatalytic perfor-mance of PtCu/CeO2, the composition and redox behavior of cata-lysts was studied by X-ray photoelectron spectroscopy (XPS). Forsimplification, fresh PtCu/CeO2 and the samples after a 10 h stabil-ity test for photocatalytic, thermocatalytic, and photothermocat-alytic mineralization of n-pentane are respectively denoted asPtCu-fresh, PtCu-PC, PtCu-TC, and PtCu-PTC. The Ce3d XPS spectrawere analyzed to ascertain the electronic state of cerium. Eight

Fig. 3. (a) UV–vis diffuse reflectance spectra and the dependence of (ahm)2 on the photon energy (inset); (b) XPS valence band spectra; (c) photoluminescence spectra withexcitation by a Xe lamp (wavelength 380 nm); (d) time-dependent photocurrents of various electrodes under on-off simulated sunlight (k = 300–800 nm) exposure pulse of80 s with a constant bias of 0.2 V vs. Ag/AgCl electrode; (e) DMPO–�OH and (f) DMPO–�O2

� EPR spectra of the catalysts.

Fig. 4. H2 TPR of the samples in the darkness or under simulated solar irradiation(k = 300–800 nm).

J. Kong et al. / Journal of Catalysis 370 (2019) 88–96 93

peaks can be found in Ce3d XPS spectra of all the tested samples,which are divided into two groups a1, a2, a3, a4 and b1, b2, b3, b4(Fig. 5a). The XPS binding energies (BE) at 882.2, 888.4, and

898.2 eV corresponding to a1, a3, a4 and the XPS binding energiesat 900.6, 907.1, and 916.5 eV indexed to b1, b3, b4 refer to theCe3d5/2 and Ce3d3/2 of Ce4+, respectively, while the binding ener-gies at 885.1 and 903.1 eV assigned to a2 and b2 stand for theCe3d5/2 and Ce3d3/2 of Ce3+ [39]. The presence of Ce3+ implies adefect structure of CeO2-x due to oxygen vacancies [40,41]. Thevalence state of Ce after the photo/thermoreaction was checkedto further reveal the stability of PtCu/CeO2. The relative amountsof Ce3+ and Ce4+ in the samples were estimated by calculatingthe peak area ratio a2/b4 of Ce3+ (BE = 885.1 eV) to Ce4+

(BE = 916.5 eV) and are summarized in Table S3. The Ce3+/Ce4+

ratio increases from 1.033 for CeO2 to 1.228 for PtCu-fresh due tothe reduced process in hydrogen treatment, but there is no appar-ent Ce3+/Ce4+ composition difference among PtCu-fresh, PtCu-PC,PtCu-TC, and PtCu-PTC. The Pt4f XPS spectrum displayed inFig. 5b can be fitted by the two group peaks, where the Pt4f7/2 BEof 71.6 and 72.2 eV belong to Pt0 and Pt2+, respectively, confirmingthe co-presence of Pt0 and Pt2+ in the samples [42]. The relativeamount of metallic Pt is 67%, and no obvious difference occurredafter the photo/thermoreaction. The above results indicate theexcellent chemical stability of the samples. Cu2p3/2 characteristicpeaks can be fitted into three Gaussian peaks at 932.0, 932.7, and

Fig. 5. XPS spectra of the samples: (a) Ce3d, (b) Pt4f, and (c) Cu2p (fresh PtCu/CeO2 and the samples after 10 h stability test for photocatalytic, thermocatalytic, andphotothermocatalytic mineralization of n-pentane are respectively denoted as PtCu-fresh, PtCu-PC, PtCu-TC, and PtCu-PTC). (d) EPR spectra of PtCu/CeO2.

94 J. Kong et al. / Journal of Catalysis 370 (2019) 88–96

933.6 eV, assigned to Cu0, Cu1+, and Cu2+, respectively [43]. The rel-ative amount of Cu0 was estimated by calculating the peak arearatio of Cu0 to the total Cu, as displayed in Fig. 5c. The percentageof Cu0 content increases from 35% in PtCu-fresh to 53% in PtCu-PC,suggesting that Cu1+/2+ is reduced to Cu0 by acquiring electronsunder simulated solar illumination, but decreases to 27% forPtCu-TC, due to the oxidation of reactive oxygen species createdin the thermocatalytic process [44]. More interestingly, there isno apparent difference in Cu0 composition between PtCu-freshand PtCu-PTC, indicating that the synergistic function of photoin-duced electron reduction and oxidation of reactive oxygen speciesleads to reversible conversion between Cu0 and Cu1+/2+. The chargetransport process between PtCu alloy and CeO2 is characterizedusing EPR. As shown in Fig. 5d, the EPR signals at g = 1.965 and2.114 are assigned to Ce3+ and Cu2+, respectively [45,46]. Undersimulated solar irradiation, both Ce3+ and Cu2+ EPR signals weakenquickly, but the Ce4+-�O2

� signal strengthens, due to part of elec-trons transferring from CeO2 to Cu2+, which would improve thetransfer capability of lattice oxygen in CeO2, and thus promotethe generation of reactive oxygen species.

3.6. Discussion

Photocatalytic mineralization of VOCs is mainly composed ofthree basic steps: (1) photogeneration of charge carriers; (2) elec-tron–hole pair transfer and separation; and (3) redox reactions [7].The thermocatalytic oxidation of VOCs on CeO2 follows the Mars–van Krevelen redox cycle, where VOCs is first oxidized by the lat-tice oxygen of the catalyst, and the reduced CeO2�x is reoxidizedlater by gaseous oxygen [16]. According to the above discussion,during the photothermocatalytic process, PtCu alloy clustersnot only form a Schottky barrier with CeO2 to capture the

photogenerated electrons and then hinder charge recombination,but also act as excellent active phases to dissociate the adsorbedO2 into O (Os) adatoms (highly reactive oxygen species), resultingin the efficient oxidation of n-pentane. Moreover, some electronsin the valence band of CeO2 are excited to Cu1+/2+ species throughthe interfacial charge transfer (IFCT) process [46,47], which resultsin the transformation of Cu1+/2+ into Cu0 and promotes the transfercapability of lattice oxygen in CeO2, therefore improving reducibil-ity of catalysts (H2 TPR) and decreasing the activation energy ofn-pentane oxidation by the lattice oxygen. The generated Cu0 spe-cies are then easily oxidized by oxygen back to Cu1+/2+ under thethermocatalytic reaction system to achieve a dynamic balance ofCu species over PtCu/CeO2. The spillover Os adatoms from Pt andthe strong oxidizing species (such as h+, �O2

�) generated duringthe photocatalytic process can not only mineralize n-pentanedirectly under photothermocatalytic conditions, but also greatlyaccelerate the reoxidation of the reduced CeO2-x catalyst. Thus,the Mars–van Krevelen redox cycle was synergistically accelerated,which resulted in the excellent photothermocatalytic activity ofPtCu/CeO2 (Fig. 6).

As is well known, coke accumulation at the catalyst surfacewould block the active sites and thus cause the deactivation ofthe catalyst [48]. The carbon deposit of the catalytic process overPtCu/CeO2 was investigated by the C1s XPS spectrum, displayedin Fig. 7. All the samples have the same composition of peaks,which includes three peaks at 284.5, 285.9, and 289.0 eV, attribu-ted to sp2 C (CAC, C@C), sp3 C (CAO), and oxidized C (C@O), respec-tively [49], and the content of assigned C1s is summarized inTable S4. For PtCu-fresh, the detected carbon species are mainlyderived from surface contamination, such as CO2 absorption andorganic residues during the preparation process. The total contentof carbon species on PtCu-fresh changes from 20.0 to 25.3% for

Fig. 6. Mechanism of photothermocatalytic degradation of VOCs and stability overPtCu/CeO2.

282 284 286 288 290 292 294 296

Inte

nsity

(a.u

.)

Binding Energy (eV)

C1s

C=O

C−O

C−C, C=C

Fig. 7. XPS C1s spectra of PtCu/CeO2.

J. Kong et al. / Journal of Catalysis 370 (2019) 88–96 95

PtCu-PC, but decreases to 18.7% and 18.4% for PtCu-TC and PtCu-PTC, respectively. The increase of CAC/C@C in PtCu-PC indicatesthat more coke would be accumulated on the catalyst surface aftera 10 h stability test for photocatalytic mineralization of n-pentane.However, the obvious increase of CAO/C@O but reduction ofCAC/C@C is also observed on both PtCu-TC and PtCu-PTC, suggest-ing that the surface coke is transformed into other oxidationproducts during the thermocatalytic process. Hence it could easilybe concluded that thermocatalysis also brings benefits to themineralization of coke for enhancing the photocatalytic efficiency.During the 10 h stability test for photocatalytic performance ofPtCu-fresh, 5.5% n-pentane mineralization is obtained in the first2 h, and then progressively drops to 1.0%, indicating the deactiva-tion of the catalyst after photocatalysis (Fig. S6a). Moreover, thesignificantly lower photocatalytic efficiency of PtCu-PC confirmsthat coke accumulation during the photocatalytic process wouldcause the deactivation of the catalyst, and the negligible changein photocatalytic activity of PtCu-PTC further verifies the regener-ation of the catalyst achieved by in situ coke elimination of the

thermocatalytic process (Fig. S6b). All these resultsdirectly demonstrated the enhanced coke-resistance inphotothermocatalysis.

4. Conclusions

A PtCu/CeO2 ordered porous catalyst was developed for pho-tothermocatalytic mineralization of paraffinic VOCs. Simultaneousphotocatalytic and thermocatalytic activation by PtCu/CeO2

resulted in unique synergistic effects of more effective mineraliza-tion of n-pentane; it can completely mineralize n-pentane (>95%)at 400 �C under simulated solar irradiation, 200 �C lower than thatfor thermocatalytic treatment alone. The synergetic mechanism ofphotothermocatalysis over PtCu/CeO2 can be summarized as fol-lows: (1) Low loadings of PtCu alloy clusters on the CeO2 surfacelead to enhanced light harvesting, improved charge separation,and increased capacity for reactive oxygen generation, whichtogether with the dynamic balance of Cu1+,2+ and Cu0 results inhigh photothermocatalytic performance stability; (2) the intensifi-cation of thermocatalytic efficiency by photocatalysis contributesto the acceleration of the Mars–van Krevelen redox cycle and thelowering of activation energy under simulated solar irradiation;(3) enhancing the photocatalytic efficiency by thermocatalysiscan facilitate deep oxidation of coke and moderate the negativeeffect of carbon deposits on photocatalysis. These results may pro-vide a practical application of PtCu/CeO2 catalysts in photothermo-catalytic oxidation of other refractory VOCs, even soot particles.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (41425015 and 41731279), the LocalInnovative and Research Teams Project of the Guangdong PearlRiver Talents Program (2017BT01Z032), the Team Project fromthe Natural Science Foundation of Guangdong Province, China(S2012030006604), and the Leading Scientific, Technical and Inno-vation Talents of Guangdong special support program(2016TX03Z094), as well as the China Postdoctoral Science Foun-dation Funded Project (2018M630925).

Appendix A. Supplementary material

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.jcat.2018.12.003.

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