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Received: 28 November 2018; Accepted: 18 December 2018; Published: 21 December 2018. Abstract: The performance of plasma-modified Pt/CeO2 for ...

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Plasma-Assisted Surface Interactions of Pt/CeO2 Catalyst for Enhanced Toluene Catalytic Oxidation Bingxu Chen 1,2 , Bangfen Wang 1,2 , Yuhai Sun 1,2 , Xueqin Wang 1,2 , Mingli Fu 1,2 , Junliang Wu 1,2 , Limin Chen 1,2 , Yufei Tan 3 and Daiqi Ye 1,2, * 1

2 3

*

School of Environment and Energy, South China University of Technology, Guangzhou 510006, China; [email protected] (B.C.); [email protected] (B.W.); [email protected] (Y.S.); [email protected] (X.W.); [email protected] (M.F.); [email protected] (J.W.); [email protected] (L.C.) National Engineering Laboratory for VOCs Pollution Control Technology and Equipment, Guangzhou 510006, China Chinese Research Academy of Environmental Sciences, Beijing 100012, China; [email protected] Correspondence: [email protected]; Tel.: +86-20-39380516

Received: 28 November 2018; Accepted: 18 December 2018; Published: 21 December 2018

 

Abstract: The performance of plasma-modified Pt/CeO2 for toluene catalytic oxidation was investigated. Pt/CeO2 nanorods were prepared by wet impregnation and were modified by thermal (PC-T), plasma (PC-P), and combined (PC-TP and PC-PT) treatments. The modified catalysts were characterized by TEM (transmission electron microscope), BET (Brunauer-Emmett-Teller), H2 -TPR, O2 -TPD, XPS, UV-Raman, and OSC tests. The significant variation of the surface morphologies and surface oxygen defects could have contributed to the modification of the Pt/CeO2 catalysts via the plasma treatment. It was found that plasma could promote the surface interaction between Pt and CeO2 , resulting in the thermal stability of the catalyst. The Pt-Ce interaction was also conducive to an increase in the number of oxygen vacancies. Furthermore, PC-PT and PC-TP showed a significant difference in oxygen vacancy concentrations and catalytic activities, which illustrated that the treatment sequence (plasma and thermal treatment) affected the performance of Pt/CeO2 . The PC-PT sample showed the highest catalytic activity with T100 at 205 ◦ C. This work thus demonstrates that plasma in combined treatment sequences could assist surface interactions of catalysts for enhanced toluene catalytic oxidation. Keywords: non-thermal plasma; Pt/CeO2 ; oxygen vacancies; treatment Sequence

1. Introduction Volatile organic compounds (VOCs) are high-risk contaminants, which can cause a significant threat to the nervous system and the environment [1,2]. Toluene is a representative VOC, which is of high toxicity and very difficult to naturally degrade. In the past few years, the use of noble metals (Pt, Au, Pd) and redox supports (CeO2 , Co3 O4 , ZrO2 ) [3–6] to improve toluene oxidation has been prevalent. In particular, Pt-based materials have been widely adopted for toluene oxidation because of their excellent toluene dissociative adsorption abilities [7]. Meanwhile, the CeO2 redox support has attracted considerable attention due to its high oxygen storage ability. During the traditional preparation of these catalysts, calcination is employed to activate the catalyst, but they are at risk of sintering at high temperatures. To avoid this risk during Pt/CeO2 activation, non-thermal plasma has been proposed as an alternative technique. Non-thermal plasma (NTP) is a promising technology for catalyst surface treatment, which could alter the morphology and chemical states of a catalyst, create more defects on the catalyst surface, and Catalysts 2019, 9, 2; doi:10.3390/catal9010002

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activate the catalyst in facile reaction conditions (fast response at room temperature). It could also improve the activity and thermal stability of the catalyst [8,9]. Di et al. [10] reported that the Pd/P25 catalyst reducing by plasma treatment had a smaller Pd particle size and showed a better activity for CO oxidation. Gulyaev et al. [11] prepared Pd/CeOx by plasma synthesis and found an increase of Pd dispersion and turnover frequency (TOF) after discharge. Notably, previous discussions of catalyst modification during discharge mainly concentrated on the surface morphology changes, dispersion of active metals, and the reduction extent of the catalysts, but its effect on surface defects such as oxygen vacancies should not be neglected. Ye et al. [12] introduced plasma and thermal annealing to create various defects on MoS2 to promote the hydrogen evolution reaction. Xu et al. [13] employed plasma to engrave Co3 O4 nanosheets, which produced more oxygen vacancies on the Co3 O4 surface after discharge, and the activity improvement was attributed to a larger number of exposed active defects. Although plasma-induced catalyst surface defects have gained research attention, there have been few studies on the effect of plasma on CeO2 surfaces. Furthermore, the more efficient use of plasma should be studied. Qi et al. [14] used a single plasma to reduce Pd species on activated carbon at low temperatures to avoid sintering, but the reduction of PdOx was insufficient. Lee et al. [15] claimed that sintering is an issue for catalysts at high temperatures. Hence, single methods (plasma or thermal) have not met the demands for catalyst modification. Thus, a combination of plasma and thermal methods may address these issues. Laura et al. [16] attempted to modify Pt catalysts by using a combination of plasma and thermal reduction and suggested that catalysts prepared via this method exhibited higher activities in the water-gas shift reaction than catalysts treated by only single plasma or calcination. Thus, the combination of plasma and thermal treatment is prominent for active catalyst modification. Currently, the synergetic effect of combining plasma and thermal treatment is still unclear, and the role of plasma in combined treatment also should be clarified. In this study, Pt loaded on CeO2 nanorods was employed as a catalyst, and the catalytic performances of Pt/CeO2 with plasma and/or thermal treatments were compared. To investigate the effects of plasma on physical/chemical changes of the catalyst, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-Raman, H2 -temperature programmed reduction (H2 -TPR), O2 -temperature programmed desorption (O2 -TPD), and oxygen storage capacity (OSC) tests were used. Furthermore, the TOFs based on Pt particles and oxygen vacancies were calculated. Through these characterizations, the effect of combining catalyst treatments in various sequences was uncovered. 2. Results The catalytic oxidation of toluene was evaluated. As shown in Figure 1, the catalytic activities of the samples were in the following order: PC-PT > PC-TP > PC-P >PC-T > PC-U. Compared to pure CeO2 whose T100 had reached 328 ◦ C, the catalytic activities of Pt/CeO2 was improved. The lowest T100 and T50 were found for PC-PT at 205 ◦ C and 167 ◦ C, respectively, while the T100 of PC-TP was 236 ◦ C, 30 ◦ C higher than that of PC-PT. Therefore, the treatment sequence in combined treatment could affect the catalytic activity. Moreover, Arrhenius plots were used to further compare the chemical kinetic activities of the samples, as shown in Figure 2 and Table 1. The Arrhenius plots were obtained when the toluene oxidation conversion was below 10%, and a linear correlation was found. As shown in Table 1, the activation energy (Ea ) values for PC-PT, PC-TP, PC-P, PC-T, and PC-U are 74.5, 83.1, 89.2, 102.6, and 116.8 kJ mol−1 . The Ea values of PC-PT and PC-TP are lower, indicating an easier activation of toluene oxidation.

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Figure Conversion of toluene over catalysts with various treatment. Figure1.1. 1.Conversion Conversionof oftoluene tolueneover overcatalysts catalystswith withvarious varioustreatment. treatment. Figure −1 . –1 Catalyst amount: 200 mg; Toluene concentration: 200 ppm; gas flow: 100 ml Catalyst amount: 200mg; mg;Toluene Toluene concentration: 200ppm; ppm; gasflow: flow: 100min mlmin min–1. . Catalyst amount: 200 concentration: 200 gas 100 ml

Figure2. Arrheniusplots plotsfor forthe theoxidation oxidationof oftoluene tolueneover overdifferent differentPt/CeO Pt/CeO222catalysts. catalysts. Figure 2.2.Arrhenius Arrhenius plots for the oxidation of toluene over different Pt/CeO catalysts. Figure

Evidently, the catalysts treated with plasma and thermal processes exhibited better catalytic Evidently, the the catalysts catalyststreated treated with with plasma plasmaand andthermal thermal processes processesexhibited exhibited better better catalytic catalytic Evidently, activities, which deserves further discussion. Generally, a Pt particle is regarded as an active site activities, which deserves further discussion. Generally, a Pt particle is regarded as an active site of activities, which deserves further discussion. Generally, a Pt particle is regarded as an active site of of Pt-based materials. Greater Pt dispersion can expose Pt atoms, is associated with Pt-based materials. Greater Ptdispersion dispersion canexpose expose moremore Ptatoms, atoms, whichwhich associated withactive active Pt-based materials. Greater Pt can more Pt which isisassociated with active toluene oxidation due to the excellent toluene adsorption [17]. To investigate this effect, TOF toluene oxidation due to the excellent toluene adsorption [17]. To investigate this effect, TOF Pt values Pt toluene oxidation due to the excellent toluene adsorption [17]. To investigate this effect, TOFPt values –3, 7.47.9 values based on Pt dispersion were calculated. As listed in Table 1, the TOF values were × based on Pt dispersion were calculated. As listed in Table 1, the TOF Pt values were 7.9 × 10 × 10 Pt 7.9 × 10–3, 7.4 × 10– – based on Pt dispersion were calculated. As listed in Table 1, the TOFPt values were − 3 − 3 − 3 − 3 − 3 − 1 3 –3 –3 –3 –1 6.2 10 3.1 , 6.1 × ,for and 3.1 ×PC-TP, 10 s PC-P, for PC-PT, PC-TP, PC-P, PC-T, and 6.2, ×7.4 × 10 10× 6.1 ×,× 10 10–3,× , and and 3.1 10–310ss–1 for PC-PT, PC-TP, PC-P, PC-T, and and PC-U, PC-U, respectively. 310 –3, ,10 , , 6.2 6.1 ×× 10 PC-PT, PC-T, respectively. PC-U, respectively. Compared to the samples without plasma modification (PC-T and PC-U), Compared to the samples without plasma modification (PC-T and PC-U), the catalytic activities of Compared to the samples without plasma modification (PC-T and PC-U), the catalytic activitiesthe of catalytic activities of the plasma-treated samples (PC-PT and PC-TP) and TOF values increased. theplasma-treated plasma-treatedsamples samples(PC-PT (PC-PTand andPC-TP) PC-TP) and and TOF TOFPtPt values valuesincreased. increased.Since Since TOFPtPtwas wasthe the Pt the TOF Since TOF was the turnover frequencies based on the Pt particles, and a positive correlation exists turnover frequencies based on the Pt particles, and a positive correlation exists between the TOF Pt turnover frequencies based on the Pt particles, and a positive correlation exists between the TOFPtPt between the TOF and the activity, itPt was inferred thatactive Pt particles were active sites. In valueand and the catalytic activity, wasinferred inferred that Ptparticles particles were active sites.In Inaddition, addition, Asgari Pt value value the catalytic activity, ititcatalytic was that were sites. Asgari addition, Asgari et that al. [18] reported that oxygen vacancies played a catalytic vital roleoxidation in the catalytic oxidation etal. al.[18] [18]reported reported that oxygen vacancies played vitalrole role inthe the catalytic oxidation bycontrolling controlling et oxygen vacancies played aavital in by by controlling the consumption and supplementation of surface activated oxygen. TOF the consumption and supplementation of surface activated oxygen. TOF ov values were also calculated ov values the consumption and supplementation of surface activated oxygen. TOFov values were also calculated − 4 s−1 ), PC-TP (3.4 − 4 s−1 ), PC-P –4 s–1), PC-TP –410 –1 –4 s–1× were also calculated (Table 1) as follows: PC-PT (3.6 × 10 (Table 1) as follows: PC-PT (3.6 × 10 (3.4 × 10 s ), PC-P (2.8 × 10 ), PC-T (2.6 10–4–4ss– – (Table 1) as follows: PC-PT (3.6 × 10–4 s–1), PC-TP (3.4 × 10–4 s–1), PC-P (2.8 × 10–4 s–1), PC-T (2.6 ××10 1), and PC-U (2.0 × 10–4 s–1). The similar trends between the catalytic activities and TOFov values 1), and PC-U (2.0 × 10–4 s–1). The similar trends between the catalytic activities and TOFov values

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(2.8 × 10− 4 s−1 ), PC-T (2.6 × 10− 4 s−1 ), and PC-U (2.0 × 10− 4 s−1 ). The similar trends between the catalytic activities and TOFov values confirmed that oxygen vacancies were also crucial active sites that determined the performances of the samples. Table 1. Catalytic performances of various samples. Catalytic activity

Sample

T50 CeO2 PC-U PC-T PC-P PC-TP PC-PT

(◦ C)

T100

262 214 204 183 178 167

(◦ C)

Ea (kJ mol−1 )

TOFPt 1 (×10−3 s−1 )

TOFOV 1 (×10−4 s−1 )

116.8 102.6 89.2 83.1 74.5

3.1 6.1 6.2 7.4 7.9

2.0 2.6 2.8 3.4 3.6

328 279 256 247 236 205 1

The TOF values were calculated at 150 ◦ C.

The specific surface areas (SBET ) of the samples are shown in Table 2. The SBET of PC-PT was increased by nearly 10 m2 g−1 as compared to that of PC-T, which indicated that catalysts’ morphologies were modified by the plasma. This might be ascribed to the fact that plasma-excited species etched the catalyst surface [19] and some of the CeO2 nanorods were cracked and became shorter, exposing more of the catalysts’ surface area. The decrease of SBET was attributed to sintering of the catalysts at high temperatures. However, the slight decreases of SBET from PC-P to PC-PT/PC-TP suggested that plasma treatment was conducive to inhibiting the sintering of catalyst. The Pt loading amounts of the samples ranged from 0.79–0.81 wt %, as shown in Table 2. Table 2. Structural parameters of various samples. Sample PC-U PC-T PC-P PC-TP PC-PT

Pt loading (%) 1 0.81 0.80 0.80 0.80 0.79

Pt mean size(nm) d1Pt 2.8 3.2 2.3 2.5 2.4

2

d2Pt

Pt dispersion (%)

3

D1Pt

3.2 3.6 2.6 2.8 2.7

1 Determined by ICP-OES. 2 Determined by TEM. isotherm. 5 Determined by XRD.

40.2 35.1 48.9 45.0 46.9 3

2

D2Pt 35.2 31.2 45.0 40.2 41.7

3

SBET (m2 g−1 ) 4

Crystalline size (nm) 5

90.02 87.88 99.31 95.55 96.94

7.0 9.0 6.8 8.0 7.3

Determined by CO-chemisorption.

4

Determined by N2

TEM images of the catalysts are presented in Figure 3. The images of the samples showed the spacing of 0.32 nm for the (111) crystal planes of CeO2 and identical Pt lattices with a Pt (111) interplanar distance of 0.23 nm. Samples were rod-shaped, as expected, but partial cracking occurred on the CeO2 nanorods modified by plasma. Thus, the plasma treatment yielded shorter CeO2 nanorods. The Pt particle sizes were determined from the images, and the results are listed in Table 1. For PC-P, PC-TP, and PC-PT, the mean Pt particle sizes were 2.3 nm, 2.4 nm, and 2.5 nm, respectively, but the size of PC-T was 3.2 nm. Compared to the Pt particle size of PC-T, PC-P showed a decreased Pt particle size, and the particle sizes of PC-TP and PC-PT were also obviously smaller than that of PC-T. This was attributed to the etching effect of the plasma: Pt particles that aggregated into clusters were split by the plasma in the discharge area and dispersed uniformly on the catalyst surface. The Pt particle sizes estimated by CO-chemisorption were as follows: PC-T (3.6 nm), PC-P (2.6 nm), PC-TP (2.8 nm), and PC-PT (2.7 nm). These calculated sizes were larger than those determined from the TEM images, which is consistent with previous reports [20,21]. The small differences between the Pt particle sizes obtained by TEM and CO-chemisorption proved the accuracy of the TEM test. The corresponding platinum particle dispersions are also listed in Table 2. The Pt dispersion for PC-PT was 46.9%, followed by PC-TP (45.0%). Notably, the PC-PT and PC-TP samples had similar Pt dispersions, indicating that the

surface. The Pt particle sizes estimated by CO-chemisorption were as follows: PC-T (3.6 nm), PC-P (2.6 nm), PC-TP (2.8 nm), and PC-PT (2.7 nm). These calculated sizes were larger than those determined from the TEM images, which is consistent with previous reports [20,21]. The small differences between the Pt particle sizes obtained by TEM and CO-chemisorption proved the accuracy of the TEM test. The corresponding platinum particle dispersions are also listed in Table 2. Catalysts 2019, 9, 2 5 of 18 The Pt dispersion for PC-PT was 46.9%, followed by PC-TP (45.0%). Notably, the PC-PT and PC-TP samples had similar Pt dispersions, indicating that the different treatment sequences had little effect different treatment sequences little effect on the between particle size. The variety catalytic on the particle size. The varietyhad of catalytic activities PC-PT and PC-TPofshould notactivities only be between PC-PT and PC-TP should not only be attributed to Pt dispersion. attributed to Pt dispersion.

Figure Figure 3. 3. TEM TEM image image images images of of samples samples and and size size distribution distribution of of Pt Pt particles particles on on CeO CeO22 support. support.

The XRD XRD patterns patterns of of the the five five samples samples are are shown shown in in Figure Figure 4. 4. The The diffraction diffraction patterns patterns of of all all the the The samples exhibited face-centered cubic fluorite structures (JCPDS card No. 34-0394) [22]. The deficiency samples exhibited face-centered cubic fluorite structures (JCPDS card No. 34-0394) [22]. The ◦ ) was due to the high dispersion or low loading of Pt. The of the diffraction peak of Ptpeak metal deficiency of the diffraction of(39.8 Pt metal (39.8°) was due to the high dispersion or low loading of average crystallite size of each material was calculated from the Scherrer equation based on the full Pt. The average crystallite size of each material was calculated from the Scherrer equation based on width at half maximum (FWHM) of the CeO2 (111) plane, and the results are displayed in Table 2. PC-P showed a decrease of the mean crystal size compared to that of PC-U, suggesting that the plasma discharge had a shrinkage effect on the catalyst morphology. Some studies found that as the crystal size decreased with a lower Ce–O symmetry, the number of surface oxygen sites/oxygen vacancies in the CeO2 sample increased [23]. In addition, thermal treatment at high temperatures could induce catalyst sintering [24], as is evidenced by the difference in the crystal sizes between PC-U and PC-T. However, compared to the remarkable increase of the crystal size from PC-U to PC-T, the slight change from PC-U to PC-PT suggested that the plasma might inhibit the catalyst sintering [25,26]. Moreover,

Table 2. PC-P showed a decrease of the mean crystal size compared to that of PC-U, suggesting that the plasma discharge had a shrinkage effect on the catalyst morphology. Some studies found that as the crystal size decreased with a lower Ce–O symmetry, the number of surface oxygen sites/oxygen vacancies in the CeO2 sample increased [23]. In addition, thermal treatment at high temperatures could induce catalyst sintering [24], as is evidenced by the difference in the crystal sizes between PCCatalysts 2019, 9, 2 6 of 18 U and PC-T. However, compared to the remarkable increase of the crystal size from PC-U to PC-T, the slight change from PC-U to PC-PT suggested that the plasma might inhibit the catalyst sintering [25,26]. et Moreover, Farmer etthat al. [27] suggested that sintering was affected byinteractions. the metalFarmer al. [27] suggested sintering was strongly affected by strongly the metal-support support interactions. et al. [28] proved that plasma couldreaction enhancebetween the interface reaction between Yang et al. [28] provedYang that plasma could enhance the interface the support and metal, the support and metal, and stabilize the plasma-activated Pt catalyst in sintering tests. Therefore, and stabilize the plasma-activated Pt catalyst in sintering tests. Therefore, the sintering inhibitionthe of sintering inhibition of to PC-PT likely reactions, due to Pt-Ce interface reactions, Pt was inserted into CeO 2, PC-PT was likely due Pt-Cewas interface as Pt was inserted intoas CeO , and made the crystal 2 and mademore the crystal more stable at high temperatures The crystal sizes of PC-PT PC-TP (7.3 (8.0 structure stable structure at high temperatures [15]. The crystal sizes[15]. of PC-TP (8.0 nm) and nm) indicated and PC-PT (7.3 nm) indicated that differentcould treatment in changes ofand the nm) that different treatment sequences resultsequences in changescould of theresult catalyst structure, catalyst and thus, the thermal stability varied. thus, thestructure, thermal stability varied.

Figure samples. Figure4.4.XRD XRDpattern patternof ofdifferent differentPt/CeO Pt/CeO2 samples.

XPS XPS analysis analysis was was employed employed to to estimate estimate the the surface surface composition composition of of the the catalysts, catalysts, and and the the results results are shown in Figure 5 and Table 3. Figure 5A shows the spectra of the Ce species, which fit are shown in Figure 5 and Table 3. Figure 5A shows the spectra of the Ce species, which werewere fit with with eight peaks, corresponding to the pairs of spin-orbit doublets. Ce 3d and Ce 3d spin–orbit 3/2 eight peaks, corresponding to the pairs of spin-orbit doublets. Ce 3d5/2 5/2 and Ce 3d 3/2 spin–orbit components were denoted as U and V, respectively. Based on previous reports, three pairs components were denoted as U and V, respectively. Based on previous reports, three pairs of of peaks, peaks, V (881.9 eV), U (899.3 eV); V” (888.5 eV), U” (905.9 eV); and V”’ (897.6 eV), U”’ (915.0 eV), V (881.9 eV), U (899.3 eV); V” (888.5 eV), U” (905.9 eV); and V”’ (897.6 eV), U”’ (915.0 eV), were were 4+ [29]. The peaks V’ (884.2 eV) and U’ (901.6 eV) were assigned to Ce3+ . The data in ascribed ascribed to to Ce Ce4+ [29]. The peaks V’ (884.2 eV) and U’ (901.6 eV) were assigned to Ce3+. The data in 3+ ratios of the combined-treated sample PC-TP (27.07%) and PC-PT (30.30%) Table Table33shows showsthat that the the Ce Ce3+ ratios of the combined-treated sample PC-TP (27.07%) and PC-PT (30.30%) 3+ were were higher higher than than that that of of the the single-treated single-treatedsamples samplesPC-T PC-T(22.35%). (22.35%).Since Sincethe therelative relativecontents contentsof ofCe Ce3+ were were regarded regarded as as an an indicators indicators of of oxygen oxygen vacancies vacancies on on the the CeO CeO22 surfaces, surfaces, it it was was inferred inferred that that the the samples modified by combined treatments obtained a greater number of oxygen vacancies than samples modified by combined treatments obtained a greater number of oxygen vacancies than those those modified method. Moreover, whenwhen PC-UPC-U was compared with PC-P, thePC-P, Ce3+ ratio increased 3+ ratio modifiedby bya single a single method. Moreover, was compared with the Ce 3+ from 20.44% to 26.44%, and the Ce ratio of PC-PT had a further increase to 30.30%. The increased 3+ increased from 20.44% to 26.44%, and the Ce ratio of PC-PT had a further increase to 30.30%. The 3+ ratio indicated that the catalyst oxygen vacancies increased stepwise due to the plasma plus Ce increased Ce3+ ratio indicated that the catalyst oxygen vacancies increased stepwise due to the plasma thermal treatments. plus thermal treatments. The The O1s O1s spectra spectra are are shown shown in in Figure Figure 5B. 5B. The The O1s O1s spectra spectra were were deconvoluted deconvoluted into into two two peaks: peaks: peaks in the 532.1–532.4 eV range were assigned to surface oxygen (O ), and peaks in the 529.6–529.9 peaks in the 532.1–532.4 eV range were assigned to surface oxygenads(Oads), and peaks in the 529.6– eV range were assigned to lattice oxygen (Olatt )(O [30]. The relative proportions of O of O could reflect the 529.9 eV range were assigned to lattice oxygen latt) [30]. The relative proportions ads ads could reflect concentration of surface oxygen species over the catalyst. As shown in Table 3, Oads ratio increased in the following order: PC-U, 51.07%; PC-T, 53.69%; PC-P, 63.78%; PC-TP, 64.20%; PC-PT, 70.11%. The relative ratios of Oads for plasma-treated samples ranged from 63.78% to 70.11% when the ratios of PC-T and PC-U were less than 55%. Hence, the concentration of surface active oxygen was increased through plasma treatment. Furthermore, the Oads proportion increased from 63.78% (PC-P) to 70.11% (PC-PT) due to the combined treatment, which might explain the better catalytic activity of PC-PT

the concentration of surface oxygen species over the catalyst. As shown in Table 3, Oads ratio increased in the following order: PC-U, 51.07%; PC-T, 53.69%; PC-P, 63.78%; PC-TP, 64.20%; PC-PT, 70.11%. The relative ratios of Oads for plasma-treated samples ranged from 63.78% to 70.11% when the ratios of PC-T and PC-U were less than 55%. Hence, the concentration of surface active oxygen was Catalysts 2019, 9, 2 7 of 18 increased through plasma treatment. Furthermore, the Oads proportion increased from 63.78% (PC-P) to 70.11% (PC-PT) due to the combined treatment, which might explain the better catalytic activity of PC-PT thanbecause PC-P, because more surface-active oxygenon species on thewould catalyst would its enhance its than PC-P, more surface-active oxygen species the catalyst enhance catalytic catalytic performance foroxidation toluene oxidation [31]. performance for toluene [31]. The Pt 4f spectra, comprising Pt 4f 7/2 and Pt 4f 4f5/2 5/2, ,are mainly The Pt 4f spectra, comprising Pt 4f7/2 and Pt areshown shownin in Figure Figure 5C. 5C. Pt Pt was was present present mainly 0 and platinum oxide Ptδ+ 2+,2+ 4+).4+ 0 species 0 δ+ 0 in metallic states Pt (Pt Pt The peaks of the Pt appeared at 70.6 in metallic states Pt and platinum oxide Pt (Pt , Pt ). The peaks of the Pt species appeared at 2+ 4+ 2+ appeared 4+ appeared and and 73.973.9 eV, eV, the the peaks of Pt appeared at 72.0 andand 75.275.2 eV,eV, andand thethe peaks of of Pt Ptappeared at 73.7 eV 70.6 peaks of Pt at 72.0 peaks at 73.7 0 in 0PC-T, PC-TP, and PC-PT were greater than 75%, while it was only andand 77.077.0 eV.eV. TheThe content of Pt eV content of Pt in PC-T, PC-TP, and PC-PT were greater than 75%, while it was 62.5% in PC-P. It was that Pt species of theofPC-P sample contained lower lower amounts of Pt0. only 62.5% in PC-P. It confirmed was confirmed that Pt species the PC-P sample contained amounts Hence, it wasit necessary to employ a combined modification method catalyst, of Pt0 . Hence, was necessary to employ a combined modification methodtotoactivate activate the the catalyst, combining thermal treatment with H 2 as a supplement to plasma reduction. The relative surface combining thermal treatment with H2 as a supplement to plasma reduction. The relative surface platinumconcentrations concentrationsof ofthe thevarious variouscatalysts catalystsmight mightimply implythe theinward inwarddiffusion diffusionof ofPt Pt[32]. [32]. The The low low platinum Pt/Ce ratios shown in Table 3 for PC-PT, PC-TP, and PC-P were attributed to the incorporation of Pt Pt/Ce ratios shown in Table 3 for PC-PT, PC-TP, and PC-P were attributed to the incorporation of into the CeO 2 surface layer, which enhanced the interaction between Pt and CeO 2 . However, the Pt into the CeO2 surface layer, which enhanced the interaction between Pt and CeO2 . However, the significantly lower lower ratio ratio in in PC-T PC-T was was due due to toPt Ptincorporation incorporationinto intothe theCeO CeO22 bulk bulk lattice lattice as as aa result result of of significantly catalyst sintering [33]. Therefore, when the Pt/Ce ratios of PC-PT and PC-TP were compared with catalyst sintering [33]. Therefore, when the Pt/Ce ratios of PC-PT and PC-TP were compared with that of of PC-T, PC-T,ititwas wasindicated indicatedthat thatthe theplasma plasmain incombined combinedtreatments treatments likely likelyinhibited inhibited sintering sintering and and that promoted metal-support interactions. promoted metal-support interactions.

Figure 5. Cont.

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Figure 5. 5. XPS XPS spectra spectra for for different different Pt/CeO Pt/CeO22 samples: Figure samples:(A) (A)Ce Ce3d, 3d,(B) (B)O O1s, 1s,(C) (C)Pt Pt4f. 4f. Table 3. XPS data of various samples. Table 3. XPS data of various samples. Sample

Pt/Ce

PC-U PC-U PC-T PC-P PC-T PC-TP PC-P PC-PT

0.0196 0.0196 0.0104 0.0183 0.0104 0.0165 0.0183 0.0162

Sample

Pt/Ce

Pt0 /(Pt0 +Pt2+ + Pt4+ ) %

Ce3+ /(Ce3+ +Ce4+ ) %

Oads /(Oads +Olat ) %

Pt0/(Pt0+Pt2++ Pt4+) %

Ce3+/(Ce3++Ce4+) %

20.13 76.37

20.44 22.35

51.07 53.69

62.50 76.37

26.40 22.35

63.78 53.69

20.13

75.26 62.50 75.10

20.44

27.07 26.40 30.30

Oads/(Oads+Olat) % 51.07

64.20

63.78 70.11

PC-TP 0.0165 75.26 27.07 64.20 H2 -TPR was employed the reducibilities of30.30 the samples with various PC-PT 0.0162 to analyze 75.10 70.11 treatments. As shown in Figure 6, the profile of CeO2 only consisted of one wide peak, and the peak temperature H2-TPR was◦ C employed analyze reducibilities of the samples with various treatments. As ranged from 250 to 500 ◦ C,towhich wasthe assigned to the reduction of oxygen species on the surface and shown in Figure the profile of CeO2 only consisted of one wide peak, and the peak temperature subsurface of CeO6, 2 . After the impregnation of Pt on CeO2 , two reduction peaks appeared at around 100 ranged from 250 °C 500 indicated °C, whichthe was assignedoftoPtO the reduction of oxygen species on the surface ◦ and 400 C. The firsttopeak reduction x or ceria oxygen adjacent to Pt species, and and subsurface CeO2. After the oxygen impregnation PtPton[34]. CeO 2, two reduction peaks appeared at the second peak of is assigned to ceria far fromofthe Compared to PC-U, the peak positions around 100 and 400 °C. The first peak indicated the reduction of PtO x or ceria oxygen adjacent to Pt of the samples with plasma treatment shifted to lower temperatures significantly, with the peak species, and the second peak is assigned to ceria oxygen far from the Pt [34]. Compared to PC-U, the peak positions of the samples with plasma treatment shifted to lower temperatures significantly, with

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the peak temperatures of PC-P, PC-TP, and PC-PT shifted from 138 °C (PC-U) to 103, 101, and 94 °C, ◦ C, respectively. temperatures Higher of PC-P,temperature PC-TP, and PC-PT from 138°C◦ Cshowed (PC-U)similar to 103, 101, and respectively. peaks shifted of around 400 shifts. It 94 was reported that ◦ Higher temperature peaks of around 400 C showed similar shifts. It was reported that species Pt species could spill over H2 to the neighboring Ce–O bond [35,36], which caused the Pt reduction could spill over2 Hto the neighboring bond [35,36], caused the reduction peaks of CeO2 peaks of CeO at lowerCe–O temperatures. Towhich further compare the low-temperature 2 toappear to appear at lower temperatures. To further compare the reducibilities,inwe calculated reducibilities, we calculated the total H2 consumption oflow-temperature the samples, as summarized Table 4. The ◦C the total H consumption of the samples, as summarized in Table 4. The H consumption below 200 H2 consumption below 200 °C showed a sharp rise for PC-P, PC-TP, and2 PC-PT to 128, 135, and 182 2 − 1 –1 showed sharp rise forwhile PC-P,the PC-TP, andofPC-PT to 128, 182 µmol g samples , respectively, while the μmol g a, respectively, values H2 uptake for135, the and PC-U and PC-T were around 50 −1 . Note that the nominal –1. H –1. valuesgof uptake for the PC-U and PC-T samples were around 50 µmol g μmol Note that the nominal value of H 2 consumed by loaded Pt species should be 28 μmol g 2 − 1 value of by loaded specieswas should be 28 to µmol . Thus,effect, the excess H2 the uptake of the Thus, theHexcess H2 uptake of thePt samples ascribed the gspillover causing reduction 2 consumed samples wasthat ascribed to the spillover effect, Hence, causingthe thefirst reduction of Ce4+beions adjacent to of Ce4+ ions were adjacent to Pt species. peak should notthat onlywere assigned to the Pt species. Hence, the first peak should be not only assigned to the reaction of PtO with H at low reaction of PtOx with H2 at low temperatures but also the reduction of active oxygenx species derived 2 temperatures also the with reduction of Therefore, active oxygen species derived from ceria of that interacted with from ceria thatbut interacted Pt [37]. the increased H2 consumption PC-PT compared Pt that [37]. of Therefore, increased consumption of PC-PT compared to that of PC-T may indicate to PC-T maythe indicate thatHthe Pt-Ce interaction was enhanced due to the plasma treatment. 2 that the Pt-Ce interaction was enhanced due to the plasma treatment. Theofdifferences of the reduction The differences of the reduction temperature and H2 consumptions PC-PT and PC-TP were temperaturetoand H2 different consumptions of PC-PT and PC-TP attributed to their different reducibilities attributed their reducibilities caused by were the various treatment sequences, and the caused by the variouswith treatment sequences, and the combined treatment withofa certain could combined treatment a certain sequence could facilitate the reduction surface sequence oxygen on the facilitate the reduction of surface oxygen on the Pt/CeO2 catalyst. Pt/CeO 2 catalyst.

Figure 6. 6. H H22-TPR -TPR profiles profiles of of different different Pt/CeO Pt/CeO22samples. Figure samples. Table 4. H2 -TPR, O2 -TPD, Raman and OSC data of various samples. Table 4. H2-TPR, O2-TPD, Raman and OSC data of various samples. Sample

Sample

H2 -TPR

O2 -TPD

H2 peak position H2-TPRconsumption (◦ C) (µmol H2 g−1 )

desorption peak position O2-TPDamount (◦ C) (µmol O g−1 )

CeO2

470 peak

PC-U PC-T PC-P PC-TP PC-PT

position 116,443

H128 2

138,461

consumption 58,91

103,429

(μmol135,48 H2 g–1)

94,414

(°C) 101,432

46,94

128,52

182,44

peak

Raman spectra I /I

D F2g spectra

desorption

119,262

amount 87,59

106,249 (°C) 106,258 104,254

(μmol O g–1) 125,72

position 107,265

Raman

OSC (µmol O g−1 )

OSC g–1) OSCO OSCPt(μmol surface

-

32.8

-

39,104

ID/I1.73 F2g

1.55

OSC 28.7Pt

124,69

2.49 2.52 2.82

40.1 36.9 38.4

40.1 OSCsurface 58.2 99.3 102.9 125.1

-

161,81

-

-

46,94

119,262

39,104

1.55

32.8

40.1

182,44

104,254

161,81

2.82

38.4

125.1

CeO2

470

128

PC-U

138,461

PC-PT

94,414

-

-

To further investigate the redox properties of the samples, the O2 -TPD profiles are shown in PC-T 87,59 28.7 58.2254, Figure 7. There116,443 are three peaks58,91 in the traces107,265 of the samples. The peaks1.73 of PC-PT appeared at 104, ◦ andPC-P 436 C. The peak temperatures samples124,69 were higher compared to40.1 that of PC-PT, 103,429 128,52 of the other 106,249 2.49 99.3and the PC-TP lower peak101,432 position indicated that oxygen is more likely to migrate over the catalyst. In general, 135,48 106,258 125,72 2.52 36.9 102.9 the oxygen adsorbed by the catalyst underwent the following process: O2 (ad) → O2 − (ad) → O−

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To further investigate the redox properties of the samples, the O2-TPD profiles are shown in Figure 7. There are three peaks in the traces of the samples. The peaks of PC-PT appeared at 104, 254, and 436 °C. The peak temperatures of the other samples were higher compared to that of PC-PT, and Catalysts 2019, 9, 2 10 of 18 the lower peak position indicated that oxygen is more likely to migrate over the catalyst. In general, the oxygen adsorbed by the catalyst underwent the following process: O2 (ad) → O2− (ad) → O− (sur) 2− (lat) → O→ were assigned as follows: the peak °C was O2− (ad), (sur) O2− [38]. (lat) The [38].peaks The peaks were assigned as follows: thebelow peak 200 below 200 ◦related C was to related to − − ◦ − peak in the 200–400 °C range was related to chemically adsorbed oxygen O (sur), and the peak Othe (ad), the peak in the 200–400 C range was related to chemically adsorbed oxygen O (sur), and 2 − (sur) are − ◦ C was to 2− (lat). above °C was O2− (lat). amounts of O2− (ad) Table 4. in As the peak400 above 400 related related to OThe The amounts ofand O2 −O(ad) and O listed (sur)inare listed − − shown, theshown, intensity the O2 of (ad) variationsvariations among the samples. The total Table 4. As the of intensity thepeak O2 showed (ad) peaksignificant showed significant among the samples. O2−total (ad)/O can−be used index the oxygen ability ofability the catalysts The O2−−(sur) (ad)/O (sur) canasbeanused as to an compare index to compare theutilization oxygen utilization of the − (ad)/O− − (sur) value − [39]. As calculated from Table 4, the O 2 of PC-PT was highest at 2.0 and followed catalysts [39]. As calculated from Table 4, the O2 (ad)/O (sur) value of PC-PT was highest at 2.0 and by PC-TP 1.7, while thewhile O2− (ad) of PC-T 1.5, which indicated that the combined treatment followed byatPC-TP at 1.7, the ratio O2 − (ad) ratiowas of PC-T was 1.5, which indicated that the combined could enhance the catalysts’ oxygen utilization ability to various extents. It was consistent with XPS treatment could enhance the catalysts’ oxygen utilization ability to various extents. It was consistent results PC-PT higher aOhigher ads ratios than PC-TP. oxygen vacancy was regarded with XPSthat results thatshowed PC-PT ashowed Oads ratios thanMoreover, PC-TP. Moreover, oxygen vacancy was as oxygen and desorbed center, which acted a bridge gaseous oxygen [40]. regarded asadsorbed oxygen adsorbed and desorbed center, which actedtoaadsorb bridgeactive to adsorb active gaseous Thus, the O 2 adsorption of each sample could be related to various oxygen vacancies, which might oxygen [40]. Thus, the O2 adsorption of each sample could be related to various oxygen vacancies, influence theinfluence catalyticthe performance. which might catalytic performance.

Figure 7. 7.OO profiles Figure 2-TPD profilesofofdifferent differentPt/CeO Pt/CeO2 2samples. samples. 2 -TPD

To Tostudy studythe therole roleofofoxygen oxygenvacancies vacanciesfor forthe thecatalysts catalystsand andinvestigate investigatethe thecrystallinities crystallinitiesofofthe the samples, UV-Raman was performed, and the results are shown in Figure 8. Three typical peaks samples, UV-Raman was performed, and the results are shown in Figure 8. Three typical peaks −1 Generally, the first peak corresponded appeared appearedininthe theRaman Ramanspectra spectraat at457, 457, 588, 588, and and 1174 1174 cm cm–1. .Generally, the first peak corresponded to tothe theF2gF2g mode, due to the symmetrical stretching vibrations of Ce–O in fluorite [41].second The mode, due to the symmetrical stretching vibrations of Ce–O in fluorite CeO2CeO [41].2 The second peak was attributed to a defect-induced mode (D), which was related to oxygen vacancies peak was attributed to a defect-induced mode (D), which was related to oxygen vacancies caused by caused by Ce3+The defects. third was distinguished as a second-order longitudinal Ce3+ defects. thirdThe peak waspeak distinguished as a second-order longitudinal opticaloptical (2LO)(2LO) mode mode [42]. To identify the samples’ relative oxygen vacancy concentrations, the ratios of the F and 2g [42]. To identify the samples’ relative oxygen vacancy concentrations, the ratios of the F2g and defectdefect-induced modes (I /I ) were determined, and the values were in the following order: PC-U F2gdetermined, and the values were in the following order: PC-U (1.55) < induced modes (ID/IF2g) Dwere (1.55) PC-T