In situ DRIFT spectroscopy insights into the reaction mechanism of CO and toluene co-oxidation over Pt-based catalysts

Qi Zhanga, Shengpeng Moa, Jiaqi Lia, Yuhai Suna, Mingyuan Zhanga, Peirong Chena, Mingli Fuabc, Junliang Wuabc, Limin Chenabc and Daiqi Ye*abc
aSchool of Environment and Energy, South China University of Technology, Guangzhou 510006, China. E-mail: cedqye@scut.edu.cn
bNational Engineering Laboratory for VOCs Pollution Control Technology and Equipment, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR China
cGuangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control (SCUT), Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR China

Received 23rd April 2019 , Accepted 17th July 2019

First published on 17th July 2019


Considering that the mutual inhibition between CO and hydrocarbon (HC) co-oxidation over platinum-group metal (PGM) catalysts is a universal problem, in this study, a series of Pt-supported catalysts (Pt–Al2O3, Pt–Co3O4 and Pt–CeO2) were synthesized by combining glycol reduction and electrostatic chemical adsorption, and then were evaluated for the catalytic oxidation of CO and toluene. The Pt–CeO2 catalyst exhibited the best catalytic performance for individual CO and toluene oxidation, and effectively slowed down the competitive reaction in the presence of both CO and toluene. This superior performance over the Pt–CeO2 catalyst was associated with the formation of strong metal–support interactions (SMSIs) between the surface oxygen and adjacent Pt species. The formation of SMSIs not only enhanced the activation and migration of oxygen species and the formation of surface oxygen vacancies, but also improved the lower temperature reducibility of the catalyst. The in situ DRIFTS spectra revealed that CO was first transformed into carbon-related species (bicarbonates and carbonates), and then completely decomposed into CO2. Moreover, the reaction route for toluene oxidation may follow only one successive step: benzyl radical → benzaldehyde → benzoate → formate species, and finally completely oxidized to CO2 and H2O. The reaction pathways between CO and toluene oxidation may be independent when CO and toluene co-exist, although their reaction rates slow down due to the competitive adsorption between CO and toluene molecules at the same sites.


Introduction

The emission of volatile organic compounds (VOCs), including acetone, benzene and toluene, continues to grow, which is mainly caused by industrial production and automobile exhaust emissions. Many VOCs not only possess offensive odors and toxicity, but are also teratogenic, carcinogenic, and mutagenic hazards, posing threats to human health. Also, VOCs can cause some environmental problems under the action of sunlight, such as ozone and photochemical smog.1,2 In recent years, various treatment technologies for VOC abatement have been developed, such as adsorption, plasma catalysis, and catalytic oxidation.3 Among these technologies, catalytic oxidation is considered a highly efficient technique to remove VOCs because of its potential advantages, such as better processing efficiency, lower reaction temperature, less harmful pollutants, and no need for additional fuel, which can greatly reduce the treatment energy consumption.4 For the catalytic oxidation technology, the key issue is to prepare highly active catalysts. Currently, traditional catalysts can be divided into two categories: precious metal and transition metal oxide catalysts.5 Although precious metal catalysts show good catalytic activity for VOC combustion at low temperatures and can maintain remarkable stability under water vapor conditions, they are prone to surface poisoning, which is influenced by the presence of CO.6–8 Due to the competitive adsorption of CO and hydrocarbons (HCs) at the active sites, the catalytic performance of CO on precious metal catalysts would be strongly inhibited by the introduction of HCs.9 Therefore, there is an urgent need to develop efficient strategies for precious metal catalysts to improve their CO toxicity resistance.

For the effective removal of reactants and the effective use of active ingredients, it is usually necessary to highly disperse precious metals on a support. The active components and used supports are the important factors to determine the catalytic oxidation of VOCs. Some studies have reported that there is a strong metal–support interaction between precious metal nanoparticles (NPs) and reducible supports, because the strong interaction can induce more surface oxygen species and a higher dispersion of precious metal NPs as well as a good sintering resistance toward water vapor and other interfering substances (SO2 and so on).10,11 Meanwhile, by combining the special advantages of precious metals and reducible supports, it is easy to prepare a unique catalyst with adsorption sites and abundant oxygen vacancy sites.12,13

Al2O3 is widely used in catalytic combustion technology due to its low price, good stability, and high specific surface area.3,14 Co3O4 as typical spinel structure has received significant attention in many catalysis fields13,15–17 because of its abundant availability through natural resources, good reduction properties, high concentration of oxygen vacancies, and electrophilic oxides. Many reports have shown that CeO2 can also be widely used in heterogeneous catalysis because CeO2 has a good oxygen storage capacity, strong interaction with deposited metals, and acts as an oxygen buffer for oxidation reactions.12,18,19 There are few reports on Pt-based catalysts with different supports applied to a competitive oxidation of CO and HCs, especially studies of the function of such catalysts in the simultaneous presence of CO and HCs.

Based on the above strategy, we successfully synthesized a series of supported-Pt catalysts using Co3O4 nanosheets, CeO2 nanorods, and commercial Al2O3 as supports via a single method combining glycol reduction and electrostatic chemical adsorption, which were evaluated for the catalytic oxidation of CO/toluene. The structure–activity relationship was fully analyzed by several techniques, with an aim to investigate the differences in the strength of the interaction and anti-inhibition toward CO and toluene co-oxidation on different Pt-based catalysts. Moreover, in situ DRIFTS technology was used to explore the possible reaction mechanism of CO and toluene oxidation on the surface of different Pt-based catalysts.

Experimental section

Synthesis of Pt NPs

Pt NPs were prepared via a glycol reduction method. In detail, 2.5 mL chloroplatinic acid (20 g L−1 H2PtCl6 solution) and 220 mg polyvinylpyrrolidone (PVP, K29–32, Mw = 58[thin space (1/6-em)]000) were dissolved in a 100 mL round-bottom flask with 25 mL glycol to obtain a yellow solution after stirring at room temperature for 15 min. Then, the yellow solution was heated to about 120 °C, keeping it under magnetic stirring for 1 h, whereby the yellow solution changed to a dark-brown solution. Finally, a certain quantity of acetone (about 60–80 mL) was slowly added to the dark-brown solution to extract the Pt NPs, and the Pt NPs were collected by centrifugation (6000 rpm, 5 min). The as-obtained precipitates were washed with n-hexane several times, and then dispersed in ethanol for later loading in further experiments.

Synthesis of Co3O4 nanosheets

In a typical procedure, Co(NO3)2·6H2O (6 mmol) and HMT (45 mmol) were dissolved in a round-bottom flask with a 200 mL mixture solution of water and ethanol (Vwater/Vethanol = 9/1). Then, the reaction solution was heated at about 95 °C for 12 h under magnetic stirring. The as-obtained precursor was washed with deionized water and ethanol several times. Finally, the precursors were dried and annealed at 400 °C for 3 h to obtain the Co3O4 nanosheets.

Synthesis of CeO2 nanorods

CeO2 nanorods were prepared by an ordinary hydrothermal approach. Typically, 3.6 g Ce(NO3)3·6H2O and 16.80 g NaOH were dissolved in 10 mL and 70 mL deionized water, respectively. Then, the NaOH solution was rapidly added to the Ce(NO3)3·6H2O solution, and the resulting solution was stirred for 30 min, and then transferred into a 100 mL stainless steel autoclave at 100 °C for 24 h. The as-obtained precipitates were washed with deionized water and ethanol several times until the liquid supernatant was neutral.

Synthesis of Pt-based catalysts

Pt-Supported catalysts were synthesized via an electrostatic chemical adsorption method. Typically, 1.0 g of the supports (Co3O4 nanosheets, CeO2 nanorods, and commercial Al2O3 with a high specific surface area) were dispersed in 30 mL ethanol. Then, a certain amount of pre-synthetic Pt NPs (loading content of about 0.5 wt%) dispersed in ethanol was added to the former solution. After stirring for 4 h, the powders were collected and washed twice. Finally, the products were dried in oven at 80 °C.

Catalyst characterization

All of the as-prepared Pt-based catalysts were characterized by X-ray diffraction (XRD), ICP-OES, N2 adsorption–desorption, transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS).

The detailed characterization and catalytic evaluation of the Pt-based catalyst are described in the ESI.

Results and discussion

TEM analysis

The size of the as-prepared Pt nanoparticles (NPs), and the morphology and microstructure of the Pt-based catalysts were examined by transmission electron microscopy (TEM), as shown in Fig. 1 and 2. It can be clearly observed that the as-prepared Pt NPs were monodispersed, and the average size of the diameter was mainly distributed in the range of 3.3–3.7 nm. The lattice fringe of the Pt NPs was 0.230 nm, corresponding to the (111) plane of the Pt species. After loading Pt NPs onto the Al2O3, Co3O4, and CeO2 supports, the Pt NPs were highly dispersed on the surfaces of these supports, as demonstrated in Fig. 2. It could be seen that the Pt NPs anchored on the supports displayed the same size in these catalysts, and all of the nanoparticles had an average diameter of 3.5 ± 0.2 nm, which matched well with the size of the as-prepared Pt NPs. From the HRTEM images of all the samples, the interplanar distance of Pt NPs was measured to be approximately 0.230 nm, corresponding to the (111) crystal plane of metal Pt. In addition, it could be clearly observed that the commercial Al2O3 support had no regular shape, and the Co3O4 support with ultrathin nanosheets exhibited rough surfaces and embedded pores. The CeO2 support exhibits a nanorod morphology, and the average diameter of nanorods is 10 nm.
image file: c9cy00751b-f1.tif
Fig. 1 (a) Low-, (b) medium-, (c) high-magnification TEM images, and (d) particle-size distribution of Pt nanoparticles.

image file: c9cy00751b-f2.tif
Fig. 2 TEM images of (a and b) Pt–Al2O3, (d and e) Pt–Co3O4, and (g and h) Pt–CeO2, (c, f, and i) corresponding to the HRTEM images in (a, d, and g) TEM images.

XRD patterns

Fig. S1 shows the XRD patterns of the Pt–Al2O3, Pt–Co3O4, and Pt–CeO2 samples, respectively. For the Pt–Al2O3 sample, the spectrum contained four distinct diffraction peaks at 37.6°, 39.5°, 45.8°, and 66.8°, corresponding to the (311), (222), (400), and (440) planes of γ-Al2O3 (JCPDS card no. 29-0063). For the Pt–Co3O4 sample, characteristic diffraction peaks at 19°, 31.3°, 36.9°, 44.8°, 59.4°, and 66.8° could be observed, ascribed to the (111), (220), (311), (400), (511), and (440) planes of Co3O4 (JCPDS card no. 42-1467), respectively. In the XRD pattern of the Pt–CeO2 sample, there were nine distinct diffraction peaks at 28.6°, 33.1°, 47.5°, 56.3°, 59.1°, 69.4°, 76.7°, 79.1°, and 88.4°, attributed to the (111), (200), (220), (311), (222), (400), (331), (420), and (422) planes of CeO2 (JCPDS card no. 34-0394), respectively. However, the characteristic peaks assigned to the Pt or PtOx phase (approximately 33°, 44°) are not observed, which indicates that the relatively low amount of loading of Pt species with high dispersion on the surface of the supports was difficult to detect in the XRD measurements.

N2 adsorption–desorption measurements

The N2 adsorption–desorption technique was used to evaluate the microstructure of these samples. The N2 adsorption–desorption isotherms and pore-size distributions of all the samples are shown in Fig. S2. All the samples showed typical type IV isotherms with an H3-type hysteresis loop in the relative pressure between 0.5 and 1.0, which suggested that these samples were mesoporous materials. It is believed that one excellent mesoporous structure could significantly improve the total pore volume and specific surface area of catalysts, which is conducive to the dispersion of Pt species and the adsorption/diffusion of reactants.20 Information on the specific surface areas, pore diameters, and pore volumes of all the samples is summarized in Table 1. The Pt–Al2O3 sample showed the largest specific surface area (214 m2 g−1), which was significantly larger than those of Pt–Co3O4 (39 m2 g−1) and Pt–CeO2 (105 m2 g−1). The specific surface area may not be the only influencing factor for the catalytic performance of catalysts, especially for catalysts with different structures.
Table 1 BET data and surface elemental compositions of Pt–Al2O3, Pt–Co3O4, and Pt–CeO2 samples
Sample Pt loading (wt%)ICP SBET (m2 g−1) Vpore (cm3 g−1) Pore diameter (nm) Pt0/(Pt0 + Pt2+) (%) Olatt (%) Oads (%) O-OH (%)
Pt–Al2O3 0.49 214 0.72 10.0 54.4 28.2 17.4
Pt–Co3O4 0.48 39 0.11 10.9 71.7 61.5 21.6 16.9
Pt–CeO2 0.50 105 0.68 27.6 81.7 31.8 31.2 33.7


H2-Temperature programmed reduction (H2-TPR)

According to the relevant literature reports,19–22 surface oxygen species play an important role in promoting the activity of catalysts. To better understand the reduction behavior over these samples, these samples were determined by H2-TPR. The two reduction peaks of CeO2 nanorods at 198 °C and 572 °C represent the reduction of surface adsorbed oxygen and bulk lattice oxygen species (Fig. S3). The H2-TPR profile of Co3O4 nanosheets displayed one overlapping reduction peak in the temperature range of 200–400 °C, which was assigned to the reduction of Co3+ → Co2+ → Co0.23 After anchoring Pt NPs onto the supports, the reduction peaks of the as-obtained Pt-based catalysts were shifted to a lower temperature, as shown in Fig. 3. In the H2-TPR profile of Pt–CeO2, there were three reduction peaks centered at 97.5 °C, 382 °C, and 743 °C. The first low-temperature reduction peak was identified as the reduction of a low amount of PtOx and the contribution of the surface oxygen adjacent to Pt NPs, forming a strong metal–support interaction (the formation of Pt–O–Ce).21,22 The second reduction peak of 200–500 °C was considered to relate to the reduction of oxygen species due to the redox transformation of Ce3+ and Ce4+. The third reduction peak at 743 °C was attributed to the reduction of lattice oxygen of pure CeO2 nanorods.22 For the Pt–Co3O4 sample, two main peaks at 93 °C and 297 °C could be observed, belonging to the reduction of surface oxygen adjacent to Pt NPs because of the spillover effect of the metal–support interaction between Pt species and Co3O4,23 and the overlap reduction of both Co3+ → Co2+ and the Co2+ → metallic Co0,24,25 respectively. Compared to the Pt–CeO2 and Pt–Co3O4 samples, there was a distinct reduction profile in the Pt–Al2O3 sample. Its reduction peak centered at 412 °C was ascribed to the reduction of surface oxygen species or the reduction of PtOxCly species interacting strongly with the Al2O3 support, but the metal–support interaction between the Pt NPs and Al2O3 support is relatively difficult to form. The low-temperature reduction peak area of Pt–CeO2 was larger than those of the Pt–Al2O3 and Pt–Co3O4 samples, indicating that the rich reactive oxygen species on the surface of the Pt–CeO2 sample could be reduced at low temperature. Therefore, this result implies that the Pt–CeO2 sample will exhibit a higher concentration of surfactant oxygen species and excellent catalytic activity, for which the oxygen vacancies play important roles in determining the activity of the catalysts.
image file: c9cy00751b-f3.tif
Fig. 3 H2-TPR profiles of the as-synthesized Pt-based samples.

Surface element composition

To further understand the surface elemental composition of these samples, XPS measurements were performed. All the XPS spectra of these samples were calibrated using the adventitious carbon (Fig. S4). Fig. 4 shows the XPS spectra of the Pt 4f, Co 2p, Ce 3d, and O 1s XPS spectra of these as-synthesized Pt-based samples. Since the binding energies (BEs) of Al 2p and Pt 4f overlap in the range of 73–75 eV, the Pt 4f of Pt–Al2O3 sample is inconvenient to fit in this study. The Pt 4f7/2 peak of the Pt–Co3O4 sample could be divided into two peaks at 70.66 eV and 71.55 eV, while its Pt 4f5/2 peak could be divided into two peaks at 74.01 eV and 74.9 eV. The peaks at 70.66 and 74.01 eV were assigned to Pt0 species, and the other peaks at 71.55 and 74.9 eV were assigned to Pt2+ species.21,26 There was a distinct change in the binding energy of Pt due to the effects of different supports. By calculating the peak areas of the different Pt species, it was found that Pt0 species mainly dominated in the surface composition, and the ratio of Pt0/(Pt0 + Pt2+) on Pt–Co3O4 reached 71.68%, as shown in Table 1. Similarly, the Pt 4f XPS of Pt–CeO2 with the same Pt chemical states could be divided into four peaks, and the ratio of Pt0/(Pt0 + Pt2+) reached 81.72%. The Pt0/(Pt0 + Pt2+) ratio of the Pt–CeO2 sample was relatively higher than that of Pt–Co3O4, which suggested that the Pt–CeO2 sample could display outstanding catalytic performance due to Pt0 species being the active sites in the oxidation reaction.
image file: c9cy00751b-f4.tif
Fig. 4 (a) Pt 4f, (b) Co 2p, (c) Ce 3d, and (d) O 1s XPS spectra of the as-synthesized Pt-based samples.

The Co 2p XPS spectrum of the Pt–Co3O4 sample was composed of Co 2p3/2 and Co 2p1/2, as shown in Fig. 4b. The Co 2p3/2 spectrum could be divided into four peaks, where the binding energies at 779.66 and 781.38 eV are attributed to Co3+ and Co2+ species, respectively.13,25,27 The surface Co3+/Co2+ molar ratio of the Pt–Co3O4 sample was about 1.29. Fig. 4c illustrates the Ce 3d spectrum of the Pt–CeO2 sample, which could be deconvoluted into 10 peaks, resulting from the five pairs of the spin–orbit doubles of cerium oxides (Ce3+ and Ce4+). According to the relevant research reports,19–21 the six peaks at binding energies of 916.28 (U′′′), 897.72 (V′′′), 901.88 (U), 884.00 (V), 906.50 (U′′), and 888.20 eV (V′′) could be assigned to Ce4+ species. Also, the other four peaks at binding energies of 903.68 (U′), 900.11 (U0), 885.93 (V′), and 881.65 eV (V0) are associated with Ce3+ species, which reflects the presence of oxygen vacancies.20,21 The proportion of Ce3+/Ce on the Pt–CeO2 sample is about 35.02%, and the higher concentration of Ce3+ ions indicates the relatively higher concentration of oxygen vacancies on the surface of the catalyst.

Fig. 4d shows the O 1s XPS spectra of these catalysts. It can be seen that the binding energies of O 1s on the three samples were obviously different. According to the relevant literature,13,14,19 the binding energies of CeO2, Co3O4, and Al2O3 are about 529, 530, and 531.5 eV, respectively, thus the results of the O 1s XPS spectra could be credible. There exist three oxygen chemical states on the surface of these samples, which are ascribed to surface lattice oxygen (Olatt), surface adsorbed oxygen (Oads), and adsorbed hydroxyl/water molecules (O-OH), respectively.13,28,29 The characteristic peaks of the three oxygen chemical states on the Pt–Al2O3 sample were detected at 530.7, 531.8, and 532.9 eV. The binding energies of oxygen species on the Pt–Co3O4 sample were at 529.9, 531.3, and 532.8 eV, respectively, whereas the binding energies of oxygen species on the Pt–CeO2 sample were located at 528.8, 530.2, and 531.85 eV, respectively. The oxygen vacancy (Oads + O-OH) ratios of the samples were calculated and the results are given in Table 1. The oxygen vacancy (Oads + O-OH) ratios were 45.6% (Pt–Al2O3), 38.5% (Pt–Co3O4), and 64.9% (Pt–CeO2), respectively, indicating that the Pt–CeO2 sample could provide a higher number of surface oxygen vacancies.

Catalytic activity measurement

The catalytic activities of CO/toluene oxidation over the Co3O4, CeO2, and Pt-based catalysts were studied under a total flow rate of 100 mL min−1 and a weight hourly space velocity (WHSV) of 60[thin space (1/6-em)]000 mL g−1 h−1. The catalytic behaviors of these catalysts were conveniently compared using T10, T50, and T99 (the temperatures for when 10%, 50%, and 99% conversion of CO/toluene oxidation were reached, respectively) as references and the results are summarized in Table S1. Fig. S5 shows the catalytic activities for CO and toluene oxidation over the Co3O4 and CeO2 supports. Lower catalytic activities for CO/toluene oxidation over CeO2 nanorods were observed under simple and mixture conditions. The Co3O4 nanosheets displayed one good catalytic activity for single CO oxidation (T99 = 160 °C), while its corresponding T99 value for single toluene oxidation was 270 °C. After loading Pt NPs into the CeO2 support, its catalytic activities for CO/toluene oxidation were enhanced. The Pt–CeO2 catalyst exhibited the highest catalytic activity for individual CO oxidation, as shown in Fig. 5a, among these catalysts. The T99 value for CO conversion over the Pt–CeO2 catalyst was achieved at 125 °C, which was 25 °C lower than that found over Pt–Co3O4 and Pt–Al2O3 catalysts. The catalytic performance of these catalysts for individual toluene oxidation was evaluated, as displayed in Fig. 5b. Similarly, the Pt–CeO2 catalyst also presented superior activity for toluene oxidation among these catalysts. The T99 value for toluene conversion with the Pt–CeO2 catalyst occurred at 180 °C, which was better than the other two samples in simple conditions. Pt–CeO2 had a lower apparent activation energy (Ea) in individual CO oxidation (Fig. S6); however, Pt–Al2O3 exhibited the lowest Ea value for individual toluene oxidation compared to Pt–Co3O4 and Pt–CeO2, which may be due to its higher specific surface area (214 m2 g−1). The Pt–Co3O4 catalyst showed relatively lower catalytic activities for individual CO/toluene oxidation. The carbon balance was guaranteed in the whole reaction.
image file: c9cy00751b-f5.tif
Fig. 5 (a) CO and (b) toluene conversions in simple conditions, (c) CO and (d) toluene conversion in mixture conditions of the Pt–Al2O3, Pt–Co3O4, and Pt–CeO2 samples. Simple conditions: 1.0 vol% CO or 1000 ppm toluene balanced with air; mixture conditions: 1.0 vol% CO and 1000 ppm toluene balanced with air. All the reactions were kept at WHSV = 60[thin space (1/6-em)]000 mL g−1 h−1.

When CO and toluene gases were simultaneously vented into the reactor, the catalytic activities for CO and toluene oxidation over the Pt-based catalysts decreased due to the competitive adsorption of CO and toluene molecules on the surface-active sites, as shown in Fig. 5(c and d). Under the co-existence of CO and toluene, the T99 values of complete CO oxidation over the Pt–CeO2, Pt–Co3O4, and Pt–Al2O3 catalysts were approximately 190 °C, 210 °C, and 210 °C, respectively. Their T99 values for complete toluene oxidation were about 200 °C, 210 °C, and 220 °C, respectively. In the mixture conditions, the Pt–CeO2 catalyst exhibited excellent resistance toward the mutual inhibition between CO and toluene molecules, while the Pt–Al2O3 catalyst displayed inferior activity for CO and toluene oxidation. Moreover, the Pt–Al2O3 catalyst, after a hydrogen reduction at 250 °C for 3 h, was further checked for its catalytic behaviors toward CO/toluene oxidation, with comparing its activities with those of fresh Pt–Al2O3, as shown in Fig. S7. It could be noted that the activity for individual CO/toluene oxidation over the Pt–Al2O3 catalyst after reduction was equal to that of fresh Pt–Al2O3, whereas its catalytic activities for CO and toluene co-oxidation were higher than those of fresh Pt–Al2O3. The resistance of the Pt-based catalysts to CO and toluene competitive oxidation can be relevant to the surface oxygen vacancies and the strong metal–support interactions between the Pt NPs and the supports. Hence, the Pt–CeO2 catalyst exhibited superior catalytic activities for CO/toluene oxidation in both the simple/mixture conditions, compared with the other two Pt-based catalysts.

The stability and the effect of water vapor on CO and toluene oxidation over the Pt–CeO2 catalyst were further evaluated, as shown in Fig. 6. It could be seen that the T50 and T99 of each cycle are no significant changes (Fig. 6a), indicating that Pt–CeO2 catalyst demonstrates one well stability. Fig. 6b shows the effect of water vapor for individual toluene oxidation. It can be observed that the water vapor slightly increases the toluene conversion. Unexpectedly, the conversions of CO and toluene are enhanced with increasing humidity under the mixture conditions, as shown in Fig. 6c and d. The conversion of toluene was significantly higher than that found in simple conditions. After introducing 10 vol% moisture into the reaction system, the T99 values of CO and toluene under mixture conditions were 20 °C and 15 °C lower than those in simple conditions, respectively. These results indicate that moisture could promote the conversion of CO/toluene on the Pt–CeO2 catalyst, especially in CO and toluene co-oxidation.30,31 The reason for the positive effect may be due to the water-gas shift reaction (CO + H2O → CO2 + H2), as shown in Fig. S8, which reduces the competitive adsorption at the active sites between CO and toluene molecules. In addition, the effects of changing the CO and toluene concentration were also monitored to explore the different competitive adsorptions on the Pt–CeO2 catalyst, as shown in Fig. S9. Along with the decrease in the CO concentration in the mixture air stream, toluene conversion was enhanced at the same temperature. Changing the toluene concentration led to a small improvement in the CO oxidation activity. In general, CO concentration had a greater impact on competitive adsorption.


image file: c9cy00751b-f6.tif
Fig. 6 (a) Cycling test of toluene oxidation, (b) effect of water vapor on toluene conversion, and (c) CO conversion in simple conditions, (d) toluene conversion in mixture conditions over the Pt–CeO2 catalyst. All the reactions were kept at WHSV = 60[thin space (1/6-em)]000 mL g−1 h−1.

In situ DRIFTS study

In order to monitor the intermediate species and verify the reaction mechanism of CO and toluene oxidation on the catalyst surface, the in situ DRIFTS spectra were recorded for 1.0 h at each temperature point under three different conditions, namely CO/air, toluene/air, CO + toluene/air. The Pt–Al2O3 catalyst was exposed to CO/air at different temperatures, and the collected in situ DRIFTS spectra are shown in Fig. 7a. The bands located at 2300–2400 cm−1 are characteristic peaks of CO2 species. It can be seen that CO molecules are adsorbed on the surface of Pt–Al2O3 at 50 °C due to the characteristic absorbance bands of CO, whereby the characteristic CO stretch vibrational peak at 2086 cm−1 is observed. The peak centered at 2086 cm−1 can be assigned to CO molecules linearly adsorbed onto Pt0.22,32 The major types of various carbon-related species in the frequency region 1700–1000 cm−1 were observed over the Pt–Al2O3 sample:33 bicarbonate species (HCO3, 1248 cm−1), bidentate carbonate species (b-CO32−, 1250–1300 and 1581 cm−1), and monodentate carbonates species (m-CO32−, 1300–1400 cm−1). The adsorption bands at 1296 and 1581 cm−1 were related to the νs(OCO) and νas(OCO) stretching vibration of bidentate carbonate species (b-CO32−), whereas the band at 1388 cm−1 was assigned to the νs(OCO) stretching vibration of monodentate carbonates species (m-CO32−).34,35 The adsorption band at 1248 cm−1 was assigned to the presence of asymmetric bicarbonate species (HCO3), which were formed via the interaction of CO molecules with the Pt nanoparticles and the surface hydroxyl groups of the Al2O3 support.35 The adsorption bands at 1581 and 1296 cm−1 are assigned to the carbonate bidentate species.36 When the reaction temperature was increased to 140 °C, the adsorption band at about 2086 cm−1 gradually disappeared. However, the amount of bicarbonate species on the catalyst surface (one major peak at 1248 cm−1) slightly decreased, while the amount of bidentate carbonate (major peak at 1550–1600 cm−1) species remained unchanged. According to the relevant literature data,33,35 the bicarbonate (HCO3) species are more easily to decompose into gaseous CO2 at lower temperature, followed by the decomposition of b-CO32− and m-CO32− species into gaseous CO2 at higher temperature. Based on the above studies, it could be found that the oxidation pathway of CO on the surface of the Pt–Al2O3 catalyst could be carried out: after the rapid transformation of CO to various carbon-related species, the steps for the transformation of the bicarbonate species to gaseous CO2 and the decomposition of b-CO32− and m-CO32− species into gaseous CO2 may occur simultaneously.
image file: c9cy00751b-f7.tif
Fig. 7 In situ DRIFTS spectra of (a) 1.0 vol% CO oxidation; (b) 500 ppm toluene oxidation; and (c) 1.0 vol% CO and 500 ppm toluene co-existence over Pt–Al2O3 catalyst.

Fig. 7b shows the in situ DRIFTS spectra of the Pt–Al2O3 catalyst exposed to toluene/air at different temperatures. When gaseous toluene was introduced into the reaction system, the bands of ν(C[double bond, length as m-dash]C) (at 1498 and 1595 cm−1) corresponding to the skeleton vibrations of the typical aromatic ring could be immediately observed.37–40 The bands located at 2941, 2844, and 1303 cm−1 were ascribed to the bending vibrations of νas(C–H), νs(C–H) of methylene (–CH2) groups, and a CH2 deformation mode, respectively, while the asymmetric C–H stretching of methyl (–CH3) group was observed at 2981 cm−1, suggesting that CH3 groups were adsorbed and cleaved on the surface of the Pt–Al2O3 catalyst to further form benzyl species.40–42 The weak band located at 1150 cm−1 was ascribed to a C–O vibration mode, which resulted in the formation of the benzoyl oxide species (C6H5–CH2–O) on the catalyst surface, while the band at 1237 cm−1 could be attributed to the vibration of C–O from the surface phenolate.43 The bands at 2737 and 2834 cm−1 could be possibly assigned to a ν(C–H) stretching vibration of adsorbed benzaldehyde (C6H5–HC[double bond, length as m-dash]O), while the band at 1667 cm−1 was attributed to a ν(C[double bond, length as m-dash]O) stretching vibration peak of aldehyde groups, which demonstrated the generation of benzaldehyde as an important intermediate. The bands at 1411, 1569, and 2717 cm−1 could be attributed to the asymmetric and symmetric vibrations of typical carboxylate species,44,45 indicating that benzaldehyde species could be further oxidized to benzoate species in the toluene oxidation. The bands centered at 1180, 1290, 1441, and 1535 cm−1 were related to the C–C vibration of formate species (–COO), indicating the destruction of the aromatic ring. With the increase in temperature, the characteristic bands of toluene species located at 1498 and 1595 cm−1 became much weaker and gradually disappeared, and the intensities of the bands at 2324 and 2367 cm−1 attributed to CO2 species gradually increased, indicating that toluene was heavily oxidized to CO2 and H2O. On the contrary, the formation of key intermediates (formate and benzoate species) were further accelerated with the increase in temperature, and the intensity of the bands increased to a maximum at 180 °C, and then the amount of these intermediates gradually declined. However, the intermediates at 2700–2950 cm−1 associated with benzaldehyde and benzoate species further accumulated on the catalyst surface with the increase in temperature, implying that the Pt–Al2O3 catalyst had a weak ability to activate oxygen involved in the oxidation of the key intermediates. According to the previous reported speculations and the in situ DRIFTS characterization in this work,40,41,46,47 the toluene catalytic oxidation on the surface of the Pt–Al2O3 catalyst could follow the reaction pathway: benzyl radical → benzaldehyde → benzoate → formate species, and finally completely oxidized into CO2 and H2O.

When gaseous CO and toluene simultaneously existed in the reaction system, the characteristic bands of CO and aromatic ring are observed, as shown in Fig. 7c. The band located at 2077 cm−1 was attributed to adsorbed CO on Pt0, while the bands at 1495 and 1596 cm−1 are usually assigned to aromatic ring species. In the frequency region 1000–1700 cm−1, the various intermediate species resulting from the CO and toluene co-oxidation were similar to those from individual toluene oxidation over the Pt–Al2O3 catalyst, while the band at 1372 cm−1 related to monodentate carbonates species (m-CO32−) in the individual CO oxidation was also observed. With the rise in the temperature, the intensity of the characteristic bands of CO and toluene decreased gradually, confirming that CO and toluene species were completely oxidized. Meanwhile, it can be seen that a new band at 2173 cm−1 relating to the CO species adsorbed on Pt2+ appeared at 100 °C,32 then gradually disappeared as the reaction temperature was increased to 220 °C. Significantly, in the co-oxidation process of CO and toluene on the Pt–Al2O3 catalyst, the intensity of the various adsorbed species became weak, indicating that the competitive adsorption at the same active sites restrained the accumulation of distinct intermediates, compared to those seen in individual CO/toluene oxidation. In addition, the intensity of the bands at 1407 and 1565 cm−1 increased to a maximum at 200 °C. The temperature of complete CO/toluene oxidation shifted to a higher region, which is in accord with the results for the catalytic activity for simultaneous CO and toluene oxidation. These results imply that the presence of CO affects the oxidation of toluene, as both CO and toluene undergo competitive adsorption at the same active sites. In addition, the transformation between Pt2+ and Pt0 on the surface of Pt–Al2O3 could not be achieved quickly at relatively low temperature due to the adsorption of plentiful pollutant molecules, resulting in the formation of a new band at 2173 cm−1. Hence, it is reasonable to deduce that the oxidation of toluene still follows the reaction path involving the aldehydic and carboxylate route, and the reaction pathways between CO and toluene oxidation may be independent of each other.

Fig. 8a shows the in situ DRIFTS spectra exposed to CO/air on the surface of the Pt–Co3O4 catalyst at different temperatures. The vibrational band of CO–Pt0 species could be clearly observed at 2086 cm−1, while the two characteristic bands at 2120 and 2173 cm−1 could be assigned to the CO–CoOx or CO–Pt2+ species, with the band of CO adsorbed on Pt2+ overlapping with that of CO–CoOx.48 The band of CO–Pt0 species is shifted to a higher wavenumber than that with Pt–Al2O3, suggesting the formation of an interface between the Pt NPs and Co3O4 nanosheets. The intensities of these bands located at 2000–2200 cm−1 decreased with the increase in temperature and completely disappeared at above 140 °C. For the DRIFTS spectra of the Pt–Co3O4 catalyst, the characteristic bands of bicarbonate species at about 1220, 1420, and 1505 cm−1 can be observed, while the band at 1581 cm−1 corresponding to bidentate carbonate species disappeared, compared to the Pt–Al2O3 catalyst. Clearly, the intensities of the reaction intermediates gradually decreased as the temperature rose, albeit weaker than for those of the Pt–Al2O3 catalyst. Meanwhile, the reaction intermediates on various carbon-related species slightly accumulated on the surface of the Pt–Co3O4 catalyst, indicating that CO could be rapidly transformed into bicarbonate and carbonate species, and then completely oxidized into gaseous CO2.


image file: c9cy00751b-f8.tif
Fig. 8 In situ DRIFTS spectra of (a) 1.0 vol% CO oxidation; (b) 500 ppm toluene oxidation; and (c) co-existence of 1.0 vol% CO and 500 ppm toluene over the Pt–Co3O4 catalyst.

When the Pt–Co3O4 catalyst was exposed to toluene/air at 50 °C, the stretching vibration peaks of the intermediate/by-product and toluene species could be rapidly detected, as displayed in Fig. 8b. The bands at 1495 and 1601 cm−1 are associated with the typical aromatic ring vibration bands, implying that toluene species are adsorbed on the surface of the Pt–Co3O4 catalyst. These characteristic bands decreased and even disappeared with the increase in temperature, indicating that toluene was oxidized on the catalyst surface. The bands at 2839, 2933, and 2944 cm−1 were also detected, which correspond to the C–H stretching of methylene (–CH2) groups. At temperatures above 140 °C, two weak peaks at 2732 and 2746 cm−1 appeared and are attributed to the production of benzaldehyde (C6H5–HC[double bond, length as m-dash]O) species. The band at 1286 cm−1 could be assigned to the symmetric vibration of C–C,43 but it quickly disappeared above 120 °C. There were obvious differences in the stretching variations of the reaction intermediates on the surfaces of the Pt–Co3O4 and Pt–Al2O3 samples for individual toluene oxidation. For the various carbon-related species in the range of 1200 to 1700 cm−1 on the surface of the Pt–Co3O4 sample, the intensities were weaker than for the Pt–Al2O3 sample. Furthermore, the major differences are the bands at 1408 and 1569 cm−1 on the surface of the Pt–Co3O4 and Pt–Al2O3 samples. Among all the characteristic bands, the band at 1569 cm−1 has the highest peak strength in the Pt–Al2O3 sample, while the Pt–Co3O4 sample displays a strongest peak at 1408 cm−1 and the band at 1569 cm−1 seems invisible, implying that a faster breakage of the aromatic ring occurred and was oxidized to different carboxylate species due to the different activation abilities of surface oxygen species on the surfaces of the Pt–Co3O4 and Pt–Al2O3 samples.

Fig. 8c shows the in situ DRIFTS spectra in the co-oxidation process of CO and toluene over the Pt–Co3O4 sample. A series of characteristic peaks of intermediates in the range of 1200 to 1700 cm−1 can also be observed, whereas bands at 2114 and 2173 cm−1 related to the CO adsorbed on the Co3O4 nanosheet can be observed below 140 °C. At above 140 °C, the vibrational band of CO–Pt0 species at 2086 cm−1 is not detected. The characteristic bands in the co-oxidation process of CO and toluene are analogous to those of single CO or toluene oxidation, but there were distinct differences in the band intensities and temperatures of complete CO/toluene oxidation. The above results imply that surface low unsaturated Pt species represent an important active site of catalysts due to strong metal–support interaction, and that the competitive adsorption between CO and toluene molecules leads to the coverage of these active sites.

Fig. 9a presents the in situ DRIFTS spectra of CO oxidation on the surface of the Pt–CeO2 catalyst at different temperatures. When gaseous CO was introduced into the reaction system, the vibrational bands of the intermediates (at about 1300–1700 cm−1) and CO adsorption (at about 2000–2200 cm−1) could be immediately observed. The adsorption band appearing at 2086 cm−1 was attributed to the linear CO molecules adsorbed on the surface of Pt0.22,32 The other two weak adsorption bands at 2125 and 2173 cm−1 are identified as the CO–CeOx or CO–Pt2+ species, in which the band of CO adsorbed on Pt2+ overlapped with that of CO–CeOx at 2173 cm−1.22,34 The band of CO–Pt0 species also shifted to a higher wavenumber, compared to that on the Pt–Al2O3 sample, suggesting the formation of strong metal–support interactions (SMSIs) between the surface oxygen and adjacent Pt species. The band at 1369 cm−1 is assigned to the monodentate carbonate species, while the band at 1554 cm−1 is assigned to the surface bidentate carbonate species. The bands appearing at 1449 and 1638 cm−1 are ascribed to the presence of bicarbonate species (HCO3). Clearly, the intensities of the two bands at 1449 and 1554 cm−1 gradually increased with the increase in temperature, suggesting that the formation of carbon-related species was accelerated with the increase in the temperature. Therein, the major differences are the band regions and the accumulation of various surface carbon-related species, compared to the Pt–Al2O3 and Pt–Co3O4 samples. For the Pt–Al2O3 sample, the maximal accumulation peak on the catalyst surface appeared at 1248 cm−1, which is assigned to the asymmetric bicarbonate species. In the case of the Pt–Co3O4 sample, the strongest peak identified as bicarbonate species is observed at 1420 cm−1. However, the intensities of the two bands at 1449 and 1554 cm−1 (bicarbonate and carbonate species, respectively) simultaneously increased to a maximum in the DRIFTS spectra of the Pt–CeO2 sample. In addition, a clear change was observed for the bidentate carbonate species with the reaction temperature increasing from 50 °C to 200 °C, implying that the bidentate carbonate species acted as the important intermediates during the CO oxidation over the Pt–CeO2 sample. Moreover, two main peaks of CO2 are observed in the range of 2300–2400 cm−1, with the CO adsorption peak gradually decreasing with the increase in temperature.


image file: c9cy00751b-f9.tif
Fig. 9 In situ DRIFTS spectra of: (a) 1.0 vol% CO oxidation; (b) 500 ppm toluene oxidation; and (c) co-existence of 1.0 vol% CO and 500 ppm toluene over the Pt–CeO2 catalyst.

Fig. 9b shows the DRIFTS spectra of the single toluene oxidation process over the Pt–CeO2 catalyst at different temperatures. The two weak peaks at 1498 and 1603 cm−1 are the typical phenyl ring vibration bands, which rapidly decreased or even disappeared when the temperature was increased from 120 °C to 240 °C. At the same time, one new peak at 1376 cm−1 appeared in the Pt–CeO2 sample, which is related to the νs(COO) stretching vibration of bidentate carbonate species, but this band is not observed in the Pt–Al2O3 and Pt–Co3O4 samples. The bands at 1754 and 1667 cm−1 are attributed to a ν(C[double bond, length as m-dash]O) stretching vibration peak of aldehyde groups,46 which implies the formation of benzaldehyde species. The bands appearing at 2929 and 2856 cm−1 are ascribed to the bending vibrations of methylen (–CH2) groups, while the bands at 2737 and 2840 cm−1 are assigned to a stretching vibration of benzaldehyde species. In addition, the bands at 1180, 1290, 1376, and 1435 cm−1 are related to a stretching vibration of formate species (–COO), indicating the breakage of the aromatic ring. The band at 1551 cm−1 is attributed to the asymmetric vibration of carboxylate species, implying the formation of benzoate species on the surface of the Pt–CeO2 sample. The bands located at 1290, 1375, 1435, 1551, 1667, and 1754 cm−1 indicate a gradual accumulation on the catalyst surface with the increase in temperature. Meanwhile, the intensities of the methylen groups also increased when the temperature was increased from 140 °C to 180 °C. Therefore, the reaction pathway of toluene oxidation on the surface of the Pt–CeO2 sample may follow a consecutive step via rapid transformation to benzyl radical, then aldehydic and carboxylate species, and finally involves breakage of the aromatic ring into CO2 and H2O.

Fig. 9c shows the DRIFTS spectra of the co-oxidation process of CO and toluene over the Pt–CeO2 sample. The characteristic bands of adsorbed CO and toluene on the surface of the Pt–CeO2 sample are also be observed. With increasing the temperature, the bands located at 2086, 2125, and 2173 cm−1, corresponding to adsorbed CO, gradually decreased. Significantly, the bands of adsorbed CO disappeared at above 180 °C, which represents a shift to a higher region than those from individual CO oxidation, indicating that the presence of toluene has an important effect on the CO oxidation. Moreover, it can be seen that the intensity and wavenumber of the other bands in the range of 1000 to 1800 cm−1 have obvious changes in comparison to those for individual CO and toluene oxidation (Fig. 9a and b). The intensities of the bands at 1000–1800 cm−1 become weaker than those from individual toluene oxidation, and the main bands observed at 1435 and 1551 cm−1 in the individual toluene oxidation shifted to 1400 and 1541 cm−1 in the CO and toluene co-oxidation, respectively. The peaks at 2700–3000 cm−1 are similar to those of single toluene oxidation. Furthermore, the adsorbed CO peaks on the surface of the Pt–CeO2 sample were slightly inhibited by the toluene molecules, while the intensities of the adsorbed CO peaks over the Pt–Al2O3 and Pt–Co3O4 samples were significantly decreased, which is consistent with the results of the catalytic performances for simultaneous CO and toluene oxidation.

Discussion

Combining the above DRIFTS analysis and physico-chemical characterization, it can be seen that there are significant differences in the physico-chemical properties and in the important intermediates of CO/toluene oxidation over the Pt-based catalysts. It could be observed that the Pt–Al2O3 catalyst using commercial Al2O3 as a support showed the largest specific surface area (214 m2 g−1) among all the Pt-based catalysts. The Pt–Al2O3 catalyst with a higher specific surface area could provide a greater adsorption capacity for pollutants and increased residence time. Therefore, the Pt–Al2O3 catalyst exhibited enhanced activity for individual CO or toluene oxidation compared to that of the Pt–Co3O4 catalyst with the lowest specific surface area. Certainly, the specific surface area is not the only key parameter for the improved catalytic activities of catalysts. In the H2-TPR results, the Pt–CeO2 catalyst showed the first lower-temperature reduction peak due to the strong metal–support interactions (SMSIs) between the surface oxygen and adjacent Pt species. SMSIs can induce the chemical state of the surface compositions, which further leads to an enhanced effect on the catalytic activity and a change in the dominant reaction pathway.49,50 Among all the Pt-based catalysts, the Pt–CeO2 catalyst exhibited the highest catalytic activities for CO and toluene oxidation. According to the previous reported literature,30,51,52 the catalytic performance in redox reactions is significantly affected via the surface oxygen species of catalysts. Combining the XPS analysis and performance tests, the binding energies and content concentration of surface oxygen species showed distinct differences among the various Pt-based catalysts with different carriers. The Pt0/Pt ratio of the Pt–CeO2 catalyst is relatively higher, implying that the Pt–CeO2 catalyst presents outstanding catalytic performances for CO and toluene oxidation due to Pt0 species as active sites in the oxidation reaction. In addition, the oxygen vacancy (Oads + O-OH) ratio of the Pt–CeO2 sample is higher than that of Pt/Al2O3 and Pt–Co3O4, confirming that the Pt–CeO2 catalyst can provide a higher content of surface oxygen vacancies, which is conducive to the activation and migration of oxygen in the oxidation reactions.

The activation and migration of oxygen species are also related to the reaction mechanism. Oxygen migration facilitates the formation of more surface oxygen vacancies in the reaction process. Meanwhile, surface oxygen vacancies could be replenished by gaseous O2 molecules, and this plays an important role in the formation of surface intermediate species. It is well known that various surface carbon-related species (carbonates or bicarbonate species) are formed in the surface of catalysts during CO or toluene oxidation, and that the adsorbed intermediate species often result in the presence of surface poisons, preventing further redox reactions.53,54 Experimental studies have confirmed that catalysts with dual-active sites (noble metal sites and reducible transition metal oxide sites with abundant oxygen vacancies) could be favorable for forming a synergistic effect in target reactant adsorption/activation.22,53,55–57 According to the previous research,32 a reaction pathway of CO oxidation is as follows: (I) the gaseous CO molecules are linearly adsorbed on the noble metal site; (II) gas-phase O2 is adsorbed/activated on the oxygen vacancies generated by the supports, and at the same time, the activated oxygen atoms are able to migrate to the interface sites due to the contribution between the oxygen atoms of the supports and the adjacent noble metal; (III) adsorbed CO on the noble metal sites is rapidly oxidized by the adjacent activated oxygen species to form crucial intermediates (carbonate species or bicarbonate species), and then completely decomposed into gaseous CO2. In this work, the Pt–CeO2 catalyst showed the presence of strong metal–support interactions, inducing the formation of metal-oxide (Pt–O-M) bonds. In situ DRIFTS spectra recorded during CO oxidation also confirmed that the accumulation of surface carbon-related species was distinctly different on the surfaces of the different Pt-based catalysts. For the Pt–Al2O3 catalyst, the maximal accumulation peak appears at 1248 cm−1, which is assigned to the asymmetric bicarbonate species, while the bidentate carbonate (the major peak at 1581 cm−1) species showed inconspicuous changes with the increase in temperature. For the Pt–Co3O4 catalyst, the strongest peak is observed at 1420 cm−1, which is identified as bicarbonate species. The two peaks at 1449 and 1554 cm−1 (bicarbonate and carbonate species, respectively) simultaneously increased to a maximum in the surface of the Pt–CeO2 catalyst. These changes indicated that the bonding mode between the crucial intermediates and the surface atoms of catalysts is different, and that the accumulation of intermediates may further prevent subsequent reactions. Therefore, the carbonate species on the surface of the Pt–Al2O3 catalyst are easier to form and are difficult to turn into bicarbonate species. On the contrary, both bicarbonate and carbonate species (occupying the surface sites) on the surface of the Pt–CeO2 catalyst could be transformed to facilitate CO oxidation, because bicarbonate species are more easily decomposed into gaseous CO2 at lower temperature, followed by the decomposition of carbonate species into gaseous CO2 at higher temperature. The reaction mechanism of CO oxidation over the Pt–CeO2 catalyst is presented in Fig. 10. In summary, the Pt–CeO2 catalyst had abundant surface oxygen vacancies, and the formation of strong metal–support interactions accelerated the rate of the key reaction, the activation and migration of oxygen species, and the catalytic activity for individual CO oxidation.


image file: c9cy00751b-f10.tif
Fig. 10 Reaction mechanism of CO and toluene oxidation over the Pt–CeO2 catalyst.

In addition, in the individual toluene oxidation, and simultaneous CO and toluene oxidation, the Pt–CeO2 catalyst also demonstrated outstanding catalytic performances compared to the other two Pt-based catalysts. Although the catalytic performances of these Pt-based catalysts were hindered due to the competitive adsorption of both CO and toluene molecules at the same active sites, the dual-active-site catalysts (Pt–CeO2 and Pt–Co3O4) immediately embodied unique advantages in the mixture conditions. The dual-active-site catalysts mitigated the mutual inhibition of both CO and toluene molecules; however, the Pt–Al2O3 catalyst using a conventional indifferent oxide as a support could not withstand its inhibition, resulting in the deactivation of the catalytic activity for simultaneous CO and toluene oxidation. It is well known that the reaction mechanism of toluene oxidation may follow successive steps to form the various intermediates, such as benzyl radicals, benzaldehyde, benzoate, and so on.40,44,46 In this study, it was clearly observed that there were some common pathways on these Pt-based catalysts:toluene molecules were first adsorbed on the surface of catalysts to form benzyl species due to the C–H bond on the methyl (–CH3) groups being broken, then these reacted with the active oxygen species to produce the crucial intermediates, such as benzaldehyde and benzoate, and finally were completely oxidized into CO2 and H2O. During the toluene oxidation, the active oxygen species are formed on the generated oxygen vacancies, which are beneficial for the rapid activation of O2 molecules. The reaction mechanism of toluene oxidation over Pt–CeO2 catalyst is shown in Fig. 10. Nevertheless, major differences were seen in the rate of catalytic toluene oxidation and the ephemeral intermediates (carboxylate species) after the breakage of the benzene ring. Cerium dioxide (CeO2) had an excellent oxygen storage capacity, while alumina (Al2O3) shows a poor one-oxygen-release capacity. Therefore, the as-obtained Pt-based catalysts with different supports directly affected the activity in catalytic toluene oxidation and the accumulation of the crucial intermediates. Forming more surface oxygen vacancies can improve the oxygen-transfer performances that directly provide active oxygen species for the reaction, which would improve the efficiency of catalytic toluene oxidation. Based on the results from the DRIFTS analysis, it could be seen that the accumulation of intermediates on the surface of Pt–Al2O3 catalyst was higher than with the other two catalysts, indicating that the reaction intermediates on the Pt–Al2O3 were not able to further oxidize into H2O and CO2 due to the limited rate of oxygen supply. Certainly, the breaking of the benzene ring to form carboxylic acid was also different in the different catalysts, which was confirmed via the changes in the bands in the DRIFTS spectra (Fig. 7–9).

In the CO and toluene co-oxidation, CO and toluene have competitive adsorption at the same active sites, but the reaction pathway of each other cannot be changed, only their reaction rate slowed down on the surface of the catalysts. Moreover, compared to the DRIFTS spectra, it was found that the intensity of the important intermediates on the catalyst surface became weak under CO and toluene co-oxidation, suggesting that the presence of CO and toluene competitive reactions. A new band appeared at 2173 cm−1 related to CO species adsorbed on Pt2+, implying that the transformation between Pt2+ and Pt0 on the surface of Pt–Al2O3 could not be formed quickly at a relatively low temperature. However, CO was linearly adsorbed on the surface of Pt and CeO2, thus Pt–CeO2 could make full use of the advantages of being a dual-active-site catalyst (adsorption sites and oxygen-rich vacancies) to simultaneously oxidize CO and toluene. The catalytic activity of Pt–CeO2 for CO and toluene co-degradation was superior to those of the other two catalysts, because the Pt–CeO2 catalyst with good strong metal–support interactions had a higher concentration of surface adsorbed oxygen species. Meanwhile, it is reasonable to deduce that the reaction pathways between CO and toluene oxidation may be independent of each other, that is, the reaction route of toluene oxidation is toluene is first transformed to benzyl radicals due to the breakage of the C–H bond of the methyl group, and then forms aldehydic and carboxylate species, and finally is completely oxidized into CO2 and H2O.

Conclusions

In summary, Pt NPs with an average size of 3.3–3.7 nm were successfully synthesized via a glycol reduction method, anchored on CeO2 nanorods, Co3O4 nanosheets, and commercial Al2O3 to acquire a series of Pt-based catalysts (Pt–Al2O3, Pt–Co3O4, and Pt–CeO2). Based on the catalytic activity measurements, it was found that the as-synthesized Pt–CeO2 catalyst exhibited the best catalytic performance for CO and toluene oxidation under single or co-existence conditions. It is concluded that the metal–support interactions between Pt NPs and different supports is different. From the results of the physico-chemical characterization, it was clear that the Pt–CeO2 catalyst with adsorption sites and oxygen-rich vacancies not only induced the formation of rich surface oxygen vacancies, but also improved its lower temperature reducibility. When gaseous CO and toluene were simultaneously introduced into the reaction system, the Pt–CeO2 catalyst with oxygen-rich vacancies could effectively slow down the competitive reaction between CO and toluene molecules, but the Pt–Co3O4 and Pt–Al2O3 catalysts displayed the distinct deactivation of catalytic activity for simultaneous CO and toluene oxidation, due to the competitive adsorption of both CO and toluene molecules at the same sites. Furthermore, the in situ DRIFTS spectra revealed the possible reaction routes toward CO and toluene oxidation on the surface of Pt-based catalysts. The reaction pathway of CO oxidation followed that gaseous CO is first transformed into carbon-related species (bicarbonates and carbonates), and then completely decomposed into CO2. In addition, the reaction route of toluene oxidation may follow a single successive step: toluene can be rapidly adsorbed, activated, and transferred to form benzyl radical species, then forming aldehydic and benzoate species, and finally is completely oxidized into CO2 and H2O. The benzaldehyde and benzoate species are the key intermediates in toluene oxidation. In the mixture conditions, both CO and toluene are competitively adsorbed at the same active sites, but the reaction pathway of both CO and toluene oxidation may be independent of each other, and only their reaction rates are slowed down due to the CO and toluene competitive oxidation. Thus, the Pt–CeO2 catalyst was shown to be able to effectively alleviate the mutual inhibition between CO and toluene oxidation, which could pave the way to a straightforward strategy for the fabrication of high-efficiency catalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research received support from the National Natural Science Foundation of China (No. 51878292, 51108187, 51578245, 51378218, 21401200, 51672273), National Key R & D Plan (2017YFC0211503), China Postdoctoral Science Foundation (No. 2018M643090), and Guangdong Natural Science Foundation, China (No. 2016A030311003).

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Footnotes

Electronic supplementary information (ESI) available: Experimental section, Fig. S1–S9 and Table S1. See DOI: 10.1039/c9cy00751b
Qi Zhang and Shengpeng Mo are co-first authors.

This journal is © The Royal Society of Chemistry 2019