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. 2023 Feb 8;8(7):6791–6800. doi: 10.1021/acsomega.2c07399

Catalytic Oxidation of n-Decane, n-Hexane, and Propane over Pt/CeO2 Catalysts

Xiaohui Gao 1, Yuting Bai 1, Hao Zhang 1, Xingyi Wang 1,*
PMCID: PMC9948155  PMID: 36844556

Abstract

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Pt species with different chemical states and structures were supported on CeO2 by solution reduction (Pt/CeO2-SR) and wet impregnation (Pt/CeO2-WI) and investigated in catalytic oxidation of n-decane (C10H22), n-hexane (C6H14), and propane (C3H8). Characterization by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, H2-temperature programming reduction, and oxygen temperature-programmed desorption showed that Pt0 and Pt2+ existed on Pt nanoparticles of the Pt/CeO2-SR sample, which promoted redox, oxygen adsorption, and activation. On Pt/CeO2-WI, Pt species were highly dispersed on CeO2 as the Pt–O–Ce structure, in which surface oxygen decreased significantly. The Pt/CeO2-SR catalyst presents high activity in oxidation of C10H22 with a rate of 0.164 μmol min–1 m–2 at 150 °C. The rate increased with oxygen concentration. Moreover, Pt/CeO2-SR presents high stability on feed stream containing 1000 ppm C10H22 at gas hour space velocity = 30,000 h–1 as low as 150 °C for 1800 min. The low activity and stability of Pt/CeO2-WI were probably related to its low availability of surface oxygen. In situ Fourier transform infrared results showed that the adsorption of alkane occurred through the interaction with Ce–OH. The adsorption of C6H14 and C3H8 was much weaker than that of C10H22, which resulted in the decrease in activity for C6H14 and C3H8 oxidation of Pt/CeO2 catalysts.

1. Introduction

Volatile organic compounds (VOCs) are considered to be a major source of air pollution, mainly from unburned hydrocarbons (HCs), certain industrial workplaces,1 and automotive engines.2 These compounds contribute to the greenhouse effect and photochemical smog.3 Catalytic combustion is the most promising technology for eliminating VOCs, due to its low reaction temperature and low energy consumption compared to thermal technology.4

The common hydrocarbon oxidation catalysts are platinum group metals (Pt, Pd, Ru, Rh, Ag)5−7 and transition metal oxides (CeO2, MnO2, Co3O2, Fe2O3).8−10 Among them, Pt catalysts possess high activity, due to their excellent ability to activate oxygen and C–H bonds except methane oxidation, in which Pd is more suitable. Santos et al.11 compared the activity of noble metal catalysts supported on TiO2 for ethanol and toluene oxidation and found the activity order to be Pt/TiO2 ∼ Rh/TiO2 > Pd/TiO2 > Au/TiO2 ∼ TiO2. Specific activities depend on O2 adsorption strengths on metals. Gololobov12 found that the catalytic activity of Pt/Al2O3 for total oxidation of C1–C6n-alkanes was promoted with the increase in the Pt particle size. In total oxidation of n-decane and 1-methylnaphthalene,13 the Pt/Al2O3 catalyst with 0.39 Pt dispersion presented high activity. However, the increase in Pt dispersion was not favorable for the activity of Pt/ZrO2 catalysts. Fabrice14 found that the reactivity of n-alkane on the Pt/Al2O3 catalyst increases with the carbon atoms. In methane oxidation, the reaction rate increased and the reaction order changed from zero-order to negative-order in the oxygen partial pressure during decreasing O2/CH4 ratio.15 The O2/alkane ratio at the maximum rate for C2H6 oxidation was higher than that for CH4 oxidation, due to the difference in the consumption of chemisorbed oxygen.16 In the case of propane oxidation, Pt species were saturated with oxygen at high O2/C3H8 ratios, and thus the reaction rate was zero-order with respect to the oxygen partial pressures.17,18 Hasan compared the oxidation of propane on ZrO2, TiO2, and CeO2 catalysts and found that the activity of CeO2 was higher than that of Pt/Al2O3 catalyst.19 As is known, CeO2 has high oxygen storage capacity, due to quick transformation between Ce4+ and Ce3+ states under reductive and oxidative conditions.20 Thus, Pt catalysts on CeO2-based supports were widely used in various reactions, such as the water gas shift reaction21 and catalytic oxidation.22 At present, the relation among the catalytic performance and structure of Pt/CeO2 catalysts and catalytic activity for the oxidation of n-alkanes with different numbers of carbon atoms needs to be still investigated. In this work, Pt/CeO2-SR and Pt/CeO2-WI catalysts were prepared by solution reduction and wet impregnation. Using the oxidation of n-decane (C10H22), n-hexane (C6H14), and propane (C3H8) as model reactions, the relation between the surface performance of Pt/CeO2 and catalytic activity was studied.

2. Results and Discussion

2.1. Characterization of Catalysts

The nitrogen adsorption/desorption isotherms and pore size distributions of samples are presented in Figure S1. All samples show the classical shape of an IV-type isotherm. The Brunauer–Emmett–Teller (BET) area of the CeO2 sample is estimated to be 116 m2/g (Table 1) and decreases to 98–101 m2/g after loading Pt. Figure 1 shows X-ray diffraction (XRD) patterns of samples. The diffraction peaks appearing at 28.6, 33.3, 47.5, 56.5, and 59.2° are ascribed to <111>, <200>, <220>, <311>, and <400> crystal planes of cubic fluorite CeO2, respectively (PDF #34-0394). For the Pt/CeO2-SR sample, a new peak at 39.8° corresponds to metal Pt <111>.23 However, no diffraction peak of crystal Pt is observed for the Pt/CeO2-WI sample, suggesting that Pt species are highly dispersed into CeO2. Pt content was determined by ICP-AES to be 0.97 and 0.91 wt % for Pt/CeO2-WI and Pt/CeO2-SR samples, respectively. Additionally, the former presents 37% Pt dispersion, and the latter, 5%, which suggests that Pt species of the Pt/CeO2-SR sample exist as Pt nanoparticles. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of the samples (Figure S2) show that small CeO2 particles aggregate into a large plane with the lattice spacing of 0.32 nm, corresponding to the <111> plane of CeO2 (JCPDS 34–0394). With Pt addition, the samples present similar morphology. HAADF-STEM-EDX element-mapping analyses (Figure S3a,b) show that the average size of Pt nanoparticles on the Pt/CeO2-SR sample is about 30 nm nanoparticles, consistent with XRD results (Table 1). For the Pt/CeO2-WI sample, Pt species are highly dispersed on the CeO2 surface (Figure S3c,d).

Table 1. Structure and Physicochemical Properties of Pt/CeO2 Samples.

sample SBETa/m2 g–1 Pt loadingb/wt % Pt dispersionc/% Pt size/nm
 Pt composition/at %
COd XRDe Pt0 Pt2+ Pt4+
CeO2 116              
Pt/CeO2-SR 98 0.91 5 20.6 24.7 44.0 46.7 9.3
Pt/CeO2-WI 101 0.97 37 2.8 n.d.f 0 75.9 24.1
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a

Determined from the N2 adsorption isotherm.

b

Obtained by ICP-AES.

c

Measured by pulsed CO chemisorption at −50 °C.

d

Calculated by the method reported by the literature.24

e

Crystallite size estimated by the Scherrer equation, applied to the (111) plane of Pt.

f

n.d. No data can be calculated partially because the particle is too small.

Figure 1.

Figure 1

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XRD patterns of Pt/CeO2 samples.

2.2. Raman Spectra

Before recording spectra, the samples were pretreated with He flow for 1 h at 400 °C, following the treatment with 5% O2/Ar flow (30 mL/min) for 30 min at 100 °C. As shown in Figure 2, a prominent band at 460 cm–1 can be attributed to triply degenerate F2g mode of CeO2 with the fluorite structure, while the peaks at 594 and 1175 cm–1 are assigned to defect-induced (D) mode and second-order longitudinal optical (2LO) mode of CeO2, respectively.25 With Pt addition, these bands become weak and broad, and there appear two additional bands at 560 and 690 cm–1, which can be attributed to the bridging Pt–O–Ce and Pt–O of Pt–O–Ce, respectively.26 Metallic Pt is not Raman-active.27 Neither amorphous PtO2 (610 cm–1) nor crystalline PtO (438 cm–1 (Eg)), 657 cm–1 (B1g)) appears.27 The intensity of Pt–O–Ce bonds is much stronger for Pt/CeO2-WI, although Ptδ+ content in the Pt/CeO2-SR sample is 56.0% (calculated based on X-ray photoelectron spectroscopy (XPS) data, Table 1). This phenomenon indicates that Pt species on Pt/CeO2-WI exist mainly in the form of Pt–O–Ce bonds. In other words, there exists a strong metal-support interaction between Pt and CeO2, but the ionic state Pt on the surface of the Pt/CeO2-SR sample mainly exists in the aggregation state. There should appear two bands at 833 and 1175 cm–1 which can be ascribed to peroxide (O22–) species and superoxide (O2–), due to them being Raman-active.28 In this test, almost no band at 833 cm–1 on CeO2 appears, probably due to the desorption of reversibly adsorbed peroxy-like species. The band at 833 cm–1 becomes strong with Pt loading (Figure 2), probably because Pt in contact with the CeO2 domain promotes the formation of O22–, especially for Pt/CeO2-SR. In contrast, there is no significant change in the ratio of I1175/I460 (Table S1), indicating that superoxide (O2–) species come mainly from CeO2.

Figure 2.

Figure 2

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Raman spectra of Pt/CeO2 samples.

2.3. Chemical States

XPS spectra of Pt 4f, Ce 3d, and O 1s for the samples are shown in Figures 3a and S4. There are two Pt species on the surface of the Pt/CeO2-WI sample with a binding energy (BE) of Pt2+ 4f7/2 and Pt2+ 4f5/2 of 72.6 and 75.8 eV, and BE of Pt4+ 4f7/2 and Pt4+ 4f5/2 of 74.2 and 77.5 eV, respectively.29 Pt0 species appear on the surface of the Pt/CeO2-SR sample with a BE of 70.7 and 74.0 eV,30 and the ratio of Pt0/Pt is estimated to be up to 44.0%. While in the Pt/CeO2-WI sample, Pt exists as Pt2+ and Pt4+ species, which strongly interacts with CeO2 to form Pt–O–Ce. IR spectra of CO adsorption on Pt/CeO2 samples are used to investigate the Pt chemical state (Figure 3b). The linear CO-adsorption peak of Pt/CeO2-SR at 2091 cm–1 is attributed to the reduced Pt, 10 cm–1 lower than that on the Pt/CeO2-WI sample (2101 cm–1). The red shift of linear CO vibrational frequency obviously indicates that the electronic abundance of the loaded Pt over Pt/CeO2-SR sample is high.31

Figure 3.

Figure 3

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XPS spectra of Pt 4f for Pt/CeO2 samples (a) and Fourier transform infrared (FTIR) spectra of CO-adsorption (b).

In Ce 3d spectra, the identified peaks with the deconvolution method are caused by the pairs of spin-orbit doublets (Figure S4a). Ce4+ contributes to six peaks denoted as v, v″, v″′, u, u″, u″′, and Ce3+, to peaks as v′, and u′.32 In O 1s spectra (Figure S4b), O 1s peaks are deconvoluted into three peaks at 529.2–529.4, 531.0–531.2, and 533.0–533.2 eV, which are ascribed to lattice oxygen, surface oxygen (O22–, O2–, O– species), and oxygen in water, hydroxyl, and CO2.33 Here, Osurface should not include reversibly adsorbed O22– species which is removed due to desorption under XPS ultrahigh vacuum conditions (oxygen temperature-programmed desorption (O2-TPD) test in Figure 5 confirms significant desorption of O22– species at low temperature). Neither the Ce3+/Ce ratio nor the Osurface/Ototal ratio shows significant changes (Table S1).

Figure 5.

Figure 5

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O2-TPR profiles of Pt/CeO2 samples.

2.4. H2-Temperature Programming Reduction (H2-TPR) and O2-TPD

H2-TPR is used to investigate the reducibility of the samples (Figure 4). For the CeO2 sample, a broad reduction peak appears at 250–580 °C, which can be ascribed to the reduction of surface Ce4+ species on small or highly defective nanoparticles (low temperature) and on large CeO2 particles (high temperature), respectively. With Pt addition, there appears a strong narrow reduction peak at 120 °C for Pt/CeO2-SR and at 290 °C Pt/CeO2-WI, respectively, which can be ascribed to the reduction of Ce4+ and Pt2+ or Pt4+ species. The promoted reduction of Ce4+ can be ascribed to high ability of Pt species for H2 dissociation at low temperature. The increase in reduction temperature of Pt/CeO2-WI should be related to weak H2 dissociation on Pt2+ or Pt4+ species and to the formation of Pt–O–Ce species with strong metal-support interaction.34 The corresponding H2 consumption of two samples is higher than that of theoretical hydrogen consumption (Table S2), which is calculated based on the mole number of H2 converted for both CeO2 reduction (normalized by surface Ce4+%) and Pt reduction (Table S2). This phenomenon could be ascribed to the H2 spillover effect induced by SMSI. It should be noted that the bulk reduction of CeO2 (higher than 800 °C) is not affected by Pt addition, which is consistent with a previous report that Pt-ceria interaction was limited to the support surface.35

Figure 4.

Figure 4

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H2-TPR profiles of Pt/CeO2 samples.

O2-TPD tests were carried out to determine the mobility of oxygen species of Pt/CeO2 (Figure 5). Generally, the desorption sequence of surface oxygen species is reversibly and chemically adsorbed oxygen (O22–), irreversibly and chemically adsorbed oxygen (O2–), surface oxygen (O–), and surface lattice oxygen (O2–). The O2-TPD profile of CeO2 is identified by deconvolution as three desorption peaks with maxima at 330, 460, and 590 °C, which should correspond to the desorption of O2–, O–, and O2–, respectively.36 For the Pt/CeO2-SR sample, an additional strong desorption peak appears at 235 °C, which can be ascribed to O22– species, probably because Pt in contact with the CeO2 domain promotes the formation of O22–.37 This phenomenon suggests that Pt species on Pt nanoparticles greatly promote the chemical adsorption of oxygen, probably through the transfer of electrons of the Pt d orbital to oxygen molecules. In the case of the Pt/CeO2-WI sample, all oxygen desorption peaks become weak. Generally, surface oxygen of Ce-based catalysts is situated on the surface oxygen vacancy of CeO2. According to Raman spectra, the difference in the band at 259 cm–1 (related to oxygen vacancy) for Pt/CeO2-WI and Pt/CeO2-SR is small, that is to say, a significant decrease in surface oxygen should be related to the other cause. Pt–O–Ce exists on CeO2 as Pt2+(−O–Ce)2 or Pt4+(−O–Ce)4, which may shield oxygen vacancy, resulting in a decrease in the availability of oxygen vacancy and oxygen mobility.

2.5. In Situ FT-IR of C10H22, C6H14, and C3H8 Adsorption and Oxidation

For C10H22 adsorption on CeO2 (Figure 6a), the bands at around 2956, 2926, and 2854 cm–1 can be ascribed to the vibration of C-H of adsorbed C10H22,38 while in the hydroxyl section, there appears a strong positive band at 3622 cm–1, along with a broad negative band in 3714–3647 cm–1. These bands become weak with the increase in temperature, and the bands at 1650 (C=O), 1577, 1559, and 1418 cm–1 (carboxylate species) become significant, indicating that the hydroxyl on the catalyst surface interacts with C10H22, resulting in the break of C–H, while the formation of intermediates containing oxygen (at 200 °C) is involved in surface oxygen. With oxygen addition, the bands ascribed to −OH and C–H quickly disappear at 200 °C, and the band at 1559 cm–1 grows in intensity greatly. This phenomenon suggests that the oxidation is promoted. With Pt particles loading on CeO2 (Pt/CeO2-SR), the adsorption of C10H22 in oxygen-containing feed becomes much weaker at low temperature (Figure 6d), probably due to the completion of oxygen with C10H22 for adsorption sites. The adsorption and activation of oxygen occur on Pt particles with Pt0 species as low as 50 °C. The produced surface oxygen species transfer to C10H22 adsorption sites, the oxidation occurs. Moreover, the bands to carboxylate species appear significantly below 150 °C, where C10H22 of conversion approaches 100% in kinetic reaction (see later). Obviously, the activity of Pt/CeO2-SR increases through the promotion of oxygen adsorption and activation. Similar C10H22 adsorption spectra on Pt/CeO2-WI with Pt–O–Ce species are observed (Figure 6e,f), but oxygen addition cannot promote the oxidation, indicating that Pt of Pt–O–Ce species is not favorable for the oxygen activation on CeO2. In the cases of C6H14 and C3H8, the intensity of adsorption decreases significantly (Figures S5 and S6). Obviously, the number of carbon atoms in alkanes affects the behavior of adsorption on CeO2. Probably, C10H22 adsorption involves in the interaction with multiple sites, which promotes the activation of C10H22, while the interaction with multiple sites becomes difficult with the decrease in carbon atoms.

Figure 6.

Figure 6

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In situ FTIR in 1200–4000 cm–1 region for the adsorption and oxidation of C10H22 over CeO2 (a,b), Pt/CeO2-SR (c,d) and Pt/CeO2-WI (e,f).

2.6. Activity

The light-off curves of alkanes (HCs) over CeO2 and Pt/CeO2 catalysts are shown in Figure 7. The reactivity of HCs is characterized by T50 and T90 (at which the conversions reach 50 and 90%, respectively (Table 2)). It can be seen that the longer the chain length of HCs, the lower the temperature required for their oxidation. CeO2 presents considerable activities with T50/T90 of 182/198 °C for C10H22 (the rate at 150 °C of 0.031 μmol min–1 m–2), and 220/257 °C for C6H14 (the rate at 200 °C of 0.058 μmol min–1 m–2), respectively. Pt addition by the solution reduction method (Pt/CeO2-SR) greatly promotes the activity of CeO2, and T50/T90 decrease to 138/158 °C for C10H22 oxidation (the rate at 150 °C of 0.164 μmol min–1 m–2), 163/200 °C for C6H14 oxidation (the rate at 200 °C of 0.321 μmol min–1 m–2). However, the activity of the Pt/CeO2-WI catalyst for the oxidation of C10H22 and C6H14 becomes lower, even than that of the CeO2 catalyst. T50/T90 values rise to 220–250/264–299 °C, and the rate significantly decreases to 0.014 μmol min–1 m–2 (at 150 °C) and 0.016 μmol min–1 m–2 (at 200 °C). Moreover, in C3H8 oxidation, the activity of CeO2 is poor with T50 at 384 °C, and T90 is not available below 400 °C and the rate at 350 °C reaches 0.257 μmol min–1 m–2. C3H8 conversion curves over Pt/CeO2-SR and Pt/CeO2-WI shift to low temperature with T50/T90 values of 306/385 °C and 334/389 °C, respectively, and the rates at 350 °C increase to 0.485 and 0.418 μmol min–1 m–2. The activity difference for C3H8 oxidation between two catalysts becomes small with the increase in temperature. It should be noted that Pt/CeO2 catalysts have similar activity of C3H8 oxidation in the temperature range blow 300 °C. Considering the fact that the adsorption behavior of HCs on Pt/CeO2-SR and Pt/CeO2-WI catalysts is similar to that on CeO2, it can be deduced that the exposed CeO2 is responsible for the activation of HCs, probably through the interaction between C–H and Ce–OH, as shown in in situ FT-IR spectra of HC adsorption. Therefore, the activity order for C10H22 and C6H14 oxidation of Pt/CeO2-SR > Pt/CeO2-WI can be ascribed to that the Pt/CeO2-SR catalyst possesses better reducibility and high dissociation and activation of oxygen molecules. As reported,39 the increase in the Pt particle size could decrease the energy of bonds between oxygen and Pt and promoted the mobility of oxygen species, due to low degree of coordination unsaturation of Pt atoms of Pt crystal planes on the surface of large particles. Moreover, the adsorption and activation of C10H22, C6H14 and C3H8 on CeO2 are dependent of their number of carbon atoms, which cooperate on multisites for adsorption with favorable geometry. Here, HCs with a small number of carbon atoms are often difficult for cooperation on CeO2, probably due to unsuitable angle or chain length, and their adsorption and activation are rate-steps. As a result, Pt addition into CeO2 almost cannot affect the activity of CeO2 catalyst for the oxidation of HCs with small number of carbon atoms such as C3H8. In addition, the Pt/CeO2-WI catalyst was treated with 10% H2/N2 at 450 °C for 4 h (noted as Pt/CeO2-WI-R). The activity of the Pt/CeO2-WI-R catalyst for C10H22 oxidation is promoted (Figure S7), and T50/T90 decreases to 180/240 °C. However, raising temperature up to 180 °C, the conversion curve shifts to high temperature, and the temperature of 100% conversion is close to that obtained on the Pt/CeO2-WI catalyst, which is probably related to the reduction of Pt species with C10H22. Indeed, the reduced Pt species of Pt–O–Ce can be reoxidized easily, which is confirmed by XPS spectra (Figure S8).

Figure 7.

Figure 7

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Catalytic oxidation of C10H22 (a), C6H14 (b), and C3H8 (c) on CeO2 and Pt/CeO2 catalysts; gas compositions: 1000 ppm C10H22, or 1500 ppm C6H14, or 3000 ppm C3H8; 20% O2 and N2 balance; gas hour space velocity (GHSV): 30,000 h–1; catalyst amount: 200 mg.

Table 2. Activities of Pt/CeO2 Catalysts for C10H22, C6H14, and C3H8 Oxidation.

catalyst C10H22     C6H14     C3H8    
  T50/°C T90/°C ratea /μmol min–1 m–2 T50/°C T90/°C ratea/μmol min–1 m–2 T50/°C T90/°C ratea/μmol min–1 m–2
CeO2 182 198 0.031 220 257 0.058 384   0.257
Pt/CeO2-SR 138 158 0.164 163 200 0.321 306 385 0.485
Pt/CeO2-WI 220 264 0.014 250 299 0.016 334 389 0.418
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a

The apparent reaction rates at 150 °C for C10H22, at 200 °C for C6H14 and at 350 °C for C3H8 under the reaction condition described in Figure 7.

2.7. Stability Tests

The stability of catalysts was tested on the feed stream containing 1000 ppm C10H22, 1500 ppm C6H14 and 3000 ppm C3H8, respectively (here, maintaining the number of C atom to be almost same) (Figure 8). In C10H22 oxidation at 150 °C, the Pt/CeO2-SR catalyst shows 80% stable conversion within 1800 min duration, while Pt/CeO2-WI and CeO2 catalysts deactivate with the decrease in conversion from 20% to 6% within initial 300 min, and from 38 to 22% within 1800 min, respectively. As seen in FT-IR spectra during C10H22 adsorption, the band at 1582 cm–1 ascribed to C=C of benzene appears. If the oxidation of benzene-like compounds is not quick, and their condensation can occur easily, finally resulting in carbon deposition on the catalysts. The Pt/CeO2-SR catalyst is active for oxidation at low temperature, due to high availability of active surface oxygen. The deactivation resulting from benzene-like compounds can be avoided. Stable conversions of C6H14 (200 °C) and C3H8 (350 °C) maintain at 6, 90 and 25%, and 42, 60 and 70% within 1800 min over CeO2, Pt/CeO2-SR, and Pt/CeO2-WI catalysts, respectively. These results are related to quick oxidation at high temperature. XPS analyses of used catalysts in C10H22 oxidation show that there is no significant change in binding energies of Pt and Ce, but Pt0 species of Pt catalysts increases obviously (Figure S9a), indicating that oxidized Pt species is reduced to Pt0 species partially with C10H22, as seen in C3H8 catalytic combustion.40 With 2% V/V H2O into the feed, the Pt/CeO2-SR catalyst presents the slightly decreased activity for C10H22 (150 °C) and C6H14 (200 °C) oxidation, and stable conversion within 400 min duration decreases to 71 and 87% from 80 and 90% respectively (Figures S10 and 9), while the deactivation for C3H8 oxidation becomes significant with stable conversion at 350 °C within 300 min decreases to 57% from 70% in dry feed. Turning off water, the conversion quickly returns to the level in dry feed. These results suggest that the deactivation caused by water is reversible. In addition, the reported results of metal oxide catalysts for C10H22, C6H14 and C3H8 oxidation were listed in Table S3. The Pt/CeO2-SR catalyst presents excellent performance in a broad range of reactant concentrations, including high activity, stability, will be desirable for industrial applications.

Figure 8.

Figure 8

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Activity for HC catalytic oxidation over CeO2 and Pt/CeO2 catalysts on stream feed at 150 °C (1000 ppm C10H22) (a), 200 °C (1500 ppm C6H14) (b), and 350 °C (3000 ppm C3H8) (c); 20% O2 and N2 balance; GHSV = 30,000 mL g–1 h–1; catalyst amount: 200 mg.

Figure 9.

Figure 9

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Stability of the Pt/CeO2-SR catalyst in C10H22, C6H14, and C3H8 oxidation in dry and wet feed (containing 2% V/V water); gas compositions: 1000 ppm C10H22, or 1500 ppm C6H14, or 3000 ppm C3H8; 20% O2 and N2 balance; GHSV = 30,000 mL g–1 h–1; catalyst amount: 200 mg.

2.8. Effect of Reactant Concentration on the Activity

Maintaining C10H22, C6H14, and C3H8 concentration at 1000, 1500, and 3000 ppm individually in the corresponding feed, oxygen concentration ([O2]) is changed in a range of 2–20%. Considering that the stoichiometric [O2] values are 1.55%, 1.425%, and 1.5%, respectively, for C10H22, C6H14, and C3H8 oxidation, the effect of [O2] on the activity of CeO2, Pt/CeO2-SR, and Pt/CeO2-WI catalysts in the fuel-lean region is investigated (Figure 10). At 2, 5, 10, and 20%, T90 of C10H22 over the Pt/CeO2-SR catalyst is 237, 196, 166, and 158 °C (Figure S11), while the conversion at 150 °C is 10, 18, 52, and 74%. Obviously, the reaction at 150 °C in 1000 ppm C10H22 feed is kinetically controlled, and the dependency of reaction on [O2] is near the first order. That is to say, C10H22 adsorption is quick enough to meet the oxidation step. For the Pt/CeO2-WI catalyst, T90 rises up to 264–313 °C, and the conversion at 150 °C decreases to 0–6% in the same [O2] range. As above mentioned, the adsorption and activation of C10H22 can occur at 150 °C, where the adsorption of oxygen molecules is limited on CeO2 with Pt–O–Ce species, even raising [O2], because of strong bonding between Pt and oxygen. Raising temperature, the different phenomenon is observed in the oxidation of C6H14 and C3H8. Over the Pt/CeO2-SR catalyst, in the 2–20% [O2] range, C6H14 conversion at 200 °C decreases to 97–90%, and C3H8 conversion at 350 °C, to 93–70%. With the activation of oxygen at high temperature, surface oxygen competes with HC molecules for adsorption sites on the catalyst surface, resulting in the decrease in activated HC molecules. While for Pt/CeO2-WI catalyst, because oxygen activation capacity is not large enough to compete with HC molecules for adsorption sites, the increase in [O2] is favorable for the oxidation. Increasing C10H22, C6H14, and C3H8 concentration from 500 ppm to 2000, 2000, and 4000 ppm, respectively, and maintaining [O2] at 20%, the reactant conversion curves shift to high temperature (Figure S12), but the reaction rates of HCs (at 150, 200, and 350 °C for C10H22, C6H14, and C3H8, respectively) increases to a different extent (Figure 10). Considering that O2/HCs ratios are far greater than the theoretical ratio, the increase in the reaction rate with HC concentration can be ascribed to the dependency of reaction on reactant.

Figure 10.

Figure 10

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C10H22 (a), C6H14 (b), and C3H8 (c) conversion at various O2 concentrations and the rate at various HCs concentrations maintaining 20% O2 over Pt/CeO2-SR and Pt/CeO2-WI catalysts; GHSV = 30,000 mL g–1 h–1; catalyst amount: 200 mg.

3. Conclusions

Pt catalysts supported on CeO2 prepared by the solution reduction method and wet impregnation method were used in the oxidation of C10H22, C6H14, and C3H8. Pt species on the Pt/CeO2-SR sample present Pt nanoparticles with Pt2+ and Pt0, while highly dispersed Pt2+ and Pt4+ species exist on the Pt/CeO2-WI sample in the form of Pt–O–Ce. High reducibility and mobility of oxygen species are observed on Pt nanoparticles, which results from high activity for the adsorption and activation of oxygen. The reducibility of the Pt/CeO2-WI sample with Pt–O–Ce decreases to a significant extent, compared with CeO2. In situ FT-IR shows that C10H22 as a long chain alkane model can adsorb as low as 50 °C on the CeO2 domain through the interaction with Ce–OH (which is confirmed by the negative hydroxyl band). Surface active oxygen can compete with C10H22 for adsorption sites. The Pt/CeO2-SR sample presents high activity for C10H22 oxidation, due to high ability of Pt nanoparticles for adsorption and activation of oxygen. The adsorption of C6H14 and C3H8 on the CeO2 domain becomes much weaker, resulting in low activity for their oxidation. In kinetic oxidation of C10H22, C6H14, and C3H8, the Pt/CeO2-SR catalyst shows the highest activity with T90 of 158, 200, and 385 °C, respectively, really due to its high availability of active surface oxygen. The difference in reactivity of C10H22, C6H14, and C3H8 can be ascribed to their different adsorption behavior, which results from the number of carbon atoms contacted with CeO2 domain. At 20% oxygen content, the rates of C10H22, C6H14 and C3H8 oxidation increase with their contents. CeO2 and Pt/CeO2-WI catalysts present the decreased activity, due to their low availability of surface oxygen. Stability investigation of catalysts in feed stream shows that 80% C10H22 conversion on Pt/CeO2-SR catalyst at 150 °C can maintain for 1800 min duration. While the deactivation of CeO2 and Pt/CeO2-WI catalysts suggests that slow oxidation of C10H22 adsorption intermediates results in carbon deposition. Raising temperature to 200 °C or higher, the stable activity for C6H14 and C3H8 oxidation over three catalysts can be obtained.

4. Experimental Section

4.1. Catalyst Preparation

CeO2 was prepared by a hydrothermal method. Ce(NO3)3·6H2O (10 g) and urea (3 g) were dissolved in deionized water (40 mL) under stirring for 30 min at room temperature, and then the obtained solution was moved into a Teflon-lined stainless-steel autoclave (100 mL) and heated up to 140 °C for 300 min. After cooling to room temperature, the formed precipitate was washed with 2 L deionized water and ethanol, dried at 110 °C overnight, and finally thermally treated at 450 °C in an air atmosphere for 240 min.

Pt/CeO2 catalysts were prepared by two different methods. For the solution reduction technique, typically, 2 g of the above obtained CeO2 was dispersed in 10 mL of deionized water, and then 1 mL of 0.2 M H2PtCl6 aqueous solution was added (1.0 wt % Pt to CeO2) to form a suspension. The mixture was magnetically stirred for 2 h at room temperature. Subsequently, 1 mL of hydrazine hydrate (60%) was dropwise added as a reducing agent. The mixture solution was stirred for 20 min at 70 °C. After that, the precipitate was washed with 1 L of deionized water and dried at 110 °C overnight. For the wet impregnation method, 2 g of the above obtained CeO2 was immersed in 1 mL aqueous solution of H2PtCl6 (0.2 M). Then, the sample was dried at 110 °C overnight. Finally, the samples were thermally treated at 450 °C in an air atmosphere for 240 min. The obtained powders were denoted as Pt/CeO2-SR and Pt/CeO2-WI, respectively.

4.2. Catalyst Characterization

The X-ray diffraction (XRD, D/MAX 2550 VB/PC, Cu Ka radiation) technique was used to study the crystal structure and phase of the samples. The diffractogram was recorded in a 2θ range of 10–80° with a 2θ step of 0.01° and a time step of 10 s. The actual Pt loading was determined by ICP-OES. The nitrogen adsorption–desorption isotherm was measured statically at 77 K on an ASAP 2460 system. The sample was pretreated thermally at 200 °C for 12 h. The surface area of the sample was estimated with the BET model. TEM/HRTEM images were taken on a JEM-2100F field emission transmission electron microscope. Raman spectra were recorded with a laser micro-Raman spectrometer system equipped with a confocal microscope, notch filter (532 nm), and a single-stage monochromator. XPS was conducted on a Thermo ESCALAB 250XI spectrometer using Al Kα (1486.6 eV) radiation as the excitation source to obtain the spectra Pt 4f, Ce 3d, and O 1s of the samples.

The Pt dispersion was determined on a Micromeritics AutochemII2920 by the cryogenic CO pulse adsorption method. For prereduced procedure, 5% H2/Ar was fed to sample at 30 mL/min at 300 °C for 30 min. Then the sample was cooled to −50 °C in Ar flow with a liquid nitrogen device. Then, 0.5173 mL of 1% CO/Ar was pulsed every 5 min until the intensity of the CO peak was constant to obtain the curves. Pt dispersion and mean particle diameter were calculated according to the literature.24

H2-TPR was investigated by heating catalysts (100 mg) in H2/Ar (5%) flow (30 mL·min–1) at a heating rate of 10 °C·min–1 from −50 to 800 °C. Before the H2-TPR analyses, samples were heated for 60 min in Ar flow at 200 °C. The quantitative result of the area under a reduction peak was calibrated on the basis of hydrogen consumption from the reduction of CuO to Cu.

The O2-TPD was tested by Micromeritics AutochemII 2920 equipped with a quadrupole mass detector. Typically, the sample (100 mg) was pretreated at 400 °C with He flow (30 mL/min) for 60 min and then cooled down to 90 °C, followed by treatment with O2/He (2%) flow (30 mL/min) for 30 min. After being purged with He flow (30 mL/min) for 30 min, the sample was heated from 90 to 700 °C in He flow (30 mL/min) at a heating rate of 10 °C/min.

A Nicolet 6700 FT-IR equipped with a liquid nitrogen-cooled mercury-cadmium-telluride (MCT) detector was used in the in situ DRIFTS test. The DRIFTS cell (Harrick, HVC-DRP) equipped with CaF2 windows was used as the reaction chamber that allowed the samples to be heated to 600 °C. All the spectra were within the range of 4000–1200 cm–1 at a resolution of 4 cm–1 and 64 scans. Before CO adsorption, the sample was pretreated at 200 °C for 60 min in Ar atmosphere.

After the pretreatment, the temperature was cooled to 30 °C and the background spectra were recorded, then CO adsorption was carried out under the atmosphere of CO/Ar (1%) until the adsorption signal did not change. The gas was switched to Ar to remove most of the gaseous CO and then adsorption spectra was collected.

For hydrocarbon adsorption, the sample was pretreated by flowing 20% O2/Ar flow at 400 °C for 60 min with the subsequent exposure to 1000 ppm C10H22/Ar or 1500 ppm C6H14/Ar or 3000 ppm C3H8/Ar feed to reach saturation at various temperature and then to Ar for 30 min, and finally FT-IR spectra were recorded. For hydrocarbon oxidation, the sample was pretreated at 400 °C by flowing 20% O2/Ar for 2 h with subsequent exposure to the feed of 20% O2/Ar containing 1000 ppm C10H22 or 1500 ppm C6H14 or 3000 ppm C3H8 to reach saturation at various temperatures. After purging the sample with 20% O2/Ar for 30 min, the corresponding spectrum was recorded.

4.3. Catalyst Activity Measurements

The activity and stability of catalysts for catalytic oxidation of hydrocarbon were investigated in a tubular quartz reactor (4 mm diameter). The particle mixture (40–60 mesh) of 200 mg catalyst was placed on the reactor bed, through which the feed composed of 2–20% O2/N2 containing 500–4000 ppm reactant flowed at 100 mL min–1 at 30,000 mL g–1 h–1 GHSV. The temperature was controlled at 50–500 °C. On-line analyses for organic compounds were conducted with GC (GC2060) equipped with a flame ionization detector (FID) and a ffap-30 m × 0.32 mm × 0.50 μm capillary column. Considering negligible change in the feed volume during the reaction, the conversion was calculated by the difference between initial and final reactant concentrations divided by initial reactant concentration. No byproducts other than H2O and CO2 were detected by GC.

Acknowledgments

We would like to acknowledge the National Natural Science Foundation of China (no. 21976056, 21777043, and 21922602).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c07399.

  • Materials characterization; the reported activities of metal oxide catalysts for C10H22, C6H14 and C3H8 oxidation; N2 sorption isotherms and pore size distributions; TEM, HRTEM images; HAADF-STEM-EDX element-mapping analyses of Pt; XPS spectra of Ce 3d and O 1s; FTIR spectra of C6H14 and C3H8 adsorption and oxidation; XPS spectra of Pt 4f and Ce 3d of the used catalysts; C10H22 conversion curves over the Pt/CeO2-WI-R catalyst; XPS spectra of the Pt 4f of Pt/CeO2-WI-R catalyst; the activity of C10H22, C6H14, and C3H8 oxidation over the Pt/CeO2-SR catalyst in wet feed; C10H22, C6H14, and C3H8 conversion curves at various O2 concentrations and HC concentrations (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao2c07399_si_001.pdf (1.6MB, pdf)

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