Catalytic Combustion of Methane over Pt–Ce Oxides under Scarce

Nanocrystalline Pt–Ce oxides were prepared by citric acid method and impregnation method, and then mounted on cordierites to investigate the catalyt...
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Catalytic Combustion of Methane over Pt−Ce Oxides under Scarce Oxygen Condition Jianhui Jin,† Chuang Li,† Chi-Wing Tsang,‡ Bin Xu,‡ and Changhai Liang*,† †

Laboratory of Advanced Materials and Catalytic Engineering, Dalian University of Technology, Dalian 116024, China ECO Environmental Energy Research Institute Limited, Hong Kong, China

‡

ABSTRACT: Nanocrystalline Pt−Ce oxides were prepared by citric acid method and impregnation method, and then mounted on cordierites to investigate the catalytic behavior for methane oxidation under scarce oxygen environment. XRD, H2-TPR, TEM, and XPS were employed to investigate the relationship between physicochemical characteristics and catalytic performances. Pt−CeO2 showed good activity for oxygen conversion in fuel rich condition, especially during extinction step, with a huge hysteresis loop. By varying the oxygen concentration, Pt−CeO2 catalysts were poisoned by oxygen along ignition, and the competitive adsorption of oxygen and methane inhibits the activation of methane. Owing to the interaction between Pt and CeO2, and a more homogeneous dispersion of Pt in CeO2 compared with that of Pt/CeO2 prepared by impregnation, Pt−CeO2 catalysts maintained high activity during calcination and pretreatments.

1. INTRODUCTION Catalytic oxidation of methane has received much attention because of its practical significance as an alternative to thermal combustion of methane for energy production, emission control of industrial and utility gas turbines, and hydrogen production for fuel cell applications.1,2 Understanding the reaction mechanism at different fuel conditions is important in making full use of methane resources.3 One of the major challenges in catalytic methane oxidation process is the relatively high temperature needed to initiate the reaction,4 since methane is the most stable hydrocarbon to oxidize catalytically. The rate-determining step in CH4 oxidation over catalysts is considered as the abstraction of the first H through dissociative adsorption.5 Both noble metals and transition metal oxides catalysts have been extensively studied to develop catalytic combustion applications for methane.6 Comparatively, platinum-based metal catalysts have high efficiency in the oxidation process for aromatic compounds.7 Supported Pt, Pd noble metal catalysts are commonly used as three way catalysts for emission control in automotive exhausts. However, each one of them is suitable for different applications. Pd-based catalysts with changeable valences are recognized as the most active catalysts for the oxidation of methane under lean-burn conditions, where Pd is generally considered in an oxide state8 and is more active than Pt,9−11 whereas metallic state is generally considered to be the active phase for methane oxidation by Pt-based catalysts.12 Thus, a Pt-based catalyst is preferable at methane rich conditions like reforming and partial oxidation of methane. In terms of Pt, adsorbed oxygen decreases the activity via selfpoisoning, inhibiting the ability of the catalyst surface to dissociate methane.5 So, the selection of catalysts depends on © 2016 American Chemical Society

reaction conditions because the rate of combustion of methane on Pt or Pd catalysts correlate with the specific state of the catalyst surface. One strategy to promote the oxidation of CH4 is to facilitate the transport of oxygen to/from metal by support materials.13 CeO2 is a critical component in three-way catalysts, both as a support and as a catalyst itself. The extensive use of CeO2 is attributed to its feature of high oxygen storage capacity (OSC),14 efficient oxygen exchanging among different conditions,15 and the ability to stabilize precious metal dispersion.16−18 Substitution of noble metals for Ce at lattice sites has been identified as a promising route to improve catalyst activity and durability, leading to the more efficient precious metal utilization.19 The presence of noble metal in the lattice of CeO2 could lead to the highest possible dispersion for a given loading and higher chemical and structural stability due to the formation of NM−O−Ce bond.20−22 Therefore, the migration and agglomeration of noble metal at high temperature can be inhibited through the strong interaction between noble metal and CeO2 surface.22−24 Both experiments and calculation have been reported for enhanced catalytic activity of Pd, Pt, Rhdoped CeO2 toward CO oxidation, hydrocarbon oxidation, and hydrogen adsorption, compared to that of metals over CeO2.25,26 Several studies have been conducted on the Pt sintering and Pt-support interaction in Pt/CeO2 catalysts.23,24,27 Received: Revised: Accepted: Published: 2293

November 5, 2015 February 14, 2016 February 15, 2016 February 15, 2016 DOI: 10.1021/acs.iecr.5b04202 Ind. Eng. Chem. Res. 2016, 55, 2293−2301

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Industrial & Engineering Chemistry Research

4f, and O 1s lines were monitored. All core-level spectra were corrected by referring the binding energy of the C 1s neutral carbon peak at 284.6 eV. Morphology and microstructure were investigated by TEM. Oxides prepared were dispersed in ethanol and sonicated to disperse sufficiently. A single drop of sample was placed onto a 200-mesh copper grid coated with an amorphous holey carbon film. The images of oxides were recorded by FEI Tecnai 20 instrument operated at 200 kV. 2.3. Catalyst Evaluation. The reaction was carried out in a fixed-bed tubular reactor (i.d. 10 mm) under atmospheric pressure. Monolith catalyst of 1 cm3 was placed at the middle of the reactor tube. The reactor tube was placed vertically inside a tubular furnace, and the reaction temperature was measured by K-type thermocouples. Two thermocouples were positioned at top and bottom end of the monolith. The third was inserted in the furnace just beside the outlet face of the catalyst and placed at the middle of the monolith to control the reaction temperature. Catalytic performance was evaluated without pretreatment. For oxygen elimination experiments, operating conditions were as follows: 50 vol % CH4, 3 vol % O2, and Ar as the balance gas. Reactants were fed to the catalyst bed through mass flow controllers, with a total flow rate of 500 mL/min, corresponding to a GHSV of 30000 h−1. Light-off experiments were performed at temperature range between 200 and 500 °C in a heating or cooling process at rate of 2 °C/min. Oxygen conversion was analyzed as the main indicator for methane oxidation activity. And the concentrations of oxygen in the inlet and outlet gases were analyzed using an online oxygen detector equipped with amperometric electrochemical sensor (JRC1020, JUNFANGLIHUA Technology-research Institute) with detect limit of 0.01%. The oxygen conversion was calculated by

The present study concerns the catalytic behavior of methane oxidation under minimal concentration of oxygen. Regarding the circumstance, Pt−CeO2 catalysts supported on cordierite could be a good candidate. Using a citric acid method, homogeneous Pt−Ce oxides were prepared. The effects of Pt loading, calcination, and pretreatments were investigated. And the catalytic behavior was compared with that of impregnated Pt/CeO2 samples.

2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. The Ce1−xPtxO2(x = 0−0.3) catalysts were prepared by a citric acid method. Stoichiometric amounts of Ce(NO 3 ) 3 ·6H 2 O and H 2 PtCl 6 ·6H 2O were dissolved in deionized water. After stirring for 30 min, citric acid (molar ratio of citric acid/metals = 1.5) was added. The solution was concentrated by heating in oil bath at 80 °C under constant stirring to produce viscous syrup-like materials. It was then dried by heating at 120 °C overnight. The dried powders were calcined at 350 °C for 30 min to promote citrate decomposition, and then they were washed thoroughly and calcined at different temperatures for 4 h. The resulting samples were named as Pt-Cex, where x represents Pt molar ratio or calcination temperature. Pt/CeO2 samples with 1 mol % Pt by conventional impregnation method were also investigated for comparison. CeO2 was prepared by citric acid method as above, and calcined in air for 4 h at 500 to 800 °C, then impregnated with H2PtCl6 and calcined at 500 °C for 2 h. The powders were labeled as Impx, where x represents calcination temperature of CeO2 support. For catalytic reactions, monolith catalysts were made. The oxides were ball-milled in H2O for 4 h to obtain homogeneous slurry. Prior to impregnation, honeycomb cordierite (length = 17 mm, ⌀ = 9 mm) was cut out from a commercial honeycomb with 400 cpsi and pretreated with 10 wt % HNO3 to adjust surface attachment; it was then dipped in the oxides suspension and the monolith was blow-dried. The coating procedure was repeated times to achieve a desire loading of 0.1 g, and the coated monolith was calcined in muffle at 400 °C for 2 h to stabilize the coating. The resulting monoliths were labeled as the same name of oxides. 2.2. Catalyst Characterization. Surface area measurement was performed by multipoint BET method on Quantachrome Autosorb-iQ. Nitrogen sorption isotherms were obtained at 77 K on samples degassed in vacuum at 250 °C for 8 h. XRD data were recorded on a Rigaku D/Max-RB diffractometer with a Cu Kα radiation (λ = 0.154 18 nm). The working voltage was 40 kV and the current was 100 mA. The patterns were collected in the 2θ range between 5° and 90° with a step of 0.02°. The phase composition, average crystallite size of CeO2 and lattice parameter was estimated with Jade 5 using Scherrer equation and Rietveld refinement method. H2-TPR experiments of the synthesized oxides were performed using Quantachrome ChemBET PULSAR. The powders were first purged under He (100 mL/min) at 300 °C for 30 min and then cooled to room temperature. TPR measurement was carried out in a flow of 10% H2/Ar at a rate of 10 °C min−1 up to 900 °C. X-ray photoelectron spectroscopy (XPS, Escalab250, Thermo Corp.) was carried out to investigate the surface compositions and valence of the oxides. Samples of calcined and used were obtained by scraping the coating of monolith surface. The XPS measurements were performed using an X-ray source of Mg Kα (1253.6 eV) with a power of 150 W. Ce 3d, Pt

the expression: XO2 =

COin2 − COout2 COin2

, with CO2 being methane

concentration corresponding to the inlet and outlet. Gas chromatograph equipped with TCD detectors was also applied to identify gas components and compositions of the product gas at intervals of 10 min.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The X-ray powder diffraction patterns of Pt−Ce samples are shown in Figure 1a. All diffraction lines are indexed to typical cubic fluorite structure of CeO2 (PDF 34-0394) without the identification of peaks belong to Pt metal or Pt oxides, indicating the small particle size of Pt or Pt has been penetrated into the lattice of CeO2.28 A gradual increase in the intensity of the lines due to better crystallization of CeO2 could be noticed with increasing calcination temperatures. Using the most intense (111) line of the CeO2 pattern, the crystallite sizes of CeO2 as a function of calcination temperatures are calculated to be 12, 19, 24, and 29 nm, respectively. The crystallite size of CeO2 increased gradually with increasing calcination temperatures, with the drop of surface area from 33 m2/g for Pt-Ce500 to only 5 m2/g for Pt-Ce800. The diffraction patterns of Pt-Ce500 and Imp500 in Figure 1b showed that the particle size of Imp500 is larger (15 to 12 nm), indicating the doping of Pt during synthesis inhibited the crystallization of CeO2. The redox properties for Pt loaded on CeO2 as Pt−Ce and Imp are studied by H2-TPR, and the profiles are displayed in Figure 2. For Pt−Ce samples, about four hydrogen consumption peaks were observed. The major peak at about 2294

DOI: 10.1021/acs.iecr.5b04202 Ind. Eng. Chem. Res. 2016, 55, 2293−2301

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Figure 1. XRD patterns of (a) Pt−Ce calcined oxides, (b) Pt-Ce500 and Imp500.

Figure 2. H2-TPR profiles of (a) Pt−Ce calcined oxides, (b) Imp oxides.

200−300 °C ascribes to the reduction of combination of fully oxidized Pt and surface oxygen of CeO2 in the vicinity of Pt site. Peaks at 400−500 °C ascribed to the active oxygen reduction far away from Pt.24 Peaks at 500−600 °C associated with the removal of bulk oxygen from near surface CeO2. Peaks above 800 °C were attributed to the reduction of bulk CeO2. Pt-Ce500 presents the maximum amount of active oxygen. The intensity of all peaks became much weaker with increase of calcination temperatures. The peaks shifted closer and shrunken to 3. It can be induced that the small surface area of CeO2 supports and the particle growth of CeO2 leads to the decrease of active oxygen. As for Imp samples in Figure 2b, three reduction zones at below 200 °C, 400−500 °C, and above 800 °C were observed. The reduction temperatures were much lower than Pt−Ce samples. For Imp500, the peak intensity below 200 °C was much higher, with a big H2 consumption peak at 200 °C similar to Pt−Ce samples, which was attributed to the interaction between Pt and the larger surface of CeO2. When the CeO2 was calcined at higher temperature, the intensity of peaks below 500 °C weakens significantly, and the peak at 200 °C disappeared. It can be induced that the small surface area of CeO2 supports and the particle growth of CeO2 lead to the decrease of the interaction between Pt and CeO2. The reduction of PtOx of

those impregnation catalysts was much easier, indicating different strength of interaction between Pt and CeO2 through preparation. The difference of hydrogen consumption between two kinds of catalysts may be due to the difference in Pt dispersion and metal−support interaction.7 Pt−Ce samples were supposed to present homogeneous Pt dispersion, whereas in Imp catalysts Pt particles were dispersed heterogeneously and likely to form relatively large particles. It is referred that metallic states cause a significant enhancement of surface active oxygen reduction, which presents lower reduction temperature, and Pt species strongly bound to the support are less reducible.29 Compared with the reduction temperature and peak intensity, the reducibility of Pt species on CeO2 in Imp samples is much higher and in a more oxidized state after calcination. TEM images of samples calcined at 500 to 800 °C were shown in Figure 3. Analysis of the images revealed that the samples contained well-defined nanocrystals of CeO2. Mean crystallite size was calculated, and the results were compared with mean crystallite sizes calculated from XRD. The results are in good agreement between TEM and XRD. Various crystal planes of the crystallites were exposed without exhibiting any specific crystal form. For samples of Pt-Ce500−700, the 2295

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theoretical value. However, for calcined Pt−Ce samples with Pt molar ratio fixed at 0.01, more Pt moved out of CeO2 at higher temperature, causing the enrichment of Pt at surface. As for Imp catalysts, much higher percent of Pt was dispersed at the surface, indicating a heterogeneous dispersion in CeO2 and weaker interaction with support. By the deconvolution of Pt 4f spectra, oxides mainly presented Pt2+. However, Pt0 existed at some samples, and the contents varied with conditions. The existence of Pt0 after oxidation could be ascribed to a shell− core structure with the PtO serving as the shell and metal as the core,7 or the wrapping of Pt by CeO2, thus preventing further oxidation by the compact PtO layer. After doping more Pt, the Pt0 content kept at 0.2, similar to calcination process. However, for Pt-Ce800, the Pt0 ratio decreased, so as the Imp samples. The balance between Pt2+ and Pt0 is considered to be related to the encapsulation of Pt particles by the reducible CeO2. From TEM images, aggregation of Pt emerged at the surface of PtCe800, close to the condition of Imp oxide, so the outer PtO layer is more easily influenced by oxygen. The Pt in CeO2 lattice was stabilized by the strong interaction, anchoring Pt at specific place and maintaining Pt at an appropriate valence state. 3.2. Light-off Activities for Pt−Ce Oxides. During the temperature-programmed experiments, the temperatures were similar around the monolith before light-off. However, once ignition occurred, the temperature at the bottom increased rapidly by the heat generated during reaction, leading the temperature gradient across the bed. But due to the short length of the monolith (1.7 cm) and the thermal conductivity of cordierites, the largest temperature difference between top and bottom of the monolith is around 15 °C. Catalysts with four different loadings of Pt, i.e., 0.005, 0.01, 0.02, and 0.03 mol %, were tested through programmed temperature reaction in methane−oxygen mixtures with fixed feed ratio (CH4/O2 = 50/3). Figure 5 displays the oxygen

Figure 3. TEM images of (a) Pt-Ce500, (b) Pt-Ce600, (c) Pt-Ce700, and (d) Pt-Ce800.

background of CeO2 particles made it difficult to discriminate lattice fringes of PtOx or Pt form, which was indicative of highly dispersion of Pt species on the surface of CeO2 or in CeO2 lattice. In contrast, when samples were thermally treated at 800 °C, uniform particles of PtOx were observed on the CeO2 surface, with particles size range between 2 and 3 nm. Hightemperature calcination in air resulted in segregation of PtOx from CeO2 lattice. XPS spectra of the Pt 4f core level region of Pt-Ce500−800 are shown in Figure 4, and the surface composition was

Figure 4. Pt 4f spectra of Pt-Ce500−800 samples.

analyzed and listed in Table 1. For Pt−Ce samples with different molar contents, a linear increase of Pt on surface was observed with the increased doping of Pt, and was close to Table 1. Surface Compositions of Catalysts Analyzed by XPS Samples

Pt0/Pt

Pt/Ce+Pt

Pt-Ce0.03 Pt-Ce0.02 Pt-Ce0.01 Pt-Ce0.005 Pt-Ce500 Pt-Ce600 Pt-Ce700 Pt-Ce800 Imp500 Imp700

0.295 0.313 0.240 0 0.240 0.357 0.346 0.177 0.153 0.067

0.040 0.021 0.013 0.009 0.013 0.032 0.033 0.028 0.058 0.103

Figure 5. Oxygen conversion as a function of temperature for Pt-Cex with different Pt ratios during ignition (solid lines) and extinction (dotted lines).

conversion curves obtained during the heating and cooling ramps. Catalytic activity had linear relation with Pt loadings, and light-off temperatures increased with decreasing amount of Pt. Measurable rates were observed above 280 °C, and a sharp increase of oxygen conversion occurred, with a narrow temperature gap between ignition and total oxygen conversion. 2296

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Industrial & Engineering Chemistry Research For Pt-Ce0.005, the light-off temperature (T10) is 420.4 °C and total conversion occurs at 442.2 °C for the condition used. Great improvements in activity were observed by increasing Pt loading to 1 mol %, the temperature for complete oxygen consumption dropped to 363.1 °C, which is about 79.1 °C lower than that of Pt-Ce0.005. However, no marginal difference was found by further doping of Pt up to 3 mol %. Significantly steep light-off curves took place between 20% and 80% oxygen conversion regardless of doping extent of noble metal, probably due to a faster depletion of molecular oxygen at light-off temperature, which could help ignite and sustain the reactions by in situ heat generation. The increase in exothermic heat flux would lead to a self-acceleration of the reaction rate toward and beyond the light-off temperature. Same condition was found on LMR catalysts for fuel-rich methane combustion.30 Extinction behavior was studied after ignition process with the same cooling rates. Complete conversion was maintained until the temperature dropped to lower than 250 °C, and the extinction temperature was inversely proportional with Pt loadings. Similar steep conversion curves were found as ignition process. A clear ignition−extinction hysteresis of activity was observed in every case during experiments, with the extinction process occurred at considerably lower temperatures than that in the corresponding ignition process. To evaluate the extent of this phenomenon, ΔT50 values are calculated and compared in Table 2. It can be seen that the extent for the gap of the

100% oxygen conversion, there is lack of oxygen and catalyst was exposed in reducing atmosphere. And metallic state of Pt is supposed to be active for methane adsorption and dissociation. The reducing atmosphere keeps Pt in metallic state to sustain good activity in a long temperature range, until to the temperature when Pt is unable to activate CH4. The catalyst surface state and heat generated by reaction may attribute to the hysteresis loop between heating and cooling. 13,35 Furthermore, by referring to our previous work on PdO− CeO2, a similar hysteresis loop was observed. But temperatures for extinction were much higher (300 to 250 °C).3 So, we consider the main cause of the hysteresis is the change of surface state of catalyst, though not excluding heat effects. Generally, high temperature may cause the deactivation of catalysts owing to the agglomeration of active sites. However, exception exists, especially for catalysts supported by valence variable oxides, which can inhibit the sintering of noble metals via either regeneration or formation of specific structure. PtCe0.01 was chosen as sample to examine thermal aging effect on oxygen elimination performance. Results were shown in Figure 6. Calcination did not inhibit the activity for oxygen

Table 2. T50 and ΔT50 Values of Pt-Cex and Imp Catalysts Pt-Ce0.03 Pt-Ce0.02 Pt-Ce0.01 Pt-Ce0.005 Pt-Ce500 Pt-Ce600 Pt-Ce700 Pt-Ce800 Imp500 Imp600 Imp700 Imp800

T50 Ignition

T50 Extinction

ΔT50

327.8 337.7 350.8 429.3 350.8 348.2 337.3 338.2 386.1 376.0 352.3 341.6

143.9 159.0 177.1 196.1 177.1 152.2 170.6 170.1 194.2 130.0 158.5 159.0

183.9 178.7 173.7 233.2 173.7 196.0 166.7 168.1 191.9 246.0 193.8 182.6

Figure 6. Oxygen conversion as a function of temperature for Pt-Cex with different calcination temperatures during ignition (solid lines) and extinction (dotted lines).

conversion significantly. Regardless of calcination, all the asprepared Pt−CeO2 initiated the reaction at around 280 °C, showed a sharp increase at about 330 °C and eliminated oxygen completely below 360 °C. For comparison, T50 and ΔT50 along heating and cooling are listed in Table 2. During ignition, catalysts calcined at higher temperatures had a T50 of 1 °C lower than Pt-Ce500 and -600, which means thermal treatment has no marginal effect on the performance. The results may indicate that particles with high dispersion would not necessarily increase the catalytic conversion. Unlike heating process, extinction trends were slightly different, Pt-Ce600 showed the lowest temperature for extinction and having the largest ΔT50. Besides, the extinction behaviors were almost the same for the other 3 catalysts. The outstanding result for PtCe600 may be attributed to the appropriate Pt distribution, and interaction with CeO2 supports. For fuel lean methane combustion, the size of the crystallite plays an important role as methane oxidation over Pt is a structure-sensitive reaction.2 However, under current reaction conditions, activity did not change much even though larger

hysteresis is much larger for Pt-Ce0.01 than for the other 3 samples having similar gaps. Such hysteresis behavior is typical for exothermic oxidation reactions, and exothermic reaction heat generated at the ignition point heats the catalyst so that the inlet gas temperature can be decreased below the temperature required for ignition without influencing the reaction rate significantly.31 Another reason for hysteresis is the self-poisoning by reactant oxygen, and the oxidation of Pt can inhibit the adsorption of methane. Previous studies have shown that the oxidation of methane over CeO2 supported platinum is sensitive toward the surface O/Pt ratios, adsorbed oxygen decreases the catalytic activity of the Pt surface.32,33 Calcined catalysts were used directly without any treatment, so Pt was mostly in its oxidation state before reactants were introduced. During heating at temperatures below 250 °C, a less active oxide-like platinum phase can be formed, blocking the active sites by oxygen. At low temperature below light-off, surface active sites are covered by oxygen due to the higher sticking coefficient of O2 than CH4, leaving few sites for CH 4 dissociation.34 However, during the cooling procedure from 2297

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Industrial & Engineering Chemistry Research particle size, lower BET surface and lower dispersion would happened and CeO2 is postulated to lose its redox ability at higher temperature after calcination. Catalysts maintained their stability at a broad temperature range. Methane oxidation is not significantly influenced by the Pt distribution. The better activity for Pt−Ce calcined at higher temperatures is due to higher Pt ratios at the platinum−CeO2 boundary on CeO2 surface and the weaker of the transport of oxygen between oxides and Pt. And the interaction makes the reaction on Pt− Ce sample structure-insensitive, regardless of particle size of noble metal.13 Because calcination had little effect on activity, to investigate further the determining factors, Pt-Ce500 was pretreated at different conditions. Samples of as prepared, reduced by hydrogen at 400 °C, and after the first ignition were compared and the ignition curves are illustrated in Figure 7. Catalytic

Figure 8. Effect of oxygen concentration for oxygen conversion activity on Pt-Ce500 catalyst.

sites available for the hydrocarbon to adsorb, thus a higher temperature is needed for ignition. A relatively oxygen-rich surface seems to suppress the dissociative adsorption of methane, resulting in low methane oxidation activity in the preignition region. With the increase of temperature, the adsorption of O2 on Pt was weaker, leaving more vacancies for the adsorption of methane. For methane activation, metallic Pt was more effective. After ignition, O2 is rapidly consumed by the reaction.38,39 Moreover, the sharp curve of ignition happened at all the oxygen range, indicating that methane dissociation is the limiting procedure. Once methane was dissociated at higher temperature, oxygen can react with fragments immediately. The effect of flow rates on the catalytic activity for Pt-Ce500 catalyst is shown in Figure 9. At the increase of flow rates, oxygen conversion rates decreased below light-off temperature. However, once light-off happened, the conversion curves seemed to overlap, having the same T50 and T100 under flow rates from 300 to 500 mL/min. Unlike most reactions that conversion decrease along with shortened contact time, activity for oxygen conversion maintained at high space velocity.

Figure 7. Effect of pretreatment on Pt-Ce500 catalyst for oxygen conversion activity.

activity was promoted after reducing treatments, either by hydrogen or reactants, to have a lower ignition temperature than fresh catalyst. However, fresh and reduced samples had the same T50, which was 8 °C higher than the second ignition. The reducing treatment at 400 °C mainly affects the surface properties of the catalyst. Therefore, only at the lower temperature did it show better performance than fresh catalysts. When reaction temperature increased gradually to the light-off temperature, the surface was deoxidized again, thus showing little difference with fresh sample. However, after light-off, reactants and products caused some irreversible structure rearrangement in bulk catalyst, and those changes benefit the catalytic activity.28,36 Figure 8 shows the influence of different oxygen concentration at constant methane concentration. Monotonic decrease in oxygen conversion took place with increasing oxygen concentration. When the oxygen concentration was decreased from 5% to 1%, the extent of improvement in activity increased. These data indicated that higher oxygen pressure inhibits the activity for methane oxidation, forming a more oxide-like platinum on surface. Catalytic activity is related to Pt surface coverage and Pt chemical state. Oxygen is adsorbed preferentially on the catalyst surface at low temperature, since the sticking coefficient of O2 is considerably higher than that of CH4.34,37 The adsorption and further Pt oxidation limit the free

Figure 9. Effect of flow rates for oxygen conversion activity on PtCe500 catalyst. 2298

DOI: 10.1021/acs.iecr.5b04202 Ind. Eng. Chem. Res. 2016, 55, 2293−2301

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Industrial & Engineering Chemistry Research Strangely, as methane combustion generates significant amount of heats, more hot spots take place on surface of catalysts at higher velocity, which may boost the elimination of oxygen. By referring the effect of oxygen concentration experiments, we consider the activity is controlled by oxygen concentration below light-off temperature. Oxygen inhibits the dissociation of CH4, the reaction rates is limited by oxygen desorption. After light-off, the cracked methyl consumes oxygen immediately, regardless of the range of flow rates used in experiments. Therefore, the possible explanation is that competitive adsorption between O2 and CH4 limits the reaction below light-off. When reaching light-off temperature, the limiting effect disappeared and the methane oxidation reaction proceeds fast. 3.3. Comparison between Pt−Ce Oxides and Imp Catalysts. To evaluate the interaction between Pt and CeO2 supports, CeO2 were calcined at temperatures as Pt−Ce, and then impregnated by Pt. Pt is considered as heterogeneous distributed on CeO2 compared with Pt−Ce samples. The activity was examined through programed temperature reaction, and the results are shown in Figure 10. The catalysts exhibited a

Figure 11. Effect of reduction for oxygen conversion activity on (a) PtCe500 and Imp500, (b) Pt-Ce700 and Imp700. Figure 10. Oxygen conversion as a function of temperature on impregnated catalysts during ignition.

induce the partial embedding of Pt in CeO2 at the interface between Pt and CeO2, forming the structure similar to Pt−Ce, which could promote the activity.36 Weaker interaction of Pt and CeO2 as in Imp samples made Pt more easily to be poisoned by oxygen. But after prereduction, the activity is improved greatly by the formation of interaction at interface. And the interaction at interface prevents the poison of Pt by oxygen. In Pt−Ce catalysts, Pt is anchored in CeO2 lattice, keeping Pt at a more stable state during pretreatment and reaction. But a more oxide-like Pt was formed after calcination at lower temperature due to the higher oxygen storage of CeO2, inhibiting the reduction of Pt and exposing fewer Pt at surface for methane activation. Higher temperature promotes the segregation of Pt to the surface, the interaction between Pt and CeO2 became weaker, and thus Pt was much easier to be reduced. By comparing the activity of the two kinds of catalysts, it is the interaction of Pt and CeO2 at the interface neither too weak in Imp nor too strong in Pt−Ce, which benefits the activity. And it can prevent further oxidation of Pt by the oxygen spillover between Pt and CeO2 during reaction.

linear increase in activity with the increase in calcination temperature of CeO2. Catalysts with Pt loaded on larger CeO2 particles showed better oxygen conversion. And Imp700 showed a similar activity as Pt-Ce500. Generally, CeO2 shows better oxygen storage capacity and transportation of oxygen ability at larger specific surface and smaller particles. The unexpected trend in the studied condition indicated that the ability of oxygen transportation on CeO2 had negative effects for methane activation, making Pt at more oxide-like state through oxygen spillover. So the strength of interaction between Pt and CeO2 has great impact on the catalytic activity. Imp500 and 700 were selected to investigate the reduction effects on activity as shown in Figure 11. Unlike the minor change for Pt−Ce samples, great improvement took place for Imp catalysts by reduction, with a decrease in ΔT50 of 28.8 °C for Imp500 and 33.2 °C for Imp700 before and after reduction. After reduction, Imp500 showed similar activity as Pt-Ce500, and Imp700 had the best activity. The activity followed the sequence as Imp-red > Pt−Ce-red > Pt−Ce > Imp. By using impregnation method, Pt was mostly dispersed on support surface. The prereduction by hydrogen on Imp catalysts would 2299

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Article

Industrial & Engineering Chemistry Research

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4. CONCLUSION Pt−Ce oxides have been prepared by a citric acid method and wash-coated onto cordierites to study the performance in methane oxidation under scarce oxygen environment. Samples with different Pt loading and calcination temperatures, plus Imp catalysts were used to investigate the correlation between structural, chemical properties of oxides and oxygen conversion activity. Hysteresis loops occurred during temperature programed ignition and extinction experiments, whereas showing better performance at cooling stage due to the change of surface state of Pt. The Pt−Ce catalysts exhibited excellent stability after calcination and pretreatment, which is mainly attributed to the homogeneous distribution of Pt in CeO2 lattice and the anchor of Pt by CeO2 prevents the segregation and agglomeration of Pt. By varying the oxygen concentration, metallic Pt was proposed to be the active site, and can be poisoned by oxygen, thus inhibiting the adsorption of methane. For Imp catalysts which Pt was heterogeneously distributed on support surface, Pt was more prone to be oxidized when supported by CeO2 with smaller particles. But the interaction between Pt and CeO2 induced due to the prereduction by H2 promoted the activity. Therefore, it is the interaction of Pt and CeO2 at the interface, rather than the OSC property, which is responsible for the good activity of methane oxidation under oxygen lean condition.

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AUTHOR INFORMATION

Corresponding Author

*C. Liang. E-mail: [email protected]. Fax: +86 411 84986353. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research is supported by the National Natural Science Foundation of China (21573031 and 21428301) and the Fundamental Research Funds for the Central Universities (DUT15ZD106).

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REFERENCES

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DOI: 10.1021/acs.iecr.5b04202 Ind. Eng. Chem. Res. 2016, 55, 2293−2301

Article

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DOI: 10.1021/acs.iecr.5b04202 Ind. Eng. Chem. Res. 2016, 55, 2293−2301