CeO2 (M= Fe, Ni, and Cu) Catalysts: In

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C: Surfaces, Interfaces, Porous Materials, and Catalysis x

2

Reaction of Methane with MO/CeO (M= Fe, Ni, and Cu) Catalysts: In-Situ Studies with Time-Resolved X-ray Diffraction Feng Zhang, Siyu Yao, Zongyuan Liu, Ramon A Gutierrez, Dimitriy Vovchok, Jiajie Cen, Wenqian Xu, Taejin Kim, Pedro J Ramirez, Sanjaya D. Senanayake, and Jose A. Rodriguez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09319 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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The Journal of Physical Chemistry

Reaction of Methane with MOx/CeO2 (M= Fe, Ni, and Cu) Catalysts: In-situ Studies with Time-resolved X-ray Diffraction

Feng Zhanga, Siyu Yaob, Zongyuan Liub, Ramón A. Gutiérrezc, Dimitriy Vovchokd, Jiajie Cena, Wenqian Xue, Pedro J. Ramírezc, Taejin Kima, Sanjaya D. Senanayakeb,*, and José A. Rodriguez a,b,d,*

a Materials

Science and Chemical Engineering Department, State University of New York at Stony

Brook, New York, 11794, United States b

Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United

States c Facultad

d

de Ciencias, Universidad Central de Venezuela, Caracas 1020-A, Venezuela

Department of Chemistry, State University of New York at Stony Brook, New York 11794,

United States e

X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne,

Illinois 60439, United States

*

Corresponding authors contact: [email protected] ; [email protected]

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Abstract Changes in chemical state and structural transformations occurring in a series of MOx/CeO2 (M= Cu, Ni and Fe) powder catalysts upon reaction with methane were investigated using in-situ time-resolved

X-ray

diffraction

(TR-XRD),

ex-situ

X-ray

absorption

spectroscopy

(XANES/EXAFS) and X-ray photoelectron spectroscopy (XPS). XPS shows the presence of adsorbed CHx and COx species after exposing the powder catalysts to methane at room temperature. Temperature-programed reduction (TPR) measurements point to reaction of the samples with methane and formation of CO, CO2 and H2O gas at temperatures as low as 100 °C. The TR-XRD results show that all the transition metal oxides in the as-prepared catalysts can be reduced to their metallic phase during the CH4-TPR process with the ceria support undergoing significant reduction from surface to bulk, yet the reduction temperature varies for different MOx/CeO2 samples. Among these samples, CuOx/CeO2 shows the lowest reduction temperature (below 260 °C) for both the oxide overlayer and the ceria support. The NiOx/CeO2 and CoOx/CeO2 powder catalysts also activate CH4 at relatively low temperatures (below 350 °C), and the oxide overlayers undergo NiO  Ni and Co3O4  CoO  Co transformations. In the case of FeOx/CeO2, a series of complex changes in chemical state is observed for the oxide overlayer, Fe2O3  Fe3O4  FeO  Fe  Fe3C. The ceria support of all the three MOx/CeO2 (M= Cu, Ni and Fe) samples undergoes severe reduction and form Ce2O3 at high temperature (> 650 °C) during the CH4-TPR reaction. The in-situ TR-XRD results highlight a rich and complex chemistry for methane on these MOx/CeO2 mixed oxide systems.


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Introduction Methane is the main component of natural gas and from a technological viewpoint is attractive as a low-cost source of energy and CHx chemical groups.1 The non-polar character of the molecule and the high strength of its CH bonds (104 kcal/mol) make the activation of CH4 a challenging task.1 In current industrial operations, methane is usually transformed into syngas (CO + H2) at high temperatures (> 600 oC) and in subsequent catalytic processes the syngas is used to synthesize oxygenates or other hydrocarbons.2 This approach is energy intensive and has a high cost.1-2 A direct transformation of methane into valued chemicals, e.g. methanol or other oxygenate, is thermodynamically feasible, but it must be achieved at moderate temperatures to avoid the decomposition of products and competing reactions.2-3 For example, the selective partial reaction of O2 with CH4 can follow two different reaction pathways:2-3 CH4 + ½ O2 → CH3OH

(1)

CH4 + ½ O2 → CO + 2H2

(2)

These two pathways are both exothermic, but at temperatures higher than 300 oC, reaction (2) is more favourable.3 Thus, to avoid the formation of syngas (CO/H2), one must identify catalytic materials which can achieve the activation of methane at the lowest possible temperatures.2-7 The enzyme methane monooxygenase is able to perform the direct conversion of methane to methanol at low temperature with high efficiencies3, 8but it is very difficult to use this biological system in technical or commercial applications. Different types of materials have been tested as potential catalysts for the activation of methane.5-7,

9-20

Metal-exchanged zeolites can activate

methane at moderate temperatures (25-200 oC).5-7, 9 Within the framework of a zeolite, methanol can be synthesized by sequential dosing of O2 and CH4, and at the end the alcohol is flushed out 3 ACS Paragon Plus Environment


with water.5-7 The dynamics of the dissociation of methane on surfaces of pure metals has been studied in detail using several experimental and theoretical methods.17-19In general, metal surfaces interact poorly with methane and only activate the molecule at high temperatures.17-19 In contrast, dissociation of the methane molecule has been observed at a temperature as low as 150 K on IrO2(110).15 On this substrate, cations and O centers work in a cooperative way to dissociate the C-H bond in methane.15 This kind of cooperative interaction has also been seen for the activation of the methane on zeolites,9 O-covered metals,10 and metal/oxide systems.12, 16, 21Previously our group reported that Ni and Co atoms dispersed on a CeO2(111) surface display strong metalsupport interactions and methane undergoes partial (CHx formation) and full decomposition (COx formation) on these systems at room temperature.16, 21 The interaction of CH4 with CoOx/CeO2 powder catalyst has also been investigated and the result shows that both cobalt oxides and ceria can be gradually reduced starting from 270 °C, while the full decomposition of CH4 (CO and H2 production) is evident at a temperature near 500 °C.22 In this article, we shift our attention from Ni and Co/CeO2(111) model catalysts and powder CoOx/CeO2 catalyst to other ceria supported earth abundant transition metal (Cu, Ni and Fe) catalysts which can be used in technical applications. Here, in-situ time-resolved X-ray diffraction (TR-XRD) is used to study the reaction of methane with MOx/CeO2 powders (M = Fe, Ni and Cu). These powders undergo massive chemical and structural transformations when exposed to methane at moderate temperatures (100-400 °C). The oxide of the transition metal is fully reduced and, in the case of iron, upon interaction with carbon, the formation of a Fe3C phase is observed. The strong-metal support interactions seen in the Ni and Co/CeO2(111) model catalysts16,21 also determine the good performance of the MOx/CeO2 powders.


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Experimental Sample synthesis. The 10 wt% FeOx/CeO2, 10 wt% NiOx/CeO2, and 10 wt% CuOx/CeO2 were synthesized by the incipient wetness impregnation method. A commercial cerium oxide (CeO2, HAS 5, manufactured by Rhodia specific surface area ~230 m2/g) was used as the support and it was impregnated by the required amount of aqueous solutions of Fe(NO)3·9H2O or Ni(NO)2·6H2O or Cu(NO)2·3H2O for the synthesis of FeOx/CeO2, NiOx/CeO2 and CuOx/CeO2 powder catalysts, respectively. The mixed slurries were aged at room temperature for 12 h, followed by a 12 h drying at 120 °C and a 6 h calcination at 400 °C. Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES). The ICP-OES analyses were performed at Stony Brook University. Elemental concentrations (Fe, Co, Ni, Cu, and Ce) within the solutions were determined with an iCAP 6300 radial view Inductively Coupled Plasma-Optical Emission Spectrometer. Standards were matrix matched with nitric acid for elemental analysis. The sample solutions were prepared through dissolving the powder samples in HNO3:H2O2 (10:1) solution at 60 °C. Methane temperature programmed reduction (TPR). CH4-TPR experiments were performed on the 10 wt% FeOx/CeO2, NiOx/CeO2 and CuOx/CeO2 catalysts to investigate the methane activation process as a function of temperature. 0.2 grams of each sample were loaded in a quartz flow reactor. The catalysts were first pretreated in a 20 cc/min He at 120 °C for 20 min and then heated from room temperature to 700 °C with a 10 °C/min ramping rate under a 10 cc/min flow of 5% CH4 in He. The weight hourly space velocity (WHSV) was 3000cm3/(h.g). The composition of the eﬄuent gas (CH4, H2, CO, CO2 and H2O) was monitored using a mass spectrometer.



Ex-situ X-ray photoelectron spectroscopy (XPS). The experiments of XPS were carried out in a system that combines an ultrahigh vacuum chamber (UHV) for surface characterization and a microreactor for batch studies.16, 21 Carbon was removed from the sample surfaces by annealing in 10 Torr of O2 at 400 °C for 5 minutes to mimic the last step in the synthesis process (see above). After this treatment, no C or C-containing species was detected in the C 1s XPS region. In the batch microreactor, the cleaned catalysts were exposed to 1 Torr of methane at 25 °C for 5 minutes. Then, the methane gas was removed and, without exposure to air, each sample was transferred to the UHV chamber to record the corresponding C 1s XPS spectra. Ex-situ X-ray absorption spectroscopy (XAS). The ex-situ XAFS (X-ray absorption fine structure spectroscopy) measurements of the 10 wt% FeOx/CeO2, CoOx/CeO2, NiOx/CeO2 and CuOx/CeO2 samples were conducted at 9 BM-C of the Advance Photon Source (APS), Argonne National Laboratory (ANL). The Fe, Ni and Cu K edge spectra were collected in fluorescence mode using a four-channel vortex detector. Fe foil, FeO, Fe3O4, Fe2O3, Ni foil, NiO, Cu foil, Cu2O, and CuO data were also collected in transmission mode as references. The data pretreatment and spectra fitting of the EXAFS (extended X-ray absorption fine structure spectroscopy) spectra were processed using IFEFFIT package.23 Linear combination fitting (LCF) was performed using ATHENA software. In-situ Time-resolved X-ray diffraction (TR-XRD). The TR-XRD measurements of CH4-TPR reaction on 10 wt% FeOx/CeO2, NiOx/CeO2, CuOx/CeO2 were conducted at 17 BM-B (λ = 0.45226 Å) of the Advance Photon Source (APS), Argonne National Laboratory (ANL). Powder samples were loaded into a Clausen cell reactor and heated from room temperature to 700 °C with a 2 °C/min ramping rate to capture the catalysts’ structural transformation during the reaction.24 A 10 cc/min pure CH4 was used for the CH4-TPR experiments. The two-dimensional (2D) XRD images 6 ACS Paragon Plus Environment

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were collected continuously with a Perkin Elmer amorphous Si panel detector through the reaction. These 2D images were subsequently integrated using GSAS-II software, and Rietveld refinement were also performed with the GSAS-II software.25 Results & Discussion Reaction of CH4 with FeOx/CeO2. The ex-situ Fe K-edge XANES spectrum for FeOx/CeO2 is shown in Figure 1a and compared with a set of standards with iron in different oxidation states. Linear combination fitting (LCF) using Fe2O3 and Fe3O4 models were utilized for the FeOx/CeO2 catalysts (see Table S1), which shows that the oxidation state of Fe species in the catalyst is mainly Fe3+. The similar near-edge features of the XANES spectrum with the Fe2O3 standard demonstrates the existence of Fe2O3 crystallites in the as-prepared fresh FeOx/CeO2 sample, which can also be verified by the ex-situ XRD profiles (Figure 1b). The EXAFS fitting results for this sample (Figure S1 and Table S2) show a Fe-O bond length at 1.99 Å. The relatively small coordination number of the Fe-Fe shell to the Fe-O shell indicates that the small Fe2O3 particles are highly dispersed on the ceria support. C 1s XPS spectra collected after exposing CeO2 and FeOx/CeO2 to 1 Torr of methane at room temperature are shown in Figure 2. In the case of plain ceria there was no adsorption of methane. All the features seen in the C 1s were detected before exposing the oxide to the hydrocarbon. On the other hand, in the case of FeOx/CeO2, there are clear features at ~ 284.5 and 290 eV that can be assigned to CHx and COx groups produced by the adsorption of methane on the catalyst surface.16, 21 Part of the adsorbed methane fully decomposed into C atoms that reacted with oxygen sites of the surface to produce COx groups. By the intensity of the C 1s species present on



the catalysts one can estimate that the amount of methane that reacted with the surface was below half a monolayer. This adsorption at room temperature did not produce changes in the catalyst. In CH4-TPR studies we observed consumption of methane at temperatures between 100200 o C and 400-650 o C, see Figure 3. The amount of methane that reacted at 100-200 o C was relatively small. Massive consumption of the molecule was seen above 400 o C. Figure 4a depicts the sample structural change during the CH4-TPR reaction probed by the TR-XRD technique. Several representative XRD patterns which show the iron phase transformation during the reaction were selected and are presented in Figure 4b to identify the evolution of the iron phase during the reaction. From Figure 4a and 4b one can see that when exposing the sample to a CH4 atmosphere and steadily heating it from room temperature to 700 °C, the Fe2O3 phase was gradually reduced to Fe3O4 starting at ~440 °C, and then further reduced to FeO at 560 °C. At 570 °C, part of the freshly-formed FeO phase was reduced to metallic α-Fe which swiftly reacted with methane and formed Fe3C around 590 °C. As the temperature was raised up to 650 °C, additional diffraction peaks at 5.7°, 9.9°, 10.8°, 11.8°,14.3°, 17.0° and 19.5° was observed and denoted by the red dots in Figure 4a and 4b. This set of peaks also appears in the ceria supported NiOx and CuOx samples during CH4-TPR reaction above 650 °C and can be assigned to a cubic Ce2O3 (space group: Ia-3) phase. The amount of the Ce2O3 phase at 700 °C was quantified by Rietveld refinement method using GSAS-II25 software and estimated to be approximately 14 wt%. The ceria lattice parameter changes during the CH4-TPR reaction was also obtained by the Rietveld refinement method using the GSAS-II software25 and is shown as the solid line in Figure 5. Upon comparison with the thermal expansion of the lattice, provided as the dashed line in Figure 5 (the linear thermal expansion coeﬃcient was determined to be ∼1.21 × 10−5 K-1)26, the steeper increase of the ceria lattice parameter indicates a partial reduction of the Ce4+ to Ce3+ in the ceria support. 26-27 The 8 ACS Paragon Plus Environment

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reduction of Ce4+ creates larger Ce3+ cations and oxygen vacancies which repulse against the surrounding cations and expand the ceria lattice abruptly.27 The expansion of the ceria lattice is observed in two temperature regimes shown in Figure 5, with one starting at 300 °C and the other significant expansion appearing near 580 °C, closely following the formation of the metallic α-Fe phase ( ~ 570 °C). These two expansions can be attributed to the partial reduction of the ceria in the near surface region at lower temperature and the deep bulk reduction of ceria at higher temperature, respectively.28-30 It is evident that the rapid reduction of the ceria support ( at ~ 580 °C) can be stimulated by the formation of the Fe metal ad-layers possibly through the hydrogen spill-over process, where the H2 is produced from the direct CH4 dissociation on the freshly-formed metal particles. Additionally, the ceria particle size was also estimated using Rietveld refinement method and it shows that the ceria particle was increased from 5 nm (at room temperature) to 21 nm (at 700 °C) after the CH4-TPR reaction. The gas profile of the CH4-TPR reaction products is provided in Figure 6. FeOx/CeO2 reacted with methane at temperatures above 100 °C with CO, CO2, H2 and H2O evolving into the gas phase. Three CO2 desoprtion regions can be observed. In the first broad CO2 yielding region, between 100 and 300 °C, there is also evolution of CO and H2O species that can be associated with the partial reduction of ceria near the surface by reaction with methane.28-30 The second CO2 peak between 400 and 530 °C can be associated with the reduction of Fe2O3 to Fe3O4, and the third region of CO2 desorption occurs between 530 and 600 °C, that corresponds to the rapid transition of Fe3O4 to FeO and further to metallic α-Fe according to the TR-XRD result. The appearance of the large CO2 peak accompanying the CO/H2 production and CH4 production at around 600 °C could also come from the dissociation of CH4 on the reduced metallic Fe or Fe-Fe3C surfaces and the bulk reduction of the ceria support.28-29, 31 However, as more CH4 is dissociated on the reduced 9 ACS Paragon Plus Environment


surface, more surface carbon from complete CH4 dissociation diffuses into the iron lattice, which results in a decrease of the metallic iron feature, together with the formation of the highly reduced Ce2O3 phase above 650 °C, the continuous CH4 dissociation and CO/H2 production were eventually impeded. This could possibly be the reason for the drop of the CO and H2 levels after their initial increased production.

(a)

(b)

Figure 1. (a) Ex-situ XANES spectra of synthesized FeOx/CeO2 sample and standard references. (b) Ex-situ XRD patterns of the synthesized FeOx/CeO2 sample. Indexed crystal planes of ceria and identified Fe2O3 phase are labeled in black and red, respectively.


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Figure 2. C 1s XPS spectra collected after exposing CeO2 and MOx/CeO2 catalysts (M= Cu, Ni or Fe) to 1 Torr of methane at 25 oC for 5 minutes.



Figure 3. Consumption of methane during CH4-TPR on MOx/CeO2 catalysts (M= Cu, Ni and Fe).


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(a)

(b)



Figure 4. (a) In-situ XRD profile of FeOx/CeO2 sample during CH4-TPR reaction; (b) representative XRD patterns showing iron phase transformations during the reaction; the temperatures listed on the right side correspond to temperatures where a new phase appears.

Figure 5. Rietvelt refinement resulsts for the ceria lattice parameter as a function of temperature in the FeOx/CeO2 sample during CH4-TPR reaction. The dashed line represents the thermal expansion of the plain ceria support.


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Figure 6. Evolution of CO, CO2, H2 and H2O products during CH4-TPR on FeOx/CeO2. Reaction of CH4 with NiOx/CeO2. The Ni K-edge XANES spectra of the as-prepared NiOx/CeO2 sample, in Figure 7a, is displayed together with NiO and Ni foil references. A linear combination fitting showed that the catalyst contained 100% Ni2+. Figure 7b shows the ex-situ XRD pattern of the NiOx/CeO2 sample. Both XANES and XRD profiles identify Ni2+ as the initial state of nickel species in the NiOx/CeO2 catalyst. Based on the EXAFS fitting results (Figure S1 and Table S2), the small NiO particles were the major Ni species supported on the ceria substrate. 15 ACS Paragon Plus Environment


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The C 1s XPS data shown in Figure 2 indicates that a room temperature methane adsorbs on the NiOx/CeO2 catalyst producing CHx and COx groups. In CH4-TPR, see Figure 3, consumption of methane is observed around 100, 310, 450 and 600

o

C. As in the case of

FeOx/CeO2, reaction with methane produced CO, CO2, H2O and H2 as desorption products. The in-situ XRD profiles for the CH4-TPR reaction on NiOx/CeO2 catalyst are shown in Figure 8. A phase transformation from NiO to a metallic Ni (hcp) phase is observed at 340 °C. The consumption feature observed for CH4 on NiOx/CeO2 at 300-350 °C in Figure 3 is probably associated with this NiOx  Ni(hcp) transformation. Further transformation to a Ni (fcc) phase occurred at 360 °C, which remained stable until 700 °C without clear evidence for nickel carbide formation.32 Only a small amount (8 wt%) of the Ce2O3 phase was detected in the NiOx/CeO2 sample near 700 °C and the characteristic peaks of the Ce2O3 phase were denoted by the red dots in Figure 8 and S2. Figure 9 provides the ceria lattice parameter evolution of the NiOx/CeO2 catalyst during the CH4-TPR reaction, and a thermal expansion line was also plotted as a reference. The ceria support started to be gradually reduced from ~260 °C and a sharp increase of the ceria lattice parameter appeared at ~340 °C, correlating with the formation temperature of the metallic Ni phase. The rapid reduction of the ceria support facilitated by the metallic Ni formation proceeds continuously from the surface into the bulk until 700 °C, and the ceria particle was slightly sintered from 5 nm to ~ 14 nm after the TPR reaction.


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(a)

(b)

Figure 7. (a) Ex-situ XANES spectra of the synthesized NiOx/CeO2 sample and standard references (NiO and Ni foil). (b) Ex-situ XRD patterns of the synthesized NiOx/CeO2 sample. Indexed crystal planes of ceria and identified NiO phase are labeled in black and red, respectively.



Figure 8. In-situ XRD profile of the NiOx/CeO2 sample during the CH4-TPR reaction. Red dots in figure represent the Ce2O3 phase.


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Figure 9. Rietvelt refinment of the ceria lattice parameter as a function of temperature in the NiOx/CeO2 sample during the CH4-TPR reaction. The dashed line represents the thermal expansion of the plain ceria support. Reaction of CH4 with CuOx/CeO2. The Cu K-edge XANES spectrum of the as-prepared CuOx/CeO2 sample is compared with those of CuO, Cu2O and Cu foil references in Figure 10a. A linear combination fitting showed that the catalyst contained 100% Cu2+. Indeed, the near edge features of the catalyst were much more similar to CuO, rather than Cu2O and Cu metal (foil). The weak adsorption pre-edge feature near 8980 eV corresponds to the quadruple allowed 1s→3d transition which is typical characteristic of Cu2+ owing to its d9 electronic configuration, while Cu1+ and Cu0 with no electron hole in the 3d orbital do not exhibit similar feature.33-36 The less pronounced 1s→4pz transition at ~8990 eV of the CuOx/CeO2 sample and the discrepancies between CuOx/CeO2 sample and CuO reference in the range of 9010–9030 eV suggests a distorted CuO structure and metal-support interactions in the CuOx/CeO2 sample.36-37 In the XRD pattern (Figure 10b), the CuO phase was not observed, indicating the CuO clusters are well-dispersed on the ceria support or partially incorporated into the ceria lattice, which is below the detection limit of the XRD measurement.36, 38 In the EXAFS fitting of the CuOx/CeO2 sample (Figure S1 and Table S2), there is only a little higher Cu-Cu shell coordination than the Cu-O shell coordination in the spectrum, confirming the fine dispersion of CuO in the CuOx/CeO2 fresh catalyst. Results of XPS showed that CuOx/CeO2 reacts with methane at room temperature but it is less active than FeOx/CeO2 or NiOx/CeO2, see Figure 2. A lower reactivity is also observed in the corresponding CH4-TPR data, Figure 3. In Figure 11, the in-situ XRD profiles of the CuOx/CeO2 sample during CH4-TPR illustrate the reduction of CuO to a metallic Cu phase when the temperature reached 260 °C. Thus, the consumption of methane seen for CuOx/CeO2 at 180-250 19 ACS Paragon Plus Environment


°C in Figure 3 is associated with a CuOx  Cu conversion. The reaction with methane at temperatures above 450 oC leads to the emergence of extra diffraction peaks (marked with the red dots in Figure 11), which are located at the same position as those detected for FeOx/CeO2 and NiOx/CeO2 samples, confirming the formation of the highly reduced cubic Ce2O3 phase (space group: Ia-3) in the ceria support at high temperatures. Approximately 20 wt% of the CeO2 support was reduced to the cubic Ce2O3 phase after CH4-TPR reaction at 700 °C, which is higher than that was detected in the FeOx/CeO2 (14 wt%) and NiOx/CeO2 (8 wt%) samples. The reduction process of the ceria support in the CuOx/CeO2 sample is also evidenced by the ceria lattice expansion shown in Figure 12. Apart from the effect of the thermal expansion (dashed line in Figure 12), the rapid increase of the ceria lattice parameter starting from ~ 250 °C can be ascribed to the partial reduction of the ceria near the surface, while the massive expansion above 650 °C manifests from the deep bulk reduction of the ceria support.28-30 Noticeably, a ceria lattice contraction also exists in the temperature range between 550 and 650 °C and this could be explained by the incorporation of Cu into the Ce site which creates a compressive strain and contracts the ceria lattice. The sintering of the ceria support was also detected in the CuOx/CeO2 sample, as can be seen from the sharp diffraction peaks at high temperatures in Figure 11, and the average particle size of ceria increased from the initial 5 nm to 29 nm after CH4-TPR reaction at 700 °C.


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(a)

(b)

Figure 10. (a) Ex-situ XANES spectra for the synthesized CuOx/CeO2 sample and standard references of CuO, Cu2O and Cu foil. (b) Ex-situ XRD pattern for the synthesized CuOx/CeO2 sample. Only characteristic peaks from the ceria phase are observed, and peaks for the copper phase are absent owing to the high dispersion of small CuO particles on the ceria support.



Figure 11. In-situ XRD profiles for CuOx/CeO2 during CH4-TPR. Red dots in figure indicates the formation of the Ce2O3 phase.


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Figure 12. Rietvelt refinment results for changes in the ceria lattice parameter as a function of temperature in the CuOx/CeO2 sample during CH4-TPR reaction. The dashed line represents the thermal expansion for the plain ceria support. A comparison of the methane interaction with MOx/CeO2 (M= Fe, Co, Ni, Cu) catalysts. As the ICP-OES results in Table S3 show that the loading of the different metals on the ceria support is always close to 10 wt%, and the EXAFS fitting results (Table S2 and Figure S1) indicate that the MOx particles are all small and well-dispersed on the ceria substrate. Thus, it is valid to compare the interaction of methane with different ceria supported inexpensive transition metal oxide (FeOx, CoOx, NiOx and CuOx) catalysts. The MOx ad-layer affects the reducibility of the ceria surface which starts at temperatures above 100 oC with CO, CO2 and H2O as reaction products. The activation of methane in this temperature range is remarkable and occurs at 23 ACS Paragon Plus Environment


temperatures much lower than seen on pure metals.39 In the MOx/CeO2 systems, there is clear evidence for the existence of the strong-metal support interactions which facilitate the interaction of the catalysts with methane. In principle, the ability of the MOx/CeO2 systems to dissociate methane at low temperature (< 200 oC) makes them attractive for conversion of this molecule into value chemicals instead of following a full decomposition path to yield syngas (CO/H2) at high temperatures.3 The XPS spectra in Figure 2 and the CH4-TPR profiles in Figure 3 show that the reactivity of the catalysts towards methane increases following the sequence: CuOx/CeO2 < NiOx/CeO2 < FeOx/CeO2. The relatively low reactivity seen for CuOx/CeO2 is in agreement with previous experimental and theoretical studies reported for model Cu/CeO2(111) surfaces.21The in-situ XRD results for CH4-TPR over FeOx/CeO2, NiOx/CeO2, and CuOx/CeO2 catalysts shown above plus the recently published CH4-TPR result for CoOx/CeO2 catalysts22 indicate that both the MOx ad-layers and the ceria support of the four catalysts are reduced during the CH4-TPR process, but the reduction temperature varies for each catalysts. The corresponding transformation temperatures in the samples are summarized in Table 1. As can be seen from Table 1, the temperature of the M0 formation increases following the order: CuOx < NiOx < CoOx < FeOx, which reflects a decreasing oxophilicity of the Fe, Co, Ni, Cu transition metals.40 The onset temperature of the apparent ceria reduction manifested by the ceria lattice expansion in different catalysts also varies slightly with an order of CuOx/CeO2 < NiOx/CeO2 < CoOx/CeO2 < FeOx/CeO2. The lowest reduction temperature of CuOx/CeO2 under CH4 atmosphere (~ 250 °C) indicates the advantage of CuOx/CeO2 for breaking C-H bonds in CH4 through an oxygen promoted process in a low temperature regime, which could possibly be utilized for low temperature CH4 conversion reactions (e.g. partial oxidation of CH4) if an oxidant reactant gas was available in the reaction


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atmosphere to maintain the proper oxygen level on the catalyst surface.3, 12 However, the methane consumption on the reduced Cu0/CeO2-x sample at high temperature was not as significant as that was seen in other reduced M0/CeO2-x samples, and this supports the reported literature that reports that a metallic Cu surface is ineffective in fully breaking C-H bonds and forming CO/H2.21, 41-42 The significant consumption of methane on the reduced Fe0/CeO2-x and Ni0/CeO2-x catalysts indicates the capacity of metallic Fe and Ni to fully dissociate CH4 and release H2 (reaction 3). CH4  CHx  C + H2

(3)

Meanwhile, part of the deposited carbon will further react with oxygen atoms from the ceria support to form CO/CO2 (reaction 4). C + O  CO/CO2

(4)

This feature of Ni0 and Co0/CeO2-x catalysts has led to their application in the methane dry reforming (DRM) reaction at relatively high temperature (above 500 °C)

22, 43-44.

With a mild

oxidizing agent (CO2) offering oxygen weakly, the deposited carbon created from complete CH4 dissociation could continuously be dissipated and form CO either through the reverse Boudouard reaction (reaction 5) or by oxygen atoms from the ceria support where CO2 could be activated, and oxygen vacancies could be restored.22,

44

The reduced Fe0/CeO2-x catalyst displays a massive

CO/H2 production during CH4-TPR around 600 °C, Figure 6, showing that the metallic α-Fe is highly efficient in dissociating methane. However, the created carbon atoms on the metal surface will quickly dissolve into the Fe lattice and form iron carbides (reaction 6) and the loss of the metallic iron phase eventually suppresses the continuous reaction of CH4 with the Fe metal surface. C + CO2  CO

(5)

C + Fe  Fe3C

(6)

Table 1. Reduction temperatures of MOx/CeO2 (M= Fe, Co, Ni, Cu) under CH4.



Metal phase transformations

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FeOx/CeO2

CoOx/CeO2

NiOx/CeO2

CuOx/CeO2

440 °C

-

-

-

M3+/2+ → M2+

560 °C

270 °C

-

-

M2+

570 °C

350 °C

340 °C

260 °C

300 °C

280 °C

260 °C

250 °C

M3+

→ M3+/2+

→ M0

Ceria lattice expansion onset temperature

Conclusion The reaction of methane with ceria supported transition metal oxides MOx (M = Fe, Ni, Cu) was studied as a function of temperature (25-700 °C). Time-resolved XRD measurements pointed to clear phase transformations in all the MOx/CeO2 catalysts with distinctly different behavior. The transition metal oxides and ceria support were reduced with the reaction temperature increasing in the order: CuOx/CeO2 < NiOx/CeO2 < CoOx/CeO2 < FeOx/CeO2. The production of CO/CO2 during the CH4-TPR reaction indicated complete CH4 decomposition, followed by the oxidation on these powder catalysts at a temperature as low as 100 °C. For the NiOx/CeO2 and CuOx/CeO2 powder catalysts, the oxide overlayers underwent NiO  Ni and CuO  Cu transformations with the CeO2 being partially reduced and partially transformed to cubic Ce2O3 (Ia-3) phase when the temperature is above 650 °C. In the case of FeOx/CeO2, a series of complex changes in chemical state was observed for the oxide overlayer, Fe2O3  Fe3O4  FeO  Fe  Fe3C, and for the ceria substrate, CeO2  CeO2-x and Ce2O3 was observed. The in-situ TR-XRD results highlight a rich and complex chemistry for methane on these MOx/CeO2 mixed oxide systems. Associated Content


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Supporting Information Linear combination fitting results of XANES spectrum for FeOx/CeO2 sample; EXAFS fitting results of MOx/CeO2 (M = Fe, Co, Ni, and Cu) catalysts; XRD pattern of 10 wt% NiOx/CeO2 catalysts at 700 °C during CH4-TPR reaction and ICP-OES results of metal phase composition in the prepared MOx/CeO2 samples. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements The work carried out at Brookhaven National Laboratory was supported by the US Department of Energy under contract no. DE-SC0012704. S.D.S. is supported by a US Department of Energy Early Career Award. This research used resources of the Advanced Photon Source (Beamlines 17BM (XRD) and 9BM-C (XANES/EXAFS), at Argonne National Laboratory, which is a DOE Office of Science User Facility under contract no. DE-AC02-06CH11357. Competing financial interests: The authors declare no competing financial interests.



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TOC Graphic


CeO2 (M= Fe, Ni, and Cu) Catalysts: In

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