Defect-Induced Efficient Partial Oxidation of Methane over

May 20, 2015 - Provas Pal†, Rajib Kumar Singha‡, Arka Saha†, Rajaram Bal‡, and Asit Baran Panda†. † Inorganic Materials and Catalysis Divi...
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Defect-Induced Efficient Partial Oxidation of Methane over Nonstoichiometric Ni/CeO2 Nanocrystals Provas Pal,† Rajib Kumar Singha,‡ Arka Saha,† Rajaram Bal,*,‡ and Asit Baran Panda*,† †

Inorganic Materials and Catalysis Division and AcSIR, Central Salt and Marine Chemicals Research Institute (CSIR), G.B. Marg, Bhavnagar 364002, Gujarat, India ‡ Nanocatalysis Area, Refining Technology Division, CSIR-Indian Institute of Petroleum, Dehradun, 248005, India S Supporting Information *

ABSTRACT: We report the development of a highly crystalline Ni/CeO2 catalyst with varying amounts of Ni content using ammonium carbonate complex solution of cerium(IV) at a low temperature. The catalyst was characterized by XRD, XPS, BET-surface area, TEM, and H2-TPR analysis. We have observed that the maximum incorporation of Ni in CeO2 crystal system as the substitution point defect took place up to impregnation of 2.5% Ni, and an almost maximum reduction of unit cell parameter was observed. Further increase in the amount of Ni, the additional Ni may create interstitial point defects with surface defects. The synthesized catalysts showed defect-dependent catalytic activity for low temperature (∼450 °C) methane activation to form synthesis gas. The 7.5 wt % Ni/CeO2 catalyst showed 98% conversion of methane with 73 and 71% selectivity of CO and H2, respectively, at 800 °C without any deactivation until 50 h on time on stream. We also believe that with enhancement of Ni loading, the interstitial point defects and the surface defects due to the formation of the Ce−O−Ni−O− layer, with under-coordinated oxygen atom, on the surface may be the possible reason for the high activity of the catalyst with Ni loading between 5 and 7.5 wt %. With further increase of Ni loading, the Ni nanoparticles were formed with the expense of the Ce−O−Ni−O− layer and in-turn decreased the catalytic activity.

1. INTRODUCTION Methane, the most abundant and predominant component of the natural gas, is forecast to outlast oil by a significant margine.1 Its plentiful abundance in many locations around the globe has driven the researchers to work on the utilization of methane by its activation to other value-added chemicals. Natural gas can be utilized for the purpose of producing fuel.2 The current mean projection of remaining recoverable resources of natural gas is 16200 trillion cubic feet (Tcf), 150 times greater than that of the current annual global gas consumption. Natural gas contains about 85−95% of methane,3 with other impurities such as ethane, propane, nitrogen, sulfur, and carbon dioxide. In recent time, it is uneconomical to use natural gas as a feedstock to synthesize chemicals or fuels because of the costly storage process and transportation system from the remote areas of the globe where it is mostly available.4−6 Hence, researchers have been investigating various methods to enhance the value addition of the methane either by synthesizing more valuable chemicals or more easily transportable fuels.7 However, the reactivity of the desired products are more than that of methane itself, which resulted in low yield and therefore unable to compete with oil.8,9 Synthesis gas can be produced by steam reforming of methane, CO2 reforming of methane, partial oxidation of methane, and decomposition of methanol (mainly used in hydrogen production in fuel cells because methanol is high in energy density and easy to transport). Till date, steam reforming is the only large scale syngas production process.10 Steam reforming © 2015 American Chemical Society

is highly endothermic, and nickel-based catalysts are used in the current industrial processes. However, nickel promotes carbon formation which deactivates the catalyst and leads to reactor plugging. The H2/CO ratio produced in steam reforming is 3, whereas the desired ratio for producing hydrocarbon liquids (in gas-to-liquid process) is lower than that. Therefore, an alternative process can be applied such as partial oxidation of methane where the H2/CO ratio is 2, which is suitable for the downstream processes, particularly for methanol synthesis and the Fischer−Tropsch process. Partial oxidation of methane is likely to become more important in the near future. The work reported in this paper discusses the catalytic partial oxidation of methane to get high yields of synthesis gas at above 450 °C; below this temperature nonequilibrium product distribution is obtained. As per thermodynamic calculations, higher temperature is favorable for partial oxidation of methane and H2, CO selectivity where high pressure is unfavorable for the process and H2, CO selectivity.11 The conventional supported nickel catalyst used for methane reforming are very active for carbon formation that leads to rapid deactivation of the catalyst, whereas the cokeresistance alternative catalysts based on Rh, Ru, Pt, etc. are excellent12−14 but discouraged due to their low availability and high cost. There are also several reports where researchers have Received: February 20, 2015 Revised: May 20, 2015 Published: May 20, 2015 13610

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synthesized catalysts were denoted as xNi/CeO2, where x = 2.5, 5, 7.5, and 10. 2.3. Characterizations. Powder X-ray diffraction patterns of the samples were obtained from a Rigaku MINIFLEX-II (FD 41521) powder diffractometer. The thermogravimetric analysis was performed using a Mettler-Toledo (TGA/SDTA 851e) instrument. The nitrogen sorption measurements were carried out using an ASAP 2020 Micromeritics. To determine the morphology of samples, a scanning electron microscope (SEM; Leo series 1430 VP) and a transmission electronic microscope (TEM; JEOL JEM 2100 microscope) were used. X-ray photoelectron spectroscopy (XPS) was performed using a Multilab-2000 (Thermo-Scientific) spectrometer using a monochromic MgKR X-ray source (1256 eV) with an analyzer pass energy of 10 eV samples were mounted on a SS sample holder with silver paint. H 2-TPR measurements were conducted on Micromeritics Autochem-II Chemisorption analyzer. Details of the procedure were followed as described in our previous report.33 Beside these, Raman spectra were obtained using a Renishaw InVia Reflex micro Raman spectrometer with excitation of argon ion (514 nm) lasers. The laser power was kept sufficiently low to avoid heating of the samples, and spectra were collected with a resolution of 1 cm−1. 2.4. Catalytic Measurements. Catalytic experiments were carried out in a fixed bed down flow quartz reactor (internal diameter = 4 mm). The reactions were carried out with 0.2 g of the catalyst in the form of granules, with the size range of 0.345−1.0 μm. The reactor zone (32 cm long) above the catalyst bed packed with ceramic beads served as the preheater. The reactor was placed in a temperature-controlled furnace with a thermocouple (K-type) placed at the center of the catalyst bed for measuring the reaction temperature. Methane, oxygen, and helium were introduced into the reactor using thermal mass flow controllers. The products were analyzed with an online gas chromatograph (Agilent 7890 GC) with a TCD by using a Porapak Q column. The C-balance as well as material balance was carried out for most of the experiments, and it was found between 98−102%.

tried to reduce the coke deposition during the partial oxidation of methane using different catalysts.15−30 The drawback of the partial oxidation of methane processes reported so far is that although they exhibit sufficiently high conversions of methane for high selectivity of syngas of H2/CO ratio of almost 2, the temperature reported for those results are very high at around 800 °C. To overcome the drawback of high temperatures and carbon deposition, researchers are working on new catalysts using cheaper transition metals like Ni over expensive metals like Pt, Ru, etc. For development of an efficient non-noble metal Ni-based catalyst, selection of support is the most important and prime step. Ceria (CeO2) shows significant reversible oxygen storage capacity (OSC) originating from its inherent Ce3+/Ce4+ red-ox cycle and widely used as active catalytic component or as support.31,32 Its activity is highly dependent to that of its surface area, particle size, and extent of defects (i.e., Ce3+/Ce4+ ratio). Recently, we have observed that CeO2 nanoparticles synthesized by the carbonate intermediate showed enhanced catalytic activity.33,34 In continuation of our previous work,33−36 herein we report the synthesis of Ni supported CeO2 catalyst and the partial oxidation of methane to synthesis gas over the synthesized catalyst which can activate methane at 450 °C and did not deactivate even after 50 h on stream at 800 °C. The catalyst is also stable at a very high reaction temperature.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Ammonium ceric nitrate AR [(NH4)2Ce(NO3)6], nickel nitrate [Ni(NO3)2·6H2O], zirconium oxychloride LR [ZrOCl2·8H2O] and ammonium carbonate were purchased from S. D. Fine Chemical, India. All the chemicals were used without further purification. For all applications, water with a resistivity of 18 MΩ cm was obtained from a Millipore water purifier and used. 2.2. Synthesis of Ceria and Nickel Impregnated Ceria Nanoparticles. Ceria nanoparticles were prepared by reflux condition using the aqueous ammonium carbonate complex solution of cerium as precursor.33 In a typical synthesis for ceria nanoparticles, 1 g ceric ammonium nitrate [(NH4)2Ce(NO3)6] dissolved in 3 mL water was added dropwise to 7 mL of saturated aqueous ammonium carbonate solution (pH 9) under stirring (500 rpm). Immediately after the addition of ceric ammonium nitrate, a white precipitate appeared that redissolved on stirring. During the addition process, the pH of the solution was maintained at 9 by adding solid ammonium carbonate. Finally, it resulted in a straw yellow clear aqueous ammonium carbonate solution of cerium(IV). After that, 10 mL of the ammonium carbonate complex solution of cerium(IV) was added to 100 mL of two-necked round-bottom flask (kept in an oil bath) fitted with a condenser containing 40 mL of water in stirring and continued the heating in reflux conditions with constant stirring for 6 h. The particles were recovered by centrifuge, thoroughly washed and dried at 90 °C/12 h. The Ni-impregnated CeO2 has been synthesized using assynthesized CeO2 by the above mention procedures and nickel nitrate. In a typical procedure, to a 35 mL aqueous solution of nickel nitrate with the desired amount (0, 2.5, 5, 7.5, and 10 wt % with respect to CeO2) of 2 g of calcined CeO2 nanoparticles was added. The resultant slurry was evaporated to dryness with continuous stirring (500 rpm) at 80 °C under atmospheric pressure. The dried powder was calcined at 500 °C for 6 h. The

3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction. All the XRD patterns of the synthesized calcined CeO2 and Ni/CeO2 nanoparticles showed identical well-resolved X-ray diffraction peaks and can be indexed to the face-centered cubic (FCC) fluorite structure of CeO2 (JCPDS 34-0394) with a space group of Fm3m (Figure 1). However, in the XRD patterns of calcined 7.5% Ni/CeO2 and 10% Ni/CeO2 samples, additional low intense peaks at 37.1 and 43.2 can be indexed to (111) and (200) planes of NiO. The crystallite sizes of calcined samples were calculated from X-ray line broadening of all the diffractions using Scherrer’s equation, taking into account the instrumental line broadening and were found in the range of 3−8 nm (Table 1). The lattice parameter (a) of the samples was calculated from the XRD date (Table 1). From Table 1, it is evident that the a of pure CeO2 is higher (0.54197 nm) than that of bulk CeO2 (0.54113 nm). The enhancement of a is due to the grain surface relaxation37−39 and presence of the enhanced amount of Ce3+ with high radius (rCe3+= 0.1143 nm, rCe4+= 0.1110 nm), in agreement with the Vegard’s rule. Whereas, the a of the 2.5% Ni/CeO2 samples are less (0.53979 nm) than that of pure CeO2 and increased gradually with an increase in the Ni content. For 7.5% Ni/CeO2 and 10% Ni/CeO2, the a is even 13611

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the surface, which may form the O−Ni−O−Ce super structure on the surface. In higher Ni-content samples, the excess Ni may create NiO particles on the surface. As a result, the diffraction peaks for NiO in the Ni/CeO2 samples up to 7.5% were not identifiable due to the formation of defects by impregnated Ni or the formed isolated NiO particles are much less. Here, it is essential to mentioned that the significant presence of defects increase the reflection widths in the powder diffraction patterns. However, the crystallite size (as mentioned above) was calculated from the X-ray diffraction line broadening using Scherrer’s equation without considering the contribution from the defect in width broadening, and the actual crystallite size will be higher, to a certain extent, than that calculated. To confirm the formation of defects predicted from XRD results, we performed the Raman spectroscopy, XPS, and H2-TPR analysis. 3.2. Raman Spectroscopic Study. From the discussion based on XRD results, it is evident that there is the presence of some additional defects, predicted as interstitial point defects and surface defects for the formation of Ni−O−Ce super structure, in addition to the substitution defect in the Ni/CeO2 samples. Raman spectroscopy is a useful tool to identify the defects of the nanocrystal. In visible, Raman spectra of the synthesized nanocrystalline pure ceria, the intense band at ∼460 cm−1 can be assigned to F2g Raman active phonon band for the symmetric breathing mode of oxygen atoms around the cerium ions, which is generally observed at 470 cm−1 for undoped bulk CeO2 (Figure 2).44 This shifting of band can be

Figure 1. XRD pattern synthesized CeO2 and Ni/CeO2 samples calcined at 500 °C/6 h.

Table 1. Textural, Structural, and Red-ox Characteristics of the Synthesized CeO2 and Ni/CeO2 Samples Calcined at 500 °C/6 h sample

dXRD (nm)a

a (Å)b

SBET (m2/g)c

H2 (mmol/g)d

CeO2 2.5% Ni/CeO2 5% Ni/CeO2 7.5% Ni/CeO2 10% Ni/CeO2

6.6 4.7 4.9 5.1 8

5.4197 5.3979 5.4053 5.4185 5.4273

167.8 165.7 162.1 145.05 118.5

0.1547 0.2357 0.6872 0.9914 0.3997

a Crystallite size calculated from XRD line broadening. bUnit cell parameters calculated from XRD results. cBET surface area. dAmount H2 utilized during H2-TPR.

higher than that of bulk CeO2. The reduction of the a on incorporation of Ni is due to the substitution of Ce4+ by small nickel ion (rNi2+ = 0.069 nm) in the CeO2 unit cell, which is in agreement with the Vegard’s rule and evidenced the substitution point defect. Thus, a gradual reduction of a is expected with an increase in the Ni content due to the extent of the enhanced substitution defect. Sparingly, the observed result is opposite to that of the expected one. This contradicting result forced one to think in other way (i.e., toward other defects for which the unit cell parameters are generally increased). It is reported that both the interstitial point defects and surface defects originate the enhancement in a of CeO2.40,41 Fabris et al.42 explored defects of Ni/CeO2 through DFT calculation and showed that on shifting of the Ni+ ion to an interstitial position of CeO2, crystal interstitial point defects were generated. Then, one electron charge transfer from the Ni to the Ce site, resulted in transformation from Ce4+ to Ce3+ ion. This effect reflects the increase in the value of a due to higher radius of Ce3+ and incorporation of Ni in interstitial position. Colussi et al.43 showed, experimentally and supported by DFT calculation, that PdO forms a stable Pd−O−Ce super structure on the surface. Ni having the same group of Pd is expected to form a O−Ni−O−Ce super structure on the surface and in turn increase the unit cell parameter. Thus, it can be presumed that the maximum incorporation of nickel as substitution defect (i.e, maximum reduction of a), with nominal interstitial point defects took place on 2.5% nickel impregnation. With a further increase in the amount of impregnated Ni, the additional Ni increased the interstitial point defect. However, the interstitial point defect introduced the strain in the crystal system, and to minimize the strain, the additional nickel was pulled out toward

Figure 2. Visible Raman spectra of synthesized CeO2 and Ni/CeO2 samples calcined at 500 °C/6 h.

explained by the enlargement of Ce−O bond lengths for lattice distortions originating from oxygen vacancy for the presence of a reasonably high amount of Ce3+.45 However, for the Ni/CeO2 samples, the further shifting of the bands toward lower wavenumber (∼444 cm−1) indicates an increase of lattice distortion for nickel incorporation and supports the substitution and interstitial point defects.45 Another phonon D band observed at ∼570 cm−1 is characteristic of oxygen vacancies in the ceria lattice.44 This band for oxygen vacancy was observed in all the samples, including pure CeO2; however, the intensity of the band was increased with an increase in the amount of impregnated nickel (Figure 2). This observation implies that with the gradual increase in the amount of impregnated nickel, the oxygen vacancy was increases gradually. 13612

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The Journal of Physical Chemistry C Other two low intense bands at 225 and 1160 cm−1 only in the Ni/CeO2 samples can be assigned to the rhombohedral NiO. In general, rhombohedral NiO gave a one-phonon band at about 200 cm−1 and 2LO vibrational bands at 1100 cm−1.46 However, in the synthesized Ni-CeO2 samples, it shifted toward a higher wavenumber and implies that the present NiO is not free; there are some interaction/bonding with the CeO2 moiety. This Raman spectroscopic observation established our prediction on the formation of the O−Ni−O−Ce super structure on the surface to some extent as presumed from the XRD result. 3.3. XPS Study. XPS of the pure CeO2 and Ni/CeO2 samples were performed for further clarification, as the XPS is a powerful tool for the study of the surface oxidation state of present atoms and their probable interactions. The deconvoluted (using Gaussian fitting) 3d core level spectra of Ce, in the range of 880−925 eV, can be indexed to the two sets of peaks for both the Ce3+ and Ce4+ ionic states (3d5/3, 3d3/2) and confirm the presence of the mixed oxidation state of cerium (Figure 3). The peaks labeled as v0, v′, u0, and u′ correspond to

The calculated Ce3+ concentration in pure CeO2 was 24.7%. The % of Ce3+ was increased on nickel incorporation, and it was 29.7% for 7.5% Ni-CeO2. The small intense peak at lower binding energy (∼852.4 eV) in the deconvoluted Ni 2p spectra in the range from 850 to 880 eV can be attributed to metallic Ni48,49 and indicate the formation of a nominal amount of metallic nickel during high temperature calcination. Other several intense peaks within the range from 858.2 to 876.9 eV correspond to the components Ni 2p (2p3/2 and 2p1/2) of Ni2+48,49 and indicate that most of nickel present in the Ni/CeO2 samples are in the form of Ni2+ (Figure 3). It is difficult to predict the position of nickel (type of defect) in the Ni/CeO2 samples. The O 1s spectra of the pure CeO2 sample can be deconvoluted in two individual peaks at 529.4 and 532.2 eV, whereas deconvoluted O 1s spectra of pure Ni/CeO2 resulted in three peaks at 530.6, 532.6, and 535.87 eV (Figure 3). Higher intense peaks at a low-energy level in both the sample (530.6 and 529.4 eV) can be attributed to the oxygen atoms in the lattice coordinated to Ce4+. The second peak at higher energy level of both the samples (532.6 and 532.2 eV) can be attributed to the oxygen atoms in the lattice coordinated to Ce3+.33,34,47 However, the third higher energy peak at 535.87 eV in the Ni/CeO2 sample, which is absent in pure CeO2, can be attributed to the oxygen atoms in the more electrondeficient region. This can be endorsed to the exposed highly under-coordinated (highly electron deficient) “O” atom of NiO in the form of the −O−Ni−O−Ce super structure formed on the surface. Similar under-coordinated O atoms of the −O− Pd−O−Ce surface structure in the Pd−CeO2 system was reported by Colussi et al.,43 and they predicted that these highly electron deficient oxygen atoms are likely to play an important role in the catalytic activity. 3.4. TEM and HR-TEM Analysis. The results obtained from the XRD pattern, Raman spectroscopy, XPS analysis, and their corresponding discussions evidenced the presence of substitution and interstitial defect with surface defect origination from the −O−Ni−O−Ce super structure with exposed highly under-coordinated O atoms on the surface in Ni/CeO2 samples. The formed surface defect was further confirmed by TEM and HR-TEM analysis. Figure 4 represents the typical TEM and HR-TEM images of synthesized Ni/CeO2 samples. TEM image of pure CeO2 revealed the presence of very small aggregated particles (Figure S1 of the Supporting Information). In a broad aspect, no such microstructural change was observed after nickel incorporation (Figure 4a). In the HR-TEM images of CeO2 and Ni/CeO2 samples, distinct lattice planes demonstrate their high degree of crystallinity, and the identified inter planar distance of 0.32 and 0.275 nm can be assigned to {111} and {200} planes, respectively, of fluorite cubic structure of CeO2 and supports the XRD results (Figure 4, panels a−d) and Figure S1 of the Supporting Information). The average particle sizes of the synthesized samples varied in the range of 5−8 nm. No such distinct identifiable change in TEM images of the CeO2 and Ni/CeO2 samples were observed. EDS and STEM-XEDS elemental mapping studied confirmed the presence of homogeneously distributed nickel and suggested the homogeneous distribution of Ni throughout the sample (Figure S2 of the Supporting Information). However, in the HR-TEM images of 7.5% Ni/CeO2 samples, in some places inconsistent brightness in fringes of a single particle and generation of a new fringe indicate the presence of the thin layer of some other

Figure 3. Ce 3d, O 1s, and Ni 2p high resolution XPS spectra of the pure CeO2 and 7.5% Ni/CeO2 catalyst.

Ce3+, while peaks labeled as v, v″, v‴, u, u″, and u‴ correspond to Ce4+.33,34,47 The concentration (%) of Ce3+ in the synthesized samples were calculated using the equation, A v + A v ′ + A uo + A u ′ × 100% [Ce3 +]% = o ∑ Av + Au where Ai is the area of the corresponding peaks. 13613

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was identified in these samples, whereas the image of the 10% Ni/CeO2 individual particle with inter planar distance of 0.19 nm, corresponding to the {002} plane of NiO over the CeO2 particle, confirmed the formation of phase-separated NiO particles (Figure 4d, and its inset), which was also supported by the distinct diffraction peaks for NiO in the XRD pattern of the 10% Ni/CeO2 sample. 3.5. Sorption Study. Nitrogen sorption measurements were performed to study the textural properties (i.e., surface area and pore properties of the synthesized samples). Nitrogen sorption isotherm of the synthesized CeO2 and Ni/CeO2 correspond to type IV, and H2 and H3 type hysteresis loop, and indicates that the materials have mesopores (Figure 5). In the entire isotherm, the hysteresis closed at around 0.42 PP0−1 regions, due to the tensile strength failure of the nitrogen meniscus. Except hysteresis of 2.5% Ni/CeO2, there is a certain extension of hysteresis below PP0−1 0.42, which might be attributed to the tensile strength of liquid adsorbent and generally this extraneous effect arises from swelling.50 However, it’s not possible for CeO2, and thus, it is difficult to explain the origin of the extension of hysteresis. A distinct hysteresis in the middle of the isotherm with low slope indicates the first few multilayer adsorption and presence of small mesoporous structure and maximum surface area is contributed from mesopores. A very narrow pore size distribution of ∼3.6 nm, calculated using BJH method from the desorption part of the isotherm, was observed and supports the hysteresis results [i.e., presence of small mesoporous (as mentioned before)]. Surface area of CeO2, 2.5% Ni/CeO2, and 5% Ni/CeO2 was found comparable, in the range of 165 m2 g−1 and decreased gradually with the further increased amount of nickel (Table 1), which is more prominent in 10% Ni/CeO2. Surface area was decreased due to the partial reduction of pore size by formed −O−Ni−O−Ce super structure in the pore surface and phase separated NiO on the pore, evidenced by all the abovediscussed characterization results.

Figure 4. TEM and HR-TEM imaged synthesized (a, b) 5% Ni/CeO2, (c) 7.5% Ni/CeO2, (d) 10% Ni/CeO2, and (c-i, c-ii, and c-iii) image of selected area of “c” as marked showing the presence of fringes corresponing to both the CeO2 and NiO.

materials in the CeO2 surface (Figure 4c). Figure 4c, panels i, ii, and iii, represents the enlarged images of different parts of Figure 4c as marked in the image. In the enlarged images presence of inter planar distance of 0.19 nm assigned to {200} planes of NiO with fringes correspond to CeO2 (planar distance of 0.32) confirmed the formation of the NiO layer on the surface. In the samples low Ni content (>7.5%), the amount of surface layer of NiO is too low/thin to identify by TEM imaging. No individual phase separated particle of Ni species

Figure 5. Nitrogen sorption isotherms and corresponding pore size distributions (inset) of the synthesized (a) CeO2, (b) 2.5% Ni/CeO2, (c) 5% Ni/CeO2, and (d) 7.5% Ni/CeO2. 13614

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ranging between 400 and 800 °C for a period ranging between 1 and 50 h. All the pure as well as Ni/CeO2 catalysts were inactive below 50−400 °C and increased activity with increasing the temperature after 400 °C. Pure CeO2 showed very small conversion of methane and selectivity toward CO and H2, whereas Ni/CeO2 gave better activity with respect to conversion, selectivity, and methane activation temperature. It was also observed that the activity of the catalyst increases when the prereduced catalyst is used in the reaction. This trend only followed up to 2.5% of the Ni/CeO2 catalyst. Surprisingly, conversion and selectivity diminished with increasing the Ni quantity, and activity shows lower than nonreduced Ni/CeO2 catalyst (Figure S3 of the Supporting Information). Thus, we used only nonreduced Ni/CeO2 catalysts for further methane activation studies. Activity of the Ni/CeO2 catalyst increases with increasing Ni loading, and the optimum Ni loading is in between 5−7.5% (Figure S4 of the Supporting Information). The observed conversion of methane was 88, 90, 98, and 98% with 75, 63, 73, and 66% selectivity of CO and 16.5, 74, 59, 71, and 61% selectivity of H2 at 800 °C for 2.5%, 5%, 7.5%, and 10% Ni/CeO2 samples, respectively (Figure 7). 7.5% Ni/CeO2

3.6. H2-TPR Study. The effect of the generated defects with varying extent on Ni impregnation, as identified and discussed above, on the red-ox reaction was identified by H2-TPR. Figure 6 represents the H2-TPR profile in the temperature range of

Figure 6. H2-TPR profile the synthesized CeO2 and Ni/CeO2 samples.

50−650 °C of synthesized CeO2 and Ni/CeO2 samples. In the TPR profile of pure ceria, the only one peak at 477 °C can be assigned to the reduction from Ce4+ to Ce3+ situated at the surface. However, the TPR profile of Ni/CeO2 gave mainly three reduction peaks. In the profile, the initial two peaks at low temperature (150−275 °C) can be assigned to the reduction of active oxygen adsorbed on defect (oxygen vacant) sites originating from Ni2+ incorporation in the crystal structure (substitution and interstitial).51 The third peak of Ni/CeO2 samples can be assigned to reduction of NiO, whose peak area was gradually increased with increased amount of Ni content.52 The third peak intensity of 2.5% Ni/CeO2 is very negligible and indicates the incorporation of most of the Ni in crystal structure of CeO2. The peak ∼325 °C of 5% and 7.5% Ni/CeO2 can be ascribed to the reduction of Ni2+ of the surface NiO present as the −O−Ni−O−Ce superstructure, whereas the peak position of 10% Ni-CeO2 was shifted toward higher temperature (∼350 °C) and can be assigned to the reduction of Ni2+ of phaseseparated NiO particles with the surface −O−Ni−O−Ce superstructure. TPR results also further support the formation of low temperature reducible −O−Ni−O−Ce superstructure on the CeO2 surface. Any high temperature reducible peak in between 400 and 650 °C was not observed in Ni/CeO2 samples. This indicates that due to the incorporation of Ni2+ in the CeO2 fluorite crystal structure conversion of Ce4+ to Ce3+ took place to a large extent and the reduction of remaining Ce4+ or the reduction of incorporated structural Ni2+, which are most stable, are not possible in the preformed temperature range. 3.7. Catalytic Results. After synthesis and successful characterization, we performed partial oxidation of methane to synthesis-gas over synthesized pure and Ni/CeO2 samples. The activity was studied with a gas mixture (comprised of O2:CH4:He mixture with a molar ratio from 1:2:2 to 1:2:7) at gas hourly space velocity (GSHV), ranging between 5000 and 500000 mL g−1 h−1, at atmospheric pressure at a temperature

Figure 7. Effect of Ni loading and temperature on partial oxidation of methane over synthesized pure CeO2 and Ni/CeO2 catalyst at a GSHV of 50000 mL g−1 h−1. The error of all the catalytic measurement is ±2%.

showed 98% conversion at 800 °C, when GHSV was 50000 mL g−1 h−1, and the conversion decreased with an increase in the GHSV. It was observed that with increasing GHSV from 50000 mL g−1 h−1 to 200000 mL g−1 h−1, the conversion was decreased from 98% to 88% (Figure S5 of the Supporting Information). The GHSV is inversely proportional to contact time (g h mL−1). However, the obtained change of conversion on increase in the GHSV, by a factor of 4, is much smaller. To resolve this, similar experiments were performed in lower 13615

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The Journal of Physical Chemistry C temperature (450 °C) [i.e., lower conversion (Figure S6 of the Supporting Information)] and quite a sharp change in conversion was observed. With an increase in the GHSV, the contact time of reactants on the catalyst surface decreased and normally conversion decreases. We also observed this phenomenon over the Ni-CeO2 catalyst. We also observed that at lower GHSV, predominantly CO2 was formed (over oxidation of methane) due to an increase in the contact times of reactants over the catalyst surface. We observed that GHSV 50000 mL g−1 h−1 was the optimum value for this catalyst. It was also observed that the 7.5% Ni/CeO2 catalyst shows 98% conversion of methane with 73% selectivity (Figure S4 of the Supporting Information) at 800 °C without any deactivation until 50 h on time on stream, which is the most strong point of the catalyst (Figure 8). Even at a lower temperature, the 7.5%

content mainly enhances the interstitial and surface (structural) defect (i.e., formation of the −O−Ni−O−Ce super structure with exposed highly under-coordinated O atoms). This undercoordinated O atoms of the −O−Ni−O−Ce super structure is the main controlling factor for the enhanced catalytic activity. Adsorbed methane with Td symmetry having an equivalent C− H bond changed its symmetry to a lower C3v symmetry on interaction with undercoordinated O atoms. In that condition, all C−H bond are nonequivalent and one of them strongly interacts with highly undercoordinated O atoms and facilitates the cleavage of weakened C−H bond (Scheme 1).54 Thus, due Scheme 1. Schematic Representation of the Interaction of CH4 with Pure CeO2 Surface and Ni-CeO2 Surface having Surface Defects with Undercoordinated O Atoms

to the lower affinity of CH4 toward the “Ce center”, the presence of a lower extent of point defect and absence of surface defects, originating from the −O−Ni−O−Ce super structure, pure CeO2 showed limiting catalytic activity in the performed temperature range from 300 to 800 °C. Whereas Ni/CeO2 catalysts showed improved catalytic activity and increased with the increase in the Ni content. This trend was observed until 7.5% Ni/CeO2 and catalytic activity was decreased in a 10% Ni/CeO2 catalyst. In the 10% Ni/CeO2 catalyst, the excess Ni creates NiO particles (clearly observed in XRD and TEM) over the active −O−Ni−O−Ce super structure, which causes less activity due to the cover-up of the active sites of the Ni/CeO2 catalyst. The 7.5% Ni/CeO2 catalyst showed low temperature CH4 activation, as low as 450 °C, most probably due to the presence of this undercoordinated O atoms of the −O−Ni−O−Ce super structure. On the basis of our knowledge, there is no such report in the literature, where a Ni catalyst can activate methane to get such a high activity (Table S3 of the Supporting Information).

Figure 8. Time on stream curve of 7.5% Ni/CeO2 catalyst at 800 °C in a GSHV of 50000 mL g−1 h−1. The error of all the catalytic measurement is ±2%.

Ni/CeO2 catalyst shows almost constant conversion until 50 h (Figure S7 of the Supporting Information). To see the reusability of the catalyst, a fresh experiment was carried out with the spent catalyst without any regeneration. We found that the catalyst shows the same activity as compared to the fresh catalyst (Table S1 of the Supporting Information). The spent catalyst also showed constant conversion and selectivity up to 50 h. Thus, it can be presumed that the synthesized catalysts are highly reusable. The Ni-CeO2 catalysts, synthesized by the developed method, are reproducible. For instance, the 7.5% Ni/ CeO2 catalyst synthesized from three different batches showed almost similar activity (Table S2 of the Supporting Information). The typical carbon balance for most of the experiments is between 98 and 102%. Specifically, the carbon balance for the catalyst 7.5 wt %Ni/CeO2 catalyst was better than 99%. The CHN analysis was carried out for the spent catalyst (after 50 h) and found that the C present is below 0.01 wt %. Difference in catalytic activity of pure CeO2 and Ni/CeO2 is mainly based on two reasons: (i) The affinity of CH4 toward the “Ce” and “Ni” center. Affinity of CH4 toward the “Ni center” is higher than that of the “Ce center”,53 which facilitates the adsorption of CH4 on the surface of the Ni/CeO2 catalyst and in-turn enhances the catalytic activity. (ii) Defect induced enhancement of catalytic activity, which increased with the rise in the amount of Ni in Ni/CeO2 catalysts. Characterization results reveal that with the increase in the Ni content in the CeO2 moiety mainly a substitution defect took place in the initial stage (i.e., up to 2.5% Ni/CeO2). Further increase in Ni

4. CONCLUSION In conclusion, we report the development of a highly crystalline Ni/CeO2 catalyst with varying amounts of Ni content using an ammonium carbonate complex solution of cerium(IV) at low temperature. XRD, XPS, BET-surface area, SEM, and TEM characterization results confirmed that during impregnation of 2.5% nickel, the maximum incorporated Ni creates the substitution point defect with maximum reduction of unit cell parameter. Further increase in the amount of Ni, the additional nickel created both the interstitial point defects and surface defects through the formation of the −O−Ni−O−Ce super structure over CeO2 crystal, and enhancement of the unit cell parameters took place in comparison to the 2.5% Ni/CeO2. The synthesized catalysts showed low-temperature methane activation to form synthesis gas. The enhanced catalytic activity is due to its enhanced high surface area, Ce3+/Ce4+ ratio, and 13616

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defect. The 7.5 wt % Ni/CeO2 catalyst shows 98% conversion of methane with 73 and 71% selectivity of CO and H2, respectively, at 800 °C without any deactivation until 50 h on time on stream. We believe that the formed surface −O−Ni− O−Ce super structure with under-coordinated O atoms is the main controlling factor for the enhanced catalytic activity. The methane adsorbed on the under-coordinated O atoms, with Td symmetry having an equivalent C−H bond, changed to lower C3v symmetry with a nonequivalent C−H bond and facilitates the cleavage of the weakened C−H bond. Thus, 7.5 wt % Ni/ CeO2 catalyst having maximum surface defect showed superior partial methane activation at a lower temperature.

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ASSOCIATED CONTENT

* Supporting Information S

TEM images of pure CeO2 and 2.5% Ni/CeO2, SAED pattern, STEM-XEDS elemental mapping images, and additional catalytic experimental results. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b01724.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] and [email protected]. Tel: + (91)278-2567760, ext 704. Fax: + (91) 278-2567562. *E-mail: [email protected]. Tel: +91-1352525917. Fax: +911352660202. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS CSIR-CSMCRI Communication no. 27/2015. The authors are thankful to the Department of Science and Technology, India (DST, SR/S1/IC-33/2011), for financial support for this work. The authors also acknowledge the Analytical Discipline and Centralized Instrument Facility of CSMCRI for materials characterization. P.P., R.K.S., and A.S. acknowledge CSIR, UGC, and UGC (RGNF), India, respectively, for a research fellowship.

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REFERENCES

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