Facile synthesis of Ruthenium decorated Zr0.5Ce0.5O2 nanorods for

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Facile synthesis of Ruthenium decorated Zr0.5Ce0.5O2 nanorods for catalytic partial oxidation of methane Subhasis Das, Rishi Gupta, Ashok Kumar, Mumtaj Shah, Manideepa Sengupta, sahil Bhandari, and Ankur Bordoloi ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00567 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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ACS Applied Nano Materials

Facile

Synthesis

of

Ruthenium

Decorated

Zr0.5Ce0.5O2 Nanorods for Catalytic Partial Oxidation of Methane

Subhasis Das, Rishi Gupta, Ashok Kumar, Mumtaj Shah, Manideepa Sengupta, Sahil Bhandari, and Ankur Bordoloi* Nanocatalysis area, Refinery Technology Division, CSIR- Indian Institute of Petroleum, Dehradun-248005, Uttarakhand, India. *Corresponding author: [email protected]; Tel: (+91) -135-2525898

ACS Paragon Plus Environment

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Abstract:

Nanosized Ru (1 wt.%) supported over CeO2-ZrO2 solid solution nanorods have been prepared via a one step hydrothermal approach and verified its activity in partial oxidation of methane at 800 oC with CH4: O2 = 2:1 for 200 h in a downflow, fixed-bed, microreactor. The so prepared catalysts have been characterized with a variety of analytical tools. Characterization results confirm higher dispersion of nanosized Ru over CeO2-ZrO2 solid solution possess strong metal support interaction and higher oxygen storage capacity of the CeO2-ZrO2 solid solution in comparison to only ceria or zirconia. We do not observe any deactivation during 200 h TOS study with 1% Ru supported over CeO2-ZrO2 solid solution, while a significant activity loss owing to coke deposition, metal oxidation observes with only ceria or zirconia supported catalysts.

Keywords: Partial Oxidation, Oxygen Storage Capacity, Metal support interaction, CeO2-ZrO2 solid solutions, Activation energy.

1. Introduction: Increasing methane reserves explores its use in various energy related applications. Methane reforming for the production of synthesis gas with different oxidants get researchers attention1-7. Among the diverse methane reforming processes, Low temperature methane activation via catalytic partial oxidation of methane (CPOX) is one of a most attractive process for the high techno-economic opportunity8-10, operated at high temperature (greater than 1200 ℃) and high pressure (higher than 40 Bar)11. As compared to other reforming techniques, CPOX is mildly exothermic and thermodynamically favorable, if such process can be operated in low temperature with low pressure or no pressure by using appropriate catalyst system without


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compromising with the product yield and syngas ratio (CO: H2 = 1: 2)9, it will be widely acceptable for monetization of standard gases (shale gas, coalbed methane, biogas) to H2, methanol, F-T products, ammonia/urea and petrochemicals in refineries12-14. Although, it is a highly essential process, however, due to lack of appropriate catalyst system and technoeconomic challenges, there is neither a suitable catalyst nor a process available in the market. In the long history for the search of a proper catalyst for CPOX, there are three front runners in this race, supported Ni, Co, Fe based catalysts, supported noble metal based catalysts and metal carbide catalysts15. Nickel based catalysts gained a substantial interest among researchers due to low cost. However, it fails to meet the expectations due to high coke formation and sintering. Although, Transition metal carbides potentially show higher activity like noble metals still they suffer from catalyst deactivation via oxidation. Noble metals are widely used owing to their high activity with long term stability15-20. Ru is the least expensive among the noble metals, appears to be stable and very active and selective in methane activation with strong resistance towards carbon deposition21-24. Coke deposition over the catalyst surface reduce the active metal sites and cause an increasing pressure at the feed sites by blocking the surface pores in CPOX25-26. Suitable support also be essential with the active species to design an efficient catalyst system. In this regard, higher oxygen storage capacity (OSC) of ceria makes it appropriate support in different energy related applications27-32. However, degradation of OSC occurs at a higher temperature. Introduction of zirconia into the ceria matrix can limit the degradation of OSC by reducing the loss of crystal defects at higher temperature33-34. Therefore, Ru supported ceria zirconia supposed to be good candidate towards CPOX. Herein, we report, 1 wt.% Ru-CeO2-ZrO2 (Ru/CeZr) with 1 wt.% Ru-CeO2 (Ru/Ce) and 1 wt.% Ru-ZrO2 (Ru/Zr) solid solution nanorods prepared via a simple hydrothermal approach using


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CTAB and hydrazine as structure directing agent. The material has been found to be brilliantly active for methane activation under low temperature and ambient pressure. A detailed kinetic experiment has also been performed in search of the activation energy of the feed over these catalyst systems. The reported catalyst system may able to overcome the techno-economical barrier related to CPOX process.

Scheme 1: Schematic diagram of Ru fabrication over Ce0.5Zr0.5O2.


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2. Experimental: 2.1.Catalysts synthesis: Ruthenium (III) chloride hydrate, Cerium (III) chloride heptahydrate, Zirconium (IV) chloride, Hexadecyltrimethylammonium bromide (CTAB), Hydrazine monohydrate (NH2NH2·H2O) used in this study were purchased from Sigma-Aldrich Co. Double-distilled water, and high-purity (> 99%) ethanol (Merck) were used in this work. Methane, Argon, and Oxygen gases were obtained from Sigma Gas services. The purity of all gases was higher than 99.9 %. Ru nanoparticles with a size of ∼5 nm supported on the CeO2-ZrO2 solid solution nanorods were synthesized using, RuCl3·xH2O, CeCl3·7H2O, ZrCl4, CTAB, and NH2NH2·H2O, maintaining a molar ratio of Ru: CTAB: NH2NH2: H2O = 0.009: 0.55: 0.99: 278. An aqueous solution of 0.02 g of RuCl3·xH2O added dropwise to an aqueous solution of 2 g CTAB at 40℃. After 2 h, of rotation, ceria and zirconia salts were added slowly in this solution maintaining a ratio of Ce: Zr = x: (1-x), where x = 1, 0.5 and 0. Subsequently, gradual addition of NH2NH2·H2O made the pH of the solutions 8, and the so obtained homogeneous solution was subjected to hydrothermal treatment at 180 °C for 60 h in a Teflon-lined autoclave vessel under autogenous pressure. The autoclave was then cooled until it reached room temperature. Then the mixture was filtered and washed several times with ethanol and dried at 100 °C, followed by calcination at 750 °C for 6 h under an air atmosphere (ramping rate 1 °C/min).

The so prepared catalysts have been

designated as Ru/Ce (1% Ru over ceria), Ru/Zr (1% Ru over zirconia) and Ru/CeZr (1% Ru over ceria-zirconia). A probable route of Ru fabrication over Ce0.5Zr0.5O2 has been shown in Scheme 1. A detailed procedure including catalyst characterization techniques and catalyst activity test procedure has been given in the supporting information.


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3. Result and discussion: 3.1. Analysis of fresh catalysts: The different phases present in the fresh calcined catalysts have been confirmed from X-Ray Diffraction analysis (Fig. 1a). Presence of a predominant tetragonal primitive RuO2 phase (ICDD card no: 88-0322) has been observed over the catalyst surfaces in XRD analysis. With the RuO2 phase, a cubic face centered ceria phase has been also prominent in the Ru/Ce catalysts (ICDD card no: 81-0792) and a monoclinic primitive zirconia phase (ICDD card no: 83-0944) in Ru/Zr catalyst. However, in Ru/CeZr with RuO2 phase, the additional peaks belongs to ceria, but its crystal structure is changed due to the incorporation of Zr, indicating the formation of a primitive tetragonal phase of Zr0.5Ce0.5O2 solid solution (ICDD card no: 38-1436). The presence of different metallic phase has been further confirmed by X-ray photoelectron spectroscopy analysis of fresh samples (Fig 1b and 1c). The deconvulated XPS peaks of Ru 3d5/2 at 280.6 eV and Ru 3d3/2 at 284.7 eV with the satellite peaks at 282.8 and 286.8 eV in all the fresh catalysts indicates the presence of Ru (IV) in the fresh samples 35(Fig 1b). In the fresh catalysts, the bands at cerium region leveled “u” and “v” collectively represent Ce 3d5/2 and Ce 3d3/2 ionization (Fig. 1c) which confirms the existence of Ce(IV) with Ce(III) in the Ru/Ce as well as the bands “x” and “z” at Zirconium region designate Zr 3d5/2 and Zr 3d3/2 ionization, suggests the presence of Zr (IV) in the fresh Ru/Zr catalyst. The incorporation of ceria into zirconia matrix shifts ceria and zirconia photoelectron bands toward higher binding energy, which indicates the formation of mixed metal oxide Zr0.5Ce0.5O236 (Fig 1c). The surface Composition has been also confirmed from XPS analysis shown in Table S1 (supporting info).


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Surface areas of the fresh catalysts with the corresponding supports are shown in Table S1. Due to the low concentration of metal (1% Ru) the physisorption parameters remain unaltered with respect to the support. Hence, the physisorption behavior of the catalysts predominantly governed by the support properties.

Fig. 1. a) X-ray diffraction pattern of the fresh catalysts b) X-ray photoelectron spectra of ruthenium c) X-ray photoelectron spectra of Ceria and Zirconia d) TPR pattern of the fresh catalysts 1) Ce02 2) ZrO2 3) Zr0.5Ce0.5O2 4) Ru/Ce 5) Ru/Zr 6) Ru/CeZr.

Nature of the reduction pattern has been confirmed from H2-Temperature Programmed Reduction data, shown in Fig. 1d. Presence of a reduction peak at 190 ℃ corresponding to the reduction of RuO2 to metallic Ru in Ru/Zr catalysts. This peak slightly shifted to higher temperature 195 ℃ in the Ru/Ce catalysts, owing to oxygen storage capacity of ceria, which renders metal reduction. The incorporation of zirconia into ceria matrix further shift the RuO2


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reduction peak to 205 ℃, suggesting incorporation of zirconia into ceria matrix improves the oxygen storage capacity (OSC) with higher metal support interaction, that shifts the reduction peak to higher temperature region. The broad peak at 750 ℃ in Ru/Ce, Ru/CeZr, and ceria support is due to the reduction of bulk phase ceria37-38.

Fig. 2. a) FESEM image of Ru/CeZr b) TEM image of Ru/CeZr c) SAED Pattern of Ru/CeZr and d) elemental mapping of Ru/CeZr

A rod like morphology of CeO2-ZrO2 solid solutions having diameter ~45 nm is visible in the Scanning electron microscope (SEM) and Transmission electron microscopy (TEM) image (Fig. 2a and 2b) upon which Ru nanoparticles (~5 nm) are well distributed. The presence of


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Zr0.5Ce0.5O2 solid solutions and RuO2 phases in the fresh Ru/CeZr catalysts has been further confirmed by Selected area electron diffraction (SAED) pattern with lattice fringes calculation (Fig. 2c). The homogeneous distribution of the metal nanoparticles over the Zr0.5Ce0.5O2 surface has been confirmed from the elemental mapping analysis (Fig. 2d). The oxygen storage capacity of the prepared supports, i.e., ceria, zirconia, and ceria-zirconia solid solution has been confirmed from the O2-TPD analysis (supporting info Table S2). The high oxygen storage capacity of the ceria-zirconia solid solution is supposed to be the critical factor towards its higher activity and long term stability.

Fig. 3: Thermogravimetric analysis of the fresh catalysis a) Ru/Ce b)Ru/Zr and c) Ru/CeZr.

Thermogravimetric analysis (TG and DTG) have been performed to check the stability of prepared catalysts. Only a little mass loss (~0.5-2 %), due to physisorbed water observed with the fresh catalysts (Fig. 3). Above 200 ℃ no mass loss observed in TG and DTG experimental


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curve confirms no phase changes happen over these catalysts beyond 200 ℃, and they are necessarily stable up to 900 ℃.

3.2. Activity study: Direct route:

CH4 + 12 O2 = CO + 2H2

∆H = -36 kJ mol-1

Indirect route:

CH4 + O2 = CO2 + 2H2O

∆H = -803 kJ mol-1

CH4 + CO2 = 2CO + 2H2

∆H = 247 kJ mol-1

CH4 + H2O =CO + 3H2

∆H = 206 kJ mol-1

Catalytic CPOX follows two pathways, direct route, and an indirect route39. It is tough to distinguish the actual route followed by a catalyst, as CO2 and H2O also form from direct oxidation of CO and H2. Previous reports conclude small metal particles with strong metal support interaction favors direct route of CPOX, while large metal particles in the catalyst follow indirect route39-40. Even though in indirect route, steam and dry reforming are endothermic but the first step, i.e., complete oxidation of methane is strongly exothermic that initiate indirect route at low temperature region over the mildly exothermic direct route that favors only at higher temperature region41-42. In addition, partial oxidation strongly favored over metallic species while complete oxidation favors over metal oxides43-44. The direct route of CPOX is only suitable towards long-term catalysts stability with constant activity, over the indirect route, as a large amount of heat evolved in the indirect route, which enhances the rate of metal sintering and causes catalyst deactivation. The performance of Ru supported catalysts has been verified regarding gas hourly space velocity (5000 to 100000 mL h-1 g-1) at constant temperature 800 ℃, as well as regarding temperature (300 to 900 ℃) at constant GHSV 50,000 mL h-1 g-1,and the corresponding feed conversion and product selectivity has been given in Fig 4a and 4b. Over Ru/CeZr solid solution nanorods, the


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activation of methane gets started only at 300 ℃ (Fig. 4c). Methane conversion increases gradually with increase in reaction temperature, and at 800 ℃ (Fig. 4d), methane conversion is 99.8 % with no detectable amount of oxygen present in the product stream. At low temperature, syngas ratio is generally below the optimum value 2, while it increases with the rise in temperature. In the product stream at 800 ℃, the H2/CO ratio in syngas has been observed 1.98. At low temperature, carbon deposition due to Boudouard reaction predominates over Ru/Ce and Ru/Zr catalysts, which decreases methane conversion rate with time. However, in case of Ru/CeZr catalysts in 10 h TOS study, we do not observe any variation in methane conversion with time (Fig. 4c). With the increase in feed flow, the conversion tends to decline for all the catalysts45-46(Fig .4b). The deviation in the conversion of feeds and selectivity of products over different catalysts mainly arise due to different metal support interaction. At lower GHSV, complete oxidation has been observed instead of partial oxidation with a significant amount of water, up to 10,000 GHSV yields a syngas with the low H2/CO ratio in the product. However, with increasing GHSV, H2 and CO dominants in the product and at above 20,000 we do not find any vapor or CO2 in the product stream giving syngas with H2/CO = 2.


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Fig. 4. Variation of feed conversion and product selectivity with a) temperature, b) with feed flow rate in terms of gas hourly space velocity (GHSV) over Ru/CeZr, c) activity study at 300 ℃ and d) at 800 ℃ with a GHSV of 50000 mL h-1 g-1 over the different catalysts.

Now from the thermodynamic and economic point of view, i.e., higher activity with higher selectivity towards desired syngas with a more extended lifetime, we select 800 ℃ as an optimum temperature with a GHSV of 50000 mL h-1 g-1 as an optimal flow rate keeping CH4: O2 ratio in feed gas 2. Under these conditions, activity, and stability of the Ru decorated fresh catalysts have been tested in a 200 h activity study (Fig 4d). The activity of the Ru/CeZr catalysts remains nearly constant throughout the observed duration with high methane conversion 99.6% and the H2/CO ratio of syngas ~1.985. In Ru/Ce and Ru/Zr catalyst, the initial methane conversion was 78% and 74% with syngas ratio 1.75 and 1.95 respectively. However, with increasing time, their activity decreases along with an increase in H2/CO ratio. After the prolonged 200 h activity study, an 8 % and 6% deactivation occur in Ru/Zr and Ru/Ce (calculated from Eq. 9, supporting info), while a 0.01 % deactivation observed with Ru/CeZr. Homogeneous distribution of ruthenium over the Zr0.5Ce0.5O2, strong metal support interaction and high oxygen storage capacity of Zr0.5Ce0.5O2 are the critical factor for high activity and


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superior stability of Ru/CeZr solid solution nanorods. CPOX being an exothermic reaction, the Weisz-Prater Criterion and the Mears Criterion calculation indicated that there was no internal and external diffusion over prepared catalyst during catalysis (supporting info). 3.3. Kinetic study of the prepared catalysts: The kinetic response of methane conversion rates to CH4 and O2 pressures has been measured over the prepared reduced catalyst in a fixed bed reactor by varying inlet gas concentrations. Without a catalyst, no reaction products were observed. As the temperature increases, the molecules move faster and therefore collide more frequently. The molecules also carry more kinetic energy. Thus, the proportion of collisions that can overcome the activation energy increases with temperature. Therefore, at higher temperature calculation of the minimum energy required activating methane molecule over these catalysts (Ru/Ce, Ru/Zr, Ru/CeZr) are difficult, due to higher conversion rate. Hence, we choose the temperature at which conversion is ≥10%. Measured CH4 conversion rates (-rCH4) are proportional to PCH4 and PO2½, where PCH4 is the partial pressure of methane and PO2½ is the partial pressure of oxygen. CH4 + 12 O2 = CO + 2H2

∆H = -36 kJ mol-1

The experimental reaction rate with respect to methane was determined using the continuity equation (Eq. 1) for the reference component in a differential plug flow tubular reactor. -ri=

(/ )

………… (1)

Where ri is the reaction rate with respect to component i, Xi is the conversion of the respective reactant, Fi is the flow of the respective component, and Wcat is the weight of the catalyst.


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Further, the ri is also a function of partial pressure of the components, and for the present case, these can be expressed through Eq. 2 47. RCH4 =k1 (PCH4) a1 (PO2) b1

…..(2)

Where k1, a1, b1 are the parameters to be determined. In this approach, the reaction kinetics of CPOX over Ru/CeZr, Ru/Ce, and Ru/Zr catalyst have been examined by changing the flow of methane and oxygen in terms of partial pressure. Reaction rates at different flow rates were determined by using Eq. 1 48. Kinetic parameters are obtained by using Polymath 5.1 software and minimizing the sum of squarer of errors. The reaction rates of the conversion of methane with a change in partial pressure are shown in Fig. 5, in the form of RCH4 =f(PCH4) and RCH4 =f(PO2). The rate Eq governed the methane conversion (Table.1), and the reaction orders with respect to methane were observed in the range of 0.6 to 0.9 over the tested catalysts.


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Fig. 5: Variation of the reaction rates of methane RCH4, as a function of the partial pressures of methane PCH4 and oxygen PO2, at the indicated temperatures over the Ru/Ce catalyst (a,b), over Ru/Zr catalyst (c,d) and over Ru/CeZr catalyst (e,f). The points are experimental data of the catalysts. The lines are the fitting using Eq. (2).

The Arrhenius model was applied to estimate the apparent activation energy (Ea) for the overall reforming reaction through Eq. 3. ln k = -Ea/RT + ln A ………………. (3) Where k is the apparent rate constant, and R is the gas constant (8.314 J K-1 mol-1). The values of Ea for CPOX over Ru/CeZr, Ru/Ce, and Ru/CeZr catalyst were predicted by linear regression of


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ln k against 1/T, as shown in Fig. 6. The Ea values of methane are 17.34, 20.89, 23.8 kcal/mole for Ru/CeZr, Ru/Ce, and Ru/Zr. The lower value of activation energy of Ru/CeZr corresponds to the high dispersion of ruthenium nanoparticle over the higher OSC of the ceria zirconia solid solution. Table 1: The parameters k1–a1-b1 used for the fitting of Eq. (2) to the experimental points in Fig. 5, as well as the corresponding apparent activation energies obtained from k1 values, for the catalysts using the Arrhenius model Eq. (3).

Catalysts Temperature RCH4 = k1(PCH4 )a1(PCO2 )b1 (℃) k1 a1 b1 Ru/Ce 300 2.1 0.61 0.09 310 2.5 0.69 0.08 320 3.0 0.74 0.08 330 5.5 0.76 0.09 Ea of CH4 =20.89 Kcal/mol Ru/Zr 300 2.1 0.03 0.74 310 3.0 0.06 0.79 320 3.8 0.07 0.85 330 6.2 0.09 0.91 Ea of CH4 = 23.80 Kcal/mol Ru/CeZr 300 2.0 0.84 0.01 310 2.6 0.85 0.01 320 2.9 0.88 0.04 330 4.5 0.98 0.06 Ea of CH4 = 17.34 Kcal/mol

3.4.Analysis of the spent catalysts: In CPOX, coke deposition, metal oxidation and metal sintering are the crucial factors for catalyst deactivation9. To support the high stability of the studied catalyst system, the spent catalyst after 200 h of TOS, has been thoroughly examined by using various characterization techniques. The surface area of Ru/CeZr catalyst remains unchanged after 200 h long TOS study which indicates no change in the surface morphology and no coke deposition occurred during the reaction. While


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both of Ru/Ce and Ru/Zr catalysts showed a decrease in surface area after the reaction (Table S1), which is further

Fig. 6: Arrhenius plots ln k = f (1/T) for the estimation of the apparent activation energy of methane over the tested catalyst a) Ru/Ce b) Ru/Zr c) Ru/CeZr.

confirmed by XRD analysis of spent catalyst. Partial oxidation of methane is a redox reaction where oxygen has oxidized metal to metal oxide, and subsequently, the metal oxide also reduced by surface hydrogen, which has been evidenced from the occurrence of both metallic ruthenium, and ruthenium oxide in all the three spent catalysts (Fig. 7a). Lattice fringe calculation of the HRTEM image also reinforces this observation (Fig 7b). The presence of both Ru and RuO2 phase visible in all the Ru/CeZr sample (Fig. 7b) In Ru/CeZr catalyst the presence of ceria-zirconia mixed phase confirms, no phase change occurs during the harsh reaction condition. Although in the spent Ru/Ce and Ru/Zr samples along with ceria, and


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zirconia phase, an additional small peak of hexagonal primitive graphitic carbon at 2θ = 27° is observed for due to carbon deposition. The absence of carbon deposition in HRTEM image of

Fig. 7: a) XRD of spent catalysts b) TEM image of the spent Ru/CeZr catalyst

Ru/CeZr indicates the superiority of the ceria-zirconia supported catalysts over neat ceria or zirconia supported catalysts. Low activation barrier of methane over Ru/CeZr catalyst compared to Ru/Ce and Ru/Zr in activation energy calculations also support the dominance. Now, on the basis of above understanding, a mechanism of CPOX over Ru/CeZr has been proposed and demonstrated in scheme 2. At the first step, methane dissociates upon the metallic ruthenium to adsorbe carbon and hydrogen. Oxygen is also dissociatively adsorbed over ruthenium as well as over the solid solution surface. Adsorption of O2 over the metallic ruthenium oxidizes it to ruthenium oxide. In the second step, ejection of hydrogen molecule occurs from the metal surface by associative desorption. In the third step, the deposited carbon oxidizes by the surface adsorbed oxygen as well as by the oxygen present in ruthenium oxide and desorbed as CO49-54.


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Scheme 2: Probable mechanism of CPOX over Ru/CeZr.

4. Conclusion: In conclusion, we have used a very facile and one-step hydrothermal approach to synthesize Ru/CeZr solid solution nanorods. a) This unique material possesses a strong metal support interaction along with homogeneous dispersion of Ru over Zr0.5Ce0.5O2 nanorods. b) The higher oxygen storage capacity of Zr0.5Ce0.5O2 nanorods in comparison to only ceria or zirconia helps in reduction of coke deposition during CPOX. c) The decoration of Ru over Zr0.5Ce0.5O2 nanorods follows the direct route of CPOX, with higher feed conversion and high selectivity of the desired syngas without any deactivation during activity analysis.


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These attractive characteristics of this newly designed material favor low temperature methane activation at 300 ℃ and unparallel stability at 800 ℃ for more than 200 h.

Acknowledgment: S.D and RG sincerely acknowledges University Grants Commission, New Delhi, India and M.S. acknowledge CSIR, India for the research fellowship. A.B sincerely acknowledges DST India for AISRF-GC grant. The Director, CSIR-IIP, also gratefully acknowledged for his encouragement and ASD for kind support.

Supporting info available: A detailed catalyst characterization technique with the experimental procedure of CPOX. N2 physisorption analysis of the catalysts (Table S1), Oxygen Storage Capacity and H2 chemisorption during H2-TPR (Table S2), CHN analysis (Table S3) with a detailed mass and heat transfer calculation. References: 1. Pakhare, D.; Spivey, J., A review of dry (CO2) reforming of methane over noble metal catalysts. Chemical Society Reviews 2014, 43 (22), 7813-7837. 2. Das, S.; Thakur, S.; Bag, A.; Gupta, M. S.; Mondal, P.; Bordoloi, A., Support interaction of Ni nanocluster based catalysts applied in CO2 reforming. Journal of Catalysis 2015, 330, 4660. 3. Das, S.; Sengupta, M.; Bag, A.; Shah, M.; Bordoloi, A., Facile synthesis of highly disperse Ni-Co nanoparticles over mesoporous silica for enhanced methane dry reforming. Nanoscale 2018, 10 (14), 6409-6425. 4. Das, S.; Sengupta, M.; Patel, J.; Bordoloi, A., A study of the synergy between support surface properties and catalyst deactivation for CO2 reforming over supported Ni nanoparticles. Applied Catalysis A: General 2017, 545, 113-126. 5. Shah, M.; Das, S.; Nayak, A. K.; Mondal, P.; Bordoloi, A., Smart designing of metalsupport interface for imperishable dry reforming catalyst. Applied Catalysis A: General 2018, 556, 137-154. 6. Van Hook, J. P., Methane-Steam Reforming. Catalysis Reviews 1980, 21 (1), 1-51. 7. Iulianelli, A.; Liguori, S.; Wilcox, J.; Basile, A., Advances on methane steam reforming to produce hydrogen through membrane reactors technology: A review. Catalysis Reviews 2016, 58 (1), 1-35.


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8. Pitchai, R.; Klier, K., Partial Oxidation of Methane. Catal. Rev. 1986, 28 (1), 13-88. 9. York, A. P. E.; Xiao, T.; Green, M. L. H., Brief Overview of the Partial Oxidation of Methane to Synthesis Gas. Top. Catal. 2003, 22 (3), 345-358. 10. Ashcroft, A. T.; Cheetham, A. K.; Foord, J. S.; Green, M. L. H.; Grey, C. P.; Murrell, A. J.; Vernon, P. D. F., Selective oxidation of methane to synthesis gas using transition metal catalysts. Nature 1990, 344, 319. 11. Korobitsyn, M.; Van Berkel, F.; Christie, G.; Huijsmans, J.; van der Klein, C., SOFC as a gas separator. ECN-Fuels, Conversion & Environment 2000. 12. Karakaya, M.; Avcı, A. K.; Aksoylu, A. E.; Önsan, Z. Đ., Steady-state and dynamic modeling of indirect partial oxidation of methane in a wall-coated microchannel. Catalysis Today 2009, 139 (4), 312-321. 13. Liu, Y.; Huang, F.-Y.; Li, J.-M.; Weng, W.-Z.; Luo, C.-R.; Wang, M.-L.; Xia, W.-S.; Huang, C.-J.; Wan, H.-L., In situ Raman study on the partial oxidation of methane to synthesis gas over Rh/Al2O3 and Ru/Al2O3 catalysts. Journal of Catalysis 2008, 256 (2), 192-203. 14. Donazzi, A.; Michael, B. C.; Schmidt, L. D., Chemical and geometric effects of Ce and washcoat addition on catalytic partial oxidation of CH4 on Rh probed by spatially resolved measurements. Journal of Catalysis 2008, 260 (2), 270-275. 15. Christian Enger, B.; Lødeng, R.; Holmen, A., A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts. Appl. Catal., A 2008, 346 (1), 1-27. 16. Donazzi, A.; Beretta, A.; Groppi, G.; Forzatti, P., Catalytic partial oxidation of methane over a 4% Rh/α-Al2O3 catalyst Part II: Role of CO2 reforming. Journal of Catalysis 2008, 255 (2), 259-268. 17. Donazzi, A.; Beretta, A.; Groppi, G.; Forzatti, P., Catalytic partial oxidation of methane over a 4% Rh/α-Al2O3 catalyst: Part I: Kinetic study in annular reactor. Journal of Catalysis 2008, 255 (2), 241-258. 18. Horn, R.; Williams, K. A.; Degenstein, N. J.; Schmidt, L. D., Syngas by catalytic partial oxidation of methane on rhodium: Mechanistic conclusions from spatially resolved measurements and numerical simulations. Journal of Catalysis 2006, 242 (1), 92-102. 19. Bruno, T.; Beretta, A.; Groppi, G.; Roderi, M.; Forzatti, P., A study of methane partial oxidation in annular reactor: activity of Rh/α-Al2O3 and Rh/ZrO2 catalysts. Catalysis Today 2005, 99 (1), 89-98. 20. Hannemann, S.; Grunwaldt, J.-D.; van Vegten, N.; Baiker, A.; Boye, P.; Schroer, C. G., Distinct spatial changes of the catalyst structure inside a fixed-bed microreactor during the partial oxidation of methane over Rh/Al2O3. Catalysis Today 2007, 126 (1), 54-63. 21. Nagaoka, K.; Okamura, M.; Aika, K.-i., Titania supported ruthenium as a cokingresistant catalyst for high pressure dry reforming of methane. Catal. Commun. 2001, 2 (8), 255260. 22. Ferreira-Aparicio, P.; Rodrı́guez-Ramos, I.; Anderson, J. A.; Guerrero-Ruiz, A., Mechanistic aspects of the dry reforming of methane over ruthenium catalysts. Applied Catalysis A: General 2000, 202 (2), 183-196. 23. Rostrupnielsen, J. R.; Hansen, J. H. B., CO2-Reforming of Methane over Transition Metals. Journal of Catalysis 1993, 144 (1), 38-49. 24. Poirier, M. G.; Trudel, J.; Guay, D., Partial oxidation of methane over ruthenium catalysts. Catalysis Letters 1993, 21 (1), 99-111.


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Page 22 of 25

25. De Groote, A. M.; Froment, G. F., The role of coke formation in catalytic partial oxidation for synthesis gas production. Catal. Today 1997, 37 (3), 309-329. 26. Basini, L.; Aasberg-Petersen, K.; Guarinoni, A.; Østberg, M., Catalytic partial oxidation of natural gas at elevated pressure and low residence time. Catal. Today 2001, 64 (1), 9-20. 27. Sudarsanam, P.; Hillary, B.; Amin, M. H.; Hamid, S. B. A.; Bhargava, S. K., Structureactivity relationships of nanoscale MnOx/CeO2 heterostructured catalysts for selective oxidation of amines under eco-friendly conditions. Applied Catalysis B: Environmental 2016, 185, 213224. 28. Putla, S.; Amin, M. H.; Reddy, B. M.; Nafady, A.; Al Farhan, K. A.; Bhargava, S. K., MnOx Nanoparticle-Dispersed CeO2 Nanocubes: A Remarkable Heteronanostructured System with Unusual Structural Characteristics and Superior Catalytic Performance. ACS Applied Materials & Interfaces 2015, 7 (30), 16525-16535. 29. Sudarsanam, P.; Hillary, B.; Mallesham, B.; Rao, B. G.; Amin, M. H.; Nafady, A.; Alsalme, A. M.; Reddy, B. M.; Bhargava, S. K., Designing CuOx Nanoparticle-Decorated CeO2 Nanocubes for Catalytic Soot Oxidation: Role of the Nanointerface in the Catalytic Performance of Heterostructured Nanomaterials. Langmuir 2016, 32 (9), 2208-2215. 30. Hillary, B.; Sudarsanam, P.; Amin, M. H.; Bhargava, S. K., Nanoscale Cobalt– Manganese Oxide Catalyst Supported on Shape-Controlled Cerium Oxide: Effect of Nanointerface Configuration on Structural, Redox, and Catalytic Properties. Langmuir 2017, 33 (8), 1743-1750. 31. Zhang, D.; Du, X.; Shi, L.; Gao, R., Shape-controlled synthesis and catalytic application of ceria nanomaterials. Dalton Transactions 2012, 41 (48), 14455-14475. 32. Wang, W.; Stagg-Williams, S. M.; Noronha, F. B.; Mattos, L. V.; Passos, F. B., Partial oxidation and combined reforming of methane on Ce-promoted catalysts. Catalysis Today 2004, 98 (4), 553-563. 33. Silva, F. A.; Resende, K. A.; da Silva, A. M.; de Souza, K. R.; Mattos, L. V.; Montes, M.; Souza-Aguiar, E. F.; Noronha, F. B.; Hori, C. E., Syngas production by partial oxidation of methane over Pt/CeZrO2/Al2O3 catalysts. Catal. Today 2012, 180 (1), 111-116. 34. Singha, R. K.; Das, S.; Pandey, M.; Kumar, S.; Bal, R.; Bordoloi, A., Ni nanocluster on modified CeO2-ZrO2 nanoporous composite for tri-reforming of methane. Catalysis Science & Technology 2016, 6 (19), 7122-7136. 35. Chastain, J.; King, R. C.; Moulder, J., Handbook of X-ray photoelectron spectroscopy: a reference book of standard spectra for identification and interpretation of XPS data. Physical Electronics Division, Perkin-Elmer Corporation Eden Prairie, Minnesota: 1992. 36. Reddy, B. M.; Reddy, G. K.; Ganesh, I.; Ferreira, J. M. F., Microwave-assisted Synthesis and Structural Characterization of Nanosized Ce0.5Zr0.5O2 for CO Oxidation. Catal. Lett. 2009, 130 (1), 227-234. 37. Yao, H. C.; Yao, Y. F. Y., Ceria in automotive exhaust catalysts: I. Oxygen storage. J. Catal. 1984, 86 (2), 254-265. 38. Shyu, J. Z.; Weber, W. H.; Gandhi, H. S., Surface characterization of alumina-supported ceria. J. Phys. Chem. 1988, 92 (17), 4964-4970. 39. Singha, R. K.; Shukla, A.; Yadav, A.; Sivakumar Konathala, L. N.; Bal, R., Effect of metal-support interaction on activity and stability of Ni-CeO2 catalyst for partial oxidation of methane. Applied Catalysis B: Environmental 2017, 202, 473-488.


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Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40. Kondratenko, V. A.; Berger-Karin, C.; Kondratenko, E. V., Partial Oxidation of Methane to Syngas Over γ-Al2O3-Supported Rh Nanoparticles: Kinetic and Mechanistic Origins of Size Effect on Selectivity and Activity. ACS Catalysis 2014, 4 (9), 3136-3144. 41. Lanza, R.; Järås, S. G.; Canu, P., Partial oxidation of methane over supported ruthenium catalysts. Applied Catalysis A: General 2007, 325 (1), 57-67. 42. Choudhary, T. V.; Choudhary, V. R., Energy‐Efficient Syngas Production through Catalytic Oxy‐Methane Reforming Reactions. Angew. Chem. Int. Ed. 2008, 47 (10), 1828-1847. 43. Elmasides, C.; Kondarides, D. I.; Neophytides, S. G.; Verykios, X. E., Partial Oxidation of Methane to Synthesis Gas over Ru/TiO2 Catalysts: Effects of Modification of the Support on Oxidation State and Catalytic Performance. Journal of Catalysis 2001, 198 (2), 195-207. 44. Rabe, S.; Nachtegaal, M.; Vogel, F., Catalytic partial oxidation of methane to synthesis gas over a ruthenium catalyst: the role of the oxidation state. Physical Chemistry Chemical Physics 2007, 9 (12), 1461-1468. 45. Singha, R. K.; Shukla, A.; Yadav, A.; Sasaki, T.; Sandupatla, A.; Deo, G.; Bal, R., PtCeO2 nanoporous spheres - an excellent catalyst for partial oxidation of methane: effect of the bimodal pore structure. Catalysis Science & Technology 2017, 7 (20), 4720-4735. 46. Singha, R. K.; Ghosh, S.; Acharyya, S. S.; Yadav, A.; Shukla, A.; Sasaki, T.; Venezia, A. M.; Pendem, C.; Bal, R., Partial oxidation of methane to synthesis gas over Pt nanoparticles supported on nanocrystalline CeO2 catalyst. Catalysis Science & Technology 2016, 6 (12), 46014615. 47. Pichas, C.; Pomonis, P.; Petrakis, D.; Ladavos, A., Kinetic study of the catalytic dry reforming of CH 4 with CO 2 over La 2− x Sr x NiO 4 perovskite-type oxides. Applied Catalysis A: General 2010, 386 (1), 116-123. 48. Fogler, H. S., Elements of chemical reaction engineering. 4th ed.; U.S.A., 2006. 49. Mallens, E. P. J.; Hoebink, J. H. B. J.; Marin, G. B., The Reaction Mechanism of the Partial Oxidation of Methane to Synthesis Gas: A Transient Kinetic Study over Rhodium and a Comparison with Platinum. Journal of Catalysis 1997, 167 (1), 43-56. 50. Horn, R.; Williams, K. A.; Degenstein, N. J.; Bitsch-Larsen, A.; Dalle Nogare, D.; Tupy, S. A.; Schmidt, L. D., Methane catalytic partial oxidation on autothermal Rh and Pt foam catalysts: Oxidation and reforming zones, transport effects, and approach to thermodynamic equilibrium. Journal of Catalysis 2007, 249 (2), 380-393. 51. Jia, A.-P.; Jiang, S.-Y.; Lu, J.-Q.; Luo, M.-F., Study of Catalytic Activity at the CuO−CeO2 Interface for CO Oxidation. The Journal of Physical Chemistry C 2010, 114 (49), 21605-21610. 52. Genuino, H. C.; Dharmarathna, S.; Njagi, E. C.; Mei, M. C.; Suib, S. L., Gas-Phase Total Oxidation of Benzene, Toluene, Ethylbenzene, and Xylenes Using Shape-Selective Manganese Oxide and Copper Manganese Oxide Catalysts. The Journal of Physical Chemistry C 2012, 116 (22), 12066-12078. 53. Özacar, M.; Poyraz, A. S.; Genuino, H. C.; Kuo, C.-H.; Meng, Y.; Suib, S. L., Influence of silver on the catalytic properties of the cryptomelane and Ag-hollandite types manganese oxides OMS-2 in the low-temperature CO oxidation. Applied Catalysis A: General 2013, 462463, 64-74. 54. Chen, J.; Li, J.; Li, H.; Huang, X.; Shen, W., Facile synthesis of Ag–OMS-2 nanorods and their catalytic applications in CO oxidation. Microporous and Mesoporous Materials 2008, 116 (1), 586-592.


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Facile synthesis of Ruthenium decorated Zr0.5Ce0.5O2 nanorods for catalytic partial oxidation of methane


Facile synthesis of Ruthenium decorated Zr0.5Ce0.5O2 nanorods for

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