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Catalysis Science & Technology, 2017, 00, 1-3 | 1. Please do not adjust ... reactivity of carbon formed during methane dry reforming over. NiCo/CeO2-ZrO2 ...

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The effect of CeO2-ZrO2 structural differences on the origin and reactivity of carbon formed during methane dry reforming over NiCo/CeO2-ZrO2 catalysts studied by transient techniques†

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Michalis A. Vasiliades , Petar Djinović , Albin Pintar , Janez Kovač and Angelos M. Efstathiou

Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/

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Nickel (1.2 wt.%) and cobalt (1.8 wt.%) were dispersed over Ce0.75Zr0.25O2-δ solid solution (3NiCo EG) or over a mixture of CeO2 and ZrO2 single phases (3NiCo HT) and tested towards dry reforming of methane (DRM) at 750 oC. The structural, morphological, textural and redox differences between 3NiCo EG and 3NiCo HT catalysts were probed by powder XRD, HAADF/STEM and SAED, N2 adsorption/desorption at 77 K, H2-chemisorption, H2-TPR and H2 transient isothermal reduction (H2-TIR) techniques. The origin, concentation and reactivity of “carbon” deposits formed in DRM (via the CH4 and CO2 activation routes) towards gaseous H2 and O2 but also towards support’s labile oxygen species in the prepared catalysts were analyzed by a series of various transient and SSITKA experiments (use of 18O2 and 13CO2). Regardless of the EG or HT support, the %-contribution of CH4 and CO2 to the “carbon” deposition is very similar but the amount and reactivity towards oxidation is largely different. On the other hand, the concentration of active carbon formed in the carbon path of CO2 activation route is very small (θC < 0.16% after 2 h in DRM). Participation of mobile lattice oxygen species towards gasification of deposited “carbon” to CO(g) does occur to a large extent on both EG- and HT-supported NiCo catalysts. Catalyst deactivation rate during the first 5 h of DRM was found to depend on the structure of support (EG vs. HT). The faster deactivation observed on the 3NiCo EG catalyst cannot be linked to existing differences in the rates of inactive “carbon” formation and depletion or the concentration of active carbon but rather to the different rates of encapsulation of NiCo bimetallic particles by carbon layers formed (∼30 nm thick) as revealed by HAADF/STEM.

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1. Introduction Syngas, which is a mixture of H2 and CO in a broad range of compositions, represents the backbone of modern chemical industry and is expected to keep its central role also in the near future. Syngas is currently the platform for methane and coal upgrading to synfuels via Fischer-Tropsch synthesis1, as well as the main source of H2 and CO for hydrocarbon upgrading via hydrogenation and carbonylation reactions.2 In the near future, 3 hydrogen is expected to play a central role as fuel in PEM fuel cells. Considering the limited reserves of fossil fuels and their pollution related impact on environment, renewable resources, like biomethane and waste gas streams, especially CO2-rich, represent a possible surrogate feedstock for syngas production. As a result, methane dry reforming (DRM) reaction (CH4 + CO2 ↔ 2H2 + 2CO) represents a possible utilization pathway for simultaneous CH4 and CO2 valorization to syngas. Methane dry reforming reaction can be performed over supported noble metal (Rh, Pd, Ru and Pt), as well 4-8 as transition metal (Ni, Co, Fe) catalysts. Among transition metals, which represent the most economically viable solution, Ni is the most extensively investigated as it exhibits the highest activity. However, due to the fact that nickel-based catalysts are prone to fast carbon accumulation and deactivation, the catalyst needs to be appropriately designed in order to minimize this severe drawback. For example, the metal component can be alloyed or passivated to modulate the activity, or its most active coordinatively unsaturated

sites could be selectively poisoned. It is also known that carbon 10 accumulation is strongly related to the size of metallic clusters, which can be controlled by the use of ordered mesoporous 11 supports with narrow pore size distribution, as well as by the 12 chemical nature of support. The use of basic promoters (MgO, 13,14 15,16 CaO) and especially redox, mainly as CeO2-based supports, has proven a viable solution. The latter approach relies on the high mobility of labile oxygen within the CeO2 lattice at DRM reaction temperatures, which can be further enhanced by doping ceria with 15,17,18 Zr, Pr or Sm. In particular, carbon deposition on the 5 wt.% Ni/Ce0.8Pr0.2O2-δ catalyst with an average Ni particle size larger than 50 nm was reduced by more than two orders of magnitude 19 compared to the case of 5 wt.% Ni/CeO2 catalyst. Oxygen species originating from the doped-CeO2 lattice can migrate to the metallic clusters, where they react with carbon precursors and gasify them 19,20 mainly to carbon monoxide. Despite the noticeable and undisputed positive role of dopedCeO2 supports towards minimization of carbon accumulation during the DRM reaction, dedicated experimental studies which would correlate the morphological differences of doped-CeO2 solid solutions and their consequences on the oxygen mobility, the 19,20 To extend nature and origin of carbon deposits, are very limited. the knowledge in this field and enable in-depth understanding of the actual redox promotional role of support, two structurally different NiCo/CeZrO2 catalysts were subjected to a variety of 18 13 transient experiments coupled with the use of O2 and CO2 isotopes, enabling detailed analysis of carbon-path as well as oxygen pool participation in relevant DRM reaction conditions, for the first time over supported NiCo bimetallic catalysts. The obtained information along with that related to the deposited carbon morphology and location and NiCo surface composition changes with TOS in DRM (HRTEM and XPS studies) was used to explain the dry reforming of methane activity behaviour of the NiCo/CeZrO2 catalysts with TOS at the reaction conditions applied.

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2.1. Materials The CeO2-ZrO2 supports (Ce/Zr = 2.87) were prepared by two different methods. During the hydrothermal method (coded HT), 4.04 g of Ce(NO3)3.6H2O (Fluka, p.a.) and 1.1 g ZrO(NO3)2.6H2O (Sigma Aldrich, >99% purity) were dissolved in 100 ml of distilled water. The solution was added drop-wise to 250 ml of 25 % NH4OH (Merck, p.a.) under stirring. The produced suspension was o transferred to PTFE-lined autoclave and aged for 6 h at 120 C. The aged product was filtered, washed with distilled water and ethanol o and dried overnight in a laboratory drier at 70 C. During the ethylene glycol method (coded EG), 4.04 g of Ce(NO3)3.6H2O and 1.1 g of ZrO(NO3)2.6H2O were dissolved in 5 ml of ultrapure water and mixed with 5 ml of propionic acid (Merck, >99% purity) and 154 ml of ethylene glycol (Merck, >99% purity). The solution was transferred to PTFE-clad autoclave and aged for o 200 min at 180 C. The precipitated solid was separated from the solution by 15-min centrifugation at 8950 rpm, washed with distilled water and ethanol and dried overnight in a laboratory drier o o at 70 C. After drying, the material was calcined at 400 C for 4 h in air with a heating ramp of 5 oC min-1. A total amount of 3 wt.% cobalt and nickel metals (1.8 wt.% Co, 1.2 wt.% Ni) was deposited over both supports in an identical manner, namely: 0.129 g of Ni(NO3)2.6H2O (Merck, p.a.), 0.193 g of Co(NO3)2.6H2O (Merck, p.a.), 2.1 g of CeZrO2 powder and 1.68 g urea (Merck, p.a.) were dispersed in 100 ml of distilled water. One drop of concentrated HNO3 was added in order to decrease the initial pH value below 3. The suspension was heated in a slow and controlled manner from room T to 90 oC and maintained under reflux for 22 h. Afterwards, the suspension was filtered, washed with ethanol and water and dried overnight in air at 70 oC. The o catalysts were calcined at 750 C for 4 h in air. For simplicity, the two prepared catalysts are named 3NiCo HT and 3NiCo EG, repsectively, the supports of which were prepared by the hydrothermal and ethylene glycol sol-gel methods. 2.2. Catalysts characterization XRD analyses were performed on a PANalytical X’pert PRO diffractometer using Cu Kα radiation (λ = 0.15406 nm). The Scherrer equation and the (111) diffraction peak of CeZrO2 solid support (Ce0.75Zr0.25O2-δ solid solution and CeO2 phases) were used for the calculation of the average scattering domain size. 2 -1 3 The BET specific surface area (m g ), total pore volume (cm g 1 ) and average pore size (dp, nm) were determined using N2 adsorption/desorption isotherms at 77 K (Micromeritics, model TriStar II 3020). The samples were degassed before measurements using a SmartPrep degasser (Micromeritics) in N2 stream. The amount of reducible and labile oxygen species in the CeZrO2 mixed metal oxide of support and its redox characterisitcs were determined using the H2-TPR technique (AutoChem II 2920 apparatus). During the experiment, 100 mg of sample was positioned inside the quartz tube and pretreated in 10% O2/He at o o 500 C for 20 min. After sample cooling to 50 C, it was degassed in Ar for 30 min. TPR analysis was performed between 50 and 750 oC, -1 o -1 using 25 mL min of 5% H2/Ar and a heating rate of 10 C min . The extent of CeO2 (or Ce0.75Zr0.25O2-δ) reduction in the CeZrO2 mixed metal oxide was calculated based on the amount of consumed H2 during the H2-TPR experiment. Ni and Co were assumed to be completely reduced and that ZrO2 did not contribute to the observed H2 consumption.

Transient isothermal reduction by hydrogen (H2-TIR) was o applied at 750 C (DRM reaction T) to study in more detail the kinetic features of the redox process of the solid by recording the 4+ 3+ transient evolution rate of Ce to Ce reduction and the amount of labile oxygen removed. For this, the gas switch He → 1% H2/1% o Kr/Ar/He (750 C, t) was conducted following treatment of the solid o (25 mg) at 800 C for 2 h in 20% O2/He and He purge. The transient response curves of Kr and H2 were continuously followed by online mass spectrometer (MS). The transient rate of reduction as a function of time was estimated on the basis of an appropriate material balance. Metal dispersion was evaluated using the H2-TPD technique. The catalyst was first reduced in 5% H2/Ar stream for 30 min at 750 o C. The flow was switched to Ar and the temperature increased to 780 oC and maintained for 20 min to completely desorb H2. The o sample was cooled to 30 C in Ar gas flow, saturated in a 5% H2/Ar gas stream for 30 min, degassed in Ar gas flow at the same o temperature for 20 min, followed by temperature ramp to 800 C. o o The low-temperature H2 desorption peak (35 C