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In Situ X‑Ray Microscopy Reveals Particle Dynamics in a Nico Dry Methane Reforming Catalyst Under Operating Conditions

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In Situ X‑Ray Microscopy Reveals Particle Dynamics in a Nico Dry Methane Reforming Catalyst Under Operating Conditions This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. pubs.acs.org/acscatalysis Research Article In Situ X‑ray Microscopy Reveals Particle Dynamics in a NiCo Dry Methane Reforming Catalyst under Operating Conditions Abbas Beheshti Askari, Mustafa al Samarai, Bruno Morana, Lukas Tillmann, Norbert Pfander,̈ Aleksandra Wandzilak, Benjamin Watts, Rachid Belkhou, Martin Muhler,* and Serena DeBeer* Cite This: ACS Catal. 2020, 10, 6223−6230 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information γ ABSTRACT: Herein, we report the synthesis of a -Al2O3-supported NiCo catalyst for dry methane reforming (DMR) and study the catalyst using in situ scanning transmission X-ray microscopy (STXM) during the reduction (activation step) and under reaction conditions. During the reduction process, the NiCo alloy particles undergo elemental segregation with Co migrating toward the center of the catalyst particles and Ni migrating to the outer surfaces. Under DMR conditions, the segregated structure is maintained, thus hinting at the importance of this structure to optimal catalytic functions. Finally, the formation of Ni-rich branches on the surface of the particles is observed during DMR, suggesting that the loss of Ni from the outer shell may play a role in the reduced stability and hence catalyst deactivation. These findings provide insights into the morphological and electronic structural changes that occur in a NiCo-based catalyst during DMR. Further, this study emphasizes the need to study catalysts under operating conditions in order to elucidate material dynamics during the reaction. KEYWORDS: in situ, heterogeneous catalysis, nanoreactor, methane reforming, X-ray spectroscopy, microscopy ■ INTRODUCTION In the literature, various classes of materials, including noble metals, spinels, hydrotalcites, and supported base metals, have The emission of greenhouse gases associated with the 9−11 combustion of fossil fuels is believed to be the main cause of been reported as catalysts for the DMR process. Among global warming.1,2 Lowering the concentration of these gases is these catalysts, noble metals show the highest activity and stability toward this reaction but are economically not therefore a matter of utmost importance. An attractive 12 approach would be the development of environmentally attractive. In recent years, various studies focused on the development of late 3d transition metal-based systems as an friendly processes capable of reusing these gases as feedstock − alternative to costly noble metals for DMR catalysis.9 11,13 Downloaded via LIB4RI on July 23, 2020 at 10:04:34 (UTC). for industrial processes. In this context, a reaction that has − received much attention is dry methane reforming (DMR).3 5 Within this context, nickel (Ni)-based catalysts are promising In this reaction, two major greenhouse gases, methane (CH ) candidates due to their relatively high activity and earth 4 abundance. Nevertheless, the major drawback of monometallic and carbon dioxide (CO2), are converted into hydrogen (H2) and carbon monoxide (CO), otherwise known as syngas. The Ni-based catalysts when compared to noble metal-based DMR reaction (eq 1) together with the side reactions (eqs catalysts is their lower activity and higher susceptibility to See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. 2−5) proceeding under non-ideal conditions are reported deactivation due to coke formation via either methane cracking − 14−19 below6 8 or the reverse Boudouard reaction. In order to increase catalyst activity and stability, researchers have implemented CH++ COF 2H 2CO various approaches including (i) depositing the catalytic active 422 ff 20,21 01− phase on di erent support materials, (ii) reducing the Δ°=+H (25 C) 247 kJmol (1) catalyst, particle size,22,23 (iii) developing bimetallic catalysts − based on transition metals,24 27 and (iv) transition metal- 01− − 28−30 H222++Δ°=+ COF H O COH (25 C) 41 kJmol based core shell nanoarchitectures. It has been shown (2) that core-shell systems have the ability to prevent metal sintering and decrease carbon formation, while the production H242 O++ CHF 3H CO 01− Received: December 20, 2019 Δ°=+H (25 C) 206 kJmol (3) Revised: April 30, 2020 01− Published: May 1, 2020 CH42F 2H+Δ CH (25 °=+ C) 75 kJmol (4) 01− 2COF C+Δ°=− CO2 H (25 C) 171 kJmol (5) © 2020 American Chemical Society https://dx.doi.org/10.1021/acscatal.9b05517 6223 ACS Catal. 2020, 10, 6223−6230 ACS Catalysis pubs.acs.org/acscatalysis Research Article of bimetallic catalysts has been very successful in increasing the ■ EXPERIMENTAL SECTION catalyst activity, stability, and selectivity. Among 3d transition Catalyst Synthesis. For the synthesis, cobalt(II) acetate metals, the addition of cobalt (Co) to Ni-based DMR catalysts tetrahydrate (>98%, Sigma Aldrich), nickel(II) acetate results in the highest increase in activity. This has been tetrahydrate (99.998%, Sigma Aldrich), ammonium oxalate ascribed to the optimal catalytic performance of the Ni/Co 31,32 monohydrate (98%, Alfa Aesar), methanol (99%, Alfa Aesar), alloy. Within this context, Zhang et al. investigated NiCo 1-hexanol (99%, Alfa Aesar), and n-hexane 99% gamma catalyst samples with various Ni and Co loadings and alumina catalyst support (surface area of 200 m2/g, Alfa Aesar) established that a lower Co concentration results in an were employed. increased catalytic activity and stability and prevents the coke The NiCoO /γAl O catalyst was synthesized according to 33 x 2 3 formation side reaction to a large extent. An operando X-ray the modified reverse micellar method.46,47 The advantage of absorption spectroscopy (XAS) study by Takanabe et al. this synthesis method is that it enables the production of a revealed that Co increases the capacity of CO2 adsorption, bimetallic catalyst with controlled size and elemental ratios which eventually leads to carbon elimination from the catalyst incorporating late transition metals. Other methods are mainly 34 surface. Furthermore, in the same study, the presence of a focused on the synthesize of small particles with the aim of homogeneous NiCo alloy was directly correlated to the DMR increasing the activity22,23,48 and limiting coke formation.49,50 activity. This finding was supported by a combined in situ In contrast, for STXM characterizations, where the spatial transmission electron microscopy (TEM) and X-ray photo- resolution is limited, larger particles are desired. By using the electron spectroscopy (XPS) study by Bonifacio and co- modified micellar method, we successfully produced highly workers, which revealed that NiCo core−shell particles tend to active and stable particles, which have the ideal size for STXM form a homogeneous alloy at high temperatures (600 °C).35 In characterizations. The first step in the production of the addition, an ex situ TEM and in situ XPS study by Carenco et bimetallic catalyst involved preparing four different mixtures. al. revealed that under reducing conditions and low temper- Herein, 2.3 g of ammonium oxalate monohydrate, 2.0 g of atures (270 °C) the morphology of NiCo particles changes nickel acetate tetrahydrate, and 0.71 g of cobalt acetate into a structure consisting of NiCo alloy shell and a nickel-rich tetrahydrate were separately dissolved in 15 mL of water and core.36 From these studies and many others reported in the stirred for 15 min to obtain three different solutions. The − literature,36 40 the formation of a NiCo alloy seems to play a fourth mixture was prepared by mixing and stirring 0.9 g of crucial role in optimizing the DMR activity. However, none of cetyltrimethylammonium bromide, 9 mL of hexanol, and 12.6 these studies were performed at optimal operating temper- mL of hexane for 20 min. This mixture was divided into three 34 fi atures for DMR (>700 °C). The differences between the equal parts, and each of these was then added to the rst three particle morphology at low and high temperatures35,36 suggest mixtures and stirred for another 20 min. In the last stage, all that it is necessary to follow the electronic structure and solutions were added together and stirred for 48 h to obtain fi elemental distributions of Co and Ni during both the reductive the bimetallic catalyst. The produced catalyst was puri ed by activation step and under DMR conditions. centrifuging at 5500 rpm for 5 min and washed with a mixture In general, the scarce information regarding the role of of 30 mL of methanol and 30 mL of chloroform. This procedure was followed by centrifugation at 5500 rpm for 10 transition metals in methane reforming catalysis is mainly due ° to the lack of experimental techniques capable of simulta- min and drying at 50 C for 12 h. To deposit the bimetallic catalyst on a support, 2.5 mL of water was added to the neously probing the changes in the electronic structure and γ visualizing the morphology of the active phase under operating prepared catalyst and 0.8 g of -Al2O3 was added to the solution. This was then stirred for 20 min and dried at 90 °C conditions. Within this context, scanning transmission X-ray for 12 h. The final phase of this process included calcining the microscopy (STXM) is a promising technique whereby soft X- supported catalyst at 400 °C for 10 h in an argon atmosphere. ray spectroscopy is combined with 2D microscopy. Recently, it In the study by Fakeeha and Al-Fatesh, it was shown that a low has been reported that by employing microfabricated 41 fi calcination temperature results in a large surface area of Ni- monolithic nanoreactors, together with a speci cally based DMR catalysts.51 This simple method allows the designed in situ gas-phase setup, the changes in the particle production of a wide variety of catalysts by selecting the morphology and electronic structure of the catalytic active desired metal acetates.
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