In Situ X-ray Microscopy Reveals Particle Dynamics in a NiCo Dry Methane Reforming Catalyst under Operating Conditions

Abstract

Herein, we report the synthesis of a γ-Al 2 O 3 -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.

Probing Physical Oxidation State by Resonant X‐ray Emission Spectroscopy: Applications to Iron Model Complexes and Nitrogenase

“…In recent years,t he literature has seen increasing applications of Kb XES to awide range of TM systems,including in situ and operando studies of catalysts, [74][75][76][77] transformations of materials under high pressure, [78,79] and time-resolved (TR) studies of enzymatic systems. [80,81] However,inmany of these cases the information conveyed by Kb XES is ambiguous and their conclusions are at times controversial.…”

In situ X-ray emission and high-resolution X-ray absorption spectroscopy applied to Ni-based bimetallic dry methane reforming catalysts

“…The NiCoO/γ-Al 2 O 3 catalyst was synthesized according to a previously reported protocol. 24,34 The NiO/γ-Al 2 O 3 catalyst was prepared by an adaptation of the reverse micellar method. 35 In this approach, two different aqueous mixtures were prepared by dissolving 2.3 g ammonium oxalate monohydrate and 3 g nickel acetate tetrahydrate in 15 mL demineralized water and stirring for 15 minutes.…”

“…In an earlier study, the details of the TPR experimental setup and the reduction profile for the NiCoO/γ-Al 2 O 3 catalyst were reported. 24 Additionally, the TPR profile of the NiO/γ-Al 2 O 3 was measured by filling a quartz cell with 64 mg of the calcined catalysts. In this experiment, the temperature was increased to 850°C, with a temperature ramp of 6°C min −1 in a flow of 4.6% H 2 in Ar, 84.1 mL min −1 total.…”

“…22 It was also found that among 3d transition metals, Co most efficiently promotes the DMR activity of Ni-based catalysts. 23,24 However, to date the synergistic interaction between Co and Ni in optimizing DMR activity is not fully understood. In a previous study, we utilized in situ Scanning Transmission X-ray Microscopy (STXM) to follow the changes in the morphology and electronic structure of multiple NiCoO/γ-Al 2 O 3 DMR catalyst particles during activation and under DMR reaction conditions.…”

“…In a previous study, we utilized in situ Scanning Transmission X-ray Microscopy (STXM) to follow the changes in the morphology and electronic structure of multiple NiCoO/γ-Al 2 O 3 DMR catalyst particles during activation and under DMR reaction conditions. 24 It was found that through the activation step, Co and Ni oxides were gradually reduced by heating the catalyst under 5% H 2 in Ar. Further, the formation of a segregated structure with a Ni-rich shell was observed.…”