Unveiling the Structure–Property Relationship of MgO-Supported Ni Ammonia Decomposition Catalysts from Bulk to Atomic Structure by In Situ/Operando Studies

Abstract

Ammonia is currently being studied intensively as a hydrogen carrier in the context of the energy transition. The endothermic decomposition reaction requires the use of suitable catalysts. In this study, transition metal Ni on MgO as a support is investigated with respect to its catalytic properties. The synthesis method and the type of activation process contribute significantly to the catalytic properties. Both methods, coprecipitation (CP) and wet impregnation (WI), lead to the formation of Mg 1−x Ni x O solid solutions as catalyst precursors. X-ray absorption studies reveal that CP leads to a more homogeneous distribution of Ni 2+ cations in the solid solution, which is advantageous for a homogeneous distribution of active Ni catalysts on the MgO support. Activation in hydrogen at 900 °C reduces nickel, which migrates to the support surface and forms metal nanoparticles between 6 nm (CP) and 9 nm (WI), as shown by ex situ STEM. Due to the homogeneously distributed Ni 2+ cations in the solid solution structure, CP samples are more difficult to activate and require harsher conditions to reduce the Ni. The combination of in situ X-ray diffraction (XRD) and operando total scattering experiments allows a structure−property investigation of the bulk down to the atomic level during the catalytic reaction. Activation in H 2 at 900 °C for 2 h leads to the formation of large Ni particles (20− 30 nm) for the samples synthesized by the WI method, whereas Ni stays significantly smaller for the CP samples (10−20 nm). Sintering has a negative influence on the catalytic conversion of the WI samples, which is significantly lower compared to the conversion observed for the CP samples. Interestingly, metallic Ni redisperses during cooling and becomes invisible for conventional XRD but can still be detected by total scattering methods. The conditions of activation in NH 3 at 650 °C are not suitable to form enough reduced Ni nanoparticles from the solid solution and are, therefore, not a suitable activation procedure. The activity steadily increases in the samples activated at 650 °C in NH 3 (Group 1) compared to the samples activated at 650 °C in H 2 and then reaches the best activity in the samples activated at 900 °C in H 2 . Only the combination of complementary in situ and ex situ characterization methods provides enough information to identify important structure−property relationships among these promising ammonia decomposition catalysts.

Ni-Ru supported on CeO2 obtained by mechanochemical milling for catalytic hydrogen production from ammonia

“…This enhanced stability is likely attributed to the calcination step used in the preparation of the IWI catalyst. Comparing the hydrogen production achieved at 450°C in the stable phase of the catalyst prepared by MM (305.8 mmol H2 g cat −1 h −1 ) with results obtained in the literature under similar or more favorable reaction conditions (WHSV of 136000 mL NH3 g met −1 h −1 or lower), the prepared catalyst demonstrates superior activity compared to nickel-based catalysts, such as Ni/MgO (19.1 mmol H2 g −1 h −1 ), 5 or promoted Ni catalyst, such as 2wt.%K-15wt.%Ni/Al 2 O 3 (20.5 mmol H2 g −1 h −1 ), 86 and bimetallic configurations like Ni-Co supported on Y 2 O 3 (168.6 mmol H2 g −1 h −1 at 500°C). 87 However, the MM-catalyst exhibits lower activity compared to catalysts containing ruthenium.…”

“…Currently, research on the catalysts for ammonia decomposition, aimed at hydrogen production, still focuses on ruthenium, with nickel and cobalt emerging as primary alternatives due to their cost-effectiveness. Alternative preparation techniques, including coprecipitation 5 or sol-gel 6 , 7 methods, are being investigated, together with the effect of different promoters, 8 , 9 , 10 supports, 11 , 12 or active phase precursors 13 on the catalyst activity. Moreover, the utilization of oxide combinations, such as Al-La-Ce 14 or CeO 2 -promoted MgAl 2 O 4 , 15 as catalyst supports, has allowed the enhancement of the stability of ruthenium, and this effect has been ascribed to the high presence of oxygen vacancies in the support structure.…”