Al2O3-Supported Transition Metals for Plasma-Catalytic NH3 Synthesis in a DBD Plasma: Metal Activity and Insights into Mechanisms

Abstract

N2 fixation into NH3 is one of the main processes in the chemical industry. Plasma catalysis is among the environmentally friendly alternatives to the industrial energy-intensive Haber-Bosch process. However, many questions remain open, such as the applicability of the conventional catalytic knowledge to plasma. In this work, we studied the performance of Al2O3-supported Fe, Ru, Co and Cu catalysts in plasma-catalytic NH3 synthesis in a DBD reactor. We investigated the effects of different active metals, and different ratios of the feed gas components, on the concentration and production rate of NH3, and the energy consumption of the plasma system. The results show that the trend of the metal activity (common for thermal catalysis) does not appear in the case of plasma catalysis: here, all metals exhibited similar performance. These findings are in good agreement with our recently published microkinetic model. This highlights the virtual independence of NH3 production on the metal catalyst material, thus validating the model and indicating the potential contribution of radical adsorption and Eley-Rideal reactions to the plasma-catalytic mechanism of NH3 synthesis.

Nitrogen fixation with renewable electricity: Plasma catalysis as an alternative for small-scale ammonia synthesis?

“…The plasma-catalytic ammonia synthesis activity for various H2:N2 ratios is shown in Figure 47. The metals show the highest activity for N2-rich plasmas, in line with literature [49,82,180] . UV Vis spectroscopy shows a higher concentration of plasma-activated nitrogen species for a higher N2 fraction in the feed (see Figure 54).…”

“…Chen et al [349] , Engelmann et al [179] , Rouwenhorst et al [223] , Shah et al [126] , and Yamijala et al [181] proposed that plasma-catalytic ammonia synthesis proceeds via N and NHX radicals, especially at high plasma powers. The dominant contribution of radical species was demonstrated by various authors for SEIs in the range of 10 to 19 kJ L -1 on Rucatalysts [223] , and at 40 kJ L -1 on Fe-, Ru-, Co-, and Cu-catalysts [180] .…”

“…Such reactions are typically negligible under thermal conditions and it is not fully understood to what extent they contribute to the total conversion under plasma conditions. However, Engelmann et al [179,180] and Yamijala et al [181] proposed that Eley-Rideal mechanisms can be dominant plasma-catalytic ammonia synthesis at high plasma powers, which is discussed in Chapter 6. These mechanisms and how they impact the total conversion and energy efficiency will be discussed in more detail in section 2.4.3.…”

“…The correction factor 0.0429 kmol-NH3 Nm -3 accounts for the molar gas density of ammonia. The normalized plasma-catalytic ammonia synthesis rate (rpc-norm in μmol m -2 metal min -1 , Equation 5) is obtained by subtracting the rate for plasma-chemical ammonia synthesis on MgO (rMgO in μmol min -1 ) from the total ammonia synthesis rate in the reactor (rM/MgO in μmol min -1 ), divided by the quantity of metal adsorption sites (nmetal in m 2 ), as measured with XRD or H2 chemisorption (see Table 7) [49,82,180] .…”

“…Thus, various authors have experimentally investigated the plasma-catalytic activity on transition metals in dielectric barrier discharge (DBD) reactors [49,76,79,82,180,336,349,350] . However, the evidence for the dominant reaction mechanisms remains inconclusive.…”