Structures and ammonia synthesis activity of hexagonal ruthenium iron nitride phases

Summary A series of ruthenium iron nitride phases with Ru:Fe ratios of ca. 1:3 were synthesized by ammonolysis. When the ammonolysis temperature was above 500°C, the obtained RuxFe3Ny materials had a ε-Fe3N (P6322) structure, while two similar phases were present when the ammonolysis was lower than 500°C. Powder neutron diffraction identified one phase as relating to the ε-Fe3N structure, while the other had a disordered NiAs-type (P63/mmc) structure. These ternary metal nitrides show ammonia synthesis activity at low temperature (200°C–300°C) and ambient pressure, which can be related to the loss of lattice nitrogen. Steady state catalytic performance at 400°C is associated with ruthenium-iron alloy. Additionally, density functional theory calculations were performed using an approximate model for the disordered hexagonal phase, revealing that this phase is more stable than a cubic anti-perovskite phase which has been previously investigated computationally, and corroborating the experimental findings of the present work.

Figure S1: XRD patterns of the RuxFe3Ny samples prepared from the solution with different ruthenium and iron molar ratios, related to Table 1.A.) at ammonolysis temperature of 500 °C; the patterns were obtained from scanning for 24 hours.B.) and ammonolysis temperature of 900 °C; the patterns were obtained from scanning for 2 hours.
The ammonolysis durations were also controlled as 12 h, 24 h and 168 h; the XRD patterns are shown in Figure S2.The ruthenium and iron molar ratio in the precursor solution is 1.4:3.The ammonolysis temperature was 500 °C.Table S4 presents the lattice parameters and nitrogen content of the samples.
When ammonolysis is increased from 12 hours to 24 hours, the elemental analysis shows that slightly more nitrogen can be accommodated into the final structure, and lattice parameters of the crystal structure therefore increase.For longer ammonolysis durations of over 168 h, Ru1.15Fe3N0.82-500°C-168 h starts losing nitrogen, and lattice parameters decrease.
Figure S2: XRD patterns of RuxFe3Ny samples prepared under different ammonolysis durations ranging from 12 hours to 168 hours, related to Table 1.

Figure S4 :
Figure S4: Conductivity profiles for Ru0.82Fe3N0.78-600°C-12 h reacted with 3:1 H2/N2, related to Table 3. A.) 200 °C for 4 h 10 min and then, 250 °C for 3 h 30 min.B.) 300 °C for 3 h 10 min and then, 400 °C for 4 h 40 min.The conductivity relates to that of the standard dilute sulfuric acid solution through which the reactor effluent is flowed.Decreasing conductivity is associated with the formation of ammonia.

Table S1 :
Synthetic process of ruthenium iron nitrides, nitrogen mass fractions in samples and sample compositions expressed as RuxFe3Ny, related to Table1.

Table S2 :
Lattice parameters, phase ratio and nitrogen contents of the RuxFe3Ny samples prepared under different ammonolysis temperatures, related to Table1 and Figure 2.

Table S3 :
Ru and Fe molar ratio in solution, lattice parameters, phase ratio and nitrogen contents of the RuxFe3Ny samples prepared from the solution with different ruthenium and iron molar ratios and ammonolysis temperatures, related to Table1.

Table S4 :
Lattice parameters, phase ratio and nitrogen contents of the RuxFe3Ny samples prepared under different ammonolysis durations, related to Table1.

Table S6 :
The results of Rietveld refinements of Ru1.03Fe3N1.08-500°C-24hfittedwithε-Fe3Ntype P6322 structure and P63/mmc structure, related to Figure3.The space groups, lattice parameters and phase fractions are shown in Table2.A.) The atom positions, fractions and Uiso in Phase 1 P6322.B.)The atom positions, fractions and Uiso in Phase 2 P63/mmc.(a)The atom positions, fractions and Uiso in Phase 1 P6322: