<p>To
realize a sustainable, carbon-free society, catalysts for the synthesis of
ammonia using renewable energy under mild reaction conditions (<400 °C,
<10 MPa) are needed. Ru-based
catalysts are currently the most promising candidates; however, Ru is expensive and of low abundance. Here,
we discovered that encapsulation of Co
nanoparticles with BaO enhanced the ammonia synthesis activity of the
Co, and that a simple Ba-doped Co/MgO catalyst pre-reduced at an
unusually high temperature of 700 °C (Co@BaO/MgO-700red)
showed outstanding ammonia synthesis activity. <a>The ammonia synthesis rate (24.6 mmol g<sub>cat</sub></a><sup>−</sup><sup>1</sup> h<sup>−</sup><sup>1</sup>)
and turnover frequency (0.255 s<sup>−</sup><sup>1</sup>)
of the catalyst at 350 °C and 1.0 MPa were 22 and 64 times higher,
respectively, than those of the non-doped parent catalyst. At the same temperature but
higher pressure (3.0 MPa), the ammonia
synthesis rate was increased to 48.4 mmol g<sub>cat</sub><sup>−</sup><sup>1</sup>
h<sup>−</sup><sup>1</sup>, which is higher than that of active Ru-based catalysts. Scanning
transmission electron microscopy and energy dispersive X-ray spectrometry investigations revealed that after
reduction at 700 °C the Co
nanoparticles had become encapsulated by a nano-fraction of BaO. The mechanism
underlying the formation of this unique structure was considered to comprise
reduction of oxidic Co to metallic Co, decomposition of BaCO<sub>3</sub> to
BaO, and migration of BaO to the Co nanoparticle surface. Spectroscopic and density-functional theory investigations revealed that
adsorption of N<sub>2</sub> on the Co atoms at the catalyst surface weakened
the N<sub>2</sub> triple bond to the strength of a double bond due to electron
donation from the Ba atom of BaO <i>via</i> adjacent Co atoms; this weakening
accelerated cleavage of the triple bond, which is the rate-determining step for
ammonia synthesis.</p>
The Role of H2:N2 Ratio on the NH3 Synthesis Rate and on Process Economics over the Co3Mo3N Catalyst