Industrial ammonia synthesis through the Haber–Bosch
process
operated under harsh reaction conditions leaves ample room for improvement
through material design. Designing a catalyst with high activity and
low cost is considered as the key to enable large-scale operation
under mild conditions. In this work, dilute metal alloys are studied
using density functional theory (DFT) calculations and microkinetic
modeling to investigate their catalytic performance for ammonia synthesis.
Thermochemical scaling relations developed between reaction intermediates
and Brønsted–Evans–Polanyi (BEP) relations developed
for *N2 dissociation and *NH
x
hydrogenation form the basis of microkinetic simulations of reaction
rates. A degree of rate control analysis shows that the overall reaction
is rate-controlled by either *N2 dissociation or *NH2 hydrogenation, resulting in a volcano plot for single-atom
alloys (SAAs) with Nb-doped Ag(111) SAA siting at the volcano peak.
The BEP relationship for N2 dissociation derived on dimer
alloys is closer to the ideal limit in comparison to that obtained
on SAAs, leading to higher activities of dimer alloys for ammonia
synthesis. Among the dimer alloys, Mo2/Ag(111) is not only
more active than the commercial Ru catalysts but also very stable
under real reaction conditions and could potentially be used in industrial
processes.
B-based catalysts are widely studied in the oxidative dehydrogenation of propane (ODHP) owing to their high selectivity. Correspondingly, two species, B−O oligomers and rings, have been recognized as active centers in B-based catalysts. To answer the essential question of whether B−O oligomers or ring species are more selective for ODHP, two AlB 2 catalysts enriched with B−O rings (R-AlB 2 ) and abundant B(OH) x O 3−x oligomers (O-AlB 2 ) were designed and compared herein. When tested in ODHP, R-AlB 2 exhibits an olefin yield of 30.2% at 500 °C, which is 2.3 times that of O-AlB 2 . Additionally, R-AlB 2 was stable for up to 200 h without deterioration. Multiple characterizations, including in situ Fourier transform infrared spectroscopy and theoretical calculations, demonstrate that B−O rings are more advantageous for producing propylene (C 3 = ) via a dehydration pathway with lower energy barriers and ethylene (C 2 = ) via two other reaction pathways (direct cracking of propane and oxidative coupling of methyl) B(OH) x O 3−x is primarily responsible for producing few C 3 = .
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