Abstract
Ammonia decomposition is often used as an archetypical reaction for predicting new catalytic materials and understanding the very reason of why some reactions are sensitive on material's structure. Core–shell or surface-segregated bimetallic nanoparticles expose outstanding activity for many heterogeneously catalysed reactions but the reasons remain elusive owing to the difficulties in experimentally characterizing active sites. Here by performing multiscale simulations in ammonia decomposition on various nickel loadings on platinum (111), we show that the very high activity of core–shell structures requires patches of the guest metal to create and sustain dual active sites: nickel terraces catalyse N−H bond breaking and nickel edge sites drive atomic nitrogen association. The structure sensitivity on these active catalysts depends profoundly on reaction conditions due to kinetically competing relevant elementary reaction steps. We expose a remarkable difference in active sites between transient and steady-state studies and provide insights into optimal material design.
Highlights
Ammonia decomposition is often used as an archetypical reaction for predicting new catalytic materials and understanding the very reason of why some reactions are sensitive on material’s structure
It is clear that Ni–Pt–Pt and Ni patches on Pt (Ni/Pt) patches are more active in dehydrogenation than Pt and Ni surfaces owing to lower dissociation barrier of NH3* and the stronger binding of NHx species on these surfaces
H2 molecule desorption is 1.04 eV endothermic on the Ni terrace of Ni/Pt and Ni–Pt–Pt, it is not rate limiting since reactors for ammonia decomposition are usually operating above 600 K
Summary
Ammonia decomposition is often used as an archetypical reaction for predicting new catalytic materials and understanding the very reason of why some reactions are sensitive on material’s structure. Core–shell or surface-segregated bimetallic nanoparticles expose outstanding activity for many heterogeneously catalysed reactions but the reasons remain elusive owing to the difficulties in experimentally characterizing active sites. By performing multiscale simulations in ammonia decomposition on various nickel loadings on platinum (111), we show that the very high activity of core–shell structures requires patches of the guest metal to create and sustain dual active sites: nickel terraces catalyse N À H bond breaking and nickel edge sites drive atomic nitrogen association. We report differences in the structure sensitivity under steady state and transient conditions and that experimental results on the effect of size and shape of patches combined with multiscale simulations as those reported can reveal mechanistic insights into the RDS and active site(s). Our patched bimetallic surface model captures key features of core–shell and surface-segregated nanoparticles and sheds light on design principles of microstructure of bimetallic catalysts
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