Controlling the Shapes of Nanoparticles by Dopant-Induced Enhancement of Chemisorption and Catalytic Activity: Application to Fe-Based Ammonia Synthesis

We showed recently that the catalytic efficiency ofammonia synthesis on Fe-based nanoparticles (NP) for Haber-Bosch(HB) reduction of N2 to ammonia depends very dramatically on thecrystal surface exposed and on the doping. In turn, the stability of eachsurface depends on the stable intermediates present during thecatalysis. Thus, under reaction conditions, the shape of the NP isexpected to evolve to optimize surface energies. In this paper, wepropose to manipulate the shape of the nanoparticles through dopingcombined with chemisorption and catalysis. To do this, we consider therelationships between the catalyst composition (adding dopantelements) and on how the distribution of the dopant atoms on thebulk and facet sites affects the shape of the particles and therefore thenumber of active sites on the catalyst surfaces. We use our hierarchical,high-throughput catalyst screening (HHTCS) approach but extend the scope of HHTCS to select dopants that can increasethe catalytically active surface orientations, such as Fe-bcc(111), at the expense of catalytically inactive facets, such as Febcc(100). Then, for the most promising dopants, we predict the resulting shape and activity of doped Fe-based nanoparticlesunder reaction conditions. We examined 34 possible dopants across the periodic table and found 16 dopants that canpotentially increase the fraction of active Fe-bcc(111) vs inactive Fe-bcc(100) facets. Combining this reshaping criterion withour HHTCS estimate of the resulting catalytic performance, we show that Si and Ni are the most promising elements forimproving the rates of catalysis by optimizing the shape to decrease reaction barriers. Then, using Si dopant as a workingexample, we build a steady-state dynamical Wulff construction of Si-doped Fe bcc nanoparticles. We use nanoparticles with adiameter of ~10 nm, typical of industrial catalysts. We predict that doping Si into such Fe nanoparticles at the optimal atomiccontent of ~0.3% leads to rate enhancements by a factor of 56 per nanoparticle under target HB conditions.