Dehydrogenation of ammonia on free-standing and epitaxial hexagonal boron nitride

We report a thermodynamically feasible mechanism for producing H2 from NH3 using hBN as a catalyst. 2D catalysts have exceptional surface areas with unique thermal and electronic properties suited for catalysis. Metal-free, 2D catalysts, are highly desirable materials that can be more sustainable than the ubiquitously employed precious and transition metal-based catalysts. Here, using density functional theory (DFT) calculations, we demonstrate that metal-free hexagonal boron nitride (hBN) is a valid alternative to precious metal catalysts for producing H2via reaction of ammonia with a boron and nitrogen divacancy (VBN). Our results show that the decomposition of ammonia proceeds on monolayer hBN with an activation energy barrier of 0.52 eV. Furthermore, the reaction of ammonia with epitaxially grown hBN on a Ru(0001) substrate was investigated, and we observed similar NH3 decomposition energy barriers (0.61 eV), but a much more facile H2 associative desorption barrier (0.69 eV vs 5.89 eV). H2 generation from the free-standing monolayer would instead occur through a diffusion process with an energy barrier of 3.36 eV. A detailed analysis of the electron density and charge distribution along the reaction pathways was carried out to rationalise the substrate effects on the catalytic reaction.

2 Figure S1. Adsorption of ammonia over a VBN defect of a hBN monolayer with an E Ads of -0.25 eV. The nitrogen atom is closer to the surface than the hydrogens (with the three N-H bonds point upwards) at a height of 2.76 Å.
Slight tilting of the ammonia molecule allows the lone pair on the nitrogen to interact with the electron-deficient B-B bond, forming a weak intermolecular bond. The ammonia adsorbs almost directly above the vacant nitrogen position of VBN. This geometry is the first step of the deportation reaction which mechanism is illustrated in Figure 6 of the main text.  Figure S2. Concentration of NH3(gas) + hBN(surf) (R), NH3(ads) + hBN(surf) (I) and, NH2(ads) + H(ads) + hBN(surf) (P) as a function of time at 600 K, 800 K, 1000 K and 1200 K. I remains at a very low steady-state concentration throughout the reaction. The intersection of the red and black lines gives the time it takes for approximately half or R to be changed into P. The equilibrium was reached roughly at 400 s, 12 s, 3.5 s and 0.54 s, at 600 K, 800 K, 1000 K and 1200 K, respectively. At 600 K the reaction is expected to take 400 s (6.6 minutes) and only 0.4 s at 1200K. Therefore, NH3 is expected to dissociate into NH2 and H in a reasonable amount of time. Here the B-H bonds shift to bring two hydrogen atoms within 1.45 Å of each other an event that would easily happen even at moderate surface temperatures. forming a weak intermolecular bond. The ammonia adsorbs almost directly above the vacant nitrogen position of VBN. This geometry is the first step of the deportation reaction which mechanism is illustrated in Figure 6 of the main text.

Concentration of NH3 as a function of time
As the barriers to NH3 desorption and dissociation are 0.25 eV and 0.52 eV respectively both processes are competitive. Therefore, the kinetics of the reactions we considered to suggest the formation rate of the NH2 and H species on the surface. The -point phonons for each of the 5 stationary points (isolated NH3, VB, NH3 adsorbed on the divacancy, TS1 and NH2 + H chemisorbed on VBN was calculated. Due the absence of the adsorption barrier, a Morse potential was adopted for the movement of NH3 alongside a perpendicular coordinate between NH3 and VBN. The respective imaginary frequency of the transition state for the NH3 dissociation was obtained at a value of 454.11 cm -1 . Rate coefficients were obtained by means of the Transition State Theory, at the temperatures of 600 K, 800 K, 1000 K and 1200 K which were possible operation temperature values of catalytic H2 production. In this regard, the Variational Transition State Theory was adopted for the adsorption/desorption and the Conventional Transition State Theory was considered for the NH3 dissociation on the divacancy. Internal modes of the hBN surface, NH3 in gas-phase and the adsorbed NH3, were considered for the adsorption/desorption and internal modes respective to the adsorbed system and transition state were assumed for the NH3 dissociation. The lattice-gas thermodynamic model was adopted in all calculations. Concentration of the NH3(gas) + hBN(surf) (R), NH3(ads) + hBN(surf) (I) and NH2(ads) + H(ads) + hBN(surf) (P), as a function of time, were obtained and reported in Figure   S2.
It was possible to suggest that, at 600 K, the complete convergence of the adsorbed NH3 into NH2 and H, was obtained, at roughly 400 seconds (6.6 minutes), whereas, complete convergence was achieved in 0.4 seconds at 1200 K. Therefore, although the equilibrium was dislocated towards NH 3 desorption, the step determinant reaction (NH 3(ads) → NH3(ads) + H(ads)) was able to proceed, being kinetically viable at the studied temperatures and NH3 is expected to dissociate into NH2 and H in a reasonable amount of time.
5 Figure S2. Concentration of NH3(gas) + hBN(surf) (R), NH3(ads) + hBN(surf) (I) and, NH2(ads) + H(ads) + hBN(surf) (P) as a function of time at 600 K, 800 K, 1000 K and 1200 K. I remains at a very low steady-state concentration throughout the reaction. The intersection of the red and black lines gives the time it takes for approximately half or R to be changed into P. The equilibrium was reached roughly at 400 s, 12 s, 3.5 s and 0.54 s, at 600 K, 800 K, 1000 K and 1200 K, respectively. At 600 K the reaction is expected to take 400 s (6.6 minutes) and only 0.4 s at 1200K. Therefore, NH3 is expected to dissociate into NH2 and H in a reasonable amount of time.