Understanding and predicting the nature of transition-metal surfaces under realistic pressure and temperature conditions is crucial for optimizing their catalytic, mechanical, or electronic properties. We focus here on the stability of transition-metal surfaces submitted to a pressure of NH3 and H2 and on the potential formation of metastable or stable surface nitrides. Our leading example is a Ni-based alcohol amination catalyst, studied by a combination of DFT, thermodynamic modeling, and experiments. Initial N-coverage on Ni nanoparticles selectively occurs on (100) facets, which become the most stable terminations. Concomitantly, the equilibrium shape of the particle becomes modified under a realistic gas-phase environment of NH3 and H2. Extreme conditions favor the genesis of metastable Ni3N nanoparticles, mainly exposing (101) terminations. Transformation into Ni and gas-phase N2, favored by thermodynamics, is kinetically hindered. H2 controls the catalyst nitridation by the competition between H-covered and N-covered surfaces. Extension to 15 transition metals unveils a huge spectrum of nitridation behaviors arising from very reactive Mo to almost inert Au. Nonetheless, in several cases, a moderate H2 pressure is sufficient to prevent nitridation under a pressure of NH3. The approach presented in this study gives insight into the surface nitridation behavior of transition metals, paving the way to in silico design under real conditions for applications in materials science and heterogeneous catalysis.