We study the adsorption energetics and quantum properties of the molecular hydrogen isotopes H-2, D-2, and T-2 onto the surface of rigid ammonia nanoclusters with quantum simulations and accurate model potential energy surfaces (PES). A highly efficient diffusion Monte Carlo (DMC) algorithm for rigid rotors allowed us to accurately define zero-point adsorption energies for the three isotopes, as well as the degree of translational and rotational delocalization that each affords on the surface. From the data emerges that the quantum adsorption energy (F-ads) of T-2 can be up to twice the one of H-2 at 0 K, suggesting the possibility of exploiting some form of solid ammonia to selectivity separate hydrogen isotopes at low temperatures (similar or equal to 20 K). This is discussed by focusing on the structural motif that may be more effective for the task. The analysis of the contributions to E-ads, however, surprisingly indicates that the average kinetic energy (E-kin) and rotation energy (E-rot(kin))of T-2 can also be, respectively, 2 times and 20 times higher than those of H-2; this finding markedly deviates from what is predicted for hydrogen molecules inside carbon nanotubes (CNT) or metallic-organic frameworks (MOP), where E-kin and E-rot(kin). is higher for H-2 due to the unavoidable effects of confinement and hindrance to its rotational motion. The rationale for these differences is provided by the geometrical distributions for the rigid rotors, which reveal an increasingly stronger coupling between rotational and translational degrees of freedom upon increasing the isotopic mass. This effect has never been observed before on adsorbing surfaces (e.g., graphite) and is induced by a strongly anisotropic and anharmonic bowl-like potential experienced by the rotors.