  Chemical processes taking place on ice-grain mantles are pivotal to the
complex chemistry of interstellar environments. In this study, we conducted a
comprehensive analysis of the catalytic effects of an amorphous solid water
(ASW) surface on the reaction between ammonia (NH$_3$) and formaldehyde
(H$_2$CO) to form aminomethanol (NH$_2$CH$_2$OH) using density functional
theory. We identified potential catalytic sites based on the binding energy
distribution of NH$_3$ and H$_2$CO reactants, on a set-of-clusters surface
model composed of 22 water molecules and found a total of 14 reaction paths.
Our results indicate that the catalytic sites can be categorized into four
groups, depending on the interactions of the carbonyl oxygen and the amino
group with the ice surface in the reactant complex. A detailed analysis of the
reaction mechanism using Intrinsic Reaction Coordinate and reaction force
analysis revealed three distinct chemical events for this reaction: formation
of the C--N bond, breaking of the N--H bond, and formation of the O--H hydroxyl
bond. Depending on the type of catalytic site, these events can occur within a
single, concerted, albeit asynchronous, step, or can be isolated in a step-wise
mechanism, with the lowest overall transition state energy observed at 1.3 kcal
mol$^{-1}$. A key requirement for the low-energy mechanism is the presence of a
pair of dangling OH bonds on the surface, found at 5\% of the potential
catalytic sites on an ASW porous surface.
