  Due to their ability to reversibly absorb/desorb hydrogen without hysteresis,
Pd--Au nanoalloys have been proposed as materials for hydrogen sensing. For
sensing, it is important that absorption/desorption isotherms are reproducible
and stable over time. A few studies have pointed to the influence of short and
long range chemical order on these isotherms, but many aspects of the impact of
chemical order have remained unexplored. Here, we use alloy cluster expansions
to describe the thermodynamics of hydrogen in Pd--Au in a wide concentration
range. We investigate how different chemical orderings, corresponding to
annealing at different temperatures as well as different external pressures of
hydrogen, impact the behavior of the material with focus on its hydrogen
absorption/desorption isotherms. In particular, we find that a long-range
ordered L1$_2$ phase is expected to form if the \ce{H2} pressure is
sufficiently high. Furthermore, we construct the phase diagram at temperatures
from \unit[250]{K} to \unit[500]{K}, showing that if full equilibrium is
reached in the presence of hydrogen, phase separation can often be expected to
occur, in stark contrast to the phase diagram in para-equilibrium. Our results
explain the experimental observation that absorption/desorption isotherms in
Pd--Au are often stable over time, but also reveal pitfalls for when this may
not be the case.
