  Estimating the heat loads on re-entry vehicles is a crucial part of preparing
for atmospheric re-entry manoeuvres. Re-entry flows at high altitudes are in
the rarefied regime and are governed by high enthalpies and thermodynamic
non-equilibrium. Additionally, catalytic gas-surface reactions change the gas
flow composition and can have a major influence on the heat transfer. Our goal
is to estimate the heat loads without a priori fitting of simulation parameters
to experiments. We use the tool PICLas for simulations of such rarefied gas
flows. It combines different particle methods, including the Direct Simulation
Monte Carlo method, for modelling of gases. Recently it has been extended to
include different catalysis models to treat reactions on surfaces. We evaluate
a kinetic Monte Carlo approach to model catalytic gas-surface interactions in
combination with flow simulations using particle methods. Here, the adsorbate
distribution is modelled by reproducing a surface system using a kinetic Monte
Carlo approach and estimating the necessary parameters using model assumptions.
This catalytic model is compared to a simple recombination model. We present
simulations that show the capability of the implemented models for a
$\mathrm{SiO_2}$ surface in an Oxygen flow. Furthermore, simulation results are
compared to heat fluxes and recombination coefficient obtained from the
respective experiment. The results show that simulations using the kinetic
Monte Carlo approach match the experimentally obtained values. Thus, the
approach can be used to estimate the reactivity of oxygen flows over
$\mathrm{SiO_2}$ surfaces.
