Incorporation of very little Ge into a Si surface significantly increases deposition during chemical vapor deposition. This is due to the fact that hydrogen and chlorine desorb faster from the SiGe surface making available additional surface sites for adsorption. Two mechanisms are discussed to explain the observed catalytic effect : (i) the diffusion model where surface diffusion of H and Cl atoms from Si to Ge sites opens up an energetically more favorable path for H and Cl desorption via Ge surface sites and (ii) the collective model where incorporation of Ge into Si stimulates an overall change of the electronic structure of the surface, thus leading to increased desorption. Ab initio cluster calculations are used in this work to evaluate both models. Binding energies of H and Cl atoms on Si, Ge, and SiGe surfaces are calculated. It is observed that Si-H, Ge-H and Si-Cl, Ge-Cl binding energies do not change whether their neighboring surface atoms are Si atoms or Ge atoms. An overall change of the electronic structure of the surface due to Ge incorporation cannot be observed, making the collective model highly unprobable. To evaluate the diffusion model transition states for migration between different surface sites need to be located and the activation barriers need to be calculated. Surface diffusion of H and C1 atoms from Si to Ge is found to be energetically more favorable than desorption of H-2, HCl, or SiCl2 from Si. Surface diffusion on mixed SiGe surfaces leads to enhanced desorption via Ge surface sites. Thus the diffusion model is considered a valid description. Macroscopic reactor simulations prove that the diffusion model can accurately describe enhanced deposition to explain the observed catalytic effect encountered during growth of SiGe heterolayers.