Karsten Reuter Technical University Munich (TUM) Understanding the detailed structure and nature of the active site is a central paradigm in modern molecular-level catalysis. One important endeavor in this quest has been to first establish this insight for well-defined low-index single-crystal model catalysts, initially under controlled ultra-high vacuum (UHV) conditions and increasingly at higher pressures. A central goal in corresponding in-situ studies of defined model catalysts has been to obtain (at best) as resolved spectro- or microscopic information as had been established in UHV surface science. With this focus possible heat and mass transport limitations in the ambient environments have not received much attention. In parts they may even be unavoidable, considering the often significant constraints that in-situ techniques impose on the design of the reactor chamber. We investigate such effects with our newly developed first-principles based multi-scale modeling approach integrating detailed kinetic Monte Carlo simulations into a fluid dynamical treatment [1]. Apart from the conceptual discussion in idealized reactor geometries [2,3], it is in particular the recent integration of our scheme into the general purpose Catalytic Foam solver [4] that now enables us to also explicitly address the complex flow profiles in actual in-situ reactor chambers. Our simulations generally demonstrate the crucial role of heat and mass transfer limitations in the emerging field of in-situ model catalyst studies, in particular for the unselective and therefore high turnover CO oxidation that is prominently used as allegedly simple test reaction. For lateral flows over the model catalyst as e.g. in reactor STM realizations the observed substantial variation in the gas-phase pressures and temperature between inlet and catalyst surface extends even to the lateral position at the active surface, i.e. the transport limitations lead to pronounced lateral changes in surface composition across the catalyst surface. This prevents the aspired direct relation between measured activity and defined catalyst structure, and therewith underscores the importance of carefully designed reactor geometries in in-situ studies. [1] S. Matera and K. Reuter, Catal. Lett. 133 (2009) 156. [2] S. Matera and K. Reuter, Phys. Rev. B 82 (2010) 085446. [3] S. Matera and K. Reuter, J. Catal. 295 (2012) 261. [4] M. Maestri and A. Cuoci, CatalyticFOAM, www.catalyticfoam.polimi.it