Local Catalytic Ignition during CO Oxidation on Low-Index Pt and Pd Surfaces: A Combined PEEM, MS, and DFT Study**

Shedding light on light-off : Photoemission electron microscopy, DFT, and microkinetic modeling were used to examine the local kinetics in the CO oxidation on individual grains of a polycrystalline sample. It is demonstrated that catalytic ignition (“light-off”) occurs easier on Pd(hkl) domains than on corresponding Pt(hkl) domains. The isothermal determination of kinetic transitions, commonly used in surface science, is fully consistent with the isobaric reactivity monitoring applied in technical catalysis.

. Sequence of PEEM images during the ignition  B * on Pt foil at constant p CO = 6.6x10 -6 mbar and p O2 = 1.3x10 -5 mbar. The temperature is ramped with a heating rate of ~0.5 K/s from 483 K in frame (a) (CO-covered), to 492 K and 506 K in frame (b) and (c) (ignition on (110) and (100) domains) and to 568 K in frame (d) (oxygen covered). The orientation of the individual domains is indicated in frame a).

S3. Density Functional Theory Calculations
Density functional theory (DFT) was used in a real space grid implementation [S2, S3] of the projector augmented wave (PAW) method [S4] with a grid spacing of 0.18 Å. The frozen core and projectors were generated with scalar relativistic corrections for Pd and Pt. Exchange and correlation contributions were described by the spin-polarized Perdew-Burke-Ernzerhof (PBE) functional. [S5] Reciprocal space integration over the Brillouin zone was approximated with finite sampling. [S6, S7] An effective temperature of 0.1 eV was used to smear the Fermi discontinuity. Activation energies were evaluated with the nudged-elastic band method [S8] or constrained optimization.
Three surfaces were considered for Pd and Pt, namely (111) Table S1. Only adsorption of atomic oxygen was considered. Thus, the E ads (O 2 ) value in Table S1 corresponds to the adsorption energy of two oxygen atoms with respect to O 2 in the gas phase. The results are in good agreement with previous reports. It should be noted that the reaction barrier is sensitive to the assumed reaction path. For the (111) surfaces, the barrier is obtained from CO close to an atop position and O in a bridge configuration.

S4. Micro-Kinetic Model
The micro-kinetic model is based on the conventional Langmuir-Hinshelwood mechanism for CO oxidation: CO + * ↔ CO* (R1) O 2 + 2* ↔ 2O* (R2) O* + CO* ↔ CO 2 + 2* (R3) Here, * denotes a free surface site and X* an adsorbate X bonded to a surface site. The corresponding rate equations are: T is the temperature, k B is the Boltzmann constant, M is the mass of the molecule under consideration, and N 0 is the number of sites per area. A linear coverage dependence is introduced for the adsorption energies and the barrier.  and  are in all cases set close to 0.5. The coverage dependence (strength and functional form) could, in principle, be evaluated from first principles.
However, here we have followed the strategy in Ref. [S26]. The desorption of O 2 is included above for completeness. The kinetics is insensitive to this step and it was not included in the simulations. The pre-exponential factors are set to 15 10  d CO  and 14 10  r  , respectively [S26].