Chemical-state specific atomic mapping of MoO_x catalyst on α-TiO₂(110) using X-ray standing wave excited XPS

X-ray standing wave (XSW) excited photoelectron emission was used to study the atomic-scale structure of a one monolayer molybdenum oxide catalyst supported on a rutile TiO₂(110) single crystal substrate. The experiments were performed at the Advanced Photon Source DND-CAT 5IDC beamline. The modulations in the Mo 2p and O 1s XPS signals were monitored while scanning through 3 different substrate Bragg peaks. This was done for different stages of a redox reaction allowing us to track the surface sites for chemically distinct Mo and O atoms, important for understanding their catalytic behaviour. Mo⁶⁺ and Mo⁴⁺ 2p XPS peaks were found to be separated by 1 eV and have slightly different XSW hkl Fourier amplitudes and phases. Summation of these Fourier components generates chemical-state sensitive 3D atomic maps of Mo and O. These results are compared with DFT calculations to give us a more complete picture of this catalytically relevant interface.