Photon Factory Activity Report 2019 #37 (2020)

BL-9A/2018G600

Development of operando PTRF-XAFS technique for elucidating 3D structure-activity
relationship in heterogeneous catalysis
Satoru TAKAKUSAGI*, Lu BANG, Daiki KIDO and Kiyotaka ASAKURA
Hokkaido University, Sapporo, 001-0021, Japan
1 Introduction
Polarization-dependent total reflection fluorescence(PTRF)-XAFS is a powerful technique which candetermine 3D structure of highly dispersed metal species
deposited on a single-crystal surface by measuring
polarization-dependent XAFS of the metal species. We
have determined the precise 3D structures of single metal
atoms and metal clusters deposited on single-crystal oxidesurfaces such as TiO2(110) and Al2O3(0001) by ultra-highvacuum (UHV) PTRF-XAFS apparatus in order to obtainfundamental aspects of metal/oxide-support interaction inheterogeneous catalysis.[1,2]
In this project, we have developed a novel PTRF-XAFS
technique which can measure PTRF-XAFS of active metal
species dispersed on a single-crystal surface under reaction
conditions (high temperatures and the presence of reactantgases). This “operando PTRF-XAFS technique” is thenapplied to CO oxidation reaction on a Pt/Al2O3(0001)
model catalyst surface. Our technique will offer a novel
methodology to elucidate the relationship between 3D
structure of active metal species and their catalytic
performance in heterogeneous catalysis.

suggesting reduction of the Pt atoms probably due topartial desorption of CO and aggregation of the Pt atoms.
CO2 production was confirmed at >473K by QMS,
showing that creating vacant sites for oxygen adsorption
by partial CO desorption might be needed to initiate COoxidation reaction. Thus we have succeeded in measuring
operando PTRF-XAFS spectra.
We also succeeded in obtaining PTRF-EXAFS spectra
during CO oxidation reaction and the detailed 3D structureof the active Pt species will soon be reported.

2 Experiment
Pt/Al2O3(0001) model catalyst surface was prepared byvacuum deposition of Pt on an α- Al2O3(0001) surface in aseparate UHV chamber (Pt coverage: 5.0×1014 /cm2), andthen transferred to the operando PTRF-XAFS apparatus(Fig.1) without exposure to air. The apparatus was attachedto the 5-axis goniometer and PTRF-XAFS measurements(Pt L3-edge) were conducted after optimization of totalreflection conditions for incident X-rays using the 5-axisgoniometer.

Fig. 1: Schematic of the operando PTRF-XAFS
apparatus.

3 Results and Discussion
Fig.1 shows the schematic of our apparatus. It consistsof a compact vacuum chamber connected to an inlet line
for reactant gases and an outlet line for products analysis
by a quadrupole mass spectrometer (QMS), and thischamber works as both the PTRF-XAFS cell and the batchtype reactor. The sample can be transferred to the chamberfrom another UHV chamber where the sample preparation(ion sputtering, annealing and metal deposition) and its
surface characterization (LEED, XPS) are carried out. Thesample in the PTRF-XAFS apparatus can be heated at hightemperatures (< 800 K) in the presence of reactant gases.
Fig. 2 shows Pt L3-edge PTRF-XANES spectra of thePt/Al2O3(0001) surface measured at room temperature(RT), 353 K, 393 K, 423 K and 473 K in the presence ofreactant gases (CO+O2). The white line intensity decreased
with increase of the temperature in both polarizations,Fig. 2: Pt L3-edge PTRF-XANES spectra of thePt/Al2O3(0001) surface obtained at RT, 353K, 393K, 433
K and 473K in the presence of reactant gases (CO:O2:Ar= 0.3:1.0:0.1, total pressure 100 Pa).
Acknowledgement
We thank the PF staffs for their kind technical support.
References[1] S. Takakusagi et al., Chem. Rec. 18, 1244 (2019).
[2] S. Takakusagi et al., Top. Catal. 56, 1477 (2013).
* takakusa@cat.hokudai.ac.jp