Probing Catalytic Sites and Adsorbate Spillover on Ultrathin FeO2–x Film on Ir(111) during CO Oxidation

The spatially resolved identification of active sites on the heterogeneous catalyst surface is an essential step toward directly visualizing a catalytic reaction with atomic scale. To date, ferrous centers on platinum group metals have shown promising potential for low-temperature CO catalytic oxidation, but the temporal and spatial distribution of active sites during the reaction and how molecular-scale structures develop at the interface are not fully understood. Here, we studied the catalytic CO oxidation and the effect of co-adsorbed hydrogen on the FeO2–x/Ir(111) surface. Combining scanning tunneling microscopy (STM), isotope-labeled pulse reaction measurements, and DFT calculations, we identified both FeO2/Ir and FeO2/FeO sites as active sites with different reactivity. The trilayer O–Fe–O structure with its Moiré pattern can be fully recovered after O2 exposure, where molecular O2 dissociates at the FeO/Ir interface. Additionally, as a competitor, dissociated hydrogen migrates onto the oxide film with the formation of surface hydroxyl and water clusters down to 150 K.

We observed a desorption peak at 600 K denoting the weakly bonded oxygen layer, which is lower than other reports 2 (peak at 700-800 K).This is mainly due to the lower heating speed (3 K/min) we applied compared with others 2 (120 K/min).Pristine FeO films decompose at temperatures as high as 1170 K 3 .CO molecules start to desorb from Ir(111) surface at 400 K and the desorption peak appears at ca. 500 K.The interaction between CO and Ir surface determines the CO 2 performance with three significant stages: 1, Below 400 K, the CO 2 generation is prohibited due to strong CO adsorption, also known as poison effects; 2, between 400-550K, there are still active sites for O 2 adsorption on CO-dominant surface where CO 2 generation is more sensitive to O 2 pulse; 3, above 550 K, CO-dominant surface turn to O-dominate surface where CO 2 generation is more sensitive to CO pulse.In order to minimize the interference of CO 2 (CO) generation from the Ir surface, we choose 500 K as the reaction temperature on FeO x /Ir.As shown in Fig. 1a, for FeO 2-x / Ir, the distribution of trilayer WBOs as active phases on the surface is relatively inhomogeneous.The interfacial fast reactive region is gradually resumed during the O 2 dose period, as seen in Fig. S5 0-400 s period (blue shade).Once the interfacial WBOs fully recovered, the CO 2 (CO) generation reaches a fast plateau: a cycle of interfacial WBOs consumption and recovery.Before that, WBOs on terraces contributed to CO 2 generation with a lower reaction rate, which we regard as the silence phase or low-reactivity phase.Compared with the surface morphology in Fig. 3B (30 min CO treatment), the coverage of the trilayer oxygen moiré pattern further decreases after another 30 min CO treatment not only at the FeO x /Ir interface but also at the FeO terrace.Consistent with the CO 2 performance, we confidently identify that both weakly bonded oxygen at the interface and terrace are active phases for CO oxidation.We notice that similar to images in Fig. 3B, even though the WBOs at the interface are fully consumed, there are still a large amount of WBOs remaining at the terrace, indicating that the WBOs at the terrace possess a lower reactivity than one at the interface.As mentioned, due to the exchange between dissociated 18 O atom and the remaining 16 O atom on oxide during oxygen treatment at 570 K, the 13 C 16 O 18 O products gradually dominate compared with 13 C 16 O 16 O.Furthermore, when the amount of oxygen increase, the ratio changes faster.As shown in Fig, S3 and S4, the CO molecules start to desorb from the Ir surface with remaining sites for oxygen dissociative adsorption.Based on classical collision theory, with increasing oxygen molecules, the frequency of collisions on the surface consequently turns faster as well as the dissociation rate.The directly positive (and nearly linear) correlation between oxygen pressure and ratio dynamics indicates that the rate-limiting step for WBO re-generation is the dissociation of oxygen on the surface rather than surface O atoms reaction with interfacial Fe-O.After hydrogen treatment, we observe larger protrusions on the surface, especially at the FeO 2-x /Ir interface.According to the height profile, there are mainly two different types of protrusions: one is small (ca. 1 Å, #1 in Fig. S9) and not only appears at the interface but also at the terrace, the other is much larger (ca.2.5 Å for monolayer and ca. 5 Å for bilayer) and mostly located at the interface.According to reference 6,7 , we regard the former as surface OH species and the latter as water clusters which may form a framework due to hydrogen bonds.Limited by the current spatial resolution, it is a challenge for us to resolve their fine structure.Here we dosed D 2 on fresh-oxidized FeO 2 /Ir at five temperature set points.A strong D 2 18 O production was observed above 400 K resulting from the dissociation of surface hydroxyl species and desorption of surface water cluster.The results match with former STM images and can be rationalized with the hydrogen spillover mechanism as shown in Fig. 6.

Fig. S3 .
Fig. S3.CO-TPD experiment on Ir(111) with different amounts of CO dose at 300 K.The heating rate is 0.5 K/s.

Fig. S7 .
Fig. S7.STM images of 0.6 ML FeOx/Ir in Fig. 3B after another 30 min CO treatment (60 mins CO treatment in total) at 500 K. Arrow indication the the reaction initially begin at FeO2/Ir boundary, and cycle indicate the reaction start at the FeO 2 terrance.

Fig. S8 .
Fig. S8.The initial CO 2 isotopic ratio ( 13 C 16 O 18 O: green, 13 C 16 O 16 O: brown) develops with reaction cycles where FeO and CO are the sources of 16 O and O 2 is the source of 18 O with different amounts of O 2 during the re-oxidation period a) ~80 L and b) ~300 L.

Fig. S9 .
Fig. S9.STM image (2.0 V, 0.1 nA) of the FeO 2 film on Ir(111) with the existence of surface hydroxyl species and water clusters, acquired at 300 K after ca. 100 L hydrogen treatment at 500 K.

Fig
Fig. S10.(a) Potential energy along a designed reaction pathway for CO reaction with a WBO on FeO 2 terrace, starting from physical adsorption and ending with the state of free carbon dioxide with the slab without an O atom.The insets show the geometries along the reaction path.(b) The reaction pathway is defined using two reaction coordinates: the distance between C and the surface O (RCO) and the height of the CO molecule (hCO).The black points mean that the geometry is relaxed with the two reaction coordinates fixed, while the blue points represent linear interpolation between the geometries represented by two black points.