Monitoring the Interaction of CO with Graphene Supported Ir Clusters by Vibrational Spectroscopy and Density Functional Theory Calculations

Abstract

The interaction of carbon monoxide (CO) with graphene supported Ir cluster (Ir/graphene/Ir(111)) and a Ir(111) single crystal surface was studied by infrared reflection–adsorption spectroscopy (IRRAS). The cluster morphology was characterized by scanning tunneling microscopy and density functional theory (DFT) calculations predicted the adsorption frequencies of CO molecules on the Ir single crystal surface and clusters. After exposing the clean Ir(111) surface to CO at 195 K, one intense vibrational band is observed at 2043 cm–1, which is assigned to on top CO species. This band shifts to a much higher frequency at 2082 cm–1 at higher CO exposure. After exposing clean graphene/Ir(111) to CO at 195 K, no CO band was observed in the IR spectra, which confirms a full graphene layer over the Ir(111) surface. However, CO molecules adsorb on Ir clusters supported on graphene/Ir(111) at 195 K. For the 0.05, 0.1, 0.15, and 0.2 ML Ir clusters, two IR bands were observed at 2060 and 2088 cm–1, 2050 and 2070 cm–1, 2048 and 2070 cm–1, and 2052 and 2070 cm–1, respectively. The IR bands at lower frequencies are assigned to the CO on one-layer high clusters, and the IR bands at higher frequencies are assigned to the CO adsorption on two- or several-layer high clusters. The IR frequencies of CO adsorbed on clusters are shifted to lower wavenumbers compared to those observed on the single crystal surface, which is in agreement with DFT calculations. The IRRAS data recorded after CO adsorption on Ir clusters at different temperatures demonstrate that CO species are stable up to 350 K, although the intensity of CO on top one-layer high cluster reduces largely, indicating CO-induced cluster sintering.

Regioselective Generation of Single‐Site Iridium Atoms and Their Evolution into Stabilized Subnanometric Iridium Clusters in MWW Zeolite

“…As displayed in Figure , a clear shift of the terminal CO adsorption band on Ir clusters from 2020 to 2065 cm −1 takes place when the reduction temperature increases from 400 to 650 °C. This shift corresponds to an increase of the size of the clusters, which is consistent with the electron microscopy characterization results . The CO adsorption band at about 1900 cm −1 can be associated to bridged CO adsorbed on very small Ir clusters present in the 0.24Ir@MWW‐400H 2 sample .…”

“…This shift corresponds to an increase of the size of the clusters,which is consistent with the electron microscopy characterization results. [33] TheC O adsorption band at about 1900 cm À1 can be associated to bridged CO adsorbed on very small Ir clusters present in the 0.24Ir@MWW-400H 2 sample. [34] On the other hand, the variation of the size distribution of the confined Ir clusters in the 0.24Ir@MWW samples is not as significant as the shift of the IR bands.O ne possible reason could be due to the change of the geometric structures of Ir clusters when being reduced at different temperatures.…”

CO Adsorption and Disproportionation on Smooth and Defect-Rich Ir(111)

“…In the 10 –6 mbar range, the observed peak positions are well in line with IRAS studies of CO on Ir(111) and graphene-supported Ir clusters. 24 …”

“…In the 10 −6 mbar range, the observed peak positions are well in line with IRAS studies of CO on Ir(111) and graphene-supported Ir clusters. 24 A single peak, characteristic of on-top CO, was observed on both surfaces. As pressure increased from 10 −7 to 1.0 mbar, the CO vibrational frequency moved from 2084 to 2094 cm −1 (coverage 0.62 to 0.77 ML) and from 2079 to 2090 cm −1 (coverage 0.56 to 0.70 ML) on "perfect" (Figure 3a,b) and "defect-rich" Ir(111) (Figure 3c,d), respectively.…”

“… 24 , 27 , 28 Two distinct LEED patterns indicated an ordered structure at 1/3 ML 1 , 17 − 19 and a (diffuse) pattern at high coverage (7/12 ML 19 and 2/3 ML 1 , 18 ). With increasing CO coverage, FT-IRAS showed that the IR spectral intensities of linearly (on-top) bonded CO increased and vibrational frequencies blue-shifted, 20 , 24 while in TPD, the desorption peaks shifted to lower temperature and different adsorption states formed, especially close to saturation. 18 − 20 Such observations/trends are quite common for CO adsorption on metals, but we could recently demonstrate that upon increasing the coverage, CO was tilted on Ir(111) [at 0.77 ML by about 20° (DFT) or 36° (SFG)].…”