Infrared Spectroscopy of Mixed Nitric-Oxide−Carbon-Monoxide Adlayers on Ordered Iridium(111) in Aqueous Solution:  A Model Study of Coadsorbate Vibrational Interactions

Abstract

In-situ infrared reflection−absorption spectra are reported for mixed nitric-oxide−carbon-monoxide adlayers along with the constituent chemisorbates separately as a function of coverage on ordered Ir(111) at 0.4−0.45 V vs standard hydrogen electrode in aqueous 0.1 M HClO4, with the objective of assessing the composition-dependent nature of the coadsorbate vibrational interactions. This substrate−coadsorbate combination provides an informative model system since both chemisorbates appear to bind exclusively in atop (or near-atop) surface sites on the basis of their simple N−O (νNO) and C−O (νCO) vibrational fingerprints, and composition-dependent mixed adlayers can readily be formed via partial replacement of saturated irreversible adsorbed NO layers by exposure to dilute CO solutions. Increasing the CO coverage, θCO, both in the absence and presence of coadsorbed NO, yields marked progressive blueshifts in the νCO band frequency (ca. 2020−2075 cm-1), attributable chiefly to enhanced dipole−dipole coupling. While adsorption of NO alone exhibited virtually coverage (θNO)-independent νNO frequencies, ca. 1835 cm-1, indicative of chemisorbate island formation, dilution within mixed CO/NO adlayers yields progressive νNO redshifts (down to ca. 1790 cm-1). The composition-dependent νCO and νNO frequencies within the CO/NO adlayers are consistent with molecular intermixing, as supported by comparison with numerical simulations extracted from conventional dipole-coupling theory, although the observed nonlinear νCO−θCO dependence suggests the formation of locally enriched CO regions at intermediate compositions. Evidence supporting coadsorbate intermixing is obtained by comparing the composition-dependent CO and NO band absorbances with the dipole-coupling predictions. In particular, the presence of coadsorbed CO yields marked (up to 3-fold) decreases in the NO band absorbance, especially toward lower νNO values, which arise from band-intensity transfer to neighboring higher-frequency (CO) oscillators. Despite the large (ca. 250 cm-1) difference in νNO and νCO singleton frequencies, this striking effect is in approximate agreement with dipole-coupling theory, again presuming molecular CO/NO intermixing. The observed marked increases in the νCO bandwidth toward lower θCO values, along with pronounced asymmetric band shapes, are in good agreement with theoretical predictions that include stochastic fluctuations of the local adsorbate population density. Moreover, the larger intermediate-θCO νCO bandwidths observed in the presence of coadsorbed NO are also quantitatively accounted for on this basis in terms of coadsorbate intensity transfer. The more broad-based utility of such dipole-coupling analyses for elucidating local interactions within mixed adlayers is considered in light of these findings.

Reduction of NO by propene over supported iridium catalysts under lean-burn conditions: an in situ FTIR study

“…5 shows the infrared spectra of the reduced 1 wt.% Ir/Al 2 O 3 solid contacted with 1000 vpm NO in He at increasing temperatures. At room temperature mainly one band at 1878 cm −1 has been detected, being attributed to νNO on Ir 0 in agreement with literature data [35] on single crystals [30][31][32]34] reporting a νNO band in the 1870-1800 cm −1 range for NO linearly adsorbed on Ir 0 faces. A broad band of lower intensity is observed at ca.…”

“…The 2090 cm −1 band can be attributed to CO linearly adsorbed on Ir 0 surface sites of fully reduced supported particles [25]. Such a band has been observed upon CO adsorption on Ir surface sites obtained by decomposition and H 2 reduction of Ir 4 (CO) 12 on Al 2 O 3 , SiO 2 or zeolites [26][27][28][29], and upon CO adsorption on various faces of Ir single crystals [30][31][32]. The exact position is function of the CO coverage.…”

In situ FTIR studies of coadsorption of CN− and CO on Pt(110) electrode surface

“…Studying the effect and nature of the intermolecular interactions involved in the coadsorption is of particularly significance in fundamental of electrocatalysis and other relevant fields, because of its distinct effect on adsorbate structure and reactivity at the metal-solution interface [1].…”

Interpretation of IR Spectra of Ultrathin Films

“…This effect retains when the substrate is placed in an aqueous environment ( Fig. 3.37a) [84] (see also Ref. [85]).…”

“…Coadsorption of highly polarizable species (e.g., solvent molecules or metal adatoms) can cause a red shift of the absorption band of the adsorbate by up to 50-80 cm −1 [65,84,85,109,110] and an attenuation of the band intensity by a factor of 2-20 [110]. When an oscillator is surrounded by such species, the local electric field acting on it decreases due to dielectric screening.…”