Operando CO Infrared Spectroscopy and On-Line Mass Spectrometry for Studying the Active Phase of IrO2 in the Catalytic CO Oxidation Reaction

Abstract

We combine operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with on-line mass spectrometry (MS) to study the correlation between the oxidation state of titania-supported IrO2 catalysts (IrO2@TiO2) and their catalytic activity in the prototypical CO oxidation reaction. Here, the stretching vibration of adsorbed COad serves as the probe. DRIFTS provides information on both surface and gas phase species. Partially reduced IrO2 is shown to be significantly more active than its fully oxidized counterpart, with onset and full conversion temperatures being about 50 °C lower for reduced IrO2. By operando DRIFTS, this increase in activity is traced to a partially reduced state of the catalysts, as evidenced by a broad IR band of adsorbed CO reaching from 2080 to 1800 cm−1.

Active Phase of IrO2 in the Catalytic CH4 Combustion Reaction: Operando Infrared Spectroscopy and Online Gas Analysis

“…When CO is added to the reaction feed during cooling of the catalysts under reducing methane oxidation conditions (1% CH 4 /1% O 2 /1% CO/97% Ar; cf. Figure S5), the band at 2085 cm –1 dominates at low temperatures (<140 °C) where CH 4 oxidation is suppressed, supporting the assignment of CO being adsorbed on oxidic IrO 2 as reported in ref . In the temperature region from 280 to 200 °C no CO band can be detected in Figure S5 because adsorbed CO is efficiently removed from the surface by further oxidation to CO 2 .…”

“…The present study investigates the catalytic methane oxidation on IrO 2 supported on rutile TiO 2 , employing operando DRIFTS and online gas analysis. Operando DRIFTS is able to monitor the oxidation state of IrO 2 via specific CO vibration frequency as recently demonstrated for the CO oxidation over the same catalyst . Our study demonstrates that the most active phase in the methane oxidation is the partially reduced IrO 2 , rather than the oxidized IrO 2 phase as found for single crystalline IrO 2 (110).…”

“…Operando DRIFTS is able to monitor the oxidation state of IrO 2 via specific CO vibration frequency as recently demonstrated for the CO oxidation over the same catalyst. 24 Our study demonstrates that the most active phase in the methane oxidation is the partially reduced IrO 2 , rather than the oxidized IrO 2 phase as found for single crystalline IrO 2 (110). This observation manifests an apparent materials gap that in some conclusions drawn from a single crystalline surface may not be transferable to supported powder samples due to the materials adapting differently to the reaction conditions.…”

“…The presence of CO on the surface under methane oxidation reaction conditions has strong implications on the reaction pathway. Especially for red-IrO 2 @TiO 2 under oxidizing condition it is surprising that CO remains observable in DRIFTS at high temperatures above 200 °C, as CO oxidation should be rather fast under these conditions . Consequently, CO must be produced at a comparably high rate to be observable.…”

“…Preparation and characterization of the samples employed in this work was published in previous work. , To briefly summarize, supported IrO 2 on TiO 2 with a loading of 2 mol % was prepared via the Pechini synthesis. DRIFTS needs a reflective matrix (TiO 2 ) when the material under investigation (IrO 2 ) absorbs the IR radiation.…”