Theoretical Prediction and Experimental Verification of IrOx Supported on Titanium Nitride for Acidic Oxygen Evolution Reaction.

Reducing iridium (Ir) catalyst loading for acidic oxygen evolution reaction (OER) is a critical strategy for large-scale hydrogen production via proton exchange membrane (PEM) water electrolysis. However, simultaneously achieving high activity, long-term stability, and reduced material cost remains challenging. To address this challenge, we develop a framework by combining density functional theory (DFT) prediction using model surfaces and proof-of-concept experimental verification using thin films and nanoparticles. DFT results predict that oxidized Ir monolayers over titanium nitride (IrOx/TiN) should display higher OER activity than IrOx while reducing Ir loading. This prediction is verified by depositing Ir monolayers over TiN thin films via physical vapor deposition. The promising thin film results are then extended to commercially viable powder IrOx/TiN catalysts, which demonstrate a lower overpotential and higher mass activity than commercial IrO2 and long-term stability of 250 h to maintain a current density of 10 mA cm-2. The superior OER performance of IrOx/TiN is further confirmed using a proton exchange membrane water electrolyzer (PEMWE), which shows a lower cell voltage than commercial IrO2 to achieve a current density of 1 A cm-2. Both DFT and in situ X-ray absorption spectroscopy reveal that the high OER performance of IrOx/TiN strongly depends on the IrOx-TiN interaction via direct Ir-Ti bonding. This study highlights the importance of close interaction between theoretical prediction based on mechanistic understanding and experimental verification based on thin film model catalysts to facilitate the development of more practical powder IrOx/TiN catalysts with high activity and stability for acidic OER.

Adsorption of acrolein and its hydrogenation products on Cu(111).

The adsorption of acrolein and its hydrogenation products propanal, 1-propanol, and 2-propenol on Cu(111) was studied by reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The experimental RAIR spectra were obtained by adsorbing multilayers of each molecule at 85 K and then annealing the surface up to 200 K to desorb the multilayer and produce the most stable monolayer structure on the surface. Each of the four molecules adsorbs weakly to the surface and desorbs at temperatures below 225 K. Compared to acrolein and propanal, the two alcohols, 2-propenol and 1-propanol, have notably higher desorption temperatures and broadened and redshifted O-H stretches that reveal strong hydrogen bonding in the multilayers. Upon annealing to 160 K, the OH stretches of both 2-propenol and 1-propanol disappear, indicating the hydrogen bonding in the multilayers is not present in the monolayers. For 2-propenol, the hydrogen bonding in the multilayer correlates with the observation of the CC stretch at 1647 cm-1, which is invisible for the monolayer. This suggests that the CC bond is parallel to the surface for monolayer coverages of 2-propenol. Similarly, for propanal, the CO stretch peak at 1735 cm-1 compared to those at 1671 and 1695 cm-1 is very weak at low coverages but becomes the most prominent peak for the multilayer, indicating a change in molecular orientation. For acrolein, the out-of-plane bending modes are more intense than the CO stretch at submonolayer coverages, indicating that the molecular plane is mainly parallel to the surface. In contrast, the opposite intensity trend was observed for multilayer acrolein, suggesting that the CO bonds are tilted away from the surface.

Adsorption properties of acrolein, propanal, 2-propenol, and 1-propanol on Ag(111).

Reflection absorption infrared spectroscopy and temperature programmed desorption were used to study the adsorption of acrolein, its partial hydrogenation products, propanal and 2-propenol, and its full hydrogenation product, 1-propanol on the Ag(111) surface. Each molecule adsorbs weakly to the surface and desorbs without reaction at temperatures below 220 K. For acrolein, the out-of plane bending modes are more intense than the C[double bond, length as m-dash]O stretch at all coverages, indicating that the molecular plane is mainly parallel to the surface. The two alcohols, 2-propenol and 1-propanol, have notably higher desorption temperatures than acrolein and display strong hydrogen bonding in the multilayers as revealed by a broadened and redshifted O-H stretch. For 1-propanol, annealing the surface to 180 K disrupts the hydrogen-bonding to produce unusally narrow peaks, including one at 1015 cm-1 with a full width at half maximum of 1.1 cm-1. This suggests that 1-propanol forms a highly orderded monolayer and adsorbs as a single conformer. For 2-propenol, hydrogen bonding in the multilayer correlates with observation of the C[double bond, length as m-dash]C stretch at 1646 cm-1, which is invisible for the monolayer. This suggests that for monolayer coverages, 2-propenol bonds with the C[double bond, length as m-dash]C bond parallel to the surface. Similarly, the C[double bond, length as m-dash]O stretch of propanal is very weak for low coverages but becomes the largest peak for the multilayer, indicating a change in orientation with coverage.