Kinetics of H₂ Adsorption at the Metal–Support Interface of Au/TiO₂ Catalysts Probed by Broad Background IR Absorbance

Abstract

H 2 adsorption on Au catalysts is weak and reversible,m aking it difficult to quantitatively study.W e demonstrate H 2 adsorption on Au/TiO 2 catalysts results in electron transfer to the support, inducing shifts in the FTIR background. This broad background absorbance (BBA) signal is used to quantify H 2 adsorption;a dsorption equilibrium constants are comparable to volumetric adsorption measurements.H 2 adsorption kinetics measured with the BBAs how al ower E app value (23 kJ mol À1)f or H 2 adsorption than previously reported from proxy H/D exchange (33 kJ mol À1). We also identify ap reviously unreported H-O-H bending vibration associated with proton adsorption on electronically distinct Ti-OH metal-support interface sites,p roviding new insight into the nature and dynamics of H 2 adsorption at the Au/TiO 2 interface.

Hydrogen Adsorption at the Au/TiO2 Interface: Quantitative Determination and Spectroscopic Signature of the Reactive Interface Hydroxyl Groups at the Active Site

“…Our previous study focused on quantifying the BBA signal and included only a cursory examination of changes to the MSI hydroxyls on two catalysts; 57 the extension of this study to four catalysts allows for a more complete examination of the hydroxyl groups involved in H 2 activation. Upon H 2 adsorption, an additional feature near 3300 cm −1 appears, corresponding to a new O−H stretching frequency.…”

“…The details on catalyst synthesis and characterization, as well as scanning transmission electron microscopy (STEM) images, are available in the SI; these methods and results are comparable to what we have described previously. 57 Selected characterization data (particle sizes, elemental analysis, BET surface areas, and H 2 adsorption measurements) are available in Table 1. Three catalysts (a−c) were prepared with similar average particle sizes (2.7−3.0 nm) but with different Au loadings.…”

“…31,57,64−67 When correlated with volumetric H 2 adsorption measurements, the BBA signal can be used to quantify H 2 adsorption on Au/TiO 2 catalysts. 57 A new feature centered near 1580 cm −1 , which we previously assigned to the δ HOH vibration of protonated MSI hydroxyl groups ( MSI TiOH 2 + ), also appears. These features occur during H 2 adsorption on all four catalysts, as shown in Figure S3.…”

“…39,40 Our own work has provided an abundance of kinetic evidence for the importance of heterolytic H 2 activation in hydrogenation reactions 38,54−56 and allowed us to quantify H 2 adsorption with in situ FTIR spectroscopy. 57 Important fundamental questions remain regarding the nature of the H 2 activation active site(s) and how they might be modified to improve broader hydrogenation activity. Particle size effects have been implicated in several high-profile Au-catalyzed reactions 21,23,31,58−61 and have been suggested to be important in selective hydrogenations.…”

“…62 We previously showed that H 2 adsorption on Au/TiO 2 results in electron injection into the support; this can be quantified via a broad-background absorbance (BBA) and correlated with volumetric adsorption measurements. 57 In this work, we extend our study to include a full examination of the support hydroxyls directly at the interface. These hydroxyls have very low surface concentrations, making them exceptionally difficult to probe and characterize.…”