Interests in noble metal single-atom catalysts (SACs) have been on the rapid rise. A factor that complicates single-atom catalysis is the instability and mobility of single metal atoms, which often leads to dynamic, condition-dependent catalyst structures. Understanding of such behaviors, especially regarding their catalytic consequences, is limited. Here we present the evolution of anatase-TiO2-supported Pd single atoms under various catalysis-related atmospheres that alters their activity for the reverse water-gas shift (rWGS) reaction. A combination of kinetic, microscopic, and spectroscopic investigations revealed that at &gt; 350 °C under H2, the gradual reduction, i.e., hydrogenation, of TiO2 leads to electron-rich Pd single atoms with elongated Pd−O interfacial bonds. The new Pd chemical environment activates Pd/TiO2 SAC for the rWGS, despite the aggregation of a fraction of Pd single atoms into small (~1 nm) clusters, and is not observed with other noble metals (Pt, Rh, or Ru) or supports (CeO2, SiO2, or rutile TiO2). Air exposure completely oxidizes Pd, restores the normal Pd−O interface, reversing the activation at room temperature, and also re-disperses small Pd clusters into single atoms at 400 °C. The 400 °C air treatment also enhances the reducibility of TiO2, leading to more effective activation of Pd/TiO2 by a subsequent second H2 treatment. On the other hand, large (~5 nm), stable Pd particles are formed under CO, deactivating Pd/TiO2 SAC. Under the rWGS condition, H2 and CO in the reaction stream affects the Pd structure simultaneously, and hence the rate increases with time-on-stream at low CO2 conversion. This work systematically depicts the dynamic responses of Pd/TiO2 (anatase) SAC to H2, CO, rWGS stream, and air at different temperatures, and demonstrates the profound catalytic impacts of these events. It enhances the understanding of the behaviors of supported single atoms during catalysis, and contributes to the mechanistic inquiry of the reaction.

Dynamic response of Pd/TiO2 (anatase) single-atom catalyst to atmospheres and its catalytic consequences for the reverse water-gas shift

ACS Fall 2021

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