Local Electronic Modification in TiO₂-Supported Pd Nanocatalysts for Carbon Dioxide Hydrogenation: A Density Functional Theory Study

Abstract

The excessive consumption of fossil fuels has led to critical issues such as energy crises and global climate change. CO₂ hydrogenation offers an appealing solution by converting CO₂ into valuable chemicals like CO and CH₄, thereby establishing an artificial carbon cycle. However, the intrinsic stability of CO₂ and the complex kinetic processes result in unfavorable activity and selectivity to desirable product, necessitating the development of high-performance catalysts.

Among various advanced catalytic systems, Pd-based catalysts have been extensively studied for CO₂ hydrogenation, primarily due to the exceptional H₂ splitting ability and long-term stability of Pd. Despite significant progress, there remains two major challenges associated with Pd-based catalysts in CO₂ hydrogenation: limited catalytic activity arising from insufficient mechanistic understanding of interfacial Pd species and inferior selectivity caused by the inherent limitations from linear adsorption relationship. To overcome these limitations, this thesis focuses on local electronic modification strategies, guided by density functional theory (DFT) calculations, to screen high-performance catalyst systems. Specifically, two representative catalyst systems are explored accordingly, TiO₂-supported bimetallic and muti-valent Pd nanocatalysts, designed to tune the adsorption strength of key reaction species and ultimately achieve efficient and selective CO₂ hydrogenation.

First, bimetallic Pd-based catalysts are anticipated to break the scaling relationship and steer exclusive CO production by leveraging the synergy of electronic and geometric effect. Herein, PdCu nanoalloys supported on TiO₂ were developed to maximize the bimetallic effect and investigate the underlying reaction mechanism. The enhanced reverse water-gas shift (RWGS) activity observed in the PdCu/TiO₂ system is attributed to the synergy between the ligand effect and ensemble effect. The ligand effect induces a downshift in the Pd d-band of PdCu compared to pure Pd, weakening the binding of reaction intermediates such as *CO₂, *COOH, and *CO to varying degrees. Upon forming the PdCu nanoalloy, the Pd-Cu hybrid sites combine the high reactivity of Pd for C-anchored intermediates and the stability of Cu for O-anchored intermediates. As a result, Pd₂₈Cu₂₈/TiO₂, with a balanced PdCu ratio, effectively activates CO₂ for efficient conversion while facilitating the *CO desorption and the *OH hydrogenation. This rational design overcomes the inherent trade-off between activity and selectivity within parent Pd and Cu catalysts, providing a promising strategy for efficient RWGS catalysis.

Secondly, to clarify the role of interfacial Pd species in CO₂ hydrogenation, we proposed a series of Pd-PdO_x@TiO₂ model catalysts to demonstrate the critical role of multivalent Pd⁰-Pd^δ+ species and their synergy in boosting CO₂ methanation. Pd⁰ sites are found to facilitate H₂ dissociation, while interfacial Pd^δ+ sites are identified as the primary active sites for CO₂ activation after examination of various adsorption configurations. Investigations into charge relocation behavior and bonding mechanisms reveals that varying the Pd⁰/Pd^δ+ ratios on the TiO₂ support can effectively manipulate the electronic states of Pd at interfacial sites. The Pd₂₁-Pd₈O₉@TiO₂ nanocatalyst is featured with enhanced electron transfer to adsorbed CO₂, thus favoring CO₂ activation. Detailed simulations of the stepwise reaction pathway suggest that CO₂ methanation proceeds via the *CO hydrogenation route. Notably, the Pd₂₁-Pd₈O₉@TiO₂ delivers the lowest kinetic barrier of 1.64 eV for the rate-determining step, significantly lower than other models. This improvement is attributed to the synergistic effect of multi-valent Pd⁰-Pd^δ+ sites in stabilization of the key intermediate *CH₃. In conclusion, this work underscores the potential of interfacial Pd^δ+ sites in CO₂ methanation, particularly highlighting the synergy with zero-valent Pd counterparts. These findings open new perspectives for rational interface engineering towards efficient and selective CO₂ hydrogenation.

Date of Award	26 May 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Tsan-Yao Chen (External Supervisor) & Alice HU (Supervisor)