Ru/ZrO2-based catalysts for CO2 hydrogenation and investigation on reaction mechanism
Creators: name: Zhao, Jiajia
Publisher: name: RMIT University
Year: 2025
Subjects: classificationCode: 340204
subject: Inorganic green chemistry

description: The rising concern about extensive depletion of fossil fuels and resulting high CO2 concentration urges the shift into the use of renewable fuels. Under this critical circumstance, CO2 utilisation presents outstanding advantages to mitigate the record-high CO2 concentration in the atmosphere. One approach of CO2 utilisation is CO2 hydrogenation, which CO2 and H2 react over a catalyst to produce carbon-neutral fuels like methane (CH4). Thermal-catalytic CO2 hydrogenation into hydrocarbons have been widely investigated as one promising CO2 hydrogenation strategy and potentially can be adapted to the existing industrial sectors. However, their cost-effective industrial applications will highly depend on the design of efficient catalysts that stably operated at high temperature and elevated pressure, whilst providing high conversion rate and selectivity. Commercial Ni-supported catalysts for large- scale CH4 production are restrained due to the deactivation by coke formation. Ruthenium (Ru) is known for its durability but bares high cost. Controlling Ru loading and introducing other metal promoters has been studied intensively in literature. To achieve desirable catalytic performance, it’s critical to understand the synergy amongst all the catalyst components and disclose the structure-activity relationship for optimising the catalyst systems. In this study, Ru was selected as active metals for H2 dissociation and spillover, while ZrO2 was selected as support materials for providing CO2 adsorbing sites and building up the strong metal-support interaction (SMSI). The structure modifications of Ru/ZrO2-based catalysts in this work include doping ZrO2, ZrO2 phase control and bimetallic RuGa supported ZrO2.

For dopant study, Ru/SnxZr1-xO2 catalysts were prepared using co-precipitated Sn-doped ZrO2. Ru/ZrO2 and Ru/Sn0.01Zr0.99O2 both comprised tetragonal (t-) and monoclinic (m-) ZrO2 nanoparticles decorated by RuO2·H2O clusters. Ru/Sn0.2Zr0.8O2 exhibited similar textural properties and metal speciation, however, X-ray diffraction evidenced new crystalline phases wherein Sn atoms substituted for Zr. Operando XRD evidenced minimal of ZrO2 and Sn-doped ZrO2 phases, despite which 20 atom% Sn switched product selectivity from 99 % CH4 for Ru/ZrO2 to 67 % CO, due to increased support CO2 adsorption capacity/strength. Operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) identify linear surface CO* as a catalytic spectator with (bi)carbonate a precursor to reactive HCOO*/carboxylate intermediates to methane. Sn0.2Zr0.8O2 promotes CO2/(bi)carbonate/formate decomposition, favouring a reverse water gas shift pathway.
 
For support phase study, monoclinic (m-) and tetragonal (t-) ZrO2 supports were prepared via hydrothermal method with subsequent Ru supported at different concentrations (0.5, 1.1 and 1.7 wt%) using incipient wetness impregnation (IWI) method. Ru species demonstrate different dispersions and oxidation states on m- and t-ZrO2 surfaces. High-resolution transmission electron microscopy (HRTEM) evidence that Ru exhibit similar atomic dispersion on m-ZrO2, while Ru cluster sizes increase on t-ZrO2 as Ru concentration increasing. X-ray photoelectron spectroscopy (XPS) demonstrates the lower binding energy of RuO2 species on t-ZrO2, indicating more reducible Ru species on t-ZrO2 than on m-ZrO2. Operando DRIFTS study confirm the formate species are key intermediates for CH4 production over Ru/m-ZrO2 catalysts, while carboxylate species are key intermediates for CH4 formation over Ru/t-ZrO2. And linear surface CO* as a catalytic spectator are easier to form up on smaller Ru clusters as more CO* species are observed on Ru/m-ZrO2 than on Ru/t-ZrO2 samples.

For bimetallic study, RuGa/ZrO2 and Ru/ZrO2were prepared using precipitation with Ga/ZrO2 as control samples. XPS shows that Ga addition increases Ru0/(Ru0 + Ru(Ⅳ)) ratio and improves the Ru reducibility induced by electron transfer between Ru and Ga species. For sample RuGa(4:1)/ZrO2, small amount of Ga improves Ru dispersion and the CO2 conversion with high CH4 selectivity. For sample RuGa(1:5)/ZrO2, higher amount of Ga demonstrates more efficient electron transfer and increases the strong basic sites in samples. Operando DRIFTS-MS confirms that more formate species form during CO2 hydrogenation for RuGa(4:1)/ZrO2, thereby enhancing CH4 production. However, RuGa(1:5)/ZrO2 sample exhibits product selectivity shifts to CO formation. Operando DRIFTS-MS reveals that higher Ga concentration inhibits the formation of formate species and promote the decomposition of carboxylate species into CO.

This work has contributed to the catalyst engineering for CO2 hydrogenation. The understanding of the obtained materials provides the insights to establish the structure-activity relationship for optimising CO2 reduction catalysts and can lead to opportunities of carbon- neutral fuel production.