Comparative Analysis of Photocatalytic CO2 Reduction Catalysts Across Synthesis Routes

Thesis Abstract

The escalating urgency of mitigating atmospheric CO2 while enabling sustainable chemical feedstocks has intensified interest in photocatalytic CO2 reduction (PCR) and its sensitivity to catalyst synthesis routes. This study investigates how different synthesis routes—sol-gel, hydrothermal, electrospinning, and solvothermal—influence the activity, selectivity, stability, and mechanistic pathways of PCR catalysts composed of layered oxide and metal oxide composites (e.g., TiO2-based, ZnO-CuO, and BiVO4) under comparable testing conditions. The aim is to provide a cross-sectional assessment that identifies which synthesis methods yield the most favorable balance between turnover frequency, product selectivity toward CO and CH4, and photostability, informing practical catalyst design. Specific objectives include (i) to quantify photoreduction performance (TOF, TOFCO, and selectivity towards CO, CH4, and H2) for each catalyst synthesized by the four routes; (ii) to correlate structural, electronic, and surface properties (crystallinity, phase composition, surface area, defect density, and band alignment) with PCR activity using X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), and transmission electron microscopy (TEM); (iii) to elucidate reaction mechanisms and active-site roles via operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and isotopic labeling with 13CO2; and (iv) to perform a multivariate statistical analysis to identify the most significant synthesis-route factors influencing activity and selectivity. The methodology employs an explanatory sequential mixed-methods design. The population comprises facilely synthesized photocatalysts in four synthesis routes (sol-gel, hydrothermal, electrospinning, solvothermal), each prepared under standardized precursor ratios and calcination protocols to yield comparable particle sizes. A total of 24 catalyst samples (6 per route) are evaluated under identical photocatalytic reaction conditions 10 vol% CO2 in Ar, 1 sun AM1.5G illumination, 25°C, with a 4-hour test period per run. Data collection instruments include gas chromatography with flame ionization and thermal conductivity detectors for product quantification, high-resolution XRD for phase identification, SEM/TEM for morphology, BET for surface area, XPS for surface composition and oxidation states, UV-Vis DRS for band-gap estimation, and operando DRIFTS for mechanistic insights. Reproducibility is ensured by triplicate runs for each sample, and calibration standards for CO, CO2, CH4, and H2 are employed to validate measurements. Validity and reliability are addressed through instrument cross-calibration, blinded sample labeling, and standard reference materials. Data analysis employs descriptive statistics and ANOVA to compare performance across synthesis routes, followed by regression analysis to correlate physicochemical descriptors (surface area, crystallinity, defect density, band gap, and copper/nickel loading where present) with PCR activity metrics. A response surface methodology (RSM) is applied to model the influence of synthesis variables on key outcomes, and principal component analysis (PCA) is used to reduce dimensionality and highlight dominant factors. Theoretical framing integrates the semiconductor photocatalysis theory with the Marcus–Hush electron-transfer framework and defect-chemistry concepts to interpret how synthesis-induced microstructure governs charge separation and surface reaction steps. 13CO2 isotopic labeling and operando DRIFTS provide corroborative evidence for reaction pathways, distinguishing carbonate, formate, and methane intermediates, while density functional theory (DFT) calculations supplement experimental findings with adsorption energies and favorable reaction coordinates for the most promising catalyst classes. Key expected findings include (i) a measurable variation in PCR activity and selectivity across synthesis routes, with sol-gel- and electrospun-derived catalysts demonstrating enhanced surface active sites and photostability relative to hydrothermal and solvothermal counterparts; (ii) a robust positive correlation between high surface area, appropriate TiO2 phase composition, and moderate defect density with higher TOFCO and CO selectivity, and (iii) operando spectroscopy identifying distinct reaction intermediates consistent with CO and CH4 production pathways that are favored by specific defect profiles and band alignments. The study contributes to knowledge by delivering a definitive cross-route framework linking synthesis strategies to photocatalytic performance in CO2 reduction, enabling tailored catalyst design with reproducible performance metrics. The main conclusion anticipates that optimization of defect engineering and phase composition through electrospinning and sol-gel routes offers superior PCR performance, and recommendations include adopting route-specific post-synthesis treatments (calcination atmosphere, temperature, and grain growth control) and standardizing evaluative metrics to enhance comparability across studies.