Vertically aligned gallium nitride (GaN) nanowires were synthesized on 4-inch silicon (111) wafers using molecular beam epitaxy under nitrogen-rich conditions, producing structures 600–900 nm in length and 50–100 nm in diameter. Their high surface area and one-dimensional morphology provided an ideal scaffold for catalytic sites. Controlled annealing in air at 150, 200, and 250 °C partially substituted nitrogen atoms with oxygen, forming gallium oxynitride (GaN₁₋ₓOₓ). X-ray photoelectron spectroscopy revealed a distinct N–O peak at 399.9 eV in the annealed samples, confirming oxynitride formation and an oxidation degree that increased with temperature.
Rhodium nanoparticles (Rh NPs), 9.1–11.8 nm in size, were deposited via photo-deposition. Even after annealing at 250 °C, Rh NP size and nanowire morphology remained stable. Energy dispersive X-ray spectroscopy mapping confirmed Rh distribution across the GaN scaffold. High-resolution XPS indicated strong electronic interactions between Rh and the GaN₁₋ₓOₓ support, with Rh existing as both Rh⁰ (~306.5 eV) and Rh³⁺ (308.8 eV). Notably, Rh 3d peaks shifted by 0.2 eV in Rh/GaN₁₋ₓOₓ compared to Rh/GaN, evidencing altered Rh electronic properties due to surface engineering.
Catalytic testing under atmospheric pressure showed that Rh loading critically influenced CO production rates. At 0.047 μmol·cm⁻² Rh, CO activity peaked at 32.0 mmol·g_cat⁻¹·h⁻¹ before declining at higher loadings due to particle agglomeration. Feedstock composition also shaped performance: a CO₂/H₂ ratio of 10/1 yielded 62.7 mmol·g_cat⁻¹·h⁻¹ CO with 94% selectivity, while excess hydrogen favored methane formation.
Annealed supports markedly improved activity. Rh/GaN₁₋ₓOₓ–250 achieved 127.0 mmol·g_cat⁻¹·h⁻¹ at 290 °C, 47.8% higher than unannealed Rh/GaN, and maintained >94% CO selectivity across 170–290 °C. At 260 °C, its CO activity was 6259 times greater than commercial Rh/Al₂O₃. Turnover frequency reached 270.2 mol CO per mol Rh per hour, with stability sustained over nine cycles before Rh NP agglomeration reduced performance.
Activation barrier measurements underscored the effect of surface engineering: Rh/GaN exhibited an Eₐ of 1.96 eV, decreasing to 1.38 eV for Rh/GaN₁₋ₓOₓ–250. Temperature-programmed desorption showed CO desorbing at 233 °C from Rh/GaN₁₋ₓOₓ–250, far lower than the 368 °C for Rh/GaN, indicating easier product release.
In situ DRIFTS spectroscopy identified *COOH as a key intermediate, with stronger signals in annealed samples suggesting enhanced intermediate formation. Peaks corresponding to adsorbed water confirmed H₂O generation alongside CO. Density functional theory models of Rh(111), GaN(10\bar{1}0), Rh/GaN, and Rh/GaN₁₋ₓOₓ revealed that pristine GaN binds CO₂ strongly (E_ad ≈ −1.71 eV), which hinders *COOH formation due to a high barrier (1.67 eV). Oxygen substitution reduced CO₂ adsorption energy to −1.38 eV, lowering the *COOH formation barrier and shifting the potential-determining step to *CO desorption. Rh decoration further weakened CO₂ binding, facilitating hydrogenation.
The Gibbs free energy change for CO₂-to-CO conversion remained positive at 0.14 eV, consistent with an endergonic process. Nevertheless, the synergy between Rh NPs and GaN₁₋ₓOₓ reduced energy barriers and improved desorption kinetics, aligning theoretical predictions with experimental activity gains.
The proposed mechanism begins with CO₂ and H₂ adsorption and activation to *CO₂ and *H. Hydrogenation on GaN₁₋ₓOₓ yields *COOH, which Rh NPs reduce to *CO. Surface engineering weakens CO₂ and H adsorption, accelerates *COOH formation, and shifts the rate-limiting step to CO desorption, resulting in significantly enhanced CO evolution. This integration of nanostructured GaN supports with tailored surface chemistry and Rh catalysis demonstrates a viable pathway for efficient CO₂ hydrogenation under moderate conditions.