Enhanced methanol synthesis via CO₂ hydrogenation under mild conditions: Tailoring Cu/ZnO/Al₂O₃ catalysts through precursor selection for optimized Cu-ZnO interface interaction

Methanol synthesis via CO₂ hydrogenation is a promising route for sustainable chemical production, providing an alternative to fossil-based methanol. In this study, Cu/ZnO/Al₂O₃ (CZA) catalysts were synthesized via wet co-impregnation using different Cu and ZnO precursor salts (nitrate, acetate, and chloride) to investigate their impact on catalytic performance. Characterization by BET, XRD, Raman spectroscopy, UV–Vis DRS, TPR, XPS, FESEM, and STEM revealed significant differences in Cu dispersion, metal-support interaction, and Cu phase distribution. CO₂ hydrogenation tests were conducted over a temperature range of 180–350 °C and a pressure range of 1–7 bar to evaluate methanol selectivity and productivity. Characterization results showed that CZA_nitrate exhibited the strongest Cu/ZnO interaction and the highest Cu/ZnO interface content, evidenced by a prominent Cu2+/ZnO charge transfer band in UV–Vis DRS, a lower-temperature reduction peak in TPR, and Cu species quantification at the Cu/ZnO interface via TPR. In contrast, CZA_chloride had significantly lower Cu/ZnO interface content, as confirmed by TPR peak deconvolution, and exhibited CuAl₂O₄ species formation (evidenced also by Raman and XRD), which hindered Cu-ZnO interactions. Among the catalysts tested, CZA_nitrate demonstrated the best catalytic performance, achieving 100 % methanol selectivity at 180–200 °C under atmospheric pressure along with the highest methanol productivity. A strong correlation between Cu/ZnO interface content and methanol productivity was further validated by linear regression analysis. Stability tests confirmed that CZA_nitrate maintained high performance over time, whereas CZA_chloride deactivated due to coke formation, likely caused by CuAl₂O₄ phases limiting Cu/ZnO synergy. Comparison with a commercial Cu/ZnO/Al₂O₃ catalyst (CZA_commercial) revealed that while both catalysts were active, CZA_nitrate significantly outperformed the commercial catalyst, achieving higher methanol selectivity (100 % vs. 23 %) at WHSV of 10 h−1, and 1 bar. At elevated pressures (up to 7 bar), methanol selectivity remained high at lower temperatures, while at 250 °C, increased pressure mitigated RWGS side reactions, enhancing selectivity. Methane formation remained negligible across all conditions, confirming that the primary side reaction was the RWGS reaction. A comparison with literature data demonstrated that CZA_nitrate outperformed previously reported Cu-based catalysts at atmospheric pressure, achieving superior methanol selectivity even at low temperatures (180–200 °C). This study highlights CZA_nitrate as an optimized catalyst for CO₂ hydrogenation and emphasizes the crucial role of precursor selection in designing highly efficient Cu-based catalysts for selective CO₂ conversion to methanol.