Preparation And Characterization Of Ru/CeO₂ Catalysts For Ammonia Synthesis

Ru/CeO₂ catalysts for ammonia synthesis have been prepared by precipitation method, and cheap RuCl₃ rather than Ru₃(CO)₁2 was selected as Ru precursor. In the present work, catalysts are characterized by means of N₂ physical adsorption, XRD, H₂-TPR, XPS and H₂-TPD technique, for better understanding of the effect of preparation parameters, La doping and promoters on catalytic performance of Ru/CeO₂ catalysts.The effect of the preparation parameters including of preparation methods, precipitants, reduction pretreatment and so on are systemically investigated on the Ru/CeO₂ catalyst performance under various conditions. The optimal preparation parameters are determined as follows: normal precipitation method, NH₄HCO₃ precipitant, 60℃aging 1h and reducing with H₂ and N₂ mixture at 500℃for 4 h.The amount of residual chloride, microstructure, and the ease of reduction of cerium of Ru/CeO₂ catalyst were determined by preparation methods, thus the activities of the catalysts have obvious differences. The activity of Ru/CeO₂ catalyst prepared by normal precipitation was much high since it contained only trace of chloride and cerium was easier to be reduced. Instead, the samples that prepared using deposition precipitation had large residual chloride and the dispersion of ruthenium was much lower than others, which led to the lower activities for ammonia synthesis.The precipitating agent type's influence on the activities of the Ru/CeO₂ catalysts because that the Ru loading, metal dispersion, and reduction degree of cerium dioxide are different. Compared to use KOH and NH₃·H₂O as precipitants, K₂CO₃, h, NH₄HCO₃ and (NH₄)₂CO₃ were favorable for the precipitation of ruthenium, and then the ruthenium loading was higher. Due to with higher ruthenium dispersion and the cerium is relatively easy to be reduced, the Ru/CeO₂ catalysts prepared using NH₄HCO₃, NH₃·H₂O and (NH₄)₂CO₃ as precipitants have better catalytic activities for ammonia synthesis than using KOH, K₂CO₃ and KHCO₃. The reduction degree of cerium and ruthenium dispersion of Ru/CeO₂ catalysts were also determined by the reduction time. With the degree of reduction for cerium has gradually deepened, more trivalent cerium produced, which resulting in some ruthenium on cerium surface were covered by Ce₂O₃, thus the ruthenium dispersion decreased.The activity test shows that ammonia concentration of the catalyst with 10% La is 13.9% at 10 MPa, 10000 h¹,450℃, which is 17% higher than that of Ru/CeO₂. TPR and XPS proved that La doping can improve the activity of Ru-ceria catalyst for ammonia synthesis by facilitating the reduction of oxygen which subsists in the cerium oxide surface. In addition, it can be realized that the test of catalyst stability proves the stability performance of Ru/CeO₂-La₂O₃ catalyst within the reaction time of 55 h. Potassium, Barium and samarium are able to enhance the activities of catalysts. Highest catalytic activity is obtained when the molar ratio of Potassium, barium and samarium to ruthenium are 2, 1 and 0.6, respectively. And K is the optimal promoter of Ru/CeO₂ catalyst.