CeO₂ Promoted Noble Metal Catalyst On Hydrodesulfurization Reaction

Noble metal catalysts are widely used on two-stage hydrodesulfurization(HDS)process because of their predominant activities. Since the main reason that thiophenicsulfurs are hard to be removed is their cyclic structure hindering the adsorption onactive sites of catalysts. Then, it is suggested that selective adsorption of thiophene onceria, together with the excellent ability of hydrogenation of noble metal, lead to anew kind of effective deep HDS catalyst. In this paper, the Pd metal was used as mainactive sites and theγ-Al₂O₃ as support, the activities of catalysts were detected whilethe thiophene HDS as probe reaction. The catalysts were also characterized by BET,TPR, XPS, NH₃-TPD, XRD and adsorption of thiophene. We studied the activities ofcatalysts, the interaction of Pd-Ce, and also the influence of CeO₂ on the structure ofcatalysts. Further studies included the effect of Ce contents, reduction temperature,catalyst precursors, preparation methods, and the sulfur tolerance of the catalysts.1. Sulfide Ru-CeO₂/Al₂O₃ and reduced Pd-CeO₂/Al₂O₃ catalysts were prepared byimpregnation method and their activities on HDS were detected. Comparing withRu/Al₂O₃ catalyst, the activity of sulfide Ru-CeO₂/Al₂O₃ on HDS was lower. TheHDS activity of the reduced Pd-CeO₂ /Al₂O₃ (re) prepared by staged-impregnationwas higher than that of Pd/Al₂O₃ catalyst, and while no obviously change of activitywas observed on co-impregnation method prepared Pd-CeO₂/Al₂O₃ (co) catalyst. Thedetection of adsorptive retention volume of thiophene on catalysts indicated that astrong adsorption occurred on Ru-CeO₂/Al₂O₃ and Pd-CeO₂/Al₂O₃ (re) catalyst, butthis phenomenon was not found on Pd-CeO₂/Al₂O₃ (co) catalyst. Based on theanalysis of the characterization of catalysts, the varies of the activities can beexplained by the different mechanism of sulfide catalyst and reduced catalyst. Theaddition of ceria caused a completely sulfide on Ru-CeO₂/Al₂O₃ catalyst anddecreased the amount of sulfur vacancies which acted as the active sites on HDSreaction. As to Pd based catalysts, a new kind of adsorptive and active sites ofthiophene was created on the surface of catalyst, namely the Ce³⁺,to which wastestified by the activities of Pd/SiO₂ mechanically mixed with Ce₂O₃ or CeO₂, it wasalso revealed that Ce³⁺ was produced by the interaction between Pd and Ce. 2. The influence of Ce content on HDS activity of Pd-CeO₂ /Al₂O₃ was studied. It wasfound that excellent promoting effect was only exhibited on the catalysts having lowcontent, activity showed the highest at 2% Ce content although the TOF value onsurface palladium atom had little discrepancy on all promoted catalysts. Theadsorption of thiophene on catalysts was effectively strengthened even at low Cecontent, but this trend was weakened at high Ce content. It was also observed thatwith the increasing of Ce content, the dispersion of palladium was decreased and theaggressive of CeO₂ on Al₂O₃ was appeared. Judged by the data of catalysts activities,we proposed that with the increasing of Ce content, the adsorbing and activatingthiophene were no more the rate-determining step. Instead of it, the low dispersion ofpalladium and corresponding the low adsorbing and activating hydrogen wereresponsible for the decreasing activity of Pd-CeO₂/Al₂O₃ catalyst.3. The surface structures of Pd-CeO₂/Al₂O₃ catalysts and the interaction betweenPd-Ce were studied to describe their relationship with the properties and activities ofcatalysts. Texture of surface property of catalysts, such as the surface area and porevolume, were mainly affected by the addition of the Ce, but the effect on the porediameter was neglectable. NH₃-TPD indicated that new acidic sites were created onPd-CeO₂/Al₂O₃ due to the interfacial effect, and the Pd (111) faces were predominantfaces on those catalysts which had good promoting effect, leading to a strong Pd-Ceinteraction and interfacial effect. The promoting effect of Ce was directly correlatedwith the amount of the interfacial acidic sites, catalyst with highest value ofinterfacial acid on unit surface palladium atom had the highest activity. Hence, it wasconcluded that interfacial structure and interfacial acid, together with the Ce³⁺produced by the interaction, were two essential conditions for good promoting effectof CeO₂.4. The influence of reduction temperature on catalyst was studied. It was observedthat the reduction temperature showed minor effect on the activity and structure ofPd/Al₂O₃ catalyst. As to Pd-CeO₂/Al₂O₃ catalyst, the activities showed a remarkabledecreasing with the increase of reduction temperature. High reduction temperatureresult the decrease of interfacial acid amount and also the aggressive and crystallizedpalladium particles. More important, the migrations of Ce made part of surface palladium atoms were covered. Consequently the activity of the catalyst wasdecreased due to the low rate of hydrogen activation. The pretreatment of reduction-reoxidize made the palladium redispersed and the interaction between Pd and Cereinforced, these were all advantageous to a high activity.5. Sulfur tolerance of Pd-CeO₂/Al₂O₃ prepared by impregnation method was detected,the data of activities of catalysts with time on stream indicated that thePd-CeO₂/Al₂O₃ catalyst possessed good sulfur tolerance and corresponding the lowsulfide degree of palladium metal. The result of FT-IR detection revealed that theaddition of Ce lead to a weak bond of Pd-S and increased liable sulfur amount duringthe reacion, suggesting a low partial pressure of H₂S around the palladium particles.From the date of TG, we also found H₂S interacted with CeO₂, forming a species witha structure like CeOx-S. Because the heat of H₂S adsorbed on CeO₂ is larger than thaton the Pd, then H₂S was easier to be adsorbed on Ce³⁺ but not on Pd particle due tothe competitive adsorption, it lead to a low partial pressure of H₂S around thepalladium particles, the sulfide of Pd was therefore inhibited.6. The catalysts were prepared by different methods to reveal the influence of thechlorine ion on the activity and sulfur tolerance of catalyst. The detection of XPSverified the existence of residual Cl^-1 on the surface of Pd/Al₂O₃ and thePd-CeO₂/Al₂O₃ (IM) catalyst prepared by impregnation method, but no Cl^-1 wasclearly observed on Pd-CeO₂/Al₂O₃ (DP) prepared by deposition-precipitationmethod. All Palladium atoms were positive valence after the addition of Ce even theywere reduced under H₂. The strong positive valence on Pd-CeO₂/Al₂O₃ (DP) catalystimplied a strong interaction of Pd and Ce. Although high metal dispersion onPd-CeO₂/Al₂O₃ (DP) catalyst was found because of the exclusion of the effect of Cl^-1,the low activity and the disappearance of interfacial acid on the catalyst suggested thenecessity of chlorine ions on the creation of the interfacial acid and the interfacialstructure. No deactivated and sulfide palladium on the Pd-CeO₂/Al₂O₃ (DP) catalystimplied a good sulfur tolerance of this catalyst. It is because the stronger interactionof Pd and Ce made Pd atoms exist in electron-deficient state, which inhibited thecreation of Pd-S species.