Understanding the Catalytic Behavior of Pt-based Bimetallic Catalysts

Abstract

Pt-based catalysts have been well known for its high activities in various reactions. However, due to its scarcity in Earth's crust, high cost, and poor thermal stability, various ways for an effective utilization of platinum have been developed and investigated. One of the well-known methods is adding the secondary metal to Pt-based catalyst like alkali, alkali earth, and transition metals etc.., leading to the improved catalytic activity and stability of Pt supported catalysts. Each added the secondary metal has various roles in the catalytic behavior like changing the physio-chemical properties of loaded metal and support, changing metal-support interaction, or providing oxygen. Therefore, it’s important to choose the right additive for the desired purpose of catalytic behavior based on the understanding the catalyst’ properties in both monometallic and bimetallic catalysts. And thanks to many devotes to develop the Pt-based bimetallic catalysts for the desired purpose by showing an improved activity and durability, the bimetallic catalysts have been widely used in commercial including propane dehydrogenation (PDH), reforming, natural gas vehicle (NGV), and diesel oxidation catalyst (DOC). In this thesis, various Pt-based bimetallic catalysts were prepared by the addition of secondary metals that Sn, Mn, and Pd to Pt/Al2O3 catalysts. And PDH, HC/CO/NO oxidation, and CH4 oxidation were conducted as a model reaction with PtSn/Al2O3, Pt/Mn-Al2O3, and Pt-Pd/Al2O3 catalysts, respectively. I tried to correlate the properties of Pt-based bimetallic catalyst with the catalytic behavior in each reaction. In order to investigate the surface properties of the bimetallic catalysts, I used diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) study with CO as a probe molecule. Through DRIFTS study by editing adsorption procedure and controlling pretreatment condition, the roles of alumina surface properties, interfacial sites, and the surface composition of bimetals were elucidated in each chapter, which were the decisive properties of each catalyst for determining the catalytic behavior in each reaction. In chapter 2, we studied the effect of acid-base properties of alumina surface on metal sintering and coke deposition in PDH using PtSn/Al2O3 catalysts. For this, two different kinds of γ-Al2O3 that synthesized from ammonium aluminum carbonate hydroxide (A750) or commercial (P200) were used, which exhibited significantly different surface properties, despite the same bulk properties. Then, 0.5 wt.% of Pt and 0.9 wt.% of Sn were loaded on these aluminas (PtSn/A750 and PtSn/P200) and PDH was conducted on these catalysts. Two PtSn/Al2O3 catalysts showed totally different catalytic behavior in metal sintering and coke deposition, which were affected by the surface properties of γ-Al2O3. PtSn/A750 showed a significantly improved stability by inhibiting metal sintering and coke deposition due to the stronger but less amounts of Lewis acid sites on A750 after metal loading than those on P200. Finally, through IR spectra of adsorbed CO on PtSn/Al2O3 catalyst obtained at -150 °C after saturation of the metal surface, we clearly demonstrated that coke precursors were initially induced on Lewis acid sites on the alumina surface. Strong but less amount of residual Lewis acid sites on alumina surface are needed to inhibit the metal sintering and reduce coke deposition for designing stable PtSn/Al2O3 catalyst. In chapter 3, we report significantly improved activities of Mn modified Pt/Al2O3 catalyst in HC/CO/NO oxidation. Mn-modified Pt/Al2O3 showed much lower light-off temperatures in the oxidation of HC, CO, and NO than those of the unmodified Pt/Al2O3 catalyst, by as much as 30 °C. Through XRD and TEM analysis, the size of Pt particles was estimated, resulting in reduced Pt particle size after Mn modification. The interaction between CO and Pt on Mn-doped catalysts was also weakened confirmed by CO-TPD. Furthermore, DRIFTS study demonstrated weakly adsorbed CO on Pt was readily oxidized even at room temperature, which was induced at Pt-Mn interfacial sites through provided oxygen nearby manganese oxide. The results give an insight towards the improvement of the activity of Pt-based DOCs used for HC, CO, and NO oxidation, by enhancing the fundamental understanding of the Pt nature promoted with Mn. In chapter 4, we report partially oxidized palladium (PdOx) in Pd and PtPd bimetallic catalysts shows a linear correlation with CH4 oxidation activity. We characterized the amount of surface PdOx through DRIFTS study using CO as a probe molecule. With a careful consideration that the redox cycle of Pd particles occurs continuously during CH4 combustion, the Pd/Al2O3 catalysts were re-oxidized before obtaining IR spectra of CO adsorption to minimize the discrepancy between catalytically relevant phase and the characterized surface composition. From IR spectra of CO adsorption on re-oxidized Pd/Al2O3 catalysts, we quantified surface PdOx, of which peak appears at 2135~2145 cm-1, and correlate with the steady-state CH4 oxidation activities at 300 °C. Furthermore, by applying this method to Pt-Pd bimetallic catalysts, we could generalize the correlation between the surface PdOx and CH4 oxidation activity irrespective of composition, preparation method, and support. These results suggest that CH4 oxidation takes place on PdOx through redox cycle and the amount of surface PdOx gives a critical role of determining CH4 oxidation activity rather than electronic properties of Pd. By mimicking the catalytically relevant phase of Pd and PtPd bimetallic particles, we could obtain the general correlation between surface PdOx and CH4 oxidation activity. This work can greatly help the fundamental understanding of CH4 oxidation reaction on Pd-based catalysts and further the development of Pd-based catalysts in methane combustion with better activity.