Oxy-coal combustion has been developed to effectively capture CO
2 in power plants. It uses nearly pure oxygen (O
2) to combust coal, and thereby a concentrated CO
2 stream (~63%) can be achieved in its exhaust [1]. The concentrated CO
2 stream can be utilized for enhanced oil recovery (EOR), however, the O
2 level in the stream should lower to 100 ppmv for satisfying the requirement for EOR [2]. Catalytic O
2 removal system with methane (CH
4) has been recently applied to solve this problem, and a high conversion of O
2 was reported with noble metal (Pd) doped metal oxide catalysts [3, 4]. In this study, Pd-TiO
2 catalysts with different Pd loadings (< 1.5 wt%) were synthesized by using a flame aerosol reactor which is a continuous one-step process [5]. The catalytic properties and kinetic characteristics of the synthesized Pd-TiO
2 catalysts were evaluated for O
2 removal. The crystal structure and the size of the bulk Pd-TiO
2 were not changed by varying the Pd loading, while the size of the Pd subnano clusters/nano particles and the fraction of the three Pd species (metallic Pd, intermediate PdO
x (0<x<1), and PdO) were significantly affected by changing the Pd loading. Scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS) were utilized to determine the size of the Pd subnano clusters/nano particles and the oxidation states of Pd. Based on the analytical results, the total surface area of the different Pd species were calculated, and a correlation to the apparent reaction rate constant was evaluated. Among the three different Pd species, the total surface area of the metallic Pd showed the strong correlation to the apparent reaction rate constant, which reveals that the metallic Pd is a major active phase in the O
2 removal reaction. Our finding emphasizes a pivotal role of metallic Pd for the effective O
2 removal, and a further modification of the catalyst could be achieved based on this finding to enhance the O
2 removal efficiency.
References:
[1] A. Gopan, B.M. Kumfer, J. Phillips, D. Thimsen, R. Smith, R.L. Axelbaum, Process design and performance analysis of a Staged, Pressurized Oxy-Combustion (SPOC) power plant for carbon capture, Appl Energ, 125 (2014) 179-188.
[2] DOE/NETL, Quality guidelines for energy system studies: CO2 impurity design parameters, (2013).
[3] Q. Zheng, S. Zhou, M. Lail, K. Amato, Oxygen Removal from Oxy-Combustion Flue Gas for CO2 Purification via Catalytic Methane Oxidation, Ind Eng Chem Res, 57 (2018) 1954-1960.
[4] A. N. Kuhn, Z. Chen, Y. Lu, H. Yang, Sequential oxygen reduction and adsorption for carbon dioxide purification for flue gas applications, Energy Technol-Ger, (2018).
[5] V. Tiwari, J. Jiang, V. Sethi, P. Biswas, One-step synthesis of noble metal-titanium dioxide nanocomposites in a flame aerosol reactor, Appl Catal a-Gen, 345 (2008) 241-246.
