1 Biochar of Corn Stover: Microwave-assisted Pyrolysis Condition 1 Induced Changes in Surface Functional Groups and 2 Characteristics 3 Lei Zhu, Hanwu Lei * , Lu Wang, Gayatri Yadavalli, Xuesong Zhang, Yi Wei, Yupeng Liu, Di Yan, 4 Shulin Chen, and Birgitte Ahring 5 Bioproducts, Sciences and Engineering Laboratory, Department of Biological Systems Engineering, 6 Washington State University, Richland, WA 99354-1671, 7 *Corresponding author, Phone: 509-372-7628, Fax: 509-372-7690, E-mail: hlei@wsu.edu 8 Abstract: Biochar was prepared by microwave-assisted pyrolysis of corn stover at 9 atmospheric pressure and different reaction temperatures and time. A central composite 10 experimental design was used in the optimization of volatiles (bio-oil and syngas), biochar 11 production, and the amount of carbon surface functional groups. Various types of oxygenated 12 functional groups on the biochar were quantified by means of titrimetric techniques using a 13 modified Boehm’s method. The range of surface carbonyl groups amount was from 0.27 to 14 1.70 mmol/g carbon. The produced biochars were also characterized in terms of the elemental 15 composition, BET surface area and chemical structure. The maximum surface area of the 16 biochar was 45 m 2 /g. The result indicates that the surface functional groups of biochar were 17 significantly influenced by the pyrolysis temperature and residence time. An adequate 18 prediction model was developed to describe the development of carbon surface functional 19 groups. Modification and characterization of biochar surface functional groups is a new way 20 to improve or extend their catalytic application in biomass conversion and bio-oil upgrading. 21 © 2015. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/