Biochar supported nickel (Ni/BC) has been widely studied as a cheap and easy-to-prepare catalyst with potential applications in tar reforming during the gasification of low-rank fuels,such as brown coal and biomass.However,the role and behaviors of inherent K species,especially their interactions with Ni particles and the biochar support,are not well understood yet.In this work,three Ni/BC catalysts with varying K amount were prepared from raw,water-washed,and acid-washed biomass.They were used in steam reforming of toluene as a tar model compound to elucidate the effects of inherent K on the catalytic activity and stability.Detailed characterization indicated that K enhanced water adsorption due to its hydroscopicity and lowered the condensation and graphitization degrees of biochar,but the alteration to the electronic state of Ni was not observed.These effects together led to a temperature-dependent role of K.That is,at relatively low temperatures of 450 and 500 ℃,toluene conversion was increased in the presence of K,due to the increased concentration of adsorbed water around Ni particles.By contrast,at relatively higher temperatures of 550 and 600 ℃,although initial high activity was achieved,Ni/BC with K deactivated rapidly because of the accelerated consumption of the biochar support.