Reactivity enhancement of gasification biochars for catalytic applications

To enhance the catalytic properties of woody biochars, an O2 gas-phase treatment was performed at different temperatures (280, 340 and 400 °C) and times (2 h, 4 h, 8 h, 16 h) post-gasification. In fact the development of o-containing carbons based materials to increase their reactivity is gaining momentum. The efficacy and selectivity of the oxygen chemisorption on the biochars surface were investigated. FTIR revealed that the oxygenation has been efficient. Temperature Programmed Desorption (TPD) confirmed FTIR results and led to go further by providing quantitative results and insights into the selectivity. In fact the oxygen content at the surface was increased 1.3, 1.7 and 2.1 times after 2 h at 280, 340 and 400 °C respectively. At 280 °C, above 4 h of treatment the surface became saturated and the oxygen content was increased by 40%. The formation of weak acid functional groups has been enhanced without removing the basic functional groups already present on the surface. Carboxylic acids (strongest acids) were removed since they were not stable at the treatment temperature. The higher the total amount of o-containing functions, the more acid the pHpzc. The pHpzc of the raw biochars is of 3.1. At 280 °C even if the oxygenation was efficient it did not impact the global surface acidity. At highest temperature the higher amount of oxygenated functions decreased the pHpzc to reach and go below 2. Thus the chemisorption was efficient and selective. BET analyses showed that at highest temperatures (340 and 400 °C) the treatment increased the surface area and porosity. At 400 °C the specific surface area gained 15% and the porosity was increased 1.5 times whereas at 280 °C the specific surface area decreased to reach 80% of the initial surface area. These results highlight that the combustion has been enhanced at 340 and 400 °C which burnt carbon atoms and free the pores. Thus oxygenation at 340 or 400 °C is more interesting than at 280 °C since results have shown that the higher efficiency, specific surface area and porosity have been obtained under those conditions.