Study On The Role Of Oxidant During Phosphoric Acid Activation

In the present work, fir sawdust, acid-pretreatment fir sawdust, ligninwas used as raw materials to prepare activated carbon by phosphoric acid with theadditive of ammonium persulfate to investigate the role of the oxidative additiveduring phosphoric acid activation. The elemental analysis, XPS, Boehm titrationand nitrogen adsorption were employed to analyze the element contents,chemical combination states, surface functional groups and pore structures ofactivated carbons. In addition, the activated carbons prepared by phosphoric acidactivation at a high temperature between700~900℃were used to modify glassycarbon electrodes, and Cyclic voltammetry was employed to test theelectrocatalytic activity of the modified electrodes to p-nitrophenol andp-dihydroxybenzene to investigate the effect of acidic surface functional groups onthe electrocatalytic activity.The results showed that ammonium persulfate as an oxidative additive playstwo roles:(1) promotion of crosslink between phosphoric acid and biopolymers and(2) oxidation to carbon. The former predominates when a less addition ratio ofammonium persulfate was added; the latter predominates at when ammoniumpersulfate with a higher addition ratio was added. The former leads to an increase inthe yield of activated carbon, and benefit the development of pore structure and theformation of surface functional groups. The latter role results into a decrease inyield of activated carbons, produce a negative effect on the crosslink structure in theactivated carbons and reduce the contents of surface functional groups.Therefore, the addition ratio of the oxidants is one of the key factors thataffects the yield of activated carbons, carbon conversion ratio of raw materials, porestructure and the surface functional groups of activated carbons. When ammoniumpersulfate with the most suiTablele addition ratio was added, phosphoric acidactivated carbon with highly developed pore structure could be prepared at thehighest yield. The study found that the value of the most suiTablele addition ratioused depends on the content of the carbon and oxygen content in raw materials. Thehigher the carbon content and the lower the oxygen content is, the more the additionratio of ammonium persulfate used is. For the fir sawdust without pretreatment, theyield (79.8%) of activated carbons and carbon conversion ratio (45.9%) of rawmaterials will attain their maximum when no ammonium persulfate was added; Forthe fir sawdust by an acid-pretreatment, the yield (74.7%) of activated carbons andcarbon conversion ratio (46.3%) of raw materials could reach their maximum, and the activated carbon obtained has the largest surface area (1541m2/g) and stronglyacidic surface functional group (1.889mmol/L) when5%ammonium persulfate wasused. For the lignin, the yield of activated carbons and carbon conversion ratio ofraw materials reach their maximum,87.4%and69.4%respectively, and the activatedcarbon has the largest surface area (1032m2/g) and strongly acidic surface functionalgroup (2.748mmol/L) when10%ammonium persulfate was used.The electrochemical measurements of phosphoric acid activated carbonsmodified electrodes showed that phosphoric acid activated carbons present the highelectrocatalytic activity to p-nitrophenol and p-dihydroxybenzene. The acidicsurface functional groups in activated carbon can obviously promote the reductionreaction to p-nitrophenol but has no effect on the redox reaction ofp-dihydroxybenzene. Moreover, the peak current of electrocatalytic reaction isrelated to adsorptive properties of activated carbons: the stronger the adsorptiveproperty is, the larger the peak current is.