Chemical pathways in the partial oxidation and steam reforming of acetic acid over a Rh-Al2O3 catalyst

The catalytic partial oxidation (CPO) and the steam reforming (SR) of acetic acid (HAc) over a 2wt% Rh/α-Al2O3 were investigated in an isothermal annular reactor. The experiments were performed at GHSV values up to 2.0×106 l(NTP)/kgcat/h by feeding highly diluted HAc/O2 and HAc/H2O mixtures; temperature was varied from 200 to 850°C. In analogy with the CPO of hydrocarbons, also the CPO of HAc showed a low temperature regime characterized by the oxidation of the acid, while the production of synthesis gas occurred above 450°C. Deep oxidation was observed below 300°C. In between 300 and 450°C, a progressive decline of O2 conversion was observed (a very unique feature), accompanied by decrease of CO2 production and formation of CO with traces of H2. C-accumulation was observed starting from 400°C. The data suggest that an increasing fraction of the surface sites was active in an oxygen-lean chemistry. This could be explained by assuming that competitive adsorption displaced oxygen in favor of acetic acid.Dedicated SR tests confirmed that the production of CO and H2 became significant only above 450-500°C, while the process was hindered, likely by C-poisoning, at lower temperatures.Raman measurements confirmed that at the intermediate temperatures (450°C) the surface was enriched by unsaturated C-species. The TPO profiles after CPO test at 450°C indicated a major peak at 350°C, suggesting the presence of C-species with intermediate reactivity or intermediate proximity to the metal particles, in between the highly reactive CHX fragments on metal sites and the polymeric C-species stored on the support.