H2 from biofuels and carriers: a kinetic investigation of formic acid decomposition on Rh/Al2O3 in the annular reactor

Vapor-phase decomposition of formic acid on Rh/Al2O3 catalyst was studied performing experiments in an isothermal annular microreactor. Effects of operating conditions on formic acid conversion and composition of products mixture were explored. Formic acid conversion was activated at 150 °C and two parallel decomposition routes were involved: dehydrogenation (predominant reaction) and dehydration (minor reaction). It was shown that water gas shift reaction played a kinetic role only above 300 °C. At T > 500 °C, the system reached chemical equilibrium, with formic acid achieving complete conversion around 400 °C. The kinetic relevance of each reaction step of this triangular network was singled out to derive a molecular kinetic scheme. This kinetic analysis demonstrated that, despite being a minor reactivity, formic acid dehydration is characterized by a lower activation energy with respect to dehydrogenation (50 vs 67 kJ/mol). Unfavorable energetics of the competing reactions and unfavorable low-temperature water gas shift are the kinetic factors behind the impossibility to obtain CO-free H2 in the low temperature region on Rh catalyst. Conversely, a sensitivity analysis on water gas shift kinetics showed that the selectivity can be optimized at intermediate temperatures by selectively promoting the consecutive consumption of CO to CO2, which opens to catalyst design and bifunctional catalysts