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H 2 O 2 direct synthesis over a palladium catalyst based on sulfated ceria (Pd-CeS) has been studied in a trickle-bed reactor at -10°C. The combined effect of liquid and gas flow rates was studied by independent variations. The maximum productivity and selectivity was always found at flow rate combinations intermediate within the investigated range. The reactor operated under pressure and its effect was investigated (10 and 20 bar), resulting in a significant gain in selectivity. Selectivity up to 80% has been measured at the highest pressure tested (20 bars), liquid flow rate of 2 mL/min, and 6 mL/min gas flow rate. The maximum production rate measured was 0.0035 mmol/min with 0.5 mL/min liquid flow rate and 2.7 mL/min gas flow rate. Relevance of direct water formation has been isolated by independently investigating H 2 O 2 decomposition and hydrogenation. Results indicate directions of further performance improvements and the importance of reactor type and operation.

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a b s t r a c t H 2 O 2 direct synthesis over two bimetallic, palladium-gold catalysts based on sulfated ceria (PdAu-CeS) and on sulfated zirconia (PdAu-ZS) has been studied in a continuous, trickle-bed reactor, at −10• C and 10 bar. Three different liquid flow rates and seven different gas flow rates were used. The combined effect of liquid and gas flow rates, with two different H 2 /O 2 ratio (2/18 and 4/16) in the gas feed, has been studied, by independent variations. The highest H 2 /O 2 ratio always increases production rate and selectivity. In the case of 4/16 ratio, PdAu-CeS catalyst shows an enhancement in selectivity only, while the increase in H 2 O 2 production rate is not significant compared to the 2/18 ratio case. PdAu-ZS catalyst shows a remarkable gain in both selectivity and production rate when the 4/16 ratio in gas feeding is used, outperforming the PdAu-CeS catalyst. The enhancement of the catalytic activity is more pronounced in the case of PdAu-ZS, compared to PdAu-CeS, when the highest H 2 /O 2 ratio is used. A selectivity up to 90% has been measured on PdAu-ZS with the higher H 2 /O 2 ratio and larger liquid flow rates. The maximum production rate measured is 6 mol/min (i.e. a productivity 0.18 mol H 2 O 2 g Pd −1 h −1 ) with 1 ml/min liquid flow rate and 2.7 ml/min gas flow rate again on PdAu-ZS.