Kinetics and Mechanism of H 2 O 2 Direct Synthesis over a Pd/C Catalyst in a Batch Reactor Pierdomenico Biasi,* ,†,‡ Nicola Gemo, ‡,† Jose ́ Rafael Herna ́ ndez Carucci, †,§ Kari Era ̈ nen, † Paolo Canu, ‡ and Tapio O. Salmi † † Process Chemistry Centre (PCC), Laboratory of Industrial Chemistry and Reaction Engineering Åbo Akademi, Biskopsgatan 8, 20500, Turku, Finland ‡ Dipartimento di Ingegneria Industriale, University of Padova, via Marzolo 9, 35131 Padova, Italy § BiCHEM Technology BV, High Tech Campus 48-2, 5656 AE, Eindhoven, The Netherlands * S Supporting Information ABSTRACT: Kinetic experiments of the decomposition, hydrogenation, and direct synthesis of hydrogen peroxide were performed on a commercial Pd/C catalyst. Temperature effects and subsequent hydrogen addition were investigated without using promoters. The hydrogen amount in the liquid phase was measured online by using a Fugatron Instrument to investigate the effect of the gas on the direct synthesis. Decomposition and hydrogenation reactions were affected differently by the temperatures used during the experiments. The formation of hydrogen peroxide showed different behaviors with different hydrogen feeding policies. The hydrogen dissolved in the liquid phase measured experimentally was correlated with the hydrogen peroxide production. As the amount of dissolved hydrogen increases in the liquid phase the direct synthesis rate increases, while the reaction slows down as the hydrogen pressure is decreased. The selectivity is also affected by the H 2 recharges. Every time that hydrogen is recharged in the reactor (during the direct synthesis) the selectivity toward H 2 O 2 increases. Two different methods to recharge H 2 during the reaction were analyzed. The first method consists in feeding the hydrogen when it is totally consumed, the second one in refilling hydrogen in the reactor before its total consumption. The hydrogen solubility was found as an important parameter for the direct synthesis. An explanation on hydrogen peroxide formation was given taking into account the H 2 /Pd ratio. 1. INTRODUCTION Hydrogen peroxide is an attractive oxidizing agent and its direct synthesis has been studied for many decades. Despite the efforts to find an innovative process to manufacture hydrogen peroxide from the elements, such technology has not been accomplished yet, due to problems related to flammability limits 1-4 between hydrogen and oxygen and consecutive/ parallel reactions. 3,5,6 The direct synthesis consists principally in one reaction: hydrogen and oxygen are dissolved in a liquid phase and react on a solid catalyst. Although this process seems to be so simple, two consecutive reactions (i.e., decomposition and hydrogenation of hydrogen peroxide) take place over the same catalyst used for the direct synthesis. 7,8 Moreover, a parallel reaction forming water from hydrogen and oxygen can take place over the catalyst surface. Despite numerous studies on the direct synthesis of H 2 O 2 ,a general understanding of the mechanism, solubility problems, and how to avoid consecutive and parallels reactions without promoters has not been achieved. 9,10 The studies on the direct synthesis have mainly been focused on the catalyst design and screening; however, such tests are almost impossible to compare due to different conditions used (i.e., solvent, halides, acids, inert gases, pressure, temperature, and many others). 2,4,5,11-15 Numerous catalysts based on Pd and PdAu have been investigated 12,15-18 and different ideas have been published on the reaction mechanism and the role of each promoter, including halides and acids. 8,13,19-23 Still there is no agreement on the mechanism and conditions favoring the synthesis due to the complicated character of the system. This is the reason why a precise kinetic analysis is needed to understand the real mechanism of the direct synthesis. Kinetic studies are important to improve the operation conditions in continuous reactors, for which a complete understanding of the regimes and mechanism is needed to enhance the catalyst performance. 21,24,25 Kinetic analyses have lately started to appear in the field of direct synthesis of hydrogen peroxide. 21,24-26 However, these studies remain scarce, and more effort has to be focused on this area if the correct mechanism of the direct synthesis is to be understood. To understand better the difficult to analyze results of different groups on hydrogen peroxide direct synthesis we report a table (Table 1) where a comparison of different works 2,12,15,23,27-32 is done. In this table, catalysts, reactors, operative conditions, additives, and results are compared. Batch reactors are adopted for catalytic tests, 12,15,29-31 semibatch reactors to investigate the operation conditions and enhance the quantity of H 2 O 2 obtained, 15,29,30,32 membrane reactors and microreactors 2,27,28 to operate safely within flammability and, Special Issue: CAMURE 8 and ISMR 7 Received: September 21, 2011 Revised: April 9, 2012 Accepted: April 9, 2012 Published: April 24, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 8903 dx.doi.org/10.1021/ie2021398 | Ind. Eng. Chem. Res. 2012, 51, 8903-8912