Preparation, performances and reaction mechanism for the synthesis of H2O2 from H2 and O2 based on palladium membranes

Abstract

A series of new tubular catalytic membranes (TCM's) have been prepared and tested in the direct synthesis of H 2 O 2 . Such TCM's are asymmetric a-alumina mesoporous membranes supported on macroporous a-alumina, either with a subsequent carbon coating (CAM) or without (AAM). Pd was introduced by two different impregnation techniques. Deposition-precipitation (DP) was applied to CAM's to obtain an even Pd particles distribution inside the membrane pore network, whereas electroless plating deposition (EPD) was successfully applied to AAM's to give a 1-10 mm thick nearly-dense Pd layer. Both type of membranes were active in the direct synthesis of H 2 O 2 . Catalytic tests were carried out in a semi-batch re-circulating reactor under very mild conditions. Concentrations as high as 250-300 ppm H 2 O 2 were commonly achieved with both CAM's and AAM's after 6-7 h time on stream, whereas the decomposition rate was particularly high in the presence of H 2 . Important features are the temperature control and pre-activation. In order to slow down the decomposition and favor the synthesis of H 2 O 2 a smooth metal surface is needed. #

The influence of the nanostructure on the effect of CO2 on the properties of Pd–Ag thin-film for H2 separation

“…Later, various authors have also investigated similar catalytic membranes for removing nitrates from groundwater [34,35]. We also used these membranes for the direct synthesis of H 2 O 2 from H 2 and O 2 [36,37], and recently other research groups have investigated them in the same reaction [38,39]. Very interesting results have been also obtained using similar membranes for the catalytic direct oxidation of benzene to phenol [40][41][42][43].…”

The issue of selectivity in the direct synthesis of H2O2 from H2 and O2: the role of the catalyst in relation to the kinetics of reaction

“…Table 1 reports a summary of the prepared catalytic membrane and their code, which will be used hereinafter. For further details reference is given to previous papers [5,6,30,31]. Scanning and transmission electron microscopy (SEM and TEM, respectively) characterization of the samples (not reported here for conciseness) indicate that the deposition-precipitation method (DP) on carbon-coated ceramic membrane (CA) leads to Pd or Pd-Me particles with mean dimension of around 8-10 nm which slightly decrease increasing the Pd:Pt ratio.…”

“…However, in the most suitable configuration from the safety point of view, e.g. the H 2 which diffuse through the membrane which is in contact with the solution saturated with O 2 , we observed earlier [5][6][7]30,31] that the membrane show low selectivity due to fast reduction of Pd. We observed also that the nature of Pd particles influences the selectivity and productivity to H 2 O 2 , but a better knowledge of the reaction network and mechanism, and the identification of the role and nature of active sites in the different reactions could allow to further improve the performances.…”

“…Pd loading was typically around 2 mg, but using more concentrated solutions of the noble metals, the noble metal loading could be increased up to 30 mg, even though these membranes show lower stability during the H 2 O 2 direct synthesis. Further details on both the preparation methods were given elsewhere [5,6,30,31]. Table 1 reports a summary of the prepared catalytic membrane and their code, which will be used hereinafter.…”

“…a technology in which there is a separate contact of O 2 and H 2 with the reaction solution in order to avoid the possibility of explosion, even during anomalous operations. This possibility is offered by the use of catalytic membranes [5][6][7][8] instead of powder-type catalysts as those used in most of the patents and publications [4,.…”