Performances of Pd Nanoparticles on Different Supports in the Direct Synthesis of H2O2 in CO2-Expanded Methanol

Abstract

Catalysts based on Pd supported on SiO 2 , SBA-15, c-Al 2 O 3 and N-CNT were investigated for the direct H 2 O 2 synthesis and decomposition/hydrogenolysis in CO 2 -expanded methanol in batch and semi-batch reactors working at room temperature and a pressure of 6.5 bar. The order of reactivity for the synthesis of H 2 O 2 per g of Pd or for the rate of H 2 O 2 decomposition, or the selectivity to H 2 O 2 does not correlate with the acidity of the support. A relation was observed with the Pd particle sizes, i.e. smaller particles lead to a higher specific activity, but the sample supported on carbon nanotube shows an order of magnitude higher specific activity per g of Pd, indicating the presence of an additional factor attributed to the interaction with the carbon support which determines an intrinsic higher reactivity. However, this higher reactivity corresponds also to a higher activity in the consecutive conversion of H 2 O 2 , probably by hydrogenolysis reaction. A rough relation between activity in H 2 O 2 synthesis and in H 2 O 2 conversion was observed in the samples with the different supports, indicating the similar nature of the active sites responsible for the two reactions. Finally, the analysis of the recyclability of the catalysts and transmission electron microscopy (TEM) characterization data after the catalytic tests indicate that the use of CO 2 -expanded methanol allows improving the catalytic performances in H 2 O 2 direct synthesis due to the enhanced solubility of H 2 and O 2 , but induces a fast catalyst deactivation due to an enhanced mobility and sintering of the supported Pd particles. The effect is present in all the four type of support used.

Nitrogen-doped carbon nanomaterials: To the mechanism of growth, electrical conductivity and application in catalysis

“…A decrease in the size of deposited metallic particles and their more uniform distribution in the case of nitrogen-doped carbon materials were demonstrated in [105][106][107][108][109][110][111]. A comparison of the 10%Pd/CNTs and 10%Pd/N-CNTs (0-5 at.% N) catalysts in hydrogenation of cinnamaldehyde with respect to activity and selectivity of C C bond hydrogenation demonstrated an evident advantage of N-CNTs [105].…”

“…The advantages of N-CNTs and other N-doped nanocarbons (1.1 wt.% N in the bulk and 8 wt.% N on the surface) for the deposition of Pd catalysts were demonstrated in the synthesis of hydrogen peroxide [106][107][108]. Doping with nitrogen made it possible to obtain a uniform distribution of palladium particles 2-3 nm in size over the carbon surface as compared with undoped carbon nanomaterials, silica and alumina, Fig.…”

Dynamics of Palladium on Nanocarbon in the Direct Synthesis of H2O2

“…The Sumitomo-Eni process for the synthesis of caprolactam and the Dow-BASF process for the synthesis of propene oxide are ex-amples of oxidation processes that aim to use H 2 O 2 produced on-site through direct synthesis from H 2 and O 2 . region, [12][13][14][15][16][17] which depends on the reactant gas-phase chemical potential, [11][12][13] the fluid-dynamic conditions, [11,18,19] and the morphological transformations of the nanoparticles after deprotection. [5][6][7] The major problems of the direct synthesis toward commercialization are related to the safety of the process as well as the selectivity.…”

“…[1,2] From the large variety of reports in the literature and patents on the topic, [3,4] it is revealed that effective catalytic systems for the direct synthesis of H 2 O 2 from H 2 /O 2 involve the use of supported palladium catalysts in a slurry of H 2 O/CH 3 OH and in the presence of H 2 SO 4 and halides to achieve good activity and selectivity to H 2 O 2 . [19][20][21] Scheme 1 illustrates how the desired selectivity of the hydrogenation reaction of oxygen depends on two chains of consecutive reactions. The apparent simplicity of the synthetic reaction is accompanied, in reality, by a complex network of thermodynamically favored deep hydrogenation reactions to H 2 O, as shown in Scheme 1.…”

Direct synthesis of H2O2 over Pd supported on rare earths promoted zirconia

“…Acidic supports were found very promising also in others studies, specifically depositing Pd on a mesoporous ion exchange resin functionalized with sulfonic groups [9,10]. More recently, Abate et al [11,12] investigated Pd supported on SiO 2 , SBA15-15, ␥-Al 2 O 3 and N-CNT and found no apparent relationship between reactivity and selectivity and the acidity of the support, concluding that their data better correlated with the metal particle size [11]. Pd-based catalysts supported on silica were also successfully tested by Strukul et al [13].…”