8566
Ind. Eng. Chem. Res. 2007, 46, 8566-8573
Direct Oxidation of Hydrogen to Hydrogen Peroxide over Pd (or PdO)/Al2O3 in Aqueous Reaction Medium: Influence of Different Acids and Halide Anions in Reaction Medium on Formation and Destruction of H2O2 Vasant R. Choudhary,* Yugesh V. Ingole, Chanchal Samanta, and Prabhas Jana Chemical Engineering & Process DeVelopment DiVision, National Chemical Laboratory, Pune-411 008, India
Effects of different mineral acids (viz. H2SO4, H3PO4, HNO3, HCl, HBr, and HI), acid (H3PO4) concentration, different halide anions (viz. F-, Cl-, Br-, and I-), and halide anion concentration in aqueous reaction medium on the H2O2 formation (in H2-to-H2O2 oxidation) and/or on the H2O2 destruction (by H2O2 decomposition and hydrogenation) activities of Pd (or PdO)/Al2O3 catalyst (at 300 K and atmospheric pressure) have been thoroughly investigated. Among the different halide anions, Br- anions are most effective for promoting the H2O2 formation and inhibiting the H2O2 destruction by both the H2O2 decomposition and/or hydrogenation. The cations associated with the halide anions, however, have only a little or no influence on both the H2O2 formation and destruction. The concentration of the different halide anions has a strong influence on the H2 conversion and H2O2 formation and destruction activities of the Pd/Al2O3. The H2O2 formation activity in the presence of Br- or Cl- anions is highest at the optimum concentration of halide (about 1.0 mmol/dm3). It is also highest at the optimum concentration of phosphoric acid (between 0.1 and 0.3 mol/dm3). In general, the H2O2 formation is increased with decreasing the H2O2 destruction activity of the catalyst, indicating a close relationship between the two. Br- anions act as an excellent catalyst promoter for Pd/Al2O3 catalyst, but they show only a small promoting effect for PdO/Al2O3 catalyst. The next choice for halide promoter for Pd/ Al2O3 catalyst is Cl- anions. F- and I- anions are, however, catalyst inhibitor and strong poison, respectively, for the H2O2 formation. At the same concentration, Br- anions are more effective than Cl- anions for inhibiting the H2O2 destruction reactions over Pd/Al2O3 catalyst. For both halide promoters (Cl- and Br-), the net H2O2 formation is controlled by the H2O2 hydrogenation rather than by the H2O2 decomposition. Introduction Catalytic direct oxidation of H2 to H2O2 is a practically important environmentally friendly process (which produces only environmentally benign water as a byproduct) for meeting the increasing demand for hydrogen peroxide (a versatile and clean oxidizing, decolorizing, and disinfecting agent). This process is known to be catalyzed by palladium for a long time,1 and use of alumina supported Pd catalysts for the process has been reported earlier in a number of studies2-8 and inventions.9-15 In the H2-to-H2O2 oxidation over Pd/Al2O3, the H2O2 formation was earlier found to be drastically increased by the oxidation of surface/subsurface Pd to PdO5,14,15 or by adding Br, as the catalyst promoter in the aqueous medium8 or in the catalyst.7 The increase in the H2O2 yield/selectivity was more drastic when using the halide promoter. This was attributed mostly to a large decrease in the H2O2 destruction (by H2O2 decomposition and hydrogenation) activity of the catalyst due to the halide promoter.7,8 Our recent earlier studies8 also reveal that the halide promoter is effective in drastically increasing the net H2O2 formation in the process only in the presence of an acid (protons). It is, therefore, of both practical and scientific interest to carry out a detailed investigation for knowing the effects of different mineral acids, concentration of acid, different halide anions, concentration of halide promoters in acidic reaction medium, and oxidation state of palladium (Pd° or PdO) on the H2-to-H2O2 oxidation and also on the H2O2 destruction (both by the H2O2 decomposition and hydrogenation reactions) activities of Pd/Al2O3 catalyst. The present investigation was undertaken for the above purpose. Apart from the H2-to-H2O2 oxidation, the H2O2 * Corresponding author. Tel.: +91-20-25902318. Fax: +91-2025902612. E-mail:
[email protected].
decomposition (H2O2 f H2O + 0.5O2) and hydrogenation (H2O2 + H2 f 2H2O) reactions over the Pd/Al2O3 catalyst under conditions similar or close to that used in the H2-to-H2O2 oxidation have also been investigated for obtaining a correlation between the net H2O2 formation and H2O2 destruction activities of the catalyst. Experimental Section The Pd (5.0 wt %)/Al2O3 catalyst in its reduced form was obtained from Lancaster (U.K.). The catalyst in its oxidized form (PdO/Al2O3) was prepared from the Pd/Al2O3 by calcining it in air at 500 °C for 4 h. The presence of Pd in its reduced form (Pd°) and oxidized form (PdO) in the catalyst was confirmed by X-ray diffraction (XRD) (using a Holland Phillips PW/1730 X-ray generator with Cu KR radiation). The catalyst was also characterized by XPS (X-ray photoelectron spectroscopy), using a VG-Scientific ESCA-3MKII electron spectrometer and assuming the binding energy of C1s as 285 eV, for its relative surface composition and also by TEM (transmission electron microscope), using a JEOL model 1200 EX instrument, for its Pd particle size. The catalytic oxidation of H2 to H2O2 over the catalysts was carried out in a magnetically stirred glass reactor (capacity ) 250 cm3) by continuously passing a mixture of H2 and O2 (4.6 mol % H2) through an aqueous medium, containing 0.5 g of catalyst in fine powder form, with or without acid and/or halide anions, at the following reaction conditions: volume of aqueous reaction medium ) 150 cm3, amount of catalyst ) 0.5 g, total gas flow rate ) 15.5 cm3/min, temperature ) 300 K, pressure ) atmospheric (100 kPa), and reaction period ) 3.0 h. The detailed reaction procedure was described earlier.8 The H2O2 decomposition (in air) and hydrogenation (under static H2) reactions over the catalysts in an aqueous reaction
10.1021/ie061653c CCC: $37.00 © 2007 American Chemical Society Published on Web 06/28/2007
Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8567 Table 1. Surface and Bulk Properties of the Pd/Al2O3 Catalyst in its Reduced and Oxidized Form binding energy (eV)
relative surface composition
form of Pd/Al2O3 (oxidized/reduced)
XRD phase (Pd° or PdO)
surface area (m2/g)
O (2s)
Pd (3d3/2)
Pd (3d5/2)
Al (2p)
oxidized reduced reduceda
PdO Pd° Pd°
173.5 198.0
531.8 531.7 531.8
342.2 340.6 340.7 & 343.5
336.9 335.4 335.6
74.7 74.5 74.6
a
Br (3p1/2)
Pd(II)/Pd(0)
191.7
∝ 0.00 0.24
ABr3pl/2/APd3d5/2 0.00 0.00 0.30
After use in the H2-to-H2O2 oxidation in the presence of KBr.
Table 2. Results of the Direct Oxidation of H2 to H2O2 and H2O2 Decomposition (in Air) over the Pd/Al2O3 (Reduced) Catalyst in Aqueous Reaction Medium Containing Different Mineral Acids (0.1 N) H2-to-H2O2 oxidation acid in reaction medium
H2 conversion (%)
H2O2 yield (%)
H2O2 selectivity (%)
time for half of the H2O2 decomposition, t1/2 (min)
distilled watera 0.1 N H3PO4a 0.1 N H2SO4b 0.1 N HNO3b 0.1 N HClb 0.1 N HBrb 0.1 N HIb
47.9 55.7 50.5 49.3 43.9 18.8
