Devices and materials
Sodium Formate (HCOONa), Formic Acid (HCOOH), NiCl2.6H2O, RuCl3.3H2O, Pd(NO3)2·2H2O (∼40% Pd), activated carbon(C), sodium borohydride (NaBH4), sodium hydroxide (NaOH), potassium dichromate (K2Cr2O7) and ethanol (C2H5OH) from Sigma-Aldrich received®. Nuve FN 300 oven (0-25000C), Heidolph MR-3004 magnetic stirrer, Lab Companion RW-0525 temperature bath, Shimadzu UV-2600 DR/UV-vis
Preparation of the catalysts ([email protected], [email protected], [email protected])
Nanocatalysts were synthesized in a simple and reproducible manner by conventional impregnation and simultaneous reduction method of Pd, Ru and Ni metals on activated carbon (C) (Zhu and Xu 2015; Yang et al. 2010; Celebi et al. 2016).
- [email protected] synthesis; Pd metal was supported on 100 mg C in 5.0 mg water (Pd(NO3)2.2H2O (4.95 mg, 19.20 µmol Pd) (mixed at 700 rpm for 2 hours)) and reduced with NaBH4 (11.12 mg, 0.3 mM).
- [email protected] synthesis; Ru metal was supported on 100 mg C in 5.0 mg water (RuCl3.3H2O (5.28 mg, 20.18 µmol Ru) (mixed at 700 rpm for 2 hours)) and reduced with NaBH4 (11.69 mg, 0.32 mM).
- [email protected] synthesis; Ni metal was supported on 100 mg C in 5.0 mg water (NiCl2.6H2O (8.32 mg, 35 µmol Ni) (mixed at 700 rpm for 2 hours)) and reduced with NaBH4 (20.16 mg, 0.54 mM).
Then the mixtures were filtered and washed with plenty of water (3x10 mL), nanocatalysts were obtained in powder form, dried in a vacuum oven at 150 0C for 1 hour.
K2Cr2O7 (10 mL, 2.0 mM, 5.89 mg) solution was transferred to the reaction vessel, 10.0 mg of Pd(0)@C nanocatalyst was added, and the reaction equilibrated at 298 K for 15 minutes. Then, 1.0 mL solution (HCOONa; 312.3 mg, 450 M + HCOOH; 173.2 µL 0.45 M) was transferred to the reaction vessel with a 1.0 mL gas-insulated syringe. Immediately after, the catalyzed reaction was initiated with turning on the stirrer (> 700 rpm), at 0 min and then at certain time intervals, 0.9 mL of solution was taken from the sample cup, diluted to 1.0 mL with water, read in UV (Shimadzu UV-2600 spectrometer).
Identification of gaseous products from the decomposition of formic acid over [email protected] catalyst
Before beginning the catalytic reaction of FA, the hot water bath (Lab Companion RW-0525) was adjusted to a constant temperature of 25 0C, and a Schlenk type jacketed reaction vessel (50.0 mL) fixed onto a magnetic stirrer (Heidolph MR-3004). The [email protected] catalyst (10.0 mg) was transferred to the reaction vessel Schlenk, and 9.0 mL of H2O was added to it, and thermal equilibrium was achieved after 15 minutes. Next, 1.0 mL of formic acid + sodium formate solution (450 mM FA + 450 mM SF) was transferred to the reaction vessel and the catalyzed reaction was initiated by operating the stirrer (> 700 rpm) (t = 0 minutes). The resulting gas was collected in the GC flask and analyzed with the Shimadzu TCD-2014 GC.
NaOH trap test
The trap test was carry out to determine the selectivity of the catalyst used in the catalytic decomposition reaction of FA. The trap experiment will give information about whether the reaction is proceeding through dehydrogenation or dehydration. Some researchers (Yadav et al. 2012; Celebi et al. 2016; Gu et al. 2011) have performed the NaOH trap experiment. The gas released from the dehydrogenation of FA is passed through a saturated NaOH (10.0 M) solution before the gas burette. When the reaction takes place with high selectivity, since all of the CO2 gas formed together with the H2 gas will react with NaOH in the trap, the gas volume to be measured should be halved (2NaOH(aq) + CO2(g) → Na2CO3(aq) + H2O(s)).
UV/vis spectroscopic studies
The absorption peak of hexavalent chromium was measured with UV/vis spectroscopy at a fixed wavelength at 350 nm. Calculation of the concentration of remaining hexavalent chromium was made with a calibration curve obtained from the absorbance of standard solutions. The degree of catalytic reduction, also referred to as cycle, was calculated from the equation Conversion=[Cr(VI)]/[Cr(V1)]0. Here, [Cr(IV)]0 and [Cr(VI)] are the initial and specific time, respectively.
Confirmation of the presence of Cr(III) as a catalytic reaction product
Extra NaOH was transferred to the solution resulting from the conversion of hexavalent chromium, forming a green colored hexahydroxochromat(III) solution, confirming the formation of Cr(III) in solution (Zhu and Xu 2015; Yang et al. 2010; Celebi et al. 2016; Bulut et al. 2015).
After the first catalytic cycle, the catalyst ([email protected]) was was purged from the solution medium, dried and used in the next cycle. The number of cycles was repeated up to 5.
Effects of sodium formate [HCOONa], formic acid [HCOOH], dichromate [Cr2O72-], catalyst [Pd(0)@C] concentrations and temperature on the catalytic reduction of Cr(VI) to Cr(III)
The effects of FA (0, 122.5, 225, 450, 675 mM), SF (0, 122.5, 225, 450, 675 mM), Dichromate ( 1.0, 2.0, 4.0, 8.0 mM), catalyst (5.0, 7.5, 10.0, 15.0 mg Pd(0)@C, 1.37% by weight Pd loadings correspond to 0.065, 0.098, 0.129, 0.196 mM Pd) initial concentrations and temperature (298-318 K) onto the catalytic reduction of Cr(VI) to trivalent chromium were investigated.
The superiority of [email protected]
To determine the superiority of Pd(0)@C catalyst over Ru(0)@C and Ni(0)@C nanocatalysts, The reduction of Cr(VI) was investigated in the same test standards (10.0 mL water; 312.3 mg, 450 mM HCOONa; 173.2 µL, 450 mM HCOOH; 2.0 mM, 5.89 mg K2Cr2O7) and 298 K.