Εκλεκτική καταλυτική απονίτρωση υδατικών μέσων με χρήση Η2 παρουσία οξυγόνου σε στηριζόμενους καταλύτες Pd-Cu σε σύστημα συνεχούς ροής

Abstract

Up-to-date, practically all the research conducted regarding the catalytic reduction of nitrates with H2 concerns experimental work in a batch or semi-batch mode. However, the catalytic reduction of nitrates in a continuous flow reactor may present several advantages over the conventional semi-batch setup. The continuous mode offers more flexibility concerning the choice of several operating conditions (e.g., liquid flow rate), while it minimizes possible pH problems that are attributed to the continuous alternation of reactants and products concentration with time on stream, a phenomenon observed only in the case of batch or semi batch modes. In the present Doctoral Thesis an attempt is being made, for the first time ever, to study the influence of various experimental parameters on the catalytic reduction of nitrates in a continuous flow stirred tank reactor. Catalytic experiments aiming at the assessment of the catalytic behaviour of Pd-Cu metals supported on series of coated alumina spheres were carried out in a state-of-the-art three-phase reactor. The present research involves a detailed fundamental study of the selective catalytic reduction of nitrates (NO3-) in water mediums towards N2 formation in the presence of oxygen (5 vol% O2) using H2 as reducing a agent (hydrogen selective catalytic reduction, H2-SCR) over Pd-Cu supported on various mixed metal oxides, x wt% MxOy^-Al2O3 (MXOY = CeO2, MgO, Mn2O3, Cr2O3, Y2O3, MoO2, Fe2O3 and TiO2) at room temperature. In particular, an in-depth investigation of the effects of the type of reactor, the support chemical composition, the liquid flow rate, the presence of oxygen in the gaseous feed, the initial concentration of nitrates, the presence of foreign ions in the liquid medium, the active phase, the particle size of alumina etc. on the activity, N2 selectivity and stability of catalysts with time on stream was conducted. For the first time a detail comparison between the specific catalytic activity observed and the main physico-chemical properties (e.g., support chemical composition) of the catalysts were attempted in order to establish correlations between these parameters their specific catalytic activity and selectivity (SN2, %). It was found that the chemical composition of the support can significantly affect both the catalytic activity and selectivity of supported Pd-Cu catalysts towards the NO3-/H2/O2 reaction. The Pd-Cu clusters supported on TiO2, CeO2 and Y2O2 coated γ-Al^ spheres showed the best catalytic behavior compared with the rest of supports examined. The most active solid 1wt% Pd-0.5wt% Cu/TiO2-Al2O3 was studied thoroughly for its physico- chemical characteristics and their relation to its catalytic activity towards the reactions at hand. Moreover, this catalyst presents an excellent stability with reaction time of NO;- /H2/O2. In order to elucidate the promoting role of T1O2 in suppressing the unwanted NH4+ production in water, DRIFTS and the other transient studies were performed. The difference in the reactivity of the various adsorbed N-species observed when varying the support chemical composition explains in an adequate degree the large effect of the chemical composition of the support on the supported Pd-Cu catalysts behaviour. The current work reports, for the first time ever, that the selectivity of the reaction at hand can by adjusted by regulating the flow rate of the liquid medium (e.g., nitrates solution) in a CSTR process. Reaction selectivity towards N2 as high as 96%, can be obtained under certain experimental conditions. In particular, the same NO3- conversion with NH4+ selectivity values up to sixteen times lower can be obtained when increasing the liquid flow rate from 2 to 6 mL/min. Moreover, it was proven for the first time, in a continuous flow process, that the presence of oxygen in the gaseous feed has a remarkable positive effect on the reaction's selectivity towards nitrogen. In particular, the reaction's selectivity towards NH4+ can be reduced by almost 60% after introducing 4.2 vol.% oxygen (or 20 vol.% air) in the reductive feed stream. In situ DRIFTS studies have shown that the oxygen feed concentration has a big impact on the reactivity of active adsorbed N-species, and no impact on their chemical structure. TPSR studies have also shown that the formation of NH3 is favored when oxygen is not present in the feed gas stream, a fact that explains the above results. The reduction rate of nitrates and reaction's selectivity towards NH4+ strongly depends on the initial concentration of nitrates in the liquid medium. It was found that NO3- reduction rate is increased by six times while NH4+ selectivity is decreased by five times when the initial concentration of nitrates is increased from 10 to 100 mg/L. The apparent reaction order with respect to nitrates was found to be 0.9. Also, the presence of HCO3- in the liquid feed of the NO3-/H2/O2 reaction can lead to the significant decrease of both catalysts activity and selectivity. On the contrary, the presence of Cl- slightly increases catalysts activity, while the presence of Na+, SO42- and PO43- do not affect significantly the catalysts behavior. As previously mentioned, the present work provides also fundamental mechanistic information for the NO3-/H2/O2 reaction over supported Pd-Cu catalysts. Detailed mechanistic studies (SSITKA-DRIFTS) have been performed on 1wt% Pd-0.5wt% Cu/γ- Al2O3 and 1wt% Pd-0.5wt% Cu/TiO2-Al2O3 catalysts concerning the reaction system at hand (NO3-/H2 and NO3-/H2/O2). Experiments revealed that the mechanism of N2 production strongly depends on the nature of the support and presence of O2 (air) in the gas feed. In particular, these parameters were found to significantly affect the formation of different adsorbed active intermediate N-species on the support or Pd metal surface. In more detail, a comparison of the catalysis examines reveals the presence of several similarities and differences which can be summarized as follows: i. Four N-species adsorbed on the catalyst surface were reported to be active in the case of Pd-Cu^-Al2O3 catalyst, while five N-species were found to be active in the case of Pd-Cu/TiO2-Al2O; catalyst, depending on reaction conditions. ii. Adsorbed ionic nitrates on Cu were found to be very reactive, and finally determined to be active species both catalysts, under any reaction conditions. iii. Bidentate nitrates on Pd were found to be active only when oxygen was present in the feed gas stream for both catalysts. iv. Adsorbed bidentate nitrates on the support were determined to be active species for the NO;7H2 reaction over Pd-Cu^-A^O; and Pd-Cu/T^-A^O; catalysts. The latter species were also found to be very reactive but not active in the case of the NO3TH2/O2 reaction over Pd-Cu/TiO2-Al2O3. The enhanced activity was due to hydrogen spillover from metallic Pd on the support surface. v. One active intermediate N-species (linear NO) on Pd was identified to be present in the nitrogen-reaction path towards N2 formation on Pd-Cu^-Al2O3 catalyst. The latter species were also observed in the case of Pd-Cu/TiO2-Al2O3. vi. Adsorbed nitrosyls (NO+) on the support were found to be active in the case of the NO37H2/O2 reaction over Pd-Cu/T^-AhO;, vii. Adsorbed NOH on Lewis acid sites of TiO2 (Ti4+-NO/NOH) have been considered as the intermediate active species finally leading to the undesired NH4+ in the case of Pd- Cu/TiO2-Al2O3, while the same species seem to be also reactive in the case of Pd-Cu/γ- AhO;. viii. Adsorbed NH4+ on Bronsted acid sites of TiO2 were observed only in the case of the NO;7H2 reaction over Pd-Cu/T^-A^O;.