Hybrid Materials Based on Silica Matrices Impregnated with Pt-Porphyrin or PtNPs Destined for CO2 Gas Detection or for Wastewaters Color Removal

Multifunctional hybrid materials with applications in gas sensing or dye removal from wastewaters were obtained by incorporation into silica matrices of either Pt(II)-5,10,15,20-tetra-(4-allyloxy-phenyl)-porphyrin (PtTAOPP) or platinum nanoparticles (PtNPs) alone or accompanied by 5,10,15,20-tetra-(4-allyloxy-phenyl)-porphyrin (TAOPP). The tetraethylorthosilicate (TEOS)-based silica matrices were obtained by using the sol-gel method performed in two step acid-base catalysis. Optical, structural and morphological properties of the hybrid materials were determined and compared by UV-vis, fluorescence and FT-IR spectroscopy techniques, by atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM) and by Brunauer–Emmett–Teller (BET) analysis. PtTAOPP-silica hybrid was the most efficient material both for CO2 adsorption (0.025 mol/g) and for methylene blue adsorption (7.26 mg/g) from wastewaters. These results were expected due to both the ink-bottle mesopores having large necks that exist in this hybrid material and to the presence of the porphyrin moiety that facilitates chemical interactions with either CO2 gas or the dye molecule. Kinetic studies concerning the mechanism of dye adsorption demonstrated a second order kinetic model, thus it might be attributed to both physical and chemical processes.


 Time course measurements
In order to establish decoloration of the dyes as a function of contact time, the samples were prepared in quarz cuvettes by adding 3 mL 0.05 M NaOH solution containing 0.5 x 10 -4 M MB over 0.01 g adsorbent (representing a loading of 3.33 g/L). The pH was 13. The mixture was stirred on vortex for 5 seconds and the measurements always take place after 15 seconds from the contact of the dye solution with the adsorbent. Figure S1 presents the time course measurement determination. During a contact time of 20 minutes, the silica control adsorbed 2.837 mg MB/g and the PtTAOPPsilica hybrid material 3.452 mg MB/g, respectively. This significant difference is also noticed from the allure of the time course measurement spectra, where it can be observed that a slight desorption phenomenon takes place in the case of silica control after 656 seconds measurement, whereas the PtTAOPP-silica hybrid does not present such trend and is able to continue the adsorption process.
 The effect of silica adsorbent materials loading upon the adsorption of methylene blue Three different silica adsorbent materials loadings: 0.83 g/L, 1.66 g/L and 3.33 g/L were used to investigate the influence of the adsorbent weight quantity upon the adsorption of MB having a fixed initial concentration of 5 x 10 -5 M (16 mg/L). In each case, time course measurements of the intensity of absorption of methylene blue were performed at the wavelength of 664 nm, for 1200 seconds. Figure S2 (a and b) shows the variation in time of the amount of MB dye adsorbed for the three adsorbent loadings investigated, for silica control (a) and for PtTAOPP-silica hybrid (b).
where: qe represents the amount of adsorbed dye (mg/g); Co represents the initial concentration of dye in solution (mg/L); Ce represents the equilibrium concentration of dye (mg/L), that could be calculated by means of Lambert-Beer law and m represents the mass of sorbent (g/L). The percentage removal of dye at various times is calculated according to the equation ( From Figure S1 corroborated with Table S1 it can be observed that the PtTAOPP-silica hybrid can act as a better sorbent for MB than silica control during the time interval of 1200 seconds, at all the investigated loadings. In addition, the process of adsorbtion is continuing its ascendent alure after 1200 seconds in case of using as adsorption material the PtTAOPP-silica hybrid. Table S1. Influence of adsorbent mass upon the adsorption capacity of silica control and PtTAOPP-silica hybrid, MB having a fixed initial concentration of 5 x 10 -5 M (16 mg/L). In theory [2], the increase in removal percentage with the increase in adsorbent mass is expected, due to the increase in the number of sites available for adsorption. Our experimental results clearly show that the maximum of MB uptake was obtained for both investigated adsorbents at a quantity of 0.83 g/L. For this reason, this loading was further used for testing the effect of dye concentration and contact time upon the adsorption. From the data presented in Table S2, the influence of initial MB concentration on the adsorption capacities of silica control and PtTAOPP-silica hybrid for 0.83 g/L adsorbent loading, it can be observed that the percentage of dye removal decreases with the increase of the initial dye concentration, probably because of the rapid saturation of the binding sites of the adsorbent [1]. When using lower MB concentrations the silica control has a better adsorption capacity than PtTAOPP-silica hybrid, but at higher MB concentrations the PtTAOPP-silica hybrid is capable to adsorb a higher amount of dye (7.261 mg/g) than silica control (6.453 mg/g), after 20 minutes contact. Kinetic studies provide information concerning the mechanism of dye adsorption. According to [3] the pseudo-first-order kinetic of adsorption is represented by equation (3):

Mass of adsorbent [g/L]
where qt is the amount of dye adsorbed at time t (mg/g); qe is the adsorption capacity at equilibrium (mg/g); kt is the pseudo first order rate constant (min -1 ); t is the contact time (min). The integration of this equation with initial conditions (qt = 0 at t = 0) leads to the following equation (4): The value for k1 rate constant is calculated from the linear plots of log(qe-qt) versus t -Lagergren plots, as the slope of the plots [4].
The plots of log(qe -qt) vs. t (Figure S4 a,b) for the two adsorbent materials, silica control and PtTAOPP-silica hybrid respectively, at different loadings, show that the adsorption on silica control does not follow a first-order kinetic, but the straight-line plots for the case of PtTAOPP-silica hybrid indicate the validity of Lagergren equation. The calculated values of qe differ from the experimental results, therefore the first-order kinetic model is not appropriate to explain the rate process. As a consequence, the pseudo-second order adsorption kinetic model was applied for both adsorbent materials and the rate constant of pseudo-second order adsorption was calculated from equation (5), representing the integrated pseudo-second order adsorption kinetic rate to initial conditions qt = 0 at t = 0 [4]: where k2 is the rate constant of pseudo-second order adsorption (g x min -1 x mg -1 ). The rate constant for the second-order kinetic model, k2 can be also calculated as the intercept of the linear plot t/qt vs t, as can be seen in Figure S5  The calculated values of qe in the case of pseudo-first order kinetics differ from the experimental results therefore the first-order kinetic model is not appropriate to explain the rate process. The calculated qe values for pseudo-second order kinetic model fit better with the obtained experimental data for both materials. This observation leads to the conclusion that the adsorption of MB on the investigated silica materials is accompanied by chemical interactions between adsorbent and adsorbate [7]. It can also be noticed that PtTAOPP-silica hybrid material is a better adsorbent for methylene blue than silica control, probably due to the presence of the porphyrin moiety that facilitates chemical interactions with the dye molecule.

Desorption studies of MB from PtTAOPP-silica hybrid
The desorption studies were performed as follows: portions of 0.0046 g PtTAOPP-silica hybrid after methylene blue adsorption were centrifuged, filtered and dried (6 h at 90 °C). These were further exposed to 2.5 mL eluent solutions: water, acetone, hydrochloric acid (0.5N), sodium hydroxide solution (0.5N), tetrahydrofurane and ethanol. The mixtures were stirred for two hours, then centrifuged and the UV-vis spectra of the supernatant solutions were recorded. The amount of dye desorbed qe,desorption (mg/g) was calculated according to equation (7): where V-eluent solution volume (L), Cf -the dye concentration in the desorbing solution (mg/L), M-the saturated adsorbent weight (g). The desorption efficiency was calculated according to equation (8)  We can conclude that the desorption efficiency of this material is slightly higher (27.48 %) than the best result reported in the literature (27.11 %).