2.2. Experimental section
2.2.4. Catalyst characterization
Prior to characterization with various techniques, the sample preparation was performed as follows, for p-XRD analysis necessary grinding of the catalyst to fine powder was carefully done to avoid destruction of the mesoporous structure of the material. Thereafter, the catalyst was analyzed with a Rigaku MiniFlex 600 difractometer equipped with Cu Kα1 (λ = 0.1542 nm) radiation source. Low angle measurements were performed between 0.5 and 10° 2θ at a step rate of 0.1 °/min and high angle measurements were performed between 10 and 90° 2θ with a step rate of 0.4 °/min. The resulting p-XRD pattern was matched using the Match! 2 software.
Prior to analysis with HRTEM, the catalyst was dispersed in ethanol and placed on a carbon coated Cu-grid, dried at room temperature and analyzed with a Jeol-Jem 2100F electron microscope operating at 200 kV.
Porosity and surface properties of the catalyst were investigated using a Micromeritics Tristar surface area and porosity analyzer. A 30 mg sample of the catalyst was weighed and degassed with nitrogen gas at 90 °C for 12 hours prior to analysis. The pore volume, pore diameter, and BET surface area measurements were carried out at -196 °C using N2 gas. Pore volume and
pore diameter were evaluated from the desorption branches using the BJH model.
Catalytic oxidation of morin was performed in a 100 ml round-bottomed flask. Mesoporous manganese oxide catalyst (0.005 g) was dispersed in 10 ml de-ionized water in a separate round-bottomed flask and stirred until uniformly distributed. At first, the catalyst concentration was varied from 4.8 × 10-5 M to 1.1 × 10-4 M while keeping the concentration of morin and peroxide constant at 5.4 × 10-5 M and 1.3 × 10-4 M, respectively. In this work catalyst
concentration implies the amount of catalyst, in moles of the solid catalyst per liter, which can be directly correlated to the mass of the catalyst. Kinetic runs were performed by variation of morin concentration between 3.6 × 10-5 M and 8.1 × 10-5 M while keeping the concentration
of the catalyst and peroxide constant at 7.6 × 10-7 M and 1.3 × 10-4 M, respectively.
Furthermore, variation of peroxide concentration was performed between
3.3 × 10-3 M and 19.6 × 10-3 M while keeping the catalyst and morin concentration constant at 5.4 × 10-7 M and 7.6 × 10-5 M, respectively. Oxidation with air was performed by bubbling air from an air cylinder at different flow rates into a round-bottomed flask containing an aqueous solution of morin and the catalyst.
Catalyst recycling was performed by centrifuging the reaction mixture after completion of the catalytic reaction and separation of the two phases. After separation of the two phases, the recycled catalyst was washed with plenty of de-ionized water and reused. For reusability studies, the concentrations of reactants and catalyst were increased to allow for better catalyst recyclability. A 1.9 × 10-4, 0.16 and 4.8 × 10-5 M concentrations of morin, peroxide and catalyst, respectively were used.
2.3. Results
The long range ordered nature of KIT-6 can be clearly observed from the peak at 2° 2θ in the low angle XRD measurement in figure 1(a). The peak at 22° in wide angle measurements is a characteristic peak for amorphous silica, Figure 2.1(b). The crystallographic results from the low angle measurements did not reveal a highly crystalline mesoporous manganese oxide as can be seen from the absence of the characteristic peak below 2° in Figure 2.1(a) and the lack of high intensities in the wide angle measurements signifies a nanoparticulate nature of the synthesized mesoporous manganese oxide, Figure 2.1(b). However, with the use of Match! 2 software we could identify the material as MnO2 with a JCPD (96-900-3477) card.
To confirm the mesoporous nature of both KIT-6 and manganese oxide, type IV hysteresis loops were obtained in nitrogen sorption measurements. The surface area of the template appears to be higher than that of the corresponding metal oxide replica while the pore volume of the template is smaller than that of the corresponding metal oxide replica. Due to the thick wall structure of KIT-6 the resulting pore diameter of the mesoporous metal oxide replica is larger than that of KIT-6. Figure 2.1 also shows the nitrogen sorption results of KIT-6 and manganese oxide and determination of average pore size by cumulative pore volume analysis.
Figure 2.1:(a) Low angle and (b) high angle patterns of KIT-6 and its mesoporous manganese oxide replica, (c) nitrogen sorption results showing type IV loops and (d) cumulative pore size distribution for determination of actual average pore diameter.
TEM micrographs revealed the mesoporous nature of both KIT-6 and mesoporous manganese oxide. In both samples the channels that reveal the mesoporous nature can be observed. It can also be seen from the HRTEM micrograph of mesoporous manganese oxide, Figure 2.2(b), that the structure of the synthesized manganese oxide looks like interconnected bicontinuous structures.
Figure 2.2: HRTEM images of (a) templating KIT-6 and (b) the corresponding mesoporous manganese oxide.
Oxidation of morin was monitored at λ 410 and 270 nm. These two absorption bands decreased over time with the introduction of air or peroxide as oxidant in the presence of the catalyst. An absorption band at λ 310 nm increased over time signifying the formation of the product. In the first few minutes isosbestic points could be clearly observed. This implied the formation of only one product. However, over time they disappeared due to over-oxidation, which indicates the formation of multiple products. The formation of the first product has been detected using HPLC-MS [3]. For kinetic analysis, only the first few minutes of the reaction were taken into consideration. Figure 2.3 shows the reaction pathway of morin oxidation and time based spectra of its degradation.
Figure 2.3: (a) Spectra of oxidation of morin as model reaction signified by formation of isosbestic points, circled in red and (b) its over-oxidation to more than one product, shown by
disappearance of isosbestic points, circled in red, (c) reaction scheme for the oxidation of morin to the first product and over-oxidation to multiple products.
To study the kinetics of the reaction it is important to first establish the order of the reaction. Figure 2.4 shows graphs used in the determination of reaction orders. The linearity of the ln(A0-
Af/At-Af) versus time is the basic tool for determination of the order of the reaction. This was
performed using peroxide concentration variation experiment and we observed linear behavior, which signifies pseudo-first order kinetics with respect to morin concentration.
Figure 2.4: Kinetic plots showing oxidation of morin over time and (b) verification of pseudo first order kinetics shown by linear plots.
To confirm that the reaction is a catalytic reaction, variation of the catalyst concentration was performed. The reaction rates increased with increasing concentration of the catalyst. Figure 2.5 shows increasing rates with increasing catalyst concentration.
Figure 2.5: Variation of catalyst concentration at constant morin and peroxide concentrations shown by plot of observed rates versus concentration of the catalyst.
Variation in concentration of the substrate and oxidant were used to determine whether the reaction is a surface reaction. This type of analysis has been reported in many studies [2, 3, 5, 18, 19] and proved to be correct with a Langmuir-Hinshelwood model. We observed a decrease in observed rates with increase in the morin concentration and increase in observed rates with increasing amount of oxidant, both peroxide and air were used as oxidants. Figure 2.6 shows the variation in substrate and oxidant concentrations.
Figure 2.6: Variation of (a) morin concentration at constant peroxide and catalyst concentrations and (b) variation of oxidant concentration at constant morin and catalyst concentrations.
Variation of substrate and oxidant concentration made it possible to fit the kinetic data. With the use of the Langmuir-Hinshelwood kinetics model, given by equation 1, kinetic parameters such as the adsorption constants of morin and oxidant were obtained. The morin adsorption constant, Kmorin, is higher than adsorption constant of the oxidant, Koxidant, this is true for both cases, when air or peroxide is used as an oxidant. Thus, this suggests competition for reactive sites of the catalyst. The use of air in oxidation of morin results in a higher adsorption constant than when peroxide is used as an oxidant. Table 2.1 shows kinetic parameters obtained from fitting experimental data with equation 1. One other observation is the higher rate at the surface of the catalyst than the rates of disappearance of morin from solution. This relates to the rate at which the intense yellow colour disappears, given by observed rate constant, to the rate of actual morin conversion to morin oxide at the surface of the catalyst, k obtained by fitting experimental data in the Langmuir-Hinshelwood model. Figure 2.7(a and b) shows fitting curves obtained from the Langmuir-Hinshelwood model.
Table 2.1: Surface kinetic parameters obtained by fitting data with Langmuir-Hinshelwood mechanism and comparison with literature data.
Catalyst Oxidant k mol/(m2s) K
morin L/mol Koxidant L/mol x y Ref
MnO2 H2O2 9.0 × 10-2 90 79 0.3 1 This work
MnO2 Air 3.0 × 10-2 2037 0.0032 0.1 2.5 This work
AgDENs H2O2 7.5 × 10-6 3 0.15 1 0.9 [3] MnOXNP H2O2 1.9 × 10-6 3434 255 1 0.9 [5] Au/γ − Al2O3 H2O2 1.1 × 10-9 1827 117 0.9 0.6 [19] Au147DENs H2O2 5.9 × 10-6 763 30 1 0.8 [2]
Kinetic data fitting with equation 2 gave the rate of formation of the product on the catalyst surface, k1, and the rate of catalyst reoxidation, k3. Figure 2.7(c and d) shows the variation of two different oxidant concentrations, air and peroxide, used in kinetic data fitting with the modified Mars-van Krevelen model. The solid lines are the fitting lines obtained by fitting equation 2. The values for the two surface reactions rates, the formation of the product and the surface reoxidation of the catalyst are given in Table 2.1.
Figure 2.7: Variation of oxidant concentrations used in data fitting. In (a and c) plots of observed rates against peroxide concentration. In (b and d) plots of observed rates against variation in air pressure bubbled into the reaction. The solid lines are obtained from the Langmuir-Hinshelwood model, equation 1, graphs (a and b) and the modified Mars-van Krevelen model, equation 2, graphs (c and d).
Table 2.2: Surface kinetics parameters obtained by fitting data with a modified Mars-van Krevelen mechanism.
Oxidant k1 mol/(m2s) k3 mol/(m2s)
H2O2 6.31 ± 1.54 480± 50
Air 6.56×10-5 ± 0 539 ± 174
In Table 2.2 we observe that the formation of the product, that is, the reaction with lattice oxygen of the catalyst is faster when peroxide is used as an oxidant compared to air. Furthermore, the rates of surface reoxidation are not significantly different.
One of the most important aspects of catalysis is the ability of the catalyst to be reused for as many catalytic cycles as possible. The MnO2 catalyst showed good reusability with 100%
morin conversions up to the fourth catalytic cycle, shown in Table 2.3. There was a difference in the rates of conversion from the first to the fourth catalytic cycles. The difference in the observed rates in all four catalytic cycles is less than 40% and is attributed to experimental error and minimal loses of the catalyst during the recycling process. Figure S2.1(a) shows catalyst recyclability study.
Table 2.3: Catalytic reusability study showing changes in apparent rate constants. Entry Catalytic Cycle Conversion (%) Rate (s-1)
100 − (rxi r0 × 100) (Δs -1 / %) 1 First 100 1.3 × 10-3 - 2 Second 100 9.2 × 10-4 31 3 Third 100 8.3 × 10-4 36 4 Fourth 100 7.8 × 10-4 40
For investigation of the role played by the oxidant in the oxidation process, morin oxidation was performed without any oxidant in a purged solution. The reaction proceeded slowly after the addition of MnO2. This effect may be due to surface adsorbed oxygen species. However,
the reaction stopped after a few minutes indicating complete consumption of adsorbed oxygen species. Figure 2.8 shows degradation of morin by oxygen species adsorbed on the catalyst surface.
Figure 2.8: Proof of morin oxidation by surface adsorbed oxygen indicating a Langmuir-Hinshelwood mechanism.
2.4. Discussions
From the kinetic data gathered in this work, it is clear from the appearance and disappearance of isosbestic points over time that morin oxidation is at one stage during the reaction a model reaction, thus, produces one product and at one stage it deviates from being a model reaction as multiple products are formed. Thus, kinetics of morin oxidation as a model reaction should only be studied for the first few minutes of the reaction.
In the case of the Langmuir-Hinshelwood model, the model gives surface reaction rates which are greater than the rate of disappearance of morin from solution, indicating activation of reacting species on the oxide catalyst’s surface. Furthermore, one interesting observation is that the oxide catalyst prefers peroxide on its surface over oxygen from air. The adsorption rate constant of peroxide is greater than that of oxygen from air, Table 2.2. Thus, the values in Table 2.2 suggest a higher adsorption rate constant, kads, than desorption rate constant, kdes, for peroxide and the opposite is true for reactive oxygen from air. However, the use of Langmuir-Hinshelwood model is only applicable when oxidation results from the surface absorbed oxidant. In this work, in the absence of any oxidant, the reaction still shows oxidation of morin which may be due to surface adsorbed oxygen. Still this suggests the Langmuir- Hinshelwood type of kinetics.
The quality of the Langmuir-Hinshelwood data validated by calculating the product of morin concentration and observed rate constants, kobs[morin] and surface coverages of both reactants,
θmorinθoxidant on the catalyst surface indicates that the model can appropriately be used in kinetics
interpretation. The interpretation stems from the assumption that both reactants must adsorb on the catalyst surface prior to the reaction. Thus, the rate of morin disappearance from solution can be given by the product of the rate constant and the concentration of morin, equation 3.
−d[morin]
dt = kobs∙ [morin]
n= k
1∙ S ∙ [morin]n
(3)
Because of the assumption of reaction of adsorbed species, equation 3 can be rewritten as equation 4, which was used for data verification.
where θmorin and θoxidant denote surface coverages by morin and the oxidant, respectively.
The slopes of the plots of the product of morin concentration and observed rates versus the product of the surface coverages give the product of apparent rate and surface area of the catalyst. The values of kobsS obtained from the slopes in Figure 2.9 are in agreement with the
calculated values of kobsS when taking into consideration the large error associated with
isotherms used to calculate surface coverages. The isotherms for the calculation of surface coverages are given by equations 5a and 5b.
θmorin= (Kmorin[morin])
x
1 + (Kmorin[morin])x+ (K
oxidant[oxidant])y
(5a)
θoxidant = (Koxidant[oxidant])
y
1 + (Kmorin[morin])x+ (K
oxidant[oxidant])y
(5b)
Figure 2.9. Plots of the product of morin concentration and observed rate constants versus the product of surface coverages for data verification of the Langmuir-Hinshelwood model.
2.5. Conclusions
We have successfully demonstrated the use of both the Langmuir-Hinshelwood and Mars-van Krevelen models for interpretation of kinetics of morin oxidation as a model reaction. Both the models are fundamental in kinetics studies and can be used as mathematical tools in surface kinetic studies. The use of Langmuir-Hinshelwood model seems to be the best model as
depicted by the experimental data fitting and trends observed in the experimental results. The assumption of oxidation by surface adsorbed species best described the Langmuir- Hinshelwood kinetics.
The Mars-van Krevelen model is a purely mathematical tool, which requires additional techniques to further elucidate surface kinetics.
The Langmuir-Hinshelwood model successfully described competitive adsorption of the reactants on the catalyst surface. It also relates the observed rate constant, kobs and the surface
rate constant, k. Oxidation of morin by lattice oxygen of the oxide catalyst and reoxidation of the catalyst was described by the modified Mars-van Krevelen model which gave the rates of morin oxidation on the surface, k1, and the catalyst reoxidation rate, k3.
2.6. References
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Chapter 3
Effect of alkali and alkaline earth metal dopants on
catalytic activity of mesoporous cobalt oxide
Abstract
Herein we report the synthesis of mesoporous cobalt oxide (Co3O4), and alkali and alkaline
earth metal doped-cobalt oxide (Li-, Ca-, Cs-, and Na-, K-, and Mg/Co3O4) catalysts via the
inverse micelle method. The as-prepared catalysts were characterized by powder X-ray diffraction (p-XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen sorption (BET), hydrogen-temperature-programmed reduction (H2-TPR), and
X-ray photoelectron spectroscopy (XPS). Preliminary characterization results suggest that the as-synthesized materials are of amorphous and mesoporous nature. Their catalytic activity was