*2.5.2. Degradation of MTBE by catalytic wet air oxidation over noble and base metals*

This research studied the degradation of the fuel oxygenated MTBE through CWAO occupying Ru, Au, and Ag as the catalysts, which will be responsible for mineralizing the pollutant.

 **Figure 6** shows the results of the catalytic activity for CWAO of MTBE with the sets of catalysts Ru, Au, and Ag. The best activity for the set of rutheniumsupported catalysts was for the one named RuAlCe1IH, since it presented a 68% conversion of MTBE and a 63% degradation of TOC; the most favorable results for this particular catalyst were attributed to a particle size of 9 nm measured by TEM, and to the contribution of ceria, due to the oxygen storage capacity phenomenon. Ceria is known for having the capacity to exchange oxygen, through its vacancies of oxygen, which promotes the increase of selectivity in all cases with the Ru-synthesized catalysts from a chlorinated salt, by the formation of a species of type Ce4+ − O2<sup>−</sup> − M+ , contributing to improve the reducibility of ruthenium.

With respect to the Au-supported catalysts, AuAlCe5DPU was the one that stood out for its catalytic performance in comparison to the rest of its counterparts. AuAlCe5DPU reached a maximum of 73% MTBE conversion and a TOC degradation of 72%, indicating that this catalyst was efficient both to have a good *Nonconventional Wastewater Treatment for the Degradation of Fuel Oxygenated… DOI: http://dx.doi.org/10.5772/intechopen.84250* 

conversion and to transform to CO2. This behavior was explained by the fact of presenting the best distribution of particles on the surface of the catalyst, according to the TEM analysis, and confirmed by TPR. The largest amount of active particles for this catalyst was below 2 nm. It was observed that the other catalysts had a similar activity attributable to the distribution of particle sizes ranging between 2 and 10 nm. According to the performed analysis, well-dispersed Au nanoparticles and the oxidation state of Au play an important role in this type of oxide-reduction reactions. The excess of CeO2 does not allow a good selectivity toward CO2 since it interferes in the exchange of O2 at the time of oxidation, giving an excess of oxygen that causes the metal particle to change its oxidation state on the surface of the catalyst and confirming the theory made by Imamura et al. [26] that a balance of metal particles in oxidized and reduced state is needed to obtain satisfactory results in terms of activity and selectivity; this theory is fulfilled in molecules that are strongly adsorbed on the surface of the catalyst such as acetic acid, and according to this study, this principle can be also applied with MTBE.

 The results of the catalytic activity for CWAO of MTBE for the set of silver catalysts indicated that the best conversion obtained corresponds to the AgCeDPNa catalyst with 66%, due to the presence of CeO2. In this case, we can only mention an effect of CeO2 because the particle sizes obtained by HRTEM and TPD-CO revealed different distributions but did not significantly impact the catalytic activity. This effect has been explained by several researchers as the formation of a bridge M-O-Ce where M means silver metal. The AgCeDPNa catalyst was the one that has a better behavior toward mineralizing CO2 due to the oxide-reducing properties of this support. However, it was observed that the catalyst with 5% of CeO2 has a very close TOC value with respect to the AgCeDPNa catalyst. In this case, the effect between the particle size, the activity, and the selectivity toward CO2 was not possible to distinguish because, as the HRTEM histograms showed, the range of sizes was

#### **Figure 6.**

*MTBE conversion and TOC degradation, at 100°C and 10 oxygen bar over Ru-, Au-, and Ag-supported catalysts.* 

 very broad in all cases; nonetheless, the effect of CeO2 appeared in the activity and also in the selectivity. Imamura proposed a theory stating that a balance of metal particles in oxidized and reduced state is needed to obtain satisfactory results in terms of activity and selectivity, which is observed here by TPR of H2 in the case of AgCeDPNa catalyst; Ag particles remain oxidized even with the passage of H2 which makes them more selective toward CO2. It is not possible in this case to account for the proportion of Ag+ /AgO particles because this can only be done by XPS, a very expensive technique and not available in this case; however, it can be concluded that, in terms of activity, the optimal catalyst is the one that only contains ceria.

## *2.5.3. Degradation of ETBE by catalytic wet air oxidation over noble and base metals*

We also analyzed the degradation process of the ETBE molecule through CWAO using Cu synthesized by three different synthesis methods, with the main characteristic of being carried out in a single step. The aim of the application of these methods is to avoid the leaching of the metal, which has occurred in other previous experiments by different investigators, after having passed a certain time of the reaction.

 The analysis results of the ETBE-treated solutions by CWAO are presented in **Table 2**; for each of the three synthesis methods of the Cu catalysts, Cu10AlSG and Cu10AlIHU catalysts were more active, obtaining 88 and 89% ETBE conversion, respectively, after 1 h of oxidation. But the highest values for TOC degradation were obtained with the catalysts prepared by sol-gel method, as Cu5AlSG reached 84% and Cu10AlSG 89%. This last result confirmed that Cu catalysts synthesized by sol-gel are more effective catalysts for the mineralization process, which allows to degrade the organic matter, coming from the contaminant present, by almost 90% until obtaining CO2 in the treated solutions. In addition, we can affirm that the optimum percentage of Cu was 10%, as well as the commercial catalyst used for this type of reactions and reported in the literature.

Another discovery in this study with copper nanoparticles was realized by measuring copper concentration through atomic absorption in the ETBE-treated solutions, since no copper concentrations were obtained, particularly in the catalysts prepared by sol-gel method, opposite situation for the other catalysts prepared


#### **Table 2.**

*ETBE conversion, TOC and SCO2 at 100°C and 10 bar of pressure during 1 h of reaction with one of [ETBE]0 = 1000 ppm.* 

*Nonconventional Wastewater Treatment for the Degradation of Fuel Oxygenated… DOI: http://dx.doi.org/10.5772/intechopen.84250* 

by wet impregnation and wet impregnation with urea. Therefore, we can encapsulate the copper in the alumina, by sol-gel method, thus avoiding contamination by the metal, leaching, and, in turn, obtaining an improvement in the catalytic performance of the metal.

## *2.5.4. Degradation of TAME by catalytic wet air oxidation over noble and base metals*

 Another series of experiments was conducted under the same conditions described below over Cu synthetized in three different methods and Au-supported catalyst but in this experiments with TAME as a target molecule by CWAO.

 **Table 3** presents the analysis results of the treated solutions of TAME by CWAO; for each of the three synthesis methods of the Cu catalysts, the catalysts of Cu15AlSG and Cu10AlIHU were more active, obtaining 78% of TAME conversion, for both catalysts after 1 h of reaction. But the highest values for TOC degradation were obtained with the catalysts prepared by sol-gel method; Cu15AlSG reached 78% and Cu10AlIHU 75%. These results sustained that the Cu catalysts synthesized by sol-gel were more effective catalysts for TAME mineralization process, which allows them to degrade the organic matter, coming from the existing contaminant, by almost 80% in the treated solutions.

 The results of the catalytic activity for TAME CWAO in Au-supported catalysts are shown in **Figure 7**. It was observed that the best activity happened with the catalyst at AuAlCe10DPU with 80% conversion of TAME, although all the remaining catalysts showed good activity except for the catalyst with AuAlCe1DPU; this could be explained by the fact that this molecule may not be very sensitive to particle size due to its structure.

 **Figure 7** also shows the abatement of TOC of the supported Au catalysts; as can be seen the best carbon transformation toward CO2 was obtained for the AuAlCe3DPU catalyst, with a 77% conversion, and for the AuCeDPU catalyst with 80%, although all the remaining catalysts showed a remarkable performance, without exceeding these, except for AuAlCe1DPU.

**Table 4** shows the selectivity to CO2, and we can say that the supported Au catalysts containing Ce 5 and 10% were the least selective to CO2. This is because there is


#### **Table 3.**

*TAME conversion, TOC degradation, and SCO2 at 100°C and 10 bar of pressure during 1 h of reaction with one of [TAME]0 = 1000 ppm.* 

#### **Figure 7.**

*TAME conversion and TOC degradation % at 100°C and 10 bar over Au-supported catalysts.* 


#### **Table 4.**

*TAME selectivity at 100°C over Au-supported catalysts.* 

 a poisoning by CeO2 that affects the selectivity when it is in excess due to the interaction of M-Ce-O; this case shows that the optimal percentage of CeO2 for the TAME is 3%. It is important to note that the AuAlCe3DPU catalyst is equally active and selective to the AuCeDPU catalyst, so the alumina-ceria support with low concentrations of CeO2 is presented as an alternative for the wet oxidation process of TAME.
