*3.3. Catalytic Activity Determination*

The catalytic performance was examined in a fixed bed reactor (Microactivity by PID Eng & Tech S.L., Alcobendas, Madrid). A multipoint K type thermocouple was fixed to the middle of the catalyst bed in order to control the reaction temperature. Of the catalyst 1 g (sieve fraction of 0.25–0.3 mm) diluted with 1 g of inert quartz (sieve fraction of 0.5–0.8 mm) was used. A gaseous mixture (500 cm<sup>3</sup> min−<sup>1</sup> ) of CH<sup>4</sup> (1 vol.%), O<sup>2</sup> (10%) and N<sup>2</sup> (89%) was continuously supplied at a space velocity of around 30,000 h−<sup>1</sup> (300 mL CH<sup>4</sup> g <sup>−</sup><sup>1</sup> h −1 ) under atmospheric pressure.

Catalytic conversion was evaluated in the 200–600 ◦C range each 25 ◦C. The products were analyzed with an on-line gas chromatography (Agilent Technologies 7890N) equipped with thermal conductivity detector (TCD), using a PLOT 5A molecular sieve column (analysis of CH4, O2, N<sup>2</sup> and CO) and a PLOT U column (CO<sup>2</sup> analysis). The methane conversion is referred to the yield of CO2. Kinetic results were checked not to be controlled by both mass and heat transfer limitations, following the criteria proposed by Eurokin [53,54] (see Table S1, Supplementary Material).

#### **4. Conclusions**

Three strategies for enhancing the behavior of alumina-supported Co3O<sup>4</sup> catalysts for oxidation of lean methane were compared. These approaches focused on two main objectives, namely minimizing the formation of inactive cobalt aluminate and promoting the intrinsic activity of the deposited cobalt oxide. Thus, our attention was focused on the surface protection of alumina with magnesia, redox promotion of Co3O<sup>4</sup> with nickel oxide and surface protection of alumina with ceria, which eventually may also act as a redox promoter for Co3O4. These samples were extensively characterized by WDXRF, BET measurements, XRD, Raman spectroscopy, XPS, H2-TPR, CH4-TPRe and STEM-EELS/EDX.

Firstly, as for the evaluation of the influence of MgO on the catalytic behavior, magnesia was loaded onto the alumina support prior to Co3O<sup>4</sup> addition. The incorporation of magnesia hardly affected the textural properties of the blank alumina support, probably due to notable surface area of this promoter. After incorporating cobalt, deposited MgO prevented Co3O<sup>4</sup> from reacting with the alumina, thereby limiting the generation of inactive cobalt aluminate. On the other hand, a cobalt–magnesium interaction was favored, thereby resulting in better redox properties of the cobalt oxide with a marked shift of the reduction onset temperature by around 30 ◦C.

Secondly, a bimetallic cobalt-nickel catalyst supported over alumina was synthesized in order to examine the effect of coprecipitating small amounts of nickel (5 wt %) along with the cobalt precursor. The resulting Ni-Co catalyst exhibited good textural properties, with only a slight loss of specific surface with respect to the bare alumina. Combined results from XRD, XPS, Raman spectroscopy and STEM-EELS evidenced that nickel was homogeneously present on the surface and induced the formation of trace amounts of NiCo2O4, because of the partial insertion of Ni2<sup>+</sup> cation into the lattice of Co3O4. The strong cobalt-nickel interaction promoted the redox properties of the resulting Ni-Co samples. Thus, when compared with the unmodified cobalt catalyst, the reduction onset temperature was noticeably shifted (around 50 ◦C) to lower temperatures and the specific H<sup>2</sup> uptake in the low temperature range increased to a considerable extent. Furthermore, the higher mobility of active oxygen species over this sample was also evidenced by the temperature-programmed reaction with methane in the absence of gaseous oxygen.

Finally, the cobalt addition over a cerium-coated alumina was examined. Ceria was efficiently dispersed on the support in view of the reduced impact on the textural properties. A dual effect of ceria on the properties of deposited cobalt was evidenced. On one hand, ceria, like magnesia, partially inhibit the formation of undesired cobalt aluminate. More interestingly, a strong interaction between cobalt oxide and ceria was found that ultimately resulted in the insertion of cerium atoms into the spinelic lattice of Co3O4. Consequently, a higher abundance of Co3<sup>+</sup> species at the cost of Co2<sup>+</sup> was evidenced, thereby promoting the mobility of active lattice oxygen species.

The comparison of the catalytic behavior of the modified catalysts revealed that the most suitable strategy was the addition of cerium to the alumina, prior to the deposition of the cobalt precursor. The resulting optimal catalyst reduced its T<sup>50</sup> value by 70 ◦C with respect to the reference catalyst supported over bare alumina, and exhibited a specific reaction rate around three times higher in comparison with the reference Co/Al catalyst.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/10/7/757/s1, Figure S1: CH<sup>4</sup> -TPRe profiles of the supported cobalt catalysts, Figure S2: Additional HAADF-STEM images of the Co-Ni/Al (left) and Co/Ce-Al (right) catalysts coupled to EELS (Co (red) and Ni (blue)) and EDX (Co (red) and Ce (green)) elemental distribution, Figure S3: Pseudo-first order fit for the experimental data over the supported cobalt catalysts, Table S1: Series of recommendations and criteria for accurate analysis of intrinsic reaction rates (as evaluated for the Co/Ce-Al catalyst at 400 ◦C).

**Author Contributions:** Conceptualization, A.C. and R.L.-F.; Methodology, A.C., B.d.R. and J.I.G.-O.; Formal Analysis, A.C., B.d.R. and R.L.-F.; Investigation, A.C.; Writing—Original Draft Preparation, A.C., B.d.R. and R.L.-F.; Writing—Review and Editing, A.C., J.I.G.-O. and R.L.-F.; Supervision, J.I.G.-O. and R.L.-F.; Funding Acquisition, B.d.R., J.I.G.-O. and R.L.-F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry of Economy and Competitiveness (CTQ2016-80253-R AEI/FEDER, UE), Basque Government (IT1297-19) and the University of The Basque Country UPV/EHU (PIF15/335).

**Acknowledgments:** The author wish to thank the technical and human support provided by SGIker (UPV/EHU) and the Advanced Microscopy Laboratory of the University of Zaragoza.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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