Evaluating the reactivity of CuO-TiO2 oxygen carrier for energy production technology with CO2 capture

Chemical looping combustion (CLC) processes have been shown to be promising and effective in reducing CO2 production from the combustion of various fuels associated with the growing global demand for energy, as it promotes indirect fuel combustion through solid oxygen carriers (SOC). Thus, this study aims to synthesize, characterize and evaluate mixed copper and titanium oxide as a solid oxygen carrier for use in combustion processes with chemical looping. The SOC was synthesized based on stoichiometric calculations by the polymeric precursor method and characterized by: X-ray fluorescence (XRF), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM-FEG) with EDS, and Programmed Temperature Reduction (PTR). The oxygen carrying capacity (ROC) and the speed index of the reduction and oxidation cycles were evaluated by Thermogravimetric Reactivity (TGA). The main reactive phase identified was: The CuO phase for the mixed copper and titanium oxide were identified and confirmed by X-ray diffraction using the Rietveld refinement method. The reactivity of the CuO-TiO2 system was high, obtaining a CH4 conversion rate above 90% and a speed index of 40%/min. Due to the structural characteristics and the reactivity tests of this material, it is concluded that mixed copper and titanium oxide have the necessary requirements to be used in chemical looping combustion (CLC) processes.


Introduction
Climate change has caused significant impacts on natural and human systems on all continents and across the oceans in recent decades, and a large part of these changes are caused by one of the environmental issues that most concern humanity: the intensification of global warming. Due to the increase in CO2 emissions, the new IPCC report (2021) states that the global temperature temporarily exceeds or surpasses 1.5°C and no longer 2°C as described at COP21 in 2015 (International Energy Agency, 2019;IPCC, 2018;Page et al., n.d.).
Due to the huge dependence on the use of fossil fuels in generating energy, CO2 Capture and Storage (CCS) technologies have emerged as an important option to reduce global CO2 emissions. In order to achieve this goal, Chemical Looping Combustion (CLC) has emerged as a promising technology for CO2 capture in power plants and industrial applications, with low energy penalties compared to other competing CO2 Capture and Storage (CCS) techniques (Juan Adánez & Abad, 2019). The CLC process scheme is shown in Figure. 1. CLC technology describes cyclical redox processes between two interconnected reactors, as shown in Figure 1, and its principle is the use of a metal oxide (MxOy), known as a solid oxygen carrier (SOC), to transfer the necessary oxygen from the air (Reaction 1) and oxidize the fuel into CO2 and H2O (Reaction 2) in order to avoid direct contact between the air and the fuel.
Its main advantage is the inherent CO2 capture, bypassing the energy penalty.
Correctly selecting the solid oxygen carrier (SOC) is the key to proper functioning of the chemical looping combustion system. This carrier must have favorable thermodynamics and stability through several reduction/oxidation cycles, high oxygen carrier capacity, high fuel conversion with selectivity for the intended product, zero or low carbon deposition, good mechanical properties in fluidized beds, not present agglomeration and still be of low cost to obtain and be environmentally safe. Based on these characteristics, some possible oxides (such as oxides based on Ni, Fe, Cu, Co and Mn) supported in inert materials (such as SiO2, Al2O3, TiO2, ZrO2) were evaluated to improve their reactivity and lifetime of solid oxygen carriers (J. Adánez et al., 2018a). In our previous works we evaluated other systems such as La2NiO4, NiO-Fe2O3/MgAl2O4, Mg6MnO8 and Mn2O3-MgAl2O4, in which we observed new structural characteristics that helped to develop and study new materials and processing forms (Costa et al., 2021;Medeiros et al., 2017Medeiros et al., , 2020Melo et al., 2018;Nascimento et al., 2020).
In the literature, solid oxygen carriers based on Cu and TiO2 have good reaction rates and much higher oxygen carrier capacity than other carriers based on Fe and Mn, in addition to not having thermodynamic restrictions for complete conversion of fuel to CO2 and H2O. In addition, TiO2 has been used as an inert support, which can increase the reactivity of oxygen carriers by increasing porosity, surface area and mechanical strength. Moreover, copper is cheaper than other materials used for CLC such as nickel and cobalt, and its use in oxygen carriers has fewer environmental problems (J. Adánez et al., 2018a;Tian et al., 2018;Tijani et al., 2018;Xu et al., 2015).
Therefore, in order to contribute to the development of materials for CLC technology aiming to evolve to industrial applications, this work aims to evaluate the reactivity of the CuO-TiO2 system as a solid oxygen carrier.
The citric acid was first dissolved in distilled water heated to 80°C in a beaker. The solution was subjected to constant stirring for 30 minutes. After the acid had dissolved, copper II nitrate was slowly added to produce the mixed copper and titanium oxide. Ethylene glycol was subsequently added for solution polymerization under 1 hour of agitation and a temperature of approximately 110°C, forming a polymeric gel. The resins obtained in the syntheses were initially subjected to a thermal treatment in a dryer at 100°C for 24 hours to evaporate excess water. Then, the resins were calcined in a muffle furnace at 350°C for 2h at a rate of 5°C/min to eliminate organic components. The obtained powders were then calcined in an oven at 900°C for 4 h at a heating rate of 10 °C/min in order to achieve high mechanical strength.
The composition of the calcined TCu SOC was determined by X-ray fluorescence. Table 1 shows the XRF analysis results of the mixed copper and titanium oxide (TCu) with the percentages by mass.

Characterization of the solid oxygen carrier
Physical and chemical characterizations were performed on the solid oxygen carrier particles. The SOC chemical composition was determined by X-ray fluorescence in a Bruker S2 Ranger instrument using Pd or Ag anode radiation, maximum power of 50 W, maximum voltage of 50 kV, maximum current of 2 mA, and a XFlash ® Silicon Drift Detector. The crystalline chemical phases were detected by X-Ray Diffraction (XRD) using a Shimadzu XRD-7000 X-ray diffractometer with Cukα radiation (λ = 1.5409 Å). The Joint Committee on Power Diffraction Standards (JCPDS) was used to designate the crystalline phases with the Inorganic Crystal Structure Database (ICSD) database. The refinement of the structure was carried out by applying the Rietveld treatment (Rietveld, 1969), using the MAUD software program.
The temperature programmed reduction (TPR) profile of the SOC was evaluated on a Micromeritics AUTOCHEM II 2920 equipped with a TCD (Thermal Conductivity Detector). The analyzes were performed by varying the temperature from 100 to 900°C under a flow of 50 mL.min -1 of a mixture of 10% H2 in Argon. It was then possible to estimate the oxygen carrier capacity of metal oxides (RO) through the consumption of H2 according to the equations below. is the volume of H2 consumed experimentally, moxy and mred is the mass of the solid oxygen carrier when it is fully oxidized and reduced, respectively, ROC is the oxygen carrying capacity of the materials, XTO is the fraction of active phases present in the carrier. The morphology and distribution of the CuO and TiO2 phases in the solid was evaluated by scanning electron microscopy (SEM) on a SHIMADZU SSX-550 microscope with a voltage of 15KV, equipped with an Oxford Link-Isis energy dispersive X-ray spectroscopy (EDS) analyzer.

Solid oxygen carrier reactivity test
The reactivity tests of the solid oxygen carrier were carried out using a thermogravimetric analyzer (TGA, CI Eletronics), with the experimental configuration shown in Figure 2. Approximately 50 mg of sample was used in the experiments, which was heated to the study temperature in synthetic air flow. The experiments started when the desired temperature was reached and the system stability was verified, with the oxygen carriers being submitted to successive reduction and oxidation stages. Nitrogen was introduced for 2 minutes after the reduction and oxidation steps to avoid mixing the reduction and oxidation gases.
Research, Society and Development, v. 10, n. 12, e514101220596, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i12.20596 6 Table 2 details the conditions of the experiments performed on the thermogravimetric analyzer sample. Gas flows were maintained at 25 L/hour during the experiments with the aim of reducing oscillations when gas changes occurred and minimizing the resistance to external diffusion of the gases. In addition, CH4 was used as a reducing gas, synthetic air as an oxidizer and pure nitrogen to purge the system and avoid contact between the reducing and oxidizing gases. The water vapor used is to simulate the conditions existing in the reduction reactor in a CLC system and/or to prevent carbon deposition.
For studying and treating the data, it was considered that the 100% mass would correspond to the sample in its highest oxidation state at the analyzed temperature, and that any loss of mass is due to the reaction of active phase oxygen (copper oxides) with the reducing gas. It was possible to determine several important parameters through the thermogravimetry results, such as: amount of effective metal oxide, speed index and the oxygen carrier capacity of each carrier. Figure 3 shows the conversion of the mixed copper and titanium oxide solid oxygen carrier as a function of time over three reactivity test cycles in TGA, reacting with methane (CH4) during the reduction step, and synthetic air in the oxidation step. Research, Society andDevelopment, v. 10, n. 12, e514101220596, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i12.20596 7 In the reduction of TCu with CH4:

CH4 (g) + 4 CuO (s) → CO2 (g) + 2 H2O(g) + 4 Cu (s):
Reaction 3 In the oxidation of TCu with O2: Cu (s) + ½ O2 (g) → CuO (s): Reaction 4 According to (J. Adánez et al., 2018b), the reduction and oxidation rate in CLC processes must be sufficiently fast, which was observed in the thermogravimetric experiments for the TCu solid oxygen carrier shown in Figure 3, as its total oxidation occurred in less than 1 minute. The TCu solid oxygen carrier showed better reactivity than the copper oxide carriers supported on diatomite (Cu-D) and kaolin particles (Cu-K) (Costa et al., 2021), making this material very promising for its application in chemical looping combustion (CLC) processes.
The evolution of the oxygen carrier capacity obtained by the TCu SOC was evaluated over time for the three cycles according to Table 3. It was found that there are generally small variations with an increase in ROC at each cycle. These obtained values correspond to the amount of oxygen needed to completely convert the fuel into CO2 and H2O. The CuO-TiO2 system has CuO as its active phase, so the mass loss suffered by this material corresponds to the reduction of copper oxide to metallic copper, according to the XRD result in Figure 4 and the TPR pattern in Figure 5 presented later.

Oxygen carrier characterizations
The XRD patterns corresponding to the mixed copper and titanium oxide (TCu) calcined at 900°C reduced and oxidized after the third cycle in the thermobalance are shown in Figure 4. Source: Authors (2021). Table 4 presents the reference sheets, chemical formulas and the crystal structure of each phase used in the identification of the diffractograms, the percentage of phases and the Sig of the Rietveld refinement of the mixed copper and titanium oxide XRD. We can see in Figure 4 that the calcined TCu diffractogram shows the Ti3O5, TiO2 and CuO phases with main peaks at 25.64°, 27.44° and 32.45° with monoclinic, tetragonal and monoclinic structures, respectively. Table 4 presents a mass percentage of 47% Ti3O5 and 21% TiO2 which differs from the XRF result, which presents a percentage for the TiO2 phase of 72.68%; as the XRF is a semi-quantitative analysis of the oxides presented in sample, this does not distinguish between titanium oxides.
According to Stem et al. (2014), rutile titanium dioxide and near-stoichiometric TiO(2-x) are stable forms of TiO2 with a small number of point defects. However, as the number of point defects increases, rearrangements in the crystal structure (crystallographic shear planes -CSPs) are observed to accommodate them. Point defects are often correlated with oxygen deficiencies, such as Ti interstitials and oxygen vacancies or a combination of both, and are also associated with oxygen diffusion or doping. Oxygen vacancies can be doubled or ionized, resulting in titanium interstitials in Ti(III) or Ti(IV) states, depending on the reaction. However, when the concentrations of these vacancies in the CSP increase enough, there is formation of TinO(2n-1) compounds (Ti3O5, Ti4O7, Ti5O9, Ti6O11), which justifies the formation of the Ti3O5 phase (Stem et al., 2014). The reducible crystalline phases correspond to rutile (TiO2) and metallic copper (Cu) with the main peaks at 27.40° and 43.29°, and tetragonal and monoclinic structures, respectively. However, there were considerable changes in the regenerated TCu particles compared to the calcined particles. The Ti3O5 phase was not regenerated, and according to Regarding the active phase found in mixed copper and titanium oxide, CuO acts as the active phase and titanium oxides as inert supports. Likewise, there are transitions between their oxidation states when subjecting these solid oxygen carriers to reducing conditions, depending on the concentration of fuel gas and the reaction temperature, according to Reaction 3 (Edelmannová et al., 2018).
For the reduction with H2 we have: CuO + H2 Cu + H2O Reaction 5 Therefore, the mixed copper and titanium oxide diffractograms reveal that the choice of uses with a calcination temperature of 900°C may be relevant for CLC applications due to the presence of a greater predominance of the CuO oxidation state than when reduced to Cu, providing complete fuel conversion. Figure 5 shows the H2-TPR profiles of the solid oxygen carrier from Cu. According to the XRD results presented in Figure 4 and Table 4, the mixed copper and titanium oxide presents titanium oxide (Ti3O5), titanium oxide (TiO2) and copper oxide (CuO) as the most oxidized phase. The active phase for this mixed oxide is copper oxide (CuO). The copper oxide in this temperature programmed reduction experiment takes place following the reaction below: CuO + H2 → Cu + H2O Reaction 6 The Figure 5 shows the reduced SOC reduction profile. The presence of a single reduction peak is observed with a maximum temperature of 273°C, which corresponds to the reduction of copper oxide (CuO) to metallic copper (Cu).
The TPR results of Figures Figure 6 and 7, respectively. In the analyzed region, the TCu morphology ( Figure 6-(a) and (b)) illustrates heterogeneous, rounded and regular surfaces without sintering signs, indicating thermal stability, which in turn suggests that there was no change in the surface area.
Microstructures with geometric characteristics of quadratic and hexagonal shape can be seen after enlarging the regions  The EDS of Figure 6 (e) and (f) show the characteristic copper, titanium and oxygen peaks corresponding to TCu without any contamination. A variation in mass of these elements was observed, whose values are shown in Figure 5. This variation occurred due to the reduction and oxidation reactions of the elements.

Conclusion
Reactivities during reduction and oxidation reactions of solid oxygen carriers based on mixed copper and titanium oxide were analyzed by thermogravimetry (TGA) and their physicochemical properties by XRD, SEM-EDS, and TPR. The copper oxide (CuO) particles synthesized with titanium oxide, according to the SEM-EDS results, presented appropriate mechanical strength and uniform distribution of the CuO particles on the oxygen carrier surface. It showed an increase in reactivity and its oxygen carrying capacity when subjected to three redox cycles, in addition to good stability after the third cycle. This reveals that there were no changes in the structure of oxygen carriers in this reaction period. The regeneration of CuO after the third cycle with CH4 led to the full formation of copper oxide, so that the CuO-TiO2 system diffractograms reveal that the choice of uses with a calcination temperature of 900°C may be pertinent for CLC applications due to the presence of a greater predominance of the CuO oxidation state which provides complete fuel conversion when reduced to Cu. From the structural characteristics and the reactivity tests reflective in methane conversion above 90% and ROC of 11.3 of this material, it is concluded that the CuO-TiO2 system has the necessary requirements to be used in chemical looping combustion processes (CLC).
We can synthesize a larger amount of mixed copper and titanium oxide to act as solid oxygen carriers in reactors in future works in order to carry out many tests in multiple redox cycles, verifying the stability of samples in fluidized beds.