XPS study on calcining mixtures of brucite with titania

Citation: Sánchez-Zambrano, K.S.; Hernández-Reséndiz, M.; Gómez-Rodríguez, C.; García-Quiñonez, L.V.; Aguilar-Martínez, J.A.; Rodríguez-Castellanos, E.A.; Verdeja, L.F.; Fernández-González, D.; Castillo-Rodríguez, G.A. XPS Study on Calcining Mixtures of Brucite with Titania. Materials 2022,15, 3117. https://doi.org/10.3390/ ma15093117 Academic Editor: Thomas Niendorf Received: 4 April 2022 Accepted: 22 April 2022 Published: 26 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article XPS Study on Calcining Mixtures of Brucite with Titania Karla Sofía Sánchez-Zambrano 1, Marina Hernández-Reséndiz 1, Cristian Gómez-Rodríguez 2,3, Linda Viviana García-Quiñonez 4, JosuéAmilcar Aguilar-Martínez 1, Edén Amaral Rodríguez-Castellanos 1, Luis Felipe Verdeja 3, Daniel Fernández-González 5and Guadalupe Alan Castillo-Rodríguez 1,* 1Facultad de Ingeniería Mecánica y Eléctrica (FIME), Universidad Autónoma de Nuevo León (UANL), San Nicolás de los Garza 66450, Mexico; [email protected] (K.S.S.-Z.); [email protected] (M.H.-R.); [email protected] (J.A.A.-M.); [email protected] (E.A.R.-C.) 2Faculty of Engineering, University of Veracruz, Coatzacoalcos 96535, Mexico; [email protected] 3 Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica, Escuela de Minas, Energía y Materiales, Universidad de Oviedo, 33004 Oviedo, Asturias, Spain; [email protected] 4CONACYT-Centro de Investigación Científica y de Educación Superior de Ensenada B.C. (CICESE), Unidad Monterrey, Apodaca 66629, Mexico; [email protected] 5Centro de Investigación en Nanomateriales y Nanotecnología (CINN), Consejo Superior de Investigaciones Científicas (CSIC), Universidad de Oviedo (UO), Principado de Asturias (PA), Avda. de la Vega, 4–6, 33940 San Martín del Rey Aurelio, Asturias, Spain; [email protected] *Correspondence: alan.castillo.r[email protected]; Tel.: +52-81-8329-4020 Abstract: In this work, we studied the phases in a Mg-Ti-O system using a 1:1 formulation of MgO:TiO 2 , mixing synthetic brucite of Mexican origin with TiO 2 microparticles of high purity, with a heat treatment at 1100 ◦ C for 1 h. Due to its valence electrons, TiO 2 can contribute to the sintering process to improve density in MgO products. The raw materials and formulation by XPS and X-RD techniques were characterized. The results demonstrate the presence of different oxidation states in titania and the formation of different oxides in the Mg-Ti-O system when mixed and calcined at 1100 ◦ C; additionally, we estimated the formation of vacancies in the crystal lattice during the transformation from hexagonal brucite to magnesia with a cubic structure centered on the faces. Its thermal behavior is indicated by the MgO-TiO2phase diagram. Keywords: brucite; magnesia; titania; Mg(OH)2; MgO; TiO2; XPS; X-RD; refractories; ceramics 1. Introduction Sintered MgO is one of the most important ceramic materials for the manufacture of basic refractory products; it has been used for many years in the production of steel, cement, and many other products on an industrial scale [ 1 ]. One of the sources of raw material to obtain sintered MgO is brine [ 1 , 2 ]; 14% of the world’s MgO production is synthetic magnesia and comes from the precipitation of magnesium hydroxide from seawater sources and brines. In Mexico, MgO is produced in the form of hydroxide, caustic, sintered, and melted from the precipitation of brine combined with calcined dolomite. The initial material for obtaining sintered magnesia is synthetic Mg(OH) 2 , which is precipitated from brine combined with doloma; thus, in the form of impurities, this material contains other oxides from the composition of the brine, mainly from dolomite, for example, SiO 2 , CaO, Fe 2 O 3 , and Al2O3[2]. Magnesium hydroxide is a chemically defined compound with hexagonal/rhombohedral crystal structure; the product obtained during thermal decomposition at 1100 ◦ C, caustic MgO, occurs in a cubic crystalline transformation centered on the faces [ 2 , 3 ]. During the thermal decomposition of magnesium hydroxide and crystallographic transformation, the presence of impurity ions in the brucite from dolomite can be an influence; this influence finally impacts the last stage to sinter and obtain the sintered MgO [ 2 ]. Regarding the Materials 2022,15, 3117. https://doi.org/10.3390/ma15093117 https://www.mdpi.com/journal/materials Materials 2022,15, 3117 2 of 20 above, it is feasible to incorporate different ions into the crystal lattice of the base material, mainly in the form of microparticles in brucite, and evaluate their influence on properties such as the melting point of MgO [ 4 ]. It is feasible that the addition of Ti 4+ cations modifies the structure of raw materials for MgO-based refractories when they are added to Mg(OH) 2 brucite before calcination at low a temperature for hydroxylation. It is intended to demonstrate how the addition of Ti 4+ cations added to brucite and their calcination at low a temperature modifies the conditions of caustic MgO for the manufacture of raw materials for MgO-based refractories [4]. The novelty of this study is the focus on obtaining a microstructural study and surface composition analysis to determine and compare the chemical oxidation states of the generated phases by determining bond energies prior to the sintering treatment. To obtain high-density, industrial-grade MgO from synthetic brines, the brucite is first calcined at a relatively high temperature to obtain caustic MgO, and then compacted and calcined at a high temperature to obtain dense MgO. There are many studies on the addition of oxides to caustic MgO to obtain magnesia with better density, but there are no references on studies adding oxides to the brucite prior to the step of obtaining caustic MgO. In this way, the current work highlights the importance and novelty of adding micro-TiO 2 in brucite and opens the possibility of new work in the future, using this route of preparation of MgO-micro-oxide composites to evaluate other physical, mechanical, and chemical properties of the compounds obtained. In this work, prior to the sintering of MgO, Mg 2+ cations were replaced by Ti 4+ cations during the calcination of brucite; this substitution is expected to generate crystalline imperfections in the MgO as vacancies due to differences in ionic radius and valence number. This work contributes to the knowledge of the formation of dense refractory ceramic phases that can occur when we add TiO 2 to brucite during the manufacture of dense MgO, as well as its possible implications for performance during its application in industrial furnaces at a high temperature, since one of the main problems of the refractory industry is obtaining refractory bricks with dense phases. We used X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques. Through these techniques, we analyzed and compared the chemical states of the elements involved, determining the specific binding energies of each emitting element, elementary composition, and generation of new phases. Other researchers have also studied MgO composites using the XPS technique [ 5 ]. Garcia et al. studied sintered MgO with the addition of nanoparticles in different percentages by weight (iron oxide and aluminum oxide) using the XPS technique, to determine the oxidation states generated after laser irradiation with different wavelengths (532 and 1064 nm). The results showed that the binding energies varied according to the wavelength, energy fluence, and concentration of nanoparticles used after the samples were irradiated [ 6 , 7 ]. In another work, the influence of the preparation procedure of MgO nanopowders on the surface properties was investigated by XPS. The powders were obtained via two methods: (1) by gelation/precipitation from a solution containing Mg 2+ , and (2) by a micellar liquid which was created by adding the Mg2+ solution to a surfactant. The XPS analysis showed the presence of magnesium carbonate in the outermost layers, and the presence of Mg(OH) 2 was evident [ 8 ]. Sun et al. prepared Mn 2 CoAl thin films on MgO substrates by magnetron sputtering and heat treatment at 300 ◦ C. They used the XPS technique to study the evolution of the chemical states of Mn 2 CoAl/MgO after annealing. The reported binding energy for MgO was 1303.80 eV, and there were changes in binding energies, indicating Mg oxidation at the interface [9]. Based on the above, in this work, the phases in the Mg-Ti-O system were studied using a 1:1 formulation of MgO:TiO 2 , mixing synthetic brucite of Mexican origin with high purity TiO 2 microparticles, with a heat treatment at 1100 ◦ C for 1 h. The raw materials and formulation were characterized by XPS and XRD techniques. The results demonstrate the presence of different oxidation states in titania and the formation of different oxides in the Mg-Ti-O and Ca-Ti-O systems when mixed and calcined at 1100 ◦ C; additionally, we Materials 2022,15, 3117 3 of 20 estimated the formation of vacancies in the crystal lattice during the transformation from hexagonal brucite to magnesia with a cubic structure centered on the faces. Its thermal behavior is indicated by an MgO-TiO2phase diagram produced. 2. Materials and Methods 2.1. Materials The magnesia used in this work is high purity and industrial grade, produced in México from brines with the addition of doloma by Grupo Peñoles company (Laguna del Rey, Coahuila, Mexico). Although there are different production methods for obtaining MgO [ 2 , 10 ], the following explains how MgO is obtained in Mexico, starting from dolomite. Doloma is obtained from the calcination of dolomite, which provides 40% of the final magnesium ions. The doloma is mixed with MgCl 2 salts in aqueous solution obtained by crystallization from a natural brine mantle, according to the following chemical reaction: MgSO4(ac)+2NaCl(ac)→Na2SO4↓+MgCl2(ac)(1) The aqueous solution of MgCl 2 is mixed with the doloma at room temperature, producing the following consecutive chemical reactions: MgO·CaO +H2O→Mg(OH)2↓+Ca(OH)2(ac)(2) Ca(OH)2(ac)+MgCl2(ac)→Mg(OH)2↓+CaCl2(ac)(3) The aqueous CaCl2is treated with the depleted brine of reaction (1), which results in more magnesium ions: MgSO4(ac)+CaCl2(ac)+2H2O→MgCl2(ac)+CaSO42H2O↓(4) The aqueous MgCl 2 is subsequently treated with reaction (3) to obtain more magnesium ions. The magnesium hydroxide obtained from reactions (2) and (3) is calcined in a multi-home Herreshoff furnace at 1100 ◦ C, and the industrial grade caustic MgO is finally obtained with 99.9% purity. The reactions of the process are outlined in Figure 1. Mg(OH)2 ∆ →MgO +H2O↑(5) The brucite obtained has a chemical composition as shown in Table 1, with high MgO content; loss on ignition (LOI) corresponds to the high content of chemical water in the form of ions (OH) 2− associated with Mg and the content of water in physical form from the brine solution. Table 1. Chemical composition of the brucite produced from synthetic brine in Mexico used in the present work. MgO (% Weight) CaO (% Weight) SiO2 (% Weight) Fe2O3 (% Weight) Al2O3 (% Weight) LOI (% Weight) 46.00 0.31 0.04 0.02 0.04 53.59 A thermogravimetric analysis of brucite reveals that physical water mass loss and chemical water loss (dehydration) occur at temperatures of 105.01 ◦ C and 450.3 ◦ C, respectively (Figure 2). The loss of mass at 755.93 ◦ C is due to the loss of residual chlorides from synthetic brine. The weight loss corresponds to 2.4%. Materials 2022,15, 3117 4 of 20 Materials 2022, 15, x FOR PEER REVIEW 3 of 21 Mg-Ti-O and Ca-Ti-O systems when mixed and calcined at 1100 °C; additionally, we estimated the formation of vacancies in the crystal lattice during the transformation from hexagonal brucite to magnesia with a cubic structure centered on the faces. Its thermal behavior is indicated by an MgO-TiO 2 phase diagram produced. 2. Materials and Methods 2.1. Materials The magnesia used in this work is high purity and industrial grade, produced in México from brines with the addition of doloma by Grupo Peñoles company (Laguna del Rey, Coahuila, Mexico). Although there are different production methods for obtaining MgO [2,10], the following explains how MgO is obtained in Mexico, starting from dolomite. Doloma is obtained from the calcination of dolomite, which provides 40% of the final magnesium ions. The doloma is mixed with MgCl 2 salts in aqueous solution obtained by crystallization from a natural brine mantle, according to the following chemical reaction: MgSO󰇛ac󰇜2NaCl 󰇛ac󰇜→Na SO↓MgCl 󰇛ac󰇜 (1) The aqueous solution of MgCl 2 is mixed with the doloma at room temperature, producing the following consecutive chemical reactions: MgO∙CaOH O→Mg 󰇛OH󰇜↓Ca 󰇛OH󰇜󰇛ac󰇜 (2) Ca󰇛OH󰇜󰇛ac󰇜MgCl 󰇛ac󰇜→Mg 󰇛OH󰇜↓ CaCl󰇛ac󰇜 (3) The aqueous CaCl 2 is treated with the depleted brine of reaction (1), which results in more magnesium ions: MgSO󰇛ac󰇜CaCl 󰇛ac󰇜2H O→MgCl 󰇛ac󰇜CaSO 2HO↓ (4) The aqueous MgCl 2 is subsequently treated with reaction (3) to obtain more magnesium ions. The magnesium hydroxide obtained from reactions (2) and (3) is calcined in a multi-home Herreshoff furnace at 1100 °C, and the industrial grade caustic MgO is finally obtained with 99.9% purity. The reactions of the process are outlined in Figure 1. Mg󰇛OH󰇜 ∆ →MgOH O↑ (5) Figure 1. Representative diagram of the process for obtaining caustic MgO in Mexico [11]. Figure 1. Representative diagram of the process for obtaining caustic MgO in Mexico [11]. Materials 2022, 15, x FOR PEER REVIEW 4 of 21 The brucite obtained has a chemical composition as shown in Table 1, with high MgO content; loss on ignition (LOI) corresponds to the high content of chemical water in the form of ions (OH) 2− associated with Mg and the content of water in physical form from the brine solution. Table 1. Chemical composition of the brucite produced from synthetic brine in Mexico used in the present work. MgO (% Weight) CaO (% Weight) SiO 2 (% Weight) Fe 2 O 3 (% Weight) Al 2 O 3 (% Weight) LOI (% Weight) 46.00 0.31 0.04 0.02 0.04 53.59 A thermogravimetric analysis of brucite reveals that physical water mass loss and chemical water loss (dehydration) occur at temperatures of 105.01 °C and 450.3 °C, respectively (Figure 2). The loss of mass at 755.93 °C is due to the loss of residual chlorides from synthetic brine. The weight loss corresponds to 2.4%. Figure 2. Results of the thermogravimetric and differential thermal analysis performed on brucite. Titania of high purity (99.99%) from Sigma Aldrich, in the form of microparticle powder, was used as an additive in the development of the present work. For sample preparation, the chemical reactions expected during the thermal process are as follows: Mg󰇛OH󰇜 ∆ →MgOH O↑ (6) MgO  TiO ∆ →MgTiO  (7) That is, the expected complete reaction is as follows: Mg󰇛OH󰇜TiO  ∆ →MgTiO H O↑ (8) Table 2 shows the percentages by weight of the amount of brucite used in the formulations of the present work, based on the molecular weights of the original substances and the product of the expected reaction. Figure 2. Results of the thermogravimetric and differential thermal analysis performed on brucite. Titania of high purity (99.99%) from Sigma Aldrich, in the form of microparticle powder, was used as an additive in the development of the present work. For sample preparation, the chemical reactions expected during the thermal process are as follows: Mg(OH)2 ∆ →MgO +H2O↑(6) MgO +TiO2 ∆ →MgTiO3(7) That is, the expected complete reaction is as follows: Mg(OH)2+TiO2 ∆ →MgTiO3+H2O↑(8) Materials 2022,15, 3117 5 of 20 Table 2shows the percentages by weight of the amount of brucite used in the formulations of the present work, based on the molecular weights of the original substances and the product of the expected reaction. Table 2. Percentage by weight for bruciteand titania-based formulations. Compound Molar Weight gr % Weight Mg(OH)258.3197 42.20 TiO279.8658 57.80 Total 138.1855 100 2.2. Sample Preparation Weight percentages of TiO 2 microparticles were added to Mg(OH) 2 powders considering the following relation: (100 − X) wt.% Mg(OH) 2 + X wt.% of TiO 2 , where X = 0, 100, and 50. The formulations studied in this work are presented in Table 3. Table 3. List of formulations developed. Mg(OH)2 % Mol TiO2 % Mol Calcined Brucite M4 (not calcined) 100 0 0 M3 (not calcined) 0 100 0 M2 (calcined) 0 0 100 M1 (calcined) 50 50 0 For the preparation of the mixtures, TiO 2 and brucite were mixed; the brucite was mixed with the titania in a porcelain mortar and homogenized manually. Subsequently, the mixture was placed in high-alumina crucibles and placed in an oven, where they were calcined at a maximum temperature of 1100 ◦ C for 1 h. Finally, the powder samples of caustic MgO mixed with TiO 2 particles were obtained. The mixtures were calcined out in a Lindberg Blue M/1700 Thermo Fisher Scientific electric furnace (Facultad de Ingeniería Mecánica y Eléctrica, UANL, San Nicolás de los Garza, Nuevo León, Mexico), using a heating rate of 5 ◦ C/min and a dwell time of 1 h at maximum temperature. Cooling down to room temperature was carried out in the furnace. 2.3. Methods 2.3.1. Characterization by Spectrometry of X-ray-Induced Photoelectrons (XPS) The samples were placed on carbon-conductive tapes to perform X-ray-Induced Photoelectron Spectroscopy (XPS) analysis on Thermo Scientific Inc. Model K-Alpha equipment (Facultad de Ingeniería Mecánica y Eléctrica, UANL, San Nicolás de los Garza, Nuevo León, Mexico). This analysis was performed with a monochromatic Al K radiation with energy E = 1486.68 eV. Cleaning by a soft surface etching step was performed to remove superficial impurities from the sample during the analysis. Binding energies of all the peaks were corrected using C 1 s energy at 284.6 eV, corresponding to adventitious carbon. Moreover, the charge compensation was corrected by the flood gun associated with the spectrometer. The peaks were deconvoluted using a Shirley-type background calculation and peak fitting using the Gaussian–Lorentzian sum function. 2.3.2. X-ray Diffraction Characterization X-ray diffraction characterization was performed with a Panalytical Empyrean model diffractometer, with Co radiation with a wavelength of 1.79 Å (Facultad de Ingeniería Mecánica y Eléctrica, UANL, San Nicolás de los Garza, Nuevo León, Mexico). The samples were analyzed with a scanning range of 10 to 144 ◦ at a scan speed of 1 ◦ /s, using a voltage of Materials 2022,15, 3117 6 of 20 40 Kv and current of 40 mA, to investigate the crystallographic information. Data analysis and the peak profile fitting were carried out using the XPowder program. 3. Results and Discussion 3.1. Analysis of Chemical State by XPS For all experiments, the electron bonding energy in carbon was adjusted to 284.6 eV; this is suggested to be a carbon pollutant on the samples due to their handling. The XPS technique provides information on the change in the chemical status [ 12 ] of the species that make up the mixtures. In this work, variations in the chemical states of “O”, “Mg”, “Ca”, and “Ti” in the different samples obtained were analyzed. Figure 3shows the spectra obtained by XPS from the formulations M1, M4, M2, and M3. The intensities of the peaks of O1s and Ti2p decrease when TiO 2 is added to the brucite, indicating a decrease in these chemical states with the addition of titania and after the treatment of calcination of the samples at 1100 ◦C for 1 h. Materials 2022, 15, x FOR PEER REVIEW 6 of 21 3. Results and Discussion 3.1. Analysis of Chemical State by XPS For all experiments, the electron bonding energy in carbon was adjusted to 284.6 eV; this is suggested to be a carbon pollutant on the samples due to their handling. The XPS technique provides information on the change in the chemical status [12] of the species that make up the mixtures. In this work, variations in the chemical states of “O”, “Mg”, “Ca”, and “Ti” in the different samples obtained were analyzed. Figure 3 shows the spectra obtained by XPS from the formulations M1, M4, M2, and M3. The intensities of the peaks of O1s and Ti2p decrease when TiO 2 is added to the brucite, indicating a decrease in these chemical states with the addition of titania and after the treatment of calcination of the samples at 1100 °C for 1 h. Figure 3. High-resolution spectrum of XPS of M1, M2, M3, and M4 formulations. For the M1 formulation (Brucite + TiO 2 treated at 1100 °C 1 h with final ratio 1:1 Molar of MgO:TiO 2 ), the presence of ions of Mg, Ti, Ca, and O was detected. For the M2 (calcined brucite at 1100 °C for 1 h) and M4 formulations (uncalcined brucite), the presence of Mg, Ca, and O ions was detected. For the M3 formulation (TiO 2 ), the presence of Ti and O ions was detected. Figure 4a shows the deconvoluted high-resolution XPS spectrum of Ti in pure TiO 2 . In this spectrum, the Ti2p 3/2 doublet with binding energy 458.53 eV and Ti2p 1/2 with binding energy 464.23 eV arises from the division of the spin orbit. These results are consistent with Ti 4+ and are the characteristic features of the TiO 2 crystallographic structure [13–15]. Furthermore, the calculated difference in binding energy (BE) of Ti2p 3/2 and Ti2p 1/2 (∆BE = BE Ti2p 3/2 − Ti2p 1/2 ) was equal to 5.7 eV, which can be assigned to the typical Ti 4+ –O bonds in TiO 2 . The 2p doublet peaks after deconvolution exhibited a tail in the region of lower binding energy, indicating the presence of lower Ti valence states, observed at the peak at a binding energy of 457.08 eV (Ti2p 3/2 ) and 463.28 eV (Ti2p 1/2 ) corresponding to Ti 3+ in the Ti 2 O 3 lattice [12] (details are shown in Figure 4b). This means that both TiO 2 and Ti 2 O 3 are present in pure titania. The existence of Ti 3+ in TiO 2 indicates that oxygen vacancies are generated to maintain electrostatic equilibrium according to the following chemical equation: 4Ti O  →4Ti  2e ”□ ⁄0.5O →2Ti  2Ti  □0.5O  (9) Figure 3. High-resolution spectrum of XPS of M1, M2, M3, and M4 formulations. For the M1 formulation (Brucite + TiO 2 treated at 1100 ◦ C 1 h with final ratio 1:1 Molar of MgO:TiO 2 ), the presence of ions of Mg, Ti, Ca, and O was detected. For the M2 (calcined brucite at 1100 ◦ C for 1 h) and M4 formulations (uncalcined brucite), the presence of Mg, Ca, and O ions was detected. For the M3 formulation (TiO2), the presence of Ti and O ions was detected. Figure 4a shows the deconvoluted high-resolution XPS spectrum of Ti in pure TiO 2 . In this spectrum, the Ti2p 3/2 doublet with binding energy 458.53 eV and Ti2p 1/2 with binding energy 464.23 eV arises from the division of the spin orbit. These results are consistent with Ti 4+ and are the characteristic features of the TiO 2 crystallographic structure [ 13 – 15 ]. Furthermore, the calculated difference in binding energy (BE) of Ti2p 3/2 and Ti2p 1/2 ( ∆ BE = BE Ti2p 3/2 − Ti2p 1/2 ) was equal to 5.7 eV, which can be assigned to the typical Ti 4+ –O bonds in TiO 2 . The 2p doublet peaks after deconvolution exhibited a tail in the region of lower binding energy, indicating the presence of lower Ti valence states, observed at the peak at a binding energy of 457.08 eV (Ti2p 3/2 ) and 463.28 eV (Ti2p 1/2 ) corresponding to Ti 3+ in the Ti 2 O 3 lattice [ 12 ] (details are shown in Figure 4b). This means that both TiO 2 and Ti 2 O 3 are present in pure titania. The existence of Ti 3+ in TiO 2 indicates that oxygen vacancies are generated to maintain electrostatic equilibrium according to the following chemical equation: 4Ti4++O2−→4Ti4++2e00 /+0.5O2→2Ti4++2Ti3+++0.5O2(9) Materials 2022,15, 3117 7 of 20 Materials 2022, 15, x FOR PEER REVIEW 7 of 21 (a)(b) Figure 4. (a) High-resolution XPS spectra of Ti2p in high-purity titania; (b) details for sample M3. The □ represents an empty position that originates from the removal of O 2− from the crystalline structure. From the equation, it can be deduced that a generated vacancy of oxygen is accompanied by two Ti 3+ ions. Therefore, with the areas obtained at each peak of binding energy by XPS, it is feasible to determine the percentage of vacancies of O with the following equations [16]: %Ti Ti  Ti area Ti  area Ti ⁄⁄ %Ti 1Ti  OTi ⁄2 in TiO  OTi ⁄2%Ti  3 2%Ti %O  OTi ⁄ 2 % Vacancies O  1  %O (10) We calculate that the Ti 3+ /Ti 4+ ratio yields approximately 6% of the peak areas, and the percentage of oxygen vacancies in the high-purity titania crystalline structure used in this work is 2%. Table 4 presents the data obtained from the measurements of pure titania by XPS and the calculations to obtain the percentage of oxygen vacancies in the crystalline structure. Figure 4. (a) High-resolution XPS spectra of Ti2p in high-purity titania; (b) details for sample M3. The  represents an empty position that originates from the removal of O 2− from the crystalline structure. From the equation, it can be deduced that a generated vacancy of oxygen is accompanied by two Ti 3+ ions. Therefore, with the areas obtained at each peak of binding energy by XPS, it is feasible to determine the percentage of vacancies of O with the following equations [16]: %Ti3+=Ti3+/Ti4+=area Ti3+/area Ti4+ %Ti4+=1−Ti3+ O/Ti =2 in TiO2 O/Ti =2%Ti4++3 2%Ti3+ %O =O/Ti 2 % Vacancies O =1−%O (10) We calculate that the Ti 3+ /Ti 4+ ratio yields approximately 6% of the peak areas, and the percentage of oxygen vacancies in the high-purity titania crystalline structure used in this work is 2%. Table 4presents the data obtained from the measurements of pure titania by XPS and the calculations to obtain the percentage of oxygen vacancies in the crystalline structure. Table 4. Data from XPS measurements on the pure titania used in this work and calculations to determine the precentage of oxygen vacancies in the crystal lattice. Ion Peak Binding Energy eV FWHM eV Area CPS eV % Ti3+ % Ti4+ O/Ti % O % Vacancies of O Ti4+ •458.53 1.87 58,355.26 4% 96% 1.979966759 99% 1% Ti3+ •457.08 0.62 2338.09 Materials 2022,15, 3117 8 of 20 After mixing brucite with titania and a treatment at 1100 ◦ C for 1 h, the deconvoluted high-resolution XPS spectrum in Figure 5shows a slight change in position along with a variation in area of the peaks with respect to those of pure titania, showing a negative shift of 0.56 eV (Figure 5). The peaks in the mixed samples of brucite with TiO 2 are now at the binding energies 457.68 eV (Ti2p 3/2 ) and 463.41 eV (Ti2p 1/2 ) respectively. The calculated ∆ BE between Ti2p 3/2 and Ti2p 1/2 was 5.73 eV, which cannot be ascribed to the normal Ti 4+ state in TiO 2 and is an indication of the formation of Ti 3+ species and/or mixtures of magnesium-titanium-oxygen Oxides Mg-Ti-O and calcium-titanium-oxygen Oxides Ca-Ti-O are formed with different oxidation states and/or stoichiometries, as demonstrated below with the X-ray diffraction results. Materials 2022, 15, x FOR PEER REVIEW 9 of 21 (a) (b) Figure 5. (a) XPS high-resolution spectra of Ti2p from TiO 2 mixed with brucite and calcined at 1100 °C for 1 h; (b) details for sample M1. Figure 6 shows the XPS high-resolution spectrum of O1s in high purity TiO 2 , which is composed of a peak at BE 530.68 eV, which was deconvoluted with three peaks located at 529.48 eV, 532.38 eV, and 533.38 eV. The highest binding energy at 533.38 eV is generally attributed to oxygen or hydroxyl (OH) species chemically absorbed or dissociated at the sample surface, such as adsorbed H 2 O [17]. The 532.38 eV bond energy component of O1s is associated with O 2 ions found in the compound Ti 2 O 3 [18], which is consistent with the XPS spectrum for Ti2p in Figure 7. The 529.48 eV bond energy component of O1s is associated with O 2− ions found in oxygen-deficient regions within the TiO 2 matrix, promoted by the present chemical state of Ti 3+ . As a result, changes in the intensity of this component may be related to variations in the concentration of oxygen vacancies (VO) [19], which is consistent with the peak BE at 532.38 eV in the same spectrum by the chemical state of Ti 3+ , as well as with the XPS spectrum for Ti2p in Figure 4a. The peak intensity with a BE of 530.68 eV exceeds all other peaks, indicating the strong Ti-O binding in the pure TiO 2 compound; this value is consistent with reference [14] and is further consistent with the XPS spectrum for Ti2p in Figure 4a. This indicates the formation of TiO 2 and some mixed oxides. Figure 7 shows the high-resolution XPS spectrum of O1s from the mixture of brucite with heat-treated titania. It consists of three peaks with BE of 530.09 eV, which in peaks with BE of 531.28 eV and 532.48 eV were deconvoluted. The calculated difference in BE of O1s (Figure 7) and Ti2p 3/2 (Figure 5) (530.09 eV) − (457.68 eV) = 72.41 eV is in reasonable agreement with that of typical Ti 3+ containing oxides (72.9 to 73.1 eV) [15]. The peaks at BE 457.68 eV (Ti2p 3/2 ) and 463.41 eV (Ti2p 1/2 ) were deconvoluted with two peaks located at 459.28 eV (Ti2p 3/2 ) and 464.48 eV (Ti2p 1/2 ) respectively, which can be attributed to the formation of Ti 4+ species in mixtures of magnesium-titanium-oxygen Oxides Mg-Ti-O and Figure 5. ( a ) XPS high-resolution spectra of Ti2p from TiO 2 mixed with brucite and calcined at 1100 ◦C for 1 h; (b) details for sample M1. After mixing and heat treatment, the peak area of Ti2p 3/2 has a BE of 457.68 eV, which is very close to 457.08 eV of Ti 3+ in pure titania, increased by 6.17 times; similarly, the peak area of Ti2p 3/2 in the mixture of brucite and titania has a BE of 459.28 eV, which is very close to 458.24 eV of the Ti 4+ of pure titania, decreased by 99%. This suggests that the oxidation state present in the sample of brucite mixed with titania after heat treatment may correspond to Ti 4+ , but the difference of 0.6 eV may be due to the presence of mixtures of oxides with different stoichiometry. The change in stoichiometry was estimated by the change in area of relative peaks. The change from the Ti 3+ peak area indicates that after brucite doping and heat treatment, oxygen is removed from the crystalline structure, showing a relative increase to Ti 3+ in the XPS spectrum. On the other hand, with the decrease in the area of the Ti 4+ new peak at 459.28 eV, the reaction of Mg 2+ ion substitutions in the TiO 2 crystalline structure can be inferred due to the reaction of Ti 4+ ion substitutions in the MgO crystalline structure from the transformation of brucite at 1100 ◦ C; on the other hand, mixtures of magnesium–titanium–oxygen oxides (Mg-Ti-O) and calcium–titanium– oxygen oxides (Ca-Ti-O) are formed with different oxidation states and/or stoichiometries, as demonstrated below with the X-ray diffraction results. This means that some mixed Materials 2022,15, 3117 9 of 20 oxide structures are formed in large quantities, either with Mg or Ca, with a Ti 4+ oxidation state after doping. Similarly, the new area of Ti 4+ means that some mixed oxide structures are formed in large quantities, either with Mg or Ca, with a Ti 4+ oxidation state after doping. Figure 6shows the XPS high-resolution spectrum of O1s in high purity TiO 2 , which is composed of a peak at BE 530.68 eV, which was deconvoluted with three peaks located at 529.48 eV, 532.38 eV, and 533.38 eV. The highest binding energy at 533.38 eV is generally attributed to oxygen or hydroxyl (OH) species chemically absorbed or dissociated at the sample surface, such as adsorbed H 2 O [ 17 ]. The 532.38 eV bond energy component of O1s is associated with O 2 ions found in the compound Ti 2 O 3 [ 18 ], which is consistent with the XPS spectrum for Ti2p in Figure 7. The 529.48 eV bond energy component of O1s is associated with O 2− ions found in oxygen-deficient regions within the TiO 2 matrix, promoted by the present chemical state of Ti 3+ . As a result, changes in the intensity of this component may be related to variations in the concentration of oxygen vacancies (VO) [ 19 ], which is consistent with the peak BE at 532.38 eV in the same spectrum by the chemical state of Ti 3+ , as well as with the XPS spectrum for Ti2p in Figure 4a. The peak intensity with a BE of 530.68 eV exceeds all other peaks, indicating the strong Ti-O binding in the pure TiO 2 compound; this value is consistent with reference [ 14 ] and is further consistent with the XPS spectrum for Ti2p in Figure 4a. This indicates the formation of TiO 2 and some mixed oxides. Materials 2022, 15, x FOR PEER REVIEW 10 of 21 calcium-titanium-oxygen Oxides Ca-Ti-O with different oxidation states and/or stoichiometries. Table 5 presents details on the data obtained and adjusted from XPS analysis of Ti2p for pure titania and brucite with titania mixture samples. Table 5. Details of the data obtained from the XPS analysis of Ti for samples of pure titania and mixture of brucite with titania, with the latter heat treated at 1100 °C for 1 h. Sample Ion Peak Binding Energy eV Area CPS eV Area Ratio FWHM eV High-purity titania •Ti 4+ 458.24 63,407.41 1 1.27 •Ti 4+ 463.68 30,579.04 0.48 2.18 •Ti 3+ 457.08 3991 0.06 0.84 •Ti 3+ 463.28 2056.9 0.03 0.84 Brucite + TiO 2 mixture calcined at 1100 °C 1 h (1:1 Molar MgO:TiO 2 ) •Ti 3+ 457.68 20,655.55 1 1.29 •Ti 3+ 463.41 9098.07 0.44 2.04 •Ti 4+ 459.28 465.95 0.02 0.46 •Ti 4+ 464.48 730.39 0.04 0.52 Figure 6. XPS high-resolution spectra of O1s in high-purity titania for formulation M3. Figure 6. XPS high-resolution spectra of O1s in high-purity titania for formulation M3. Figure 7shows the high-resolution XPS spectrum of O1s from the mixture of brucite with heat-treated titania. It consists of three peaks with BE of 530.09 eV, which in peaks with BE of 531.28 eV and 532.48 eV were deconvoluted. The calculated difference in BE of O1s (Figure 7) and Ti2p 3/2 (Figure 5) (530.09 eV) − (457.68 eV) = 72.41 eV is in reasonable agreement with that of typical Ti 3+ containing oxides (72.9 to 73.1 eV) [ 15 ]. The peaks at BE 457.68 eV (Ti2p 3/2 ) and 463.41 eV (Ti2p 1/2 ) were deconvoluted with two peaks located at 459.28 eV (Ti2p 3/2 ) and 464.48 eV (Ti2p 1/2 ) respectively, which can be attributed to the formation of Ti 4+ species in mixtures of magnesium-titanium-oxygen Oxides Mg-TiO and calcium-titanium-oxygen Oxides Ca-Ti-O with different oxidation states and/or Materials 2022,15, 3117 16 of 20 Materials 2022, 15, x FOR PEER REVIEW 17 of 21 Figure 14. Diffractogram of the M4 formulation (uncalcined brucite). For Ca(OH) 2 , which is present at only 0.20% by weight, the following diffraction planes were detected: (001), (100), (011), (012), (110), (111), (201), (103), (121), and (122), corresponding to angles 21.124°, 33.479°, 39.925°, 43.012°, 55.576°, 59.849°, 64.209°, 66.719°, 70.343°, 74.382°, 76.498°, 76.742°, 86.122°, 94.311°, 95.881°, 99.281°, 103.212°, 103.941°, 105.573°, 115.478°, 119.558°, 124.083°, 124.945°, 132.84°, 137.611°, 140.004°, 147.766°, and 172.165°, respectively, in agreement with reference ICDD 01-076-0570. Comparing these compounds with the XPS results, we can confirm that in the M8 formulation, the compound Ca(OH) 2 is present. Figure 15 shows the diffractogram of TiO 2 . According to this diagram, in the mixture of the M3 formulation, the compound TiO 2 is present. In the same diffractogram, the planes with respective angles 2θ belonging to this compound are shown to be in agreement with ICDD 04-008-4342, which supports the presence of TiO 2 . For the M3 formulation, the presence of the compound Ti 2 O 3 was also found; the diffractogram of this compound is shown in Figure 16. Figure 15. Diffractogram of the M3 formulation (high-purity titania), showing the identification of TiO 2 . Figure 14. Diffractogram of the M4 formulation (uncalcined brucite). Figure 15 shows the diffractogram of TiO 2 . According to this diagram, in the mixture of the M3 formulation, the compound TiO 2 is present. In the same diffractogram, the planes with respective angles 2 θ belonging to this compound are shown to be in agreement with ICDD 04-008-4342, which supports the presence of TiO 2 . For the M3 formulation, the presence of the compound Ti 2 O 3 was also found; the diffractogram of this compound is shown in Figure 16. Materials 2022, 15, x FOR PEER REVIEW 17 of 21 Figure 14. Diffractogram of the M4 formulation (uncalcined brucite). For Ca(OH) 2 , which is present at only 0.20% by weight, the following diffraction planes were detected: (001), (100), (011), (012), (110), (111), (201), (103), (121), and (122), corresponding to angles 21.124°, 33.479°, 39.925°, 43.012°, 55.576°, 59.849°, 64.209°, 66.719°, 70.343°, 74.382°, 76.498°, 76.742°, 86.122°, 94.311°, 95.881°, 99.281°, 103.212°, 103.941°, 105.573°, 115.478°, 119.558°, 124.083°, 124.945°, 132.84°, 137.611°, 140.004°, 147.766°, and 172.165°, respectively, in agreement with reference ICDD 01-076-0570. Comparing these compounds with the XPS results, we can confirm that in the M8 formulation, the compound Ca(OH) 2 is present. Figure 15 shows the diffractogram of TiO 2 . According to this diagram, in the mixture of the M3 formulation, the compound TiO 2 is present. In the same diffractogram, the planes with respective angles 2θ belonging to this compound are shown to be in agreement with ICDD 04-008-4342, which supports the presence of TiO 2 . For the M3 formulation, the presence of the compound Ti 2 O 3 was also found; the diffractogram of this compound is shown in Figure 16. Figure 15. Diffractogram of the M3 formulation (high-purity titania), showing the identification of TiO 2 . Figure 15. Diffractogram of the M3 formulation (high-purity titania), showing the identification of TiO2. Materials 2022, 15, x FOR PEER REVIEW 18 of 21 Figure 16. Diffractogram of the M3 formulation (titania of high purity), showing the identification of Ti 2 O 3 . As can be seen in the diffractograms and in the XPS spectra, it is confirmed that by mixing TiO 2 with industrial-grade brucite of national origin and calcining at a low temperature (1100 °C) for a short period of time (1 h), it is possible to form ceramic phases mainly in the Mg-Ti-O system. The compounds that are obtained are mainly MgTi 2 O 5 , MgTiO 3 , MgO, and TiO 2 . The percentages obtained are relatively high: approximately 21, 13, 14, and 38% for MgTi 2 O 5 , MgTiO 3 , MgO, and TiO 2 , respectively. When analyzing the phase diagram in Figure 17, the compounds between MgTi 2 O 5 and MgTiO 3 have melting points between 1605 °C and 1660 °C. Figure 17. Phase diagram of MgO and TiO 2 showing the phases formed with different contents of the substances at different temperatures [35]. Figure 16. Diffractogram of the M3 formulation (titania of high purity), showing the identification of Ti2O3. Materials 2022,15, 3117 17 of 20 As can be seen in the diffractograms and in the XPS spectra, it is confirmed that by mixing TiO 2 with industrial-grade brucite of national origin and calcining at a low temperature (1100 ◦ C) for a short period of time (1 h), it is possible to form ceramic phases mainly in the Mg-Ti-O system. The compounds that are obtained are mainly MgTi 2 O 5 , MgTiO 3 , MgO, and TiO 2 . The percentages obtained are relatively high: approximately 21, 13, 14, and 38% for MgTi 2 O 5 , MgTiO 3 , MgO, and TiO 2 , respectively. When analyzing the phase diagram in Figure 17, the compounds between MgTi2O5and MgTiO3have melting points between 1605 ◦C and 1660 ◦C. Materials 2022, 15, x FOR PEER REVIEW 18 of 21 Figure 16. Diffractogram of the M3 formulation (titania of high purity), showing the identification of Ti 2 O 3 . As can be seen in the diffractograms and in the XPS spectra, it is confirmed that by mixing TiO 2 with industrial-grade brucite of national origin and calcining at a low temperature (1100 °C) for a short period of time (1 h), it is possible to form ceramic phases mainly in the Mg-Ti-O system. The compounds that are obtained are mainly MgTi 2 O 5 , MgTiO 3 , MgO, and TiO 2 . The percentages obtained are relatively high: approximately 21, 13, 14, and 38% for MgTi 2 O 5 , MgTiO 3 , MgO, and TiO 2 , respectively. When analyzing the phase diagram in Figure 17, the compounds between MgTi 2 O 5 and MgTiO 3 have melting points between 1605 °C and 1660 °C. Figure 17. Phase diagram of MgO and TiO 2 showing the phases formed with different contents of the substances at different temperatures [35]. Figure 17. Phase diagram of MgO and TiO 2 showing the phases formed with different contents of the substances at different temperatures [35]. Considering that MgO-based refractory materials produced from double-calcined brucite are used in melting processes with temperatures above 1537 ◦ C in steel production, for example, it is appropriate to take care of the formation of these compounds during the sintering of MgO if it is doped with titanium ions by micro or TiO2microparticles. Additionally, Table 7shows that the reticular values of the titania and magnesium titanate phases are exactly consistent with the values of the crystal structure of these compounds, as well as the MgO lattice parameters in the calcined brucite (M2) sample; on the other hand, it is observed that for the reticular values of MgO in the mixture of brucite with heat-treated titania at a temperature of 1100 ◦ C for 1 h (M1), the crystallographic parameters of MgO are modified as a result of the presence of Ti ions in its crystal structure and possible vacancies generated by the greater number of valence electrons between Mg and Ti. Materials 2022,15, 3117 18 of 20 Table 7. Cristallographic parameters obtained from XRD analysis for samples of pure calcined brucite (M2) and brucite mixed with titania (M1), both heat-treated at 1100 ◦C for 1 h. Sample Compound Crystal Structure Space Group a Å b Å c Å α ◦ β ◦ γ ◦ M2 MgO Cubic F m −3 m (225) 4.213313 4.213313 4.213313 90 90 90 M1 MgO Cubic F m −3 m (225) 4.21156 4.21156 4.21156 90 90 90 MgTiO3Rhombohedral R −3 (148) 5.0549 5.0549 13.8939 90 90 90 MgTi2O5Orthorhombic C m c m (63) 3.7428 9.7387 9.9976 90 90 90 TiO2Tetragonal P 42/m n m (136) 4.59327 4.59327 2.95892 90 90 90 4. Conclusions It is concluded that the incorporation of Ti 4+ ions by mixing TiO 2 microparticles in the brucite slightly modifies the crystallographic structure of the caustic MgO obtained after its calcination at 1100 ◦ C 1 h, forming compounds of the Mg-Ti-O system. In addition, it is concluded that Ti 4+ modifies the size of the crystal structure, possibly due to the demand of twice as many O ionsgenerating vacancies in the crystal structure. It is concluded that the presence of TiO 2 in brucite promotes the formation of MgTi 2 O 5 and MgTiO 3 compounds, which have relatively low melting points; care must be taken during the addition of said oxides in the densification of the double calcined MgO. Additionally, it is concluded that the low oxidation states in TiO 2 generate oxygen vacancies in the crystal lattice structure. Finally, it is concluded that, of the impurities of Ca, Fe, Al, and Si in the Mexican brucite, only the presence of Ca influences calcination in the interaction of TiO2, forming CaTiO3compounds. Author Contributions: Conceptualization, G.A.C.-R., C.G.-R. and K.S.S.-Z.; methodology, G.A.C.-R., K.S.S.-Z., C.G.-R., D.F.-G., L.F.V., L.V.G.-Q. and M.H.-R.; validation, G.A.C.-R., C.G.-R., D.F.-G. and L.F.V.; formal analysis, G.A.C.-R., C.G.-R., D.F.-G., J.A.A.-M. and L.F.V.; investigation, K.S.S.-Z., M.H.-R., G.A.C.-R., C.G.-R. and E.A.R.-C.; resources, G.A.C.-R.; writing—original draft preparation, G.A.C.-R., C.G.-R. and L.V.G.-Q.; writing—review and editing, G.A.C.-R., C.G.-R., J.A.A.-M., D.F.-G. and L.V.G.-Q.; visualization, G.A.C.-R., C.G.-R., D.F.-G. and L.F.V.; supervision, D.F-G. and L.F.V.; project administration, G.A.C.-R., C.G.-R., E.A.R.-C. and J.A.A.-M.; funding acquisition, G.A.C.-R. and C.G.-R. All authors have read and agreed to the published version of the manuscript. Funding: Guadalupe Alan Castillo-Rodríguez and Cristian Gómez-Rodríguez thank for the support to Universidad Autónoma de Nuevo León PAICYT-UANL agreement number IT1382-20. Daniel Fernández-González acknowledges the grant (Juan de la Cierva-Formación program) FJC2019-041139I funded by MCIN/AEI/10.13039/501100011033 (Ministerio de Ciencia e Innovación, Agencia Estatal de Investigación). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors acknowledge the support of Grupo Peñoles (Laguna del Rey, Coahuila, México). The authors acknowledge to Alberto Toxqui Terán, for his support in the DSC/TGA thermal analyses, carried out at CIMAV. Conflicts of Interest: The authors declare no conflict of interest. References 1. Landy, R. Magnesia Refractories. In Refractories Handbook, 2nd ed.; Schacht, C., Ed.; Marcel Dekker, Inc.: New York, NY, USA, 2004; Volume 1, pp. 109, 112. 2. Shand, M.A. Formation and Ocurrence of Magnesite and Brucite. In The Chemistry and Technology of Magnesia, 1st ed.; Shand, M.A., Ed.; John Wiley & Sons, Inc. 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