**3. Materials and Methods**

Four BaTi0.8B0.2O3 catalysts (being B Mn, Fe, Co, or Cu), named BTMnO\_2, BTFeO\_2, BTCoO\_2, and BTCuO\_2 respectively, were prepared by the sol–gel method as previously described [11]. Summarizing, first of all, the hydrolysis of titanium isopropoxide (Ti) was carried out, dissolving the resulting species in an aqueous solution of citric acid (CA) (Ti:CA = 1:2) and hydrogen peroxide (Ti:H2O2 = 2:1), and obtaining the citrate–peroxo–titanate (IV) complex. Subsequently, NH3 was used in order to adjust the pH to 8.5, and the addition of an stoichiometric (BaTi0.8B0.2O3) amount of barium (Ba:Ti = 1:1) and metals precursors (barium acetate and Fe, Co, Cu, and Ni nitrates), took place. During 5 h, until the obtention of a gel, the temperature of the mixture remained 65 ◦C. Afterwards, a temperature of 90 ◦C or 24 h was used to dry the sample, which was in the end calcined at 850 ◦C for 6 h.

To measure the metal content in the samples by ICP-OES, a Perkin-Elmer device model Optima 4300 DV was used. An Autosorb-6B instrument from Quantachrome served to determine, by N2 adsorption at −196 ◦C, the BET surface area of the samples. To identify different phases and crystalline structures, X-ray diffraction (XRD) and Raman spectroscopy were employed. XRD tests were performed with a Rigaku Miniflex II powder diffractometer, using Cu Kα (0,15418 nm) radiation with the 2θ angle in the range 20 to 80◦, with a step of 0.025◦ and a time per step of 2 s. Raman scattering spectra were obtained on a Jobin-Ivon dispersive Raman spectrometer (model LabRam) with a variable power He:Ne laser source (633 nm) in the range of 100–1000 nm. To register the XPS spectra, a K-Alpha photoelectron spectrometer by Thermo-Scientific, with an Al Kα (1486.6 eV) radiation source, was used in the following conditions: 5 <sup>×</sup> 10−<sup>10</sup> mbar pressure in the chamber and setting the C1s transition at 284.6 eV, and the binding energy (BE) and kinetic energy (KE) values then determined with the peak-fit software of the spectrophotometer, to regulate the BE and KE scales.

The catalytic activity of the samples (80 mg of catalyst diluted in 300 mg SiC) was tested using two different experiments in a fixed-bed quartz reactor at atmospheric pressure and under a gas flow (500 mL/min): i) temperature programmed reaction (TPR-NOx) tests (10 ◦C/min, 800 ◦C) in a gas mixture of 500 ppm NOx and 5 % O2 and ii) NOx storage cyclic tests at 400 ◦C, with a gas mixture composed of: i) for lean (storage) cycle (5 min), 500 ppm NOx and5%O2 balanced with N2, and ii) for rich (regeneration) cycle (3 min), 10% H2 balanced with N2. To achieve the stability of the catalysts, and then determine the NSC, 10 consecutive storage–regeneration cycles were accomplished. The gas composition was controlled by specific NDIR-UV gas analyzers for NO, NO2, CO, CO2, and O2 (Rosemount Analytical Model BINOS 1001, 1004, and 100).

NOx conversion profiles as a function of temperature were obtained using the next equation

$$\text{NOx conversion} \left( \% \right) = \frac{\text{NOx}\_{\text{in}} - \text{NOx}\_{\text{out}}}{\text{NOx}\_{\text{in}}} \times 100$$

where 'NOxin' is the concentration of NOx (=NO + NO2) feed to the reactor and 'NOxout' is the concentration of NOx that leaves the reactor.

The percentage of NO2 generated during TPR was determined with the equation

$$\text{NO}\_2(\%) = \frac{\text{NO}\_{2,\text{out}}}{\text{NO}\_{2,\text{in}}} \times 100$$

where 'NO2out' is the concentration of NO2 that leaves the reactor.

The NSC was obtained as the difference between the NOx signal when the reactor is unfilled and the NOx signal when the reactor is full of catalyst with

$$\text{NOx storage} = \int\_{t0}^{t \text{f}} \text{NOx}\_{\text{inlet}}(\mathbf{t}) - \text{NOx}\_{\text{exp}}(\mathbf{t})\,\text{dt}$$

where 'NOxinlet' is the concentration of NOx (=NO + NO2) measured when the reactor is empty, and 'NOxexp' is the concentration of NOx during the NOx storage test.
