Novel Ni / Ce(Ti)ZrO 2 Catalysts for Methane Dry Reforming Prepared in Supercritical Alcohol Media

: To achieve a high activity and coking stability of nickel catalysts in dry reforming of methane, materials comprised of ceria–zirconia doped by Ti were investigated as supports. Ceria–zirconia supports doped with titanium were prepared either via the Pechini method or by synthesis in supercritical alcohol media. Ni-containing catalysts were prepared by two techniques: standard incipient wetness impregnation and one-pot synthesis. The catalytic reaction of DRM to synthesis gas was carried out in the 600–750 ◦ C range over 5% wt. Ni / Ce(Ti)ZrO 2 . Dried and calcined supports and catalysts were characterized by physicochemical methods including N 2 adsorption, XRD, Raman, H 2 -TPR, and HRTEM. Both preparation methods led to formation of solid solution with cubic ﬂuorite-like structure, as well as after addition of Ti. Introduction of Ti should provide improved oxygen storage capacity and mobility of support oxygen. The highest activity was observed with the catalyst of 5% wt. Ni / Ce 0.75 Ti 0.2 Zr 0.05 O 2 − δ composition due to optimized oxide support structure and support oxygen mobility. by N 2 adsorption, X-ray di ﬀ raction (XRD), Raman high-angle annular dark ﬁeld scanning transmission electron microscopy (HAADF-STEM), high-resolution transmission electron (HRTEM), temperature-programmed reduction by and in


Introduction
Dry reforming of methane (DRM) is reaction (CH 4 + CO 2 = 2H 2 + 2CO) for biogas utilization into two useful resource gases-Hydrogen and carbon monoxide. The obtained syngas is characterized by a ratio of H 2 /CO ≤ 1, which makes it suitable for oxygenate synthesis [1][2][3].
Over the last several years, a large number of studies have been focused on the Ni-based DRM catalysts, which proved to be very promising for the practical application due to their high activity and low cost [1,[4][5][6][7][8][9][10]. However, the main problem of these catalysts is the rapid deactivation caused by nickel sintering and coke formation. Many research studies aimed to increase catalytic activity and stability by controlling the morphology of the active component and using it in the form of highly dispersed nano-particles, which should minimize carbon formation [11,12]. Since the process of coking is sensitive to the support structure and dispersion of active metal deposited on the catalyst surface is very important, activity and stability of catalysts can be provided by developing supports with required characteristics. Fibrous carbon deposition occurs on relatively large active metal particles. Agglomeration of metal clusters and coking can be prevented using supports with a strong metal-support interaction as well as a high reactivity and mobility of the surface/lattice oxygen. Ceria has been extensively studied in the literature, and the oxygen storage capacity of ceria can be increased

Support Preparation
Ce 0.75 Zr 0.25 O 2−δ and Ce 0.75 Ti x Zr 0.25-x O 2−δ (x = 0.1, 0.2) oxides were prepared by the solvothermal continuous synthesis using isopropanol along with acetylacetone as a complexing agent, which was added into the Zr solution at a 2:1 molar ratio to Zr amount. Zr(OBu) 4 (Alfa Aesar, 80% solution in n-butanol) and Ce(NO 3 ) 3 ·6H 2 O (Vecton, pure for analysis grade), Ti[O(CH 2 ) 3 CH 3 ] 4 (Acros Organics) isopropanol solutions were used as Zr and Ce precursors. The experimental custom-built setup used for preparation of Ce(Ti)ZrO 2 supports and one-pot catalyst series in SC conditions is shown in Figure 1. The mixture of the Ce and Zr precursor solutions was fed into a U-shaped reactor (l = 1 m, inner d = 4 mm) at a rate of 5 mL/min along with isopropanol preheated to 150 • C, which flow rate was 9 mL/min at T = 400 • C and P = 120 atm.
For comparison, Ce 0.75 Zr 0.25 O 2−δ sample was synthesized by a modified polymerized precursor (Pechini) route using Ce(NO 3 ) 3 ·6H 2 O (Vecton, pure for analysis grade) and Zr(OCl) 2 (Vecton, pure for analysis grade). Ethylene glycol (EG) and citric acid (CA) were used as complexing agents. The reagents were taken in the following molar ratios CA: EG: metal (Ce + Zr) = 3.75:11.25:1. Citric acid in the ratio 1:3 was dissolved in EG; the corresponding salts were added until completely dissolved. The solution was heated and thickened, becoming a dark yellow one. The obtained gel was dried at a temperature of 80 • C for 12 h and calcined in static air by heating at a rate of 5 • C/min from room temperature to 700 • C. For comparison, Ce0.75Zr0.25O2−δ sample was synthesized by a modified polymerized precursor (Pechini) route using Ce(NO3)3·6H2O (Vecton, pure for analysis grade) and Zr(OCl)2 (Vecton, pure for analysis grade). Ethylene glycol (EG) and citric acid (CA) were used as complexing agents. The reagents were taken in the following molar ratios CA: EG: metal (Ce + Zr) = 3.75:11.25:1. Citric acid in the ratio 1:3 was dissolved in EG; the corresponding salts were added until completely dissolved. The solution was heated and thickened, becoming a dark yellow one. The obtained gel was dried at a temperature of 80 °C for 12 h and calcined in static air by heating at a rate of 5 °C/min from room temperature to 700 °C.

Catalysts Preparation
In the case of obtaining catalysts by both impregnation and one-pot synthesis, the loading amount of Ni was fixed at 5 wt%.
For one-pot synthesis via the Pechini route, the required amount of Ni(NO3)2·6H2O (Vecton, pure for analysis) was added into EG solution containing all other elements (vide supra).
For one-pot synthesis in supercritical conditions, the required quantities of Ni(NO3)2·6H2O (Vecton, pure for analysis) and Ce(NO3)3·6H2O were combined in isopropanol and mixed with Zr(OBu)4 and Ti[O(CH2)3CH3]. The mixed solution was fed into the reactor and SCS was performed under the same conditions as for obtaining carriers. All precursors were dried and calcined at 700 °C for 2h.

Characterization
All materials were characterized using a complex of methods. X-ray diffraction analysis was carried out using a D8 Advance (Bruker, Germany) diffractometer with Cu Ka radiation and LynxEye detector. XRD pattern were collected in 2Θ range 20-85 °C with 0.05 step size and 3 s accumulation times. The fraction of oxygen vacancies was counted from refinement of occupancy of oxygen positions in the

Catalysts Preparation
In the case of obtaining catalysts by both impregnation and one-pot synthesis, the loading amount of Ni was fixed at 5 wt%.
The For one-pot synthesis in supercritical conditions, the required quantities of Ni(NO 3 ) 2 ·6H 2 O (Vecton, pure for analysis) and Ce(NO 3 ) 3 ·6H 2 O were combined in isopropanol and mixed with Zr(OBu) 4 and Ti[O(CH 2 ) 3 CH 3 ]. The mixed solution was fed into the reactor and SCS was performed under the same conditions as for obtaining carriers. All precursors were dried and calcined at 700 • C for 2 h.

Characterization
All materials were characterized using a complex of methods. X-ray diffraction analysis was carried out using a D8 Advance (Bruker, Germany) diffractometer with Cu Ka radiation and LynxEye detector. XRD pattern were collected in 2Θ range 20-85 • C with 0.05 step size and 3 s accumulation times. The fraction of oxygen vacancies was counted from refinement of occupancy of oxygen positions in the structure by the Rietveld method using the TOPAS (Total Pattern Analysis System) program (Bruker, Germany). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained with a JEM-2200FS transmission electron microscope (JEOL Ltd., Tokyo, Japan, acceleration voltage 200 kV, lattice resolution 1 Å) equipped with a Cs-corrector and an EDX spectrometer (JEOL Ltd., Tokyo, Japan). The minimum spot diameter for the step-by-step line or mapping elemental EDX analysis was~1 nm with a step of about 1.5 nm. FT-Raman spectra were recorded in the 3600-100 cm −1 range with 300 scans using an RFS 100/S Bruker spectrometer. Experimental spectra were gained in the backscattering geometry using the 514.5 nm line of an Ar+ laser. The spectral resolution was 4 cm −1 and 180 • geometry. The texture of samples was studied by N 2 adsorption isotherms at −196 • C using an ASAP-2400 instrument (Micromeritics, Norcross, GA, USA). Prior to analysis, samples were degassed at 150 • C under vacuum for 16 h. The micropore volume was determined by t-plot analysis. The mean pore size was evaluated as 4V/A. The specific surface area of materials was estimated with a SORBI N.4.1 apparatus by the four-point BET method. Prior to measurements, samples were pretreated at 200 • C in vacuum for 1 h. Redox properties of supports and catalysts were studied by the temperature programmed reduction by H 2 (H 2 -TPR) with the temperature ramp 10 • /min from 60-900 • C in the mixture of argon (90%) with hydrogen (10%) content recorded by GC Tsvet 500 (Russia).

Catalytic Reaction
Methane dry reforming reaction was carried out in a tubular quartz plug flow reactor with catalyst sample diluted with quartz (particle size fraction of 0.5-0.25 mm) in a 1:1 ratio using the feed of 15% CH 4 + 15% CO 2 + N 2 balance at 600-750 • C and contact time 10 ms. Concentrations of initial reagents and products were detected on-line with a Test-201 (Bonair, Russia) gas analyzer. Before reaction, catalysts were pretreated in 10 vol% O 2 /N 2 at 600 • C for 30 min, then reduced in 5 vol% H 2 /N 2 at 600 • C for one hour. The variation of temperature in the range of 600-750-600 • C was realized stepwise (temperature rise and decline with an interval of 50 • C) for comparison of effects of thermal cycling. Concentrations of reagents at each step were registered during 30 min with contact time 10 ms. Compositions and abbreviations of prepared samples are presented in Table 1. As follows from the XRD patterns (Figures 2 and 3), Ce 0.75 Zr 0.25 O 2−δ samples prepared by both synthesis methods have the same fluorite crystal structure. However, there is an increase in the lattice constants for the CZsc sample prepared by the supercritical method (5.369 Å) compared to the CZ P sample (5.356 Å), suggesting a higher disordering of its structure.

Results and Discussion
As follows from the XRD patterns (Figures 2 and 3), Ce0.75Zr0.25O2−δ samples prepared by both synthesis methods have the same fluorite crystal structure. However, there is an increase in the lattice constants for the CZsc sample prepared by the supercritical method (5.369 Å) compared to the CZP sample (5.356 Å), suggesting a higher disordering of its structure.     Figure 1a shows the XRD patterns of Ni/Ce0.75Zr0.25O2−δ catalysts with the support obtained using the Pechini route. Both samples show the characteristic reflections corresponding to cubic fluorite-like structure and NiO.
The unit cell parameter a of impregnated Ni/Ce0.75Zr0.25O2−δ (Ni/CZP-im) and one-pot sample (Ni/CZPop) is equal to 5.357 Å and 5.358 Å, respectively, which coincides with the value for the initial Ce0.75Zr0.25O2−δ support (5.357 Å). This fact implies that Ni is not incorporated into the fluorite lattice. According to the X-ray diffraction data, the average crystallite size of the fluorite-like phase did not constants for the CZsc sample prepared by the supercritical method (5.369 Å) compared to the CZP sample (5.356 Å), suggesting a higher disordering of its structure.     Figure 1a shows the XRD patterns of Ni/Ce0.75Zr0.25O2−δ catalysts with the support obtained using the Pechini route. Both samples show the characteristic reflections corresponding to cubic fluorite-like structure and NiO.
The unit cell parameter a of impregnated Ni/Ce0.75Zr0.25O2−δ (Ni/CZP-im) and one-pot sample (Ni/CZPop) is equal to 5.357 Å and 5.358 Å, respectively, which coincides with the value for the initial Ce0.75Zr0.25O2−δ support (5.357 Å). This fact implies that Ni is not incorporated into the fluorite lattice. According to the X-ray diffraction data, the average crystallite size of the fluorite-like phase did not  The unit cell parameter a of impregnated Ni/Ce 0.75 Zr 0.25 O 2−δ (Ni/CZ P-im ) and one-pot sample (Ni/CZ P-op ) is equal to 5.357 Å and 5.358 Å, respectively, which coincides with the value for the initial Ce 0.75 Zr 0.25 O 2−δ support (5.357 Å). This fact implies that Ni is not incorporated into the fluorite lattice. According to the X-ray diffraction data, the average crystallite size of the fluorite-like phase did not depend on the preparation method and varied within 9-12 nm ( Table 1). The NiO phase was detected for both samples, the size of its crystallites varied within the 18-25 nm range.
The Raman spectrum of Ce 0.75 Zr 0.25 O 2−δ samples prepared by both methods shows a strong peak at 470 cm −1 , corresponding to the main cubic fluorite phase (Figure 2b) [50]. This peak corresponds to the fundamental mode with F 2g symmetry, which can be assigned to a symmetric O-Ce-O stretching. In addition to the F 2g mode, the Raman spectra of impregnated samples show another strong band at 600 cm −1 , whereas for the one-pot sample, there is one more additional weak band at 232 cm −1 . These bands are related to the formation of lattice defects, mostly oxygen vacancies [51]. The intensity of defects induced bands I(D1 + D2) in the range 500-700 cm −1 was higher for samples with a larger defects concentration, therefore, the ω = I(D1 + D2)/I F2g ratio, reported in Figure 1b in the frame, was used as a defect sensitive parameter [6,52,53]. In the case of Ce 0.75 Zr 0.25 O 2−δ , regardless of the preparation method, oxygen defects concentration is low, defects can be related only to the difference of the ionic radii of Zr 4+ and Ce 4+ . For catalysts, the I(D1 + D2)/I F2g ratios are orders of magnitude larger than Energies 2020, 13, 3365 7 of 24 those of supports. The appearance of a broad band at about 230 cm −1 was also revealed in the case of Ni/CZ P-op samples. The peak at 230 cm −1 corresponded to the second-order transverse acoustic mode of ceria, and that at 1180 cm −1 to the second-order longitudinal mode of ceria [53,54]. Figure 3a shows the XRD patterns of Ni/Ce 0.75 Zr 0.25 O 2−δ catalysts prepared using SCS. In distinction to other samples, the diffraction pattern of the CZsc sample synthesized in supercritical media without calcination shows only the broad reflections which were indexed within space group Fm3m as a cubic phase of the CeO 2 -ZrO 2 solid solution ( Figure 3a, Table 2). The Raman spectrum of this sample shows the presence of weak broad peaks of a low intensity at 450-700 cm −1 , also indicating that the crystal structure is not completely formed (Figure 3b). XRD patterns of 5% Ni/Ce 0.75 Zr 0.25 O 2−δ (Ni/CZ sc-im ) prepared by the impregnation of the support CZ sc showed the presence of crystalline NiO, while for the 5% Ni/Ce 0.75 Zr 0.25 O 2−δ (Ni/CZ sc-op ) catalyst obtained by the one-pot route in supercritical conditions, crystalline NiO is practically not observed ( Figure 3a). The presence of crystalline nickel oxide in the Ni/CZ sc-im indicates that the particles are not uniformly distributed over the surface, which leads to the formation of bulk crystalline structures. This phenomenon negatively affects the process of dry reforming of methane, since it catalyzes such reactions as the Boudouard reaction and the decomposition of methane [16,45]. The average crystallite size of NiO for Ni/CZ sc-op is reduced to 19.5 nm, compared to Ni/CZ sc-im (30 nm). According to the Raman data ( Figure 3b) for all SCS samples, the presence of a single peak at 470 cm −1 confirms formation of the main crystal phase with cubic fluorite-like structure, which agrees with X-ray structural data. As in the case of Pechini samples, the Raman spectra of impregnated and one-pot samples have other strong bands at 500-600 cm −1 . The formation of the band at about 560 cm −1 (oxygen vacancies) is caused by various oxidation states of the introduced cation, while the band at about 600 cm −1 appears due to dissimilar ionic radius of the dopant compared to that of Ce 4+ [52]. The parameter ω of catalysts in the case of samples obtained by the Pechini method were orders of magnitude larger than those of the support. In the case of the Ni/CZ P-op sample, its value is higher than that for the Ni/CZ sc-im .  Figure 4). After the introduction of titanium, the overall mixed oxide structure of the solid solution with the cubic lattice is retained. As also confirmed by Raman spectroscopy (Figure 4b), the incorporation of dopant Ti cations into the Ce 0.75 Zr 0.25 O 2-δ lattice occurs. 500-600 cm −1 . The formation of the band at about 560 cm -1 (oxygen vacancies) is caused by various oxidation states of the introduced cation, while the band at about 600 cm -1 appears due to dissimilar ionic radius of the dopant compared to that of Ce 4+ [52]. The parameter ω of catalysts in the case of samples obtained by the Pechini method were orders of magnitude larger than those of the support. In the case of the Ni/CZP-op sample, its value is higher than that for the Ni/CZsc-im.

Structural Properties of Supports Doped by Ti and Catalysts Ni/Ce0.75TixZr0.15-xO2−δ
The XRD patterns of all Ni/Ce0.75TixZr0.15-xO2−δ catalysts are mainly similar to the patterns of Ni/Ce0.75Zr0.25O2−δ showing reflections of CeO2-ZrO2 complex oxides ( Figure 4). After the introduction of titanium, the overall mixed oxide structure of the solid solution with the cubic lattice is retained. As also confirmed by Raman spectroscopy (Figure 4b), the incorporation of dopant Ti cations into the Ce0.75Zr0.25O2-δ lattice occurs.  In accordance with the Vegard law, by increasing the dopant (having a lower ionic radius) amount, a gradual decrease of the unit cell volume should be observed. This is evident when Zr is introduced into the CeO2 lattice since there is a difference between the ionic radii of Ce 4+ (0.97Å) and Zr 4+ (0.84Å) [55]. The lattice constant of the initial CZsc support is 5.368 Å and this value is less than for CeO2 (a = 5.411Å; PDF#340394). However, when Ce0.75Zr0.25O2 Zr 4+ cations were replaced by Ti 4+ , the volume of the cubic unit cell increased. The unit cell parameter a of Ce0.75Ti0.1Zr0.15O2−δ is 5.378Å. With increasing the amount of titanium, the unit cell parameter is further increased and for CT0.2Zsc-op a = 5.393 Å ( Table 1). The question is why it happens, since the ionic radius of Ti 4+ (the coordination number (c.n.) = 8) r = 0.74 Å and the ionic radius of Zr 4+ (c.n. = 8) r = 0.84 Å.
This dependence can be clarified by the number of oxygen vacancies estimated using the Rietveld refinement. Figure 5 shows the dependence between the unit cell parameter and oxygen deficiency (δ) for initial supports, and it is clear that the parameter is increased with increase of the oxygen vacancies number. It is known that the formation of oxygen vacancies is associated with a partial reduction of Ce 4+ to Ce 3+ [56,57]. Since the radius of Ce 3+ ions (1.14 Å) is greater than that of Ce 4+ (0.97 Å), the appearance In accordance with the Vegard law, by increasing the dopant (having a lower ionic radius) amount, a gradual decrease of the unit cell volume should be observed. This is evident when Zr is introduced into the CeO 2 lattice since there is a difference between the ionic radii of Ce 4+ (0.97Å) and Zr 4+ (0.84Å) [55]. The lattice constant of the initial CZ sc support is 5.368 Å and this value is less than for CeO 2 (a = 5.411Å; PDF#340394). However, when Ce 0.75 Zr 0.25 O 2 Zr 4+ cations were replaced by Ti 4+ , the volume of the cubic unit cell increased. The unit cell parameter a of Ce 0.75 Ti 0.1 Zr 0.15 O 2−δ is 5.378Å. With increasing the amount of titanium, the unit cell parameter is further increased and for CT 0.2 Z sc-op a = 5.393 Å ( Table 1). The question is why it happens, since the ionic radius of Ti 4+ (the coordination number (c.n.) = 8) r = 0.74 Å and the ionic radius of Zr 4+ (c.n. = 8) r = 0.84 Å.
This dependence can be clarified by the number of oxygen vacancies estimated using the Rietveld refinement. Figure 5 shows the dependence between the unit cell parameter and oxygen deficiency (δ) for initial supports, and it is clear that the parameter is increased with increase of the oxygen vacancies number. It is known that the formation of oxygen vacancies is associated with a partial reduction of Ce 4+ to Ce 3+ [56,57]. Since the radius of Ce 3+ ions (1.14 Å) is greater than that of Ce 4+ (0.97 Å), the appearance of a trivalent cation leads to an increase in the lattice constant. For a series of catalysts prepared using one-pot synthesis, an increase of the lattice parameter with an increasing titanium amount and number of oxygen vacancies is also observed. It could not be excluded that a part of the titanium cations can also be in 3+ state, thus having a higher cationic radius and increasing the lattice constant as well.
Energies 2020, 13, 3365 9 of 24 of a trivalent cation leads to an increase in the lattice constant. For a series of catalysts prepared using one-pot synthesis, an increase of the lattice parameter with an increasing titanium amount and number of oxygen vacancies is also observed. It could not be excluded that a part of the titanium cations can also be in 3+ state, thus having a higher cationic radius and increasing the lattice constant as well. As in the case of Ce0.75Zr0.25O2−δ, the oxygen defects concentration in Ce0.75Ti0.1Zr0.15O2−δ is low, so generation of defects can only be explained by the difference between the ionic radii of Ti 4+ and Ce 4+ .
The ω = I(D1+D2)/IF2g ratio of the intensities of induced bands I (D1 + D2) in the range 500-700 cm −1 is shown in Figure 4a. This ratio was higher for the catalysts with a larger defect concentration. Furthermore, just as in the case of samples Ni/Ce0.75Zr0.25O2−δ, the maximum value of this parameter is reached for the one-pot catalyst.
The XRD patterns of all catalysts are mainly similar. For one-pot catalysts, the broad reflections of NiO are observed. This is an indication of the high dispersion of nickel oxide phase achieved in these samples. The introduction of nickel does not have any noticeable effect on the particle size of supports (8.5-10 nm). The calculated NiO crystallite size for Ni/CT0.1Zsc-im is found to be 20 nm, but it is decreased for the one-pot catalyst (13-15 nm).

Textural Characterization
The isotherms of nitrogen adsorption-desorption for the catalysts are shown in Figure 6a,b. The isotherms for the most samples correspond to the II type according to the IUPAC classification [58], which is characteristic for macroporous solids. Regardless of the preparation method of the catalysts, for samples prepared in supercritical alcohols multimodal pore size distribution is observed ( Figure  6c,d). It depends on the support composition and includes micropores (d~4 nm) and macropores (50-200 nm). The introduction of titanium cations to Ce0.75Zr0.25O2−δ led to an increase of pore size.
Only the Ni/CZP-im isotherm fits to the IV type with H3 shaped hysteresis loop, characterizing the mesoporous structure. This type is observed for solids consisting of agglomerates of particles with nonuniform size and shape. The XRD patterns of all catalysts are mainly similar. For one-pot catalysts, the broad reflections of NiO are observed. This is an indication of the high dispersion of nickel oxide phase achieved in these samples. The introduction of nickel does not have any noticeable effect on the particle size of supports (8.5-10 nm). The calculated NiO crystallite size for Ni/CT 0.1 Z sc-im is found to be 20 nm, but it is decreased for the one-pot catalyst (13-15 nm).

Textural Characterization
The isotherms of nitrogen adsorption-desorption for the catalysts are shown in Figure 6a,b. The isotherms for the most samples correspond to the II type according to the IUPAC classification [58], which is characteristic for macroporous solids. Regardless of the preparation method of the catalysts, for samples prepared in supercritical alcohols multimodal pore size distribution is observed (Figure 6c,d). It depends on the support composition and includes micropores (d~4 nm) and macropores (50-200 nm). The introduction of titanium cations to Ce 0.75 Zr 0.25 O 2−δ led to an increase of pore size.
Only the Ni/CZ P-im isotherm fits to the IV type with H3 shaped hysteresis loop, characterizing the mesoporous structure. This type is observed for solids consisting of agglomerates of particles with nonuniform size and shape.  Table 2 shows the BET specific surface area (SSA) of fresh ceria-zirconia mixed oxides prepared using supercritical synthesis and the Pechini method and impregnated and one-pot catalysts. For SCS supports, the introduction of titanium cations into the Ce-Zr oxide structure leads to increase in the specific surface area. The addition of Ni results in decreasing SSA of all catalysts irrespective of the preparation method. For one-pot samples, this decrease may be caused by the influence of nickel cations on the formation of CeZrO framework during the synthesis and by the blocking a part of pores with NiO particles formed after calcination. So, in the case of catalysts obtained by impregnation, there is a slight decrease in the specific surface area due to the blocking of pores by NiO particles. Thus, the SSA of one-pot samples was lower than that of catalysts prepared by the traditional impregnation. However, preparation method (SCS or Pechini) almost has no additional influence on the value of their surface area.

H2-TPR Characterization
H2-TPR experiments for supports and Ni-containing catalysts were performed to investigate the redox characteristics; their results are presented in Figures 7 and 8 and in Table 2. Deconvolution of TPR curves into individual peaks and calculation of value of hydrogen consumption for each peak was carried out using the asymmetric Gauss decomposition method (AGDM). For all supports prepared using both synthesis methods, three peaks were observed after deconvolution, where the first two peaks (up to 550 °C) correspond to reduction of active surface oxygen, while the high-temperature hydrogen consumption is related to reduction of the bulk oxygen [13,59]. The introduction of titanium cations to Ce0.75Zr0.25O2−δ led to the shift of the reduction peak toward higher temperatures, while the value of hydrogen consumption decreases as expected due to increase of the oxygen deficiency. The Ce(Ti)ZrO2  Table 2 shows the BET specific surface area (SSA) of fresh ceria-zirconia mixed oxides prepared using supercritical synthesis and the Pechini method and impregnated and one-pot catalysts. For SCS supports, the introduction of titanium cations into the Ce-Zr oxide structure leads to increase in the specific surface area. The addition of Ni results in decreasing SSA of all catalysts irrespective of the preparation method. For one-pot samples, this decrease may be caused by the influence of nickel cations on the formation of CeZrO framework during the synthesis and by the blocking a part of pores with NiO particles formed after calcination. So, in the case of catalysts obtained by impregnation, there is a slight decrease in the specific surface area due to the blocking of pores by NiO particles. Thus, the SSA of one-pot samples was lower than that of catalysts prepared by the traditional impregnation. However, preparation method (SCS or Pechini) almost has no additional influence on the value of their surface area.  Table 2. Deconvolution of TPR curves into individual peaks and calculation of value of hydrogen consumption for each peak was carried out using the asymmetric Gauss decomposition method (AGDM). For all supports prepared using both synthesis methods, three peaks were observed after deconvolution, where the first two peaks (up to 550 • C) correspond to reduction of active surface oxygen, while the high-temperature hydrogen consumption is related to reduction of the bulk oxygen [13,59]. The introduction of titanium cations to Ce 0.75 Zr 0.25 O 2−δ led to the shift of the reduction peak toward higher temperatures, while the value of hydrogen consumption decreases as expected due to increase of the oxygen deficiency. The Ce(Ti)ZrO 2 samples prepared by the supercritical synthesis did not show better reducibility compared to the Pechini ones. The H2-TPR spectra of Ni/Ce(Ti)ZrO2 catalysts prepared by both methods are presented in Figure  8. It is a well-known fact that peaks in lower temperature regions are associated with the reduction of relatively free nickel oxide species, while peaks at higher temperatures indicate the reduction of complex NiO species, as a result of strong interaction with the support. [60]. It is noticeable that the redox behavior of Ni particles is quite different depending on the method of metal introduction. The H2-TPR curves for the impregnated catalysts are shifted to lower temperatures compared to the pure supports. Two main peaks were observed when Ce0.75Zr0.25O2-δ oxides were used as supports. The first peak (311 °C) is related to the reduction of free NiO and the second higher intensity peak (455 °C) corresponds to NiO crystallites interacting with support. The profiles of the one-pot Ni/Ce0.75Zr0.25O2-δ catalysts differ from that of impregnated samples. Thus, for the one-pot SC samples, the first clearly marked peak is shifted to lower temperature and new peak with the maxima at ~345-360 °C is observed. Shape of the TPR profiles for one-pot samples suggests existence of nickel oxide species with a wide range of particle sizes and interacting with support with different strength [51]. Hence, it could be concluded that the lattice oxygen in catalysts prepared using one-pot supercritical synthesis could migrate more easily than in the case of the Pechini method.

H 2 -TPR Characterization
As shown in Figure 8, after the addition of Ti, the reduction peak is shifted toward higher temperatures, which implies a stronger interaction between NiO and the support. According to [1], the introduction of magnesium oxide shifted the reduction peak considerably towards higher temperatures as well. This obviously improved stability to nickel sintering and coking, and for the MgO-promoted Ni-Ce0.8Zr0.2O2 catalyst, equilibrium conversions of methane and carbon dioxide were observed for 200 h without noticeable deactivation. The H2-TPR spectra of Ni/Ce(Ti)ZrO 2 catalysts prepared by both methods are presented in Figure 8. It is a well-known fact that peaks in lower temperature regions are associated with the reduction of relatively free nickel oxide species, while peaks at higher temperatures indicate the reduction of complex NiO species, as a result of strong interaction with the support [60]. It is noticeable that the redox behavior of Ni particles is quite different depending on the method of metal introduction. The H 2 -TPR curves for the impregnated catalysts are shifted to lower temperatures compared to the pure supports. Two main peaks were observed when Ce 0.75 Zr 0.25 O 2-δ oxides were used as supports. The first peak (311 • C) is related to the reduction of free NiO and the second higher intensity peak (455 • C) corresponds to NiO crystallites interacting with support. The profiles of the one-pot Ni/Ce 0.75 Zr 0.25 O 2-δ catalysts differ from that of impregnated samples. Thus, for the one-pot SC samples, the first clearly marked peak is shifted to lower temperature and new peak with the maxima at~345-360 • C is observed. Shape of the TPR profiles for one-pot samples suggests existence of nickel oxide species with a wide range of particle sizes and interacting with support with different strength [51]. Hence, it could be concluded that the lattice oxygen in catalysts prepared using one-pot supercritical synthesis could migrate more easily than in the case of the Pechini method.
As shown in Figure 8, after the addition of Ti, the reduction peak is shifted toward higher temperatures, which implies a stronger interaction between NiO and the support. According to [1], the introduction of magnesium oxide shifted the reduction peak considerably towards higher temperatures as well. This obviously improved stability to nickel sintering and coking, and for the MgO-promoted Ni-Ce 0.8 Zr 0.2 O 2 catalyst, equilibrium conversions of methane and carbon dioxide were observed for 200 h without noticeable deactivation.    The values of the hydrogen consumption calculated for all deconvoluted peaks are presented in Table 2. The hydrogen consumption below 300 • C for the one-pot catalysts suggested more effective reduction of the surface oxygen, its amount being substantially larger than that for impregnated samples. The deconvoluted peaks of Ni/CT 0.1 Z sc-op and Ni/CT 0.2 Z sc-op corresponding to the main hydrogen consumption in the region of 300-800 • C are shifted to higher temperatures as compared with impregnated catalysts (Figure 8, Table 2). It can be explained by more difficult reduction of Ni 2+ in the one-pot catalysts due to its stronger interaction with the support. According to XRD data, in one-pot samples, Ni is partially incorporated into ceria-zirconia solid solution [51], but is mainly present as NiO in the impregnated catalysts. The highest amount of removed oxygen, and, hence, the oxygen mobility was found for the Ni/CT 0.2 Z sc-op sample ( Table 2).

Morphology of Ceria-Zirconia Mixed Oxides
Particle morphology of the 5%Ni/Ce 0.75 Zr 0.75 O 2−δ and 5%Ni/Ce 0.75 Ti x Zr 0.25−x O 2−δ samples was analyzed using a high-resolution TEM. Figure 9 shows TEM images of catalysts obtained using both preparation techniques and with both methods of Ni addition.  The average particle size of all samples is about 10-15 nm; that is consistent with XRD data. Particles of catalysts prepared by the Pechini method (Figure 9a,c) have a disordered shape and are assembled into chaotically formed agglomerates. Synthesis in supercritical alcohols results in formation of particles with a more spherical shape. They are packed into large spherical aggregates with diameter more than 100 nm. Formation of such aggregated CeO 2 and Ce 1−x Zr x O 2 in bigger spherically-shaped nanostructured assemblies (20-100 nm in diameter) was shown in [61].
The addition of nickel at the stage of oxide preparation in the one-pot method, as well as doping with titanium, leads to formation of agglomerates with more ideal spherical morphology.
The images of HAADF-STEM with EDX analysis demonstrate distribution of elements in samples ( Figure 10). Ce and Zr cations of the initial support, as well as Ti cations in doped supports, are uniformly distributed in the oxide structure of all samples, irrespective of the preparation method. The nickel distribution nature is more complex, partially depending on the method of preparation. For impregnated SCS samples, it can be assumed that addition of titanium to the support promotes a more uniform distribution of nickel after deposition (Figure 10 a,b).
Generally, it could be expected that addition of nickel during the oxide synthesis will contribute to its more homogeneous distribution. However, in the case of the Pechini method, this assumption turned out to be wrong and nickel is present as separate large particles (Figure 11a) (even at low nickel content in the investigated area), which is consistent with large NiO crystallite size estimated from XRD data. One-pot synthesis in supercritical alcohols allows one to obtain more uniform distribution of Ni species (Figure 11b), which confirms better mixing of components in these conditions.
The images of HAADF-STEM with EDX analysis demonstrate distribution of elements in samples ( Figure 10). Ce and Zr cations of the initial support, as well as Ti cations in doped supports, are uniformly distributed in the oxide structure of all samples, irrespective of the preparation method. The nickel distribution nature is more complex, partially depending on the method of preparation. For impregnated SCS samples, it can be assumed that addition of titanium to the support promotes a more uniform distribution of nickel after deposition (Figure 10 a,b).
. Generally, it could be expected that addition of nickel during the oxide synthesis will contribute to its more homogeneous distribution. However, in the case of the Pechini method, this assumption turned out to be wrong and nickel is present as separate large particles (Figure 11a) (even at low nickel content in the investigated area), which is consistent with large NiO crystallite size estimated from XRD data. One-pot synthesis in supercritical alcohols allows one to obtain more uniform distribution of Ni species (Figure 11b), which confirms better mixing of components in these conditions.
All catalysts after tests in methane dry reforming have also been investigated using TEM. Figure  12a,b demonstrates images of impregnated 5% Ni/Ce0.75Zr0.25O2 catalysts. It can be seen that the surface of SCS sample is covered with carbon filaments, while Ni/CZP-im is free from carbon, which could explain its higher activity. For the Ti-doped SCS catalyst, carbon filaments are also observed (not shown). During catalytic tests, nickel is segregated to the surface in the form of metal particles with sizes 10-20 nm. Some All catalysts after tests in methane dry reforming have also been investigated using TEM. Figure 12a,b demonstrates images of impregnated 5% Ni/Ce 0.75 Zr 0.25 O 2 catalysts. It can be seen that the surface of SCS sample is covered with carbon filaments, while Ni/CZ P-im is free from carbon, which could explain its higher activity. For the Ti-doped SCS catalyst, carbon filaments are also observed (not shown). During catalytic tests, nickel is segregated to the surface in the form of metal particles with sizes 10-20 nm. Some metal particles are breaking away from the oxide support during the growth of carbon fibers, which might lead to a drop in catalytic activity.
Energies 2020, 13, x FOR PEER REVIEW 17 of 24 metal particles are breaking away from the oxide support during the growth of carbon fibers, which might lead to a drop in catalytic activity.  In the case of one-pot 5% Ni/Ce 0.75 Zr 0.25 O 2 catalysts, there are no coke depositions for both preparation methods (Figure 12c,d). It can be explained by a stronger interaction between the nickel particles and oxide support. Figure 11d demonstrates a nickel particle strongly interacting with support and covered by loose oxide layers, mainly containing nickel oxide, though traces of Ce and Zr cations could be present at the surface as well.

Ni/CZP-im
Carbon filaments are also observed over the surface of Ni/CT 0.2 Z sc-op catalyst (Figure 12e), despite its high activity in DRM. However, there is not necessarily a relationship between carbon formation and activity decrease since this type of carbon does not block catalyst surface and, thus, does not inhibit catalytic activity [62].

Methane Dry Reforming Studies
The evolution over time of CH 4 and CO 2 conversions over impregnated catalysts at reaction temperatures 600-750 • C is presented in Figure 13. A summary of catalytic data is given in Table 3. For all the catalysts, conversion of reagents increases significantly with the temperature due to the endothermic nature of this reaction. The reagent conversions are higher over the impregnated catalyst 5%Ni/Ce 0.75 Zr 0.25 O 2 prepared using the Pechini method (Figure 13a,b) as compared with the sample prepared using supercritical synthesis. The Ni/CZ sc-im catalyst with support obtained under supercritical conditions is characterized by a sufficiently higher catalytic activity at 600-650 • C, but at higher temperatures, its catalytic activity is lower than that for Ni/CZ p-im . However, for the one-pot sample prepared using the supercritical route, CH 4 and CO 2 conversions and product concentrations are somewhat higher than for the impregnated catalyst Ni/CZ sc-im , while catalytic activity of the one-pot Pechini sample is much lower (Table 3, Figure 14c,d); that may be due to the low dispersion of nickel species. Table 3. Conversion of CH 4 (X) and CO 2 , selectivity (S), concentration of H 2 and CO and H 2 /CO revealed during DRM (T = 700-750 • C; contact time 10 ms; feed composition-15 vol% CH 4 + 15 vol% CO 2 + N 2 ) over impregnated and one-pot catalysts 5 wt% Ni/CZ(T).    It was shown that introduction of 20 at% of Ti instead of Zr results in a significant improvement in DRM catalytic performance (Table 3). Short-term stability tests ( Figure 15) reveal that methane conversion on a one-pot SCS ceria-zirconia sample doped by Ti immediately reaches maximum values at 750 °C and then gradually decreases during the experiment. An increase in the amount of introduced titanium leads to a noticeable increase in stability ( Figure 16) as well. This fact apparently correlates with the highest oxygen mobility and storage capacity of Ni/CT0.2Zsc-op catalyst revealed by H2 TPR (Figure 8,  It was shown that introduction of 20 at% of Ti instead of Zr results in a significant improvement in DRM catalytic performance (Table 3). Short-term stability tests ( Figure 15) reveal that methane conversion on a one-pot SCS ceria-zirconia sample doped by Ti immediately reaches maximum values at 750 °C and then gradually decreases during the experiment. An increase in the amount of introduced titanium leads to a noticeable increase in stability ( Figure 16) as well. This fact apparently correlates with the highest oxygen mobility and storage capacity of Ni/CT0.2Zsc-op catalyst revealed by H2 TPR (Figure 8,   Table 3). This fact is explained by the side competitive reaction of RWGS, which leads to an additional increase in the conversion of CO 2 compared to the CH 4 conversion and, as a result, to decrease of the content of hydrogen formed and a low H 2 /CO ratio. With increasing temperature, there is a slight increase in the H 2 /CO ratio for the Ni-CZsc-im sample, followed by a decrease at 700 • C and a drop in this ratio in the reverse cycle. In the temperature range of 700-750 • C, the H 2 /CO ratio for the one-pot SCS sample was 0.65-0.68, which is comparable with the impregnated sample (0.69-0.71).
It was shown that introduction of 20 at% of Ti instead of Zr results in a significant improvement in DRM catalytic performance (Table 3). Short-term stability tests ( Figure 15) reveal that methane conversion on a one-pot SCS ceria-zirconia sample doped by Ti immediately reaches maximum values at 750 • C and then gradually decreases during the experiment. An increase in the amount of introduced titanium leads to a noticeable increase in stability ( Figure 16) as well. This fact apparently correlates with the highest oxygen mobility and storage capacity of Ni/CT 0.2 Z sc-op catalyst revealed by H 2 TPR (Figure 8, Table 2), which helps to prevent coking. From the structural point of view, it is determined by the largest oxygen nonstoichiometry in heavily Ti-doped mixed ceria-zirconia oxide.
Energies 2020, 13, x FOR PEER REVIEW 20 of 24 Table 2), which helps to prevent coking. From the structural point of view, it is determined by the largest oxygen nonstoichiometry in heavily Ti-doped mixed ceria-zirconia oxide.

Conclusions
The mixed ceria-zirconia oxides also doped by Ti with cubic fluorite phase have been prepared using either the Pechini method or using continuous solvothermal flow synthesis in supercritical alcohols. For the first time using the supercritical synthesis method, a series of new one-pot Ni/Ce(Ti)ZrO2 catalysts was prepared for the production of hydrogen and synthesis gas by means of methane dry reforming reaction. We compared catalysts prepared by two techniques: the standard incipient wetness impregnation and the one-pot synthesis. Catalysts prepared in supercritical isopropanol exhibited a higher catalytic activity and coking stability in CH4 dry reforming reaction than catalysts with a similar chemical composition prepared by the modified Pechini route. Based on the results of the catalytic tests, it was concluded that the highest conversion and maximum yield of products were achieved on the one-pot catalyst doped with titanium cations. The addition of Ni at the  Table 2), which helps to prevent coking. From the structural point of view, it is determined by the largest oxygen nonstoichiometry in heavily Ti-doped mixed ceria-zirconia oxide.

Conclusions
The mixed ceria-zirconia oxides also doped by Ti with cubic fluorite phase have been prepared using either the Pechini method or using continuous solvothermal flow synthesis in supercritical alcohols. For the first time using the supercritical synthesis method, a series of new one-pot Ni/Ce(Ti)ZrO2 catalysts was prepared for the production of hydrogen and synthesis gas by means of methane dry reforming reaction. We compared catalysts prepared by two techniques: the standard incipient wetness impregnation and the one-pot synthesis. Catalysts prepared in supercritical isopropanol exhibited a higher catalytic activity and coking stability in CH4 dry reforming reaction than catalysts with a similar chemical composition prepared by the modified Pechini route. Based on the results of the catalytic tests, it was concluded that the highest conversion and maximum yield of products were achieved on the one-pot catalyst doped with titanium cations. The addition of Ni at the stage of oxide synthesis (so-called one-pot synthesis), according to TEM, resulted in a more uniform distribution of NiO particles on the surface of the catalyst, which confirms better mixing of components

Conclusions
The mixed ceria-zirconia oxides also doped by Ti with cubic fluorite phase have been prepared using either the Pechini method or using continuous solvothermal flow synthesis in supercritical alcohols. For the first time using the supercritical synthesis method, a series of new one-pot Ni/Ce(Ti)ZrO 2 catalysts was prepared for the production of hydrogen and synthesis gas by means of methane dry reforming reaction. We compared catalysts prepared by two techniques: the standard incipient wetness impregnation and the one-pot synthesis. Catalysts prepared in supercritical isopropanol exhibited a higher catalytic activity and coking stability in CH 4 dry reforming reaction Energies 2020, 13, 3365 21 of 24 than catalysts with a similar chemical composition prepared by the modified Pechini route. Based on the results of the catalytic tests, it was concluded that the highest conversion and maximum yield of products were achieved on the one-pot catalyst doped with titanium cations. The addition of Ni at the stage of oxide synthesis (so-called one-pot synthesis), according to TEM, resulted in a more uniform distribution of NiO particles on the surface of the catalyst, which confirms better mixing of components under these conditions. This uniform distribution of nickel provides a high activity and stability of one-pot catalysts. At the same time, after the reaction, the surfaces of many prepared catalysts are covered with carbon fibers, which, however, do not block the surface of the catalyst and do not lead to activity decrease.