Evaluation of Properties Changes by the Addition of Surfactant in the Synthesis of Ni/CeO₂

Abstract

This work evaluates how adding cetyltrimethylammonium bromide (CTAB) in the synthesis affects the physical and chemical features of Ni/CeO₂ catalysts. The samples were characterized by the Rietveld refinement from X-ray diffraction (XRD) and showed a decrease in the crystallite size and an increase in the microstrain from 5.9 to 7.5 x 10^-3. Nitrogen physisorption analysis was used to obtain the pore size distribution by the BJH method, varying the pore size between 8 and 12 nm. Raman spectroscopy was employed to confirm the fluorite structure arrangement of ceria and the number of vacancies in the bulk phase, where the ratio from vibrational modes related to defects and oxygen vacancies increased from 0.488 to 0.516 ratio. With the spectra obtained from X-ray photoelectron spectroscopy (XPS), it was possible to observe the distribution of metals on the surface of the samples and their oxidative states, providing clues about the quantity of vacancies. Samples with higher concentrations of CTAB obtained higher values of Ce³⁺, reaching a 0.45 ratio on the surface. The scanning electron microscopy (SEM) analysis showed that the addition of CTAB formed more irregular shape grains, reaching 20 μm of medium size for all the samples.

Keywords:  Sol-gel synthesis; Nickel catalysts; CeO₂ support; Oxygen vacancy; Crystal defects

1. Introduction

Ni/CeO₂, as heterogeneous catalysts, with nickel as the active phase and CeO₂ as the support, exhibit enormous potential for selectively converting CO₂ into methane¹, methane into methanol², and in the dry reforming of methane, converting CO₂ and methane into CO and H₂³. These catalysts have many applications and are significant in sustainable catalysis and energy conversion processes. The challenge of heterogeneous catalysis is the development of catalysts resistant to coke formation and sintering so that they remain active throughout the reforming reaction, providing higher conversions and selectivity.

Nickel is a transition metal that stands out due to its high activity, low cost, and availability. However, because nickel has a high potential to form coke and due to the severe reaction conditions of temperature and pressure, catalyst deactivation occurs through carbon deposition on its surface and sintering due to the mobility of the active phase on the support⁴. An alternative to reduce such issues would be using catalysts with lower nickel content, supported on materials that provide stability to the catalyst, and with the addition of promoters that, besides being selective to the reaction, inhibit coke formation⁵.

CeO₂-based materials have a wide range of industrial applications. Being the most abundant rare earth metal on Earth, its cost for application is feasible for large-scale use. Its physical and chemical characteristics, such as thermal resistance and variable oxidation states, allowing for significant oxygen storage, have made it a focus of study in catalytic reactions. For instance, the catalytic hydrogenation of CO₂, turning CO₂ into an energy resource, is one area of interest. Additionally, methane can be synthesized into valuable organic reagents⁶.

Cerium oxide prevents carbon deposition on the catalyst surface due to its oxygen transfer capacity⁷. It is an essential support as it promotes oxygen vacancies due to the change in oxidation state between Ce⁺⁴ and Ce⁺³. However, its effectiveness may be restricted in reducing environments or under high-temperature conditions, promoting a tendency for coke formation⁸. In this context, the synthesis of the catalyst can promote a greater quantity of oxygen vacancies in the support due to alterations caused in the oxide’s crystal lattice and a better dispersion of nickel loads on the surface of CeO₂⁹, which may improve the catalytic conversion.

Textural characteristics have a fundamental role in its catalytic performance. A higher specific area improves the distribution of the active phase in the support, decreasing the sintering and coke formation. This can affect the quantity of catalytic sites in area per gram, which depends on the composition of the surface¹⁰. The size of the metallic crystallite also directly affects carbon deposition on the catalyst, with smaller crystallite-size particles desirable to provide higher active phase dispersion. Larger crystallites lead to a reduction in specific area¹¹.

These features can be induced by the formation of deformations in the structure, primarily caused by oxygen vacancies. Vacancy sites enhance the catalytic performance of these catalysts, increasing the mobility and adsorption of CO₂. Increasing this chemical species in this catalyst’s structure makes improving its catalytic activity at lower temperatures possible, reducing the application cost¹²,¹³.

The formation of structural defects due to oxygen vacancies may be induced by adding a promoter during synthesis, which can be a surfactant, forming micelles during synthesis¹⁴. The surfactant agent is crucial in producing the synthesized catalyst’s structure. Cetyltrimethylammonium bromide (CTAB) is a commonly used cationic surfactant in sol-gel synthesis due to the formation of silica micelles, creating a mesoporous material that contributes to increasing the material’s surface area¹⁵,¹⁶.

The sol-gel synthesis technique can be more effectively employed industrially, as the reaction does not require elevated temperatures or abrupt pressure changes, significantly reducing energy costs compared to other methods. Its application in the synthesis of Ni/CeO₂ is crucial due to its ability to produce more than one type of nanoparticle, synthesizing a ceramic material from two types of metals environment (a metal and a metal oxide) in a defined ratio, acting as an intermediary between them and forming homogeneous particles according to the followed proportion with a high degree of purity¹⁷. Some of these characteristics are due to the synthesis method, which involves creating a homogeneous solution in the sol phase, gelatin it using different techniques, such as the addition of surfactants to a colloidal state with well-defined structures, and then drying the gel, transforming it into a ceramic material with unique porous properties. Changes in each synthesis stage can give the particle different properties, such as the type or quantity of surfactant agent¹⁴.

In this article, the structural effects caused by the concentration of the surfactant cetyltrimethylammonium bromide (CTAB) were studied when added during the sol-gel synthesis, aiming to produce a catalyst with favorable characteristics for catalytic reactions.

2. Experimental

2.1. Synthesis of the materials

The catalysts were synthesized according to Luo et al.¹⁸ using the sol-gel method, varying the concentration of cetyltrimethylammonium bromide (CTAB, Sigma-Aldrich 98%) for the samples named Ni/CeO₂-X, where X is the amount in mmol of CTAB used in the synthesis. One synthesis was conducted without CTAB, and three others were carried out, varying the concentration to 6 mmol, 12 mmol, and 24 mmol of this surfactant 200 mL of distilled water was added with stirring, into which CTAB, Ni(NO₃)₂.3H₂O (2 mmol, Sigma-Aldrich 98%), and Ce(NO₃)6H₂O (9.5 mmol, Sigma-Aldrich 99%) were added. After 0.5 hours, NaOH (50 mmol, Sigma-Aldrich, 98%) in 300 mL of distilled water was added and stirred for an additional 16 hours. Subsequently, the material was placed in an oven at 90°C for 5 hours, washed, and then transferred to an oven at 110°C for 16 hours. The material was calcined in a muffle furnace with a ramp of 3°C for 4 hours at 450°C.

2.2. Characterization of the materials

The crystalline phases in the calcined samples were analyzed by X-ray diffraction using a Rigaku DMAX Ultima+ diffractometer with Cu Kα radiation (40 kV/20 mA). Data were collected in the 2θ range from 5 to 90°, with a scan step of 0.03°. The Match! Software¹⁹ was used to obtain the standard powder diffraction file and to identify the phases. The refinement of the crystalline structure was performed using the Rietveld method with the MAUD software²⁰. Instrumental broadening correction and standard measurement were made using a metallic Si powder as a reference. Maud manages the instrumental and sample aberrations independently rather than combining the two to express the broadening change with 2θ as a sum of the instrumental and sample widening using a special classical Caglioti and related formulations. While the sample widening is calculated for each phase based on the crystallite size and microstrain (and eventually additional microstructure/defects parameters), the instrumental broadening is stored in the instrument object using a Caglioti-like formula²⁰. For the crystallite size determination, Maud considers the Scherrer equation. Scale factors, zero shifts, peak backgrounds, and lattice parameters were refined using a sixth-degree Chebyshev polynomial. Pseudo-Voigt functions were employed for peak profile refinements.

Using a Micromeritics ASAP 2010 physisorption system, nitrogen adsorption-desorption isotherms were measured at the liquid nitrogen temperature (~77 K) in the relative pressure range of 0.002 to 0.998. Before the measurements, the samples were degassed for 12 hours at 200 °C under 10 μPa vacuum. The Brunauer-Emmett-Teller (BET) equation²¹ determined the specific surface area, and the BJH method was used to determine the mesopore size distribution²².

The Raman spectra were collected using an i-Raman Plus 532H Portable Spectrometer. The spectra were acquired with an excitation wavelength of 532 nm, with 60 measures of 80 seconds of integration time. Vibrational modes from the Raman spectra were deconvolved using the Fityk software²³.

The overall micromorphology of the catalysts was determined using a Carls Zeiss EVO-LS15 scanning electron microscope. The samples were prepared in isopropanol suspension, deposited on copper strips, and coated with gold. The grain size distribution was cataloged using the ImageJ software²⁴, where the histogram was made using the Origin Pro 2018 software²⁵. The medium size and the deviation value were also determined by Origin Pro 2018, using the ANOVA function.

The IPE beamline at Sirius Synchrotron, in LNLS, Campinas, Brazil, was the XPS end station for the X-ray photoelectron spectroscopy (XPS) experiment. With 45° incidence and take-off angles, the kinetic energy was maintained at around 400 eV, while the photon energy was varied for the XPS experiments. SPECS Phoibos 150 was the type of electron analyzer utilized. XPS Data was analyzed using CasaXPS software²⁶. The Shirley function was employed to subtract the background from all the signal deconvoluted, and Multiple Voigt profiles were used to get the deconvoluted spectra without any limitations. The calibration was made using the Au 4f_7/2-line reference, and the peak positions were accurately determined to be within ±0.1 eV, with the width at half maximum (FWHM) varying between 1.5 and 2.2 eV.

3. Results and Discussion

The X-ray diffraction technique was applied to verify the crystalline structure of the samples. CeO₂ has a cubic system (space group $Fm\overline{3}m$) with a lattice parameter of 5.3623 Ų⁷. The characteristic reflection planes of the cubic phase of CeO₂ are (111), (220), and (311). NiO has a cubic crystal system (space group $Fm\overline{3}m$), and the characteristic reflection planes are (111), (220) and (311)²⁸. There is no evidence of a segregate phase between NiO and CeO₂ (Figure 1). An attempt was made to quantify the crystallographic parameters through Rietveld refinement. There are no significant differences between the lattice parameter values with the addition of CTAB, which may be related to changes in the atomic radius of some species that change depending on the oxidative state. There is a minor increase in crystallite size from 99.3 Å on Ni/CeO₂-0 to 101.1 Å on Ni/CeO₂-12 sample, and when the maximum concentration is reached, the size of crystallite decay to 95.1 Å on Ni/CeO₂-24 sample. The increase in microstrain can be related to atomic distortions caused by the length difference between Ce³⁺ and Ce⁴⁺ ions, where the Ce³⁺ induces a shortening of Ce⁴⁺ bond with oxygen, inducing this increase in microstrain (Table 1).

Figure 1
X-ray diffraction patterns of Ni/CeO₂ samples varying in CTAB concentration.

Table 1
Structural parameters obtained from the Rietveld refinement method from the XRD patterns.

Sample	Lattice Parameter (Å)	Crystallite Size (Å)	Microstrain ξ (x10-3)	Rwp (%)	σXRD
Ni/CeO2-0	5.4153 ± 0.0005	99.3 ± 0.4	5.9	9.3	1.9
Ni/CeO2-6	5.4097 ± 0.0005	100.8 ± 0.3	6.4	9.2	1.7
Ni/CeO2-12	5.4073 ± 0.0005	101.1 ± 0.5	7.1	9.5	1.3
Ni/CeO2-24	5.4088 ± 0.0005	95.1 ± 0.9	7.5	8.8	1.6

The sample’s nitrogen adsorption/desorption isotherms (Figure 2) exhibit a well-defined hysteresis of a Type IV isotherm, indicating the presence of mesopores according to IUPAC²⁹. The coprecipitation synthesis created mesopores with an average size of 9.1 nm and a mesopore volume of 0.031 cm³/g for the Ni/CeO₂-0 sample (Table 2). The presence of the CTAB surfactant in the synthesis decreased the pore volume by 0.003 cm³/g and increased the average mesopore size by 0.4 nm from the Ni/CeO₂-0 to the Ni/CeO₂-6 sample. The mesopore volume remained constant for the Ni/CeO₂-12 sample, while the average mesopore size decreased by 0.5 nm. With the maximum concentration in the Ni/CeO₂-24 sample, the mesopore volume coincided with the value of the sample without CTAB, at 0.031 cm³/g, but with a smaller average size of 8.7 nm. Similarly to the pore information, there were no significant changes in the specific surface area of the samples, with an average area of 124 m²/g. The values with slight differences in specific areas reflect the crystal size values obtained from the XRD analysis, which, despite an increase in microstrain, the structural changes are subtle, having a faint effect on textural characteristics dependent on the crystallographic arrangement.

Figure 2
Nitrogen adsorption-desorption isotherms (filled points correspond to nitrogen adsorption and empty points to desorption), (b) BJH pore size distribution based on the desorption branch of the isotherms for Ni/CeO₂-0, Ni/CeO₂-6, Ni/CeO₂-12, and Ni/CeO₂-24.

Table 2
Textural Properties of the samples.

Sample	SBET (m2/g)	Vol.meso (cm3/g)	Pore Size DistributionBJH (nm)
Ni/CeO2-0	132	0.031	9.1
Ni/CeO2-6	116	0.028	9.5
Ni/CeO2-12	119	0.028	9.0
Ni/CeO2-24	131	0.031	8.7

The Raman spectroscopy also allowed the comprehension of crystallography profile changes involving the CTAB synthesis (Figure 3). An attempt was performed to quantify some of the vibrational modes reported in the literature. The band around 226 cm^-1 is associated with a vibrational mode of NiO. It can indicate crystal defects (CD) caused by interactions between Ni's active phase and Ce's support or a defect caused by the Ce³⁺ Ce⁴⁺ Redox couple³⁰. The band around 276 cm^-1 is associated with the transversal mode of F₁u symmetry³¹. The band at 329cm^-1 is related to the vibrational mode from an O-Ce-O trylayer³¹. The mode around 386 cm^-1 can be associated with NiO species³⁰ and some Ce modes³¹,³², nominated M band (M for multiple) for this quantification. The mode at 409 cm^-1 is associated with the modes from E_g and A₁u symmetry³¹. The band around 444 cm^-1 can be related to a NiO mode from 1P symmetry³². The band at 466 cm^-1 corresponds to the F₂g mode, the fluorite structure of the CeO₂ support in the Raman scattering spectrum³³ (Figure 3). A disturbance in this band is noted, causing the formation of shoulders next to 487 cm^-1, indicating the presence of structural defects in CeO₂. The defect in the band located near 480 cm^-1 indicates a vibrational mode for a Ce³⁺O₈ specie³³. The bands at 550-600 cm^-1 highlight the vacancies in the catalyst, referred to as the defect band. The D₁ band (540-570 cm^-1) is related to extrinsic oxygen vacancies and increases with the concentration of CTAB³³. The D₂ band (around 600 cm^-1) is related to oxygen vacancies in the CeO₂ support³³. The range reported as the statistical residual from the deconvolutions were considered to make the FHWM for each vibrational mode.

Figure 3
Raman spectra of Ni/CeO₂ samples.

The bands related to structural vibrations, as the F₁u, O-Ce-O, M, and 1P were used to deconvolve the peaks in the most grounded way possible. Interactions between nickel and cerium and defects induced by CTAB may differentiate them from what is reported in the literature, requiring a specific study to relate these bands with other data.

Some of the relevant changes are described in Table 3. The CD band has increased from 0.014 on Ni/CeO₂-0 to 0.028 on Ni/CeO₂-12 and Ni/CeO₂-24 samples. The changes in intensity on this band can be associated with the increase in intensity of bands related to Ce³⁺ species, mainly causing a strain effect in structure, as the Ce³⁺O₈ that increases from 0.14 on Ni/CeO₂-0 to .017 on Ni/CeO₂-12 and Ni/CeO₂-24. The D₁ band did not follow the same variability patterns, decreasing 0.016 on Ni/CeO₂-0 to 0.007 on Ni/CeO₂-6, and increasing to 0.02 on Ni/CeO₂-12, and again to 0.022 on Ni/CeO₂-24 sample. The D₂ band followed the same type of variation, decreasing from 0.07 on Ni/CeO₂-0 to 0.054 on Ni/CeO₂-6, increasing to 0.075 on Ni/CeO₂-12, where a slight decrease happened on Ni/CeO₂-24 sample to 0.074.

Table 3
The phonons ratio of the Raman spectra from the samples.

Samples	CD	F2g	Ce3+O8	D1	D2	Total Ratio
Ni/CeO2-0	0.013	0.248	0.140	0.016	0.071	0.488
Ni/CeO2-6	0.019	0.210	0.174	0.007	0.054	0.464
Ni/CeO2-12	0.028	0.211	0.170	0.020	0.075	0.504
Ni/CeO2-24	0.028	0.222	0.170	0.022	0.074	0.516

These profiles can indicate the effectiveness of the synthesis, which, even without CTAB caused a deformation in the ceria fluorite structure evidenced by the formation of shoulders, confirming the existence of vacancies in the bulk of the catalyst, with an increase in band related to defects on structure as the CTAB concentration increased.

The X-ray photon electron spectroscopy (XPS) confirmed that the behavior observed in the structure by Raman is also reflected on the surface. In the cerium spectrum (Ce3d), the peaks attributed to Ce⁴⁺ (u^III, u^II, u, v^III, v^II, and v) and Ce³⁺ (u^I, u⁰, v^I, and v⁰) are observed, where u corresponds to the spin-orbit coupling 3d_3/2 and v to 3d_5/2 with a distance between u and v of 18.6 eV, and the intensity difference Ce⁴⁺/Ce³⁺ is 1.5³⁴,³⁵. These parameters were used to fit the photoemission curves. (Figure 4).

Figure 4
XPS spectra in the Ce3d energy region of Ni/CeO₂ samples.

The spectra of the Ni/CeO₂-0 and Ni/CeO₂-6 samples are more like the characteristic cerium peak, but as the concentration of CTAB increases, the peaks become more dislocated. In the highest concentration in the sample Ni/CeO₂-24, it is possible to see a distortion of the spectrum shape, but it still follows the distance u and v species.

The Ce³⁺ ratio indicates the amount of oxygen vacancies on the surface, varying irregularly, decreasing from 0.38 on Ni/CeO₂-0 to 0.35 on Ni/CeO₂-6 and increasing considerably at Ni/CeO₂-12 and 24 samples (Table 4) coinciding with the values reported from the quantification made from the Raman, where Ce³⁺ related bands decreased on Ni/CeO₂-6 sample but increased at of Ni/CeO₂-12 and Ni/CeO₂-24 samples.

Table 4
Ce³⁺ ratio in the structure of the synthesized samples.

Sample	Ce3+/Ce3++Ce4+
Ni/CeO2-0	0.38
Ni/CeO2-6	0.35
Ni/CeO2-12	0.44
Ni/CeO2-24	0.45

In the nickel XPS spectra (Ni2p), (Figure 5), three peaks corresponding to the Ni2p_3/2 signal are located at 854.4 eV, associated with Ni²⁺ in the form of NiO, 855.8 eV, associated with Ni³⁺ in the form of NiOOH which could correspond to defects at the interface between the active phase and the support due to its strong interaction, and 857.3 eV, associated with Ni²⁺ in the form of Ni(OH)₂. Due to the charge transfer mechanism effect, the satellite signals are located at approximately 6.1 eV of distance³⁶. The strong interaction between nickel and cerium, evidenced by the concentration of Ni³⁺, indicates the degree of nickel detachment from ceria during a catalytic reaction. Lower values of Ni³⁺ concentration indicate a higher degree of nickel detachment in ceria, but a high Ce³⁺ and Ni²⁺ ratio is preferable to dry reforming of methane mechanism³⁰. The Ni²⁺ ratio changes intermittently with the addition of CTAB, decreasing from 0.6 to 0.53 from the Ni/CeO₂-0 to the Ni/CeO₂-6 samples, and there was a sharp increase from 0.53 to 0.81 in the Ni²⁺ ratio from Ni/CeO₂-6 to Ni/CeO₂-12 samples. Finally, the Ni/CeO₂-24 sample shows a 0.7 ratio of Ni²⁺ (Table 5).

Figure 5
XPS spectra in the Ni2p energy region of Ni/CeO₂ samples.

Table 5
Ni²⁺ ratio in the structure of the synthesized samples.

Sample	Ni2+/Ni2++Ni3+
Ni/CeO2-0	0.60
Ni/CeO2-6	0.53
Ni/CeO2-12	0.81
Ni/CeO2-24	0.70

On the oxygen spectrum, the range from 527.7 to 530.6 eV is associated with O^2- ions. From 530.6 to 531.1 eV can be associated as oxygen with ionization of OH^- species. The range from 531.1 to 532 eV is associated with O^- species, part of oxygen from the subsurface, and some weakly adsorbed species on the surface; higher than 532 eV corresponds to absorbed species (AS) (Figure 6)³⁷.

Figure 6
XPS spectra in the O1s energy region of NiCeO₂ samples.

The concentration values of oxygen species (Table 6) can be associated with the formation of oxygen vacancy defects on the surface, decreasing the O^2- concentration by the increase of Ce³⁺ ratio from 0.29 on Ni/CeO₂-0 to 0.66 on Ni/CeO₂-6 sample; this increase can be related to the decrease in Ce³⁺ ratio. The same happens to Ni/CeO₂-12 sample, where the O^2- ratio reaches 0.07, the lowest value from Table 5, associated with the highest value of Ce³⁺ ratio in Table 4. By a survey analysis, the O^2- concentration on Ni/CeO₂-24 sample can be compared better.

Table 6
O^2-, OH^-, and O^- ratios in the structure of the synthesized samples.

Sample	O2-	OH-	O-
Ni/CeO2-0	0.29	0.07	0.31
Ni/CeO2-6	0.66	0.05	0.23
Ni/CeO2-12	0.07	0.08	0.20
Ni/CeO2-24	0.36	0.29	0.27

The Ce3d, Ni2p, O1s, and C1s bands were quantified on surveys spectrums using a Shirley function to subtract the background (Figure 7). There is an error on the survey from NiCeO2-6 sample, where the data on the region from 900 to 1000 eV was not collected properly, making the quantification from this sample less precise. It is possible to see a typical profile from Ni/CeO₂-0 to Ni/CeO₂-12 samples, with a different profile on Ni/CeO₂-24, where the format of Ce3d and Ni2p region changed, with a huge intensity decrease on O1s and C1s energy region.

Figure 7
XPS surveys of Ni/CeO₂ samples.

The concentration of cerium, as the peak profiles kept the same, from 0.36 Ni/CeO₂-0 to 0.35 Ni/CeO₂-12 samples, with a considerable increase to 0.51 in Ni/CeO₂-24 sample. The nickel concentration from all samples shows a close value of 0.06. The oxygen concentration decreased with the rise of CTAB concentration on synthesis, which can be associated with the oxygen vacancy concentration formation. The changes in carbon concentration may be attributed to some organic compounds on the surface that are not related to CTAB (Table 7).

Table 7
Ce, Ni, O and C ratios in the structure of the synthesized samples.

Sample	Ce	Ni	O	C
Ni/CeO2-0	0.36	0.05	0.31	0.28
Ni/CeO2-6	0.35	0.07	0.30	0.28
Ni/CeO2-12	0.35	0.05	0.25	0.35
Ni/CeO2-24	0.51	0.07	0.09	0.33

With the images obtained from the SEM, the morphology and size distribution of the samples were analyzed, which showed similar appearance and size distribution, except for the Ni/CeO₂-6 sample, which exhibited a larger size variability, and a more brittle morphology compared to the other samples. The Ni/CeO₂-12 sample also has a brittle morphology but with a more defined size dispersion, which may be attributed to the effect caused by the microstrain from Rietveld refinement, as cited before (Figure 8).

Figure 8
SEM Images of the samples (a) Ni/CeO₂-0, (b) Ni/CeO₂-6, (c) Ni/CeO₂-12 and (d) Ni/CeO₂-24.

No significant change in grain size was observed between the samples, with a small decrease but a high standard deviation. The changes were predominantly morphological (Figure 9).

Figure 9
Grain size count histogram of the samples (a) Ni/CeO₂-0, (b) Ni/CeO₂-6, (c) Ni/CeO₂-12 e (d) Ni/CeO₂-24.

4. Conclusions

By adding CTAB in the sol-gel synthesis was possible to see the increase in the characteristics of Ni/CeO₂ necessary to improve the catalytic reaction, as the oxygen vacancy, which may increase the microstrain and defects in bulk and on surface phase from the catalyst. The presence of CTAB causes slight differences in specific areas, maintaining it high, with significant differences in oxidative states from the metals, causing a formation of a less oxidate state for cerium and nickel at high concentrations. The bulk characteristics resulted in a fluorite structure deformed to all the samples synthesized, with an increase in defects intrinsically from oxygen vacancies evidenced by the ratio from the defect band. The XPS spectra gave some characteristics composition from the surface, with an irregular increase in oxygen vacancies from the cerium support evidenced by the Ce³⁺ concentration and variability on nickel-cerium interaction evidenced by the Ni³⁺ concentration. An increase in reduced species on the surface was observed when CTAB reached concentrations of 12 mmol and 24 mmol. The concentration of the 6 mmol sample was not high enough to cause a difference in reduced species on the surface from this sample, but it had a more substantial effect in the bulk phase. The bulk characteristics follow the same pattern from the surface, which increases the reduced species in the bulk phase.

5. Acknowledgements

The São Paulo State Research Foundation (FAPESP, grants #2023/03754-8, #2023/04143-2, #2022/14265-5, and #2021/05246-4) provided financial support for this work.

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Publication Dates

• Publication in this collection
  11 Oct 2024
• Date of issue
  2024

History

• Received
  02 Apr 2024
• Reviewed
  04 Aug 2024
• Accepted
  01 Sept 2024