Sulfur in transportation fuels such as diesel, gasoline, and jet fuel remains a major source of air pollution ^[1]. More stringent environmental regulations for the oil refining industry have been enacted worldwide, and the Euro V emission standards require the sulfur content in gasoline to be less than 10 ppm. However, it is difficult to reduce the sulfur of liquid fuels to a very low level with conventional hydro desulfurization methods without a decrease in octane number. Therefore, new strategies for desulfurization have been explored to meet the urgent need to produce cleaner gasoline ^[2].

Adsorptive desulfurization (ADS) is one of the most promising complementary and alternative desulfurization techniques. Reactive adsorption refers to processes using metal-based adsorbents to capture sulfur to form metal sulfides in the presence of hydrogen ^[3]. The sulfur atom is retained on the adsorbent, while the hydrocarbon portion of the fuel molecule is released back into the process stream. This process can achieve ultra-deep desulfurization with low octane number loss. During the past decade, several reports have been published on fuel desulfurization by reactive adsorption ^{[4, 5, 6]}. The Conoco Phillips Petroleum Company developed what is known as a S-Zorb process using Ni/ZnO as an effective reactive adsorbent for the production of low sulfur gasoline ^{[4, 5]}. In this adsorbent, Ni functions as a hydro desulfurization site, while ZnO takes up the produced H₂S and is simultaneously converted into ZnS. Gao’s group ^[6] studied the use of Ni/ZnO-SiO₂- Al₂O₃ as an adsorbent for the ADS of fluid catalytic cracking (FCC) gasoline in a fixed-fluidized bed reactor at low pressures in the presence of hydrogen. Their results indicated that high temperature, high pressure, high hydrogen-to-oil molar ratio, and low weight hourly space velocity (LHSV) are favorable for improving the desulfurization ability of the adsorbent but are not conducive to maintaining the octane number of FCC gasoline.

Additionally, kinetic experiments in a thermobalance and performance tests in different types of reactors have shown that ferrites can be used as regenerable adsorbents for hot coal gas desulfurization in integrated gasification combined cycle (IGCC) technologies for clean and efficient power generation ^[7]. The H₂S concentration in the outlet gas from a fixed-bed reactor can be as low as a few ppm and the ferrite adsorbent conversion at breakthrough may be above 70%, indicating a high adsorbent reactivity for the sulfidation process ^{[8, 9]}. However, these features are frequently associated with structural changes occurring both in the fresh or regenerated and the sulfided adsorbents, and, unfortunately, these changes are not easy to detect with the usual characterization techniques.

In this study, nanocrystalline ferrite adsorbents were synthesized from stoichiometric metal nitrate and urea mixtures using a microwave assisted combustion method. To evaluate the ADS reactivity over the ferrites, a series of ADS experiments with a model gasoline feedstock (n-heptane containing thiophene) were performed using a fixed-bed reactor. The structural changes occurring in the ferrites during calcination at high temperature or regeneration in oxidizing atmosphere and the metallic sulfides formed during the sulfidation process were investigated in detail by powder X-ray diffraction (XRD) and ⁵⁷Fe Mössbauer spectroscopy.

Nanocrystalline ferrites AFe₂O₄ (A = Mg, Co, Ni, Zn, and CuZn) were prepared by the exothermic reaction of mixtures of metallic nitrates (metallic A and Fe) and urea (CO(NH₂)₂) (as reducing agent) ^{[10, 11]}. Stoichiometric ratios of metallic nitrate (A(NO₃)₂·nH₂O), ferric nitrate (Fe(NO₃)₃·9H₂O), and urea were dissolved in a small amount of doubly distilled water. The molar ratios in the mixture were 1.7(U):1(M):16.65(W), where U, M, and W denote the molar ratios of urea, metal nitrates, and water, respectively. The vessel containing the solution was placed in a microwave oven that was then operated at a maximum power of 800 W for 20 min. The solution boiled and underwent dehydration followed by decomposition with the evolution of large amounts of gas. After the solution reached the point of spontaneous combustion, it instantly became a solid powder accompanied by the release of large amounts of heat and vapor.

XRD analyses of the samples were carried out using a Rigaku D/Max-RB diffractometer with a Cu K_α monochromatized radiation source operated at 40 kV and 100 mA. The XRD patterns were compared with the calculated patterns obtained from the Inorganic Crystal Structure Database (ICSD) using Jade 6.0 software. The crystallite sizes of the prepared and annealed samples were estimated using the Scherrer equation.

⁵⁷Fe Mössbauer spectra were recorded on a Topologic 500 A spectrometer with a proportional counter at room temperature. ⁵⁷Co (Rh) moving in constant acceleration mode was used as the radioactive source. All spectral analyses were conducted assuming a Lorentzian line shape for computer fitting. The Doppler velocity of the spectrometer was calibrated with respect to α-Fe foil.

The adsorption experiments were performed in a fixed-bed reactor at 500 ℃ under the following operating conditions: 0.3 g catalyst diluted to 3 mL with SiO₂, H₂ partial pressure of 1.0 MPa, LHSV of 2 h⁻¹, and H₂/oil of 600. The model gasoline feedstock was n-heptane containing thiophene, with a total sulfur concentration of 3000 mg/L. All of the sulfur contents reported in this work were determined by gas chromatography (GC) analysis. The sulfur removal was expressed as the equation ^[12]: Sulfur removal = (1 − C_t/C₀) × 100%, where C₀ is the sulfur concentration in the feedstock (mg/L) and C_t is the transient effluent sulfur concentration (mg/L) at any time t (min). Additionally, a regeneration experiment for the ferrite adsorbents was conducted at 500 ℃ in air and was considered to be complete when SO₂ was undetectable in the outlet gas.

Figure 1 shows the XRD patterns of representative samples of ferrites synthesized by the microwave combustion method using various metallic nitrates. The pattern of the MgFe₂O₄ sample indicated a low crystallinity, with two broad peaks (around 2θ = 35° and 63°) corresponding to the position of the two main peaks of spinel ferrite. The diffraction peaks of the Co, Ni, and Zn ferrite samples were perfectly indexed to a single phase with cubic spinel structure of space group Fd-3m (standard JCPDS Card Nos. 22-1086 of cubic CoFe₂O₄, 10-0325 of cubic trevorite NiFe₂O₄, and 22-1012 of cubic ZnFe₂O₄), and no secondary phases were detected in the XRD patterns. This confirmed the phase purity of the final products. The broadening of the peaks for these samples indicates the fine particle nature of the ferrite powder ^[13]. Additionally, all the peaks of the CuZnFe₂O₄ sample matched well with those of standard JCPDS Card No. 22-1012 for the cubic ZnFe₂O₄ phase. The positions of the XRD peaks were shifted to higher 2θ owing to the doping of Cu in the zinc ferrite. This shift can be explained by the fact that the doping cation Cu²⁺ (0.073 nm) has a smaller ionic radius than that of Zn²⁺ (0.083 nm) and by a possible redistribution of Zn²⁺ and Fe³⁺ ions caused by Cu²⁺ ions in the tetrahedral/octahedral ionic sites ^[11]. Furthermore, using the Scherrer equation, the particle size of the ferrites was calculated 5.0, 27.6, 44.6, 72.3, and 92.0 nm for the MgFe₂O₄, CoFe₂O₄, NiFe₂O₄, ZnFe₂O₄, and CuZnFe₂O₄ samples, respectively. A small particle size may be expected to reduce the diffusion path and promote the adsorption desulfurization performance of the ferrites to some extent. The doping of Cu in the zinc ferrite resulted in a measurable progressive increase in the degree of crystallinity of the produced cubic phase. The formation of Cu-doped zinc ferrites is more endothermic than the formation of pure Zn-ferrite. When Cu was introduced into the system, less heat would have been liberated, thereby increasing the molecular concentration of Cu at the crystal surface and degree of crystallinity of the product.

Fig. 1. Powder XRD patterns of AFe2O4 ferrites obtained by the microwave combustion method. (1) MgFe2O4; (2) CoFe2O4; (3) NiFe2O4; (4) ZnFe2O4; (5) CuZnFe2O4.

The desulfurization reactivity over the AFe₂O₄ adsorbents was compared under the reaction conditions with model gasoline (n-heptane containing thiophene to a total sulfur concentration of 3000 mg/L). The breakthrough curves of sulfur over the adsorbents are shown in Fig. 2. Generally, there are two basic components in a reaction ADS adsorbent: the reactive part (Ni, Cu, Co, Pt, and Pd), which has a relatively high affinity for sulfur compounds, and the adsorptive part (ZnO, MnO₂, FeO, and CaO) that accepts the sulfur and regenerates the reactive part ^[14]. However, there have only been a few studies of ferrites as ADS adsorbents until now. In our study, the ferrites may have been partially reduced in the H₂ atmosphere during the ADS process, forming metal or alloy and metallic oxide composites to serve as the adsorptive and reactive part, respectively. In a blank experiment, the sulfur removal reached 27%, which indicated that the thiophene was partially thermally-decomposed at the chosen reaction temperature (500 ℃). The MgFe₂O₄ adsorbent initially gave a high sulfur removal of 95%, but was partially deactivated to 75% by 40 h, while the ZnFe₂O₄ adsorbent also exhibited a high initial sulfur removal of 100% but was deactivated by 50% within the same period. This indicated that structural change of the ferrites and the formation of iron sulfides occurred during the desulfurization process.

Fig. 2. Breakthrough curves for adsorptive desulfurization of model gasoline on AFe2O4 adsorbents at 500 ℃, H2 partial pressure = 1.0 MPa, LHSV = 2 h−1, and H2/oil = 600. (1) Blank; (2) CoFe2O4; (3) ZnFe2O4; (4) CuZnFe2O4; (5) NiFe2O4; (6) MgFe2O4.

The results in Fig. 2 show that the desulfurization capability of the ferrite adsorbents during the 40 h continuous ADS process had the following sequence: MgFe₂O₄ > NiFe₂O₄ > CuZnFe₂O₄ > ZnFe₂O₄ > CoFe₂O₄, which may have been caused by the different types of ferrite with probable site migration and/or change in oxidation state. MgFe₂O₄ exhibited the highest ADS properties, which may have been because its spinel structure was not destroyed in the reducing atmosphere used and because the doped Mg increased the interaction between Fe and S compounds, leading to a significant improvement in the desulfurization capability of the adsorbent. Additionally, basic/acid sites provided by adsorbents may promote the decomposition of thiophene ^{[15, 16]}. Mg is a generally basic element which can dramatically promote the decomposition of thiophene, which may also have given MgFe₂O₄ the best desulfurization performance. Furthermore, doping with Cu can substantially increase the overall sulfidation rate of zinc ferrite, because Cu increases the amount of reactive part and thereby the desulphurization performance. This result is similar to those observed for Cu-doped zinc ferrites used as regenerable adsorbents for hot coal gas desulfurization ^[9].

To understand the relationship between the structures of the ferrites and their desulfurization properties in depth, a detailed characterization of the ferrites was carried out. The structure of the ferrites was found to have changed significantly after the ADS experiments, as shown in Fig. 3. These structural changes were observed through the formation of different types of metallic sulfides during the sulfidation process. Typical diffraction peaks at 2θ = 29.9°, 33.8°, 43.6°, and 53.1° corresponding to those of Fe_1-xS (JCPDS No. 29-0725) were detected in the XRD pattern of the MgFe₂O₄ sample. Trace amounts of FeS were also found. The crystalline phases of the AFe₂O₄ (A = Ni, Zn, and CuZn) adsorbents were identified by comparison with standard iron sulfide JCPDS files (hexagonal FeS, No. 37-0477 and cubic FeS, No. 23-1123), which confirmed that the ferrite adsorbents had been sulfided during the ADS process. Especially, the presence of ZnS phase was found in the pattern of the ZnFe₂O₄ sample, which means that ZnO in the ZnFe₂O₄ system acted as a sulfur acceptor via diffused H₂S from the thiophene ^[17]. Additionally, crystalline Co₃Fe₇ (JCPD No. 48-1816) or CoFe (JCPD No. 44-1433) phase was detected in the CoFe₂O₄ sample, and Fe_0.64Ni_0.36 phase (JCPD No. 47-1405) was detected in the NiFe₂O₄ sample. The formation of Fe-base alloy may have protected the metallic phase from agglomeration during the subsequent reduction treatment and desulfurization process ^[12]. However, the formation of Fe-base alloy may weaken the Fe-S bond and adversely affect reactive ADS. Combined with the structural changes and the ADS properties, it can be speculated that the formation of Fe-base alloy works against desulfurization, suppressing the metal to metallic sulfide transformation.

Fig. 3. Powder XRD patterns of AFe2O4 ferrite adsorbents after ADS experiments. (1) MgFe2O4; (2) CoFe2O4; (3) NiFe2O4; (4) ZnFe2O4; (5) CuZnFe2O4.

In spinel ferrites AFe₂O₄ (A = Mg²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, etc.), there are twice as many octahedral B cationic sites as tetrahedral cationic A sites. In normal spinel, A divalent ions occupy the tetrahedral sites and only Fe³⁺ ions occupy the octahedral sites. In contrast, in the inverse spinel, A divalent ions occupy the octahedral sites and Fe³⁺ ions are equally distributed in octahedral and tetrahedral sites ^[18]. ⁵⁷Fe Mössbauer spectroscopy is a tool for clarifying the chemical state and relative amount of iron species in a material, which can assist in understanding the structural changes of ferrites during the ADS process.

Room temperature Mössbauer spectra of representative ferrite samples are shown in Fig. 4. The spectral parameters such as isomer shift (IS, δ), quadrupole splitting (QS, Δ) and hyperfine field (H) were computed and are summarized in Table 1. As shown in Fig. 4, the spectrum of fresh MgFe₂O₄ was fitted well by two doublets with IS = 0.16 mm/s, QS = 0.63 mm/s and IS = 0.34 mm/s, QS = 0.66 mm/s, which can be ascribed to tetrahedrally and octahedrally oxygen-coordinated Fe³⁺ ions in (Mg_1-x²⁺Fe_x³⁺)_A[Mg_x²⁺Fe_2-x³⁺]_BO₄²⁻ magnesium ferrite, where round and square brackets denote tetrahedral (A) and octahedral [B] coordination sites, respectively, and where x represents the degree of inversion ^[19]. Based on the Mössbauer parameters of the doublets for MgFe₂O₄, it was concluded that spinel MgFe₂O₄ had been successfully synthesized by the microwave assisted combustion method. The structural change of the MgFe₂O₄ during the ADS process was determined from measurements of the chemical state of Fe in the spent adsorbent, as shown in Fig. 4(b) and Table 1. The Mössbauer spectrum could be fitted with one doublet and three sextets. The doublet with IS = 0.41 mm/s and QS = 0.94 mm/s associated with the MgFe bimetallic sulfide (MgFeS) accounted for 32.6% of the total spectral area. The sextets with IS = 0.61 mm/s, QS = 0.00 mm/s, and H = 29.7 T could be ascribed to A sites in Fe_1-xS pyrrhotite. The other two sextets, which had IS = 0.11 mm/s, QS = 0.08 mm/s, and H = 21.8 T and IS = 0.05 mm/s, QS = −0.07 mm/s, and H = 19.5 T were attributed to B- sites and C-sites in Fe_1-xS pyrrhotite, respectively. The A-, B-, and C-sites in Fe_1-xS were assigned according to the hyperfine field distribution. The high ratio of Fe_1-xS phase in the spent MgFe₂O₄ sample (ca. 67.4% of the total spectral area) was not sulfurized completely, which explained why the MgFe₂O₄ maintained a high sulfur removal activity after 40 h continuous ADS. Combined with the XRD data for spent MgFe₂O₄, it can be speculated that the MgFe₂O₄ was decomposed and reduced by H₂ during the ADS process, leading to its structural distortion. The sulfur content on the surface of the MgFe₂O₄ increased significantly with elapsed ADS time, which led to it taking on the general formula of ferrous sulfide, Fe_1-xS. This result is similar to the thermodynamic behavior and phase transformation of transition metal-sulfur systems, especially in the Fe-S system ^[20].

Fig. 4. 57Fe Mössbauer spectra of (a) fresh, (b) spent, and (c) regenerated MgFe2O4 adsorbent.

Table 1 57Fe Mössbauer parameters of MgFe2O4 samples. Sample A (%) δ a (mm/s) H (T) Δ (mm/s) 2Γ (mm/s) Site b Fresh 68.6 31.4 0.16 0.34 — — 0.63 0.66 0.47 0.37 A-site in MgFe2O4 B-site in MgFe2O4 Spent 25.9 27.8 32.6 13.6 0.05 0.61 0.41 0.11 19.5 29.7 — 21.8 -0.07 0.00 0.94 0.08 0.22 0.32 1.29 0.30 C-site in Fe1-xS A-site in Fe1-xS MgFeS B-site in Fe1-xS Regenerated 9.2 0.58 — 0.80 0.46 A-site in MgFe2O4 43.0 0.38 51.0 -0.21 0.46 α-Fe2O3 39.9 0.27 — 0.75 0.58 B-site in MgFe2O4 7.9 0.46 — 2.40 0.58 MgFe2O4 with oxygen vacancies a Relative to α-Fe at room temperature. b Site identification considered to be the most probable case.

Table 1 57Fe Mössbauer parameters of MgFe2O4 samples.

For the ADS process, it is crucial to regenerate the adsorbent. In this work, the regeneration of sulfurized MgFe₂O₄ was conducted at 500 ℃ in air and was considered to be complete when SO₂ was undetectable in the outlet gas. As shown in Fig. 4(c), the Mössbauer spectrum of regenerated MgFe₂O₄ could be fitted with a sextet and three doublets. The sextet associated with α-Fe₂O₃ accounted for 43.0% of the total spectral area. The doublet having Mössbauer parameters of IS = 0.46 mm/s and QS = 2.40 mm/s accounted for about 7.9% and was attributed to MgFe₂O₄ with oxygen vacancies, which means that the sulfurized MgFe₂O₄ was re-oxidized after treatment at 500 ℃ for 3 h in air. Moreover, about 50% of the MgFe₂O₄ was regenerated completely. As shown in Table 1, the two doublets with IS = 0.27 mm/s, QS = 0.75 mm/s and IS = 0.58 mm/s, QS = 0.80 mm/s were assigned to the A-sites and B-sites in MgFe₂O₄, respectively, and were similar to those detected in fresh MgFe₂O₄. Therefore, the regeneration process was effective in eliminating all traces of iron sulfides in the MgFe₂O₄ adsorbent.

The Mössbauer spectra and parameters of fresh, spent, and regenerated NiFe₂O₄ adsorbent are shown in Fig. 5 and Table 2, respectively. The spectrum of NiFe₂O₄ consisted of two clearly split Zeeman sextets. The isomer shifts of the sextets were in the range of 0.27-0.37 mm/s, which is typical of that for Fe³⁺ ions. Bulk NiFe₂O₄ has an inverse spinel structure in which Ni²⁺ occupies B-sites totally while the Fe³⁺ ions are distributed between A- and B-sites as (Fe³⁺)_A[Fe³⁺,Ni²⁺]_BO^{2− [21]}. As shown in Table 2, the fraction of iron ions in (A) and [B] sites in the NiFe₂O₄ sample were 59% and 39%, respectively, which indicated that spinel NiFe₂O₄ had been successfully synthesized by the microwave assisted combustion method.

Fig. 5. 57Fe Mössbauer spectra of (a) fresh, (b) spent, and (c) regenerated NiFe2O4 adsorbent.

Table 2 57Fe Mössbauer parameters of NiFe2O4 samples. Sample A (%) δ a (mm/s) H (T) Δ (mm/s) 2Γ (mm/s) Site b Fresh 61.2 38.8 0.26 0.37 49.6 52.7 0.00 -0.01 0.38 0.29 A-site in NiFe2O4 B-site in NiFe2O4 Spent 36.0 9.8 23.6 7.40 23.2 0.03 0.18 0.17 0.30 0.75 30.7 21.0 19.5 — — 0.03 0.06 0.08 0.68 -0.18 0.67 0.21 0.44 0.48 0.32 A-site in Fe1-xS B-site in Fe1-xS C-site in Fe1-xS Fe3+ in Fe2S3 Fe2+ in NiFeS Regenerated 33.0 0.29 49.5 -0.03 0.39 A-site in NiFe2O4 45.3 0.37 51.9 -0.14 0.32 B-site in NiFe2O4 21.7 0.62 46.1 0.02 0.38 α-Fe2O3 a Relative to α-Fe at room temperature. b Site identification considered to be the most probable case.

Table 2 57Fe Mössbauer parameters of NiFe2O4 samples.

The Mössbauer spectrum of the spent NiFe₂O₄ sample showed three sextets and two doublets. The sextet with the Mössbauer parameters of IS = 0.03 mm/s, QS = 0.03 mm/s, and H = 30.7 T was attributed to A-sites in Fe_1-xS. The other two sextets, which had IS = 0.18 mm/s, H = 21.0 T, and IS = 0.17 mm/s, H = 19.5 T, were attributed to Fe atoms at nonequivalent lattice sites B and C of Fe_1-xS, respectively. The two doublets, which had IS = 0.30 mm/s, QS = 0.68 mm/s and IS = 0.75 mm/s, QS = −0.18 mm/s, were attributed to Fe³⁺ in Fe₂S₃ and Fe²⁺ in NiFeS violarite, respectively ^{[22, 23]}. According to the above analysis, the spinel NiFe₂O₄ adsorbent was almost completely transformed into iron sulfides during the 40-h ADS process. In the dynamic balance of ADS, iron sulfide was reduced by the H₂ to form non-stoichiometric Fe_1-xS phase. Additionally, nickel ferrites have excellent reaction ADS performance, which may be attributed to Ni functioning as a hydrodesulfurization site ^[6].

The spent NiFe₂O₄ adsorbent was regenerated at 500 ℃ for 3 h in air. The Mössbauer spectrum of regenerated NiFe₂O₄ showed three sextets. The two sextets having the Mössbauer parameters of IS = 0.29 mm/s, H = 49.5 T and IS = 0.37 mm/s, H = 51.9 T were attributed to the Fe³⁺ ions distributed between the A- and B-sites of NiFe₂O₄, respectively. The other sextet was attributed to α-Fe₂O₃. Comparison of the Mössbauer spectra of fresh NiFe₂O₄ and regenerated NiFe₂O₄ indicates that the nickel ferrite adsorbent was also successfully regenerated.

As shown in Fig. 6 and Table 3, the Mössbauer spectrum of fresh ZnFe₂O₄ adsorbent showed a doublet with IS = 0.35 mm/s, QS = 0.40 mm/s, which corresponded to Fe³⁺ ions occupying octahedral sites in the normal spinel phase ZnFe₂O₄. The structure of the ZnFe₂O₄ also changed significantly during the ADS process. The appearance of one doublet and four sextets for spent ZnFe₂O₄ may be reasonably assigned to FeZnS sphalerite and Fe_1-xS, respectively. There were four nonequivalent iron sites, A, B, C, and D, in the Fe_1-xS. Under the reducing atmosphere well as the presence of thiophene, the zinc ferrite adsorbent was unstable and readily decomposed into the single oxides ZnO and Fe₂O₃. The ferric oxide was further reduced to metallic iron, which led to the formation of a Fe_1-xS structure mixed with sphalerite during the sulfidation process. However, when the sulfided adsorbent was regenerated at 500 ℃ for 3 h in air, its Mössbauer spectrum (Fig. 6(c)) showed a well-defined doublet and a sextet. The Mössbauer parameters shown in Table 3 enabled these sites to be assigned to the presence of ZnFe₂O₄ and α-Fe₂O₃. Consequently, these Mössbauer results indicate that the regeneration process was effective in eliminating all traces of iron sulfides, although the regenerated adsorbent was incompletely converted to the spinel phase owing to apparent diffusional limitations in the solid-state reaction ^[8].

Fig. 6. 57Fe Mössbauer spectra of (a) fresh, (b) spent, and (c) regenerated ZnFe2O4 adsorbent.

Table 3 57Fe Mössbauer parameters of ZnFe2O4 samples. Sample A (%) δ a (mm/s) H (T) Δ (mm/s) 2Γ (mm/s) Site b Fresh 100 0.35 — 0.40 0.37 Fe3+[O] Spent 26.2 28.4 8.6 24.3 12.5 0.25 0.17 0.80 0.60 0.22 21.5 18.5 31.4 — 11.0 0.08 0.08 -0.23 0.65 0.13 0.32 0.38 0.28 0.65 0.24 B-site in Fe1-xS C-site in Fe1-xS A-site in Fe1-xS Fe2+ in FeZnS D-site in Fe1-xS Regenerated 50.2 0.38 51.5 -0.21 0.38 α-Fe2O3 49.8 0.34 0.46 0.45 Fe3+[O] a Relative to α-Fe at room temperature. b Site identification considered to be the most probable case.

Table 3 57Fe Mössbauer parameters of ZnFe2O4 samples.

Nanocrystalline AFe₂O₄ ferrite adsorbents (A = Mg, Zn, Ni, Co, and CuZn) were successfully synthesized from stoichiometric metal nitrate and urea mixtures using a microwave assisted combustion method. The as-prepared AFe₂O₄ were effective adsorbents for the reactive ADS of model gasoline. The ADS reactivity over the ferrites was found to have the following order: MgFe₂O₄ > NiFe₂O₄ > CuZnFe₂O₄ > ZnFe₂O₄ > CoFe₂O₄. This effect is explained by the fact that the low degree of alloying of Mg-Fe and the doped Mg increased the interaction between Fe and S compounds, leading to a significant improvement in the desulfurization capability of the adsorbent. Additionally, Mg is a typical basic element which can dramatically promote the decomposition of thiophene. XRD and Mössbauer spectroscopy were used to characterize the fresh, regenerated and sulfided adsorbents in detail. The obtained results indicated a change in oxidation state of the adsorbents. During the adsorption process, the ferrites were partially sulfided to the monometallic and/or bimetallic sulfides. However, the ferrite adsorbents were found to be successfully regenerated under oxidizing conditions.