Al₂O₃ Nanorod With Rich Pentacoordinate Al³⁺ Sites Stabilizing Co²⁺ for Propane Dehydrogenation

1. Introduction

Propane dehydrogenation (PDH) reaction is a production process used in industry to produce propylene. This process has several advantages over the traditional steam cracking/catalytic cracking process, including high propylene selectivity, simple raw material and product composition, and easy separation[1,2,3]. The development of this process can help address the issue of increasing demand for propylene while also providing significant economic benefits.

Currently, the industry primarily utilizes Pt-Sn catalysts and Cr-based catalysts for propane dehydrogenation. The former is used in the Oleflex process while the latter is utilized in the Catofin process [1,4]. However, due to their high cost and toxicity (in the case of CrO_X), there is ongoing research focused on developing low-cost and environmentally friendly alternatives [5,6].

Co-based catalysts are a promising candidate due to their ability to activate C-H bonds and high selectivity towards olefins [7]. It is believed that the active site of the reaction is Co(II) on the carrier, which promotes propylene desorption. Under high loading conditions, Co₃O₄ can be easily reduced to metallic Co, leading to propylene cracking [8,9,10]. Stabilizing Co(II) has received significant attention. Yihu Dai et al. [11] synthesized Co/Al₂O₃ using a one-step hydrothermal method. The separated Co²⁺ sites in the resulting catalyst showed high intrinsic activity (specific reaction rate >16 mmol g^-1· h^-1). The tetrahedral Co²⁺ isolation site was stabilized by γ-Al₂O₃ carrier, promoting propylene desorption and inhibiting coke formation and reduction of active Co sites. Sun et al.[12] found that at low loadings, "surface spinel" produced by tetrahedral Co²⁺ during the reaction was the active site; large particles of crystalline-Co₃O₄ were formed when loading was high, leading to increased cracking reactions and methane formation. Designing a more suitable carrier for stable Co(II) species and reducing carbon deposition is necessary.

γ-alumina (γ-Al₂O₃) is commonly used as a catalyst carrier in thermal catalysis due to its large specific surface area, good thermodynamic and chemical stability, among other properties [13,14]. Al₂O₃ contains three different states of Al³⁺: tetrahedral Al³⁺ (Al³⁺_tetra), octahedral Al³⁺ (Al³⁺_oct), and pentacoordinate Al³⁺ (Al³⁺_penta). The unsaturated pentacoordinate Al³⁺ is located on the surface of Al₂O₃ and can effectively anchor the active phase of the catalyst [15,16,17,18]. Liu et al. [19] synthesized stable Ru-CeO_X supported on nano-flower flake Al₂O₃ spheres rich in unsaturated pentacoordinate Al³⁺ which were then used to catalyze the oxidation of propane. Therefore, this paper aims to stabilize Co(II) through the anchoring effect of pentacoordinate Al³⁺ with the goal of improving stability.

Here, a successful synthesis of Al₂O₃ nanorods (Al₂O₃-NR) rich in pentacoordinate Al³⁺ ions was achieved to serve as a support for Co in the PDH reaction. The pentacoordinate Al³⁺ present in the Al₂O₃-NR effectively anchors Co(II), resulting in the formation of the CoAl₂O₄ spinel phase that exhibits high intrinsic activity and stability. In comparison, samples without pentacoordinate Al³⁺, such as Co(II) in Al₂O₃-NS, tend to form the Co₃O₄ phase which can be easily reduced to Co0, leading to rapid deactivation of the PDH reaction.

2. Methods and Materials

2.1. Catalyst Preparation

Al₂O₃ nanosheet was created using a hydrothermal method that had been previously reported. Aluminum nitrate (Al(NO₃)₃·9H₂O) and urea (CO(NH₂)₂) were dissolved in deionized water and stirred for 15 minutes until a uniform solution was formed. The concentration of Al(NO₃)₃ was 0.09 mol/L, with the ratio of Al(NO₃)₃ to CO(NH₂)₂ being 1:9. The solution, which had a pH of approximately 4, was then transferred to a PTFE lined stainless steel autoclave and kept at 100℃ for 48 hours. After natural cooling, the white precipitate was filtered and washed several times with deionized water and anhydrous ethanol. The powder was dried overnight at 100℃ before being calcined in air at a heating rate of 1℃/min for two hours at a temperature of 600℃. This resulted in the creation of Al₂O₃ nanosheet which is referred to as Al₂O₃-NS.

The process for creating Al₂O₃ nanorods is similar to that of Al₂O₃-NS. The ratio of Al(NO₃)₃ to CO(NH₂)₂ remains at 1:9, but the concentration of Al(NO₃)₃ has been adjusted to 0.2 mol/L. The resulting product is referred to as Al₂O₃-NR.

The method used to load Co is the impregnation method. To do this, a specific amount of cobalt nitrate (Co(NO₃)₂·6H₂O) was dissolved in a small quantity of deionized water and loaded onto two different Al₂O₃ carriers using the constant volume impregnation method. The sample was then left at room temperature for 2 hours, dried overnight at 70℃, and roasted in air at 600℃ for 2 hours with a heating rate of 1℃/min. The amount of Co that was impregnated was 5 wt%. The resulting materials were named Co/Al₂O₃-NS and Co/Al₂O₃-NR respectively.

2.2. Catalyst Preparation

The X-ray diffraction (XRD) measurement was conducted on a Rigaku C/max-2500 diffractometer using CuKα radiation filtered through graphite (λ=1.5406 Å), with a 2θ range of 5 to 90°.

The Raman spectrum was acquired using a RenishawinViaReex Raman spectrometer with an Ar ion laser beam of 532 nm at room temperature. The data collection process involved an exposure time of 10 s and 1 accumulation. In the in-situ reaction tank, the inner reaction tank is heated while the outer shell is cooled by cooling water. The gas enters into the void inside the sample tank, flows over the surface of the catalyst in a flooding manner, and then exits. Spectra are collected through the quartz glass window located on top of the sample.

Thermogravimetric analysis (TGA) is conducted using STA449F3 NETZSCH Corp. to examine the carbon deposition of used catalysts. The sample is first preheated in N2 (50 mL/min) at 80℃ for half an hour, and then heated in air (100 mL/min) at a speed of 10℃/min until it reaches 800℃.

The laboratory uses self-made equipment to test H₂-TPR and NH₃-TPD. A desiccant is added before the reaction tail gas enters the TCD detector to absorb water produced by the reaction. For H₂-TPR, an appropriate amount of catalyst is treated at 350℃ under 30 ml/min N₂ for 20 minutes during the pretreatment stage. Then, high-purity helium is used and the sample is lowered to 50℃. During program temperature reduction stage, a helium switch with a 10% H₂/N₂ mixture maintains a flow rate of 30 ml/min and cuts into the inlet channel of the TCD detector after stabilization of its baseline signal. The heating furnace heats up at a rate of 10℃/min until it reaches 900℃ while detecting changes in H₂ signals online using TCD. For NH₃-TPD, dry pretreatment involves placing the sample in a reaction tube and programming temperature from room temperature to reach up to 300℃ at a rate of 10℃/min. He gas flow (30 mL/min) purges for about twenty minutes then cools down to around fifty degrees Celsius before introducing mixed gas consisting of ten percent NH₃/He (30 mL/min) for thirty minutes until saturation occurs; finally, desorption attached takes place in He atmosphere heated at ten degrees Celsius per minute until reaching seven hundred degrees Celsius while detecting outgassing through TCD detection method.

A ThermoFisherK-Alpha X-ray photoelectron spectrometer (XPS) from the United States was utilized to examine the valence state and relative content of elements in the catalyst.

The ²⁷Al MAS NMR was measured using a Bruker 400M high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy with a MAS spin rate of 10 Khz, recovery time of 4 seconds, and pre-scan delay of 6.5 μs.

2.3. Catalyst Tests

The PDH catalytic reaction was conducted in a fixed bed reactor under atmospheric pressure. The quartz tubular reactor had an inner diameter of 6 mm and a wall thickness of 1 mm. A catalyst loading mass of 0.2 g was used, with the catalyst being loaded in the middle of the quartz tube. Quartz cotton was filled at both ends of the catalyst, and 0.5 g of quartz sand was added to the front end. The catalyst underwent heating in nitrogen at a rate of 15℃/min until it reached 580℃, followed by pretreatment in H₂ atmosphere for one hour at 580℃ before conducting the PDH reaction. During this reaction, C₃H₈:N₂=1:19 and total gas flow rate were maintained at 20 mL/min respectively. An on-line gas chromatograph equipped with TCD detector and flame ionization detector (FID) analyzed C₃H₈, C₃H₆, C₂H₄, C₂H₆ and CH₄ gases; propane conversion rates as well as propylene selectivity were calculated accordingly.

Chromatographic data were analyzed using the FID area normalization method. The propane conversion rate X(C₃H₈), propylene selectivity S(C₃H₆), propylene yield Y(C₃H₆) and propylene space-time yield STY(C₃H₆) were determined using the following formula.

$$\mathrm{X}\left({\mathrm{C}}_3{\mathrm{H}}_8\right)=\frac{\mathrm{n}{\left(C_3{H}_8\right)}_{\mathrm{in}}-\mathrm{n}{\left(C_3{H}_8\right)}_{\mathrm{out}}}{\mathrm{n}{\left(C_3{H}_8\right)}_{\mathrm{in}}}\times 100\%$$

$$S\left(C_3{H}_6\right)=\frac{\mathrm{n}{\left(C_3{H}_6\right)}_{\mathrm{out}}}{\mathrm{n}{\left(C_3{H}_8\right)}_{\mathrm{in}}-\mathrm{n}{\left(C_3{H}_8\right)}_{\mathrm{out}}}\times 100\%$$

$$Y\left(C_3{H}_6\right)=S\left(C_3{H}_6\right)\times X\left(C_3{H}_8\right)$$

$$STY\left(C_3{H}_6\right)=\frac{\mathrm{n}{\left(C_3{H}_8\right)}_{in}\times Y(C_3{H}_6)\times M(C_3{H}_6)}{V_m\times m_{cat}}$$

Where n(C₃H₈)in represents the molar concentration of propane in the reactants, n(C₃H₈)out represents the molar concentration of propane in the product, n(C₃H₆)out represents the molar concentration of propylene in the product, M(C₃H₆) represents the relative molecular weight of propylene and Vm represents the molar volume of gas (22400 mL/mol). Mcat represents the mass of catalyst used in reaction and STY is measured in kg(C₃H₆)·h^-1·kg^-1.

3. Results and Discussion

3.1. Catalytic Performances of Co/Al₂O₃ Catalysts

Figure 1 displays the catalytic efficiency of Co/Al₂O₃-NS and Co/Al₂O₃-NR in PDH reaction. During the initial 4 hours of the reaction, propylene selectivity for both catalysts was between 90.5% to 93.5% and 96% to 97.5%, respectively. Methane, ethane, and ethylene were identified as primary by-products with methane content being relatively high. The Co/Al₂O₃-NR catalyst exhibited an induction period of approximately one hour where conversion increased from 11% to 40.2%, followed by a slight decrease from 40.2% to 29.8%. (the deactivation constant kd was found to be at a rate of 0.15 h^-1). In comparison with Co/Al₂O₃-NR, the conversion rate peaked at around 35% for Co/Al₂O₃-NS catalyst before rapidly decreasing down to 17.3% (the deactivation constant kd is measured at 0.34 h^-1). Conversion rates for Al₂O₃-NR were slightly higher than those observed in Al₂O₃-NS while also exhibiting better stability characteristics; indicating that carbon deposits on Al₂O₃-NR surfaces are less or have better carbon storage capacity when compared with their NS counterparts.

Table 1. Physico-chemical and catalytic properties of Co/Al₂O₃ catalysts.

Catalyst	Ypropene, %[a]	kd, h-1[b]	Carbon Deposition, wt%[c]
Co/Al2O3-NS	32.4	0.34	9.16
Co/Al2O3-NR	38.6	0.15	7.54

[a] Ypropene represents the yield of propylene, and the maximum yield is listed in the table. [b] The deactivation rate constant, kd, was calculated based on the equation, ln [(1 - X_final) / X_final] = kd * t + ln [(1 - _Xinitial) / X_initial][20]. [c] Carbon deposition was calculated was calculated based on the TGA profile.

Table 1. Physico-chemical and catalytic properties of Co/Al₂O₃ catalysts.

Catalyst	Ypropene, %[a]	kd, h-1[b]	Carbon Deposition, wt%[c]
Co/Al2O3-NS	32.4	0.34	9.16
Co/Al2O3-NR	38.6	0.15	7.54

[a] Ypropene represents the yield of propylene, and the maximum yield is listed in the table. [b] The deactivation rate constant, kd, was calculated based on the equation, ln [(1 - X_final) / X_final] = kd * t + ln [(1 - _Xinitial) / X_initial][20]. [c] Carbon deposition was calculated was calculated based on the TGA profile.

Figure 2 displays the activation energies of Co/Al₂O₃-NS and Co/Al₂O₃-NR during the PDH reaction. The apparent activation energy for each was measured at different reaction temperatures: 520°C, 550°C, 580°C, 600°C and 610°C. The results indicate that the apparent activation energy for Co/Al₂O₃-NS was found to be 147.2 kJ/mol while that of Co/Al₂O₃-NR was found to be 202.9 kJ/mol. It is evident from these values that the dehydrogenation energy barrier for Co/Al₂O₃-NR was lower than that of Co/Al₂O₃-NS, making it more favourable towards propane dehydrogenation.

A Thermogravimetric TGA analysis was conducted on the used catalyst to determine its coke content. The findings are presented in Figure 3. The temperature range of 100℃ to 300℃ is attributed to the desorption of water molecules from the surface of the catalyst, while 300℃ to 600℃ corresponds to carbon removal from the catalyst surface[21]. After reaching a temperature above 600℃, the curve remains relatively stable. As shown in Figure 3, Co/Al₂O₃-NS has a higher carbon deposition rate at 9.16% compared with Co/Al₂O₃-NR's rate of only7.54%. This suggests that high levels of Co₃O₄ on Co/Al₂O₃-NS's surface lead to severe carbon accumulation and rapid deactivation of the catalyst.

3.2. Bulk and Surface Characterization

To determine the crystal structure and composition information of the material, XRD spectra was used to analyze the catalyst. Figure 4 shows that both Al₂O₃-NS and Al₂O₃-NR detected a peak associated with γ-Al₂O₃ (PDF#29-0063), indicating that the synthesized nanorod Al₂O₃ and nanosheet Al₂O₃ are in γ-Al₂O₃ phase. Additionally, it was observed that the peak height of Al₂O₃-NS is higher than that of Al₂O₃-NR, suggesting that the crystallinity of Al₂O₃-NS is higher. After loading Co, only a peak associated with γ-Al₂O₃ can be detected which indicates that either introduced Co species are uniformly dispersed on the surface of carrier or beyond detection range due to small amount loaded by X-ray diffraction.

The UV-Vis spectra (refer to Figure 4) provides details about the oxidation states and coordination geometry of cobalt species on different morphologies. In the case of Co/Al₂O₃-NR catalysts, two distinct peaks are observed at around 587 nm and 631 nm, which are attributed to the ⁴A₂→⁴T₁(P)d-d transition of tetrahedral (Td) Co²⁺ (the actual number of characteristic peaks is 3, which may be caused by the insufficient resolution of UV-Vis spectrometers)[8,22]. No other peaks associated with other forms of Co were detected in the spectrum, indicating a normal CoAl₂O₄ spinel structure for the sample. Apart from these two peaks mentioned earlier, the Co/Al₂O₃-NS sample also exhibits two broad absorption bands at around 409 nm and 703 nm respectively. These bands are related to coordination-metal charge transfer between O^2-→Co²⁺ and O^2-→Co³⁺ in Co₃O₄. Therefore, it can be concluded that while Co(II) exists mainly as CoAl₂O₄ in Co/Al₂O₃-NR samples; in addition to a small amount of CoAl₂O₄ present in them; large amounts of CO₃O₄ species along with some traces of CO(II) exist within the structure of Co/Al₂O₃-NS samples.

Figure 4. XRD patterns and UV-Vis spectras of Co/Al₂O₃-NS and Co/Al₂O₃-NR catalysts:(a) XRD; (b) UV-Vis.

Figure 4. XRD patterns and UV-Vis spectras of Co/Al₂O₃-NS and Co/Al₂O₃-NR catalysts:(a) XRD; (b) UV-Vis.

To analyze the element composition and chemical state of Co/Al₂O₃, X-ray photoelectron spectroscopy (XPS) was used to intercept information related to Co 2p. Figure 5 displays the XPS spectra of the Co 2p electronic energy levels in both catalysts, while Table 2 presents the binding energy and relative surface concentration of Co analyzed by XPS, along with the calculated ratio of Co²⁺/Co³⁺. No peak corresponding to metal Co0 was observed on either catalyst. The four separate characteristic peaks in all Co 2p nuclear energy level spectra include two spin-orbit double peaks of Co²⁺ and Co³⁺ at positions Co 2p3/2 and 2p1/2, as well as two satellite peaks. The double peaks at position Co 2p3/2 correspond respectively to characteristic peaks of both states [11]. Peak fitting results indicate that surfaces for both types of catalysts are mainly in a state of Co²⁺; however, the surface content is higher for Co/Al₂O₃-NR than for Co/Al₂O₃-NS. Additionally, there is a higher ratio between Co²⁺/Co³⁺ on the former's surface compared to that on latter's surface which suggests that it tends more towards forming CoAl₂O₄.

Figure 5. XPS profiles of Co/Al₂O₃-NS and Co/Al₂O₃-NR catalysts.

Figure 5. XPS profiles of Co/Al₂O₃-NS and Co/Al₂O₃-NR catalysts.

Table 2. XPS results of Co/Al₂O₃-NS and Co/Al₂O₃-NR catalysts.

Catalyst	Surface Concentration		Co2+/Co3+ Molar Ratio	Binding Energy (eV)
	Co, mol %	Co, wt%
Co/Al2O3-NS	1.89	5.90	3.03	779.5
Co/Al2O3-NR	3.29	11.31	3.56	779.4

Table 2. XPS results of Co/Al₂O₃-NS and Co/Al₂O₃-NR catalysts.

Catalyst	Surface Concentration		Co2+/Co3+ Molar Ratio	Binding Energy (eV)
	Co, mol %	Co, wt%
Co/Al2O3-NS	1.89	5.90	3.03	779.5
Co/Al2O3-NR	3.29	11.31	3.56	779.4

Figure 6 displays the Raman spectrum of each catalyst. Co/Al₂O₃-NS exhibits a clear Co₃O₄ structure and five Raman activity modes: A_1g (680 cm^-1), F_2g (616 cm^-1, 523 cm^-1, 191 cm^-1), E_g (480 cm^-1) [23,24]. No peaks related to Al₂O₃ or other Co phases are present. In comparison, Co/Al₂O₃-NR also has peaks corresponding to the Co₃O₄ structure at 680 cm^-1 and 191 cm^-1. Additionally, there are peaks belonging to the CoAl₂O₄ structure that correspond to F_2g (607 cm^-1, 191cm ^-1) and E_g (480cm ^-1) [25]. The peak of CoAl₂O₄ is difficult to observe due to its weak intensity and coverage by Co₃O₄. The A_1g peak in the figure shows that the peak value of Co/Al₂O₃-NR is significantly smaller than that ofCo/Al₂O₃- NS indicating that less surface area on Co/Al₂O₃-NR containsCo₃O₄and its main component is CoAl₂O₄.

Figure 6. Raman spectra of Co/Al₂O₃-NS and Co/Al₂O₃-NR catalysts.

Figure 6. Raman spectra of Co/Al₂O₃-NS and Co/Al₂O₃-NR catalysts.

From the above characterization, it is easy to see that in the Co/Al₂O₃-NS catalyst, Co(II) mainly exists in the form of Co₃O₄, which leads to the rapid deactivation of the catalyst. In contrast, the Co/Al₂O₃-NR catalyst has a small amount of Co₃O₄ and mostly consists of stable spinel structure (CoAl₂O₄), leading to improved stability. To understand the difference between these two catalysts, ²⁷Al MAS NMR spectra were analyzed. Pure Al₂O₃ without Co showed only two different Al species in Al₂O₃-NS: octahedral Al³⁺ at about 4.13 ppm and tetrahedral Al³⁺ at about 61.8 ppm. However, in addition to these peaks, there was also a characteristic peak at about 30.7 ppm in Al₂O₃-NR attributed to pentacoordinate Al³⁺ ions (~18.5%)[26,27]. With the addition of Co, the peak corresponding to the pentacoordinate Al³⁺ ion disappeared and most of the peaks were converted into tetrahedral Al³⁺ ions. This indicates that after being impregnated, the Co particles first occupied the pentacoordinate Al³⁺ position on the surface of Al₂O₃. The disappearance of the Co-loaded NMR peak confirms that this peak is designated as Al³⁺ ions located on the surface of the Al₂O₃ support rather than lattice defect sites uniformly distributed in bulk phase[28]. The pentacoordinate Al³⁺ can anchor active centers, maintain high dispersion and thermal stability. Co(II) loaded onto a pentacoordinate Al³⁺ site on an Al₂O₃ surface is well anchored to prevent formation of a Co₃O₄ phase, which is consistent with Raman spectrum results.

Figure 7. ²⁷Al MAS NMR spectra of Al₂O₃ and Co/Al₂O₃ catalysts.

Figure 7. ²⁷Al MAS NMR spectra of Al₂O₃ and Co/Al₂O₃ catalysts.

Table 3. The proportion of different coordination Al³⁺ calculated from the peak area of ²⁷Al MAS NMR spectra.

Catalysts	Al3+ Coordination/%
	Tetrahedral Al	Pentacoordinate Al	Octahedral Al
Al2O3-NS	31.5	－	68.4
Al2O3-NR	17.2	18.5	64.3
Co/Al2O3-NS	33.8	－	66.1
Co/Al2O3-NR	29.4	－	70.6

Table 3. The proportion of different coordination Al³⁺ calculated from the peak area of ²⁷Al MAS NMR spectra.

Catalysts	Al3+ Coordination/%
	Tetrahedral Al	Pentacoordinate Al	Octahedral Al
Al2O3-NS	31.5	－	68.4
Al2O3-NR	17.2	18.5	64.3
Co/Al2O3-NS	33.8	－	66.1
Co/Al2O3-NR	29.4	－	70.6

The SEM images of the Al₂O₃ support and Co-based catalyst after loading are presented in Figure 8. The Al₂O₃-NR support used in this study has a regular long rod-shaped structure, while the Al₂O₃-NS exhibits a regular sheet-like structure. Upon impregnation with 5%wt Co, there was no significant change observed on the surface of the Co/Al₂O₃-NS catalyst, and its main layered structure remained intact. On the other hand, for Co/Al₂O₃-NR, the main structure still consisted of nanorods with aggregated Co oxide particles appearing at both ends of these nanorods.

3.3. Reducing Ability and Acidity of the Catalysts

H₂-TPR was used to examine how oxidized Co species in two Co/Al₂O₃ catalysts (Figure 9) behave during reduction. The Co/Al₂O₃-NS catalyst has two reduction peaks at 450℃ and 640℃, which correspond to the reduction of Co₃O₄ to CoO and the reduction of CoO to Co0, respectively. On the other hand, the peaks of Co/Al₂O₃-NR that correspond to the reduction of Co₃O₄ are located at lower temperatures 420℃ and 598℃. A comparison between these two catalysts shows that there is a stronger interaction between Co and Al₂O₃-NR under pentacoordinate Al³⁺ ions' influence as evidenced by the shift in position for peak temperature towards higher values for Co/Al₂O₃-NS. Additionally, it is evident that there is less content of Co₃O₄ on the surface of Co/Al₂O₃-NR compared with that on Co/Al₂O₃-NS since its peak temperature for reducing this compound is much lower; thus indicating a tendency for formation into a spinel structure consisting mainly of Co(Ⅱ) and CoAl₂O₄.

Figure 10 displays the results obtained from NH₃-TPD, while Table 4 presents the ratios of various acid strengths. The Al₂O₃ support exhibits weak and strong acids at approximately 160℃ and 450℃, respectively. Upon Co loading, a medium-strength acid emerges on the surface of Co/Al₂O₃-NR and Co/Al₂O₃-NS around 215℃. Furthermore, the strong acid present on Co/Al₂O₃-NS's surface is significantly stronger than that found on Co/Al₂O₃-NR's surface, which could be another explanation for their varying carbon deposition rates.

3.4. Discussion

The Co/Al₂O₃-NR catalyst exhibits superior selectivity and stability in the propane dehydrogenation reaction compared to Co/Al₂O₃-NS. During a 4-hour activity test, the inactivation rate of Co/Al₂O₃-NR was only 0.15 h^-1, significantly lower than that of Co/Al₂O₃-NS, indicating higher stability. To investigate this difference in stability between the two catalysts, we analyzed carbon deposition on spent catalyst using thermogravimetry (TGA). The results showed that carbon deposition was 7.54% for Co/Al₂O₃-NR and 9.16% for Co/Al₂O₃-NS. Additionally, NH₃-TPD characterization revealed that Co/Al₂O₃-NS had a higher proportion of strong acid sites which led to greater carbon deposition and faster deactivation compared to Co/Al₂O₃-NR. These characterizations demonstrate that surface carbon deposition is the primary cause of catalyst deactivation.

The UV-Vis and Raman spectra indicate that Co(II) in Co/Al₂O₃ undergoes partial oxidation to form Co₃O₄. This is subsequently reduced to CoO, then to metallic Co0 (as evidenced by H₂-TPR data). The presence of metallic Co0 leads to propylene cracking and carbon deposition on the catalyst surface. In contrast, when interacting with Al₂O₃, Co(II) in Co/Al₂O₃-NR forms a stable spinel phase called CoAl₂O₄. Compared to Co₃O₄, this spinel phase is less easily reduced (the reduction peak for the spinel in H₂-TPR occurs above 700℃). The UV-Vis spectrum shows that while there is some amount of the spinel phase present on the surface of the catalyst in both types (NS and NR), it mainly exists as Co₃O₄. This conclusion is supported by an A_1g peak at 680 cm^-1 observed in the Raman spectrum. Based on H₂ consumption during H₂-TPR analysis, only a small amount of Co₃O₄ exists in the NR catalyst; most of it exists as stable structured-CoAl₂O₄. Therefore, two states exist for cobalt: one as unstable metallic-Co0 from easily reducible-Co₃O₄ found mostly in NS catalysts leading to deactivation due to carbon deposition; and another state as stable structured-CoAl₂O₄ from less-reducible-spinels found mostly in NR catalysts which do not cause deactivation.

We have analyzed the reasons behind the formation of different Co(Ⅱ) species. Our analysis suggests that the variation in surface coordination of Al₂O₃ carriers results in distinct binding energies between Co(Ⅱ) and Al₂O₃, leading to two different structures: CoAl₂O₄ and Co₃O₄. The ²⁷Al MAS NMR spectrum indicates two states of Al³⁺ on the Al₂O₃-NS carrier - tetrahedral and octahedral - while pentacoordinate Al³⁺ is present as a defect site on the surface of the Al₂O₃-NR carrier. Upon loading, Co(Ⅱ) preferentially binds to pentacoordinate Al³⁺, resulting in spinel phase formation of CoAl₂O₄. This phase is more stable than the Co₃O₄ phase as an active catalyst and less susceptible to reduction by H₂ gas.

4. Conclusions

Two different morphologies of Al₂O₃ were synthesized using the hydrothermal method and then loaded with 5 wt% Co for propane dehydrogenation reaction. The results showed that Co/Al₂O₃-NR exhibited higher selectivity and stability compared to Al₂O₃-NS. This is because Al₂O₃-NR contains a higher amount of pentacoordinate Al³⁺ which can stabilize Co(Ⅱ) particles, forming more stable active phase CoAl₂O₄ spinel that is not easily reduced by H₂.On the other hand, the surface of Al₂O₃-NS does not contain pentacoordinate Al³⁺, resulting in mainly Co₃O₄ as an active phase. However, this phase is easily reduced to metallic Co0 leading to cracking reactions and carbon deposition.In conclusion, modifying the support morphology can further improve the activity and stability of Co-based PDH catalysts.