Surface active holmia/ γ -alumina nanocatalyst: Preparation and characterization

Abstract

Holmia supported γ-alumina nanocatalyst was prepared by impregnation of γ-alumina with aqueous solution of holmium acetate hydrate Ho(CH₃COO)₃.3.5 H₂O. The physicochemical characteristics of the nanocatalyst calcined at 600°C were established by different techniques, using surface adsorption–desorption of N₂ (S_BET), thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and UV–vis diffuse reflectance spectroscopy (DRS). The recorded optical reflectance of the sample showed that the new self-assembled nanocatalyst is excellent as host material for advanced optical applications. Moreover, the catalyst showed enhanced catalytic activity toward Isopropyl alcohol decomposition.

Keywords

Nano-supported catalyst holmium acetate self- assembly band gap energy characterization

1 Introduction

Heterogeneous catalysis, whereby a gas- or liquid-phase reaction is performed over a solid catalyst, is at the core of the modern energy and chemical industries. Most chemical processes both established [1 –3] and emerging [4 –7], are performed using functional nanomaterials as catalysts. Metal oxide nanoparticles (NPs) can exhibit catalytic properties that differ significantly from bulk materials due to geometric and electronic effects that arise when the particles become smaller than 10 nm [8, 9]. Additionally, a clear advantage in catalyst activity and utilization achieved with nanoparticles because as the size decreases, the fraction of surface atoms increases. More interestingly, selectivity and/or activity increase for some reactions when the particle diameters drop below 10 nm, or even further as the diameters approach 2 nm [9, 10].

Among the different transition alumina known, γ-alumina (γ-Al₂O₃) is the most significant with direct use as a catalyst and catalyst support in both the automotive and petroleum industries [11, 12]. The usefulness of γ-alumina can be traced to a favorable combination of its textural characteristics, such as pore volume, and pore size distribution, surface area, and its acid/base properties, which are mainly linked with surface chemical composition, local microstructure, and phase composition [13].

Nevertheless, the chemical and hydrothermal stability of γ-Al₂O₃ is still a critical point for catalytic applications. On the other hand, holmium oxide Ho₂O₃, like other lanthanide sesquioxides (Ln₂O₃), finds its use as a catalyst with acid- base properties [14] and in various optical, ceramic, and chemical applications [15].

Recently, many efforts have been performed to develop an active, selective and stable alumina-based catalyst. Nevertheless, modification with another oxide is considered as one of the most powerful approaches to improve the performance of γ-Al₂O₃ [16]. Therefore, our aim in the present study is to (i) prepare and characterize a supported nano alumina catalyst using a well-known catalytically active rare earth oxide holmium oxide which obtained from the acetate precursor. (ii) Find out the new interface properties that may arise from processing and self-assembly of metal and/or metal oxide NPs over a conventional catalyst γ-Al₂O₃. Therefore, the obtained catalyst was characterized by employing TG/DTA, XRD, FTIR, S_BET, and UV-Vis diffuse reflectance spectroscopy. Moreover, the preliminary test for the catalytic performance of the new nanocatalyst toward isopropanol dehydration at a relatively low temperature of 200°C correlates between the activity and the acid properties of the prepared catalysts.

2 Experimental

2.1 Reagents

Holmium acetate Ho (CH₃COO)_{3 .} 3.5 H₂O precursor was used as provided by Wako-Japan. aluminum nitrate nonahydrate, Al(NO₃).₃9H₂O was used as provided by LOBA Chemie, ammonia solution (33% product of El-Nasr Company, Egypt) and Isopropyl alcohol was 99% pure from Global Specialty Chemicals- Egypt.

2.2 Preparation of γ- alumina support and holmia/ γ- alumina nano-catalyst

Scheme 1

Schematic diagram of the synthesis of nanocrystalline holmia/ γ- Alumina catalyst.

γ- Al₂O₃ as a support material used in this study has been prepared according to a well-established conventional method described in detail elsewhere [17]. First, γ- Al₂O₃ nanoparticles were synthesized by precipitation method. Calculated amount of aluminum nitrate nonahydrate with concentration of 0.5 M was dissolved in bidistilled water at 60°C with continuous stirring; ammonia solution was added dropwise until the pH was equal to 7. The precipitate was dried in an oven at 100°C for 24 h and calcined at 500 °C in a static air atmosphere for 3 hours. The white solid product thus obtained has been structurally verified by X-ray diffraction analysis.

Second, Holmia/ γ-Al₂O₃ catalysts have been prepared applying the impregnation method by dissolving calculated amounts of holmium acetate hydrate (Ho(CH₃COO)₃·3.5H₂O) corresponding to loading levels loading levels of 5 wt% of Ho₂O₃ / 95 wt % of Al₂O₃ (5H/A) and loading levels of 10 wt% of Ho₂O₃ / 90 wt % of Al₂O₃ (10H/A)in an appropriate volume of water (100 ml). The γ-Al₂O₃ support powder particles (5grams) were sprayed slowly (10 min) into the appropriate solution while being continuously stirred. Following the complete addition of the support, the beaker’s contents were maintained stirred for 30 minutes. The suspension thus produced was left in the drying oven at 110°C for 24 hours. (See scheme 1). Calcination of portions of the dried products was performed in a muffle at 600°C for 4 hours in static air atmosphere. The samples were denoted in the text as 5H/A and 10H/A.

2.3 Methods of characterization of the nano-catalyst-

2.3.1 XRD, FTIR, TGA, SEM-EDX, TEM, UV-VIS and N₂- sorption analyses

The thermal studies of the oven dried precursor powder were characterized by TGA/DTA (thermogravimetry/ differential scanning calorimetry) by TA 60H shimadzu thermal analyzer (Japan), on heating small 5 mg up to 600°C at a heating rate of 10°C/min in air atmosphere. The phase composition of the as prepared sample was characterized by X-ray powder Diffraction (XRD). The XRD pattern was carried out by means of a model JSX-60 PA Jeol diffractometer (Japan) equipped with Ni-filtered CuKα radiation (λ= 0.15416 nm) and a generator operated at 40 kV and 30 mA. Based on scans in the range 4°≤2θ≤80°, the 2θ and relative intensity (I/I°) values were obtained for the observed diffraction peaks and matched with those filed in the JCPDS database [18] for phase identification purposes.

Moreover, the elemental composition of the samples was examined by the EDX (Energy-dispersive X-ray spectroscopy) line profiles of the samples. EDX analyses were performed on a model QUANTA FEG 250. FTIR (FT-IR Spectroscopy) spectra were measured at 4000-400 cm^-1 with the resolution of 4 cm^–1, using a model 410 Jasco FT-IR spectrophotometer (Japan). The spectra were taken of lightly loaded (<1%) thin discs of KBr-supported test materials.

The morphology of the particles, as well as the actual composition of the Holmia/ γ- Alumina oxides, were obtained by scanning electron microscopy (SEM) Quanta (FEG 250) and (Joel SEM 5004 LV) and the Transmission electron microscope (TEM and HRTEM: Techni G2 Sprit Twin). UV–vis DRS (Diffuse Reflectance Spectroscopy) spectra of the samples were taken from 3 mm thick, using a UV-3600 plus spectrometer (Shimadzu, Japan). The spectra were collected at 200–900 nm using BaSO₄ as a reflectance reference. The N₂- sorption isotherms were determined volumetrically at –196°C using a micro-apparatus based on the design of Lippens et al. [19]. The test samples were out-gassed at 200°C for 2 h under evacuation at 10^–5 torr. The decomposition of (Isopropyl Alcohol) IPA was carried out in a conventional fixed-bed flow type reactor at atmospheric pressure using nitrogen as a carrier gas. The reaction conditions were: A 500 mg catalyst, 2% reactant of IPA in the gas feed, 50 ml/ min total flow rate, and 190°C reaction temperature.

3 Results and discussion

3.1 Thermogravimetry

The thermal behavior of the uncalcined acetate impregnated alumina sample 10H/A was tested, and the result revealed by TG and DTA analysis is exhibited in Fig. (1). The TG curve show steep several mass loss stages over the temperature range 50–650°C. The first step represents the gradual removal of adsorbed and coordinated water of the crystal lattice near 180°C with a mass loss of about 6%. The corresponding DTA curve Fig. (1), shows that these thermal events to be endothermic in nature. Low temperature, endothermic mass losses are usually ascribed to desorption of physisorbed water molecules. On the other hand, the subsequent mass losses could be attributed to the decomposition of the impregnated Holmium acetate species [20]. The total mass loss consistent with heating the catalyst up to 700°C reaches 16.5%. The very weak and ill-defined DTA- events following the decomposition of the acetate precursor species may refer to strong dispersion of the Ho⁺³ species, i.e. strong precursor support interactions. The removal of volatile species from the acetate precursor during heating of the sample acetate loaded γ-Al₂O₃ may result in the creation of porous oxide with smaller particle size.

Fig. 1

TG and DTA curves recorded for holmia/ γ- Alumina catalyst at the heating rate 10°C/min, in a dynamic (20 ml min^–1) atmosphere of air.

3.2 Characterization of materials

The structure of γ-Al₂O₃ is traditionally considered as a cubic defect spinel- type in which Aluminum atoms occupy the octahedral and tetrahedral sites. The precise distribution of aluminum atoms (and vacancies) is debatable and seems to depend on the preparation conditions of γ-Al₂O₃ [21 –24]. According to our preparation method, the characterization study of the dried aluminum hydroxide and γ-Al₂O₃ revealed that the hydroxide is mainly oxy-hydroxide (AlOOH) and its calcination product is XRD verified as γ-Al₂O₃. The obtained γ-Al₂O₃ support exhibits a relatively high surface area (S_BET = 257.6 m²/g). However, the S_BET of AlOOH was measured and found to be relatively small (S_BET = 16.4 m²/g). On the thermal treatment of the oxyhydroxide (AlOOH), successive dehydration and dehydroxylation processes occur, which leaves the surface more porous and the particles smaller and smaller. Such created pores and smaller nano-like particles lead to an increase in surface area of the formed γ-Al₂O₃.

In our study, The XRD patterns of the two alumina supported Holmia catalysts with 5 wt% and 10 wt% loadings and heat treated at 600°C for 3 hours as well as the XRD pattern of γ–Alumina are shown in (Fig. 2A) and (Fig. 2A-a). The results give evidence that the non-impregnated γ- Alumina with cubic phase could be observed at 2θ values of 37.6°, 45.7° and 66.7° [13 , 21] The XRD patterns of pure and supported γ-Al₂O₃ matched well with that identified in the (JCPDS No. 290063) [18].

Fig. 2

XRD patterns of Holmia/γ-Al₂O₃ catalyst with different loadings heat treated at 600°C for 4 hours Inset XRD pattern of γ-Al₂O₃ (A), FTIR spectra for holmia /γ- Al₂O₃ at different loadings heat treated at 600°C for 4 hours (B), Inset (a) FTIR for γ-Al₂O₃.

On impregnating γ-Alumina with Holmium salt, acetate precursor species, the calcined samples 5H/A and 10H/A do not display any characteristic diffraction peaks in the XRD patterns exhibited by the catalyst (Fig. 2A). These results are in line with the DTA results in suggesting a strong dispersive interaction at the acetate/ γ-Alumina interfaces [25]. The calcination product at 600°C (Fig. 2A) is shown to display nothing but the featureless XRD of the nanocrystalline support. The addition of Holmia as well as the calcination temperature affected the rearrangement of the crystal lattice in such a way that ordered periodic arrangement produced narrower and sharper peaks attributable to the minor changes in the support structure dimensions.

This phenomenon is due to the tendency for minimization of the interfacial surface energy [26].

In support to the XRD analysis, FTIR spectrum of the pure nano γ-Al₂O₃ calcined at 500°C for 3 hours (Fig. 2A) and the different loadings (5 and 10 %) of Holmium acetate on nano γ-Al₂O₃ calcined at 600°C for 4 hours as well as the untreated dried mixture are shown in (Fig. 3). The FTIR spectra are shown in (Fig. 2B) exhibit strong, broad bands in the range of 3448–3450 cm^–1 which are believed to be associated with the stretching vibrations of hydrogen-bonded surface water molecules and hydroxyl groups. Additionally, the bands at ≈ 1634 and 1384 cm^–1 correspond to the existence of a large number of residual hydroxyl groups, which implies the O–H vibration mode of the traces adsorbed water. The bands located at 883-610 cm^–1 can be ascribed to the Al–O vibration of γ-Al₂O₃.These results are in good agreement with the results obtained by Huber et al. [26]. The FTIR spectra of the original mixture (the uncalcined Holmia supported Alumina) shows the same absorption peaks displayed by the Alumina spectrum in addition to few peaks with less intensity due to holmium acetate hydrate Ho(CH₃COO)₃.3.5 H₂O [19]. The bands corresponds to Ho–O is in the range of 561 to 400 cm^–1 and was masked due to the stronger absorption of Al-O bands.

Fig. 3

Uv-Vis Diffuse reflectance spectrum of 5% holmia/ γ- Alumina nano-Catalyst obtained at 600°C.

The FTIR spectrum (Fig. 2B) of the alumina supported catalyst is similar to that of γ-Al₂O₃. The similarity is related to the strong lattice absorption of alumina over the frequency region, where characteristic lattice vibrations of Ho₂O₃ are expected.

Many researchers [27 –31] have been studied extensively nanostructured oxides to realize the effect of crystal structure and particle size on the tunability of specific electronic and photonic properties. The Uv-Vis Diffuse reflectance spectra (DRS) of Holmia / γ- Alumina and γ-Al₂O₃ in the range 200–1000 nm region are shown in (Fig. 3) and (Fig. 3 A), respectively. Comparing with the DRS of the support, the two spectra were found to be not identical, thus revealing different co-ordination symmetries of Ho⁺³ ions. The UV–Vis DRS spectrum of γ-Al₂O₃ (Fig. 4A) shows an absorption band of low intensity at around 270 nm, assigned to electronic charge transfer between the ligands (O²^–, OH^–) and aluminum [32, 33].

Fig. 4

and table 1: EDX patterns (Table 1 inset) and SEM micrographs of 5% holmia/ γ- Alumina nano- Catalyst (5H/A) obtained at 600°C.

However, UV- Vis DRS (Fig. 3) shows the spectrum of Holmia/ γ-Al₂O₃ consists of several absorption sharp peaks approximately at 369, 434, 450, 490, 540, 620 nm. The assignments of the different absorption band were made on the basis of energy level scheme of Ho^{3 +} originating from the ground level 5I₈ to various excited levels within the 4f shell [27, 28]. Simultaneously, Surface modification of the γ-Al₂O₃ expanded its UV-vis wavelength response from 415 nm to 550 nm. It is clear that the spectrum shows a red shift in the band gap transition up to >100 nm compared to the alumina spectrum. Redshifts may be attributed to the charge-transfer transition between the lanthanide f electrons and the γ-Al₂O₃ conduction or valence band, revealing strong interaction between holmium oxide species and the support. The estimated band gap energy of the supported catalyst (Fig. 3) was found to be 2.25 eV approximated using λ (nm) at the UV-Vis absorption edge, according to bensalem et al. [29]. Moreover, the presence of sharp peaks in the spectrum (Fig. 3) may be assigned to the excitonic feature, which means the existence of a very narrow size distribution of small particles. It is well known that the presence of exciton effects reduces the overall energy gap of the compounds [30, 31].

It is worth mentioning that, the tiny blue-shift in the present work might be ascribed to quantum size effective due to the decrease of the crystallite size. These small crystallites are expected to be formed as a result of the evolution of the volatiles during the thermal treatment (up to 600°C) of the dried mixture in order to prepare the supported catalyst. Figure 3 presents digital images of the sample dispersion taken under ambient light and under irradiation with a UV light (365 nm), highlighting the strong, blue fluorescence of the catalyst dispersion resulting from such excitation. Such brightness may be due to the surface efficient donor-acceptor pair recombination. The above findings provide a new chance for the practical display applications. This interesting phenomenon requires further experimental validation, which is under investigation.

Table 1

The elemental identification and quantitative compositional information of the as prepared sample

Element	Weight	Atomic %
C K	22.65	31.63
O K	49.26	51.64
Al K	26.67	16.58
Ho L	1.42	0.14
Total	100	99.99

The elemental identification and quantitative compositional information are further confirmed by energy dispersive x-ray (EDX) analysis. The profile in (Fig. 4) and the values in Table (1) display the relative atomic abundance of O, Al and Ho species present in the surface layers of the as prepared sample. The local composition distribution shows that Oxygen and aluminum are the main constituent elements of the surface and oxygen is the most abundant element. Clearly, the EDX detected values of carbon content in the samples are owing to the carbon tape used in the procedure for samples processing and / or traces of the organic ligands.

3.3 Surface: Morphology and area (SEM –TEM - S_BET)

Fig. 5

TEM micrographs of 5% holmia/ γ- Alumina nano-Catalyst at different magnifications.

The SEM and TEM analysis support the above mentioned results. The overview morphology of the nanocatalyst as well as the particle size of the catalyst, was clearly observed from scanning (SEM) and transmission electron microscopy (TEM) (Fig. 4) and (Fig. 5), respectively. It can be clearly observed from the low magnification SEM micrographs that the powders show many agglomerates with an irregular morphology of highly porous fluffy sample.

The morphology and the crystallites have no uniform size and shape where Small particles are distributed over the larger one. This non uniformity in size and shape is believed to be related to the non- uniform distribution of temperature and mass flow during the solid state reaction and the calcination process. The TEM micrograph (Fig. 5) of the powder catalyst also confirms the presence of agglomerates. Although it is quite difficult to estimate the exact particle size due to the aggregations, the TEM micrograph confirms the nano size crystalline nature of Holmia/γ- Alumina catalyst. The produced alumina particles are mostly nanosized and a close observation by (Fig. 5) indicates that the shape of particles is a quasi-spherical shape. The particles are typically around ≤10–30 nm in diameter. It is worth mentioning that our method of preparation was successful in obtaining fine powder of holmia that spread over alumina particles on heating the mixture up to 600°C for 4 hours.

The N₂ -adsorption-desorption isotherms were measured to investigate the specific surface area. The N₂- adsorption studies at –196°C on 5H/A and 10H/A are shown in (Fig. 6a) and (Fig. 6b), the isotherms generally belong to type IV of the BET classification [34]. The hysteresis loops are nearly of type H3. The general features primarily indicated that the pore structures are slit-shaped and or interplating pores [34]. The BET surface area of γ- Alumina was found to be 257 m² g^–1, While, The BET surface area of 5H/A and 10H/A are about 113.9 and 104 m² g^–1, respectively. The decrease in surface area with increasing the holmia percent loaded on alumina surface may be attributed to the different particle size of holmia in the pores of Alumina as well as different distribution of holmia on the surface of Alumina. Increasing the content of Ho₂O₃ causes agglomeration or grain growth of the particles which leads to enlargement of the particle size and blocking of the pores as a result the surface area of 10H/A decrease [34].

Fig. 6

N₂- adsorption/desorption isotherms of the holmia/ γ-Alumina nano- Catalyst 5 and 10% loadings obtained at 600°C.

3.4 Dehydration of isopropyl alcohol (IPA)

Isopropanol (IPA) decomposition has long been considered as chemical prop reaction for catalytic activity [35], it has been reported that IPA undergoes dehydrogenation to acetone over basic sites or dehydration to propylene over- acidic sites. The supported oxides produced at 600°C were subjected to a preliminary test as a catalyst for the decomposition of IPA vapor at 200°C. The catalytic dehydration of IPA over pure γ-Al₂O₃ calcined at 500°C and over the different loadings of Ho₂O₃ on γ-Al₂O₃ catalysts calcined at 600°C is shown in Table 2. Under our working conditions, it was found that IPA reacts on γ-Al₂O₃ and selectively on the different loadings catalysts to propene. The results indicated that pure γ-Al₂O₃ calcined at 500°C exhibit conversion and yield of propene 70 %. Moreover, as summarized in Table 2, the 5% holmia / γ-Al₂O₃ lead to an increase in the yield of propene to ≈80 %.

Table 2
The catalytic dehydration of IPA over the different loadings of Ho₂O₃ on γ-Al₂O₃ catalysts calcined at 600°C

Catalyst Conversion (%) Selectivity to Propene (%)

5H/A 76 100

10H/A 40 100

Catalyst	Conversion (%)	Selectivity to Propene (%)
5H/A	76	100
10H/A	40	100

The decrease in the activity of the catalysts in higher holmia content (10%) may be related to the fact that pure Ho₂O₃ is an inactive catalyst toward IPA dehydration. The difference between the performances of 5H/A and γ-Al₂O₃ catalysts is explained on the basis of Ho₂O₃ interacting with γ-Al₂O₃ via creation of more acidic sites, which is responsible for the dehydration reaction.

Some important properties and characterization data of the catalyst compared to that for similar compounds known in the literature are given in Table 3.

Table 3

Comparison of some properties, the total conversion% and selectivity% of various catalysts toward the IPA conversion

Catalyst	Cal. Temp. (°C) and Method	TC %	S%	Reaction temp.(°C)	S_BET (cm²/gm)	B.G.E (eV)	Particle size (nm)	Ref.
γ-Al₂O₃	650,5h, (im)	60	60	200	145	–	8	[36]
	550, (Sol-gel)	–	–	–	250	3.35		[37]
	800,1h (pr.)				131.3	7–11		[17]
	900 (pr.)	58	100	400	114	–	–	[38]
0.5% MoO₃/γ-Al₂O₃	900	83	>100	400	62	–	–
	700	70	>100	400	96	–	–
5% Nd₂O₃/ γ-Al₂O₃	650,4h	–	–	–	266	3.54	8–10	[37]
10% Nd₂O₃/ γ-Al₂O₃	650,4h	–	–	–	≥266	3.2	8–10	[37]
19.3% Co₂O₃/ γ-Al₂O₃	400, 4h, (im)	5	80	250	153	–	–	[39]
20% NiO/ γ-Al₂O₃	500 (im)	–	24	280	89.5	–	34	[40]
20% MoO₃/ γ-Al₂O₃	500 (im)	–	60	280	104	–	–
8–12% NiO-MoO₃/	500 (im)	–	38	280	85	–	55
γ-Al₂O₃
10% Ho₂O₃/ γ-Al₂O₃	700	3	78	250	147.7	–	–	[41]
		isobutane to isobutene
γ-Al₂O₃	600, (im)	70	100	200	257	–		This Paper
5% Ho₂O₃/ γ-Al₂O₃	600, (im)	76	100	200	113.9	–		This Paper
10% Ho₂O₃/ γ-Al₂O₃	600, (im)	40	100	200	104	2.25		This Paper

TC=total conversion %, S% =selectivity%, S_{BET =} surface area (cm²/gm), pr.=precipitation, im = impregnation, B.G.E=Band Gap Energy (eV).

4 Conclusion

In this study, thermogravimetry coupled with spectroscopic techniques, were used to characterize Holmia/ γ- Alumina nanocatalyst. The synthesis approach was simple and successful, applying the impregnation method using the acetate precursor of holmia and calcination of the samples at 600°C. Obtaining holmia from holmium acetate as precursor was helpful in creating very small particles whose size detected to be in the range of ≤8–30 nm. TEM and Surface area measurements indicate will dispersion of holmium oxide in the alumina matrix.

The catalyst (5% loading of Holmia/ γ- Alumina) exhibit 100% selectivity and showed better performance for the conversion of Isopropyl alcohol to propene at relatively low temperature 200°C. On the other hand, optical characteristics reveal the synergism that arises at the holmium- alumina interface. The nanosized crystallites with band gap energy 2.25 eV and high surface area of about 113.9 and 104 m² g^–1 of the 5% and 10% loadings of Holmia/ γ- Alumina nanocatalyst, respectively; may expect excellent properties for catalytic and photo-assisted applications.

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