Combined hydrothermal and mechanochemical control of structural modifications of zirconium dioxide for catalytic applications
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As featured in: See J. E. Olszowka et al., RSC Mechanochem., 2025, 2, 209. Showcasing research from Dr. J.E. Olszówka’s team, Department of Nanocatalysis, J. Heyrovsky Institute of Physical Chemistry of the Czech Academy of Sciences, Prague, Czech Republic. Combined hydrothermal and mechanochemical control of structural modifi cations of zirconium dioxide for catalytic applications In this work the infl uence of hydrothermal treatment and subsequent mechanochemical treatment on the porous and crystalline structure of precipitated zirconium dioxide was studied. Such new synthetic protocol enabled to obtain purephase materials with the stable defected structure for catalytic applications as revealed by as indicated by the results obtained using XRD and UV-vis spectroscopy. Image reproduced by permission of J. Olszówka from RSC Mechanochem., 2025, 2, 209. Image created by Karolina Simkovicova Image partly generated with AI. Registered charity number: 207890 rsc.li/RSCMechanochem
Combined hydrothermal and mechanochemical control of structural modifications of zirconium dioxide for catalytic applications† V. Sydorchuk, a S. Levytska, b O. Kiziun, b L. Vasylechko, c K. Simkovicova, a S. Valtera, a B. E. Billinghurts, d S. Vajda a and J. E. Olszowka * a The influence of hydrothermal treatment (HTT) and subsequent mechanochemical treatment (MChT, milling) on the porous and crystalline structure of precipitated zirconium dioxide was studied. It has been established that HTT at 300 °C promotes the transformation of amorphous ZrO 2 into a pure monoclinic phase, as well as the formation of a uniform mesoporous structure which has higher thermal stability. Soft dry milling (300 rpm, 0.5–1 h) of hydrothermally modified monoclinic ZrO 2 causes the introduction of defects into its structure without a noticeable change in the phase composition. The presence of defects is confirmed directly using UV-vis spectra and indirectly by the manifestation of photocatalytic activity of milled samples under visible irradiation. Importantly, it is found that after calcination of milled samples at 500 °C a high fraction of defects remains preserved which opens up the potential of using zirconium dioxide modified in this way as a catalyst or catalytic support with added specific properties offered by defects. 1. Introduction Zirconium dioxide (ZrO 2 ) is a versatile material thanks to the tunability of its physicochemical characteristics. ZrO 2 has been receiving increasing attention both as a heterogeneous catalyst and catalyst support since it has adjustable phase composition, redox and acid–base properties, and defects, notably oxygen vacancies, which affect its own catalytic performance and/or of the catalytic moieties dispersed on it. 1–5 In particular, ZrO 2 has been used in a wide spectrum of industrially relevant processes such as water–gas shi, selective reduction of NO 2 , or biodiesel production. ZrO 2 combined with active metals or metal oxides dispersed on its surface is used in hydrogenation 6,7 or ketonization 7 reactions, as well as in CO 2 and CH 4 conversion 1,8 via CO 2 methanation or dry methane reforming (DMR). 9 Its role is still not fully understood, but theory calculations indicate that oxygen vacancies signicantly lower the adsorption energy of CO 2 and promote its dissociation 10,11 with the added benetof suppressing coke formation. 12,13 One of the most important properties is the phase composition of ZrO 2 along its porous vs. crystalline structure, both highly stable in a wide range of applied temperatures. Coking resistance and hydrothermal stability are other major challenges for catalytic materials. 1,4,9 Bulk zirconia is hardly reducible, as demonstrated by the high oxygen vacancy formation energies observed by EPR; 14 however, reducibility can be altered by lowering ZrO 2 particle size. Under-coordination and nanostructuring can strongly affect the properties of ZrO 2 making non reducible bulk zirconia reducible at the nanoscale, impacting its role as an oxide support as well. 1,4,9 Thus, tailored synthesis protocols offer ways of tuning the structure, morphology and chemical nature of ZrO 2 for catalytic applications. 12,15–21 The inuence of ZrO 2 precipitation conditions on its porosity and specic surface area value is relatively well described in the literature. 17,22–24 However, the disadvantages of precipitated ZrO 2 are the signicant content of micropores and an underdeveloped mesoporous structure as well as an amorphous state. 25,26 Also, the rapid sintering of micropores and a dramatic decrease in the specic surface area with increasing temperature have been observed. 27–30 Post-synthetic modication is necessary to transform amorphous ZrO 2 into a crystalline state. Calcination in air, which is most oen used for this purpose, results in the formation of an unstable tetragonal phase at low temperature. The complete transformation of tetragonal ZrO 2 into a monoclinic phase does not occur even at 500 °C 31,32 and is a Department of Nanocatalysis, J. Heyrovsk´ y Institute of Physical Chemistry v.v.i., Czech Academy of Sciences, Dolejˇ skova 2155/3, 18223 Prague, Czech Republic. E-mail: [email protected]; Tel: +420 266053535 b Institute for Sorption and Problems of Endoecology of the NAS of Ukraine, 13 Naumova St., Kyiv 03164, Ukraine c Semiconductor Electronics Department, Lviv Polytechnic National University, 12 Bandera Str., 79013 Lviv, Ukraine d Canadian Light Source Far-Infrared Beamline, 44 Innovation Blvd, Saskatoon, SK S7N 2V3, Canada †Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4mr00094c Cite this: RSC Mechanochem.,2025,2, 209 Received 19th August 2024 Accepted 29th November 2024 DOI: 10.1039/d4mr00094c rsc.li/RSCMechanochem © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Mechanochem.,2025,2,209–223 | 209 RSC Mechanochemistry PAPER Open Access Article. Published on 11 December 2024. Downloaded on 12/9/2025 2:02:17 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue
also accompanied by a signicant decrease in the specic surface area. 33 It appears that it is more advantageous to use hydrothermal treatment (HTT) as a post-synthetic modication of precipitated ZrO 2 by either processing the wet gel or the dried xerogel. 34 Hydrothermal processes are distinguished as a technique for the synthesis and modication of the structure of porous oxides and nanomaterials 31–34 and excel as simple and cost-effective alternative methods. 35–44 A specic feature of wet gels HTT is the possibility of obtaining materials that simultaneously have high specic surface area and accessible pore size as was shown for zirconium oxide. 26,45 Such modied materials are moreover resistant to sintering, 46 as a co-precipitated composition, e.g., CeO 2 –ZrO 2 . 47,48 Ball milling is considered another environmentally friendly and effective method used for the preparation of oxide materials, including catalysts 46,49–51 with an added important feature that nanostructured milled oxides contain various defects (e.g., oxygen vacancies). The latter can act as active sites in different catalytic processes, 52–55 e.g., in test photocatalytic reactions of safranin degradation. 27 It should be noted that studies on ZrO 2 milling are not very numerous, do not have a systematic character and are not focused on obtaining catalysts. 56–60 As a rule, they are devoted to the milling of monoclinic ZrO 2 with its transformation into unstable cubic and tetragonal phases. The effect of milling of ZrO 2 on its porous and surface structure has not been studied, to the best of our knowledge. The purpose of this work is to ll the gap in the synthesis of ZrO 2 structures with controllable and thermostable porosity, crystallinity, specic surface area, surface chemistry, redox properties and defects by a unique combination of hydrothermal and mechanochemical routes and explore the properties of this new class of materials. 2. Experimental 2.1 Precipitation An 11.5 M ammonium hydroxide solution (28%, Sigma-Aldrich, analytical grade) was gradually added to the 0.35 M solution of zirconium(IV) oxynitrate dihydrate (99%, Sigma-Aldrich, analytical grade) under intensive stirring (500 rpm) while monitoring the pH value. During this process, a white gelatinous precipitate was formed. The pH values at the end of the precipitation were chosen to be at the edges of the interval favorable for the formation of the monoclinic phase, 9.5 and 7.0, respectively. The resulting gel was aged in the mother solution for 24 hours at room temperature. Aerward, it was separated by decantation and then washed on a lter to the neutral pH value with distilled water and compacted to a water content of approximately 85% resulting in a wet gel. Part of the obtained wet gel was dried for 48 hours at room temperature to form a dried xerogel. 2.2 Modication procedures The obtained ZrO 2 in the form of a wet gel and a dried xerogel was subsequently subjected to hydrothermal treatment (HTT) at 260–320 °C and equilibrium vapor pressure for 5–7 hours. HTT Scheme 1 Preparation of ZrO 2 using procedures (a) previously described and (b) used in this work. 210 |RSC Mechanochem.,2025,2,209–223 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Mechanochemistry Paper Open Access Article. Published on 11 December 2024. Downloaded on 12/9/2025 2:02:17 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online
was carried out in Teon-lined steel autoclaves with a volume of 45 mL. In both cases, the wet gel and dried xerogel were loaded in a quartz tube that was placed in an autoclave. 15 mL of water was added to the bottom of the autoclave in the processing of the dried xerogel so that HTT was carried out in the vapor phase. 34 Hydrothermally treated samples were dried at 100 °C for 6 hours. The difference between the HTT option proposed in this paper and the previously described treatments 40–45,47–49 can be seen in Scheme 1 presented below. In the rst case (Scheme 1a), HTT of the initial reaction mixture is used, resulting in the formation of powders. Their particles can be porous but in general, they are loosely dispersed materials. 61 In the second case (Scheme 1b), washed wet gels or the dried xerogel can be subjected to HTT, the products of which are consolidated or porous materials. 61 Hydrothermally treated dried xerogels were subjected to mechanochemical treatment (MChT) in air (dry milling) at 300– 500 rpm for 30–60 min. MChT was performed using a planetary ball mill Pulverisette-7, premium line (Fritsch Gmbh) with a vessel of zirconium dioxide. 134 Zirconium dioxide balls with a diameter of 5 mm and a total mass of 91.5 g were used as working bodies. The use of a zirconia ball-milling assembly was chosen to reduce the introduction of contaminants into the product. 62 The ratio of the mass of the balls to the mass of the sample (ball-to-powder, BPR) was 10, 13, and 25. The prepared samples, their designations and synthesis conditions are listed in Table 1. 2.3 Physicochemical characterization The porous structure of ZrO 2 samples was characterized using the adsorption–structural method. The isotherms of nitrogen adsorption–desorption were obtained using an automatic gas adsorption analyzer ASAP 2405N (Micromeritics Instrument Corp.) aer outgassing the samples at 150 °C for 20 h. The specic surface area S, volume of mesopores V me , and volume of micropores V mi were calculated from these isotherms using the Brunauer–Emmett–Teller (BET), Barrett–Joyner–Halenda (BJH), and t-methods, respectively. Sorption pore volume Vwas determined at a relative pressure of nitrogen p/p 0 close to 1. The curves of pore size distribution (PSD) were then plotted using the desorption branches of isotherms. X-ray phase analysis and structural and microstructural characterization of the samples were performed by using a DRON-3M diffractometer (CuKaradiation, l=0.154 nm), fullprole Rietveld renement was performed by using WinCSD soware 63 to obtain lattice parameters of the monoclinic ZrO 2 structure, and microstructural parameters of the samples (average grain size Dand microstrains h3i) were derived by full prole Rietveld renement. The Garvie–Nicholson method allows determination of the molar content of the monoclinic phase X m according to the following equation: 64 X m =[I m (111) + I m (−111)]/[I m (111) + I m (−111) + I t (101)], where I m (111) and I m (−111) denote the intensity of peaks of the monoclinic phase at 28.2 and 31.3°, respectively, and I t (101) denotes the intensity of the peak of the tetragonal phase at 30.4°. The volumetric fractions of the monoclinic phase V m were calculated using the so-called Toraya method based on only some X-ray diffraction (XRD) peak intensity values: 56,57 V m =1.311X m /(1 + 0.311X m ) Table 1 List of samples with their method of preparation. The symbols H, M and T denote hydrothermal treatment, milling and thermal treatment, X and G –xerogel and gel, and the numbers after them –the conditions of these treatments in terms of time and temperature that were used for the designation Sample designation Method of preparation N1-9.5 Initial pH 9.5 N2-9.5_HX HTT xerogel 260 °C 5 h N3-9.5_HXM_300 HTT xerogel 260 °C + MChT 300 rpm 0.5h BPR 10 N4-9.5_HXM_450 HTT xerogel 260 °C + MChT 450 rpm 0.5h BPR 10 N5-9.5_HX HTT xerogel 320 °C 5 h N6-9.5_HXM_300 HTT xerogel 320 °C 5 h + MChT 300 rpm 0.5h BPR 10 N7-9.5_HG HTT gel 260 °C 5 h N8-9.5_HGM_300 HTT gel 260 °C 5 h + MChT 300 rpm 0.5h BPR 10 N9-7 Initial pH 7 N10-7_HX HTT xerogel 260 °C 5 h N11-7_HXM_300_1h_10 HTT xerogel 260 °C 5 h + MChT 300 rpm 1h BPR 10 N12-7_HXM_300_0.5h_13 HTT xerogel 260 °C 5 h + MChT 300 rpm 0.5 h BPR 13 N13-7_HXM_300_1h_13 HTT xerogel 260 °C 5 h + MChT 300 rpm 1h BPR 13 N14-7_T Initial pH 7 +TT 500 °C N15-7_HG HTT gel 300 °C 5 h N16-7_HG_T HTT gel 300 °C 5 h + TT 500 °C N17-7_H7hG HTT gel 300 °C 7 h N18-7_H7hG_T HTT gel 300 °C 7 h + TT 500 °C N19-7_H7hGM_500_0.5_25 HTT gel 300 °C 7 h + MChT 500 rpm 0.5h BPR 25 N20-7_H7hGM_500_0.5_25_T HTT gel 300 °C 7 h + MChT 500 rpm 0.5h BPR 25 + TT 500 °C N21-7_ H7hGM_400_0.5_10 HTT gel 300 °C 7h + MChT 400 rpm 0.5 h BPR 10 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Mechanochem.,2025,2,209–223 | 211 Paper RSC Mechanochemistry Open Access Article. Published on 11 December 2024. Downloaded on 12/9/2025 2:02:17 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online
Accordingly, the content of the tetragonal phase X t (V t ) was calculated as follows: X t (V t )=1−X m (V m ) The size of the crystallites (D t and D m ) was calculated according to Scherrer's formula using the most intense reexes of each phase. The curves of differential thermal analysis (DTA) and thermogravimetry (TG) were recorded using the Derivatograph-C apparatus (F. Paulik, J. Paulik, L. Erdey) in the temperature range 20–1000 °C at a heating rate of 10 °C min −1 . The initial mass of the sample was about 200 mg. The UV-visible diffuse reectance spectra were recorded with a UV-vis/DRS Lambda 35 spectrometer (PerkinElmer Instruments) in the range of 200–600 nm. MgO was the reference sample. The absorption edge lwas determined based on the spectra plotted in the Kubelka–Munk equation coordination. The bandgap E g was determined using a Tauc plot. 58 The morphology of the ZrO 2 powders was characterized with a Hitachi S4800 scanning electron microscope (SEM) equipped with a Nanotrace electron diffraction EDX detector (Thermo Electron). To study the lattice vibrations of ZrO 2 ,diffuse reectance measurements were conducted in the Far-Infrared (Far-IR) region (30–600 cm −1 ), at the Canadian Light Source (CLS) farinfrared beamline using a Bruker IFS125HR spectrometer equipped with a T222 Mylar beamsplitter. The infrared beam was directed out of the sample compartment into a Pike Instruments diffuse IR accessory and then to a QMC superconducting niobium Transition-Edge Sensor (TES) bolometer. A synchrotron was used as a source because of its high ux of photons below 200 cm −1 . The ZrO 2 samples were packed into ceramic sample cups and placed into the environmental chamber of the DiffusIR accessory. The chamber with an internal volume of 63 cm 3 was equipped with an Si window. Argon gas was continuously owing through the chamber at a rate of 200 mL min −1 , regulated by a mass ow controller (Brooks SLA5850). All spectra were collected with 4 cm −1 resolution and averaged over 5 sets of measurements each averaging over 2048 scans. Data processing was done using the OPUS soware package to obtain the reectance spectra, which were produced by dividing the ZrO 2 spectra by the spectra of polyethylene powder. The spectra of polyethylene powder were collected directly before and aer the collection of ZrO 2 spectra and using the same conditions and served as a background. Selected samples with a pure monoclinic phase were tested in the photocatalytic degradation of safranin T (ST), a dye which is commonly used as a model compound due to its high stability. 65 Along with the UV-vis spectra, the reactivity is considered an indirect measure for assessing the formation of defects in the structure of the zirconium dioxide at different stages of its preparation (hydrothermal treatment, milling, and post-calcination). Testing of pure monoclinic ZrO 2 samples under visible irradiation was carried out in an aqueous medium using 1 ×10 −5 mol dm −3 solution of ST 59 with a Philips LED cool daylight (100 W) lamp as an irradiation source. A Shimadzu spectrophotometer UV-2450 was used for the analysis of changes in the concentration of dye during the photocatalytic degradation (band at 520 nm). The optimal dose of catalysts, 1 g dm −3 , was used as it was established in other studies. 59 Prior to irradiation, the ZrO 2 dispersed in the ST-containing solution was stirred until an adsorption–desorption equilibrium was established for 1 h; conditions were applied as those in the testing of other oxide photocatalysts. 27,65 3. Results and discussion 3.1 Effect of pH during precipitation 3.1.1 Structural analysis of the samples prepared at pH 9.5. X-ray diffraction was used to determine the crystal structure of the materials. The initial sample precipitated at pH 9.5 is amorphous (sample N1-9.5, diffraction pattern not shown). For the xerogel sample aer HTT (N2-9.5_HX), the reections of the tetragonal phase are very intense (Fig. 1). The content of the particular phase determined based on the diffractograms showed that the volumetric percentage of monoclinic ZrO 2 is 38% for this modied xerogel, see Table 2 (the indicated content refers to the crystalline part of zirconium dioxide). As mentioned above, the transition from the tetragonal to the monoclinic phase occurs already at low temperatures in the case of HTT reaction mixtures of zirconium salt with a precipitant. This is due to the mineralizing effect of anions in the case of direct hydrothermal synthesis of zirconium dioxide. 35,41 On the other hand, these anions can interfere with the preparation of pure ZrO 2 . Simultaneously, dry milling of the ZrO 2 samples, which had been hydrothermally processed as xerogels, gave a very promising result in the formation of the monoclinic phase. For the series of samples with a pH of 9.5 the initial material HTT promotes the formation of crystalline ZrO 2 in all cases, which is represented by a mixture of tetragonal and monoclinic phases; Fig. 1 XRD patterns for ZrO 2 , precipitated at pH 9.5, after HTT and post-milling at 300 and 450 rpm (T –tetragonal phase, peak from plane [101]; M –monoclinic phase, peaks from planes [111] and [−111]). 212 |RSC Mechanochem.,2025,2,209–223 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Mechanochemistry Paper Open Access Article. Published on 11 December 2024. Downloaded on 12/9/2025 2:02:17 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online
however, small content of the amorphous material cannot be excluded. Signicant transformation of the tetragonal phase into the monoclinic one (Fig. 1) reaching 93% was achieved already with soand short ball milling of the xerogel in sample N39.5_HXM_300. An increase in the intensity of milling (sample N4-9.5_HXM_450) does not promote further transformation (Table 2). A similar effect of reexes' intensication can be observed for the gel and its milled counterpart (ESI materials, Fig. S1 and Table S1,†samples N7-9.5_HG and N89.5_HGM_300). At the same time, increasing the temperature of the HTT to 320 °C does not give an additional effect (ESI materials, Table S1,†samples N5-9.5_HX and N6-9.5_HXM_300). It should be noted that in earlier studies, the milling of the tetragonal phase was scarcely studied. 60,66,67 It was suggested that this transformation (tetragonal into monoclinic) becomes possible in the presence of water vapor. 68,69 Indeed, hydrothermal samples, which are subjected to subsequent milling, contain a lot of adsorbed water, as is shown below based on thermogravimetric measurements. Also, there is a certain critical crystallite size (∼46 nm) above which the transformation of the tetragonal phase into the monoclinic phase may occur. 70 However, this mechanism refers to the transformation that occurs during calcination and it is hardly applicable for milling. In this work, the effect is rather attributed to the presence of water and ammonia impurities, which is further discussed in Subsection 3.5. The size of the crystallites was calculated according to Scherrer's formula using the half-width (FWHM) of the most intense peaks of both phases (D t and D m ). As can be seen, the crystallites of both monoclinic and tetragonal ZrO 2 are reduced aer milling (Table 2). To gather further information about changes in the ZrO 2 structure aer milling, FIR spectra were acquired for samples N2-9.5_HX, N3-9.5_HXM_300, and N4-9.5_HXM_450, which have different contents of monoclinic phase and defects. Generally, in a crystalline structure, molecules are arranged in a repeating, three-dimensional pattern, leading to well-dened and sharp peaks in the IR spectrum. Commercial ZrO 2 was used as a reference material. In general, spectra conrm the results of XRD analysis in terms of the tetragonal and monoclinic mixture of phases. Also, the signal in the lattice vibration range which is characteristic of the monoclinic phase becomes more pronounced with milling (Fig. 2). The spectrum of the reference (commercial monoclinic ZrO 2 ) shows all 9 peaks characteristic of the ZrO 2 structure observed by Maczka et al., 73 El Boutaybi et al. 74 and previously by Hirata et al. 75 with doublets at 494 cm −1 and 524 cm −1 . The doublets at 452 and 444 cm −1 and the shoulder at 375 cm −1 are less dened for commercial ZrO 2 compared to the ones presented in the cited work 73 where highly crystalline ZrO 2 , calcined at 1200 °C, was studied. The band at 348 cm −1 is the most prominent feature of monoclinic ZrO 2 . The spectrum of the reference sample also contains intense bands at 411, 262 and 230 cm −1 . For the selected synthesized samples with different tetragonal/monoclinic phase ratios, starting with N2-9.5_HX (the as-prepared hydrothermal material), the features of the spectrum are difficult to distinguish. One possible cause of the loss of spectral details may be a strong scattering on this sample with ne crystallite size. According to the XRD results, the crystal structure of sample N2-9.5_HX is characterized as a mixture of tetragonal and monoclinic phases, reaching about 62% of the former and the rest of the latter (Table 2). The spectrum of this hydrothermal sample is poorly resolved, possibly due to the presence of an amorphous component, as well as adsorbed water and surface hydroxyl groups. Also, the more random distribution of molecular vibrations, leading to a less sharp and oen at IR spectrum, 76 may indicate the lack of a well-dened lattice. With the milling of N2-9.5_HX, the effect of tetragonal into monoclinic transformation is visible already aer treatment at 300 rpm for 30 min (N3-9.5_HXM_300) and even more intense bands appear for sample N4-9.5_HXM_450 (appearance of features originating from translations and vibration of the ZrO 2 molecules). The spectra of these milled samples, containing about 90% of the monoclinic phase, are similar to each other and the bands characteristic for monoclinic ZrO 2 are present at Table 2 Volume content of the monoclinic and tetragonal phases and their crystallite size, calculated using the Scherrer formula Samples Tetragonal Monoclinic V t ,% D t ,nm V m ,% D m ,nm N1-9.5 a —— — — N2-9.5_HX 62 9.9 38 14.2 N3-9.5_HXM_300 7 9.5 93 13.8 N4-9.5_HXM_450 15 8.9 85 12.7 a Amorphous. Fig. 2 Far IR spectra of samples N2-9.5_HX, N3-9.5_HXM_300, N49.5_HXM_450 and commercial ZrO2 (reference). The spectra were obtained at 20 °C with an Si window, and polyethylene was used as a background. Marked +are the bands of monoclinic ZrO 2 (ref. 71) and ;denote bands of tetragonal ZrO 2 , 72 and Abands of tetragonal ZrO 2 . 71 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Mechanochem.,2025,2,209–223 | 213 Paper RSC Mechanochemistry Open Access Article. Published on 11 December 2024. 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406, 347, 262, and 230 cm −1 . The shape of the spectra is strongly reminiscent of that of the commercial sample, suggesting that milling of the N2-9.5_HX hydrothermal sample leads to a more ordered crystal structure with a monoclinic phase. 3.1.2 Structural analysis of the samples prepared at pH 7. It was noted that the monoclinic phase of ZrO 2 is stable in the high-temperature range (200–900 °C), which is favorable for industrial catalytic processes. 4 The usage of a tetragonal phase, which is unstable under these conditions, or a mixture of these two phases is undesirable because the phase shifrom tetragonal to monoclinic is accompanied by an increase in volume by 3–5%, which may cause cracking of ZrO 2 granules. 27,60 Since modication of ZrO 2 obtained at pH 9.5 did not lead to the formation of a pure monoclinic phase even aer HTT at 320 °C, an attempt to reach this by lowering the precipitation pH was made. In terms of pH inuence, it was established in previous studies that monoclinic zirconia is formed from precursor precipitated in the 6.5 to 10.5 pH range while the tetragonal precursor is obtained for the 3 to 4 or 13 to 14 pH range. 65,77,78 Precipitation from nitrate solutions also contributes to the formation of the monoclinic phase during post-synthetic modication. 79 For further investigations, pH 7 was chosen for ZrO 2 precipitation since its lowering should promote the formation of monoclinic ZrO 2 , 21,31,32,34 albeit with a lower specic surface area. 17,23,25,31 HTT of the xerogel, precipitated at pH 7 and 260 °C (N10-7_HX) results in the formation of a sample with a monoclinic phase content reaching 96% but the diffractogram of this sample contains very broad peaks (Fig. 3). This indicates that an imperfect crystal structure with a crystallite size of about 4.2 nm is formed. Subsequent milling of this hydrothermal sample at 300 rpm (N11-7_HXM_300_1h_10, N12-7_HXM_300_0.5h_13, and N13-7_HXM_300_1h_13) practically does not change the diffraction patterns of the resulting samples (Fig. 3). Finally, the elevation of HTT to 300 °C (N15-7_HG) allows for obtaining a pure monoclinic phase with an improved structure and crystallite size of about 12.2 nm. The negligible presence of an amorphous component in the hydrothermal samples can be conrmed by comparing the results of thermogravimetric analysis (DTA-TG) for the initial sample and the sample aer HTT at 300 °C (Fig. S2a and b†). It is known that crystallization of the amorphous precipitated ZrO 2 is accompanied by an intense exo-effect on the DTA curve at 420–450 °C. 26 Indeed, the acquired DTA curve of the initial sample (sample N9-7) contains a sharp exo-effect at 436 °C which corresponds to the crystallization of amorphous ZrO 2 (Fig. S2a†). In comparison, this effect is absent on the DTA curve recorded for the sample aer HTT at 300 °C (N15-7_HG, Fig. S2b†). To completely transform the tetragonal phase into a monoclinic phase, HTT at 300 °C was increased to 7 hours (N177_H7hG). The diffraction pattern of sample N17-7_H7hG contains only peaks attributed to the monoclinic phase (Fig. 4). The milling of this hydrothermal sample at 400 rpm and BPR =10 does not cause a change in the phase composition and the diffraction pattern of the milled sample N217_H7hGM_400_0.5_10 shows only reexes of monoclinic ZrO 2 . Besides, the slight background in its diffraction pattern and calculations indicate the absence of a noticeable amount of amorphous phase. Also interestingly, even harsh milling (500 rpm, BPR 25) of this sample does not lead to broadening of the peaks in the diffraction pattern (N19-7_H7hGM_500_0.5_25 in Fig. 4). This indicates that the crystallite size does not change for this series of samples. The diffractogram shows a lowintensity peak at 2Q=30.18°, indicating the presence of a cubic rather than a tetragonal phase. This can be concluded based on the absence of the peak at 43.2° and two peaks in the range of 73–75° by which tetragonal and cubic phases can be distinguished. 80 Also, the monoclinic phase can be transformed into a cubic phase during milling, as reported in earlier studies. 60,62,66,72,81 It is also important to compare the thermal behavior of the Xray amorphous initial sample (N9-7) with the samples aer HTT Fig. 3 XRD patterns for ZrO 2 , precipitated at pH 7, after HTT additional MChT under different conditions. Fig. 4 XRD patterns for ZrO 2 , precipitated at pH 7, after HTT, additional milling and calcination. 214 |RSC Mechanochem.,2025,2,209–223 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Mechanochemistry Paper Open Access Article. Published on 11 December 2024. Downloaded on 12/9/2025 2:02:17 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online
and milling. It can be seen that the initial sample aer calcination at 500 °C (N14-7_T) contains a predominant monoclinic phase and about 12% of a tetragonal phase (Fig. 5). Calcination of the hydrothermally treated sample (N15_7_HG) at 500 °C results in conservation of the pure monoclinic phase (N167_HG_T). The phase composition of the milled sample does not change aer its post-calcination at 500 °C. Moreover, the position of the main peak of the monoclinic phase also does not change in all these diffractograms and the values of FWHM for this peak are within 0.55–0.57°. Increasing the time of HTT from 5 to 7 h (N17-7_H7hG) allowed a material to be obtained that contains only the monoclinic phase. Therefore, for a more precise analysis of the crystal structure of the pure phase samples presented in Fig. 4, lattice parameters and microstructural parameters (average grain size D and microstrains h3i) were derived by full prole Rietveld renement, Table 3. It should be noted that the lattice parameters of the sample aer HTT at 300 °C are very close to the tabulated data for monoclinic ZrO 2 (JCPDS, no. 65-1025). 71 A comparison of the results presented in Table 3 shows that the most detectable decrease in all lattice parameters, monoclinic angle b, and unit cell volume V(by 0.1–0.4%) is observed aer milling (samples N19-7_H7hGM_500_0.5_25 and N207_H7hGM_500_0.5_25_T). Also, an increase in the concentration of the microstrain value h3iby 52% occurs, similarly as reported in the work 62 in which h3iis approximately 0.00555 aer milling commercial monoclinic ZrO 2 for 30 min at 500 rpm using a stainless-steel assembly. This indicates the introduction of defects into the crystal structure, which takes place during milling. It is also important that post-calcination of the milled sample (N19-7_H7hGM_500_0.5_25) at 500 °C (as in ref. 60) –sample N20-7_H7hGM_500_0.5_25_T, leads to minimal changes in lattice parameters (less than 0.1%), the microstrain values, and average grain size D. The results provide evidence that the introduced defects are quite stable at this temperature. 3.2 Porous structure of the samples prepared at pH 9.5 and 7 It is known that zirconium dioxide, precipitated from aqueous solutions, is a micro-mesoporous material. 26,27 The parameters of the porous structure for all prepared samples are presented in Table 4. It should be noted that only the as-precipitated samples (N1-9.5 and N9-7) contain micropores, the volume of which is given in brackets in Table 4: 0.04 and 0.03 cm3 g −1 , respectively. This is 40–50% of the total pore volume (column 3 of Table 4). HTT of both as-precipitated samples promotes the formation of a developed and uniform mesoporous structure: the volume of mesopores increases to 0.17–0.23 cm 3 g −1 and their size is 7.9–9.7 nm, as can be seen for samples N2-9.5_HX, N5-9.5_HX, N7-9.5_HG, N15-7_HG, and N17-7_H7hG (Table 4). For all Fig. 5 Nitrogen adsorption–desorption isotherms and PSD curves (insets) for samples of ZrO 2 , precipitated at pH 7, after HTT of the xerogel (a) and gel (b). Table 3 Lattice and microstructural parameters (average grain size Dand microstrains h3i) for samples containing a pure monoclinic phase Samples Lattice parameters Microstructural parameters N17-7_H7hG a=5.1484(7) Å b=5.2096(8) Å h3i=0.00208 c=5.3228(8) Å b=99.437(7) o D=26.9 nm V=140.83(6) Å 3 N18-7_H7hG_T a=5.1488(6) Å b=5.1969(8) Å h3i=0.00228 c=5.3193(7) Å b=99.418(6) o D=26.5 nm V=140.41(6) Å 3 N19-7_H7hGM_500_0.5_25 a=5.143(2) Å b=5.203(2) Å h3i=0.00318 c=5.315(2) Å b=99.03(1) o D=32.7 nm V=140.5(2) Å 3 N20-7_H7hGM_500_0.5_25_T a=5.146(1) Å b=5.197(2) Å h3i=0.00325 c=5.319(2) Å b=99.122(9) o D=30.1 nm V=140.5(1) Å 3 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Mechanochem.,2025,2,209–223 | 215 Paper RSC Mechanochemistry Open Access Article. Published on 11 December 2024. Downloaded on 12/9/2025 2:02:17 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online
modied samples, the total pore volume Vcoincides with the volume of mesopores V me . This coincidence, as well as the calculations of microporosity using the t-method, indicates the absence of micropores in all modied samples. One can see that the isotherms recorded for hydrothermal samples of both series contain pronounced capillarycondensation hysteresis which is characteristic of mesoporous materials. This is related to the samples precipitated at pH 9.5 (Fig. S3†). Similar isotherms were obtained for samples precipitated at pH 7 (Fig. 5) which have larger sizes of mesopores –9.7 nm (Table 4, N15-7_HG and N17-7_H7hG). The specic surface area of the sample as-precipitated at pH 9.5 (N1-9.5) is higher than that of the as-precipitated sample at pH 7 (N9-7) which is consistent with the results of earlier studies. 17,23,25 Its HTT leads to a reduction in specic surface area: all hydrothermal samples modied at 260–320 °C have a specic surface area within 60–87 m 2 g −1 (N2-9.5_HX, N59.5_HX, and N7-9.5_HG). In contrast, the HTT of the sample, precipitated at pH 7, in the form of a dried xerogel at 260 °C (N10-7_HX), causes an increase in specic surface area to 184 m 2 g −1 . This is obviously due to the fact that a ne crystalline structure is formed under these conditions, as was shown above according to the XRD data. As a result, the coalescence of these small crystallites leads to the formation of a nely porous structure with a much smaller size of mesopores –3.7 nm –and, accordingly, a higher specic surface area. On the other hand, an increase in HTT temperature to 300 °C leads to the reduction of specic surface area to 60 m 2 g −1 (N15-7_HG). Milling at low and medium intensity (300–450 rpm, BPR 10– 13) causes partial destruction and transformation of the porous structure of all hydrothermally modied samples due to changes in particle morphology and pore shape (N39.5_HXM_300, N4-9.5_HXM_450, N6-9.5_HXM_300, N89.5_HGM_300, N11-7_HXM_300_1h_10, N127_HXM_300_0.5h_13, and N13-7_HXM_300_1h_13). For samples precipitated at pH 9.5, the specic surface area increases slightly aer milling and the volume of mesopores, on the contrary, decreases (Table 4, N3-9.5_HXM_300, N49.5_HXM_450, N6-9.5_HXM_300, and N8-9.5_HGM_300). Milling of the hydrothermal sample with high specic surface area (precipitation at pH 7 aer HTT of the xerogel at 260 °C) is accompanied by its reduction (samples N117_HXM_300_1h_10, N12-7_HXM_300_0.5h_13, and N137_HXM_300_1h_13). The volume of mesopores for this sample also slightly decreases aer milling. But the diameter of the mesopores d me remains unchanged. The change in the shape of nitrogen adsorption–desorption isotherms for milled samples of both series conrms this (Fig. S3†and 5). This can also be seen from the pore volume distribution (PSD) curves by size shown in the inset of Fig. 5a. Increasing the intensity of milling up to 500 rpm at BPR 25 results in more signicant destruction of porosity and a corresponding decrease in specic surface area, volume, and size of mesopores (N19-7_H7hGM_500_0.5_25, Table 4 and Fig. 5b). The PSD curve for this sample contains a second diffuse maximum centered at around 13 nm, which is visible in Fig. S4.†Mesopores with a size greater than 5 nm make a signicant contribution to the total pore volume of the milled samples N11-7_HXM_300_1h_10 and N13-7_HXM_300_1h_13, while for the hydrothermal sample N10-7_HX this contribution is minimal. Consequently, the bi-porous structure is formed under milling. The presence of pores with a larger size – 5–50 nm –should contribute to the acceleration of the diffusion of reagents and products in catalytic processes. It should be noted that the results obtained –multidirectional changes in porosity parameters –coincide with the general trends that are observed during dry milling of porous oxides with different values of specic surface area. 50,76,82 As mentioned above, the porous structure of hydrothermal samples has higher thermal stability, i.e. they are sintered to a lesser extent with increasing temperature. This is mainly because they have a larger pore size and a more uniform pore structure. Thus, if the specic surface area of the as-precipitated sample N9-7 aer post-calcination at 500 °C decreases almost 5 times and is 27 m 2 g −1 (N14-7_T), then it does not change for hydrothermal sample N15-7_HG and has a value of 61 m 2 g −1 (N16-7_HG_T). Moreover, the volume of mesopores in this hydrothermal sample is signicantly higher aer postcalcination. A comparison of samples N197_H7hGM_500_0.5_25 and N20-7_H7hGM_500_0.5_25_T also indicates that the sample, successively modied by HTT and milling, does not change the porous structure aer postcalcination. The described results are well illustrated in Fig. 5b, which shows corresponding isotherms and PSD curves. As can be seen, the pairs of isotherms and PSD curves for samples before and aer post-calcination practically coincide. The monoclinic ZrO 2 phase and the developed uniform mesoporous structure are optimal from the point of view of their application in catalysis. In fact, mesoporous catalysts are the most demanded for many processes because they provide a balance between a good diffusion rate of reactants and Table 4 Parameters of the porous structure of prepared samples Samples S,m 2 g −1 V,cm 3 g −1 d me ,nm N1-9.5 166 0.06(0.04) 3.5 N2-9.5_HX 61 0.20 7.9 N3-9.5_HXM_300 66 0.16 6.5 N4-9.5_HXM_450 67 0.15 3.9 N5-9.5_HX 66 0.23 9.5 N6-9.5_HXM_300 74 0.19 6.5 N7-9.5_HG 87 0.175 6.6 N8-9.5_HGM_300 100 0.165 3.7 N9-7 130 0.07(0.03) 2.8 N10-7_HX 184 0.18 3.7 N11-7_HXM_300_1h_10 149 0.215 3.7 N12-7_HXM_300_0.5h_13 144 0.205 3.7 N13-7_HXM_300_1h_13 139 0.20 3.7 N14-7_T 27 0.07 6.5 N15-7_HG 60 0.17 9.7 N16-7_HG_T 61 0.24 9.7 N17-7_H7hG 48 0.18 9.7 N18-7_H7hG_T 44 0.17 9.7 N19-7_H7hGM_500_0.5_25 28 0.07 3.8; 13.0 N20-7_H7hGM_500_0.5_25_T 28 0.07 3.8; 13.0 216 |RSC Mechanochem.,2025,2,209–223 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Mechanochemistry Paper Open Access Article. Published on 11 December 2024. 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