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Article

Design Control of Copper-Doped Titania–Zirconia Catalysts for Methanol Decomposition and Total Oxidation of Ethyl Acetate

by
Tanya Tsoncheva
1,*,
Gloria Issa
1,
Radostina Ivanova
1,
Momtchil Dimitrov
1,
Daniela Kovacheva
2,
Genoveva Atanasova
2 and
Jiří Henych
3,4
1
Centre of Phytochemistry, Institute of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
2
Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
3
Institute of Inorganic Chemistry, Czech Academy of Science, 25068 Husinec-Řež, Czech Republic
4
Faculty of Environment, Jan Evangelista Purkyně University, Pasteurova 3632/15, 40096 Ústí nad Labem, Czech Republic
*
Author to whom correspondence should be addressed.
Symmetry 2022, 14(4), 751; https://doi.org/10.3390/sym14040751
Submission received: 7 March 2022 / Revised: 20 March 2022 / Accepted: 1 April 2022 / Published: 6 April 2022
(This article belongs to the Special Issue Heterogeneous Catalysis: Topics and Advances)
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Abstract

:
This study is focused on the design control of Cu–Zr–Ti oxide composites by the variation of the Zr/Ti ratio and the copper deposition procedure used. For the first time, these ternary composites were obtained by a combination of template-assisted hydrothermal techniques for the preparation of mesoporous ZrO2–TiO2 mixed oxides with diverse compositions, followed by the consecutive chemisorption and hydrolysis of copper ammonia complexes on them. The nitrogen physisorption, XRD, SEM, HRTEM, TPR, XPS, UV-Vis, and Raman spectroscopies were applied for the catalysts’ characterization. Methanol decomposition and the total oxidation of ethyl acetate, both of which with potential for sustainable environmental protection, were used as catalytic tests. The complex relationship between the phase composition, structure, and morphology of titania–zirconia mixed oxides and the state and catalytic behavior of the copper oxide species supported on them was investigated. In comparison with the conventional impregnation technique, the novel preparation procedure revealed the generation of more uniform and homogeneously dispersed needle-like copper oxide crystallites in the mesoporous TiO2–ZrO2 host matrix, which typically ensure improved catalytic performance. The synergistic activity between the loaded copper species and TiO2–ZrO2 support was discussed. All ternary composites exhibited superior catalytic activity in total oxidation of ethyl acetate. The specific behavior of the catalysts in methanol decomposition was related to the irreversible phase transformations by the influence of the reaction medium.

1. Introduction

The deterioration of the environment and the depletion of fossil fuels arising from the accelerated urbanization and industrialization has been a subject of long-term interest [1]. In response, more and more countries have established carbon neutrality targets in the transport sector and hydrogen has been considered as the most promising carbon-free, renewable, and recyclable fuel [2]. However, due to the relatively low volumetric energy density, novel strategies for hydrogen production from renewable sources and its safe storage, transportation, and supply, are needed [3,4]. Liquid organic substances could be an appropriate solution in this aspect and, among them, methanol has received much attention due to its high H/C ratio, abundance, and possibility to release hydrogen at significantly low temperatures [5,6,7,8,9,10,11,12]. The catalytic decomposition of methanol is a simple strategy for hydrogen production without using additional reactants and has the potential for recovering the extra costs related to the endothermicity by the thermochemical recuperation of engine-exhausted gases [6]. The Convention on Long-range Transboundary Air Pollution [13] declared volatile organic compounds (VOCs) as the dominant air pollutants due to their toxic, volatile, and carcinogenic features, also leading to serious climate changes via the formation of ground-level ozone, aerosol, and smog. Recently, the application of catalytic and catalytic–hybrid technologies is recognized as the most effective approach for VOCs elimination, and a number of valuable reviews for the removal of a variety of VOCS have been already published [14,15,16].
For the wide application of the catalytic processes both under industrial and mobile conditions, the design of the catalysts has been determined as the most important factor. Transition metal-based catalysts are considered as a promising alternative to noble metals due to their lower price, resistance to poisons, capability of regeneration, and possibility to produce composites with an unusual texture and certain morphological and electronic characteristics [6,14,15]. Among them, copper-containing materials have been widely studied because of their low cost, abundance, and high activity at relatively low temperatures [15,17,18,19]. However, operating at higher temperatures, the copper catalysts rapidly deactivate due to thermal sintering, coke deposition, or changes in the oxidation state. These disadvantages could be overcome to a high extent using diverse morphologies [20,21] and promotors [22,23,24], porous supports, and appropriate techniques for copper deposition on them [15,25,26,27]. The application of metal oxides as catalyst supports provides an opportunity for either their direct participation in the catalytic process, supplying acid–base or redox-active sites, or indirectly supporting the active component in a highly dispersed state, initiating facile electron transfer and synergistic effects [27]. Many authors have reported that there is a limit in the copper content, which ensures the formation of a monolayer on the support, without the formation of less active bulk CuO particles [19,28]. It is well-established that this threshold depends on the textural and structural properties of the support, especially on the specific surface area and lattice distortion.
Titania is an active reducible support providing strong interaction with the active phase and, more importantly, it is a low-cost, widely available, and environmentally friendly material [6,29,30]. Hu et al. reported improved copper dispersion on titania supports [31]. Larson suggested the presence of monomeric Cu+ and polymeric Cu2+-containing species [32]. In line with [29], the stabilization of two types of Cu2+ ions in different coordination due to the interaction with the oxygen ions in the titania lattice is responsible. Morales et al. [33] reported the formation of isolated, strongly interacting with the support Cu2+ ions, highly dispersed, and in good interaction with the support CuO species, amorphous aggregates, and crystalline CuO, the proportions of which varied with copper loading. Fang et al. [34] demonstrated the tuning of the CuOx–TiO2 interaction by the engineering of the titania’s surface morphology. Okamoto et al. [35] proposed that the copper–titania interaction and the related reductive properties are strongly affected by the titania’s crystal structure, being optimal in the anatase phase.
As compared to titania, ZrO2 possesses better thermal stability and unique surface acidity. Experimental studies have demonstrated that zirconia improves the copper dispersion, stimulates the reducibility, and prevents its sintering [6,33,36,37,38,39,40,41,42]. Wang et al. [39] considered strong interactions between the copper oxide clusters and ZrO2, which not only limited their migration and sintering, but also made the CuO species more active in CO oxidation due to the electron donation from the ZrO2 support. Bianchi et al. [43] reported the participation of the zirconia support in the reverse reaction of methanol synthesis from CO due to its own high hydrogen storage capacity and possibility to provide adsorption sites for the intermediate species. Kundakovic et al. [44] considered the higher catalytic activity of small copper clusters and negligible contribution of bulk CuO particles during the oxidation of CO and CH4. Aguila et al. [18] considered a significant impact of the zirconia surface area on the production of highly dispersed species.
Mixed TiO2–ZrO2 oxides provide new prospects in the improvement of the surface area and mechanical strength of individual oxides with a simultaneous fine-tuning of their acid–base properties [19,45,46,47]. However, to the best of our knowledge, there have been few studies related to CuO supported on mixed TiO2–ZrO2 oxides [19,48,49,50], which motivated expanding the investigations in this field. Recently, Nomura et al. [51] reported that, during the hydrogenation of CO2, the ZrO2 additives to Cu/TiO2 catalysts promoted the formation of surface formate species. Kikugawa et al. [49] observed the co-existence of isolated Cu2+, CuO layers, and CuO particles on the TiO2–ZrO2 support, the ratio of which varied with the copper content. Among them, the CuO layer exhibited the highest reducibility and catalytic activity in CO oxidation.
In our previous study using a template-assisted hydrothermal procedure, successful preparation of mesoporous TiO2–ZrO2 oxides in a wide range of Zr/Ti ratios was achieved [52]. The advantages of the novel “chemisorption–hydrolysis” strategy for the deposition of uniform, finely dispersed, and highly active CuO particles on mixed TiO2–CeO2 supports were also reported [53]. The current study is aimed at the elucidation of the complex effect of the variation in the microstructure of the TiO2–ZrO2 binary oxides with diverse Ti/Zr ratios and copper loading procedures on the state of the supported CuO species. Low-temperature nitrogen physisorption, XRD, SEM, HRTEM, XPS, Raman, and UV-Vis spectroscopies, as well as TPR with hydrogen, were applied for sample characterization. Methanol decomposition and ethyl acetate oxidation were used as catalytic tests, both of which have potential for environmental protection.

2. Materials and Methods

2.1. Materials

Zirconia–titania mesoporous oxides with Zr/Ti molar ratios of 2:8; 5:5; 8:2 as well as pure TiO2 and ZrO2, were synthesized using TiCl4 and/or ZrCl4 as precursors. For this purpose, 12.0 g of N-hexadecyl-N, N, N-trimethylammoniumbromide (CTAB) was dissolved in 100 mL of distilled water and mixed with 50 mL of an aqueous solution of the Ti and Zr precursors under stirring. The temperature was adjusted at 323 K for 30 min, and then 20 mL of a 25% solution of ammonia was added drop-wise. After stirring overnight at 323 K, the obtained product was added to a polypropylene closed container and treated at 373 K for 24 h. After filtration, washing with water, and drying at room temperature, the solid was calcined at 773 K for 10 h. The samples were denoted as xZryTi, where x/y was the molar Zr/Ti ratio [52].
Two series of copper modifications of the thus obtained titania–zirconia materials were prepared. Incipient wetness impregnation (WI) of the solids with an aqueous solution of Cu(NO3)2.3H2O, followed by drying at room temperature overnight and calcination at 773 for 4 h, was applied for the preparation of the samples from the CuxZryTi_WI series.
Alternatively, a copper ammonia complex was prepared from Cu(NO3)2.3H2O and an aqueous solution of ammonia as described in [53]. Then, the zirconia–titania solid was added and, after 30 min of stirring, was cooled in an ice bath up to 273 K. The product was diluted with water and, after filtration, washing, and drying at 383 K overnight, it was subjected to calcination at 773 K for 4 h. The materials obtained by using the thus described chemisorption–hydrolysis (CH) procedure were denoted as CuxZryTi_CH.
For all modifications, the overall amount of copper was 8 wt.%.

2.2. Methods of Characterization

Nitrogen physisorption measurements were performed on a Beckman Coulter SA 3100 apparatus (Beckman Coulter Inc., California, USA). The samples were outgassed at 423 K for 6 h before measurement. The specific surface area was determined using the Brunauer−Emmett−Teller (BET) equation, the total pore volume was obtained at a relative pressure of 0.99, and the pore size distribution was obtained by using the non-local density functional theory (NLDFT) method.
A Bruker D8 Advance diffractometer (Bruker AXS GmbH, Bremen, Germany) with Cu Kα radiation and a LynxEye detector with a constant step of 0.02°, 2θ, and counting time of 17.5 s per step was applied for the powder X-ray diffraction analyses. The mean crystallite sizes were determined by the Topas-4.2 software.
UV–Vis spectra were collected on a Jasco V-650 UV-Vis spectrophotometer (JASCO Corporation, Japan). A Raman spectroscopic study was performed on a DXR Raman microscope (Thermo Fischer Scientific, Inc., Waltham, MA, USA) equipped with a 532 nm laser.
The SEM investigation was carried out on an FEI Nova NanoSEM450 (Electron Nanoscopy Instrumentation, NE, USA) in a High-Vacuum mode with an acceleration voltage of 5 kV.
An FEI Talos F200X (Thermo Fisher Scientific, NC, USA) transmission electron microscope was applied for the HRTEM analyses.
The XPS measurements were carried out on an AXIS Supra electron spectrometer (Kratos Analytical Ltd. Trafford Park, Manchester, UK) using monochromatic AlKα radiation with a photon energy of 1486.6 eV. The energy calibration was performed by normalizing the C1s line of the adsorbed adventitious hydrocarbons to 284.6 eV. The concentrations of different chemical elements (O1s, Zr3d, Ti2p, and Cu2p3/2) were calculated by normalizing the areas of the photoelectron peaks to their relative sensitivity factors. The peak deconvolution was conducted using ESCApeTM of Kratos Analytical Ltd. software.
The TPR/TG (temperature-programmed reduction/thermo-gravimetric) analyses were performed on a Setaram TG92 (SETARAM Instrumentation, Caluire, France) instrument in a 100 cm3.xmin−1 flow of 50% H2 in Ar with a heating rate of 5 K.min−1.

2.3. Catalytic Tests

The total oxidation of ethyl acetate was carried out in a flow-type apparatus. Typically, 30 mg of the catalysts were put into the reactor and subjected to treatment with a step-wise increase in the temperature (1 K/min) in a flow of 1.21% ethyl acetate (EA) in argon. The obtained products were analyzed online by HP 5890 GC equipped with a flame ionization detector.
The methanol decomposition was studied in a flow-type micro reactor in a temperature-programmed regime under a flow of 1.57 kPa methanol vapors in argon. The analyses were performed on a SCION 456-GC apparatus equipped with flame ionization and thermo-conductivity detectors using a PORAPAC-Q column.
Before the catalytic tests, the catalysts were treated in situ per 1 h at 373 K in argon. The absolute calibration method and carbon-based material balance were used for the elucidation of conversion in both catalytic tests. The products’ selectivity (Si) at 50% conversion for EA oxidation and at 30% for methanol decomposition, respectively, was calculated as Si = Yi/X*100, where Yi is the yield of product (i) and X is the conversion. The specific catalytic activity (SA) was calculated as X/SBET, where X is the conversion at the selected temperature and SBET is the specific surface area.

3. Results and Discussion

3.1. Copper-Zirconia Composites

In Figure 1, the nitrogen physisorption isotherms and pore size distribution for the Cu-modified TiO2–ZrO2 composites are presented. For comparison, data for the parent ZrO2–TiO2 supports are illustrated in Figure S1. The isotherms of both copper modifications of zirconia were of type IV with the H1 hysteresis loop above 0.7 P/P0 (Figure 1). According to the IUPAC classification, they are indicative of the materials with mesoporous texture, developed by cylindrical-like pores with a relatively wide pore size distribution. The shape of the isotherms and pore size distribution were almost similar to the corresponding ones of pure zirconia (Figure S1), indicating that the copper loading did not significantly affect the texture of the support. This assumption was quantitatively confirmed by the negligible changes in the BET surface area and total pore volume (Table 1), which proposed the predominant deposition of copper phases on the external surface.
The latter suggestion is in line with the XRD data (Figure 2, Table 2), where reflections at 35.5° and 38.6° correspond to the well-crystallized tenorite phase (PDF 48-1548) with average crystallite sizes of 57 and 32 nm for the CuZr_WI and CuZr_CH samples, respectively, are observed. According to Liu et al. [54], the formation of bulk CuO particles was related to the higher copper loading than the “dispersion capacity” of the ZrO2 support. The additional well-resolved reflections at 2ϴ of 30.2°, 34.5°, 50.3°, and 60.1° correspond to the tetragonal t–ZrO2 phase (PDF 79–1768), while the strong reflections at ca. 28° reveal the co-existence of the monoclinic m–ZrO2 phase.
For the visualization of the samples’ microstructures, SEM analyses were carried out (Figure 3). The CuZr_WI micrographs (Figure 3e) showed the presence of a small number of CuO spherical-like aggregates in a wide range of sizes situated on amorphous ZrO2 support. In contrast, needle-like CuO particles were highly penetrated with the amorphous ZrO2 phase in the CuZr_CH sample (Figure 3f).
In Figure 4a, the Raman spectra of the copper modifications are shown and the spectra of the corresponding supports [52] are also presented for comparison. In the spectrum of the initial ZO2 support, the Raman shifts were at 175, 193, 340, 382, and 476 cm−1, corresponding to m–ZrO2 [55], while the Raman shifts at 138 and 266 cm−1 are characteristic of t–ZrO2.
These features were not resolved in the copper modifications, indicating the presence of highly distorted ZrO2, which was probably related to its high dispersion [56]. Besides, the weak signal at ca. 290 cm−1, which was only detectable in the spectrum of CuZr_CH, revealed the presence of a well-crystallized CuO phase.
More information about the state and the environment of the metal ions was obtained from the UV-Vis spectra (Figure 4b). The referent ZrO2 support exhibited an absorption band situated at ca. 210 nm, which is indicative of Zr–O–Zr bonds [46]. The additional bands at ca. 260 nm in the spectra of its copper modifications corresponded to ligand-to-metal charge transfer (CT) between the oxygen ions of the support and isolated Cu2+ ions. The broad band between 300 and 600 nm could be assigned to the superimposition of absorption features, related to the CT transitions of octahedrally coordinated Cu2+ ions in (Cu-O-Cu)2+ clusters [18,38,57] and Cu1+ in Cu2O [58,59,60]. The wide absorption band above 600 nm, which was well-detected for CuZr_WI, was attributable to the 2Eg→2T2g spin-allowed transitions of octahedrally coordinated Cu2+ ions in bulk CuO particles [38,57].
The obtained XRD (Figure 2, Table 2), Raman (Figure 4a,) and UV-Vis (Figure 4b) data were clarified by XPS analyses (Figure S2). The high-resolution Zr 3d photoelectron spectra (Figure S2a) were deconvoluted in two spectra, with binding energies (BEs) of the Zr 3d5/2 peaks at ca. 182.2 and 183.6 eV, corresponding to Zr4+ ions in the m–ZrO2 and t–ZrO2 phases, respectively [61]. The BEs were slightly lower for CuZr_WI, indicating the strong interaction of ZrO2 support with the loaded copper species. The m–ZrO2/t–ZrO2 ratios were 2.5 and 10.5 for the WI and CH copper modifications, respectively, which significantly differed from the corresponding 0.7 ratio for the pure ZrO2 (Table 3). Obviously, the copper modification procedure provided the stabilization of m–ZrO2 polymorph, which was more pronounced for CuZr_CH. Stefani et al. [62] also confirmed that the presence of differently sized and shaped CuO particles, as was the case of CuZr_CH (Figure 2 and Figure 3), prevented their interaction with zirconia and decreased the stability of t–ZrO2. The stabilization of the t–ZrO2 polymorph could be achieved by the incorporated Cu2+ ions, as was the case for CuZr_WI.
The core-level XPS spectra of Cu 2p3/2 are presented in Figure S2c. The main Cu 2p3/2 peak was well-fitted with two components. The peak located at 934.2 eV with a shake-up satellite at ca. 943 eV was characteristic of Cu2+ ions [63,64].
The Cu 2p3/2 peak with a maximum at 932.5 eV was due to the presence of copper ions in oxidation states below 2+. Because of the similarity in the BEs, the recognition of the Cu+ and Cu0 species only on the base of the Cu 2p3/2 peaks was difficult. For this purpose, Cu LMM Auger spectra were also recorded (Figure S2d).
Despite the partial overlapping of the Cu LMM Auger region with the Ti 3s photoelectron peak, it is evident that the kinetic energies of the Cu LMM electrons corresponded to mixed Cu2+ and Cu+ states. The overall surface concentration of copper was more than twice higher for CuZr_CH (Table 3). This result is in line with the XRD (Figure 2, Table 2), UV-Vis (Figure 4b), and SEM (Figure 3) analyses, indicating higher copper dispersion in this sample. Besides, the significantly higher Cu+/Cu2+ ratio for CuZr_WI indicates facile electron donation from the ZrO2 support to the Cu2+ ions existing in close contact with it. However, we could not fully ignore the generation of Cu+ ions by the photoreduction of the easily reducible copper species in the spectrometer [65].
In order to prove this suggestion, a TPR study in hydrogen was also carried out (Figure 5). Only one strong reduction feature, centered at 407 K, was observed for CuZr_CH. Here, the weight loss corresponded to approximately 100% transition of Cu2+ ions to Cu0 [66].
These results are in line with the spectral and structural study (see above), indicating the presence of finely dispersed and almost uniform CuO crystallites, which are also easily reducible. In comparison, two wider reduction effects, centered at 385 and 447 K with a ratio of approximately 1:3, were detected for CuZr_WI. The hydrogen consumption corresponding to these features indicated about 80% reduction of Cu2+ ions to metallic copper. On the basis of the physicochemical results reported above, the low-temperature effect in the TPR profile of CuZr_WI could be assigned to the reduction of Cu–O–Cu entities stabilized on the ZrO2’s surface [18,19]. The wide high-temperature peak probably originated from the reduction of bulk CuO particles with different dispersions [33,67]. In agreement with [40] and in line with the XPS analyses (Table 3), we can partially assign the low-temperature and high-temperature effects to the reduction of copper species in close contact with m–ZrO2 and t–ZrO2, respectively. Taking into account the UV-Vis and XPS data (Figure 4a, Table 3), the misbalance in the hydrogen consumption could be assigned to the presence of Cu+ ions and/or a small portion of isolated Cu2+ ions strongly interacting with the support [33].
Thus, the physicochemical characterization clearly indicates variations in the microstructure of the samples, depending on the modification procedure used. In line with our previous study [53], it could be attributed to different mechanisms of copper deposition on the support. Obviously, during the CH procedure, the interaction of copper ammonia precursor with the zirconia surface acidic sites provided the formation of uniform, well-crystallized, needle-like particles, combined with transformations of t–ZrO2 to the more stable m–ZrO2 under ambient conditions (Table 3). In contrast, following the mechanism described in [53], the WI procedure ensured limited insertion of Cu2+ ions into the zirconia lattice. This provoked the stabilization of the t–ZrO2 polymorph [62] and simultaneous formation of a Cu–O–Cu layer and CuO agglomerates.
Figure 6 illustrates the catalytic behavior of the CuZr samples in ethyl acetate oxidation.
Both materials represented much higher catalytic activity as compared to the commercial Pt-containing catalyst as well as to the corresponding pure ZrO2 support [52]. The CH modification achieved slightly higher catalytic activity (Figure 6a). The nitrogen physisorption data (Figure 1, Table 1) and the specific catalytic activity (SA), normalized per unit surface area (Figure 5), clearly indicated that the observed effect was not simply related to the variations in the textural parameters of the catalysts. This assumption was also confirmed by the lower selectivity to CO2 for CuZr_CH due to the formation of ethanol and negligible amounts of acetaldehyde, acetic acid, and ethylene as by-products (Figure 6b). As was previously reported [52,53], EA oxidation is a step-wise process, which is realized by the initial hydrolysis of EA to acetic acid and ethanol, and their further oxidation to CO2 following the Mars van Krevelen mechanism. The promotion effect of the higher acidity of m–ZrO2, which was stabilized in CuZr_CH (Table 3) [56], on the hydrolysis of EA is expected. At the same time, the total oxidation of the produced intermediates could be improved by the higher oxygen mobility in the easily reducible Cu–O–Cu species (Figure 5) in CuZr_WI [33].
More complicated was the catalytic behavior of the CuZr catalysts in methanol decomposition (Figure 7). There was a well-defined maximum in the conversion curves, situated in the moderate-temperature region, followed by a further conversion increase above 600–630 K (Figure 7a).
Besides the production of about 80–90% CO, by-products, such as methyl formate (MF) and CO2, with higher selectivity for CuZr_WI were also registered (Figure 7c). The complex catalytic performance of the CuZr materials could be related to the reduction transformations of the loaded copper species by the influence of the reaction medium, which was well illustrated during the TPR study (Figure 5). The catalytic activity of both composites was significantly higher than that of the corresponding ZrO2 support [53]. The synergistic activity of ZrO2 and loaded copper species could be proposed [43,68,69]. Obviously, the acid–base sites of the support not only facilitated the adsorption of methanol molecules [52], but also promoted the formation and stabilization of methoxy, formate, carbonate, and oxymethylene intermediates [43]. Their further decomposition to CO, CO2, MF, or CH4 could be promoted by the spillover of hydrogen to the existing nearby copper species (Figure 7b). The rapid reduction transformations of the active copper species under the reaction medium (Figure 5) probably provoked a diverse mechanism of methanol conversion [17,43], where the reverse hydrogenation of the surface intermediates to methanol occurred. Consistent with this suggestion, the observed improved SA, stability, and selectivity of CO for CuZr_CH could be related to the cooperative activity of the high number of uniform and finely dispersed CuO particles hosted by the more acidic m–ZrO2 matrix.

3.2. Copper-Titania Composites

The XRD patterns of the CuTi composites (Figure 2) exhibited reflections at 2ϴ = 25.5°, 37.9°, 38.5°, 48.2°, 54.0°, 55.2°, and 62.7°, related to the (101), (004), (112), (200), (105), (211), and (204) crystal planes of anatase (PDF 00-021-1272). Small reflections of the tenorite phase were also observed. The unit cell parameters of the anatase phase in both copper-containing samples were almost similar to those of the pure titania [53], indicating the absence of significant structural changes with the support during the modification procedure. The average crystallite size of the observed phases was larger for the sample obtained by the incipient wetness impregnation technique (Table 2).
In line with the XRD analyses (Figure 2, Table 2), the Raman spectra (Figure 4a) of both composites exhibited vibration bands at ca. 143, 201, 392, 508, and 637 cm−1, typical of the Raman active modes of anatase [58]. The Raman mode at 637 cm−1 slightly shifted to lower values for CuTi_WI, which could be attributed to the strong interactions between titania support and loaded copper species [50]. The Raman shifts typical of CuO were not detected. However, the strong absorption in the whole UV-Vis region (Figure 4b) revealed the co-existence of isolated copper ions, oligomer Cu–O–Cu species, and bulk CuO crystallites [70]. The higher absorption above 700 nm in the spectrum of CuTi_WI (Figure 4b) indicated the presence of a higher amount of less-dispersed copper phase, which is consistent with the XRD analyses (Table 2).
The variation in the samples’ morphology was well visualized by the SEM investigation (Figure 3a,f). Long parallelepiped-like CuO aggregates were observed on the coral-like titania support in the SEM image of CuTi_WI, while the copper particles were sharper and significantly smaller in CuTi_CH.
The higher dispersion of the CuTi_CH composite was confirmed by its higher BET surface area and pore volume (Figure 1, Table 1). Here, the change in the shape of the isotherm desorption branch (Figure S1a) combined with negligible variations in the pore size distribution (Figure S1b) after the copper loading suggests the almost homogeneous deposition of copper particles into the mesoporous titania matrix. In contrast, the observed significant decrease in the BET surface area and pore volume (Table 1), combined with a shift in the pore size distribution profile, especially in the range of the smaller mesopores (Figure S1b) for CuTi_WI, propose significant pore-blocking due to copper deposition.
Figure S2b shows the high-resolution Ti 2p photoelectron spectra of the samples. The spectral region was deconvoluted into two doublets with Ti 2p3/2 binding energies of 458.5 and 457.6 eV. The former peak corresponded to Ti4+ in TiO2 [71,72], while the latter was attributable to Ti3+ ions in Ti2O3 [73]. Despite the TiO2 phase being the main component in both samples (Table 3), a slight tendency for the Ti3+ defects to decrease after copper loading was observed. In agreement with the XRD data (Table 2), this could be assigned to the agglomeration of the anatase phase. Besides, the co-existence of copper species in different oxidative states was also detected (Figure S2c,d). The Cu+/Cu2+ ratio was lower for the TiO2-based composites than that for the corresponding ZrO2 analogs (Table 3), which could be assigned to the lower electron donation effect from TiO2 to the supported copper species. Among the titania modifications, the overall surface concentration of copper was slightly higher for CuTi_WI. Taking into account the XRD (Table 2), UV-Vis (Figure 4b), and SEM (Figure 3) data, this could be attributed to the higher exposure of larger CuO crystallites located on the external titania surface.
The TPR profiles of the CuTi composites (Figure 5) consisted of one well-determined DTG effect, centered at 386 K and 440 K, for the samples obtained by the CH and WI procedure, respectively. It represented 100% conversion of Cu2+ to metallic copper for CuTi_CH and about 62% for CuTi_WI. For the latter, continuous weight loss with the increase in the reduction temperature up to 770 K was also observed. The oxygen defects in the titania lattice (Table 3) probably provoked the stabilization of isolated, hardly reducible Cu2+ ions when the traditional incipient wetness impregnation technique was used. In contrast, the deposition of smaller, more uniform, and more easily reducible CuO particles during the modification of titania by the CH procedure was assumed. As mentioned above, their formation was controlled by the interaction of the copper ammonia precursor with the titania’s surface acidic sites [53]. Obviously, the higher density of the acidic sites in TiO2 in comparison with the ZrO2 support [52] was responsible for the higher dispersion and easier reduction of the copper phase loaded on it (Figure 5).
Figure 6 and Figure 7 are demonstrate the catalytic behavior of the CuTi composites in EA oxidation and methanol decomposition, respectively. As compared to the ZrO2 analogs, TiO2 modifications possessed higher catalytic activity (Figure 6a) and selectivity (Figure 6b) in the total oxidation of EA to CO2. The facile hydrolysis of EA on the more acidic TiO2 surface [52], combined with the easier oxidation of the intermediates by the more labile oxygen in the finely dispersed CuO entities (Figure 5), could be a reasonable explanation for the observed effect. The higher SA (Figure 6c) for CuTi_WI combined with the slightly lower selectivity to CO2 (Figure 6b) could be related to the presence of a larger portion of isolated Cu2+ ions (Figure 5), which behaved as additional Lewis acidic sites and promoted the hydrolysis of EA. Consistent with the nitrogen physisorption data (Table 1, Figure 1), the higher accessibility of the copper particles, situated on the external titania surface, could additionally contribute to the observed effect.
The CuTi catalysts exhibited significantly different behavior in methanol decomposition (Figure 6a). Here, the CuTi_WI modification demonstrated much lower SA (Figure 7c) and selectivity (Figure 7c) to the most important CO product accompanied by the formation of a large fraction of dimethyl ether (DME) and CH4 (Figure 7b). According to [43], their formation is provoked by the interaction of surface methoxy groups between themselves or with hydroxyl groups in their vicinity. As proposed above, this is promoted by the higher acidity of the Cu2+-doped TiO2 support combined with a small catalytic contribution of the larger CuO particles in CuTi_WI. In contrast, the finely dispersed copper particles in CuTi_CH provided the decomposition of the intermediates formed with the activity of the TiO2 support in a synergistic mode. Based on the TPR results (Figure 5), an additional impact of the reduction transformations of the copper species under the reaction medium could be expected. As in the case of the ZrO2-based composites (See Section 3.1), these transformations were responsible for the complex course of the conversion curves (Figure 7a).

3.3. Copper–Zirconia–Titania Composites

The XRD patterns of copper-modified ZrO2–TiO2 mixed oxides varied significantly with the changes in the Zr/Ti ratio (Figure 2, Table 2). Well-defined reflections of anatase with an average crystallite size of 18 nm were detected for Cu2Zr8Ti, while a negligible amount of t–ZrO2 was observed for Cu8Zr2Ti. The absence of any reflections in the XRD patterns of both Cu5Zr5Ti composites indicates their highest dispersion. The deviations of the unit cell parameters of the observed phases as compared to the corresponding ones in CuTi and CuZr (Table 2) could be assigned to the penetration of the individual TiO2 and ZO2 oxides in the binary supports [52]. For all samples, no reflections of any copper-containing phases were detected, indicating a low degree of agglomeration.
The preservation of the anatase features only in Cu2Zr8Ti was also confirmed by the Raman spectrum (Figure 4a). The slight shift of the main 3Eg Raman active mode to lower values as compared to the CuTi analogs indicates structural changes of the titania lattice, probably originating from the strong interaction of the copper species with the support [74].
The improved texture parameters of the ternary composites as compared to the corresponding binary ones (Table 1) could be an adequate explanation for the higher copper dispersion on them (Figure 2). All samples possessed the main characteristics of the mesoporous materials (Figure 1). The decrease in the BET surface area and total pore volume after the modification procedures (Table 1), combined with the preservation of the shape of the isotherms and the pore size distribution profiles (Figure S1), revealed an almost homogeneous distribution of the loaded copper phase into the porous matrix without significant pore blocking.
The variations in the morphology of the samples with diverse compositions were well-illustrated by the SEM images (Figure 3). The microstructure of the samples produced by the CH procedure (Figure 3b–d) was looser and more homogeneous than that of their WI analogs (Figure 3g–i). A small number of spherical- or cubic-like aggregates were detected in Cu2Zr8Ti_WI and Cu8Zr2Ti_WI, respectively, while a coral-like structure was observed for Cu5Zr5Ti_WI. Besides, the SEM images verified the assumption above, that the copper dispersion in the three-component materials was significantly improved as compared to the copper modifications of the individual TiO2 and ZrO2 supports.
The microstructure of both Cu5Zr5Ti samples was visualized in detail by the HRTEM study (Figure 8 and Figure 9). The presence of m–ZrO2, TiO2, and CuO in highly dispersed states was detected in the ED-HRTEM patterns. The EDX measurements confirmed the homogeneous distribution of copper species within the 5Zr5Ti mixed oxide matrix. Note that the Zr/Ti ratio was higher than the expected one. This could be related either to the deposition of the zirconia phase over the titania as a result of the different rates of hydrolysis of the corresponding precursors [75], or to the preferable covering of the titania entities by the copper species. The lower oxygen content for the Cu5Zr5Ti_WI was attributable to the higher concentration of oxygen deflects that most probably generated during the insertion of isolated Cu2+ ions into the support matrix and/or deposition of CuO entities on it.
More information for the surface elemental distribution was obtained by the XPS analyses (Table 3, Figure S2). The deconvolution of the Zr 3d (Figure S2a) and Ti 2p (Figure S2b) photoelectron spectra of the ternary composites revealed the appearance of additional peaks with BE of 182.8 and 459.5 eV, respectively. Consistent with [73,76], they corresponded to Zr4+ and Ti4+ ions in the highly distorted crystal lattice of penetrated TiO2 and ZrO2 oxides. This interpretation was confirmed by the quantitative analyses of the surface composition (Table 3).
Note that the modification of the ZrO2-TiO2 support with copper provided higher exposure of shared Ti–O–Zr structures, combined with a simultaneous increase in the relative part of the Cu+ and Ti3+ ions. This was more pronounced for the WI modification and proposed strong interactions of the isolated copper ions with the TiO2–ZrO2 interface. The copper species were in a higher amount on the Cu5Zr5Ti_CH’s surface, which was in agreement with the structural study (see above), and was indicative of its higher dispersion. The stabilization of t–ZrO2 polymorph for this sample (Table 3), which was probably related to the deposition of small CuO crystallites on the zirconia “shell” of the composites, should be stressed. Nevertheless, slight deviations for both ternary composites were observed; the Cu/Zr and Cu/Ti surface ratio was about 0.5, while that of Zr/Ti was close to 1. This indicates an almost random distribution of copper species in the 5Zr5Ti matrix and their preferable localization near the Ti–O–Zr defects.
For all ternary composites, the UV-Vis spectra (Figure 4b) clearly demonstrated the presence of significantly uniform and finely dispersed Cu–O–Cu and CuO species. However, their environment and proportion varied with the support composition, and the relative part of the bulk CuO particles seemed to be negligible for the Cu5Zr5Ti, which well correlated with the SEM (Figure 3), TEM (Figure 8 and Figure 9), and XPS (Table 3) analyses.
More information on the state and distribution of various copper species in relation to the composition of the support was obtained by TPR analysis (Figure 5). Only one reduction effect, which was also shifted to lower temperatures as compared to the bi-component CuTi and CuZr composites, was observed for the CH-obtained materials, which corresponded to 100% transition of Cu2+ to Cu0.
These results evidence the advantages of the chemisorption–hydrolysis procedure for the deposition of more uniform and finely dispersed copper oxide species into the TiO2–ZrO2 mixed oxide matrix, and this was facilitated for the supports with an equimolar Zr/Ti ratio. For comparison, the traditional incipient wetness impregnation method provided the formation of copper species different in their size and environment, which was reflected in the appearance of two reduction features in the TPR profiles (Figure 5). This assumption was in line with the data reported in [49]. The reduction features not only shifted to lower temperatures, as compared to their binary analogues, but also corresponded to the 100% reduction of Cu2+ to Cu0. The observed additional weight loss in the 500–700 K region proposes the partial reduction of Ti4+ to Ti3+. Following the mechanism described above for the WI procedure [53], this could be related to the higher mobility of the O ions in the shared Ti–O–Zr bonds situated in the vicinity of the isolated Cu2+ ions.
Figure 6 shows the catalytic performance of the ternary composites in EA oxidation. All materials possessed stable and improved catalytic activity as compared to the corresponding ZrO2–TiO2 supports and also to the commercial Pt-containing catalyst (Figure 6a), which was most pronounced for Cu8Zr2Ti_WI. The fluctuation in the specific activity of the composites (Figure 6c) indicated that their catalytic activity was not in simple dependence on the corresponding textural characteristics and probably originated from the diverse acid–base and redox properties of the samples. In accordance with the XPS (Table 3) and TEM studies (Figure 8 and Figure 9), the lower SA for all ternary composites as compared to the copper modifications of the individual oxides could be related to the significant impact of the less active zirconia shell covering the titania core via the development of the Zr–O–Ti interface. This effect was complicated by the stabilization of different ZrO2 polymorphs (Table 3). Obviously, the contribution of these structures was regulated by the tuning of the Zr/Ti ratio in the support and the choice of the modification procedure. Indeed, the XPS results (Table 3) indicate the predominant exposition of the more acidic m–ZrO2 to the surface of the WI-obtained samples. For this series of samples, lower and inhomogeneous dispersion of the copper phase was also found (Figure 3). Thus, as a result of the improved acidity and suppressed redox ability, these materials represented much lower selectivity in terms of EA oxidation (Figure 6b). The facile electron transfer between the shared Zr–O–Ti structures and isolated Cu2+ ions probably located in their vicinity provided the stabilization of Cu1+ and Ti3+ ions (Table 3). This decreased the number of active Cu2+–Cu redox pairs and suppressed the catalytic activity, which was most pronounced for the Cu5Zr5Ti composites. This problem could be avoided by the application of the CH modification procedure. Here, the generation of uniform and finely dispersed CuO entities improved the redox ability of the composites (Figure 5) and ensured superior selectivity in the total oxidation of EA to CO2 (Figure 6b). However, the lower accessibility of the copper species to the reactants due to their blocking of the support mesopores (Table 1) provided a decrease in the overall catalytic activity.
As in the case of the binary CuTi and CuZr samples, the ternary composites exhibited the complex temperature dependences of methanol conversion. In line with the TPR data (Figure 5), they were probably related to the reduction transformations with the loaded copper species under the reaction medium. In contrast to the corresponding ZrO2–TiO2 supports, some of their copper modifications at lower temperatures represented similar or even higher catalytic activity as compared to the commercial Cu-containing catalyst (Figure 7a). The higher SA (Figure 7c) for most of the CH modifications could be easily understood when taking into account the higher dispersion of the copper phase within them (see above). The variations in the SA (Figure 7c) with the support composition were likely related to the superimposition of the effects originating from the diverse structure, morphology, polymorph, and texture of the supports. All of these could influence the electron charge densities around the metal ions in the TiO2–ZrO2 supports and the situated nearby copper ions [19]. In line with the physicochemical analyses (see above), the higher selectivity to methyl formate, which was registered for Cu5Zr5Ti_WI and all CH modifications (Figure 7b), could be assigned to the increase in the number of Lewis acidic sites on the TiO2–ZrO2’s surface, which co-existed with finely dispersed copper species. By the interaction of the acidic sites with methanol, methoxy groups with a high surface density were formed on the support. They easily transformed to formate species by the spillover of hydrogen to the copper particles finely dispersed in their vicinity and either by the further dimerization of the neighboring formate species or by their interaction with the adsorbed methanol molecules located nearby, thereby providing the formation of methyl formate [43].

4. Conclusions

This study demonstrates simple control of a catalyst’s design by the simultaneous variation of their composition and the preparation procedure used. For the first time, high-quality mesoporous copper–zirconium–titanium oxide composites were prepared using the template-assisted hydrothermal synthesis of ZrO2–TiO2 mixed oxides with diverse composition, followed by a sequence of chemisorption and hydrolysis of a copper ammonia complex. As compared to the traditional incipient wetness impregnation procedure, the novel approach ensured the formation of more uniform, finely dispersed, and easily reducible needle-like copper oxide particles, which typically improved the catalytic behavior in methanol decomposition and ethyl acetate oxidation. The development of a high surface area in the binary TiO2–ZrO2 oxide supports increased the catalytic activity and selectivity by increasing the dispersion of the loaded copper species and, in a synergistic mode with them, providing active sites for the adsorption of the reagents and their further transformation to the intermediates. However, the catalytic properties of the composites were in a complex dependence on their texture, structure, and morphology. and on the related surface acidity and electron density around the metal ions. They could be optimized by the variation of the Zr/Ti ratio in the support and the procedure of copper deposition. All ternary composites exhibited superior catalytic activity in the total oxidation of ethyl acetate. During methanol decomposition, their behavior was complicated due to the phase- transformations under the influence of the reduction reaction medium.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/sym14040751/s1. Figure S1: Nitrogen physisorption isotherms, shifted by the y axis (left) and pore size distribution (right) for the copper-modified titania and zirconia mesoporous supports. For comparison, the texture parameters for the corresponding zirconia–titania supports are also presented; Figure S2: Zr 3d (a), Ti 2p (b), and Cu 2p3/2 (c) photoelectron spectra, and Auger Cu LMM spectra (d) of selected composites.

Author Contributions

Conceptualization, T.T.; methodology, T.T.; investigation, R.I., D.K., G.I., J.H., M.D., and G.A.; writing—original draft preparation, T.T.; writing—review and editing, G.A. and J.H.; visualization, R.I., G.I., G.A., D.K., M.D., and J.H.; supervision, T.T.; project administration, T.T.; funding acquisition, T.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Bulgarian Scientific Fund: Project KP-06-H 29/2 and Project BG05M2OP001-1.002-0019: “Clean technologies for sustainable environment—water, waste, energy for circular economy” (Clean & Circle).

Acknowledgments

The authors acknowledge the assistance provided by the Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project No. LM2018124 and Bulgarian–Czech Academies of Sciences bilateral project (BAS-20-11).

Conflicts of Interest

The authors declare no conflict of interest.

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Symmetry 14 00751 g001 550
Figure 1. Nitrogen physisorption isotherms (shifted by the y axis) and pore size distribution (inset) of the CH (a) and WI (b) obtained CuZrTi oxide composites.
Figure 1. Nitrogen physisorption isotherms (shifted by the y axis) and pore size distribution (inset) of the CH (a) and WI (b) obtained CuZrTi oxide composites.
Symmetry 14 00751 g001
Symmetry 14 00751 g002 550
Figure 2. XRD patterns of CuTiZr oxide compositions prepared by: (a) incipient wetness impregnation and (b) chemisorption–hydrolysis techniques.
Figure 2. XRD patterns of CuTiZr oxide compositions prepared by: (a) incipient wetness impregnation and (b) chemisorption–hydrolysis techniques.
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Symmetry 14 00751 g003 550
Figure 3. SEM images of Cu-modified (a,f) TiO2, (b,g) 2Zr8Ti, (c,h) 5Zr5Ti, (d,i) 8Zr2Ti, and (e,j) ZrO2 prepared by incipient wetness impregnation (left images) and the chemisorption–hydrolysis (right images) method.
Figure 3. SEM images of Cu-modified (a,f) TiO2, (b,g) 2Zr8Ti, (c,h) 5Zr5Ti, (d,i) 8Zr2Ti, and (e,j) ZrO2 prepared by incipient wetness impregnation (left images) and the chemisorption–hydrolysis (right images) method.
Symmetry 14 00751 g003
Symmetry 14 00751 g004 550
Figure 4. Raman (a) and UV-Vis (b) spectra of different copper modifications of the ZrO2-TiO2 composites. For comparison, the spectra of the corresponding pure supports are presented.
Figure 4. Raman (a) and UV-Vis (b) spectra of different copper modifications of the ZrO2-TiO2 composites. For comparison, the spectra of the corresponding pure supports are presented.
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Symmetry 14 00751 g005 550
Figure 5. TPR-DTG profiles of copper–titania–zirconia composites obtained by different techniques (solid line, incipient wetness impregnation; dashed line, chemisorption–hydrolysis).
Figure 5. TPR-DTG profiles of copper–titania–zirconia composites obtained by different techniques (solid line, incipient wetness impregnation; dashed line, chemisorption–hydrolysis).
Symmetry 14 00751 g005
Symmetry 14 00751 g006 550
Figure 6. Ethyl acetate conversion (a) vs. temperature, (b) products distribution at 50% conversion, and (c) specific activity per unit surface area at 573 K for various CuZrTi composites.
Figure 6. Ethyl acetate conversion (a) vs. temperature, (b) products distribution at 50% conversion, and (c) specific activity per unit surface area at 573 K for various CuZrTi composites.
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Symmetry 14 00751 g007 550
Figure 7. Methanol conversion vs. temperature (a), products selectivity at 30% conversion (b), and specific activity at 500 K (c) for various CuZrTi composites.
Figure 7. Methanol conversion vs. temperature (a), products selectivity at 30% conversion (b), and specific activity at 500 K (c) for various CuZrTi composites.
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Symmetry 14 00751 g008 550
Figure 8. TEM images of Cu5Zr5Ti_CH.
Figure 8. TEM images of Cu5Zr5Ti_CH.
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Symmetry 14 00751 g009 550
Figure 9. TEM images of Cu5Zr5Ti_WI.
Figure 9. TEM images of Cu5Zr5Ti_WI.
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Table
Table 1. N2 physisorption data for the specific surface area (SBET) and total pore volume (Vp) of copper modified zirconia–titania materials. For comparison, the parameters of the initial supports [52] are also presented.
Table 1. N2 physisorption data for the specific surface area (SBET) and total pore volume (Vp) of copper modified zirconia–titania materials. For comparison, the parameters of the initial supports [52] are also presented.
SampleZrTi SupportCuZrTi_WICuZrTi_CH
SBET m2.g−1Vp mL.g−1SBET m2.g−1Vp mL.g−1SBET m2.g−1Vp mL.g−1
TiO2850.29400.24660.26
2Zr8Ti1570.441190.371170.35
5Zr5Ti2480.691870.601930.59
8Zr2Ti1570.481220.441160.43
ZrO2670.32610.31580.31
Table
Table 2. Phase composition of copper-doped titania–zirconia materials.
Table 2. Phase composition of copper-doped titania–zirconia materials.
SampleIncipient Wetness Impregnation
(WI)		Chemisorption–Hydrolysis
(CH)
Preparation TechniquePhase CompositionUnit Cell Parameters, ÅCrystallite Size, nmPhase CompositionUnit Cell Parameters, ÅCrystallite Size, nm
CuTiAnatase

Tenorite


3.7861(2)
9.4881(9)
4.691(3)
3.419(1)
5.138(4)
99.59(4)		20.8(1)

36(4)		Anatase

Tenorite		3.7871(2)
9.487(1)
4.683(7)
3.417(4)
5.145(9)
99.67(9)		17.5(1)

14(1)
Cu2Zr8TiAnatase3.7979(9)
9.540(2)		18Anatase3.7958(9)
9.535(2)		18
Cu5Zr5Tiamorphous amorphous
Cu8Zr2TiZrO2-tetragonal3.558(2)
5.243(4)		20 fixZrO2-tetragonal3.579(2)
5.243(4)		20 fix
CuZrZrO2-tetragonal

ZrO2-monoclinic



Tenorite		3.600(1)
5.203(7)
5.327(5)
5.150(4)
5.230(5)
Beta-99.21(2)
4.703(4)
3.443(3)
5.120(4)
99.49(7)		13

14



57(25)		ZrO2-tetragonal

ZrO2-monoclinic



Tenorite		3.598(2)
5.19(1)
5.321(6)
5.151(5)
5.219(6)
Beta-99.17(3)
4.691(4)
3.433(6)
5.122(6)
99.56(7)		13

14



32(7)
Table
Table 3. XPS data for the selected Cu–Ti–Zr composites.
Table 3. XPS data for the selected Cu–Ti–Zr composites.
SampleZr4+
(m–ZrO2)
at%		Zr4+
(t–ZrO2)
at%		Zr4+
(TiOZr)
at%		Cu1+
at%		Cu2+
at%		Ti4+ (TiO2)
at%		Ti4+
(TiOZr)
at%		Ti3+
at%		O,
at%
ZrO212.819.10 68.1
CuZr_WI20.98.303.44.0 63.4
CuZr_CH20.11.901.714.2 62.1
5Zr5Ti10.45.31.1 9.91.90.670.7
Cu5Zr5Ti_WI6.63.44.43.12.47.43.02.667.1
Cu5Zr5Ti_CH5.57.02.53.93.19.32.41.165.2
TiO2 32.201.466.4
CuTi_WI 3.07.920.702.366.1
CuTi_CH 2.07.722.401.366.6
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Tsoncheva, T.; Issa, G.; Ivanova, R.; Dimitrov, M.; Kovacheva, D.; Atanasova, G.; Henych, J. Design Control of Copper-Doped Titania–Zirconia Catalysts for Methanol Decomposition and Total Oxidation of Ethyl Acetate. Symmetry 2022, 14, 751. https://doi.org/10.3390/sym14040751

AMA Style

Tsoncheva T, Issa G, Ivanova R, Dimitrov M, Kovacheva D, Atanasova G, Henych J. Design Control of Copper-Doped Titania–Zirconia Catalysts for Methanol Decomposition and Total Oxidation of Ethyl Acetate. Symmetry. 2022; 14(4):751. https://doi.org/10.3390/sym14040751

Chicago/Turabian Style

Tsoncheva, Tanya, Gloria Issa, Radostina Ivanova, Momtchil Dimitrov, Daniela Kovacheva, Genoveva Atanasova, and Jiří Henych. 2022. "Design Control of Copper-Doped Titania–Zirconia Catalysts for Methanol Decomposition and Total Oxidation of Ethyl Acetate" Symmetry 14, no. 4: 751. https://doi.org/10.3390/sym14040751

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