Ammonia Chemisorption Research Articles

MWCNT-conducting polymer composite based ammonia gas sensors: A new approach for complete recovery process

Highly sensitive ammonia gas sensors were fabricated using Multi-walled Carbon Nanotubes (MWCNTs) reinforced electrically conducting polymer composites following solution casting method. Two types of conducting polymers like poly(3,4-ethylenedioxythiophene)–polystyrene sulfonic acid (PEDOT:PSS) and polyaniline (PANI) were used and compared for their ammonia gas sensing properties at room temperature (RT).Both the sensors were found to exhibit excellent sensitivity and poor recovery for ammonia gas at room temperature, but as compared to PANI, PEDOT:PSS polymer composite was found to be more sensitive (with sensitivity of ∼16%) with less response time (∼15min). In addition, thermal behavior of both the composites was investigated in detail, where MWCNT–PEDOT:PSS composite showed significantly better thermal stability than MWCNT–PANI composite. Sensor recovery posed a great problem at room temperature and a new approach is proposed to get complete recovery, exclusively tested in polymer–CNT composite sensor, to lessen the heat dependence and improve the cycling behavior. A trial experiment is conducted in combination of heat and DC electric field to optimize the complete recovery of the MWCNT–PEDOT:PSS composite based sensor where the recovery time was reduced from 48h to 20min. It is believed that such stimulation process provides sufficient energy to desorb chemisorbed ammonia from CNT surface completely.

Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate

Multiwall carbon nanotubes were sulfonated at different temperatures (150, 180, 210, 230, 250 and 280°C) and used as catalysts in the esterification of levulinic acid with ethanol. The materials sulfonated between 150 and 230°C presented almost the same acidity (measured by ammonia chemisorption), which was higher than that obtained for samples sulfonated at 250 and 280°C. Raman spectroscopy revealed that the treatment with sulfuric acid did not lead to the destruction of the carbon nanotubes structure, even for the higher temperature treatment. The activity results have shown that the nanotubes sulfonated below 250°C presented a specific activity higher than those sulfonated at 250 and 280°C. The association of these results with those obtained by temperature-programmed desorption of ammonia indicate that the activity in the esterification reaction is related to the number of acidic sites that desorb ammonia in temperatures around 220°C. Temperature-programmed desorption experiments suggest that there is a strong adsorption of the levulinic acid on the active sites therefore not allowing the reuse of the materials.

Ti-incorporated SBA-15 mesoporous silica as an efficient and robust Lewis solid acid catalyst for the production of high-quality biodiesel fuels

One-pot synthesized Ti-SBA-15 mesoporous materials with various Ti loadings of 0.808–6.78mol% were applied as heterogeneous solid acid catalysts for simultaneous esterification and transesterification of vegetable oils with methanol into high-quality biodiesel fuel (BDF) at 200°C under autogeneous pressure. According to the diffuse-reflectance (DR) UV–vis spectra, diffuse-reflectance infrared Fourier transform (DRIFT) spectra and pulsed ammonia (NH3) chemisorption studies combined with other conventional characterizations, the catalytically active site for high-quality BDF synthesis was mostly related to the tetrahedral Ti4+ species with weak Lewis acid character, which differential heat of NH3 adsorption was lower than 90kJmol−1. Due to that the tetrahedral Ti4+ species were accessible on largely mesoporous framework, the Ti-SBA-15 catalyst gave much higher activity in transesterification of crude Jatropha oil (CJO) with methanol than microporous titanosilicate of TS-1 and commercial TiO2 nanocrystallites. Among them, the 3Ti-SBA-15 catalyst with a Ti loading of 2.46mol% showed a highest fatty acid methyl ester (FAME) content of 90mass% at 200°C for 3h using a methanol-to-oil molar ratio of 27. When the reaction period and methanol-to-oil molar ratio were increased to 3–6h and 108, respectively, a great variety of edible and non-edible vegetable oils with various acid values (0.06–190mgKOHg−l), including refined soybean oil (RSO), refined rapeseed oil (RRO), waste cooking oil (WCO), crude palm oil (CPO), CJO and palm fatty acid distillates (PFAD), was directly transformed into high-quality BDFs, which met with a European standard (EN 14214:2009), over 3Ti-SBA-15 catalyst at 200°C. The used 3Ti-SBA-15 catalyst was easily regenerated by calcination and its high activity was maintained. Most importantly, the 3Ti-SBA-15 catalysts could resist 5wt% of water or 30wt% of free fatty acid (FFA), which tolerance levels were several ten times better than those of homogeneous and heterogeneous catalysts in the current BDF production technology.

Effect of ammonia gas addition to the synthesis environment of single-walled carbon nanotubes on their surface chemistry

This study evaluates the effect of ammonia gas addition on synthesis environment and surface chemistry of single-walled carbon nanotubes (SWCNTs) produced by induction thermal plasma system (ITP). The experiments were carried out based on the opposed-flow injection of ammonia into the ITP reactor used for mass production of SWCNTs containing soot. The influence of in situ ammonia injection on the SWCNT final products was closely examined by Raman spectroscopy at three wavelengths, coupled thermogravimetry-infrared spectroscopy (TG-FTIR) and solid-phase infrared spectroscopy. The results clearly indicated the surface modification of SWCNTs where the metallic tubes had a greater tendency than the semiconducting tubes to react with ammonia at high temperature. To better understand the reaction system and predict the concentration profile of the intermediates and the product species from thermal decomposition of ammonia in the ITP reactor, a numerical simulation was developed including both the detailed kinetic of ammonia decomposition and turbulent mixing in the reaction system. By comparing the experimental and simulation results, a two-step reaction mechanism is suggested for this in situ SWCNTs chemistry in which the reaction of atomic hydrogen and amidogen radicals with SWCNTs, is followed by the ammonia chemisorption.

Effect of water and ammonia on surface species formed during NOx storage–reduction cycles over Pt–K/Al2O3 and Pt–Ba/Al2O3 catalysts

The effect of water, in the temperature range 25-350 °C, and ammonia at RT on two different surface species formed on Pt-K/Al2O3 and Pt-Ba/Al2O3 NSR catalysts during NO(x) storage-reduction cycles was investigated. The surface species involved are nitrates, formed during the NO(x) storage step, and isocyanates, which are found to be intermediates in N2 production during reduction by CO. FT-IR experiments demonstrate that the dissociative chemisorption of water and ammonia causes the transformation of the bidentate nitrates and linearly bonded NCO(-) species into more symmetric species that we call ionic species. In the case of water, the effect on nitrates is observable at all the temperatures studied; however, the extent of the transformation decreases upon increasing temperature, consistent with the decreased extent of dissociatively adsorbed water. It was possible to hypothesize that the dissociative chemisorption of water and ammonia takes place in a competitive way on surface sites able to give bidentate nitrates and linearly bonded NCO(-) that are dislocated, remaining on the surface as ionic species.

Comprehensive Study on the Dissociative Chemisorption of NH3 on the Sidewalls of Stone–Wales Defective Armchair (5,5) Single-Walled Carbon Nanotubes

Dissociative chemisorption of NH3 into H and NH2 on the sidewalls of (5,5) armchair Stone–Wales defective single-walled carbon nanotubes (SWCNTs) has been investigated using B3LYP/6-31G(d) and M06-2X/6-31G(d) levels of theory. In particular, H• and NH2• have been attached to the adjacent carbon atom sites on and around the Stone–Wales defect (SWD). Here, we consider SWD with two different orientations in finite H-terminated (5,5) SWCNTs. The reaction energy data suggest that the dissociative chemisorption of NH3 on the surface of the Stone–Wales defective SWCNTs is mostly endothermic except the adsorption at very few defect sites, wherein the chemisorption process is exothermic at the M06-2X/6-31G(d) level. Our systematic study reveals that the SWD generated by 90° rotation of nearly axial C–C bond in (5,5) SWCNT is more favorable for the chemisorption of ammonia than the nanotube containing SWD with another orientation. The HOMO–LUMO energy gap values of defect-free and Stone–Wales defective SWCNTs are s...

Toward Understanding Reactive Adsorption of Ammonia on Cu-MOF/Graphite Oxide Nanocomposites

The adsorption of ammonia on HKUST-1 (a metal-organic framework, MOF) and HKUST-1/graphite oxide (GO) composites was investigated in two different experimental conditions. From the isotherms, the isosteric heats of adsorption were calculated from the Clausius-Clapeyron equation following the virial approach. The results on HKUST-1 were compared with those obtained using molecular simulation studies. All materials exhibit higher ammonia adsorption capacities than those reported in the literature. The ammonia adsorption on the composites is higher than that measured separately on the MOF component and on GO. The strong adsorption of ammonia caused by chemical interactions on different adsorption sites is evidenced by the trends in the isosteric heats of adsorption. The molecular simulations conducted on HKUST-1 support the trends observed experimentally. In particular, the strong chemisorption of ammonia on the metallic centers of HKUST-1 is confirmed. Nevertheless, higher adsorption capacities are predicted compared with the experimental results. This discrepancy is mainly assigned to the partial collapse of the MOF structure upon exposure to ammonia, which is not accounted for in the simulation study.

Reactions of Cyclohexane on Platinum, Palladium, or Iridium-loaded H-ZSM-5 Zeolite Hydrohalogenated Catalysts

The catalytic hydroconversion of cyclohexane using catalysts containing H-ZSM-5 zeolite loaded with 0.35 wt% platinum, palladium, or iridium was studied in a pulse-type microreactor/GC system at atmospheric pressure. These catalysts were also either doped with 3.0% of HCl or HF. The activities of these catalysts and the distribution of the products formed were found to depend on the dispersion of the metallic component as well as on the acidity, acid sites number, and strength in the catalysts. TPD of ammonia and hydrogen chemisorption were applied to evaluate acid site strength distribution and metals dispersion, respectively, in the catalysts. In case of the Pt- and Pd-containing catalysts, hydrochlorination enhanced the isomerization and dehydrogenation activities and selectivities of cyclohexane, but decreased its hydrocracking activity. However, catalyst hydrofluorination resulted in the reverse effects. Nevertheless, for the Ir-loaded catalyst, both hydrohalogenation treatments decreased the isomerization and dehydrogenation of cyclohexane. The Ir/H-ZSM-5 catalyst exhibited higher hydrogenolysis activities than did those acquired by the Pt- and Pd-containing catalysts.

Study on the Preparation of Ultramicro Molybdenum Nitride Powder and Wear Properties of Its Coating

Using the specially designed mechanochemical ball-mill equipment, the ultramicro molybdenum nitride powders were prepared from pure molybdenum powders in ammonia atmosphere at room temperature by high-energy ball milling. The results show that the mass ratio of grinding media to powder was 4:1 and after milling for 30 h, the Mo2N of fcc structure was obtained, and the average particle size of powders was around 100 nm. It is found that the chemisorption of ammonia onto the fresh molybdenum surfaces created by milling was the predominant process during solid–gas reaction，and the change of Mo electronic undersaturation induced by the grain refining also accelerated the bonding between Mo and N. The coating was formed on the 40Cr steel base using plasma spray method by mix Ni60 alloy powders and ultramicro Mo2N powders of 5wt%, 10wt% and 15wt%, respectively. Coating abrasion test under the condition of dry-grinding , 2h wear time and 300N test load showed that the wear-resisting property of coating added with ultramicro Mo2N powders could be improved greatly, and the wear-resisting property of coating increased with the increase of Mo2N content. In the abrasion process, the evenly distributed ultramicro Mo2N particles play the dispersion strengthening and self-lubricating role in the coating.

Dissociative Chemisorption of Trimethylgallium, Trimethylindium, and Ammonia on Gallium and Indium Nitride Substrates. A Computational Study

The methyl or hydrogen dissociations of trimethylgallium [Ga(CH3)3], trimethylindium [In(CH3)3], and ammonia (NH3), adsorbed onto model substrates for gallium nitride (GaN) and indium nitride (InN), were studied using the hybrid program ONIOM. The model substrates consisted of 44 GaN units and 35 InN units, with no capping hydrogen atoms, and were simulated using a quantum mechanical semiempirical approach (PM6). The adsorbates were represented using density functional theory (DFT) and basis sets containing polarization functions [B3LYP/6-311G(d,p)/3-21G(d,p)]. Adsorption rate constants, as well as methyl and hydrogen unimolecular homolytic dissociation rate constants, were computed using a semiclassical procedure, for pressures between 1 and 100 atm and for temperatures ranging from 300 to 1400 K. Time-dependent dissociative chemisorption processes of Ga(CH3)3, In(CH3)3, and NH3 adsorbing onto GaN and InN were modeled by solving sets of coupled differential equations using stiff methods.

Macroporous WO3 Thin Films Active in NH3 Sensing: Role of the Hosted Cr Isolated Centers and Pt Nanoclusters

Macroporous WO(3) films with inverted opal structure were synthesized by one-step procedure, which involves the self-assembly of the spherical templating agents and the simultaneous sol-gel condensation of the semiconductor alkoxide precursor. Transition metal doping, aimed to enhance the WO(3) electrical response, was carried out by including Cr(III) and Pt(IV) centers in the oxide matrix. It turned out that Cr remains as homogeneously dispersed Cr(III) centers inside the WO(3) host, while Pt undergoes reduction and aggregation to form nanoclusters located at the oxide surface. Upon interaction with NH(3), the electrical conductivity of transition metal doped-WO(3) increases, especially in the presence of Pt dopant, resulting in outstanding sensing properties (S = 110 ± 15 at T = 225 °C and [NH(3)] = 74 ppm). A mechanism was suggested to explain the excellent electrical response of Pt-doped films with respect to the Cr-doped ones. This associates the easy chemisorption of ammonia on the WO(3) nanocrystals, promoted by the inverted opal structure, with the catalytic action exerted by the surface Pt nanoclusters on the N-H bond dissociation. The overall results indicate that in Pt-doped WO(3) films the effects of the macroporosity positively combine with the electrical sensitization promoted by the metal nanoclusters, thus providing very lightweight materials which display high functionality even at relatively low temperatures. We expect that this synergistic effect can be exploited to realize other functional hierarchical metal oxide structures to be used as gas sensors or catalysts.

Mesoporous Tin-Triflate Based Catalysts for Transesterification of Sunflower Oil

Unimodal porous system (denoted as SnTf-MCM-41) and bimodal porosity (denoted as SnTf-UVM-7) were prepared in a two-step synthesis. The triflic acid was incorporated into previously synthesized Sn-containing porous silicas. Chemical analysis was carried out by electron probe microanalysis (EPMA), textural properties were determined by transmission electron microscopy (TEM) and from nitrogen adsorption–desorption isotherms (77 K), while structural characterization by X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) measurements. The structural characterization provided evidences on the direct interaction of triflate with incorporated tin species. The acidity of these catalysts was determined from the ammonia chemisorption measurements. The catalysts have been tested for transesterification of sunflower oil under both microwave and ultrasound activation. These experiments showed a direct reaction between the acidity of the catalysts and catalytic performances. No leaching of the triflate species or catalyst deactivation have been detected over several runs.

Effects of nanoparticle and porous metal oxide supports on the activity of palladium catalysts in the oxidative coupling of 4-methylpyridine

The oxidative coupling of 4-methylpyridine to 4,4′-dimethyl-2,2′-bipyridine over palladium oxide is a simple, environmentally friendly, one-step process to produce bipyridines, which are commonly used with transition metal ions to form complexes with interesting properties. However, the reaction is slow and the palladium catalyst deactivates during reaction, which means that catalyst improvements are needed for large-scale production of more economically viable bipyridine products. In this study, a number of metal oxides were investigated as catalyst supports and compared to the best performing catalysts to date, i.e. Pd/C and Pd/n-Al 2O 3(+). Catalysts supported on several nanoparticle oxides with varying properties as well as some conventional supports were prepared and characterized in an attempt to determine properties that lead to high catalytic activities in the oxidative coupling of 4-methylpyridine. It was found that two general categories of active catalysts can be prepared; (1) palladium supported on very high surface area materials, such as Pd/n-Al 2O 3(+) and Pd/MgO, and (2) palladium supported on metal oxides known to induce strong palladium-support interactions, e.g. Pd/ZrO 2, Pd/(n-ZrO 2 + n-CeO 2) and Pd/n-ZnO. While there is no simple correlation between the palladium surface area and the catalytic activity, higher palladium dispersions generally gave higher yields compared to lower dispersion catalysts. The results indicate that the reaction is structure sensitive, i.e. not all the palladium on the surface is equivalent and some palladium species are more active than others. The acidic and basic properties of the supports were determined via chemisorption of ammonia and carbon dioxide, respectively. The data indicate that there is no correlation between the acidic or basic sites of the supports and the palladium dispersion or the catalytic activity, although highly acidic or highly basic supports should be avoided as they resulted in lower dispersions than expected from their corresponding surface areas. In terms of economic viability the porous TiO 2 support was determined to be the most competitive with the nanoparticle alumina support as it results in a catalyst with comparable yields and is less expensive compared with nanoparticle alumina. The palladium supported on nanoparticle ZrO 2 and MgO are also promising catalysts.

Preparation, characterisation and catalytic evaluation of MgF2 supported V2O5 catalysts for ammoxidation of 3-picoline

Non-conventional MgF2 supported V2O5 catalysts with different vanadia contents (2–15 wt.%) were prepared by impregnation (using NH4VO3), characterised and catalytically evaluated for selective ammoxidation of 3-picoline to nicotinonitrile. Oxygen and ammonia chemisorption uptakes increased continuously from 60 to over 600 μmol g−1 and 275 to >750 μmol g−1, respectively, with rise in V2O5 proportion indicating that the redox as well as acidic sites are increasing with increase in V2O5 content. Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) analysis revealed endothermic as well as exothermic thermal effects mainly due to liberation of water and ammonia, and also due to structural changes. XRD patterns showed the formation of crystalline V2O5 in the fresh solids having 8 wt.% V2O5 and above and NH4VO3 phase in the spent samples. The conversion of 3-picoline is observed to increase continuously with increase in V2O5 loading. However, the selectivity of nicotinonitrile is found to be independent on conversion of 3-picoline. The catalyst with the highest V2O5 loading (15.7 wt.%) displayed the best activity (X > 90%) and selectivity (S > 95%) compared to all other catalysts of this series. The 3-picoline conversion of 10% at 548 K is increased to almost 100% with rise in temperature to 663 K. Increase in 3-picoline feed rate and NH3: 3-picoline ratio exhibited an inhibiting effect on the conversion, while an increase in air: 3-picoline ratio has no significant influence on the performance.

Ultramicro molybdenum nitride powder prepared using high-energy mechanochemical method

Using the specially designed mechanochemical ball-mill equipment, ultramicro molybdenum nitride powders were prepared from pure molybdenum powders in ammonia atmosphere at room temperature by high-energy ball milling. The structure and the particle size of the powders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that the mass ratio of grinding media to powder was 8:1, after milling for 30 h the Mo 2N of fcc structure was obtained, and the average particle size of the powders was around 100 nm. It is found that the chemisorption of ammonia onto the fresh molybdenum surfaces created by milling was the predominant process during solid-gas reaction, and the energy input due to introduction of highly dense grain boundaries and lattice defects offered the activation energy for the transition from Mo—N chemisorption to molybdenum nitride. In addition, the change of Mo electronic undersaturation induced by the grain refining accelerated the bonding between Mo and N. The mechanism model of whole nitriding reaction was given. During the high-energy ball milling processing, the rotational speed of milling played a critical role in determining the overall reaction speed.

Environmentally friendly solid acid catalyst prepared by modifying TiO2 with cerium sulfate for the removal of volatile organic chemicals

An environmentally friendly solid acid catalyst, Ce(SO4)2/TiO2 was prepared simply by modifying TiO2 with Ce(SO4)2 for acid catalysis of volatile organic chemicals, 2-propanol and cumene. The characterization of prepared catalysts was performed using FTIR, XRD and DSC. The surface area of 7-Ce(SO4)2/TiO2 calcined at 300°C was very high (206.0m2/g) compared to that of unmodified TiO2 (115.2m2/g) due to the interaction between Ce(SO4)2 and TiO2. 7-Ce(SO4)2/TiO2 containing 7wt% Ce(SO4)2 and calcined at 300°C exhibited maximum catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation. The catalytic activities for both reactions were correlated with the acid amounts of catalysts measured by an ammonia chemisorption method. The role of Ce results in an increase in the thermal stability of the surface sulfate species and consequently the acid amount of Ce(SO4)2/TiO2 is increased. The asymmetric stretching frequency of the SO bonds for Ce(SO4)2/TiO2 catalysts was related to the acidic properties and to the catalytic activity for acid catalysis to remove volatile organic chemicals, 2-propanol and cumene.

Ammoxidation of 3-picoline over V2O5/MgF2 catalysts: Correlations between nicotinonitrile yield and O2 and NH3 chemisorption properties

Abstract V2O5/MgF2 catalysts with varying vanadia contents (V/Mg = 0.02–0.13) were prepared by impregnation and tested towards ammoxidation of 3-picoline to nicotinonitrile. The solids were characterised by BET-surface area determination, high temperature oxygen chemisorption and ammonia chemisorption. Both the O2 and NH3 uptakes were found to increase continuously with increase in V/Mg molar ratio of the catalysts. The catalytic activity changes in a similar fashion, while the nicotinonitrile selectivity is varied over a small range. Thus, direct correlations between the yield of nicotinonitrile and O2 and NH3 uptakes could be obtained. The catalyst with the highest V/Mg molar ratio displayed the best result giving 96% selectivity of nicotinonitrile at 91% conversion of 3-picoline.

Characterization of NiSO4Supported on Fe2O3and Catalytic Properties for Ethylene Dimerization

The NiSO4 supported on Fe2O3 catalysts were prepared by the impregnation method. No diffraction line of nickel sulfate was observed up to 30 wt %, indicating good dispersion of nickel sulfate on the surface of Fe2O3. The addition of nickel sulfate to Fe2O3 shifted the phase transition of Fe2O3 (from amorphous to hematite) to higher temperatures because of the interaction between nickel sulfate and Fe2O3. 20-NiSO4/Fe2O3 containing 20 wt % of NiSO4 and calcined at 500 oC exhibited a maximum catalytic activity for ethylene dimerization. The initial product of ethylene dimerization was found to be 1-butene and the initially produced 1-butene was also isomerized to 2-butene during the reaction. The catalytic activities were correlated with the acidity of catalysts measured by the ammonia chemisorption method.

Characterization and Acid Catalytic Properties of NiO Supported on ZrO<sub>2</sub> and Modified with MoO<sub>3</sub>

Nickel oxide supported on zirconia and modified with MoO3 for acid catalysis was prepared by drying powdered Ni(OH)2-Zr(OH)4 with ammonium heptamolybdate aqueous solution, followed by calcining in air at high temperature. The characterization of prepared catalysts was performed using FTIR, Raman, XRD, and DSC. MoO3 equal to or less than 15 wt% was dispersed on the surface of catalyst as two-dimensional polymolybdate or monomolybdate, while for MoO3 above 15 wt%, crystalline orthorhombic phase of MoO3 was formed. The high acid strength and acidity were responsible for the Mo=O bond nature of the complex formed by the interaction between MoO3 and ZrO2. The catalytic activity for cumene dealkylation was correlated with the acidity of the catalyst measured by the ammonia chemisorption method

Effect of TiO2–ZrO2 composition on catalytic activity of supported NiSO4 for ethylene dimerization

A series of catalysts: NiSO4/TiO2–ZrO2, having different TiO2–ZrO2 compositions, for ethylene dimerization was prepared by the impregnation method using an aqueous solution of nickel sulfate. No diffraction line of nickel sulfate was observed up to 30 wt%, indicating good dispersion of nickel sulfate on the surface of the supported NiSO4 catalysts. The surface area and acidity of TiO2–ZrO2 binary metal oxide increased remarkably compared with the values for single metal oxides, TiO2 or ZrO2. The binary oxide TiO2–ZrO2 calcined above 600 °C resulted in the formation of a crystalline orthorhombic phase of ZrTiO4. Therefore, NiSO4/TiO2–ZrO2 calcined at 500 °C exhibited a maximum catalytic activity, and then the catalytic activity decreased with the calcination temperature. NiSO4 supported on 50TiO2-50ZrO2 (TiO2/ZrO2 ratio = 1) exhibited the highest catalytic activity for ethylene dimerization among TiO2–ZrO2 binary oxides. The catalytic activities of supported NiSO4 catalysts were correlated with the acidity of catalysts measured by the ammonia chemisorption method, regardless of the kind of support (TiO2, ZrO2, or TiO2–ZrO2).