Anatase Support Research Articles

Green Thermo-Photo Catalytic Production of Syngas Using Pd/Nb-TiO2 Catalysts.

In this contribution, a series of Pd-promoted Nb-doped titania samples were essayed in the gas-phase thermo-photo production of syngas from methanol/water mixtures. The Pd loading was tested in the 0.1 to 2.5 wt % range, leading to the presence of metallic nanoparticles under reaction. Reaction rates exceeding 52 mmol H2 g-1 h-1 and quantum efficiencies above 33% were obtained. The optimum sample having a 0.5 wt % of Pd provided an outstanding synergy between light and heat under reaction conditions, facilitating the boost of activity with respect to the single-source processes and achieving high selectivity to syngas. The spectroscopic analysis of the physico-chemical ground of the activity unveiled that the noble metal interaction with the Nb-doped anatase support triggers a cooperative effect, promoting the evolution of formic acid-type methanol-derived carbon-containing species and rendering a significant enhancement of syngas production. The proposed thermo-photo system is thus a firm candidate to contribute to the new green circular economy.

Extracting Turnover Frequencies of Electron Transfer in Heterogeneous Catalysis: A Study of IrO2-TiO2 Anatase for Water Oxidation Using Ce4+ Cations

Within the context of electron transfer during the catalytic water oxidation reaction, the Ir-based system is among the most active. The reaction, mimicking photosynthesis II, requires the use of an electron acceptor such the Ce4+ cation. This complex reaction, involving adsorbed water at the interface of the metal cation and Ce4+, has mostly been studied in homogenous systems. To address the ambiguity regarding the gradual transformation of a homogenous system into a heterogeneous one, we prepared and studied a heterogeneous catalyst system composed of IrO2, with a mean particle size ranging from about 5 Å to 10 Å, dispersed on a TiO2 anatase support, with the objective of probing into the different parameters of the reaction, as well as the compositional changes and rates. The system was stable for many of the runs that were conducted (five consecutive runs with 0.18 M of Ce4+ showed the same reaction rate with TON > 56,000) and, equally importantly, was stable without induction periods. Extraction of the reaction rates from the set of catalysts, with an attempt to normalize them with respect to Ir loading and, therefore, to obtain turnover frequencies (TOF), was conducted. While, within reasonable deviations, the TOF numbers extracted from TPR and XPS Ir4f were close, those extracted from the particle shape (HR-STEM) were considerably larger. The difference indicates that bulk Ir atoms contribute to the electron transfer reaction, which may indicate that the reaction rate is dominated by the reorganization energy between the redox couples involved. Therefore, the normalization of reaction rates with surface atoms may lead to an overestimation of the site activity.

Insight into the effect of support crystal form on semi-continuous oxidation of glycerol

In this work, two Pt catalysts using anatase and rutile as supports were prepared, and then the support effect on catalytic performance in glycerol oxidation was investigated. Different Pt/TiO2 catalysts exhibited not so negligible distinction on the product distribution. On one side, the C=O bonds of intermediate glyceraldehyde (GLYAD) highly dissociated in the form of bidentate species through C atom bonding to O atom and O atom bonding to Ti atom of rutile support, which largely promoted the transformation from GLYAD to glyceric acid (GLYA). While, in the case of anatase support, the adsorption form of C=O was monodentate. On another side, the formed GLYA was weakly adsorbed through two O atoms bridging/bonding to two Ti atoms. Due to the longer Ti–Ti distance in anatase, C–C cleavage of GLYA was obviously inhibited. The rutile support with shorter Ti–Ti distance inevitably led to obvious C–C cleavage. The discovery of the support effect could offer an alternative method to controllably regulate product distribution and offers specific perspective on designing efficient catalysts in semicontinuous oxidation reactions.

Modification of O and CO binding on Pt nanoparticles due to electronic and structural effects of titania supports.

Metal oxide supports often play an active part in heterogeneous catalysis by moderating both the structure and the electronic properties of the metallic catalyst particle. In order to provide some fundamental understanding on these effects, we present here a density functional theory (DFT) investigation of the binding of O and CO on Pt nanoparticles supported on titania (anatase) surfaces. These systems are complex, and in order to develop realistic models, here, we needed to perform DFT calculations with up to ∼1000 atoms. By performing full geometry relaxations at each stage, we avoid any effects of "frozen geometry" approximations. In terms of the interaction of the Pt nanoparticles with the support, we find that the surface deformation of the anatase support contributes greatly to the adsorption of each nanoparticle, especially for the anatase (001) facet. We attempt to separate geometric and electronic effects and find a larger contribution to ligand binding energy arising from the former. Overall, we show an average weakening (compared to the isolated nanoparticle) of ∼0.1 eV across atop, bridge and hollow binding sites on supported Pt55 for O and CO, and a preservation of site preference. Stronger effects are seen for O on Pt13, which is heavily deformed by anatase supports. In order to rationalize our results and examine methods for faster characterization of metal catalysts, we make use of electronic descriptors, including the d-band center and an electronic density based descriptor. We expect that the approach followed in this study could be applied to study other supported metal catalysts.

Toward the Green Production of H2: Binary Pt–Ru Promoted Nb-TiO2 Based Photocatalysts

Hydrogen photoproduction using methanol:water mixtures was essayed with PtRu titania-based materials. All materials contain a high-surface-area Nb-doped anatase support. Six bimetallic formulations with Pt:Ru ratios from 4:1 to 1:4 and two (Pt and Ru) monometallic reference samples were utilized. The catalytic response of the materials to three main experimental variables (methanol:water molar ratio, catalyst concentration, and light intensity) was studied through a Box-Benhken factorial design and the calculation of the true quantum efficiency. The optimum activity is achieved using a bimetallic PtRu (2:1) composition, which provides outstanding efficiency under ultraviolet (UV), visible, and sunlight illumination, as well as long-term stability. The interpretation of the activity was performed using characterization results, the outcome of an analysis of charge recombination with photoluminescence spectroscopy and of an in situ infrared study under reaction conditions. This proves that the photophysical and chemical properties of metallic Pt particles in contact with Ru oxidized particles are key to render highly active systems.

Enhanced CO2 Methanation Activity of Ni/Anatase Catalyst by Tuning Strong Metal–Support Interactions

Strong metal–support interaction (SMSI) has been widely recognized for platinum-group metals on reducible oxide supports. Herein we report that the catalytic activity of Ni catalyst in CO2 methanation is significantly suppressed over conventional anatase (a-TiO2) support due to the SMSI-induced formation of a titania overlayer around the Ni nanoparticles. Furthermore, CO is the only product . In contrast, the NH3-treatment and H2-treatment of the a-TiO2 support enhance remarkably the activity of Ni, i.e., CO2 conversion increases by 1 order of magnitude and CO2 is hydrogenated almost exclusively to CH4. X-ray photoelectron spectroscopy (XPS), H2 and CO chemisorption, and low temperature electron paramagnetic resonance (EPR) reveal that the enhanced CO2 methanation activity may be related with the Ti3+ species in the bulk that are generated by reduction treatment, which likely have altered the SMSI between Ni and a-TiO2 support. This simple reduction treatment approach may be applicable to modulate the SM...

Striking effects of a titania support on the low-temperature activities of Ir catalysts for the dehydrogenative synthesis of benzimidazole and indole

The crystalline structure of titania supports for iridium catalysts markedly affected their low-temperature activity for the dehydrogenative synthesis of N-containing heteroaromatics, namely benzimidazole and indole. While solid iridium catalysts supported on anatase showed moderate to poor activity for the synthesis of 2-phenylbenzimidazole (3) from o-phenylenediamine (1) and benzyl alcohol (2) at 100°C, the reaction in the presence of rutile-supported catalysts proceeded smoothly to give 3 in high yields of up to 88%. Similar results were observed for the dehydrogenative conversion of 2-(2-aminophenyl)-ethanol (4) to indole (5). The reaction at 100°C for 18h in the presence of 1.0mol% iridium on rutile gave 5 in a yield of 73%, while the use of anatase-supported catalysts resulted in significantly lower yields. TEM analysis showed the formation of small (ca. 2nm in diameter), homogeneously-dispersed iridium nanoparticles on rutiles, while the inhomogeneous loading of iridium species was observed on anatase supports. CO pulse experiments revealed that there is a strong correlation between CO uptake by iridium nanoparticles and the activities at 100°C. These results suggest that the predominant formation of small, well-reduced iridium nanoparticles is one major reason for the excellent activities of rutile-supported catalysts at low temperatures.

Controlled Growth of Ceria Nanoarrays on Anatase Titania Powder: A Bottom-up Physical Picture.

The leading edge of catalysis research motivates physical understanding of the growth of nanoscale oxide structures on different supporting oxide materials that are themselves also nanostructured. This research opens up for consideration a diverse range of facets on the support material, versus the single facet typically involved in wide-area growth of thin films. Here, we study the growth of ceria nanoarchitectures on practical anatase titania powders as a showcase inspired by recent experiments. Density functional theory (DFT)-based methods are employed to characterize and rationalize the broad array of low energy nanostructures that emerge. Using a bottom-up approach, we are able to identify and characterize the underlying mechanisms for the facet-dependent growth of various ceria motifs on anatase titania based on formation energy. These motifs include 0D clusters, 1D chains, 2D plates, and 3D nanoparticles. The ceria growth mode and morphology are determined by the interplay of several factors including the role of the common cation valence, the interface template effect for different facets of the anatase support, enhanced ionic binding for more compact ceria motifs, and the local structural flexibility of oxygen ions in bridging the interface between anatase and ceria structures.

In situ FT-IR investigation of CO oxidation on CuO/TiO2 catalysts

The present work tentatively investigated anatase- or rutile-supported copper oxide catalysts. 5% copper oxide can be well dispersed on anatase or rutile surfaces. The copper oxide dispersed on rutile surface exists in Cu2+ and Cu+ ions, which is more readily reduced, and show higher catalytic activity of CO oxidation. The mechanism of CO oxidation over rutile support is different from anatase support.

Facile synthesis, characterization and catalytic activity of nanoporous supports loaded with monometallic and bimetallic nanoparticles

Abstract When an aqueous platinum chloride or mixed rhodium and platinum chloride solutions are added to a mixture of lecithin, AOT and titanium isopropoxide (TIP) in isooctane, the surfactants partition the aqueous and oil phases into a nanoscale bicontinuous microstructure. Hydrolysis and condensation of the TIP generates a highly connected porous anatase support. Reduction of the metal salt/s in the confined aqueous nanodomains of the bicontinuous template results in monometallic or bimetallic alloy NPs. The NPs are well-distributed throughout the interior surfaces of the support. This route, where monometallic and bimetallic nanoparticles are synthesized together with the support material, can be exploited for generating diverse catalytically active materials for various applications.

Computational Study of Structure and Reactivity of Oligomeric Vanadia Clusters Supported on Anatase and Rutile TiO2 Surfaces

We use density functional theory to examine structure–activity relationships of small vanadia clusters supported on anatase TiO2(001) and rutile TiO2(110) surfaces. A thermodynamic analysis indicates that the vanadia monomer cluster can be stabilized on the anatase TiO2(001) surface in a catalytically relevant oxygen environment. On the other hand, vanadia clusters tend to aggregate into dimers on the rutile TiO2(110) surface because this surface binds the monomer less strongly as compared to anatase. Hydrogen adsorption is found to be exothermic on the vanadia monomer adsorbed on both supports, enhanced by a charge transfer between the adsorbate and the substrate. There is no such charge transfer on vanadia dimers and tetramers, where the hydrogen adsorption energies are similar to that on the single crystal V2O5(001) surface. The improved catalytic performance of the anatase support can be attributed to the ability of this surface to stabilize the catalytically active vanadia monomer clusters.

Design and Preparation of Supported Au Catalyst with Enhanced Catalytic Activities by Rationally Positioning Au Nanoparticles on Anatase.

A synergistic effect between individual components is crucial for increasing the activity of metal/metal oxide catalysts. The greatest challenge is how to control the synergistic effect to obtain enhanced catalytic performance. Through density functional theory calculations of model Au/TiO2 catalysts, it is suggested that there is strong interaction between Au nanoparticles and Ti species at the edge/corner sites of anatase, which is favorable for the formation of stable oxygen vacancies. Motivated by this theoretical analysis, we have rationally prepared Au nanoparticles attached to edge/corner sites of anatase support (Au/TiO2-EC), confirmed by their HR-TEM images. As expected, this strong interaction is well characterized by Raman, UV-visible, and XPS techniques. Very interestingly, compared with conventional Au catalysts, Au/TiO2-EC exhibits superior catalytic activity in the oxidations using O2. Our approach to controlling Au nanoparticle positioning on anatase to obtain enhanced catalytic activity offers an efficient strategy for developing more novel supported metal catalysts.

Supercritical antisolvent precipitation of TiO2 with tailored anatase/rutile composition for applications in redox catalysis and photocatalysis

TiO2 with tailored anatase/rutile composition has been prepared from the supercritical antisolvent (SAS) precipitation of a range of titanium alkoxides. The calcination of the SAS TiO2 was monitored by in situ powder X-ray diffraction to determine the optimal calcination conditions for the formation of a mixed anatse/rutile phase TiO2. The SAS precipitated material calcined at 450°C produced a predominantly anatase support while calcination at 750°C resulted in a 90wt% anatase and 10wt% rutile TiO2. 5wt% AuPd was added to the SAS TiO2 using an impregnation technique, with exceptional dispersion of the metals being observed by transmission electron microscopy. Mean metal particle sizes were determined to be below 1nm for both anatase and anatase/rutile SAS TiO2 materials. These catalysts were found to be highly active for the selective oxidation of benzyl alcohol and the direct synthesis of hydrogen peroxide. In addition the anatase/rutile SAS TiO2 was found to have comparable activity to commercial anatase/rutile mixed phase TiO2 for the photocatalytic splitting of water for hydrogen production.

Titania-Supported Catalysts for Levulinic Acid Hydrogenation: Influence of Support and its Impact on γ-Valerolactone Yield.

A series of titania-supported ruthenium and platinum catalysts was investigated in the levulinic acid hydrogenation towards γ-valerolactone, a key reaction for the catalytic transformation of biomass. It was shown that various morphologies and phases of titania strongly influence the physicochemical and catalytic properties of supported Ru and Pt catalysts in different ways. In the case of the catalyst supported on mixed TiO2 phases, Ru particles are exclusively located on the minority rutile crystallites, whereas such an effect was not observed for platinum. The platinum catalyst activity could be increased when the metal was dispersed on the large surface-area anatase, which was not the case for ruthenium as a result of its agglomeration on this support. The activity of ruthenium on anatase could be increased in two ways: a) when RuO2 formation during catalyst preparation was avoided; b) when pure anatase support material was modified so that it exhibited no microporosity. The obtained results allow a better understanding of the role of the support for Ru and Pt catalysts.

One-pot Synthesis of Pt Catalysts Supported on Al-modified TiO2

A facile, industrially viable, one-pot synthesis of 0.5-8 wt% Pt supported on 22 mol% Al-modified ana-tase with high surface area and thermal stability is presented. Four pathways were studied to deter-mine the effects of support properties on catalyst dispersion, and the highest dispersions were observed for high surface area materials containing 5-coordinate anatase. Systematic study of preparation vari-ables shows that low drying temperatures, slow calcination ramp rates, and slow reduction ramp rates further increased Pt dispersion and resulted in a more uniform Pt size distribution. Pt dispersions as high as 54% have been obtained using the one-pot method and 59% for Pt catalysts synthesized by dry impregnation. Statistically designed studies are needed to more completely determine the effects of synthesis variables and to optimize the dispersion and reduction of Pt supported on Al-modified ana-tase. Results presented in this paper show that this one-pot method and dry impregnation method us-ing our Al-modified anatase support are promising syntheses of highly dispersed Pt supported on stabi-lized titania. Our results demonstrate that the alumina-stabilized anatase support is superior to other anatase supports for (1) obtaining high Pt dispersions, i.e. more efficiently utilizing this expensive pre-cious metal, and (2) processes in which thermal stability is important due to its constant phase and pore structures at high temperatures. © 2014 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

A DFT + U study of acetylene selective hydrogenation over anatase supported PdaAgb (a + b = 4) cluster.

It is well known that the addition of Ag into Pd can promote the selectivity of acetylene hydrogenation to ethylene, and early theoretical studies focus on ideal single crystal model catalysts, so it is worth studying relatively realistic catalyst models, such as metal oxide supported PdAg systems. In this work, the reaction mechanisms for acetylene selective hydrogenation on the anatase TiO2(101) supported PdaAgb (a + b = 4) cluster are studied by density functional theory calculations with a Hubbard U correction. The results show that Ag addition to the Pd4 cluster reduces the interaction between the PdAg cluster and the support, and the possible reason is that the amount of electron transfer from the TiO2 support to the PdAg cluster decreases with increasing number of Ag atoms. Consequently the adsorption energies of acetylene and ethylene would become smaller on the anatase supported PdAg cluster as compared to that on the anatase supported Pd4 cluster, and this may help to enhance the selectivity of ethylene formation. Moreover, the reaction kinetics study of acetylene hydrogenation on anatase TiO2(101) supported PdAg cluster shows that the activation energy of the hydrogenation step is higher on the PdAg cluster than that on the pure Pd4 cluster, and thus reduces its catalytic activity. Importantly, the present calculation results suggested that the selectivity of ethylene formation, which is defined as the energy difference between the adsorption energy of ethylene and the barrier for its further hydrogenation, varies with the ratio of Pd/Ag in the PdAg cluster: the Pd3Ag system shows relatively low selectivity compared to that of the pure Pd4 cluster, whereas Pd2Ag2/PdAg3 displays higher selectivity than that of the pure Pd4 cluster. Furthermore, our present results demonstrated that the anatase support plays a key role in the acetylene hydrogenation processes: on one hand, it reduces the reaction activity of acetylene hydrogenation processes compared to the Pd2Ag2/Pd(111) and Pd2Ag2 clusters; on the other hand, it enhances the selectivity of ethylene due to its lower desorption energy. It was also found that the carbon species inside the Pd2Ag2 cluster has little effect on the catalytic selectivity towards ethylene formation, whereas the hydrogenation catalytic activity is enhanced significantly. Finally the role of the Pd2Ag2-anatase interface on the catalytic properties of acetylene hydrogenation was studied, and it was found that the interface can increase the activity of acetylene hydrogenation but the selectivity is not improved.

A theoretical investigation on the influence of anatase support and vanadia dispersion on the oxidative dehydrogenation of propane to propene

The oxidative dehydrogenation of propane (ODP) on the anatase supported vanadia catalysts (VOx/TiO2) have been investigated using periodic DFT calculations. Free energy profiles indicate that the first C–H activation step is the rate-determining (RD) step and the transition state (TS) of the propene formation step is the RD–TS. ODP activity can be tuned by vanadia dispersion and support surface via the modification of the electronic structure of the active oxygen sites. For the RD step, on both dimer VOx/TiO2 catalysts terminal sites have higher activity. On monomer VOx/TiO2 (100) terminal and interface sites exhibit similar activity, while on monomer VOx/TiO2 (001) interface sites have higher activity. With increasing vanadia loading, the formation of propene changes from propyl radical mechanism to a concerted propoxide one. The results suggest that TiO2 (100) is a better support surface. Terminal and interface oxygen sites act cooperatively as the first and second C–H bond activation centers, respectively.

A DFT+U study of acetylene selective hydrogenation on oxygen defective anatase (101) and rutile (110) TiO2 supported Pd4 cluster

The reaction mechanisms for selective acetylene hydrogenation on three different supports, Pd(4) cluster, oxygen defective anatase (101), and rutile (110) titania supported Pd(4), cluster are studied using the density functional theory calculations with a Hubbard U correction (DFT+U). The present calculations show that the defect anatase support binds Pd(4) cluster more strongly than that of rutile titania due to the existence of Ti(3+) in anatase titania. Consequently, the binding energies of adsorbed species such as acetylene and ethylene on Pd(4) cluster become weaker on anatase supported catalysts compared to the rutile supported Pd(4) cluster. Anatase catalyst has higher selectivity of acetylene hydrogenation than rutile catalyst. On the one hand, the activation energies of ethylene formation are similar on the two catalysts, while they vary a lot on ethyl formation. The rutile supported Pd catalyst with lower activation energy is preferable for further hydrogenation. On the other hand, the relatively weak adsorption energy of ethylene is gained on anatase surface, which means it is easier for ethylene desorption, hence getting higher selectivity. For further understanding, the energy decomposition method and micro-kinetic analysis are also introduced.

Periodic density functional theory studies of the VOx/TiO2 (anatase) catalysts: Structure and stability of monomeric species

Periodic density functional theory has been utilized to investigate the structure and stability of monomeric HVOx species on anatase support. The three most stable surfaces of anatase were investigated, namely the (001), (100) and (101) surfaces. Unlike previous theoretical studies it was found that on the (001) surface vanadia species with five-coordinated vanadium atom are more stable than those with tetrahedrally coordinated vanadium atom. On the other hand, on the (100) and (101) surfaces, the vanadium atom in the vanadia species is still tetrahedrally coordinated. The stability of different VOx/TiO2 structures which are not fully dehydrated has been systematically studied and the results show that the vanadia species on the three surfaces follow an order of TiO2 (001)>TiO2 (100)>TiO2 (101). This can be understood from the acidity and basicity of the three anatase surfaces. The results suggest that monomeric VOx species may be better stabilized if the support exposes more (001) surfaces. Our analyses on electronic structure of the most stable VOx/TiO2 structure (D001) suggest that its bridging V–O–Ti oxygen atoms may have higher reactivity than the terminal vanadyl oxygen atoms.

The structure and catalytic activity of anatase and rutile titania supported manganese oxide catalysts for selective catalytic reduction of NO by NH3

The structure and catalytic properties of anatase and rutile supported manganese oxide catalysts prepared by impregnation method have been studied by using X-ray diffraction (XRD), laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy (XPS), H(2) temperature-programmed reduction (H(2)-TPR) and BET surface area measurements combined with activity testing of selective catalytic reduction (SCR) of NO by NH(3). It has been shown that the manganese oxide loadings on the two TiO(2) supports exert great influences on the SCR activity. For the rutile supported manganese oxide catalysts, increasing manganese oxide loading leads to the increase of reducibility of dispersed manganese oxide species and the rate constant k, which reaches a maximum around 9.6 × 10(-6) mol g(Mn)(-1) s(-1) at 0.5 mmol Mn per 100 m(2) TiO(2). When the manganese oxide loading is beyond this value, the existence of amorphous MnO(x) multiple layers will certainly reduce the ratio of manganese oxide species exposed on the surface and the reducibility of dispersed manganese oxide species, resulting in the rapid decrease of rate constant k. The LRS and XPS results have revealed that for the anatase supported manganese oxide catalysts manganese oxide species exist in Mn(+4) as a major species with Mn(+3) species and partially undecomposed Mn-nitrate as the minor species. Under the SCR reaction conditions, Mn(+3) species on anatase are oxidized to Mn(+4) species, inserting in the surface of anatase and promoting the anatase-to-rutile transformation in the surface layers of the anatase support. Since these Mn(4+) cations are actually dispersed on the support with a rutile shell-anatase core structure and its concentration is very near to that of MnO(x)/TiO(2) (R) catalyst, the relation between the rate constant k and the MnO(x) loading on the anatase support is similar to that on the rutile support, and that the rate constant k values for anatase and rutile supported manganese oxide catalysts are very close at the same MnO(x) loading.