Fundamental Studies of Well-Defined Surfaces of Mixed-Metal Oxides: Special Properties of MOx/TiO2(110) {M = V, Ru, Ce, or W}

Dynamic control and quantification of active sites on ceria for CO activation and hydrogenation

Ceria (CeO2) is a widely used oxide catalyst, yet the nature of its active sites remains elusive. This study combines model and powder catalyst studies to elucidate the structure-activity relationships in ceria-catalyzed CO activation and hydrogenation. Well-defined ceria clusters are synthesized on planar CeO2(111) and exhibit dynamic and tunable ranges of Ce coordination numbers, which enhance their interaction with CO. Reduced ceria clusters (e.g., Ce3O3) bind CO strongly and facilitate its dissociation, while near-stoichiometric clusters (e.g., Ce3O7) adsorb CO weakly and promote oxidation via carbonate formation. Unlike planar ceria surfaces, supported ceria clusters exhibit dynamic properties and enhanced catalytic activity, that mimic those of powder ceria catalysts. Insight from model studies provide a method to quantify active sites on powder ceria and guide further optimization of ceria catalysts for syngas conversion. This work marks a leap toward model-guided catalyst design and highlights the importance of site-specific catalysis.

Enthalpy Change of V-Ti Interface as an Indicator of High Catalytic Activity

Vanadium-supported TiO2 is one of the most widely used catalysts. In previous reports, most researchers focused on the performance of a formed catalyst and almost no work was devoted to understanding the activation process from a precursor to a catalyst. In this work, differential scanning calorimetry was used to calculate the enthalpy change (ΔH, kJ·mol-1) during the transition from a precursor to a catalyst. When the V-loading amount was increased from 0.1 to 5 wt %, more polymeric V were formed and ΔH of V-supported anatase was decreased from 10.13 to 4.13 kJ·mol-1. At the same loading amount of 1 wt %, anatase showed a higher ΔH value of 8.71 kJ·mol-1 than rutile and brookite. When the ratio of the {001} facet was increased in the anatase, ΔH was increased to 9.65 kJ·mol-1. A theoretical calculation proved that V embedding into {001} facet resulted in a bigger energy difference in comparison to {101} and {100} facets. A bigger ΔH stood for forming a more active V species during catalyst preparation, which further stood for a higher turnover frequency (TOF, s-1) during the catalysis. The anatase with the biggest ratio of the {001} facet resulted in the biggest ΔH as well as the largest TOF. These results help to understand the interaction between loaded active species and catalyst support, which is in favor of designing an effective catalyst.

CeO2 nanorods supported CuOx-RuOx bimetallic catalysts for low temperature CO oxidation

Bimetallic catalysts often outperform monometallic catalysts due to changeable structural orientation, synergistic effects, and integration of two different metal or metal oxide properties. Here, a series of CeO2 nanorods (NR) supported bimetallic CuOx and RuOx catalysts (Cu: Ru ratios of 9:1, 7:3, and 5:5) were prepared using a wet impregnation method. In situ DRIFTS, H2 temperature programmed reduction (H2-TPR), CO temperature programmed desorption (CO-TPD), and other characterization techniques were used to investigate the effect of the Cu:Ru ratio on the activity of low-temperature CO oxidation. Among three catalysts, CeO2 NR supported 7 wt% Cu-3 wt% Ru catalyst after a reduction activation treatment showed the best performance with 100 % CO conversion at 166 °C and the lowest activation energy of 18.37 kJ mol-1. Raman and XPS profiles revealed that the origin of the superior performance is at least partially related to the high surface oxygen vacancy concentration and other distinct oxygen species (physi-/chemi-sorbed oxygen and bulk lattice oxygen), leading to outstanding adsorption and oxidation property of CO.

Theoretical Perspective of Promoting Direct Methane-to-Methanol Conversion at Complex Metal Oxide-Metal Interfaces

Direct methane conversion to methanol has been considered as an effective and economic way to address greenhouse effects and the current high demand for methanol in industry. However, the process has long been challenging due to lack of viable catalysts to compromise the activation of methane that typically occurs at high temperatures and retaining of produced methanol that requires mild conditions. This Perspective demonstrates an effective strategy to promote direct methane to methanol conversion by engineering the active sites and chemical environments at complex metal oxide - copper oxide - copper interfaces. Such effort strongly depends on extensive theoretical studies by combining density functional theory (DFT) calculations and kinetic Monte Carlo (KMC) simulations to provide in-depth understanding of reaction mechanism and active sites, which build a strong basis to enable the identification of design principles and advance the catalyst optimization for selective CH₄-to-CH₃OH conversion.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Praseodymia–titania mixed oxide supported gold as efficient water gas shift catalyst: modulated by the mixing ratio of oxides

Modulating the active sites for controllable tuning of the catalytic activity has been the goal of much research, however, this remains challenging. The O vacancy is well known as an active site in reducible oxides. To modify the activity of O vacancies in praseodymia, we synthesized a series of praseodymia–titania mixed oxides. Varying the Pr : Ti mole ratio (2 : 1, 1 : 2, 1 : 1, 1 : 4) allows us to control the electronic interactions between Au, Pr and Ti cations and the local chemical environment of the O vacancies. These effects have been studied study by X-ray photoelectron spectroscopy (XPS), CO diffuse reflectance Fourier transform infrared spectroscopy (CO-DRIFTS) and temperature-programmed reduction (CO-TPR, H₂-TPR). The water gas shift reaction (WGSR) was used as a benchmark reaction to test the catalytic performance of different praseodymia–titania supported Au. Among them, Au/Pr₁Ti₂O_x was identified to exhibit the highest activity, with a CO conversion of 75% at 300 °C, which is about 3.7 times that of Au/TiO₂ and Au/PrO_x. The Au/Pr₁Ti₂O_x also exhibited excellent stability, with the conversion after 40 h time-on-stream at 300 °C still being 67%. An optimal ratio of Pr content (Pr : Ti 1 : 2) is necessary for improving the surface oxygen mobility and oxygen exchange capability, a higher Pr content leads to more O vacancies, however with lower activity. This study presents a new route for modulating the active defect sites in mixed oxides which could also be extended to other heterogeneous catalysis systems.

Schematic illustration of H₂O activation on the Pr-TiO_x support and the following reaction with CO in the Au–oxide interface.

Impacts of the Catalyst Structures on CO2 Activation on Catalyst Surfaces

Utilizing CO2 as a sustainable carbon source to form valuable products requires activating it by active sites on catalyst surfaces. These active sites are usually in or below the nanometer scale. Some metals and metal oxides can catalyze the CO2 transformation reactions. On metal oxide-based catalysts, CO2 transformations are promoted significantly in the presence of surface oxygen vacancies or surface defect sites. Electrons transferable to the neutral CO2 molecule can be enriched on oxygen vacancies, which can also act as CO2 adsorption sites. CO2 activation is also possible without necessarily transferring electrons by tailoring catalytic sites that promote interactions at an appropriate energy level alignment of the catalyst and CO2 molecule. This review discusses CO2 activation on various catalysts, particularly the impacts of various structural factors, such as oxygen vacancies, on CO2 activation.

Oxidative dehydrogenation of ethane: catalytic and mechanistic aspects and future trends

Ethane oxidative dehydrogenation (ODH) is an attractive, low energy, alternative route to reduce the carbon footprint for ethene production, however, the commercial implementation of ODH processes requires catalysts with improved selectivity.

Mesoporous WO3/TiO2 spheres with tailored surface properties for concurrent solar photocatalysis and membrane filtration

The strategical integration of membrane water filtration with semiconductor photocatalysis presents a frontier in deep purification with a self-cleaning capability. However, the membrane fouling caused by the cake layer of the reclaimed TiO₂ nanoparticles is a key obstacle. Herein, mesoporous WO₃/TiO₂ spheres (∼450 nm in diameter) consisting of numerous self-assembled WO₃-decoated anatase TiO₂ nanocrystallites are successfully prepared via a facile wet-chemistry route. The decoration of monolayered WO₃ significantly affects the surface, photocatalytic, and optical properties of original mesoporous TiO₂ spheres. XRD and Raman analyses show the presence of monolayered WO₃ suppresses the crystal growth of TiO₂ during the calcination process, significantly improves the surface acidity, and causes an obvious red shift in absorption edge. These favorable textural properties, coupling the enhanced interfacial charge carrier separation, render mesoporous WO₃/TiO₂ spheres with a superior photocatalytic activity in degradation of methylene blue under UV, visible, and solar light irradiations. The optimal molar ratio of W/Ti is examined to 6%. The synthesized mesoporous WO₃/TiO₂ spheres also show much higher flux during membrane filtration in both dead-end and cross-flow modes, suggesting a promising photocatalyst for concurrent membrane filtration and solar photocatalysis.

SO2-Tolerant NOx Reduction by Marvelously Suppressing SO2 Adsorption over FeδCe1-δVO4 Catalysts

SO₂-tolerant selective catalytic reduction (SCR) of NO_x at low temperature is still challenging. Traditional metal oxide catalysts are prone to be sulfated and the as-formed sulfates are difficult to decompose. In this study, we discovered that SO₂ adsorption could be largely restrained over Fe_δCe_1-δVO₄ catalysts, which effectively restrained the deposition of sulfate species and endowed catalysts with strong SO₂ tolerance at an extremely low temperature of 240 °C. The increasing oxygen vacancies, enhanced redox properties, and improved acidity contributed to the SCR activity of the Fe_δCe_1-δVO₄ catalyst. The reaction pathway changed from the reaction between bidentate nitrate and the NH₃ species over CeVO₄ catalysts via the Langmuir-Hinshelwood mechanism to that between gaseous NO_x and the NH₄⁺/NH₃ species over Fe_δCe_1-δVO₄ catalysts via the Eley-Rideal mechanism. The effective suppression of SO₂ adsorption allowed Fe_δCe_1-δVO₄ catalysts to maintain the Eley-Rideal pathways on account of the reduced formation of sulfate species. This work demonstrated an effective route to improve SO₂ tolerance via modulating SO₂ adsorption on Ce-based vanadate catalysts, which presented a new point for the development of high-performance SO₂-tolerant SCR catalysts.

Amorphous TiO2 nanostructures: synthesis, fundamental properties and photocatalytic applications

In this review, we mainly highlight the advances made in the development of amorphous TiO₂ nanostructures for photocatalysts. Some perspectives on the challenges and new direction are also discussed.

Mechanism of heterogeneous catalytic oxidation of organic compounds to carboxylic acids

The results of studies on the mechanism of heterogeneous catalytic oxidation of organic compounds of different chemical structure to carboxylic acids are analyzed and generalized. The concept developed by Academician G.K.Boreskov, according to which the direction of the reaction is governed by the structure and bond energy of surface intermediates, was confirmed taking the title processes as examples. Quantitative criteria of the bond energies of surface compounds of oxidizable reactants, reaction products and oxygen that determine the selective course of the reaction are presented.

The bibliography includes 195 references.

Metal Cluster Models for Heterogeneous Catalysis: A Matrix-Isolation Perspective

Metal cluster models are of high relevance for establishing new mechanistic concepts for heterogeneous catalysis. The high reactivity and particular selectivity of metal clusters is caused by the wealth of low-lying electronically excited states that are often thermally populated. Thereby the metal clusters are flexible with regard to their electronic structure and can adjust their states to be appropriate for the reaction with a particular substrate. The matrix isolation technique is ideally suited for studying excited state reactivity. The low matrix temperatures (generally 4-40 K) of the noble gas matrix host guarantee that all clusters are in their electronic ground-state (with only a very few exceptions). Electronically excited states can then be selectively populated and their reactivity probed. Unfortunately, a systematic research in this direction has not been made up to date. The purpose of this review is to provide the grounds for a directed approach to understand cluster reactivity through matrix-isolation studies combined with quantum chemical calculations.

Photodegradation of spent wash, a sugar industry waste, using vanadium-doped TiO2 nanoparticles

A sol–gel-derived V-TiO₂ photocatalyst exhibited high photocatalytic activity for the degradation of spent wash and Jakofix red dye under natural sunlight with high stability.

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.

Facile embedding of single vanadium atoms at the anatase TiO2(101) surface

The interaction of V with anatase TiO₂(101) is studied by high-resolution STM at various temperatures, and embedding of monomeric V is found.

Mesoporous Ni/Ce1−xNixO2−y heterostructure as an efficient catalyst for converting greenhouse gas to H2 and syngas

A heterostructure of highly dispersed Ni nanoparticles in pore channels of Ni–CeO₂ solid solution, having excellent thermo-stability, redox properties, and metal/support synergy, is identified as an efficient nanocatalyst for converting greenhouse gas into H₂ energy and syngas.

A high energy output and low onset temperature nanothermite based on three-dimensional ordered macroporous nano-NiFe2O4

3DOM Al/NiFe₂O₄ nanothermite with significantly improved heat output and lower onset temperature.

Three-dimensional ruthenium-doped TiO2 sea urchins for enhanced visible-light-responsive H2 production

Ru-doped rutile TiO₂ composed of radially aligned nanorods exhibits good H₂ production from water under visible light irradiation.

Highly efficient and stable Au/CeO2-TiO2 photocatalyst for nitric oxide abatement: potential application in flue gas treatment

In the present work, highly efficient and stable Au/CeO2-TiO2 photocatalysts were prepared by a microwave-assisted solution approach. The Au/CeO2-TiO2 composites with optimal molar ratio of Au/Ce/Ti of 0.004:0.1:1 delivered a remarkably high and stable NO conversion rate of 85% in a continuous flow reactor system under simulated solar light irradiation, which far exceeded the rate of 48% over pure TiO2. The tiny Au nanocrystals (∼1.1 nm) were well stabilized by CeO2 via strong metal-support bonding even it was subjected to calcinations at 550 °C for 6 h. These Au nanocrystals served as the very active sites for activating the molecule of nitric oxide and reducing the transmission time of the photogenerated electrons to accelerate O2 transforming to reactive oxygen species. Moreover, the Au-Ce(3+) interface formed and served as an anchoring site of O2 molecule. Then more adsorbed oxygen could react with photogenerated electrons on TiO2 surfaces to produce more superoxide radicals for NO oxidation, resulting in the improved efficiency. Meanwhile, O2 was also captured at the Au/TiO2 perimeter site and the NO molecules on TiO2 sites were initially delivered to the active perimeter site via diffusion on the TiO2 surface, where they assisted O-O bond dissociation and reacted with oxygen at these perimeter sites. Therefore, these finite Au nanocrystals can consecutively expose active sites for oxidizing NO. These synergistic effects created an efficient and stable system for breaking down NO pollutants. Furthermore, the excellent antisintering property of the catalyst will allow them for the potential application in photocatalytic treatment of high-temperature flue gas from power plant.

Exceptional thermal stability of undoped anatase TiO2 photocatalysts prepared by a solvent-exchange method

A new solvent-exchange method for preparation of undoped anatase TiO₂ nanoparticles with unprecedented thermal stability has been presented.

Catalytic oxygen activation versus autoxidation for industrial applications: a physicochemical approach

The activation and use of oxygen for the oxidation and functionalization of organic substrates are among the most important reactions in a chemist's toolbox. Nevertheless, despite the vast literature on catalytic oxidation, the phenomenon of autoxidation, an ever-present background reaction that occurs in virtually every oxidation process, is often neglected. In contrast, autoxidation can affect the selectivity to a desired product, to those dictated by pure free-radical chain pathways, thus affecting the activity of any catalyst used to carry out a reaction. This critical review compares catalytic oxidation routes by transition metals versus autoxidation, particularly focusing on the industrial context, where highly selective and "green" processes are needed. Furthermore, the application of useful tests to discriminate between different oxygen activation routes, especially in the area of hydrocarbon oxidation, with the aim of an enhanced catalyst design, is described and discussed. In fact, one of the major targets of selective oxidation is the use of molecular oxygen as the ultimate oxidant, combined with the development of catalysts capable of performing the catalytic cycle in a real energy and cost effective manner on a large scale. To achieve this goal, insights from metallo-proteins that could find application in some areas of industrial catalysis are presented, as well as considering the physicochemical principles that are fundamental to oxidation and autoxidation processes.

Dehydration, dehydrogenation, and condensation of alcohols on supported oxide catalysts based on cyclic (WO3)3 and (MoO3)3 clusters

The review summarizes recent synthesis and reactivity studies of model oxide catalysts prepared by the deposition of gas phase cyclic (WO₃)₃ and (MoO₃)₃ clusters.