Catalyst design using an inverse strategy: From mechanistic studies on inverted model catalysts to applications of oxide-coated metal nanoparticles

Metal-oxide interfaces are of great importance in catalytic applications since each material can provide a distinct functionality that is necessary for efficient catalysis in complex reaction pathways. Moreover, the synergy between two materials can yield properties that exceed the superposition of single sites. While interfaces between metals and metal oxides can play a key role in the reactivity of traditional supported catalysts, significant attention has recently been focused on using “inverted” oxide/metal catalysts to prepare catalytic interfaces with unique properties. In the inverted systems, metal surfaces or nanoparticles are covered by oxide layers ranging from submonolayer patches to continuous films with thickness at the nanometer scale. Inverse catalysts provide an alternative approach for catalyst design that emphasizes control over interfacial sites, including inverted model catalysts that provide an important tool for elucidation of mechanisms of interfacial catalytic reactions and oxide-coated metal nanoparticles that can yield improved stability, activity and selectivity for practical catalysts.This review begins by providing a summary of recent progress in the use of inverted model catalysts in surface science studies, where oxides are usually deposited onto the surface of metal single crystals under ultra-high vacuum conditions. Surface-level studies of inverse systems have yielded key insights into interfacial catalysis and facilitated active site identification for important reactions such as CO oxidation, the water-gas shift reaction, and CO₂ reduction using well-defined model systems, informing strategies for designing improved technical catalysts. We then expand the scope of inverted catalysts, using the “inverse” strategy for preparation of higher-surface area practical catalysts, chiefly through the deposition of metal oxide films or particles onto metal nanoparticles. The synthesis techniques include encapsulation of metal nanoparticles within porous oxide shells to generate core-shell type catalysts using wet chemical techniques, the application of oxide overcoat layers through atomic layer deposition or similar techniques, and spontaneous formation of metal oxide coatings from more conventional catalyst geometries under reaction or pretreatment conditions. Oxide-coated metal nanoparticles have been applied for improvement of catalyst stability, control over transport or binding to active sites, direct modification of the active site structure, and formation of bifunctional sites. Following a survey of recent studies in each of these areas, future directions of inverted catalytic systems are discussed.     

Effects of bimetallic modification on the decomposition of CH3OH and H2O on Pt/W(110) bimetallic surfaces

The decomposition pathways of methanol and water on Pt-modified W(110) bimetallic surfaces have been investigated using density functional theory (DFT), temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). The reaction of methanol on submonolayer and monolayer Pt-modified W(110) surfaces is compared to that on clean Pt(111) and W(110). Similar to clean W(110), the Pt/W(110) bimetallic surfaces remain active toward the dissociation of methanol, although the reaction pathway leading to the production of CH₄ is reduced on the bimetallic surfaces. The Pt/W(110) surfaces are also active toward the decomposition of water. These results are compared with previous studies of the reactions of H₂ and ethylene on Pt/W(110) bimetallic surfaces to reveal the different Pt-modification effects for the dissociation of oxygen-containing molecules.     

Effect of combination of noble metals and metal oxide supports on catalytic reduction of NO by H2

We investigated the effects of combination of noble metals M (Rh, Pd, Ir, Pt) and metal oxide supports S (Al₂O₃, SiO₂, ZrO₂, CeO₂) on the NO + H₂ reaction using planar catalysts with M/S two layered thin films on Si substrate. In this study, NO reduction ability per metal atom were evaluated with a specially designed apparatus employing pulse valves for the injection of reactant molecules onto catalysts and a time-of-flight mass spectrometer to measure multiple transient products: NH₃, N₂ and N₂O simultaneously as well as with an atomic force microscopy to observe the surface area of metal particles. The catalytic performances of Rh and Ir catalysts were hardly affected by a choice of a metal oxide support, while Pd and Pt catalysts showed different catalytic activity and selectivity depending on the metal oxide supports. This assortment is consistent with ability to dissociate NO depending on metals without the effect of any support materials. There, the metals to the left of Rh and Ir on the periodic table favor dissociation of NO and those to the right of Pd and Pt tend to show molecular adsorption of NO. Therefore, the catalytic property of noble metals could be assorted into two groups, i.e. Rh and Ir group whose own property would mainly dominate the catalytic performance, and Pd and Pt group whose interaction with metal oxides supports would clearly contribute to the reaction of NO with H₂. NO reduction activity of Pd and Pt was found to be promoted above that of Rh and Ir, provided that Pd and Pt were supported by CeO₂ and ZrO₂.     

Studies of the activity of catalysts based on heteropolyacids

The catalytic activity of samples such as PPy(H₄SiW₁₂O₄₀), PPy(H₅PMo₁₀V₂O₄₀), PPy(H₂Fe(III)PMo₁₀V₂O₄₀), PPy(H₃Cu(II)PMo₁₀V₂O₄₀) has been examined in two different test reactions. The acid-base and oxidation-reduction properties were studied using the conversion of isopropyl alcohol to propene and acetone. Redox ability of catalysts was examined in the reaction of oxidation of allyl alcohol to glycidol. It was found that the activity of catalysts in the oxidation of allyl alcohol increases as the oxidation properties determined from the conversion of isopropyl alcohol increase. It was also observed that stronger oxidation-reduction properties of the catalyst result in a high rate of the consecutive reaction of glycidol to 3-hydroxypropanone.The phase composition of catalysts was determined by means of X-ray diffraction (XRD).     

Regulating the surface of nanoceria and its applications in heterogeneous catalysis

Ceria (CeO₂) as a support, additive, and active component for heterogeneous catalysis has been demonstrated to have great catalytic performance, which includes excellent thermal structural stability, catalytic efficiency, and chemoselectivity. Understanding the surface properties of CeO₂ and the chemical reactions occurred on the corresponding interfaces is of great importance in the rational design of heterogeneous catalysts for various reactions. In general, the reversible Ce³⁺/Ce⁴⁺ redox pair and the surface acid-base properties contribute to the superior intrinsic catalytic capability of CeO₂, and hence yield enhanced catalytic phenomenon in many reactions. Particularly, nanostructured CeO₂ is characterized by a large number of surface-bound defects, which are primarily oxygen vacancies, as the surface active catalytic sites. Many efforts have therefore been made to control the surface defects and properties of CeO₂ by various synthetic strategies and post-treatments. The present review provides a comprehensive overview of recent progress in regulating the surface structure and composition of CeO₂ and its applications in catalysis.     

Mössbauer Study of 119Sn-Pd Catalysts for Selective Hydrogenation of Hexa-1,5-Diene

The addition of tin to palladium-alumina catalysts induces significant modifications of the catalytic properties of palladium in the hydrogenation of hexa-1,5-diene. The influence of the support, of the metal loading and of the preparation method on the chemical state of tin in Pd-Sn/Al₂O₃ catalysts are studied by Mössbauer spectroscopy. In reduced catalysts, about 60% of tin is present as Sn(0), which forms different Pd-Sn compounds, that are responsible for the changes in the catalytic performances.     

Reactions of molybdenum atoms with NO,O<Subscript>2</Subscript>, N<Subscript>2</Subscript>O,and CO<Subscript>2</Subscript> molecules behind shock waves

Experimental results on the interaction of Mo atoms with various oxygen-containing molecules (NO, O₂, N₂O, and CO₂) at high temperatures (>1200 K) are presented, which are in close agreement with measurements at moderate and low temperatures. It is demonstrated that the height of the activation barrier is additionally increased for spin-forbidden reactions and that an increase in the heat of reaction causes an increase in the rate constant for a given type of reaction. For the reactions of Mo atoms with O₂ and N₂O, interpolated temperature dependences of the rate constants, based on the high-temperature measurements conducted in the present work and the published low-temperature data, are proposed.     

Dealloyed PtCu catalyst as an efficient electrocatalyst in oxygen reduction reaction

Pt-transition metal alloy catalysts with an active Pt surface have exceptional properties for use in oxygen electro-reduction reactions in fuel cells. Herein, we report the simple synthesis of dealloyed PtCu catalysts and their catalytic performance in oxygen reduction. The dealloyed PtCu catalysts consisted of a Pt-enriched shell with a Pt–Cu alloy core and were synthesized through a chemical co-reduction process followed by thermal annealing and chemical dealloying. During synthesis, thermal annealing leads to a high degree of formation of PtCu alloy particles (e.g., PtCu or PtCu₃), and chemical dealloying causes selective dissolution of unstable Cu species from the surface layers of the PtCu alloy particles, resulting in a PtCu alloy@Pt-enriched surface core–shell configuration. Our PtCu₃/C catalyst exhibits a great improvement in the oxygen reduction reaction with a mass activity of 0.501 A/mg_Pt, which is 2.24 times greater than that of a commercial Pt catalyst. In this article, the synthesis details, characteristics and performance improvements in ORR of chemically dealloyed PtCu catalysts are systemically explained.     

Application of Mössbauer Spectroscopy to the Carbon Oxides Hydrogenation Reactions

Iron-based catalysts have favorable activity and selectivity properties for the CO and CO₂ hydrogenation reactions. Several Fe phases (oxides and carbides) can be present in these catalysts. The interaction of Fe with the other components of the catalyst (support, promoters) can affect the ease of reduction and also its transformation during the reactions. In this work, the relationship between catalytic behavior in the CO and CO₂ hydrogenation reactions and the Fe phase composition of fresh and reacted catalysts was studied. Two types of catalysts were tested: a laterite and the other one made of iron supported on alumina, both unpromoted and promoted with K and Mn. Only those Fe species which can be reduced-carburized, by means of a pretreatment or by an in situ transformation under the reaction, seem to be able to perform the CO or CO₂ hydrogenation. The reoxidation of the Fe carbide to magnetite was not associated to deactivation. The selectivity seems to be more affected by Fe species difficult to reduce than by magnetite produced by reoxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Quantitative EPR studies of transition metal ions in oxide,aluminosilicate and polymer matrices

The importance of the proper choice of diamagnetic diluent used for preparation of standards for quantitative EPR measurements is shown by the example of CuSO₄ and VOSO₄ standards. The results of determination of the stability of chemical composition of VOSO₄-K₂SO₄ standards stored for various periods of time, performed by different analytical methods, are compared. The examples are given to illustrate application of quantitative EPR measurements in the studies of structure and properties of transition metal ions dispersed in various matrices. Changes in the coordination sphere of surface transition metal ions occurring upon adsorption of gas molecules, the degree of dispersion of these ions and the extent of Mⁿ⁺?Mⁿ⁺ interactions derived from quantitative EPR measurements are described. The results of investigation of the mechanism of adsorption and catalytic reactions occurring on dispersed transition metal ions are presented.     

Influence of water on HFO-1234yf oxidation pyrolysis via ReaxFF molecular dynamics simulation

ABSTRACT

The effect of water molecules on HFO-1234yf oxidation pyrolysis was investigated by ReaxFF-molecular dynamics simulation from 1900 to 4200?K. The initial pyrolysis of HFO-1234yf starts around 2500?K and the water molecules participate in chemical reactions at 2800?K when the reactants pyrolysis reached the highest reaction rate. The primary products including HF, COF₂ and CO₂ are observed at 2600, 2700 and 2900?K, respectively. The influence of water molecules on products is mainly reflected in the promotion activity on the conversion from COF₂ to CO₂ and the generation of HF molecules. Four formation pathways are observed and calculated to further elucidate the procedure of pyrolysis. The main conversion process from H₂O to HF is the ?F?+?H₂O?=?HF+?OH reaction, and the paths from H₂O to ?OH radical and COF₂ to ?CFO radical which are promoted by ?F and ?H radical, respectively, have relatively low energy barriers of 10.44 and 40.29?kJ/mol, and both reaction processes released HF molecules.     

Physicochemical characteristics of ZSM-5/SAPO-34 composite catalyst for MTO reaction

SAPO-34 and ZSM-5 are the most well-known catalyst for MTO reaction. A combination of ZSM-5 and SAPO-34 might give rise to optimal catalyst to meet a change of market demand for ethylene, propylene and butadiene. In this study, we have developed ZSM-5/SAPO-34 composite catalysts to control the composition of light olefins in MTO reaction. ZSM-5/SAPO-34 composite catalysts showed very different physicochemical and catalytic properties with respect to ZSM-5 and SAPO-34 synthetic procedure. The physicochemical properties of the composite catalysts have been compared by XRD, SEM, N₂ isotherm, FT-IR and NH₃-TPD. Their catalytic performances were also evaluated for MTO reaction. The series composite catalyst synthesized by successive crystallization of SAPO-34 synthetic gel after ZSM-5 crystallization exhibited relatively high catalytic performance.     

A versatile sonication-assisted deposition–reduction method for preparing supported metal catalysts for catalytic applications

This work aims to develop a rapid and efficient strategy for preparing supported metal catalysts for catalytic applications. The sonication-assisted reduction–precipitation method was employed to prepare the heterogeneous mono- and bi-metallic catalysts for photocatalytic degradation of methyl orange (MO) and preferential oxidation (PROX) of CO in H₂-rich gas. In general, there are three advantages for the sonication-assisted method as compared with the conventional methods, including high dispersion of metal nanoparticles on the catalyst support, the much higher deposition efficiency (DE) than those of the deposition–precipitation (DP) and co-precipitation (CP) methods, and the very fast preparation, which only lasts 10–20 s for the deposition. In the AuPd/TiO₂ catalysts series, the AuPd(3:1)/TiO₂ catalyst is the most active for MO photocatalytic degradation; while for PROX reaction, Ru/TiO₂, Au–Cu/SBA-15 and Pt/γ-Al₂O₃ catalysts are very active, and the last one showed high stability in the lifetime test. The structural characterization revealed that in the AuPd(3:1)/TiO₂ catalyst, Au–Pd alloy particles were formed and a high percentage of Au atoms was located at the surface. Therefore, this sonication-assisted method is efficient and rapid in the preparation of supported metal catalysts with obvious structural characteristics for various catalytic applications.     

Water formation on oxygen exposure of some hydrides of intermetallic compounds

It is found that a large number of metal hydrides such as hydrides of materials in Ca−Mg−Ni can form water when exposed to O₂ largely without signs for concomitant substrate oxidation. One can speak of a catalytic reaction in this connection as the H-depleted metal matrix can in most cases be rehydrided and water synthesis repeated on exposure to O₂. In some cases a considerable portion of the H combustion reaction takes place in a matter of seconds. Some of the metal hydrides such as hydrides of CaNi₄B, CeNi₃, and YbNi₂, are relatively stable concerning decomposition of the alloy by the product water, while others, for example hydrides of CaNi₃, and CaNi₂, are rapidly decomposed by the product water forming the hydroxide of the electropositive metal. They also produce fine ferromagnetic Ni-particles which could be interesting for other catalytic purposes. Guest professor, work also performed at University of California at San Diego where support by a grant from DOE, Basic Energy Sciences, is acknowledged.     

Raman investigation of chemical reaction product in thermal‐treated SiCf/C/Mo/Ti6Al4V composite

SiC fiber‐reinforced titanium matrix composite (TMC) is an interesting material for aerospace industry because of its excellent properties. Characterization of their interfacial reaction product is principal to further optimization of the TMCs. Here we report application of Raman spectroscopy to identify reaction products and their microstructural details in thermal‐treated SiC_f/C/Mo/Ti6Al4V composite. Meanwhile TEM is performed to help explain phenomena in Raman result. Raman line scanning along interface indicates thickness of two sublayers (Ti₅Si₃(C_x) layer next to SiC fiber and TiC_x layer next to matrix). The main Raman result of phase distribution is confirmed by TEM. While a 300‐nm Ti₃SiC₂ layer whose Raman features are similar with nearby Ti₅Si₃(C_x) layer is also detected. Raman peakshift in Ti₅Si₃(C_x) layer possibly results from residual stress or/and microstructural evolution caused by carbon solution. No evidence indicates Mo participation of interfacial reactions. However, Mo diffusion changes phase distribution of matrix alloy and affects interfacial reaction indirectly. Meanwhile, TEM and Raman results indicate that particles are TiC_x and defective Ti₃AlC₂. Raman features of Ti₃AlC₂ particles differ from that of bulk material, which is attributed to defects. Although Ti₃AlC₂ formation mechanism is ambiguous, it possibly relates to TiC_x formation nearby. Copyright © 2014 John Wiley & Sons, Ltd.     

Ultrasonication-assisted synthesis of 2D porous MoS2/GO nanocomposite catalysts as high-performance hydrodesulfurization catalysts of vacuum gasoil: Experimental and DFT study

In this study, a novel, simple, high yield, and scalable method is proposed to synthesize highly porous MoS₂/graphene oxide (M−GO) nanocomposites by reacting the GO and co-exfoliation of bulky MoS₂ in the presence of polyvinyl pyrrolidone (PVP) under different condition of ultrasonication. Also, the effect of ultrasonic output power on the particle size distribution of metal cluster on the surface of nanocatalysts is studied. It is found that the use of the ultrasonication method can reduce the particle size and increase the specific surface area of M−GO nanocomposite catalysts which leads to HDS activity is increased. These nanocomposite catalysts are characterized by XRD, Raman spectroscopy, SEM, STEM, HR-TEM, AFM, XPS, ICP, BET surface, TPR and TPD techniques. The effects of physicochemical properties of the M−GO nanocomposites on the hydrodesulfurization (HDS) reactions of vacuum gas oil (VGO) has been also studied. Catalytic activities of MoS₂-GO nanocomposite are investigated by different operating conditions. M9-GO nanocatalyst with high surface area (324 m²/g) and large pore size (110.3 Å), have the best catalytic performance (99.95%) compared with Co-Mo/γAl₂O₃ (97.91%). Density functional theory (DFT) calculations were also used to elucidate the HDS mechanism over the M−GO catalyst. It is found that the GO substrate can stabilize MoS₂ layers through weak van der Waals interactions between carbon atoms of the GO and S atoms of MoS₂. At both Mo- and S-edges, the direct desulfurization (DDS) is found as the main reaction pathway for the hydrodesulfurization of DBT molecules.     

The mechanism and kinetics of water formation on Pt(111)

We use modulated molecular beam relaxation spectroscopy (MMBRS) to identify the sequence of elementary reaction steps and to quantify the rate parameters for catalytic water (D₂O) formation on Pt(111). This investigation is restricted to surface temperatures in the range 373 ⩽ T_S ⩽ 723 K and oxygen and deuterium pressures on the order of 10⁻⁵ mbar, where coverages of oxygen-containing species are low (ϑ ⩽ 0.04 monolayer), and rate parameters can be assumed coverage-independent. Under these conditions, we find that adsorbed hydroxyl (OD_a) formation is rapid and (nearly) irreversible; the rate-limiting, majority pathway for water production is D_a + OD_a → D₂O (E_a = 16 kcal/mol); and water production by OD_a disproportionation. i.e. 2 OD_a → D₂O + O_a, contributes as a minority pathway (E_a = 18 kcal/mol). From the results we construct potential energy diagrams that account for the energetics of nearly all elementary steps in these reactions.     

Preparation and characterization of hybrid nanoparticles based on chitosan and poly(methacryloylglycylglycine)

CeO₂–MnO _x composites possessing rod-like morphology (fixed mole proportion of Ce/Mn) were synthesized through hydrothermal method and chosen as supporters to load PdO nanoparticles (PdO/Ce _x Mn_1–x ). The size of loaded PdO nanoparticles is about 2 nm. The catalytic behaviors of supported catalysts were examined through the complete catalytic oxidation of benzene. The results illustrated that the activities of supported catalysts were enhanced greatly as compared to unsupported, and the completely conversion temperature of benzene was reduced to ca. 250 °C. The effect of noble metal species (PdO) addition on the catalytic property and crystal structure of composites was researched in detail. The data revealed that the interaction between PdO and supporter, and intrinsic properties of supporter resulted in the enhancement of catalytic abilities.     

Surface chemistry and catalysis of oxide model catalysts from single crystals to nanocrystals

Fundamental understandings of surface chemistry and catalysis of solid catalysts are of great importance for the developments of efficient catalysts and corresponding catalytic processes, but have been remaining as a challenge due to the complex nature of heterogeneous catalysis. Model catalysts approach based on catalytic materials with uniform and well-defined surface structures is an effective strategy. Single crystals-based model catalysts have been successfully used for surface chemistry studies of solid catalysts, but encounter the so-called “materials gap” and “pressure gap” when applied for catalysis studies of solid catalysts. Recently catalytic nanocrystals with uniform and well-defined surface structures have emerged as a novel type of model catalysts whose surface chemistry and catalysis can be studied under the same operational reaction condition as working powder catalysts, and they are recognized as a novel type of model catalysts that can bridge the “materials gap” and “pressure gap” between single crystals-based model catalysts and powder catalysts. Herein we review recent progress of surface chemistry and catalysis of important oxide catalysts including CeO₂, TiO₂ and Cu₂O acquired by model catalysts from single crystals to nanocrystals with an aim at summarizing the commonalities and discussing the differences among model catalysts with complexities at different levels. Firstly, the complex nature of surface chemistry and catalysis of solid catalysts is briefly introduced. In the following sections, the model catalysts approach is described and surface chemistry and catalysis of CeO₂, TiO₂ and Cu₂O single crystal and nanocrystal model catalysts are reviewed. Finally, concluding remarks and future prospects are given on a comprehensive approach of model catalysts from single crystals to nanocrystals for the investigations of surface chemistry and catalysis of powder catalysts approaching the working conditions as closely as possible.     

Non-faradaic electrochemical modification of catalytic activity in solid electrolyte cells

The catalytic activity and selectivity of metal catalysts used as electrodes in high temperature solid electrolyte cells can be altered dramatically and in a reversible manner. This is accomplished by electrochemically supplying oxygen anions onto catalytic surfaces via polarized metal-solid electrolyte interfaces. Oxygen anions, forced electrochemically to adsorb on the metal catalyst surface, alter the catalyst work function in a predictable way and lead to reaction rate increases as high as 4000%. Changes in catalytic rates typically exceed the rate of O^2– transport to or from the catalyst surface by 10²-3 · 10⁵. Significant changes in product selectivity have been also observed. The case of several catalytic reactions in which this new phenomenon has been observed is presented and the origin of the phenomenon is discussed.