Low-Temperature Methane Combustion Using Ozone over Coß Catalyst.

Catalytic methane (CH4) combustion is a promising approach to reducing the release of unburned methane in exhaust gas. Here, we report Co-exchanged ß zeolite (Coß) as an efficient catalyst for CH4 combustion using O3. A series of ion-exchanged ß zeolites (Co, Ni, Mn, Fe, and Pd) are subjected to the catalytic test, and Coß exhibits a superior performance in a low-temperature region (<100 °C). The results of X-ray absorption spectroscopy (XAS) and catalytic tests for Coß with different Co loadings indicate the isolated Co species is the plausible active site. The reaction mechanism of CH4 combustion over the isolated Co2+ cation is theoretically investigated by the single-component artificial force-induced reaction (SC-AFIR) method to thoroughly search for possible reaction routes. The resulting path toward CO2 formation shows an activation energy of 73 kJ/mol for the rate-determining step and an exothermicity of 1025 kJ/mol, which supports the experimental results. During a long-term catalytic test for 160 h without external heating, the CH4 conversion gradually decreases from 80 to 40%, but the conversion fully recovers after dehydration at 500 °C (0.5 h). The copresence of H2O and CO exhibits a negative impact on the catalytic activity, while NO and SO2 do not markedly change the catalytic activity.

Exploring Catalytic Intermediates in Pd-Catalyzed Aerobic Oxidative Amination of 1,3-Dienes: Multiple Metal Interactions of the Palladium Nanoclusters.

Metal nanoclusters (NCs) have unique properties because of their small size, which makes them useful as catalysts in reactions like cross-coupling. Pd-catalyzed oxidative amination, which involves dehydrogenative C-N bond formation, uses Pd complexes as the active species. It is known that the catalytic conditions involve the formation of Pd(0) species from Pd NCs, but the precise role of Pd NCs in the transformations has not been established. In this study, we investigated the characteristic properties of Pd NCs in oxidative amination of 1,3-dienes. The reaction achieved direct amination of commercially accessible 1,3-dienes with secondary aromatic amines, providing a variety of nitrogen containing 1,3-dienes. The compound was applicable to radical polymerization to provide the nitrogen-fabricated 1,3-diene-based polymer, which exhibited a different thermal stability compared to aliphatic nitrogen-fabricated diene polymers. In addition to the synthetic utility, by combining X-ray absorption fine structure and small-angle X-ray scattering analysis, we revealed amines and 1,3-dienes affected metal leaching from the Pd nanoparticles and stabilization of Pd NCs in the catalytic reaction. Additionally, DFT calculation suggested that the catalytic intermediate contained multiple adjacent Pd atoms and was responsible for formation of an σ-allylic intermediate that is difficult to form with the use of Pd complexes. These results could be used to understand the underlying phenomenon in the oxidative coupling reaction and develop Pd NCs-based dehydrogenation.

Microporosity and Catalytic Activity for Hydrodesulfurization of Pharmacosiderite Mo4P3O16 Synthesized at a Moderate Temperature.

Pharmacosiderite Mo4P3O16 (Pharma-MoPO) consists of [Mo4O4] cubane unit and [PO4] tetrahedral to form an open framework with a microporous structure similar to that of LTA-type zeolite. Although attractive applications are expected due to its microporous structure and redox-active components, its physicochemical properties have been poorly investigated due to the specificity of its synthesis, which requires a high hydrothermal synthesis temperature of 360 °C. In this study, we succeeded in synthesizing Pharma-MoPO by hydrothermal synthesis at 230 °C, which can be applied using a commercially available autoclave by changing the metal source. Through the study of the solids and liquids obtained after hydrothermal syntheses, the formation process of Pharma-MoPO under our studied synthesis conditions was proposed. Advanced characterizations provided detailed structural information on Pharma-MoPO, including the location site of a countercation NH4+. Pharma-MoPO could adsorb CO2 with the amount close to the number of cages without removing NH4+. Pharma-MoPO exhibited stable catalytic activity for the hydrodesulfurization of thiophene while maintaining its crystal structure, except for the introduction of sulfide by replacing lattice oxygens. Pharmacosiderite Mo4P3O16 was successfully obtained by hydrothermal synthesis at a moderate temperature, and its microporosity for CO2 adsorption and catalytic properties for hydrodesulfurization were discovered.

Interstitial Carbon Dopant in Palladium-Gold Alloy Boosting the Catalytic Performance in Vinyl Acetate Monomer Synthesis.

Vinyl acetate monomer (VAM), an important chemical intermediate in industry, is produced by the well-established commercial process of acetoxylation of ethylene with Pd-Au/SiO2 and a KOAc promoter. No paper has since decades defined the true effects of Au and KOAc, despite numerous attempts to clarify them. The role of subsurface carbon as a catalyst booster for enhanced catalytic performance in VAM synthesis was found by us for the first time. X-ray diffraction and X-ray absorption fine structure studies revealed that carbon atoms spontaneously doped into the Pd-Au alloy lattice while maintaining the alloy's size, metallic state, and alloy composition. Additionally, during the process, the KOAc addition dramatically raised the equilibrium carbide fraction. Because of the high carbide fraction, KOAc/Pd0.8Au0.2/SiO2 had a 5.6-fold higher formation rate (89.0% selectivity) than Pd0.8Au0.2/SiO2 (69.2% selectivity) due to high carbide fraction. Surprisingly, kinetic and theoretical analyses showed that the coupling of acetate and ethylene, which is a rate-determining step, is effectively promoted by the synergistic contributions of Au (electronic/geometric effects) and interstitial carbon (electronic effect). Additionally, the synergy inhibits ethylene dehydrogenation, which ultimately slows the formation of CO2. The contentious debates about the roles of Au and KOAc in the acetoxylation of ethylene have been resolved thanks to experimental and theoretical insights into the roles of Pd-Au formation, Au/Pd ratio, and interstitial carbon atoms. These insights also open the door for the logical design of catalysts with desirable catalytic performance.

Dynamic Behavior of Intermediate Adsorbates to Control Activity and Product Selectivity in Heterogeneous Catalysis: Methanol Decomposition on Pt/TiO2(110).

Dynamic behavior of intermediate adsorbates, such as diffusion, spillover, and reverse spillover, has a strong influence on the catalytic performance in oxide-supported metal catalysts. However, it is challenging to elucidate how the intermediate adsorbates move on the catalyst surface and find active sites to give the corresponding products. In this study, the effect of the dynamic behavior of methoxy intermediate on methanol decomposition on a Pt/TiO2(110) surface has been clarified by combination of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. The methoxy intermediates were formed by the dissociative adsorption of methanol molecules on Pt nanoparticles at room temperature followed by spillover to the TiO2(110) support surface. TPD results showed that the methoxy intermediates were thermally decomposed at >350 K on the Pt sites to produce CO (dehydrogenation) and CH4 (C-O bond scission). A decrease of the Pt nanoparticle density lowered the activity for the decomposition reaction and increased the selectivity toward CH4, which indicates that the reaction is controlled by diffusion and reverse spillover of the methoxy intermediates. Time-lapse STM imaging and DFT calculations revealed that the methoxy intermediates migrate on the five-fold coordinated Ti (Ti5c) sites along the [001] or [11¯0] direction with the aid of hydrogen adatoms bonded to the bridging oxygens (Obr) and can move over the entire surface to seek and find active Pt sites. This work offers an in-depth understanding of the important role of intermediate adsorbate migration in the control of the catalytic performance in oxide-supported metal catalysts.

Molybdenum Oxide Constructed by {Mo6O21}6- Pentagonal Unit Assembly and Its Redox Properties.

Molybdenum oxides are widely used in various fields due to their electronic and structural characteristics. These materials can generate lattice oxygen defects by reduction treatments, which sometimes play central roles in various applications. However, little has been understood about their properties since it is difficult to increase the amount of lattice oxygen defects due to the crystal structure changes in most cases. Here, we report a new class of high-dimensionally structured Mo oxide (HDS-MoOx) constructed by the random assembly of {Mo6O21}6- pentagonal units (PUs). Since the PU is a stable structural unit, the structural network based on the PU hardly caused structural changes to make the lattice oxygen defects vanish. Consequently, HDS-MoOx could generate a substantial amount of lattice oxygen defects, and their amount was controllable, at least in the range of MoO2.64-MoO3.00. HDS-MoOx was more redox active than typical Mo oxide (α-MoO3) and demonstrated an oxidation ability for gas-phase isopropanol oxidation under the reaction conditions, whereas α-MoO3 affords no oxidation products.

Continuous Unsteady-State De-NOx System via Tandem Water-Gas Shift, NH3 Synthesis, and NH3-SCR under Periodic Lean/Rich Conditions.

The development of urea-free and platinum group metal (PGM)-free catalytic systems for automotive emission control is a challenging task. Herein, we report a new de-NOx system using cyclic feeds of rich and lean gas mixtures with PGM-free catalysts. Initial catalyst screening tests showed that Cu/CeO2 with 5 wt % Cu loading was the most suitable for the water-gas shift reaction (WGS, CO + H2O → CO2 + H2), followed by the selective NH3 synthesis by the NO + H2 reaction. The unsteady-state system under alternating feeds of rich (0.1% NO + 0.5% CO + 1% H2O) and lean (0.1% NO + 2% O2 + 1% H2O) gas mixtures over a mixture of Cu/CeO2 and Cr-exchanged mordenite (CrMOR) showed higher NOx conversion than the steady-state (0.1% NO + 0.35% CO + 0.6% O2 + 1% H2O) reaction between 200 and 500 °C. The de-NOx mechanism under periodical rich/lean conditions was studied by operando infrared (IR) experiments. In the rich period, the WGS reaction on the Cu/CeO2 catalyst yield H2, which reduces NO to NH3 on the Cu/CeO2 catalyst. NH3 is then captured by the Brønsted acid sites of CrMOR. In the subsequent lean period, the adsorbed NH3 acts as a reductant for the selective catalytic reduction of NOx catalyzed by the Cr sites of CrMOR. This study demonstrates a new urea-free and PGM-free catalytic system that can provide an alternative de-NOx technology for automotive catalysis under periodic rich/lean conditions.

Mechanism of NH3-SCR over P/CeO2 Catalysts Investigated by Operando Spectroscopies.

This study reports a comprehensive investigation into the active sites and reaction mechanism for the selective catalytic reduction of NO by NH3 (NH3-SCR) over phosphate-loaded ceria (P/CeO2). Catalyst characterization and density functional theory calculations reveal that H3PO4 and H2P2O6 species are the dominant phosphate species on the P/CeO2 catalysts under the experimental conditions. The reduction/oxidation half-cycles (RHC/OHC) were investigated using in situ X-ray absorption near-edge structure for Ce L3-edge, ultraviolet-visible, and infrared (IR) spectroscopies together with online analysis of outlet products (operando spectroscopy). The Ce4+(OH-) species, possibly adjacent to the phosphate species, are reduced by NO + NH3 to produce N2, H2O, and Ce3+ species (RHC). The Ce3+ species is reoxidized by aqueous O2 (OHC). The results from IR spectroscopy suggest that the RHC initiates with the reaction between NO and Ce4+(OH-) to yield Ce3+ and gaseous HONO, which then react with NH3 to produce N2 and H2O via NH4NO2 intermediates.

An automated reaction route mapping for the reaction of NO and active species on Ag4 clusters in zeolites.

A computational investigation of the catalytic reaction on multinuclear sites is very challenging. Here, using an automated reaction route mapping method, the single-component artificial force induced reaction (SC-AFIR) algorithm, the catalytic reaction of NO and OH/OOH species over the Ag42+ cluster in a zeolite is investigated. The results of the reaction route mapping for H2 + O2 reveal that OH and OOH species are formed over the Ag42+ cluster via an activation barrier lower than that of OH formation from H2O dissociation. Then, reaction route mapping is performed to examine the reactivity of the OH and OOH species with NO molecules over the Ag42+ cluster, resulting in the facile reaction path of HONO formation. With the aid of the automated reaction route mapping, the promotion effect of H2 addition on the SCR reaction was computationally proposed (boosting the formation of OH and OOH species). In addition, the present study emphasizes that automated reaction route mapping is a powerful tool to elucidate the complicated reaction pathway on multi-nuclear clusters.

In- and Ga-oxo clusters/hydrides in zeolites: speciation and catalysis for light-alkane activations/transformations.

Metal-exchanged zeolites have great potential to form unique active metal species and develop their catalysis by promoting small molecules such as light alkanes. Ga-exchanged zeolites have attracted attention as promising heterogeneous catalysts for dehydrogenative light-alkane transformations. The speciation of active Ga species in reduced and oxidized Ga-exchanged zeolites and their reaction mechanisms have been discussed in several studies based on experimental and theoretical investigations. In contrast, studies on In-exchanged zeolites have been far less explored, and thus active In-species have rarely been investigated. In this perspective, we summarized our investigations on In- and Ga-exchanged zeolites for light-alkane transformations. Our research group reported the formation of In-oxo clusters using the O2 treatment of In-CHA and their potential for the partial oxidation of CH4 (POM) at room temperature. We also observed the formation of In-hydrides in CHA zeolites during the preparation through reductive solid-state ion-exchange (RSSIE) and revealed their catalysis for non-oxidative C2H6 dehydrogenation (EDH). Their detailed structures and reaction mechanisms are discussed in combination with spectroscopic, kinetic, and theoretical studies. Furthermore, comparative studies on the formation of Ga-oxo clusters for POM at room temperature and the controlled formation of Ga-hydrides for selective EDH were conducted. The obtained results and insights are comprehensively discussed, including the relationship between the local structure of the active In/Ga species and reaction selectivity, as well as the influence of different zeolite frameworks on the formation of active species.

Surface Engineering of Titania Boosts Electroassisted Propane Dehydrogenation at Low Temperature.

Electric field catalysis using surface proton conduction, in which proton hopping and collision on the reactant are promoted by external electricity, is a promising approach to break the thermodynamic equilibrium limitation in endothermic propane dehydrogenation (PDH). This study proposes a catalyst design concept for more efficient electroassisted PDH at low temperature. Sm was doped into the anatase TiO2 surface to increase surface proton density by charge compensation. Pt-In alloy was deposited on the Sm-doped TiO2 for more favorable proton collision and selective propylene formation. The catalytic activity in electroassisted PDH drastically increased by doping an appropriate amount of Sm (1 mol % to Ti) where the highest propylene yield of 19.3 % was obtained at 300 °C where the thermodynamic equilibrium yield was only 0.5 %. Results show that surface proton enrichment boosts alkane dehydrogenation at low temperature.

Super Mg2+ Conductivity around 10-3 S cm-1 Observed in a Porous Metal-Organic Framework.

We first report a solid-state crystalline "Mg2+ conductor" showing a superionic conductivity of around 10-3 S cm-1 at ambient temperature, which was obtained using the pores of a metal-organic framework (MOF), MIL-101, as ion-conducting pathways. The MOF, MIL-101⊃{Mg(TFSI)2}1.6 (TFSI- = bis(trifluoromethanesulfonyl)imide), containing Mg2+ inside its pores, showed a superionic conductivity of 1.9 × 10-3 S cm-1 at room temperature (RT) (25 °C) under the optimal guest vapor (MeCN), which is the highest value among all Mg2+-containing crystalline compounds. The Mg2+ conductivity in the MOF was estimated to be 0.8 × 10-3 S cm-1 at RT, by determining the transport number of Mg2+ (tMg2+ = 0.41), which is the level as high as practical use for secondary battery. Measurements of adsorption isotherms, pressure dependence of ionic conductivity, and in situ Fourier transform infrared measurements revealed that the "super Mg2+ conductivity" is caused by the efficient migration of the Mg2+ carrier with the help of adsorbed guest molecules.

High-Entropy Intermetallics Serve Ultrastable Single-Atom Pt for Propane Dehydrogenation.

Propane dehydrogenation has been a promising propylene production process that can compensate for the increasing global demand for propylene. However, Pt-based catalysts with high stability at ≥600 °C have barely been reported because the catalysts typically result in short catalyst life owing to side reactions and coke formation. Herein, we report a new class of heterogeneous catalysts using high-entropy intermetallics (HEIs). Pt-Pt ensembles, which cause side reactions, are entirely diluted by the component inert metals in PtGe-type HEIs. The resultant HEI (PtCoCu) (GeGaSn)/Ca-SiO2 exhibited an outstandingly high catalytic stability, even at 600 °C (kd-1 = τ = 4146 h = 173 d), and almost no deactivation of the catalyst was observed for 2 months for the first time. Detailed experimental studies and theoretical calculations demonstrated that the combination of the site-isolation and entropy effects upon multi-metallization of PtGe drastically enhanced the desorption of propylene and the thermal stability, eventually suppressing the side reactions even at high reaction temperatures.

Oxidation Catalysis over Solid-State Keggin-Type Phosphomolybdic Acid with Oxygen Defects.

Keggin-type phosphomolybdic acid (PMo12O40), treated with pyridine (Py), forms a crystalline material (PyPMo-HT) following heat treatment under an inert gas flow at ∼420 °C. Although this material is known to have attractive catalytic properties for gas-phase oxidation, the origin of this catalytic activity requires clarification. In this study, we investigated the crystal structure of PyPMo-HT. PyPMo-HT comprises a one-dimensional array of Keggin units and pyridinium cations (HPy), with an HPy/Keggin unit ratio of ∼1.0. Two oxygen atoms were removed from the Keggin unit during crystal structure transformation, which resulted in an electron being localized on the Mo atom in close contact with the adjacent Keggin unit. Upon the introduction of molecular oxygen, electron transfer from this Mo atom resulted in the formation of an electrophilic oxygen species that bridged two Keggin units. The electrophilic oxygen species acted as a catalytically active oxygen species, as confirmed by the selective oxidation of propylene. PyPMo-HT showed excellent catalytic activity for the selective oxidation of methacrolein, with the methacrylic acid yield being superior to that obtained with PMo12O40 and comparable to that obtained with an industrial Keggin-type polyoxometalate (POM) catalyst. The oxidation catalysis observed over PyPMo-HT provides a deeper understanding of POM-based industrial catalytic processes.

The reducibility and oxidation states of oxide-supported rhenium: experimental and theoretical investigations.

The activity and stability of supported metal catalysts, which exhibit high efficiency and activity, are significantly influenced by the interactions between the metal and the support, that is, metal-support interactions (MSIs). Here, we report an investigation of the MSIs between supported rhenium (Re) and oxide supports such as TiO2, SiO2, Al2O3, MgO, V2O5, and ZrO2 using experimental and computational approaches. The reducibility of the Re species was found to strongly depend on the oxide support. Experimental studies including temperature-programmed reduction by H2 as well as Re L3- and L1-edge X-ray absorption near edge structure (XANES) analysis revealed that the valency of the Re species started to decrease upon H2 reduction in the 200-400 °C range, except for Re on MgO, where the shift occurred at temperatures above 500 °C. The dependence of the Re L3- and L1-edge XANES spectra of the oxide-supported Re catalysts on the size of Re was also examined.

Nickel-Based High-Entropy Intermetallic as a Highly Active and Selective Catalyst for Acetylene Semihydrogenation.

Acetylene semihydrogenation is a key technology for producing polymer-grade ethylene from crude ethylene. Ni-based catalysts are promising alternatives to noble-metals for this process. However, achieving high catalytic activity and selectivity remains a big challenge. We report a novel catalyst design based on high-entropy intermetallics (HEI), which provide thermally stable isolated Ni without excess counterpart metals and achieve exceptionally high performance. Intermetallic NiGa was multi-metalized to a (NiFeCu)(GaGe), where the Ni and Ga sites were partially substituted with Fe/Cu and Ge, respectively, without altering the parent CsCl-type structure. The NiFeCuGaGe/SiO2 HEI catalyst completely inhibited ethylene overhydrogenation even at complete acetylene conversion, and exhibited five-times higher activity than other 3d-transition-metal-based catalysts. The DFT study showed that the surface energy decreased by multi-metallization, which drastically weakened ethylene adsorption.

Surface activation by electron scavenger metal nanorod adsorption on TiH2, TiC, TiN, and Ti2O3.

Metal/oxide support perimeter sites are known to provide unique properties because the nearby metal changes the local environment on the support surface. In particular, the electron scavenger effect reduces the energy necessary for surface anion desorption, and thereby contributes to activation of the (reverse) Mars-van Krevelen mechanism. This study investigated the possibility of such activation in hydrides, carbides, nitrides, and sulfides. The work functions (WFs) of known hydrides, carbides, nitrides, oxides, and sulfides with group 3, 4, or 5 cations (Sc, Y, La, Ti, Zr, Hf, V, Nb, and Ta) were calculated. The WFs of most hydrides, carbides, and nitrides are smaller than the WF of Ag, implying that the electron scavenger effect may occur when late transition metal nanoparticles are adsorbed on the surface. The WF of oxides and sulfides decreases when reduced. The surface anion vacancy formation energy correlates well with the bulk formation energy in carbides and nitrides, while almost no correlation is found in hydrides because of the small range of surface hydrogen vacancy formation energy values. The electron scavenger effect is explicitly observed in nanorods adsorbed on TiH2 and Ti2O3; the surface vacancy formation energy decreases at anion sites near the nanorod, and charge transfer to the nanorod happens when an anion is removed at such sites. Activation of hydrides, carbides, and nitrides by nanorod adsorption and screening support materials through WF calculation are expected to open up a new category of supported catalysts.

Factors determining surface oxygen vacancy formation energy in ternary spinel structure oxides with zinc.

Spinel oxides are an important class of materials for heterogeneous catalysis including photocatalysis and electrocatalysis. The surface O vacancy formation energy (EOvac) is a critical quantity for catalyst performance because the surface of metal oxide catalysts often acts as a reaction site, for example, in the Mars-van Krevelen mechanism. However, experimental evaluation of EOvac is very challenging. We obtained the EOvac for (100), (110), and (111) surfaces of normal zinc-based spinel oxides ZnAl2O4, ZnGa2O4, ZnIn2O4, ZnV2O4, ZnCr2O4, ZnMn2O4, ZnFe2O4, and ZnCo2O4. The most stable surface is (100) for all compounds. The smallest EOvac for a surface is the largest in the (100) surface except for ZnCo2O4. For (100) and (110) surfaces, there is a good correlation, over all spinels, between the smallest EOvac for the surface and bulk formation energy, while the ionization potential correlates well in (111) surfaces. Machine learning over EOvac of all surface sites in all orientations and for all compounds to find the important factors, or descriptors, that decide the EOvac revealed that bulk and surface-dependent descriptors are the most important, namely the bulk formation energy, a Boolean descriptor of whether the surface is (111) or not, and the ionization potential, followed by geometrical descriptors that are different in each O site.

Local structure and NO adsorption/desorption property of Pd2+ cations at different paired Al sites in CHA zeolite.

Recently, Pd-exchanged CHA zeolites (Pd-CHA) have attracted attention as promising passive NOx adsorbers (PNAs) for reducing NOx emissions during the cold start period of a vehicle engine. In this work, the relationship between the local structures and the NO adsorption/desorption properties of the Pd cations in CHA zeolites was investigated. Pd cation formation and NO adsorption were theoretically explored by density functional theory (DFT) calculations for different paired Al sites in six-/eight-membered rings (6MR/8MR). Furthermore, we prepared a series of Pd-CHAs with different Pd loadings (0.5-5.4 wt%) and evaluated their NO adsorption/desorption properties by in situ infrared (IR) spectroscopy and temperature-programmed desorption (TPD) measurements. The increase in the Pd loading resulted in a shift in the NO desorption temperature toward a higher temperature regime. This phenomenon was ascribed to the increase in the proportion of less stable Pd cations, resulting in improved NO adsorption. Furthermore, the effect of Al distribution on the NO adsorption property of Pd-CHA was examined using CHA zeolites containing different proportions of paired Al sites in 6MR while maintaining similar Si/Al ratios (Si/Al = 12.0-16.5). The present study, based on a combination of theoretical and experimental techniques, shows that the NO adsorption/desorption properties over Pd-CHA can be tuned by controlling the Pd loading amount and the type of paired Al sites.

Doubly Decorated Platinum-Gallium Intermetallics as Stable Catalysts for Propane Dehydrogenation.

Propane dehydrogenation (PDH) is a promising chemical process that can satisfy the increasing global demand for propylene. However, the Pt-based catalysts that have been reported thus far are typically deactivated at ≥600 °C by side reactions and coke formation. Thus, such catalysts possess an insufficient life. Herein, we report a novel catalyst design concept, namely, the double decoration of PtGa intermetallics by Pb and Ca, which synergize the geometric and electronic promotion effects on the catalyst stability, respectively. Pb is deposited on the three-fold Pt3 sites of the PtGa nanoparticles to block them, whereas Ca, which affords an electron-enriched single-atom-like Pt1 site, is placed around the nanoparticles. Thus, PtGa-Ca-Pb/SiO2 exhibits an outstandingly high catalytic stability, even at 600 °C (kd =0.00033 h-1 , τ=3067 h), and almost no deactivation of the catalyst was observed for up to 1 month for the first time.