Palladium species in Pd/H-ZSM-5 zeolite catalysts for CH4-SCR

The active state of palladium for NO reduction with methane (CH₄-SCR) was investigated by comparing the catalytic activity of Pd/H-ZSM-5 with that of PdO/SiO₂. High catalytic activity for CH₄-SCR was given by Pd/H-ZSM-5 in the temperature range of 300–500 °C. PdO/SiO₂ catalyzed the reaction between NO₂ and CH₄ in the absence of oxygen, which retarded the reaction by consuming CH₄ in combustion. CH₄ combustion occurred on either zeolite-supported or silica-supported catalyst, while NO preferentially retarded the combustion on Pd/H-ZSM-5. NO was found to be chemisorbed on the palladium sites in zeolite, while it was hardly chemisorbed on PdO/SiO₂. NaCl titration showed that the palladium species in zeolite are Pd²⁺ cations content, on which NO is strongly chemisorbed resulting in high selectivity for CH₄-SCR.     

Selective methanation of CO in the presence of CO<Subscript>2</Subscript> in hydrogen-containing mixtures on nickel catalysts

The screening of commercial nickel catalysts for methanation and a series of nickel catalysts supported on CeO₂, γ-Al₂O₃, and ZrO₂ in the reaction of selective CO methanation in the presence of CO₂ in hydrogen-containing mixtures (1.5 vol % CO, 20 vol % CO₂, 10 vol % H₂O, and the balance H₂) was performed at the flow rate WHSV = 26000 cm³ (g Cat)⁻¹ h⁻¹. It was found that commercial catalytic systems like NKM-2A and NKM-4A (NIAP-07-02) were insufficiently effective for the selective removal of CO to a level of <100 ppm. The most promising catalyst is 2 wt % Ni/CeO₂. This catalyst decreased the concentration of CO from 1.5 vol % to 100 ppm in the presence of 20 vol % CO₂ in the temperature range of 280–360°C at a selectivity of >40%, and it retained its activity even after contact with air. The minimum outlet CO concentration of 10 ppm at 80% selectivity on a 2 wt % Ni/CeO₂ catalyst was reached at a temperature of 300°C.     

Palladium Encapsulated by an Oxygen-Saturated TiO2 Overlayer for Low-Temperature SO2-Tolerant Catalysis during CO Oxidation

The development of oxidation catalysts that are resistant to sulfur poisoning is crucial for extending the lifespan of catalysts in real-working conditions. Herein, we describe the design and synthesis of oxide-metal interaction (OMI) catalyst under oxidative atmospheres. By using organic coated TiO₂, an oxide/metal inverse catalyst with non-classical oxygen-saturated TiO₂ overlayers were obtained at relatively low temperature. These catalysts were found to incorporate ultra-small Pd metal and support particles with exceptional reactivity and stability for CO oxidation (under 21 vol % O₂ and 10 vol % H₂O). In particular, the core (Pd)-shell (TiO₂) structured OMI catalyst exhibited excellent resistance to SO₂ poisoning, yielding robust CO oxidation performance at 120 °C for 240 h (at 100 ppm SO₂ and 10 vol % H₂O). The stability of this new OMI catalyst was explained through density functional theory (DFT) calculations that interfacial oxygen atoms at Pd−O−Ti sites (of oxygen-saturated overlayers) serve as non-metal active sites for low-temperature CO oxidation, and change the SO₂ adsorption from metal(d)-to-SO₂(π*) back-bonding to much weaker σ(Ti−S) bonding.     

Preparation and characterization of W/γ-Al2O3 and Pd–W/γ-Al2O3 catalysts from organometallic precursors. The catalytic activity for NO decomposition

The photochemical reaction of W(CO)₆ with triphenylphosphine (PPh₃) in the presence of γ-Al₂O₃ and Pd/γ-Al₂O₃ has been used to prepare W/γ-Al₂O₃ and Pd–W/γ-Al₂O₃ catalysts. Adsorbed mono- and disubstituted W species have been identified by FTIR spectroscopy. There is evidence of the adsorption of W(CO)_6−x L_x species on both the alumina and the Pd surface. After thermal decomposition and reduction at 573 K the catalysts have been characterized by FTIR spectroscopy of adsorbed NH₃, CO and NO. The retention of W and P suppresses the Lewis acidity of the alumina support. On Pd–W/γ-Al₂O₃, the W is present in a partially reduced state in close association with Pd. This interaction modifies the chemisorptive properties of NO relative to those of the monometallic Pd and W catalysts. In line with these observations the Pd–W/γ-Al₂O₃ catalyst presents an enhanced activity for NO decomposition at 473 K.     

Activation and properties of Mo=O bonds in Mo/SiO2 catalysts

The existence of the charge transfer excited triplet state [Mo⁵⁺-O^-] produced by UV-irradiation of Mo/SiO₂ catalysts, and its reactivity are evidenced by experiments of photoluminescence, photoinduced metathesis, and photoreduction of CO. Mo⁵⁺ ions can be produced separately by thermal activation and O^- ions by further adsorption of N₂O on those Mo⁵⁺ ions. The latter of which are adsorbed on Mo⁶⁺ ions are found to be more reactive than O^2- of [Mo⁶⁺ =O^2-] bond. They are able either to add a molecule such as CO or C₂H₄, or to abstract hydrogen from H₂, CH₄ or trans-dicyanoethylene, or a CN group form tetracyanoethylene (TCNE). The Mo⁵⁺ ions are able to coordinate gas phase ligands when their coordination sphere possesses vacant sites. This is the case for tetracoordinated Mo⁵⁺ _4c ions arising from reduction of tetrahedral Mo⁶⁺ ions (Eq. (7)). These Mo⁵⁺ _4c ions are similar to those produced by UV-irradiaiion (Eq. (2)). In addition, if the adsorbed molecule has a sufficiently large electron affinity, such as TCNE or O₂, an electron transfer can occur (Eq. (9) and (17)). The [Mo⁵⁺-O^-] bond obtained by thermal activation is more difficult to evidence than that obtained with UV-activation because it is not detectable by EPR. However, the EPR results obtained at low temperature show that the O^- ions adsorbed on Mo/SiO₂ catalysts as well as the [Mo⁵⁺-O^-] excited triplet state obtained by UV-irradiation of 1Mo⁶⁺=O²] interact with methanol (Eq. (16)). They are consistent with the mechanism of methanol oxidation occurring at high temperature (Eq. (4)).     

The state of metals in the Pt/Al2O3 and (Pt-Cu)/Al2O3 catalysts as indicated by IR spectroscopy with isotope dilution of 12C16O with 13C18O molecules

Adsorption of ¹³C¹⁸O+¹²C¹⁶O mixtures on the Pt(2.9%)/γ-Al₂O₃, (Pt(2.6%)+Cu(2.7%))/γ-Al₂O₃, and (Pt(2.6%)+Cu(5.1%))/γ-Al₂O₃ catalysts was studied by FTIR spectroscopy. On the metallic Pt surface at coverages close to saturation, CO is adsorbed both strongly and weakly to form linear species for which the vibrational frequencies of the isolated ¹³C¹⁸O molecules adsorbed on Pt are ∼1940 and ∼1970 cm⁻¹, respectively. The redistribution of intensities of the high-and low-frequency absorption bands in the spectra of adsorbed ¹³C¹⁸O indicates that these linear forms are present on the adjacent metal sites. The weak adsorption is responsible for the fast isotope exchange between the gaseous CO and CO molecules adsorbed on metal. The Pt-Cu alloys, in which the electronic state of the surface Pt atoms characteristic of monometallic Pt remains unchanged, are formed on the surface of the reduced Pt-Cu bimetallic catalysts. The decrease in the vibrational frequencies of the isolated C=O bonds in the isolated Pt-CO complexes suggests that the CO molecules adsorbed on the Cu atoms affect the electronic properties of Pt. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 831–836, May, 2007.     

Supported catalysts based on Pd–In nanoparticles for the liquid-phase hydrogenation of terminal and internal alkynes: 1. formation and structure

The formation of Pd–In catalysts synthesized from the heteronuclear acetate complex PdIn(CH₃COO)₅ was studied by temperature-programmed reduction, electron microscopy, IR spectroscopy of adsorbed CO and hydrogen temperature-programmed desorption (H₂-TPD). IR spectroscopy of adsorbed CO and H₂-TPD confirmed the formation of bimetallic Pd–In nanoparticles. It was found that the Pd–In nanoparticle surface contains predominantly Pd atoms separated from one another by indium atoms, which is evidenced by the disappearance of the CO band shift resulting from the lateral dipole–dipole interaction between adsorbed CO molecules and by a significant decrease in the band intensity of CO adsorbed in bridged form. Almost complete inhibition of palladium hydride (PdH_x) provides additional evidence of the formation of Pd–In bimetallic particles.     

Formation of Pd–Ag nanoparticles in supported catalysts based on the heterobimetallic complex PdAg<Subscript>2</Subscript>(OAc)<Subscript>4</Subscript>(HOAc)<Subscript>4</Subscript>

The formation of Pd–Ag nanoparticles deposited from the heterobimetallic acetate complex PdAg₂(OAc)₄(HOAc)₄ on α-Al₂O₃, γ-Al₂O₃, and MgAl₂O₄ has been investigated by high-resolution trans-mission electron microscopy, temperature-programmed reduction, and IR spectroscopy of adsorbed CO. The reduction of PdAg₂(OAc)₄(HOAc)₄ supported on γ-Al₂O₃ and MgAl₂O₄ takes place in two steps (at 15–245 and 290–550°C) and yields Pd–Ag particles whose average size is 6–7 nm. The reduction of the Pd–Ag catalyst supported on α-Al₂O₃ occurs in a much narrower temperature range (15–200°C) and yields larger nanoparticles (~10–20 nm). The formation of Pd–Ag alloy nanoparticles in all of the samples is demonstrated by IR spectroscopy of adsorbed CO, which indicates a marked weakening of the absorption band of the bridged form of adsorbed carbon monoxide and a >30-cm^–1 bathochromic shift of the linear adsorbed CO band. IR spectroscopic data for PdAg₂/α-Al₂O₃ suggest that Pd in this sample occurs as isolated atoms on the surface of bimetallic nanoparticles, as is indicated by the almost complete absence of bridged adsorbed CO bands and by a significant weakening of the Pd–CO bond relative to the same bond in the bimetallic samples based on γ-Al₂O₃ and MgAl₂O₄ and in the monometallic reference sample Pd/γ-Al₂O₃.     

Fe- and Cu-oxides supported on γ-Al2O3 as catalysts for the selective catalytic reduction of NO with ethanol. Part I: catalyst preparation,characterization, and activity

Fe- and Cu-oxides supported on γ-alumina (γ-Al₂O₃; metal loading of 3 mass %) were investigated as alternative catalysts to the conventional Ag-based system in the selective catalytic reduction of NO with ethanol (EtOH-SCR). The catalysts were characterized by elemental analysis, N₂ sorption, X-ray diffraction, temperature-prgrammed desorption of NH₃, temperature-programmed reduction with H2, diffuse reflectance UV-VIS (DR-UV-VIS) spectroscopy, and compared with 3 mass % Ag/γ-Al₂O₃ as a reference catalyst. Catalytic experiments were carried out between 423 K and 773 K in the steady state and by temperature-programmed surface reaction (TPSR) experiments. For all catalysts, the highest NO conversion (900 ppm (ppm = parts of the mixture component per million parts of all mixture components) NO, 900 ppm EtOH, 0.5 vol. % H₂O, 4 vol. % O₂ in He) was found at 573 K. While 84 % of NO were converted over the Ag-based catalysts, only 20–60 % NO conversion was observed for the Fe- and Cu-containing catalysts. Total oxidation of ethanol as an unwanted side reaction occurs over 3 mass % Cu on γ-Al₂O₃ already at 573 K, whereas the highest activity of 3 mass % Fe on γ-Al₂O₃ for this conversion was reached at 743 K. For lower temperatures, partial oxidation of ethanol leads to organic by-products which can act as active intermediates in EtOH-SCR. TPSR experiments show that ethanol reacts over both the Fe- and the Cu-based catalysts to organic by-products, such as ethene or acetaldehyde, which affect the EtOH-SCR reaction.     

The influence of hydrogen-containing molybdenum and tungsten bronzes on the catalytic activity of palladium composite catalysts for the oxidation of H<Subscript>2</Subscript>, CO,and CH<Subscript>4</Subscript>

The influence of hydrogen-containing molybdenum and tungsten bronzes on the catalytic activity of palladium composite catalysts for the oxidation of H₂, CO, and CH₄ was studied. It was found that the composite catalysts containing H _x MO₃ phases (M = W or Mo), which were formed by the reduction of MoO₃ and WO₃ oxides with hydrogen in the presence of deposited Pd, showed higher catalytic activity in the oxidation of small molecules (H₂, CO, and CH₄) with excess oxygen than the traditional Pd/Al₂O₃ deposited catalyst with the same content of the deposited metal. It was shown that the thermal stability of the H _x MO₃ phases was the limiting factor influencing the activity of these composite catalysts.     

Stabilization of sensing performance for mixed-potential-type zirconia-based hydrocarbon sensor

The recently reported sensing characteristics of the mixed-potential-type yttria-stabilized zirconia (YSZ)-based hydrocarbon (HC) sensor attached with ZnCr₂O₄-sensing electrode (SE) were found to be changed after the 10-day operation at 550 °C under the wet condition (5 vol.% water vapor). To improve the stability of the present sensor, the several modifications of the SE material by adding YSZ powder were examined. As a result, the sensor using the laminated (ZnCr₂O₄/YSZ)-SE gave the stable electromotive force (emf) response against 100 ppm C₃H₆ at 550 °C for about one month examined. Based on the scanning electron microscopy (SEM) observation and the AC complex-impedance measurements, it was concluded that the stable behavior of the sensor using the laminated (ZnCr₂O₄/YSZ)-SE was provided by the stabilization of the interface between ZnCr₂O₄ grains and YSZ particles. The fabricated sensor exhibited the linear dependence of sensitivity on the logarithm of either C₃H₆ concentration (in the range of 20-800 ppm) or mixtures of various hydrocarbons (HCs) (in the range of 90-2600 ppmC). In addition, the emf response was not altered by the change of O₂ (2-20 vol.%), H₂O (0-10.8 vol.%) and CO₂ (0-20 vol.%) concentrations, and no interference of other gases (CO, NO, NO₂, H₂, and CH₄) was observed.     

Metal oxide semiconductors based on tin dioxide: Gas-sensitivity to methane in a wide range of temperatures,concentrations and humidities of the gas phase

The response of metal oxide semiconductors (MOSs) made by the thick-film technology on the basis of SnO₂ with various catalytic additives was studied in dry gaseous media containing 200 ppm CH₄ in the temperature range 100–600° C. Concentration dependence was studied for four sensors (on the basis of pure SnO₂ and with three catalytic additives, 3% Pd, 1% Sb₂O₅ + 3% La₂O₃, and 1% Pt + 3% Pd) in the concentration range 1–20600 ppm CH₄ at the humidity of the gas phase varied from 0 to 100%. It was found out that, in the strength of all performance and technical characteristics, the structure SnO₂ + 1% Pt + 3% Pd working at 400°C and consuming ～180 mW was the best for recording CH₄.     

Autothermal reforming of methane to syngas for Fischer-Tropsch synthesis with promoted palladium and a fast start-up device

In this study, a Pd catalyst was prepared with promoters such as CeO₂, BaO and SrO in a washcoated form on a metallic monolith for autothermal reforming of methane to syngas for the Fischer-Tropsch synthesis. A reactor was installed with an electric heater in the form of the metallic monolith as a start-up device instead of a burner with which stable and fast start-ups (within 4 min) were achieved. Gas hourly space velocity and O₂/CH₄ governed, methane conversion, while H₂O/CH₄ controlled H₂/CO ratio. A methane conversion of approx. 96%, H₂+CO selectivity of approx. 85%, and H₂/CO of approx. 2.6 were obtained under the conditions of gas hourly space velocity (GHSV) at 103000 h^?1, O₂/CH₄=0.7 and H₂O/CH₄=0.35.     

The oxidation of carbon monoxide on a catalyst with a spinel structure containing Mg ferrite

The oxidation of carbon monoxide on xMgO · yFe₂O₃ catalysts was studied over the temperature range 440–660 K. The catalysts contained 5, 40, and 75 at % Fe and the spinel MgFe₂O₄ and MgO phases. Complete CO conversion on compact and deposited (on γ-Al₂O₃) catalysts containing 75 at. % Fe occurred at 650–660 K. The kinetics of interaction of CO with adsorbed oxygen was studied on catalysts with 5 and 40 at % Fe below and above the Curie point (T _c = 593 ± 10 K). The differences in the reaction orders with respect to gaseous CO and adsorbed oxygen below and above T _c were explained as follows. In the ferromagnetic state of the active MgFe₂O₄ phase, oxidation involved adsorbed oxygen localized at oxygen vacancies in the environment of Fe³⁺ ions, whereas, in the paramagnetic state, superexchange interactions were absent in spinel structure fragments.     

Catalytic activity of bimetal-containing Co,Pd systems in the oxidation of carbon monoxide

The catalytic activity of low-percentage Co,Pd systems on ZSM-5, ERI, SiO₂, and Al₂O₃ supports in the oxidation of CO was studied. The activity of bimetal-containing catalysts was shown to depend on the nature of the catalyst and the amount and ratio of their active components. According to the results of thermoprogrammed reduction with H₂ (H₂ TPR) and X-ray photoelectron spectroscopy (XPS) data, the metals are distributed as isolated cations or Co^δ+-O-Pd^δ+ clusters with cobalt and palladium cations surrounded by off-lattice oxygen in Co,Pd systems. The 0.8% Co,0.5% Pd-ZSM-5 bimetal catalysts were found to be more active due to the presence of clusters.     

Influence of the Support on the Catalytic Characteristics of the Deposited Palladium in the Liquid-Phase Hydrogenation of Diphenylacetylene

Palladium catalysts on various types of supports were studied in the liquid-phase hydrogenation of diphenylacetylene. Samples of Pd/SiO₂–Al₂O₃, Pd/MgAl₂O₄, Pd/Al₂O₃, and Pd/TiO₂ were characterized by the chemisorption of the CO and IR spectroscopy of adsorbed CO. The use of n-hexane as the solvent increases the reaction rate, which can be explained by the better solubility of hydrogen in the liquid phase. It is established that the acid–base properties of the support do not affect the activity and selectivity of the catalysts in the reaction under study. However, they alter the electronic state of palladium. According to the catalytic tests, Pd/TiO₂ has the highest activity (turnover frequency) and selectivity to alkene. The comparison of the obtained catalytic data and the results of IR spectroscopy made it possible to conclude that this is due to the electron density redistribution between the palladium and TiO _x particles, which is caused by the strong metal–support interaction.     

Catalytic Methane Oxidation Over Pd Supported on Nanocrystalline and Polycrystalline TiO2 Mn3O4, CeO2 and ZrO2

The catalytic oxidation of methane has been examined over Pd supported on nanocrystalline (n-) and polycrystalline (p-) TiO₂, Mn₃O₄, CeO₂ and ZrO₂. In all cases the Pd supported on the nanocrystalline oxides performs better on a mass basis than Pd supported on the polycrystalline oxides. Conversion vs temperature curves indicate that n-ZrO₂ is more active than p-ZrO₂ and that calcining both n-ZrO₂ and p-ZrO₂ at 500°C produces better catalysts than calcining at 280°C. n-CeO₂ is a very good catalysts for methane oxidation, while p-CeO₂ is not, and Pd supported on n-CeO₂ performs much better than bare n-CeO₂ and somewhat better than Pd supported on p-CeO₂; Pd supported on n-Mn₃O₄ or p-Mn₃O₄ does not perform as well as CeO₂-supported Pd catalysts. The 5 wt.% Pd/n-ZrO₂ catalyst calcined at 500°C performs very well, achieving 100% conversion at 320°C for the reactor conditions used, while 5 wt.% Pd/n-CeO₂ exhibits initial activity at the lowest temperature of about 100°C. The best catalyst tested in this study is 30 wt.% Pd/n-TiO₂, which achieves 100% conversion at 300°C.     

15N NMR studies of the No?NH3?O2 reaction over ZSM-5 type zeolites and vanadia supported catalysts

¹⁵N nuclear magnetic resonance spectra produced by adsorption of NO together with O₂ and NH₃ on H-ZSM-5 and V₂O₅/Al₂O₃ catalysts have been examined. Several surface species and reaction intermediates have been identified by their characteristic¹⁵N NMR spectra. The intermediate complexes between the products of disproportionation and partial oxidation of NO, on one hand, and ammonia, on the other hand, were found to be formed in the reaction. On V₂O₅/Al₂O₃ catalyst the reaction proceeds more rapidly compared with ZSM-5 but some details of the mechanism are similar for both catalysts.     

Catalytic Active Site for NO Decomposition Elucidated by Surface Science and Real Catalyst

Comprehensive studies combining surface science and real catalyst were performed to get further insight into catalytic active site and reaction mechanism for NO decomposition over supported palladium and cobalt oxide-based catalysts. On palladium single-crystal model catalysts, adsorption, dissociation and desorption behavior of NO was found to be closely related to the surface structures, the stepped surface palladium being active for dissociation of NO. In accordance with this result, the activity of powder Pd/Al₂O₃ catalysts for NO decomposition was directly related to the number of step sites exposed on the surface, suggesting that the step sites act as the catalytic active site for NO decomposition on Pd/Al₂O₃. NO decomposition over cobalt oxide was found to be significantly promoted by addition of alkali metals. Surface science study and catalyst characterization led to the same conclusion that the interface between the alkali metal and Co₃O₄ serves as the catalytic active site. From the results of in situ Fourier transform infrared (FT-IR) spectroscopy and isotopic transient kinetic analysis, a reaction mechanism was proposed in which the reaction is initiated by NO adsorption onto alkali metals to form NO₂⁻ species and then NO₂⁻ species react with the adsorbed NO species to form N₂ over the interface between the alkali metal and Co₃O₄.     

制备方法对Co-MOR催化剂CH4选择还原NO性能的影响

采用离子交换法、浸渍法制备一系列的Co-MOR 催化剂, 并将其用于CH₄选择性催化还原 NO_x(CH₄-SCR)反应. 运用X 射线衍射(XRD)、X 射线荧光光谱(XRF)、扫描电子显微镜(SEM)、紫外-拉曼(UVRaman)光谱、X射线光电子能谱(XPS)、NO程序升温脱附(NO-TPD)等手段对催化剂进行了表征. 结果表明, 浸渍法制备的催化剂, Co以Co₃O₄形式存在; 而离子交换法制备的催化剂, Co以离子形式进入丝光沸石(MOR)骨架之中, 在催化剂上形成更多的Co²⁺和[Co-O-Co]²⁺, 形成更均匀NO吸附中心和CH₄-SCR反应活性中心. 催化剂活性评价表明离子交换法制备的催化剂具有更宽的活性温度区间, Co(0.30)-MOR 催化剂在327-450℃温度范围内NO转化率大于50%.