Reaction paths of the formation and consumption of nitroorganic complex intermediates in the selective catalytic reduction of nitrogen oxides with propylene on zirconium dioxide according to in situ Fourier transform IR spectroscopic data

Nitrate, acetate, and nitroorganic complexes were detected on the surface of ZrO₂ under the reaction conditions of nitrogen oxide reduction with propylene using Fourier transform IR spectroscopy. The nitroorganic complex was formed in the reaction between acetate and nitrate complexes by the replacement of the carboxyl group in the acetate complex by the nitro group. Monodentate nitrate was the most reactive species in this process. The adsorption of various nitroorganic substances was studied. It was found that the nitroorganic complex was structurally analogous to the nitromethane molecule bound to the surface through the nitro group. The experimental data led us to a conclusion that nitroorganic compounds were subsequently consumed in reactions with nitrate complexes. In this surface reaction, monodentate nitrate was also the most reactive species. The presence of oxygen had no effect on the consumption of the nitroorganic complex.     

Metal–Organic Framework (MOF) Template Based Efficient Pt/ZrO2@C Catalysts for Selective Catalytic Reduction of H2 Below 90 °C

Selective catalytic reduction (SCR) of NO_x with H₂ as a reductant is the most promising denitration technology at low temperature. Achieving the conversion of NO_x into N₂ at ambient temperature not only prolongs the service life of the catalyst, but also provides more freedom for the arrangement of denitration units throughout the flue gas treatment equipment. However, the development of highly efficient, stable, and environmentally benign supported platinum‐based catalysts for H₂‐SCR at ambient temperature is still a major challenge. Herein, a 0.5 wt % Pt/ZrO₂@C catalyst, which was composed of carbon‐coated octahedral ZrO₂ with highly dispersed Pt particles, was prepared by using a new stabilization strategy based on UiO‐66‐NH₂ (a zirconium metal–organic framework) as a template. The catalytic performance of this Pt/ZrO₂@C in H₂‐SCR was tested and confirmed to achieve near 100 % NO_x conversion at 90 °C. Also, 70 % N₂ selectivity of the catalyst was achieved. The morphology, structure, and porous properties of the as‐synthesized nanocomposites were characterized by using data obtained from field‐emission SEM, TEM, XRD, Raman spectroscopy, thermogravimetric analysis, X‐ray photoelectron spectroscopy, and N₂ adsorption–desorption isotherms. The results show that residual carbon formed by pyrolysis treatment is coated on octahedral ZrO₂, and effectively prevents the agglomeration of platinum particles on the surface.     

Spectrokinetic study of the mechanism of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> reduction with propylene over ZrO<Subscript>2</Subscript> in excess oxygen

It has been demonstrated by quantitative spectrokinetic measurements that, on the surface of zirconia stabilized as a tetragonal phase, the rate-limiting step of the selective catalytic reduction of nitrogen oxides (SCR of NO _x ) with propylene is the interaction of surface nitrates with C₃H₆ yielding organic nitro compounds. It is hypothesized that propylene reacts not with the nitrates themselves but with the activated complex NO₂ ^ads whose structure is intermediate between the structures of the monodentate NO₃ ^? and NO₂ species. Deep C₃H₆ oxidation exerts an adverse effect on the rate of the SCR of NO _x with propylene, and the interaction between O₂ and NO, which yields NO₂ and NO₃ ^? stimulates further nitrogen reduction to N₂. The effect of the reaction between oxygen and O₂N?C _n H _m on the NO _x reduction rate is variable and is determined by the C₃H₆/NO _x ratio. A generalized scheme of the SCR of NO _x with propylene on the surface of ZrO₂ partially stabilized as a tetragonal phase has been developed by comparing experimental data of this study and data available from the literature.     

Interaction between NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> and the surface of supported heteropoly compounds: In situ IR spectroscopic data

The mechanism of activation of nitrogen oxides on unsupported heteropoly compounds and the composition, location, stability, and interconversion mechanisms of adsorption complexes on supported heteropoly compounds have been investigated by in situ IR spectroscopy under thermal desorption conditions. Supporting a small amount of a heteropoly compounds (1% or below) increases NO _x adsorption relative to the adsorption observed for the pure support. This effect is most pronounced for CeO₂ and least pronounced for ZrO₂. The increase in NO _x adsorption is due to NO oxidation to NO₂ on the supported heteropoly compound. The main adsorption species are nitrite and nitrate complexes, which are located on the support. As the temperature is raised, the nitrite complexes turn into the nitrate complexes. The presence of variable-valence ions in the Keggin anion reduces the nitrate complex-surface binding strength. The ions that are not components of the Keggin anion increase the binding strength. The changes in the nitrate complex-support surface binding strength are due to the support modification taking place via the destruction of part of the supported heteropoly compound.     

Effect of copper precursors on the catalytic activity of Cu/ZSM-5 catalysts for selective catalytic reduction of NO by NH<Subscript>3</Subscript>

The Cu/ZSM-5 catalysts prepared by different copper precursors were used for the selective catalytic reduction (SCR) of NO _x with NH₃. The Cu/ZSM-5 catalyst prepared by the copper nitrate (Cu/ZSM-5-N) presented the best performance among the Cu/ZSM-5 catalysts and showed above 90 % NO _x conversion at 225–405 °C. The average particle size of CuO was 5.82, 9.20, and 11.01 nm over Cu/ZSM-5-N, Cu/ZSM-5-S (prepared by copper sulfate), and Cu/ZSM-5-C (prepared by copper chloride), respectively. The Cu/ZSM-5-N catalyst showed the highly dispersed copper species, the strong surface acidity, and the excellent redox ability compared with the Cu/ZSM-5-C and Cu/ZSM-5-S catalysts. The Cu⁺ and Cu²⁺ existed in the Cu/ZSM-5 catalysts and the abundant Cu⁺ over Cu/ZSM-5-N might be responsible for the superior SCR activity.     

Review of Ag/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Reductant System in the Selective Catalytic Reduction of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Ag/Al₂O₃ is a promising catalyst for the selective catalytic reduction (SCR) by hydrocarbons (HC) of NO _x in both laboratory and diesel engine bench tests. New developments of the HC-SCR of NO _x over a Ag/Al₂O₃ catalyst are reviewed, including the efficiencies and sulfur tolerances of different Ag/Al₂O₃-reductant systems for the SCR of NO _x ; the low-temperature activity improvement of H₂-assisted HC-SCR of NO _x over Ag/Al₂O₃; and the application of a Ag/Al₂O₃-ethanol SCR system with a heavy-duty diesel engine. The discussions are focused on the reaction mechanisms of different Ag/Al₂O₃-reductant systems and H₂-assisted HC-SCR of NO _x over Ag/Al₂O₃. A SO₂-resistant surface structure in situ synthesized on Ag/Al₂O₃ by using ethanol as a reductant is proposed based on the study of the sulfate formation. These results provide new insight into the design of a high-efficiency NO _x reduction system. The diesel engine bench test results showed that a Ag/Al₂O₃-ethanol system is promising for catalytic cleaning of NO _x in diesel exhaust.     

X-ray photoelectron spectroscopic study of the interaction of supported metal catalysts with NO<Subscript>x</Subscript>

The reactions of the platinum and rhodium model catalysts applied to aluminum oxide with NO_x (10 Torr NO + 10 Torr O₂) were studied by X-ray photoelectron spectroscopy. The reaction conducted at room temperature formed on the surface of the oxide support the NO _3,s ^? nitrate ions characterized by the N1s line at 407.4 eV and O1s line at 533.1 eV and the NO _2,s ^? nitrite ions characterized by the N1s line with a binding energy of 404.7 eV. At the same time, the Pt4f and Rh3d lines of the supported platinum particles are shifted toward higher binding energies by 0.5–1.0 eV and 0.7–1.2 eV, respectively. It is assumed that the binding energies increase due to changes in the chemical state of the platinum metal in which oxygen is dissolved. The reaction of NO_x with Pt/Al₂O₃ at 200°C forms platinum oxide defined by the Pt4f _7/2 line with a binding energy of 72.3 eV.     

Catalytically Promoted NO x Sorption by ZrO2-Based Oxides

Metal promoted zirconia-based oxide sorbents, such as Pt–ZrO₂/Al₂O₃ for NO _x have been investigated. To clarify the role of the catalyst component, sorption of NO and NO₂ was compared using the samples with and without Pt. The catalytic oxidation of NO to NO₂ and successively to nitrate ions is an important role for the Pt catalyst. The experimental results indicate that a high-temperature calcination is essential to remove residual Cl from Pt–ZrO₂–Al₂O₃ prepared from H₂PtCl₆ in order to provide more active NO _x sorption sites. Of M–ZrO₂–Al₂O₃ samples investigated, ruthenium as well as Pt demonstrated relatively good performance as a catalyst component in the sorbent. The FT-IR spectra after sorption of NO and NO₂ demonstrated a strong band attributed to stored nitrate ions. The Pt catalyst was more resistant to sulfur poisoning than a base metal catalyst. However, the NO _x sorptive capacities of the Pt–ZrO₂/Al₂O₃ sorbents were expected to be deteriorated in dilute SO₂ as far as observed from FT-IR spectra.     

Reaction paths of the formation and consumption of nitroorganic complex intermediates in the selective catalytic reduction of nitrogen oxides with propylene on zirconia-pillared clays according to in situ spectroscopic data

It was found that the adsorption and catalytic properties of nanosized ZrO₂ particles as the pillar constituents of ZrO₂-pillared clay and bulk ZrO₂ are essentially different. The interaction of NO with the surface of bulk ZrO₂ resulted in the formation of three types of nitrate complexes. Only two nitrate species were formed on ZrO₂-pillared clay (the monodentate species was absent). Only an acetate complex was formed in the interaction of a mixture of propylene and oxygen with the surface of bulk ZrO₂, whereas an isopropoxide complex was the main propylene activation species on ZrO₂-pillared clay. On the surface of ZrO₂-pillared clay, isopropoxide and nitrate intermediates formed a complex structurally similar to adsorbed dinitropropane. On the surface of bulk ZrO₂, acetate and monodentate nitrate complexes formed a complex structurally similar to adsorbed nitromethane. The dinitropropane complex on ZrO₂-pillared clay was consumed in reactions with surface nitrates. The decomposition reaction of a dinitropropane compound with the formation of acetate complexes and ammonia predominated on the surface containing no nitrate complexes in the absence of NO + O₂ from a gas phase. The found differences in reactant activation species and their thermal stabilities explained differences in the activities of bulk ZrO₂ and nanosized ZrO₂ particles as pillars in pillared clay in the course of the selective catalytic reduction of nitrogen oxides with propylene in an excess of oxygen.     

NO2对Cu/SAPO-34分子筛催化剂上NH3选择性催化还原NO性能的影响

主要考察了NO₂对Cu/SAPO-34 分子筛催化剂在整个温度范围内（100-500 ℃）NH₃选择性催化还原（SCR）NO性能的影响. 研究所使用样品为新鲜Cu/SAPO-34 催化剂在750 ℃下水热处理4 h 的稳定期样品.通过X射线衍射（XRD）和扫描电子显微镜（SEM）对样品的结构以及形貌进行表征,采用SCR活性评价、动力学实验以及原位漫反射傅里叶变换红外光谱（in situ-DRIFTS）表征催化剂的性能以及催化剂表面物种的变化. 活性评价实验结果表明,NO₂会抑制催化剂的低温（100-280 ℃）活性,但其存在会提高催化剂的高温（280 ℃以上）活性. 与此同时,随着反应物中NO/NO₂的摩尔比例减少,由于NH₄NO₃物种的分解,副产物（N₂2O）的浓度增大. 动力学结果表明,Cu/SAPO-34 催化剂上快速SCR反应的表观活化能（E_a=64.02 kJ·mol^-1）比标准SCR反应的表观活化能（E_a=48.00 kJ·mol^-1）更大. In situ-DRIFTS实验结果表明NO比NO₂更容易在催化剂表面形成硝酸盐,并且NO₂更容易与吸附在Brønsted 酸性位上的NH₃物种反应生成NH₄NO₃. 低温下,催化剂表面的NH₄NO₃物种会覆盖SCR反应的活性位,造成活性降低,但在高温时,形成的NH₄NO₃物种一部分会被NO还原为N2,而另一部分会直接热分解为N₂O,造成催化剂的选择性降低.     

Films of certain oxides of rare-earth elements as the activators of platinum electrode on ZrO<Subscript>2</Subscript> + 10 mol % Y<Subscript>2</Subscript>O<Subscript>3</Subscript> electrolyte

The characteristics of porous Pt/YSZ (ZrO₂ + 10 mol % Y₂O₃) electrodes activated with small amounts of either oxides of rare-earth elements (REE) of the cerium subgroup (CeO₂, PrO _x , TbO _x ) or a mixed oxide with the Сe₂Tb₄O₁₁ composition by the procedure of impregnating the electrodes with ethanol solutions of REE nitrates and subsequent heating at 850°С are studied by the impedance method. The studies are carried out for those cases where the REE oxides after thermal treatment form a film on the electrolyte and also where no activator film is formed. The characteristics of films and activated electrodes are compared. Film-activated Pt/YSZ electrodes are discussed within the framework of the model of compact oxide electrodes.     

The mechanism of selective NO<Subscript>x</Subscript> reduction by hydrocarbons in excess oxygen on oxide catalysts: VI. Spectroscopic and kinetic characteristics of surface complexes on a Ni-Cr oxide catalyst

The reaction mechanism of the selective catalytic reduction of NO_x by propane in the presence of O₂ on a commercial Ni-Cr oxide catalyst was studied using in situ IR spectroscopy. It was found that nitrite, nitrate, and acetate surface complexes occurred under reaction conditions. Considerable amounts of hydrogen were formed in the interaction of NO + C₃H₈ + O₂ or C₃H₈ + O₂ reaction mixtures with the catalyst surface. The rates of conversion of the surface complexes detected under reaction conditions were measured. The resulting values were compared to the rate of the process. It was found that, at temperatures lower than 200°C, nitrate complexes reacted with the hydrocarbon to form acetate complexes; in this case, the formation of reaction products was not observed. In the temperature region above 250°C, two reaction paths took place. One of them consisted in the interaction of acetate and nitrate complexes with the formation of reaction products. The decomposition of NO on the reduced surface occurred in the second reaction path. Nitrogen atoms underwent recombination, and oxygen atoms reoxidized the catalyst surface and reacted with the activated hydrocarbon to form CO₂ and H₂O in a gas phase.     

Catalytically Promoted NOx Sorption by ZrO2-Based Oxides

Metal promoted zirconia-based oxide sorbents, such as Pt–ZrO₂/Al₂O₃ for NO _x have been investigated. To clarify the role of the catalyst component, sorption of NO and NO₂ was compared using the samples with and without Pt. The catalytic oxidation of NO to NO₂ and successively to nitrate ions is an important role for the Pt catalyst. The experimental results indicate that a high-temperature calcination is essential to remove residual Cl from Pt–ZrO₂–Al₂O₃ prepared from H₂PtCl₆ in order to provide more active NO _x sorption sites. Of M–ZrO₂–Al₂O₃ samples investigated, ruthenium as well as Pt demonstrated relatively good performance as a catalyst component in the sorbent. The FT-IR spectra after sorption of NO and NO₂ demonstrated a strong band attributed to stored nitrate ions. The Pt catalyst was more resistant to sulfur poisoning than a base metal catalyst. However, the NO _x sorptive capacities of the Pt–ZrO₂/Al₂O₃ sorbents were expected to be deteriorated in dilute SO₂ as far as observed from FT-IR spectra.     

Mechanism of methanol conversion on ZrO<Subscript>2</Subscript> and 5% Cu/ZrO<Subscript>2</Subscript> according to in situ IR spectroscopic data

It is demonstrated by in situ IR spectroscopy that, in methanol conversion on ZrO₂ and 5% Cu/ZrO₂ catalysts, methoxy groups are present on the catalyst surface, which result from O-H or C-O bond breaking in the methanol molecule. Two types of formate complexes, localized on ZrO₂ and CuO, are also observed. The formate complexes form via the oxidative conversion of the methoxy groups. There are two types of linear methoxy groups. First-type linear methoxy groups condense with the formate complex located on CuO to yield methyl formate and then CO and H₂. Second-type methoxy groups appear as intermediate products in the formation of dimethyl ether. The main hydrogen formation reactions are the recombination of hydrogen atoms (which result from the interconversion of surface complexes) on copper clusters and the decomposition of methyl formate. The source of CO₂ in the gas phase is the formate complex, and the source of CO is methyl formate. The effect of water vapor and oxygen the surface reactions and product formation is discussed.     

Use of the differential charging effect in XPS to determine the nature of surface compounds resulting from the interaction of a Pt/(BaCO<Subscript>3</Subscript> + CeO<Subscript>2</Subscript>) model catalyst with SO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Changes in the chemical composition of the surface of a Pt/(BaCO₃ + CeO₂) model NO _x storage-reduction catalyst upon its interaction with SO _x (SO₂ (260 Pa) + O₂ (2600 Pa) + H₂O (525 Pa)) followed by regeneration in a mixture of CO (2100 Pa) with H₂O (525 Pa) were studied by X-ray photoelectron spectroscopy (XPS). Model catalyst samples were prepared as a thin film (about several hundreds of angstrom units in thickness) on the surface of tantalum foil coated with a layer of aluminum oxide (～100 Å). It was found that the Pt/BaCO₃ and Pt/CeO₂ catalyst constituents acquired different surface charges (differential charging) in the course of photoelectron emission; because of this, it was possible to determine the nature of surface compounds formed as a result of the interaction of the catalyst with a reaction atmosphere. It was found that barium carbonate was converted into barium sulfate as a result of reaction with SO _x on the surface of BaCO₃ at 150°C. As the treatment temperature in SO _x was increased to 300°C, the formation of sulfate on the surface of CeO₂ was observed. The sulfatization of CeO₂ was accompanied by the reduction of Ce(IV) to Ce(III). The regeneration reaction of the catalyst treated in SO _x at 300°C resulted in the consecutive decomposition of cerium(III) sulfate at ≤500°C and then barium sulfate at 600–700°C. Upon the decomposition of BaSO₄, a portion of sulfur was converted into a sulfide state, probably, because of the formation of BaS.     

The mechanism of reduction of NO with H<Subscript>2</Subscript> in strongly oxidizing conditions (H<Subscript>2</Subscript>-SCR) on a novel Pt/MgO-CeO<Subscript>2</Subscript> catalyst: Effects of reaction temperature

Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments using on-line Mass Spectrometry (MS) and in situ Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) have been performed to study essential mechanistic aspects of the Selective Catalytic Reduction of NO by H₂ under strongly oxidizing conditions (H₂-SCR) in the 120–300°C range over a novel 0.1 wt % Pt/MgO-CeO₂ catalyst. The N-path of reaction from NO to the N₂ gas product was probed by following the ¹⁴NO/H₂O₂ → ¹⁵NO/H₂/O₂ switch (SSITKA-MS and SSITKA-DRIFTS) at 1 bar total pressure. It was found that the N-pathway of reaction involves the formation of two active NO _x species different in structure, one present on MgO and the other one on the CeO₂ support surface. Inactive adsorbed NO _x species were also found on both the MgO-CeO₂ support and the Pt metal surfaces. The concentration (mol/g cat) of active NO _x leading to N₂ was found to change only slightly with reaction temperature in the 120–300°C range. This leads to the conclusion that other intrinsic kinetic reasons are responsible for the volcano-type conversion of NO versus the reaction temperature profile observed.     

FT-IR study of the nature and stability of NOx surface species on ZrO2, VOx/ZrO2 and MoOx/ZrO2 catalysts

The adsorption of NO, NO/O₂ mixtures and NO₂ on pure ZrO₂ and on two series of catalysts supported on ZrO₂, one containing vanadia and the other molybdena (ZV and ZMo, respectively), has been investigated. The V and Mo surface contents of the latter were ≤3 atoms nm⁻² and ≤5 atoms nm⁻², respectively. All samples had been previously submitted to a standard oxidation treatment. On all samples, only extremely minor amounts of NO_x surface species are formed by NO interaction at room temperature (RT). NO_x surface species are formed in greater amounts on pure ZrO₂ when NO and O₂ are coadsorbed at RT; they are mainly nitrites, small amounts of nitrates, and small amounts of (O₂NO−H)^δ− species; when ZrO₂ is warmed to 623 K in the NO/O₂ mixture, nitrites decrease, nitrates and (O₂NO−H)^δ− species increase. The same NO_x species as on the ZrO₂ surface free from V (or Mo) are formed on ZV (or ZMo) samples with surface V (or Mo) density <1.5 atoms nm⁻²; however, they occur in decreased amount with increasing V (or Mo) coverage. On ZV samples with a surface V density of 1.5–3 atoms nm⁻² (or ZMo samples with a surface Mo density of 1.5–5 atoms nm⁻²) when NO and O₂ are coadsorbed at RT, there is formation of small amounts of nitrites, nitrates (both on ZrO₂ surface free from V (or Mo) and at the edges of V- or Mo-polyoxoanions) and NO₂ ^δ+ species, associated with V⁵⁺ (or Mo⁶⁺) of very strong Lewis acidity; when samples are warmed up 623 K in the NO/O₂ mixture, nitrites disappear, nitrates increase, NO₂ ^δ+ species remain constant or slightly decrease. When NO₂ is allowed into contact at RT with oxidized samples, surface situations almost identical to those obtained for each sample warmed to 623 K in NO/O₂ mixture is reached. The NO_x surface species stable at 623 K, the temperature at which catalysts show the best performance in the selective catalytic reduction (SCR) of NO by NH₃, are nitrates, both on ZrO₂ and on polyvanadates or polymolybdates at high nuclearity. On the contrary, nitrites and NO₂ ^δ+ species are unstable at 623 K.     

Ni基催化剂催化燃油重整耦合选择性催化还原NOx反应

分别以La₂O₂CO₃, CeO₂, ZrO₂和Al₂O₃为载体, 采用浸渍法制备了Ni基重整催化剂, 并以正十二烷模拟车载燃油进行催化重整反应以同时制备小分子碳氢化合物(HCs)和H₂, 考察了其在4wt%Ag/Al₂O₃上选择性催化还原(HC-SCR)氮氧化物(NO_x)的性能. 采用N₂吸附-脱附、X射线粉末衍射、H₂程序升温还原和热重等手段对Ni基催化剂进行了表征. 结果表明, 随着重整催化剂氧化还原性能增强, 产物中H₂浓度增加, 可参与SCR反应的HCs含量减少, 从而导致重整-SCR耦合体系上NO_x净化活性温度窗口向低温移动, NO_x最高转化率降低. Ni/ZrO₂+Ag/Al₂O₃耦合体系中H₂/HCs符合SCR反应所需的最优比例, 在柴油车典型排气温度范围内表现出良好的NO_x净化能力. 同时, 在Ni/ZrO₂+Ag/Al₂O₃耦合体系上考察了其燃油重整-SCR的活性稳定性. 结果显示, 重整催化剂的耐久性有待进一步提高.     

Zr 改性MnOx/MWCNTs催化剂的结构特征与低温SCR活性

以ZrO（NO₃）₂·2H₂O为前驱体对多壁碳纳米管（MWCNTs）进行了改性并负载MnO_x制备了MnO_x/ZrO₂/MWCNTs 催化剂. 考察了Zr 对催化剂低温选择性催化还原（SCR）反应活性的影响,并通过多种分析手段对催化剂的结构进行了表征. 结果表明Zr 的添加对催化剂的低温SCR活性具有显著的促进作用,当Zr 负载量为30%时,催化剂活性最佳. X射线衍射（XRD）、拉曼（Raman）光谱、透射电镜（TEM）、N₂吸附-脱附的表征结果分析表明,适量的Zr 改性促进了MnO_x在载体表面的分散,增强金属氧化物与MWCNTs 之间的作用,也能增加催化剂的比表面积、孔容和孔径. X 射线光电子能谱（XPS）、H₂程序升温还原（H₂-TPR）和NH₃程序升温脱附（NH₃-TPD）的分析结果则显示,Zr 能提高催化剂表面化学吸附氧浓度,促进Mn³⁺转化为Mn⁴⁺,从而使催化剂表面的活性位点增多,氧化还原能力增强,同时还提高了催化剂表面酸性位点的数量和强度,促进了NH₃的吸附,是MnO_x/ZrO₂/MWCNTs 催化剂低温SCR活性提高的主要原因.     

An XPS study of the oxidation of noble metal particles evaporated onto the surface of an oxide support in their reaction with NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The interaction of the model catalysts Rh/Al₂O₃, Pd/Al₂O₃, Pt/Al₂O₃, and Pt/SiO₂ with NO _x (mixture of 10 Torr of NO and 10 Torr of O₂) was studied by X-ray photoelectron spectroscopy (XPS). Samples of the model catalysts were prepared under vacuum conditions as oxide films ≥100 Å in thickness on tantalum foil with evaporated platinum-group metal particles. According to transmission electron microscopic data, the platinum-group metal particle size was several nanometers. It was found by XPS that the oxidation of Rh and Pd nanoparticles in their interaction with NO _x occurs already at room temperature. The particles of platinum were more stable: their oxidation under the action of NO _x was observed at elevated temperatures of ～300°C. At room temperature, the interaction of platinum nanoparticles with NO _x hypothetically leads to the dissolution (insertion) of oxygen atoms in the bulk of the particles with the retention of their metallic nature. It was found that dissolved oxygen is much more readily reducible by hydrogen than the lattice oxygen of the platinum oxide particles.