NO + H2 reaction on Pt-Ru/SiO2 catalysts: correlation between nanostructure and catalytic activity and selectivity

Nitric oxide reduction by hydrogen has been studied on Pt-Ru/SiO₂ catalysts of various Pt/Ru atomic compositions in the temperature range 298–673 K. Physical characterization showed the presence of bimetallic particles which tend to be Pt-rich. The overall activity of the bimetallic catalysts suggests a dilution of the active component (Pt) in the range 373–523 K. The addition of Ru results in a general improvement of the N₂ selectivity and significant modifications in the product distribution are observed as a function of the catalyst composition. A bimetallic particle model is proposed in which various types of surfaces are exposed including those with pure Pt atoms and/or Pt-Ru mixture. This model allows to explain the overall activity and selectivity in the whole series of catalysts.     

Oscillatory behaviour of the reduction of NO by H2 over Rh

The NO reduction by H₂ on Rh has been studied by field emission microscopy (FEM). It has been observed that this reduction shows oscillatory behaviour at 460 K andP _NO = –1.5 ×10^–1 Torr andP _H2 =1×10^-6 Torr, Unique features of FEM are the very high spatial resolution and the presence of, in principle, an indefinite number of different crystal planes. The oscillatory behaviour is reflected by periodic changes in the emission current and in the images observed. The communication between different surfaces present on the field emitter is shown on a fluorescent screen. Diffusion and gas phase coupling seem to play a role. Many of the features reported earlier for the oscillatory behaviour of the NO-H₂ and NO-NH₃ reactions over Pt(100) are observed on Rh as well, including the surface explosion. The vacancy model proposed earlier for the oscillations over Pt(100), can be applied to the reactions described in this paper as well.     

An overview: Comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO and NO + H2 reactions

This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H₂ over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H₂ reaction Pt/Al₂O₃>Pd/Al₂O₃>Rh/Al₂O₃ and for the NO+CO reaction Rh/Al₂O₃>Pd/Al₂O₃> Pt/Al₂O₃. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H₂ reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al₂O₃ during the NO+H₂ reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N₂O) irrespective of the nature of the reducing agent (CO or H₂). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N₂ via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H₂+O₂ reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.     

NO2 inhibits the catalytic reaction of NO and O2 over Pt

The rate equation for the overall reaction of NO and O₂ over Pt/Al₂O₃ was determined to be r=k_f[NO] ^1.05±0.08[O₂]^1.03±0.08[NO₂]^0.92±0.07(1-), with k_f as the forward rate constant, =([NO₂]/K[NO][O₂]^1/2), and K as the equilibrium constant for the overall reaction. An apparent activation energy of 82 kJ mol^–1 ± 9 kJ mol^–1 was observed. The inhibition by the product NO₂ makes it imperative to include the influence of NO₂ concentration in any analysis of the kinetics of this reaction. The reaction mechanism that fits our observed orders consists of the equilibrated dissociation of NO₂ to produce a surface mostly covered by oxygen, thereby inhibiting the equilibrium adsorption of NO, and the non-dissociative adsorption of O₂, which is the proposed rate determining step.     

Photocatalytic reaction of H2O+CO2 over pure and doped Rh/TiO2

In the photocatalytic reduction of carbon dioxide to formic acid, formaldehyde and methanol in aqueous suspensions of TiO₂ and Rh/TiO₂, the effects of doping the TiO₂ with W⁶⁺ were investigated.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged.     

The role of Rh on Pt-based catalysts: structure sensitive NO + H2 reaction on Pt(110) and Pt(100) and structure insensitive reaction on Rh/Pt(110) and Rh/Pt(100)

The catalytic activity of the Pt(110) surface for the reaction of NO + H₂ was much less than that of the Pt(100) surface. However, the catalytic activity of the Rh deposited Pt(1l0) surface was almost equal to that of the Rh deposited Pt(100) surface. That is, the catalytic reaction of NO + H₂ on Pt(110) and Pt(100) surfaces is highly structure sensitive, but it changes to structure insensitive by the deposition of Rh atoms. These results are rationalized by formation of an active overlayer on the Pt(110) and Pt(100) surfaces, which is very analogous to the Rh-O/Pt-layer formed on Rh/Pt(100), Pt/Rh(100) and Pt-Rh(100) alloy surfaces during catalysis. The formation of the common overlayer of Rh-O/Pt-layer during catalysis is responsible for the structure insensitive catalysis of Rh deposited Pt-based catalysts, which is an important role of Rh in a three way catalyst.     

Imaging and probing catalytic surface reactions on the nanoscale: Field Ion Microscopy and atom-probe studies of O2–H2/Rh and NO–H2/Pt

We present dynamic studies of surface reactions using video-Field Ion Microscopy (FIM) along with Pulsed Field Desorption Mass Spectrometry (PFDMS). Catalytic water formation is followed using rhodium and platinum 3D field emitter crystals for the oxidation of hydrogen with either oxygen (Rh) or NO (Pt). Strongly non-linear dynamics are observed with nanoscale spacial resolution. For both reactions quasi-oscillatory behaviour exists under certain conditions of temperatures and partial pressures. An influence of the probing electric field is observed and possibly essential in establishing oscillatory behaviour. Local chemical probing of selected surface areas with up to 400 atomic surface sites proves catalytic water formation to take place. Since water ions (H₂O⁺/H₃O⁺) cause image formation of the O₂–H₂ reaction on Rh, respective videos provide space-time resolved information on the catalytically active sites. Atom-probe data also reveal that the surface of the Rh sample reversibly switches from a metallic to an oxidized state during oscillations. As to the NO–H₂ reaction on Pt, fast ignition phenomena are observed to precede wave fronts. After catalytic water formation, NO molecules diffuse into emptied areas and cause high image brightness. Depending on the size of the Pt crystal, the reaction may ignite in planes or kinked ledges along the <100> zone lines. Thus FIM provides clear experimental evidence that kinks are more reactive than steps in the catalytic NO + H₂ reaction. Pt surface oxidation occurs and has probably been underestimated in previous FIM studies.     

Effect of synthetic method on CeZr support and catalytic activity of related Rh catalyst in the oxidative reforming reaction

The production of hydrogen by oxy-reforming is a process coupling steam reforming and catalytic partial oxidation allowing a balance of endothermic and exothermic reaction and smoothing the temperature profile. Nevertheless, deactivation of the catalyst by carbon formation is still an issue favored by low O/C and Steam/C ratio used in the process. The development of a catalyst for the oxy-reforming process started with the study of the synthesis by water-in-oil microemulsion of Ce_0.5Zr_0.5O₂, to obtain high oxygen storage capacity, a property that helps the removal of eventually formed carbon. The studied synthesis was able to provide the desired Ce_0.5Zr_0.5O₂ with a good stability with temperature, while classical coprecipitation gives Ce_0.6Zr_0.4O₂ phase. The catalysts were impregnated with 2.7% of Rh and tested in oxy-reforming using low residence time and reaction conditions able to discriminate the differences in catalyst activity. The catalyst prepared using microemulsion CeZr showed higher conversion and stability when compared with the one synthesized by coprecipitation. In particular, differently from the coprecipitated sample, the carbon formation was not present in the one prepared by microemulsion. The activity and stability were linked to the characterization features of the mixed oxides and the impregnated catalysts.     

标准曲线法测定催化加氢反应中2-甲基呋喃和2-甲基四氢呋喃

采用气相色谱,建立标准曲线法测定催化加氢反应中2-甲基呋喃和2-甲基四氢呋喃含量。在建立的色谱条件下,催化反应体系中乙醇、2-甲基呋喃和2-甲基四氢呋喃分离较好。2-甲基呋喃和2-甲基四氢呋喃质量分数均不超过21.00%,标准曲线法测定二者的峰面积与质量分数线性关系良好,线性相关系数均大于0.999,回收率95.72%~105.00%,相对标准偏差小于2.10%。建立的标准曲线法分析催化加氢反应中2-甲基呋喃和2-甲基四氢呋喃浓度变化,操作简单,结果准确可靠,是一种便捷的分析方法。     

Catalytic properties of La2CuO4 in the CO + NO reaction

La₂CuO₄ is an active catalyst for the reduction of NO by CO. Under reaction conditions, the catalyst exhibits an activation which results in a lowering of the light‐off temperature by 80°C. XRD, TEM and EDX analysis carried out after the catalytic test indicate that the mixed oxide has been reduced to form a La₂O₃, Cu binary system. It seems that metallic copper species are the most active sites in the CO + NO reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

CuO/Al2O3催化剂上CO还原NO反应条件的研究

为减小工业废气中氮氧化物对环境的污染,以多价态金属助剂修饰的Cu/Al₂O₃为催化剂,采用以CO为还原剂的催化还原法进行消除氮氧化物的研究,考察工艺条件对催化剂反应性能的影响。结果表明,反应温度240 ℃即可实现NO的完全转化,较低空速有利于催化剂长周期稳定运行,助剂最佳质量分数为6%,降低或提高助剂含量均不利于NO的完全转化,体系中CO含量影响催化剂性能,CO含量过高会打破催化剂活性中间体的动态平衡,导致催化剂快速失活,应严格控制CO含量。     

Diffusional behavior of amino acids in solid-phase reaction field as studied by H pulsed-field-gradient spin-echo NMR method

The diffusion coefficients (D) of tert-butyloxycarbonyl-l-phenylalanine (Boc-Phe) in Merrifield network polystyrene gels, used as a solid-phase reaction field have been determined as a function of the amino acid concentration over the temperature range from 30 to 50 °C by means of the ¹H pulsed-field-gradient spin-echo NMR method. From these experimental results, it was found that the D value of Boc-Phe in DMF-d₇ solution, in DVB 1 and 2% cross-linked network polystyrene gels depends on the amino acid concentration. The D value of Boc-Phe·Cs(tert-butyloxycarbonyl-l-phenylalanine cesium) salt in the solid-phase reaction field under chemical reaction was determined at 50 °C. Further, it was found that the D value depends on the NMR observation time, that is the applied two field-gradient pulse interval. Details of its analysis were discussed.     

Bimetallic catalysts as dissipative structures: stationary concentration patterns in the O2 + H2 reaction on a composite Rh(110)/Pt surface

The O₂ + H₂ reaction has been studied under low pressure conditions (10^-5 mbar) employing a microstructured Rh(110)/Pt surface as catalyst. Photoemission electron microscopy (PEEM) and scanning photoelectron microscopy (SPEM) were used as spatially resolving in situ methods. Under reaction conditions stationary concentration patterns (Turing‐like structures) of the adsorbates develop inside the Pt domains which are associated with a compositional change of the metallic substrate. This revised version was published online in July 2006 with corrections to the Cover Date.     

A comparative study of the selective catalytic reduction of NO by propylene over supported Pt and Rh catalysts

The catalytic performance of Pt and Rh catalysts for the selective catalytic reduction (SCR) of NO by propylene in the presence of excess oxygen has been investigated over catalysts supported on six different metal oxide carriers (CeO₂, Al₂O₃, TiO₂, YSZ, ZrO₂ and W⁶⁺-doped TiO₂). It has been found that the nature of the dispersed metal affects strongly the light-off temperature of propylene, the maximum NO conversion to reduction products and the selectivity towards nitrogen. For a given support, Pt catalysts are always more active for both NO reduction and propylene oxidation, but are much less selective towards N₂, compared to Rh catalysts. Rhodium catalysts are able to selectively reduce NO even in the absence of oxygen in the feed. However, their activity is suppressed with increasing oxygen feed concentration possibly due to the formation of less reactive rhodium oxides. In contrast, oxygen promotes the de-NO_x activity of platinum catalysts but decreases selectivity towards nitrogen. Results are explained by considering the effects of the nature of the metallic phase and the support on the elementary steps of the propylene-SCR reaction. It is concluded that the catalytic performance of both metals may be improved by proper selection of the support.     

Structural regeneration of LaCoO3 perovskite-based catalysts during the NO + H2 + O2 reactions

In situ X-ray diffraction (XRD) analysis was used to investigate structural evolutions of LaCoO₃ catalysts and then further modified by palladium (Pd) addition, under various controlled atmospheres, particularly during the reduction of NO by hydrogen in lean conditions. Complementary, XPS measurements provided information about changes in the chemical environments of Pd, Co and nitrogen during sequential temperature-programmed reactions. A preactivation thermal treatment under hydrogen led to the destruction of the perovskite structure while in the course of the NO + H₂ + O₂ reactions, the regeneration of the perovskite structure evidenced by XRD at 873 K started at lower temperature (573 K) at the surface. Palladium has been incorporated in order to evidence its effective role in the surface modifications of LaCoO₃ and its consequence on the catalytic activity.     

Improving the Activity of Rh/Al2O3 Catalyst for NO Reduction by Na Addition in the Presences of H2O and O2

The effect of Na addition on the performance of Rh/Al₂O₃ catalyst for NO reduction with CO in the presence of H₂O and O₂ was investigated. The reacted catalysts were analyzed by the FTIR technique to identify the products for further investigation on the possible catalytic reaction mechanisms and the reasons behind the H₂O poisoning. Experimental results show that the removal efficiency of NO by Rh/Al₂O₃ catalyst was 63% at 250 °C but that decreased as the H₂O content increased. Adding Na to modify the Rh/Al₂O₃ catalyst significantly enhanced the conversion of NO to 99% at 250–300 °C even as the H₂O content was 1.6 vol%. The FTIR analyses results reveal that the abundant H₂O in the flue gas can compete with NO to adsorb on the surfaces of Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts and further enhance the formation of NO₃ that reacts with H. The effects of H₂O on Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts can be eliminated by increasing the reaction temperature to higher than 300 °C. Rh-Na/Al₂O₃ is a feasible catalyst for NO reduction at such condition with relative high H₂O and O₂ contents.     

Novel Catalytic Properties of Rh Sulfide for the Synthesis of Methanol from CO + H2 in the Presence of H2S

Rh sulfide yielded 800 gkg-cat^–1h^–1 of methanol at 593 K and 5.1 MPa from CO + H₂ (syngas) even in the presence of H₂S 100 ppm in concentration. The obtained space-time yield of methanol was comparable with that obtained with a commercial Cu/Zn/Al catalyst at a conventional reaction condition (523 K and 5.1 MPa) from a feed containing both syngas and CO₂.     

Stabilization of various CO + H2 catalysts,studied by in situ mössbauer spectroscopy

The carbon formed during the reaction of CO + H₂ on iron catalysts plays an essential role in determining the catalytic activity, product selectivity and deactivation. Different forms of carbon can be distinguished in the reaction: (i) reactive or mobile carbon, already identified for iron and ruthenium catalysts; (ii) immobile carbon which is responsible for methane formation, and (iii) bulk, inactive carbide which makes the catalysts deactivated.Various possibilities are available for stabilizing the catalytically active metallic component. One of the important factors is the particle size. The smaller the iron particle, the less amount of inactive carbon is formed. Other factors such as the type of support (e.g. zeolite) which maintains the iron in highly dispersed state, or the use of second metal or non-reducible promoter by which high dispersion of iron can be ensured and stabilized, favourably affect the activity and selectivity of the catalysts, as well. In the present paper examples are presented for the above mentioned cases.     

无模板法合成的Phi分子筛在NO选择性催化还原中的应用

氮氧化物(NO_x)是大气中的一种主要污染物。采用具有菱沸石结构(CHA)的铜基分子筛作为催化剂,通过选择性催化还原(SCR)技术可有效去除NO_x。采用一种经济环保的制备方法,在不使用模板剂的条件下水热合成一种具有结构缺陷的低硅铝比CHA型分子筛(Phi,Si/Al=4.7)。结果表明,经Cu离子交换制备的Cu/Phi具有最丰富的表面酸性与孤立态Cu²⁺,表现出较好的低温活性、较宽的工作温度窗口以及良好的水热稳定性。Na或Mg的存在降低了Cu/Phi的表面酸性及孤立态Cu²⁺的含量,水热老化后的Na,Cu/Phi和Mg,Cu/Phi均呈现出不同程度的骨架坍塌,相应导致了催化剂失活。