Impedance spectroscopic investigation of a Rh/YSZ catalyst under polarization

Electrochemical impedance spectra at 450–600 °C and kPa of a rhodium catalyst interfaced with yttria-stabilized-zirconia (Rh/YSZ) were compared with a model based on the mechanism of electrochemical promotion. In the proposed equivalent electric circuit, existence of an “effective” double layer at the gas-exposed catalyst surface and its potential-controlled modification via diffusion of oxygen ions between the O²⁻ conducting solid electrolyte support (YSZ) and the catalyst are represented by two additional elements: adsorption capacitance and Warburg impedance. Under positive polarization, the adsorption capacitance increases dramatically indicating reinforcement of the “effective” double layer at the catalyst/gas interface, in agreement with the observation known from electrochemical promotion practice that positive polarization of a rhodium electrode leads to rhodium oxide reduction, hence, to dramatic increase in catalytic reaction rate.     

Electrochemical promotion of NO reduction by C2H4 in 10% O2 using a monolithic electropromoted reactor with Rh/YSZ/Pt elements

The reduction of NO by C₂H₄ in high excess of O₂ and temperatures 200−300 °C was investigated using a monolithic electropromoted reactor (MEPR) with twenty-two Rh/YSZ/Pt parallel plate elements. It was found that at 220–240 °C and 10% O₂ the selective catalytic reduction (SCR) of NO can be electropromoted by 450% with near 100% selectivity to N₂ and Λ_NO values up to 2.4. The corresponding rate enhancement ratio of complete C₂H₄ oxidation is up to 900% with Faradaic efficiency, , values up to 350. The system appears promising for practical applications.     

The decomposition of NO on CNTs and 1 wt% Rh/CNTs

Carbon nanotubes (CNTs) and CNTs-supported rhodium were tested as catalysts for NO decomposition. For the fresh catalysts, 100% NO conversion was achieved at 600°C over CNTs; when 1 wt% Rh was loaded on CNTs, 100% NO conversion was achieved at 450°C. If the catalysts were pre-reduced in H₂ at or above 300°C, 100% NO conversions were observed at 300°C. XPS investigation indicated that there was still metallic rhodium (BE=307.2 eV) on Rh/CNTs after heating in air at 500°C for 2 h and after the NO decomposition reaction. As for a 1 wt% Rh/Al₂O₃ sample, the rhodium (BE = 308.2 eV) was completely in the form of Rh₂O₃ after similar treatments. These results suggest that compared to γ-Al₂O₃, the CNTs material is more capable of keeping the rhodium in its metallic state. The results obtained in H₂-TPR studies support this conclusion. In addition, TEM investigation revealed that the rhodium particles distributed rather evenly over CNTs with a particle diameter of around 8 nm. We propose that CNTs can be used as a material for the facilitation of NO decomposition. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electrochemical promotion of the oxidation of propane on Pt/YSZ and Rh/YSZ catalyst-electrodes

The effect of electrochemical promotion (EP) or non-faradaic electrochemical modification of catalytic activity (NEMCA) was studied in the catalytic reaction of the total oxidation of propane on Pt and Rh films deposited on Y₂O₃-stabilized-ZrO₂ (or YSZ), an O²⁻ conductor, in the temperature range 420–520 °C. In the case of Pt/YSZ and for oxygen to propane ratios lower than the stoichiometric ratio it was found that the rate of propane oxidation could be reversibly enhanced by application of both positive and negative overpotentials (“inverted volcano” behavior), by up to a factor of 1350 and 1130, respectively. The induced rate increase Δr exceeded the corresponding electrochemically controlled rate I/2F of O²⁻ transfer through the solid electrolyte, resulting in absolute values of the apparent faradaic efficiency Λ=Δr/(I/2F) up to 2330. The Rh/YSZ system exhibited similar EP behavior. Abrupt changes in the oxidation state of the rhodium catalyst, accompanied by changes in the catalytic rate, were observed by changing the O₂ to propane ratio and catalyst potential. The highest rate increases, by up to a factor of 6, were observed for positive overpotentials with corresponding absolute values of faradaic efficiency Λ up to 830. Rate increases by up to a factor of 1.7 were observed for negative overpotentials. The observed EP behavior is explained by taking into account the mechanism of the reaction and the effect of catalyst potential on the binding strength of chemisorbed reactants and intermediates and on the oxidative state of the catalyst surface.     

Activity during reduction of NO by CO over bimetallic palladium-rhodium/silica catalysts

A study of the activity of bimetallic Pd-Rh catalysts supported on silica in the reduction of NO by CO is presented. The catalysts were prepared by three different methods: (1) Pd and Rh were coimpregnated on the support, (2) Rh was impregnated first and, after calcining, the sample was impregnated with Pd, (3) the monometallic Pd and Rh catalysts were physically mixed. The results showed that the activity of the catalysts prepared by coimpregnation was much lower than that of the other two catalysts.     

Cu/海泡石——新型NO还原催化剂

以改性海泡石为载体,用液相浸渍法负载Cu,制备用于富氧条件下CO还原NO的催化剂.海泡石的改性条件由正交实验方法确定.考察了反应气体中氧含量和空速条件等对Cu/海泡石催化剂性能的影响,并用XRD、TGA、H2-TPR、BET等对该催化剂进行表征.研究表明,灼烧温度为400 ℃、负载量为5%（质量）的Cu/海泡石催化剂对NO还原有较高的活性.Ce 和 Sm 的加入可以明显改善Cu/海泡石催化剂的性能.在实验条件基本相同的情况下,Cu/海泡石的催化活性好于Cu/ZSM-5催化剂.     

丙烯选择还原一氧化氮反应铜基交联黏土催化剂的研究

采用聚合羟基阳离子合成交联蒙脱土(PILC),经SO2-4改性,制备了应用于C3H6选择还原NO的铜基交联黏土催化剂。考察了交联剂种类、Cu担载量及水蒸气存在对催化剂性能的影响,并采用DTA、XRD对PILC进行表征。研究发现,Al-PILC较Zr-PILC具有较好的热稳定性;350℃时Cu/Al-PILC(Cu质量分数为3%)上NO转化率达52.0%;由于金属氧化物交联柱表面的疏水特性,Cu/Al-PILC较Cu/ZSM-5具有较强的抗水蒸气能力,10%水蒸气存在仅使NO最大转化率下降了13.7%,NO和C3H6转化曲线较不含水蒸气时向高温方向移动。     

Electrochemical promotion of bimetallic RhAg/YSZ catalysts for the reduction of NO under lean burn conditions

At positive overpotentials, RhAg bimetallic electrodes interfaced with yttria-stabilised zirconia (YSZ) exhibit strong electrochemical promotion in the catalytic reduction of NO by propene, even in the presence of a large excess of gaseous oxygen. Compared with the unpromoted system, at positive overpotentials, the N₂ production rate can be accelerated by a factor of eight: this is the key figure of merit. In addition the N₂ selectivity improves from 28 to 55%. The effect on reaction rates is strongly non-faradaic and persists to a significant degree in the absence of applied potential. At negative overpotentials the nitrogen-containing products exhibit faradaic behaviour, possibly reflecting self-poisoning of the system due to excessive adsorption of oxygen. The overall performance of the RhAg/YSZ bimetallic system is substantially better than that of Rh/YSZ, suggesting that the former are promising candidates for use in novel approaches to NO_x reduction under fuel lean conditions.     

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.     

Selective catalytic reduction of NO by ammonia with fly ash catalyst

This paper investigates the selective catalytic reduction (SCR) of NO with NH₃ using fly ash catalyst. The catalytically active elements investigated here included Fe, Cu, Ni and V. The results indicated that fly ash, after pre-treatment, can be reasonably used as the SCR catalyst support to remove NO from flue gas. Cu gave the highest catalytic activity and NO conversion, compared with Fe, Ni and V. In the pre-treatment process, the nitric acid treatment and drying temperatures for the fly ash particles had little effect on the NO conversion. However, the calcination temperature had an important effect on the catalyst preparation process.     

NO reduction performance of Rh paste catalyst on YSZ under steady state and forced oscillation electropromotion conditions

The technique of cyclic voltammetry was applied in conjunction with on-line catalytic product analysis to investigate the electrochemical promotion of NO reduction by C₃H₆ in presence of O₂ on Rh catalyst-electrode films on YSZ at temperatures 350–490 °C. Cyclic linear potential sweep amperometry under catalytic reaction conditions leads to cyclic non-Faradaic electrochemical modifications in the CO₂ formation and NO reduction rates which are compared to those obtained under steady state potentiostatic operation.     

四种增产丙烯催化工艺的技术经济比较

对四种增产丙烯催化工艺进行了评述，其中丙烷脱氢工艺已成为全球第三大丙烯来源。尽管炼厂FCC装置的升级仍是增产丙烯的重要途径，技术经济比较表明，烯烃歧化和C₄/C₅烃的选择性裂解具有良好的经济性。     

Electrochemical promotion in a monolith electrochemical plate reactor applied to simulated and real automotive pollution control

The novel monolithic-type electrochemical promoted reactor (MEPR), filled with thick Rh and Pt coated films on thin yttria-stabilized zirconia (YSZ) plates, has been tested in simulated and real automotive exhaust gas. Ethylene oxidation and NO reduction by C₂H₄ in presence of oxygen were investigated in laboratory scale while the automotive bench testing was performed in the exhaust of a diesel engine. In all cases electropromotion was achieved, an advance which in addition to their compact and simple design, makes electropromoted units quite promising for automotive exhaust pollution control.     

Competition between NO reduction and NO decomposition over reduced V–W–O catalysts

The SCR of NO and NO decomposition were investigated over a V–W–O/Ti(Sn)O₂ catalyst on a Cr–Al steel monolith. The conversions of NO and NH₃ over the reduced and oxidised catalysts were determined. The higher conversion of NO than of NH₃ was observed in SCR over the reduced catalyst and very close conversions of both substrates were found over the oxidised one. The increase of the pre-reduction temperature was found to cause an increase in catalyst activity and its stability in direct NO decomposition. The surface tungsten cations substituted for vanadium ones in vanadia-like active species are considered to be responsible for the direct NO decomposition. The results of DFT calculations for the 10-pyramidal clusters: V₁₀O₃₁H₁₂ (V–V) and V₉WO₃₁H₁₂ (V–W) modelling (0 0 1) surfaces of vanadia and WO₃–V₂O₅ solid solution (s.s.) active species, respectively, show that preferable conditions for NO adsorption exist on W sites of s.s. species and that reduction causes an increase in their ability for electron back donation to the adsorbed molecule. Electron back donation is believed to be responsible for the electron structure reorganisation in the adsorbed NO molecule resulting in its decomposition. The high selectivity of NO decomposition to dinitrogen was considered to be connected with the formation of the tungsten nitrosyl complexes solely via the W–N bond.     

Relation between potential and catalytic activity of rhodium in propylene combustion

The relation between the catalyst potential and the catalytic performance has been investigated in the gas-phase combustion of propylene with oxygen over rhodium catalysts at 375 °C. The rhodium catalyst, deposited on yttria-stabilized zirconia (YSZ) solid electrolyte, also served as working electrode in the electrochemical cell. Under open-circuit conditions, the measured catalyst potential was found to be a sensitive indicator of the oxidation state of the rhodium catalyst, which influences the catalytic reaction rate dramatically and depends strongly both on the method of catalyst film preparation and on the composition of the reacting gas mixture. In turn, under closed-circuit conditions, the applied catalyst potential is a convenient tool to maintain the catalyst in its more active, reduced form and to control its catalytic performance. The activity of atomic oxygen at the three-phase boundary (tpb) during open-circuit catalytic reaction was estimated from solid electrolyte potentiometric (SEP) measurements, in good agreement with the average surface oxidation state obtained from XRD and XPS analyses. O/Rh atomic ratios higher than stoichiometric were found by XPS at the outer surface of the catalysts suggesting a strong open circuit O^2– spillover due to strong metal support interactions (SMSI) and a concomitant extension of the electric double layer to the gas-exposed catalyst surface, similarly to emersed electrodes in aqueous electrochemistry. Applying potentials up to several hundreds of mV, highly nonfaradaic promotion of propylene combustion was achieved. Electrochemical promotion of catalysis (EPOC) was most efficient at stoichiometric gas composition, that is, close to the limit of surface reduction, and with the catalyst exhibiting the smallest O^2– spillover population at open-circuit conditions.     

Steady-state multiplicity phenomena during the electrochemical promotion of NO reduction by C2H4 in presence of O2 on thin Rh and Pt catalyst-electrodes in a monolithic electropromoted reactor

The electrochemical promotion of catalysis (EPOC) was used to promote the selective reduction of NO by hydrocarbons in presence of oxygen using thin (40 nm) porous Rh and Pt catalyst layers sputtered on the opposite surfaces of thin (0.25 mm) solid electrolyte (YSZ) plates serving as electrocatalytic elements of a monolithic electrochemically promoted reactor (MEPR). Using 22 Rh/YSZ/Pt type cells it was found that the reduction of NO in presence of 1.1 kPa O₂ and 0.36 kPa C₂H₄ can be efficiently electropromoted with 340% rate enhancement, reaching 95% NO conversion with 100% selectivity to N₂ in the temperature range from 280 to 340 °C. The apparent Faradaic efficiency is larger than unity for both the NO reduction and the C₂H₄ oxidation reaction.At elevated temperatures (≥300 °C) and high reactant conversions it was found that after current interruption, the catalytic rates do not return to their initial values but remain in a new highly active steady state. It appears that this highly active state is not a genuine intrinsic permanent NEMCA state but is manifestation of steady-state multiplicity in the monolithic reactor resulting from near complete gaseous O₂ consumption. Thus the low and high activity steady states corresponding to zero applied potential appear to correspond to high and low average P_O2 in the reactor. The latter is the result of the near complete reactant conversion under the preceding electropromoted operation. These highly active permanent NEMCA states may be quite useful for practical applications.     

Highly dispersed MgO-supported model Pd-Mo catalysts prepared from bimetallic clusters: Chemisorption and selective catalytic reduction of NO

Model supported palladium and palladium-molybdenum catalysts prepared from organometallic precursors and previously characterized by a variety of chemical and physical methods were examined by IR spectroscopy of NO chemisorption and tested for their activities as catalysts in the competitive NO + CO + O₂ reaction. The IR results reveal distinctive behavior of the catalyst made from a bimetallic precursor, and the activity results show that this catalyst is more selective for NO reduction than the other catalysts, but its stability is vulnerable to the reaction conditions. The high selectivity is attributed to Pd-Mo interactions, which are inferred to be stronger in the catalyst prepared from a bimetallic precursor than in catalysts prepared from monometallic precursors.     

Decomposition of nitric oxide and its reduction by CO over superconducting and related cuprate catalysts

Catalytic activities of five non-conducting and three superconducting cuprates were measured for the decomposition of NO and the reduction of NO by CO. The concentration of the reactants and the space velocities approximate the conditions of automotive catalysts. In contrast to earlier results, obtained at 20 to 30 times higher partial pressures of NO and 20 to 100 times lower space velocities, none of the catalysts, including five perovskite-like cuprates, showed significant activity for the decomposition of NO at reaction temperatures up to 700 °C. All catalysts were fairly active for the reduction of NO. At temperatures above about 400 °C on the perovskite-like cuprates YBa₂Cu₃O_7-x and Ba₂CuO_3.5-x , the rates for NO reduction were higher than on CuO. All catalysts lost activity for NO reduction in the presence of oxygen (oxidizing conditions).     

Pretreatment effects and NO x decomposition on alumina supported Rh/MoO3 catalysts

Rh-MoO₃/Al₂O₃, Rh/Al₂O₃ and MoO₃/Al₂O₃ catalysts have been prepared and subjected to various pretreatments including high temperature reduction, high temperature oxidation and oxidation/ reduction cycles. After each series of treatments, surfaces were analysed by XPS and FTIR of adsorbed NO. The effectiveness of these surfaces in dissociating NO was studied by TPRS in the temperature range 300–773 K. Reduction rates of Mo oxides in H₂ were determined gravimetrically while rhodium dispersion was determined from H₂ adsorption isotherms at 298 K. Results indicate that molybdenum and rhodium exist in close contact in both oxidised and reduced forms. H₂ chemisorption was suppressed following HTR of the catalyst due to coverage of rhodium by molybdenum oxide species but this could be reversed by HTO (773 K) followed by low temperature reduction. Although a small proportion of Mo could be reduced following HTR, cycles of HTR/HTO produced Rh/Mo oxide phases in which a proportion of Mo could be reduced to Mo° at 473 K. The presence of reduced Mo would appear to play an important role in the improved performance of Rh-MoO₃/Al₂O₃ catalysts.     

一步法制备丙烯氢甲酰化制丁醛铑膦络合催化剂研究

采用一步法制备铑膦络合催化剂,考察了反应温度、反应时间、铑膦比、还原剂用量、氢源用量等因素对催化剂制备过程铑利用率的影响。在优化的工艺条件下,铑利用率达99%以上。在适宜的评价条件下,丙烯转化率为97.1%,丁醛选择性为98.1%,产物醛正异构比15∶1以上。