In situ FT-infrared investigation of CO or/and NO interaction with CuO/Ce0.67Zr0.33O2 catalysts

In situ FT-IR was employed to investigate CO or/and NO interaction with CuO supported on Ce_0.67Zr_0.33O₂ (hereafter denoted as CZ) catalysts. The physicochemical properties of CuO–CZ were also studied by combination of XRD, TPR and NO + CO activity tests. The results indicated that the dispersed CuO species were the main active components for this reaction. The catalysts showed different activities and selectivities at low and high temperatures, which should be resulted from the reduction of dispersed copper oxide species. This reaction went through different mechanisms at low and high temperatures due to the change of active species. FT-IR results suggested: (1) CO was activated by oxygen originating from CZ support, which led to surface carbonates formation, and partial dispersed CuO was reduced to Cu⁺ species above 150 °C; (2) NO interacted with the dispersed CuO and formed several types of nitrite/nitrate species, whereas crystalline CuO made little contribution to the formation of new NO adsorbates; (3) NO was preferentially adsorbed on CuO–CZ catalysts compared with CO in the reactants mixture. These adsorbed nitrite/nitrate species exhibited different thermal stability and reacted with CO at 250 °C. As a result, a possible mechanism was tentatively proposed to approach NO reduction by CO over CuO–CZ catalyst.     

A comparative study of Ag/Al2O3 and Cu/Al2O3 catalysts for the selective catalytic reduction of NO by C3H6

The selective catalytic reduction (SCR) of NO by C₃H₆ in excess oxygen was evaluated and compared over Ag/Al₂O₃ and Cu/Al₂O₃ catalysts. Ag/Al₂O₃ showed a high activity for NO reduction. However, Cu/Al₂O₃ showed a high activity for C₃H₆ oxidation. The partial oxidation of C₃H₆ gave surface enolic species and acetate species on the Ag/Al₂O₃, but only an acetate species was clearly observed on the Cu/Al₂O₃. The enolic species is a more active intermediate towards NO + O₂ to yield—NCO species than the acetate species on the Ag/Al₂O₃ catalyst. The Ag and Cu metal loadings and phase changes on Al₂O₃ support can affect the activity and selectivity of Ag/Al₂O₃ and Cu/Al₂O₃ catalysts, but the formation of enolic species is the main reason why the activity of the Ag/Al₂O₃ catalyst for NO reduction is higher than that of the Cu/Al₂O₃ catalyst.     

Model bimetallic Pd-Ni automotive exhaust catalysts: Influence of thermal aging and hydrocarbon self-poisoning

Bimetallic Pd-Ni catalysts supported on Al₂O₃ and (Ce,Zr)O_x/Al₂O₃ were examined with respect to their catalytic performance for the elimination of CO, NO and C₃H₆ under stoichiometric conditions. The effects of a thermal aging treatment at 1273 K, reactant competition in the presence of the hydrocarbon and the influence of the presence of nickel in the catalyst have been analysed by XRD, HREM, catalytic activity measurements and in situ DRIFTS spectroscopy. Self-poisoning effects, induced by the presence of the hydrocarbon in the reactant mixture, were identified as the main factor affecting the light-off activity. While a Ni-induced preferential interaction between Pd and the Ce-Zr mixed oxide component appears, in general terms, to be beneficial for the catalytic performance of the fresh (Ce,Zr)O_x/Al₂O₃-supported bimetallic catalyst, it is shown to be detrimental for the aged system as a consequence of a facilitated degradation of the (Ce,Zr)O_x component and encapsulation of the active palladium particles.     

M/SBA-15(M=Cu、Fe、Cr)介孔分子筛的制备、表征及其催化NO+CO反应研究

以介孔分子筛SBA-15为载体,采用浸渍法制备M/SBA-15(M=Cu、Fe、Cr) 介孔分子筛催化剂。采用XRD、BET、FT-IR、H₂-TPR和XPS等对样品进行分析表征,在固定床微型反应器中评价M/SBA-15(M=Cu、Fe、Cr)分子筛催化剂催化NO+CO的反应性能。结果表明,负载金属的SBA-15分子筛仍保持高度有序的二维六方介孔结构,比表面积和孔径略有减少,负载的活性金属组分在SBA-15分子筛表面具有较高的分散度。Cu/SBA-15、Cr/SBA-15和Fe/SBA-15催化剂对NO+CO反应体系均有一定活性,但由于活性金属自身的特性及其在载体表面负载量的差异,3种催化剂上呈现的NO还原活性不同,顺序为：Cr/SBA-15＞Cu/SBA-15＞Fe/SBA-15。     

Structure–reactivity relationships in VOx/TiO2 catalysts for the oxyhydrative scission of 1-butene and n-butane to acetic acid as examined by in situ-spectroscopic methods and catalytic tests

Different VO_x/TiO₂ catalyst have been catalytically tested and studied by in situ-spectroscopic methods (FT-IR, UV/vis, EPR) in the oxyhydrative scission (OHS) of 1-butene and n-butane to acetic acid (AcOH). While 1-butene OHS follows the sequence butene → butoxide → ketone → AcOH/acetate with a multitude of side products also formed, n-butane OHS leads to AcOH, CO_x and H₂O only. Water vapour in the feed improves AcOH selectivity by blocking adsorption sites for acetate. The admixture of Sb₂O₃ was found to improve AcOH selectivity which is due to deeper V reduction under steady state conditions and lowering of surface acidity. VO_x/TiO₂ catalysts with sulfate-containing anatase are the most effective ones. Covalently bonded sulfate at the catalyst surface causes specific bonding of VO_x, stabilizes active V species and ensures their high dispersity.     

Pd–Ce interactions and adsorption properties of palladium: CO and NO TPD studies over Pd–Ce/Al2O3 catalysts

We have examined the adsorption of CO and NO on powder Pd/Al₂O₃, Pd–Ce/Al₂O₃ and CeO₂/Al₂O₃ catalysts, using temperature-programmed desorption (TPD). For CO adsorption on oxidized and pre-reduced Pd–Ce/Al₂O₃ TPD profiles are identical to those observed for Pd/Al₂O₃, suggesting that interactions between ceria and Pd have a negligible effect on the adsorption properties of CO. It does, however, affect the oxidation state of the palladium particles. For NO, there are differences between Pd/Al₂O₃ and Pd–Ce/Al₂O₃. On oxidized catalysts, Pd/Al₂O₃ is more efficient for NO dissociation. However, pre-reduction increases the amount of NO that can adsorb on Pd–Ce/Al₂O₃ and react to N₂O and N₂. In comparison with Pd/Al₂O₃, reduced Pd–Ce/Al₂O₃catalysts dissociate NO at relatively high temperatures but they are more reactive and favor N₂ over N₂O.     

PdO/Al2O3–(Ce1-X Zr X )O2 catalysts: effect of the sol-gel support composition

The contribution of (Ce_1-X Zr _X )O₂ additives to alumina supports prepared by sol-gel and the catalytic properties of PdO/Al₂O₃–(Ce_1-X Zr _X )O₂ catalysts (~0.3 wt% Pd, ~5 wt% (Ce_1-X Zr _X )O₂) in CO oxidation was herein investigated. The addition of (Ce_1-X Zr _X )O₂ to the support enhanced the surface area and decreased the size of Al₂O₃ particles. The UV–Vis bands of PdO particles and Pd²⁺ ions indicate that zirconia in (Ce_1-X Zr _X )O₂ promotes palladium-support interactions by forming highly dispersed PdO particles. Temperature-programmed reduction (TPR) in hydrogen revealed that ceria enhanced the redox capacity of the supports while zirconia lowered the reduction temperature of palladium oxide species. The comprehensive study revealed that the Ce/Zr ratio was a key factor influencing the catalytic activity of samples in CO oxidation, because palladium oxide-support interactions had a significant effect in changing of the reducibility of samples. So, the PdO/Al₂O₃–(Ce_0.5Zr_0.5)O₂ exhibited the highest catalytic activity.     

CO oxidation over CuOx-CeO2-ZrO2 catalysts: Transient behaviour and role of copper clusters in contact with ceria

The effects of ZrO₂ content on the CO oxidation activity in a series of CuO_x/Ce_xZr_1−xO₂ (x = 0, 0.15, 0.5, 0.7 and 1) catalysts were investigated, both in the absence and in the presence of H₂, i.e. preferential CO oxidation—PROX. The investigation was performed under light-off conditions to focus the effects of transients and shut-down/start-up cycles on the performance; such phenomena are expected to affect the activity of PROX catalysts in small/delocalised fuel reformers. Evidence has been obtained for a transition from an “oxidized” towards a “reduced” state of the catalyst under the simulated PROX reaction conditions as a function of the reaction temperature, leading to different active species under the reaction conditions. Both CO oxidation activity and PROX selectivity appear to be affected by this process. IR characterisation of the surface copper species suggests an important role of reduced cerium sites in close contact with copper clusters on the CO oxidation activity at low temperatures.     

Pd/Ce0.6Zr0.4O2/Al2O3 as advanced materials for three-way catalysts: Part 1. Catalyst characterisation, thermal stability and catalytic activity in the reduction of NO by CO

Pd-loaded Ce_0.6Zr_0.4O₂ solid solutions supported on Al₂O₃ are investigated as catalysts for the reduction of NO by CO. The attention is focused on the role of the Ce_0.6Zr_0.4O₂ and of the Pd dispersion on the catalytic activity. The system shows a very high activity below 500 K, which is almost independent on the Pd dispersion. The high activity is attributed to a promoting effect of the Ce_0.6Zr_0.4O₂ on the NO conversion. Investigation of the influence of high temperature treatments disclosed a thermal stabilisation of both Ce_0.6Zr_0.4O₂ and Al₂O₃ in the Ce_0.6Zr_0.4O₂/Al₂O₃ system.     

Reduction of NO by CO on Cu/ZrO2/Al2O3 catalysts: Characterization and catalytic activities

ZrO₂, γ-Al₂O₃ and ZrO₂/γ-Al₂O₃-supported copper catalysts have been prepared, each with three different copper loads (1, 2 and 5 wt%), by the impregnation method. The catalysts were characterized by nitrogen adsorption (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR) with H₂, Raman spectroscopy and electronic paramagnetic resonance (EPR). The reduction of NO by CO was studied in a fixed-bed reactor packed with these catalysts and fed with a mixture of 1% CO and 1% NO in helium. The catalyst with 5 wt% copper supported on the ZrO₂/γ-Al₂O₃ matrix achieved 80% reduction of NO. Approximately the same rate of conversion was obtained on the catalyst with 2 wt% copper on ZrO₂. Characterization of these catalysts indicated that the active copper species for the reduction of NO are those in direct contact with the oxygen vacancies found in ZrO₂.     

NO Reduction with CO in the Presence of O2 over Cu/Al2O3 (3) – Structural Analysis of Active Species by Means of XAFS and UV/VIS/NIR Spectroscopy

The local structure around Cu supported on Al₂O₃ was determined by UV/VIS/NIR and XAFS spectroscopic techniques. The relationship between catalytic performance for NO–CO–O₂ reaction and the state of supported Cu species is discussed. A new method for estimating the fraction of aggregated to isolated Cu species is proposed.     

An IR study of thermally stable V2O5-WO3 -TiO2 SCR catalysts modified with silica and rare-earths (Ce, Tb, Er)

The catalytic activity of silica-free and silica-modified rare earth (Ce, Tb, Er) containing V₂O₅-WO₃ -TiO₂ catalysts in the selective catalytic reduction of NO by ammonia has been investigated as a function of ageing temperatures. The adsorption of ammonia on the catalysts and the behavior of their surface hydroxy groups and of bulk vibrations has also been studied by IR spectroscopy. Rare earths slightly decrease the catalytic activity of catalysts in a fresh state, and this has been attributed to the perturbation, observed by IR, of the vanadyl groups with a likely lowering of their Lewis acidity. However, rare earths (in particular Tb and Er) increase strongly the catalytic activity of catalysts after ageing. Silica only does not seem to have a positive effect on thermal stability and activity when vanadium is present. It has been concluded that rare earths strongly increase the thermal resistance of the catalysts and inhibit rutilization and surface area loss because they do not penetrate the anatase bulk while tend to cover the external surface. In addition the negative action of free vanadium on phase stability is decreased due to formation of rare earth vanadates.     

Water vapor sensitivity of nanosized La(Co, Mn, Fe)1−x(Cu, Pd)xO3 perovskites during NO reduction by C3H6 in the presence of oxygen

A series of La(Co, Mn, Fe)_1−x(Cu, Pd)_xO₃ perovskites having high specific surface areas and nanosized crystal domains was prepared by reactive grinding. The solids were characterized by N₂ adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed desorption (TPD) of O₂, NO + O₂, C₃H₆, in the absence or presence of 5% H₂O, Fourier transform infrared (FTIR) spectroscopy, as well as activity tests towards NO reduction by propene under the conditions of 3000 ppm NO, 3000 ppm C₃H₆, 1% O₂, 0 or 10% H₂O, and 50,000 h⁻¹ space velocity. The objective was to investigate the influence of H₂O addition on catalytic behavior. A good performance (100% NO conversion, 77% N₂ yield, and 90% C₃H₆ conversion) was achieved at 600 °C over LaFe_0.8Cu_0.2O₃ under a dry feed stream. With the exposure of LaFe_0.8Cu_0.2O₃ to a humid atmosphere containing 10% water vapor, the catalytic activity was slightly decreased yielding 91% NO conversion, 51% N₂ yield, and 86% C₃H₆ conversion. A competitive adsorption between H₂O vapor with O₂ and NO molecules at anion vacancies over LaFe_0.8Cu_0.2O₃ was found by means of TPD studies here. A deactivation mechanism was therefore proposed involving the occupation of available active sites by water vapor, resulting in an inhibition of catalytic activity in C₃H₆ + NO + O₂ reaction. This H₂O deactivation was also verified to be strictly reversible by removing steam from the feed.     

介孔CuCe/M/CuCe(M=Co,Mn,Zr)催化剂富H_2中优先氧化CO的催化性能

优先氧化是去除富H2中CO最有效的方法,铜铈催化剂是该领域的研究热点。以SBA-15为模板剂,采用纳米刻蚀法合成系列介孔Cu Ce/M/Cu Ce(M=Co,Mn,Zr)催化剂,采用XRD、N2吸附-脱附、TEM、H2-TPR和O2-TPD对催化剂结构及形貌进行表征,并对其在富H2中CO优先氧化性能进行研究。结果表明,Mn有利于催化剂表面吸附氧的增加,有助于大量氧空位的产生,进而促进CO优先氧化性能的提高;Zr的加入抑制了Cu O的还原,且其表面氧脱附温度范围过宽,不利于催化剂催化氧化性能的释放。掺杂Co与Mn可以形成Ce-Cu-M-O固溶体,促进了催化剂表面氧和晶格氧之间的相互转化,最终有利于铜铈催化剂CO优先氧化性能的提高。     

[SmNi(pic)3(H2O)5]n(ClO4)2n · 3nH2O, the first Sm–Ni heterometallic complex of picolinic acid ligand showing novel basket weave topology: Synthesis, structure and magnetics

The first basket weave framework topology of Sm–Ni heterometallic complex with picolinic acid as bridging ligand showing two different coordination modes of picolinic acid has been characterized; The studies on temperature-dependent magnetization indicate the title complex follows the Curie–Wiess paramagnetic behavior down to 5 K.     

Do Cu(II) ions need Al atoms in their environment to make CuSiBEA active in the SCR of NO by ethanol or propane? A spectroscopy and catalysis study

The two-step postsynthesis method (creation of vacant T-sites and associated SiOH groups by dealumination of BEA zeolite with nitric acid followed by incorporation of copper in the resulting SiBEA by impregnation with an aqueous solution of copper nitrate) allows to obtain a CuSiBEA zeolite which contains 0.8 Cu wt%. The incorporation of Cu(II) into the lattice of SiBEA is evidenced by XRD while the concomitant consumption of SiOH groups is monitored by FTIR. The presence of mainly isolated mononuclear Cu(II) in D_2d-distorted tetrahedral symmetry is evidenced by diffuse reflectance UV–vis-NIR, EXAFS and XANES. The CuSiBEA zeolite is active in the selective catalytic reduction (SCR) of NO with ethanol or propane with maximum NO conversion of 40 and 20% and selectivity toward N₂ close to 80–90 and 90–100%, respectively. These results suggest that the SCR process occurs on isolated mononuclear Cu(II) in D_2d-distorted tetrahedral symmetry after Al atoms have been removed from the zeolite structure. Thus, Cu(II) ions do not need Al atoms in their environment to be catalytically active. The lack of correlation between the SCR activity in presence of ethanol and the oxidation of NO to NO₂ suggests that the two reactions are more competitive than sequential. The higher activity of CuSiBEA with ethanol than with propane may be due to different activation energies and/or reaction mechanisms.     

Dynamic “operando” observation of 1 wt% Pd-based TWCs: Simultaneous XAS/DRIFTS/mass spectrometry analysis of the effects of Ce0.5Zr0.5O2 loading on structure, reactivity and performance

The behavior of 1 wt% Pd-TWCs (three-way catalysts), containing up to 33 wt% Ce_0.5Zr_0.5O₂ is followed under reducing (CO) and oxidizing (NO) cycling conditions. The dynamic behavior of these systems is analyzed using a synchronous, time-resolved energy dispersive X-ray absorption spectroscopy (XAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS) set-up with subsecond time resolution. Two main physico-chemical phenomena corresponding to noble metal morphological (size/shape) changes and the redox behavior of the noble metal–promoter interface are shown to control the TWC response to NO/CO cycling conditions. Metal-only aspects strongly influence N–O dissociation and N–N coupling steps while the metal–promoter interface has a global influence on both N₂ and CO₂ formation via oxygen handling (storage/release) properties. The relative importance of these two phenomena is studied as a function of the Ce_0.5Zr_0.5O₂ promoter content of the catalysts.     

Co–ZSM-5 catalysts in the decomposition of N2O and the SCR of NO with CH4: Influence of preparation method and cobalt loading

Co–ZSM-5 prepared via different methods with Co/Al ratios ranging from 0.03 to 0.83 are investigated in both the direct N₂O decomposition and the selective catalytic reduction (SCR) of NO with CH₄. UV–vis and H₂-TPR are used to get an insight in the active species in these reactions. It is observed that in catalysts with low Co loadings (Co/Al < 0.3) Co is predominantly present as mono-atomic Co species, located at ion exchange positions in ZSM-5. Higher Co loadings result in the formation of different kinds of Co-oxides, which constitute the majority of species in the over-exchanged catalysts (Co/Al > 0.5). The mono-atomic species show the highest activity in the direct decomposition of N₂O, whereas the oxidic Co species do not seem to contribute much to the overall decomposition. In the SCR, the Co-oxide species catalyze the combustion of CH₄ whereas the selectivity towards NO reduction is much increased at low Co loadings. Therefore, over-exchange of Co–ZSM-5 does not seem to be favorable for both the direct N₂O decomposition and the SCR of NO with CH₄. Co/Al ratios <0.3 give the best results both in terms of conversion and activity per Co atom in both reactions.     

Catalytic activities of Co(Ni)Mo/TiO2–Al2O3 catalysts in gasoil and thiophene HDS and pyridine HDN: effect of the TiO2–Al2O3 composition

The effect of the TiO₂–Al₂O₃ mixed oxide support composition on the hydrodesulfurization (HDS) of gasoil and the simultaneous HDS and hydrodenitrogenation (HDN) of gasoil+pyridine was studied over two series of CoMo and NiMo catalysts. The intrinsic activities for gasoil HDS and pyridine HDN were significantly increased by increasing the amount of TiO₂ into the support, and particularly over rich- and pure-TiO₂-based catalysts. It is suggested that the increase in activity be due to an improvement in reducing and sulfiding of molybdena over TiO₂. The inhibiting effect of pyridine on gasoil HDS was found to be similar for all the catalysts, i.e., was independent of the support composition. The ranking of the catalysts for the gasoil HDS test differed from that obtained for the thiophene test at different hydrogen pressures. In the case of gasoil HDS, the activity increases with TiO₂ content and large differences are observed between the catalysts supported on pure Al₂O₃ and pure TiO₂. In contrast, in the case of the thiophene test, the pure Al₂O₃-based catalyst appeared relatively more active than the catalysts supported on mixed oxides. Also, in the thiophene test the difference in intrinsic activity between the pure Al₂O₃-based catalyst appeared relatively more active than the catalysts supported on mixed oxides. Also in the thiophene test, the difference in intrinsic activity between the pure Al₂O₃- and pure TiO₂-based catalysts is relatively small and dependent on the H₂ pressure used. Such differences in activity trend among the gasoil and the thiophene tests are due to a different sensitivity of the catalysts (by different support or promoter) to the experimental conditions used. The results of the effect of the H₂ partial pressure on the thiophene HDS, and on the effect of H₂S concentration on gasoil HDS demonstrate the importance of these parameters, in addition to the nature of the reactant, to perform an adequate catalyst ranking.     

Selective reduction of NOx by hydrogen and methane in natural gas stationary sources over alumina supported Pd, Co and Co/Pd catalysts: Part B: On the effect of bimetallic catalyst preparation

The aim of the present work is to study the selective reduction of NO_x from natural gas sources using unburned methane or hydrogen as reducing agents. The results suggest that the NO_x are reduced by H₂ at low temperature, when methane is not activated and at higher temperature the methane is then the main reducing agent. Similar results are obtained for alumina supported palladium and alumina supported cobald-palladium catalysts at low temperature in presence of hydrogen suggesting that the active phase for the reaction NO/H₂ is the palladium. However, at high temperature the higher activity is obtained on bimetallic catalyst. The presence of cobalt enhances the catalytic activity. This result suggests that cobalt and palladium both in cationic form are the active sites when the reducing agent is the methane.