Catalyst Structure-Performance Relationship Identified by High-Throughput Operando Method: New Insight for Silica-Supported Vanadium Oxide for Methanol Oxidation

The reaction mechanism of methanol oxidation catalyzed by vanadium oxides on a silica support (V₂O₅/SiO₂) was investigated in a high-throughput operando reactor coupled with a Fourier transform-infrared (FT-IR) imaging system for rapid product analysis and six parallel, in situ Raman spectroscopy probes for catalyst characterization. Up to six V₂O₅/SiO₂ catalysts with different vanadium loadings (i.e., from 0 to 7%) were simultaneously monitored under identical experimental conditions. The specific Raman bands of the different catalysts in the six parallel reaction channels are quantitatively determined in this work. Under steady-state reaction conditions, the Raman intensities of C–H stretch in Si–O–CH₃ and V–O–CH₃ were extensively studied at different reaction temperatures and different vanadium loadings. For the first time, we observed enhanced Si–O–CH₃ formation on V₂O₅/SiO₂ catalysts with low vanadium loadings. We attribute this phenomenon to surface cluster edge activation. Careful comparison of the in situ Raman intensity of V–O–CH₃ on V₂O₅/SiO₂ catalysts revealed different methoxy formation mechanisms in different reaction temperature regimes.     

Physicochemical characterization and catalytic applications of MoO3–ZrO2 composite oxides towards one pot synthesis of amidoalkyl naphthols

A series of MoO₃–ZrO₂ composite oxide catalysts were prepared by coprecipitation and impregnation methods and characterized by XRD, Raman, UV–Vis, TEM and sorptometric techniques. Characterization studies indicated the presence of tetragonal zirconia phase and well dispersed MoO₃ species as isolated and polymolybdate clusters in the composite oxide. The MoO₃(20 mol%)–ZrO₂ material was used as efficient catalyst for synthesis of amidoalkyl naphthols under solvent free conditions using conventional as well as microwave heating. The results obtained clearly showed that the composite oxide catalyst was recyclable and highly efficient for the reaction giving good yield and purity of the products.     

Nanodispersed Au/Al2O3 catalysts for low-temperature CO oxidation: Results of research activity at the Boreskov Institute of Catalysis

This paper presents some important results of the studies on preparation and catalytic properties of nanodispersed Au/Al₂O₃ catalysts for low-temperature CO oxidation, which are carried out at the Boreskov Institute of Catalysis (BIC) starting from 2001. The catalysts with a gold loading of 1–2 wt.% were prepared via deposition of Au complexes onto different aluminas by means of various techniques (“deposition-precipitation” (DP), incipient wetness, “chemical liquid-phase grafting” (CLPG), chemical vapor deposition (CVD)). These catalysts have been characterized comparatively by a number of physical methods (XRD, TEM, diffuse reflectance UV/vis and XPS) and catalytically tested for combustion of CO impurity (1%) in wet air stream at near-ambient temperature. Using the hydroxide or chloride gold complexes capable of chemical interaction with the surface groups of alumina as the catalyst precursors (DP and incipient wetness techniques, respectively) produces the catalysts that contain metallic Au particles mainly of 2–4 nm in diameter, uniformly distributed between the external and internal surfaces of the support granules together with the surface “ionic” Au oxide species. Application of organogold precursors gives the supported Au catalysts of egg shell type which are either close by mean Au particle size to what we obtain by DP and incipient wetness techniques (CVD of (CH₃)₂Au(acac) vapor on highly dehydrated Al₂O₃ in a rotating reactor under static conditions) or contain Au crystallites of no less than 7 nm in size (CLPG method). Regardless of deposition technique, only the Cl-free Au/Al₂O₃ catalysts containing the small Au particles (d_i ≤ 5 nm) reveal the high catalytic activity toward CO oxidation under near-ambient conditions, the catalyst stability being provided by adding the water vapor into the reaction feed. The results of testing of the nanodispersed Au/Al₂O₃ catalysts under conditions which simulate in part removal of CO from ambient air or diesel exhaust are discussed in comparison with the data obtained for the commercial Pd and Pt catalysts under the same conditions.     

In situ Raman spectroscopy studies of catalysts

In situ Raman spectroscopy is rapidly becoming a very popular catalyst characterization method because Raman cells are being designed that can combine in situ molecular characterization studies with simultaneous fundamental quantitative kinetic studies. The dynamic nature of catalyst surfaces requires that both sets of information be obtained for a complete fundamental understanding of catalytic phenomena under practical reaction conditions. Several examples are chosen to highlight the capabilities of in situ Raman spectroscopy to problems in heterogeneous catalysis: the structural determination of the number of terminal M=O bonds in surface metal oxide species that are present in supported metal oxide catalysts; structural transformations of the MoO₃/SiO₂ and MoO₃/TiO₂ supported metal oxide catalysts under various environmental conditions, which contrast the markedly different oxide–oxide interactions in these two catalytic systems; the location and relative reactivity of the different surface M–OCH₃ intermediates present during CH₃OH oxidation over V₂O₅/SiO₂ catalysts; the different types of atomic oxygen species present in metallic silver catalysts and their role during CH₃OH oxidation to H₂CO and C₂H₄ epoxidation to C₂H₄O; and information about the oxidized and reduced surface metal oxide species, isolated as well as polymerized species, present in supported metal oxide catalysts during reaction conditions. In summary, in situ Raman spectroscopy is a very powerful catalyst characterization technique because it can provide fundamental molecular‐level information about catalyst surface structure and reactive surface intermediates under practical reaction conditions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Low temperature selective catalytic reduction of NO by NH3 over V2O5 supported on TiO2–SiO2–MoO3

The V₂O₅ catalysts supported on TiO₂–SiO₂–MoO₃ (TSM) prepared by the coprecipitation method were investigated for the selective catalytic reduction (SCR) of NO by NH₃ at low temperatures. The V₂O₅/TSM catalyst with 7–13 wt% SiO₂ was found to exhibit a superior SCR activity and a good sulfur tolerance at low temperatures (<250 °C). The presence of highly active polymeric vanadates formed by the incorporation of MoO₃ to TiO₂–SiO₂ and superior redox properties seems to enhance SCR activity, and furthermore the very lower SO₂ oxidation activity due to the higher acidity leads to the remarkable improvement of sulfur tolerance.     

Comparison Between a Pt–Rh/Ba/Al2O3 and a Newly Formulated NOX-Trap Catalysts Under Alternate Lean–Rich Flows

In situ FT-IR spectroscopy coupled with mass spectrometry have been used to study the mechanism of nitrates formation and reduction over a common Pt–Rh/Ba/Al₂O₃ NO _x storage catalyst, compared with a different alumina-based compound.The experimental device used consists of a transmission reactor cell (having a very small dead volume) dedicated to the evolution of surface species, and of a mass spectrometer combined with a FT-IR micro-cell for gas analysis, allowing time resolved analysis in stationary and transient conditions.At the first time the nitration properties of the catalysts under a lean flow have been studied in the appropriate temperature window (473–673 K). The dynamics of nitrates formation has been pointed out, as well as the different coordination sites on the compounds surface. Then the catalysts have been alternatively exposed to rich and lean flows very close to the real exhaust composition. This has allowed the identification of reduction pathway, active sites, intermediate species and by-products for NO _X -trap reaction. In particular, we have differentiated the role of the support and of the noble metal in the mechanism, as well as of isocyanate adspecies and ammonia among the detected species. The very high NO _X storage properties and the selectivity (near 100%) in nitrogen of the newly designed catalyst have been pointed out.     

Comparative preparation of MoO3/SiO2 catalysts using conventional and slurry impregnation method and activity in transesterification of dimethy oxalate with phenol

Silica-supported MoO₃ catalyst prepared by slurry impregnation method exhibits higher activity and dispersion capacity compared to the MoO₃/SiO₂ prepared conventionally. Slurry MoO₃/water is used instead of the solution ammonium heptamolybdate. Highly dispersed amorphous Mo catalysts are obtained, which is closely related the catalytic activities, without calcination, waste solutions, and calcining nitrogeous gases. The dependence of catalytic activity on Mo loading for the slurry prepared catalysts was similar to the samples prepared by the conventional impregnation method, indicating the slurry method is a simple and clean alternative to the conventional one.     

Use of Double Wash-Coatings of Platinum and Zeolite Catalysts to Improve Selective Reduction of NOx by Hydrocarbons

The selective catalytic reduction by hydrocarbons (HC-SCR) of NO _x under lean conditions has been improved by the use of double-layered catalysts with a lower layer of Pt/SiO₂ and an upper layer of a zeolite such as H-, Ce-, and Cu-ferrierite (-FER). H-FER wash-coated over Pt/SiO₂ (H-FER//Pt/SiO₂) performed best among the samples examined. The promotional effect was attributed to the synergy of the oxidation catalyst (Pt/SiO₂) in converting NO into NO₂, which is more reactive to C₃H₆, and the HC-SCR catalyst (H-FER). Cu-FER//Pt/SiO₂ was also effective at widening the temperature window, but with this combination the performance was attributed to a simple summation of the activity of two HC-SCR catalysts that were active at different temperatures.     

Spectroscopic Study on the Nature of Active Entities in Copper–Ceria CO-PROX Catalysts

A series of oxidised copper-ceria catalysts with varying copper loadings and prepared by two different methods (impregnation of ceria and coprecipitation of the two components within reverse microemulsions) have been examined with the aim of determining the nature of active entities for the two main (CO and H₂) competing oxidation reactions that take place during preferential oxidation of CO in H₂-rich streams over this type of catalysts. The analysis is mainly based on operando spectroscopic exploration by DRIFTS and XANES as well as in situ analysis by Raman. The results allow establishing a model of the catalytic behaviour of this type of catalysts which can provide keys to control their CO-PROX catalytic properties.     

From Electron Density Flow Towards Activation: Benzene Interacting with Cu(I) and Ag(I) Sites in ZSM-5. DFT Modeling

The effects of support pretreatment with nC₁–C₅ alcohols on the performance of Rh–Mn–Li/SiO₂ catalyst in the synthesis of C₂-oxygenates from syngas have been investigated by CO hydrogenation reaction, transmission electron microscopy (TEM), pulse adsorption of CO and H₂, and Fourier Transform infrared (FT-IR) spectroscopy. The catalysts prepared from the pretreated silica supports exhibited higher space time yields of C₂-oxygenates (STY_C2-oxy) and selectivities towards C₂-oxygenates (S_C2-oxy) than that prepared from the untreated silica support. The enhancement in the hydrophobicity of the pretreated silica supports would be favorable for increasing Rh dispersion and ratio of Rh⁺/Rh⁰ sites, therefore increasing the number of active sites, especially the active sites for CO insertion. Such variations are responsible for the improvements in the catalytic performance of the Rh–Mn–Li/SiO₂ catalyst.     

The molybdenum species of MoO3/SiO2 and their catalytic activities for the epoxidation of propylene with cumene hydroperoxide

The MoO₃/SiO₂ catalysts containing different surface molybdenum species were prepared by a sol–gel method, and the effects of the preparation condition and MoO₃ loading on the surface molybdenum species and property of MoO₃/SiO₂ were studied. The XRD, FT-IR, UV–vis and Raman spectroscopies were used to characterize the surface molybdenum species, and temperature-programmed desorption of NH₃ adsorbed on a catalyst was used to detect the surface acidic properties. The results show that, there were the dispersed polymolybdate, α-MoO₃, β-MoO₃, monomeric molybdenum species and silicomolybdic acid on the MoO₃/SiO₂ catalyst, and their distributions and subsistence states were affected by the preparation condition and MoO₃ loading. Different molybdenum species exhibit different catalytic activities for the epoxidation of propylene with cumene hydroperoxide. In the 15 wt% MoO₃/SiO₂ catalyst synthesized at pH 9.1 and dried appropriately, there are the small size β-MoO₃ and monomeric molybdenum species that they are mainly effective catalyst components for the epoxidation of propylene. Using this catalyst, the ～100% conversion of cumene hydroperoxide and ～100% selectivity to propylene oxide can be obtained in the tert-butyl alcohol solvent at 2.6 MPa and 80 °C for 4 h.     

An exploratory study of diesel soot oxidation with NO2 and O2 on supported metal oxide catalysts

A number of supported metal oxide catalysts were screened for their catalytic performance for the oxidation of carbon black (CB; a model diesel soot) using NO₂ as the main oxidant. It was found that contact between the carbon and catalyst was a key factor in determining the rate of oxidation by NO₂. Oxides with low melting points, such as Re₂O₇, MoO₃ and V₂O₅ showed higher activities than did Fe₃O₄ and Co₃O₄. The activities of MoO₃ and V₂O₅ on various supporting materials were also examined. MoO₃/SiO₂ was the most active catalyst among the supported MoO₃ examined, whereas, V₂O₅/MCM-41 showed the highest activity among the supported V₂O₅. Different performances of the supported MoO₃ catalysts were explained by the interaction of MoO₃ with the supports: a strong MoO₃/support interaction may result in a poor mobility of MoO₃ and a poor activity for oxidation of carbon by NO₂. The high activity of V₂O₅/MCM-41 was associated with its catalysis of the oxidation of SO₂ by NO₂ to form SO₃, which substantially promotes oxidation of carbon by NO₂. Addition of transition metal oxides or sulfates to supported MoO₃ and V₂O₅ was also investigated. Combining MoO₃ or V₂O₅ with CuO on SiO₂, adding VOSO₄ to MoO₃/SiO₂ or MoO₃/Al₂O₃ and adding TiOSO₄ or CuSO₄ to V₂O₅/Al₂O₃ improved the catalytic performance.     

Support modification to improve the sulphur tolerance of Ag/Al2O3 for SCR of NOx with propene under lean-burn conditions

Ag/Al₂O₃ catalysts with 1 wt% SiO₂ or TiO₂ doping in alumina support have been prepared by wet impregnation method and tested for sulphur tolerance during the selective catalytic reduction (SCR) of NO_x using propene under lean conditions. Ag/Al₂O₃ showed 44% NO_x conversion at 623 K, which was drastically reduced to 21% when exposed to 20 ppm SO₂. When Al₂O₃ support in Ag/Al₂O₃ was doped with 1 wt% SiO₂ or TiO₂ the NO_x conversion remained constant in presence of SO₂ showing the improved sulphur tolerance of these catalysts. Subsequent water addition does not induce significant deactivation. On the contrary, a slight promotional effect on the activity of NO conversion to nitrogen is observed after Si and Ti incorporation. FTIR study showed the sulphation of silver and aluminum sites of Ag/Al₂O₃ catalysts resulting in the decrease in the formation of reactive intermediate species such as –NCO, which in turn decreases NO_x conversion to N₂. In the case of Ag/Al₂O₃ doped with SiO₂ or TiO₂, formation of silver sulphate and aluminum sulphate was drastically reduced, which was evident in FTIR resulting in remarkable improvement in the sulphur tolerance of Ag/Al₂O₃ catalyst. These catalysts before and after the reaction have been characterized with various techniques (XRD, BET surface area, transmittance FTIR and pyridine adsorption) for physico-chemical properties.     

An effective heterogeneous WO3/TiO2–SiO2 catalyst for selective oxidation of cyclopentene to glutaraldehyde by H2O2

TiO₂–SiO₂ mixed oxide with large pore size was synthesized by the xerogel method and it was then used to prepare the WO₃/TiO₂–SiO₂ catalyst by an incipient wetness method. The as‐prepared WO₃/TiO₂–SiO₂ sample was employed as the first heterogeneous catalyst in the liquid‐phase cyclopentene oxidation by aqueous H₂O₂, which exhibited higher selectivity (about 75%) to glutaraldehyde (GA) and, in turn, higher GA yield than the WO₃/SiO₂ heterogeneous catalyst and even the tungstic acid homogeneous catalyst under the same reaction conditions. The amorphous WO₃ phase was identified as the active sites and the loss of the active sites was proved to be not important. The lifetime of the catalyst was determined and its regeneration method was proposed. The effects of various factors on the catalytic behaviors, such as the WO₃ loading, the calcination temperature, the surface acidity and the reaction media, were investigated and discussed based on various characterizations including BET, XRD, XPS, FTIR, EXAFS and Raman spectra etc. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oxidation of CH4 over Pd supported on TiO2-doped SiO2: Effect of Ti(IV) loading and influence of SO2

Titania-modified silicas with different weight% of TiO₂ were prepared by sol–gel method and used as supports for Pd (1 wt%) catalysts. The obtained materials were tested in the oxidation of methane under lean conditions in absence and in presence of SO₂. Test reactions were consecutively performed in order to evaluate the thermal stability and poisoning reversibility. Increasing amounts of TiO₂ improved the catalytic activity, with an optimum of the performance for 10 wt% TiO₂ loading. Moreover, the titania-containing catalysts exhibited a superior tolerance towards SO₂ by either adding it to the reactants or feeding it as a pure pretreatment atmosphere at 350 °C. Catalysts were characterized by XPS, XRD, FT-IR and BET measurements. According to the structural and surface analyses, the mixed oxides contained Si–O–Ti linkages which were interpreted as being responsible for the enhanced intrinsic activity of supported PdO with respect to PdO on either pure SiO₂ or pure TiO₂. Moreover, the preferential interaction of the sulfur molecule with TiO₂ and the easy SO_x desorption from high surface area silica were the determining factors for the superior SO₂ tolerance of the TiO₂-doped catalysts.     

Partial oxidation of methane over ruthenium catalysts

The catalytic partial oxidation of methane with oxygen to produce synthesis gas was studied under a wide range of conditions over supported ruthenium catalysts. The microreador results demonstrated the high activity of ruthenium catalysts for this reaction. A catalyst having as little as 0.015% (w/w) Ru on Al₂O₃ gave a higher synthesis gas selectivity than a catalyst having 5% Ni on SiO₂. XANES measurements for fresh and used catalyst samples confirmed that ruthenium is reduced from ruthenium dioxide to ruthenium metal early during the experiments. Ruthenium metal is thus the active element for the methane partial oxidation reaction.     

Low-temperature catalytic combustion of chlorobenzene over MnOx–CeO2 mixed oxide catalysts

MnO_x–CeO₂ mixed oxide catalysts prepared by sol–gel method were tested for the catalytic combustion of chlorobenzene (CB), as a model of chlorinated aromatic volatile organic compounds (CVOCs). MnO_x–CeO₂ catalysts with the different ratio of Mn/Ce + Mn were found to possess high catalytic activity for catalytic combustion of CB, and MnO_x(0.86)–CeO₂ was the most active catalyst, on which the complete combustion temperature (T_90%) of chlorobenzene was 236 °C. The stability of MnO_x–CeO₂ catalysts in the CB combustion was investigated. MnO_x–CeO₂ catalysts with high Mn/Ce + Mn ratios present high stable activity, which is related to their high ability to remove Cl species adsorbed and a large amount of active surface oxygen.     

Deactivation of V2O5-WO3-TiO2 SCR catalyst at biomass fired power plants: Elucidation of mechanisms by lab- and pilot-scale experiments

In this work, deactivation of a commercial type V₂O₅-WO₃-TiO₂ catalyst by aerosols of potassium compounds was investigated in two ways: (1) by exposing the catalyst in a lab-scale reactor to a layer of KCl particles or fly ash from biomass combustion; (2) by exposing full-length monolith catalysts to pure KCl or K₂SO₄ aerosols in a bench-scale reactor. Exposed samples were characterized by activity measurements, SEM-EDX, BET/Hg-porosimetry, and NH₃ chemisorption. The work was carried out to support the interpretation of observations of a previous study in which catalysts were exposed on a full-scale biomass fired power plant and to reveal the mechanisms of catalyst deactivation.Slight deactivation (about 10%) was observed for catalyst plates exposed to a layer of KCl particles at 350 °C for 2397 h. No deactivation was found for catalyst plates exposed for 2970 h to fly ash (consisting mainly of KCl and K₂SO₄) collected from an SCR pilot plant installed on a straw-fired power plant. A fast deactivation was observed for catalysts exposed to pure KCl or K₂SO₄ aerosols at 350 °C in the bench-scale reactor. The deactivation rates for KCl aerosol and K₂SO₄ aerosol exposed catalysts were about 1% per day and 0.4% per day, respectively.SEM analysis of potassium-containing aerosol exposed catalysts revealed that the potassium salt partly deposited on the catalyst outer wall which may decrease the diffusion rate of NO and NH₃ into the catalyst. However, potassium also penetrated into the catalyst wall and the average K/V ratios (0.5–0.75) in the catalyst structure are high enough to explain the level of deactivation observed. The catalyst capacity for NH₃ chemisorption decreased as a function of exposure time, which reveals that Brønsted acid sites had reacted with potassium compounds and thereby rendered inactive in the catalytic cycle. The conclusion is that chemical poisoning of active sites is the dominating deactivation mechanism, but physical blocking of the surface area may also contribute to the loss of activity in a practical application. The results support the observation and mechanisms of deactivation of SCR catalysts in biomass fired systems proposed in a previous study [Y. Zheng, A.D. Jensen, J.E. Johnsson, Appl. Catal. B 60 (2005) 253].     

An Investigation of Catalytic Activity for CO Oxidation of CuO/Ce x Zr1–x O2 Catalysts

A series of CuO/Ce _x Zr_1–x O₂ catalyst powders with different Ce/Zr ratio were prepared via an impregnation method and characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman), H₂-Temperature-programmed reduction (TPR) and X-ray photoelectron spectra techniques. The catalytic properties of the catalysts were evaluated by means of a microreactor-GC system. XRD results showed that the addition of CuO had no effect on the crystalline lattice of the support. The structures of the Ce _x Zr_1–x O₂ samples were confirmed by XRD analyses and FT-Raman results. The H₂-TPR profiles for these catalysts had three peaks, which could be attributed to the reduction of three kinds of CuO species, i.e., the highly dispersed CuO, the larger CuO species and the bulk CuO. The TPR analyses and catalytic property tests indicated that the Ce/Zr ratio of CuO/Ce _x Zr_1–x O₂ had an effect on the dispersion degree of CuO and the catalytic activity of the catalysts.     

Influence of H4SiW12O40 loading on hydrocracking activity of non-sulfide Ni-H4SiW12O40/SiO2 catalysts

The effect of H₄SiW₁₂O₄₀ loading on the catalytic performance of the reduced Ni-H₄SiW₁₂O₄₀/SiO₂ catalysts for hydrocracking of n-decane with or without the presence of thiophene and pyridine is studied. The catalysts were characterized by BET, XRD, Raman, XPS, H₂-TPR, H₂-TPD, NH₃-TPD and FT-IR of pyridine adsorption. It was found that addition of H₄SiW₁₂O₄₀ to the system increases the catalytic activity and the promoting effect is a function of the H₄SiW₁₂O₄₀ loading. The best result was obtained on 5%Ni-50%H₄SiW₁₂O₄₀/SiO₂ catalyst which shows the highest activity for hydrocracking of n-decane and excellent tolerance to the sulfur and nitrogen compounds in the feedstock. The results showed that a suitable amount of H₄SiW₁₂O₄₀ loading on the 5%Ni/SiO₂ catalyst increases the amount of both hydrogen adsorbed and Brønsted acid and Lewis acid sites on the catalyst. The high catalytic performance of the catalyst can be related to the nature of H₄SiW₁₂O₄₀ and the proper balance between metal and acid functions.