Surface changes in Ru/KL supported catalysts induced by the preparation method and their effect on the selective hydrogenation of citral

Several 2 wt.% Ru/KL supported catalysts were prepared by various methods with different ruthenium precursors and characterized by CO and H₂ chemisorption, N₂ adsorption, TPD of NH₃, TEM and XPS. Furthermore, CO chemisorbed species have been studied by FT-IR and microcalorimetry. Characterization measurements of catalyst IWI-Ru, prepared by incipient wetness impregnation from ruthenium acetylacetonate, evidence metal nanoparticles of 1 nm placed inside the zeolite channels, thus blocking the accessibility to part of ruthenium loading inside the micropores. Catalyst prepared by treating the KL zeolite with RuCl₃·xH₂O aqueous solution (I-Ru) exhibits nanoparticles in the range 6–8 nm at the external surface and clusters smaller that 1 nm, inside the micropores. These latter do not significantly affect the diffusion of probe molecules through the channels. Catalytic performances in the selective hydrogenation of citral in the liquid phase, at 323 K and 5 MPa, show that IWI-Ru is less active than I-Ru, but more selective towards unsaturated alcohols. Furthermore, for IWI-Ru, selectivity increases with the increasing conversion. On the other hand, removal of acid sites of the I-Ru catalyst enhances the hydrogenation activity and increases the selectivity towards citronellal. All these results are analyzed and discussed in terms of the size, shape and location of ruthenium particles in the catalysts, as well as of the metal–support interaction.     

Catalytic hydrogenation of benzene to cyclohexene on Ru(0 0 0 1) from density functional theory investigations

The catalytic hydrogenation of benzene on transition metal surfaces is of fundamental importance in petroleum industry. With the aim to improve its efficiency and particularly the selectivity to cyclohexene, in this contribution we perform periodic density functional theory calculations to determine the potential energy surface in the hydrogenation of benzene on Ru(0 0 0 1). By following the Horiuti–Polanyi mechanism with a step-wise addition of hydrogen adatoms, we investigate the adsorption of all the possible reaction intermediates and identify the most favored adsorption configuration for each intermediate. In particular, the most stable isomer for the same C₆H_n (n = 8, 9, 10) species are revealed as the most conjugated isomers, which are consistent with those in the gas phase. The elementary hydrogenation reactions of the most stable intermediates are then investigated under different H coverage conditions: the reaction barriers are calculated to be 0.68–0.97 eV at the low H coverage and 0.32–1.14 eV at the high H coverage. The high H coverage reduces significantly the overall barrier height of hydrogenation. With the determined pathway, we propose that the hydrogenation of benzene on Ru(0 0 0 1) follows the mechanism with the step-wise hydrogenation of neighboring C atoms in the ring, i.e., 1–2–3… hydrogenation. The selectivity to cyclohexene on Ru is also discussed, which highlights the importance of the π mode adsorption of benzene and also the adverse effect of secondary reaction process involving the readsorption and hydrogenation of cyclohexene.     

Preparation of egg-shell nanonickel catalyst for CO hydrogenation

Egg-shell like Ni-based catalyst was prepared on a porous hollow silica (PHS) support to synthesize methanol from CO hydrogenation. It was found from Transmission Electron Microscope (TEM) that Ni particles with a size of 9–11 nm were loaded on PHS to obtain the egg-shell catalysts. X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) analyses indicated that the active site of the catalysts was metallic Ni, which would not be oxidized during the reaction. When the as-prepared Ni/SiO₂ egg-shell catalyst was used for CO hydrogenation at 220 °C and 2.0 MPa, the conversion of CO and selectivity to methanol were about 69.5 mol% and 92 mol%, respectively, showing higher activity and selectivity than conventional catalysts. The thermodynamic analysis of CO hydrogenation for methanol synthesis indicated that the conversion of CO is 69.5 mol% under these conditions.     

Benzene hydrogenation over Ni-Cu/SiO2 catalysts prepared by aqueous hydrazine reduction

Non conventional nickel (1%) and nickel (1%)-copper (0.2%-0.75%) catalysts supported on silica have been prepared by aqueous hydrazine reduction of nickel acetate at 70 °C. They were characterized by TEM, H₂-adsorption, H₂-TPD and tested in the gas phase hydrogenation of benzene at atmospheric pressure in the temperature range 75 °C-230 °C. The obtained results show that nickel is in a whisker-like shape or as a film of low density for the Ni/SiO₂ and Ni-Cu/SiO₂ catalysts respectively. Copper is in the shape of facetted particles in the mono or bimetallic systems with a mean particle size varying from 25 to 100 nm. The presence of copper decreased the nickel phase dispersion as well as the conversion whereas it increased carbon deposit in benzene hydrogenation. The results obtained are ascribed to nickel phase shape changes and Ni-Cu interactions. A kinetic reaction mechanism model is proposed. The comparative study of a pair of classical/non-classical Ni/SiO₂ catalysts showed much higher surface and catalytic properties of the hydrazine catalyst.     

Properties and performance of Pd based catalysts for catalytic oxidation of volatile organic compounds

Catalytic oxidations of volatile organic compounds (VOCs) (benzene, toluene and o-xylene) over 1 wt% Pd/γ-Al₂O₃ catalyst were carried out to assess the properties and performance of the Pd based catalyst. The properties of the prepared catalysts were characterized by the Brunauer Emmett Teller (BET) surface area, H₂ chemisorption, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) analyses. The experimental results revealed a significant increase in VOCs conversion with the lapse of the reaction time at certain reaction temperatures. On the other hand, the hydrogen pretreated 1 wt% Pd/γ-Al₂O₃ catalyst, whose shape of conversion curve is similar to the non pretreated catalyst, led the conversion curves for the total oxidation of VOCs to be shifted to lower temperature. It was also found that such increases in VOCs conversion were highly dependent on the oxidation state of Pd and the growth of Pd particles in the catalyst. In addition, in the case of the catalyst consisting of the same oxidation state (PdO/Pd²⁺ or Pd⁰), the particle sizes possibly play a more important role in the catalytic activity. The activity order of 1 wt% Pd/γ-Al₂O₃ catalyst with respect to the VOC molecule was o-xylene > toluene > benzene.     

Microwave-Assisted Synthesis and Properties of Sodium Glycol Bis-(Isooctyl)Sulfosuccinate

An anionic gemini surfactant was synthesized under microwave irradiation conditions. The optimum reaction conditions were obtained by the reaction of maleic anhydride, glycol, isooctyl alcohol and sodium sulfite. The first step esterification reaction was carried out at 90 °C and 100 W microwave power for 20 min with a ratio a n _{maleic anhydride}:n _glycol = 2.1:1, and 1.0 wt% of catalyst. The second step esterification reaction was carried out at 80 °C for 25 min and 100 W microwave power. Optimum sulfonation was carried out with n _{sodium sulfite}:n _{glycol bismaleic anhydride bisester} = 2.25:1 and 1.5% catalyst for 25 min at 60 °C. It is found that microwave-assisted synthesis is an efficient means of preparation of this anionic gemini surfactant with shorter reaction times and higher yields. The title compound structure was confirmed by IR. The critical micelle concentration of the title compound is 6.2 × 10⁻⁴ mol L⁻¹, γ_CMC is 25.7 mN m⁻¹, and benzene emulsion persistence was 2 days.     

Simplified treatment of mass transfer for gas-phase hydrogenation/dehydrogenation of heavy compounds

Using single catalyst pellets (5 mm) 15% Ptγ–Al₂O₃, we experimentally studied gas-phase benzene hydrogenation at normal pressure by thermocouple measurements of gas flow and the pellet center. Temperature of gas flow was varied in the range of 20 ‡C / 350 ‡C for three molar fractions of benzene vapor (0.1, 0.2, and 0.3) mixed with hydrogen. The ignition/extinction behavior of the flow-pellet temperature rise (maximum values up to 100 ‡C/ 200 dgC) is explained by internal-external mass transport limitations of the reaction rate and reaction reversibility at high pellet temperature. A simplified pseudobinary treatment of both multicomponent intrapellet mass transfer (in bimodal porous media) and multicomponent external mass transfer (under forced convection) is proposed on the basis of the analytical estimation. The validity of the suggested approach is confirmed by comparing the experimental data for benzene hydrogenation with rigorous (multicomponent) and approximated (pseudobinary) calculations obtained by using a mathematical model of a spherically symmetric pellet. The simplified approach appears to be quite accurate for reactions A+nH₂=B of hydrogenation (n>0) or dehydrogenation (n<0) of sufficiently heavy compounds, i.e. if D _AH ≈D _BH >>D _AB      

Characterisation and catalytic properties of MCM-41 and Pd/MCM-41 materials

Pure silica MCM-41 mesoporous molecular sieve material was synthesised and characterised by in situ synchrotron XRD, TEM, TGA/DTA and DRIFTS techniques. In situ energy dispersive XRD (EDXRD) confirmed the exact nature of the pore diameter of MCM-41 and the change in crystal structure on calcination. The IR band at 1057 cm^-1 of as-synthesised MCM-41 was shifted by 14 cm^-1 on heating to 673 K due to increased condensation of silanol groups to form Si-O-Si bridges. Calcined MCM-41 materials were used to support Pd, and the catalytic activities for 1-hexene and benzene selective hydrogenation were investigated. The Pd/MCM-41 catalyst showed high activity in hydrogenation of 1-hexene at an inlet reaction temperature of 298 K, but did not show any activity in hydrogenation for benzene. TEM results for the reduced Pd/MCM-41 catalysts revealed that the average Pd particle size was around 2-2.5 nm and these particles were located in the pores of MCM-41 and showed good distribution. TPR measurements showed that about 70% of palladium oxide (PdO) loading in the calcined catalysts was reduced at sub-ambient temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selective methanation of CO over supported Ru catalysts

The catalytic performance of supported ruthenium catalysts for the selective methanation of CO in the presence of excess CO₂ has been investigated with respect to the loading (0.5–5.0 wt.%) and mean crystallite size (1.3–13.6 nm) of the metallic phase as well as with respect to the nature of the support (Al₂O₃, TiO₂, YSZ, CeO₂ and SiO₂). Experiments were conducted in the temperature range of 170–470 °C using a feed composition consisting of 1%CO, 50% H₂ 15% CO₂ and 0–30% H₂O (balance He). It has been found that, for all catalysts investigated, conversion of CO₂ is completely suppressed until conversion of CO reaches its maximum value. Selectivity toward methane, which is typically higher than 70%, increases with increasing temperature and becomes 100% when the CO₂ methanation reaction is initiated. Increasing metal loading results in a significant shift of the CO conversion curve toward lower temperatures, where the undesired reverse water–gas shift reaction becomes less significant. Results of kinetic measurements show that CO/CO₂ hydrogenation reactions over Ru catalysts are structure sensitive, i.e., the reaction rate per surface metal atom (turnover frequency, TOF) depends on metal crystallite size. In particular, for Ru/TiO₂ catalysts, TOFs of both CO (at 215 °C) and CO₂ (at 330 °C) increase by a factor of 40 and 25, respectively, with increasing mean crystallite size of Ru from 2.1 to 4.5 nm, which is accompanied by an increase of selectivity to methane. Qualitatively similar results were obtained from Ru catalysts supported on Al₂O₃. Experiments conducted with the use of Ru catalyst of the same metal loading (5 wt.%) and comparable crystallite size show that the nature of the metal oxide support affects significantly catalytic performance. In particular, the turnover frequency of CO is 1–2 orders of magnitude higher when Ru is supported on TiO₂, compared to YSZ or SiO₂, whereas CeO₂- and Al₂O₃-supported catalysts exhibit intermediate performance. Optimal results were obtained over the 5%Ru/TiO₂ catalyst, which is able to completely and selectively convert CO at temperatures around 230 °C. Addition of water vapor in the feed does not affect CO hydrogenation but shifts the CO₂ conversion curve toward higher temperatures, thereby further improving the performance of this catalyst for the title reaction. In addition, long-term stability tests conducted under realistic reaction conditions show that the 5%Ru/TiO₂ catalyst is very stable and, therefore, is a promising candidate for use in the selective methanation of CO for fuel cell applications.     

Pd-Decorated CNT-Promoted Pd-Ga2O3 Catalyst for Hydrogenation of CO2 to Methanol

Abstract  

A type of Pd-decorated CNT-promoted Pd-Ga catalysts was developed. The catalyst displayed excellent performance for CO₂ hydrogenation to methanol. Under the reaction conditions of 5.0 MPa and 523 K, the observed specific reaction rate of CO₂ hydrogenation reached 2.23 μmol s⁻¹ (m²-Pd)⁻¹, which was 1.39 times that (1.60 μmol s⁻¹ (m²-Pd)⁻¹) of the non-promoted Pd-Ga host. The addition of a small amount of the Pd-decorated CNTs to the Pd-Ga host catalyst did not cause a marked change in the E _a of the CO₂ hydrogenation reaction. The function of the CNT-promoter was mainly in increasing the molar percentage of the catalytically active Pd⁰-species in the total Pd-amount at the surface of the functioning catalyst, and in improving the capability of the catalyst to adsorb and activate H₂ (one of the reactants). Compared to the “Herringbone-type” CNTs, the “Parallel-type” CNTs possess less active surface (with less dangling bonds), and thus, lower capacity for adsorbing H₂, resulting in the rather limited promoter effect.     

Highly efficient complete oxidation of dilute benzene over ultrafine Cu0.1Ce0.5Zr0.4O2−δ catalyst in a fluidized bed reactor

Ultrafine Cu_0.1Ce_0.5Zr_0.4O_2−δ catalyst operated in a fluidized bed reactor was found to be very effective for complete oxidation of dilute benzene in air. The complete conversion of benzene could be achieved at reaction temperature as low as 220 °C. The mechanism of benzene oxidation over the Cu_0.1Ce_0.5Zr_0.4O_2−δ catalyst was investigated by conducting pulse reaction of pure benzene in the absence of O₂ over the catalyst and the results indicated the involvement of lattice oxygen from the catalyst in benzene oxidation.     

Preparation,characterization and performance of Pd/SiO2 catalyst for benzene catalytic hydrogenation

Palladium nanoparticles supported on silica were prepared by hydrazine reduction in aqueous medium at room temperature. They were characterized by XRD, TEM, EDX, H₂-adsorption, and H₂-TPD. The catalytic properties were evaluated in the gas-phase hydrogenation of benzene in the temperature range of 75–250 °C. Metal particles with a size range of 4.0–25.8 nm were obtained. The metal surface area and hydrogen storage increase with decreasing metal particle size. The H₂-TPD profiles exhibited a main peak appeared at 540 °C with two shoulders at lower (445 °C) and higher (605 °C) temperatures. These peaks were ascribed to strongly adsorbed hydrogen on the surface catalyst. The catalytic activity of the catalysts strongly depends on the metal loading. It increases with decreasing Pd loading. This is ascribed to metal surface area, which increases with decreasing Pd content.     

The effect of TiO2 particle size on the characteristics of Au–Pd/TiO2 catalysts

The nanocrystalline TiO₂ materials with average crystallite sizes of 9 and 15 nm were synthesized by the solvothermal method and employed as the supports for preparation of bimetallic Au/Pd/TiO₂ catalysts. The average size of Au–Pd alloy particles increased slightly from sub-nano (< 1 nm) to 2–3 nm with increasing TiO₂ crystallite size from 9 to 15 nm. The catalyst performances were evaluated in the liquid-phase selective hydrogenation of 1-heptyne under mild reaction conditions (H₂ 1 bar, 30 °C). The exertion of electronic modification of Pd by Au–Pd alloy formation depended on the TiO₂ crystallite size in which it was more pronounced for Au/Pd on the larger TiO₂ (15 nm) than on the smaller one (9 nm), resulting in higher hydrogenation activity and lower selectivity to 1-heptene on the former catalyst.     

Deep desulphurization of diesel fuels on bifunctional monolithic nanostructured Pt-zeolite catalysts

The preparation of Pt-zeolite catalysts, including choice of the noble metal precursor and loading (1.0–1.8 wt.%), was optimized for maximizing the catalytic activity in thiophene hydrodesulphurization (HDS) and benzene hydrogenation (HYD). According to data obtained by HRTEM, XPS, EXAFS and FTIR spectroscopy of adsorbed CO, the catalysts contained finely dispersed Pt nanoparticles (2–5 nm) located on montmorillonite and zeolite surfaces as: Pt⁰ (main, ν_CO = 2070–2095 cm⁻¹), Pt^δ+ (ν_CO = 2128 cm⁻¹) and Pt²⁺ (ν_CO = 2149–2155 cm⁻¹). It was shown that the state of Pt depended on the Si/Al zeolite ratio, montmorillonite presence and Pt precursor. The use of H₂PtCl₆ as the precursor (impregnation) promoted stabilization of an oxidized Pt state, most likely Pt(OH)_xCl_y. When Pt(NH₃)₄Cl₂ (ion-exchange) was used, the Pt⁰ and hydroxo- or oxy-complexes Pt(OH)₆²⁻ or PtO₂ were formed. The addition of the Ca-montmorillonite favoured stabilization of Pt^+δ. The Cl⁻ ions inhibit reduction of oxidized Pt state to Pt particles. The Pt-zeolite catalyst demonstrated high efficiency in ultra-deep desulphurization of DLCO. The good catalyst performance in hydrogenation activity and sulphur resistance can be explained by the favourable pore space architecture and the location and the state of the Pt clusters. The bimodal texture of the developed zeolite substrates allows realizing a concept for design of sulphur-resistant noble metal hydrotreating catalyst proposed by Song [C. Song, Shape-Selective Catalysis, Chemicals Synthesis and Hydrocarbon Processing (ACS Symposium Series 738), Washington, 1999, p. 381; Chemtech 29(3) (1999) 26].     

Characterization of sepiolite as a support of silver catalyst in soot combustion

Turkish sepiolite–zirconium oxide mixtures were applied as a support for the silver catalyst in a soot combustion. Sepiolite–Zr–K–Ag–O catalyst was characterized by XRD, N₂ adsorption, SEM, TPR-H₂ and EGA-MS. The combustion of soot was studied with a thermobalance (TG-DTA). The modification resulted in a partial degradation of the sepiolite structure, however, the morphology was preserved. The adsorption of N₂ of the modified sepiolite is a characteristic for mesoporous materials with a wide distribution of pores. The specific surface area S_BET equals 83 m²/g and the pores volume is 0.23 cm³/g. The basic character of the surface centers of sepiolite is indicated by CO₂ desorption (TPD-MS) at 170 °C and at about 620 °C due to a surface carbonates decomposition. The thermodesorption of oxygen at 650–850 °C indicates the decomposition of AgO_x phases at the surface. The presence of AgO_x phases is also confirmed by TPR-H₂ spectrum (low temperature reduction peak at 130 and 180 °C). The high-temperature reduction at about 570 °C is probably related to Ag–O–M phases on the support.The soot combustion takes place at T₅₀ = 575 °C. Without silver (sepiolite–Zr–K–O) T₅₀ = 560 °C but sepiolite modified with silver (sepiolite–Zr–K–Ag–O) undergoes the same process at T₅₀ = 490 °C.     

Effect of sintering on the catalytic activity of a Pt based catalyst for CO oxidation: Experiments and modeling

The goal of this paper was to make the link between sintering of a 1.6% Pt/Al₂O₃ catalyst and its activity for CO oxidation reaction. Thermal aging of this catalyst for different durations ranging from 15 min to 16 h, at 600 and 700 °C, under 7% O₂, led to a shift of the platinum particle size distributions towards larger diameters, due to sintering. These distributions were studied by transmission electron microscopy. The number and the surface average diameters of platinum particles increase from 1.3 to 8.9 nm and 2.1 to 12.8 nm, respectively, after 16 h aging at 600 °C. The catalytic activity for CO oxidation under different CO and O₂ inlet concentrations decreases after aging the catalyst. The light-off temperature increased by 48 °C when the catalyst was aged for 16 h at 600 °C. The CO oxidation reaction is structure sensitive with a catalytic activity increasing with the platinum particle size. To account for this size effect, two intrinsic kinetic constants, related either to platinum atoms on planar faces or atoms on edges and corners were defined. A platinum site located on a planar face was found to be 2.5 more active than a platinum site on edges or corners, whatever the temperature. The global kinetic law {r (mol m⁻² s⁻¹) = 103 × exp(−64,500/RT)[O₂]^0.74[CO]^−0.5)} related to a reaction occurring on a platinum atom located on planar faces allows a simulation of the CO conversion curves during a temperature ramp. Modeling of the catalytic CO conversion during a temperature ramp, using the different aged catalysts, allows prediction of the CO conversion curves over a wide range of experimental conditions.     

Effect of Temperature Gradient Direction in the Catalyst Nanoparticle on CNTs Growth Mode

To improve the understanding on CNT growth modes, the various processes, including thermal CVD, MP-CVD and ECR-CVD, have been used to deposit CNTs on nanoporous SBA-15 and Si wafer substrates with C₂H₂ and H₂ as reaction gases. The experiments to vary process parameter of ΔT, defined as the vector quantities of temperature at catalyst top minus it at catalyst bottom, were carried out to demonstrate its effect on the CNT growth mode. The TEM and TGA analyses were used to characterize their growth modes and carbon yields of the processes. The results show that ΔT can be used to monitor the temperature gradient direction across the catalyst nanoparticle during the growth stage of CNTs. The results also indicate that the tip-growth CNTs, base-growth CNTs and onion-like carbon are generally fabricated under conditions of ΔT > 0, <0 and ~0, respectively. Our proposed growth mechanisms can be successfully adopted to explain why the base- and tip-growth CNTs are common in thermal CVD and plasma-enhanced CVD processes, respectively. Furthermore, our experiments have also successfully demonstrated the possibility to vary ΔT to obtain the desired growth mode of CNTs by thermal or plasma-enhanced CVD systems for different applications.     

Infrared Study of Benzene Hydrogenation on Pt/SiO2 Catalyst by Co-adsorption of CO and Benzene

FT-IR spectra of the co-adsorption of benzene and CO have been performed to identify the preferred adsorption sites of hydrogen and benzene on a Pt/SiO₂ catalyst for hydrogenation of benzene. Results of CO adsorbed on atop sites on Pt/SiO₂ includes: an α peak at 2091 cm⁻¹, a β peak at 2080 cm⁻¹ and a γ peak at 2067 cm⁻¹ indicating three kinds of adsorption sites for dissociative hydrogen on Pt/SiO₂. The site of lowest CO stretching frequency offers stronger adsorbates–metal interaction for benzene and hydrogen. Hydrogen binding on the site of lowest CO stretching frequency before benzene adsorption significantly enhances the reaction rate of benzene hydrogenation.     

An experimental investigation of Fischer–Tropsch synthesis over washcoated metallic structured supports

In principle, the application of monolithic catalysts to the Fischer–Tropsch synthesis can solve many of the problems related to the classical Fischer–Tropsch reactors, in particular concerning the necessity to operate with short diffusion distances and low pressure drops, preferably according to the ideal plug-flow behavior, while still maintaining a reasonable inventory of catalytic material in the reactor volume.The preparation of prototype cobalt-based catalysts, washcoated onto metallic structured supports with different geometries, is described herein, together with the evaluation of the catalytic properties of such systems in the Fischer–Tropsch synthesis at industrially relevant process conditions (220–235 °C, 20 bar, 2.1  mol_H2/mol_CO,  5000 cm³(STP)_CO+H2/h/g_cat). Comparative tests with the same catalyst in the powdered form were also carried out at the same process conditions.It was found that the structured catalysts maintained the activity and the selectivity of the original powdered catalyst, provided that the washcoat thickness is sufficiently thin.     

The effect of sulfur compound on the hydrogenation of tetralin over a Pd–Pt/HDAY catalyst

A composite metal catalyst with a Pd/Pt of 4:1 and an H-form dealuminated Y zeolite support for the hydrogenation of tetralin was investigated. The materials were characterized by XRD, NH₃-TPD, N₂ adsorption, TPR and FTIR. The activity for the hydrogenation of tetralin was investigated with a high-pressure fixed-bed continuous-flow reactor operating at 4.0 MPa. The thiotolerance was tested with feeds containing 200 ppm sulfur in the form of 4,6-dimethyldibenzothiophene and thiophene, respectively. It was found that thiophene was more effective for deactivating the catalyst than the former one. The diffusion barrier of the pores for the sulfur compound is likely the major reason for the different deactivation rate of catalysts. The products other than decalins were mainly decalin isomers and ring-opened products. Selectivity of cracking products for the feed with thiophene was lower than that for the other feed, while that of ring-opened and ring-contracted products with the two feeds was similar.