The influence of the precursor form on the ammonia synthesis activity of ruthenium zeolite catalysts

At temperatures ranging from 300 to 450°C and under atmospheric pressure, Ru₃/KY and Ru₃/NaY catalysts prepared from sorbing Ru(CO)₅ generated by Ru₃(CO)₁₂ on KY and NaY zeolites were found to be much more active for ammonia synthesis than Ru/KY and Ru/NaY prepared from cation exchange with Ru(NH₃)₆Cl₃. Dynamic secondary ion mass spectrometric and X-ray photoelectron spectroscopic studies disclosed that the reduced Ru metal particles were mainly entrapped within the zeolite framework and that their nature is related to the precusor forms.     

Reducibility and CO hydrogenation over Pt and Pt-Co bimetallic catalysts encaged in NaY-zeolite

Temperature programmed techniques (TPR, TPD) and X-ray diffraction (XRD) have been used to study ion migration and location as well as reducibility of platinum and cobalt ions encapsulated in Pt/NaY, Co/NaY and Pt-Co/NaY zeolites prepared by ion exchange. The temperature required to reduce Co²⁺ in NaY was significantly lowered by the presence of Pt and dependent upon the relative locations of Pt and Co ions in zeolite cages. The exact location was controlled by the calcination condition and the metal contents. For bimetallic catalyst with low Pt content (0.5 wt% Pt and 0.9 wt% Co), the TPR results indicated that reduction of Co²⁺ ions in the vicinity of Pt shifted toward lower temperature, while that of Co²⁺ staying alone was not affected. With high Pt loading (4.5 wt% Pt, 0.7 and 2.6 wt% Co), however, most of the Co²⁺ ions were reduced by means of Pt at temperature below 723 K after calcination at 573 K. The temperature for Pt reduction in bimetallic catalysts was somewhat higher than Pt/NaY and increased with Co atomic fraction, indicating that mixed oxide, PtCo _x O _y , might be formed during calcination. After reduction in hydrogen at 723 K, highly dispersed metal particles were formed. These fine particles were most probably confined inside zeolite cages as indicated by the absence of XRD peak for all samples after calcination and reduction. Surface composition of the bimetallic particles may be different for catalysts with similar Pt content but different Co loading. Accordingly, H/Pt ratios of 1.0 and 0.72 for catalysts with low and high Co content, respectively, were shown by hydrogen chemisorption. It was further supported by the increase in TPD peak intensity with Co loading in the high temperature range, which was related to the reoxidation of Co in bimetallic particles by surface hydroxyl groups. Preliminary results on CO hydrogenation demonstrated that activity and methanol selectivity were higher on Pt-Co bimetallic catalysts than either over monometallic Pt or Co catalyst, which was consistent with the Pt enhanced Co reduction and formation of Pt-Co bimetallic particles.     

Ship-in-Bottle synthesis of Pt9-Pt15 carbonyl clusters inside NaY and NaX zeolites,in-situ FTIR and EXAFS characterization and the catalytic behaviors in13CO exchange reaction and NO reduction by CO

[Pt₉(CO)₁₈]^2–/NaY (orange-brown, 2056 and 1798 cm^–1), [Pt₁₂(CO)₂₄]^2–/NaY (dark-green, 2080 and 1824 cm^–1 and [Pt₁₅(CO)₃₀]^2–/NaX (yellow-green, 2100 and 1865 cm^–1) were stoichiometrically synthesized by the reductive carbonylation of [Pt(NH₃)₄]²⁺/NaY, Pt²⁺/NaY and Pt²⁺/NaX, respectively. The IR bands characteristic of their linear carbonyls shift to higher frequencies whereas the bridging CO bands to lower frequencies, compared with those on the external zeolites and in solution. In-situ FTIR studies suggested that the subcarbonyl species such as PtO(CO) and Pt₃(CO)₃(₂ –CO)₃ are formed as the proposed intermediates towards [Pt₁₂(CO)₂₄]^2–/NaY in the reductive carbonylation of Pt²⁺/NaY.¹³CO exchange reaction preceded with the different intrazeolite Pt carbonyl species in the following order of activity at 298–343 K: Pt₃(CO)₃(₂ –CO)₃/NaY PtO(CO)/NaY>[Pt₉(CO)₁₈]^2–/NaY >[Pt₁₂(CO)₂₄]^2–/NaY. Pt-L₃-edge EXAFS measurment for these synthesized samples demonstrated that they are consistent with the Pt carbonyl clusters having trigonal prismatic Pt₉ and Pt₁₂ frameworks infered to a series of the Chini complexes such as [NEt₄]₂[Pt₃(CO)₆] _n ( _n = 3–5). The intrazeolite Pt₉ and Pt₁₂ carbonyl clusters exhibited higher cataytic activity in NO reduction by CO towards N₂ and N₂O at 473 K, compared with those on the conventional Pt/Al₂O₃ catalysts. The mechanism of intrazeolite Pt₉-Pt₁₅ carbonyl cluster formation are discussed in terms of the intrazeolite basicity and acidity.On leave from National Laboratory for Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 129 Street, China.     

Partial oxidation of methane to synthesis gas over α-Al2O3-supported bimetallic Pt–Co catalysts

The partial oxidation of methane to synthesis gas was studied at atmospheric pressure and in the temperature range of 550–800°C over -Al₂O₃-supported bimetallic Pt–Co, and monometallic Pt and Co catalysts, respectively. Both methane conversion and CO selectivity over a bimetallic Pt_0.5Co₁ catalyst were higher than those over monometallic Pt_0.5 and Co₁ catalysts. Furthermore, the addition of platinum in Pt–Co bimetallic catalysts effectively improved their resistance to carbon deposition with no coking occurring on Pt_0.5Co₁ during 80 h reaction. The FTIR study of CO adsorption observed only linearly bonded CO on bimetallic Pt–Co catalysts. TPR and XPS showed enhanced formation of a cobalt surface phase (CSP) in bimetallic Pt–Co catalysts. The origins of the good coking resistivity of bimetallic Pt–Co catalysts were discussed.     

Carbon Monoxide Hydrogenation on Cobalt/Zeolite Catalysts

The synthesis of hydrocarbons from catalytic hydrogenation of CO/H₂ was investigated over Co/zeolite catalysts at 1 atm, 493–553 K, H₂/CO = 2, and GHSV = 1200. Various zeolites, such as NaA, NaX, NaY, KL and NaMordenite, were used as the supports. The catalysts were prepared by impregnation and were characterized by H₂/CO chemisorption and temperature-programmed reduction (TPR). Based on TPD measurements, the CO/H₂ adsorption ratio can be used as an index for the extent of metal-zeolite interaction. The stronger the metal-zeolite interaction is, the higher the Co/H₂ adsorption ratio on metal is. The activity and selectivity of cobalt supported in zeolites were affected by complex factors such as framework structure, Si/Al ratio, and the complementary cations. The activity of the catalyst is in the order: Co/KL > Co/NaX > Co/NaY > Co/NaMordenite > Co/NaA. All of the Co/zeolite catalysts had a very high selectivity to C₂–C₄ olefins, which would decrease with increasing reaction temperature. Cobalt oxide supported in zeolite was difficult to reduce. Increasing the reduction temperature could increase the reducibility of cobalt and resulted in the increase of activity.     

Synergy of ruthenium and cobalt in SiO2-supported catalysts on ethylene hydroformylation

Dynamic hydroformylation of ethylene at atmospheric pressure and 150°C has been studied in a fixed bed reactor over ruthenium- and cobalt-containing SiO₂-supported catalysts (1% Ru loading). Any combination of ruthenium and cobalt precursors leads to significant improvement of hydroformylation activity with respect to those of monometallic catalysts. The optimal atomic ratio of Co:Ru is estimated to be 3:1 for ideal catalytic activity. A catalyst derived from Ru₃(CO)₁₂ and Co₂(CO)₈ is most active. A catalyst derived from metal carbonyls is generally more active than a catalyst prepared from metal salts. Metal chlorides retard the preparation of active catalysts in most cases. The catalysts studied exhibit fairly good catalytic stability. The determined rate enhancement of ethylene hydroformylation suggests a synergy of ruthenium and cobalt, which is understood as catalysis by bimetallic particles or ruthenium and cobalt monometallic particles in intimate contact. The synergy causes high ethylene hydrogenation activity while giving enhanced ethylene hydroformylation activity. Meanwhile, the potential of the ruthenium-based catalysts is evaluated from both catalytic performances and cost by comparison with the corresponding rhodium-based ones.     

Structure of Pt–Co/Al2O3 and Pt–Co/NaY Bimetallic Catalysts: Characterization by In Situ EXAFS,TPR, XPS and by Activity in Co (Carbon Monoxide) Hydrogenation

Temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and in situ extended X-ray absorption fine structure (EXAFS) studies were performed to investigate Pt-Co/NaY and Pt-Co/Al₂O₃ bimetallic catalysts. The EXAFS experiments were carried out at the Pt L_III and Co K edges of the same sample. This particular approach allows a precise determination of the electronic and structural characteristics of the metallic part of the catalyst. For both systems in situ reduction under pure H₂ results in the formation of nanometer-scale metallic clusters. For both Co and Pt, nearest neighbors are Co atoms. The complete set of parameters implies the presence of two families of nanometer-scale metallic clusters: monometallic Co nanosized particles and Pt-Co bimetallic clusters, in which only Pt-Co bonds exist (no Pt-Pt bonds). TPR and XPS results indicating a reduction of Co²⁺ ions in Pt-Co/NaY to a greater extent than in Pt-Co/Al₂O₃ give evidence of a facilitated reduction. XRD also shows the presence of nanometer-scale particles with only a very small fraction of larger bimetallic particles. In subsequent mild oxidation of the reduced systems the Co nanoparticles are still present inside the supercage of NaY zeolite in bimetallic form and the oxidation of the metallic particles is slowed down. Catalytic behavior is in good agreement with the structure of the Pt-Co bimetallic system.     

In situ FTIR and UV/VIS spectroscopic study of the generation of anionic Pt-carbonyl complexes within a faujasite matrix

The temporal generation of anionic platinum-carbonyl complexes in platinum ionexchanged zeolites X and Y by reductive carbonylation at 10⁵ Pa and 363 K is monitored by in situ UV/VIS and FTIR spectroscopy. A monomer [Pt₃(CO)₆]^2–, exhibiting bands at 318 and 456 nm in the UV/VIS spectra and at 1790 and 2025 cm^–1 in the FTIR spectra, is the only platinum/species formed in NaX. The monomer as well as oligomers are generated in NaY, where the formation of the latter species is due to the stronger acidity in the NaY as compared to NaX. The decomposition of the complexes results in the generation of Pt clusters of the size 1 nm.     

Preparation and characterization of Co/SiO2, Co-Mg/SiO2 and Mg-Co/SiO2 catalysts and their activity in CO hydrogenation

Co/SiO₂, Mg-Co/SiO₂ and Co-Mg/SiO2 catalysts were prepared from acetate, nitrate or carbonyl precursors. The catalysts were characterized by XRD, XPS, SIMS and TGA. The steady-state activity and product distribution of the catalysts were evaluated in synthesis gas reactions at 0.5 MPa and 235-290°C using 3 : 1 : 3 molar ratio of Ar : CO : H₂. The activity in CO hydrogenation decreased in the precursor order Co₂(CO)₈>Co(NO₃)₂> Co(CH₃COO)₂, and the probability of chain growth decreased in the precursor order Co(NO₃)₂>Co₂(CO)₈>Co(CH₃COO)₂. Alcohol yields were highest with Co₂(CO)₈, and lowest with Co(NO₃)₂, Magnesium promotion influenced the catalyst activity and decreased the CO₂ formation, but the promotion effects were less profound than those of the precursor. Surface studies on partially magnesium covered cobalt foil model catalysts suggested that magnesium promotes CO dissociation and chain growth, neither of which were, however, observed in the supported catalysts.     

Ru/MgO Sol-Gel Prepared Catalysts for Ammonia Synthesis

A new method of preparation of alkali-promoted Ru/MgO catalysts, based on a sol-gel procedure, starting from magnesium ethoxide, Ru₃(CO)₁₂ and a cesium compound has been designed. The gels were subjected to an activation/reduction procedure to substantially obtain Ru-CsOH/MgO. The activated catalysts were tested in ammonia synthesis at atmospheric pressure. It was clear that the sol-gel prepared Cs-promoted Ru/MgO catalysts are much more active, under the same reaction conditions, than the analogous catalysts prepared by impregnation procedures.     

Fe-Promoted selective methanol synthesis in CO hydrogenation catalyzed on SiO2-supported Ir4Fe and Pd6Fe6 bimetallic carbonyl cluster-derived catalysts

The bimetallic catalysts prepared from Ir₄Fe and Pd₆Fe₆ carbonyl clusters exhibited high activities and selectivities towards methanol from CO + H₂. In contrast, hydrocarbons and CO₂ were preferentially obtained on the catalyst prepared from Fe-rich PdFe₄ cluster. The promotion towards methanol synthesis on the SiO₂-supported Ir₄Fe and Pd₆Fe₆ cluster catalysts was proposed to be associated with Pd-Fe³⁺ and Ir-Fe³⁺ sites located at the metal-support interface.     

High energy ball-milled Pt and Pt–Ru catalysts for polymer electrolyte fuel cells and their tolerance to CO

High energy ball milling, an industrially amenable technique, has been used to produce CO tolerant unsupported Pt–Ru based catalysts for the oxidation of hydrogen in polymer electrolyte fuel cells. Nanocrystalline Pt_0.5–Ru_0.5 alloys are easily obtained by ball-milling but their performances as anode catalysts are poor because nanocrystals composing the material aggregate during milling into larger particles. The result is a low specific area material. Improved specific areas were obtained by milling together Pt, Ru and a metal leacheable after the milling step. The best results were obtained by milling Pt, Ru, and Al in a 1:1:8 atomic ratio. After leaching Al, this catalyst (Pt_0.5–Ru_0.5 (Al₄)) displays a specific area of 38 m²g^–1. Pt_0.5–Ru_0.5 (Al₄) is a composite catalyst. It consists of two components: (i) small crystallites (4 nm) of a Pt–Al solid solution (1–3 Al wt%) of low Ru content, and (ii) larger Ru crystallites. It shows hydrogen oxidation performance and CO tolerance equivalent to those of Pt_0.5–Ru_0.5 Black from Johnson Matthey, the commercial catalyst which was found to be the most CO tolerant one in this study.     

SiO2‐supported bimetallic Rh–Co catalysts derived from [Rh(CO)2Cl]2 and cobalt carbonyls

According to the results of IR characterization and catalytic study in ethylene hydroformylation, bimetallic Rh–Co catalysts can be efficiently prepared from [Rh(CO)₂Cl]₂ and cobalt carbonyls by co‐impregnation on SiO₂. The reaction of Co₂(CO)₈ with [Rh(CO)₂Cl]₂ (Rh : Co = 1 : 3 atomic ratio) gives rapidly RhCo₃(CO)₁₂ on the surface of SiO₂. Although Co₄(CO)₁₂ is not reactive with [Rh(CO)₂Cl]₂ on SiO₂ to form directly RhCo₃(CO)₁₂, an equivalent bimetallic catalyst can be easily obtained from ([Rh(CO)₂Cl]₂ + Co₄(CO)₁₂)/SiO₂ or its derivative (Rh⁺ + Co²⁺)/SiO₂ (Rh : Co = 1 : 3 atomic ratio) under reducing conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the nature of catalyst promotion by manganese in CO hydrogenation to oxygenates over RhMn/NaY

CO hydrogenation over Mn promoted Rh/NaY catalysts was studied at 10 bar and 250°C. Significant selectivity to oxygenates, mainly ethanol and ethyl acetate, was obtained after neutralizing the protons that are formed during reduction of Rh ions. Layered bed experiments show that protons act as sites catalyzing secondary reactions. Protons also convert Mn(OH)₂ to Mn²⁺ ions; the catalysts with highest selectivity to oxygenates contain MnO particles and Rh clusters. The results suggest chemical interaction of adsorbates on Rh_n clusters with those on MnO.     

A triheterometallic cluster: synthesis and structural characterisation of a cyclopentadienyl-substituted pentanuclear cluster based on an Os3CoRu metal core

The ionic coupling of the carbonyl cluster anion [Os₃Co(CO)₁₃]⁻ (1) with [Ru(η⁵-C₅H₅)(NCMe)₃]⁺ affords the new pentanuclear triheterometallic cluster Os₃CoRu(CO)₁₃(η⁵-C₅H₅) (2) as well as the known bimetallic cluster compounds HOs₃Ru(CO)₁₁(η⁵-C₅H₅) and Os₃Ru₂(CO)₁₁(η⁵-C₅H₅)₂. The crystal structure of cluster 2 shows that the metal framework is based on a trigonal bipyramid (approximate C_s symmetry) with the Ru, Co and an Os atom occupying the equatorial metal plane.     

Catalysts by design: genesis and CO hydrogenation of novel Pd carbonyl clusters in zeolite 5A

The potential of catalyst synthesis by design is demonstrated by comparing Pd/5A to Pd/NaY. While supercage dimensions are similar for both zeolites, the diameter of the supercage windows not only determines the Pd nuclearity of entrapped Pd carbonyl clusters, but also restricts their growth under CO hydrogenation conditions. While Pd₁₃(CO) _x clusters prevail in zeolite Y as a result of migration and coalescence of primary Pd carbonyl clusters after CO exposure at room temperature, cluster growth in zeolite 5A is confined to Pd₆(CO) _x . Under the conditions of syngas conversion, small Pd clusters are stabilized in the supercages of 5A, in contrast to the agglomeration of Pd particles to size larger than 60 Å in NaHY. The catalytic activity of Pd/5A is twice that of Pd/NaHY. The selectivities of CO hydrogenation on both catalysts are also drastically different: on Pd/5A, methanol and dimethylether are the sole products besides methane, but on Pd/NaHY, production of C₂₊ hydrocarbons is significant.     

Characterization of Fischer–Tropsch Cobalt-Based Catalytic Systems (Co/SiO<Subscript>2</Subscript> and Co/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) by X-ray Diffraction and Magnetic Measurements

Results of the characterization of six Co-based Fischer–Tropsch (FT) catalysts, with 15% Co loading and supported on SiO₂ and Al₂O₃, are presented. Room temperature X-ray diffraction (XRD), temperature and magnetic field (H) variation of the magnetization (M), and low-temperature (5 K) electron magnetic resonance (EMR) are used for determining the electronic states (Co⁰, CoO, Co₃O₄, Co²⁺) of cobalt. Performance of these catalysts for FT synthesis is tested at reaction temperature of 240 °C and pressure of 20 bars. Under these conditions, 15% Co/SiO₂ catalysts yield higher CO and syngas conversions with higher methane selectivity than 15% Co/Al₂O₃ catalysts. Conversely the Al₂O₃ supported catalysts gave much higher selectivity towards olefins than Co/SiO₂. These results yield the correlation that the presence of Co₃O₄ yield higher methane selectivity whereas the presence of Co²⁺ species yields lower methane selectivity but higher olefin selectivity. The activities and selectivities are found to be stable for 55 h on-stream.     

Influence of Ru segregation on the activity of Ru–Co/γ-Al2O3 during FT synthesis: A comparison with that of Ru–Co/SiO2 catalysts

γ-Al₂O₃ and SiO₂ supported Co catalysts, with varying amounts of Ru, were prepared and evaluated for Fischer–Tropsch synthesis (FTS). The composition of Ru for optimum activity was found to be support-dependent. The reducible Co₃O₄ was high in the region of 0–1.64 wt.% of Ru in Co/SiO₂ catalysts. Co/γ-Al₂O₃ displayed a maximum for reducible Co species at 0.42 wt.% Ru. Segregation of Ru occurred beyond this composition decreasing the extent of reduction. Co/γ-Al₂O₃ catalysts showed lower activity and olefin selectivity, in spite of higher Co dispersion, than Co/SiO₂ catalysts. The catalytic performance depends on the amount of reducible Co species, which again depends upon the optimum content of Ru.     

Support effect on the low-temperature hydrogenation of benzene over PtCo bimetallic and the corresponding monometallic catalysts

PtCo bimetallic and Co, Pt monometallic catalysts supported on γ-Al₂O₃, SiO₂, TiO₂ and activated carbon (AC) were prepared and evaluated for the hydrogenation of benzene at relatively low temperatures (343 K) and atmospheric pressure. Results from flow reactor studies showed that supports strongly affected the catalytic properties of different bimetallic catalysts. AC supported PtCo bimetallic catalysts exhibited significantly better performance than the other bimetallic catalysts, and all the bimetallic catalysts possessed higher activity than the corresponding monometallic catalysts. Results from CO chemisorption and H₂-temperature-programmed reduction (H₂-TPR) studies suggested that different catalysts possessed different properties in chemisorption capacity and reduction behavior, and AC supported PtCo catalysts possessed significantly higher CO chemisorption capacity compared to the other catalysts. Extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) analysis provided additional information regarding the formation of Pt–Co bimetallic bonds and metallic particle size distribution in the PtCo bimetallic catalysts on different supports.     

Characterization of zeolite-encapsulated iron phthalocyanines and butadiene hydrogenation catalyzed by their electron donor-acceptor complexes with sodium

NaY zeolite-encapsulated iron phthalocyanine (FePc) was synthesized from [HFe₃(CO)₁₁]^– in NaY. The structural properties of FePc in NaY were studied by EXAFS, Mössbauer, FT-IR, and diffuse reflectance UV-VIS. The spectroscopic results suggest that NaY-encapsulated FePc (FePc/NaY) is slightly distorted from the original planar structure of a phthalocyanine molecule due to the intrazeolitic constraint. Electron donor-acceptor (EDA) complex (Na⁺)₄(FePc^4–)/NaY was prepared from FePc/NaY by the reaction with sodium-naphthalene (Na⁺)(C₁₀H ₈ ^– ). They gave a higher trans/cis ratio of 2-butenes in butadiene hydrogenation compared with the EDA complex on the external surface of NaY.