Surface structure and metathesis activity of photoreduced allyl‐based Mo/SiO2 catalysts

XPS and metathesis activity studies were performed on oxidic allylbased Mo/SiO₂ catalysts (0.5 wt% Mo) and the oxidic catalyst following photoreduction to various extents. XPS results and average oxidation state measurements from the amount of CO₂ formed during photoreduction, indicated that controlled photoreduction of the oxidic catalyst, in CO atmosphere, produces a mixture of Mo⁶⁺ and Mo⁴⁺ oxidation states with Mo⁴⁺ in abundances ranging from 0 to 100%. Propene metathesis activity studies were performed on the oxidic and photoreduced allylbased Mo/SiO₂ catalysts. The results indicated that metathesis activity appears on photoreduction and increases linearly with increasing abundance of Mo⁴⁺ present on the catalyst. The linear relationship between the % propene conversion and the % Mo⁴⁺ present on the catalyst is consistent with the proposed uniformity of the Mo species and with the hypothesis that the active site (or active site precursor) is associated with Mo⁴⁺.     

The Identification of Different Active Sites on Mo/Al2O3 Metathesis Catalysts

Mo/Al₂O₃ catalysts prepared via fixation of Mo(³-C₃H₅)₄ on Al₂O₃ or by conventional impregnation (2.2 or 2.9 wt% Mo) have been compared with regard to their catalytic behavior in the metathesis of propene in different temperature ranges (293-323 K, 473 K). Different active sites have been distinguished. A site derived from a Mo(VI) precursor by thermal activation in inert gas exhibits stable activity, with a propene reaction order near 1. Other sites that are derived from a reduced Mo precursor, probably Mo(IV), are of higher activity but unstable with time-on-stream and also at elevated temperatures (>323 K). These sites support the metathesis at a propene reaction order of 0.5 and with activation energies between 10 and 25 kJ/mol depending on unknown structural details. Due to their instability, they cannot contribute to the high-temperature (T > 373 K) metathesis activity observed with Mo/Al₂O₃ catalysts. The latter is supported by Mo(VI)-derived sites or, at after reduction of catalysts with higher Mo contents, by Mo(IV)-derived sites that are different from those identified in the present study.     

First IR spectroscopic observation of the stable Mo=CH2 carbene complex on the surface of an active catalyst for olefin metathesis: Photoreduced silica-molybdena with chemisorbed cyclopropane

We report in the first IR observation of carbene complex Mo=CH₂ on the surface of Mo/SiO₂ metathesis catalyst. Mo=CH₂ species were produced by cyclopropane chemisorption on catalysts, photoreduced in CO (Mo(IV)/SiO₂), and are characterized in IR by C-H stretching vibrations at 3080 and 2945 cm^–1.     

Structure and reactivity of MoO3-MgO catalysts

Surface of OH groups on reduced MoO₂-MgO catalysts such as $$ - - Mg - - O - - \begin{array}{*{20}c} {||} \\ {Mo} \\ | \\ \end{array} - - OH$$ may act as an active site for hydrogenation of propene. The surface hexa-coordinated Mo⁵⁺ ion (MO _6c ⁵⁺ ) was reduced to a lower number of cation such as Mo⁴⁺ or Mo³⁺ which act as an active site for metathesis of propene.     

DRIFT Studies of Adsorbed CO Over Sulfided K–Rh–Mo/Al2O3 Catalysts: Detection of Rh–Mo–S Phase

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was used to study the nature of active species in K–Rh–Co–MoS₂/Al₂O₃ catalyst by means of probing with CO molecule. The effects of K addition to Rh and interaction between Mo and Rh were studied with varying K and Mo loadings over 1 wt% Rh/Al₂O₃ catalyst. In sulfided Rh–Mo/Al₂O₃, the formation of Rh–Mo–S phase was evidenced first time by a band at 2,095 cm^?1. The introduction of Co to K–Rh–MoS₂/Al₂O₃ catalyst showed the existence of both Rh and Co promoted MoS₂ sites, but the CO absorption frequencies in DRIFT spectra are significantly at lower side compared to Co free Rh–Mo catalyst. The stabilities of CO band from Rh and Co promoted and unpromoted MoS₂ sites are studied at different temperatures. When activated carbon used as support, bands for both promoted and unpromoted MoS₂ sites were appeared, but the intensity of these bands were decreased largely compared to alumina based catalyst, resulted from the coverage of added K not only on the support surface but also on the active metal components due to the neutral nature of activated carbon.     

Organometallic Rhodium Complexes Encapsulated in Mesoporous Molecular Sieves

Rh(III) complexes both dimmer [Cp*RhCl]₂(μ-Cl)₂ and monomer ([RhCp*(S)₃]²⁺) were encapsulated into MCM-41 channels. All silica MCM-41 molecular sieve and aminosililated MCM-41 matrix were used for rhodium complexes accommodation. Reactivity of Cp* rhodium complexes encapsulated in meso structure was estimated on the grounds of their susceptibility to interaction with CO molecules resulting in the formation of carbonyl complexes. Formation of Cp*Rh carbonyls was recorded by means of FTIR spectra. It was found that accommodation of Rh(III) complexes in MCM-41 molecular sieves activated the complex and led to the formation of Rh(III)Cp* carbonyls as a result of contact with CO. Contact of rhodium (III) complexes encapsulated in MCM-41 matrix with CO did not result in rhodium (III) reduction, whereas in the presence of amine groups in aminosililated MCM-41 the reduction of Rh(III) to Rh(I) occurred relatively easily and formation of Cp*Rh(CO)₂ complex containing Rh(I) was noted. Encapsulated rhodium complexes showed some activity in methanol carbonylation reaction carried out under heterogeneous conditions. For the most active catalyst the amount of methyl acetate reached about 8 mol.%, however, deactivation of catalyst occurred and after 2 h on stream methyl acetate was not found in the product.     

Mo+TiO2(110) Mixed Oxide Layer: Structure and Reactivity

We present a STM/XPS/TPD/LEED study of the structural and electronic properties of Mo+Ti mixed oxide layers on TiO₂(110), and of their interaction with water, methanol and ethanol. Several different preparation procedures were tested and layers with different degrees of Mo/Ti mixing were prepared. Ordered mixed oxide surface phases with distinct LEED patterns could not be found; for all investigated Mo concentrations a TiO₂(110) like pattern was observed. Mo tends to agglomerate on the surface where it is found predominantly as Mo⁶⁺ at low coverages and as Mo⁴⁺ at high coverages. Mo⁴⁺ was also identified in the bulk of the mixed oxide layer. The Mo3d binding energies categorize the Mo⁴⁺ species as being dimeric. A third Mo3d doublet is attributed to a Mo species (Mo ⁿ⁺) with an oxidation state between those reported for Mo in MoO₂ and metallic Mo. Two types of Mo-induced features could be identified in the STM images for low Mo concentrations (in the range of 1 %). At higher Mo concentrations (~50 %) the surface is characterized by stripes with limited lengths in [001] direction. The concentration of bridging oxygen vacancies, which are common defects on TiO₂(110), is reduced significantly even at low Mo concentrations. Methanol and ethanol TPD spectra reflect this effect by a decrease of the intensity of the features related to these surface defects. At elevated MoO _x coverages, the yield of reaction products in methanol and ethanol TPD spectra are somewhat smaller than those found for clean TiO₂(110) and the reactions occur at lower temperature.     

Investigations on the photoluminescence properties of Mo-MCM-41 and the photocatalytic decomposition of NO in the presence of CO

The photocatalytic decomposition of NO into N₂ and CO₂ on Mo-MCM-41 was found to proceed efficiently in the presence of CO. In situ photoluminescence measurements demonstrated that this reaction proceeds in a redox cycle between alternating Mo⁶⁺ and Mo⁴⁺ ions. The yields of N₂ formation in the photocatalytic reaction correspond with the yields of the photoluminescence of the tetrahedrally coordinated Mo oxide species, indicating that the charge transfer excited triplet state of the tetrahedral Mo oxide species plays a significant role in this reaction, leading to the formation of N₂ and CO₂ with a good stoichiometry. This revised version was published online in July 2006 with corrections to the Cover Date.     

Flame-made MoO3/SiO2–Al2O3 metathesis catalysts with highly dispersed and highly active molybdate species

MoO₃/SiO₂–Al₂O₃ catalysts are prepared via flame spray pyrolysis and evaluated in the self-metathesis of propene to ethene and butene. Their specific surface area ranges between 100 and 170 m² g^?1 depending on the MoO₃ loading (1–15 wt.%, corresponding to Mo surface density between 0.3 and 6.1 Mo atoms per nm²). The catalysts were characterized by N₂-physisorption, X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS). The silica–alumina matrix condenses first in the flame and forms non-porous spherical particles of 5–20 nm, followed by the dispersion of Mo oxide at their surface. Depending on the MoO₃ loading, different MoO_x species are stabilized: dispersed and amorphous molybdates (mono- and oligomeric) at low loadings (<5 wt.%, <1.5 Mo nm^?2) and crystalline MoO₃ species at higher loadings. Raman spectroscopy suggests the presence of monomeric species for surface densities of 0.3, 0.5 and 0.8 Mo nm^?2. The formation of MoOMo bonds is, however, clearly established by ToF-SIMS from surface densities as low as 0.5 Mo nm^?2. At 1.5 Mo nm^?2, crystallites of β-MoO₃ (2–3 nm) are detected and further increasing the loading induces the formation of bigger α- and β-MoO₃ crystals (around 20 nm). The speciation of Mo proves to have a marked impact on the metathesis activity of the catalysts. Catalysts with high Mo loading and exhibiting MoO₃ crystals are poorly active, whereas catalysts with low Mo loading (<5 wt.%) perform well in the reaction. The catalyst loaded with only 1 wt.% of MoO₃ (0.3 Mo nm^?2) is the most active, reaching turn over frequencies seven times higher than reference catalysts reported in the literature. Moreover, the specific metathesis activity is clearly inversely correlated to the degree of condensation of the molybdenum oxide phase (as evaluated by ToF-SIMS). The latter finding indicates that monomeric MoO_x species are the main active centres in the olefin metathesis.     

Preparation, Characterisation and Catalytic Behaviour of a New TeVMoO Crystalline Phase

TeM_xMo_1.7O mixed oxides (M = V and/or Nb; x = 0-1.7) have been prepared by calcination of the corresponding salts at 600 °C in an atmosphere of N₂. A new crystalline phase, with a Te/V/Mo atomic ratio of 1/0.2-1.5/1.7, has been isolated and characterised by XRD and IR spectroscopy. This phase is observed in the TeVMo or TeVNbMo mixed oxide but not in the TeNbMo mixed oxide. The new crystalline phase shows an XRD pattern similar to Sb₄Mo₁₀O₃₁ and probably corresponds to the M1 phase recently proposed by Aouine et al. (Chem. Commun. 1180, 2001) to be present in the active and selective MoVTeNbO catalysts. Although these catalysts present a very low activity in the propane oxidation, they are active and selective in the oxidation of propene to acrolein and/or acrylic acid. However, the product distribution depends on the catalyst composition. Acrolein or acrylic acid can be selectively obtained from propene on Nb-free or Nb-containing TeVMo catalysts, respectively. The presence of both V and Nb, in addition to Mo and Te, appears to be important in the formation of acrylic acid from propene.     

Comparison of the reactivity of bulk and surface oxides on Pd(100)

The reactivity of bulk PdO clusters produced by plasma oxidation of Pd(100) towards propene oxidation was characterized using temperature programmed desorption (TPD) and isothermal oxygen titration. The TPD results were dominated by simultaneous CO₂ and water desorption in a peak at 490 K. The only other product observed was a small amount of CO near saturation propene coverages that also desorbed at 490 K. The propene coverage saturated at exposures between 0.5–1 l, indicating a sticking coefficient close to one. In the titration experiments, CO₂ production peaked almost immediately upon exposure to propene, indicating that the propene oxidation rate fell as the surface was reduced. Above 450 K, virtually all of the propene was completely oxidized to CO₂ and water, while at lower temperatures small amounts of CO were observed and unreacted propene fragments accumulated on the surface. In comparison, previous results for a well-ordered surface oxide on Pd(100) were similar in that CO₂ and water also desorbed simultaneously indicating a similar mechanism, but different in that the sticking coefficient on the surface oxide was a factor of 20 lower, and the desorption peaked 60 K lower. These differences cause the bulk oxide to be far more active at higher temperatures than the surface oxide, but the surface oxide displays some activity down to lower temperatures where propene simply accumulates on the bulk oxide surface.     

Influence of alkali metal (Li and Cs) addition to Mo2N catalyst for CO hydrogenation to hydrocarbons and oxygenates

The aim of this work is to understand the catalytic behaviour of Li and Cs promoted Mo₂N for CO hydrogenation to hydrocarbons and oxygenates at the reaction conditions 275–325 °C, 7 MPa, and 30 000 h^?1 GHSV. Molybdenum nitrides were synthesized via temperature programmed treatment of ammonium heptamolybdate (AHM) and alkali metal (AM) precursors under continuous gaseous ammonia flow. Unpromoted Mo₂N and AM‐Mo₂N catalysts were characterized using BET‐pore size, X‐ray diffraction, TPD‐mass of CO, HR‐TEM, and XPS techniques. Nominal loadings of 1, 5, and 10 wt% of Li and Cs were selected for these studies. At a 10 % CO conversion level, the total oxygenate selectivity of 28, 11, and 6.5 % was observed on 5Cs‐Mo₂N, 5Li‐Mo₂N, and unpromoted Mo₂N, respectively. The decreased oxygenate selectivity for unpromoted Mo₂N was mainly associated with CO dissociative hydrogenation on Mo^δ+ sites. On the other hand, improved molecular CO insertion into ?C_xH_y intermediate accelerates the total oxygenate formation on the Cs‐Mo‐N catalyst. However, during nitridation, crystal structure changes were observed in Li‐Mo‐N and the obtained oxygenates selectivity was attributed to the Li₂MoO₄ phases. At lower AM loadings, the active sites corresponding to oxygenates formation were inadequate, and at higher AM loadings, surface metallic molybdenum decreased the total oxygenate selectivity.

     

Redox and photo‐redox properties of isolated Mo5+ ions in MoH‐ZSM‐5 and MoH‐beta zeolites: in situ ESR study

Redox and photo‐redox properties of isolated Mo⁵⁺ ions stabilized in H‐ZSM‐5 and H‐beta zeolites are studied by in situ ESR in flowing O₂, NO, H₂, and C₃H₆. Upon oxidation of pre‐reduced samples at 20 °C, NO demonstrates a higher oxidative ability, as compared with O₂. Interaction of Mo⁵⁺ ions with propene at 20 °C results in formation of a chemisorption complex with enhanced reactivity of Mo(V) toward NO. Illumination of the Mo⁵⁺/HZSM‐5 sample with UV‐visible light causes measurable acceleration of Mo(V) oxidation by NO at 20 °C. Therefore, photochemical activation of the oxidation step could be realized, in principle, for Mo/zeolite catalysts. At 500 °C in the reaction mixture NO + H₂, the step of the catalytic site reduction is fast, and the dynamic equilibrium of the redox reaction Mo(VI) ↔ Mo(V) for MoH‐ZSM‐5 and MoH‐beta seems to be strongly shifted to Mo⁵⁺. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of Mo on the catalytic and surface properties of Rh-Mo/ZrO2 catalysts

A series of Rh-Mo/ZrO₂ catalysts with fixed Rh and different Mo loadings were prepared and characterized by H₂ chemisorption, XRD, TPR, TEM and XPS. The catalysts were studied in two reactions: the hydrogenation of carbon monoxide and the hydrogenation of toluene. The results suggest that the increase in the Mo content produces a partial coverage of the support and the Rh particles. Moreover, at high Mo coverage, an increase of the MoO₃ layer thickness is produced. After being treated in hydrogen, the molybdenum oxide remains as slightly reduced particles, while Rh is essentially as Rh⁰, with only a small contribution of Rh+ species. The Mo promotes the formation of oxygenates in the CO hydrogenation and it does not affect the activity in the hydrogenation of toluene.     

In situ carburization of metallic molybdenum during catalytic reactions of carbon-containing gases

Supported molybdenum clusters were prepared by sublimation of Mo(CO)₆ onto dehy-droxylated alumina followed by decomposition in flowing dihydrogen at 970 K. These alumina-supported molybdenum clusters were found by XAFS to transform into Mo₂C if heated in a 20% methane/H₂ mixture at 950 K. For the hydrogenolysis ofn-butane at 510 K and CO-H₂ reactions at 570 K, both at atmospheric pressure, molybdenum and carburized molybdenum showed similar, but different for each reaction, turnover rates. The product distribution was the same for each reaction on Mo and Mo₂C. In both reactions, in situ XAFS data for fresh and used catalysts indicated that Mo clusters progressively transformed into Mo₂C under the reaction conditions     

The promoter function of molybdenum in Rh/Mo/SiO2 catalysts for CO hydrogenation

A series of Rh/Mo/SiO₂ catalysts with fixed Rh and different Mo contents were studied by FT-IR, chemisorption and CO hydrogenation. The FT-IR results at room temperature under CO atmosphere indicate that the addition of Mo to Rh/SiO₂ suppresses the linear and bridged CO species and promotes the twin CO species, which is consistent with the chemisorption results. It is suggested that the Mo promoter works via stabilization of Rh^1– ions and the coverage of Rh sites. The molybdenum promotes the formation of oxygenates and shifts the selectivity from hydrocarbons to oxygenates.     

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

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

The hydrogenation of CO over molybdenum/alumina in the presence of ethylene; coupling of olefin metathesis and CO hydrogenation

Three reactions, CO hydrogenation, metathesis of C₂H₄ and CO hydrogenation in the presence of C₂H₄, have been investigated on Mo(+2)/Al₂O₃ under identical experimental conditions. The products of hydrogenation reactions were methane, ethylene and propylene. The results show that the initial rate of propylene formation for CO hydrogenation in the presence of ethylene is much larger, by a factor of 7, than the sum of the rate of propylene formation for CO hydrogenation and metathesis alone. Furthermore, it was found that¹³C labelled propylene was formed in the hydrogenation of¹³CO in the presence of ethylene. These results, taken as a whole, suggest that the same intermediate is formed in both CO hydrogenation and metathesis of ethylene, and that these intermediates can be either incorporated into ethylene and higher hydrocarbons by polymerization or incorporated into ethylene to form propylene via olefin metathesis.     

Study on Reduction of MoO2 Powders with CO to Produce Mo2C

Molybdenum hemicarbide (Mo₂C), which is widely applied in steel and metal ceramics as well as catalysts, was successfully synthesized by using a simple method of reducing MoO₂ powders by CO. It was found that the final reduction product was all Mo₂C under both isothermal and nonisothermal conditions. However, the reduction mechanisms were significantly different at lower and higher temperatures: at lower temperatures (1070 to 1342 K), reduction of MoO₂ to Mo₂C followed a one‐step reaction (simultaneous reduction and carburization), while at higher temperatures (1423 to 1485 K), MoO₂ was first reduced to metallic Mo, and then Mo was carburized to Mo₂C. Mo₂C particles obtained at higher temperatures contained micrometer‐sized surface features which formed during the MoO₂ to Mo reduction step.     

EXAFS characterization of supported metal-complex and metal-cluster catalysts made from organometallic precursors

Nearly uniform (nearly molecular) supported metals made from molecular organometallic precursors are ideally suited to characterization by EXAFS spectroscopy at the metal edge. Among the most thoroughly investigated mononuclear metal complexes on metal oxide and zeolite supports are MgO-supported rhenium subcarbonyls, approximately Re(CO)₃{OMg}₃ (where the braces denote groups terminating the bulk of the support). These were made, e.g., from [HRe(CO)⁵] and from [H₃Re₃(CO)₁₂]; the Re–O_surface distance determined by EXAFS spectroscopy is 2.15 ± 0.03 Å the support is a tridentate ligand. The Re–O_surface distances in related supported complexes of Groups 7 and 8 metals are all in the range of 2.1–2.2 Å, matching those in molecular analogues. HTa{OSi}₂, prepared from [Ta(CH₂C(CH₃)₃)₃(=CHCCH₃)₃] on SiO₂, catalyzes a new reaction, propane metathesis. Supported complexes made from [HRe(CO)⁵] catalyze alkene hydrogenation but not cyclopropane hydrogenolysis, whereas catalysts made from [H₃Re₃(CO)₁₂] catalyze both these reactions, and EXAFS data indicate neighboring Re centers on the latter (but not the former), which are implicated in the catalysis. EXAFS data similarly indicate supported Mo and W pair sites as catalysts. Supported metal clusters made by decarbonylation of metal carbonyl clusters, e.g., Ir₄/γ-Al₂O₃ and Ir₆/γ-Al₂O₃ or Rh₆/zeolite NaY, are indicated by EXAFS data to be tetrahedra and octahedra, respectively. Such clusters are the species detected by EXAFS spectroscopy at 298 K in the presence of propene and H₂ undergoing catalytic hydrogenation, and they are identified as the catalytically active species. The catalytic activities of the clusters for toluene hydrogenation and alkene hydrogenation are almost unaffected by changes in metal oxide support composition, but they depend on the cluster size, although the catalytic reaction is structure insensitive. Thus, supported metal clusters offer new catalytic properties.