Rh SiO Research Articles

Preparation of Rh–SiO2 fiber catalyst with superior activity and reusability by electrospinning

Rh–SiO2 fiber catalyst prepared by electrospinning for room temperature hydrogenation of alkenes with superior catalytic activity and reusability.

Marked capability for hydrogen occlusion of hollow silica nanospheres containing group 8–10 metal clusters

Hollow silica nanospheres containing various precious metal clusters such as Rh, Ir and Ru were synthesized by solubilizing corresponding metal ammine complexes into NP-6/cyclohexane reversed micelle system. The condensation and nucleation of the metal ammine complex in the reversed micelles took place and the resulting nanocrystals of the ammine complexes acted as a structure-directing agent for the formation of hollow cavity in silica spheres. The nano-hollow spheres were stable for thermal treatment up to 1073 K. By this method, bimetallic nanoparticles, i.e. Rh–Co–SiO 2 and Ir–Co–SiO 2, were also easily synthesized and both metal elements containing in the hollows may be alloyed after thermal treatment. The Ir– and Rh–SiO 2 nano-hollows exhibited excess hydrogen uptake. In particular, over Ir–SiO 2 sample, the H/Ir ratios reached 3.3 under 200 Torr of hydrogen at room temperature. At higher pressure, the H/M value reached 11.6 at 2.6 MPa.

Performance and Kinetic Behavior of a New SiO 2-Attached Molecular-Imprinting Rh-Dimer Catalyst in Size- and Shape-Selective Hydrogenation of Alkenes

A new size- and shape-selective Rh-dimer catalyst was designed by combination of a metal-complex-attaching method and a molecular-imprinting method on an Ox.50 surface. Coordinatively unsaturated, air-stable Rh dimers with a Rh–Rh bond of 0.268 nm were prepared in 0.74-nm micropores of SiO 2-matrix overlayers on the Ox.50 silica surface by attaching a Rh 2Cl 2(CO) 4 precursor on the surface (Rh 2Cl 2(CO) 4/SiO 2), followed by coordination of template ligands P(OCH 3) 3 to the attached Rh species (Rh sup catalyst), and then by surface imprinting of the template with SiO 2-matrix overlayers formed by hydrolysis–polymerization of Si(OCH 3) 4 (Rh imp catalyst). The Rh dimers, micropores, and SiO 2-matrix overlayers in the molecular-imprinting Rh imp catalyst were characterized by EXAFS, BET analysis, and 29Si solid-state MAS NMR, respectively. It was found that activity of the Rh sup catalyst for hydrogenation of alkenes was promoted remarkably (35–51 times) after the imprinting. The alkene hydrogenation proceeded on the imprinted Rh dimers with a monohydride without any breaking of the Rh–Rh bond. Size and shape selectivities of the molecular-imprinting Rh imp catalyst were examined by measuring the hydrogenation rates of eight alkenes of different sizes and shapes. It was also found that the Rh imp catalyst exhibited not only high activity and stability but also size and shape selectivities for the alkene molecules, probably due to a template-size cavity, created behind the removed template ligand, being used as a reaction site. Activation energies and activation entropies for the hydrogenation of large and branched alkenes were much smaller than those for small alkenes, which implies a shift in the rate-determining step in the reaction sequence for alkene hydrogenation. The performance of the molecular-imprinting Rh-dimer catalyst is discussed from structural and kinetic viewpoints.

Design of catalytic sites at oxide surfaces by metal-complex attaching and molecular imprinting techniques

This paper attempts to summarize our recent studies on the chemical design and characterization of catalytic sites at oxide surfaces such as Nb monomer, dimer and monolayer on SiO 2, Rh dimers on SiO 2, TiO 2, Al 2O 3, and MgO, and Au nanoparticles on TiO 2(1 1 0), prepared by using suitable metal-complex precursors in a molecular scale. The paper also reports performances of new molecular imprinting catalysts, SiO 2 overlayers/α-Al 2O 3 and Rh–SiO 2 overlayers/SiO 2, designed by surface molecular imprinting techniques.

Unique hydrogenation activity of supported tin catalyst: Selective hydrogenation catalyst for unsaturated aldehydes

Selective hydrogenation of unsaturated aldehydes, crotonaldehyde (CH 3CHCHCHO) and cinnamaldehyde (C 6H 5CHCHCHO), has been studied over SiO 2-supported monometallic Sn and bimetallic RhSn catalysts in the liquid phase. Over a silica-supported monometallic Rh catalyst, Rh SiO 2 , no unsaturated alcohol (crotyl alcohol or cinnamyl alcohol) was formed, whereas considerable amounts of the corresponding saturated aldehyde and saturated alcohol were obtained. The selectivity to the unsaturated alcohol was improved over the RhSn bimetallic catalyst. The selectivity to the corresponding unsaturated alcohol attained ca. 65% over the RhSn bimetallic catalysts. On the other hand, The supported Sn catalyst showed markedly high selectivity to the unsaturated alcohols. The selectivity of the Sn SiO 2 , attained 95% to crotyl alcohol and 100% to cinnamyl alcohol, respectively. Although the conversion of each unsaturated aldehyde over RhSn SiO 2 catalysts was greater than that over Sn SiO 2 catalysts, the selectivity of Sn SiO 2 catalysts to the corresponding unsaturated alcohols was superior to that over RhSn SiO 2 . The selectivity of Sn SiO 2 was also compared with that of RhSn SiO 2 at a similar conversion of the unsaturated aldehydes. The selectivity of Sn SiO 2 was significantly greater than that of the RhSn bimetallic catalyst. These results indicate that the high selectivity over Sn SiO 2 was ascribed not to low conversion but to intrinsic selectivity of the Sn catalyst.

Unique selectivity for the dimerization of propene on [formula omitted] catalysts: implications on the mechanism of CC bond formation and cleavage on bimetallic catalysts

A series of bimetallic catalysts RhSn x SiO 2 ( x = 0.4, 0.7, 0.9, and 1.4) were synthesized by the reaction of the monometallic catalyst Rh SiO 2 with Sn( n-butyl) 4 under hydrogen. Various chemical and spectroscopic methods indicated that the metals present were fully reduced, and that tin atoms rest on the surface, very slightly increasing particle size and producing isolated rhodium sites. The catalytic reactions of propylene/hydrogen mixtures in the presence of these bimetallic catalysts are compared with those of the monometallic Rh SiO 2 catalysts. The mechanistically interesting reactions observed are those of carbon-carbon bond formation and cleavage. For the monometallic catalyst, olefin homologation and hydrogenolysis were observed, reactions which invoked the transfer of C 1 fragments from one olefin to another. For the bimetallic catalysts, a marked increase in the selectivity for C 6 products was observed. The presence of hydrogen is necessary to this reaction but selectivity for C 6 is enhanced when hydrogen is in deficit with respect to propylene. Selectivity for C 6 increases with the surface rhodium to tin, Rh s Sn , ratio to a maximum at 0.9. Low temperature favors the formation of C 6 and C 2 products.

Sn-promoting mechanism for ketone hydrogenation over [formula omitted

Hydrogenation of acetophenone, 2-butanone, styrene and 1-hexene over RhSn SiO 2 in heterogeneous liquid phase reaction systems was studied by in situ EXAFS, FT-IR, TEM, analytical TEM, CO and H 2 adsorption measurements. The catalytic activity of Rh SiO 2 for hydrogenation of acetophenone and 2-butanone increased by a factor of 5–500 by Sn addition, showing a maximum activity at surface composition Sn s Rh s = 1.5 , whereas hydrogenation of styrene and 1-hexene decreased monotonously and drastically by Sn addition. In situ Sn K-edge EXAFS of the well characterized CVD-RhSn SiO 2 catalyst prepared by using Sn(CH 3) 4 vapor suggested that oxygen of CO group makes a bond with Sn atom upon acetophenone adsorption.

Temperature-programmed reaction and in situ infrared study of CO adsorbed on [formula omitted] and [formula omitted] catalysts

The temperature-programmed reaction (TPR) technique coupled with infrared (IR) spectroscopy has been employed to study the reactivity of adsorbed CO towards hydrogen. The combination of the two techniques provides information on the structure and reactivity of adsorbates, activation energy and kinetic data for the CO hydrogenation reaction. CO adsorption on Rh SiO 2 at 298 K produced mainly linear and bridged CO on the surface. Linear CO appears to be more reactive than bridged CO in the formation of methane during TPR over Rh SiO 2 . The addition of silver to Rh SiO 2 promotes the formation of gem-dicarbonyl CO and suppresses the formation of bridged CO during CO adsorption at 298 K. The disproportionation of gem-dicarbonyl CO and the reduction of Rh + by oxidation of the CO ligand of Rh +(CO) 2 produce CO, CO 2, and surface carbon on the Ag Rh SiO 2 catalyst at temperatures above 310 K. Hydrogenation of surface carbon begins at 390 K leading to methane formation. The methane peak temperature was observed at 442 K on the Ag Rh SiO 2 and at 420 K on Rh SiO 2 . The higher hydrogenation temperature of adsorbed CO and surface carbon over Ag Rh SiO 2 as compared to Rh SiO 2 is attributed to the suppression of hydrogenation by Ag.

Effects of surface modifiers on the liquid-phase hydrogenation of alkenes over silica-supported platinum, palladium and rhodium catalysts I. Quinoline and carbon tetrachloride

Hydrogenation of 1-hexene and cyclohexene was studied over silica-supported Pt, Pd and Rh catalysts in the liquid phase, in the presence of quinoline or carbon tetrachloride at room temperature. As the amount of surface modifiers increased in the reactor, the rate of hydrogenation decreased over Pd SiO 2 and Rh SiO 2 . The activity of Pt SiO 2 could also be lowered with quinoline; however, it remained constant for cyclohexene hydrogenation or decreased just slightly for 1-hexene even when excess CCl 4 was supplied. The inhibition by CCl 4 was substantial for palladium and rhodium; nevertheless, complete deactivation did not occur. Quinoline effectively deactivated the palladium and rhodium catalysts in cyclohexene hydrogenation, but residual activity for 1-hexene hydrogenation was above 50% for each of the catalysts used.

Vibrational control of chaotic self-sustained oscillations during CO oxidation on a RhSiO 2 catalyst

Vibrational control was applied to suppress the self-sustained oscillations during CO oxidation on a Rh/SiO 2 catalyst wafer. Using the appropriate external sinusoidal vibration on the CO and O 2 flow rate, the amplitude of CO 2 concentration oscillations was reduced to one-tenth of the constant feed case and the time-averaged CO 2 production increased by 30%. The decrease in the amplitude of the oscillations was accompanied by the suppression of the movement of the thermal patterns, measured by IR thermography, occurring during oscillations. The optimum control is attained at a specific frequency and amplitude of the forced oscillations.

Bimetallic catalysts derived from heteronuclear clusters; CO hydrogenation and olefin hydroformylation on [formula omitted] catalysts

Molybdenum-promoted Rh SiO 2 catalysts have been prepared from rhodium and molybdenum salts and from a heteronuclear Rh-Mo cluster as precursors. These catalysts have been compared with unpromoted Rh SiO 2 in CO hydrogenation and in ethene hydroformylation reactions. Mo promotes the CO hydrogenation both on salt- and cluster-derived catalysts, but the product distribution is different with a significantly enhanced selectivity to oxygenated products on cluster-derived samples. To test the CO insertion capability of the catalysts, ethene hydroformylation was performed under mild conditions (393 K-473 K, 0.1 MPa) and at CO hydrogenation conditions (473 K-573 K, 1 MPa). Only with the conditions of CO hydrogenation was it possible to show that the higher selectivity of the cluster-derived catalyst towards alcohols in the CO hydrogenation is due to the higher CO insertion capability of the cluster-derived catalyst.

Effects of high-temperature oxidation and reduction on the structure and activity of [formula omitted] catalysts

The effects of high-temperature oxidizing and reducing treatments on the structure and activity of Rh Al 2O 3 and Rh SiO 2 catalysts were examined using the techniques of temperature-programmed reduction, H 2 chemisorption, infrared spectroscopy, transmission electron microscopy, and CO oxidation kinetic analysis. Sequential treatments in O 2 and H 2 at 1075 K had little effect on the Rh SiO 2 catalysts beyond increasing the mean particle size from ≈4 nm to greater than 12 nm. No interactions between the Rh and the support were observed for the Rh SiO 2 catalysts. In contrast, 1075 K oxidation of the Rh Al 2O 3 catalysts caused most of the Rh to diffuse into the bulk of the alumina and resulted in a five- to tenfold decrease in CO oxidation activity. Nearly all of the Rh could be restored to the surface of the alumina by a subsequent 1075-K 1-h H 2 reduction followed by a reoxidation at 775 K for 1 h. The reactivated catalyst oxidized CO at greater rates than the fresh catalyst and with an apparent activation energy of ≈22 kcal/g mol compared to ≈30 kcal/g mol for the fresh catalyst. The data suggest that the most active form of Rh in the reactivated catalyst is small clusters whereas most of the active Rh in the fresh catalyst is present in larger Rh particles which are more bulk-like in character.

Structural transformation in Nb 2O 5-promoted Rh catalysts during calcination and reduction treatments

The extent of RhNb 2O 5 interaction in the niobia-promoted Rh SiO 2 catalyst depends on parameters such as calcination temperature. For the Nb 2O 5-promoted Rh SiO 2 catalyst with the Nb Rh atomic ratio of unity, no significant SMSI effects were observed after air calcination at 773 K. However, this catalyst exhibits significant SMSI behaviors after calcination at higher temperatures (973 and 1173 K). After high-temperature reduction at 773 K, the capacity of H 2 chemisorption typically diminished almost to zero and the catalytic activity for ethane hydrogenolysis was suppressed by ca. 3.5 orders of magnitude compared with that after low-temperature reduction at 373 K. X-ray diffraction showed that the RhNbO 4 compound was formed, with the amount increasing with increasing calcination temperature. After air calcination at 1173 K, the RhNbO 4 compound was formed almost exclusively on the SiO 2 surface. The RhNbO 4 compound was reduced to Rh and NbO 2 during HTR treatment at 773 K, accompanied by splitting of the large RhNbO 4 particle into a number of smaller Rh particles. The mechanism of RhNb 2O 5 interaction is discussed. It is proposed that the NbO 2 species formed during the HTR treatment block surface Rh atoms to induce strong RhNb 2O 5 interaction (SMSI).

Reversibility of strong metal-support interactions on [formula omitted

Hydrogenolysis of n-butane and CO hydrogenation were used to study the reversibility of the SMSI state in Rh supported on model nonporous TiO 2 spheres. High-temperature reduction (HTR) caused a three-order-of-magnitude drop in hydrogenolysis activity and led to increased selectivity toward ethane. Oxidation at 573 K was sufficient to restore the hydrogenolysis activity in these catalysts. Variations in hydrogenolysis selectivity on Rh TiO 2 were similar to those on Rh SiO 2 subjected to comparable treatments. This suggests that site blocking by TiO x may be primarily responsible for the suppressed hydrogenolysis activity on Rh TiO 2 . CO hydrogenation activity, on the other hand, was only marginally affected by the onset of SMSI and either increased or decreased after HTR depending on the prior treatment of the catalyst. High-resolution transmission electron microscopy showed the presence of 2- to 4-Å amorphous overlayers on both the Rh and the support after high-temperature reduction. The thickness of these overlayers was nonuniform and oxidation followed by low-temperature reduction led to partial removal of the overlayers and a roughening of the Rh metal surfaces.

Characterization of Zn-promoted [formula omitted] catalysts by chemisorption, infrared spectroscopy, and temperature-programmed desorption

Rh SiO 2 samples with various atomic ratios of Zn to Rh were prepared by impregnation of the chlorides, followed by reduction in hydrogen. X-ray photoelectron spectra indicate that for low Zn Rh ratios, the Zn ions are preferentially located on the surface of the Rh particles, whereas the Cl ions are efficiently removed. For high Zn Rh ratios, ZnCl 2 is deposited on the surface of the SiO 2 support. The Chemisorption of CO and H 2 decreases with increasing Zn loading. Fourier transform infrared spectroscopy reveals that added Zn leads to a decrease in the proportion of bridging CO. Temperature-programmed desorption (TPD) shows that reactive desorption of CO to form methane is strongly depressed by Zn. These results are consistent with a model that assumes that Zn blocks the rhodium ensembles for CO dissociation and subsequent methane formation.

Lanthana-promoted [formula omitted]: II. Studies of CO hydrogenation

A study of the influence of lanthana promotion on the hydrogenation of CO over Rh SiO 2 has been conducted. Lanthana-promoted Rh SiO 2 exhibits higher turnover frequencies for the synthesis of CH 4, C 2C 4 hydrocarbons, CH 3OH, and C 2 oxygenates than unpromoted Rh SiO 2 . The turnover frequency for each product goes through a maximum with increasing lanthana addition. Lanthana promotion is also found to increase the selectivity for the formation of CH 3OH, C 2 oxygenates, and C 2C 4 hydrocarbons and to decrease the selectivity for CH 4. The selectivity for C 2 oxygenates is optimized at low levels of lanthana addition. In situ infrared observations show that lanthana promotion blocks the chemisorption of CO onto surface Rh sites. Of particular interest is the observation of a band at 1725 cm −1. This feature is assigned to CO adsorbed on a Rh site immediately adjacent to an LaO x island. The low frequency of this vibration is attributed to a weakening of the CO bond caused by the interaction of lanthana cations with the oxygen end of the adsorbed CO. It is postulated that the interaction of the lanthana with the CO is responsible for the enhanced rates of CO dissociation and the formation CH 4 and C 2+ products. Infrared spectroscopy also provides evidence for acyl, formate, and acetate groups on the surface of lanthana-promoted Rh SiO 2 . The possible role of these species in the synthesis of products is discussed.

A kinetic study on the effects of support on the catalytic performances of [formula omitted], [formula omitted], and [formula omitted] in a COH 2 reaction

The catalytic hydrogenation of CO with hydrogen over Rh TiO 2 , Rh Al 2O 3 , and Rh SiO 2 reduced at 673 K was studied kinetically and by monitoring the behavior of adsorbed CO species with in situ FT-IR to reveal the origins of their markedly different activities. The higher activity of Rh TiO 2 is related to the much higher reactivity of its irreversibly adsorbed CO against hydrogen. The reactivity of 17% of irreversibly adsorbed CO on Rh Al 2O 3 was comparable to the rate of catalytic reaction, suggesting that its very limited portion is the reactive intermediate. The reactivity of the irreversibly adsorbed species on Rh SiO 2 was much less, indicating that the reversibly adsorbed species is the reaction intermediate. The rate of carbon deposition and its reactivity against hydrogen were much less than the rate of catalytic reaction, ruling out a major contribution to the catalytic sequence. Thus, the extent of CO activation which is strongly influenced by the supports is the basis for providing markedly different activities for these catalysts.

Lanthana-promoted [formula omitted]: I. Studies of CO and H 2 adsorption and desorption

Promotion of a Rh SiO 2 catalyst with lanthana results in a partial coverage of the supported Rh crystallites with LaO x islands. These moieties reduce the capacity of Rh to adsorb CO but have little effect on the chemisorption of H 2 because of spillover of H atoms from the exposed Rh sites onto the surface of the LaO x islands. At elevated temperatures, the LaO x islands promote the dissociation of CO. It is proposed that this process occurs preferentially at Rh sites located along the perimeter of the LaO x islands. Support for this interpretation is provided by infrared spectra of CO adsorbed on lanthana-promoted Rh SiO 2 .

Adsorption and reaction of CO and H 2 on K-promoted [formula omitted] catalysts

The adsorption of CO and H 2 on a series of alkali-promoted Rh SiO 2 catalysts was investigated by IR spectroscopy and volumetric chemisorption. The characteristics of the support as well as the method of addition of the alkali species were found to influence the adsorptive properties of the catalysts. Alkali species on wide-pore Rh SiO 2 tended to partition to the support and did not interact strongly with the Rh crystallites. When alkali and metal salts were coimpregnated onto a nonporous SiO 2 support, intimate alkali-metal contact resulted in significant electronic interactions between the alkali species and the metal. When alkali species were added to a prereduced Rh SiO 2 (nonporous) catalyst, a chemical interaction between a tilted adsorbed CO and the alkali species was suggested. The nature and location of the alkali species were suggested to be important parameters in determining the effect of alkali promoters on Rh SiO 2 catalysts. The rate of CO conversion decreased substantially with promotion for all of the promoted catalysts. An unusually low apparent activation energy was found for the sequentially impregnated (nonporous SiO 2) promoted catalyst, and it was suggested that this might be related to the unusually low frequency peak seen in the IR spectrum of adsorbed CO on this catalyst.

Frequency response study of the effect of alkali promotion on [formula omitted] catalysts: Evidence for kinetically distinct H 2-sorbing sites

The role of support and alkali promoter on the kinetics of hydrogen chemisorption on supported rhodium catalysts has been investigated using frequency response chemisorption (FRC). In all catalysts studied, several kinetically distinct chemisorbing sites were observed; with the exact number and kinetic parameters depending on the exact combination of promoter and support used. Comparison of the FRC spectra of Rh SiO 2 and Rh TiO 2 revealed the existence of new chemisorbing sites in the latter. One of the Rh TiO 2 sites may be attributed to H 2 spillover onto the support. The addition of alkali promoters suppressed the H 2 chemisorption of specific sites.