Silica-supported Ru Research Articles

The role of alkali metals as promoters in the methanation and Fischer-Tropsch reaction: An in situ study

The effect of alkali metals as catalytic modifiers on the methanation and Fischer-Tropsch reaction over a series of silica-supported Ru catalysts has been studied. When hydrocarbon product distributions were compared at constant CO conversion or at constant CH 4 formation turnover frequencies, rather than at constant temperature, differences in selectivity were small for the series of catalysts studied. The temperature-programmed desorption of CO from these catalysts showed the presence of three desorption maxima, centered at 100, 220, and 450 °C, respectively. The relative intensity of the maximum centered at 220 °C was observed to increase slightly on the catalysts modified by the addition of alkali metals. However, this slight increase in binding energy did not appear to promote the formation of surface carbon. The high-temperature maximum centered at 450 °C was assigned to CO formed by the reaction between surface carbon and H 2O from the support. An “ in situ” infrared study of the CO surface species present on the catalyst surface during reaction showed only one band centered at 2030 cm −1 on supported Ru. This was in contrast to the two bands centered at 2020 and 1950 cm −1, which were observed for the K- and Cs-promoted catalysts. The results of this study suggest a depression in the rate of hydrogenation of the surface carbon species as a result of the addition of the alkali metal modifier. This is explained by invoking site blocking by the alkali metal adatoms rather than by an electronic effect.

The coadsorption of CO and H 2 on silica-supported Ru catalysts

The coadsorption of CO and H 2 on silica-supported Ru catalysts

Methanation and Fischer-Tropsch studies on potassium-promoted silica-supported Ru catalysts

The role of potassium as a promoter in the methanation and Fischer-Tropsch reaction has been studied. When the potassium content of a series of silica-supported Ru catalysts was increased, turnover numbers for both methane and hydrocarbon formation decreased. Significant increases in the selectivity for higher hydrocarbon and olefin formation occurred as the potassium concentration was increased. The results of a temperature-programmed desorption study showed that the binding sites of CO on Ru were significantly changed due to the presence of the potassium. When the potassium concentration was increased, the high-temperature desorption maxima centered at 370 °C increased relative to the low-temperature desorption maxima at 200 °C. This result is interpreted in terms of the stronger CRu bond resulting from the presence of the potassium. The results of the temperature-programmed desorption study were verified using in situ infrared spectroscopy. As the potassium content of the RuSiO 2 catalysts was increased, additional CO infrared bands assigned to more strongly bound CO were observed. An in situ infrared temperature-programmed reaction study for the potassium-promoted catalyst showed that the more weakly bound CO desorbed without undergoing reaction to form methane.

Evidence for [formula omitted] isotope effects on fischer-tropsch synthesis over supported ruthenium catalysts

The effects of using D 2 rather than H 2 during Fischer-Tropsch synthesis were investigated using alumina- and silica-supported Ru catalysts. For the alumina-supported catalysts, the rate of CD 4 formation was 1.4 to 1.6 times faster than the formation of CH 4. A noticeable isotope effect was also observed for higher molecular weight products. The magnitude of the isotope effects observed using the silica-supported catalyst was much smaller than that found using the alumina-supported catalysts. The formation of olefins relative to paraffins was found to be higher when H 2 rather than D 2 was used, independent of the catalyst support. The observed isotope effects are explained in terms of a mechanism for CO hydrogenation and are shown to arise from a complex combination of the kinetic and equilibrium isotope effects associated with elementary processes occurring on the catalyst surface.

Synthesis of hydrocarbons from CO and H 2 over silica-supported Ru: Reaction rate measurements and infrared spectra of adsorbed species

The synthesis of hydrocarbons from CO and H 2 was studied on a silica-supported Ru catalyst, and the species present on the catalyst surface were characterized by infrared spectroscopy. Initial rates of methane formation were correlated by the expression N CH 4 = 5.6 × 10 6 exp (−24,000/RT) P H 2 1.5 P CO 0.6 . The synthesis of ethane, propylene, and propane, the principle products observed in addition to methane, was favored at high CO partial pressures, low H 2 CO ratios, and low temperatures. The primary feature observed in the infrared spectra was a band at 2030 cm −1, associated with chemisorbed CO. Neither the position nor intensity of this band was affected by the CO partial pressure or the H 2 CO ratio. The CO band intensity did decrease with increasing temperature due to a decrease in CO surface coverage. Bands were also observed at 2950, 2910, and 2845 cm −1 and were assigned to hydrocarbon structures surrounded by chemisorbed CO. These structures could be removed from the catalyst surface by hydrogenation, but do not appear to be intermediates in the synthesis of stable products. Reduction of the catalyst following steady-state reaction revealed that the working catalyst maintains a reservoir of carbon several Ru monolayers in magnitude. Carbon deposition mechanisms and the role of carbon in the synthesis of hydrocarbons are discussed.

Infrared study of the reactions between NO and CO and NO and H 2 on a silica-supported Ru catalyst

Observations of the interactions of NO with CO and H 2 on a silica-supported Ru catalyst have been performed using infrared spectroscopy. The formation of an isocyanate species was found when NO reacted with preadsorbed CO. When NO and CO interacted with the surface simultaneously, N 2O and CO 2 were formed in addition to the isocyanate species. Mechanisms for the formation of the isocyanate structure, N 2O, and CO 2 are proposed and discussed. Results of the interaction of NO with hydrogen (as D 2) showed that adsorbed NO reacted with D 2 to produce species containing ND and OD bonds.

An infrared study of NO and CO adsorption on a silica-supported Ru catalyst

Infrared spectra were obtained for NO and CO adsorbed on reduced and oxidized Ru supported on silica. Adsorption of NO on a fully reduced sample produced bands at 1810 and 1860 cm −1. With prolonged catalyst exposure to NO, the latter band shifted to higher frequencies. Spectra of NO adsorbed on an oxidized sample showed only a single band at 1880 cm −1. Both the band at 1810 cm −1 and that occurring between 1860 and 1880 cm −1 are assigned to NO adsorbed as NO +. The low frequency band is associated with adsorption on partially oxidized Ru sites and the high frequency band with adsorption on more fully oxidized sites. On a fully reduced sample, CO adsorption produced a single band at 2040 cm −1 which is assigned to linearly bonded CO. Oxidation of Ru and subsequent adsorption of CO produced bands at 2130 and 2070 cm −1. This pair of bands is assigned to the symmetric and asymmetric stretches of two CO molecules bonded to a single oxidized Ru site.

An infrared study of the adsorption of CO and NO on silica-supported RuPt bimetallic clusters

The infrared spectra of CO and NO adsorbed on a supported bimetallic RuPt sample (62 at% Ru) have been recorded. In addition, the stability of adsorbed CO to NO (g) and the stability of adsorbed NO to CO (g) have been determined using infrared spectroscopy. These studies reveal that there are both Ru and Pt surface sites present on the bimetallic RuPt sample. The following band assignments have been made: for CO adsorbed on the RuPt sample a broad asymmetric band at 2055 cm −1 and a weak high frequency bank at 2145 cm −1 are observed. The broad asymmetric bank at 2055 cm −1 was found to have two components, a sharp band at 2055 cm −1 assigned to ν (co) for PtCo and a broad band on the low frequency side of this band assigned to ν (CO) for RuCO. The band at 2145 cm −1 was assigned to ν (CO) for CO adsorbed on a RuO surface site. For NO adsorbed on a bimetallic RuPt sample two bands are observed in the infrared spectra, a strong band at 1805 cm −1 assigned to ν (NO) for Ru δ−NO δ+ and a band of medium intensity at 1590 cm −1 assigned to ν (NO) for Ru δ+NO δ−. The results of this infrared study of the adsorption of CO and NO on a bimetallic RuPt sample are compared with earlier studies of the adsorption of CO and NO on supported monometallic Ru and Pt samples.

An infrared study of the adsorption of NO on the reduced and oxidized forms of silica-supported Ru

The infrared spectra of NO adsorbed on the reduced and oxidized forms of supported Ru have been recorded. (An oxidized Ru sample was made by exposing a reduced sample to 50 Torr of O 2 at 325 °C.) For NO adsorbed on an oxidized 6% Ru-silica sample, a strong sharp band at 1880 cm −1 is observed, and this band is assigned to v( no) for NO adsorbed on an oxygen-perturbed Ru surface site. NO adsorbed on the oxidized form of Ru is stable both to evacuation up to temperatures of 325 °C and to H 2 (g) up to temperatures of 200 °C. When NO is adsorbed on a reduced 6% Ru-silica sample, N 2O (g) is detected, and the oxygen species formed in this reduction is adsorbed on the Ru-silica surface. Three bands are observed in the infrared spectra: a band at 1630 cm −1 assigned to v( no) for Ru δ+-NO δ−, a band at 1820 cm − assigned to v ( no) for Ru δ−-NO δ+, and a band at 1860 cm −1 assigned to v( no) for NO adsorbed on an oxygen-perturbed Ru surface site. When a reduced Ru-silica sample with full NO surface coverage is heated, we observe an intensification of the high-frequency NO band; however, if this sample is heated to 125 °C in the presence of a reducing gas, i.e., H 2, the surface species react, and the frequencies of the infrared bands are shifted to lower wave numbers.

An infrared study of the interaction between adsorbed CO and adsorbed NO on supported Ru and supported Pt

The interaction between CO and NO adsorbed on 6% Ru-silica and 6% Pt-silica samples at 25 °C has been studied using infrared spectroscopy. When adsorbed CO is slowly displaced from a 6% Ru-silica surface by NO, we observe a new band at 2180 cm −1 in the infrared spectrum. The intensity of the band at 2180 cm −1depended on the relative amounts of CO and NO adsorbed on the surface. In studies made using C 13O and N 15O, we find that this new surface species contains both C and N atoms and that it is bound to the Ru surface through the N atom; thus, we conclude that the band at 2180 cm −1 is the antisymmetric stretching vibration of a surface isocyanate, Ru-NCO. When CO and NO are both adsorbed on a 6% Pt-silica sample, we do not observe a strong band in the 2200 cm −1 region. A mechanism is proposed for the formation of a surface isocyanate at 25 °C on silica-supported Ru; the failure to observe isocyanate formation on silica-supported Pt is also discussed.