CO Disproportionation Research Articles

Adsorption Studies of Gases on Pt−Rh Bimetallic Catalysts by Reversed-Flow Gas Chromatography

In the present work, the relatively new technique of reversed-flow gas chromatography was applied for the study of adsorption of carbon monoxide, oxygen, and carbon dioxide on Pt-Rh bimetallic catalysts. Using suitable mathematical analysis, equations were derived by means of which rate constants for adsorption, desorption, and disproportionation reaction were calculated. From the variation of these rate constants with temperature and the nature of the catalyst (Pt content), as well as from the finding that the CO adsorption is a dissociative process, useful conclusions concerning the mechanism for the CO oxidation reaction over Pt-Rh bimetallic catalysts were extracted. The catalytic fractional conversions for the CO disproportionation reaction were found to be higher for the Pt-Rh bimetallic catalysts than those for the pure Pt catalyst, indicating the presence of beneficial Pt-Rh synergism.

Coupling of alkynes promoted by Ru(CO) 3(PPh 3) 2 and carbon dioxide: syntheses and structures of (η 4-C 4R 2R′ 2CO)Ru(CO) 2(PPh 3) (R=R′=Ph; R=Ph, R′=CCPh) and {Ph 2C 4(CCPh) 2}Ru(CO)(PPh 3) 2

Dimerization of alkyne by Ru(CO) 3(PPh 3) 2 proceeded under carbon dioxide in refluxing toluene or C 6H 6 to afford a cyclopentadienone Ru(0) complex (R 2C 4R′ 2CO)Ru(CO) 2(PPh 3) (R=R′=Ph; R=Ph, R′=–CCPh) and cyclobutadiene Ru(0) complex (R 2C 4R′ 2)Ru(CO) 2(PPh 3) (R=Ph, R′=–CCPh) in moderate yields, each compound being formed as mixtures of isomers, 2,4-and 3,4-diethynylated cyclobutadiene-1-one and 1,2-and 1,3-diethynylated cyclobutadiene, from MS, IR, 13C-, 31P-NMR and X-ray analyses. At higher temperatures, a carbonate Ru(II) complex Ru(CO 3)(CO) 2(PPh 3) 2 was also obtained in 85% yield from the reaction in presence of diphenylacetylene. A cationic Ru(II) species supplied by (CO+CO 3 2−) formed via a reductive disproportionation of CO 2 is proposed as an intermediate in an earlier stage of the coupling reactions of the alkyne molecules.

CO interaction with zeolites studied by TPD and FTIR: transition-metal ion-exchanged FAU-type zeolites

The interaction of carbon monoxide with the cationic Lewis acid sites of transition-metal (Fe 2+ , Co 2+ , Ni 2+ ) and Cu 2+ -exchanged Y and X zeolites was investigated by Fourier transform infrared spectroscopy (FTIR) and temperature-programmed desorption (TPD). The activation of the sample, the adsorption (at room temperature) of CO and the detection of desorbed molecular species were done in situ, either with FTIR or with a mass spectrometer, in gas-flow systems. During the adsorption of CO from the gas stream, a low surface coverage (between two and seven molecules per unit cell) was achieved. In the case of CuY and CuX zeolites, the TPD/MAS spectra exhibit only CO signals. During adsorption of CO, IR absorption bands in the 2200–2000 cm −1 spectral region, characteristic for symmetric stretching of CO adsorbed on zeolites, were collected in the FTIR spectra, indicating molecular adsorption of CO on Cu 2+ -exchanged samples. In the case of all other samples investigated, disproportionation of CO was noticed during its adsorption and temperature-programmed desorption. IR absorption bands near 2350 cm −1 , characteristic for ν 3 asymmetric stretching of weakly bound CO 2 on zeolites, and IR absorption bands in the 2200–2000 cm −1 spectral region were collected during adsorption of CO on Fe 2+ -, Co 2+ - and Ni 2+ -exchanged samples. In the TPD/MAS spectra both CO and CO 2 signals were monitored. Significant amounts of carbon deposits (between two and nine carbon atoms deposited per unit cell of the zeolite) were found after adsorption and TPD procedures.

Studies on Carbon Deposition in CO2Reforming of CH4over Nickel–Magnesia Solid Solution Catalysts

Carbon formation behavior under CH4-CO2reaction and through CH4decomposition and CO disproportionation was investigated over Ni0.03Mg0.97O solid solution, supported Ni/MgO, and NiO-Al2O3catalysts by means of thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). Ni0.03Mg0.97O showed high resistance to carbon formation in CO2reforming of methane and the selectivity to carbon formation was much lower than two other catalysts. It is suggested that CO2plays an important role in the inhibition of carbon formation on Ni0.03Mg0.97O through the activation of CO2at the interface between small nickel particles and the support surface.

Effects of Titania on the Adsorptive and Catalytic Properties of Silica‐Supported Cobalt Catalyst

AbstractA series of Ti‐promoted (6 wt%) Co/SiO2 catalysts with titania content of 0 to 10 wt% were sequentially prepared by incipient wetness impregnation, and characterized with X‐ray diffraction, thermogravimetric analysis, chemisorption, temperature‐programmed desorption and infrared spectroscopy. The influences of Ti addition and reduction temperature (400–700 °C) on the adsorptive behavior and the catalytic properties for CO hydrogenation were investigated. The presence of Ti decreases the adsorption capacity of the cobalt surface for H2, but enhances activity per gram cobalt. In addition, the turnover frequency increases 2–4 times upon Ti addition at reduction temperatures of 400–700 °C. The promotion in activity is accompanied by an enhanced selectivity for higher hydrocarbons and olefins. These modifications can be rationalized by the creation of active sites for CO dissociation. The desorption of CO2 at 100 °C during temperature‐programmed desorption of CO indicates the formation of active sites for CO disproportionation. Infrared spectroscopy indicates an increase in the relative absorbance of 2060–2075 cm−1 bands upon Ti addition, which are attributed to CO adsorbed on the defect sites of the cobalt surface. Therefore, the promotion effect of Ti may be directly related to the formation of defect sites on the cobalt surface induced by the decorated titania moieties.

Development of ultra-stable Ni catalysts for CO 2 reforming of methane

Carbon deposition behavior in CO 2 reforming of methane, methane decomposition, and CO disproportionation on nickel-magnesia solid solution was investigated by means of thermogravimetric analysis and temperature programmed reaction of deposited carbon with carbon dioxide. It was found that rapid oxidation of CH x on Ni surface by oxygen species from CO 2 through dissociation at metal-support interface is a key step for the inhibition of carbon formation.

Effect of metallic cobalt particles size on occurrence of CO disproportionation. Role of fluidized metallic cobalt-carbon solution in carbon nanotube formation

The behavior of Co−MgO catalysts in the reaction of CO disproportionation was studied in the temperature range up to 800°C. Two temperature regions that differ by the morphology of graphite produced (“egg-shells” or nanotubes) were detected. The experimental data obtained were analyzed in terms of the phase equilibria “amorphous carbon”—“carbon solution in metallic Co” and “graphite”— “carbon solution in metallic Co”. Under certain assumptions, the Co° particles are considered to dissolve amorphous carbon under the reaction conditions and transform to oversaturated fluidized carbon-cobalt solution, these fluidized particles being responsible for the formation of carbon nanotubes.

Carbon formation on nickel and nickel-copper alloy catalysts

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Carbon Structures Obtained By the Disproportionation of Carbon Monoxide over Nickel Catalysts

The disproportionation of CO over 18% Ni/MgO and 18% Ni/AlPO 4 catalysts has been studied at different temperatures. The nature of carbonaceous deposits over metal particles after the reaction has been examined by transmission electron microscopy. Carbon nanotubes are obtained by the disproportionation of CO over Ni/MgO at 700°C.

Effects of Ethanol Impregnation on the Properties of Thoria-Promoted Co/SiO2Catalyst

Two Th-added Co/SiO2catalysts were prepared by pore volume impregnation of 6 wt% Co3O4/SiO2with either aqueous or ethanol solutions. Smaller ThO2crystallites were obtained for the ethanol-prepared Th/Co3O4/SiO2. The reduction behavior of cobalt oxide was modified by aqueous impregnation. Furthermore, Th addition was found to be promotional in the case of ethanol impregnation, or detrimental in the case of aqueous impregnation, for the CO oxidation activity of Co3O4/SiO2. Th addition has no effect on the extent of reduction of cobalt phase, but (1) reduces the crystallite size of cobalt by hindering the sintering of cobalt metal; (2) increases the amount of hydrogen desorbed (150 or 190%), CO adsorbed (3 or 4 times), CO2evolved (2.6 or 4 times), and carbon deposited; (3) enhances the activity for CO disproportionation; (4) induces new CO adsorption modes, subcarbonyl-like species (Co(CO)x,x> 1) and multifold CO ((Co)xCO,x≥ 2); (5) decreases methanation activity; and (6) enhances the activity and the selectivity for higher hydrocarbons and olefins for Fischer–Tropsch synthesis. A roughening of the cobalt surface, which consists of steps and kinks induced by the decorated ThO2moieties around the interface area, is the proposed origin for the new adsorption and CO disproportionation sites. The more significant effects of Th addition when ethanol was employed as impregnation solvent instead of water is attributed to a more extensive Co–ThO2interface formed due to a better dispersed Th phase.

Partial oxidation of methane to syngas over Ni/MgO, Ni/CaO and Ni/CeO2

Partial oxidation of methane to syngas at atmospheric pressure and 750°C was examined over Ni/MgO, Ni/CaO and Ni/CeO2 catalysts with nickel loading of 13 wt%. All catalysts had similar high conversion of methane and high selectivity to syngas, which nearly approached the values predicted by thermodynamic equilibrium. However, only Ni/MgO showed high resistance to carbon deposition under thermodynamically severe conditions (CH4/O2 = 2.5, a higher CH4 to O2 ratio than the stoichiometric ratio). Its catalytic activity remained stable during 100 h of reaction, with no detectable carbon deposition. The oxidation of carbon deposited from pure CH4 decomposition and from pure CO disproportionation was investigated by in situ TPO-MS study which showed that both were effectively inhibited over Ni/MgO. In addition, the catalysts were characterized by TPR, XRD and XPS. It was revealed that the excellent performance of Ni/MgO resulted from the formation of an ideal solid solution between NiO and MgO.

Effects of Ethanol Impregnation on the Catalytic Properties of Silica‐Supported Cobalt Catalysts

AbstractSilca‐supported Co3O4 (6 wt% as Co) catalysts were prepared by pore volume impregnation of ethanol or aqueous cobalt nitrate solutions, and calcined in vacuo to 300 °C. The catalytic performances of these catalysts for oxidation and hydrogenation of CO were examined. All Co3O4/SiO2 catalysts were found to be very active in catalyzing oxidation of CO to CO2 as compared to a commercial 1 wt% Pt/Al2O3. The ethanol‐prepared catalysts exhibited higher activity than those of the aqua‐prepared catalysts. Pre‐calcination of the ethanol‐prepared catalysts in oxygen at 600 °C resulted in a dramatic decrease in the activity. Temperature programmed oxidation indicated the presence of carbon deposits on the surface of used catalysts. Infrared spectra showed the continuous generation of CO2 when these catalysts were exposed to CO. These indicate the primary role of CO disproportionation in catalytic oxidation of CO on Co3O4 at low temperature and explain the sharp decrease in activity in the initial period. After reduction at 400 °C, the ethanol‐prepared catalysts were also found to be more active in catalyzing hydrogenation of CO, and produced less methane and olefin (C2‐C4) fraction. Higher turnover frequencies were observed after high temperature reduction (600 °C) as well, at which ethoxyl groups were removed from silica surface. In both reactions, the enhanced activity for the ethanol‐prepared catalysts can not be fully accounted for by the increase in the dispersion of Co3O4 or CO metal. This suggests that the surface structures of Co3O4 or CO were further modified by the carbonaceous species derived from ethanol.

Kinetics of reverse water gas shift (RWGS) reaction on metal disulfide catalysts

The hydrogenation of CO 2 was studied on disulfides of molybdenum and tungsten by comparison with that on the group VIII metals of Fe, Co, and Ni supported on Al 2O 3. The hydrogenation activity was lower on the disulfides on a per active site of catalyst basis than on the transition metals, however, the selective reverse water gas shift (RWGS) reaction (CO 2 + H 2 → CO + H 2O) was observed on the disulfides. The selectivity was close to ca. 100% in the reaction conditions examined. The rate was expressed by the equation: r = kP 0.61–0.73 CO 2 P 0.36 H 2 on the disulfides, and the activation energy was ca. 18.0–19.1 kcal mol −1. The disulfides exhibited little activities for the hydrogenation of CO (viz. CO + 3H 2 → CH4 + H 2O) and also for the disproportionation of CO (viz. 2CO → C + CO 2). These results were in contrast to those for the transition metals where high activities were observed for both reactions, suggesting that the retardation of CO bond cleavage in the CO molecule produced by the CO 2 hydrogenation must be the cause for the selective hydrogenation of CO 2 to CO on the disulfides. The effect of supports on the hydrogenation of CO 2 was also investigated for MoS 2. The reaction rate on a per active site of catalyst basis was not influenced by the supports, suggesting that the CO 2H 2 reaction is insensitive to the morphology of MoS 2 supported on metal oxides.

Study by mössbauer spectroscopy and magnetization measurement of the evolution of iron catalysts used in the disproportionation of CO

Mössbauer spectroscopy and magnetization measurement have been used to investigate the chemical state of a supported iron catalyst during the disproportionation of carbon monoxide. It was observed that the Fe 2O 3 precursor was reduced to αFe metal during the initial pretreatment under H 2 and that this metal catalyzed the decomposition of CO. At 808 K and under the reaction conditions used, the catalyst was carburized directly into Fe 3C until no other iron phase could be identified. This carburization was correlated to the decrease of reaction rate and seemed to be the process of catalytic poisoning. Under experimental conditions, no other carbides were observed during reaction time.

Co and Co 2 hydrogenation under mechanochemical treatment

The effect of mechanochemical treatment on the hydrogenation of CO and CO 2 was studied using a flow milling vial as a catalytic reactor. Traditional catalysts for the methane formation (Ni, Zr, Zr-Ni-containing samples, Zr hydride and Zr-Ni hydrides of various genesis), as well as inactive NiO and ZrO 2, were examined. The following features of the mechanically driven CO hydrogenation were observed: (i) Zr and Zr-Ni hydrides demonstrated the highest activity in the formation of CH 4; (ii) the hydrogen present in the reaction mixture suppressed the formation of CH 4; (iii) Ni was found to be ineffective in the formation of CH 4 but active in the reaction of CO disproportionation; (iv) on the oxide samples, a small amount of CH 4 was formed owing to the mechanical activation. Deep structural transformations of the metal and hydride samples under milling in the reaction mixture were found to be responsible for the changes in their catalytic activity. Only zirconium hydride exhibited catalytic activity for CO 2 hydrogenation. In this case, no phase transformation was observed.

Synthesis of Benzene from Methane and Carbon Monoxide over Silica-Supported Rh, Ru and Pd Catalysts

Abstract Benzene was formed selectively among hydrocarbons produced in CH4-CO reaction over silica supported Rh, Ru and Pd catalysts at 573-723 K under atmospheric pressures, although CO2 was the main product, which was formed by the disproportionation of CO. Addition of H2 to CH4-CO reaction accelerated the rate of benzene formation several times, and the selectivity increased considerably.

Physico-chemical characteristics and CO oxidation studies over Pd/(Mn 2O 3 + SnO 2) catalyst

The mixed oxide catalyst (Mn 2O 3 + SnO 2) prepared by the coprecipitation method has been impregnated with Pd metal and it's catalytic behaviour for CO oxidation reaction has been investigated. In the coprecipitated material, Mn 2O 3 and SnO 2 were found to crystallise at 875 K and 1175 K, respectively, which are significantly higher than the crystallisation temperatures of individual oxides prepared under similar conditions. Results of catalytic oxidation of CO, carried out using the pulse method for the mixed oxide system and the individual oxides, suggest significant synergistic effects between these two oxides. The impregnation of palladium metal facilitated CO oxidation and the catalyst Pd/(Mn 2O 3 + SnO 2) was found to be quite effective for CO oxidation even at room temperature. Further, the CO disproportionation has been observed on palladium sites in the temperature range 350 to 400 K for the individual oxide systems.

Catalytic oxidation of carbon monoxide over stoichiometric and non-stoichiometric hydroxyapatites

Calcium hydroxyapatites [Ca10−z(PO4)6−z(HPO4)z(OH)2−z; (0 ⩽ z ⩽ 1)] catalyze the oxidation of CO at 600 °C. O2− radical oxygen species can be formed on the sample of approximately stoichiometric form (z = 0.10). It is believed that oxygen molecules are activated with O2− produced by the dehydration of the hydroxy groups in the structure. No radical oxygen species can be detected with the nonstoichiometric form (z = 0.34) on which the disproportionation of CO to CO2 and carbon takes place at 600 °C.

Comparative behaviour of extremely dispersed Pt/Mg(Al)O and Pt/Al 2O 3 for the chemisorption of hydrogen, CO and CO 2

A Pt/MgAl(O) was prepared from Pt(acac) 2 and a Mg Al mixed oxide, synthesized from a hydrotalcite precursor, which contains extremely small Pt particles with H/Pt≈ 1.1. Its properties were compared to that of a Pt/Al 2O 3 of a similar Pt dispersion and prepared according to exactly the same protocol. The properties of Pt were probed by H 2 and CO chemisorption followed by IR spectroscopy and microcalorimetry, and those of basic sites of Mg(Al)O by TPD of CO 2. The surface basicity of Mg(Al)O decreases after deposition of Pt particles. When compared to Pt/Al 2O 3 a new IR absorption band of CO appears at 1930–1960 cm −1 which was tentatively assigned to CO species bridge-bonded on Pt through the carbon atom and with Mg 2+ cations by the oxygen atom. These species are most stable upon desorption at high temperature. The disproportionation of CO to adsorbed C and CO 2 was occurring on Pt/Mg(Al)O. The CO 2 formed migrated to basic sites of the support to form carbonates species.

Carbon dioxide reforming of methane to synthesis gas over Ni/La 2O 3 catalysts

Carbon dioxide reforming of methane to synthesis gas was studied employing Ni/La 2O 3 catalysts. It was found that, in contrast to the performance of other nickel-based catalysts (e.g. Ni/Al 2O 3 and Ni/CaO) which exhibit continuous deactivation with time on stream, the rate over the Ni/La 2O 3 catalyst increases during the initial 2–5 h of reaction and then tends to be essentially invariable with time on stream, displaying very good stability. X-ray diffraction (XRD) studies reveal that a large CO 2 pool, stored in the form of La 2O 2CO 3, is accumulating on the Ni/La 2O 3 catalyst, following exposure to the CH 4/CO 2 mixture at reaction conditions. Results of H 2- and CO-temperature-programmed desorption reveal that the H Ni bond is weakened and CO disproportionation is unfavoured on the Ni/La 2O 3 catalyst, as compared to the Ni/Al 2O 3 catalyst. Comparison of H 2 and CO uptake and Ni dispersion by XRD shows that H 2 and CO uptakes are significantly suppressed, by ca. 3–10 times, suggesting that a large portion of the Ni surface is blocked by lanthanum species. It is proposed that the interaction between Ni crystallites and La 2O 3 support or La species which are decorating the Ni crystallites is responsible for the unusual chemisorptive and catalytic behaviour observed over the Ni/La 2O 3 catalyst.