CO Disproportionation Research Articles

Reactivity of Pt/Al 2O 3 and Pt-CeO 2Al 2O 3 Catalysts for the Oxidation of Carbon Monoxide by Oxygen : I. Catalyst Characterization by TPR Using CO as Reducing Agent

Ceria, platinum, and platinum-ceria catalysts supported on γ-alumina have been characterized by temperature-programmed reduction (TPR) using CO as reducing agent. In all cases, CO disproportionation was found to be negligible. Nevertheless, a small CO 2 TPR signal was detected on the γ-alumina support at tempratures higher than 673 K, and has been attributed to the water-gas shift reaction between gaseous CO and the hydroxyl groups of the alumina surface. The comparison between the CO-TPR spectra of CeO 2/Al 2O 3 and Pt-CeO 2/Al 2O 3 has revealed that platinum induces a downscale shift on the temperature of the ceria reduction peak. Two possible interpretations of this platinum-ceria interaction are proposed: (i) a CO bond weakening when CO is adsorbed on platinum particles and (ii) a decrease of the CeO bond strength for ceria localized near platinum.

Magnesia-Supported Nickel Catalysts: II. Surface Properties and Reactivity in Methane Steam Reforming

Surface properties and reactivity of Ni/MgO catalysts, air-calcined at temperatures ( T C) in the range 400-800°C, have been evaluated by TEM, IR spectroscopy of adsorbed CO, and catalytic measurements in methane steam reforming. The effects of calcination and reduction temperature on the Ni particle size distribution (PSD) provide evidence of the singular structure properties of the Ni/MgO system. The "broad" PSD of samples calcined at T C ≤ 600°C well accounts for the structure model previously described (Arena, F., Horrell, B. A., Cocke. D. L., Parmaliana. A., and Giordano. N., J. Catal. 132, 58 (1991)). IR spectroscopy shows both the formation of mono- and polycarbonyls and the occurrence of the CO disproportionation reaction (2 CO ⇄ CO 2 + C). The higher reactivity towards CO of more dispersed systems has been attributed to the "defective nature" of small Ni crystallites. Catalytic measurements ( T R = 625°C: GHSV = 150,000 h −1) reveal the superior activity of the system calcined at T C = 400°C, notwithstanding that a comparable stability for the systems calcined up to 600°C has been experienced. At higher T C (>600°C) the catalytic pattern, both in terms of activity and resistance to coking, is negatively affected by the formation of the "bulk" NiO-MgO solid solution. The peculiar influence of the mean Ni particle size (20 ≤ d S ≤ 1400 Å) on the turnover frequency (TOF, s −1). resulting in a volcano-shape relationship with the maximum centred at 90-130 Å, has been explained by invoking a structure-sensitive character of the methane stream-reforming reaction on highly dispersed Ni/MgO catalysts.

CO disproportionation on Ni-based catalysts

Ni/SiO2 exhibits higher activity for CO dissociation than coprecipitated Na−Mn−Ni, which is a higher oxygenate synthesis catalyst at 303 K, while Na−Mn−Ni exhibited higher CO dissociation activities at 673 K.

Aktivierung von CO 2 an übergangsmetallzentren: Zum ablauf der homogen-katalytischen Bildung von 2-Pyron aus Kohlendioxid und Hex-3-in an Nickel(0)-Fragmenten

IR-investigations of the catalytic formation of tetraethyl-2-pyrone from hex-3-yne and CO 2 at (TMED)Ni 0- or (COD)Ni 0 centres show that in the first step of this reaction the nickel(0) complex reacts under oxidative coupling with both CO 2 and alkyne to afford the five-membered metallacycle A. The alternative route, the oxidative coupling of two alkynes at nickel(0), can be excluded. Complex A reacts in a second step with a further molecule alkyne to yield compound B, which undergoes a reductive elimination forming the 2-pyrone C. The insertion of CO 2 into A and the “reductive disproportionation of CO 2 at (TMED)Ni 0 are deactivation reactions of the catalytic cycle forming the trimeric carbonato complex D and the Ni 0 compound E bearing diethylmaleic anhydride. The structure of complex A was determined by X-ray analysis.

CO hydrogenation on Mo-promoted Rh/SiO 2 and Rh/ZrO 2 derived from metal carbonyl clusters

Support and promoter effects on molybdenum-containing Rh/SiO 2 and Rh/ZrO 2 catalysts, derived from the carbonyl precursors Rh 4(CO) 12 and Mo(CO) 6, have been studied in carbon monoxide hydrogenation at 1.1 MPa. Mo increases the total activity of the catalysts. ZrO 2 favours the formation of oxygenates, particularly ethanol. In situ FT-IR spectroscopic experiments, temperature-programmed reaction of adsorbed CO, H 2 and CO chemisorption, CO disproportionation as well as ethylene hydroformylation have been used to characterize the catalysts. Whereas the activity-promoting effect of Mo has been found to be due to the specific promotion of CO dissociation by Mo, the effect of ZrO 2 on the selectivity can be related to an increased CO insertion ability on zirconia-supported catalysts owing to the presence of activated surface carbonyl complexes, such as [Display omitted] under reaction conditions.

Methane formation in H 2,CO mixtures over carbon-supported potassium carbonate

The rates of CH 4 and CO 2 formation over K 2CO 3/carbon have been studied in H 2,CO mixtures as a function of temperature (600–1000 K), total pressure (1.5–10 bar), H 2/CO feed ratio (0.3–8), and the K 2CO 3 loading (0–20 wt%). In H 2,CO mixtures both the rate of CH 4 and CO 2 formation are enhanced by the presence of the potassium catalyst and show the same dependence on the alkali loading as that observed for gasification of carbon in H 2O and CO 2. Below 900 K the CH 4 formation has an apparent order of ≌1.2 in p H 2 and ≌0.3 in p co. Under most experimental conditions the rate of CO 2 formation ( E a(app) = 40–50 kJ mol −1) is higher than that of CH 4 formation ( E a(app) = 130–150 kJ mol −1), resulting in carbon deposition. Only at pressures above 5 bar can CO be selectively converted with H 2 into CH 4 and CO 2 above 900 K. Hydrogenation of the support and of the carbon deposited by the CO disproportionation is not catalysed by potassium. A reactive carbon intermediate is proposed, formed by catalysed dissociation of CO. This can either react with hydrogen to form methane or form the carbon deposit. The oxygen is removed by CO in a manner similar to that in potassium-catalysed oxygen exchange reactions. The observed deactivation is ascribed to a combined effect of blocking of the active sites by carbon deposition and migration of metallic potassium into the carbon matrix, leading to H 2 adsorption on the liberated carbon edge sites, as revealed by temperature-programmed desorption. Potassium is hardly lost at all during the methanation experiments.

In situ perpetual regeneration of a Ni-faujasite methanation catalyst

A prolonged lifetime of a Ni-faujasite methanation catalyst and a stable rate of methanation can be achieved by operating the fluidized catalyst bed under unsteady state conditions. A higher rate of methanation is accompanied by a shift in selectivity compared to a steady state operation due to an enhanced disproportionation of CO.

Size effect in the CO chemisorption on palladium clusters supported on magnesium oxide

In a previous paper [C. Duriez et al., Surf. Sci. 253 (1991) 190], molecular beam measurements were made of the adsorption-desorption kinetics of CO on Pd clusters, epitaxially grown on MgO(100). A large increase (∼8 kcal/mol) in the adsorption energy for clusters smaller than 3 nm grown on clean air-cleaved MgO was observed relative to clusters larger than 5 nm grown on UHV-cleaved MgO. New experimental measuremnts have shown that clusters smaller than 5 nm, grown on UHV-cleaved MgO or mica, give the same enhancement of reactivity as on a clean air-cleaved MgO surface, ruling out a possible substrate effect. The origin of this size effect is explained by the presence of a strong adsorption energy on some sites, the proportion of which increasing when the size of the clusters is decreased. Checks for carbon contamination by CO dissociation or CO disproportionation were negative.

Disproportionation of Carbon Monoxide on the Surface of Titanium and Vanadium Clusters

Abstract In the framework of CNDO (Complete Neglect of Differential Overlap method) VOIP (Valance Orbital Ionization Potential) has been evaluated using the method of Anno and Sakai and for exchange integrals Wolfsberg–Helmholtz approximation has been used. The method has been used for studying the disproportiontion of carbon monoxide on titanium and vanadium clusters having two atoms respectively. A large number of theoretical models were designed, based upon various mechanistic possibilities. The bond energies between different atoms have been plotted against the distance, for the purpose of studying the formation and desorption of CO2. The present studies suggest that the disproportionation of CO on titanium cluster may follow a dissociative path while in case of vanadium cluster this reaction may follow a disproportionation, where disproportionation involves a reaction between a ‘bridge site’ adsorbed CO molecule with a gas phase CO molecule.

A TPD and XPS investigation of palladium on modified alumina supports used for the catalytic decomposition of methanol

A series of modified palladium catalysts on alumina supports previously have been prepared and evaluated for selectivity, activity, and thermal stability in decomposing methanol into CO and H 2. In this investigation the modified palladium catalysts are characterized by X ray photoelectron spectroscopy (XPS) and CO and NH 3 temperature-programmed desorption (TPD). Average particle sizes of the palladium crystallites were also determined using chemisorption techniques. The XPS results indicate that the lithium-modified catalyst, which deactivated substantially during methanol decomposition testing, exhibited a significant build-up of carbon on the catalyst surface, while a similar accumulation was not observed for the lanthanum-modified catalyst under identical conditions. The CO TPD results show that the lithium-, sodium-, and barium-modified catalysts, which deactivate during methanol decomposition testing, produce a much greater amount of CO 2 relative to the amount of CO desorbed than do the potassium-, rubidium-, cesium-, and lanthanum-modified and the unmodified catalysts, which do not deactivate considerably under identical conditions. Ammonia TPD and chemisorption results indicate that no correlation exists between support acidity or particle size and the degree of CO dissociation. Finally, a correlation between the charge density of the modifier and the degree of CO disproportionation is presented which fits for all cases except one. This appears to support an electrostatic model for the promotion of CO bond cleavage by the alkali modifier.

The interaction of model cobalt catalysts with carbon

The interaction of discontinuous cobalt films with carbon at high temperature was investigated by transmission electron microscopy and electron diffraction. The experiments were performed with the carbon deposited during the disproportionation of CO at 653 K on cobalt films supported on SiO 2. The behavior of a Co SiO 2 + C system was compared with cobalt films supported on amorphous carbon. It was established that cobalt catalyzes graphitization of amorphous carbon above 773 K and that the conversion of amorphous to graphitic carbon proceeds through an intermediate state in which dissolution of carbon in cobalt takes place and carbide is formed. The formation Co 2C was observed after thermal treatment of the Co C and Co SiO 2 + C systems at 773 K.

Structure sensitive reactions over supported ruthenium catalysts during Fischer-Tropsch synthesis

Highly dispersed ruthenium catalysts can be prepared on alumina by aqueous impregnation of ruthenium. EXAFS at the K-edge showed that this type of catalyst, after calcination and reduction, consisted of ruthenium particles, which were about 0.8 nm in size. When highly dispersed on alumina, ruthenium appears to catalyze the water-gas shift reaction, which occurs subsequent to Fischer-Tropsch synthesis. The hydrocarbons produced had low olefinicity, possibly because ofin situ production of hydrogen via the water-gas shift reaction. Highly dispersed ruthenium was not stable on alumina during Fischer-Tropsch synthesis. The ruthenium agglomeration on the alumina surface, as well as overall ruthenium loss from the catalyst, was attributed to the formation of a volatile ruthenium carbonyl species. Catalysts with about 85% of the ruthenium in the form of 3–7 nm particles were prepared on alumina by reverse micelle impregnation of ruthenium. These larger particles were stable against ruthenium carbonyl formation and, therefore, did not exhibit ruthenium agglomeration or loss of ruthenium. Catalysts with 3–7 nm ruthenium particles displayed a higher turnover number for hydrocarbon synthesis, higher olefinicity, and chain-growth probability and did not exhibit water-gas shift activity in contrast to ruthenium particles which were about 0.8 nm in size. The CO disproportionation measurements showed much less CO dissociation over highly dispersed ruthenium relative to 3–7 nm ruthenium particles. This phenomenon is consistent with the low activity, the low chain-growth probability and may also relate to the tendency to form ruthenium carbonyl that is observed with small ruthenium particles. The apparent water-gas shift activity of highly dispersed ruthenium can be explained by the low CO dissociation efficiency as well as by the proposed ability to dissociate the water molecule.

CO disproportionation at mild temperatures over partially reduced cerium oxide

Direct dissociation and disproportionation of carbon monoxide over partially reduced cerium oxide are demonstrated, even at room temperature, by Fourier-transform IR spectroscopy.

CO disproportionation over supported Pd particles: a TPD and static SIMS study

Temperature programmed desorption (TPD) has been used to monitor the decay in CO adsorption capacity during cycles of CO adsorption/desorption over small palladium particles deposited on both MgO and alumina substrates. It is found that CO desorption is accompanied by CO 2 production as a result of CO disproportionation on the Pd surface. The decay in CO adsorption capacity is caused by a strong inhibition of CO adsorption by surface carbon which results from the CO disproportionation. The formation of surface carbon is accompanied by the appearance of Pd n C + ions in the static secondary ion mass spectroscopy (SSIMS) signal. We show, by combining TPD and SSIMS measurements, that the surface carbon coverage can be monitored using the ion intensity ratio Pd n C + Pd + n . The ability to monitor the carbon coverage in such a way makes it possible to investigate t kinetics of surface carbon formation while heating the particles in CO atmosphere. It is shown that the rate of surface carbon formation increases with both surface temperature and the CO coverage. The CO disproportionation rate depends on the particle size. The smaller the particle size, the higher the carbon formation rate.

Water-gas shift reaction over chromia-promoted magnetite. Use of temperature-programmed desorption and chemical trapping in the study of the reaction mechanism

The mechanism of WGSR has been studied on an Fe−Cr catalyst. Although after reaction (from 250 to 400°C) formate species have been characterized on the Fe−Cr catalyst, by chemical trapping, TPD studies after reaction and after HCOOH adsorption have shown that formates are not the main intermediates. The disproportionation of CO or the regenerative mechanism seems to be more probable.

The mechanism of carbon deposition during the breakaway oxidation of steels in carbon dioxide

Breakaway oxidation is a rapid form of oxidation which occurs in low Cr steels in high pressure CO 2 atmospheres. It is attributed to the deposition of carbon within the growing oxide scale which makes the oxide porous and thereby allows rapid oxidation. The carbon deposition occurs by the disproportionation of CO, a byproduct of the oxidation, according to the Boudouard reaction. Breakaway is favoured by the presence of water or hydrogen in the gas stream. These observations are shown to be consistent with catalytic behaviour found in direct studies of carbon deposition.

Temperature-programmed hydrogenation of surface carbonaceous deposits on a Ni/SiO2 methanation catalyst

Hydrogenation of surface carbonaceous deposits from CO disproportionation or methanation on a high-weight loading commercial Ni/SiO2 catalyst was investigated by temperature-programmed surface reaction (TPSR). Two types of surface carbon (Cα and Cβ)were hydrogenated following the CO disproportionation. Adsorbed carbon monoxide was probably hydrogenated after CO methanation. Hydrogenation of Cα proceeded substantially faster than hydrogenation of Cβ and faster than hydrogenation of preadsorbed CO. Significantly lower activation energy was estimated for hydrogenation of Cα than for hydrogenation of CO (50 vs 90 kJ/mol). An approach for analysis of the data from a temperature-programmed experiment is given.

Thermogravimetric reduction of lanthanum strontium iron perovskite oxides with carbon monoxide

The reduction behaviour from surface to bulk of lanthanum strontium iron perovskite oxide (La 1− x Sr x FeO 3 :0 ⩽ x ⩽ 1.0) particles by CO gas was studied by means of TG, DSC, IR, XRD and X-ray micro-analysis. Interactions with CO and oxygen species that proceed exothermally gave a large weight loss and weight increase respectively. Both increased with the increase of x in La 1− x Sr x FeO 3. A weight loss was observed in the temperature range 525–725 K as a result of the desorption and reduction by CO of surface oxygen species. A weight increase was observed in the temperature range 775–1125 K as a result of the formation of carbon and/or carbonaceous species. It was concluded that the disproportionation of CO occurs at higher x and higher temperature.

Studies on the reactivity of supported rhodium and molybdenum carbonyl complexes using temperature-programmed techniques

The reactivity of zirconia-supported RH 4(CO) 12 and Mo(CO) 6 has been studied by temperature-programmed desorption (TPD) into vacuum and temperature-programmed reaction (TPR). Thermal activation of the carbonyl precursors results in a complex reaction system including CO disproportionation, water gas shift reaction and oxidative interaction of Rh o species with hydroxyl groups on the support. The surface species formed after chemisorption of CO on the reduced samples exhibit different reactivity compared with the original metal carbonyls. K 2O added to ZrO 2 promotes the shift reaction and selectivity to methanol but suppresses CO hydrogenation; Mo(CO) 6 co-adsorbed with Rh 4(CO) 12 brings on formation of surface sites exhibiting weak adsorption properties towards CO and high hydrogenation activity. TPR experiments in flow of H 2-CO mixture show that above 500 K the thermodynamically favoured products CH 4 and CO 2 rapidly become predominant; however, with RhMo/ZrO 2, a good selectivity to oxygenated products can be obtained in a restricted temperature range by operating at high CO pressure.

The kinetics of CO dissociation on Ru(001): Time-resolved vibrational spectroscopy at elevated pressures

Time-resolved Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) has been utilized to measure the kinetics of CO dissociation on a Ru(001) surface at elevated pressures (10−3 to 10 Torr) and temperatures (500–700 K). The reaction of CO with Ru(001) is found to be a nonsteady state and results in CO disproportionation, i.e., 2CO→C+CO2. The decrease in total CO coverage follows first order kinetics and exhibits Arrhenius behavior with an activation energy of 20.6 kcal and a preexponential factor of 102 s−1. Comparison of the overall reaction rate with that of CO2 formation (O+CO→CO2) confirms that CO dissociation is the rate-limiting step in the disproportionation reaction. The in situ reaction rate constant exhibits a weak dependence on CO pressure (<first order). However, the determination of local CO coverages during reaction reveals a linear dependence of the dissociation rate with CO coverage. This confirms that the chemisorbed state of the molecule is a precursor to dissociation and that a high pressure is required to maintain a steady state surface coverage of CO at reaction temperature. In situ vibrational spectra demonstrate the formation of carbon islands under reaction conditions which prevent further CO adsorption and result in a decrease in total CO coverage at constant local CO coverage. Post-reaction spectroscopy confirms the formation of two-dimensional islands of carbon whose reactivity toward oxidation is found to be between that of amorphous carbon and three-dimensional graphite.