Low temperature CO oxidation over Au/TiO2 and Au/SiO2 catalysts

After a high-temperature reduction (HTR) at 773 K, TiO₂-supported Au became very active for CO oxidation at 313 K and was an order of magnitude more active than SiO₂-supported Au, whereas a low-temperature reduction (LTR) at 473 K produced a Au/TiO₂ catalyst with very low activity. A HTR step followed by calcination at 673 K and a LTR step gave the most active Au/TiO₂ catalyst of all, which was 100-fold more active at 313 K than a typical 2% Pd/Al₂O₃ catalyst and was stable above 400 K whereas a sharp decrease in activity occurred with the other Au/TiO₂ (HTR) sample. With a feed of 5% CO, 5% O₂ in He, almost 40% of the CO was converted at 313 K and essentially all the CO was oxidized at 413 K over the best Au/TiO₂ catalyst at a space velocity of 333 h^–1 based on CO + O₂. Half the chloride in the Au precursor was retained in the Au/TiO₂ (LTR) sample whereas only 16% was retained in the other three catalysts; this may be one reason for the low activity of the Au/TiO₂ (LTR) sample. The reaction order on O₂ was approximately 0.4 between 310 and 360 K, while that on CO varied from 0.2 to 0.6. The chemistry associated with this high activity is not yet known but is presently attributed to a synergistic interaction between gold and titania.     

High catalytic activity of PdOx/MnO2 for CO oxidation and importance of oxidation state of Mn

PdO_x/MnO₂ has been examined as a catalyst for CO oxidation using a conventional flow reactor at reaction temperatures between 50 and 150°C. In the reaction conditions of GHSV (gashourlyspacevelocity) of 1.22 × 10⁵/h and CO concentration of 2000 ppm, PdO_x/MnO₂ showed higher catalytic activity compared with PdO_x/Mn₂O₃, which had been previously reported as an effective catalyst due to the cooperative action of Pd and Mn₂O₃ for this reaction. The reason for higher activity of PdO_x/MnO₂ than PdO_x/Mn₂O₃ has been investigated using TPR (temperatureprogrammed reduction) and XPS studies. TPR showed that PdO_x/MnO₂ could be reduced by CO at much lower temperature than PdO_x/Mn₂O₃. During the experiment of reduction and oxidation, XPS showed that the valence of Mn in the PdO_x/MnO₂ was between 4+ and 3+, which is higher than that of Mn in the PdO_x/Mn₂O₃ catalyst of which the valence has been reported to be between 3+ and 2+. It is known that in this catalyst system the support supplies oxygen onto Pd, where the oxidation occurs with adsorbed CO, and the ability of the support to provide oxygen improves the performance of the catalyst. Therefore, it was concluded that the readiness of MnO₂ to be reduced with maintaining a higher oxidation state showed higher CO oxidation activity than Mn₂O₃ as support for PdO_x.     

A kinetic and DRIFTS study of low-temperature carbon monoxide oxidation over Au-TiO2 catalysts

Titania-supported gold catalysts are extremely active for room temperature CO oxidation; however, deactivation is observed over long periods of time under our reaction conditions Impregnated AuTiO₂ is most active after a sequential pretreatment consisting of high temperature reduction at 773 K, calcination at 673 K and low temperature reduction at 473 K (HTR/C/LTR); the activity after either only low temperature reduction or calcination is much lower. A catalyst prepared by coprecipitation had much smaller Au particles than impregnated AuTiO₂ and was active at 273 K after either an HTR/C/LTR or a calcination pretreatment. Deposition of TiO_x overlayers onto an inactive Au powder produced high activity; this argues against an electronic effect in small Au particles as the major factor contributing to the activity of AuTiO₂ catalysts and argues for the formation of active sites at the AuTiO_x interface produced by the mobility of TiO_x species. DRIFTS (diffuse reflectance FTIR) spectra of impregnated AuTiO₂ reveal the presence of weak reversible CO adsorption on the Au surface but not on the TiO₂; however, a band for adsorbed CO is observed on the pure TiO₂. Kinetic studies with a 1.0 wt.-% impregnated AuTiO₂ sample showed a near half-order rate dependence on CO and a near zero-order rate dependence on O₂ between 273 and 313 K with an activation energy near 7 kcal/mol. A two-site model, with CO adsorbing on Au and O₂ adsorbing on TiO₂, is consistent with Langmuir-Hinselwood kinetics for noncompetitive adsorption, fits partial pressure data well and shows consistent enthalpies and entropies of adsorption. The formation of carbonate and car☐ylate species on the titania surface was detected but it appears that these are spectator species. DRIFTS experiments under reaction conditions also show the presence of weak, reversible adsorption of CO₂ (near 2340 cm⁻¹) which may be competing with CO for adsorption sites.     

XPS evidence for molybdenum nitride formation in ZSM-5

Microwave plasma-assisted catalytic reduction of SO₂ by CO was studied over four catalysts. The activities of the four catalysts under microwave plasma decreased in the order of CoO/γ-Al₂O₃>>SnO₂> copper wires > iron wires, which was consistent with the results under conventional heating. By comparing the activity of CoO/γ-Al₂O₃ catalyst in the microwave plasma mode with that in the conventional mode, it is demonstrated that the temperature at which the full SO₂ conversion was obtained in the microwave plasma mode was about 200 °C lower than that under the conventional heating mode. Moreover, an increase of space velocity had little effect on SO₂ conversion and sulfur selectivity under microwave plasma; while under conventional heating mode, both SO₂ conversion and sulfur selectivity significantly decreased with an increase of space velocity.     

Isoprene formation from CO and H2 over CeO2 catalysts

The CO-H₂ reaction over CeO₂ catalysts at around 623 K and 67 kPa forms isoprene with about 20% and 70% selectivities in total and C₅ hydrocarbons, respectively. The formation of dienes may be due to the low and high activity of CeO₂ for alkene and CO hydrogenation, respectively.     

Evidence of a metal-support interaction in sol-gel derived Cu-ZrO2 catalysts for CO hydrogenation

Sol-gel derived Cu/ZrO₂ catalysts have recently been shown to have high activity and selectivity toward methanol synthesis. TPR, TEM, in situ XRD and N₂O decomposition have now been used to characterize the active sites in such catalysts over a wide range of Cu concentration. Copper is shown to be in two forms: surface aggregates (or particulate) and dispersed copper in the ZrO₂ substitutional sites. The proportion of the former increases with an increasing Cu content, while the overall strength of the Cu-ZrO₂ interaction simultaneously decreases. The activity in CO/CO₂ hydrogenation showed no evident correlation with the total Cu surface area, but rather with the concentration of highly-dispersed form of copper. This is taken to indicate that the copper in the substitutional sites of ZrO₂ is predominantly responsible for and associated with the active sites on Cu/ZrO₂ for CO/CO₂ hydrogenation.     

Uptake and apparent utilization of urea and ammonium nitrate in wheat seedlings

Intact wheat (Triticum aestivum cv. Quern) seedlings that were grown in presence or absence of NH₄NO₃ were exposed to solutions containing CO(NH₂)₂, NH₄NO₃, CO(NH₂)₂ + NH₄NO₃, CO(NH₂)₂ + KNO₃ and CO(NH₂)₂ + (NH₄)₂SO₄ for consecutive periods of 3, 3, 6, 12 and 24h and N uptake determined by solution depletion measurements. Differences in ethanol-soluble N and ethanol-insoluble N content of roots and shoots of control (zero time) seedlings and seedlings exposed to CO(NH₂)₂, NH₄NO₃ and CO(NH₂)₂ + NH₄NO₃ for 48 h were used to characterize N utilization during/following uptake.Regardless of initial N status, uptake of N from CO(NH₂)₂ was less than one-third of that from NH₄NO₃. Relative absorption of the CO(NH₂)₂ and NH₄NO₃ was not substantially altered by acidity control of the uptake solutions. There was a reciprocal antagonism between uptake of CO(NH₂)₂ and uptake of NH₄NO₃. Whereas CO(NH₂)₂ inhibited NH₄ absorption in each set of seedlings, it decreased NO₃ uptake only in seedlings that had been pretreated with N. Simultaneous presence of KNO₃ enhanced CO(NH₂)₂ uptake but presence of (NH₄)₂SO₄ decreased it to the same extent as NH₄NO₃. All absorption processes involving CO(NH₂)₂ and NH₄ were substantially restricted by pretreatment of the seedlings with NH₄NO₃. The results suggest that apparent utilization of ambient N was dependent on initial N status of the seedlings and on the nature of the N species to which they were exposed.     

Unique adsorption and catalytic behavior of H2 and CO over hollow Silica-Rh, -Ir, and -Ru nanocomposites prepared by Reversed Micelle Technique

Hollow silica nano spheres containing Rh, Ir or Ru metal particles were synthesized by Rh(NH₃)₆Cl₃ aq, Ir(NH₃)₃Cl₃ aq or Ru(NH₃)₆Cl₃ aq/NP-6/cyclohexane reversed micelle system. Hydrolysis of TEOS surrounding metal ammine complex crystals inside the micelle caused the formation of the hollow, which contained small metal particles inside and tiny metal clusters in the silica network. The amounts of H₂ adsorption over Rh and Ir nanocomposites were two to three times more in the cases of hollow-SiO₂ catalysts compared with those of non-hollow ones, suggesting the occlusion of hydrogen inside the hollows of Rh–SiO₂ or Ir–SiO₂. CO molecules could also permeate into the silica wall and be adsorbed on the metal clusters in the silica wall after 573 K pretreatment. Especially in the case of Ru nanocomposite the amount of adsorbed CO was much more than that of H₂, suggesting some unique character of Ru metal nanoparticles. After 773 K pretreatment, however, the amount of CO(a) decreased drastically to less than 1/10 of H(a), indicating the densification of Si–O–Si bonds and the formation of ultra-micropores in the silica wall where only H₂ can selectively permeate. Selective formation of methane was observed in the CO–H₂ reaction over these nanocomposite catalysts, provably because of the higher concentration of hydrogen inside the hollow and silica network.     

Reduction of NO by H2 and CO on PdO-MoO3/γ-Al2O3 of low molybdena loading

The activity and selectivity in the catalytic reduction of NO by a mixture of CO and H₂ of three PdO-MoO₃/-Al₂O₃ catalysts are compared in the presence of varying amounts of oxygen at reaction temperatures from 100 to 550°C. The catalysts were prepared by different methods and contain about 2% Mo and 2% Pd. Results are compared with those for PdO/-Al₂O₃, PdO-MoO₃/-Al₂O₃ containing 2% Pd and 20% Mo, and a commercial Pt-Rh catalyst. The PdO-MoO₃/-Al₂O₃ catalysts are more active for the selective reduction of NO to N₂ and N₂O than PdO/-Al₂O₃ under slightly oxidizing conditions at temperatures from 300 to 550°C. At these reaction conditions, the fresh PdO-MoO₃/-Al₂O₃ catalysts are comparable with a commercial Pt-Rh catalyst. The improved activity of PdO-MoO₃/-Al₂O₃ relative to PdO/-Al₂O₃ is believed to be due to the interaction between Pd and Mo. The effect of O₂ on the activity and selectivity of these catalysts is different in the reduction of NO by H₂, by CO, and by a mixture of H₂ and CO. The results using the mixture of reductants cannot be inferred from the results with the single reductants.     

Adsorption and reactivity during low temperature oxidation of carbon monoxide on Au/TiO2 catalysts studied by rapid response mass spectrometry

The adsorption and reaction of CO, CO₂ and O₂ on TiO₂ and Au/TiO₂ have been studied using a mass spectrometric method which can detect processes occurring on a time scale of seconds. Adsorption of CO on TiO₂ at 300 K is rapidly reversible and less on reduced samples than oxidised ones indicating that the adsorption sites are oxide ions. The amount adsorbed reversibly on reduced Au/TiO₂ is less still, consistent with enhanced reduction, but additional amounts adsorb irreversibly at a slower rate. The amount of CO₂ adsorbed under similar conditions is also greater on TiO₂ than reduced Au/TiO₂ and approximately one order of magnitude greater than that of CO. However, adsorption of O₂ is undetectable on the time scale of the measurement. Exposure of Au/TiO₂ to mixtures of CO and O₂ results in near instantaneous generation of CO₂ although its appearance is attenuated by adsorption. Adsorption of CO occurs concurrently in a way similar to that seen with CO alone except that the amount of the more slowly adsorbed form seems less. This suggests that it is the form utilised in catalysis. Oxygen uptake beyond that generating CO₂ is appreciable during the initial stages of exposure to reaction mixtures and this capacity is enhanced if one or other reactant is removed and then reintroduced, possibly due to the generation of reducible interface sites. It is concluded that the remarkable activity of Au/TiO₂ for CO oxidation at ambient temperature resides in a very high turnover frequency on sites at the interface between the metal and oxide. This revised version was published online in June 2006 with corrections to the Cover Date.     

Oxidation state of oxide supported nanometric gold

We address the structure gap between surface science and catalysis studies of the activity of oxide supported Au clusters. Reviewing the recent literature we find that surface science investigations often deal with highly reduced systems that have anionic Au clusters and oxygen vacancies in the support. The catalysis studies on the other hand consistently report on oxidized samples with traces of cationic Au. Performing density functional theory calculations we show that the effect of oxidation of oxide supported Au clusters, Au₈/MgO, Au₇/TiO₂ and Au₁₀/TiO₂, is a strong increase in the Au/support adhesion energy and a great structural transformation of the clusters. Some of the Au atoms become positively charged (cationic) in the oxidation process as evidenced indirectly by calculated vibrational stretch frequency shifts of adsorbed CO.     

Probing the bimetallic RhCo3 cluster preserved on SiO2 after thermal treatment under O2 by hydroformylation

RhCo₃(CO)₁₂/SiO₂, after decarbonylation under atmospheric O₂ at 623 K, exhibits excellent catalytic performances in atmospheric ethylene hydroformylation at 423 K, which is consistent with the corresponding catalysis by the bimetallic cluster catalyst RhCo₃/SiO₂.     

C,CO and CO2 hydrogenation on cobalt foil model catalysts: evidence for the need of CoO reduction

The hydrogenation of C, CO, and CO₂ has been studied on polycrystalline cobalt foils using a combination of UHV studies and atmospheric pressure reactions in temperature range from 475 to 575 K at 101 kPa total pressure. The reactions produce mainly methane but with selectivities of 98, 80, and 99 wt% at 525 K for C, CO, and CO₂, respectively. In the C and CO₂ hydrogenation the rest is ethane, whereas in CO hydrogenation hydrocarbons up to C₄ were detected. The activation energies of methane formation are 57, 86, and 158 kJ/mol from C, CO, and CO₂, respectively. The partial pressure dependencies of the CO and CO₂ hydrogenation indicate roughly first order dependence on hydrogen pressure (1.5 and 0.9), negative first order on CO (–0.75) and zero order on CO₂ (–0.05). Post reaction spectroscopy revealed carbon deposition from CO and oxygen deposition from CO₂ on the surface above 540 K. The reduction of cobalt oxide formed after dissociation of C-O bonds on the surface is proposed to be the rate limiting step in CO and CO₂ hydrogenation.     

Copper oxide catalysts supported on ceria for low-temperature CO oxidation

Copper oxide catalysts supported on ceria were prepared by wet impregnation method using finely CeO₂ nanocrystals, which was derived from alcohothermal synthesis, and copper nitrate dissolved in the distilled water. The catalytic activity of the prepared CeO₂ and CuO/CeO₂ catalysts for low-temperature CO oxidation was investigated by means of a microreactor-GC system. The samples were characterized using BET, XRD, SEM, HRTEM and TPR.     

Components for PEM fuel cell systems using hydrogen and CO containing fuels

Proton exchange membrane fuel cells (PEMFC) show a significant performance drop in CO containing hydrogen as fuel gas in comparison to pure hydrogen. The lower performance is due to CO adsorption at the anode thus poisoning the hydrogen oxidation reaction. Two approaches to improve the cell performance are discussed. First, the use of improved electrocatalysts for the anode, such as PtRu alloys, can significantly enhance the CO tolerance. On the other hand, CO poisoning of the anode could be avoided by the use of non-electrochemical methods. For example, the addition of liquid hydrogen peroxide to the humidification water of the cell leads to the formation of active oxygen by decomposition of H₂O₂ and the oxidation of CO. In such a way a complete recovery of the CO free cell performance is achieved for H₂/100 ppm CO.     

Recent progress in selective CO removal in a H2-rich stream

In this review, recent works related to the selective CO removal in a H₂-rich stream for the application of the low-temperature fuel cell are discussed. The membrane separation, the selective CO hydrogenation, and the preferential CO oxidation (PROX) have been generally studied to meet the requirement for the polymer electrolyte membrane fuel cell (PEMFC) where the CO concentration should be controlled to be less than 10 ppm not to degrade the electrochemical performance of Pt anode. For the membrane separation, the thin layer of Pd-based alloy metal on the porous ceramic material coupled with the catalytic purification is the most advanced method at present. For PROX catalysts, supported Ru catalysts and Pt-based alloy catalysts have been successfully developed so far. The combination of highly selective PROX catalysts and the CO methanation catalyst can provide the extended temperature range to achieve the acceptable CO removal. Because each method has presently its own weak points, the further advance is still in need. The non-noble metal-based membrane requiring smaller pressure differentials is highly plausible in the membrane separation. The highly selective catalyst for CO methanation in the presence of excess CO₂ and H₂O can simplify the CO removal step. The PROX catalyst should be operative over a wide reaction temperature as well as at low temperatures not to cause the reverse water–gas shift reaction. During the development of these catalysts, the progress on the high-temperature PEM fuel cell or the CO-tolerant anode should be carefully evaluated.     

Rate constant for CO insertion in the hydroformylation of ethene on Rh/SiO2 as determined by pulse surface reaction rate analysis

A new method of pulse surface reaction rate analysis (PSRA) was proposed for determining the rate constant of the step of CO insertion in ethene hydroformylation. On pulsing ethene onto Rh/SiO₂ via the continuous flow of a mixture of CO and H₂, propanal was found to be produced gradually. It is revealed here that rate analysis of the dynamic behavior of produced propanal allows the determination of the rate constant for CO insertion. The effect of metal carbonate and oxides, added to the catalyst, on CO insertion was discussed in terms of the rate constant determined.     

Supported copper–ceria catalysts for low temperature CO oxidation

In this investigation, CuO/CeO₂–M_xO_y (M_xO_y = Al₂O₃, ZrO₂ and SiO₂) nanocomposite oxide catalysts were prepared by deposition-precipitation and wet impregnation methods, and evaluated for CO oxidation. Catalysts were characterized by XRD, TEM, UV–vis DRS, BET surface area and H₂-TPR techniques. The synthesized catalysts exhibited high specific surface area, and uniform particle size distribution over the supports. The nanocrystalline texture of mixed metal oxides is clearly evidenced by TEM analysis. TPR and XRD results revealed synergetic interactions between copper oxide and ceria. Among various catalysts investigated, the CuO/CeO₂–Al₂O₃ combination exhibited excellent CO oxidation activity with T_1/2 = 374 K and 100% CO conversion at below 420 K.     

Separation of H2/CO by the selective sorption property of H3PMo12O40 embedded in polyvinylalcohol membrane

H₂ and CO permeabilities through H₃PMo₁₂O₄₀-blended polyvinylalcohol membrane and catalyst-free polyvinylalcohol membrane were measured. The blended membrane showed better selectivity and higher permeation flux of H₂ than the catalyst-free polyvinylalcohol membrane. Its enhanced H₂ separation ability was mainly due to the selective sorption property of PMo embedded in the polyvinylalcohol membrane.     

Kinetic Investigation of the Photocatalytic Reduction of Nitric Oxide by Carbon Monoxide at Low Pressure on Silica-Supported Molybdenum Oxide

The kinetics of photoinduced reactions that occur upon UV irradiation (<360 nm) of a MoO₃/SiO₂ catalyst (2.5 wt% Mo) in CO-NO mixtures and CO alone are studied at gas pressures from 0.05 to 2 torr and for CO/NO ratios from 0.3 to 3.0 in the temperature interval 20-150C. The data obtained are consistent with a previously proposed two-stage redox mechanism. In the first stage NO is reduced to N₂O through the reaction CO+2NO CO₂+N₂O, while in the second stage the N₂O formed is further reduced to N₂ via the reaction CO+N₂O CO₂+N₂. The ratio of rate constants for quenching of a transient excited state (Mo⁵⁺-O^-)* by NO and CO molecules is found to be 2.8. The reaction rates decrease with increasing temperature, apparently because of a lower concentration of adsorbed species and/or a reduction of the steady-state concentration of (Mo⁵⁺-O^-)*.