High Initial CO Research Articles

Ru-M (M = Fe or Co) Catalysts with high Ru surface concentration for Fischer-Tropsch synthesis

Ru-Co/Al2O3 and Ru-Fe/Al2O3 have been prepared by the reduction-deposition method. This approach favours the interaction between Ru and Fe or Co in the final catalysts, while increasing surface enrichment of Ru. Their performance for the Fischer-Tropsch Synthesis (FTS) has been evaluated in a fixed bed reactor. Ru-Co/Al2O3 displays the highest activity in the series, doubling the steady-state CO conversion rate of a Ru/Al2O3 catalyst with 3 times higher Ru loading. Ru-Fe/Al2O3 displays higher initial CO conversion and -CH2- productivity than Ru/Al2O3, but its activity declines during operation reaching lower conversion rates than Ru/Al2O3. In addition to CO2, Ru-Fe/Al2O3 produces oxygenated products, mainly alcohols. To a lesser extent, the formation of oxygenates over Ru-Co/Al2O3 and Ru/Al2O3 is also observed, and ascribed to the small crystallite size of the Ru particles. The evolution of CO2 during time on stream indicates that Fe is segregated to the surface of this catalyst during the FTS.

Fischer–Tropsch Synthesis: Influence of Acid Treatment and Preparation Method on Carbon Nanotube Supported Ruthenium Catalysts

The influences of nitric acid treatment on a carbon nanotube (CNT) support and the preparation method, incipient wetness impregnation (IWI) versus chemical vapor deposition (CVD), on catalytic performance during Fischer–Tropsch (FT) synthesis were examined using a slurry phase reactor. Acid treated CNT (ACNT) supported Ru catalysts exhibited higher activities compared to Ru supported on untreated CNTs (UCNTs). The acid-treated CVD catalyst had higher initial CO conversion (smaller average Ru particle size) but sintered more due to a greater tendency for Ru to be deposited exterior to CNT channels relative to IWI. After the initial decline and leveling off period, the ACNT IWI catalyst had the highest steady activity among the catalysts tested. The ACNT IWI catalyst also displayed greater oxygenate selectivity (∼17%) compared to ACNT CVD (∼12%) and UCNT IWI (∼10%) catalysts at similar conversions. Acid treatment created adsorption sites on the CNT surface that anchor Ru precursors and promote CO insertion ...

Oxidized Ni Nanostructures Supported by Mesoporous Al2O3: Relationship between the Structure and Reactivity for CO Oxidation Studied via Photoemission Spectroscopy

Oxidized Ni, at very small loadings, was deposited on mesoporous Al2O3 using atomic layer deposition. The prepared structures were used as catalysts for CO oxidation between 30 and 250 °C. Ni particles with a mean size less than 1–2 nm were shown to be reactive, even at room temperature, for CO oxidation. The room temperature reactivity for CO oxidation was decreased with increasing reaction time. However, when post‐annealing at 300 °C was applied to deactivated catalysts, a higher initial CO oxidation reactivity (compared with the value of the preceding catalytic operation) was observed. Furthermore, we observed that repeated cycles of room‐temperature CO oxidation and post‐annealing at 300 °C gradually increased the catalytic activity for room temperature CO oxidation. X‐ray photoelectron spectroscopy was used to determine the origin of this behavior and the results are discussed in detail.

Fischer–Tropsch synthesis: Effect of Pd, Pt, Re, and Ru noble metal promoters on the activity and selectivity of a 25%Co/Al2O3 catalyst

The effect of noble metal promoters (atomic ratio of promoter to Co=1/170) on the activity and selectivity of a 25%Co/Al2O3 catalyst was studied at a similar CO conversion level of 50% at 493K, 2.2MPa and H2/CO=2.1 using a 1-L continuously stirred tank reactor (CSTR). The results show that all promoted catalysts exhibited markedly higher initial CO conversion rates on a per gram catalyst basis than the unpromoted one, which was ascribed to increased Co site density when the promoters were present. This is because the Re, Ru, Pt and unpromoted Co catalysts give essentially the same initial Co TOF values of 0.092–0.105s−1 (based on hydrogen-chemisorption). However, the initial Co TOF value for the Pd-Co catalyst was about 40% lower, which might be caused by Pd atoms segregating on the Co surface and partially blocking Co sites.At 50% CO conversion, Re and Ru promoters decreased CH4 selectivity and increased C5+ selectivity by nearly the same extent, whereas the opposite effect was observed for Pd and Pt promoters. The Re and Ru promoters had less of an impact on C2–C4 olefin selectivity (7.5–60%), but suppressed the secondary reaction of 1-C4 olefin (from 14.3 to 9%) compared to the unpromoted one; however, the addition of Pd and Pt promoters resulted in lower olefin selectivity (4.4–55%) but higher 2-C4 olefin selectivity (14.3 to 27–31%). Pt promotion had a negligible effect on C4 olefin isomerization. The selectivity results were reproducible.Both Pt and Pd promoters slightly increased WGS activity, whereas Re and Ru promoters had a negligible effect. The Pd and Pt promoters were observed to slightly enhance oxygenate formation, while Re and Ru slightly decreased it.

Methanation in a fluidized bed reactor with high initial CO partial pressure: Part II— Modeling and sensitivity study

A model of a bench-scale methanation reactor was set-up by modifying the classical two-phase model approach and introducing an additional bulk flow from bubble to dense phase to consider the volume contraction of the methanation reaction. The model uses experimentally determined kinetics and hydrodynamic correlations from literature. It was satisfyingly validated by comparing the calculated gas concentration profiles with the experimental data, especially with respect to initial reaction rates and reactor exit concentrations. A sensitivity study with respect to different bubble size correlations, mass transfer rates and considering or neglecting the bulk flow (influence of volume contraction caused by the methanation reaction) was carried out. It showed that the bubble size correlation by Werther and the resulting gas concentration profiles fit the measured data better than the computed gas concentration profiles using the bubble size correlation by Rowe. Neither a variation of the mass transfer coefficient nor neglecting the bulk flow in the fluidized bed model did yield further improvement of the calculated concentration profiles.

Methanation in a fluidized bed reactor with high initial CO partial pressure: Part I—Experimental investigation of hydrodynamics, mass transfer effects, and carbon deposition

An extensive experimental study on the methanation reaction was carried out in a gas–solid fluidized bed reactor at 320 °C with a stoichiometric ratio of H 2/CO=3. By means of spatially resolved measurements of the axial gas species concentration and temperatures along the fluid bed the effects of different catalyst loadings, gas velocities and dilution rates were observed and analyzed. By applying this technique, it was found that most of the reaction (CO and H 2 conversion) proceeds in the first 20 mm of the bed depending on the experimental conditions. For a few cases, the temperature increases by up to 80 °C from 320 to 400 °C within the first 3 mm of the bed. By increasing the inlet volume flow only by a factor of 1.4, the temperature hotspot diminishes and isothermal behavior develops. For all experiments, a CO conversion of practically 100% was achieved. The experimental data indicate that the dense phase of the fluidized bed is probed and that mass transfer between bubble and dense phase is dominating in the upper part of the bed. It could be shown that both hydrodynamic and chemical boundary conditions influence the methanation reaction inside the fluidized bed reactor.

Applying spatially resolved concentration and temperature measurements in a catalytic plate reactor for the kinetic study of CO methanation

In this work, the successful application of spatially resolved measurements in an optically accessible catalytic plate reactor was demonstrated, which allows the detailed investigation of the reaction kinetics of the exothermic CO methanation reaction with high initial CO partial pressure. By means of a movable sampling capillary, the axial gas species concentration profiles over the catalyst plate were measured, and the catalyst surface temperature was determined simultaneously along the reactor through a quartz glass window by means of infrared thermography. A one-dimensional model of the catalytic plate reactor and a Bayesian approach were applied to estimate the kinetic model parameters of the proposed Langmuir–Hinshelwood rate expressions by comparing simulated and measured gas concentration profiles. The validity of using a computationally efficient one-dimensional model was proven by solving a two-dimensional model using the kinetic parameters determined with the one-dimensional model and by comparing the two results.

Modeling remineralization of desalinated water by limestone dissolution

Desalted waters or highly soft waters produced by desalination plants cannot be directly used as they are unpalatable, corrosive and unhealthy. Remineralization is necessary in order to overcome these problems. A commonly used operation in the remineralization process is to contact CO 2 acidified desalinated water with a bed of domestic limestone. Limestone dissolution provides two essential ingredients to the water—bicarbonate alkalinity and calcium content: CaCO 3 + CO 2 + H 2O = Ca 2+ + 2HCO 3 –. Limestone dissolution is a slow rate-controlling step. Prediction of the limestone rate of dissolution as a function of the water composition is essential for reliable design and operation of the limestone contactor. A critical comparison of various kinetic expressions proposed in the literature carried out in this study reveals major differences in results evaluated from different dissolution models. An experimental study was conducted in order to identify the most reliable kinetic dissolution model. Two series of experiments were carried out—one involving remineralization of distilled water containing low initial CO 2 concentrations (0.5–2 mM) and the other, remineralization of soft water, having high initial CO 2 concentrations (1.5–15 mM). The CO 2 acidified water was contacted in a 2 m high vertical column (32 mm I.D.), packed with 2.85 mm calcite particles. The change in water composition along the column was monitored to provide both differential and integral dissolution data. Analysis of the data showed that none of the available models fitted the experimental results. The closest agreement was with the rather complex model of Plummer et al but this agreement was rather mediocre. In the high CO 2 content range, the model predicted dissolution rates higher by a factor of 2–4 in the high CO 2 range and by a factor of 10–20 in the low CO 2 range. Based on the experimental results, two models were developed for the design of limestone dissolution column contactors. When the final composition of the remineralized water has a CO 2 content above 2 mM, the limestone bed can be designed by a very simple integral expression. However, if the dissolution depletes the CO 2 concentration to low values, well below 2 mM, the bed design requires numerical integration of the more general dissolution rate expression derived in this work.

CO dissociation on Mo(1 1 0) studied by high-resolution core-level spectroscopy

We have studied the CO/Mo(1 1 0) adsorption system at different sample temperatures by high-resolution core-level spectroscopy and near-edge X-ray-absorption fine-structure measurements. We demonstrate that CO molecules having different tilt angles with respect to the surface normal may be assigned to different components of the C 1s core-level spectra. The tilt of the CO molecules is shown to depend on the CO coverage. The thermally induced dissociation of CO is studied and is shown to differ at low and high initial CO coverages. At low coverage CO dissociation starts already at temperatures as low as 125 K whereas at saturation CO coverage no dissociation occurs below 280 K. The reasons for this difference in dissociation behavior are discussed and it is concluded that steric effects play a large role.

CO adsorption and oxidation on Au/TiO2

Preoxidized Au/TiO2 showed no initial activity during a first heating stage up to 70°C, while prereduction yielded a high initial CO conversion at room temperature. With FTIRS, two different CO absorption bands were detected. One band is usually attributed to CO on an oxidic gold species (2151 cm-1), the other one is characteristic of CO on metallic gold (2112 cm-1). The presence of the first species appears to have a detrimental effect on the CO oxidation by O2. The present results do not support a model in which the activity of supported gold catalysts in CO oxidation is ascribed to ionic Au particles.

Interactions between Ni and La2O3in Ni/La2O3Catalysts Prepared Using Different Ni Precursors

The role of the anion (NO3−or Cl−), used in the preparation via impregnation of unreduced Ni/La2O3catalysts, in the interactions between Ni and La2O3was investigated using XRD, TPR, and ESCA experiments. The following results have been obtained: In the 20 wt% Ni/La2O3prepared using Ni(NO3)2as precursor, LaNiO3and La(OH)3were the only compounds identified by XRD in the sample calcined in O2, and La2NiO4and La(OH)3were the only compounds identified in the sample calcined in He. At lower Ni contents, the La2O3phase was additionally identified. For the 20 wt% Ni/La2O3prepared using Ni chloride as precursor, LaOCl was the only compound detected by XRD; no phase containing Ni was identified by XRD in the samples calcined in O2or He. At lower Ni chloride contents, LaNiO3was additionally detected in the sample calcined in O2and La2NiO4in that calcined in He. A comparison between the XRD spectra and the TPR curves suggested the presence of amorphous NiO in the samples based on Ni chloride. It is also shown that the Ni precursor employed in the preparation of the catalyst plays a role in the CO2reforming of CH4. During the CO2reforming of methane, the unreduced Ni/La2O3catalysts, based on Ni nitrate, had a high initial CO yield but a low time stability; in contrast, the unreduced Ni/La2O3catalysts, based on chloride, had a high stability. A possible explanation of this behavior, based on ESCA analysis and on the reaction between CH4and catalysts, is provided.

The striking difference in the behaviour of Rh and Pt towards their interaction with CO 2

The interaction of CO 2 with both Rh and Pt was studied between 80 and 1200 K using field emission microscopy (FEM) and thermal desorption spectroscopy (TDS). On Rh the sticking probability at 80 K is high. Dissociation of CO 2 into CO and O starts around 165 K on (210). The influence of the surface structure on the dissociation is large. Starting with a low initial CO 2 coverage, CO recombines with O and desorbs as CO 2 between 300 and 340 K. At high initial CO 2 coverages, some O is forced to diffuse into the bulk. Using a newly developed digital FEM set-up it could be shown that oxygen diffuses out of the bulk between 600 and 1200 K. Also CO desorption around 450 K was found. CO 2 is only weakly adsorbed on Pt at 80 K with an estimated heat of adsorption of at most 25 kJ mol . No dissociation was found on Pt surfaces. CO 2 desorbs around 90 K leaving a clean surface. The surface structure dependence of the adsorption of CO 2 on Pt and of the adsorption and dissociation of CO 2 on Rh will be discussed.

CO oxidation over Au/ZrO 2 catalysts: Activity, deactivation behavior, and reaction mechanism

Gold catalysts supported on amorphous zirconia have been prepared by coprecipitation. These catalysts exhibit a high initial CO oxidation activity, which is due to a synergy between the zirconia and the supported gold particles. The activity decreases significantly over a time scale of ≈20 h. The deactivation behavior is only weakly influenced by pretreatments in O 2, CO, or CO 2. However, the long-time stability is found to depend strongly on the O 2/CO feed ratio. With stoichiometric reaction mixtures (O 2/CO = 0.5), the activity stays nearly constant over the time of observation. In the presence of excess oxygen (O 2/CO = 1 or 40, respectively), rapid deactivation is observed. From a careful monitoring of the carbon balance, and from thermal desorption/mass spectrometry experiments, a progressive covering of the surface by oxygenated carbon species is deduced. Diffuse reflectance FTIR experiments indicate the presence of carbonates, formate, and several types of differently bound CO molecules on the surface. Temperature-dependent experiments reveal that these species are involved in a series of interconnected equilibria, which are coupled to the relevant adsorption/de sorption processes. Oxidized and reduced states of the catalyst can be distinguished. The deactivated state in excess oxygen is characterized by a high concentration of surface formate. On the deactivated catalyst, oxydation of formate and of CO appears to be slow, possibly as a consequence of site blocking.

The spatial distribution of product desorption in the oxidation of carbon monoxide on platinum (110)(1×2) reconstructed surfaces

The spatial distribution of the desorption of carbon dioxide produced on platinum (110)(1×2) reconstructed surfaces was studied in the wide range of the reactant coverages by means of angle-resolved thermal desorption and low-energy electron diffraction. Heating the surface covered by oxygen atoms and carbon monoxide yields four peaks in the CO2 formation: P1-around 400 K, P2- (300 K), P3- (250 K), and P4-CO2 peaks (170 K). The desorption of each CO2 indicated a sharp angular distribution collimated along the bulk surface normal in the [11̄0] direction. On the other hand, the desorption of P1-and P2-CO2 produced at small CO coverages showed two-directional desorption collimated at either about +23 or −23 degrees off the bulk surface normal in the [001] direction. It indicates that the reactive desorption takes place on declining terraces of the reconstructed plane. The desorption of P3- and P4-CO2, and also of P2-CO2 produced at high initial CO coverages, showed a single peak in the angular distribution curve in the same direction, which was collimated along the bulk surface normal. This suggests that the reconstruction is at least partly lifted during heating procedures, and CO2 is formed on the nonreconstructed as well as the reconstructed plane. It was supported by low-energy electron diffraction observations.