Selective carbon monoxide (CO) oxidation reaction was studied over mono- and bimetallic Co, Cu and Fe, as well as Cu–Co and Cu–Fe heterogeneous catalysts, using multi-walled carbon nanotubes (MWCNT) as substrates. Materials were synthesized by wet (co-)impregnation technique and characterised. It was found that hydrophilic hydroxyl and carboxyl nanotubes’ (CNT) functional groups were favourable for a strong metal–support interaction. The catalytic conversion performance for preferential CO oxidation (PROX) process was carried out in hydrogen, water, and carbon dioxide-containing feedstock gasses. The addition of iron or cobalt to Cu/CNT improved the activity with comparison to Cu/CNT. The optimized Cu–Fe/CNT could preferentially oxidize dilute CO in H2-rich simulated WGS streams within a wide temperature range of 120–220?°C. The temperatures, where 50% CO conversion was achieved, were as follows: Cu–Fe/CNT (120?°C) ) Cu–Co/CNT (140?°C) ? Cu/CNT (140?°C). A high determined selectivity towards CO2 for Cu–Fe/CNT could be attributed to the presence of CuFe2O4 and the synergy between Co and Cu for Cu-Co/CNT. At 220?°C and in a 1% CO/1% O2/10% H2O/10% CO2/60% H2/18% He stream, Cu–Fe/CNT could achieve a 100% CO conversion, granting a low H2 conversions, not differing from those in the absence of CO2, while its turnover performance remained stable for a longer continuous time-on-stream with basically no deactivation. By comparison, Cu–Fe/CNT exhibited a higher apparent rate and lower activation energy of CO conversion (with and without H2O and CO2).
COBISS.SI-ID: 6424346
CuZnGaOx catalysts were prepared by co-precipitation synthesis method and their turnover performance was evaluated for the water–gas shift (WGS) reaction. The effects of pH on composition, structure and morphology were investigated. Materials were characterised using powder XRD, physisorption, chemisorption and electron microscopic techniques. Basic preparation conditions produced smaller, uniform and homogenously distributed particles, while a high interface concentration of bulk copper phase was observed at acidic pH. X-ray diffraction peak, corresponding to CuO(111), was broader at neutral/alkaline pH, which indicated small synthesized crystallites, disordered oxide arrangement and polymorphism. The analyses after H2-TPR revealed that all Cu was in the metallic oxidation state after reduction. H2-TPR profiles demonstrated that a stronger comparative interaction between Cu–ZnOx and Cu–GaOx existed for a higher applied syntheses pH. Continuous temperature-programmed surface reaction (TPSR) measurements showed that these also granted the thermodynamic equilibrium CO conversion at 240 °C. Time-on-stream catalytic activities were examined in a fixed bed reactor. An increase in the vaporised steam content in reactant feedstock mixture caused a rise in CO consumption and effluent hydrogen productivity. WGS process was thus structure-sensitive to the supported active metal in CuZnGa-based composite nano-catalysts. Intrinsic kinetic reactivity was proportional to Cu area, dispersion and sites, which could be altered by precipitation composite fabrication pH. The methane, methanol or ethanol reforming design with low-temperature WGS is particularly vital in high-temperature proton-exchange membrane fuel cells (PEMFC), in which thermal unit integration may be used to supply heat from the stack to WGS operation, and in the case of CH3OH, even reformer.
COBISS.SI-ID: 6264346
Empirical correlation (EC) equations are still of a great designing importance for industrial plant construction. They are an indispensable modeling tool for engineers, reducing the time to find the optimal operating conditions. Nonetheless, numerical method complexity and product yield optimisation are advancing. Computational fluid dynamics (CFD) is thus nowadays applicable for optimizing chemical reactors. In contrast to EC, CFD acknowledges specific vessel geometry, where local physical and chemical phenomena, contributing to apparent catalytic turnover, prevail. Presently, EC and CFD were compared considering the pressure drop predictions within the packed bed columns for spherical, cylindrical, trilobe and quadrilobe particle packing, in order to determine the limits of EC accuracy. 52 configurations were simulated and the estimations within EC validity range margins were in agreement with CFD ((15%), while in extremes (non-negligible entrance and exit patterns), a 70% deviation could be exceeded. Furthermore, boundary wall effects were found to be dependent on the stacked pellet shape and orientation, and did not necessarily lead to an increase of viscous friction loss, relative to the infinitely wide systems, for the column-to-particle diameter ratios, lower than 10, which is contradictory to non-mechanistic relationship models. While the induced pressure difference within realistic fixed beds is elevated due to gas or liquid surface interaction and back-mixing, it can also be decreased by channeling/tunneling, which is why the effective net influence should be analyzed with CFD simulations, particularly in novel intensified and micronized processes, in which momentum, mass and heat transfer resistances are far from bulk medium continuity.
COBISS.SI-ID: 6360858
Catalytic conversion performance of the ?-Al2O3-supported Cu-, Mn- and Ni-based materials for the selective preferential oxidation of CO was investigated. The influence of CO2 and H2O in the reactor feed gas on activity and selectivity was lastly investigated for copper bimetallic catalysts containing manganese and nickel. Cu/Al2O3 proved to be optimal among the monometallic examined. It was observed that even in the absence of H2, CO was effectively converted. Upon H2 presence, a fraction of oxygen is utilized for the latter, which results in the decrease of CO turnover. Cu–Mn/Al2O3 surpassed other single and mixed metal oxide catalysts which could be due to a combination of surface desorption capacity and redox properties. The oxygen storage capacities (OSC) were in the order of Cu–Mn/Al2O3 ) Cu–Ni/Al2O3 ) Cu/Al2O3 ) Mn/Al2O3 ) Ni/Al2O3, in an agreement with reactivity. The efficiency at low process temperatures was enhanced by adding H2O vapors, nonetheless, decreased at high-end range due to the reverse water–gas shift (RWGS). Also, the rate of CO conversion was high utilizing Cu–Mn/Al2O3; furthermore, even upon applying H2O and CO2. CO2 notably affected CO conversion, and ultimately, decreased the activity, which, conversely, remained stable upon time-on-stream.
COBISS.SI-ID: 6264858
Commercial heterogeneous Ni–Mo/Al2O3 catalyst was tested for the oxidative dehydrogenation (ODH) reaction of n-butane with different oxidant species: O2, CO2 and N2O. The effect of the lattice oxygen mobility and storage in Ni–Mo/Al2O3 on catalytic conversion performance was investigated. Experiments indicated that a high O2-storing/release is beneficial for activity, however at the expense of selectivity. A significant amount of butadiene with no oxygenated compound products was formed upon using carbon dioxide and nitrous oxide, while O2 favoured the formation of cracked hydrocarbon chains and COx. The highest turnover yield to 1,3-butadiene was achieved at the oxidant-to-butane molar ratio of 2:1 at the temperatures of 350 °C and 450 °C. With CO2, significant amounts of hydrogen and carbon monoxide were evolving due to parallel reforming pathway. A partial nickel/molybdenum oxidation was also observed under CO2 and N2O atmospheres. TPR revealed the transformation of the high valence oxides into structurally distinct metal sub-oxides. In TPRO, three distinct peaks were visible and ascribed to surface oxygen sites and two framework positions. With N2O, these shifted towards lower temperature region, indicating a better diffusional accessibility and an easier bulk-to-surface migration. XRD revealed ?-NiMoO4 active phase presence, which was used in DFT modelling as (110) plane. Theoretical ab initio calculations elucidated fundamentally different reactive chemical intermediates when using CO2/N2O or O2 as oxidant. The former promote Mo atom oxygen termination, while in O2 environment, Ni is also oxygenated. Consequently, CO2 and N2O selectively dehydrogenate C4H10 through serial hydrogen abstraction: butane › butyl › 1-butene › 1-butene-3-nyl › butadiene. With O2, butane is firstly transformed into butanol and butanal, which are prone to subsequent C–C bond cleavage. The latter is mirrored in different mechanisms and rate-determining steps, which are essential for efficient butadiene monomer process productivity and the optimisation thereof.
COBISS.SI-ID: 6183194