10 Search Results

Captured carbon dioxide (CO₂) streams contain impurities that must be removed to meet specifications for safe transport, storage, and utilization. Among these impurities, oxygen poses challenges due to its high reactivity and potential to cause corrosion, motivating stringent purity limits below 10 ppmv. Building on recent experimental demonstrations of catalytic oxygen removal using hydrogen (H₂), carbon monoxide (CO), methanol (CH₃OH), and methane (CH₄) as reducing agents, this study presents a technoeconomic (TEA) and life cycle assessment (LCA) of these four catalytic purification pathways. Process flowsheets were developed and simulated in Aspen Plus for CO₂ streams representative of both low-temperature andmore » high-temperature capture processes, with integrated heat recovery and energy optimization. Results showed that total purification costs were dominated by feedstock procurement and electricity consumption. Among the studied reducing agents, the CH₄-assisted route achieved the lowest purification cost and highest CO₂ recovery. Sensitivity analyses showed that the H₂ route became competitive at H₂ prices below $$\$$$$0.56/kg to $$\$$$$0.84/kg, depending on the CO₂ feed temperature conditions. In conclusion, environmental impacts were primarily driven by indirect CO₂ emissions from raw material production and utility consumption.« less

Streams of CO₂ from various capture processes may contain several impurities of which O₂ is often overlooked as a problematic impurity. Because of its reactivity, the National Energy Technology Laboratory has recommended a stringent limit, less than 10 ppm, for the O₂ level in treated CO₂ products for safe transport, storage, and utilization. Here, in this study, a variety of catalytic oxygen reduction approaches using commercial automotive exhaust emission catalysts are evaluated for O₂ removal from CO₂ streams using different reducing agents, including H₂, CO, CH₃OH, and CH₄. When the amount of reductant added into the feed is carefully controlled,more » H₂, CO, or CH₃OH can effectively remove O₂ from 1.5% to below 10 ppm with a single-reactor design (>99.93% removal efficiency), while simultaneously meeting the impurity limits for other species. With CH₄ as a reductant, it is challenging to simultaneously meet the the O₂ and CO specifications in a single-reactor design because CH₄ also reacts with CO₂ to produce high levels of CO in the stream (at several hundred ppm). A dual-reactor design is developed to enable the use of CH₄, a readily available and low-cost reductant, for the purification of CO₂ streams to meet the specifications for O₂ and other impurities.« less

Here, this study discusses model-based optimization strategies for CO₂ hydrogenation to dimethyl ether (DME) over aCuZnZr (CZZ) and ferrite (FER) mixed catalyst system in a packed-bed reactor configuration. A two-dimensional axisymmetric, nonisothermal packed-bed reactor model was developed using COMSOL Multiphysics 6.2 software. The model solves two-dimensional (radial and axial) heat and mass transport equations in the packed-bed and integrates intraparticle diffusion and heat transfer in a 1D approach. This powerful feature differs from a traditional porous media approach and takes into account any heat and mass transfer limitations that may exist. Analysis shows that the heat transfer limitations are negligible,more » but strong internal mass transfer limitations were observed at 10 ≤ WHSV ≤ 90 h^-1 on the FER catalyst and at 240 °C. The optimum catalyst composition (i.e., mixing ratio) varies depending on the operating regime. The FER catalyst weight in the mixture can be as low as 5 wt.%, but the ideal composition de-pends on the internal mass transfer limitation and its relationship with the operating regime (i.e., weight hourly space velocity, temperature). A catalyst composition of 80 wt.% CZZ and 20 wt.% FER was suggested; this composition can provide high CO₂ conversion and DME production rates at a wide range of temperatures and flow rates.« less

A PtPd/Al₂O₃ catalyst developed for the complete oxidation of methane from the ventilation air of underground coal mines is compared against a model PdO/Al₂O₃ catalyst. Although the PtPd/Al₂O₃ catalyst is substantially more active and stable than the model catalyst, the nature of active sites between the two catalysts is deemed to be fundamentally the same based on their response to different feed gas compositions and the evolution of surface CO adsorption complexes during time-resolved CO adsorption DRIFTS experiment. For both catalysts, coordinatively unsaturated Pd sites are considered the active centers for methane activation and the subsequent oxidation reaction. H₂O competesmore » with CH₄ for the same active sites, resulting in severe inhibition. Additionally, the CH₄ oxidation reaction also causes self-inhibition. Taking both inhibition effects into consideration, a relatively simple kinetic model is developed. The model provides a good fit of the 72 sets of kinetic data collected on the PtPd/Al₂O₃ catalyst under practically relevant reaction conditions with CH₄ concentration in the range of 0.05–0.4%, H₂O concentration of 1.0–5.0%, and reaction temperatures of 450–700 °C. Kinetic parameters based on the model suggest that the CH₄ activation energy on the PtPd/Al₂O₃ catalyst is 96.7 kJ/mol, and the H₂O adsorption energy is –31.0 kJ/mol. Both values are consistent with the parameters reported in the literature. The model can be used to develop catalyst sizing guidelines and be incorporated into the control algorithm of the catalytic system.« less

PdO/γ-Al₂O₃ catalysts suffer from gradual and irreversible catalyst deactivation under lean CH₄ oxidation conditions, especially in a wet feed. Here, time-resolved CO chemisorption DRIFTS measurements are conducted systematically on a series of PdO/γ-Al₂O₃ catalysts to probe the surface reactivity of PdO nanoparticles after various in situ pretreatments. At 80 °C, CO barely adsorbs on fully oxidized PdO surfaces but interacts with coordinatively unsaturated Pd sites, causing gradual reduction of the PdO surfaces. This results in the formation of characteristic IR bands on various metallic Pd⁰ sites. By monitoring and comparing the formation kinetics of these IR bands on samples beforemore » and after CH₄ oxidation, we theorize that the irreversible catalyst deactivation during CH₄ oxidation is caused by PdO surface reconstruction, in which coordinatively unsaturated Pd sites gradually become fully coordinated by oxygen. Effectively, the surface reconstruction leads to the formation of a passivation layer on the PdO nanoparticles, which hinders their ability in activating CH₄, and hence the subsequent oxidation reaction. Temperature-programmed reduction with CO as the reductant (CO-TPR) reveals that the passivation layer formed during CH₄ oxidation is significant enough to increase the reduction temperature of PdO nanoparticles of the 3.0% PdO/γ-Al₂O₃ samples, although such an effect is less obvious for the 0.4% PdO/γ-Al₂O₃ samples. On the other hand, it is also discovered that the passivation layer is not completely inert. Under certain reaction conditions, with some being relatively mild, such as low-temperature CO oxidation in a net lean atmosphere and in the presence of H₂O, the passivation layer can undergo structure change which results in regeneration or even activation of CH₄ oxidation activity of an already deactivated catalyst. Additionally, it is discovered that the fully coordinated Pd–O surface is a metastable phase under CH₄ oxidation conditions. In the presence of H₂O and at ambient temperatures, surfaces with coordinatively unsaturated Pd sites are thermodynamically more favorable.« less

A monolith supported bifunctional catalyst for the direct conversion of CO₂ to dimethyl ether was developed and evaluated. The catalyst consists of a layer structured configuration, in which a CuO/ZnO/ZrO₂ component for methanol synthesis using CO₂ as feedstock and a Ferrierite zeolite component for the subsequent dehydration reaction are washcoated onto the channel surfaces of a metallic monolith substrate as two consecutive layers. The metal substrate provides heat conduction to regulate the catalyst bed temperature. The layered configuration significantly improves the synergistic effects of the two components, resulting in a 20% increase in the productivity for dimethyl ether at 240more » °C as compared with the conventional catalysts with the two components being blended in various levels of proximity. Furthermore, the layer structured design minimizes the undesirable interaction between the two components and drastically improves the on-stream durability of the catalyst. No activity decline was observed in a 146-h performance test.« less

This review summarizes the recent literature reports on the development of zeolite supported Pd catalysts for the complete oxidation of methane. In-depth analysis reveals that different types of zeolite framework structures, regardless of the dimensionality, pore opening structure, and channel size, have little influence on the methane oxidation activity or the on-stream stability of the supported Pd catalysts. In contrast, the Si/Al ratio of a zeolite support plays a critical role. Both the catalytic activity and the on-stream stability of a Pd/zeolite catalyst increase with the increase of the Si/Al ratio. Catalysts supported on siliceous zeolites consistently show excellent light-offmore » activity and remarkable on-stream stability whether in a dry or wet feed. Silanol nest defect sites in a siliceous zeolite are proposed to be the anchoring sites promoting the formation and minimizing the sintering of finely dispersed Pd nanoparticles. Remaining challenges in overcoming the sulfur poisoning effect are discussed.« less

The catalytic activities of small-pore Cu-CHA and large-pore Cu-BEA catalysts for the selective catalytic reduction of NO with NH₃ were measured at a very high flow rate. Cu-CHA clearly exhibited much higher intrinsic SCR activity and lower N₂O selectivity. In situ DRIFT spectra were recorded during the adsorption and desorption following NO and (NO+O₂) exposure to fully oxidized samples in a flow cell. The results are in agreement with what we have reported previously based on in situ transmission IR studies of partially reduced samples. Both suggest that different SCR reaction pathways might exist on these two catalysts and thatmore » NO⁺ could be an important reaction intermediate for Cu-CHA. Detailed IR studies with various isotopically labeled gas mixtures of (NO+O₂), (¹⁵NO+O₂), (NO+¹⁸O₂) and (¹⁵N¹⁸O+O₂) were conducted to understand the origin of the surface adsorption complexes on Cu-CHA. Formation of NO⁺ was not the consequence of a simple charge transfer reaction, NO+Cu²⁺=NO+ + Cu⁺. Instead, O₂ was found to be essential in changing the oxidation state of N from +2 to +3 although it did not participate in new N$$-$$O bond formation. In conclusion, the majority of the adsorbed NO⁺ maintained its isotopic origin of the feed gas.« less