Open-cell metallic foams coated by electrodeposition as structured catalysts for energy and environmental applications

Structured catalysts based on open-cell metallic foams offer a great potential for process intensification of fast, highly exo-/endothermic and diffusion-controlled catalytic processes. For the development of such structured catalysts, the choice of coating technique used to deposit the active phase on the foam surface is crucial, as the coating technique strongly influences the properties of the coated layer and in turn the activity/selectivity of the obtained structured catalysts. In this regard, the electro-base generation method has been proposed as a promising option that allows to coat foams without pore clogging, a common drawback of conventional wash-coating technique. The thesis aims to improve the electrodeposition and widen its applicability in the preparation of structured catalysts. In this respect, the first part of this thesis focuses on investigation of the electrochemical processes, taking place during the electrodeposition of Mg-Al hydrotalcite, thereby studying the role of nitrate concentration and reduction. Understanding the reactions involved in the electrodeposition helps to identify the main steps that determine the properties of the coating, and provides possible solutions to improve the method. In fact, by using a new electrochemical set-up, the control on the electrodeposition is achieved in terms of homogeneity and composition of the coating. In the second part of this thesis, a modified method for the electrodeposition of active phase on metallic foams is proposed and its applicability is demonstrated in the preparation of following catalyst systems for the use in energy and environmental catalytic processes: i) Rh/Mg/Al on FeCrAl or NiCrAl foams for the catalytic partial oxidation of CH4, ii) Pd-CeO2 on FeCrAl foam for CO oxidation, and iii) Rh/Mg/Al, Rh-CeO2, and Co3O4 on FeCrAl foam for catalytic N2O decomposition. The aforementioned catalyst systems exhibit satisfactory performances in the respective test reactions, thus confirming the potential of using electrodeposition in preparing the structured catalysts

Role of ZrO2 and CeO2 support on the In2O3 catalyst activity for CO2 hydrogenation

Methanol synthesis from CO2 hydrogenation has drawn global attention as catalytic CO2 hydrogenation is an attractive choice to mitigate CO2 emissions and lessen dependency on fossil resources. In the present study, we have synthesized ZrO2 and CeO2-supported In2O3 catalysts for methanol synthesis from CO2 hydrogenation and the catalytic performances of Inx/ZrO2, and Inx/CeO2 (x = 1 % and 13 %) were compared. Specifically, the effect of the ZrO2 and CeO2 supports on In2O3 catalyst during CO2 hydrogenation was explored. This study reveals that ZrO2 support increased the catalytic activity while the CeO2 support decreased although both supports have almost the same indium loading and surface area. Various characterizations like XRD, DRIFT, CO2-TPD, H2-TPR and XPS analysis of catalysts provided insight into changes that arise after mixing the two oxides and during the reaction as well as after the reaction. The stabilities of In2O3, In13/ZrO2, and In13/CeO2 were tested for up to 50 h and we found In13/ZrO2 was stable during this time-on-stream, while In13/CeO2 lost activity after 2 h of reaction. XPS results of spent catalysts showed that In(OH)3 was observed significantly over the spent In13/CeO2. OH groups were also verified by DRIFT experiments, however at low levels due to low CO2 conversion at atmospheric pressure. XRD analysis confirmed the sintering of CeO2 support during the reaction. Thus, the hydrophilic nature of CeO2, redox properties of CeO2 and sintering of CeO2 support in the presence of water, were the main reasons for the early deactivation of In13/CeO2. A regeneration study was carried out to regenerate the catalyst and the results showed that partial regeneration of the In13/CeO2 catalyst is possible by Ar flushing. We, therefore, suggest that the build-up of OHgroups deactivate the In13/CeO2 catalyst and some of these OH groups could be removed during flushing with inert gas, causing a partial regeneration. However, the decreased surface area is not reversible, and this results in a continuous decrease in the activity of the catalyst after repeated experiments, even if the catalyst is flushed with Ar between the experiments

The Impact of Lanthanum and Zeolite Structure on Hydrocarbon Storage

Hydrocarbon traps can be used to bridge the temperature gap from the cold start of a vehicle until the exhaust after-treatment catalyst has reached its operating temperature. In this work, we investigate the effect of zeolite structure (ZSM-5, BEA, SSZ-13) and the effect of La addition to H-BEA and H-ZSM-5 on the hydrocarbon storage capacity by temperature-programmed desorption and DRIFT spectroscopy. The results show that the presence of La has a significant effect on the adsorption characteristics of toluene on the BEA-supported La materials. A low loading of La onto zeolite BEA (2% La-BEA) improves not only the toluene adsorption capacity but also the retention of toluene. However, a higher loading of La results in a decrease in the adsorbed amount of toluene, which likely is due to partial blocking of the pore of the support. High loadings of La in BEA result in a contraction of the unit cell of the zeolite as evidenced by XRD. A synergetic effect of having simultaneously different types of hydrocarbons (toluene, propene, and propane) in the feed is found for samples containing ZSM-5, where the desorption temperature of propane increases, and the quantity that desorbed increases by a factor of four. This is found to be due to the interaction between toluene and propane inside the structure of the zeolite

Hydrotreatment of lignin dimers over NiMoS-USY: effect of silica/alumina ratio

Sulfides of NiMo over a series of commercial ultra-stable Y zeolites were studied in an autoclave reactor to elucidate the effect of silica/alumina ratio (SAR = 12, 30, and 80) on the cleavage of etheric C-O (beta-O-4) and C-C (both sp(3)-sp(2) and sp(2)-sp(2)) linkages present in native/technical lignin and lignin derived bio-oils. 2-Phenethyl phenyl ether (PPE), 4,4-dihydroxydiphenylmethane (DHDPM), and 2-phenylphenol, (2PP) were examined as model dimers at 345 degrees C and 50 bar of total pressure using dodecane as the solvent. The etheric C-O hydrogenolysis activity was found to be in the order NiMoY30 > NiMoY12 > NiMoY80, despite a high initial rate of C-O cleavage over NiMoY12 owing to its high acid density. A high degree of hydrodeoxygenation (HDO) and hydrocracking reactions were observed with NiMoY30 yielding >80% of deoxygenated products of which similar to 58% are benzene, toluene, and ethylbenzenes. A similar experiment with DHDPM showed the rapid cleavage of the methylene-linked C-C dimer (sp(3)-sp(2)) to phenols and cresols even with the low acid density (high SAR) catalyst, NiMoY80. Direct hydrocracking of the recalcitrant 5-5 \u27 linkage in 2PP is very slow, however, it cleaved via a cascade of HDO, ring-hydrogenation, and hydrocracking reactions. A high degree of hydrogenolysis and hydrocracking occurs over NiMoY30 due to suitable balance between acidity and pore accessibility, enhanced proximity between acidic and deoxygenation sites leading to a slightly higher dispersion of Ni promoted MoS2 crystallites. Overall, the product spectrum consisted of a high yield of deoxygenated products. The carbon content on the recovered catalyst was in the range of 3-7 wt%. These results pave the way for effective catalysts to break recalcitrant linkages present in lignin to obtain a hydrocarbon-rich liquid transportation fuel. An experiment with Kraft lignin over NiMoY30 shows good selectivity for deoxygenated aromatics and cycloalkanes in the liquid phase

Role of the supports during phosphorus poisoning of diesel oxidation catalysts

Phosphorus (P) poisoning is one of the main factors accounting for the deactivation of diesel oxidation catalysts (DOC) apart from sulfur poisoning and sintering of the Pt active sites. This study compares the impact of P with loading up to 2.4 wt% on the catalytic performance of monometallic and bimetallic Pt-Pd catalysts using alumina and high silica BEA zeolites as the supports. P poisoning caused deactivation for CO, C3H6, C3H8 and NO oxidation; however, the degree of the impact of P in terms of temperatures at which 50% of the component is converted (T50) depends not only on the types of the active phase (Pt and Pt-Pd) but also on the types of supports (alumina and BEA zeolite). The influence of P impregnation on the textural properties of the materials is more significant for zeolite than alumina-based catalysts, which is in line with the activity measurements. A weak interaction between P and high silica zeolite resulted in the formation of a prominent fraction of P2O5 in the P-Pt/BEA, whereas a strong binding between P and alumina accounted for a dominant fraction of phosphate in the P-Pt/Al2O3 as revealed by XPS and NMR measurements. Phosphorus compounds partially covered the available surface of the active sites and this lowered the catalytic activity. For alumina-based catalysts, P mainly reacted with the support and only deactivated a part of the active noble metals. Whereas, for zeolite-based catalysts, P existed mainly in the form of phosphorus oxides that significantly blocked the catalyst surface and thereby deactivated more of the available active sites than that on alumina-based materials, which is consistent with the CO chemisorption data

The role of Pd-Pt Interactions in the Oxidation and Sulfur Resistance of Bimetallic Pd-Pt/γ-Al2O3Diesel Oxidation Catalysts

Diesel oxidation catalysts (DOC) were investigated for oxidation activity, NO conversion stability, and sulfur poisoning/regeneration on Pd/Al2O3, Pt/Al2O3, and Pd-Pt/Al2O3 catalysts. The Pd/Al2O3 catalyst was more active for CO and hydrocarbon (C3H6 and C3H8) oxidation, while the Pt/Al2O3 catalyst efficiently oxidized NO. The formation of a Pd-Pt alloy in the Pd-Pt/Al2O3 catalyst maintained Pd in a more reduced phase, resulting in the superior activity of this catalyst for the oxidation of CO, C3H6, and NO in comparison with its monometallic counterparts. The Pd-Pt alloy not only provided more low-temperature activity but also retained the stability of NO oxidation. The Pd-Pt alloy also favored the spillover of SO2 to the alumina support, resulting in significantly higher adsorption capacity of the Pd-Pt/Al2O3 catalyst, extensively prolonging its lifetime. However, the stable sulfates on Pd-Pt/Al2O3 made it difficult to completely regenerate the catalyst. The bimetallic sample showed higher activity for CO, C3H8, and C3H6 after sulfur poisoning and regeneration

Investigation of CO Deactivation of Passive NOx Adsorption on La Promoted Pd/BEA

Passive NOx adsorption (PNA) is a method, in which NOx can be stored at low temperatures and released at higher temperatures where the urea decomposition is functional during selective catalytic reduction (i.e., above 180–200 \ub0C). We have studied the promotion of Pd/BEA with La as a PNA in the presence of high CO concentration. Both the reference and promoted samples exhibited a significant loss of NOx adsorption/desorption capacity after multiple cycles using 4000 ppm CO. However, already after 5 cycles, 99% of the NOx released between 200 and 400 \ub0C was lost for Pd/BEA, compared to only 64% for Pd-La/BEA, which thereafter was stable. XPS and O2-TPD clearly showed that the Pd species were influenced by La. The PNA deactivation in the presence of CO could be related to Pd reduction followed by migration and the formation of more PdOx clusters, as observed by O2-TPD analysis. Interestingly, significantly more PdOx clusters formed on Pd/BEA after 10 cycles compared to Pd-La/BEA

CO2 hydrogenation to light olefins using In2O3 and SSZ-13 catalyst − Understanding the role of zeolite acidity in olefin production

With the aim to explore the effect of acidic properties of zeolites in tandem catalysts on their performance for CO2 hydrogenation, two types of SSZ-13 zeolites with similar bulk composition, but different arrangements of framework Al, were prepared. Their morphology, pore structure, distribution of framework Al, surface acid strength and density, were explored. The results showed that SSZ-13 zeolites with isolated aluminum distribution could be successfully synthesized, however, they contained structural defects. During calcination, the framework underwent dealumination, resulting in weaker Br\uf8nsted acidity and lower crystallinity. The morphologies were, however, well preserved. Compared with the SSZ-13 zeolites, synthesized conventionally, these low acidity SSZ-13 zeolites with isolated aluminum were good zeolite components in bifunctional catalysts for CO2 hydrogenation to light olefins. By combining with In2O3, they exhibited better catalytic performance for light olefin production during CO2 hydrogenation at low temperatures. Na+ cation exchange was used to adjust the Br\uf8nsted acid site (BAS) density with only minor changes to the cavity structure. Comparative experiments established that the BAS density of the zeolite, rather than the framework Al distribution (BAS distribution), overwhelmingly affected catalyst stability and product selectivity. A higher acid density reduced the selectivity for light olefins, while lower acid density tended to form inert coke species leading to rapid deactivation. The ideal amount of BAS density in the bifunctional catalyst was approximately 0.25 mmol/g, which exhibited 70% selectivity for light olefins among hydrocarbons, and 74% selectivity for CO without deactivation, after 12 h reaction at 325 ℃ and 10 bar

Towards stable nickel catalysts for selective hydrogenation of biomass-based BHMF into THFDM

Selective transformation of BHMF (2,5-bis(hydroxymethyl)furan) to THFDM (tetrahydrofuran-2,5-dimethanol) over a variety of structured Ni/Sx-Z1−x catalysts was investigated. The effects of support, Ni loading, solvent, temperature, pressure, and particle size on the conversion and selectivity were studied. Among them, the 10 wt% Ni catalyst supported on the SiO2:ZrO2 weight ratio of 90:10 (10NiS90Z10) exhibits the best performance in terms of BHMF conversion and THFDM selectivity. Its good performance was attributed to its well-balanced properties, that depend upon the ZrO2 content of the support in combination with SiO2, the active Ni sites-support interaction, and acidity/basicity ratio of each catalyst resulting in different Ni dispersions. Importantly, the 10NiS90Z10 catalyst showed a stable selectivity to THFDM (>94%), with 99.4% conversion of BHMF during 2 h reaction time. Poor catalytic activity resulted from excessive ZrO2 content (>10 wt%). The structural, textural, and acidity properties of NiSi100−y-Zry catalysts, tuned by selectively varying the Ni amount from 5 to 15 wt%, were critically investigated using numerous material characterization techniques. Catalyst recycling experiments revealed that the catalyst could be recycled several times without any measurable loss of catalytic activity

Enhanced CO resistance of Pd/SSZ-13 for passive NOx adsorption

Passive NOx adsorption (PNA) is a novel technology to control NOx emissions during cold start. However, the recent generation of PNA material, Pd/zeolite, suffers from major degradation under high CO concentrations. In this work, we developed a novel form of Pd/SSZ-13 by using a freeze-drying process after incipient wetness impregnation. This Pd/SSZ-13 showed a better stability than the sample synthesized by the common process. Several characterization measurements were conducted and it was found that the Pd sites on the freeze-dried sample were more resistant towards CO-induced agglomeration. By combing in-situ characterization and kinetic modeling, we found that the freeze-dried Pd/SSZ-13 had more ion-exchanged Pd sites, which provided greater resistance towards the CO-induced Ostwald ripening process, and consequently suppressed the sintering behavior under a high CO concentration. This material offers a potentially improved stability of PNAs under extremely high CO concentration pulses from incomplete diesel combustion during engine cold start