Changes In Product Selectivity Research Articles

Sulfate residuals on Ru catalysts switch CO2 reduction from methanation to reverse water-gas shift reaction

Efficient heterogeneous catalyst design primarily focuses on engineering the active sites or supports, often neglecting the impact of trace impurities on catalytic performance. Herein, we demonstrate that even trace amounts of sulfate (SO42−) residuals on Ru/TiO2 can totally change the CO2 reduction from methanation to reverse-water gas shift (RWGS) reaction under atmospheric pressure. We reveal that air annealing causes the trace amount of SO42− to migrate from TiO2 to Ru/TiO2 interface, leading to the significant changes in product selectivity from CH4 to CO. Detailed characterizations and DFT calculations show that the sulfate at Ru/TiO2 interface notably enhances the H transfer from Ru particles to the TiO2 support, weakening the CO intermediate activation on Ru particles and inhibiting the further hydrogenation of CO to CH4. This discovery highlights the vital role of trace impurities in CO2 hydrogenation reaction, and also provides broad implications for the design and development of more efficient and selective heterogeneous catalysts.

Alkaline Earth Metal Aluminate Support for Selective Oxidative Coupling of Methane

AbstractThe oxidative coupling of methane (OCM) provides a direct route to transform methane into higher value products. The alkali metal tungstate catalysts have demonstrated high selectivity towards C2 products, ethane (C2H6) and ethylene (C2H4). However, the severe sintering of the SiO2 support limits the reaction rate requiring active site densification to compete with unselective gas phase combustion reaction especially at high pressure operations. This work studies alkaline earth metal oxides as supports for K2WO4 catalysts to maintain comparatively high surface area under OCM conditions. Among Mg, Ca, and Sr aluminates, the K2WO4/MgAl2O4 catalyst exhibited the highest C2 yields. To achieve high C2 product selectivity, a relatively large loading of K2WO4 (20 wt %) was required likely to cover the unselective surface sites of MgAl2O4 support. The catalyst further showed high levels of stability when utilized at high pressures (0.9 MPa) for over 60 h, without any change in product selectivity. The Mn/K2WO4/MgAl2O4 showed multifold higher CH4 conversion rate, compared with the SiO2 counterpart. The findings showcase the potential of the MgAl2O4 support as a viable candidate for alkali metal tungstate catalysts for OCM, which introduces high density of active components in given volume in the reactor, which induces high contribution of the catalyst relative to the pure gas phase oxidation.

Controlling product selectivity during dioxygen reduction with Mn complexes using pendent proton donor relays and added base.

The catalytic reduction of dioxygen (O2) is important in biological energy conversion and alternative energy applications. In comparison to Fe- and Co-based systems, examples of catalytic O2 reduction by homogeneous Mn-based systems is relatively sparse. Motivated by this lack of knowledge, two Mn-based catalysts for the oxygen reduction reaction (ORR) containing a bipyridine-based non-porphyrinic ligand framework have been developed to evaluate how pendent proton donor relays alter activity and selectivity for the ORR, where Mn(p-tbudhbpy)Cl (1) was used as a control complex and Mn(nPrdhbpy)Cl (2) contains a pendent -OMe group in the secondary coordination sphere. Using an ammonium-based proton source, N,N'-diisopropylethylammonium hexafluorophosphate, we analyzed catalytic activity for the ORR: 1 was found to be 64% selective for H2O2 and 2 is quantitative for H2O2, with O2 binding to the reduced Mn(ii) center being the rate-determining step. Upon addition of the conjugate base, N,N'-diisopropylethylamine, the observed catalytic selectivity of both 1 and 2 shifted to H2O as the primary product. Interestingly, while the shift in selectivity suggests a change in mechanism for both 1 and 2, the catalytic activity of 2 is substantially enhanced in the presence of base and the rate-determining step becomes the bimetallic cleavage of the O-O bond in a Mn-hydroperoxo species. These data suggest that the introduction of pendent relay moieties can improve selectivity for H2O2 at the expense of diminished reaction rates from strong hydrogen bonding interactions. Further, although catalytic rate enhancements are observed with a change in product selectivity when base is added to buffer proton activity, the pendent relays stabilize dimer intermediates, limiting the maximum rate.

Carbon Supported Pd Nanostructures for Electrochemical Reduction of Carbon Dioxide – Effects of Ozonation

Out of the transition metals capable of electrochemical carbon dioxide reduction, Pd is interesting as it can convert carbon dioxide electrochemically into formate or carbon monoxide depending on the applied potential. In fact, it is capable of producing formate at the most positive known potentials that are close to zero overpotentials although at an unfortunately low activity and at the cost of deactivation by carbon monoxide poisoning. One aim is to improve the activity and stability of Pd-based electrocatalysts towards formate production in low overpotentials. As Pd is a critical raw material, we also wish to decrease the amount of Pd required while maintaining high carbon dioxide electroreduction capability. These goals can be achieved by nanostructuring and supporting the Pd catalyst. Here, we have employed a simple wet impregnation synthesis approach to prepare small nanoparticles and nanowires of Pd supported on single walled carbon nanotubes and tested the optimum loading of Pd to obtain high formate yield with improved activity and stability. Reactive sites can be created on the carbon support by subjecting it to ozonation prior to supporting the metal, which may help certain interesting nanostructures, such as nanowires, to grow. Additionally, the oxygen functional groups on the carbon surface are expected to affect the wettability of the electrode which is important for achieving an efficient carbon dioxide electroreduction and a longer-term stability of the reaction. Therefore, we also studied the effects of ozonation of the carbon supports on the electrochemical reduction of carbon dioxide into both formate and syngas (mixture of hydrogen and carbon monoxide) on Pd. Carbon atoms inevitably participate in hydrogen evolution reaction and, thus, in syngas production on Pd-supported catalysts at higher overpotentials. Our results show that ozonation greatly enhances the activity of the catalyst material and improves its stability when applying low overpotentials for formate formation in comparison to the pristine carbon support. The current density on Pd supported ozone treated carbon nanotube material remains stable over 4h of carbon dioxide electrolysis at an applied potential of -0.45 V (vs. RHE) while Pd on pristine carbon support deactivates during the initial 30 min of the experiment. Longer electrolysis times do reveal slow changes in product distribution although activity on ozone-treated single walled carbon nanotube-supported catalyst is excellent. Additionally, the different support materials cause interesting changes in product selectivity upon applying higher overpotentials for the production of syngas. Pd supported on pristine nanotubes produces syngas with carbon monoxide-to-hydrogen ratios of 0.72 and 1.38 at applied potentials of -0.85 V (vs. RHE) and -0.95 V (vs. RHE), respectively, while ozone treated material produces less than 10% of carbon monoxide. Through physico-chemical characterizations of the materials we aim at understanding the observed changes in electrochemical reduction of carbon dioxide on carbon supported Pd nanostructures.

Contaminant removal from plastic waste pyrolysis oil via depth filtration and the impact on chemical recycling: A simple solution with significant impact

Recycling of mixed plastic waste via pyrolysis and subsequent steam cracking towards light olefins is a promising solution for the ever-growing plastic waste crisis. However, the pyrolytic recycling pathway is still not established on industrial scale due to the large variety of pyrolysis oil contaminants that hamper the application in (petro-)chemical processes. In short, plastic waste pyrolysis oils are unfeasible for steam crackers without upgrading. In this work, depth filtration for the removal of particulate contamination from a post-consumer mixed polyolefin (MPO) pyrolysis oil was performed using three different filter media with different porosity. Comprehensive analyses using SEM-EDX of the retained particles as well as ICP-OES and GC × GC-FID of the filtered oils allowed to understand the efficacy of filtration. Most of the particulate contamination was removed by filtration leading to a reduction from 69 mg/L in the unfiltered sample to < 2 mg/L in the filtered samples. Particle characterization confirmed that the main contamination is composed of iron, calcium and silicon-based contaminants in combination with carbon-based species. It was confirmed by ICP-OES that after filtration, important contaminants such as nickel, vanadium and lead were reduced below contamination thresholds for steam crackers indicating that filtration is an efficient, and potentially cost-effective upgrading technique for pyrolysis oils. When steam cracking the filtered oils, results show that radiant coil coke formation was reduced by 40–60% compared to the unfiltered oil without changes in product selectivity, confirming the strong impact of particulate contamination on coke formation during steam cracking.

Assessment of the Degradation Mechanisms of Cu Electrodes during the CO2 Reduction Reaction

Catalyst degradation and product selectivity changes are two of the key challenges in the electrochemical reduction of CO2 on copper electrodes. Yet, these aspects are often overlooked. Here, we combine in situ X-ray spectroscopy, in situ electron microscopy, and ex situ characterization techniques to follow the long-term evolution of the catalyst morphology, electronic structure, surface composition, activity, and product selectivity of Cu nanosized crystals during the CO2 reduction reaction. We found no changes in the electronic structure of the electrode under cathodic potentiostatic control over time, nor was there any build-up of contaminants. In contrast, the electrode morphology is modified by prolonged CO2 electroreduction, which transforms the initially faceted Cu particles into a rough/rounded structure. In conjunction with these morphological changes, the current increases and the selectivity changes from value-added hydrocarbons to less valuable side reaction products, i.e., hydrogen and CO. Hence, our results suggest that the stabilization of a faceted Cu morphology is pivotal for ensuring optimal long-term performance in the selective reduction of CO2 into hydrocarbons and oxygenated products.

Significance and influence of various promoters on Cu‐based catalyst for synthesizing methanol from syngas: a critical review

AbstractMethanol synthesis from syngas is an effective method for producing fuel additives, oxygenated fuels, and other chemicals. Several promoted catalytic systems have been tested for the synthesis of methanol. Catalysts based on noble metals are extremely active in generating methanol from syngas. However, sintering, poisoning, and related expenses accounted for the switch to Cu‐based catalysts. The promotion of Cu‐based catalysts, even with small doses of an effective promoter, resulted in a considerable change in product selectivity. A suitable promoter is required for Cu‐based catalysts used in the synthesis of methanol because it significantly impacts the electronic, geometric, or acid/base surface properties, which in turn affect the catalyst's activity and selectivity. It also significantly impacts the interactions between the catalyst's active ingredients. However, the interactions between the active component and the promoter can be adjusted or influenced by appropriate support. The present paper primarily focuses on several promoters in the copper‐based catalyst for synthesizing methanol from syngas. We briefly deliberated on the need for copper‐based catalysts for methanol synthesis, their reaction and mechanism, the reasons for promoter addition concerning their role and influence, the importance of CO2 addition in feed synthesis gas and their impact on the catalyst. This paper mainly highlights the selectivity and activity of various promoted Cu‐based catalysts, although different reaction parameters are the cause of promotion and inhibition in the catalyst. The effects of reaction conditions, including pressure, temperature, gas hourly space velocity (GHSV), and syngas composition, are briefly reported. This paper also comprehensively compares the effects of adding various promoters in small amounts to copper‐based catalysts. This review intends to be illustrative rather than exhaustive. © 2023 Society of Chemical Industry (SCI).

Effect of acid modification of ZSM-5 catalyst on performance and coke formation for methanol-to-hydrocarbon reaction

To study effects of different acid treatments with HZSM-5 zeolites on the chemistry of framework, acid nature, performance, and coke formation for methanol-to-hydrocarbon reaction, HZSM-5 with different Si/Al ratio were post treated with two different acid solutions (tartaric acid and ammonium hexafluorosilicate). After the post treatment with hexafluorosilicate (HZn-HF), it was found the extensive decrease in all kinds of acid sites owing to the non-selective dealumination, whereas the noticeable decrease in weak Lewis acid sites was found after the post treatment with tartaric acid (HZn-TA) resulted by the selective removal of the surface alumium. The result of the distingushed removal of Al from the bare HZSM-5 zeolites (HZn) also attributed the change in product selectivity for the MTH reaction as follows; high carbon distributions to the C2–C4 range olefins and low carbon distribution to C10+ species from the HZn-HF samples whereas high carbon distribution to BTEX from the HZn-TA samples. Analysis of the spent catalysts revealed the amount and chemistry of carbon deposits formed during the MTH reaction are directly related to the acidic nature of the HZSM-5 catalysts. For example, the amount of carbon deposit from the spent catalysts were found to be increased in the order: HZn-TA > HZn > HZn-HF. Therefore, among the bare and acid modified HZ catalysts, the HZn-HF type catalyst showed the least carbon deposition despite requiring the longest reaction time for the methanol conversion to reach 80%.

Revealing the Doping Effect of Cu2+ on SrSnO3 Perovskite Oxides for CO2 Electroreduction

AbstractIn this study, SrSnO3, 0.5 wt %‐SS and 1 wt %‐SS (SrSnO3 loaded with 0.5 wt % Cu2+ and 1 wt % Cu2+, respectively) catalysts were designed using a simple and versatile strategy to investigate the changes of products selectivity in CO2 electroreduction (CER) induced by doping Cu2+ on SrSnO3 perovskite oxide. Both SrSnO3 and 0.5 wt %‐SS showed a consistent trend for the selectivity of HCOOH. While 1 wt %‐SS showed a significant change in product selectivity and the FECO reached a maximum at −1.53 V with a value of 49 %. The sufficient Cu2+ loaded on the surface of 1 wt %‐SS worked as the actual active sites during the CER and adjusted the pathway of product generation of SrSnO3 in the CER process, subsequently promoting the generation of CO and achieving regulation of the CER product selectivity. This work may stimulate more attention on the design of the Cu‐based perovskite oxide catalyst for CER.

Solvation Effects on Quantum Tunneling Reactions.

A decisive factor for obtaining high yields and selectivities in organic synthesis is the choice of the proper solvent. Solvent selection is often guided by the intuitive understanding of transition state-solvent interactions. However, quantum-mechanical tunneling can significantly contribute to chemical reactions, circumventing the transition state and thus depriving chemists of their intuitive handle on the reaction kinetics. In this Account, we aim to provide rationales for the effects of solvation on tunneling reactions derived from experiments performed in cryogenic matrices.The tunneling reactions analyzed here cover a broad range of prototypical organic transformations that are subject to strong solvation effects. Examples are the hydrogen tunneling probability for the cis-trans isomerization of formic acid which is strongly reduced upon formation of hydrogen-bonded complexes and the [1,2]H-shift in methylhydroxycarbene where a change in product selectivity is predicted upon interaction with hydrogen bond acceptors.Not only hydrogen but also heavy atom tunneling can exhibit strong solvent effects. The direction of the nearly degenerate valence tautomerization between benzene oxide and oxepin was found to reverse upon formation of a halogen or hydrogen bond with ICF3 or H2O. But even in the absence of strong noncovalent interactions such as hydrogen or halogen bonding, solvation can have a decisive effect on tunneling as evidenced by the Cope rearrangement of semibullvalenes via heavy-atom tunneling. Can quantum tunneling be catalyzed? The acceleration of the ring expansion of 1H-bicyclo[3.1.0.]-hexa-3,5-dien-2-one by complexation with Lewis acids provides a proof-of-concept for tunneling catalysis.Two concepts are central for the explanation and prediction of solvation effects on tunneling phenomena: a simple approach expands the Born-Oppenheimer approximation by separating nuclear degrees of freedom into intra- and intermolecular degrees. Intermolecular movements represent the slowest motions within molecular aggregates, thus effectively freezing the position of the solvent in relation to the reactant during the tunneling process. Another useful approach is to treat reactants and products by separate single-well potentials, where the intersection represents the transition state. Thus, stabilization of the reactants via solvation should result in an increase in barrier heights and widths which in turn lowers tunneling probabilities. These simple models can predict trends in tunneling kinetics and provide a rational basis for controlling tunneling reactions via solvation.

Methanol mediated direct CO2 hydrogenation to hydrocarbons: Experimental and kinetic modeling study

Carbon dioxide can be utilized as a feedstock to produce chemicals and renewable fuels sustainably. CO2 hydrogenation to hydrocarbons through a methanol mediated pathway requires a more detailed study, examining interactions between reaction processes leading to different product selectivities. In this particular work, we propose a kinetic model for the direct CO2 hydrogenation to different hydrocarbon products over an In2O3/HZSM-5 bifunctional catalytic bed. The model includes a CO2 hydrogenation to methanol model based on a Langmuir Hinshelwood Hougen Watson (LHHW) reaction mechanism over In2O3 catalyst combined with a lump-type methanol to hydrocarbon (MTH) model over the HZSM-5 zeolite. Interestingly, the combined model could largely predict the suppression of the reverse water gas shift (RWGS) reaction and an increase in the yield of hydrocarbons compared to the formation of methanol in case of the same reaction conditions carried out with only the methanol synthesis catalyst (In2O3). Further, by varying the mass ratio of the individual components of the bifunctional catalytic bed, it was demonstrated that a higher outlet concentration of methanol achieved with a higher mass ratio of the methanol synthesis catalyst caused less suppression of the RWGS reaction and shifted the hydrocarbon product distribution to a slightly larger share of higher hydrocarbons. These changes in product selectivity caused by variation of the catalyst mass ratio were both also successfully reproduced by the model. Therefore, a comparison between the experimental results and the model predictions shows that this model, including equilibrium effects for the reactions, can accurately predict the trends of the experimental findings for direct CO2 hydrogenation to hydrocarbons over the In2O3/HZSM-5 catalyst.

Post-Mortem Analysis of MNC Catalysts at Different Stages of CO2 Reduction Reaction

For the wellbeing of future generations, the CO2 footprint must be drastically reduced. To address this, CO2 can be used as a starting substance for the fabrication of CO or hydrocarbons as a valuable step in the materials and energy cycles. Metal-nitrogen-carbon catalysts (MNC) are known to be promising materials for the electrochemical CO2 reduction reaction (CO2RR) into various products.1,2 Their activity towards CO2RR has been attributed mainly to molecular structural motifs (MN4 centers) embedded in the carbon network.3,4 The benefit of these single site catalysts is that they offer the possibility of tuning the selectivity and activity by varying the nature of the central metal. However, their suitability for CO2 electrolyzers in the near future depends on the optimization the catalysts for achieving high current densities whilst maintaining the integrity of the active sites. And because high reduction potentials may be required to achieve this, it is important to gain insights on the behavior of MNC under these conditions, as it has been proposed that restructuration of the metal sites may occur.5 Therefore, in this study different post mortem characterization techniques are applied on a group of MNCs for gaining a better understanding of the relation between selectivity and structural changes over time. The preparation of the catalysts was chosen such, that secondary phases in the as-prepared materials were minimized. The synthesis was performed for non-precious and abundant metals such as Fe, Cu, Ni and Sn which also show the best performances for CO2RR.6 The product analysis was done by coupling a gas-fed electrochemical cell with carbon-based gas diffusion electrodes (GDEs) in line with mass spectrometry, subsequently HPLC was used for the liquid product analysis. Techniques such as post mortem XPS, Identic location TEM (IL-TEM) and Raman spectroscopy were used to understand the influence of the applied potential on the changes in product selectivity and morphology of the GDEs.By the comparison of the metal 2p3/2 and N 1s XP-spectra with the onset potentials for the different CO2 reduction products, important changes can be seen to depend on the applied reduction potential. The integrated approach offers unique insights for understanding the role of the metal center in MNC catalysts in pursuit of their adaptation for future applications.(1) Varela, A. S.; Ranjbar Sahraie, N.; Steinberg, J.; Ju, W.; Oh, H.-S.; Strasser, P. Angew. Chemie 2015, 127 (37), 10908–10912.(2) Tripkovic, V.; Vanin, M.; Karamad, M.; Björketun, M. E.; Jacobsen, K. W.; Thygesen, K. S.; Rossmeisl, J. J. Phys. Chem. C 2013, 117 (18), 9187–9195.(3) Leonard, N.; Ju, W.; Sinev, I.; Steinberg, J.; Luo, F.; Varela, A. S.; Roldan Cuenya, B.; Strasser, P. Chem. Sci. 2018, 9 (22), 5064–5073.(4) Karapinar, D.; Tran, N. H.; Giaume, D.; Ranjbar, N.; Jaouen, F.; Mougel, V.; Fontecave, M. Sustain. Energy Fuels 2019, 3 (7), 1833–1840.(5) Karapinar, D.; Huan, N. T.; Ranjbar Sahraie, N.; Li, J.; Wakerley, D.; Touati, N.; Zanna, S.; Taverna, D.; Galvão Tizei, L. H.; Zitolo, A.; Jaouen, F.; Mougel, V.; Fontecave, M. Angew. Chemie - Int. Ed. 2019, 58 (42), 15098–15103.(6) Paul, S.; Kao, Y.-L.; Ehnert, R.; Herrmann-Geppert, I.; van de Krol, R.; Stark, R. W.; Jaegermann, W.; Kramm, U. I.; Bogdanoff, P. ACS Catal. (under revision) 2021.

A Dichotomy in Cross-Coupling Site Selectivity in a Dihalogenated Heteroarene: Influence of Mononuclear Pd, Pd Clusters, and Pd Nanoparticles-the Case for Exploiting Pd Catalyst Speciation.

Site-selective dihalogenated heteroarene cross-coupling with organometallic reagents usually occurs at the halogen proximal to the heteroatom, enabled by intrinsic relative electrophilicity, particularly in strongly polarized systems. An archetypical example is the Suzuki–Miyaura cross-coupling (SMCC) of 2,4-dibromopyridine with organoboron species, which typically exhibit C2-arylation site-selectivity using mononuclear Pd (pre)catalysts. Given that Pd speciation, particularly aggregation, is known to lead to the formation of catalytically competent multinuclear Pdn species, the influence of these species on cross-coupling site-selectivity remains largely unknown. Herein, we disclose that multinuclear Pd species, in the form of Pd3-type clusters and nanoparticles, switch arylation site-selectivity from C2 to C4, in 2,4-dibromopyridine cross-couplings with both organoboronic acids (SMCC reactions) and Grignard reagents (Kumada-type reactions). The Pd/ligand ratio and the presence of suitable stabilizing salts were found to be critically important in switching the site-selectivity. More generally, this study provides experimental evidence that aggregated Pd catalyst species not only are catalytically competent but also alter reaction outcomes through changes in product selectivity.

Chlorodifluoromethane Hydrodechlorination on Carbon-Supported Pd-Pt Catalysts. Beneficial Effect of Catalyst Oxidation

Previously tested 2 wt % palladium-platinum catalysts supported on Norit activated carbon preheated to 1600 °C have been reinvestigated in CHFCl2 hydrodechlorination. An additionally adopted catalyst oxidation at 350–400 °C produced nearly an order of magnitude increase in the turnover frequency of Pd/C, from 4.1 × 10−4 to 2.63 × 10−3 s−1. This increase is not caused by changes in metal dispersion or possible decontamination of the Pd surface from superficial carbon, but rather by unlocking the active surface, originally inaccessible in metal particles tightly packed in the pores of carbon. Burning carbon from the pore walls attached to the metal changes the pore structure, providing easier access for the reactants to the entire palladium surface. Calcination of Pt/C and Pd-Pt/C catalysts results in much smaller evolution of catalytic activity than that observed for Pd/C. This shapes the relationship between turnover frequency (TOF) and alloy composition, which now does not confirm the Pd-Pt synergy invoked in the previous work. The absence of this synergy is confirmed by gradual regular changes in product selectivity, from 70 to 80% towards CH2F2 for Pd/C to almost 60% towards CH4 for Pt/C. The use of even higher-preheated carbon (1800 °C), completely free of micropores, results in a Pd/C catalyst that does not need to be oxidized to achieve high activity and excellent selectivity to CH2F2 (&gt;90%).

Hydrodechlorination of CHClF2 (HCFC-22) over Pd–Pt Catalysts Supported on Thermally Modified Activated Carbon

Commercial activated carbon, pretreated in helium at 1600 °C and largely free of micropores, was used as a support for two series of 2 wt.% Pd–Pt catalysts, prepared by impregnating the support with metal acetylacetonates or metal chlorides. The catalysts were characterized by temperature-programmed methods, H2 chemisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM) with energy dispersive spectroscopy (EDS). Overall, the results confirmed the existence of well-dispersed Pd–Pt nanoparticles in the bimetallic catalysts, ranging in size from 2 to 3 nm. The catalysts were investigated in the gas phase hydrodechlorination of chlorodifluoromethane (HCFC-22). In this environmentally relevant reaction, both the ex-chloride and ex-acetylacetonate Pd–Pt/C catalysts exhibited better hydrodechlorination activity than the monometallic catalysts, which is consistent with the previous results of hydrodechlorination for other chlorine-containing compounds. This synergistic effect can be attributed to the electron charge transfer from platinum to palladium. In general, product selectivity changes regularly with Pd–Pt alloy composition, from high in CH2F2 for Pd/C (70–80%) to the selective formation of CH4 for Pt/C (60–70%).

Seashell waste-derived materials for secondary catalytic tar reduction in municipal solid waste gasification

Catalytic tar removal from producer gas is critical for the economic feasibility of Municipal Solid Waste (MSW) gasification in the waste-to-energy(WtE) approach. Nickel- and noble-metal catalysts have the highest tar cracking activities, but they increase costs, use scarce materials, and generate dangerous byproducts. To overcome these drawbacks, naturally occurring materials should be used for tar cracking. In this paper, two nanomaterials, synthesized from oyster and mussel waste shells respectively, are used to clean syngas from MSW in a secondary tar cracking unit. We observed that they reform class 1 tar (heavy tars that condense at high temperatures at very low concentrations) into class 3 tar (light hydrocarbons that are not important in condensation) and benzene. Although both catalysts’ composition and textural properties were identical, crystallite size and especially specific surface area variation was enough to generate a change in product selectivity. A larger crystallite size and SSA shows a soot yield reduction of 95% with respect to the non-catalytic case, simultaneously increasing the H2/CO at 1000 °C.

Methanol-Based Chain Elongation with Acetate to n-Butyrate and Isobutyrate at Varying Selectivities Dependent on pH

Biomass fermentation technologies offer alternative methods to produce platform chemicals that currently originate from fossil sources. This research showed that an enriched microbiome was capable to produce iso-butyric acid via methanol based chain elongation of acetate. A long term continuous reactor experiment showed that the selectivity for isobutyrate (i-C4) and/or n-butyrate (n-C4) could be reversibly adjusted by changing the reactor pH. A reactor pH of 6.75 led to formation of (carbon per total carbon of products) of 0.78 n-C4 and 0.024 i-C4, whereas a reactor pH of 5.2 led to a selectivity of 0.25 n-C4 and 0.63 i-C4 . This shift in product spectrum was also represented by a shift in microbial composition. The results suggest that an Eubacterium genus is responsible for the formation of n-C4, whereas a Clostridium luticellarii strain is responsible for the formation of a mixture of i-C4 and n¬¬-C4. The formation of n and i-C4 at a low pH was observed to be coupled according to the thermodynamics of isomerization. At a reactor pH of 5.5 and 5.2 the product ratio of i-C4:n-C4 approached the ratio of 0.684 i-C4 : 0.316 n-C4, which is the theoretical ratio that would be achieved when i-C4 and n-C4 would be formed at amounts determined by the equilibrium of isomerization. Various batch experiments at pH 5.5 and 5.2 confirmed that addition of either n-C4 or i-C4 at the start of the batch would immediately lead to the formation of the other butyrate component. Moreover, batch experiments performed at pH 6.5 immediately produced mainly n-C4 and led to the development of a completely different microbiome. The imposed pH as a strong selective pressure can immediately facilitate changes in product selectivities for n-C4 and i-C4 during methanol based chain elongation of acetate.

Electrochemical reduction of CO2 using Germanium-Sulfide-Indium amorphous glass structures

The research in electrochemical reduction of CO2 is shifting towards the discovery of new and novel materials. This study shows a new class of material, that of Ge-S-In chalcogenide glass, to be active for reduction of CO2 in aqueous solutions. Experiments were conducted with bulk and particle form of the material, yielding different product for each structural form. Faradaic efficiency of upto 15% was observed in bulk form for CO production while formic acid with up to 26.1 % faradaic efficiency was measured in powder form. Chalcogenide studies have focused primarily on the photoelectrochemical reduction however these results provide a strong merit for introducing metal in chalcogenide glass structures for electrochemical reduction of CO2. The activity for CO2 reduction and the change in product selectivity reflects that further efforts to improve the glass structures can be undertaken in order to increase the faradaic efficiency and selectivity of the products.

Synthesis and Characterization of Degradation‐Resistant Cu@CuPd Nanowire Catalysts for the Efficient Production of Formate and CO from CO2

AbstractNot only are the product selectivity and the activity of electrocatalysts important aspects for the evaluation of their overall performance, but also their stability and resistance against structural and compositional degradation during the electrocatalytic reaction. Palladium nanoparticles (Pd‐NPs) have already been identified as superior electrocatalysts for the electrochemical conversion of CO2 into formate at extremely low overpotentials and into carbon monoxide (CO) at medium overpotentials. However, these catalysts suffer from fast degradation, owing to irreversible CO poisoning of Pd reaction sites. Herein, we report on nanoparticulate bimetallic Pd45Cu55 catalysts, demonstrating a similar selectivity and activity to the pure Pd‐NPs, but showing additional superior degradation stability and resistance against CO poisoning. Pd45Cu55 catalysts were synthesized by means of a galvanic displacement reaction using copper nanowires (Cu‐NWs) as the starting material (template) for the galvanic displacement reaction, which leaves a surface‐confined alloy film and respective nanoparticles on the Cu‐NW template (denoted as Cu@CuPd‐NWs). A faradaic efficiency (FE) up to FEformate ≈80 % can be achieved at −0.3 V vs. RHE, thus clearly proving that the ‘anomalous’ CO2 reaction mechanism via the CO2 hydrogenation pathway, discussed for pure Pd‐NPs, can also be transferred to Pd‐based bimetallic systems. At medium overpotentials, the product selectivity changes from selective formate to predominant CO formation with a maximum faradaic efficiency of FECO=86 %. Extended CO2 electrolyses demonstrate, for both formate and CO production, superior degradation stability, for example, FECO remains at 85 %±5 % for the duration of 20 h. For the first time, identical location high‐angle annular dark‐field scanning transmission electron microscopy (IL‐HAADF‐STEM) in combination with energy‐dispersive X‐ray spectrometry and identical location scanning electron microscopy (IL‐SEM) were applied to demonstrate the compositional and structural stability of the bimetallic catalyst under CO2 reduction conditions.

Alternating catalytic reactions

The application of alternating current is advantageous in energy transfer over long distances. It is a well-known fact now, but subject of long conflict in the era of pioneering works in electric power production. There are also some processes in physical chemistry, organic and inorganic chemistry, in biochemistry and related sciences, which take place in opposite directions, with consecutive alternations in time. However, the very existence of alternate reactions, now known as the oscillatory or oscillating reactions, has long been disputed because it was thought that it is contrary to the basic principles of thermodynamics. Nevertheless, according to our knowledge, there are no oscillatory reactions without catalytic loop as the essential part of a mechanism. There could be a fundamental rule that catalysis is necessary to generate oscillations in concentrations and reaction rates. Particularly, homogeneous oscillatory reactions are often subject of research as relatively simple systems with good chance to clearly define feedbacks responsible for instability phenomena. However, oscillations can at least equally often be found in heterogeneous catalytic reactions. Recently, changes in product selectivity was proved when Pd catalyzed carbonylation of phenylacetylene was moved to the oscillatory dynamic state. With this simple result, the doors are now open for wide spectrum of research projects and applications.