Showing papers in "Topics in Catalysis in 2018"
TL;DR: In this article, the electronic structure of several representative single atom alloys of gold, silver and copper doped with single atoms of platinum, palladium, iridium, rhodium and nickel in the context of the d-band model of Hammer and Norskov.
Abstract: Single atom alloys are gaining importance as atom-efficient catalysts which can be extremely selective and active towards the formation of desired products. They possess such desirable characteristics because of the presence of a highly reactive single atom in a less reactive host surface. In this work, we calculated the electronic structure of several representative single atom alloys. We examined single atom alloys of gold, silver and copper doped with single atoms of platinum, palladium, iridium, rhodium and nickel in the context of the d-band model of Hammer and Norskov. The reactivity of these alloys was probed through the dissociation of water and nitric oxide and the hydrogenation of acetylene to ethylene. We observed that these alloys exhibit a sharp peak in their atom projected d-band density of states, which we hypothesize could be the cause of high surface reactivity. We found that the d-band centers and d-band widths of these systems correlated linearly as with other alloys, but that the energy of adsorption of a hydrogen atom on these surfaces could not be correlated with the d-band center, or the average reactivity of the surface. Finally, the single atom alloys, with the exception of copper–palladium showed good catalytic behavior by activating the reactant molecules more strongly than the bulk atom behavior and showing favorable reaction pathways on the free energy diagrams for the reactions investigated.
101 citations
TL;DR: The objective of this review is to motivate researchers to design new strategies to break this strong C–C inter-unit bonds to valorise lignins, technical lignin in particular.
Abstract: Lignin depolymerisation has received considerable attention recently due to the pressing need to find sustainable alternatives to fossil fuel feedstock to produce chemicals and fuels. Two types of interunit linkages (C–C and C–O linkages) link several aromatic units in the structure of lignin. Between these two inter-unit linkages, the bond energies of C–C linkages are higher than that of C–O linkages, making them harder to break. However, for an efficient lignin depolymerisation, both types of inter-unit linkages have to be broken. This is more relevant because of the fact that many delignification processes tend to result in the formation of additional C–C inter-unit bonds. Here we review the strategies reported for the cleavage of C–C inter-unit linkages in lignin model compounds and lignin. Although a number of articles are available on the cleavage of C–O inter-unit linkages, reports on the selective cleavage of C–C inter-unit linkages are relatively less. Oxidative cleavage, hydrogenolysis, two-step redox-neutral process, microwave assisted cleavage, biocatalytic and photocatalytic methods have been reported for the breaking of C–C inter-unit linkages in lignin. Here we review all these methods in detail, focused only on the breaking of C–C linkages. The objective of this review is to motivate researchers to design new strategies to break this strong C–C inter-unit bonds to valorise lignins, technical lignins in particular.
99 citations
TL;DR: The calculations reveal that CO adsorption induces dopant atom segregation into the surface layer for all SAAs considered here, whereas aggregation and island formation may be promoted or inhibited depending on alloy constitution and CO coverage, which suggests the possibility of controlling ensemble effects in novel catalyst architectures through CO-induced aggregation and kinetic trapping.
Abstract: Platinum group metals (PGMs) serve as highly active catalysts in a variety of heterogeneous chemical processes. Unfortunately, their high activity is accompanied by a high affinity for CO and thus, PGMs are susceptible to poisoning. Alloying PGMs with metals exhibiting lower affinity to CO could be an effective strategy toward preventing such poisoning. In this work, we use density functional theory to demonstrate this strategy, focusing on highly dilute alloys of PGMs (Pd, Pt, Rh, Ir and Ni) with poison resistant coinage metal hosts (Cu, Ag, Au), such that individual PGM atoms are dispersed at the atomic limit forming single atom alloys (SAAs). We show that compared to the pure metals, CO exhibits lower binding strength on the majority of SAAs studied, and we use kinetic Monte Carlo simulation to obtain relevant temperature programed desorption spectra, which are found to be in good agreement with experiments. Additionally, we consider the effects of CO adsorption on the structure of SAAs. We calculate segregation energies which are indicative of the stability of dopant atoms in the bulk compared to the surface layer, as well as aggregation energies to determine the stability of isolated surface dopant atoms compared to dimer and trimer configurations. Our calculations reveal that CO adsorption induces dopant atom segregation into the surface layer for all SAAs considered here, whereas aggregation and island formation may be promoted or inhibited depending on alloy constitution and CO coverage. This observation suggests the possibility of controlling ensemble effects in novel catalyst architectures through CO-induced aggregation and kinetic trapping.
98 citations
TL;DR: In this paper, a small amount of Ni on either supported or unsupported Au surfaces induces resistance to sintering, along with a beneficial effect on the catalytic activity of the alloy.
Abstract: Gold is examined here as an alternative to copper for the selective dehydrogenation of ethanol to acetaldehyde and hydrogen. Despite its high selectivity, gold is only active at temperatures higher than 250 °C for this reaction. We demonstrate that addition of a small amount of Ni on either supported or unsupported Au surfaces induces resistance to sintering, along with a beneficial effect on the catalytic activity. NiAu alloys prepared here with Ni as the minority component to the limit of atomic dispersion in the gold surfaces, catalyze the reaction beginning below 150 °C. A significant decrease of the apparent activation energy from 96 ± 3 kJ/mol for the monometallic Au to 59 ± 5 kJ/mol for the alloy was found. The Ni dispersion and concentration as a function of gas environment was followed by in situ DRIFTS and by XPS. The stability of the catalyst morphology was investigated through post-reaction microscopy imaging and long-term stability tests under reaction conditions. As shown via dynamic reaction experiments, acetaldehyde and H2 were selectively produced up to 280 °C. A small drop of selectivity at higher temperatures is attributed to the formation of Ni clusters, as proven by CO-DRIFTS on the used sample. Comparison with samples of higher Ni loading, where Ni clusters are formed, clearly shows that they catalyze the undesired full decomposition of ethanol to CO, CH4, and H2.
68 citations
TL;DR: In this paper, a review of recent efforts made in the literature on the employment of WO3 for photo-assisted water splitting in the last 5 years is presented, where an attempt is made to summarize the important efforts made for WO 3 for photocatalytic and photoelectrochemical (PEC) water splitting.
Abstract: Photocatalytic and photoelectrochemical (PEC) water splitting to generate clean fuel H2 and O2 from water and solar energy using semiconductor nanomaterials is a green technology which could fulfill the growing energy need of the future and environment concerns. WOx≤3 has received considerable attention in photo-assisted water splitting due to its fascinating advantages such as absorbance in visible region up to ~ 480 nm, low cost, and stability in acidic and oxidative conditions. In this review, an attempt is made to summarize the important efforts made in the literature on the employment of WO3 for PEC water splitting in the last 5 years. Great milestones in PEC performance of WO3 have been reached with possible improvements via morphology control, crystal structure/facet, introduction of oxygen vacancy/defects and choice of suitable electrolyte. It is established that, WO3 nanostructure thin films require annealing, usually between 450 and 550 °C to attain more crystallinity and monoclinic phase of WOx≤3 is the most stable phase at room temperature and demonstrated highest photocatalytic activity when compared to other crystal phases. WO3 structures that are tightly interconnected and strongly bound to the metal collector substrate result in increased photogenerated charge collection efficiency while increase in PEC operating temperature augments the gas evolution quantity. Finally, we provide possibility for further improvements in WO3-based PCE which may be required to enhance its efficiency in water splitting.
65 citations
TL;DR: This study sheds further light onto LSC surface changes also occurring under SOFC operating conditions, and finds that formation of precipitates was found to play a less important role for the electrochemical degradation of LSC.
Abstract: Owing to its extraordinary high activity for catalysing the oxygen exchange reaction, strontium doped LaCoO3 (LSC) is one of the most promising materials for solid oxide fuel cell (SOFC) cathodes. However, under SOFC operating conditions this material suffers from performance degradation. This loss of electrochemical activity has been extensively studied in the past and an accumulation of strontium at the LSC surface has been shown to be responsible for most of the degradation effects. The present study sheds further light onto LSC surface changes also occurring under SOFC operating conditions. In-situ near ambient pressure X-ray photoelectron spectroscopy measurements were conducted at temperatures between 400 and 790 °C. Simultaneously, electrochemical impedance measurements were performed to characterise the catalytic activity of the LSC electrode surface for O2 reduction. This combination allowed a correlation of the loss in electro-catalytic activity with the appearance of an additional La-containing Sr-oxide species at the LSC surface. This additional Sr-oxide species preferentially covers electrochemically active Co sites at the surface, and thus very effectively decreases the oxygen exchange performance of LSC. Formation of precipitates, in contrast, was found to play a less important role for the electrochemical degradation of LSC.
62 citations
TL;DR: The present commercial catalyst demonstrates to be a promising candidate for the efficient in-situ hydrogen generation at mild conditions possibiliting practical applications of formic acid systems on fuel cells.
Abstract: Safe and efficient hydrogen generation and storage has received much attention in recent years. Herein, a commercial 5 wt% Pd/C catalyst has been investigated for the catalytic, additive-free decomposition of formic acid at mild conditions, and the experimental parameters affecting the process systematically have been investigated and optimised. The 5 wt% Pd/C catalyst exhibited a remarkable 99.9% H2 selectivity and a high catalytic activity (TOF = 1136 h−1) at 30 °C toward the selective dehydrogenation of formic acid to H2 and CO2. The present commercial catalyst demonstrates to be a promising candidate for the efficient in-situ hydrogen generation at mild conditions possibiliting practical applications of formic acid systems on fuel cells. Finally DFT studies have been carried out to provide insights into the reactivity and decomposition of formic acid along with the two-reaction pathways on the Pd (111) surface.
61 citations
TL;DR: In this article, the authors address case by case some recent advancements in the conversion of methane and CO2 to methanol both homogeneously and heterogeneously with emphasis on the contribution from Professor George A. Olah and his group.
Abstract: Need for clean energy is imminent and methanol is considered as a promising alternative energy source. Conventional process for the production of methanol has been achieved via syngas which is derived by the steam reforming of methane or naphtha and the gasification of coal. Methanol can also be prepared by direct oxidation of methane (natural gas) or reduction of carbon dioxide (CO2) with hydrogen. In this way, carbon-neutral cycling can be achieved and world’s dependence on fossil fuels will be alleviated. In this minireview, we will address case by case some recent advancements in the conversion of methane and CO2 to methanol both homogeneously and heterogeneously with emphasis on the contribution from Professor George A. Olah’s and our group. In the end, a short outlook is provided towards existing problems and future opportunities.
51 citations
TL;DR: In this paper, the authors performed periodic, self-consistent density functional theory (DFT-GGA) calculations to study the adsorption of five atomic species (H, C, N, O, and S), seven molecular species (NH3, CH4, N2, CO, HCN, NO, and HCOOH), and 13 molecular fragments (CH, CH2, CH3, NH, NH2, OH, CN, COH, HCO, HCOO, NOH, and HNO) on the Cu(111) surface at
Abstract: Due to the wide use of copper-based catalysts in industrial chemical processes, fundamental understanding of the interactions between copper surfaces and various reaction intermediates is highly desired. Here, we performed periodic, self-consistent density functional theory (DFT-GGA) calculations to study the adsorption of five atomic species (H, C, N, O, and S), seven molecular species (NH3, CH4, N2, CO, HCN, NO, and HCOOH), and 13 molecular fragments (CH, CH2, CH3, NH, NH2, OH, CN, COH, HCO, COOH, HCOO, NOH, and HNO) on the Cu(111) surface at a coverage of 0.25 monolayer. The preferred binding site, binding energy, and the corresponding surface deformation energy of each species were determined, as well as the estimated diffusion barrier and diffusion pathway. The binding strengths calculated using the PW91 functional decreased in the following order: CH > C > O > S > CN > NH > N > CH2 > OH > HCOO > COH > H > NH2 > NOH > COOH > HNO > HCO > CH3 > NO > CO > NH3 > HCOOH. No stable binding structures were observed for N2, HCN, and CH4. The adsorbate–surface and intramolecular vibrational modes of all the adsorbates at their preferred binding sites were deternined. Using the calculated adsorption energetics, potential energy surfaces were constructed for the direct decomposition of CO, CO2, NO, N2, NH3, and CH4 and the hydrogen-assisted decomposition of CO, CO2, and NO.
50 citations
TL;DR: A short review of recent advances to illustrate the novel understanding derived from the use of new techniques focusing on the gas-solid interface, where two barriers have been bridged: the pressure gap, and the temperature gap as mentioned in this paper.
Abstract: The rapidly increasing field of surfaces under ambient conditions of temperature, and pressure in gas and liquid environments, reflects the importance of understanding surface properties in conditions closer to practical situations. This has been enabled by the emergence in the last two decades of a number of new techniques, both spectroscopy and microscopy, that can deliver atomic scale information with the required surface/interface sensitivity. Here we present a short review of recent advances to illustrate the novel understanding derived from the use of new techniques focusing on the gas–solid interface, where two barriers have been bridged: the pressure gap, and the temperature gap. The later gap is very important when dealing with weakly bound molecules, where only by the presence of gas at a suitable pressure can a measurable coverage of adsorbed molecules be achieved. The temperature gap manifests also in the removal of kinetic barriers. Future developments to continue extending the range of pressures are also mentioned. Finally, new challenges that appear, both from X-ray and electron-induced damage to the sample, and from contamination under high pressure of desired gases, while maintaining very low pressures of undesirable ones.
47 citations
TL;DR: In this paper, a passive NOx adsorbers (PNA) system that stores NOx at low temperature and releases it during a temperature ramp (to 400°C) under various gas feed composition was investigated.
Abstract: The mitigation of cold-start emissions involves the development of passive NOx adsorbers (PNA) systems that store NOx at low temperature, being designed to release the trapped NOx at higher temperatures, where downstream NOx reduction catalysts are efficient. Pd-based zeolites (BEA, SSZ-13) with different SAR (Si-to-Al ratio) were used for PNA investigation, and Pd/Ce/Al2O3 catalyst was used as a reference for comparison. In this study, NOx adsorption is investigated at low temperature (80 °C) and it is released during a temperature ramp (to 400 °C) under various gas feed composition. Moreover, detailed characterization was performed using BET, XRD, XPS, TPO, STEM and ICP-SFMS and the stored NO species was studied using in-situ DRIFTS. The addition of CO to the storage mixture resulted in that for Pd/zeolites with low and medium SAR the binding energy for NO was increased. In addition, NO was stored in larger quantities, especially for the Pd/SSZ-13 samples. However, for Pd/BEA (SAR = 300) no such stable NO species was formed and for Pd/Ce/Al2O3 the CO addition was even negative. Moreover, in-situ NO DRIFTS showed that there was large amount of nitrosyls on ionic palladium for the Pd/zeolites with low and medium SAR, indicating that a significant fraction of the palladium was in ion-exchanged positions, while this peak was small for the Pd/BEA (SAR = 300) and non-existing for Pd/Ce/Al2O3. Thus, CO addition is beneficial for Pd species that are in ion-exchanged positions, but this is not the case for Pd particles and this can explain the observations that CO is only beneficial for Pd/zeolites with low and medium SAR. Moreover, experiments with similar SAR [Pd/BEA (SAR = 25) and Pd/SSZ-13 (SAR = 24)], showed that there is larger stability of the stored NOx in the small pore Pd/SSZ-13.
TL;DR: In this paper, it was shown that adding 1% Pt into the surface of Cu(111) to form dilute Pt-Cu single atom alloys (SAAs) increases the activity of the surface for ethanol dehydrogenation.
Abstract: The non-oxidative dehydrogenation of ethanol to acetaldehyde and hydrogen is an industrially relevant chemical conversion. Although Cu-based catalysts show high reactivity toward oxidative ethanol dehydrogenation, the flat Cu(111) surface is rather inactive for ethanol dehydrogenation in the absence of water, surface oxygen or defects. Herein we show, using experimental and theoretical studies of model systems, that adding 1% Pt into the surface of Cu(111) to form dilute Pt–Cu single atom alloys (SAAs) increases the activity of Cu(111) for ethanol dehydrogenation sixfold. The mechanism of ethanol dehydrogenation was investigated at the molecular level using scanning tunneling microscopy, temperature programmed experiments and density functional theory calculations. Our results demonstrate that Pt–Cu SAAs are much more active than Cu(111) for converting ethanol to acetaldehyde and hydrogen in the absence of surface oxygen and water. Specifically, the O–H bond of ethanol is activated at Pt sites below 160 K, followed by ethoxy spillover to Cu sites which results in a significant increase of the ethoxy intermediate yield. The C–H bond of ethoxy is then activated at 310 K, and the final product, acetaldehyde, desorbs from Cu(111) in a reaction rate limited process. Finally, we show that the Cu model surfaces exhibit stability with respect to poisoning as well as 100% selectivity in the alcohol dehydrogenation to acetaldehyde and hydrogen.
TL;DR: A review of homogeneous catalysts for methanol reforming can be found in this article, where the authors discuss the possibility to improve the activity of these catalysts by in depth mechanistic studies and rational design of the catalyst on the molecular level.
Abstract: The “Methanol Economy” is a concept that was championed by the late professor and Nobel laureate George A. Olah. Methanol can act not only as a convenient fuel and energy carrier but also as a raw material for numerous chemicals and products. While methanol is still predominantly produced from fossil fuels, it can be made from any carbon source including biomass and CO2. The capture and recycling of CO2 to methanol offers a pathway to a sustainable carbon neutral cycle that would be an anthropogenic equivalent to nature’s own carbon cycle. The required energy would come from renewable as well as nuclear sources (fission and hopefully fusion in the future). While methanol can be used directly as a fuel for example in internal combustion engines, stoves, turbines as well as direct methanol fuel cells, it can also act as a convenient hydrogen carrier. Hydrogen is the preferred fuel for fuel cells. Storing hydrogen in the form of methanol avoids the cumbersome, expensive and potentially dangerous storage of hydrogen at very high pressures or under cryogenic conditions. When needed, the hydrogen can be easily liberated by methanol reforming with water. Until now, most reforming reactions were based on heterogeneous catalysts operating at relatively high temperatures and pressures. However, in order to lower the reaction temperature and improve its selectivity, homogeneous catalysts for methanol reforming have over the past few years gained much interest. Early results are promising with TON already in excess of 300,000. In this review, the homogeneous catalysts studied and reported so far will be presented and discussed. The possibility to improve the activity of these catalysts for methanol reforming by in depth mechanistic studies and rational design of the catalyst on the molecular level is particularly intriguing.
TL;DR: In this article, the influence of calcination temperature (500-800°C) of Ni/CeO2 and Ni/ce0.8Zr0.2O2 catalysts on the specific surface area, pore volume, crystalline size, lattice parameter, chemical bonding and oxidation states, nickel dispersion and CH4/CO production rate in CO2 methanation was studied.
Abstract: Herein, we studied the influence of calcination temperature (500–800 °C) of Ni/CeO2 and Ni/Ce0.8Zr0.2O2 catalysts on the specific surface area, pore volume, crystalline size, lattice parameter, chemical bonding and oxidation states, nickel dispersion and CH4/CO production rate in CO2 methanation. In general, the catalytic performance revealed that Zr doping catalysts could increase the CH4 production rate. Combined with the production rate and the characterizations results, we found that the combination of nickel dispersion, peak area of CO2–TPD and OII/(OII + OI)) play the critical role in increasing the CH4 production rate. It is well to be mentioned that the CO production rate is strongly influenced by the nickel dispersion. Furthermore, the in-situ DRIFTS confirmed that the CO originates from the decomposition of H-assisted formate species.
TL;DR: In this article, the effects of upgrading on different fast pyrolysis bio-oil (FPBO) fractions obtained from a commercially available FPBO were studied by elemental analysis, GC-MS and 1H-NMR.
Abstract: Residual biomass shows potential to be used as a feedstock for fast pyrolysis bio-oil production for energetic and chemical use. Although environmentally advantageous, further catalytic upgrading is required in order to increase the bio-oil stability, by reducing reactive compounds, functional oxygen-containing groups and water content. However, bio-oils may separate in fractions either spontaneously after ageing or by fractionated condensation. Therefore the effects of upgrading on different fast pyrolysis bio-oil (FPBO) fractions obtained from a commercially available FPBO were studied by elemental analysis, GC-MS and 1H-NMR. Not only the FPBO was upgraded by catalytic hydrotreatment, but also the heavy phase fraction formed after intentional aging and phase separation. The reactions were conducted between 175 and 325 °C and 80–100 bar by using a nickel–chromium catalyst in batch experiments. The influence of the hydrotreatment conditions correlated with the composition of the upgraded products. Higher oxygen removal was obtained at higher temperatures, whereas higher pressures resulted in higher hydrogen consumption with no significant influence on deoxygenation. At 325 °C and 80 bar 42% of the oxygen content was removed from the FPBO. Compounds attributed to pyrolysis oil instability, such as ketones and furfural were completely converted while the number of alcohols detected in the upgraded products increased. Coke formation was observed after all reactions, especially for the reaction with the fraction rich in lignin derivatives, likely formed by polymerization of phenolic compounds mainly concentrated in this phase. Independently of the feedstock used, the upgraded bio-oils were very similar in composition, with reduced oxygen and water content, higher energy density and higher carbon content.
TL;DR: In this paper, a 4% Ru/Al2O3 catalyst was synthesized by impregnation of the support with an aqueous solution of Ru(OH)Cl3, followed by liquid phase reduction using NaBH4 and gas phase activation using the stoichiometric mixture of CO2 and H2 (1:4).
Abstract: CO2 methanation over supported ruthenium catalysts is considered to be a promising process for carbon capture and utilization and power-to-gas technologies. In this work 4% Ru/Al2O3 catalyst was synthesized by impregnation of the support with an aqueous solution of Ru(OH)Cl3, followed by liquid phase reduction using NaBH4 and gas phase activation using the stoichiometric mixture of CO2 and H2 (1:4). Kinetics of CO2 methanation reaction over the Ru/Al2O3 catalyst was studied in a perfectly mixed reactor at temperatures from 200 to 300 °C. The results showed that dependence of the specific activity of the catalyst on temperature followed the Arrhenius law. CO2 conversion to methane was shown to depend on temperature, water vapor pressure and CO2:H2 ratio in the gas mixture. The Ru/Al2O3 catalyst was later tested together with the K2CO3/Al2O3 composite sorbent in the novel direct air capture/methanation process, which combined in one reactor consecutive steps of CO2 adsorption from the air at room temperature and CO2 desorption/methanation in H2 flow at 300 or 350 °C. It was demonstrated that the amount of desorbed CO2 was practically the same for both temperatures used, while the total conversion of carbon dioxide to methane was 94.2–94.6% at 300 °C and 96.1–96.5% at 350 °C.
TL;DR: In this paper, the authors compared Ni/SiC and NiO2 catalysts prepared by both wet impregnation (WI) and deposition-precipitation (DP) methods for CO and CO2 methanation.
Abstract: Ni/SiC and Ni/SiO2 catalysts prepared by both wet impregnation (WI) and deposition–precipitation (DP) methods were compared for CO and CO2 methanation. The prepared catalysts were characterized using N2 physisorption, temperature-programmed reduction with H2 (H2-TPR), H2 chemisorption, pulsed CO2 chemisorption, temperature-programmed desorption of CO2 (CO2-TPD), transmission electron microscopy, and X-ray diffraction. H2-TPR analysis revealed that the catalysts prepared by DP exhibit stronger interaction between the nickel oxides and support than those prepared by WI. The former catalysts exhibit higher Ni dispersions than the latter. The catalytic activities for both reactions over Ni/SiC and Ni/SiO2 catalysts prepared by WI increase on increasing the Ni content from 10 to 20 wt%. The Ni/SiC catalyst prepared by DP shows higher catalytic activity for CO and CO2 methanation than that of the Ni/SiC catalyst prepared by WI. Furthermore, it exhibits the highest catalytic activity for CO methanation among the tested catalysts. The high Ni dispersion achieved by the DP method and the high thermal conductivity enabled by SiC are beneficial for both CO and CO2 methanation.
TL;DR: A series of perovskite-type manganites AMnO3 particles were investigated as electrocatalysts for the oxygen reduction reaction by means of an ionic-liquid method, yielding phase pure particles at different temperatures.
Abstract: A series of perovskite-type manganites AMnO3 (A = Sr, La, Ca and Y) particles were investigated as electrocatalysts for the oxygen reduction reaction. AMnO3 materials were synthesized by means of an ionic-liquid method, yielding phase pure particles at different temperatures. Depending on the calcination temperature, particles with mean diameter between 20 and 150 nm were obtained. Bulk versus surface composition and structure are probed by X-ray photoelectron spectroscopy and extended X-ray absorption fine structure. Electrochemical studies were performed on composite carbon-oxide electrodes in alkaline environment. The electrocatalytic activity is discussed in terms of the effective Mn oxidation state, A:Mn particle surface ratio and the Mn–O distances.
TL;DR: In this article, the authors reported on the significant promoting effects of K and La addition to Fe-Cu/TiO2 catalyst on higher hydrocarbon production from CO2 hydrogenation.
Abstract: Developing selective and active catalyst is critical for CO2 hydrogenation to higher hydrocarbons especially C5+ products. The present work reports on the significant promoting effects of K and La addition to Fe–Cu/TiO2 catalyst on higher hydrocarbon production from CO2 hydrogenation. The incorporation of both K and La promoters can improve both CO2 hydrogenation activity and selectivity to higher hydrocarbons of Fe-based catalyst. Characterization by temperature-programmed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that the presence of K promoter significantly decreased the adsorption of H2, which suppressed the CH4 formation. On the other hand, La addition can promote the moderately adsorbed CO2 species (mainly monodentate carbonate species), which leads to the enhanced C5–C7 selectivity. The simultaneous use of promoters La and K can tailor the H and C coverage on the catalyst surface, which plays an important role in altering product distribution in CO2 hydrogenation.
TL;DR: It was shown that oxidation current decreased with an increase in the rotation rate (ω) for all alcohols studied over electrodeposited Pt film electrodes, and the reversibility of the peak current on decreasing the ω could indicate that the surface state has not significantly changed due to the sluggish reaction kinetics of ethanol and n-butanol.
Abstract: Electrochemical oxidation of four different alcohol molecules (methanol, ethanol, n-butanol and 2-butanol) at electrodeposited Pt film and carbon-supported Pt catalyst film electrodes, as well as the effect of mass transport on the oxidation reaction, has been studied systematically using the rotating disk electrode (RDE) technique. It was shown that oxidation current decreased with an increase in the rotation rate (ω) for all alcohols studied over electrodeposited Pt film electrodes. In contrast, the oxidation current was found to increase with an increase in the ω for Pt/C in ethanol and n-butanol-containing solutions. The decrease was found to be nearly reversible for ethanol and n-butanol at the electrodeposited Pt film electrode ruling out the possibility of intermediate COads poisoning being the sole cause of the decrease and was attributed to the formation of soluble intermediate species which diffuse away from the electrode at higher ω. In contrast, an increase in the current with an increase in ω for the carbon supported catalyst may suggest that the increase in residence time of the soluble species within the catalyst layer, results in further oxidation of these species. Furthermore, the reversibility of the peak current on decreasing the ω could indicate that the surface state has not significantly changed due to the sluggish reaction kinetics of ethanol and n-butanol.
TL;DR: In this paper, a batch-type two-electrode cell prototype and the improvements attained with a continuous flow three-electron cell were presented for studying interfacial structural dynamics of energy materials under near ambient pressure X-ray photoelectron spectroscopy.
Abstract: In this contribution, we report the development of in situ electrochemical cells based on proton exchange membranes suitable for studying interfacial structural dynamics of energy materials under operation by near ambient pressure X-ray photoelectron spectroscopy. We will present both the first design of a batch-type two-electrode cell prototype and the improvements attained with a continuous flow three-electrode cell. Examples of both sputtered metal films and carbon-supported metal nanostructures are included demonstrating the high flexibility of the cells to study energy materials. Our immediate focus was on the study of the oxygen evolution reaction, however, the methods described herein can be broadly applied to reactions relevant in energy conversion and storage devices.
TL;DR: PdZn alloy nanoparticles supported on TiO2 via a solvent-free chemical vapour impregnation method are shown to be effective for the hydrogenation of CO2 to methanol and this synthesis technique is shown to minimise surface contaminants, which are detrimental to catalyst activity.
Abstract: Catalytic upgrading of CO2 to value-added chemicals is an important challenge within the chemical sciences. Of particular interest are catalysts which are both active and selective for the hydrogenation of CO2 to methanol. PdZn alloy nanoparticles supported on TiO2 via a solvent-free chemical vapour impregnation method are shown to be effective for this reaction. This synthesis technique is shown to minimise surface contaminants, which are detrimental to catalyst activity. The effect of reductive heat treatments on both structural properties of PdZn/TiO2 catalysts and rates of catalytic CO2 hydrogenation are investigated. PdZn nanoparticles formed upon reduction showed high stability towards particle sintering at high reduction temperature with average diameter of 3–6 nm to give 1710 mmol kg−1 h of methanol. Reductive treatment at high temperature results in the formation of ZnTiO3 as well as PdZn, and gives the highest methanol yield.
TL;DR: In this article, Pd-promoted zeolites (BEA, FER, MFI and MOR) are synthesized and investigated for potential use in the low-temperature NOx adsorption.
Abstract: In this study Pd-promoted zeolites (BEA, FER, MFI and MOR) are synthesized and investigated for potential use in the low-temperature NOx adsorption. The catalysts are characterized by BET, XRD, UV–Vis and XRF, while the adsorption/desorption characteristics of the samples are investigated by NOx adsorption/TPD, with and without water in the feed. The nature of the adsorbed NOx species has been analyzed by operando FT-IR spectroscopy. Under dry conditions at 50 °C all the investigated zeolite frameworks are able to store significant amounts of NOx, up to 1 mmol/gcat for the MOR sample; the presence of Pd has not a significant impact on the amounts of the stored NOx. In fact, under dry conditions at 50 °C the zeolite frameworks oxidize NO to NO2, and store NOx from NO2 over the zeolitic support. FTIR data indicates that NOx are stored in the form of nitrosonium ion NO+ and nitrate ions; nitrosyl species formed on Al3+ and Pdn+ sites have also been observed, although to a lower extent. Evidences have also been provided for the existence of gaseous/weakly interacting species (mostly NO2) contained in the pores of the zeolites. Water inhibits NO oxidation over all investigated samples, resulting in a strong decrease in the NOx storage capacity. Still, appreciable amounts of NOx could be stored at 50 °C (up to 60 µmol/gcat for Pd/MFI) in the presence of water over the Pd-doped carriers.
TL;DR: In this paper, a graphite-like mesoporous carbon material Sibunit was developed for the one-pot solubilization-hydrolysis-dehydration of cellulose into glucose and 5-hydroxymethylfurfural (5-HMF).
Abstract: Solid acid catalysts based on graphite-like mesoporous carbon material Sibunit were developed for the one-pot solubilization–hydrolysis–dehydration of cellulose into glucose and 5-hydroxymethylfurfural (5-HMF) The catalysts were produced by treating Sibunit surface with three different procedures to form acidic and sulfo groups on the catalyst surface The techniques used were: (1) sulfonation by H2SO4 at 80–250 °C, (2) oxidation by wet air or 32 v/v% solution of HNO3, and (3) oxidation-sulfonation what meant additional sulfonating all the oxidized carbons at 200 °C All the catalysts were characterized by low-temperature N2 adsorption, titration with NaOH, TEM, XPS Sulfonation of Sibunit was shown to be accompanied by surface oxidation (formation of acidic groups) and the high amount of acidic groups prevented additional sulfonation of the surface All the Sibunit treatment methods increased the surface acidity in 3–15 times up to 014–062 mmol g−1 compared to pure carbon (0042 mmol g−1) The catalysts were tested in the depolymerization of mechanically activated microcrystalline cellulose at 180 °C in pure water The main products 5-HMF and glucose were produced with the yields in the range of 8–22 wt% and 12–46 wt%, respectively The maximal yield were achieved over Sibunit sulfonated at 200 °C An essential difference in the composition of main products obtained with solid acid Sibunit carbon catalysts (glucose, 5-HMF) and soluble in water H2SO4 catalysts (formic and levulinic acids) as well as strong dependence of the reaction kinetics on the morphology of carbon catalysts argue for heterogenious mechanism of cellulose depolymerization over Sibunit
TL;DR: In this article, the physicochemical properties of composite catalysts were studied by numerous characterization techniques, such as X-ray powder diffraction (XRD), scanning-, as well as high-resolution transmission electron microscopy (SEM/HRTEM), and Xray photoelectron spectroscopy (XPS).
Abstract: The conversion of the biomass-derived lipid, lignocellulosic and carbohydrate resources into renewable platform intermediates, chemicals and biofuels has been lately increasing in interest. The mechanistic reaction pathways, like hydro-deoxygenation, decarboxylation and hydrocracking, of the selected palmitic acid, as a model fatty acid, over Ni/ZSM-5 zeolite catalysts were studied. The ZSM-5 material with different Al/Si molar ratios was synthesized via a green template-free hydrothermal synthesis procedure, treated and subsequent functionalised with various Ni metal loadings. However, Ni/Al molar ratio was kept stoichiometric (Ni/Al = 0.5). The characteristic physicochemical properties of composite catalysts were studied by numerous characterization techniques, such as X-ray powder diffraction (XRD), scanning-, as well as high-resolution transmission electron microscopy (SEM/HRTEM), and X-ray photoelectron spectroscopy (XPS). NiO with an average particle size of 10–20 nm was found on ZSM-5 support. The relative Ni/Al atom fraction in Ni/ZSM-5 systems influenced their Lewis/Bronsted acidic sites, as well as the external exposed area of prepared heterogeneous structures. Furthermore, the mentioned morphological parameters affected predominant catalytic routes. Species’ production mechanism, as a consequence of Lewis/Bronsted centre weak/strong acidity, as well as their integral concentration, was proposed, mirroring the observed process kinetics, selectivity and turnover. It was demonstrated that the main obtained products were esters, aldehydes, alcohols, hydrocarbons and gases (CO2, CO…), produced by deoxygenation (e.g. decarbonylation), hydrogenation and cracking, less, though, through isomerisation.
TL;DR: In this article, the authors couple electrochemical studies of epitaxial La1−xSrxCoO3−δ films with in situ and operando ambient pressure X-ray photoelectron spectroscopy to investigate the surface stoichiometry, adsorbates, and electronic structure.
Abstract: Cobalt-containing perovskite oxides are promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolyzers. However, a lack of fundamental understanding of oxide surfaces impedes rational catalyst design for improved activity and stability. We couple electrochemical studies of epitaxial La1−xSrxCoO3−δ films with in situ and operando ambient pressure X-ray photoelectron spectroscopy to investigate the surface stoichiometry, adsorbates, and electronic structure. In situ investigations spanning electrode compositions in a humid environment indicate that hydroxyl and carbonate affinity increase with Sr content, leading to an increase in binding energy of metal core levels and the valence band edge from the formation of a surface dipole. The maximum in hydroxylation at 40% Sr is commensurate with the highest OER activity, where activity scales with greater hole carrier concentration and mobility. Operando measurements of the 20% Sr-doped oxide in alkaline electrolyte indicate that the surface stoichiometry remains constant during OER, supporting the idea that the oxide electrocatalyst is stable and behaves as a metal, with the voltage drop confined to the electrolyte. Furthermore, hydroxyl and carbonate species are present on the electrode surface even under oxidizing conditions, and may impact the availability of active sites or the binding strength of adsorbed intermediates via adsorbate–adsorbate interactions. For covalent oxides with facile charge transfer kinetics, the accumulation of hydroxyl species with oxidative potentials suggests the rate of reaction could be limited by proton transfer kinetics. This operando insight will help guide modeling of self-consistent oxide electrocatalysts, and highlights the potential importance of carbonates in oxygen electrocatalysis.
TL;DR: In this article, a multivariate curve resolution (MCR) was applied in combination with principal component analysis (PCA) to assist the interpretation of X-ray absorption spectroscopy (XAS) experiments in the near-edge (XANES) region, determining the spectroscopic signatures and concentration profiles of the pure Cu-species formed.
Abstract: The Cu–CHA zeolite today represents an attractive platform to design catalysts for deNOx applications by NH3-assisted selective catalytic reduction (NH3–SCR) and for the low-temperature selective oxidation of methane to methanol (MTM). Accessing a quantitative understanding of Cu–speciation in this material is a key step to unveil structure–performance relationships for these high-impact processes. Herein, we select Cu–CHA as a case study to demonstrate the potential of chemometric approaches, such as multivariate curve resolution (MCR) applied in combination with principal component analysis (PCA). We employ these methods to assist the interpretation of X-ray absorption spectroscopy (XAS) experiments in the near-edge (XANES) region, determining the spectroscopic signatures and concentration profiles of the pure Cu–species formed. We pinpoint the composition impact on the material reducibility and highlight Cu–speciation–productivity relationships for the MTM process. Furthermore, we report novel insights on the formation of O2-derived species in Cu–CHA, obtained from MCR analysis of high energy resolution fluorescence detected (HERFD) XANES data collected during thermal treatment of Cu–CHA in both He and O2 gas flow. Multivariate analysis, in combination with the superior energy resolution adopted, allows us to identify an additional Cu(II) species. This component, different from the previously characterized Z[Cu(II)OH] moiety, is only formed at significant concentrations in O2 and it is envisaged to play an important role in the MTM conversion.
TL;DR: In this paper, the doping of palladium (Pd) into poly(N-vinyl-2-pyrrolidone) (Au:PVP) has been shown to improve catalytic performance for aerobic oxidation of benzyl alcohol.
Abstract: It is known that small gold clusters (average diameter: ~ 1.2 nm) stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) exhibit size-specific catalysis in aerobic oxidation reactions. A recent matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) study of Au:PVP revealed that Au clusters with the magic sizes of 34 and 43 were preferentially produced. Here, we reported how the doping of palladium (Pd) into Au:PVP affected the catalytic performance. MALDI-MS analysis of Pd-doped Au:PVP showed that a single Pd atom was selectively doped by co-reduction of Au and Pd precursor ions and that PdAu33 and PdAu43 were produced as the dominant species. Extended X-ray absorption fine structure (EXAFS) analysis indicated that a Pd atom was located at the exposed surface of the Au:PVP clusters. It was found that single Pd atom doping enhanced the catalytic activity for aerobic oxidation of benzyl alcohol and provided hydrogenation catalysis in a chemoselective manner to the C=C bonds over the C=O bonds.
TL;DR: In this paper, Bacterial nanocellulose (BNC), tracing paper, and polyester film were tested as substrates, since they are inexpensive, flexible, recyclable, lightweight and when associated to low temperature synthesis and absence of a seed layer, they become suitable for several low-cost applications.
Abstract: In this study, titanium dioxide (TiO2) nanostructured films were synthesized under microwave irradiation through low temperature synthesis (80 °C) and integrated in ultraviolet (UV) photodetectors and as photocatalysts. Bacterial nanocellulose (BNC), tracing paper, and polyester film were tested as substrates, since they are inexpensive, flexible, recyclable, lightweight, and when associated to low temperature synthesis and absence of a seed layer, they become suitable for several low-cost applications. The nanostructured TiO2 films and substrates were structurally characterized by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy. The optical properties of all materials were investigated. The TiO2 nanostructured films were implemented as a photoactive layer of UV photodetectors and demonstrated significant increase of conductance upon exposed to UV irradiation. The photodetection behaviour of each material was investigated by in-situ Kelvin probe force microscopy experiments, in which the contact potential difference varied under dark or UV irradiation conditions, demonstrating higher shift for the BNC-based UV photodetector. Photocatalytic activity of the films was assessed from rhodamine B degradation under solar radiation, and BNC based devices revealed to be the best photocatalyst. The structural characteristics of the TiO2 films and substrates were correlated to the differences in the UV photodetection and photocatalytic performances.
TL;DR: In this paper, first-principle density functional theory was used to calculate the work function and Volta potential differences between aluminum alloy matrix and two intermetallic phases (Mg2Si and Al2Cu) with varying surface terminations as a function of adhering monolayers (ML) of water.
Abstract: In this work, first-principle density functional theory (DFT) was used to calculate the work function and Volta potential differences between aluminum alloy matrix and two intermetallic phases (Mg2Si and Al2Cu) with varying surface terminations as a function of adhering monolayers (ML) of water. The calculated data were compared with experimental local Volta potential data obtained by the scanning Kelvin probe force microscopy (SKPFM) on a commercial aluminum alloy AA6063-T5 in atmospheric environments with varying relative humidity (RH). The calculations suggest that the surface termination has a major effect on the magnitude and polarity of the Volta potential of both intermetallic phases (IMP’s). The Volta potential difference between the IMP’s and the aluminum matrix decreases when the surface is gradually covered by water molecules, and may further change as a function of adhering ML’s of water. This can lead to nobility inversions of the IMP’s relative to the aluminum matrix. The measured Volta potential difference between both IMP’s and their neighboring matrix is dependent on RH. Natural oxidation in ambient indoor air for 2 months led to a nobility inversion of the IMP’s with respect to the aluminum matrix, with the intermetallics showing anodic nature already in dry condition. The anodic nature of Al2Cu remained with the introduction of RH, whereas Mg2Si became cathodic at high RH, presumably due to de-alloying of Mg and oxide dissolution. The DFT calculations predicted an anodic character of both IMP’s in reference to the oxidized aluminum matrix, being in good agreement with the SKPFM data. The DFT and SKPFM data were discussed in light of understanding localized corrosion of aluminum alloys under conditions akin to atmospheric exposure.