Showing papers on "Catalyst support published in 2016"

Catalytic steam reforming of biomass tar: Prospects and challenges

Abstract: Tar is unavoidable by-product during biomass gasification process. Catalytic steam reforming of tar to syngas is a promising way for the removal of tar from the gas products. However, the key issue for this way is catalyst development. To date, the developed catalysts always have advantages and disadvantages: nickel-based catalysts have high activity, but they are easily deactivated by coking; noble metal based catalysts have high catalytic activity, long-term stability and high carbon deposition resistance, but they are expensive; other transition metal catalysts such as Fe, Co and Cu exhibit a good performance, but they are also deactivated easily by carbon deposition in the case of high heavy-tar content in the tar; alkali metal catalysts also have high catalytic activity for tar reforming, but they are easy to be evaporated with the generated gases; natural catalysts have been widely applied for the steam reforming of tar due to its inexpensive, abundant and disposable, but their catalytic activities are lower than those man-made ones, and especially have low mechanical strength, making them not suitable to be used in fluidized bed reactor; zeolite is suggested to be a good catalyst support due to its high thermal/hydrothermal stability, high resistance to sulfur compounds, and easy to be regenerated; biomass char has been used as the catalyst or catalyst support in the steam reforming of tar due to its low cost and its natural production inside the biomass gasifier; even biomass ash now is considered to be a good catalyst for tar removal. In this review, to get better understanding of the mechanism of catalytic steam reforming of tar derived from biomass, tar formation, tar properties and catalytic reaction mechanism are also introduced, and prospects and challenges are summarized.

Electrochemical CO2 Reduction to Hydrocarbons on a Heterogeneous Molecular Cu Catalyst in Aqueous Solution

Abstract: Exploration of heterogeneous molecular catalysts combining the atomic-level tunability of molecular structures and the practical handling advantages of heterogeneous catalysts represents an attractive approach to developing high-performance catalysts for important and challenging chemical reactions such as electrochemical carbon dioxide reduction which holds the promise for converting emissions back to fuels utilizing renewable energy. Thus, far, efficient and selective electroreduction of CO2 to deeply reduced products such as hydrocarbons remains a big challenge. Here, we report a molecular copper-porphyrin complex (copper(II)-5,10,15,20-tetrakis(2,6-dihydroxyphenyl)porphyrin) that can be used as a heterogeneous electrocatalyst with high activity and selectivity for reducing CO2 to hydrocarbons in aqueous media. At −0.976 V vs the reversible hydrogen electrode, the catalyst is able to drive partial current densities of 13.2 and 8.4 mA cm–2 for methane and ethylene production from CO2 reduction, correspo...

Electrochemical Catalyst–Support Effects and Their Stabilizing Role for IrOx Nanoparticle Catalysts during the Oxygen Evolution Reaction

Abstract: Redox-active support materials can help reduce the noble-metal loading of a solid chemical catalyst while offering electronic catalyst–support interactions beneficial for catalyst durability. This is well known in heterogeneous gas-phase catalysis but much less discussed for electrocatalysis at electrified liquid–solid interfaces. Here, we demonstrate experimental evidence for electronic catalyst–support interactions in electrochemical environments and study their role and contribution to the corrosion stability of catalyst/support couples. Electrochemically oxidized Ir oxide nanoparticles, supported on high surface area carbons and oxides, were selected as model catalyst/support systems for the electrocatalytic oxygen evolution reaction (OER). First, the electronic, chemical, and structural state of the catalyst/support couple was compared using XANES, EXAFS, TEM, and depth-resolved XPS. While carbon-supported oxidized Ir particle showed exclusively the redox state (+4), the Ir/IrOx/ATO system exhibited ...

Hydroformylation of Olefins by a Rhodium Single-Atom Catalyst with Activity Comparable to RhCl(PPh3)3

Abstract: Homogeneous catalysts generally possess superior catalytic performance compared to heterogeneous catalysts. However, the issue of catalyst separation and recycling severely limits their use in practical applications. Single-atom catalysts have the advantages of both homogeneous catalysts, such as "isolated sites", and heterogeneous catalysts, such as stability and reusability, and thus would be a promising alternative to traditional homogeneous catalysts. In the hydroformylation of olefins, single-atom Rh catalysts supported on ZnO nanowires demonstrate similar efficiency (TON≈40000) compared to that of homogeneous Wilkinson's catalyst (TON≈19000). HAADF-STEM and infrared CO chemisorption experiments identified isolated Rh atoms on the support. XPS and XANES spectra indicate that the electronic state of Rh is almost metallic. The catalysts are about one or two orders of magnitude more active than most reported heterogeneous catalysts and can be reused four times without an obvious decline in activity.

Nickel supported on nitrogen-doped carbon nanotubes as hydrogen oxidation reaction catalyst in alkaline electrolyte

Abstract: The development of a low-cost, high-performance platinum-group-metal-free hydroxide exchange membrane fuel cell is hindered by the lack of a hydrogen oxidation reaction catalyst at the anode. Here we report that a composite catalyst, nickel nanoparticles supported on nitrogen-doped carbon nanotubes, has hydrogen oxidation activity similar to platinum-group metals in alkaline electrolyte. Although nitrogen-doped carbon nanotubes are a very poor hydrogen oxidation catalyst, as a support, it increases the catalytic performance of nickel nanoparticles by a factor of 33 (mass activity) or 21 (exchange current density) relative to unsupported nickel nanoparticles. Density functional theory calculations indicate that the nitrogen-doped support stabilizes the nanoparticle against reconstruction, while nitrogen located at the edge of the nanoparticle tunes local adsorption sites by affecting the d-orbitals of nickel. Owing to its high activity and low cost, our catalyst shows significant potential for use in low-cost, high-performance fuel cells.

Highly Tunable Selectivity for Syngas-Derived Alkenes over Zinc and Sodium-Modulated Fe5 C2 Catalyst.

Abstract: Zn- and Na-modulated Fe catalysts were fabricated by a simple coprecipitation/washing method. Zn greatly changed the size of iron species, serving as the structural promoter, while the existence of Na on the surface of the Fe catalyst alters the electronic structure, making the catalyst very active for CO activation. Most importantly, the electronic structure of the catalyst surface suppresses the hydrogenation of double bonds and promotes desorption of products, which renders the catalyst unexpectedly reactive toward alkenes-especially C5+ alkenes (with more than 50% selectivity in hydrocarbons)-while lowering the selectivity for undesired products. This study enriches C1 chemistry and the design of highly selective new catalysts for high-value chemicals.

Identification of carbon-encapsulated iron nanoparticles as active species in non-precious metal oxygen reduction catalysts.

Abstract: The widespread use of fuel cells is currently limited by the lack of efficient and cost-effective catalysts for the oxygen reduction reaction. Iron-based non-precious metal catalysts exhibit promising activity and stability, as an alternative to state-of-the-art platinum catalysts. However, the identity of the active species in non-precious metal catalysts remains elusive, impeding the development of new catalysts. Here we demonstrate the reversible deactivation and reactivation of an iron-based non-precious metal oxygen reduction catalyst achieved using high-temperature gas-phase chlorine and hydrogen treatments. In addition, we observe a decrease in catalyst heterogeneity following treatment with chlorine and hydrogen, using Mossbauer and X-ray absorption spectroscopy. Our study reveals that protected sites adjacent to iron nanoparticles are responsible for the observed activity and stability of the catalyst. These findings may allow for the design and synthesis of enhanced non-precious metal oxygen reduction catalysts with a higher density of active sites. Determining active species in non-precious metal catalysts for the oxygen reduction reaction remains a challenge due to catalyst heterogeneity. Here the authors perform gas-phase treatments on an iron-based catalyst to allow the identification of carbon-encapsulated iron nanoparticles as the active species.

Efficient hydrogen production from ethanol steam reforming over La-modified ordered mesoporous Ni-based catalysts

Abstract: This paper describes the synthesis of a group of meso-xLaNiAl catalysts with ordered mesostructure and La promoters, and their catalytic performance for hydrogen production from ethanol steam reforming. For comparison, the conventional 0LaNiAl catalyst was prepared by impregnation with γ-Al2O3. The characterization results exhibited that meso-xLaNiAl materials possessed excellent textural properties, such as high specific surface areas, large pore volumes and uniform pore sizes. The ordered mesostructure was beneficial to obtain and maintain the 4–6 nm Ni nanoparticles, which were smaller than that of the conventional 0LaNiAl catalyst (∼10 nm). Consequently, meso-xLaNiAl catalysts exhibited superior initial activity with respect to the reference 0LaNiAl catalyst, especially at higher temperatures (873 and 973 K). Particularly, the meso-3LaNiAl catalyst gained the highest amount of easily reduced Ni species and then the highest active surface areas. In addition, the highest initial activity at 873 K is exhibited over meso-3LaNiAl catalyst, which is mainly attributed to the highly dispersed nickel nanoparticles and abundant active surface areas. The meso-3LaNiAl catalyst also exhibited excellent long-term stability. Besides the excellent textural properties, the presence of La-modifiers enhanced the basicity of the catalyst, strengthened the metal-support interaction and cleaned the deposited carbon, resulting in the suppression of carbon deposition and the improvement of stability.

The use of different metal catalysts for the simultaneous production of carbon nanotubes and hydrogen from pyrolysis of plastic feedstocks

Abstract: Nickel, iron, cobalt and copper catalysts were prepared by impregnation and used to produce carbon nanotubes and hydrogen gas from a LDPE feedstock. A two stage catalytic pyrolysis process was used to enable large yields of both products. Plastics samples were pyrolysed in nitrogen at 600 °C, before the evolved gases were passed to a second stage and allowed to deposit carbon onto the catalyst at a temperature of 800 °C. Carbon nanotubes were successfully generated on nickel, iron and cobalt but were barely observed on the copper catalyst. Iron and nickel catalysts gave the largest yield of both hydrogen and carbon nanotubes as a result of metal-support interactions which were neither too strong, like cobalts, nor too weak like copper. These metal support interactions proved a key factor in CNT production. A nickel catalyst with a weaker interaction was prepared using a lower calcination temperature. Yields of both carbon nanotubes and hydrogen gas were lower on the Ni-catalyst prepared at the lower calcination temperature, as a result of sintering of the nickel particles. In addition, the catalyst prepared at a lower calcination temperature produced metal particles which were too large for CNT growth, producing amorphous carbons which deactivate the catalyst instead. Overall the iron catalyst gave the largest yield of CNTs, which is attributed to both its good metal-support interactions and irons large carbon solubility.

CuxCo1−xO Nanoparticles on Graphene Oxide as A Synergistic Catalyst for High‐Efficiency Hydrolysis of Ammonia–Borane

Abstract: Ammonia-borane (AB) is an excellent material for chemical storage of hydrogen. However, the practical utilization of AB for production of hydrogen is hindered by the need of expensive noble metal-based catalysts. Here, we report Cux Co1-x O nanoparticles (NPs) facilely deposited on graphene oxide (GO) as a low-cost and high-performance catalyst for the hydrolysis of AB. This hybrid catalyst exhibits an initial total turnover frequency (TOF) value of 70.0 (H2 ) mol/(Cat-metal) mol⋅min, which is the highest TOF ever reported for noble metal-free catalysts, and a good stability keeping 94 % activity after 5 cycles. Synchrotron radiation-based X-ray absorption spectroscopy (XAS) investigations suggested that the high catalytic performance could be attributed to the interfacial interaction between Cux Co1-x O NPs and GO. Moreover, the catalytic hydrolysis mechanism was studied by in situ XAS experiments for the first time, which reveal a significant water adsorption on the catalyst and clearly confirm the interaction between AB and the catalyst during hydrolysis.

High-Yield Production of Boron Nitride Nanosheets and Its Uses as a Catalyst Support for Hydrogenation of Nitroaromatics

Abstract: Single- or few-layered h-BN nanosheets (BNNSs) are analogous to graphene and possess unique properties. However, their technological applications were severely hindered by the low production efficiency of BNNSs. We reported here a study in which BNNSs were efficiently produced by exfoliating bulk h-BN powder in thionyl chloride without using any dispersion agents. The BNNSs yield was as high as 20%, and it could be doubled through the second round of exfoliation of the h-BN precipitate. Microscopic results revealed that the BNNSs generally consisted of 3–20 layers. Pd nanoparticles were successfully immobilized and uniformly distributed on BNNS surfaces through the deposition–precipitation method. The resultant Pd–BNNS catalyst exhibited high catalytic activity and recyclability for the hydrogenation of nitro aromatics, demonstrating that BNNSs served as a promising platform to fabricate heterogeneous catalysts.

Structural parameters of supported fuel cell catalysts: The effect of particle size, inter-particle distance and metal loading on catalytic activity and fuel cell performance

Abstract: Carbon supported platinum is commonly used as anode and cathode catalyst in low-temperature fuel cells. The need of modify the chemical characteristics of the supported catalyst has emerged due to several factors, such as reducing the price of the active catalyst and increasing its activity, selectivity, and long-term stability. Thus, pure Pt is now rapidly being replaced by oxide promoted Pt and Pd or Pt- and Pd-based alloy catalysts in low-temperature fuel cells. In addition to the chemical characteristics of the catalysts, many studies on nanocatalysts have been addressed to correlating the catalytic activity with some physical characteristics of supported fuel cell catalysts. This review article examines the role played by metal particle size, inter-particle distance and metal loading on the support in determining the catalytic activity of supported catalysts.

Mesoporous Ni60Fe30Mn10-alloy based metal/metal oxide composite thick films as highly active and robust oxygen evolution catalysts

Abstract: A major challenge in the field of water electrolysis is the scarcity of oxygen-evolving catalysts that are inexpensive, highly corrosion-resistant, suitable for large-scale applications and able to oxidize water at high current densities and low overpotentials. Most unsupported, non-precious metals oxygen-evolution catalysts require at least ∼350 mV overpotential to oxidize water with a current density of 10 mA cm−2 in 1 M alkaline solution. Here we report on a robust nanostructured porous NiFe-based oxygen evolution catalyst made by selective alloy corrosion. In 1 M KOH, our material exhibits a catalytic activity towards water oxidation of 500 mA cm−2 at 360 mV overpotential and is stable for over eleven days. This exceptional performance is attributed to three factors. First, the small size of the ligaments and pores in our mesoporous catalyst (∼10 nm) results in a high BET surface area (43 m2 g−1) and therefore a high density of oxygen-evolution catalytic sites per unit mass. Second, the open porosity facilitates effective mass transfer at the catalyst/electrolyte interface. Third and finally, the high bulk electrical conductivity of the mesoporous catalyst allows for effective current flow through the electrocatalyst, making it possible to use thick films with a high density of active sites and ∼3 × 104 cm2 of catalytic area per cm2 of electrode area. Our mesoporous catalyst is thus attractive for alkaline electrolyzers where water-based solutions are decomposed into hydrogen and oxygen as the only products, driven either conventionally or by photovoltaics.

Carbon nanotube containing Ag catalyst layers for efficient and selective reduction of carbon dioxide

Abstract: Over the last few decades significant progress has been made in the development of catalysts for efficient and selective electroreduction of CO2. These improvements in catalyst performance have been of the extent that identifying electrodes of optimum structure and composition has become key to further improve throughput levels in the electrolysis of CO2 to CO. Here we report on a simple one-step method to incorporate multi-walled carbon nanotubes (MWCNT) in the catalyst layer to form gas diffusion electrodes with different structures: (i) a “mixed” catalyst layer in which the Ag nanoparticle catalyst and MWCNTs are homogeneously distributed; and (ii) a “layered” catalyst layer comprised of a layer of MWCNTs covered with a layer of Ag catalyst. Both approaches improve performance in the electroreduction of CO2 compared to electrodes that lack MWCNTs. The “mixed” layer performed best: an electrolyzer operated at a cell potential of −3 V using 1 M KOH as the electrolyte yielded unprecedented high levels of CO production of up to 350 mA cm−2 at high faradaic efficiency (>95% selective for CO) and an energy efficiency of 45% under the same condition. Electrochemical impedance spectroscopy measurements indicate that the observed differences in electrode performance can be attributed to a lower charge transfer resistance in the “mixed” catalyst layer. This study shows that a simple optimization of electrode structure and composition, i.e. incorporation of MWCNTs in the catalyst layer of a GDE, has a profound beneficial effect on their performance in electrocatalytic conversion of CO2, while allowing for a lower precious metal catalyst loading with improved performance.

Hyper-crosslinked β-cyclodextrin porous polymer: an adsorption-facilitated molecular catalyst support for transformation of water-soluble aromatic molecules

Abstract: A hyper-crosslinked β-cyclodextrin porous polymer (BnCD-HCPP) was designed and synthesized facilely by β-cyclodextrin benzylation and subsequent crosslinking via a Friedel–Crafts alkylation route. The BnCD-HCPP shows an extremely high BET surface area, large pore volume, and high thermal stability, making it a highly efficient adsorbent for removal of aromatic pollutants from water. The adsorption efficiency in terms of distribution coefficient, defined as the ratio of adsorption capacity to equilibrium adsorbate concentration, ranged from 103 to 106 mL g−1 within a concentration of 0–100 ppm, one order of magnitude higher than that of other β-cyclodextrin-based adsorbents reported previously. The molar percentage of adsorbate to β-cyclodextrin exceeded 300%, suggesting that the adsorption occurred not only in the cyclodextrin cavities via a 1 : 1 complexation, but also in the nanopores of the BnCD-HCPP created during the hyper-crosslinking. The BnCD-HCPP can be further functionalized by incorporation of gold nanoparticles for catalytic transformation of adsorbed phenolic compounds such as 4-nitrophenol to 4-aminophenol.

Low temperature plasma-catalytic NOx synthesis in a packed DBD reactor: Effect of support materials and supported active metal oxides

Abstract: The direct synthesis of NOx from N2 and O2 by non-thermal plasma at an atmospheric pressure and low temperature is presented, which is considered to be an attractive option for replacement of the Haber-Bosch process. In this study, the direct synthesis of NOx was studied by packing different catalyst support materials in a dielectric barrier discharge (DBD) reactor. The support materials and their particle sizes both had a significant effect on the concentration of NOx. This is attributed to different surface areas, relative dielectric constants and particles shapes. The nitrogen could be fixed at substantially lowered temperatures by employing non-thermal plasma-catalytic DBD reactor, which can be used as an alternative technology for low temperature synthesis. The γ-Al2O3 with smallest particles size of 250–160 μm, gave the highest concentration of NOx and the lowest specific energy consumption of all the tested materials and particle sizes. The NOx concentration of 5700 ppm was reached at the highest residence time of 0.4 s and an N2/O2 feed ratio of 1 was found to be the most optimum for NOx production. In order to intensify the NOx production in plasma, a series of metal oxide catalysts supported on γ-Al2O3 were tested in a packed DBD reactor. A 5% WO3/γ-Al2O3 catalyst increased the NOx concentration further by about 10% compared to γ-Al2O3, while oxidation catalysts such as Co3O4 and PbO provided a minor (∼5%) improvement. These data suggest that oxygen activation plays a minor role in plasma catalytic nitrogen fixation under the studied conditions with the main role ascribed to the generation of microdischarges on sharp edges of large-surface area plasma catalysts. However, when the loading of active metal oxides was increased to 10%, NO selectivity decreased, suggesting possibility of thermal oxidation of NO to NO2 through reaction with surface oxygen species.

Electrocatalysts for hydrogen oxidation and evolution reactions

Abstract: The hydrogen economy is a clean, efficient, and sustainable energy system that delivers energy using hydrogen. Electrolyzers are used to generate hydrogen, and fuel cells consume hydrogen as a fuel. These devices rely on hydrogen evolution and oxidation reactions. Catalysts are required to accelerate the reactions. Pt is the best hydrogen oxidation/evolution reaction (HOR/HER) catalyst to date. However, the scarcity of Pt hinders its applications. Various processes have been developed to increase the catalyst activity in order to reduce the amount of Pt, or to develop non-precious metal catalysts to replace Pt. In this review, we focus on electrocatalysts for the hydrogen oxidation and evolution reactions. The reaction mechanism, factors influencing the catalyst activity, and the influence of the electrolyte (acid vs . base) are discussed. The recent advances in catalyst development, especially of non-precious metal catalysts, are also summarized. Due to the different reaction conditions and catalytic activities associated with the different electrolytes, the catalysts are classified into two categories:active in acid or in base.

Role of the Support and Reaction Conditions on the Vapor-Phase Deoxygenation of m-Cresol over Pt/C and Pt/TiO2 Catalysts

Abstract: The catalytic deoxygenation of biomass fast pyrolysis vapors offers a promising route for the sustainable production of liquid transportation fuels. However, a clear understanding of the mechanistic details involved in this process has yet to be achieved, and questions remain regarding the role of the catalyst support and the influence of reaction conditions. In order to gain insight into these questions, the deoxygenation of m-cresol was investigated over Pt/C and Pt/TiO2 catalysts using experimental and computational techniques. The performance of each catalyst was evaluated in a packed-bed reactor under two conditions (523 K, 2.0 MPa and 623 K, 0.5 MPa), and the energetics of the ring hydrogenation, direct deoxygenation, and tautomerization mechanisms were calculated over hydrogen-covered Pt(111) and oxygen vacancies on the surface of TiO2(101). Over Pt(111), ring hydrogenation to 3-methylcyclohexanone and 3-methylcyclohexanol was found to be the most energetically favorable pathway. Over TiO2(101), ta...

An efficient and easily retrievable dip catalyst based on silver nanoparticles/chitosan-coated cellulose filter paper

Abstract: Re-use of a catalyst is an important task, which is usually achieved by loading it on easily separable supports such as magnetic substrates. However, we demonstrate here the process of easy and fast catalyst separation from a reaction medium by loading it onto an economically feasible and microscopically high surface substrate of filter paper (FP) made up of cellulose microfibers as catalyst support. To achieve the goal, we coated chitosan (CH) on filter paper (CH-FP) to impart a high affinity of the substrate for metal ion absorption. AgNO3 dissolved in water with a 0.1 M concentration was used as the Ag ion carrier solution, and CH-FP strips with known rectangular dimensions were submerged into it for the metal ion absorption. The metal ion-laden CH-FP strips were dip treated with sodium borohydride (NaBH4) aqueous solution to prepare Ag-nanoparticle loaded CH-FP (Ag/CH-FP). X-ray diffraction and energy dispersive X-ray spectroscopy confirmed the formation of the Ag/CH-FP hybrid. Ag/CH-FP morphology was examined through scanning electron microscopy analysis, which showed the presence of Ag nanoparticles attached to the cellulose microfibers. The prepared Ag/CH-FP was employed as a dip catalyst for the degradation of nitroarene compounds of 2-nitophenol (2-NP) and 4-nitrophenol (4-NP) by NaBH4. Remarkably, the rate constants for 4-NP and 2-NP were 3.9 × 10−3 and 1.7 × 10−3 s−1, respectively. In addition, we discussed the ease of the catalyst retrievability from the reaction mixture and its re-usability.