Nanoscale Catalysts Research Articles

Nanotechnology's Implications for Select Systems of Renewable Energy

Developments in micro and nanotechnology within the renewable energy industry have the potential to create significant advances for the renewable energy industry. A review of selected renewable energy sectors being influenced by micro and nanotechnology was performed, finding that the most promising areas involve electricity generation, biomass technologies, and hydrogen technologies. Such technologies include: surface microtexturization and nanocrystalline films in photovoltaic and photoelectrochemical cells; nanoscale catalysts and membranes in biomass or thermochemical hydrogen generation; surface utilization of carbon nanotubes in hydrogen storage and fuel cell applications. These advances may increase process yields and efficiencies, and also may lower overall costs.

Direct synthesis of dimethyl carbonate from carbon dioxide and methanol using supported copper (Ni, V, O) catalyst with photo-assistance

The photo-catalytic effect of a copper modified (Ni, V, O) semiconductor complex catalyst on the direct synthesis of dimethyl carbonate (DMC) from CO 2 and CH 3OH was investigated. The synthesized catalysts were fully characterized by temperature programmed reduction (TPR), X-ray diffraction (XRD), Ultraviolet visible drift reflection spectra (UV–vis DRS) and transmission electron microscopy (TEM). The nano-scale catalyst particles were observed with TEM and light absorbance was then predicted by UV–vis DRS spectra. The pressure and temperature dependencies of the photo-catalytic activity were studied. The results demonstrated that the catalytic activity was enhanced with the assistance of ultraviolet (UV) irradiation compared with the pure thermal and surface catalytic reaction under the same reaction conditions.

Development of ceria-supported sulfur tolerant nanocatalysts: Rh-based formulations

The conversion of logistics fuels to hydrogen by steam reforming is attractive but poses great challenge since they contain sulfur up to about 3000 ppm leading to catalyst deactivation due to sulfur poisoning. In this paper, we report the fabrication of nominally doped nanoscale ceria-supported rhodium catalyst matrices for their performance evaluation in sulfur-laden fuel streams. Systematic structural and microstructural characterization of the catalysts was carried out before and after the steam reforming and simulated experiments in sulfur-containing streams (50 ppm < S < 1000 ppm) over a wide range of temperature and duration, to speculate and understand the deactivation mechanism and the sulfur tolerance aspects. Steam reforming of toluene as a model fuel without or with 50 ppm sulfur (as thiophene) was carried out at 825 °C and steam-to-carbon (S/C) ratio of 3. The performance of catalysts with bimetal (Rh + Pd) dispersion in small but equal concentrations was found to be the best both in terms of sulfur tolerance and percent H2 yield. It was found that the addition of metal oxide additives yielded more stable and sulfur-tolerant formulation. Rhodium-alone and the rhodium + metal-oxide formulations outperformed their palladium-bearing analogs.

SOFC Anode Performance Enhancement through Precipitation of Nanoscale Catalysts

This paper describes a new method for improving the performance of oxide anodes by precipitating metal nano- clusters on oxide surfaces during the initial stages of SOFC operation, without additional processing steps. Transmission electron microscope and x-ray photoelectron spectroscopy observations showed that nano-clusters of Ni or Ru metal precipitated onto lanthanum chromite (La0.8Sr0.2Cr1-yXyO3-d, X = Ni, Ru) surfaces, respectively, after exposure to hydrogen at 800{degree sign}C. While Ni nano-clusters coarsened over ~300 h at 800{degree sign} C, Ru nano-cluster size was stable at <= 5 nm. SOFC tests were done with the doped lanthanum chromite anodes on LSGM electrolyte-supported cells. Ru and Ni nano-cluster nucleation improved cell performance and reduced anode polarization resistance during the first 50-100 h of cell operation.

Decomposition kinetics of ammonia in gaseous stream by a nanoscale copper-cerium bimetallic catalyst

This study performance is to examine the kinetics over nanoscale copper-cerium bimetallic catalyst under selective catalytic oxidation (SCO) of ammonia to N 2 in a tubular fixed-bed reactor (TFBR) at temperatures from 150 to 400 °C in the presence of oxygen. The nanoscale copper-cerium bimetallic catalyst was prepared by co-precipitation with Cu(NO 3) 2 and Ce(NO 3) 3 at molar ratio of 6:4. Experimental results showed that the catalyst with transmission electron microscopy (TEM) revealed that copper and cerium are well dispersed and catalyst in the form of nanometer-sized particles. Moreover, the kinetic behavior of NH 3 oxidation with catalysis can be accounted by using the rate expression of the Langmuir–Hinshelwood type kinetic model. Kinetic parameters are also developed on the basis of the differential reactor data. Also, experimental results are compared with those of the model predicted.

Trends in electrocatalysis on extended and nanoscale Pt-bimetallic alloy surfaces

One of the key objectives in fuel-cell technology is to improve and reduce Pt loading as the oxygen-reduction catalyst. Here, we show a fundamental relationship in electrocatalytic trends on Pt(3)M (M=Ni, Co, Fe, Ti, V) surfaces between the experimentally determined surface electronic structure (the d-band centre) and activity for the oxygen-reduction reaction. This relationship exhibits 'volcano-type' behaviour, where the maximum catalytic activity is governed by a balance between adsorption energies of reactive intermediates and surface coverage by spectator (blocking) species. The electrocatalytic trends established for extended surfaces are used to explain the activity pattern of Pt(3)M nanocatalysts as well as to provide a fundamental basis for the catalytic enhancement of cathode catalysts. By combining simulations with experiments in the quest for surfaces with desired activity, an advanced concept in nanoscale catalyst engineering has been developed.

The formation mechanisms of multi-wall carbon nanotubes over the Ni modified MCM-41 catalysts

Multi-wall carbon nanotubes (MWCNTs) were grown by thermal chemical vapor deposition (thermal CVD) of CH4 by using Ni-MCM-41 as the catalyst. Methane pyrolysis has been performed in a quartz tube reactor over the catalyst surface to form carbon atoms via dehydrogenation process. The migration and rearrangement of the surface carbon atoms result in the formation of MWCNTs. Transmission electron microscope (TEM) and scanning electron microscope (SEM) were used to determine the morphologies and structures of CNTs, and Raman spectroscopy was exploited to analyze their purity with the relative intensity between the D-band (Disorder band) in the vicinity of 1,350 cm−1 which is characteristic of the sp3 structure and G-band (Graphitic band) in vicinity of 1,580 cm−1 which is characteristic of the sp2 structure. In addition, the controlling factors of methane pyrolysis such as the catalyst composition; the reaction temperature, and the methane flow rate on the formation of MWCNTs were investigated to optimize the structure and yield of MWCNTs. SEM/TEM results indicate that the yield of the CNTs increases with increasing Ni concentration in the catalyst. The optimized reaction temperature to grow CNT is located between 640 and 670 °C. The uniform and narrow diameter MWCNTs form at lower flow rate of methane (∼30 sccm), and non-uniform in diameter and disorder structure of MWCNTs are observed at higher flow rate of methane. This is consistent with Raman analysis that the relative intensity of I D/I G increases with increasing methane flow rate. The formation mechanisms of the MWCNTs on the Ni-MCM-41 catalyst have been determined to be a Tip-Growth mode with a nanoscale catalyst particle capsulated in the tip of the CNT.

Synthesis and Characterization of High Quality InN Nanowires and Nano-networks

ABSTRACTHigh quality InN nanowires have been synthesized in a horizontal quartz-tube furnace through direct reaction between metallic Indium and Ammonia using Nitrogen as the carrier gas. Thin film of Au on SiO2/Si substrate has been used as the catalyst layer, facilitating vapor-liquid-solid growth of the nanostructures. The nanowires were grown at a very fast rate of up to 30 μm/hr. Smooth and horizontal nanowire growth was achieved only with nanoscale catalyst patterns, while large area catalyst coverage resulted in uncontrolled and three-dimensional growth. The InN nanowires, which were usually covered with a thin shell layer of In2O3, grew along [110] direction, with overall diameters 20 - 60 nm and lengths 5 - 15 μm. The synthesized nanowires bent spontaneously or got deflected from other nanowires at multiples of 30 degrees forming nano-networks. The catalyst particles for the NWs were found mostly at the sides of the NW apex which helped them to bend spontaneously or get deflected from other NWs at angles which were multiples of 30 degrees. The NW based FETs with a back-gated configuration have already been investigated. The gate-bias dependent mobility of the NWs ranged from 55 cm2/Vs to 220 cm2/Vs, and their carrier concentration was ∼1018 cm−3.

Study of the performance of modified nano-scale ZSM-5 zeolite on olefins reduction in FCC gasoline

Nanoscale HZSM-5 zeolite (20–50 nm) was hydrothermally treated with steam containing 0.8 wt.% NH 3 at 773 K firstly and then was modified with mixed rare earth oxides and Ga 2O 3. Both the parent nanoscale HZSM-5 and the modified nanoscale HZSM-5 zeolites catalysts were characterized by TEM, XRD, IR, NH 3-TPD, and XRF techniques. IR and NH 3-TPD results showed that the total amount of acids sites in the parent nanoscale HZSM-5 zeolite decreased after the hydrothermal treatment, especially for the strong acid sites. The following modification using the mixed rare earth oxides had little effect on the acid properties of the hydrothermally treated nanoscale HZSM-5 catalyst; however, the further modification using Ga 2O 3 increased the total acid sites of nanoscale HZSM-5 catalyst, especially for the Lewis acid sites. On the other hand, the amount of total acid sites in Ga 2O 3 modified catalyst was still less than that in the parent HZSM-5 catalyst. The performance of the modified nanoscale HZSM-5 zeolite catalyst on olefins reduction in fluidized catalytic cracking (FCC) gasoline was investigated and it was found that the activity and stability of the modified catalyst was satisfactory, but the initial activity slightly decreased. After 300 h on stream, the olefins content in the FCC gasoline decreases from 49.6 to 15.1 vol.%. Further work showed that the ability of nanoscale HZSM-5 zeolite catalyst for aromatization reduced while its isomerization ability increased. The ratio of iso-paraffins to n-paraffins in FCC upgraded gasoline reached 8.2. The yield of upgraded gasoline reached 96 wt.%, while the research octane number (RON) was increased from 89.4 to 90.0. The components of the upgraded gasoline met the requirement of European III gasoline standard.

Investigation of isosynthesis via CO hydrogenation over ZrO2 and CeO2 catalysts: Effects of crystallite size, phase composition and acid–base sites

This paper investigates the isosynthesis via CO hydrogenation over zirconia and ceria catalysts. Various techniques including XRD, NH3-TPD, CO2-TPD and BET surface area were employed for the catalyst characterization. The results showed that not only acid–base properties, but also crystallite size and crystal phase essentially influenced the catalytic performance. It was found that the activity and the selectivity of isobutene in hydrocarbons on nanoscale catalysts were higher than those on the micronscale ones. Moreover, the acid–base properties were dependent on the fraction of tetragonal phase for zirconia, but independent on crystal phase for ceria. The synthesized nanoscale zirconias were more active than the commercial one but less than the nanoscale ceria. From the results, it was indicated that zirconia with 29% tetragonal phase exhibited the highest activity. Furthermore, the presence of tetragonal phase in zirconia played an important role on the selectivity of isobutene in hydrocarbons.

Alloy Electrocatalysts: Combinatorial Discovery and Nanosynthesis

Improving efficiency and reducing overall cost are two key issues to the commercialization of fuel cell powered vehicles. Electrocatalysts play an important role, particularly in the cathode, where the oxygen reduction reaction is sluggish and the noble metal loading is relatively high. To discover less expensive and more active cathode catalysts, a novel combinatorial workflow has been developed to investigate alloy-based electrocatalysts. In addition to the discovery program, various synthesis technologies have been studied and developed to engineer nanoscale catalyst particles with controllable size, monodispersity, and microcomposition. The progress of these research activities is reported with particular attention focused on the activity–stability–composition relationship for a series of platinum-based metal alloys.

The effect of specific surface area on the activity of nano-scale ceria catalysts for methanol decomposition with and without steam at SOFC operating temperatures

Nano-particulate high surface area CeO 2 was found to have a useful methanol decomposition activity producing H 2 , CO, CO 2 , and a small amount of CH 4 without the presence of steam being required under solid oxide fuel cell temperatures, 700–1000 °C. The catalyst provides high resistance toward carbon deposition even when no steam is present in the feed. It was observed that the conversion of methanol was close to 100% at 850 °C, and no carbon deposition was detected from the temperature programmed oxidation measurement. The reactivity toward methanol decomposition for CeO 2 is due to the redox property of this material. During the decomposition process, the gas–solid reactions between the gaseous components, which are homogeneously generated from the methanol decomposition (i.e., CH 4 , CO 2 , CO, H 2 O , and H 2 ), and the lattice oxygen ( O O x ) on ceria surface take place. The reactions of adsorbed surface hydrocarbons with the lattice oxygen ( C n H m + O O x → n CO + m / 2 ( H 2 ) + V O · · + 2 e ′ ) can produce synthesis gas (CO and H 2 ) and also prevent the formation of carbon species from hydrocarbons decomposition reaction ( C n H m ⇔ n C + m / 2 H 2 ) . V O · · denotes an oxygen vacancy with an effective charge 2 + . Moreover, the formation of carbon via Boudouard reaction ( 2 CO ⇔ CO 2 + C ) is also reduced by the gas–solid reaction of carbon monoxide with the lattice oxygen ( CO + O O x ⇔ CO 2 + V O · · + 2 e ′ ) . At steady state, the rate of methanol decomposition over high surface area CeO 2 was considerably higher than that over low surface area CeO 2 due to the significantly higher oxygen storage capacity of high surface area CeO 2 , which also results in the high resistance toward carbon deposition for this material. In particular, it was observed that the methanol decomposition rate is proportional to the methanol partial pressure but independent of the steam partial pressure at 700–800 °C. The addition of hydrogen to the inlet stream was found to have a significant inhibitory effect on the rate of methanol decomposition.

Optical absorption of magnesia-supported gold clusters and nanoscale catalysts: Effects due to the support, clusters, and adsorbants

Polarization-resolved optical spectra of magnesia-supported gold clusters ${\mathrm{Au}}_{N}∕\mathrm{Mg}\mathrm{O}$ $(N=1,2,4,8)$, bound at a surface color center ${F}_{s}$ of the MgO(100) face, are calculated from the time-dependent density functional theory. The optical lines for $N=1,2$ are dominated by transitions that involve strong hybridization between gold and ${F}_{s}$ states whereas for $N=4,8$ intracluster transitions dominate. The theoretical optical spectra are sensitive to cluster structure and adsorbants (here CO and ${\mathrm{O}}_{2}$ molecules on ${\mathrm{Au}}_{8}∕{F}_{s}@\mathrm{Mg}\mathrm{O}$) which suggests polarization-resolved optical spectroscopy as a powerful tool to investigate structures and functions of chemically active, supported clusters.

Growth mechanism of vapor phase CVD-grown multi-walled carbon nanotubes

The formation mechanisms involved in the growth of carbon nanotubes (CNTs) by spray pyrolysis was studied. Both iron and nickel were used as catalysts for growth, and nanotubes were also produced using thermal chemical vapor deposition for comparison. Transmission electron microscopy was used to analyze the encapsulated metal catalyst particles found within the tubes, and the dimensions and location of these particles was recorded. CNTs grown by spray pyrolysis were found to have encapsulated particles in both the middle and end of tubes, with large length to diameter ratios. As a result of these observations, it is concluded that nanotubes grown using spray pyrolysis are formed via an open-ended, root growth mechanism. Additionally, the presence of multiple, high aspect ratio particles within single tubes is explained by an additional growth theory. During the continued growth of these CNTs, metal atoms or nanoscale metal catalyst particles deposit in the open ends of growing tubes, forming new particles and helping to prevent tube closure. CNTs grown with thermal CVD did not contain similar elongated particles or particles along the middle of the tubes, indicating that this new growth mechanism is only applicable in the case of tubes grown via spray pyrolysis or other vapor phase CVD growth methods.

Deposition and electrocatalytic properties of platinum on well-aligned carbon nanotube (CNT) arrays for methanol oxidation

Well-aligned carbon nanotube (CNT) arrays were used as supporting materials for platinum (Pt) electrocatalysts. The microstructure of the resulting Pt/aligned-CNT electrode was characterized by scanning electron microscopy (SEM), and nano-scale Pt catalysts dispersed highly on CNT arrays by electro-deposition can be obtained. The electrocatalytic properties of the Pt/aligned-CNT electrode for methanol oxidation have been investigated by cyclic voltammetry (CV). Compared with Pt/graphite and Pt/tangled-CNT/graphite electrodes, higher electrocatalytic activity of the Pt/aligned-CNT electrode can be observed.

Selective hydrogenation of citral over amorphous NiB and CoB nano-catalysts

Alloy catalysts of P-1NiB and P-1CoB were prepared by reduction of their acetate salts with NaBH 4 in aqueous solution. Alloy catalysts of P-2WNiB and P-2WCoB were similarly prepared in ethanolic solution. The NiB and CoB catalysts prepared by this method were characterized as nano-scale (10–20 nm) particles with amorphous structure. These nano-scale alloy catalysts were used for the selective hydrogenation of citral to citronellal, nerol/geraniol and citronellol, which are important perfumery products. These catalysts are compared with the popular commercial catalysts: Raney nickel and Raney cobalt. The NiB and CoB catalysts were generally much more active and selective than Raney nickel or Raney cobalt. The distribution of products depended on the catalysts, the solvent used to prepare the catalysts and the solvent employed as the reaction medium. Over P-1NiB, the conjugated C C bond was preferentially reduced and citronellal was the main product in a reaction medium of cyclohexane, while citronellol was the major one in an ethanol medium. P-2WNiB was much more active than P-1NiB, but citronellol was further reduced to form the completely saturated product 3,7-dimethyloctanol. Over CoB catalysts, the conjugated C O bond was preferentially reduced to form nerol/geraniol and then consecutively reduced to citronellol; no observable 3,7-dimethyloctanol was formed. A yield of citronellal over 85% was obtained over P-1 and P-2WNiB in cyclohexane, and a 100% yield of citronellol was obtained over P-1 and P-2WCoB in ethanol. Only a 65% yield of nerol/geraniol was obtained over CoB catalysts.

Partial oxidation of polyvalent oxygen substituted compounds on nano-scale gold catalysts

Investigations of the properties of supported nano-scale Au catalysts in the partial oxidation of three different polyvalent oxygen substituted compounds are presented. The relations between Au particle size and catalytic properties of Au/Al2O3 in the partial oxidation of ethylene glycol to glycolic acid as model reaction are discussed. Moreover, influences of the nature of the alumina support and the applied precipitation agent on the catalyst properties were studied. Furthermore, the potential of these catalysts for the industrial interesting partial oxidation of glyoxal to glyoxylic acid was checked. However, it seems that the oxidation of glyoxal is dominated by a CC rupture leading to formic acid, mainly. Finally, the excellent selectivity of gold supported on alumina or titania in the partial oxidation of aldehyde groups of saccharides to carboxylic groups is briefly presented.

Molecular dynamics simulations of formation process of single-walled carbon nanotubes

Formation process of single-walled carbon nanotubes (SWNTs) by the catalytic chemical vapor deposition method is studied by molecular dynamics simulations. The random deposition process of carbon atoms to a nano-scale metal catalyst is realized with the multi-body potentials based on density functional theory calculations of small metal-carbon and metal-metal clusters A nanotube cap structure is generated when pieces of the hexagonal network structure emerges from the metal-carbon binary cluster and then coalesces. Furthermore, the tendency of graphitization of carbon atoms is compared for Ni, Co, Fe metal clusters. The Co cluster has a pseudo crystal structure where metal atoms are regularly allocated and embedded in the hexagonal carbon network. On the other hand, carbon atoms cover the entire surface of the Fe cluster. This implies stronger interaction between the graphitic lattice and Co atoms, hence higher graphitization ability. Finally, the mechanism of chirality determination by the stability of nanotube cap structure is discussed in order to explain the recent experimental finding that the near-armchair nanotubes are more dominantly produced for small diameter nanotubes.

Size Control, Metal Substitution, and Catalytic Application of Cryptomelane Nanomaterials Prepared Using Cross-linking Reagents

A novel sol−gel-assisted solid-state synthetic method has been developed to prepare pure and transition-metal-substituted cryptomelane 1D nanomaterials with controlled particle sizes. Different cross-linking reagents (PVA, glycerol, or glucose) have been used to prepare precursor powder materials and to produce nanorods, nanoneedles, or nanowires. The phase transformation, crystal structures, and properties of these nanomaterials have been investigated using a variety of characterization techniques including XRD, TEM, SEM, FT-IR, TGA, and BET surface area measurement. Both the cross-linking reagents and nitrate were found to affect the crystalline phase transformation of the nanomaterials. The cryptomelane phase was formed around 500−600 °C in the nanomaterials depending on the type of cross-linking reagent used. The nanoscale materials exhibit long-range ordered structures along the b axis. No cross-linking reagent residues were found in the final products after the reactions were complete. Several types of transition metal cations (Fe3+, Co2+, Ni2+, and Cu2+) were used to substitute into cryptomelane nanomaterials. XRD data show that Fe(III) cations have been substituted into the materials without the formation of additional amorphous or crystalline phases, while the other three cations caused the formation of impure amorphous or crystalline phases. These nanomaterials showed thermal stability up to 700−800 °C. Catalytic application of these nanoscale manganese oxide materials has been explored in the green oxidation of toluene to produce benzyl alcohol, benzylaldehyde, and benzoic acid. The nanoscale catalysts showed unique catalytic activity for this oxidation compared to those catalysts conventionally used for this reaction.

Synthesis of gallium borate nanowires

Gallium borate nanowires have been synthesized using a simple and readily scaleable solid-state reaction among boron, boron oxide and gallium oxide. The GaBO3 nanowires have diameters ranging from 90 to 150nm and lengths of several hundreds nanometer, exhibiting a poor crystallization. After a nanoscale support catalyst Fe2O3/Al2O3 was used as additive in the reaction, the diameters, lengths and crystallization were effectively improved. The nanowires exhibit a well-crystallized one-dimensional single-crystal structure with diameters ranging from 20 to 60nm and lengths up to several micrometers. The growth mechanism was also presented in this letter.