Structure-sensitive Reaction Research Articles

On Apparent Activation Energy of Structure Sensitive Heterogeneous Catalytic Reactions

The notion of apparent activation energy is discussed in relation to structure sensitivity of complex heterogeneous catalytic reactions comprising several steps. The cases of nonuniform surfaces (intrinsic and induced) are considered, showing that the apparent activation energy depends on the cluster size and surface coverage. The theoretical analysis of the apparent activation energy was also extended for the two step sequence with deactivating catalyst by poisoning or when coking occurs as a result of the reactant interactions with an intermediate.Graphical

Investigation of the interfacial properties of platinum stepped surfaces using peroxodisulfate reduction as a local probe

Peroxodisulfate (PDS) reduction has been investigated by electrochemical and FTIR techniques in a wide pH range, on platinum stepped single crystals electrodes vicinal to the (111) pole. PDS reduction is a structure-sensitive reaction and proceeds on well differentiated potential regions for terrace and step sites. This fact allows gaining local information about the individual behavior of those sites. The reaction proceeds at appreciable rate in a narrow potential range, being inhibited in the low and high potential regions of the voltammogram. The inhibition of the reaction at low potentials is most likely associated with a change in the sign of the charge on the electrode, from positive to negative. Therefore, PDS reduction gives an approximate indication of the position of the potential of zero free charge (pzfc). In this way, two local values of pzfc, associated with terrace and steps sites, can be determined. Such values of local pzfc are compared with the local values of potential of maximum entropy (pme) of double layer formation deduced from laser induced T-jump experiment. Good agreement between both magnitudes is found, especially for terraces. The local pzfc on terraces shifts to more positive potentials as the terrace length diminishes, while the local pzfc for steps remains constant. The inhibition of PDS reduction at high potentials can also be related with a second inversion in the sign of the electrode charge. Therefore, PDS reduction can be used to determine a second pzfc on the (111) terraces at high potentials, revealing a non-monotonic charge – potential relationship. Spectroscopic experiments allow the identification of the species involved in PDS reduction.

Auδ−–Ov–Ti3+ Interfacial Site: Catalytic Active Center toward Low-Temperature Water Gas Shift Reaction

The electronic metal–support interaction (EMSI) plays a crucial role in promoting catalytic performance toward interface electronic structure sensitive reactions, such as the low temperature water ...

Structure-Sensitive and Insensitive Reactions in Alcohol Amination over Nonsupported Ru Nanoparticles

The reaction rates and selectivity of many metal-catalyzed reactions depend on the size of the metal particles in the nanoscale range. Primary amines are important platform molecules in the chemical industry. In this work, the catalytic performance of nonsupported Ru nanoparticles with sizes from 2 to 9 nm was investigated in direct amination of octanol and other alcohols into primary amines in the presence of ammonia. The 90% selectivity to octylamine was obtained over small Ru nanoparticles (d = 2 nm) even at 92% conversion, whereas for larger Ru nonsupported and supported nanoparticles, the octylamine selectivity dropped as the octanol conversion approached 70–80%. The primary reaction of alcohol amination into octylamine was found to be nearly a structure-insensitive reaction. The selectivity to primary amine drops over large Ru particles at higher conversions, because of the secondary highly structure-sensitive reaction of amine self-coupling. Over small metal nanoparticles, amine self-coupling is hi...

XPS and NEXAFS study of the reactions of acetic acid and acetaldehyde over UO2(100) thin film

The reactions of two C2 oxygenates, acetaldehyde (CH3CHO) and acetic acid (CH3COOH), over the (100) terminated surface of UO2 thin film are investigated. The UO2 film (ca. 100 nm thick) was prepared on a LaAlO3 (100) substrate and its reactions were studied by high-resolution synchrotron X-ray photoelectron spectroscopy (XPS – C 1 s, O 1 s, and U 4f) and Near-Edge X-ray Absorption Fine Structure (NEXAFS – C K-edge). Results show a strong interaction of both molecules with the (100) surface at 300 K. Adsorption of acetic acid at 280 K produces acetates. Upon heating their population steadily decreased (XPS C 1 s) and some enolate or ketene-like species (such as CH2=COO(a)) are formed (NEXAFS). Acetaldehyde primarily adsorbed molecularly at 300 K and subsequent annealing to 480 K led to α-H abstraction making enolates upon interaction with surface O anions. No preferential orientation was detected for acetates while some could be seen for adsorbed acetaldehyde (NEXAFS). Results for the adsorption and reactivity of acetic acid and acetaldehyde on UO2(100) are compared with those of UO2(111) reported previously to further probe into possible structure-sensitive reactions of this actinide metal oxide.

Morphology control of Co2C nanostructures via the reduction process for direct production of lower olefins from syngas

Fischer-Tropsch to olefins (FTO) is recognized as a surface-catalyzed structure-sensitive reaction, and the catalytic performance is strongly influenced by the morphology and exposed facets of the active phase. Here we report the effect of the reduction process on the morphology of the active phase and the catalytic performance for FTO over the CoMn catalyst. For the catalysts reduced by 10% CO-300 °C, 10% H2-300 °C and 10% H2-250 °C, Co2C nanoprisms were formed after reaching the steady state. However, for the catalysts reduced by CO-300 °C and 10% H2-400 °C, Co2C nanospheres were found instead. Both Co2C nanoprisms and nanospheres were present for the spent sample reduced by 10% H2-350 °C. Kinetic study found Co2C nanospheres to possess higher activation energy, and are more sensitive to hydrogen than Co2C nanoprisms. Density functional theory (DFT) calculations were also performed to clarify the structure-performance relationship of Co2C nanostructures for syngas conversion.

Hcp-Co Nanowires Grown on Metallic Foams as Catalysts for Fischer-Tropsch Synthesis.

The Fischer-Tropsch synthesis (FTS) is a structure-sensitive exothermic reaction that enables catalytic transformation of syngas to high quality liquid fuels. Now, monolithic cobalt-based heterogeneous catalysts were elaborated through a wet chemistry approach that allows control over nanocrystal shape and crystallographic phase, while at the same time enables heat management. Copper and nickel foams have been employed as supports for the epitaxial growth of hcp-Co nanowires directly from a solution containing a coordination compound of cobalt and stabilizing ligands. The Co/Cufoam catalyst was tested for Fischer-Tropsch synthesis in a fixed-bed reactor, showing stability and significantly superior activity and selectivity towards C5+ compared to a Co/SiO2 -Al2 O3 reference catalyst under the same conditions.

Carbonylation of methanol to methyl acetate over Cu/TiO2-SiO2 catalysts: Influence of copper precursors

In the present study, three kinds of copper/titania-silica mixed oxides (Cu/TS) catalysts were prepared using copper chloride, copper nitrate, and copper acetate as copper precursors by the sol-gel method. The catalytic performance was tested in the vapor phase carbonylation of methanol free of halide. The characterization results showed that the interaction between copper and support was the strongest on the 10Cu/TS (Cl) catalyst, leading to the highest copper dispersion and the most surface Cu+ species. As a result, the 10Cu/TS (Cl) catalyst obtained the largest absorption capacity of CO and the maximum amount of Lewis acid sites. The catalytic performance results showed that the space time yield (STY) of methyl acetate (MA) was positively correlated with the content of surface Cu+ species and the specific surface area of the Cu+ cations (SCu(I)). Therefore, the 10Cu/TS (Cl) catalyst obtained the highest STYMA of 1.619 mol/h.kgcat. Moreover, the STYMA per Cu+ site was inversely proportional to the copper particle size, indicating that the carbonylation of methanol to MA is a structure-sensitive reaction. And the methanol conversion was positively correlated with the amount of surface acid sites. Furthermore, the active sites for the synthesis of methyl formate and dimethyl ether were also studied.

In Situ Characterization of Cu/CeO2 Nanocatalysts for CO2 Hydrogenation: Morphological Effects of Nanostructured Ceria on the Catalytic Activity

A combination of time-resolved X-ray diffraction (TR-XRD), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and diffuse reflectance infrared Fourier transform spectroscopy was used to carry out in situ characterization of Cu/CeO2 nanocatalysts during the hydrogenation of CO2. Morphological effects of the ceria supports on the catalytic performances were investigated by examining the behavior of copper/ceria nanorods (NR) and nanospheres. At atmospheric pressures, the hydrogenation of CO2 on the copper/ceria catalysts produced mainly CO through the reverse water–gas shift (RWGS) reaction and a negligible amount of methanol. The Cu/CeO2-NR catalyst displayed the higher activity, which demonstrates that the RWGS is a structure-sensitive reaction. In situ TR-XRD and AP-XPS characterization showed significant changes in the chemical state of the catalysts under reaction conditions, with the copper being fully reduced and a partial Ce4+ → Ce3+ transformation occurring. A more effective CO2 dissociati...

Diversity of Pd-Cu active sites supported on pristine carbon nanotubes in terms of water denitration structure sensitivity

Four Pd-Cu-based (2:1 wt.%) catalysts for water denitration, differing in physico-chemical characteristics and structural quality, were prepared, using carbon nanotubes (CNTs) as supports. The extent of lattice defects in the pristine CNTs turned out to govern resulting varieties of Pd nanoparticle size and the composition of the bimetallic (Pd:Cu) entities in the catalysts, playing an important role in nitrate reduction. Catalyst samples characterized by close proximity of Pd and Cu, as in an alloy, displayed the highest degree of nitrate conversion. At the same time, the size of the Pd particles, alone, or decorated to different extents with Cu atoms, was found to be critical for the second step of denitration reaction, directing the formed nitrite to either N2 or NH3. Water denitration is a structure sensitive reaction related to Pd particle size, and this effect is further emphasized in the decoration of the particle by the incorporation of Cu atoms into Pd particle corners and edges.

Steric Hindrance in Sulfur Vacancy of Monolayer MoS2 Boosts Electrochemical Reduction of Carbon Monoxide to Methane.

The efficient generation of methane by total electroreduction of carbon monoxide (CO) could be of benefit for a more sustainable society. However, a highly efficient and selective catalyst for this process remains to be developed. In this study, density functional theory calculations indicate that steric hindrance in monolayer molybdenum sulfide with 2 S vacancies (DV-MoS2 ) can facilitate the conversion of CO into CH4 with high activity and selectivity under electrochemical reduction at a low potential of -0.53 V vs. RHE and ambient conditions. The potential is a significant improvement on the state-of-the-art Cu electrode (-0.74 V vs. RHE), with less electrical energy. Moreover, the results suggest that such steric hindrance effects are important for structure-sensitive catalytic reactions.

Efficient catalytic properties of SO42−/MxOy (M = Cu, Co, Fe) catalysts for hydrogen generation by methanolysis of sodium borohydride

The SO42−/MxOy (M = Cu, Co, Fe) catalysts were prepared and applied to hydrogen production from methanolysis of sodium borohydride for the first time. The morphologies and properties of the as-prepared catalysts were characterized by XRD, BET, FT-IR, TEM and SEM-EDX techniques. Under our experimental conditions, SO42−/CuO exhibits much higher catalytic activity than those of SO42−/CoO and SO42−/Fe2O3 catalysts and follows the order of SO42−/CuO > SO42−/CoO > SO42−/Fe2O3, which is opposite to order of BET surface area, implying that the methanolysis of sodium borohydride is not a structure-sensitive reaction. It can be inferred that both acidic and metallic sites are responsible for the high catalytic performance of the SO42−/MxOy catalysts towards NaBH4 methanolysis. In addition, the effects of the concentrations of NaBH4 and methanol, catalyst dosage, and reaction temperature on the hydrogen generation rate have been investigated using SO42−/CuO as catalyst. The SO42−/CuO-catalyzed methanolysis reaction follows a power law, i.e. r = A·exp (−13135/RT)·[NaBH4]1.01·[CH3OH]1.60·[Catalyst]0.52. The apparent activation energy was calculated to be 13.13 kJ/mol.

Study of catalytic hydrodeoxygenation performance for the Ni/KIT-6 catalysts

Ni/KIT-6 catalysts loaded with different amounts of metallic Ni were prepared by impregnation method. The prepared catalysts and their precursors were investigated through wide- and low-angle XRD, TEM, BET, H2-TPR, and H2-TPD analyzes. The catalytic hydrodeoxygenation performance of the catalysts was evaluated using ethyl acetate as a model bio oil compound. Results indicate that the catalytic hydrodeoxygenation performance of the prepared catalysts was directly related to hydrogen storage properties, hydrogen desorption properties, dispersion of the active component Ni, and so on. The ethyl acetate conversion and ethane selectivity of 25 wt% Ni/KIT-6 catalyst were 100 and 96.8%, respectively, at 300 °C, which shows the best performance. The hydrodeoxygenation activity of ethyl acetate was higher than that of methyl acetate and isopropyl acetate because of the effect of molecular polarity and size. And, this reaction is a structure sensitive reaction.

Recent advances in the investigation of nanoeffects of Fischer-Tropsch catalysts

Fischer-Tropsch synthesis (FTS) is a structure-sensitive reaction for sustainable production of green fuels and value-added chemicals via syngas derived from coal, biomass, shale gas and natural gas. The nanostructure of a Fischer-Tropsch (FT) catalyst plays a crucial role in its catalytic performance. This review summarizes recent advances in the investigation of nanoeffects of FT catalysts, especially the effects of the active phase, particle size and exposed facet on catalytic performance. Perspectives and challenges for further research in nanocatalysis for syngas conversion are also given.

Effective Catalytic Method of Diamantane Isomer Synthesis

<p>General principles of effect of the nature of metal-catalyst, carrier, solvent, method of preparation and temperature of catalyst reduction and other parameters (P<sub>H2</sub>, T, etc) on the process of hydrogenolysis of “binor-S” into pentacyclotetradecane have been studied and stated. On the basis of the results obtained a new effective and economical method of diamantane isomer synthesis has been elaborated. The presence of supported platinum catalyst (10-15% Pt/C) allows to proceed the reaction under mild conditions (0.1-10 MPa, 60-80 °C and in non-aggressive medium (water) with a quantitative yield of the product. It was stated with the help of a complex of physico-chemical methods of investigation (RPES, IR-spectroscopy, NMR, <sup>13</sup>C, PMR, etc) that the methods of preparation and the temperature range of reduction of supported platinum catalysts contributing to the formation of highly dispersed metal particles (2.5-5.0 mm) and their uniform distribution on the carrier surface increase the activity and selectivity of platinum in the reaction of hydrogenolysis of “binor-S” into pentacyclotetradecane. It was shown that the more is the dispersity of metal and the value of the carrier surface, the higher is the activity and selectivity of supported platinum catalysts due to the peculiarity of electronic structure of small particles as well as the influence of the carrier on stabilization of the metal surface structure, <em>i.e. </em>the process of hydrogenolysis of “binor-S” belongs to the type of structure-sensitive reactions.</p>

Insight into the properties of stoichiometric, reduced and sulfurized CuO surfaces: Structure sensitivity for H2S adsorption and dissociation

A mechanistic and kinetic study about the adsorption and dissociation of H2S on different types of CuO(111) surfaces, including the stoichiometric, reduced and sulfurized surfaces, has been carried out to probe into the structure sensitivity for H2S adsorption and dissociation. Here, density functional theory calculations have been employed. Our results show that sulfur-containing species (H2S, SH and S) dominantly interact with the surface Cu via S atom, and H is mainly adsorbed at the outermost surface oxygen site. H2S dominantly exists in the form of dissociative adsorption leading to SH and H species, meanwhile, a small quantity of molecular adsorption H2S also exist on these surfaces. The complete dissociation of molecular adsorption H2S on three CuO(111) surfaces clearly show that the overall dissociation process is exothermic, the stoichiometric surface is the most favorable; the sulfurized surface suppresses the HS bond activation and dissociation. Consequently, the catalytic activity toward molecular adsorption H2S dissociation follows the order: Stoichiometric>Reduced>Sulfurized surface, which have been also confirmed by projected density of states (pDOS), d-band analysis, and the inter-atomic distances. Therefore, H2S dissociation over CuO(111) surface is a structure-sensitive reaction, the effect of surface structure on H2S adsorption and dissociation may play an important role in improving high-performance H2S gas sensors, in which molecular adsorption H2S can be detected on the sulfurized surfaces. In addition, the vibrational frequencies of the adsorbed H2S and SH species on these surfaces can provide a theoretical guidance for the experimental vibrational spectroscopy.

CO Oxidation on Ceria-Based Nanocatalysts: Cooperative and Non-Cooperative Behavior for a Structure-Sensitive Reaction

Ceria-based catalysts (namely, CeO2, CePrOx and CePrZrOx) have been prepared to investigate the structuredependency for the CO oxidation. Then, it has been proved that the CO oxidation over ceria nanocatalysts depends on the presence of highly reactive (100) and (110)-type planes. The best performance has been achieved for the Ce-NC catalyst because of the higher amount of low-index surfaces on the CeO2-nanocubes. For catalysts prepared by the Solution Combustion Synthesis (SCS), higher CO conversion values have been reached with ternary and binary oxides rather than with pure ceria, and the following activity order can be drawn: Ce-SCS < CePr-SCS < CePrZr-SCS. Therefore, the presence of Zr and Pr species into the ceria framework improves the catalytic performance, showing a cooperative behavior among the Ce/Pr/Zr/O sites. On the other hand, the oxidation activity with the nanocubic structures increases as follows: CePrZr-NC < CePr-NC < Ce- NC. This means that the presence of Pr/Zr species into the ceria lattice does not have a beneficial effect on the activity (non-cooperative behavior). These findings further confirm that the surface properties of solid catalysts depend on both the electronic and geometric factors. Moreover, CO oxidation is highly sensitive to the electronic and geometric structures of ceria-based catalysts and it appears as a ill-conditioned system, since small variations in the catalyst properties give rise to large effects on the activity.

Morphology and location manipulation of Fe nanoparticles on carbon nanofibers as catalysts for ammonia decomposition to generate hydrogen

Ammonia decomposition over Fe-based catalysts is a typical structure sensitive reaction, and the shape-controlled synthesis of Fe nanoparticles based on catalytic chemical vapor deposition (CCVD) method appears to be an effective method toward enhanced catalytic activity. The objective of this work is to understand effects of the reaction parameters on structural and textural properties of the resultant Fe catalysts and thus catalytic ammonia decomposition performance. The as-obtained catalysts are characterized by multiple techniques (N2 physisorption, XRD, SEM, TEM, and Raman spectroscopy). Both the morphology and location of Fe nanoparticles are found to strongly depend on the partial pressure of H2 as well as the growth time of CNFs. The long polyhedron shaped Fe nanoparticles on CNFs exhibit the highest activity among the prepared catalysts, which are relatively higher than the activities of the reported Fe based catalysts. The possible explanation for the good catalytic performance was proposed.

On the Structure Sensitivity of Formic Acid Decomposition on Cu Catalysts

Catalytic decomposition of formic acid (HCOOH) has attracted substantial attention since HCOOH is a major by-product in biomass reforming, a promising hydrogen carrier, and also a potential low temperature fuel cell feed. Despite the abundance of experimental studies for vapor-phase HCOOH decomposition on Cu catalysts, the reaction mechanism and its structure sensitivity is still under debate. In this work, self-consistent, periodic density functional theory calculations were performed on three model surfaces of copper—Cu(111), Cu(100) and Cu(211), and both the HCOO (formate)-mediated and COOH (carboxyl)-mediated pathways were investigated for HCOOH decomposition. The energetics of both pathways suggest that the HCOO-mediated route is more favorable than the COOH-mediated route on all three surfaces, and that HCOOH decomposition proceeds through two consecutive dehydrogenation steps via the HCOO intermediate followed by the recombinative desorption of H2. On all three surfaces, HCOO dehydrogenation is the likely rate determining step since it has the highest transition state energy and also the highest activation energy among the three catalytic steps in the HCOO pathway. The reaction is structure sensitive on Cu catalysts since the examined three Cu facets have dramatically different binding strengths for the key intermediate HCOO and varied barriers for the likely rate determining step—HCOO dehydrogenation. Cu(100) and Cu(211) bind HCOO much more strongly than Cu(111), and they are also characterized by potential energy surfaces that are lower in energy than that for the Cu(111) facet. Coadsorbed HCOO and H represents the most stable state along the reaction coordinate, indicating that, under reaction conditions, there might be a substantial surface coverage of the HCOO intermediate, especially at under-coordinated step, corner or defect sites. Therefore, under reaction conditions, HCOOH decomposition is predicted to occur most readily on the terrace sites of Cu nanoparticles. As a result, we hereby present an example of a fundamentally structure-sensitive reaction, which may present itself as structure-insensitive in typical varied particle-size experiments.

Pt/C nanocatalysts for methanol electrooxidation prepared by water-in-oil microemulsion method

Pt nanoparticles supported on Vulcan XC-72R were synthesized by water-in-oil microemulsion method. By incorporating different amounts of HCl as a capping agent in the precursor-containing water phase, nanoparticle shape was varied. Influencing the growth of certain facets leads to the changes of the particle shape depending on the preferential facets. As a result, nanoparticles exhibit some of the electrochemical features typical for single crystals. Commonly employed synthesis procedure for water-in-oil microemulsion method was altered with the addition of catalyst support in the system and changing the catalyst cleaning steps. Prepared catalysts were characterized by thermogravimetric analysis (TGA), transmission electron microscopy (TEM) and electrochemical methods. Activity and stability for methanol oxidation reaction (MOR), a structure-sensitive reaction, were tested. Electrochemical results reveal the influence of particle size, shape and exposed facets on the electrochemical processes. TEM investigations confirm electrochemical findings, while TGA verifies Pt loading in catalyst powder. Based on the results, optimal HCl concentration for cubic particle formation is determined, and structural effect on MOR activity and stability was tested. Cuboidal NPs show very good reaction activity and fair stability under applied experimental conditions.