Selective Hydrogenation of Acetylene over Pd-Boron Catalysts: A Density Functional Theory Study

Boron-modified Pd catalysts have shown excellent performance for the selective hydrogenation of alkynes experimentally. In the current work, we investigated the hydrogenation of acetylene on boron-modified Pd(111) and Pd(211) surfaces, utilizing density functional theory calculations. The activity of acetylene hydrogenation has been studied by estimating the effective barrier of the whole process. The selectivity of ethylene formation is investigated from a comparison between the desorption and the hydrogenation of ethylene as well as comparison between the ethylene and the 1,3-butadiene formation. Formation of subsurface carbon and hydrogen on both boron-modified Pd(111) and Pd(211) surfaces has also been evaluated, since these have been reported to affect both the activity and the selectivity of acetylene hydrogenation to produce ethylene on Pd surfaces. Our results provide some important insights into the Pd B catalysts for selective hydrogenation of acetylene and also for more complex hydrogenation systems, such as stereoselective hydrogenation of longer chain alkynes and selective hydrogenation of vegetable oil.

The impacts of replacing air bubbles with microspheres for the clarification of algae from low cell-density culture

Dissolved Air Flotation (DAF) is a well-known coagulation-flotation system applied at large scale for microalgae harvesting. Compared to conventional harvesting technologies DAF allows high cell recovery at lower energy demand. By replacing microbubbles with microspheres, the innovative Ballasted Dissolved Air Flotation (BDAF) technique has been reported to achieve the same algae cell removal efficiency, while saving up to 80% of the energy required for the conventional DAF unit. Using three different algae cultures (Scenedesmus obliquus, Chlorella vulgaris and Arthrospira maxima), the present work investigated the practical, economic and environmental advantages of the BDAF system compared to the DAF system. 99% cells separation was achieved with both systems, nevertheless, the BDAF technology allowed up to 95% coagulant reduction depending on the algae species and the pH conditions adopted. In terms of floc structure and strength, the inclusion of microspheres in the algae floc generated a looser aggregate, showing a more compact structure within single cell alga, than large and filamentous cells. Overall, BDAF appeared to be a more reliable and sustainable harvesting system than DAF, as it allowed equal cells recovery reducing energy inputs, coagulant demand and carbon emissions. © 2014 Elsevier Ltd.

On The Energy-Efficiency of Byte-Addressable Non-Volatile Memory

Non-Volatile Memory (NVM) technology holds promise to replace SRAM and DRAM at various levels of the memory hierarchy. The interest in NVM is motivated by the difficulty faced in scaling DRAM beyond 22 nm and, long-term, lower cost per bit. While offering higher density and negligible static power (leakage and refresh), NVM suffers increased latency and energy per memory access. This paper develops energy and performance models of memory systems and applies them to understand the energy-efficiency of replacing or complementing DRAM with NVM. Our analysis focusses on the application of NVM in main memory. We demonstrate that NVM such as STT-RAM and RRAM is energy-efficient for memory sizes commonly employed in servers and high-end workstations, but PCM is not. Furthermore, the model is well suited to quickly evaluate the impact of changes to the model parameters, which may be achieved through optimization of the memory architecture, and to determine the key parameters that impact system-level energy and performance.

Density functional study of the C atom adsorption on the α-Fe 2O3 (001) surface

The adsorption of C atoms on the α-Fe₂O₃ (001) surface was studied based on density function theory (DFT), in which the exchange-correlation potential was chosen as the PBE (Perdew, Burke and Ernzerhof) generalized gradient approximation (GGA) with a plane wave basis set. Upon the optimization on different adsorption sites with coverage of 1/20 and 1/5 ML, it was found that the adsorption of C atoms on the α-Fe ₂O₃ (001) surface was chemical adsorption. The coverage can affect the adsorption behavior greatly. Under low coverage, the most stable adsorption geometry lied on the bridged site with the adsorption energy of about 3.22 eV; however, under high coverage, it located at the top site with the energy change of 8.79 eV. Strong chemical reaction has occurred between the C and O atoms at this site. The density of states and population analysis showed that the s, p orbitals of C and p orbital of O give the most contribution to the adsorption bonding. During the adsorption process, O atom shares the electrons with C, and C can only affect the outermost and subsurface layers of α-Fe₂O₃; the third layer can not be affected obviously. Copyright © 2008 Chinese Journal of Structural Chemistry.

Harmonic Generation from Relativistic Plasma Surfaces in Ultrasteep Plasma Density Gradients

Harmonic generation in the limit of ultrasteep density gradients is studied experimentally. Observations reveal that, while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale lengths (L-p/lambda <1), the absolute efficiency of the harmonics declines for the steepest plasma density scale length L-p -> 0, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the relativistic oscillating mirror was estimated to be in the range of 10(-4)-10(-6) of the laser pulse energy for photon energies ranging from 20-40 eV, with the best results being obtained for an intermediate density scale length.

Slopes of Avian Species-Area Relationships, Human Population Density, and Environmental Factors

There is increasing interest in how humans influence spatial patterns in biodiversity. One of the most frequently noted and marked of these patterns is the increase in species richness with area, the species-area relationship (SAR). SARs are used for a number of conservation purposes, including predicting extinction rates, setting conservation targets, and identifying biodiversity hotspots. Such applications can be improved by a detailed understanding of the factors promoting spatial variation in the slope of SARs, which is currently the subject of a vigorous debate. Moreover, very few studies have considered the anthropogenic influences on the slopes of SARs; this is particularly surprising given that in much of the world areas with high human population density are typically those with a high number of species, which generates conservation conflicts. Here we determine correlates of spatial variation in the slopes of species-area relationships, using the British avifauna as a case study. Whilst we focus on human population density, a widely used index of human activities, we also take into account (1) the rate of increase in habitat heterogeneity with increasing area, which is frequently proposed to drive SARs, (2) environmental energy availability, which may influence SARs by affecting species occupancy patterns, and (3) species richness. We consider environmental variables measured at both local (10 km x 10 km) and regional (290 km x 290 km) spatial grains, but find that the former consistently provides a better fit to the data. In our case study, the effect of species richness on the slope SARs appears to be scale dependent, being negative at local scales but positive at regional scales. In univariate tests, the slope of the SAR correlates negatively with human population density and environmental energy availability, and positively with the rate of increase in habitat heterogeneity. We conducted two sets of multiple regression analyses, with and without species richness as a predictor. When species richness is included it exerts a dominant effect, but when it is excluded temperature has the dominant effect on the slope of the SAR, and the effects of other predictors are marginal.

Identifying the distinct features of geometric structures for hole trapping to generate radicals on rutile TiO2(110) in photooxidation using density functional theory calculations with hybrid functional

Using density functional theory calculations with HSE 06 functional, we obtained the structures of spin-polarized radicals on rutile TiO2(110), which is crucial to understand the photooxidation at the atomic level, and further calculate the thermodynamic stabilities of these radicals. By analyzing the results, we identify the structural features for hole trapping in the system, and reveal the mutual effects among the geometric structures, the energy levels of trapped hole states and their hole trapping capacities. Furthermore, the results from HSE 06 functional are compared to those from DFT + U and the stability trend of radicals against the number of slabs is tested. The effect of trapped holes on two important steps of the oxygen evolution reaction, i.e. water dissociation and the oxygen removal, is investigated and discussed.

Activity and coke formation of nickel and nickel carbide in dry reforming: A deactivation scheme from density functional theory

Dry reforming is a promising reaction to utilise the greenhouse gases CO2 and CH4. Nickel-based catalysts are the most popular catalysts for the reaction, and the coke formation on the catalysts is the main obstacle to the commercialisation of dry reforming. In this study, the whole reaction network of dry reformation on both flat and stepped nickel catalysts (Ni(111) and Ni(211)) as well as nickel carbide (flat: Ni3C(001); stepped: Ni3C(111)) is investigated using density functional theory calculations. The overall reaction energy profiles in the free energy landscape are obtained, and kinetic analyses are utilised to evaluate the activity of the four surfaces. By careful examination of our results, we find the following regarding the activity: (i) flat surfaces are more active than stepped surfaces for the dry reforming and (ii) metallic nickel catalysts are more active than those of nickel carbide, and therefore, the phase transformation from nickel to nickel carbide will reduce the activity. With respect to the coke formation, the following is found: (i) the coke formation probability can be measured by the rate ratio of CH oxidation pathway to C oxidation pathway (r(CH)/r(C)) and the barrier of CO dissociation, (ii) on Ni(111), the coke is unlikely to form, and (iii) the coke formations on the stepped surfaces of both nickel and nickel carbide can readily occur. A deactivation scheme, using which experimental results can be rationalised, is proposed. 

Density functional theory study on the activation of molecular oxygen on a stepped gold surface in an aqueous environment: a new approach for simulating reactions in solution

The activation of oxygen molecules is an important issue in the gold-catalyzed partial oxidation of alcohols in aqueous solution. The complexity of the solution arising from a large number of solvent molecules makes it difficult to study the reaction in the system. In this work, O-2 activation on an Au catalyst is investigated using an effective approach to estimate the reaction barriers in the presence of solvent. Our calculations show that O-2 can be activated, undergoing OOH* in the presence of water molecules. The OOH* can readily be formed on Au(211) via four possible pathways with almost equivalent free energy barriers at the aqueous-solid interface: the direct or indirect activation of O-2 by surface hydrogen or the hydrolysis of O-2 following a Langmuir-Hinshelwood mechanism or an Eley-Rideal mechanism. Among them, the Eley-Rideal mechanism may be slightly more favorable due to the restriction of the low coverage of surface H on Au(211) in the other mechanisms. The results shed light on the importance of water molecules on the activation of oxygen in gold-catalyzed systems. Solvent is found to facilitate the oxygen activation process mainly by offering extra electrons and stabilizing the transition states. A correlation between the energy barrier and the negative charge of the reaction center is found. The activation barrier is substantially reduced by the aqueous environment, in which the first solvation shell plays the most important role in the barrier reduction. Our approach may be useful for estimating the reaction barriers in aqueous systems.

A density functional theory study of hydrogen dissociation and diffusion at the perimeter sites of Au/TiO2

Reactivity of supported gold catalysts is a hot topic in catalysis for many years. This communication reports an investigation on the dissociation of molecular hydrogen at the perimeter sites of Au/TiO2 and the spillover of hydrogen atoms from the gold to the support using density functional theory calculations. It is found that the heterolytic dissociation is favoured in comparison with homolytic dissociation of molecular hydrogen at the perimeter sites. However, the surface oxygen of the rutile TiO2(110) surface at these sites can be readily passivated by the formed OH, suggesting that further dissociation of molecular hydrogen may occur at pure gold sites.

Density Functional Theory Studies of Ethanol Decomposition on Rh(211)

Density functional theory calculations were carried out to examine the mechanism of ethanol decomposition on the Rh(211) surface. We found that there are two possible decomposition pathways: (1) CH(3)CH(2)OH -> CH(3)CHOH -> CH(3)COH -> CH(3)CO -> CH(3) + CO -> CH(2) + CO -> CH + CO -> C + CO and (2) CH(3)CH(2)OH -> CH(3)CHOH -> CH(3)COH -> CH(2)COH -> CHCOH -> CHCO -> CH + CO -> C + CO. Both pathways have a common intermediate of CH(3)COH, and the key step is the formation of CH(3)CHOH species. According to our calculations, the mechanism of ethanol decomposition on Rh(211) is totally different from that on Rh(111): the reaction proceeds via CH(3)COH rather than an oxametallacycle species (-CH(2)CH(2)O- for Rh( 111)), which implies that the decomposition process is structure sensitive. Further analyses on electronic structures revealed that the preference of the initial C(alpha)-H path is mainly due to the significant reduction of d-electron energy in the presence of the transition state (TS) complex, which may stabilize the TS-surface system. The present work first provides a clear picture for ethanol decomposition on stepped Rh(211), which is an important first step to completely understand the more complicated reactions, like ethanol steam reforming and electrooxidation.

Oxygen reduction reaction mechanism on nitrogen-doped graphene:A density functional theory study

Nitrogen-doped graphene (N-graphene) was reported to exhibit a good activity experimentally as an electrocatalyst of oxygen reduction reaction (ORR) on the cathode of fuel cells under the condition of electropotential of similar to 0.04 V (vs. NNE) and pH of 14. This material is promising to replace or partially replace the conventionally used Pt. In order to understand the experimental results. ORR catalyzed by N-graphene is studied using density functional theory (DFT) calculations under experimental conditions taking the solvent, surface adsorbates, and coverages into consideration. Two mechanisms, i.e., dissociative and associative mechanisms, over different N-doping configurations are investigated. The results show that N-graphene surface is covered by O with 1/6 monolayer, which is used for reactions in this work. The transition state of each elementary step was identified using four different approaches, which give rise to a similar chemistry. A full energy profile including all the reaction barriers shows that the associative mechanism is more energetically favored than the dissociative one and the removal of O species from the surface is the rate-determining step. (C) 2011 Elsevier Inc. All rights reserved.

Acrolein hydrogenation on Pt(211) and Au(211) surfaces:a density functional theory study

Partial hydrogenation of acrolein, the simplest alpha, beta-unsaturated aldehyde, is not only a model system to understand the selectivity in heterogeneous catalysis, but also technologically an important reaction. In this work, the reaction on Pt(211) and Au(211) surfaces is thoroughly investigated using density functional theory calculations. The formation routes of three partial hydrogenation products, namely propenol, propanal and enol, on both metals are studied. It is found that the pathway to produce enol is kinetically favoured on Pt while on Au the route of forming propenol is preferred. Our calculations also show that the propanal formation follows an indirect pathway on Pt(211). An energy decomposition method to analyze the barrier is utilized to understand the selectivities at Pt(211) and Au(211), which reveals that the interaction energies between the reactants involved in the transition states play a key role in determining the selectivity difference.

A density functional theory study of CO and atomic oxygen chemisorption on Pt(111)

Ab initio total energy calculations within a density functional theory framework have been performed for CO and atomic oxygen chemisorbed on the Pt(111) surface. Optimised geometries and chemisorption energies for CO and O on four high-symmetry sites, namely the top, bridge, fee hollow and hcp hollow sites, are presented, the coverage in all cases being 0.25 ML. The differences in CO adsorption energies between these sites are found to be small, suggesting that the potential energy surface for CO diffusion across Pt(111) is relatively flat. The 5 sigma and 2 pi molecular orbitals of CO are found to contribute to bonding with the metal. Some mixing of the 4 sigma and 1 pi molecular orbitals with metal states is also observed. For atomic oxygen, the most stable adsorption site is found to be the fee hollow site, followed in decreasing order of stability by the hcp hollow and bridge sites, with the top site being the least stable. The differences in chemisorption energies between sites for oxygen are larger than in the case of CO, suggesting a higher barrier to diffusion for atomic oxygen. The co-adsorption of CO and O has also been investigated. Calculated chemisorption energies for CO on an O/fcc-precovered surface show that of the available chemisorption sites, the top site at the oxygen atom's next-nearest neighbour surface metal atom is the most stable, with the other four sites calculated bring at least 0.29 eV less stable. The trend of CO site stability in the coadsorption system is explained in terms of a 'bonding competition' model. (C) 2000 Elsevier Science B.V. All rights reserved.

A density functional theory study of CH2 and H adsorption on Ni(111)

Ab initio total energy calculations within the density functional theory framework have been used to study the adsorption of CH2 and H as well as the coadsorption of CH2 and H on Ni(111). H binds strongly at threefold hollow sites with calculated adsorption energies of 2.60 and 2.54 eV at the face-centered-cubic (fcc) and hexagonal-close-packed (hcp) hollow sites, respectively. Adsorption energies and H-Ni distances are found to agree well with both experimental and theoretical results. CH2 adsorbs strongly at all high symmetry sites with calculated adsorption energies of 3.26, 3.22, 3.14 and 2.36 eV at the fcc, hcp, bridge and top sites, respectively. Optimized structures are reported at all sites, and, in the most stable hollow sites there is considerable internal reorganization of the CH2 fragment. The CH2 molecule is tilted, the hydrogens are inequivalent and the C-H bonds are lengthened relative to the gas phase. In the CH2-H coadsorption systems the adsorbates have a tendency to move toward bridge sites. The bonding of all adsorbates to the surface is analyzed in detail. (C) 2000 American Institute of Physics. [S0021-9606(00)71213-X].