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Hydrocarbon Fragmentation Patterns on Rhodium Surfaces
Published in John R. Kosak, Thomas A. Johnson, Catalysis of Organic Reactions, 2020
Shane L. Anderson, Dinesh Kalakkad, Abhaya K. Datye
The effect of surface structure can be also be studied in a more direct fashion by performing reactions on single crystals of a metal that expose a known surface facet. For instance, Engstrom et al. [7] measured the kinetics of n-butane hydrogenolysis on the (110) and the (111) surfaces of Ir. They found that the activity and selectivity of small particles of supported Ir was very similar to that of the Ir(110) single-crystal surface, which is known to undergo a 1 × 2 surface reconstruction. On the other hand, the behavior of large Ir particles was very well modeled by the Ir(111) surface. Studies such as these provide insight into the nature of surface atoms in small metal particles. When a similar study was performed on Rh single crystals, however, the results were markedly different. Rh(110) caused nearly statistical cleavage of n-butane into methane, ethane and propane, quite unlike the behavior of highly dispersed Rh which favors internal bond hydrogenolysis by a factor of 4 both in the case of n-butane [3] and n-pentane [8]. To better understand this apparent inability to model highly dispersed Rh, we have studied hydrocarbon cleavage patterns as a function of Rh particle size and thermal treatment. Our results show that while large particles of Rh can be modeled very well by the Rh(111) surface, the hydrocarbon cleavage patterns of small Rh particles (<10 Á in diameter) do not resemble those on Rh(110). However, when the surface of large Rh metal particles is roughened by an oxidative pretreatment, the selectivity starts to approach that of Rh(110).
Photocatalytic Phenomena
Published in Debasish Sarkar, Nanostructured Ceramics, 2018
Along with the phase junction, semiconducting material and nano-structured conducting material junction have also influence to enhance the photocatalytic activity. This is attributed to the promoted charge separation caused by the formation of Schottky junction between the semiconducting photocatalyst and the nano-structured conducting material. For instance, 1 wt% graphene and TiO2 composite exhibit an excellent upgradation in H2 production rate up to 41-fold in comparison to pure TiO2 [18]. Exposure of highly active surface facet is an effective strategy to enhance the photocatalytic activity. In every semiconductor, there exists a facet, which has highest reactivity, and, thus, nano-structured semiconductor with highest reactive facets exposed to the surface is developed by Yang et al. [19]. The facet reactivity can be termed as a function of atomic alignment in distinct directions, which results in variation in optical and electronic properties. The variation of physico-chemical properties of different facets is a harmonious result of photo-adsorption activity and selectivity process of the facets.
mombe of GaAs on a patterned substrate
Published in G B Stringfellow, Gallium Arsenide and Related Compounds 1991, 2020
Y. Morishita, Y. Nomura, S. Goto, Y. Katayama, T. Isu
Epitaxial growth on patterned substrates is useful for achieving advanced structures of optoelectronic devices such as laser diodes (Dzurko et al. 1989), waveguides (Colas et al. 1989), and transistors (Asai et al. 1984). For these devices, it is important to control the shapes of the epitaxial layers, especially in the active region of the device. It is well known that crystal planes with different indices affect the growth behavior on neighboring surfaces in molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) on patterned substrates. It has been reported that facets are formed between different crystal planes during growth (Tsang and Cho 1977, Hersee et al. 1986).
Evaluation of pore re-opening after HIP in LPBF Ti–6Al–4V
Published in Powder Metallurgy, 2021
Topi Kosonen, K. Kakko, N. Raitanen
SEM images of the typical pore types found in the samples after each processing step are collected in Figure 4. In as-manufactured condition, (Figures 4(a,b)) sharp corners were found from LoF-type pores. These sharp corners can serve as initiation points for fatigue cracks. Some minor rounding of pore corners occurs during heat treatment (Figure 4(c,d)). During aging, rounding continues and LoF pores transform to more oval shape (Figure 4(e,f)). Some so-called faceting was also observed in inner surfaces of pores. Facets form during high-temperature aging most likely as the surfaces start to grow inwards towards certain low energy lattice directions. After HIP:ing, only a few pores can be found (Figure 4(g,h)). Pores are very small and transformed to a rounded or circular shape. After HIP:ing and low-temperature aging (Figure 4(i,j)), only a few rounded defects can be found. Remaining pores are predominantly small <5 µm circular pores. After HIP:ing and high-temperature aging, pore size grows, but remain rounded in shape (Figure 4(k,l)). No cracking or formation of new sharp corners was found after high-temperature aging. Faceting effect was clearly observable.
The Effect of Heat Treatment in Different Atmospheres on Tungsten-doped MnO2 for Ozone Decomposition
Published in Ozone: Science & Engineering, 2021
Yajie Yang, Dan Shen, Pengyi Zhang
High-resolution transmission electron microscopy (HRTEM) helps to learn the change of crystal structure after heat treatment. As shown in Figure 4, the original W-MnO2 showed the well-crystallized nanorods with the lattice spacing of 0.69 nm assigned to the (110) crystal planes of α-MnO2 (Yang et al. 2018), and the lattice spacing of 0.24 nm assigned to the (211) plane of could be identified from the weakly crystallized nanoparticles with somewhat amorphous area. After heated in air or helium at 200, 300 or 400 °C, there was no obvious change in nanoparticle parts with the fringe spacings of 0.24 nm (not shown). However, in the nanorod parts, the fringe spacings changed from 0.69 nm to 0.49 nm in Air-300 and 0.31 nm in Air-400, corresponding to the (200) and (310) crystal planes of α-MnO2, respectively. In addition, there were some structural defects circled in the images. These results indicate that high-temperature treatment changed the exposed facets of α-MnO2. It is well known, the high-index-exposed crystal facets usually have high surface energy characteristics (Liu et al. 2011); thus, the change of exposed facets may exert an effect on their catalytic activity. For the sample treated at 500 °C in air (Air-500), the lattice spacing of 0.27 nm was observable, which was assigned to the (222) crystal plane of Mn2O3 (Cheng et al. 2017), consistent with the XRD result.
The role of phosphorus in pore rounding of sintered steels
Published in Cogent Engineering, 2018
Walid Khraisat, Wisam Abu Jadayil, Nathir Rawashdeh, Henrik Borgström
The line integral of conservative vector fields is path independent, i.e. the work done in moving along a path on the surface depends only on the start and endpoints of the path. At equilibrium the force is zero this means that the term (k1 + k2) is either zero or constant with position. According to Equation (21) the gradient of stress will be equal to zero only in two cases the first one is if the surface is that of a sphere and the second case is that of a plane or facets. Facets with lower surface energies will be favored and exposed (Polyanin & Manzhirov, 2007; Wu & Overbury, 2015).