Above all, some of the materials used in this work were supported by the National Science Foundation under Grant CBET-1033810. Time and. the temperature of each image is at the bottom left of the corresponding image.
INTRODUCTION
From this, the system's 𝑝𝑂2 can be determined as a function of temperature with equation (5)-(9). Shapes of the crystallites have direct effects on the catalytic activity and turnover frequency (TOF) of the ensemble.
EXPERIMENTAL PROCEDURE
Sample Preparation – Focused Ion Beam Milling
A Pt precursor gas was used to deposit a protective layer on the desired particle for surface retention under milling conditions. The lift-off samples were then welded to the nanomanipulator using a Pt precursor gas, removed from the milled section and attached to an Omniprobe Cu FIB lift-off TEM grid on a shielded "B" pillar.
Electron Energy Loss Spectroscopy
Nine L-edge relative intensities were extracted by background subtraction of the 240.0 eV pre-edge, with the signal region starting at 850.0 eV and ending at 960.0 eV. The relative intensity of the O K edge was analyzed in the same way with a pre-edge background region of 28.0 eV and a signal range from 530 to 585 eV.
RESULTS AND DISCUSSION
X-ray Diffraction
Typical of solid solutions, the lattice parameter of the solid solution can be expected to obey Vegard's Law, however, slight deviations from linearity (observed in the x = 0.75 sample) arise from the inherent variation in cation site occupancy as spinel becomes more opposite XRD patterns for the series are shown in Figure 7, with Figure 8 describing Vegard's Law as it pertains to the lattice parameter. To evaluate the dual phases of ZrO2, the amount of ZrO and 20.0 wt. %) increased during the collection process and the calcined products were evaluated by Rietveld analysis (Figure 9).
High Temperature X-ray Diffraction
Therefore, the spinel is only destabilized by the loss of the CCP oxygen sublattice, i.e. however, these features are spread over the initially reduced area of the spinel particle, which is much larger than the NiO islands. All phases persisted through the maximum temperature of the experiment with no observed phase transitions.
The lack of the strongest NiO reflection between 𝜃 also suggests that no observable oxidation occurs under these conditions.
Raman Spectroscopy
As discussed in the previous diffraction sections, the phase composition of the reduced samples consists of spinel residue, which is the main phase and is analogous to the 𝛾-Al2O3 structure. If we now consider the Eg band of the reduced spinel solid solution, we observe a linear trend in composition. The vacuum-oxidized sample in this region shows band positions similar to those of the as-prepared oxide and are noticeably shifted compared to the reduced counterpart.
In addition, inspection of the main Eg band, which is diagnostic for Mo-O bond vibrations, reveals
Scanning Electron Microscopy
Metal oxide particles persist throughout the oxidation process, even at 1100 °C, as Figure 20: SEM images of solid solution end-pieces, NixMg1-xAl2O4. Surface pitting is more prevalent on the spinel surface, with some small microcracks observed. The absence of metal oxide particles is expected, as no reflections were observed in the HTSEM experiments.
The enhancement of undercoordinated surface features also provides atomically defined step edges for methane reaction or additional Ni dissolution sites during reduction.
In-situ Low Voltage High Temperature Scanning Electron Microscopy
However, it can be seen from HTSEM that the formation of large Ni particles creates a surface texture on the spinel in the form of dimples that follow the contours of the metal crystals. The enhancement of surface particle diffusion is believed to arise from the available defect content, particularly the extent of reduction of Ni from the spinel. The lattice parameter of the spinel decreases by ≈1% (≈3% volume reduction) due to Ni/Co and O removal.
Rietveld analysis (Table VIII) confirms that the variance of the spinel lattice parameter is measurable but insignificant (<0.01% difference) compared to the undoped sample.
High Temperature X-ray Diffraction – Reduction Stability
To the author's knowledge, no data are available on the reduction stability of the compound Ni1Hf, or any of the other compounds examined below. Oxygen transport from the spinel to the external atmosphere is slowed to such a degree that oxygen loss is not catastrophic and the CCP nature of the oxygen sublattice is preserved, making the Ni1Hf sample a potential candidate to study for enhanced mass transport . Additions of CeO2 to spinel gave similar results to the Ni1Hf compound; no detrimental oxygen loss occurred in the spinel resulting in the absence of 𝜃- or 𝛼-Al2O3.
Nb was found to be incorporated into the spinel structure at low doping levels (around 2 mol% Nb), where higher doping levels (6 and 10 mol%) precipitated NiNb2O6.116 By extracting Ni into the metallic state, Nb2O5 can be removed from the structure NiNb2O6 to the oxide before AlNbO4 can form.
Scanning Electron Microscopy
Regardless of phase composition, Nb doping hindered corundum formation, making the compositions viable candidates for enhanced mass transport evaluation. Spinel modified with 2.5 wt% HfO2 showed the least anomalous microstructure with the ZrO2 modified sample. In the previous sections, compositional candidates were identified through systematic evaluation of the supporting and modifying chemistry.
Comparison of the calculated Ea and tunnel speed with the literature values for bulk, grain boundary and surface diffusion in spinel.
High Temperature Scanning Electron Microscopy and Empirical Evaluation of
The modifying particle shrinks, and the bored tunnel is from the removed volume, where the components are incorporated into the spinel (O can also be lost in vacuum). To explain the tunneling of ZrO2 into spinel, diffusion rather than reactions must be considered. Surface rates account for the material at the spinel-modifier interface that diffuses away per unit time.
Mg additions to the spinel as well as the substitution of HfO2 for ZrO2 shift the onset of tunneling to higher temperatures.
Focused Ion Beam Milling & Direct Evidence of Diffusion Beneath ZrO 2 Particles
Si and C are artifacts resulting from grinding the Protochips support, which are all constant and excluded from the line scans. Intensity is consistent between lines, indicating that no concentration gradients are present within the error of EDS, as well as uniformity of the milled thin foil. The reverse side of the ZrO2 also experiences slight intensity increases, but to a much less noticeable degree.
Ga deposited by the ion milling process is usually implanted into the sample at constant concentrations due to the uniformity of the milling process.
Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy 112
The onset and near-edge features of the O K-edge are sensitive to local oxygen binding, which has been used as a diagnostic to determine between spinel and ZrO2, as well as possible oxygen deficiencies near the interface. The shoulder observed at 534 eV indicates the hybridization of the O 2p and Ni 3d states.230 The Ni L2,3 edge spectrum measured in spinel is characteristic of spinel containing Ni and Ni – 𝛼-Al2O3. The intensity of Ni decreases noticeably and is barely detectable above the background, with only an absorption edge clearly visible, which is in agreement with previous work that reported a lack of Ni near the ZrO2–spinel interface.103 The low-energy shoulders (530 and 534 eV) of common spinel O The K-edge spectrum increases while the main peak in the original spinel position loses intensity, thus changing the overall peak positions and profile.
Local inversion changes due to Ni loss in the near-boundary region can be drastically different from the bulk due to diffusional or modifying effects of ZrO2, as manifested by the growth of a low-energy shoulder and loss of the Ni signal.
Potential Mechanistic Causes for the Tunneling Phenomenon
Recall that the total phase composition of the ZrO2 on the spinel support is approximately equal parts of monoclinic and tetragonal phases. First, the dilation region shows enhanced lattice strain diffusion in the near t-ZrO2 region of the spinel. A model based solely on the local stress fields under the ZrO2 lacks a plausible explanation for the persistence of the tunneling phenomenon.
The inhomogeneous dispersion of these voids can be attributed to the local texture inhomogeneities in the ZrO2 modifier and/or the crystallographic direction of the spinel at the interface.
SUMMARY AND CONCLUSIONS
Tunneling of ZrO2 and HfO2 into the spinel surface was found to be relatively insensitive to both spinel and modifier composition, as the rates did not vary significantly. The tunnel phenomenon turned out to be thermally activated; it is clear that a reducible metal content is required to observe the tunneling effect, as no tunneling was found for MgAl2O4. The activation energy of the NiAl2O4 + 2.5 wt% ZrO2 composition resembled that of Ni diffusion in defective MgO⋅xAl2O3.30 Voids were also formed under the ZrO2 particles, and the spinel-ZrO2 interface was found to be Ni- to be deficient, which resembles the defective spinel, 𝛾-Al2O3.
All evidence obtained points to a model based on enhanced diffusion along the spinel-modifier interface, thereby enabling rapid mass transport from the bulk to the surface via this short-circuit diffusion pathway.
FUTURE WORK
In addition, HfO2 tunneling on other compositions could be further explored, although such studies may not prove fruitful, such as NiAl2O4 + 2.5 wt% ZrO2. Since the NiAl2O4 + 2.5 wt% HfO2 composition shows tunneling at 1200°C, which is the maximum temperature of the Protochips phase, tunneling cannot be observed because Mg is substituted in the spinel. With the arrival of these instruments at Alfred University, a more comprehensive evaluation could be performed in future work on all compositions exhibiting tunneling, which would provide additional insight into the legitimacy of the proposed diffusion model.
This particular strategy is not applicable to NiAl2O4 since excess NiO spinel is not possible, but it can be used in Mg-substituted versions.
Yong Sheng, H.; Jian Bao, L.; Xiao Shan, N.; Bo, C.; Johnson, D., Temperature dependence of cation distribution in nickel aluminum spinel from thermodynamics and X-rays Yamaguchi, G.; Nakano, M.; Tosaki, M., Cation vacancy effects of nickel ion diffusion in defective spinels. Yamasaki, M.; Habazaki, H.; Yoshida, T.; Akiyama, E.; Kawashima, A.; Asami, K.; Hashimoto, K.; Komori, M.; Shimamura, K., Composition dependence of CO2 methanation activity of Ni/ZrO2 catalysts prepared from amorphous NiZr alloy precursors.
Shintani, T.; Murata, Y., Evaluation of dislocation density and dislocation character in cold-rolled type 304 steel determined by X-ray diffraction profile analysis.
APPENDIX
