ABSTRACT
Understanding and controlling the structure and composition of nanoparticles in supported metal catalysts are crucial to improve chemical processes. For this, atom probe tomography (APT) is a unique tool, as it allows for spatially resolved three-dimensional chemical imaging of materials with sub-nanometer resolution. However, thus far APT has not been applied for mesoporous oxide-supported metal catalyst materials, due to the size and number of pores resulting in sample fracture during experiments. To overcome these issues, we developed a high-pressure resin impregnation strategy and showcased the applicability to high-porous supported Pd-Ni-based catalyst materials, which are active in CO2 hydrogenation. Within the reconstructed volume of 3 × 105 nm3, we identified over 400 Pd-Ni clusters, with compositions ranging from 0 to 16 atom % Pd and a size distribution of 2.6 ± 1.6 nm. These results illustrate that APT is capable of quantitatively assessing the size, composition, and metal distribution for a large number of nanoparticles at the sub-nm scale in industrial catalysts. Furthermore, we showcase that metal segregation occurred predominately between nanoparticles, shedding light on the mechanism of metal segregation. We envision that the presented methodology expands the capabilities of APT to investigate porous functional nanomaterials, including but not limited to solid catalysts.
ABSTRACT
Shale host rock and containment potential are largely determined by the connected pore network in the rock, and the connection between the pore network and the naturally present or mechanically induced fracture network together determines the total bulk permeability. Pore connectivity in shales is poorly understood because most of the porosity is present in sub-micrometer-sized pores that are connected through nanometer-sized pore throats. We have used a number of different techniques to investigate the microstructure and permeability of Early Jurassic shales from the UK (Whitby Mudstone), under intact and fractured conditions. Whitby Mudstone is a clay matrix-rich rock (50-70%), with different mineralogical layers on the sub-millimeter scale and very low natural permeability (10-19 to 10-22 m2), representative of many gas shales and caprocks present in Europe. Artificial fracturing of this shale increases its permeability by 2-5 orders of magnitude at low confining pressure (5 MPa). At high confining pressures (30 MPa), permeability changes were more sensitive to the measuring direction with respect to the bedding orientation. Given the distinct lack of well-defined damage zones, most of the permeability increase is controlled by fracture permeability, which is sensitive to the coupled hydro-chemo-mechanical response of the fractures to fluids.
Subject(s)
Minerals , Clay , Europe , Permeability , PorosityABSTRACT
To predict the behavior of the cement sheath after CO2 injection and the potential for leakage pathways, it is key to understand how the mechanical properties of the cement evolves with CO2 exposure time. We performed scratch-hardness tests on hardened samples of class G cement before and after CO2 exposure. The cement was exposed to CO2-rich fluid for one to six months at 65 °C and 8 MPa Ptotal. Detailed SEM-EDX analyses showed reaction zones similar to those previously reported in the literature: (1) an outer-reacted, porous silica-rich zone; (2) a dense, carbonated zone; and (3) a more porous, Ca-depleted inner zone. The quantitative mechanical data (brittle compressive strength and friction coefficient) obtained for each of the zones suggest that the heterogeneity of reacted cement leads to a wide range of brittle strength values in any of the reaction zones, with only a rough dependence on exposure time. However, the data can be used to guide numerical modeling efforts needed to assess the impact of reaction-induced mechanical failure of wellbore cement by coupling sensitivity analysis and mechanical predictions.
