Article pubs.acs.org/JPCC
In Situ Synthesis and Nanoscale Evolution of Model Supported Metal Catalysts: Ni on Silica Ritubarna Banerjee and Peter A. Crozier* School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287-6106, United States S Supporting Information *
ABSTRACT: The ability to disperse metal nanoparticles over a high surface area support is critical for catalyst preparation. For impregnation techniques, there are many fundamental questions about the spreading, diffusion, and coarsening behavior of metal intermediates during the drying and thermal treatments. We employ in situ environmental transmission electron microscopy to monitor the nanoscale processes taking place during the preparation of Ni on silica-supported metal model catalysts. Conventional salt impregnation techniques were employed, and the electron microscope was used to follow the spatial evolution of the material from salt impregnation through calcination and reduction. The best precursor dispersion was obtained with a nickel nitrate solution prepared in an ethanol solvent, followed by performing impregnation in an atmosphere saturated with ethanol vapor to avoid premature drying or deliquescence during mixing. In situ observations showed that no significant wetting or dispersion of the nickel precursor took place during calcination, and the final metal particles spatial distribution was controlled entirely by the initial spatial distribution of salt dispersion on the support in this case. Substantial diffusion of the nickel species took place when the precursor was directly reduced in hydrogen, resulting in high dispersion of Ni particles. The in situ approach should also yield valuable insight into the fundamental process taking place during the synthesis of other nanomaterials, especially for monitoring the structural changes taking place during thermal treatments. processes on real high-surface-area support.3,10,16 Recently, we have employed the technique of in situ environmental transmission electron microscopy (ETEM) to investigate the nanoscale changes taking place during the impregnation preparation of metallic and bimetallic catalysts.18−21 One advantage of this approach is that it allows the spatial distributions of the salt, the oxide and the final metal to be correlated from the same region of the catalytic material. From such observations, the role of the initial salt dispersion and the diffusion and chemical decomposition during the thermal treatments can be evaluated directly with imaging techniques. A challenge associated with these earlier previous studies relates to the inherent complexity that is often present in the nanostructure and composition of real working catalysts. The previous work was performed on high surface area supports such as titania P25 and γ-alumina, which made the work relevant to industrial catalysts. Such supports often possess a high degree of heterogeneity in terms of exposed crystal facets, phases present, and surface defect structures. For example, in our previous work on Ni supported on TiO2, we found that the nature of the metal−support interaction depended on whether the metal nucleated on anatase or rutile grains and on the particular crystal facet of these two different titanias.18,19 The complexity of most crystalline high-surface-area support
1. INTRODUCTION Metal-supported heterogeneous catalyst plays an important role in many areas including energy, chemical synthesis, and environmental control.1 Developing and understanding new and existing methodologies for catalyst preparation is important for developing new and improved catalysts.2−4 Many methods have been developed for fabricating well-defined nanocatalyst using a wide range of chemical and physical methods.4−12 More traditional preparation methods such as metal salt impregnation remain widely used, especially for industrial applications because of their relatively low cost and simplicity.13 These approaches typically involve impregnating a high surface area support with a salt solution and then thermally decomposing the salt via calcination and reduction steps to yield a dispersion of metal particles on the support. The desired metal dispersion depends on the application and is strongly influenced by the choice of metal salt, support, impregnation method, and thermal treatment. It is often challenging to generate uniform metal dispersions, especially when high metal dispersions are sought. Studies employing physicochemical macroscopic characterization techniques have been carried out over the years on the effect of varying preparation conditions on the final metal dispersion.10,12,14−17 However, relatively few studies have directly followed the nanoscale evolution of the precursor and resulting dispersion. In this area, de Jong and coworkers have employed ex situ transmission electron microscopy (TEM) to investigate the various stages of impregnation © XXXX American Chemical Society
Received: August 1, 2011 Revised: March 19, 2012
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dx.doi.org/10.1021/jp2073446 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
the choice of solvent (water or ethanol) and the mixing procedure (no mixing, mechanical grinding, or the use of an ultrasonic). We explored so-called dry impregnation or incipient wetness impregnation in which the volume of salt solution employed was equal to the pore volume of the silica. This was determined empirically to be the point where the powder loses its free-flowing character.27 For our sample, this corresponded to the addition of 300 μL of solution to 0.2 g of silica. We also employed so-called wet impregnation involving the addition of solution in excess of the pore volume. In this case, we added 400 μL of solution to 0.2 g of silica. The atmosphere in which the impregnation was conducted can significantly affect the salt dispersion. Impregnation was usually performed in a glovebag in an atmosphere of lab air saturated with solvent vapor (either water or ethanol). Each of these precursor dispersions were labeled as methods A through G (and abbreviated PDA, PDB, etc.), and the samples are summarized in Table 1. To improve the mixing between the
structures can lead to significant heterogeneity in the microstructure of supported monometallic catalyst and provide rich but complex nanostructure in bimetallic systems.20,21 However, for in situ work in which dynamic structural changes take place during calcinations and reduction, this structural complexity greatly complicates the interpretation of the ETEM data. These interpretational problems obscure information about the fundamental mechanisms leading to morphological, chemical, and structural changes taking place during the thermal treatments. The heterogeneity of the catalyst also makes it challenging to determine the relationship between catalyst structure and catalytic properties because of the potentially large number of different active sites. For this reason, we have chosen to work on a simple model system consisting of metal nanoparticles supported on SiO2 spheres. The surface of the sphere is homogeneous and interacts only weakly with the metal particles. Moreover, the simple geometry of this support is ideal for observing and interpreting changes in structure and morphology of the metal species during the calcination and reduction steps. This combination of well-defined model supports in combination with in situ TEM allows us to obtain new insight into the role of precursor distribution, diffusion, calcination, and reduction on the final metal dispersion. We show how the information obtained from in situ microscopy can be used to modify the catalyst preparation method to improve the metal dispersion. In this work, we follow the changes taking place during the preparation of a supported nickel model catalyst. There are many methods of impregnating the oxide support with precursor solutions of which the incipient wetness technique is still popular because of the reproducibility and low cost.2,13,22 Recently, we have investigated structural transformations taking place during the activation of model supported Ni catalysts during partial oxidation of methane.23 Ni catalysts show high conversion efficiencies at low cost,24,25 although deactivation by coke formation and sintering can be problematic. Here we explore the nanoscale structure and composition changes taking place during the model catalyst preparation process. The in situ electron microscopy provided us with new insights into the evolution and fundamental nanoscale processes taking place during salt impregnation, calcination, and reduction. With this information, we were able to optimize rapidly the catalyst preparation procedure to produce reproducible and uniform Ni dispersion using simple and inexpensive processing paths.
Table 1. Salt Impregnation Procedure Employed in Preparation of Ni/SiO2 Catalysts precursor dispersion
solvent employed
impregnation method
PDA PDB PDC PDD
water ethanol ethanol ethanol
dry dry dry wet
air air air air
PDE
ethanol
wet
air
PDF
ethanol
wet
air saturated with ethanol
PDG
water
wet
air saturated with water
atmosphere
other comment
ultrasonic for 1 h soaking in solution or 2 h grinding in mortar for 10 min grinding in mortar for 10 min grinding in mortar for 10 min
salt solution and the silica, we used an ultrasonic mixing approach or simple mechanical grinding in a mortar. The details are given in Table 1. The impregnated powder was dried in air at 120 °C for 2 h and observed in a JEOL JEM 2010F TEM. 2.2. Thermal Decomposition and Morphology Evolution of Precursor. We undertook a series of experiments to determine the morphological changes that took place in the metal species during the various calcination and reduction steps in catalyst preparation. For our initial work, we concentrated on samples with coarsely dispersed precursor (PDA and PDB) because the larger particles of nickel nitrate hexahydrate made it easier to establish the phase transformations and morphological changes that take place. Similar experiments were also performed on samples with more highly dispersed precursors (mostly dispersion PDF of Table 1). A series of in situ experiments were conducted to investigate the nanoscale changes that take place during calcination and reduction. This involved heating the samples in an ETEM in atmospheres of ∼1 Torr of O2 for calcination and 1 Torr H2 to for reduction. The samples were gently crushed between glass microscope slides, loaded onto platinum grids, and placed in a Gatan Inconel heating holder. The in situ ETEM was performed in an FEI Tecnai F20 field emission ETEM operating at 200 kV with a point resolution of 0.24 nm and an information limit of 0.13 nm. The gas reaction cell or environmental cell (essentially a
2. EXPERIMENTAL METHODS 2.1. Precursor Preparation by Conventional Impregnation Technique. Silica spheres of uniform sizes were prepared by Stober’s method, and details can be found in the original paper.26 The as-prepared spheres were calcined in air at 500 °C for 2 h to remove any residual precursor remaining from the synthesis. Different batches of spheres with average sizes in the range of 80−400 nm were employed for the current experiments. Specific surface areas were determined using single-point BET surface area analysis in an In-Situ Research Instruments RIG 150 system and were ∼5 m2/g for the larger spheres and 20 m2/g for the smaller spheres. These values were in close agreement with the surface areas determined from the TEM particle size distributions. The silica powder was impregnated with nickel nitrate hexahydrate, Ni(NO3)2·6H2O, obtained from Sigma-Aldrich using an incipient wetness technique to obtain a 7 wt % Ni/SiO2 loading. Many different physical parameters were varied during impregnation including B
dx.doi.org/10.1021/jp2073446 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
flow microreactor) of this instrument allows us to perform atomic level observations of gas−solid interactions at pressures up to 8 Torr and temperatures up to 800 °C. In situ electron microscope images and diffraction patterns were recorded with a Gatan CCD camera using Digital Micrograph 3.1 software. The instrument is also equipped with a Gatan imaging filter permitting in situ electron energy-loss spectroscopy to be performed. During in situ experiments, we often paused at an intermediate temperature during ramp-up to look for evidence of structural changes in the material. The duration and number of interruptions varied from run to run. For this reason, we do not list a ramp rate for the in situ experiments in Table 2. However, during periods when the temperature was increased, the heating rate was ∼0.5 to 1 °C/s.
morphological changes observed during in situ experiments were compared with ex situ experiments performed in traditional annealing furnaces. This further confirmed that our observations were not significantly influenced by the electron beam. After the decomposition of nitrate to oxide, the sample became much more resistant to electron irradiation and higher doses could be employed to obtain higher quality electron micrographs during subsequent thermal treatments. Ex situ experiments were conducted at atmospheric pressure in air or a 5%H2/Ar mixture. Electron microscopy of the ex situ samples was performed mostly on a JEOL JEM 2010F using high-angle annular dark-field scanning transmission electron microscopy (Z-contrast STEM). The details of the various in situ and ex situ treatments are summarized in Table 2. The level of Z-contrast available in the STEM images from the FEI ETEM was significantly lower than the contrast from the JEOL 2010F. This is because the differential apertures associated with the reaction cell of the ETEM act as angle-limiting apertures and impose a high angle cutoff of ∼50 mrad.28
Table 2. In Situ and Ex Situ Thermal Treatments for the Preparation of Different Ni/SiO2 Catalysts catalyst sample
precursor dispersion
ex situ or in situ
CB1
PDB
in situ
CF1
PDF
in situ
CF2
PDF
ex situ
CF3
PDF
ex situ
CF4
PDF
ex situ
CF5
PDF
in situ
CF6
PDF
ex situ
thermal treatment calcination at 300 °C for 2 h and reduction at 400 °C for 3h calcination at 300 °C for 2 h and reduction at 400 °C for 3h calcination at 300 °C for 2 h and reduction at 400 °C for 3h calcination at 300 °C for 2 h and reduction at 400 °C for 3h direct reduction at 300 °C for 2 h followed by 3 h at 400 °C direct reduction at 400 °C for 3h direct reduction at 400 °C for 3h
ramp rate (°C/min)
3. RESULTS AND DISCUSSIONS 3.1. Dispersion of Nitrate Salt on Silica Spheres. Figure 1a shows a typical bright-field TEM image of the catalyst precursor after performing incipient wetness impregnation with aqueous nickel nitrate solution (sample PDA). The micrograph shows large patches of nitrate precursor on the silica spheres. The coverage is not uniform and varies considerably over the surface of the silica and between individual silica spheres. The nitrate patches are not random in shape but show characteristic concave surfaces, and the radius of curvature matches the radius of curvature of the silica. The origin of this morphology is easily deduced from the well-defined geometry of the silica support and is shown schematically in Figure 1b. Capillary forces cause solution to accumulate at the points of contact between the silica spheres. When the solution dries, the nitrate salts bind neighboring spheres together, giving rise to salt bridges between silica particles. The salt binding is rather weak, and the bridge can easily fracture along the salt/silica interface, giving rise to the concave salt structures observed in the TEM images. Capillary forces will always play a role in the initial dispersion of solution over the surface of fine powders, but the volume of solution trapped at the interface will depend on the surface tension of the liquid. The surface tension of water is rather high at ∼73 dyn/cm, suggesting that employing a solvent with a lower surface tension may improve the salt dispersion. Experiments were performed with both dry and wet impregnation using ethanol as the solvent (PCB and PCD, respectively). Figure 1c shows the nitrate dispersion obtained when ethanol was used as the solvent (with a surface tension of 22 dyn/cm) corresponding to sample PDB of Table 1. The TEM image shows that salt bridges were still formed during the impregnation but the volume of salt associated with each bridge is much smaller. The lower surface tension of alcohol causes the bridges to hold less fluid, making them resemble rings of salt nanoparticles on the silica after drying. Similar results were obtained for sample PCD. The samples shown in Figure 1 were obtained under static conditions. Agitating powder/solution mixtures significantly improves the overall dispersion of salt. We investigated several approaches for improving the dispersion involving physical agitation with an ultrasonic (sample PDC) or grinding with a mortar and pestle (samples PDE). Our initial results from these
7 1.6 7
1.6
The initial nickel nitrate is sensitive to the electron beam, and moderate electron irradiation of Ni(NO3)2·6H2O resulted in pore formation in the material due to water loss and nitrate decomposition. For the first phase of this work, it was necessary to determine a safe electron dose to prevent sample damage so that our observations could be interpreted in terms of gas- and temperature-induced phase transformation. Initial pore formation was observed in the nitrate precursor when the electron dose exceeded 900 000 e/nm2. To avoid this damage, we worked with an incident current density of 3000 e nm−2 s−1. Under this condition, it took 5 min of constant exposure to initiate pore formation with the electron beam. In practice, the total dose reaching the sample never came close to the critical dose even after many hours of in situ thermal annealing because the electron beam was either turned off during annealing or parked in the vacuum away from the sample. (The beam diameter was typically 50 nm or less.) The sample was irradiated only during image collection, and we estimated the total dose during imaging (including focusing time) to be
