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Nanoparticle Synthesis via Electrostatic Adsorption using Incipient Wetness Impregnation Sonia Eskandari, Gregory Tate, Nathan Robert Leaphart, and John R. Regalbuto ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03435 • Publication Date (Web): 28 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018
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ACS Catalysis
Nanoparticle Synthesis via Electrostatic Adsorption using Incipient Wetness Impregnation
Sonia Eskandaria, Gregory Tatea, Nathan Robert Leapharta, and John R. Regalbutoa*
a
Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA
*Corresponding author ; Tel. : 803 7775501 ;
[email protected] 1 ACS Paragon Plus Environment
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Abstract In this work Charge Enhanced Dry Impregnation (CEDI), a hybrid method of supported nanoparticle synthesis which combines the advantages of electrostatic adsorption – small particle size and tight size distributions – with the simplicity of incipient wetness impregnation, is demonstrated for four different metals (Pt, Pd, Co, and Ni) at multiple metal loadings over a common silica support. CEDI is achieved by basifying the impregnating solution sufficiently to charge the silica surface at the condition of incipient wetness. The electrostatic interactions induced between cationic ammine metal precursors and the deprotonated, negatively charged support result in smaller nanoparticles with tighter size distribution than incipient wetness impregnation (or dry impregnation, DI) with no pH adjustment. The method works best when the balancing ion of the precursor salt is hydroxide, such as platinum tetraammine hydroxide, (NH3)4Pt(OH)2. Using the corresponding chloride salts with CEDI results in larger metal particles, but still smaller than DIderived particles. Washing out the chloride results in very small nanoparticles without appreciable metal loss at metal loadings corresponding to one monolayer of precursor or below. Ammine complexes with nitrate as the counterion gives small nanoparticles at lower but still relevant metal loadings (one or two weight percent) with no washing; in this way the CEDI synthesis procedure is completely parallel to incipient wetness impregnation, but gives much better metal dispersion.
Keywords: Charged Enhanced Dry Impregnation (CEDI); electrostatic adsorption; catalyst preparation; particle size; Pt; Pd; Co; Ni
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ACS Catalysis
1. Introduction Supported metal catalysts for the production of fuels, commodity and specialty chemicals, and pharmaceutical drugs, as well as for energy production and pollution abatement, underpin the global economy and standard of living. In many applications it is desirable to synthesize the smallest possible metal particles on the catalyst support so as to maximize the number of active sites per mass of metal. The predominant method of synthesizing metal particles on oxide and carbon supports is incipient wetness impregnation (or pore filling or dry impregnation) in which the pore volume of a catalyst support is just filled with a solution containing the desired amount of metal precursor1. The thick paste is dried and pretreated in oxygen (air) and/or hydrogen to remove the ligands and reduce the metal to its active, zero valent state. While this method is simple and cheap, the problem with dry impregnation is that the resultant metal particles are relatively large with wide particle size distributions2. This is due, in most cases, to a lack of interaction between the metal precursor and the support surface, such that metal precursors agglomerate in the liquid phase during drying3. The resulting metal utilization is relatively poor. A recent review of Pt nanoparticle preparation on silica, alumina, titania, and carbon supports in1500 papers from the catalysis literature of the past three years revealed that the average particle size from impregnation was 10 nm, with average particle size distributions as large or larger than the particle size itself 2. Based on the hypothesis that well dispersed metal nanoparticles require a well-dispersed metal precursor, and starting from the pioneering work of Brunelle
4
and Schwarz 5, we have
developed the method of Strong Electrostatic Adsorption (SEA)6 in which coulombic metal precursor-support interactions are induced between a charged surface and an oppositely charged metal precursor complex. This is achieved by controlling the final pH of the impregnating solution relative to the point of zero charge (PZC) of the support; values above the PZC of an acidic support
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like silica (PZC = 4) the surface hydroxyl groups are deprotonated, the surface is negatively charged, and cationic complexes such as metal ammines are strongly adsorbed. For a higher PZC support such as alumina (PZC = 8), at acidic pHs the surface hydroxyl groups are protonated and positively charged, and anionic precursors such as metal chloride complexes are electrostatically adsorbed. The particle size for SEA-derived Pt nanoparticles in the recent review was 1.8 nm with standard deviation typically about 25% of the particle size2. In the laboratory, SEA is normally employed with a great excess of solution for experimental convenience; thin slurries minimize pH shifts and make sampling of the liquid metal concentration easier. In large scale catalyst synthesis, however, the use of thin slurries necessitates an additional filtration step, and any excess metal is lost in the filtrate. There is no reason in principle why electrostatic attraction cannot be induced in the thick slurries employed in dry impregnation; our preliminary work with platinum over silica, alumina, and carbon supports has shown that a simple modification of the dry impregnation recipe – adding enough acid or base to charge up the support surface – results in a drastic reduction of nanoparticle size7. We have termed this method “Charge Enhanced Dry Impregnation” (CEDI). In the present work, we extend CEDI to other noble and base metals (Pt, Pd, Co, and Ni) over a silica support, show ways to mitigate the deleterious effects of chloride counterion8 in the impregnating solution (by washing or the use of nitrate salts) and finally show the effects of drying and the pH of the washing solution in the CEDI method.
We refine CEDI to show the simplest method to synthesize the smallest supported
nanoparticles.
2. Materials and methods
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ACS Catalysis
Platinum(II) tetraammine hydroxide (PtTAOH), platinum(II) tetraammine chloride (PtTACl), platinum(II) tetraammine nitrate (PtTAN), palladium(II) tetraammine chloride (PdTACl), palladium(II) tetraammine nitrate (PdTAN), cobalt(III) hexaammine chloride (CoHACl), and nickel(II) hexaammine chloride (NiHACl), purchased from Sigma Aldrich, were used as precursors without any purification. As support, SiO2 Aerosil 300 was used as received. The PZC, BET surface area, and water accessible pore volume of the support were 3.5, 280 m2/g, and 2.8 ml/g respectively. A spear tip pH probe was used for pH measurements at incipient wetness. All samples were prepared by the CEDI method with optimal initial and final pH values as reported in previous work:9 each precursor was dissolved into NH4OH solution at an initial pH of 11.5, which resulted, after 2.8 ml of the solution had been contacted with 1 g of the silica support, in a final pH of 10.0. The amount of metal precursors placed in solution was calculated relative to one monolayer of those precursors as determined from previous work. A monolayer (ML) of PtTAOH, PtTACl, PdTACl, NiHACl and CoHACl respectively are 0.86 µmoles/m2 (4.5wt%), 0.80 µmoles/m2 (4.2 wt%)10, 0.90 µmoles/m2 (2.5 wt%), 1.4 µmoles/m2 (2.2 wt%), and 1.3 µmoles/m2 (2.1 wt%)
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. These values correspond to a close-packed layer of precursors retaining one or two
hydration sheaths6. For each metal, precursor concentrations were employed to give metal loadings of 0.25, 0.5, 1.0, 1.5 and 2.0 monolayers. After impregnation all samples except PtTACl and PtTAN were dried in ambient air at 120 °C overnight. The PtTACl and PtTAN/silica samples were dried overnight under vacuum at room temperature since the Pt ammine precursor decomposes to a neutral Pt(NH3)2O species when drying at 120°C or more
12
. Drying the Pt ammines at ambient caused more than 70% Pt loss during a
washing step (see below) regardless of the initial metal coverage.
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The dried samples were reduced for 1 h in 10% H2/He with the ramp rate of 5°C/min at the reduction temperatures determined by temperature programmed reduction (TPR). The reduction temperatures were based on the minimum reduction temperatures from TPR studies of PtTAOH, PtTACl, PtTAN, PdTACl, NiHACl and CoHACl were 250 °C, 300 °C, 300 °C, 200 °C, 500 °C and 450 °C, respectively7,10,11. A set of samples was prepared with residual chloride removed. To do so, the CEDI-derived dried, unreduced samples were re-ground to a fine powder and washed using a dilute NH4OH solution (300 mL/g catalyst) with the pH=10.5 so that the final pH after sample addition was at or close to the final pH of adsorption. The sample and wash solution were agitated for 10 minutes by means of an orbital shaker, at a rate of 120 rpm. After the washing, a 5 ml aliquot was taken for metal loss analysis by ICP. The post-wash sample was filtered, dried and reduced at the same condition as for the unwashed samples. An additional series of samples was washed immediately after the impregnation, that is, without the drying step. Powder XRD analysis was performed on a Rigaku Miniflex-II equipped with a silicon strip detector (D/teX Ultra) and Cu Kα radiation (λ = 1.5406 Å) source, operated at 15 kV and 30 mA. Scans were made in the 2θ range of 30°−80°, with a scan rate of 2.0° 2θ/min; metal diffraction patterns of small particles (broad peaks) were obtained by background subtraction of the silica support and deconvoluted using Gaussian peak shapes. Average particle sizes were calculated using the Scherrer equation. Some patterns were best fit with a narrower and a broader peak, implying a bimodal distribution.
Average particles sizes were estimated from an area average of the
deconvoluted peak sizes.
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ACS Catalysis
Concentrations of metal in the impregnating solution and the washing solution were measured by inductively coupled plasma-optical emission spectroscopy (ICP-OES) with a Perkin Elmer Optima 2100DV instrument. Z-contrast images were obtained using an aberration-corrected JEOL 2100F scanning transmission electron microscope (STEM) equipped with a 200kV field emission gun and a Fischione Model 3000 HAADF detector with a camera length such that the inner cut-off angle of the detector was 50 mrad. Particle size analysis was performed using Particule2 software.
3. Results and Discussion Influence of pH, Counterion, and Washing with Pt A series of silica supported Pt nanoparticles were initially synthesized by CEDI from the Cl-
(a)
(c)
(b)
(d)
7 Figure 1. XRD patterns of PtTAOH and PtTACl samples: (a) CEDI – prepared series of PtTAOH, (b) CEDI – prepared series of PtTACl, (c) DI - prepared series of PtTACl, and (d) ACS Paragon Plus Environment washed, Cl free samples from CEDI – prepared PtTACl samples.
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-free precursor, PtTAOH, dried, and reduced in flowing H2. Powder XRD characterization of these samples is shown in Figure 1a. Samples were prepared at coverages of 0.25 (1.1%), 0.5 (2.3%), 1 (4.5%), 1.5 (6.8%), and 2 (9.0%) monolayers. The resulting size of Pt phase after reduction and exposure to ambient air was estimated from the deconvoluted Pt (111) and Pt3O4 (210) peaks. These are shown in Figure S1. In Figure 1a for all the loadings of Pt made by PtTAOH both metal and oxide forms of Pt can be seen (Figure S1). According an earlier work by Banerjee et.al, the oxidation of small Pt nanoparticles occurring in ambient conditions can now be detected by powder diffractometers with high sensitivity solid state detectors13 (and corroborated with STEM and x-ray photoelectron spectroscopy). With increased amounts of platinum from 1.1 to 9.0 wt%, particle size of the metallic Pt cores increased from 1.1 to 1.5 nm and the Pt oxide shells to a lesser extent from 1.0 to 1.3 nm. These are in good agreement to the core and shell sizes seen earlier on silica13 as well as with the STEM–derived average particle size seen for the representative STEM image of the 1 ML
(a)
(b)
(c)
Figure 2. STEM images and corresponding particle distributions of reduced Pt catalyst samples for: (a) 1ML PTAOH prepared by CEDI (corresponds to XRD pattern of 1 ML in Figure 1a and deconvolution in Figure S1c), (b) 1ML PTAOH prepared by SEA, (c) 2ML post-washed samples from PTACl prepared with CEDI (corresponds to top XRD pattern in Figure 1d and deconvolution in Figure S4d). sample shown in Figure 2a.
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ACS Catalysis
In a parallel series of experiments, Pt samples were prepared by CEDI from Pt tetraammine with chloride as counterion, PtTACl. XRD patterns of these reduced samples are shown in Figure 1b and deconvolutions of them are given in Figure S2. With increased loading of Pt, diffraction peaks became sharper. The deconvolutions in Figure S2 are best fit at the higher Pt loadings with one broader and one narrower peak, implying that particles have bimodal size distribution for coverages of 1.0 ML and above. (STEM corroboration of XRD-determined bimodality was shown recently13 and is not repeated here.) To isolate the effect of controlling the pH on particle size, a third series of PtTACl samples was prepared by DI (with no pH control). The XRD patterns of the reduced materials are shown in Figure 1c and the deconvolution are shown in Figure S3. At equivalent metal loadings, the Pt nanoparticles made by DI are larger than particles made by CEDI. All the DI particles exhibit a bimodal size distribution (Fig. S3). Zhu et al. earlier prepared nanoparticles at 2wt% Pt/SiO2 using PtTACl by CEDI and DI methods and reported particle sizes of
