Study of synthetic parameter effects on the formation of colloidal Pd-polyacrylamide NPs
3.2. Experimental Section
3.2.1. Material for colloidal NPs synthesis
In this chapter, all the Pd NPs were synthesized by chemical reduction method. The use of hydrogen as reducing agent, water as solvent and mild temperature and pressure conditions were preferred for future industrial economic and environmental scale up. The metal precursors used were commercially available: Na2PdCl4 and
Pd(NO3)2.
The NPs were synthesized in 200 mL Fischer Porter vessels previously cleaned with aqua regia, soap solution and rinsed with distilled water. Note that these vessels can hold a maximum pressure of 3 bar.
The heating set-up consisted in a stirrer-hot plate IKA RCT BASICTM and a 1000 mL polyethylene glycol bath. The thermocouple measured the temperature from the bath.
The metal precursors were purchased from Alfa Aesar, the stabilizers from Sigma Aldrich, the polyethylene glycol from Merck and the hydrogen gas from Linde (Hydrogen 5.0 with a purity of 99.999%). The milli-Q water used for the syntheses came from a Millipore Ultrapure Water Purification system.
3.2.2. Characterization
3.2.2.1. Transmission Electron Microscopy (TEM)
All the synthesized Pd NPs were routinely characterized by conventional TEM using bright field image modality.
The grids were prepared from the fresh suspension of Pd NPs without any pretreatment since it was noticed that the NPs were not easily observable after the isolation step. The TEM samples were prepared by adding 20 drops of the fresh
colloidal suspensions on a Cu-formvar grid. The grids were dried under air prior to analysis.
The measurements were performed at the “Servei de Recursos Científics I Tècnics” of the Rovira I Virgili University (Tarragona, Spain) with a JEOL 1011 Transmission Electron Miscroscope operating at 100 kV. The images obtained were monochromatic with a 0.2 nm of resolution. The increases oscillated between 200x and 1000000x. Images were systematically taken at a nominal magnification between 100000x and 400000x on three different points of the grid as minimum. To record images, iTEM® imaging platform from Olympus was used.
The NPs presented in sections 3.4.2. and 3.4.3. were manually measured with Image J software. A minimum of 400 measurements was systematically carried out. The data were processed with Origin Lab® software fitting a Gaussian function. The estimation of the average diameter and standard deviation of each sample was given from the arithmetic mean of the fitted curve.
Concerning the NPs presented in section 3.4.5. , the images obtained after the TEM characterization were treated automatically with Fiji Is Just ImageJ software. The precise procedure for this automatic treatment is given in appendix 3.6.1. Thanks to this treatment, information about size, area and shape descriptors could be obtained. The data were adjusted to a Gaussian function using a Matlab® routine.
3.2.2.2. X-Ray Diffraction (XRD)
The measurements were performed at the “Servei de Recursos Científics I Tècnics” of the Rovira I Virgili University (Tarragona, Spain) with a Siemens D5000 diffractometer (Bragg-Brentano parafocusing geometry and vertical θ-θ goniometer) fitted with a curved graphite diffracted-beam monochromator, incident and diffracted-beam Soller slits, a 0.06º receiving slit and scintillation counter as a detector. Unless stated, the angular diffraction range was between 25º and 95º. The data were collected with an angular step of 0.05º at 16 sec per step and sample rotation. A low background Si
(510) wafer was used as sample holder. Cukα radiation was obtained from a copper X- ray tube operated at 40 kV and 30 mA.
3.2.2.3. X-ray Photoelectron Spectroscopy (XPS)
XPS was performed to evaluate the amount of Pd2+ and Pd0 at the surface of the synthesized catalysts. The analyses were performed at the “Instituto de Carboquímica” (Zaragoza, Spain) with an ESCAPlus Omicron equipped with a hemispherical electron energy analyzer. The spectrometer operated at 225 W (15 mA, 15 kV), using a non- monochromatized MgKα anode (1253.6 eV) and under vacuum (<5.10-9) over an area of sample of 1.75x2.75 mm. A survey scan was acquired between 0 and 1000 eV at 0.5 eV/step, 0.3 s/dwell and 50 eV of pass energy. Detailed scans for C 1s, O 1s, N 1s and Pd 3d were obtained at 0.1 eV/step, 0.5 s/dwell and 20 eV of pass energy. For calibration purposes, the C 1s binding energy was referred at 284.5 eV. The data were analyzed with CASAXPS® software.
3.2.2.4. Thermogravimetric analysis (TGA)
TGA was used to estimate the percentage of water and remaining stabilizer on the synthesized catalysts. The analysis was performed at the “Chromatography, Thermal Analysis & Electrochemistry Unit” of the Research Support Area of the Institut Català d’Investigació Química (ICIQ, Tarragona, Spain) using a Mettler Toledo DSC822 with a 56-point Au-AuPd thermopile FRS5 sensor. This equipment enables the measuring range from room temperature up to 1100ºC with an accuracy of ±0.25ºC and the balance enables a measuring range up to 1 g with a resolution of 1 μg.
In a typical TGA analysis, 1-5 mg of sample was place in an alumina crucible and heated under nitrogen flow from room temperature to 900ºC with a heating ramp of 10ºC/min. The sample mass was recorded continuously during the analysis.
3.2.3. General recipe of Pd NPs stabilized by polyacrylamide (PAM)
A typical synthesis of Pd-polyacrylamide NPs (Pd-PAM NPs) consisted in the chemical reduction of a metal precursor in water using hydrogen as reducing agent and in the presence of a stabilizer as presented in Scheme 3.1.
Scheme 3.1. General recipe used for the formation of Pd-PAM NPs via chemical reduction
The precise procedure consisted in mixing the corresponding amount of metal precursor, stabilizer and solvent (only water was used in this project) in a Fischer Porter reactor. The solution was then homogenized and preheated to the desired reaction temperature. Once this temperature was reached, the reactor was charged with hydrogen. The inlet valve of the reactor was opened for 15 seconds and then the system was closed (batch mode). The reaction was stirred at the reaction temperature during the desired synthesis time. At the end of the reaction, the reactor was first cooled down using an ice or cold water bath and then depressurized. The TEM sample was taken directly from this fresh solution to prepare the grid. For XRD characterization the colloidal NPs were isolated and washed by centrifugation.