Study On Electrode Oxidation Of Palladium - Based Catalysts And Surface Infrared Spectroscopy

The investigation of electrocatalytic oxidations of Cl small molecules is essential for the clarification of fundamental electrocatalysis problems and the development of Polymer Electrolyte Membrane Fuel Cells (PEMFCs). Formic acid (FA) is an important intermediate during the oxidation of methanol and formaldehyde, hence the exploration of FA oxidation mechanism benefits the understanding of other relevant oxidation processes. Meanwhile, FA oxidation may also act as a probe reaction in search of promising catalytic materials due to its sensitivity on the surface structure and adsorption strength of adsorbates. On the other hand, Direct Formic Acid Fuel Cells (DFAFCs) possess the advantages of the non-toxic fuel, a lower membrane crossover rate and a higher open circuit voltage as compared to Direct Methanol Fuel Cells (DMFCs). Therefore, the investigation of electrocatalytic FA oxidation is of great importance from the views of both fundamental and applied sectors.In comparison to the widely investigated Pt-based catalysts, Pd-based catalysts possess a lower price and higher catalytic activity for FA oxidation. Nevertheless, there yet exists a large challenge and space for further boosting the electrocatalytic performance of Pd-based catalysts and understanding the mechanism leading to their rapid activity decay. Along this line, this thesis work is targeted on the synthesis-by-design of robust Pd-based catalysts and the mechanistic study by real-time spectroscopy to address these concerns. Our research results not only provide an insight to the deactivation issue during the FA oxidation process on Pd electrodes, but also contribute to the technical development for material fabrication and in-situ dynamic study.On one hand, with the guidelines of the relevant theories, we attempted to fabricate high-performance Pd-based catalysts by manipulating their "electronic structure","geometric structure" and "facet structure". Firstly, inspired by the "third body effect" and "d-band center-shift" principles, we synthesized carbon supported low Pt/Pd ratioed Pd-Pt alloy nanocatalysts with excellent performance for FA oxidation, clarifying the impact of geometric and electronic structures on their electrochemical behaviors. Secondly, by using Ag nanoparticles as sacrificial templates, the carbon supported hollow Pd-based Pd-Pt-Ag alloy nano-catalysts with a mean nanoparticle size less than10nm were obtained. Benefited from the hollow structure effect and electronic effect, such a nanocatalyst exhibited enhanced catalytic activity toward FA oxidation; Thirdly, with a simple one-pot synthesis, we obtained the cubic and rhombic dodecahedral Pd nanocrystals. By introducing a simple and mild CO adsorption and replacement process, we were able to remove the surface contaminants on the nanocrystals and obtained the reliable facet-dependent electrochemical responses on these Pd nanocrystals.On the other hand, we mainly applied the in-situ Electrochemical Attenuated Total Reflection Surface Enhanced IR Absorption Spectroscopy (EC-ATR-SEIRAS) to study the FA oxidation process on Pd surface. Based on earlier results in our group, we have made a further clarification of the origin of CO accumulation on Pd surfaces during FA oxidation. Furthermore, we have extended this technique to study the CO accumulation process on Pd black nano-catalysts in a concentrated FA solution, revealing the potential and time dependent CO poisoning behavior in mimic DFAFC anode conditions. We also make a preliminary EC-ATR-IR investigation on CO adsorption and stripping on Pd nanocrystals, as a concept of proof for extending this method to explore the surface electrochemical processes on metal nanocrystals.The main results and conclusions are summarized as follows:Part Ⅰ. The synthesis-by-sign of efficient Pd-based catalysts towards FA oxidation1.1Low Pt-content carbon supported Pd-Pt alloy nanocatalystsPd and Pt atoms possess close electronic structures and atomic radii, but selective FA oxidation pathways on Pt and Pd surfaces are quite different. Pt surfaces are easily to be poisoned by the COad species resulted from facile FA dehydration on three continuous Pt sites, while the catalytic activity of Pd surfaces can be further improved through an electronic structure manipulation. Keeping these two points in mind, we designed a kind of low Pt-content Pd-Pt alloy catalyst based on "third body effect" and "d-band center-shift" principles, i.e., by segregating the surface Pt sites by the Pd atoms and tuning the d-band electronic structure of Pd atoms by alloying with Pt to achieve a better electrocatalytic activity.By carefully screening the alloy compositions, we found that the peak oxidation current was almost doubled with the peak potential negatively shifted by200mV with only5%Pt addition as compared to the pure Pd catalyst. With the increment of Pt content in the alloy, the peak current and peak potential further decrease accompanied with an increased surface poisoning during the catalytic process, confirming the adjustment of electronic structure of Pd by alloying with Pt and the geometric separation of Pt sites by Pd. The Pd9Pt1/C is proved to be the best catalyst among all the Pd-Pt alloy catalysts screeened, in accordance with the original design.1.2Carbon supported hollow Pd-Pt-Ag ternary alloy nanocatalystsBuilding hollow structure is a powerful geometric manipulation approach to fabricate the novel catalysts with enhanced activity and lower cost. However, the hollow-structured nanocatalysts reported were mainly supportless with large nanosizes. Accordingly, we have synthesized by design the carbon supported hollow Pd-based alloy nanocatalysts for a better FA oxidation.Copper and silver nanoparticles were tested to be used as the sacrificial template to synthesize the hollow nanostructures by galvanic replacement reactions. It turns out that the ultrafine copper nanoparticles synthesized by a polyol process are not suitable since the surfactant PVP is hardly removed and the copper surface is easily oxidized.. With the7-nm Ag nanoparticles as the sacrificial template, we succeeded in obtaining carbon supported hollow Pd-Pt-Ag ternary alloy nanocatalysts with2-nm shell and10-nm in diameter through a new synthesis route.1/3of the original mass of Ag remained in the resulting nanocatalysts and the ratio of Pd and Pt could be easily controlled through the adjustment of the precursor solution. Electrochemical measurements proved that the hollow structure effect and the alloy effect enable such catalysts to exhibit promising electrocatalytic activity for FA oxidation, which providing a new thought for designing the high efficient nanocatalysts in the future.1.3Synthesis and electrochemical study of Pd nanocrytalsThe facet-dependent electrocatalysis is another important issue. However, few reports provided conceivable results due to the lack of suitable synthesis method and effective decontamination approaches. In this part, we obtained series of Pd nanocrystals through a simple one-pot synthesis method by using AA as the reductant, CTAB as stabilizer and KI as addictive. The preferential facet orientations of these Pd nanocrystals could be controlled from{100} to{110} ones. The cubic and rhombic dodecahedral nanocrystals with comparable sizes and well-oriented facets, are desired for further electrocatalytic studies.By taking a convenient and mild CO adsorption-replacement method, we are able to remove the surface impurities on the Pd nanocrystals effectively, and get rational electrochemical responses comparable to those of the corresponding bulk single crystal electrodes. With further determination of FA and ethanol oxidation in acidic and alkaline solution, respectively, Moreover, better electrocatalysis of FA oxidation in acidic media and ethanol oxidation in alkaline media were confirmed on Pd nanocubes than that on Pd rhombic dodecahedron ones, providing a hint for the facet manipulation on nanocatalysts.Part II. Surface IR study of FA oxidation on Pd surfaces2.1Clarification of CO accumulation and origination on Pd film electrodeTriply-bonded CO (COT) species on Pd film was detected in FA solution by EC-ATR-SEIRAS in a previous thesis work in our group. The detected CO was attributed mainly to the (electro-)reduction of the FA decomposition product CO2. In the present work, further analysis of the COT assignment was carried out by comparing time-dependent ATR-SEIRA spectra for a Pd electrode during CO bubbling in a FA-free electrolyte with those obtained in a CO-free FA solution at OCP. In addition, comparative ATR-SEIRAS measurements were also run to demonstrate the difference in CO origination on Pd and Pt electrodes in a FA solution, providing an additional evidence in support of the CO2reduction mechanism for CO accumulation on Pd electrodes..2.2CO accumulation and removal on Pd black nanocatalysts in high concentrated FA solutionEC-ATR-IR spectroscopy was extended to investigate the surface CO poisoning and accumulation in the (electro)chemical decomposition of FA on commercial Pd black catalyst in5M FA solution, mimicking to some extent the anode operation conditions of a DFAFC.EC-ATR-IR measurements were performed at an open circuit potential (OCP, ca.0.06V vs. RHE), a constant potential of0.4V (vs. RHE) and scanned potentials, corresponding to the OCP, constant voltage output and switching "on" and "off’ conditions of a real DFAFC. A higher CO coverage in the concentrated FA solution was observed through an appropriate spectral subtraction. COad coverage decreases with increasing operation potential. The CO accumulation due to CO2reduction at OCP poisoned the FA oxidation at a Pd electrode is reported according to the electrochemical and spectroscopic data.The present results suggest that anti-CO poisoning property should be considered in designing new efficient Pd-based catalysts, or one should develop a Pd-based catalyst that redirect the CO2reduction to other non-CO weakly adsorbed products. Our results also indicate that special attention to OCP time control and CO2removal during a DFAFC operation.2.3Primary spectroscopic investigation on Pd nanocrystalsBased on the above work, we carried out a preliminary surface IR spectroscopic measurement on the CO adsorption and oxidation at cubic or rhombic dodecahedral Pd nanocrystals. During dosing CO on Pd nanocrystals at a controlled potential, no clue suggests the adsorption of COM and COT in the initial stage. The difference in facet orientations mainly impacts the ratio of the COL and COB peak intensities. In the CO oxidation process, the spectroscopic results indicate a much lower CO binding energy on Pd nanocubes with an earlier oxidation as compared to the rhombic dodecahedral ones, which may be related to the better catalysis for CO oxidation on the former. The preliminary results demonstrate the feasibility of extending ATR-SEIRAS to study metal nanocrystal surfaces.