Hydrogen-Assisted 1,2-Dichloroethane Dechlorination Catalyzed by Pt−Cu/SiO2: Evidence for Different Functions of Pt and Cu Sites

Atmosphere-Induced Transient Structural Transformations of Pd-Cu and Pt-Cu Alloy Nanocrystals

We have investigated the transformations of colloidal Pd-Cu and Pt-Cu bimetallic alloy nanocrystals (NCs) supported on γ-Al₂O₃ when exposed to a sequence of oxidizing and then reducing atmospheres, in both cases at high temperature (350 °C). A combination of in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray absorption spectroscopy was employed to probe the NC surface chemistry and structural/compositional variations in response to the different test conditions. Depending on the type of noble metal in the bimetallic NCs (whether Pd or Pt), different outcomes were observed. The oxidizing treatment on Pd-Cu NCs led to the formation of a PdCuO mixed oxide and PdO along with a minor fraction of CuO _x species on the support. The same treatment on Pt-Cu NCs caused a complete dealloying between Pt and Cu, forming separate Pt NCs with a minor fraction of PtO NCs and CuO _x species, the latter finely dispersed on the support. The reducing treatment that followed the oxidizing treatment largely restored the Pd-Cu alloy NCs, although with a residual fraction of CuO _x species remaining. Similarly, Pt-Cu NCs were partially restored but with a large fraction of CuO _x species still located on the support. Our results indicate that the noble metal present in the bimetallic Cu-based alloy NCs has a strong influence on the dealloying/migrations/realloying processes occurring under typical heterogeneous catalytic reactions, elucidating the structural/compositional variations of these NCs depending on the atmospheres to which they are exposed.

Reactions of chlorinated ethenes with surface-sulfidated iron materials: reactivity enhancement and inhibition effects

Recent studies on the use of controlled sulfur amendment to improve the reactivity and selectivity of zerovalent iron (ZVI) in reductive dechlorination reactions have generated renewed interest in ZVI-based remediation materials. However, existing studies have focused on the reactions between trichloroethene (TCE) and lab-synthesized ZVI, and the applicability of sulfidation to ZVIs with different material characteristics for reductive dechlorination of chloroethenes such as tetrachloroethene (PCE) and cis-dichloroethene (cis-DCE) has not been systematically examined. In this study, four ZVI materials from commercial sources having different sizes and morphological and compositional characteristics were subjected to various sulfidation treatments and were assessed in batch reactions with PCE, TCE, or cis-DCE. Sulfur amendment induces modest increases in PCE degradation rates and steers reactions towards a cleaner pathway that has minimum accumulation of partially dechlorinated intermediates. In the case of cis-DCE, bifurcating outcomes were observed that include enhancement effects for two high-purity ZVIs and inhibitory effects for two ZVIs possessing low levels of metal impurities. Further investigations based on controlled metal dosing reveal that the trace metals commonly present in cast iron or recycled metal scraps, such as Cu and Ni, can act as adventitious catalysts for cis-DCE reduction. Sulfidation results in poisoning of these catalytic ingredients and accounts for the adverse effect observed with a subset of ZVIs. Collectively, this study confirms enhanced degradation of highly chlorinated ethenes (PCE and TCE) by sulfidation of ZVIs from diverse origins; nonetheless, the effects of sulfidation can be highly variable for the less chlorinated ethenes due to differences in the material characteristics of ZVI and the predominant dechlorination pathways.

A DFT study of chlorine coverage over late transition metals and its implication on 1,2-dichloroethane hydrodechlorination

Chlorine coverage and its impact on 1,2-dichloroethane hydrodechlorination over ten late transition metals, predicted using DFT-based phase diagrams.

Bimetallic Nanoparticles as Efficient Catalysts: Facile and Green Microwave Synthesis

This work deals with the development of a green and versatile synthesis of stable mono- and bi-metallic colloids by means of microwave heating and exploiting ecofriendly reagents: water as the solvent, glucose as a mild and non-toxic reducer and polyvinylpirrolidone (PVP) as the chelating agent. Particle size-control, total reaction yield and long-term stability of colloids were achieved with this method of preparation. All of the materials were tested as effective catalysts in the reduction of p-nitrophenol in the presence of NaBH₄ as the probe reaction. A synergistic positive effect of the bimetallic phase was assessed for Au/Cu and Pd/Au alloy nanoparticles, the latter showing the highest catalytic performance. Moreover, monoand bi-metallic colloids were used to prepare TiO₂- and CeO₂-supported catalysts for the liquid phase oxidation of 5-hydroxymethylfufural (HMF) to 2,5-furandicarboxylic acid (FDCA). The use of Au/Cu and Au/Pd bimetallic catalysts led to an increase in FDCA selectivity. Finally, preformed Pd/Cu nanoparticles were incorporated into the structure of MCM-41-silica. The resulting Pd/Cu MCM-41 catalysts were tested in the hydrodechlorination of CF₃OCFClCF₂Cl to CF₃OCF=CF₂. The effect of Cu on the hydrogenating properties of Pd was demonstrated.

In situ spectroscopy of complex surface reactions on supported Pd-Zn, Pd-Ga, and Pd(Pt)-Cu nanoparticles

It is well accepted that catalytically active surfaces frequently adapt to the reaction environment (gas composition, temperature) and that relevant "active phases" may only be created and observed during the ongoing reaction. Clearly, this requires the application of in situ spectroscopy to monitor catalysts at work. While changes in structure and composition may already occur for monometallic single crystal surfaces, such changes are typically more severe for oxide supported nanoparticles, in particular when they are composed of two metals. The metals may form ordered intermetallic compounds (e.g. PdZn on ZnO, Pd2Ga on Ga2O3) or disordered substitutional alloys (e.g. PdCu, PtCu on hydrotalcite). We discuss the formation and stability of bimetallic nanoparticles, focusing on the effect of atomic and electronic structure on catalytic selectivity for methanol steam reforming (MSR) and hydrodechlorination of trichloroethylene. Emphasis is placed on the in situ characterization of functioning catalysts, mainly by (polarization modulated) infrared spectroscopy, ambient pressure X-ray photoelectron spectroscopy, X-ray absorption near edge structure, and X-ray diffraction. In the present contribution, we pursue a two-fold, fundamental and applied, approach investigating technologically applied catalysts as well as model catalysts, which provides comprehensive and complementary information of the relevant surface processes at the atomic or molecular level. Comparison to results of theoretical simulations yields further insight. Several key aspects were identified that control the nanoparticle functionality: (i) alloying (IMC formation) leads to site isolation of specific (e.g. Pd) atoms but also yields very specific electronic structure due to the (e.g. Zn or Ga or Cu) neighboring atoms; (i) for intermetallic PdZn, the thickness of the surface alloy, and its resulting valence band structure and corrugation, turned out to be critical for MSR selectivity; (ii) the limited stability of phases, such as Pd2Ga under MSR conditions, also limits selectivity; (iii) favorably bimetallic catalysts act bifunctional, such as activating methanol AND water or decomposing trichlorothylene AND activating hydrogen; (iv) bifunctionality is achieved either by the two metals or by one metal and the metal-oxide interface; (v) intimate contact between the two interacting sites is required (that cannot be realized by two monometallic nanoparticles being just located close by). The current studies illustrate how rather simple bimetallic nanoparticles may exhibit intriguing diversity and flexibility, exceeding by far the properties of the individual metals. It is also demonstrated how complex reactions can be elucidated with the help of in situ spectroscopy, in particular when complementary methods with varying surface sensitivity are applied.

Tuning Supported Catalyst Reactivity with Dendrimer-Templated Pt−Cu Nanoparticles

The effects of particle composition on heterogeneous catalysis were studied using dendrimer-encapsulated nanoparticles (DENs) as precursors to supported Pt-Cu catalysts. Bimetallic Pt-Cu DENs with varying Pt/Cu ratios were prepared in an anaerobic aqueous solution and deposited onto a high-purity commercial alumina support. The dendrimer template was then thermally removed to yield supported nanoparticle catalysts, which were studied with toluene hydrogenation and CO oxidation catalysis as well as infrared spectroscopy of adsorbed CO. Incorporating Cu into Pt nanoparticles had opposite effects on the two test reactions. Cu acted as a mild promoter for CO oxidation catalysis, and the promoting effect was independent of the amount of Cu present. Conversely, Cu acted as a strong poison for toluene hydrogenation catalysis, and the normalized rate tracked inversely with Cu content. Infrared spectroscopy of the supported nanoparticles indicated that electronic effects (electron donation from Cu to Pt) were minimal for these materials. Consequently, the catalysis results are interpreted in terms of potential structural differences as a function of Cu incorporation and reaction conditions.

Probing Adsorption Sites of Silica-Supported Platinum with 13C16O + 12C16O and 13C18O + 12C16O Mixtures: A Comparative Fourier Transform Infrared Investigation

A comparative investigation of the adsorption of (13)C(18)O + (12)C(16)O and (13)C(16)O + (12)C(16)O mixtures on silica-supported Pt has been conducted. It is advantageous to use (13)C(18)O + (12)C(16)O mixtures rather than (13)C(16)O + (12)C(16)O to probe the adsorption sites and electronic state of supported Group VIII metals because the vibrational bands of the adsorbed (13)C(18)O and (12)C(16)O isotopic molecules do not overlap. In addition, while an intensity redistribution suppresses the lower-frequency band with adsorbed (13)C(16)O and (12)C(16)O with vibrational frequencies differing by 50 cm(-1), the intensity redistribution is less pronounced with the adsorbed (13)C(18)O and (12)C(16)O in which the frequency difference is 100 cm(-1). Moreover, the small intensity redistribution that does occur between the bands of adsorbed (13)C(18)O and (12)C(16)O still allows the detection of the vibrational band of adsorbed (13)C(18)O at (13)C(18)O gas-phase concentrations as low as 3%. At such low concentrations, the dipole-dipole interaction between adsorbed (13)C(18)O molecules is negligible, and, hence, both the singleton frequency and the dipole-dipole shift for adsorbed CO may be obtained in a single experiment. Two types of strongly bound and one type of weakly bound linear CO-Pt adsorption complexes have been identified and characterized by their singleton frequencies and dipole-dipole coupling shifts. The origin of these CO adsorption modes is discussed.