Hydrodechlorination Of Trichloroethylene Research Articles

Effect of Pt nanoparticle size on resistance to chloride-induced inhibition for hydrodechlorination of trichloroethylene in water

Catalysts used for hydrodechlorination (HDC) of trichloroethylene (TCE) undergo partial or complete loss of catalytic activity due to inhibition by inevitable reaction product HCl. Changing the size of Pt NPs could be used as a strategy to prevent inhibition of HDC catalysts if the structure sensitivity of the reaction was known. This study aims to obtain structure-sensitivity of aqueous-phase HDC of TCE and investigate the effect of Pt NPs’ size on resistance to chloride inhibition. The research encompassed the synthesis of Pt NPs, characterization studies, and catalytic activity measurements at 30 °C & 1 atm H2 in batch or discontinuous pulse mode. Characterization results showed that targeted Pt NPs were successfully synthesized with average particle sizes of 3.0 nm, 5.8 nm, 8.9 nm, and 60.9 nm. Catalytic activity experiments revealed that initial turnover frequency values exhibited a decrease with increasing particle size of Pt NPs, indicating that aqueous-phase HDC of TCE is a structure sensitive reaction. The fastest kinetics and the highest resistance to HCl inhibition were observed over the smallest Pt NPs employed in this study. This information can serve as a rational synthesis parameter for designing catalysts that could mitigate chloride inhibition caused by the reaction product HCl.

Thermal annealing effects on palladium-decorated gold nanoparticle catalysts

Palladium metal supported on gold in the form of surface ensembles have enhanced catalytic properties compared to monometallic Pd, as exemplified by Pd-decorated Au nanoparticles (Pd-on-Au NPs) for various room-temperature reactions. Whereas the catalytic properties and nanostructure of Pd-on-Au NPs are not known at higher temperatures, this work focuses on thermal annealing effects on the Pd-on-Au NP nanostructure, bimetal distribution, and room-temperature water-phase trichloroethene hydrodechlorination (TCE HDC) as the model reaction. Analysis of the average coordination environment of Pd and Au atoms through x-ray absorption spectroscopy showed that as-synthesized Pd-on-Au NPs transitioned from a Au core/Pd shell structure to Au-rich core/PdAu surface alloy or PdAu mixed alloy structures depending on the Pd surface coverage (30–150 sc%) and annealing temperature (100–400 °C). The HDC activity strongly correlated with Pd ensemble size, where the as-formed Pd islands exhibited one order of magnitude enhanced activity compared to monometallic Pd. Higher annealing temperatures led to a surface/mixed alloy structure with smaller Pd ensemble size, resulting in lower activity but still ∼3 times more active than monometallic Pd. These results illustrate the importance of catalyst structure on activity and the usefulness of metal-decorated metal catalysts for higher-temperature reactions.

Self-activated Ni(OH)2 cathode for complete electrochemical reduction of trichloroethylene to ethane in low-conductivity groundwater

This study investigates the electrochemical dechlorination of trichloroethylene (TCE) to non-toxic and value-added ethane by self-activated Ni(OH)2 cathode. The Ni(OH)2 cathode was fabricated by drop-casting the as-synthesized Ni(OH)2 suspensions on a glassy carbon electrode. At −1.0 V applied potential, 94.6% of TCE is reduced after 5 h of electrolysis. TCE is efficiently transformed to ethane at a wide pH range in simulated groundwater environment with low-conductivity. The air-exposed Ni(OH)2 cathode can still dechlorinate 49.8% TCE, showing the antioxidative capacity of Ni(OH)2 cathode is superior than that of pristine Ni electrode. Both the directly transferred electrons and the generated atomic H* at the active Ni(0) sites contributed to the hydrodechlorination of TCE. Our work not only provides an effective way to completely detoxify chlorinated organic contaminants (COCs) under environment-relevant conditions, but also proves a concept that the chemical energy in COCs can be harvested during their electrochemical reduction processes.

Generation of atomic hydrogen by Ni-Fe hydroxides: Mechanism and activity for hydrodechlorination of trichloroethylene

Atomic hydrogen (H•) is highly reactive for the hydrodechlorination of trichloroethylene (TCE). In this work, we found that the coprecipitation of Ni2+ and Fe2+ at neutral pH led to an unprecedented catalytic generation of H•. The generated H• effectively dechlorinate TCE to nontoxic ethylene and ethane, and Fe2+ is the only electron donor. The abundant adsorbed H• produced with a Ni/Fe ratio of 0.4 enhances hydrogen evolution reaction causing a low efficiency for hydrodechlorination. In contrast, the active absorbed H• is generated in the crystal lattice of Ni-Fe hydroxides with a Ni/Fe ratio of 3.0 causing highly efficient hydrodechlorination of TCE. This work not only reveals the mechanism of catalytic hydrodechlorination by Ni-Fe hydroxides at neutral pH, but also provides a novel approach to detoxify TCE in contaminated water using facile precipitated Ni-Fe hydroxides.

On the dual role of the reactant during aqueous phase hydrodechlorination of trichloroethylene (HDC of TCE) using Pd supported on swellable organically modified silica (SOMS)

Herein, the dual role of trichloroethylene (TCE) in liquid-phase hydrodechlorination with Pd catalysts supported on swellable organically modified silica (SOMS), is investigated. Batch experiments with TCE-soaked Pd/SOMS showed 45% higher conversion after 4 h than dry Pd/SOMS, suggesting that TCE acts as a reactant and as a swelling-agent. Flow experiments performed with and without ethanol (external swelling-agent) exhibited similar steady-state activities, confirming that TCE itself can cause swelling. Moreover, flow experiments, regardless of whether the SOMS-supported Pd catalysts were pre-reduced or not, yielded similar steady-state conversions, suggesting that Pd(2+) is rapidly reduced by H2 dissolved in water during approach to steady-state. In-situ characterization performed under reaction conditions using time-resolved X-ray absorption spectroscopy confirmed the reduction of Pd(2+) species to Pd(0) by dissolved H2. Overall, this study shows economic viability of hydrodechlorination of TCE using Pd/SOMS by eliminating the need for an external swelling-agent as well as a catalyst pre-reduction step.

Understanding the origin of structure sensitivity in hydrodechlorination of trichloroethylene on a palladium catalyst

Mechanistic understanding on the origin of structure sensitivity in the hydrodechlorination of trichloroethylene.

Aqueous-phase hydrodechlorination of trichloroethylene over Pd-based swellable organically-modified silica (SOMS): Catalyst deactivation due to chloride anions

Swellable-organically modified silica (SOMS) has been demonstrated to be an efficient catalyst scaffold for catalytic treatment of water contaminated with trichloroethylene (TCE). In this study, deactivation characteristics of Pd-incorporated SOMS for aqueous-phase hydrodechlorination (HDC) of TCE were investigated. Pd/SOMS catalysts were exposed to highly-concentrated chloride solutions (up to 1 M NaCl or 0.01 M HCl) to examine the deactivation resistant behavior of Pd/SOMS. The commonly used HDC catalyst Pd/Al2O3 was also studied for comparison purposes. Pd/SOMS and Pd/Al2O3 in their pristine and treated states were tested for aqueous-phase HDC of TCE and characterized by several techniques including N2 physisorption, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy, extended X-ray absorption fine structure spectroscopy (EXAFS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO. The aqueous-phase treatments had a pronounced adverse effect on the textural properties of Pd/Al2O3, although the effect was independent of the type of the chloride precursor, NaCl or HCl. Treating Pd/Al2O3 with chloride-containing solutions lowered the catalytic activity due to formation of Pd-Cl complexes and active metal leaching. The leached Pd obtained from the treatment solution was shown to be inactive for aqueous-phase HDC of TCE. While Pd/Al2O3 underwent severe deactivation due to the chloride treatments, Pd/SOMS exhibited resistance to chloride deactivation and metal leaching. The chloride treatments did not impact the textural properties of Pd/SOMS. The achieved deactivation resistance was attributed to the novel characteristics of the SOMS support.

Trichloroethene (TCE) hydrodechlorination by Ni[sbnd]Fe nanoparticles: Influence of aqueous anions on catalytic pathways

Amending bulk and nanoscale zero-valent iron (ZVI) with catalytic metals significantly accelerates hydrodechlorination of groundwater contaminants such as trichloroethene (TCE). The bimetallic design benefits from a strong synergy between Ni and Fe in facilitating the production of active hydrogen for TCE reduction, and it is of research and practical interest to understand the impacts of common groundwater solutes on catalyst and ZVI functionality. In this study, TCE hydrodechlorination reaction was conducted using fresh NiFe bimetallic nanoparticles (NiFe BNPs) and those aged in chloride, sulfate, phosphate, and humic acid solutions with concurrent analysis of carbon fractionation of TCE and its daughter products. The apparent kinetics suggest that the reactivity of NiFe BNPs is relatively stable in pure water and chloride or humic acid solutions, in contrast to significant deactivation observed of PdFe bimetallic particles in similar media. Exposure to phosphate at greater than 0.1 mM led to a severe decrease in TCE reaction rate. The change in kinetic regimes from first to zeroth order with increasing phosphate concentration is consistent with consumption of reactive sites by phosphate. Despite severe kinetic effect, there is no significant shift in TCE 13C bulk enrichment factor between the fresh and the phosphate-aged particles. Instead, pronounced retardation of TCE reaction by NiFe BNPs in deuterated water (D2O) points to the importance of hydrogen spillover in controlling TCE reduction rate by NiFe BNPs, and such process can be strongly affected by groundwater chemistry.

Unveiling the hydrodechlorination of trichloroethylene by reduced graphene oxide supported bimetallic Fe/Ni nanoparticles

Reduced graphene oxide (rGO) is a promising support for electrochemical and environmental applications. However, the role of rGO in dechlorination of chlorinated hydrocarbon by zerovalent iron-based nanoparticles remains unclear. Herein, the rGO-supported bimetallic Fe/Ni nanocomposites were fabricated by chemical reduction method and the microstructures as well as the electrochemical properties of rGO/Fe/Ni, were investigated to elucidate the dechlorination behaviors of trichloroethylene (TCE) under various environmental conditions. Results show that the 10–60 nm Fe/Ni nanoparticles with average particle size of 32 nm are homogeneously dispersed onto the rGO surface. The rGO/Fe/Ni exhibits excellent dechlorination efficiency and rate of TCE is 2.4 times higher than that of free Fe/Ni nanoparticles. Cyclic voltammetric curves and electrochemical impedance spectra indicate that the superior dechlorination activity is attributed to the strong interaction between Fe/Ni and rGO, low internal resistance and rapid diffusion rate of electrons and ions. In addition, the capacitive behavior of rGO can store and transfer 2.7–3.4 electrons produced from Fe0 to the adsorbed TCE more readily, and then converts TCE to non-toxic ethane via hydrodechlorination in the presence of Ni nanoparticles. The rGO/Fe/Ni can be recycled for at least 8 times to effective dechlorinate TCE. Moreover, the reaction rate of TCE dechlorination can be enhanced 1.26–1.49 times when 5–10 mg L−1 humic acid are added. Results obtained in this study have clearly unveiled the role of rGO in hydrodechlorination of TCE and can provide a new insight into the development of rGO−supported bimetallic Fe/Ni nanoparticles for the enhanced removal of chlorinated pollutants in water and wastewater treatment.

Effect of high-temperature on the swellable organically-modified silica (SOMS) and its application to gas-phase hydrodechlorination of trichloroethylene

Pd catalysts supported on swellable organically-modified silica (SOMS) and high-temperature-treated swellable organically-modified silica (H-SOMS) were characterized and tested for gas-phase hydrodechlorination (HDC) of trichloroethylene (TCE) conditions. The high-temperature treatment on SOMS resulted in an increase in surface area and pore diameter as well as significant improvement of Pd dispersion on H-SOMS with smaller Pd particle sizes compared to the Pd/SOMS catalyst. Although the high-temperature treatment led to some alteration of the SOMS polysiloxane network, the hydrophobicity and organic vapor adsorption characteristics of SOMS were preserved. The reduction and oxidation characteristics of Pd on SOMS and HSOMS were investigated in situ using XANES technique. It was found that the Pd sites in the pores of SOMS was accessible to small molecules such as H2, facilitating the reduction of PdOx, whereas oxidation of metallic Pd was limited even at higher temperatures when O2 was used. This effect was only observed over Pd/SOMS catalyst. For Pd/H-SOMS, because the pores were more widely open than Pd/SOMS, both reduction and oxidation of Pd were observed. Finally, the catalytic activity of Pd/H-SOMS for gas-phase HDC of TCE was significantly better than Pd/SOMS. When water was added to the reactant stream (TCE+H2O), both Pd/SOMS and Pd/H-SOMS maintained its catalytic performances due to hydrophobic property of the supports.

Turbostratic carbon supported palladium as an efficient catalyst for reductive purification of water from trichloroethylene

This work investigates the catalytic properties of turbostratic carbon supported Pd catalyst in hydrodechlorination of trichloroethylene (TCE HDC) in aqueous phase. 1.57 wt% Pd/C was thoroughly characterized by BET, TPHD, CO chemisorption, PXRD, STEM, XPS and used as the catalyst in removal of trichloroethylene from drinking water in batch and continuous-flow reactors. The studies showed that catalytic performance of Pd/C depended on the hydrophobicity and textural properties of carbon support, which influenced noble metal dispersion and increased catalyst tolerance against deactivation by chlorination. Palladium in the form of uniformly dispersed small (~3.5 nm) nanoparticles was found to be very active and stable in purification of water from TCE both in batch and continuous-flow operation.

Hydrodechlorination of trichloroethylene over Pd supported on swellable organically-modified silica (SOMS)

The catalytic activity and resistance to poisoning of Pd catalysts supported on swellable organically-modified silica (SOMS) were investigated for hydrodechlorination (HDC) of trichloroethylene (TCE). The promising catalytic activity of 1% Pd/SOMS sample was attributed to the high affinity of SOMS for organics and its high hydrophobicity. While latter characteristic repels water, the adsorptive capacity for organics allows TCE dissolved in aqueous media to concentrate inside the pores, in the vicinity of the active sites, thus helping the kinetics. In the liquid phase, using a continuous flow reactor, higher TCE conversion was obtained over the 1% Pd/SOMS compared to the commercial 1% Pd/Al2O3 catalyst. When the pores of 1% Pd/SOMS sample were fully opened by pre-treating it with ethanol prior to the reaction, HDC activity was seen to significantly increase. In the gas phase, the extent of adsorption was less, reducing the concentration of reactants near the active sites. As a result, 1% Pd/SOMS was less active than 1% Pd/Al2O3 for HDC of TCE. To determine their resistance to poisoning, 1% Pd/SOMS and 1% Pd/Al2O3 catalysts were poisoned ex-situ with Li2S. The ex-situ poisoned Pd/SOMS sample maintained its catalytic activity for HDC of TCE. However, a significant loss in catalytic activity of the Pd/Al2O3 catalyst was observed after poisoning. Protection from aqueous phase sulfide poisoning was attributed to the hydrophobicity of the Pd/SOMS, which would exclude anionic species from the embedded Pd particles. The XPS, STEM and ICP-OES results indicated that when Pd/Al2O3 and Pd/SOMS were treated with 1M HCl, most of the Pd metal was leached from the Pd/Al2O3 catalyst in contrast to Pd/SOMS, which had negligible leaching. Overall, due to hydrophobicity and high affinity for organics, SOMS has potential as a catalyst scaffold for different reactions in groundwater remediation applications.

Gold-doping of carbon-supported palladium improves reduction catalysis

Bimetallic palladium-gold (PdAu) catalysts have better catalytic performance than monometallic catalysts for many applications. PdAu catalysts with controlled nanostructures and enhanced activities have been extensively studied but their syntheses require multiple and occasionally complicated steps. In this work, we demonstrated that supported PdAu catalysts could be simply prepared by doping a supported Pd catalyst with gold through wet impregnation and calcination. Resulting PdAu-on-carbon (PdAu/C) catalysts were tested for the room-temperature, aqueous-phase hydrodechlorination of trichloroethene. The most active PdAu/C catalyst (Pd 1.0 wt%, Au 1.1 wt%, dried/air/H2 process) had an initial turnover frequency (TOF) of 34.0 × 10−2 molTCE molPd−1 s−1, which was >15 times higher than monometallic Pd/C (Pd 1.0 wt%, initial TOF of 2.2 × 10−2 molTCE molPd−1 s−1). Through X-ray absorption spectroscopy, the gold kept Pd from oxidizing under calcination at 400 °C. Probable nanostructure evolution pathways are proposed to explain the observed catalysis.

Trichloroethene hydrodechlorination by Pd-Fe bimetallic nanoparticles: Solute-induced catalyst deactivation analyzed by carbon isotope fractionation

Pd-Fe bimetallic nanoparticles (BNPs) have substantially higher reactivity for reductive dehalogenation of chlorinated ethenes (e.g., trichloroethylene, TCE) compared to monometallic zero-valent iron nanoparticles, yet the rapid deactivation of BNPs in groundwater matrices limits their large-scale use in field applications. In spite of this shortcoming, the causes of BNP deactivation have not been clearly delineated. Stable carbon isotope fractionation measurements and product distribution analysis were used in this study to investigate the mechanisms of Pd-Fe deactivation in the presence of common groundwater solutes. Based on the apparent TCE degradation rates and at a constant solute concentration of 5mM (except for humic acid, which was dosed at 20mg/L), Pd-Fe BNPs exposed to SO42−, HPO42−, and humic acid solutions showed moderate declines in TCE dechlorination rates. Aging the bimetallic particles in Cl−, SO32−, HCO3−, and NO3− solutions, however, resulted in excessive or complete loss of TCE dechlorination reactivity. Analyses of the isotope fractionation associated with TCE hydrodechlorination (εTCEHDC) as well as the yield of ethane over other dechlorination products suggest at least four distinctive causes of deactivation: (i) aging the BNPs in deionized water and humic acid induces surface passivation due to buildup of mineral or organic carbon deposits; (ii) SO32− and Cl− ions interact specifically with Pd sites and disable the catalyst functions; (iii) NO3− and HCO3− inhibits iron corrosion, thereby limiting the production of H2 as the precursor of reactive hydrogen species, and (iv) selective deactivation of surface sites involved in ethene hydrogenation was observed for BNPs aged in SO42− and HPO42− solutions. The findings suggest the Pd-on-Fe configuration of the bimetallic particles is susceptible to deactivation in a broader range of groundwater chemistry than previously expected.

Hydrodechlorination of trichloroethylene over MoP/γ-Al2O3 catalyst with high surface area

Alumina-supported molybdenum phosphide (MoP/γ-Al2O3) with high surface area was successfully synthesized by combining sol–gel and temperature-programmed reduction (SG-TPR) method and was firstly used to study the hydrodechlorination (HDC) of trichloroethylene (TCE). The properties of the catalysts were evaluated by several different characterization techniques, such as Braunner–Emmet–Teller (BET) surface area, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The results prove that a well-dispersed and high BET surface area catalyst can be obtained by SG-TPR method, which shows higher catalytic activity than those prepared by impregnation and mechanical mixing method for the HDC of TCE.

Hydrodechlorination of TCE in a circulated electrolytic column at high flow rate

Palladium-catalytic hydrodechlorination of trichloroethylene (TCE) by cathodic H2 produced from water electrolysis has been tested. For a field in-well application, the flow rate is generally high. In this study, the performance of Pd-catalytic hydrodechlorination of TCE using cathodic H2 is evaluated under high flow rate (1 L min−1) in a circulated column system, as expected to occur in practice. An iron anode supports reduction conditions and it is used to enhance TCE hydrodechlorination. However, the precipitation occurs and high flow rate was evaluated to minimize its adverse effects on the process (electrode coverage, clogging, etc.). Under the conditions of 1 L min−1 flow, 500 mA current, and 5 mg L−1 initial TCE concentration, removal efficacy using iron anodes (96%) is significantly higher than by mixed metal oxide (MMO) anodes (66%). Two types of cathodes (MMO and copper foam) in the presence of Pd/Al2O3 catalyst under various currents (250, 125, and 62 mA) were used to evaluate the effect of cathode materials on TCE removal efficacy. The similar removal efficiencies were achieved for both cathodes, but more precipitation generated with copper foam cathode (based on the experiments done by authors). In addition to the well-known parameters such as current density, electrode materials, and initial TCE concentration, the high velocities of groundwater flow can have important implications, practically in relation to the flush out of precipitates. For potential field application, a cost-effective and sustainable in situ electrochemical process using a solar panel as power supply is being evaluated.

Catalytic removal of trichloroethylene from water over palladium loaded microporous and hierarchical zeolites

The work was aimed to investigate the catalytic properties of Pd- containing ZSM-5 zeolites in hydrodechlorination of trichloroethylene (TCE HDC) in aqueous phase. Conventional purely microporous zeolite ZSM-5 and its mesostructured analogue obtained by alkaline leaching were used as the supports for palladium moieties. The resulting materials were subjected to chemical analysis, XRD characterization, CO chemisorption, and spectroscopic investigation (XPS and FTIR) and, then, used as the catalysts in removal of trichloroethylene from drinking water. The studies have shown clearly that catalytic performance of palladium loaded materials depended on acidic and textural properties of zeolites, which influenced both noble metal dispersion and Pd0/Pd2+ ratio. In a series of the zeolite catalysts the best results were obtained for palladium deposited on desilicated form of ZSM-5. Palladium in the form of uniformly dispersed species, containing large amount of Pd0 species was found to be significantly active in TCE HDC.

Catalytic hydrodechlorination of trichloroethylene in a novel NaOH/2-propanol/methanol/water system on ceria-supported Pd and Rh catalysts

The catalytic hydrodechlorination (HDC) of high concentrations of trichloroethylene (TCE) (4.9 mol%, 11.6 vol%) was studied over 1%Pd, 1%Rh and 0.5%Pd-0.5%Rh catalysts supported on CeO2 under conditions of room temperature and pressure. For this, a one-phase system of NaOH/2-propanol/methanol/water was designed with molar percentages of 13.2/17.5/36.9/27.6, respectively. In this system, the alcohols delivered the hydrogen required for the reaction through in-situ dehydrogenation reactions. PdRh/CeO2 was the most active catalyst for the degradation of TCE among the evaluated materials, degrading 85% of the trichloroethylene, with alcohol dehydrogenation rates of 89% for 2-propanol and 83% for methanol after 1 h of reaction. Fresh and used catalysts were characterized by Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), and Thermogravimetric analysis (TGA). These results showed important differences of the active phase in each catalyst sample. Rh/CeO2 had particle sizes smaller than 1 nm and the active metal was partially oxidized (Rh0/Rh+δ ratio of 0.43). This configuration showed to be suitable for alcohols dehydrogenation. On the contrary, Pd/CeO2 showed a Pd completed oxidized and with a mean particle size of 1.7 nm, which seemed to be unfavorable for both, alcohols dehydrogenation and TCE HDC. On PdRh/CeO2, active metals presented a mean particle size of 2.7 nm and more reduced metallic species, with ratios of Rh0/Rh+δ = 0.67 and Pd0/Pd+δ = 0.28, which showed to be suitable features for the TCE HDC. On the other hand, TGA results suggested some deposition of NaCl residues over the catalyst surfaces. Thus, the new reaction system using PdRh/CeO2 allowed for the degradation of high concentrations of the chlorinated compound by using in situ hydrogen liquid donors in a reaction at room temperature and pressure.

Remarkable effect of soft-templating synthesis procedure on catalytic properties of mesoporous carbon supported Ni in hydrodechlorination of trichloroethylene in liquid phase

Ordered mesoporous carbon (CST-A) obtained by soft-templating method under acidic condition was used for preparation of nickel containing catalyst by incipient wetness impregnation with an aqueous solution of NiCl2·6H2O. The reduction of as prepared catalysts by 10% H2/Ar at 673K led to formation of well dispersed active metallic phase with average nickel particles size ∼11nm. 2wt.% Ni/CST-A was examined as a catalyst in hydrodechlorination (HDC) of trichloroethylene in liquid phase, without presence of an additional organic solvent in the reaction mixture. Ethane and ethylene were the predominant product of reaction. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies for catalysts after reduction and after HDC reaction indicated that use of hydrophobic, mesoporous active carbon as the support for nickel led to formation of resistant for sintering and leaching catalyst and the effective material for catalytic removal of trichloroethylene from water. Post-reaction deposits investigated by the temperature programmed hydrogenation (TPH) showed substantial amounts of carbon-containing deposit, with C2-hydrocarbons desorbing at a relatively low temperature (∼650K).

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.