Clean Pd Surface Research Articles

Electrocatalytic CO oxidation on porous ternary PdNiO-CeO2/carbon black nanocatalysts: Effect of supports and electrolytes

The catalytic activity and durability of porous ternary Pd-based electrocatalysts are augmented by alloying with other metals and using supports, however, the effect of supports and electrolytes at varied pH on the electrochemical CO oxidation (COOxid) performance is not yet studied. Herein, a PdNiO-CeO2/CB nanosphere, composed of PdNiO nanocrystals sprouted on CeO2 flake-like and carbon black (CB) nanosheets, was synthesized by a sol-gel and impregnation methods. Remarkably, during the impregnation step, CeO2/CB serves as a reactor for the reduction of Pd/Ni precursors and supported the growth of PdNiO nanocrystals without surfactant and/or reducing agent. This endowed the formation of PdNiO-CeO2/CB with multifunctional porous structures, including pore volume (0.29 cm3/g) and specific surface area (151.86 m2/g) and a clean Pd surface. The as-prepared PdNiO-CeO2/CB delivers a superior COOxid activity than those of PdNiO-CeO2, PdNiO/CB, and commercial Pd/C in acidic (HClO4), neutral (NaHCO3), and alkaline (KOH) electrolytes, ascribable to the oxygen-enriched support (CeO2) and its interaction with CB support along with electronic effect of PdNiO, which enhances the CO adsorption/activation and simultaneously activates/splits H2O to produce active oxygenated species required for hastening COOxid kinetics. Interestingly, the COOxid performance of the PdNiO-CeO2/CB in HClO4 electrolyte was higher than those in NaHCO3 and KOH electrolytes. This study reveals that coupling oxygen-enriched support (CeO2) with CB support is preferred for the enhancement of the COOxid activity of PdNiO nanocrystals.

How decoration with Tl affects CO electro-oxidation on Pd (1 0 0) nanocubes: In situ FTIR and ab-initio insights

• Pd nanocubes were synthesized, modified by Tl and tested for CO electrooxidation. • Different amounts of Tl influence CO electrooxidation reaction on these surfaces. • CO adsorbs in a two-site geometry on clean Pd surfaces. • The adsorption geometry changes for on-top in presence of minor amounts of Tl. • DFT data reveals that high Tl coverages hinder CO adsorption on Pd. In this work we prepare Pd (1 0 0) preferentially oriented nanoparticles modified with different amounts of Tl and study the CO electro-oxidation reaction on these surfaces in alkaline medium. The analysis is based on cyclic voltammetry coupled with in situ FTIR spectroscopy, and the results are interpreted with the help of a DFT study. In situ FTIR indicates a change in the CO adsorption geometry on Pd surfaces partially covered by Tl, while DFT data reveal that for high Tl coverages CO adsorption is completely inhibited. Based on our findings, by using the proper amount of Tl, one can think in the development of Pd-based catalysts designed to present tolerance to CO poisoning, aiming their application in the electro-oxidation of small organic molecules.

Nanosheet Array-Like Palladium-Catalysts Pdx/rGO@CoAl-LDH via Lattice Atomic-Confined in Situ Reduction for Highly Efficient Heck Coupling Reaction.

A series of novel nanosheet array-like catalysts Pdx/rGO@CoAl-LDH (x = 0.0098-1.9, refers to Pd loading in wt % on ICP, rGO: reduced graphene oxide, LDH: layered double hydroxide) were first prepared via a simple and green lattice atomic-confined in situ reduction of oxidative Pd precursors by the evenly atomic-dispersed reductive Co2+ sites on LDH layers of a nanohybrid rGO@CoAl-LDH with hexagonal LDH nanoplates (∼73 × 7 nm) interdigitated vertical to the surfaces of rGO layer in both sides, fabricated through a simple citric acid-assisted aqueous-phase coprecipitation method. The as-obtained Pd catalysts possess clean Pd nanoclusters (NCs) with tunable sizes in 1.3-1.8 nm on varied Pd loadings. All the Pdx/rGO@CoAl-LDH catalysts show excellent activities for the Heck reaction, and the Pd0.0098/rGO@CoAl-LDH with the ultrafine Pd NCs of 1.3 ± 0.2 nm yields a maximum turnover frequency of 160 000 h-1 over a heterogeneous catalyst so far. The excellent activities can be attributed to the ultrasmall Pd NCs with high dispersion and clean Pd surfaces, increased electron transfer capacity and surface area, and remarkable Pd-CoAl-LDH-rGO three-phase synergistic effect of the present unique nanosheet array-like Pd NCs catalysts. Moreover, the catalyst Pd0.33/rGO@CoAl-LDH shows a broad range of substrate applicability and can be reused more than five runs without obvious loss of activity, giving the present catalysts long-term stability. These findings make the rGO@CoAl-LDH hybrid prepared by a facile and scalable synthesis route a universal green platform to support other noble or nonprecious metal NCs via lattice atomic-confined in situ reduction strategy to construct more desired heterogeneous catalysts.

Reactivity of O2 on Pd/Ru(0001) and PdRu/Ru(0001) surface alloys.

The reactivity of a Pd monolayer epitaxially grown on Ru(0001) toward O2 has been investigated by molecular beam techniques. O2 initial sticking coefficients were determined using the King and Wells method in the incident energy range of 40-450 meV and for sample temperatures of 100 K and 300 K, and compared to the corresponding values measured on the clean Ru(0001) and Pd(111) surfaces. In contrast to the high reactivity shown by Ru(0001) at 100 K, the Pd/Ru(0001) system exhibits a monotonic decrease in the sticking probability of O2 as a function of normal incident energy. At room temperature, the system was found to be inert. Thermal desorption measurements show that O2 is adsorbed molecularly at 100 K. A completely different behaviour has been measured for the Pd0.95Ru0.05/Ru(0001) surface alloy. On this surface, the O2 sticking probability increases with incident energy and resembles the one observed on the clean Ru(0001) surface, even at 300 K. Thermal desorption measurements point to dissociative adsorption of O2 in this system. Both the charge transfer from the Pd to the Ru substrate and the compressive strain on the Pd monolayer contribute to decrease in the reactivity of the Pd/Ru(0001) system well below those of both Ru(0001) and Pd(111).

Surface Structures and Electrochemical Stabilities for Pt/Pd(111) Model Electrocatalysts

Introduction Pd@Pt core-shell catalyst is one of the promising cathode electrode materials for polymer electrolyte fuel cell (PEFC) from the view-points of a great potential to reduce Pt usage and to improve the oxygen reduction reaction (ORR) activity of Pt [1]. Recently, some researchers have reported that ORR activities of the core-shell catalysts increase during applying potential cycles, as a result of structural changes at the near surface regions [2]. Their results suggest that comprehensive understandings of dynamic behavior of the surface atomic structures during potential cycles are crucial for developing highly-active and highly-durable Pd@Pt core-shell catalysts. In this study, we investigate the relation between surface structures and electrochemical stabilities for the Pt/Pd(111) model electrocatalysts prepared by molecular beam epitaxy (MBE). Experimental Sample fabrication processes of the Pt/Pd(111) were conducted in UHV. Pd(111) single crystal substrate was cleaned by repeating cycles of Ar+ sputtering and subsequent annealing at 800 °C. 4ML-thick Pt was deposited onto the cleaned Pd(111) (4ML-Pt/Pd(111)) by an electron-beam evaporation method at the substrate temperatures of 300 °C. The resulting surface structures were verified with reflection high-energy electron diffraction (RHEED), scanning tunneling microscope in UHV (UHV-STM), and low-energy ion scattering (LEIS). Then, the prepared samples were transferred without being exposed to air to an electrochemical system set in a N2-purged glove box. Cyclic voltammogram (CV) of the samples were recorded in N2-purged 0.1 M HClO4, and, then, linear sweep voltammetry (LSV) was conducted by using a rotating electrode (RDE) method after saturating the solution with O2. The ORR activities were estimated by kinetic-controlled current density (j k) at 0.9V vs. RHE by using Koutecky-Levich plots. The electrochemical stabilities were evaluated by applying potential cycles (PCs) between 0.6 V and xV (x = 0.8, 0.85, 0.9 and 1.0V) vs. RHE for each 3 seconds in O2-saturated 0.1 M HClO4 at 80 °C. The potential cycle conditions are hereafter denoted as “xV-PCs”. Results and Discussion Surface structures for the clean Pd(111) and 4ML-Pt/Pd(111) are summarized in Fig.1 (a) and (b). The UHV-STM image indicates that the clean Pd(111) surface has atomically flat terraces. On the other hand, UHV-STM image of the 4ML-Pt/Pd(111) shows islands-like structures having hexagonal-shaped terraces with ca. 20 nm width. However, the corresponding RHEED pattern indicates sharp streaks, suggesting that deposited Pt atoms have epitaxially grown (111) lattice on the substrate. Initial ORR activities for the 4ML-Pt/Pd(111), clean Pt(111) surfaces are shown in Fig.1 (c). The results indicates that the 4ML-Pt/Pd(111) exhibits ca. 4.5 times higher ORR activity than the clean Pt(111).Fig.1 (d) summarizes changes in ORR activities of the 4ML-Pt/Pd(111) during applying PCs at 80 °C. The activity of 1.0V-PCs applied sample drastically decreased with increasing PC numbers and dropped to less than that of the clean Pt(111) at 1000 PCs. However, it can be seen that decrease in upper limit potential of PCs suppress activity deterioration of the 4ML-Pt/Pd(111). In particular, the activities of 0.8V- and 0.85V-PCs applied samples after 5000 PCs were 120 % and 90 % of the initials, respectively. To investigate effects of surface structural changes in the ORR activity, the surface structures and compositions were evaluated by UHV-STM and LEIS after 5000 PCs. LEIS spectra (Fig.1 (e)) obtained after the PCs indicate that Pd surface concentration of 0.9V-PCs applied sample is higher by ca. 30 % than 0.8V-PCs applied sample, suggesting that activity deterioration during PCs is derived from increase in surface composition of Pd which show much lower ORR activity than Pt. Furthermore, judging from the corresponding UHV-STM images, although height roughness of the 0.8V-PCs applied surface is suppressed to lower than 0.7 nm, the 0.9V-PCs applied surface comprises agglomerated particle-like structures with 20 nm width and 3 nm heights. These results suggest that the structural and electrochemical stabilities for Pd@Pt core-shell catalysts strongly depend on operating conditions of PEFC. Acknowledgement This study was supported by the new energy and industrial technology development organization (NEDO) of Japan.

Palladium Nanoparticles Embedded in a Layer-by-Layer Nanoreactor Built with Poly(Acrylic Acid) Using "Electro-Click Chemistry".

Palladium nanoparticles (Pd NPs) were formed by electrochemical reduction of Pd(NH3)4(3+) ions entrapped by ion exchange in poly(acrylic acid) (PAA) multilayer films grown by the Sharpless "click reaction." The alkyne (PAAalk) and azide (PAAaz) groups were covalently bound to the PAA, and the catalyzed buildup of the multilayer film was performed by electrochemical reduction of Cu(2+) to Cu(+). The size of the Pd NPs formed in Au/(PAAalk)3(PAAaz)2 multilayer films by the click reaction, that is, 50 nm, is larger than that of similar Pd NPs formed in electrostatically bound Au/(PAA)3(PAH)2 nanoreactors, that is, 6-9 nm, under similar conditions. A combination of electrochemical methods and electrochemical quartz crystal microbalance, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), ellipsometry, and scanning electron microscopy has been used to follow these processes. Cyclic voltammetry of the resulting Pd NPs in a 0.1 M H2SO4 solution at 0.1 V·s(-1) shows the PdO reduction peak at the same potential as that on the clean Pd surface unlike the NPs formed in electrostatically self-assembled Au/(PAA)3(PAH)2 nanoreactors with a 0.2 V shift in the cathodic direction most probably because of the strong adsorption of amino groups on the Pd NP surfaces.

Theoretical study of NH3 decomposition on Pd-Cu (1 1 1) and Cu-Pd (1 1 1) surfaces: A comparison with clean Pd (1 1 1) and Cu (1 1 1)

The adsorption and successive dehydrogenation mechanisms of NH3 on Pd-Cu (111) and Cu-Pd (111) surfaces (the Pd atoms substitution of the first and second layers of Cu (111) surfaces) have been systematically investigated by density functional theory (DFT) method with a periodic slab model. All possible adsorption configurations of relevant intermediates on Pd-Cu (111) and Cu-Pd (111) surfaces are identified. It is revealed that the adsorption configurations and corresponding adsorption energies of adsorbates are slightly changed on Pd-Cu (111) and Cu-Pd (111) surfaces. The adsorption energies of NHx(x=0–3) species exhibit the following trend: NH3<NH2<NH<N. Then, the minimum energy path for the complete dehydrogenation of NH3 into adsorbed N and H is identified to explore the dehydrogenation mechanisms on different surfaces. The highest energy barrier and reaction energy on Pd-Cu (111) surface are greatly reduced to 1.56 and 0.99eV, implying that the complete dehydrogenation of NH3 on Pd-Cu (111) surface is favorable both kinetically and thermodynamically, namely, the doped-Pd atoms in the first layer are the reaction active center. Compared to that on clean Pd (111) and Cu (111) surfaces, it is found that the synergistic effect exits in different layers of catalyst surfaces. The calculated results show that the layer-substituted Pd atoms on the surface of Cu catalysts exhibit a better catalytic activity for NH3 dehydrogenation compared to the clean Cu (111) surface.

The influence of surface oxygen and hydroxyl groups on the dehydrogenation of ethylene, acetic acid and hydrogenated vinyl acetate on pure Pd(1 0 0): A DFT study

On the basis of a Langmuir–Hinshelwood-type mechanism, the dehydrogenation of ethylene, acetic acid and hydrogenated vinyl acetate (VAH) on pure Pd(100) with surface oxygen atoms (Os) and hydroxyl groups (OHs) was studied with density functional theory (DFT) method. Our calculation results show that both Os and OHs can consistently reduce the activation energies of dehydrogenation of ethylene, acetic acid and VAH to some degree with only one exception that OHs somehow increase the activation energy of VAH. Based on Langmuir–Hinshelwood mechanism, the three dehydrogenation reactions in presence of surface Os and OHs are almost consistently favored, compared with the corresponding processes on clean Pd(100) surfaces, and thus a Langmuir–Hinshelwood-type mechanism may not be excluded beforehand when investigating the microscopic performance of the oxygen-assisted vinyl acetate synthesis on Pd(100) catalysts.

Importance of surface carbide formation on the activity and selectivity of Pd surfaces in the selective hydrogenation of acetylene

A recent experimental investigation (Kim et al. J. Catal. 306 (2013) 146–154) on the selective hydrogenation of acetylene over Pd nanoparticles with different shapes concluded that Pd(100) showed higher activity and selectivity than Pd(111) for acetylene hydrogenation. However, our recent density functional calculations (Yang et al. J. Catal. 305 (2013) 264–276) observed that the clean Pd(111) surface should result in higher activity and ethylene selectivity compared with the clean Pd(100) surface for acetylene hydrogenation. In the current work, using density functional theory calculations, we find that Pd(100) in the carbide form gives rise to higher activity and selectivity than Pd(111) carbide. These results indicate that the catalyst surface is most likely in the carbide form under the experimental reaction conditions. Furthermore, the adsorption energies of hydrogen atoms as a function of the hydrogen coverage at the surface and subsurface sites over Pd(100) are compared with those over Pd(111), and it is found that the adsorption of hydrogen atoms is always less favoured on Pd(100) over the whole coverage range. This suggests that the Pd(100) hydride surface will be less stable than the Pd(111) hydride surface, which is also in accordance with the experimental results reported.

Influence of Subsurface Oxygen in the Catalytic CO Oxidation on Pd(111)

The formation of palladium subsurface oxygen, i.e., Pd(111) saturated with oxygen below the surface, has been demonstrated. This compound is much more stable than palladium surface oxide or bulk palladium oxide, which easily reacts with CO at elevated temperatures. The catalytic activity of an atomically clean Pd(111) surface and of Pd(111) affected by subsurface oxygen (coverage Θ = 0.00–0.46 ML) was studied during the CO oxidation reaction. The measurements were performed for a temperature range of 353–523 K, partial pressures of reactants in the range of 10–7–10–5 mbar, and different subsurface oxygen content. A distinct, previously not observed feature in the dependency of the reaction rate vs temperature was found. Furthermore, it was found that subsurface oxygen reduces the catalytic activity of the Pd(111) surface significantly, which is attributed to reduction of the sticking coefficient of CO and oxygen. In the temperature range applied for investigation of catalytic CO oxidation, the rate of the formation of subsurface oxygen is negligible. However, the formation of this phase starts to be pronounced above ∼600 K. The presence of subsurface oxygen was confirmed by the KLL Auger transition excited by monochromatic X-ray radiation, not by electron beam. It was observed that the classical Auger measurement with an electron beam destroys the subsurface oxygen compound. The combination of the CO titration with Auger peak intensity measurements allows the quantitative analysis of the subsurface oxygen. The problem of subsurface oxygen detection and its impact on the adsorption of CO and oxygen on Pd(111) are discussed. It was suggested that subsurface oxygen and surface oxide can be manufactured simultaneously. Then the obtained activity in the CO oxidation reaction will be a result of two competitive effects, i.e., promoting from surface oxide and inhibiting from subsurface oxide. This might be a source of the discrepancy in results on the influence of surface oxide on the CO oxidation reaction measured by different researchers.

Adsorption and dehydrogenation mechanism of methane on clean and oxygen-covered Pd (1 0 0) surfaces: A DFT study

Using density functional theory (DFT) together with periodic slab models, the adsorption and dehydrogenation mechanisms of methane on clean and oxygen-covered Pd (100) surfaces have been studied systematically. Different adsorption geometries were investigated for CH4 and related intermediates (CH3, CH2, CH, C, H, O and OH). It was found that CH4 and CH3 prefer to adsorb on the top site, CH2 and OH are favorable on the bridge site, while CH, C, O and H species adsorb preferentially on the hollow site. In addition, this work identified the stable co-adsorption configurations for the relevant co-adsorption groups. It was concluded that the effect of co-adsorbed oxygen atom tends to weaken the adsorbate–substrate interaction on the Pd (100) surface. Finally, transition states, energy barriers and reaction energies were determined to confirm the mechanism of dehydrogenation of CH4 on clean and oxygen-covered Pd (100) surfaces. The existence of oxygen atom increases the energy barriers obviously and inhibits the dissociation of CHx (x=1, 2 and 4) except for CH3 group.

Collective Descriptors for the Adsorption of Sugar Alcohols on Pt and Pd(111)

Linear-scaling relationships are powerful tools in the rational design of new catalysts. Few of these thermodynamic relationships consider multifunctionalized molecules, and none deal with intramolecular interactions such as hydrogen bonds, widely present in polyoxygenated molecules obtained from renewable sources. We have completed an adsorption database with 14 mono- and polyalcohols up to four carbon atoms, on clean Pt and Pd(111) surfaces, using periodic density functional theory, and including van der Waals terms, and we have obtained results that are in good agreement with experimental data. With this data, we propose a robust multifactorial linear-scaling relationship, which is an additive function of the contributions from hydroxyl and CHx groups and the hydrogen bonds. We have validated the model by comparison with experiments and by the prediction of adsorption energies of C4–C7 sugar alcohols on both surfaces. Our study paves the way for the study of large multifunctionalized molecules, like those derived from biomass.

PdZn Surface Alloys as Models of Methanol Steam Reforming Catalysts: Molecular Studies by LEED, XPS, TPD and PM-IRAS

The formation and stability of PdZn/Pd(111) surface alloys have been studied, with emphasis on their interaction with CO, methanol and D2O, applying complementary techniques such as low energy electron diffraction, X-ray photoelectron spectroscopy, temperature programmed desorption (TPD), and polarization–modulation infrared reflection absorption spectroscopy. PdZn surface alloys represent well-suited model systems for technological methanol steam reforming (MSR) catalysts. It could be shown that upon Zn deposition on Pd(111) at or below room temperature non-interacting Zn layers are formed first, that subsequently transform to PdZn surface alloys upon annealing above 473 K. At annealing temperatures above approximately 623 K the surface alloy starts to decompose, finally restoring the clean Pd(111) surface. TPD spectra reveal that methanol was decomposing to a significant amount on Pd(111), yielding CO and CHx (apart from H2), a process that did not occur on the PdZn surface alloys (i.e. methanol desorbed molecularly). This difference in part explains the improved catalytic properties (selectivity and stability) of PdZn catalysts for the MSR reaction.

Density functional theory study on the adsorption and decomposition of H2O on clean and oxygen-modified Pd (1 0 0) surface

Based on density functional theory together with periodic slab models, the adsorption and the corresponding dehydrogenation reaction of H2O on clean and oxygen modified Pd (1 0 0) have been investigated. The preferential sites for H2O, OH, O, and H were obtained on the surfaces. According to the optimized structural and energetic properties, it was found that H2O prefers to adsorb on the top site with weak adsorption energy (physisorption), whereas O and H atoms are prone to adsorb on the hollow site and OH occupies the bridge site. In addition, this work identified the optimum configurations for the relevant co-adsorption groups. The results confirmed that co-adsorption tends to weaken the adsorbate-substrate interaction due to the existence of oxygen atom, and that the OH group, O and H atoms are less stable on oxygen-covered Pd (1 0 0) surface than on the clean surface. Finally, the transition states and related barrier energies were ascertained to analyze the dehydrogenation mechanism of H2O. Water decomposition was found favorable on O-covered Pd (1 0 0) surface (0.49eV), in agreement with the experimental observations. This result indicated that the joining of Oads could reduce the barrier energy and facilitate the decomposition of H2O. Besides, the distinct differences over Pd (1 1 1) and Pd (1 0 0) surface implied that water decomposition over Pd-based catalysts is a structure-sensitive reaction.

Oxidation of Silanes to Silanols on Pd Nanoparticles: H2 Desorption Accelerated by Surface Oxygen Atom

The oxidation of silane to silanol on the clean and oxygen-covered Pd(111) surface is investigated with periodic density functional theory calculations to gain a better understanding of the effect of surface oxygen atom on Pd nanoparticle catalysts. The calculations confirmed that this catalytic reaction is initiated by the dissociative adsorption of silane on the Pd surface. The resultant silyl group is attacked by a water molecule to form silanol and an H atom on the Pd surface with inversion of configuration at the Si center. An activation energy of 11.3 kcal/mol is required for the water addition, and the transition state for this step is energetically highest in the entire reaction profile. These computational results are in good agreement with our stereochemical and kinetic studies. The H atoms on the Pd surface inhibit further reaction, and therefore, they should be removed to achieve the catalytic activity experimentally. We found that the role of the surface oxygen atom is to facilitate the desorption of H2 from the Pd surface without the formation of OH and H2O. The introduction of surface oxygen atoms can enhance the catalytic ability of metal nanoparticles for green organic reactions.

Surface Oxygen Atom as a Cooperative Ligand in Pd Nanoparticle Catalysis for Selective Hydration of Nitriles to Amides in Water: Experimental and Theoretical Studies

On the basis of an insight in surface science that Pd surfaces partially covered with oxygen adatoms (Oad) show higher reactivity for water dissociation than clean Pd surfaces, we studied the effec...

Surface Oxygen‐Assisted Pd Nanoparticle Catalysis for Selective Oxidation of Silanes to Silanols

Just add O(2): Based on the fact that an oxygen-adsorbed Pd metal surface shows higher reactivity for water dissociation than a clean Pd surface, carbon-supported Pd nanoparticles (NPs) with surface oxygen atoms were developed as a highly effective and reusable heterogeneous catalyst for selective oxidation of silanes to silanols with water as a green oxidant (see figure).

The decarbonylation of acetaldehyde on Pd crystals and on supported catalysts

The reaction of acetaldehyde with pure Pd (in single crystal form, Pd(1 1 0) and Pd(1 1 1)) and with nanoparticles of Pd supported on silica have been investigated. The main reactions involved are decarbonylation (to produce CO and methane) and dehydrogenation (to produce CO, H 2 and carbon), and both of these reactions are seen on the unsupported and supported forms of Pd. In general the behaviour is similar, but there is a marked contrast in the temperature dependence of the reaction on single crystals and on the catalysts. On the crystals there is a sharp transition between decarbonylation and dehydrogenation, occurring at 420 K, whereas the catalysts show little activity for dehydrogenation, even up to 600 K. The reason for this relates to the capacity of the samples to absorb the carbon product of dehydrogenation into the bulk of the sample. The single crystal samples have a very low surface:bulk ratio and C formed at the surface is quickly lost to the bulk above 420 K, leaving an apparently clean and reactive Pd surface. The catalysts on the other hand, due to their high surface:bulk ratio, quickly become carbidised and are thereby passivated towards dehydrogenation, favouring high selectivity to decarbonylation. This proves that Pd catalysts are dehydrogenation catalysts and are only more selective to other routes when passivated by bulk carbon (or other additives such as Au).

Reactivity of crotonaldehyde and propene over Au/Pd(111) surfaces

The surface chemistry of crotonaldehyde and propene, primary and secondary reaction products in the aerobic selective oxidation of crotyl alcohol, has been studied by temperature-programmed reaction over Au/Pd(111) surface alloys. Gold strongly promotes desorption versus reaction at mole fractions > or = 0.3 (crotonaldehyde) and > or = 0.8 (C(3)H(6)); only approximately 5% of the chemisorbed aldehyde or alkene react over Au-rich alloys. Surprisingly, co-adsorbed oxygen strongly suppresses crotonaldehyde decomposition over both clean Pd(111) and alloy surfaces, while C(3)H(6) combustion, an important undesired side-reaction over unpromoted Pd(111), is also moderated by Au.

Structure and Reaction Pathways of Methyl Pyruvate on Pd(111)

The surface chemistry of methyl pyruvate is studied on a clean Pd(111) surface by using a combination of temperature-programmed desorption (TPD) and reflection−absorption infrared spectroscopy (RAIRS). It is found that methyl pyruvate adsorbs molecularly at low temperatures with an orientation that changes depending on coverage. At low coverages, it adsorbs with its molecular plane parallel to the surface, while as the coverage increases it bonds perpendicular to the surface in a trans conformation. The perpendicular form desorbs molecularly at ∼205 K, while a portion of the flat-lying form desorbs at between 270 and 300 K, and the reminder thermally decomposes. This appears to take place via an initial C−OCH3 bond scission to form adsorbed pyruvate species that decompose at higher temperature to form acetyl and acyl species along with ethylidyne.