FTIR study of CO adsorption on Pd/SiO2 and Pd-Cu/SiO2 treated with sulphur dioxide

Infrared spectra are reported of Pd/SiO₂ and Pd-Cu/SiO₂ exposed at 295 K to either CO followed by SO₂ or SO₂ followed by CO. Chemisorptive interaction of SO₂ with bridging CO sites on Pd {100} faces created cationic Pd sites which adsorbed CO linearly. Multibonding CO sites on Pd {111} faces and linear CO sites were poisoned by weak non-dissociative SO₂ adsorption, and could be regenerated by evacuation. Pd/SiO₂ was more resistant than Cu/SiO₂ to SO₂ poisoning but for Pd-Cu/SiO₂ the poisoning of Pd was promoted by the presence of Cu. This revised version was published online in July 2006 with corrections to the Cover Date.     

Methyl formation from methanol decomposition on Pd{111} and Pt{111}

The decomposition of CH₃OH adsorbed on Pd{111} and Pt{111} is compared as the surface is heated between 100 and 500 K. Using secondary ion mass spectrometry (SIMS) and thermal programmed desorption (TPD) it is suggested that an anomalous CH ₃ ⁺ ion signal observed previously by Akhter and White on oxygen precovered Pt{111} arises from the formation of a surface CH₃ species resulting from activation of the C-O bond of CH₃OH. This interpretation stems from a recent observation by Levis, Zhicheng and Winograd that CH₃OH decomposes to CH₃, OH and OCH₃ on clean Pd{111} between 100 and 300 K. The results are discussed in terms of the relative ability of these metals to synthesize CH₃OH from CO and H₂.     

Study of the low-temperature reaction between CO and O2 over Pd and Pt surfaces

Field electron microscopy (FEM), high-resolution electron energy loss spectroscopy (HREELS), molecular beams (MB) and temperature-programmed reaction (TPR) have been applied to the study of the kinetics of CO oxidation at low temperature, and to determine the roles of subsurface atomic oxygen (O_sub) and surface reconstruction in self-oscillatory phenomena, on Pd(111), Pd(110) and Pt(100) single crystals and on Pd and Pt tip surfaces. It was found that high local concentrations of adsorbed CO during the transition from a Pt(100)-hex reconstructed surface to the unreconstructed 1×1 phase apparently prevents oxygen atoms from occupying hollow sites on the surface, and leads to the appearance of a weakly bound active adsorbed atomic oxygen (O_ads) state in an on-top or bridge position. It was also inferred that subsurface oxygen O_sub on the Pd(110) surface may play an important role in the formation of new active sites for the weakly bound O_ads atoms. Experiments with ¹⁸O isotope labeling clearly show that the weakly bound atomic oxygen is the active form of oxygen that reacts with CO to form CO₂ at T 140–160 K. Sharp tips of Pd and Pt, several hundreds angstroms in diameter, were used to perform in situ investigations of dynamic surface processes. The principal conclusion from those studies was that non–linear reaction kinetics is not restricted to macroscopic planes since: (i) planes as small as 200 Å in diameter show the same non-linear kinetics as larger flat surfaces; (ii) regular waves appear under conditions leading to reaction rate oscillations; (iii) the propagation of reaction–diffusion waves involves the participation of different crystal nanoplanes via an effective coupling between adjacent planes.     

Enhanced catalytic activity of ceria nanorods from well-defined reactive crystal planes

The crystal plane of ceria plays an essential role in determining its catalytic oxidation properties. In this study, single-crystalline CeO₂ nanorods with well-defined crystal planes have been synthesized by a facile solution-based hydrothermal method. HRTEM studies reveal that the predominantly exposed planes are the unusually reactive {001} and {110} in the CeO₂ nanorods rather than the stable {111} in the irregular nanoparticles. Consequently, it is demonstrated that the CeO₂ nanorods are more reactive for CO oxidation than their counterparts, irregular nanoparticles. The present results indicate that catalysts with well-defined reactive sites may be “designed” because of the recent development of morphology-controlled synthesis of nanostructured materials.     

Characterization by CO/FTIR spectroscopy of Pd/silica catalysts and its correlation with syn‐gas conversion

Well‐dispersed Pd catalysts, supported on two morphologically different silicas (meso‐ and microporous Davison G59 and G03 grades, respectively) and used for syn‐gas activation at T=493–523 K and P=1–4 MPa (CO/H₂= 1/2.5), have been studied by CO chemisorption using FTIR spectroscopy. The long‐term exposure to 760 Torr CO(g) at 298 K produces deep changes on the surface structure of Pd particles on both supports. The Pd particles become rougher and/or show more open crystal planes. This phenomenon of surface restructuring seems to depend both on the exposed metal fraction (FE) of palladium and the morphology of the support. The rate of surface restructuring but not its extent, is a function of the superimposed CO(g) pressure. On the microporous G03 silica CO chemisorbs in multicoordinated or “hollow” sites (H band), but these signals are not shown by preparations of the supported metal of comparable dispersity on mesoporous G59 grade. Terminal (L band) and di‐coordinated CO_s (B band) appear in both types of catalysts. The high‐loading preparations on the microporous support showed a higher proportion of Pd(111) planes than those of low Pd loading, which seems to contribute to the high TOF_CH4 and high selectivity to methane in syn‐gas activation of these catalysts, while the remaining ones show excellent capability for methanol production. This revised version was published online in July 2006 with corrections to the Cover Date.     

Screening of Oligopeptides that Recognize Inorganic Crystalline Facets of Metal Nanoparticles

Peptides that possess specific affinity to distinct crystal facets have been reported previously. However, their adsorption behavior in terms of the crystal sizes and shapes is less exploited. Herein, we isolate several phage clones that show the strong affinity to {100} of Pd at a neutral pH from the M13 phage library, and among them the phages that have shape selectivity to the cubic structure are identified by eliminating ones that bind randomly shaped Pd nanoparticles (NPs). Since Pd nanocube-binding phages are eluted by lowering pH values in the biopanning process, the selected phages (and their binding peptides displayed on protein pIII) can be released from Pd surfaces through pH changes. We used this feature to modulate the capping density of selected peptides on NPs. For example, when less peptides are capped on Pd nanocubes by lowering the pH values, the shape of the nanocubes is deformed and some evolve into a concave shape, indicating that Pd atoms are released from the less protected {100} facet selectively due to the higher surface energy. This type of crystalline facet-recognizing peptides can be applied for smart capping agents that not only bind target crystalline planes, but also modify their coverage on the specific surfaces with pH changes. The peptide-capping agents could be useful to fabricate NPs with characteristic shapes through etching and adsorption of atoms on specific crystalline planes of seed nanocrystals.     

Morphology effects of Co3O4 on the catalytic activity of Au/Co3O4 catalysts for complete oxidation of trace ethylene

Au/Co₃O₄ catalysts with different morphologies (nanorods, nanopolyhedra and nanocubes) were successfully synthesized and evaluated for ethylene complete oxidation. We found that support morphology has a significant effect on catalytic activity, which is related to the exposed planes of different morphological Co₃O₄. HRTEM revealed the Co₃O₄-nanorods predominantly exposes {110} planes, while the dominant exposed planes of Co₃O₄-nanopolyhedra and -nanocubes are {011} and {001} planes, respectively. Compared with {011} and {001} planes, {110} planes exhibit the maximum amount of oxygen vacancies, which play a major role in ethylene oxidation. Therefore, Au/Co₃O₄-nanorods exhibits extraordinary catalytic activity, yielding 93.7% ethylene conversion at 0 °C.     

Synthesis gas production in methane conversion using the P|yttria-stabilized zirconia|Ag electrochemical membrane system

The electrochemical membrane reactor of YSZ (yttria-stabilized zirconia) solid electrolyte coated with Pd and Ag as anode and cathode, respectively, has been applied to the partial oxidation of methane to synthesis gas (CO + H₂). The Pd|YSZ|Ag catalytic system has shown a remarkable activity for CO production at 773 K, and the selectivity to CO was quite high (96.3%) under oxygen pumping condition at 5 mA. The H₂ production strongly depended on the oxidation state of the Pd anode surface. Namely, the H₂ treatment of the Pd anode at 773 K for 1 h drastically reduced the rate of H₂ production, while air treatment enhanced the H₂ production rate. From the results of the partial oxidation of CH₄ with molecular oxygen, it is considered that the reaction site of the electrochemical oxidation of CH₄ to synthesis gas was the Pd–YSZ–gas-phase boundary (triple-phase boundary). In addition, it is found that the oxygen species pumped electrochemically over the Pd surface demonstrated similar activity to adsorbed oxygen over Pd, PdO_ad, for the selective oxidation of CH₄ to CO, when the Pd supported on YSZ was used as a fixed-bed catalyst for CH₄ oxidation with the adsorbed oxygen. The difference with respect to the H₂ formation between the electrochemical membrane system and the fixed-bed catalyst reactor results from differences in the average particle size of Pd and the way of the oxygen supply to the Pd surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of CHO during interaction of CO and H2 on alumina-supported Pd catalysts

The reaction of CO and H₂ on alumina-supported Pd catalysts (2, 5 and 10 wt% Pd) has been studied by a transient pulse technique in vacuum (Temporal-Analysis-of-Products (TAP) reactor). CHO species were formed on a hydrogen-treated catalyst surface at temperatures between 423 and 530 K by the interaction of hydrogen solved in the palladium metal and CO (adsorbed at 300 K after reduction). The CHO species were released into the gas phase when hydrogen was pulsed over the catalysts. CHO is assumed to be an intermediate in the hydrogenation of CO to CH₄ starting at 560 K. No methanol was observed under the experimental conditions applied.     

Favorable synergetic effects between CuO and the reactive planes of ceria nanorods

High-energy, more reactive {001} and {110} planes of CeO₂ nanorods were found to generate favorable synergetic effects between CuO and ceria, resulting in significant enhancement of the copper catalyst performance for CO oxidation.     

Selective hydrogenation of cyclopentadiene over supported metal catalysts

The selective hydrogenation of cyclopentadiene to cyclopentene has been studied in the liquid phase using Pd and Pd Me/Al₂O₃ bimetallic catalysts (Me = Mn, Ni, Co, W). The highest activity was obtained with Pd Co and Pd W/Al₂O₃. For these catalysts, no hydrogen or CO chemisorption was detected although Pd could be seen by XPS at 335·8 eV; it is considered that new species, more active for the selective hydrogenation of cyclopentadiene, appeared at the catalyst surface. The sulfur resistance towards thiophene has also been studied. It was observed that the highest sulfur resistance is coincident with the highest activity. XPS analysis shows that the poisoning species is thiophene adsorbed on the catalyst surface.     

The Surface Chemistry of Acetic Acid on Pd{111}

XPS, temperature-programmed reaction and HREELS have been used to study the adsorption and reactions of acetic acid on Pd{111}. At 170 K the adsorbed monolayer contains intact and dissociated acetic acid molecules, the latter consisting of a mixture of bidentate acetate and another species tentatively identified as monodentate acetate. The monodentate acetate appears to resemble closely the acetate species observed under reaction conditions at the surface of a pure palladium vinyl acetate synthesis catalyst. Thermal decomposition of the adsorbate yields CO₂, H₂O, CO, H₂ and carbon. The associated processes may be rationalised in terms of two reaction channels, one due to the monodentate and the other due to the bidentate acetate.     

The Effect of CO2 and H2O on the Kinetics of NO Reduction by CH4 Over Sr-Promoted La2O3

The influence of CO₂ and H₂O on the activity of 4% Sr-La₂O₃ mimics that observed with pure La₂O₃, and a reversible inhibition of the rate is observed. CO₂ causes a greater effect, with decreases in rate of about 65% with O₂ present and 90% in its absence, while with H₂O in the feed, the rate decreased around 35-40% with O₂ present or absent. The influence of these two reaction products on kinetic behavior can be described by assuming competitive adsorption on the surface, incorporating adsorbed CO₂ and H₂O in the site balance, and using rate expressions previously proposed for this reaction over Sr-promoted La₂O₃. In the absence of O₂, the rate expression is $$r_{N_2 } = \frac{{k'P_{{\text{NO}}} P_{{\text{CH}}_{\text{4}} } }}{{{\text{(1 + }}K_{{\text{NO}}} P_{{\text{NO}}} {\text{ + }}K_{{\text{CH}}_{\text{4}} } P_{{\text{CH}}_{\text{4}} } {\text{ + }}K_{{\text{CO}}_{\text{2}} } P_{{\text{CO}}_{\text{2}} } {\text{ + }}K_{{\text{H}}_{\text{2}} {\text{O}}} P_{{\text{H}}_{\text{2}} {\text{O}}} {\text{)}}^{\text{2}} }},$$ which yields a good fit to the experimental data and gives optimized equilibrium adsorption constants that demonstrate thermodynamic consistency. With O₂ in the feed, nondifferential changes in reactant concentrations through the reactor bed were accounted for by assuming integral reactor behavior and simultaneously considering both CH₄ combustion and CH₄ reduction of NO, which provided the following rate law for total CH₄ disappearance: $$(r_{{\text{CH}}_{\text{4}} } )_{\text{T}} = \frac{{k'_{{\text{com}}} P_{{\text{CH}}_{\text{4}} } P_{{\text{O}}_{\text{2}} }^{{\text{0}}{\text{.5}}} + k'_{{\text{NO}}} P_{{\text{NO}}} P_{{\text{CH}}_{\text{4}} } P_{{\text{O}}_{\text{2}} }^{{\text{0}}{\text{.5}}} }}{{{\text{(1 + }}K_{{\text{NO}}} P_{{\text{NO}}} {\text{ + }}K_{{\text{CH}}_{\text{4}} } P_{{\text{CH}}_{\text{4}} } {\text{ + }}K_{{\text{O}}_{\text{2}} }^{{\text{0}}{\text{.5}}} P_{{\text{O}}_{\text{2}} }^{{\text{0}}{\text{.5}}} {\text{ + }}K_{{\text{CO}}_{\text{2}} } P_{{\text{CO}}_{\text{2}} } {\text{ + }}K_{{\text{H}}_{\text{2}} {\text{O}}} P_{{\text{H}}_{\text{2}} {\text{O}}} {\text{)}}^{\text{2}} }}.$$ The second term of this expression represents N₂ formation, and it again fit the experimental data well. The fitting constants in the denominator, which correspond to equilibrium adsorption constants, were not only thermodynamically consistent but also provided entropies and enthalpies of adsorption that were similar to values obtained with other La₂O₃-based catalysts. Apparent activation energies typically ranged from 23 to 28 kcal/mol with O₂ absent and 31-36 kcal/mol with O₂ in the feed. With CO₂ in the feed, but no O₂, the activation energy for the formation of a methyl group via interaction of CH₄ with adsorbed NO was determined to be 35 kcal/mol.     

Imaging and chemical probing on the atomic scale: reconstruction and dynamics of the systems O2/Rh and NO2–H2/Pt

This paper presents two case studies of adsorbate-induced surface reconstruction, on the one hand, and dynamical reaction imaging along with local chemical probing, on the other hand. The first one deals with the oxygen-induced reshaping of 3D Rh crystals. Field ion microscopy (FIM) was applied to image in real-space the change from a nearly hemispherical shape in the absence of oxygen toward a polyhedral one in the presence of oxygen. Shape transformation occurs at temperatures of 380–550 K and is associated with the appearance of facets with {111} and {001} orientation. The only high-index planes present in the polyhedral form are of {137} symmetry. (1×2) and (1×3) missing-row reconstructions appear in the {113} and {011} planes. The polyhedral form has also been imaged under in situ conditions of the oxygen–hydrogen reaction on Rh at 505 K. The second case study deals with kinetic non-linearities occurring in the NO₂ reaction with hydrogen on the surface of a 3D Pt crystal reconstructed to a top- and edge-truncated pyramid. The reaction was found to ignite in the {012} corner planes of the crystal. One-dimensional wavefronts were subsequently observed to move along the 211 zone lines. These studies were performed by video-FIM and could be correlated with a local chemical analysis by time-of-flight mass spectrometry of ionised species. The mass spectrum provided information on water product (H₂O⁺ and H₃O⁺) and NO intermediate formation. Strong fluctuations in the NO ₂ ⁺ current indicated the occurrence of NO₂ surface diffusion. These species are most likely responsible for the field ion image formation.     

Effect of metal dispersion on CO hydrogenation over Pd/HZSM-5 catalysts

Pd/HZSM-5 catalysts prepared by ion-exchange method using Pd(NH₃) ₄ ²⁺ were calcined and reduced at different temperatures to provide different metal dispersions. The effect of Pd dispersion on CO adsorption characteristics and acidity were observed through FT-IR study. Methanol and dimethyl ether were the main products in CO hydrogénation over Pd/HZSM-5 catalyst with small Pd particles on which CO was weakly adsorbed, while the selectivity to methane increased with metal sizes.     

Intrazeolite Pd clusters: particle location

A chemical vapour deposition (CVD) procedure has been adopted for the preparation of Pd/NaY, resulting in a high dispersion of the metal phase, characterised by a bimodal distribution of particle size. The most abundant particles (80%) are about 25 Å in size, corresponding to almost twice the dimension of the zeolitic supercages. To identify their location inside or outside the zeolite matrix, the IR spectra of adsorbed CO, obtained before and after admission of NH₃, have been compared. The results obtained are in sharp contrast with those for a Pd/SiO₂ system, where Pd particles of comparable size are exclusively located on the external surface of the carrier. These differences support the conclusion that Pd particles in Pd/NaY are indeed located in the zeolitic cavities.     

Adsorption and reaction of CO on vanadium oxide–Pd(111) “inverse” model catalysts: an HREELS study

The room temperature adsorption and reaction of CO on Pd(111) surfaces decorated with submonolayer coverages of vanadium oxide – so-called inverse model catalysts – have been studied by high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The HREELS surface phonon spectra of the V oxide phases have been measured and used to monitor the changes in the oxide as a result of the interaction with CO. The intramolecular C–O stretching frequency of CO adsorbed on the V-oxide/Pd(111) surfaces displays two vibrational loss components as a function of CO coverage as it has been observed on the clean Pd(111) surface. The relative intensities of the two vibrational features as a function of V oxide coverage however suggest that the balance of CO adsorption sites is modified as compared to clean Pd(111) by the presence of the V oxide–Pd phase boundary. Preferential population of high coordination adsorption sites by CO in the vicinity of the oxide–metal interface is proposed. The analysis of the V oxide phonon spectra indicates that adsorbed CO partially reduces the V oxide at the boundaries of the oxide islands to the Pd metal. The reduction of V oxide by CO is dependent on the oxygen content of the V oxide phase. The reduction of V oxide is confirmed by the XPS V 2p core level shifts.     

Shape Control of Pt Nanoparticles

A preparative method to control the shape of platinum (Pt) nanoparticles in the presence of sodium polyacrylate (PAA) or poly(N-vinyl-2-pyrrolidone) (PVP) is described. Regardless of the kind of protective polymer used, the dominant shape of Pt nanoparticles was controlled by changing the reduction rate of Pt⁴⁺ ions. Tetrahedral particles predominated by using H₂ reduction of H₂[PtCl₆]. Methanol reduction generated mainly truncated octahedral particles. It seems that the slow H₂ reduction of Pt⁴⁺ ions favorably leads to the formation of tetrahedral Pt nuclei enclosed with four {111} planes that have the lowest surface energy. The truncated octahedral nuclei are formed by the faster reduction with methanol. The selective growth of the {111} planes takes place at a lower polymer concentration and results in the generation of cubic nanoparticles.     

Size-controlled synthesis and impedance-based mechanistic understanding of Pd/C nanoparticles for formic acid oxidation

This work provides a detailed electrochemical impedance study for formic acid electro-oxidation on size-controlled Pd/C nanoparticles, the synthesis of which was done by a simple protocol using ethylene glycol as a reducing agent. By controlling KOH concentration, this strategy provides a synthesis method for Pd nanoparticles with a selective size range of 3.9–7.5 nm. The as-prepared Pd nanoparticles exhibited size-dependent electrochemical property and electrochemical characterizations of four different Pd/C nanocatalysts (3.9, 5.2, 6.1, and 7.5 nm) showed that Pd particle with average size of 6.1 nm has the highest formic acid oxidation activity. Electrochemical impedance-based characterizations of formic acid oxidation on Pd/C suggested that at high potentials the adsorbed oxygen species could block the catalyst surface and inhibit the oxidation reaction, as reflected by the negative polarization resistance. Unlike Pd/C, the intermediate adsorbed CO species (CO_ads) plays a critical role for formic oxidation on Pt/C and thus the impedance spectra of Pd/C and Pt/C appear different potential-dependent patterns in the second quadrant. The issue of CO was investigated by an impedance investigation of Pd/C in a mixture of formic acid containing dissolved CO.     

Interaction Between Thiourea and Palladium Electrodes

Interction of thiourea (THU) with Pd electrodes (geometrical surface area 1.5 cm²,roughness factor $\text{f}\text{?}$_w =3-600) has been studied at 20°C. The roughness factor was measured by krypton adsorption. The interaction occurs by the following steps:(1) reversible adsorption of THU molecules on the surface of Pd electrode according to a simple Langmuir isotherm, in neutral medium and (2) surface hydrolosis of adsorbed THU molecules followed by the formation of a monolayer of strongly adsorbed sulphur on the surface of the Pd electrode in acidic or alkaline medium. The THU molecules are precursors of the adsorbed sulphur and an equivalence between their amounts has been found. The adsorbed sulphur has been determined by anodic oxidation by linear sweep voltammetry in 1 M NaOH. Brief comments on the interaction of Pd electrodes with other sulphur compounds (Na₂S, H₂S, CS₂, cysteine and cystine) are also given.