Experimental study of intermediates and wave phenomena in co oxidation on platinum metal (Pt,Pd) surfaces

The mechanism of catalytic CO oxidation on Pt(100) and Pd(110) single-crystal surfaces and on Pt and Pd sharp tip (～10³ Å) surfaces has been studied experimentally by temperature-programmed reaction, temperature desorption spectroscopy, field electron microscopy, and molecular beam techniques. Using the density functional theory the equilibrium states and stretching vibrations of oxygen atoms adsorbed on the Pt(100) surface have been calculated. The character of the mixed adsorption layer was established by high resolution electron energy loss spectroscopy—molecular adsorption (O_2ads, CO_ads) on Pt(100)-hex and dissociative adsorption (O_ads, CO_ads) on Pt(100)-(1×1). The origin of kinetic self-oscillations for the isothermal oxidation of CO in situ was studied in detail on the Pt and Pd tips by field electron microscopy. The initiating role of the reversible phase transition (hex) ? (1 × 1) of the Pt(100) nanoplane in the generation of regular chemical waves was established. The origination of self-oscillations and waves on the Pt(100) nanoplane was shown to be caused by the spontaneous periodical transition of the metal from the low-active state (hex) to the highly active catalytic state (1 × 1). A relationship between the reactivity of oxygen atoms (O_ads) and the concentration of CO_ads molecules was revealed for the Pd(110) surface. Studies using the isotope label ¹⁸O_ads demonstrated that the low-temperature formation of CO₂ at 150 K is a result of the reaction of CO with the highly reactive state of atomic oxygen (O_ads). The possibility of the low-temperature oxidation of CO via interaction with the so-called “hot” oxygen atoms (O_hot) appearing on the surface at the instant of dissociation of O_2ads molecules was studied by the molecular beam techniques.     

Changes in the surface structure of Pd/Ta2O5 by oxygen and CO studied using X‐ray Photoelectron Spectroscopy (XPS)

Behaviors of Pd structures with different thicknesses supported by Ta₂O₅/Ta in the reaction with oxygen and CO were studied by XPS and SEM. For the samples with a Pd thickness of 3 nm, a new low‐binding‐energy component appeared in the Pd 3d level upon O₂ exposure at ～200 °C and was reduced in intensity after a subsequent CO exposure at 150 and 200 °C. The change in the Ta 4f state could also be found upon oxygen and CO exposure, indicating that both Pd and the Ta‐oxide substrate participate in the chemical reactions. For the sample with a higher Pd thickness, a positive shift in the Pd 3d level due to the oxidation of Pd was observed after exposure to O₂ at a higher temperature (280 °C). A subsequent CO exposure at ～150 °C could not reduce Pd‐oxide layers, as confirmed by the unchanged Pd 3d spectra after CO treatment, i.e. Pd‐oxide was not reactive for CO oxidation. Copyright © 2010 John Wiley & Sons, Ltd.     

Adsorption of carbon monoxide on Pd low‐index surfaces and (110) reconstructed surface

Adsorption and diffusion of carbon monoxide on Pd low‐index surfaces and missing‐row Pd (110) reconstructed surface have been investigated by the extended London–Eyring–Polyani–Sato (LEPS) method constructed by means of a five‐parameter Morse potential. All critical characteristics, such as adsorption site, adsorption geometry, binding energy, CO vibrational frequency have been obtained and compared with the experimental and theoretical data. On these surfaces, the stable adsorption sites of CO are changed with increasing CO coverage. On the missing‐row Pd (110) reconstructed surface, there are five stable adsorption sites: H₁, H₂ (H₁ and H₂ are threefold hollow sites on (111) subsurface), B (bridge site on the second layer), SB (short‐bridge site), and T (top site). Copyright © 2010 John Wiley & Sons, Ltd.     

Pd(Cu)在MgO(100)表面吸附CO和O2的理论研究

采用固体镶嵌势能模型和DFT/B3LYP方法研究了在Pd/MgO和Cu/MgO表面吸附CO和O₂分子的电子性质. 计算结果表明, 在完美MgO(100)表面Pd原子对CO和O₂的吸附能分别为206.5和84.8 kJ/mol, 因此可知Pd原子更容易吸附CO分子; 而当Pd原子附着于有氧缺陷的MgO表面时, 它对两种分子的吸附都非常弱. 相反, 附着于MgO表面的Cu原子对O₂分子的吸附更为有利, 其吸附能在140～155 kJ/mol之间. 研究结果还表明, 对于双分子吸附体系, 即CO＋CO, CO＋O₂, O₂＋O₂体系, 双分子之间的结合力可减小完美MgO表面上Pd原子与被吸附分子的相互作用, 使吸附能减少了46～96 kJ/mol. 而对于在MgO表面上的Cu原子, 只有O₂＋O₂ 体系使吸附能减少了大约50～71 kJ/mol.     

The Adsorption of Oxygen and Coadsorption of CO and Oxygen on Structurally Well‐Defined PdAg Surface Alloys

The dissociative interaction of oxygen with structurally well‐defined monolayer Pd_xAg_1?x/Pd(111) surface alloys of different compositions, with well‐known distributions of the respective surface atoms (A. K. Engstfeld et al., Phys. Chem. Chem. Phys. 2012 , 14, 10754–10761), and the coadsorption of/reaction with CO on oxygen pre‐covered surfaces were studied by high‐resolution electron energy loss spectroscopy (HREELS) and temperature‐programmed desorption/reaction spectroscopy (TPD/TPR). The impact of geometric ensemble effects as well as electronic ligand and strain effects on the adsorption and reaction behaviour of the respective species on the bimetallic surfaces is elucidated and compared with related systems such as CO adsorption on similar surfaces and oxygen adsorption on a Pd₆₇Ag₃₃(111) bulk alloy surface. The data show a clear dominance of ensemble effects on the oxygen adsorption and CO coadsorption behaviour, with oxygen adsorption limited to threefold‐hollow sites on Pd₃ sites, while the combined electronic effects, as evident from modifications in the adsorption and reaction characteristics on the Pd sites, are small.     

Three-way catalytic converter reactions aspects at near-ambient temperatures on modified Pd-surfaces

The dissolution of oxygen in palladium plays an important role in palladium catalysis. The present study shows that the surface modification (SM) due to the dissolution of atomic oxygen into the subsurfaces of palladium can be used as a control to tune its catalytic activity. CO oxidation and NO + H₂ + O₂ reaction was separately carried out on metallic Pd and on surface modified Pd using a molecular beam instrument and the results were compared. The metallic Pd does not show activity below 400 K for both reactions, whereas the SM-Pd shows activity at near-ambient temperatures. The electronic change due to SM was investigated using ambient pressure photoelectron spectroscopy, and the investigation clearly shows the effect of subsurface oxygen in the ambient temperature activity of palladium.     

CO Oxidation by Neutral Gold-Vanadium Oxide Clusters

Oxidation of CO by gas-phase atomic clusters is being actively studied to understand the molecular-level mechanisms of heterogeneous CO oxidation over related catalytic surfaces. However, it is experimentally challenging to study CO oxidation by neutral heteronuclear metal oxide clusters because of the difficulty of cluster ionization and detection without fragmentation. Herein, the neutral AuVO_2-4 clusters were experimentally generated and their reactions with CO and O₂ were studied. The experimental results showed that CO adsorption is the dominant channel on the interactions of AuVO₄ and AuVO₃ with CO, and AuVO₂ can pick up an O₂ molecule to generate AuVO₄. Theoretical studies indicated that the oxidation of the trapped CO in AuVO_3,4CO into CO₂ is exothermic while the reaction barriers have to be overcome at the elevated temperatures. A catalytic cycle for CO oxidation by AuVO_2-4 is proposed.     

Structure Sensitivity of CO Oxidation on Co3O4: A DFT Study

The reaction mechanism of CO oxidation on the Co₃O₄ (110) and Co₃O₄ (111) surfaces is investigated by means of spin‐polarized density functional theory (DFT) within the GGA+U framework. Adsorption situation and complete reaction cycles for CO oxidation are clarified. The results indicate that 1) the U value can affect the calculated energetic result significantly, not only the absolute adsorption energy but also the trend in adsorption energy; 2) CO can directly react with surface lattice oxygen atoms (O_2f/O_3f) to form CO₂ via the Mars–van Krevelen reaction mechanism on both (110)‐B and (111)‐B; 3) pre‐adsorbed molecular O₂ can enhance CO oxidation through the channel in which it directly reacts with molecular CO to form CO₂ [O₂(a)+CO(g)→CO₂(g)+O(a)] on (110)‐A/(111)‐A; 4) CO oxidation is a structure‐sensitive reaction, and the activation energy of CO oxidation follows the order of Co₃O₄ (111)‐A(0.78 eV)>Co₃O₄ (111)‐B (0.68 eV)>Co₃O₄ (110)‐A (0.51 eV)>Co₃O₄ (110)‐B (0.41 eV), that is, the (110) surface shows higher reactivity for CO oxidation than the (111) surface; 5) in addition to the O_2f, it was also found that Co³⁺ is more active than Co²⁺, so both O_2f and Co³⁺ control the catalytic activity of CO oxidation on Co₃O₄, as opposed to a previous DFT study which concluded that either Co³⁺ or O_2f is the active site.     

Key Role of a-Top CO on Terrace Sites of Metallic Pd Clusters for CO Oxidation

Pd-based catalysts are the most widely used for CO oxidation because of their outstanding catalytic activity and thermal stability. However, fundamental understanding of the detailed catalytic processes occurring on Pd-based catalysts under realistic conditions is still lacking. In this study, we investigated CO oxidation on metallic Pd clusters supported on Al₂O₃ and SiO₂. High-angle annular dark-field scanning transmission electron microscopy revealed the formation of similar-sized Pd clusters on Al₂O₃ and SiO₂. In contrast, CO chemisorption analysis indicated a gradual change in the dispersion of Pd (from 0.79 to 0.2) on Pd/Al₂O₃ and a marginal change in the dispersion (from 0.4 to 0.24) on Pd/SiO₂ as the Pd loading increased from 0.27 to 5.5 wt %; these changes were attributed to differences in the metal-support interactions. Diffuse reflectance infrared Fourier-transform spectroscopy revealed that fewer a-top CO species were present in Pd supported on Al₂O₃ than those in Pd supported on SiO₂, which is related to the morphological differences in the metallic Pd clusters on these two supports. Despite the different dispersion profiles and surface characteristics of Pd, O₂ titration demonstrated that linearly bound CO (with an infrared signal at 2090 cm⁻¹) reacted first with oxygen in the case of CO-saturated Pd on Al₂O₃ and SiO₂, which suggests that a-top CO on the terrace site plays an important role in CO oxidation. The experimental observations were corroborated by periodic density functional calculations, which confirmed that CO oxidation on the (111) terrace sites is most plausible, both kinetically and thermodynamically, compared to that on the edge or corner sites. This study will deepen the fundamental understanding of the effect of Pd clusters on CO oxidation under reaction conditions.     

Low‐Temperature Oxidation of Carbon Monoxide with Gold(III) Ions Supported on Titanium Oxide

Au/TiO₂ catalysts prepared by a deposition–precipitation process and used for CO oxidation without previous calcination exhibited high, largely temperature‐independent conversions at low temperatures, with apparent activation energies of about zero. Thermal treatments, such as He at 623 K, changed the conversion–temperature characteristics to the well‐known S‐shape, with activation energies slightly below 30 kJ mol^?1. Sample characterization by XAFS and electron microscopy and a low‐temperature IR study of CO adsorption and oxidation showed that CO can be oxidized by gas‐phase O₂ at 90 K already over the freeze‐dried catalyst in the initial state that contained Au exclusively in the +3 oxidation state. CO conversion after activation in the feed at 303 K is due to Au^III‐containing sites at low temperatures, while Au⁰ dominates conversion at higher temperatures. After thermal treatments, CO conversion in the whole investigated temperature range results from sites containing exclusively Au⁰.     

Site- and surface species-dependent propylene oxidation with molecular oxygen on gold surface

A complete fundamental understanding of propylene oxidation with molecular O₂ on Au surface is achieved, in which site-and surface species-dependent reaction behaviors are revealed.     

Isothermal kinetic study of nitric oxide adsorption and decomposition on Pd(111) surfaces: molecular beam experiments

The kinetics of NO adsorption and dissociation on Pd(111) surfaces and the NO sticking coefficient (s(NO)) were probed by isothermal kinetic measurements between 300 and 525 K using a molecular beam instrument. NO dissociation and N2 productions were observed in the transient state from 425 K and above on Pd(111) surfaces with selective nitrogen production. Maximum nitrogen production was observed between 475 and 500 K. It was found that, at low temperatures, between 300 and 350 K, molecular adsorption occurs with a constant initial s(NO) of 0.5 until the Pd(111) surface is covered to about 70-80% by NO. Then s(NO) rapidly decreases with further increasing NO coverage, indicating typical precursor kinetics. The dynamic adsorption - desorption equilibrium on Pd(111) was probed in modulated beam experiments below 500 K. CO titration experiments after NO dosing indicate the diffusion of oxygen into the subsurface regions and beginning surface oxidation at > or = 475 K. Finally, we discuss the results with respect to the rate-limiting character of the different elementary steps of the reaction system.     

Thermal characterization of titania-modified alumina-supported palladium and catalytic properties for methane combustion

Thermal characterization of Pd/TiO₂-Al₂O₃ catalysts under oxygen or hydrogen atmosphere was investigated by means of thermogravimetry (TG), differential scanning calorimetry (DSC), temperature programmed reduction (TPR) and temperature programmed desorption (TPD). Further, the effects of titania on the catalytic property of Pd/Al₂O₃ towards methane combustion were also examined. A TG-DSC studies revealed that the heat evolved during oxygen adsorption at 25 °C depended on the crystallite diameter. TPR and TPD studies of oxidized samples also demonstrated that the coating of Pd/Al₂O₃ catalysts with titania can weaken the strength of the PdO bonds. Apparently the effect of coating Pd/Al₂O₃ catalyst with TiO₂ is the improvement of the methane combustion at lower temperatures. This is probably due to the decrease of the strength of the PdO bonds.     

Catalytic activity of small supported Pd/Al2O3 particles: CO oxidation

Thermo-progammed desorption and catalytic CO oxidation has been used to investigate the influence of the Pd/Al₂O₃ particle size on the CO adsorption and CO and O₂ reactive sticking coefficient. Both CO molecule saturation density and reactive sticking coefficients increase with average particle diameter decreasing.     

Highly Ordered and Thermally Stable FeRh Cluster Superlattice on Graphene for Low-Temperature Catalytic CO Oxidation

We report on bimetallic FeRh clusters with a narrow size-distribution grown on graphene on Ir(111) as a carbon-supported model catalyst to promote low-temperature catalytic CO oxidation. By combining scanning tunneling microscopy with catalytic performance measurements, we reveal that Fe−Rh interfaces are active sites for oxygen activation and CO oxidation, especially at low temperatures. Rh core Fe shell clusters not only provide the active sites for the reaction, but also thermally stabilize surface Fe atoms towards coarsening compared with pure Fe clusters. Alternate isotope-labelled CO/O₂ pulse experiments show opposite trends on preferential oxidation (PROX) performance because of surface hydroxyl species formation and competitive adsorption between CO and O₂. The present results introduce a general strategy to stabilize metallic clusters and to reveal the reaction mechanisms on bimetallic structures for low-temperature catalytic CO oxidation as well as preferential oxidation.     

Enhanced catalytic performance of Pd‐Ga bimetallic catalysts for 2‐ethylanthraquinone hydrogenation

A series of Pd and Pd‐Ga bimetallic catalysts were prepared by a co‐impregnation method for 2‐ethylanthraquinone (EAQ) hydrogenation to produce hydrogen peroxide. Compared with 0.6Pd catalyst, the hydrogenation efficiency of 0.6Pd1.2Ga catalyst (11.9 g L^?1) increases by 32.2%, and the stability of 0.6Pd1.2Ga catalyst is also higher than that of 0.6Pd catalyst. The structures of the samples were determined by N₂ adsorption–desorption, ICP, XRD, CO chemisorption, TEM, H₂‐TPR, in situ CO‐DRIFTS and XPS. The results suggest that incorporation of Ga species improves Pd dispersion and generates a strong interaction between Ga₂O₃ and Pd interface or between Pd and support. DFT calculation results indicate that the strong adsorption of carbonyl group on Ga₂O₃/Pd interface facilitates the activation of EAQ and promotes the hydrogenation efficiency.     

Does enhanced oxygen activation always facilitate CO oxidation on gold clusters?

We investigate the catalytic activity of the subnanometer‐sized bimetallic Au₁₉Pt cluster for oxidation of CO via first‐principles density functional theory calculations. For this purpose we consider two structurally similar and energetically close homotops of the Au₁₉Pt cluster with the Pt atom occupying an edge (Td‐E) or a facet (Td‐S) site of a 20‐atom tetrahedron. Using these homotops as catalysts we calculate the complete reaction paths and the thermodynamic functions corresponding to the oxidation of CO to CO₂. It is found that the oxidation of CO on the Td‐S isomer occurs through a smaller reaction barrier (0.38 eV) as compared with that on the Td‐E isomer (0.70 eV), although the activation of O₂ on the latter is much higher than that on the former. Therefore, a clear conclusion is that a higher O₂ activation, which is generally believed to be the key factor for CO oxidation, solely cannot determine the catalytic efficiency of the Au‐Pt bimetallic clusters. In addition, we find a stronger CO adsorption on the Td‐E isomer (2.06 eV) as compared with that on the Td‐S isomer (1.68 eV). Although stronger CO adsorption on the Td‐E isomer leads to a higher O₂ activation; however, high value of CO adsorption energy deteriorates the catalytic activity of the Td‐E isomer towards the CO oxidation reaction. © 2015 Wiley Periodicals, Inc.     

Identification of the Active Sites for Low Temperature CO Oxidation over Nanocrystalline Co3O4 Catalysts

The microstructural properties of dry‐grinding derived Co₃O₄ catalysts pretreated under different atmospheres, in relation to the activities on CO oxidation were investigated. The Co₃O₄ synthesized by soft reactive grinding and pretreated with O₂ resulted in the best activity, with 100% conversion of CO at ?52 °C, superior to that of Co₃O₄ pretreated with He. To find out the active sites on Co₃O₄ for low temperature CO oxidation, the characterizations of the cobalt oxides had been investigated by means of N₂ physisorption, XRD, TEM, H₂‐TPR, CO‐titration, XPS and O₂‐TPD technologies. XPS of Co2p results show that it is difficult to ascribe the difference in catalytic performance to the surface concentration of active Co³⁺ sites. A correlation between the activity and the CO‐titration and O₂‐TPD results for Co₃O₄ reveals that a high abundance of readily accessible superficial electrophilic oxygen (O^?) species is important for achieving a high activity. Therefore, CO oxidation takes place on the surface active oxygen sites in Co₃O₄ crystallites via the suprafacial mechanism.     

Structure of the CO bond on supported Pd particles: influence of size and surface state

We have investigated the surface of supported palladium particles by static secondary ion mass spectrometry (SSIMS). Pd particles were grown in situ on alumi na (oxide layer and sapphire surfaces) and stabilized by heating treatment. The particle size, density and crystallographic structure were determined in previous studies by transmission electron microscopy and diffraction (TEM and TED). Various ionic species are detected by SSIMS analysis which makes it possible to characterize the CO absorbed layer: Pd _n CO⁺ (n=1, 2) for molecular adsorption and Pd _n C⁺ for CO dissociation. The size dependence of the bonding state of CO (linear, bridge, ...) was monitored by: PdCO⁺/σ _n Pd _n CO⁺ signal ratio over various size particles (mean diameter in the 2–9 nm range). Investigations were performed as a function of CO coverage (adsorption at room temperature) and also under CO dissociation conditions: heating under CO atmosphere or CO+O₂ (catalysis). The data analysis shows that on clean Pd particles smaller than 3 nm the CO molecules give rise mainly to PdCO⁺ species. We have interpreted this result by the adsorption of CO on two palladium atoms, the carbon end being tightly bonded to a low coordination Pd atom and the oxygen end weakly bonded to a neighbour Pd atom. These couples of Pd atoms form the specific sites for CO dissociation, the density of which depends on the roughness of the particle surface.     

Synthesis and Performance of Polyvinylpyrrolidone‐Protected Pd Nanoparticles Supported on TiO2MWCNTs under Protection of PVP for in Alcohols Oxidation in Alkaline Media

This paper describes the synthesis and performance of Pd/p‐TiO₂‐MWCNTs for alcohol electrooxidation. Regulation of the steric repulsive forces of polyvinylpyrrolidone was used to achieve even dispersion of Pd nanoparticles over the TiO₂ surfaces. The order of the electroactivities of the Pd/p‐TiO₂? MWCNTs in alcohol oxidation was ethanol>ethylene glycol>methanol. The order of the stabilities of alcohol oxidation by Pd/p‐TiO₂? MWCNTs was ethylene glycol>ethanol>methanol. Electroimpedance spectroscopy showed that at a potential of ?0.1 V, the Pd/p‐TiO₂? MWCNTs had good tolerance of adsorbed CO in ethylene glycol oxidation, and at ?0.2 V, the Pd/p‐TiO₂? MWCNTs had good adsorbed CO tolerance in methanol oxidation.