CO Adlayer Research Articles

Computational demystification of iron carbonyls formation under syngas environment

Iron pentacarbonyl (IPC) gas forms upon the reaction of carbon monoxide with Fe containing metallic surfaces under gas reforming conditions. IPC formation can sometimes reach alarming levels that cause metal loss, pipeline thinning corrosion, catalyst poisoning, and contamination of sensitive industrial equipment. In this work, we demystify using multiscale computational modeling the mechanism of Iron pentacarbonyl formation: Density functional theory (DFT) is used to explore various catalytic reactions that involve a Fe adatom reacting with adsorbed carbon monoxide. Our calculated carbonyls desorption barriers on a perfect and clean Fe surface are too high to allow the carbonyls to form then desorb at temperatures <500 K at the rates reported experimentally. Most importantly, our calculations indicate that a high CO surface coverage, in addition to the presence of Fe adatoms, favors carbonyl formation and its desorption towards the flowing gas medium. Using insights extracted from ab initio molecular dynamics simulations, we propose that the most plausible IPC formation mechanism consists of: (1) on surface reactions of adsorbed CO molecules with an Fe adatom to form iron tricarbonyl (Fe(CO)3*) molecules; (2) an adsorbate assisted movement of iron tricarbonyl on top of the CO adlayer; and (3) the interaction of iron tricarbonyl with CO molecules from the gaseous medium eventually leading to iron adatom removal as Fe(CO)5 gas.

Investigations of mechanism, surface species and support effects in CO hydrogenation over Rh

The Rh-catalyzed CO hydrogenation to hydrocarbons and C2-oxygenates over Rh/ZrO2 and Rh/SiO2 was studied. Catalytic reaction tests show a support effect with a faster steady state reaction rate over Rh/ZrO2 compared to Rh/SiO2. Temperature programmed hydrogenation (TPH) experiments reveal that the CO dissociation on the metal surface is rate limiting, and the support effect thus accelerates the CO dissociation on the metal. Combined TPH and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies after low-temperature CO pre-adsorption reveal a H-assisted C-O bond breakage through CH3O species on the Rh surface. Transient measurements indicate that this mechanism is also likely to be in operation during the steady state reaction at higher temperatures, although here the methoxide is too short-lived to be detected at steady state. This hydrogen-assisted CO activation can help to explain that previous studies have observed an inverse H/D isotope effect for Rh despite CO dissociation being the rate limiting step. TPH studies show that both CO pre-adsorption at 30 ℃ and CO/H2 exposure at 250 ℃ lead to so high CO coverages that it restricts the CO activation, which only starts once part of the CO has desorbed. The near-complete CO coverage on the working Rh surface is restricting the rate, but is essential for the selectivity towards oxygenates. Studies of acetaldehyde conversion in various atmospheres over Rh/SiO2 and Rh/ZrO2 catalysts show that acetaldehyde decomposes over a bare Rh surface with a rate that greatly exceeds the oxygenate formation rate in CO hydrogenation. In the presence of CO the acetaldehyde decomposition is strongly inhibited. The high CO coverage on the surface of the working Rh catalysts thus prevents oxygenate decomposition and is therefore essential for the ability to produce oxygenated products. The reaction temperature is observed to play a role for the establishment of the high coverage. During exposure to CO/H2 at reaction temperatures (>200 ℃) an activated process occurs whereby both the stability and coverage of the CO adlayer increases. This activated stabilization of the CO adlayer shifts the CO activation up in temperature in a subsequent TPH. The results contribute to a fundamental understanding of the reaction mechanism and support effects in the Rh-catalyzed CO hydrogenation, which can assist the formulation of improved Rh-based catalysts. The results, such as the identification of a H-assisted mechanism of CO hydrogenenation via methoxide, could also be of general relevance for the understanding of CO hydrogenation over other metals.

Electrochemical In Situ Video-STM Studies of the Phase Transition of CO Adlayers on Pt(111) Electrodes

The potential-dependent (2 × 2)-3CO → (√19 × √19)R23.4°-13CO phase transition of CO adlayers on Pt(111) in a 0.1 M H2SO4 electrolyte containing minor amounts of CO was studied on the subsecond time...

Study on the influence of the thickness of nanostructured rhodium films toward electrooxidation of adsorbed carbon monoxide

Carbon-supported rhodium thin films with various thicknesses are grown by varying the number of laser pulses, Nlp (from 5000 to 50,000) under 2 Torr of He background atmosphere using the pulsed laser deposition method. The thin films are characterized for their morphological features and structural properties. It is observed that smooth dense 2D nanosheets of Rh are obtained with 5000 Nlp, whereas higher Nlp yields to highly porous Rh films. X-ray photoelectron spectroscopy reveals that both metallic Rh and Rh3+ are present at the surface of the Rh films. Nevertheless, the amount of metallic Rh increased as the Nlp increased. The influence of the thickness of the Rh films on the tolerance to CO, a poison that strongly adsorbs on the surface of catalysts in fuel cells reactions, was investigated through anodic stripping voltammetry. It is found that Rh film deposited with 50,000 Nlp exhibits the highest electroactive surface area (27.90 cm2 vs. 1.20 cm2 of Pt), the lowest onset potential of CO electrooxidation (0.47 V vs. 0.60 V Pt), and a net charge corresponding to electrooxidation of the CO adlayer of 424 μC cm−2 (vs. 358 μC cm−2 of Pt). Such high tolerance to CO poisoning is explained on the basis of the elevated porosity and roughness of the Rh surface. Indeed, a roughness factor of 90.5 for Rh against about 3.8 for Pt grows under analogous deposition conditions of pressure and number of laser pulses.

Electrochemical study of carbon dioxide reduction at copper–palladium nanoparticles: Influence of the bimetallic composition in the CO poisoning tolerance

A study of the CO2 electrochemical reduction reaction (CO2RR) at Pd100−xCux solid solution type nanoalloys is presented. Cyclic voltammetry and chronoamperometry are used in combination with a cavity microelectrode (CME) for the first time. Low-distorted voltammetric signals obtained with the CME evidence that for alloys with low or moderate Cu content (up to 50 at%) the typical CO2RR inhibition peak, related to CO blocking Pd active sites, becomes a current plateau. Interestingly, the plateau height matches the theoretical value corresponding to the steady state current of a microdisc. This fact indicates a better tolerance of Pd100−xCux towards CO poisoning and also the attainment of a pseudo-steady state where mass transport should be controlled by diffusion. The synergy between Cu and Pd is thus established for the first time at the level of the voltammograms. The chronoamperometric responses exhibit three well defined regions corresponding to (1) double layer charging and diffusion/reaction establishment, (2) beginning of CO poisoning by formation of a CO adlayer and (3) final collapse of the catalytic activity at a critical time, tc, which depends directly on the bimetallic composition. A plot of tc vs. Pd content exhibits a clear volcano shape, from which ~ Pd72Cu28 is identified as a close to optimal composition in terms of CO tolerance and Pd35Cu65 as the composition from which the advantage of alloying Pd with Cu is lost. An in-depth analysis of the I/V response and its evolution with composition and time is performed in the CO2RR and CO stripping regions. As a result, the possibility of developing a method to further increase the catalyst lifetime based on periodical releasing of “CO-like” intermediates by CO stripping is proposed.

The Dynamic Nature of CO Adlayers on Pt(111) Electrodes.

CO adlayers on Pt(111) electrode surfaces are an important electrochemical system and of great relevance to electrocatalysis. The potential‐dependent structure and dynamics of these adlayers are complex and still controversial, especially in the CO pre‐oxidation regime. We here employ in situ high‐speed scanning tunneling microscopy for studying the surface phase behavior in CO‐saturated 0.1 m H2SO4 on the millisecond time scale. At potentials near the onset of CO pre‐oxidation local fluctuations in the (2×2)‐CO adlayer are observed, which increase towards more positive potentials. Above 0.20 V (vs. Ag/AgCl), this leads to an adlayer where COad apparently reside on every top site, but still exhibit a (2×2) superstructure modulation. We interpret this observation as a dynamic effect, caused by a small number of highly mobile point defects in the (2×2)‐CO adlayer. As shown by density functional theory calculations, the CO lattice near such defects relaxes into a local (1×1) arrangement, which can rapidly propagate across the surface. This scenario, where a static (2×2) COad sublattice coexists with a highly dynamic sublattice of partially occupied top sites, explains the pronounced COad surface mobility during electrooxidation.

The Dynamic Nature of CO Adlayers on Pt(111) Electrodes

AbstractCO adlayers on Pt(111) electrode surfaces are an important electrochemical system and of great relevance to electrocatalysis. The potential‐dependent structure and dynamics of these adlayers are complex and still controversial, especially in the CO pre‐oxidation regime. We here employ in situ high‐speed scanning tunneling microscopy for studying the surface phase behavior in CO‐saturated 0.1 m H2SO4 on the millisecond time scale. At potentials near the onset of CO pre‐oxidation local fluctuations in the (2×2)‐CO adlayer are observed, which increase towards more positive potentials. Above 0.20 V (vs. Ag/AgCl), this leads to an adlayer where COad apparently reside on every top site, but still exhibit a (2×2) superstructure modulation. We interpret this observation as a dynamic effect, caused by a small number of highly mobile point defects in the (2×2)‐CO adlayer. As shown by density functional theory calculations, the CO lattice near such defects relaxes into a local (1×1) arrangement, which can rapidly propagate across the surface. This scenario, where a static (2×2) COad sublattice coexists with a highly dynamic sublattice of partially occupied top sites, explains the pronounced COad surface mobility during electrooxidation.

Vacancy dynamics on CO-covered Pt(111) electrodes.

In situ video-STM studies of Pt(111) electrodes in CO-saturated 0.1 M H2SO4 solution are presented, which reveal the presence of defined point defects in the CO pre-oxidation regime, where the surface is covered by a highly dynamic apparent (1 × 1)-CO adlayer. These defects are generated at the Pt steps and can switch between a mobile and an immobile state. They are assigned to diffusing vacancies in the Pt surface layer, induced by interaction of the highly mobile CO adsorbates with Pt surface atoms at potentials as low as 0.30 VAg/AgCl.

Monitoring of CO Binding Sites on Stepped Pt Single Crystal Electrodes in Alkaline Solutions by in Situ FTIR Spectroscopy.

The site geometry preference of CO binding on stepped Pt single crystals in alkaline solution was investigated by in situ FTIR spectroscopy. The surfaces of the Pt single crystals consisted of different width (111) terraces, interrupted by (110) or (100) monatomic steps. Experiments carried out with CO adsorbed exclusively on the top of the steps revealed that only linearly bonded CO formed on the (110) steps, while two CO binding geometries (linear and bridge) were observed on the (100) steps. On one hand, for CO adsorbed only on the steps, the positions of the bands corresponding to linearly bonded CO were similar, regardless of the density of steps, suggesting the existence of an interaction between COads only along the line of the steps. On the other hand, for full CO coverage, the CO stretching frequencies and the geometry of bound CO were sensitive to the width of the (111) terraces and the step orientations. Consequently, the CO binding sites favored linearly bonded CO for surfaces consisting of shorter (111) terraces and (110) steps. Bridge-bonded CO was favored on surfaces consisting of shorter (111) terraces interrupted by (100) steps. In order to understand the origin of the preference of CO binding sites, the results were compared to the corresponding behavior in acid media, which revealed that, in addition to the effect inherent to the Pt surface, the charge on the metal side in an aqueous environment should be taken into consideration. The analysis suggested that the CO adlayers formed at full coverage in acidic and alkaline media had different structures. On the other hand, the structure of the layer of CO adsorbed only at the steps was independent of pH.

Facile Fischer–Tropsch Chain Growth from CH2 Monomers Enabled by the Dynamic CO Adlayer

Numerous chain growth mechanisms, namely CO insertion and carbide, and active sites (flat and stepped surfaces) have been proposed to explain how hydrocarbons are formed from syngas during the Fischer–Tropsch reaction on Ru catalysts, particularly active and selective toward long-chain products. While these reaction pathways are supported by density functional theory (DFT) calculations, computational models often considered surfaces at rather low adsorbate coverage. A systematic comparison of chain growth mechanisms including the CO adlayer present on the catalyst’s surface under reaction conditions is therefore not available due to the challenging representation of co-adsorbate interactions in DFT models. Here, we show that the high coverage of chemisorbed CO on the metal surface favors the carbide mechanism on flat surfaces according to ab initio molecular dynamics simulations, which introduce the complex adlayer effects at the reaction temperature of 200 °C. At the considered CO and H coverages (0.50–0.72 and 0.24 monolayer, respectively) hydrocarbon formation involves CH2 monomers yielding ethylene and propylene as primary products, consistent with the selectivity observed in experiments. Such mechanism is favored by the presence of the CO adlayer. Indeed, in the absence of co-adsorbed CO, methane may be formed on the flat surface and the first C–C bond occurs preferentially on stepped surfaces via CH and CH2 monomers with a higher free-energy barrier (55 kJ mol–1) compared to the coupling of two CH2 species at high CO coverage on the flat surface (20 kJ mol–1). Therefore, the CO adlayer strongly modulates the nature of chain growth monomers and active sites and drives the formation of hydrocarbons during Ru-catalyzed Fischer–Tropsch. Overall, these results show how adsorbate–adsorbate interactions dictate reaction mechanisms operating in adlayers, ubiquitous in heterogeneous catalysis.

Elucidating the structure and dynamics of CO ad-layers on MgO surfaces.

The combination of quantum-mechanical simulations and infrared absorption spectroscopy measurements provides a clear picture for a long standing puzzle in surface science: the actual structure and vibrational dynamics of the low-temperature ordered CO monolayer adsorbed on (001) MgO surfaces. The equilibrium structure of the commensurate (4 × 2) adsorbed phase consists of three CO molecules per primitive cell (surface coverage of 75%) located at two inequivalent sites: one molecule seats upright on top of a Mg site while two molecules, tilted off the normal to the surface, are symmetrically positioned relative to the upright one with anti-parallel projections on the surface. This configuration, long believed to be incompatible with measured polarization infrared spectra, is shown to reproduce all observed spectral features, including a new, unexpected one: the vanishing anharmonicity of CO stretching modes in the monolayer.

Dynamic nanoscale imaging of enriched CO adlayer on Pt(111) confined under h-BN monolayer in ambient pressure atmospheres

Fundamental understanding of chemistry confined to nanospace remains a challenge since molecules encapsulated in confined microenvironments are difficult to be characterized. Here, we show that CO adsorption on Pt(111) confined under monolayer hexagonal boron nitride (h-BN) can be dynamically imaged using near ambient pressure scanning tunneling microscope (NAP-STM) and thanks to tunneling transparency of the top h-BN layer. The observed CO superstructures on Pt(111) in different CO atmospheres allow to derive surface coverages of CO adlayers, which are higher in the confined nanospace between h-BN and Pt(111) than those on the open Pt surface under the same conditions. Dynamic NAP-STM imaging data together with theoretical calculations confirm confinement-induced molecule enrichment effect within the 2D nanospace, which reveals new chemistry aroused by the confined nanoreactor.

Absence of diffuse double layer effect on the vibrational properties and oxidation of chemisorbed carbon monoxide on a Pt(111) electrode

In this work we investigate the effects of the diffuse double layer thickness on the electrochemical Stark tuning and oxidation of carbon monoxide at Pt(111) surfaces in perchloric acid solution. The diffuse double layer thickness was modified by changing the concentration (ionic strength) of the supporting electrolyte. The Stark tuning slope of the adsorbed CO was evaluated with Fourier Transformed Infrared Spectroscopy, and the CO oxidation was monitored with cyclic voltammetry. The results show that both electrochemical Stark tuning and oxidation are independent of the HClO4 concentration of the supporting electrolyte, revealing the absence of diffuse layer effects on the aqueous Pt(111)/CO system. By comparison to previously reported theoretical calculations, we attribute this insensitivity to the special double layer structure of Pt(111)/CO, in which the potential drop occurs primarily between the terminating oxygen of the adsorbed CO adlayer and first water layer of the electrolyte, making the properties of adsorbed CO nearly independent of the ionic strength of the electrolyte.

Underpotential deposition of Nickel on platinum single crystal electrodes

The initial stages of nickel electrodeposition on platinum single crystal electrodes have been investigated using cyclic voltammetry and CO charge displacement. While at pH=1, nickel deposition is not visible in the voltammogram, regardless of the crystal orientation, at buffered solutions with pH>3, nickel presence in solution produces clear voltammetric peaks around 0.35V RHE on Pt(111) and Pt(110). Conversely, no clear voltammetric peaks are observed at any of the studied pHs on Pt(100). CO charge displacement experiments suggest that deposited nickel is in the form of a nickel hydroxide. Coulometric analysis is used to deduce the stoichiometry of the deposited adlayer. Moreover, the effect of nickel presence on CO oxidation has been investigated and the CO adlayer characterised using FTIR reflection absorption spectroscopy.

Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces.

High CO* coverages lead to rates much higher than Langmuirian treatments predict because co-adsorbate interactions destabilize relevant transition states less than their bound precursors. This is shown here by kinetic and spectroscopic data-interpreted by rate equations modified for thermodynamically nonideal surfaces-and by DFT treatments of CO-covered Ru clusters and lattice models that mimic adlayer densification. At conditions (0.01-1 kPa CO; 500-600 K) which create low CO* coverages (0.3-0.8 ML from in situ infrared spectra), turnover rates are accurately described by Langmuirian models. Infrared bands indicate that adlayers nearly saturate and then gradually densify as pressure increases above 1 kPa CO, and rates become increasingly larger than those predicted from Langmuir treatments (15-fold at 25 kPa and 70-fold at 1 MPa CO). These strong rate enhancements are described here by adapting formalisms for reactions in nonideal and nearly incompressible media (liquids, ultrahigh-pressure gases) to handle the strong co-adsorbate interactions within the nearly incompressible CO* adlayer. These approaches show that rates are enhanced by densifying CO* adlayers because CO hydrogenation has a negative activation area (calculated by DFT), analogous to how increasing pressure enhances rates for liquid-phase reactions with negative activation volumes. Without these co-adsorbate effects and the negative activation area of CO activation, Fischer-Tropsch synthesis would not occur at practical rates. These findings and conceptual frameworks accurately treat dense surface adlayers and are relevant in the general treatment of surface catalysis as it is typically practiced at conditions leading to saturation coverages of reactants or products.

Electrochemical CO Oxidation at Platinum on Carbon Studied through Analysis of Anomalous in Situ IR Spectra.

The oxidation of adsorbed CO is a key reaction in electrocatalysis. It has been studied extensively on both extended model surfaces and on nanoparticles; however, correlation between the two is far from simple. Molecular insight into the reaction is often provided using in situ IR spectroscopy; however, practical challenges mean in situ studies on nanoparticles have yet to provide the same level of detail as those on model surfaces. Here we use a new approach to in situ IR spectroscopy to study the mechanism of CO adlayer oxidation on a commercial carbon-supported Pt catalyst. We observe bipolar IR absorption bands but develop a simple model to enable fitting. Quantitative analysis of band behavior during the oxidation prepeak using the model agrees well with previous analysis based on conventional absorption bands. A second linear CO band is observed during the main oxidation region and is assigned to the distinct contribution of CO on step as opposed to terrace sites. Analysis of the step and terrace CO bands during oxidation shows that oxidation begins on the terraces of the nanoparticles before CO on steps is removed. Further correlation of this behavior with the current shows that step CO is only lost in the first of the two main oxidation peaks.

Effect of pH and Alkali Cations on Hydroxide Adsorption on Ir, Pt, and Au (111)

Adsorbed hydroxide is an active intermediate or spectator which can block active sites in many electrocatalytic reactions, including CO oxidation [1, 2], methanol oxidation [3], hydrogen oxidation [4], and oxygen reduction [5, 6]. Therefore, understanding the thermodynamics of hydroxide adsorption are important for designing highly active catalysts for these reactions. Experimental measurements, namely cyclic voltammograms, have shown that pH can alter the onset potential and coverage of hydroxide adsorption on Pt(111) [7]. Recent work further shows that the identity of the alkali metal cation (Li, Na, K, Cs) present in an alkaline electrolyte may also impact the thermodynamics of hydroxide adsorption [8, 9]. We use density functional theory to examine the adsorption of hydroxide onto Ir(111), Pt(111), and Au(111) in the absence (acid) and presence (alkaline) of near-surface or adsorbed alkali metal cations. The predicted hydroxide adsorption potentials compare well with experiment. On Ir(111) and Pt(111), the presence of an alkali metal cation in a high pH electrolyte weakens low coverage hydroxide adsorption and strengthens high coverage adsorption. This acts to shift the onset of hydroxide adsorption to more positive potentials, increase the adsorbed hydroxide coverage, and sharpen the experimentally measured cyclic voltammetric peak in an alkaline electrolyte, relative to an acid electrolyte. We can further capture the major differences between the alkali metal cations, including the trend in hydroxide adsorption potential on Ir(111) (Li<Na<K<Cs) and lower peak potential of hydroxide adsorption in LiOH relative to the other alkali metal cations on Pt(111). On Au(111), the coverage dependence of hydroxide adsorption is significantly different than on Ir(111) and Pt(111), and the presence of a near-surface cation may have a slight promoting effect on low coverage hydroxide adsorption. These results are important for understanding the effects of pH and alkali cation on many electrocatalytic reactions which involve adsorbed hydroxide as either a spectator species which blocks active sites or an active intermediate. [1] J. S. Spendelow, G. Q. Lu, P. J. A. Kenis, and A. Wieckowski, "Electrooxidation of adsorbed CO on Pt(1 1 1) and Pt(1 1 1)/Ru in alkaline media and comparison with results from acidic media," Journal of Electroanalytical Chemistry, vol. 568, pp. 215-224, 2004. [2] N. P. Lebedeva, M. T. M. Koper, J. M. Feliu, and R. A. van Santen, "Mechanism and kinetics of the electrochemical CO adlayer oxidation on Pt(111)," Journal of Electroanalytical Chemistry, vol. 524–525, pp. 242-251, 2002. [3] S. Wasmus and A. Küver, "Methanol oxidation and direct methanol fuel cells: a selective review," Journal of Electroanalytical Chemistry, vol. 461, pp. 14-31, 1999. [4] D. Strmcnik, M. Uchimura, C. Wang, R. Subbaraman, N. Danilovic, V. van der, et al., "Improving the hydrogen oxidation reaction rate by promotion of hydroxyl adsorption," Nat Chem, vol. 5, pp. 300-306, 2013. [5] N. Markovic, H. Gasteiger, and P. N. Ross, "Kinetics of Oxygen Reduction on Pt(hkl) Electrodes: Implications for the Crystallite Size Effect with Supported Pt Electrocatalysts," Journal of The Electrochemical Society, vol. 144, pp. 1591-1597, 1997. [6] H. A. Gasteiger and P. N. Ross, "Oxygen Reduction on Platinum Low-Index Single-Crystal Surfaces in Alkaline Solution: Rotating Ring DiskPt(hkl) Studies," The Journal of Physical Chemistry, vol. 100, pp. 6715-6721, 1996. [7] G. Garcia and M. T. M. Koper, "Stripping voltammetry of carbon monoxide oxidation on stepped platinum single-crystal electrodes in alkaline solution," Physical Chemistry Chemical Physics, vol. 10, pp. 3802-3811, 2008. [8] StrmcnikD, KodamaK, D. van der Vliet, GreeleyJ, V. R. Stamenkovic, and N. M. Marković, "The role of non-covalent interactions in electrocatalytic fuel-cell reactions on platinum," Nat Chem, vol. 1, pp. 466-472, 2009. [9] C. Stoffelsma, P. Rodriguez, G. Garcia, N. Garcia-Araez, D. Strmcnik, N. M. Marković, et al., "Promotion of the Oxidation of Carbon Monoxide at Stepped Platinum Single-Crystal Electrodes in Alkaline Media by Lithium and Beryllium Cations," Journal of the American Chemical Society, vol. 132, pp. 16127-16133, 2010.

Nonuniform Synergistic Effect of Sn and Ru in Site-Specific Catalytic Activity of Pt at Bimetallic Surfaces toward CO Electro-oxidation

This work investigates the mechanisms by which Sn and Ru can improve or inhibit the site-specific catalytic activity of Pt (neighboring or distant from foreign atoms) at bimetallic surfaces. For this purpose, we decorated Pt stepped surfaces (nonequivalent sites) by site-selective electrodeposition of different coverages of either Sn or Ru on (110) Pt steps, forming Snsteps/Pt(hkl) and Rusteps/Pt(hkl) bimetallic surfaces, and we used CO adlayer electro-oxidation as a surface probe reaction, monitored by in situ FTIR and cyclic voltammetry techniques. The results showed that both Sn and Ru selectively accelerated the reaction pathway of CO electro-oxidation only at the (111) Pt terrace sites but importantly played different underlying roles in favoring activity at these active sites. In the case of Snsteps/Pt(hkl) catalysts, the CO adlayer oxidation started at potentials lower than those on Rusteps/Pt(hkl), but Sn only improved the activity at sites on atoms of the first rows of (111) Pt terraces, while th...

Mobility and Oxidation of Adsorbed CO on Shape-Controlled Pt Nanoparticles in Acidic Medium.

The knowledge about how CO occupies and detaches from specific surface sites on well-structured Pt surfaces provides outstanding information on both dynamics/mobility of COads and oxidation of this molecule under electrochemical conditions. This work reports how the potentiostatic growth of different coverage CO adlayers evolves with time on both cubic and octahedral Pt nanoparticles in acidic medium. Data suggest that during the growth of the CO adlayer, COads molecules slightly shift toward low coordination sites only on octahedral Pt nanoparticles, so that these undercoordinated sites are the first filled on octahedral Pt nanoparticles. Conversely, on cubic Pt nanoparticles, adsorbed CO behaves as an immobile species, and low coordinated sites as well as (100) terraces are apparently filled uniformly and simultaneously. However, once the adlayer is complete, irrespectively of whether the CO is oxidized in a single step or in a sequence of different potential steps, results suggest that COads behaves as an immobile species during its oxidation on both octahedral and cubic Pt nanoparticles.

Site-specific catalytic activity of model platinum surfaces in different electrolytic environments as monitored by the CO oxidation reaction

Stepped Pt surfaces having different width (111) terraces interrupted by (110) or (100) monoatomic steps were employed to evaluate the catalytic activity toward CO oxidation at Pt specific sites in HClO4, H2SO4, and H3PO4 solutions, as well as in phosphate buffer and alkaline solution. The catalytic activity at the (111) terraces was sensitive to the nature of the anions derived from the electrolyte dissociation, while no effect on activity was detected at the monoatomic steps. A change in solution pH, passing from acid to alkaline solutions, had contrasting effects on catalytic activity at the (111) terraces and the step sites, with the catalytic activity of the (111) terraces improving, while activity at the step sites deteriorated. During the CO adlayer oxidation, the release of surface sites occurred preferentially from the (111) terraces of the Pt(s)−[(n−1)(111)×(110)] surfaces, while from Pt(s)−[(n)(111)×(100)] surfaces, (111) terrace and (100) step sites were released simultaneously.