New findings on CO electrooxidation at platinum nanoparticle surfaces

3D-printed integrative probeheads for magnetic resonance

Magnetic resonance (MR) technology has been widely employed in scientific research, clinical diagnosis and geological survey. However, the fabrication of MR radio frequency probeheads still face difficulties in integration, customization and miniaturization. Here, we utilized 3D printing and liquid metal filling techniques to fabricate integrative radio frequency probeheads for MR experiments. The 3D-printed probehead with micrometer precision generally consists of liquid metal coils, customized sample chambers and radio frequency circuit interfaces. We screened different 3D printing materials and optimized the liquid metals by incorporating metal microparticles. The 3D-printed probeheads are capable of performing both routine and nonconventional MR experiments, including in situ electrochemical analysis, in situ reaction monitoring with continues-flow paramagnetic particles and ions separation, and small-sample MR imaging. Due to the flexibility and accuracy of 3D printing techniques, we can accurately obtain complicated coil geometries at the micrometer scale, shortening the fabrication timescale and extending the application scenarios.

Here, the authors combine 3D printing and liquid metal filling techniques to fabricate customised probeheads for magnetic resonance experiments. They demonstrate in situ electrochemical nuclear magnetic resonance analysis, reaction monitoring with continues-flow separation and small-sample imaging.

PAC Synthesis and Comparison of Catalysts for Direct Ethanol Fuel Cells

Pt/C, PtMOn/C (M = Ni, Sn, Ti, and PtX/C (X = Rh, Ir) catalyst systems were prepared by using the pulse alternating current (PAC) technique. Physical and electrochemical parameters of samples were carried out by x-ray powder diffraction (XRD), transmission electron microscopy (TEM), and CO stripping. The catalytic activity of the synthesized samples for the ethanol electrooxidation reaction (EOR) was investigated. The XRD patterns of the samples showed the presence of diffraction peaks characteristic for Pt, NiO, SnO2, TiO2, Rh, and Ir. The TEM images indicate that the Pt, Rh, and PtIr (alloys) particles had a uniform distribution over the carbon surface in the Pt/C, PtRh/C, PtIr/C, and PtMOn/C (M = Ni, Sn, Ti) catalysts. The electrochemically active surface area of catalysts was determined by the CO-stripping method. The addition of a second element to Pt or the use of hybrid supported catalysts can evidently improve the EOR activity. A remarkable positive affecting shift of the onset potential for the EOR was observed as follows: PtSnO2/C > PtTiO2/C ≈ PtIr/C ≈ PtNiO/C > PtRh/C ≈ Pt/C. The addition of SnO2 to Pt/C catalyst led to the decrease of the onset potential and to significantly facilitate the EOR. The long-term cyclic stability of the synthesized catalysts was investigated. Thereby, the PtSnO2/C catalyst prepared by the PAC technique can be considered as a promising anode catalyst for direct ethanol fuel cells.

Shape-controlled metal nanoparticles for electrocatalytic applications

The application of shape-controlled metal nanoparticles is profoundly impacting the field of electrocatalysis. On the one hand, their use has remarkably enhanced the electrocatalytic activity of many different reactions of interest. On the other hand, their usage is deeply contributing to a correct understanding of the correlations between shape/surface structure and electrochemical reactivity at the nanoscale. However, from the point of view of an electrochemist, there are a number of questions that must be fully satisfied before the evaluation of the shaped metal nanoparticles as electrocatalysts including (i) surface cleaning, (ii) surface structure characterization, and (iii) correlations between particle shape and surface structure. In this chapter, we will cover all these aspects. Initially, we will collect and discuss about the different practical protocols and procedures for obtaining clean shaped metal nanoparticles. This is an indispensable requirement for the establishment of correct correlations between shape/surface structure and electrochemical reactivity. Next, we will also report how some easy-to-do electrochemical experiments including their subsequent analyses can enormously contribute to a detailed characterization of the surface structure of the shaped metal nanoparticles. At this point, we will remark that the key point determining the resulting electrocatalytic activity is the surface structure of the nanoparticles (obviously, the atomic composition is also extremely relevant) but not the particle shape. Finally, we will summarize some of the most significant advances/results on the use of these shaped metal nanoparticles in electrocatalysis covering a wide range of electrocatalytic reactions including fuel cell-related reactions (electrooxidation of formic acid, methanol and ethanol and oxygen reduction) and also CO2 electroreduction.

Graphical Abstract:

Tailoring the morphology of Ni–Pt nanocatalysts through the variation of oleylamine and oleic acid: a study on oxygen reduction from synthesis to fuel cell application

Systematic study on the variation of morphology, size and composition of Ni–Pt nanoparticles with higher catalytic activity towards oxygen reduction.

Atomically-defined model catalysts in ultrahigh vacuum and in liquid electrolytes: particle size-dependent CO adsorption on Pt nanoparticles on ordered Co3O4(111) films

We have studied particle size effects on atomically-defined model catalysts both in ultrahigh vacuum (UHV) and under electrochemical (EC) conditions in liquid electrolytes.

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.

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 CO_ads 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, CO_ads 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 CO_ads behaves as an immobile species during its oxidation on both octahedral and cubic Pt nanoparticles.

Shape-selected nanocrystals for in situ spectro-electrochemistry studies on structurally well defined surfaces under controlled electrolyte transport: A combined in situ ATR-FTIR/online DEMS investigation of CO electrooxidation on Pt

The suitability and potential of shape selected nanocrystals for in situ spectro-electrochemical and in particular spectro-electrocatalytic studies on structurally well defined electrodes under enforced and controlled electrolyte mass transport will be demonstrated, using Pt nanocrystals prepared by colloidal synthesis procedures and a flow cell set-up allowing simultaneous measurements of the Faradaic current, FTIR spectroscopy of adsorbed reaction intermediates and side products in an attenuated total reflection configuration (ATR-FTIRS) and differential electrochemical mass spectrometry (DEMS) measurements of volatile reaction products. Batches of shape-selected Pt nanocrystals with different shapes and hence different surface structures were prepared and structurally characterized by transmission electron microscopy (TEM) and electrochemical methods. The potential for in situ spectro-electrocatalytic studies is illustrated for COad oxidation on Pt nanocrystal surfaces, where we could separate contributions from two processes occurring simultaneously, oxidative COad removal and re-adsorption of (bi)sulfate anions, and reveal a distinct structure sensitivity in these processes and also in the structural implications of (bi)sulfate re-adsorption on the CO adlayer.

Shape-controlled nanostructures in heterogeneous catalysis

Nanotechnologies have provided new methods for the preparation of nanomaterials with well-defined sizes and shapes, and many of those procedures have been recently implemented for applications in heterogeneous catalysis. The control of nanoparticle shape in particular offers the promise of a better definition of catalytic activity and selectivity through the optimization of the structure of the catalytic active site. This extension of new nanoparticle synthetic procedures to catalysis is in its early stages, but has shown some promising leads already. Here, we survey the major issues associated with this nanotechnology-catalysis synergy. First, we discuss new possibilities associated with distinguishing between the effects originating from nanoparticle size versus those originating from nanoparticle shape. Next, we survey the information available to date on the use of well-shaped metal and non-metal nanoparticles as active phases to control the surface atom ensembles that define the catalytic site in different catalytic applications. We follow with a brief review of the use of well-defined porous materials for the control of the shape of the space around that catalytic site. A specific example is provided to illustrate how new selective catalysts based on shape-defined nanoparticles can be designed from first principles by using fundamental mechanistic information on the reaction of interest obtained from surface-science experiments and quantum-mechanics calculations. Finally, we conclude with some thoughts on the state of the field in terms of the advances already made, the future potentials, and the possible limitations to be overcome.

Evidence of an Eley-Rideal mechanism in the stripping of a saturation layer of chemisorbed CO on platinum nanoparticles

The oxidative stripping of a saturation layer of CO(chem) was studied on platinum nanoparticles of high shape selectivity and narrow size distribution. Nanospheres, nanocubes, and nano-octahedrons were synthesized using the water-in-oil microemulsion or polyacrylate methods. The three shapes allowed examination of the CO(chem) stripping in relation to the geometry of the nanoparticles and presence of specific nanoscopic surface domains. Electrochemical quartz crystal nanobalance (EQCN) measurements provided evidence for the existence of more than one mechanism in the CO(chem) stripping. This was corroborated by chronoamperometry transient for a CO(chem) saturation layer at stripping potentials of E(strip) = 0.40, 0.50, 0.60, and 0.70 V. The first mechanism is operational in the case of CO(chem) stripping at lower E(strip) values; it proceeds without adsorption of anions or H(2)O molecules and corresponds to desorption of a fraction of CO(chem) in the form of a prepeak in voltammograms or in the form of an exponential decay in chrono-amperometry (CA) transients. The second mechanism is operational in the desorption of the remaining CO(chem) at higher E(strip) values and gives rise to at least two voltammetric peaks or two CA peaks. Analysis of the experimental data and modeling of the CA transients lead to the conclusion that the stripping of a saturation layer of CO(chem) first follows an Eley-Rideal mechanism in the early stage of the process and then a Langmuir-Hinshelwood mechanism.

Electro-oxidation of CO(chem) on Pt nanosurfaces: solution of the peak multiplicity puzzle

An understanding of the oxidation of chemisorbed CO (CO(chem)) on Pt nanoparticle surfaces is of major importance to fuel cell technology. Here, we report on the relation between Pt nanoparticle surface structure and CO(chem) oxidative stripping behavior. Oxidative stripping voltammograms are obtained for CO(chem) preadsorbed on cubic, octahedral, and cuboctahedral Pt nanoparticles that possess preferentially oriented and atomically flat domains. They are compared to those obtained for etched and thermally treated Pt(poly) electrodes that possess atomically flat, ordered surface domains separated by grain boundaries as well as those obtained for spherical Pt nanoparticles. A detailed analysis of the results reveals for the first time the presence of up to four voltammetric features in CO(chem) oxidative stripping transients, a prepeak and three peaks, that are assigned to the presence of surface domains that are either preferentially oriented or disordered. The interpretation reported in this article allows one to explain all features within the voltammograms for CO(chem) oxidative stripping unambiguously.

The effect of Bi adsorption on CO oxidation inside 1.8 nm Pt pores

Modification of the surface of H(1)-e Pt with Bi causes significant changes in the CO stripping voltammetry; the pre-wave disappears and CO and Bi oxidation peaks appear. The absence of the pre-wave suggests that Bi preferentially adsorbs on the trough sites of the concave 1.8 nm diameter pore walls preventing oxygenated species from nucleating there.

Structure sensitivity and nanoscale effects in electrocatalysis

This review discusses the role of the detailed nanoscale structure of catalytic surfaces on the activity of various electrocatalytic reactions of importance for fuel cells, hydrogen production, and other environmentally important catalytic reactions, such as carbon monoxide oxidation, methanol and ethanol oxidation, ammonia oxidation, nitric oxide reduction, hydrogen evolution, and oxygen reduction. Specifically, results and insights obtained from surface-science single-crystal-based model experiments are linked to experiments on well-defined shape-controlled nanoparticles. A classification of structure sensitive effects in electrocatalysis is suggested, based both on empirical grounds and on quantum-chemical viz. thermochemical considerations. The mutual relation between the two classification schemes is also discussed. The review underscores the relevance of single-crystal modeling of nanoscale effects in catalysis, and points to the special role of two kinds of active sites for electrocatalysis on nanoparticulate surfaces: (i) steps and defects in (111) terraces or facets, and (ii) long-range (100) terraces or facets.