Determination of Kinematic Viscosity of Mg(ClO4)2 and KOH Brines Saturated with CO2 at Sub-Zero Temperatures.

The current race for space exploration has hastened the development of electrochemical technologies for the in-situ utilisation of planetary resources for the synthesis of vital chemicals such as O2 and fuels. Understanding the physicochemical properties, such as the density and kinematic viscosity, of aqueous solutions is essential for the design of electrochemical devices for the electrolysis of water and CO2, particularly at low temperatures. The density and kinematic viscosity of highly concentrated Mg(ClO4)2 and KOH solutions have been determined, both at low temperatures and in the presence of CO2 gas. It was found that, for all of the solutions, independent of the concentration or nature of the electrolyte, as the temperature was decreased to 255 K, the density and the viscosity of the solutions increased. Upon saturation with CO2, no significant change to the density and viscosity of Mg(ClO4)2, at all of the temperatures measured, was observed. Conversely, the CO2 saturated solutions of KOH showed significant changes in density and viscosity at all temperatures, likely due to the formation of carbonates. The effects of these changes on the diffusion coefficient for dissolved CO2 is also discussed.

Investigation on the surface charge separation in Pt-supported morphology-related-TiO2 and its effect on water splitting.

Lowering Pt loading in the catalyst while maintaining its superior catalytic efficiency during hydrogen evolution reaction (HER) is essential for the large-scale application of water splitting. The utilization of strong metal-support interaction (SMSI) through morphology engineering has emerged as an effective strategy in fabricating Pt-supported catalysts. However, a simple and explicit routine to realize the rational design of morphology-related SMSI remains challenging. Here we report a protocol for the photochemical deposition of Pt, which benefits from the intrinsic difference in absorption capability of TiO2 to establish proper Pt+ species and charge separation domains on the surface. With a comprehensive investigation of the surface environment through experiments and Density functional theory (DFT) calculations, charge transfer from Pt to Ti, the separation of electron-hole pairs, and the enhanced electron transfer in the TiO2 matrix were confirmed. It is reported that H2O molecules can be spontaneously dissociated by the surface Ti and O, generating OH stabilized by adjacent Ti and Pt. Such adsorbed OH group induces changes in the electron density of Pt, consequently favours the H adsorption and enhances the HER. Benefiting from the preferable electronic state, the annealed Pt@TiO2-pH9 (PTO-pH9@A) exhibits an overpotential of 30 mV to reach 10 mA cm-2 geo and a mass activity of 3954 A g-1Pt, which is 17-fold higher than the commercial Pt/C. Our work provides a new strategy for the high-efficient catalyst design by the surface state- regulated SMSI.

Electrocatalytic reduction of CO2 on size-selected nanoclusters of first-row transition metal nanoclusters: a comprehensive mechanistic investigation.

Recycling CO2 back to fuels offers an ideal solution to control anthropogenic global CO2 emissions as well as providing a sustainable green solution to alternative energy resources from a cheap and earth-abundant carbon source. Size-selected nanoclusters open a novel area in catalysis as these atomically precise nanoclusters possess unique electronic and catalytic properties different from larger nanoparticles and traditional bulk catalysts. In this work, we have investigated the ability of first-row transition metal nanoclusters (Sc-Cu) of varying sizes (3 to 10 atoms) for CO2 electroreduction (CO2RR). Employing computational hydrogen model (CHE), we have performed detailed analyzes on various CO2RR electrocatalytic reaction pathways on all nanocluster surfaces. We have identified a general trend of decreasing adsorption energies while moving across the periodic table from Sc to Cu. Moreover, we have found a general preference for CHO* mediated pathways over COH* mediated pathways for methane formation. The CHO* mediated pathways prefer the reaction route via CHO* → CH2O* → CH2OH* → CH2* → CH3* → CH4 + * on most of the nanocluster surfaces. In addition, we have established that methanol formation is greatly disfavored on all nanocluster surfaces, and the release of CO and HCOOH is greatly suppressed on all nanoclusters. We have identified several nanoclusters as potential nanocluster-based electrocatalysts for CO2RR for methane formation with relatively lower limiting potential values below 0.50 V. CO2 electroreduction versus hydrogen evolution reaction (HER) competition was also evaluated on various nanoclusters, and we identified a number of nanoclusters (Ti6, V5, V6, Mn4, Mn7, Mn10, Fe4, Fe8, Fe10, Ni4, and Cu5) that can suppress the formation of HER over CO2RR. We have also established a linear scaling relationship between the adsorption free energies of various CO2RR adsorbates to the adsorption free energies of CO2*, O*, and C* adsorbates. We have found that scaling free energy relationships that exit on heterogeneous catalysts such as the correlation between the adsorption energies of AHx with the adsorption energies of atom A (A = C, N, O, S, etc.) often breaks on nanocluster surfaces, especially for adsorbates with more than one binding motifs.

Insight into the Activity and Selectivity of Nanostructured Copper Titanates during Electrochemical Conversion of CO2 at Neutral pH via In Situ X-ray Absorption Spectroscopy.

The electrochemical conversion of carbon dioxide (CO2) to useful chemical fuels is a promising route toward the achievement of carbon neutral and carbon negative energy technologies. Copper (Cu)- and Cu oxide-derived surfaces are known to electrochemically convert CO2 to high-value and energy-dense products. However, the nature and stability of oxidized Cu species under reaction conditions are the subject of much debate in the literature. Herein, we present the synthesis and characterization of copper-titanate nanocatalysts, with discrete Cu-O coordination environments, for the electrochemical CO2 reduction reaction (CO2RR). We employ real-time in situ X-ray absorption spectroscopy (XAS) to monitor Cu species under neutral-pH CO2RR conditions. Combination of voltammetry and on-line electrochemical mass spectrometry with XAS results demonstrates that the titanate motif promotes the retention of oxidized Cu species under reducing conditions for extended periods, without itself possessing any CO2RR activity. Additionally, we demonstrate that the specific nature of the Cu-O environment and the size of the catalyst dictate the long-term stability of the oxidized Cu species and, subsequently, the product selectivity.

High-Throughput Preparation of Metal Oxide Nanocrystals by Cathodic Corrosion and Their Use as Active Photocatalysts.

Nanoparticle metal oxide photocatalysts are attractive because of their increased reactivity and ease of processing into versatile electrode formats; however, their preparation is cumbersome. We report on the rapid bulk synthesis of photocatalytic nanoparticles with homogeneous shape and size via the cathodic corrosion method, a simple electrochemical approach applied for the first time to the versatile preparation of complex metal oxides. Nanoparticles consisting of tungsten oxide (H2WO4) nanoplates, titanium oxide (TiO2) nanowires, and symmetric star-shaped bismuth vanadate (BiVO4) were prepared conveniently using tungsten, titanium, and vanadium wires as a starting material. Each of the particles were extremely rapid to produce, taking only 2-3 min to etch 2.5 mm of metal wire into a colloidal dispersion of photoactive materials. All crystalline H2WO4 and BiVO4 particles and amorphous TiO2 were photoelectrochemically active toward the water oxidation reaction. Additionally, the BiVO4 particles showed enhanced photocurrent in the visible region toward the oxidation of a sacrificial sulfite reagent. This synthetic method provides an inexpensive alternative to conventional fabrication techniques and is potentially applicable to a wide variety of metal oxides, making the rapid fabrication of active photocatalysts with controlled crystallinity more efficient.

Role of the Adsorbed Oxygen Species in the Selective Electrochemical Reduction of CO2 to Alcohols and Carbonyls on Copper Electrodes.

The electrochemical reduction of CO2 into fuels has gained significant attention recently as source of renewable carbon-based fuels. The unique high selectivity of copper in the electrochemical reduction of CO2 to hydrocarbons has called much interest in discovering its mechanism. In order to provide significant information about the role of oxygen in the electrochemical reduction of CO2 on Cu electrodes, the conditions of the surface structure and the composition of the Cu single crystal electrodes were controlled over time. This was achieved using pulsed voltammetry, since the pulse sequence can be programmed to guarantee reproducible initial conditions for the reaction at every fraction of time and at a given frequency. In contrast to the selectivity of CO2 reduction using cyclic voltammetry and chronoamperometric methods, a large selection of oxygenated hydrocarbons was found under alternating voltage conditions. Product selectivity towards the formation of oxygenated hydrocarbon was associated to the coverage of oxygen species, which is surface-structure- and potential-dependent.

Fundamentals, achievements and challenges in the electrochemical sensing of pathogens.

Electrochemical sensors are powerful tools widely used in industrial, environmental and medical applications. The versatility of electrochemical methods allows for the investigation of chemical composition in real time and in situ. Electrochemical detection of specific biological molecules is a powerful means for detecting disease-related markers. In the last 10 years, highly-sensitive and specific methods have been developed to detect waterborne and foodborne pathogens. In this review, we classify the different electrochemical techniques used for the qualitative and quantitative detection of pathogens. The robustness of electrochemical methods allows for accurate detection even in heterogeneous and impure samples. We present a fundamental description of the three major electrochemical sensing methods used in the detection of pathogens and the advantages and disadvantages of each of these methods. In each section, we highlight recent breakthroughs, including the utilisation of microfluidics, immunomagnetic separation and multiplexing for the detection of multiple pathogens in a single device. We also include recent studies describing new strategies for the design of future immunosensing systems and protocols. The high sensitivity and selectivity, together with the portability and the cost-effectiveness of the instrumentation, enhances the demand for further development in the electrochemical detection of microbes.

Noble metal aerogels-synthesis, characterization, and application as electrocatalysts.

CONSPECTUS: Metallic and catalytically active materials with high surface area and large porosity are a long-desired goal in both industry and academia. In this Account, we summarize the strategies for making a variety of self-supported noble metal aerogels consisting of extended metal backbone nanonetworks. We discuss their outstanding physical and chemical properties, including their three-dimensional network structure, the simple control over their composition, their large specific surface area, and their hierarchical porosity. Additionally, we show some initial results on their excellent performance as electrocatalysts combining both high catalytic activity and high durability for fuel cell reactions such as ethanol oxidation and the oxygen reduction reaction (ORR). Finally, we give some hints on the future challenges in the research area of metal aerogels. We believe that metal aerogels are a new, promising class of electrocatalysts for polymer electrolyte fuel cells (PEFCs) and will also open great opportunities for other electrochemical energy systems, catalysis, and sensors. The commercialization of PEFCs encounters three critical obstacles, viz., high cost, insufficient activity, and inadequate long-term durability. Besides others, the sluggish kinetics of the ORR and alcohol oxidation and insufficient catalyst stability are important reasons for these obstacles. Various approaches have been taken to overcome these obstacles, e.g., by controlling the catalyst particle size in an optimized range, forming multimetallic catalysts, controlling the surface compositions, shaping the catalysts into nanocrystals, and designing supportless catalysts with extended surfaces such as nanostructured thin films, nanotubes, and porous nanostructures. These efforts have produced plenty of excellent electrocatalysts, but the development of multisynergetic functional catalysts exhibiting low cost, high activity, and high durability still faces great challenges. In this Account, we demonstrate that the sol-gel process represents a powerful "bottom-up" strategy for creating nanostructured materials that tackles the problems mentioned above. Aerogels are unique solid materials with ultralow densities, large open pores, and ultimately high inner surface areas. They magnify the specific properties of nanomaterials to the macroscale via self-assembly, which endow them with superior properties. Despite numerous investigations of metal oxide aerogels, the investigation of metal aerogels is in the early stage. Recently, aerogels including Fe, Co, Ni, Sn, and Cu have been obtained by nanosmelting of hybrid polymer-metal oxide aerogels. We report here exclusively on mono-, bi- and multimetallic noble metal aerogels consisting of Ag, Au, Pt, and Pd and their application as electrocatalysts.

Electrocatalysis on gold.

This perspective article reviews recent advances in the study of important catalytic reactions on gold electrodes. The paper discusses both oxidation and reduction reactions: the oxidation of carbon monoxide and alcohols as well as the oxygen reduction reaction on gold electrodes and also a brief discussion of other interesting reactions on gold electrodes such as the amine borane oxidation and the CO2 reduction. A common theme in electrocatalysis on gold is the sensitive dependence of various reaction rates on pH and gold surface structure. The electrocatalysis of redox reactions on gold is highly pH dependent, often preferring alkaline media, due to the prominent role of negatively charged reaction intermediates related to the fact that gold does not bind the neutral intermediates strongly enough. Gold also tends to be a selective catalyst, again due to its weak adsorption properties, as on gold the reaction often stops when a difficult bond breaking or making event will be the necessary next step.

Cathodic corrosion as a facile and effective method to prepare clean metal alloy nanoparticles.

The cathodic corrosion method described here is a simple, clean, and fast way of synthesizing nanoalloys with high catalytic performance. Using a series of Pt-Rh alloys as an example, we show that this one-step method can convert a bulk alloy electrode into an aqueous suspension of nanoparticles, retaining the composition and crystal lattice structure of the starting alloy. Compared to pure metals, these alloy nanocatalysts are more active toward CO and methanol oxidation and nitrate reduction reactions. Nanoparticles made of PtRu, PtIr, PtNi, AuCo, AuCu, and FeCo bulk alloys demonstrate the universality of this synthesis method.

Selective catalytic reduction at quasi-perfect Pt(100) domains: a universal low-temperature pathway from nitrite to N2.

The highly selective conversion of nitrite to N(2) at a quasi-perfect Pt(100) electrode in alkaline media was investigated with a particular emphasis on its structure sensitivity and its mechanism. High-quality (100) terraces are required to optimize the catalytic activity and steer the selectivity to N(2): defects of any symmetry dramatically reduce the N(2) evolution at [(100) × (110)] and [(100) × (111)] surfaces. On the other hand, nitrite reduction proves to be an additional example of the unique intrinsic ability of (100) surfaces to catalyze reactions involving bond breaking and successive bond formation. In the present case, (100) is able to reduce nitrite to NH(2,ads), which in a certain potential window combines with NO(ads) to give N(2) in a Langmuir-Hinshelwood reaction. Our findings are similar to those for other processes generating N(2), from bacterial anoxic ammonia oxidation ("anammox") to the high-temperature NO + NH(3) reaction at Pt(100) crystals under ultra-high-vacuum conditions, thus suggesting that the combination of these two nitrogen-containing species is a universal (low-temperature) pathway to N(2). The advantages of this pathway over other N(2)-generating pathways are pointed out.

Electrocatalytic oxidation of alcohols on gold in alkaline media: base or gold catalysis?

On the basis of a comparison of the oxidation activity of a series of similar alcohols with varying pK(a) on gold electrodes in alkaline solution, we find that the first deprotonation is base catalyzed, and the second deprotonation is fast but gold catalyzed. The base catalysis follows a Hammett-type correlation with pK(a), and dominates overall reactivity for a series of similar alcohols. The high oxidation activity on gold compared to platinum for some of the alcohols is related to the high resistance of gold toward the formation of poisoning surface oxides. These results indicate that base catalysis is the main driver behind the high oxidation activity of many organic fuels on fuel cell anodes in alkaline media, and not the catalyst interaction with hydroxide.

The promoting effect of adsorbed carbon monoxide on the oxidation of alcohols on a gold catalyst.

In heterogeneous catalysis and electrocatalysis, adsorbed carbon monoxide typically acts as a poison or poisoning intermediate in the oxidation of alcohols. However, gold as an (electro)catalyst often exhibits unexpected properties. Here we show that carbon monoxide irreversibly adsorbed on a Au(111) surface in aqueous alkaline media can act as a promoter for the electrocatalytic oxidation of certain alcohols, in particular methanol. In comparison with bare Au(111), the onset potential for methanol oxidation is significantly lower in the presence of adsorbed CO, and formation of the main methanol oxidation products--formaldehyde and formic acid--is enhanced. By studying the effect of adsorbed CO on the oxidation of other alcohols on gold, we conclude that the presence of adsorbed CO promotes beta-hydrogen elimination, that is, C-H bond breaking. Apart from its importance to gold catalysis, this is an unanticipated example of promotion effects by co-adsorbed small molecules in electrocatalysis.

Direct reduction of nitrite to N2 on a Pt(100) electrode in alkaline media.

The selective reduction of NO(2)(-) to N(2) in 0.1 M NaOH was obtained at a Pt(100) electrode in a narrow but distinct potential region. This is the first report of such selectivity for this reaction on Pt(100), which is known to be the most catalytically active platinum surface toward NO(2)(-) reduction in alkaline media. Both ammonia and nitrous oxide are ruled out as possible reaction intermediates on the basis of online electrochemical mass spectrometry. Based on earlier work on ammonia oxidation, NH(2) adsorbates are speculated to be involved in the reaction mechanism.

Promotion of the oxidation of carbon monoxide at stepped platinum single-crystal electrodes in alkaline media by lithium and beryllium cations.

The role of alkali cations (Li(+), Na(+), K(+), Cs(+), and Be(2+)) on the blank voltammetric response and the oxidative stripping of carbon monoxide from stepped Pt single-crystal electrodes in alkaline media has been investigated by cyclic voltammetry. A strong influence of the nature of the cation on both the blank voltammetric profile and the CO oxidation is observed and related to the influence of the cation on the specific adsorption of OH on the platinum surface. Especially Li(+) and Be(2+) cations markedly affect the adsorption of OH and thereby have a significant promoting effect on CO(ads) oxidation. The voltammetric experiments suggest that, on Pt(111), the influence of Li(+) (and Be(2+)) is primarily through a weakening of the repulsive interactions between the OH in the OH adlayer, whereas in the presence of steps also, the onset of OH adsorption is at a lower potential, both on steps and on terraces.

Thermodynamic evidence for K(+)-SO4(2-) ion pair formation on Pt(111). New insight into cation specific adsorption.

This work contributes to the understanding of cation specific effects on platinum electrochemistry by means of a thorough thermodynamic analysis of potassium adsorption on Pt(111) in sulfuric acid solutions. It is concluded that potassium specific adsorption is better described as the adsorption of the K(+)-SO ion pair. From the evaluation of the potassium sulfate concentration, it is found that potassium specific adsorption only takes place in the presence of coadsorbed sulfate species. Within the main sulfate adsorption state, for ∼0.3 V < E < ∼0.4 V (vs. SHE), the extent of potassium specific adsorption is small, reaching ∼0.1 × 10(14) species per cm(2) for c(K(+)) > 0.1 M. Then, at higher potentials, E > 0.55 V (vs. SHE), a second potassium adsorption process takes place, concomitant with the second sulfate adsorption process (associated to the small voltammetric feature called "the hump"). This last process involves the adsorption of an equal amount of potassium and sulfate species, leading to the adsorption of ∼0.5 × 10(14) ion pair species per cm(2) (∼0.03 ion pair species per platinum surface atom). Furthermore, the results of the formal partial charge numbers corroborate that potassium adsorption involves sulfate cooperative coadsorption, in such a way that the effective adsorbing species is anionic, rather than cationic. In conclusion, this work evidences that cation specific effects may originate from the formation of surface ion pairs, which is probably related to the presence of ion pairs in solution.

Elucidation of the chemical nature of adsorbed species for Pt(111) in H2SO4 solutions by thermodynamic analysis.

The nature of the adsorbed species for Pt(111) in sulfuric acid solutions has been elucidated by a careful thermodynamic analysis of the effect of pH on charge density data. This analysis takes advantage of the fact that, for solutions of constant total sulfate + bisulfate concentration, an increase of pH would increase the sulfate concentration, at the expense of decreasing the bisulfate concentration. As a result, sulfate adsorption would be shifted toward lower potentials, while bisulfate adsorption would follow the opposite trend. In the present work, coulostatic data for Pt(111) in (0.2 - x) M Me(2)SO(4) + x M H(2)SO(4) (Me: Li, Na; x: 10(-4) - 0.2) and (0.1 - x) M KClO(4) + x M HClO(4) + 10(-3) M K(2)SO(4) (x: 10(-4) - 0.1) solutions are carefully analyzed. It is concluded that sulfate rather than bisulfate adsorption takes place at potentials higher than the potential of zero charge. This result agrees with the fact that similar FTIRRAS bands for adsorbed sulfate species are observed for pH 0.8-3.5 in (0.2 - x) M K(2)SO(4) + x M H(2)SO(4) solutions.