Electrocatalysis of ethylene glycol oxidation (EGO) on shape-controlled Pd nanocrystals is of great interest in the pursuit of efficient biomass fuel utilization and nanomaterial application. The present work is aimed at a mechanistic study of electrocatalytic EGO in alkaline media on surface-cleaned high-index Pd concave nanocubes (Pd CNCs) with and without surface Bi modification. CO-adsorption displacement effectively removes the surfactants on as-synthesized Pd CNCs, facilitating controlled Bi adatom formation. EGO on the Pd CNCs is notably enhanced as a result of Bi modification, with the activity peak at a Bi coverage of ca. 0.31 in terms of apparent and specific oxidation current densities. Internal (ATR-SEIRAS) and external (IRRAS) reflection modes of in situ infrared spectroscopy have been used to probe the EGO process at a molecular level. High surface sensitivity ATR-SEIRAS enabled ready identification of the formation and removal of CO and 2-hydroxyacetyl surface species during EGO on Pd CNCs and Bi-modified Pd (Bi/Pd) CNCs. In comparison to that on bare Pd CNCs, the COad band is significantly stronger on Bi/Pd CNCs, suggestive of a promoted C–C bond cleavage. IRRAS results further reveal that glycolate and glyoxal are the main products of EGO on both pristine and Bi/Pd CNCs. In addition, formations of glyoxal, CO, and CO2 on Bi/Pd CNCs are relatively enhanced, in comparison to those on bare Pd CNCs. On the basis of the comprehensive spectral results and literature reports, relevant reaction pathways are proposed for EGO at Pd and Bi/Pd CNCs in alkaline media.Keywords: Bi modification; electrocatalysis; ethylene glycol oxidation; Pd concave nanocubes; surface infrared spectroscopy; surfactant removal;

Boron doping can boost the catalytic activity of palladium for diverse reactions. Precise control of the doping content is crucial but remains difficult in current synthesis, which generally involves the use of instable and costly borane-organic compounds. Herein, by taking advantage of the relatively strong solvation of N,N-dimethylformamide (DMF) to Na+ and the increased stability BH4– in DMF, we synthesize B-Pd interstitial nanocrystals in DMF, with NaBH4 acting as a reductant and boron source. The boron content, which can be readily tuned by changing the reaction time and NaBH4 concentration, can reach up to 20 at. %. Such a high boron doping results in a great beneficial effect on the catalytic capability of Pd toward the oxygen reduction reaction (ORR). The synthesized B-Pd nanoalloy exhibits a mass and specific activity for ORR that are, respectively, ca. 14 and 14.6 times higher than those of the state-of-the-art commercial Pt catalyst in alkaline solution. Density functional theory (DFT) calculations reveal three types of surface sites that are responsible for the enhanced activity, namely, Pd-BO2 assemblies, Pd atoms neighbored by the assemblies, and the Pd atoms modified with subsurface B atoms. The Pd-BO2 assembly has a Pt-like activity, while the neighboring Pd-BO2 assembly and subsurface B-modified Pd atoms could catalyze ORR much more efficiently than Pt. The facile and controllable boron doping in palladium should strengthen the power of Pd-based catalysts and, therefore, provides great prospects for their widespread application.

A new facile fabrication of Pt, Pd and their alloy films on Si, intended mainly for in situ ATR surface infrared spectroscopy and electrocatalysis study, has been achieved through an alternate electroless deposition approach from very simple acidic baths with hydrazine dihydrochloride as the reducing agent. Compositional analyses reveal that Pt and Pd elemental ratios in the alloy films are close to those in the plating baths, and higher in skin layers compared to those of bulk films. Electrochemical and IR spectroscopic characterizations suggest the Pt−Pd alloy films may exhibit synergetic effects in surface electrochemistry toward oxidation of CO adlayer and formic acid. Of particular interest is the reproducible control of internal reflection absorption responses through adjusting Pt and Pd compositions in the films. More enhanced surface IR absorption with normal band shape and direction was observed for surface species at Pt and Pd electrodes. For the as-deposited Pt−Pd alloys, in addition to less enhanced IR absorption intensity, bipolar and totally inverted bands may appear for different alloy films.

Most electrocatalysts for the ethanol oxidation reaction suffer from extremely limited operational durability and poor selectivity toward the CC bond cleavage. In spite of tremendous efforts over the past several decades, little progress has been made in this regard. This study reports the remarkable promoting effect of Ni(OH)2 on Pd nanocrystals for electrocatalytic ethanol oxidation reaction in alkaline solution. A hybrid electrocatalyst consisting of intimately mixed nanosized Pd particles, defective Ni(OH)2 nanoflakes, and a graphene support is prepared via a two-step solution method. The optimal product exhibits a high mass-specific peak current of >1500 mA mg−1Pd, and excellent operational durability forms both cycling and chronoamperometric measurements in alkaline solution. Most impressively, this hybrid catalyst retains a mass-specific current of 440 mA mg−1 even after 20 000 s of chronoamperometric testing, and its original activity can be regenerated via simple cyclic voltammetry cycles in clean KOH. This great catalyst durability is understood based on both CO stripping and in situ attenuated total reflection infrared experiments suggesting that the presence of Ni(OH)2 alleviates the poisoning of Pd nanocrystals by carbonaceous intermediates. The incorporation of Ni(OH)2 also markedly shifts the reaction selectivity from the originally predominant C2 pathway toward the more desirable C1 pathway, even at room temperature.

•10 μM Cl− already suppresses electrocatalytic oxidation of formic acid on Pd.•Relevant interfacial chemistry monitored by in situ electrochemical ATR-SEIRAS•Cl− inhibits the adsorption of ClO4− and HCO3– but not necessarily that of HCOOB.•CO coverage increases with incremental Cl− concentration.The effect of chloride anions in electrolyte on formic acid oxidation (FAO) at Pd electrode is investigated by cyclic voltammetry and in situ surface-enhanced infrared absorption spectroscopy with attenuated total reflection (ATR-SEIRAS). Electrochemical measurement indicates that 1–2 × 10− 5 M Cl− in 0.1 M HClO4 + 0.1 M HCOOH solution leads to a significant decrease of formic acid oxidation current. Molecular level results from the ATR-SEIRAS indicate that such a small amount of chloride suppresses the adsorption of ClO4− and HCO3−, increased the CO coverage and somehow stabilized the bidentate formate (HCOOB) species over the main oxidation potentials. The blocking of active Pd surface sites by Cl− and COad is mainly responsible for the oxidation current density decay, and the inconformity of υ(HCOOB) band intensity and oxidation current implies that HCOOB is probably not the active intermediate in the direct pathway of formic acid oxidation.Download high-res image (225KB)Download full-size image

•Facile aqueous phase approach developed to synthesize Pt3Ni-B/C toward ORR.•DMAB used for the reductant and the B-doping source.•Highest activity and durability achieved on Pt3Ni-B/C.•Structural and electronic effects countable for improved performance.Boron-doped platinum-nickel nanoalloy supported on carbon black (Pt3Ni-B/C) is facilely synthesized through an aqueous phase process under mild conditions with dimethylamine borane as the reducing agent. The Pt3Ni-B/C catalyst exhibits an activity toward oxygen reduction reaction (ORR) in acid media at 0.90 V vs. RHE with a mass activity 3 and 1.7 times, and a specific activity 6 and 1.4 times as high as those for a commercial Pt/C and a Pt3Ni/C synthesized using NaBH4 reductant, respectively. After 10,000 cycles of accelerated durability test the Pt3Ni-B/C demonstrates a significantly improved ORR durability in comparison to its counterparts with only 10 mV decay for its half-wave potential. The higher performance of the Pt3Ni-B/C with respect to that of the Pt3Ni/C can be ascribed to well-dispersed and relatively small nanoalloy particles, as well as slightly intensified adsorption strength toward O-containing species and structural resistance to potential cycling with boron doping.

The investigation of electrocatalysis of oxygen reduction reaction (ORR) on non-Pt electrodes is of great interest to address the current technical bottleneck of using costly and rare metal Pt in the cathodes of low-temperature fuel cells. The present work presents a comparative study of ORR on carbon supported Pd and B-doped Pd (with ca. 7 at. % B doping) nanocatalysts with well-controlled particle sizes, dispersions, and loadings (both with 20 wt % Pd). It is found that the Pd–B/C exhibits a modestly higher electrocatalytic activity toward ORR: the specific activity is enhanced by factors of ca. 2.0 and 2.7 on Pd–B/C as compared to that on Pd/C in acidic media at 0.85 and 0.90 V, respectively. In contrast, the corresponding enhancement factors are ca. 1.3 and 1.6, respectively, in alkaline media. To understand the promoted ORR activity by B-doping, density functional theory (DFT) calculations are applied, revealing weakened adsorption of the O-containing species on B-doped Pd surfaces, consistent with the XPS and CO stripping results. Despite the modest improvement at this moment, it raises the hope of further developing Pd-based ORR catalysts as well as the concern of reasonable comparison of two sets of non-Pt catalysts.

Electroless deposition of a quasi-monolayer (q-ML) of Pt and/or Pd on different Au substrates is achieved by using CO as both reducing and quenching agents, imparting Au@Pt/C or Au@Pd/C with superior electrocatalytic activity for ethanol oxidation in alkaline media.

Direct formic acid fuel cell (DFAFC) with Pd-based catalyst anode is a promising energy converter to power portable devices. However, its commercialization is entangled with insufficient activity and poor stability of existing anode catalysts. Here we initially report that a DFAFC using facilely synthesized Pd–B/C with ca. 6 at. % B doping as the anode catalyst yields a maximum output power density of 316 mW cm–2 at 30 °C, twice that with a same DFAFC using otherwise the state-of-the-art Pd/C. More strikingly, at a constant voltage of 0.3 V, the output power of the former cell is ca. 9 times as high as that of the latter after 4.5 h of continuous operation. In situ attenuated total reflection infrared spectroscopy is applied to probe comparatively the interfacial behaviors at Pd–B/C and Pd/C in conditions mimicking those for the DFAFC anode operation, revealing that the significantly improved cell performance correlates well with a substantially lowered CO accumulation at B-doped Pd surfaces.Keywords: attenuated total reflection infrared spectroscopy; boron-doped palladium; direct formic acid fuel cell; electrocatalysis; interfacial chemistry

Design of appropriate supporting materials is an alternative route to yield efficient Pt-free catalysts for ethanol oxidation reaction, which in practice may determine the conversion efficiency of direct alkaline ethanol fuel cells. In this work, graphene nanoribbons (GNRs) coated with MnO2 are used as a unique supporting material for loading and dispersing Pd nanoparticles. XRD, TEM and XPS are applied to characterize the structure of as-synthesized Pd/MnO2/GNRs nanocomposite catalyst, revealing a good dispersion as well as a modification of electronic property of Pd nanoparticles. Electrochemical measurements demonstrate that the as-synthesized nanocomposite displays largely enhanced electrocatalytic activity and durability toward ethanol oxidation in alkaline media as compared to the other tested Pd-based catalysts with various supports.

•Ethanol electrocatalysison Pd-Ni-P film enhanced through electrochemical dealloyment.•ATR-SEIRAS initially applied to probe the electrocatalysis on dealloyed film.•Molecular level evidence obtained for enhanced electrocatalysis on dealloyed Pd-Ni-P.In order to extend the dealloying systems and their electrocatalytic applications, in this work, electrocatalysis of ethanol in alkaline media on dealloyed Pd-Ni-P film is selected as a case study. Pd-Ni-P film is prepared via electro-deposition on Au substrate, and the dealloying process is carried out by repetitive potential cycling in acidic media to leach out most Ni and P components. The surface structural and electronic properties of the as-deposited film and the dealloyed film are characterized and compared using field-emission scanning electron microscopy, and X-ray photoelectron spectroscopy. Surface roughening, Pd-segregation and electronic property variation upon dealloying are confirmed. Cyclic voltammetry and chronoamperometry on the two films in ethanol-containing alkaline media are used to assess their electrocatalytic performances, demonstrating significantly enhanced and durable ethanol oxidation on the dealloyed film. More importantly, in situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) is initially applied to explore the interfacial molecular information in the electrocatalysis on these two films to provide molecular insight into the enhanced electrocatalytic activity on the dealloyed film, revealing that the enhanced electrocatalysis correlates well with enhanced formation of both COad and acetate.

•One-pot polyol synthesis of Pd–Cu/C nanocatalysts.•Formation of Pd–Cu nanoalloy with surface Pd enrichment.•Enhanced electrocatalysis of FAO on Pd1Cu1/C.Carbon supported Pd–Cu nanoparticles have been synthesized via a facile one-pot polyol reduction process (simplified hereafter as Pd–Cu/C-polyol), as electrocatalysts toward formic acid oxidation (FAO). The as-synthesized Pd–Cu/C-polyol catalysts are structurally characterized by TEM, XRD, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements. Notably, an atomic level understanding of the as-prepared bimetallic nanocatalysts is achieved by XAS, providing insights into both the interatomic distances and the coordination numbers information. The results suggest that Pd–Cu nanoalloy particles are highly dispersed on Vulcan XC-72 carbon and enriched with Pd in their skin layers. Electrochemical measurements indicate that Pd1Cu1/C-polyol exhibits the best electrocatalytic performance towards FAO in terms of Pd-mass activity and durability among commercial Pd/C, Pd2Cu1/C and Pd1Cu1/C. The comprehensive structural characterizations enable a better understanding of the enhanced electrocatalytic performance of the bimetallic catalysts: the addition of Cu is suggested to lower appropriately the d-band center of Pd and increase the electrochemical active surface area of the Pd–Cu/C nanocatalysts.

Facile production of hydrogen at room temperature is an important process in many areas including alternative energy. In this Communication, a potent boron-doped Pd nanocatalyst (Pd-B/C) is reported for the first time to boost hydrogen generation at room temperature from aqueous formic acid–formate solutions at a record high rate. Real-time ATR-IR spectroscopy is applied to shed light on the enhanced catalytic activity of B-doping and reveals that the superior activity of Pd-B/C correlates well with an apparently impeded COad accumulation on its surfaces. This work demonstrates that developing new anti-CO poisoning catalysts coupled with sensitive interfacial analysis is an effective way toward rational design of cost-effective catalysts for better hydrogen energy exploitation.

In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy in conjunction with H–D isotope replacement is used to investigate the dissociation and oxidation of CH3CH2OH on a Pd electrode in 0.1 M NaOH, with a focus on identifying the chemical nature of the pivotal intermediate in the so-called dual-pathway (C1 and C2) reaction mechanism. Real-time spectroelectrochemical measurements reveal a band at ∼1625 cm–1 showing up prior to the multiply bonded COad band. CH3CD2OH and D2O are used to exclude the spectral interference with this band from interfacial acetaldehyde and H2O, respectively, confirming for the first time that the ∼1625 cm–1 band is due to the adsorbed acetyl on the Pd electrode in alkaline media. The spectral results suggest that the as-adsorbed acetyl (CH3COad) is oxidized to acetate from approximately −0.4 V as the potential moves positively to conclude the C2 pathway. Alternatively, in the C1 pathway, the CH3COad is decomposed to α-COad and β-CHx species on the Pd electrode at potentials more negative than approximately −0.1 V; the α-COad species is oxidized to CO2 at potentials more positive than approximately −0.3 V, while the β-CHx species may be first converted to COad at approximately −0.1 V and further oxidized to CO2 at more positive potentials.Keywords: alkaline media; electrocatalysis; ethanol; mechanism; Pd electrode; surface-enhanced infrared spectroscopy

A silicon oxide film doped with fluorine was grown on a (100)-oriented Si wafer through liquid-phase deposition (LPD) as a protective mask of the wafer’s rear side in order to chemically texture the wafer’s unprotected front side in a basic etching bath, which is a new process in solar-cell manufacturing. The growth rate of the LPD-SiO2 film increased monotonically with an increase of the deposition temperature up to 60 °C for a given precursor solution. Field-emission scanning electron microscopy (FE-SEM) indicates that a pyramidal surface texture forms on the front side in the chemical texturing bath, whereas the underlying Si surface on the rear side remains intact. As a result, the average reflectivity for incident light over 450–850 nm is decreased to 11.1% on the front side, and a 5.8 μm thick Si surface on the rear side is saved per wafer. The all-wet process involved in this single-sided texturing is promising for the mass production of thinner and higher-efficiency Si-based solar cells because of its simplicity and lower cost.Keywords: liquid-phase deposition; silicon oxide film; silicon wafer; single-sided texturing; solar cells;

Formic acid as a natural biomass and a CO2 reduction product has attracted considerable interest in renewable energy exploitation, serving as both a promising candidate for chemical hydrogen storage material and a direct fuel for low temperature liquid fed fuel cells. In addition to its chemical dehydrogenation, formic acid oxidation (FAO) is a model reaction in the study of electrocatalysis of C1 molecules and the anode reaction in direct formic acid fuel cells (DFAFCs). Thanks to a deeper mechanistic understanding of FAO on Pt and Pd surfaces brought about by recent advances in the fundamental investigations, the “synthesis-by-design” concept has become a mainstream idea to attain high-performance Pt- and Pd-based nanocatalysts. As a result, a large number of efficient nanocatalysts have been obtained through different synthesis strategies by tailoring geometric and electronic structures of the two primary catalytic metals. In this paper, we provide a brief overview of recent progress in the mechanistic studies of FAO, the synthesis of novel Pd- and Pt-based nanocatalysts as well as their practical applications in DFAFCs with a focus on discussing studies significantly contributing to these areas in the past five years.

•Initial synthesis of Pd–Ni–P/C ternary nanocatalyst for ethanol electro-oxidation in alkaline media.•Significantly enhanced electrocatalytic performance on Pd–Ni–P/C compared to that on Pd/C, Pd–P/C or Pd–Ni/C.•Electronic, geometric and bifunctional effects assumed to account for the improved performance.Carbon-supported well-dispersed Pd–Ni–P ternary catalyst targeted for ethanol oxidation reaction (EOR) in alkaline media is synthesized in a simple aqueous bath containing Pd(II) and Ni(II) salts with sodium hypophosphite as the reducing agent and the source for P and sodium citrate as the complexing agent. XRD analysis on the as-prepared Pd–Ni–P/C reveals that Ni shrinks while P expands the Pd lattice structure, and XPS measurement suggests different electronic effects of the two alloying elements on Pd. Cyclic voltammetry and chronoamperometry indicate that the Pd–Ni–P/C presents a remarkably higher electrocatalytic activity than the state-of-the-art Pd/C, Pd–P/C and Pd–Ni/C catalysts. This may be ascribed to the unique electronic, geometric and bifunctional effects involved in this ternary nanoalloy.

•H–D kinetic isotope effect (KIE) of alcohol electrooxidation was studied.•Ethanol, isopropanol, and EG oxidation on Au, Pd, and Pt was explored.•Much larger KIE (> 6) was observed on Au than on Pt (< 3.5).•The cleavage of α-H from CH is the rate-determining step for Au, but not for Pt.The identification of rate-determining step (RDS) is vital for understanding reaction mechanisms and designing advanced catalysts. Here, we systemically studied the H–D kinetic isotope effects (KIEs) on electrooxidation of ethanol, isopropanol, and ethylene glycol (EG) on Au, Pd and Pt electrodes in alkaline solution by combining cyclic voltammetry and in situ FTIR spectroscopy. Large primary KIEs are observed on Au electrode: 8.4 for ethanol, 6.2 for EG and 7.9 for isopropanol when α-H of alcohols are substituted by deuterium, but much lower KIEs (< 3.2) were observed on Pt, and the values on Pd are between Au and Pt. According to the criteria (KIEs = 6–10) for CH (D) breaking/forming as RDS, we conclude that the cleavage of the α-H from CH bond is the RDS for alcohol electrooxidation on Au, but not on Pt. On Pd, the RDS depends on the structure of alcohols. The difference can be rationally correlated with the binding affinity of hydrogen on precious metal surfaces.

Real time surface enhanced infrared absorption spectroscopy in ATR configuration (ATR-SEIRAS) has been applied to monitor the self assembly process of iron(III) protoporphyrin IX (FeIIIPP or hemin) onto an alkanedithiol modified Au film (HSRS-Au) electrode through the thioether linkage. This chemical grafting assembly is a two-stage process with a rapid increase of surface coverage in the initial 1000 s followed by a gradual saturation for the HSRS-modified Au electrode in a 50 μM FeIIIPP-contained phosphate buffer solution (PBS) (pH 7.4). The cyclic voltammogram of thus obtained electrode shows clearly a pair of quasi-reversible FeIIIPP/FeIIPP redox peaks in PBS of pH 7.4 with the formal potential ca. −0.36 V (SCE), and the surface coverage of the attached FePP is estimated to be ca. 2.55 × 10−11 mol/cm2 from the charge contained in the redox peak. In situ ATR-SEIRAS is also extended to study the potential-dependent coordination of CO to the as-grafted FeIIPP monolayer. The mid-point transition potential for de-ligating CO off the (CO)FeIIPP is centered at ca. −0.21 V and 0.01 V (SCE) in PBS solutions of pH 7.4 and pH 3.8, respectively. No spectral feature indicated the formation of the bicarbonyl adduct.Highlights► Assembly process of hemin (or FePP) on alkanedithiol modified Au film electrode monitored by real time ATR-SEIRAS. ► Quasi-reversible redox behavior seen for well-defined surface FeIIIPP/FeIIPP. ► CO coordination to and decoordination off surface FePP investigated by electrochemical ATR-SEIRAS.

ATR-SEIRAS is extended for the first time to study potential-induced surface and interface structure variation of a CO-covered Pt electrode in a room-temperature ionic liquid of N-butyl-N-methyl-piperidinium bis((trifluoromethyl)sulfonyl)imide (or [Pip14][TNf2]). Owing to a wide effective potential window of [Pip14][TNf2], a gradual conversion from bridged COad (COB) to terminal COad (COL) is observed in response to positively going potentials, suggesting that [Pip14]+ may be involved in a strong electrostatic interaction with the COad. This site conversion enables the ratio of the apparent absorption coefficient of COL to that of COB to be determined. Also, the spectral results reveal the potential-dependent COad frequency variations as well as the potential-induced interfacial ionic reorientation and movement at the Pt/CO/[Pip14][TNf2] interface.Keywords: adsorption site conversion; apparent absorption coefficient; electrochemical interface; ionic rearrangement; Stark tuning rate;

The dissociative adsorption and electrooxidation of CH3OH at a Pd electrode in alkaline solution are investigated by using in situ infrared spectroscopy with both internal and external reflection modes. The former (ATR-SEIRAS) has a higher sensitivity of detecting surface species, and the latter (IRAS) can easily detect dissolved species trapped in a thin-layer-structured electrolyte. Real-time ATR-SEIRAS measurement indicates that CH3OH dissociates to COad species at a Pd electrode accompanied by a “dip” at open circuit potential, whereas deuterium-replaced CH3OH doesn’t, suggesting that the breaking of the C–H bond is the rate-limiting step for the dissociative adsorption of CH3OH. Potential-dependent ATR-SEIRAS and IRAS measurements indicate that CH3OH is electrooxidized to formate and/or (bi)carbonate, the relative concentrations of which depend on the potential applied. Specifically, at potentials negative of ca. −0.15 V (vs Ag/AgCl), formate is the predominant product and (bi)carbonate (or CO2 in the thin-layer structure of IRAS) is more favorable at potentials from −0.15 to 0.10 V. Further oxidation of the COad intermediate species arising from CH3OH dissociation is involved in forming (bi)carbonate at potentials above −0.15 V. Although the partial transformation from interfacial formate to (bi)carbonate may be justified, no bridge-bonded formate species can be detected over the potential range under investigation.

A facile one-pot tactic is developed for the selective synthesis of either rhombic dodecahedral or cubic Pd nanocrystals with high yields. By applying a mild cleaning process, we establish for the first time reasonable and distinct electrochemical features corresponding to {110} or {100} facet predominated Pd nanocrystals.

The mimetic underpotential deposition (MUPD) technique without external potential control has been newly extended for surface decorations of Pb sub-monolayer on Pt (denoted as Pt@Pb), Pt (sub)monolayer on Au (denoted as Au@Pt), and Pt–Pd mixed monolayer on Pd (denoted as Pd@Pt–Pd), respectively, by introducing appropriate reducing agents in decorating baths. The carbon black-supported Pt@Pb catalyst (denoted as Pt@Pb/C) is obtained simply through the Pb MUPD on Pt/C whereas the Au@Pt and the Pd@Pd–Pt/C catalysts are obtained through the Cu MUPD followed a subsequent galvanic redox replacement. The electro-oxidation of formic acid is used as a model reaction to verify the effectiveness of such surface modifications. The tremendously enhanced activities on various electrocatalysts demonstrate that the extended MUPD approach is promising for scale-up skin modification on nanocatalysts with foreign adatoms.Highlights► New extensions of mimetic underpotential deposition. ► Skin modification of Pb on Pt, Pt on Au and Pd–Pt on Pd surfaces. ► Enhanced electrocatalytic activity for formic acid oxidation. ► Promising for scale-up modification on nanocatalysts with foreign adatoms.

Attenuated total reflection-infrared (ATR-IR) spectroscopy is extended to investigate the surface poisoning species in the processes of (electro)chemical decomposition of formic acid (FA) on a state-of-the-art commercial Pd black catalyst in 5 M FA solution. During the FA decomposition under different potential settings including the open circuit potential (OCP, ca. 0.06 V vs. RHE), the constant potential 0.4 V (vs. RHE) and the scanned potentials between 0.1 and 0.5 V (vs. RHE), CO is clearly confirmed as a surface poisoning species with its vibrational frequencies located over ∼1845 to 2016 cm−1, featuring different CO bonding configurations (including the triple-, bridge- and linear-bonded CO species) on Pd black surfaces. COad coverage increases with increasing operation time and decreasing operation potential. Once formed, COad can only be removed at a much higher oxidation potential, corresponding to the reactivation of the Pd black surfaces. The present results provide a molecular level insight into an important aspect of the deactivation issue for a real Pd nanocatalyst in a practical FA concentration relevant to the anode operations of direct formic acid fuel cells (DFAFCs).Graphical abstractHighlights► ATR-IR spectroscopy is used to study surface poisoning of Pd black in 5 M HCOOH. ► CO accumulation and removal on Pd black surfaces is clarified at molecular level. ► Lower potential favors CO accumulation on Pd black surfaces. ► Removal of as-formed CO reactivates Pd black surfaces for HCOOH electro-oxidation.

The decomposition of HCOOH on Pd surfaces over a potential range of practical relevance to hydrogen production and fuel cell anode operation was probed by combining high-sensitivity in situ surface-enhanced IR spectroscopy with attenuated total reflection and thin-layer flow cell configurations. For the first time, concrete spectral evidence of COad formation has been obtained, and a new main pathway from HCOOH to COad involving the reduction of the dehydrogenation product of HCOOH (i.e., CO2) is proposed.

Well-dispersed Ag@Pd supported on magnetite nanoparticles have been obtained through a simple colloidal impregnation method. The as-synthesised nanocomposite exhibits greatly enhanced catalytic reactivity and reusability towards 4-nitrophenol hydrogenation.

Attenuated total reflection surface-enhanced infrared absorption spectroscopy, in conjunction with Gram−Schmidt intensity response and 2-dimensional correlation IR analyses, has been extended to study the structure and interaction of interfacial H2O at a CO-predosed platinum electrode in 0.1 M HClO4 with its potential navigated from the double-layer region to the strong hydrogen evolution reaction (HER) region. The strong HER enables at least part of the outer interfacial H2O layers to be effectively expelled by tremendous amounts of H2 bubbles dynamically formed on the Pt surface. The H2 bubbles could effectively survive during a fairly fast potential excursion to a positive limit for CO stripping at which a single-beam spectrum was then acquired and used as the reference, facilitating the analysis of “quasi-absolute” spectra of the initial interfacial H2O and CO at the Pt electrode prior to the significant HER. As a result, the broad νOH band may be deconvoluted into four peaks at ca. 3630, 3550, 3250, and 3450 cm−1, assignable, respectively, to the innermost H2O layer with hydrogen bonding broken (H2Ofree) and interacted with the underlying CO adlayer, the second layer of H2O with hydrogen bonding partially broken, the outer icelike water layers, and the liquid-like water layers. The interaction of H2Ofree with the CO adlayer is much stronger than that expected simply from the hydrophobic effect of the latter as deduced from the stability of rather stable CO and H2Ofree co-structure even with a drastic HER condition, suggesting that a sort of chemical bonding may be involved.

Adsorption and electro-oxidation of CO on a polycrystalline Pt electrode in acidic solutions were systematically revisited by in situ attenuated-total-reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) in conjunction with related Gram–Schmidt response analysis. CO was either adsorbed in the double-layer region, i.e., 0.45 V (RHE) (denoted as CO@DL) or in the hydrogen underpotential deposition region, i.e., 0.1 V (RHE) (denoted as CO@UPD). The results indicate that the CO@UPD and H2Ofree coexisted structure (or simply costructure) forms only at a sufficiently high global CO coverage (H2Ofree denotes hydrogen-bonding-broken water); In contrast, the CO@DL and H2Ofree costructure forms in an earlier adsorption phase, less dependent on the global CO coverage. The partial oxidation of CO from solution and weakly adsorbed COL at the active sites is suggested to yield a prepeak that occurs with the relaxation of the COad-H2Ofree costructure and the disorganization of the outer water net layers. In the main oxidation process, the oxidation of CO@UPD tends to proceed via the “mean-field approximation” kinetics due to the high COad mobility resulting from the oxidation prepeak. The oxidation process of CO@DL is, however, likely via the “nucleation and growth” kinetics due to the good stability of the local CO@DL and H2Ofree costructured islands. The H2Ofree can be better assigned to the “probe” of the local COad coverage rather than the main oxygenated species for COad oxidation according to the spectral results for both CO@UPD and CO@DL.

Surface-enhanced infrared absorption spectroscopy (SEIRAS) in attenuated total reflection (ATR) configuration has been extended to the Fe electrode/electrolyte interface in neutral and weakly acidic solutions for the first time. The SEIRA-active Fe film electrode was obtained through a potentiostatic electrodeposition of a virtually pinhole-free 40 nm-thick Fe overfilm onto a 60 nm-thick Au underfilm chemically predeposited on the reflecting plane of an ATR Si prism. The infrared absorption for CO adlayer at the Fe film electrode measured with ATR-SEIRAS was enhanced by a factor of larger than 34, as compared to that at a Fe bulk electrode with external infrared absorption spectroscopy in the literature. More importantly, the unipolar band shape enabled the reliable determination of the Stark tuning rates of CO adlayer at Fe electrode. In situ ATR-SEIRAS was also applied to study the electrosorption of a typical corrosion inhibitor benzotriazole (BTAH) on Fe electrode as a function of potential, providing additional spectral information at positive potentials in support of the formation of a polymer-like surface complex FeIIm(BTA)n as the corrosion-resistant layer.

The electro-oxidation of CO adlayer on Pt electrode in Cl−-containing 0.1 M HClO4 has been investigated with in situ attenuated-total-reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). Two potentials were selected for predosing CO on the Pt electrode: one is in the H-UPD region, i.e., 0.1 V (vs. RHE) and the other is in the double-layer region, i.e., 0.45 V (vs. RHE). The broadening of the prewave and the main peak for the CO oxidation is observed, in addition to the positively shifted oxidation potentials. The spectroelectrochemical data suggest the specific adsorption of Cl− starts at a potential as negative as 0.1 V which may compete with the adsorption of OH at CO-unoccupied sites (including but not limited to defect sites) and/or hinder the diffusion of CO to OH adsorption sites on Pt electrode, slowing down the CO oxidation. This competitive Cl− adsorption at lower potentials disrupts the interfacial free H2O structure on the top of CO adlayer, signaled by a reduced OH stretching band intensity.

In situ electrochemical surface-enhanced infrared absorption spectroscopy (EC-SEIRAS) together with a periodic density functional theory (DFT) calculation has been initially applied to investigate the mechanism of formic acid electro-oxidation on Sb-modified Pt (Sb/Pt) electrode. EC-SEIRAS measurement reveals that the main formic acid oxidation current on Sb/Pt electrode is ca. 10-fold enhanced as compared to that on clean Pt electrode, mirrored by nearly synchronous decrease of the CO and formate surface species, suggesting a “non-formate” oxidation as the main pathway on the Sb/Pt electrode. On the basis of the calculations from periodic DFT, the catalytic role of Sb adatoms can be rationalized as a promoter for the adsorption of the CH-down configuration but an inhibitor for the adsorption of the O-down configuration of formic acid, kinetically facilitating the complete oxidation of HCOOH into CO2. In addition, Sb modification lowers the CO adsorption energy on Pt, helps to mitigate the CO poisoning effect on Pt.

Inspired by the “third-body” effect and the d-band center theory, a series of carbon black-supported PdxPt1−x (denoted as PdxPt1-x/C, with atomic fraction x = 0.5−1) nanocatalysts were synthesized and screened in order to pinpoint the optimal composition targeted for the electrocatalytic oxidation of formic acid. The effective alloying of these two elements is better demonstrated by the compositional dependent synergetic electrocatalysis identified for the as-synthesized PdxPt1−x/C nanoparticles rather than by the XRD characterization. Preliminary screening indicates that Pd0.9Pt0.1/C is the optimum catalyst for the desired reaction among all tested PdxPt1−x/C samples. This high performance of the Pd0.9Pt0.1 nanoalloy can be ascribed to the effectively inhibited CO poisoning at largely separated Pt sites and appropriately lowered d-band center of Pd sites.

In situ ATR–FTIR spectroscopy has been extended to the study of CO and pyridine adsorptions at crystalline and amorphous Ni–P alloy film electrodes, with an emphasis on the alloying effect of P on their adsorption configurations on Ni sites. The Ni–P films were prepared with initial seeding of a catalytic Pd layer on Si, followed by chemical deposition. Transition from bridge-bonded CO (COB) dominant to linearly bonded CO (COL) dominant adsorption was found with increasing P content, together with a blue-shift in the COL vibrational frequency and an attenuation of the Stark tuning rate of COL. As for Py on Ni–P electrodes, essentially the N-end-on adsorption was revealed, in contrast to the edge-tilted adsorption on Ni electrode. Modification in the adsorption configurations as compared to that on Ni electrode is ascribed mainly to the site-blocking effect with alloying P, rather than to partial electron transfer between Ni and P.

A new facile approach towards developing superior Pt-based catalysts for HCOOH electrooxidation has been proposed, which is exemplified with a mimetic underpotential deposition (MUPD) of Sb on Pt surfaces to attain a favorable coverage. Suitable Sb modification was achieved simply through immersing a bulk Pt electrode or dispersing Pt/C powders in a Sb(III) solution mixed with ascorbic acid (AA). AA serves as the mild reducing agent to ensure freshly reduced Pt surfaces for Sb modification, as demonstrated by the negatively shifted open circuit potential. The catalytic activity towards HCOOH electrooxidation on the above Sb-modified Pt/C catalyst far exceeds that on commercial Pt–Ru/C or Sb-modified Pt/C through traditional irreversible adsorption. This electroless approach is generally applicable to all types of Pt surfaces, in particular suited for upgrading Pt/C for practical anode catalysts of direct formic acid fuel cells.

A facile chemical reduction method has been developed to fabricate ultrafine copper nanoparticles whose sizes can be controlled down to ca. 1 nm by using poly(N-vinylpyrrolidone) (PVP) as the stabilizer and sodium borohyrdride as the reducing agent in an alkaline ethylene glycol (EG) solvent. Transmission electron microscopy (TEM) results and UV–vis absorption spectra demonstrated that the as-prepared particles were well monodispersed, mostly composed of pure metallic Cu nanocrystals and extremely stable over extended period of simply sealed storage.

Highly dispersed boron-doped palladium nanoparticles supported on carbon black (Pd−B/C) with high Pd loading (ca. 40 wt % Pd) are synthesized through an aqueous process using dimethylamine borane as the reducing agent. The as-prepared Pd−B/C catalyst shows extraordinary activity toward formic acid electro-oxidation compared to that of a commercially available Pd/C catalyst and the one prepared by using NaBH4 as the reductant. Subsequent thermal treatment further enhances the durability of the electro-oxidation current on Pd−B/C, enabling this new material to be a promising anode catalyst for direct formic acid fuel cells. The superior performance of our Pd−B/C catalyst may arise from uniformly dispersed nanoparticles within optimal size ranges, the increase in surface-active sites, and the electronic modification effect of boron species.

A seeded-growth approach has been developed to fabricate a Cu nanoparticle film (simplified hereafter with nanofilm) on Si for electrochemical ATR surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). The approach comprises an initial activation of the reflecting plane of hemicylindrical Si prism by introducing a Cu seed layer in a CuSO4-HF solution and the subsequent electroless deposition of the Cu nanofilms from an electroless Cu plating bath. The as-deposited Cu nanofilm exhibited strong SEIRA effect for the CO probe and interfacial free H2O. ATR-SEIRAS was also applied to characterize the adsorbed geometries of pyridine at the Cu/electrolyte interface. Only vibrational bands assignable to the A1 symmetry modes were detected in the entire potential window investigated, suggestive of an end-on adsorption via the ring N-atom on a Cu electrode.

A versatile peripheral-pore-nanocasting method is employed to fabricate a series of novel microcapsules with mesoporous shell of noble metal or noble metal oxides, including Pt, PdO, RuO2 and IrO2. The shell thickness of these hollow microcapsules can be adjusted by controlling the depth of 3-amino-propyltriethoxysilane (APS) modification. By pre-filling the guest species or their precursors into the 3-D pores of HMS (hexagonal mesoporous molecular sieves) spheres, an interesting material with a “sphere in shell” structure could be fabricated via the same method. Both the mesoporous platinum hollow microcapsules and carbon spheres encapsulated in platinum microcapsules have shown high mass-normalized activities as a catalyst in the oxidation of methanol, a fundemantal reaction in direct methanol fuel cells (DMFC).

•High surface sensitivity ATR-IR is a powerful tool for probing interfacial electrochemistry.•Recent application and advancement of ATR-IR are reviewed and highlighted.•Coupling ATR-IR with other methods is emphasized.•Opinions on future development and application of ATR-IR are given.In situ attenuated total reflection infrared (ATR-IR) spectroscopy is a powerful analytical tool for the molecular-level study of interfacial electrochemistry. This report presents a brief overview on selected publications over the past 3 years related to the application of in situ ATR-IR to electrochemical systems of fundamental and practical interests, and gives opinions on the technical development as well as the future applications in electrochemical systems.

Real-time attenuated total-reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), in conjunction to cyclic voltammetry, was applied to investigate the adsorption and reduction of nitric oxide at Ru electrode in acidic solutions. For a NO-predosed Ru electrode, only one band located at 1840–1874 cm−1 was detected in 0.1 M HClO4, attributable to atop NO coadsorbed with oxygen-containing species [designated ν2(O)NO species]. For a Ru electrode in 0.1 M HClO4 containing 20 mM NaNO2, two IR bands were observed at 1850–1886 cm−1 and 1740–1790 cm−1. The former, predominant at relatively high potentials, is ascribable to the ν2(O)NO species; the latter, to atop NO adsorbed on nominal Ru sites at relatively low potentials (designated ν2-NO species). In addition, a very weak band at 1520–1578 cm−1 may be assigned to multicoordinated NO coadsorbed with oxygen-containing species. The real-time spectral results suggest that the reduction of NO molecules and the coadsorbed oxygen-containing species proceed simultaneously rather than separately.

Well-dispersed Ag@Pd supported on magnetite nanoparticles have been obtained through a simple colloidal impregnation method. The as-synthesised nanocomposite exhibits greatly enhanced catalytic reactivity and reusability towards 4-nitrophenol hydrogenation.

A facile one-pot tactic is developed for the selective synthesis of either rhombic dodecahedral or cubic Pd nanocrystals with high yields. By applying a mild cleaning process, we establish for the first time reasonable and distinct electrochemical features corresponding to {110} or {100} facet predominated Pd nanocrystals.

Electroless deposition of a quasi-monolayer (q-ML) of Pt and/or Pd on different Au substrates is achieved by using CO as both reducing and quenching agents, imparting Au@Pt/C or Au@Pd/C with superior electrocatalytic activity for ethanol oxidation in alkaline media.

PtRu/C material is one of the most well-known and efficient anode catalysts in direct methanol fuel cells. Nevertheless, new anode catalysts with even higher performance and lower cost are highly demanded for the further development of this fuel cell technology. Herein, we present a CoP-promoted Pt catalyst as a highly active, anti-poisoning and low-Pt loading catalyst for direct methanol fuel cells (DMFCs). The in situ attenuated total reflection surface-enhanced infrared radiation absorption spectroscopy (ATR-SEIRAS) technique revealed that the presence of CoP in the Pt-based catalyst can promote the methanol oxidation to CO2. A maximum power density of 88.5 mW cm−2 is achieved on a fuel cell based on this novel anode catalyst, which is ca. 1.4 times as high as that based on the state-of-the-art commercial PtRu/C catalyst with the same Pt loading. The present work demonstrates that the Pt–CoP/C will be a very competitive alternative to PtRu/C as the promising anode catalyst for the scale-up production of DMFCs in terms of overall performance and cost effectiveness.

Formic acid as a natural biomass and a CO2 reduction product has attracted considerable interest in renewable energy exploitation, serving as both a promising candidate for chemical hydrogen storage material and a direct fuel for low temperature liquid fed fuel cells. In addition to its chemical dehydrogenation, formic acid oxidation (FAO) is a model reaction in the study of electrocatalysis of C1 molecules and the anode reaction in direct formic acid fuel cells (DFAFCs). Thanks to a deeper mechanistic understanding of FAO on Pt and Pd surfaces brought about by recent advances in the fundamental investigations, the “synthesis-by-design” concept has become a mainstream idea to attain high-performance Pt- and Pd-based nanocatalysts. As a result, a large number of efficient nanocatalysts have been obtained through different synthesis strategies by tailoring geometric and electronic structures of the two primary catalytic metals. In this paper, we provide a brief overview of recent progress in the mechanistic studies of FAO, the synthesis of novel Pd- and Pt-based nanocatalysts as well as their practical applications in DFAFCs with a focus on discussing studies significantly contributing to these areas in the past five years.

In order to explore how and to what extent differently shaped Au nanocrystals (NCs) as the cores may affect the electrocatalytic properties of the resulting Au@Pt catalysts; a monolayer (ML) of Pt was deposited on cubic, octahedral and rhombic dodecahedral Au nanocrystals by using the well-established galvanic redox replacement method. The as-obtained Au-NCs@Pt-ML catalysts exhibited strongly inhibited COad oxidation in acidic media and a significantly enhanced ethanol oxidation reaction (EOR) in alkaline media as compared to commercial Pt/C, with an Au-shape dependent activity sequence in line with that of Pt single crystals of the corresponding orientations. Strain and facet effects were found to be jointly involved in determining the overall electrocatalytic activities at the three Au-NCs@Pt-ML catalysts, with the facet effect superimposed on the more pronounced strain effect.