Linear-bonded CO Research Articles

Electrochemical Formation of C2+ Products Steered by Bridge-Bonded *CO Confined by *OH Domains.

During the electrochemical CO2 reduction reaction (eCO2RR) on copper catalysts, linear-bonded CO (*COL) is commonly regarded as the key intermediate for the CO-CO coupling step, which leads to the formation of multicarbon products. In this work, we unveil the significant role of bridge-bonded *CO (*COB) as an active species. By combining in situ Raman spectroscopy, gas and liquid chromatography, and density functional theory (DFT) simulations, we show that adsorbed *OH domains displace *COL to *COB. The electroreduction of a 12CO+13CO2 cofeed demonstrates that *COB distinctly favors the production of acetate and 1-propanol, while *COL favors ethylene and ethanol formation. This work enhances our understanding of the mechanistic intricacies of eCO(2)RR and suggests new directions for designing operational conditions by modifying the competitive adsorption of surface species, thereby steering the reaction toward specific multicarbon products.

MXene-Regulated Metal-Oxide Interfaces with Modified Intermediate Configurations Realizing Nearly 100% CO2 Electrocatalytic Conversion.

Electrocatalytic CO2 reduction via renewable electricity provides a sustainable way to produce valued chemicals, while it suffers from low activity and selectivity. Herein, we constructed a novel catalyst with unique Ti3 C2 Tx MXene-regulated Ag-ZnO interfaces, undercoordinated surface sites, as well as mesoporous nanostructures. The designed Ag-ZnO/Ti3 C2 Tx catalyst achieves an outstanding CO2 conversion performance of a nearly 100% CO Faraday efficiency with high partial current density of 22.59 mA cm-2 at -0.87 V versus reversible hydrogen electrode. The electronic donation of Ag and up-shifted d-band center relative to Fermi level within MXene-regulated Ag-ZnO interfaces contributes the high selectivity of CO. The CO2 conversion is highly correlated with the dominated linear-bonded CO intermediate confirmed by in situ infrared spectroscopy. This work enlightens the rational design of unique metal-oxide interfaces with the regulation of MXene for high-performance electrocatalysis beyond CO2 reduction.

Investigation of CO2 photoreduction mechanism over the Zn2V2O7/Zn3V2O8/ZnO Z-scheme system

CO2 photoreduction under mild conditions remains challenging, attributing to sluggish kinetics of complex coupling process of multi-electrons-protons and inefficient electron-hole pairs utilization. Herein, with Zn3V2O8 as a phase transformation material, we construct an effective Zn2V2O7/Zn3V2O8/ZnO (OVZn) Z-scheme system by partial decomposition of Zn3V2O8 into Zn2V2O7 and ZnO. Time-resolved fluorescence decay (TRFD) spectra investigation shows that the photogenerated electrons of Zn2V2O7 and ZnO recombine with the holes of Zn3V2O8, retaining more reductive electron of Zn3V2O8 for CO2 reduction. Moreover, in-situ Fourier-transform infrared spectroscopy suggests that the heterostructure can lower the energy barrier of CO desorption by changing bridge-bonded CO to linear-bonded CO and optimizing the energy barrier of CO production. Moreover, Cu, Ag and Au loading on OVZn greatly enhance light harvesting and promote photoelectric conversion, producing much photogenerated electron-hole pairs for CO2 reduction. OVZn and metal loaded OVZn have better photocatalytic performance, showing application superiority on CO2 photoreduction.

Platinum-Enriched Ni/Pt(111) Surfaces Prepared by Molecular Beam Epitaxy: Oxygen Reduction Reaction Activity and Stability

Oxygen reduction reaction (ORR) activities and electrochemical stabilities were evaluated for Ni/Pt(111) model electrode-catalysts fabricated by molecular beam epitaxy (MBE). Exposure of clean Pt(111) to 1.0-Langmuir (i.e., fractional coverage, a = 1.0 in the Langmuir isotherm) CO at 300K produced linear-bonded and bridge-bonded CO­Pt IR bands at 2093 and 1858 cm11. In contrast, 3.0-nm-thick Ni deposition onto Pt(111) at 823K (823K-Ni3.0nm/Pt(111)) showed broad IR bands for adsorbed CO at around 2064 cm11; the separation of reflection high-energy electron diffraction (RHEED) streaks of the 823K-Ni3.0nm/Pt(111) is greater than for clean Pt(111). In contrast, 923KNi3.0nm/Pt(111) yielded a single sharp IR band because of linear-bonded CO at 2080 cm11, and the separation of the RHEED streaks is almost the same as that for the Pt(111). The results suggest that a Pt-enriched topmost surface is generated above 923K through surface segregation of the substrate Pt atoms during the Ni deposition. After transferring the sample from ultra-high vacuum to an electrochemical system, without being exposed to air, changes in ORR activities of the MBE-prepared 923K-Ni3.0nm/Pt(111) were evaluated in a 0.1-mol L11 HClO4 aqueous solution under applied potential cycles of between 0.6 and 1.0V vs. RHE. The ORR activity enhancement factor vs. clean Pt(111) for the asprepared 923K-Ni3.0nm/Pt(111) was estimated to be twelve, and this factor was reduced to four after the application of 1000 potential cycles. However, a 943K-Ni3.0nm/Pt(111) sample, which showed a relatively low ORR activity (an enhancement factor of eight), had the activity enhancement factor of six even after the application of 1000 potential cycles. These results reveal that the sub-surface structures and composition of Pt-based alloys determine not only initial ORR activity but also electrochemical durability of the ORR catalysts. [doi:10.2320/matertrans.M2013061]

In situ spectroscopic investigation of CO accumulation and poisoning on Pd black surfaces in concentrated HCOOH

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. CO ad coverage increases with increasing operation time and decreasing operation potential. Once formed, CO ad 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).

Enhanced activity of rare earth doped PtRu/C catalysts for methanol electro-oxidation

PtRuRE/C catalysts were prepared through doping the commercial JM PtRu/C catalyst with rare earth (RE = La, Eu, Gd, Y, Sm and Er). The doping was conducted by chemical reduction and sintering treatment methods. The catalysts were characterized by transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the rare earth doping did not significantly change the average size of the JM PtRu/C particles, and modified the PtRu surface with two element states of the metal and oxide. Studies of cyclic voltammetry and chronoamperometry indicated that the electrocatalytic activity of PtRuRE/C catalysts was higher than that of the JM PtRu/C for methanol oxidation (except for PtRuGd/C), generating a decreasing order of PtRuEu/C > PtRuEr/C > PtRuY/C > PtRuSm/C > PtRuLa/C > JM PtRu/C > PtRuGd/C. It has revealed that among the PtRuRE/C catalysts the PtRuEu/C exhibited the best performance. In addition, the reaction process of methanol electrocatalytic oxidation on several catalysts was investigated by in situ FTIR spectroscopy at molecule level. The results demonstrated that the adsorbates derived from methanol dissociative adsorption on the catalysts were the linear-bonded CO (CO L), and the CO-tolerance performance of PtRuRE/C catalysts was better than JM PtRu/C. It has revealed also that the electronic effect of rare earth plays an important role in the catalytic performance of PtRuRE/C catalysts for methanol electrooxidation.

Hydrodechlorination of CCl4 over Pt/γ-Al2O3 prepared from different Pt precursors

Abstract The platinum particle size on γ-Al2O3 prepared from different platinum precursors such as Pt(NH3)2(NO2)2, H2PtCl6, and K2PtCl6, and its effect on hydrodechlorination (HDC) of CCl4 with the variation of calcination temperatures was investigated. It concomitantly affects the products distribution and catalytic stability in HDC of CCl4. To verify the effects of platinum particle size with different platinum precursors on the product distribution, the catalysts have been characterized by HRTEM and CO chemisorption, FT-IR, and TP methods (TPR, TPD and TPSR). The catalysts with small platinum particles, which possess low coordination number and electron-deficient character, favor the complete dechlorination of CCl4 and produce CH4 more selectively. It could be due to the strong adsorption strength of CCl4 or the decreased activation energy of surface intermediates on small platinum particles. FT-IR studies reveal that the maximum peak position of linear-bonded CO shifted to the higher frequency with the increase of platinum particle size in all catalysts prepared from three different platinum precursors. While the selectivity to CH4 increased with the decrease in the platinum particle size, the total amount of carbonaceous species on the platinum particles was enhanced. The larger the platinum particle, the higher the selectivity to CHCl3 was obtained in all tested catalysts under the non-deactivating condition.

FTIR Studies of Zeolite Matrix Effects on the Properties of Palladium Clusters Confined in the Supercages of NaX and NaY

Palladium clusters have been synthesized by the "ship-in-a-bottle" approach in the supercages of NaX and NaY faujasite zeolites. In comparison with CO adsorbed on a bulk Pd electrode, the same molecule adsorbed on the Pd clusters electrodes evoked an enhanced IR absorption (EIRA). The enhancement factors have been determined to be about 38 and 51 in NaX and NaY, respectively. IR band centers of linear-bonded CO, bridge-bonded CO, and multi-bonded CO in NaX are measured, respectively, 12, 14, and 11 cm(-1) lower than those of the corresponding adsorption modes in NaY. The adsorption of CO and the oxidation of adsorbed CO in NaX matrix are faster than that in NaY matrix. These results suggest that part of the Pd2+ ions in NaX are located in sites III and III' that are near the 12-ring window of the supercage of zeolite, which lead to the formation of small Pd clusters. The present study is of significant importance in exploring the dependence of catalyst properties on structures, as well as in understanding and predicting the locations and properties of metal clusters in zeolites.

Vibrational analysis of chemisorbed CO on the Pt(111)/Ru bimetallic electrode

Abstract We investigated a Pt(1 1 1) electrode covered by two-dimensional nanosized Ru islands and used CO chemisorption, cyclic voltammetry and sum-frequency-generation (SFG) to probe the distribution of CO binding sites on such surfaces. Two distinctive CO species were resolved spectroscopically: Pt(1 1 1)/Ru–CO, with CO chemisorbed on the surface ruthenium sites; and Ru/Pt(1 1 1)–CO, with CO chemisorbed on the platinum sites. Our SFG assignment of these species is that the 1970 cm −1 peak corresponds to linear-bonded CO on the Ru island sites and 2070 cm −1 to the linear-bonded CO on the Pt(1 1 1) sites. Further, two current-potential voltammetric peaks from Pt(1 1 1)/Ru–CO, at ≈0.55 and 0.67 V vs. RHE, are associated with SFG resonances at 1970 and 2070 cm −1 . Upon positive going CV scan, the first peak was selectively removed, and the SFG spectrum confirmed that only the resonance from CO chemisorbed on Pt remained, although there was noticeable spectral intensity reduction and spectral tailing, which will have to be reconciled in future research. Clearly however, it is possible to remove CO from the Ru sites while the Pt sites remain occupied by CO, and the remaining CO contribute to SFG and voltammetry without participation of the CO previously present on the Ru sites.

Surface processes and kinetics of CO2 reduction on Pt(100) electrodes of different surface structure in sulfuric acid solutions

The reduction of CO2 on a Pt(100) electrode in CO2 saturated 0.5 M H2SO4 solutions was studied by in situ FTIR reflection spectroscopy and a programmed potential step technique. Different surface structures of Pt(100) electrode were prepared by different treatments including fast potential cycling (200 V s−1) for a known time. The Pt(100) surface was characterized by a parameter γ that designates the relative amplitude of the current peak of hydrogen adsorption on (100) sites distributed on the one-dimensional surface domains to that on the two-dimensional surface domains. The in situ FTIR spectroscopic results demonstrated that the reduction of CO2 on the Pt(100) dominated by two-dimensional surface domains produced only bridge-bonded CO (COB) species, which give rise to IR absorption near 1840 cm−1. However both bridge- and linear-bonded CO (COL, yielding IR absorption at around 2010 cm−1) species are found for CO2 reduction on the Pt(100) dominated by one–dimensional surface domains. The small intensity of the COL and COB bands indicates that coverage by reduced CO2 species (r-CO2, or COL and COB species) is low. The cyclic voltammetric (CV) studies confirmed quantitatively the in situ FTIRS results, and revealed that the r-CO2 species adsorb preferentially on (100) sites distributed on the two-dimensional surface domains. The initial rate of CO2 reduction υi, i.e., the rate of CO2 reduction on a clean Pt(100) surface, has been determined quantitatively from studies using a programmed potential step technique. It has been demonstrated that the maximum values of υi (υim) measured on Pt(100) electrodes with different surface structures all appeared at − 0.19 V. From analysis of the relationship between υim and γ we have determined that the υim of CO2 reduction on (100) sites distributed on the two-dimensional surface domains is 0.53 × 10−11 mol cm−2 s−1 and that on (100) sites distributed on the one-dimensional surface domains is approximately 2.66 × 10−11 mol cm−2 s−1. Based on in situ FTIRS and electrochemical studies a migration process of the r-CO2 from the one-dimensional surface domains to the two-dimensional surface domains has been proposed to be involved in CO2 reduction. The present study has thrown new light on the electrocatalytic activity of different surface structures of a Pt(100) electrode and the surface processes and kinetics of CO2 reduction.

CO adsorption on the Rh(100) surface studied by high resolution photoelectron spectroscopy

CO adsorption on the Rh(100) surface has been studied by means of high resolution photoelectron spectroscopy supplemented with low energy electron diffraction (LEED) and photoelectron diffraction measurements. From the C 1s core level spectra, as well as from photoelectron diffraction spectra, emission from CO occupying linear and bridge-bonded sites was identified. In the Rh 3d core level spectra the core level shift induced by linear-bonded CO was within 50 meV of the bulk component, whereas bridge-bonded CO gave rise to a core level component shifted about 300 meV to lower binding energy with respect to the bulk component.

NMR investigation of the binding of CO on supported Pd clusters.

The authors report an analysis of nuclear relaxation rates and Knight shifts of $^{13}\mathrm{CO}$ molecules adsorbed on supported Pd clusters. They find that the relaxation rates for on-top (linear) and bridge-bonded CO are proportional to the temperature, an indication of the mixing of molecular orbitals with conduction-band levels. They establish that the relaxation rate of the bridge-bonded CO arises from the Fermi contact interaction. The relaxation rate for the linear-bonded CO is determined, in addition to the Fermi contact interaction, by the interaction of nuclear spins with electron spins in non-s states and with the orbital moments of the electrons. These results indicate that at the Fermi surface bridge-bonded CO exhibits an antibonding combination of the 5\ensuremath{\sigma} orbital of CO with the metal d band, while linear-bonded CO presents admixture also with the CO 2${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ orbitals.

Characterization of the Rh(111) electrode by CEELS, AES, LEED, and voltammetry. Adsorption of (Bi)sulfate, perchlorate, and carbon monoxide

We studied the Rh(111) electrode in sulfuric and perchloric acid solutions, and in sulfuric acid solutions containing dissolved carbon monoxide, using voltammetry, core electron energy loss spectroscopy (CEELS), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Emersion of the electrode from the clean H{sub 2}SO{sub 4} solution to ultrahigh vacuum (UHV) produces a stable ({radical}3 x {radical}3)R 30{degree} (bi)sulfate surface structure. This and some other measurements show that adsorption of perchlorate on rhodium is strong and successfully competes with (bi)sulfate adsorption. CO replaces (bi)sulfate adsorbate irreversibly. In agreement with recent STM investigations, we observe that the main CO structure is (2 x 2). However, depending on experimental conditions, a split (2 x 2) also appears, a LEED analogue of STM`s Rh(111)(3 x {radical}3). AES shows that at a low potential the linear-bonded CO molecules from the (2 x 2)-CO surface structure is substituted in part by the bridge-bonded CO, which increases back-donation of metal d-electrons to the CO. The stronger back-bonding increases the electron screening of the orbital (core) hole, leading to a more fully relaxed final state of the Rh(111)-CO system. 58 refs., 10 figs., 2 tabs.

Structural effects on CO2 reduction at Pt single-crystal electrodes: Part 1. The Pt(110) surface

Cyclic voltammetry and Fourier transform IR spectroscopy have been employed to study the activity of Pt(110) single crystals towards CO2 reduction. Linear-bonded CO and traces of multibonded CO are the main reaction products. No differences in the nature of “reduced CO2” were observed for different adsorption potentials, adsorption times, base electrolyte composition and cooling conditions of the electrode surface. At any potential below 0.20 V the surface can be saturated, indicating that hydrogen readsorption takes place. Anionic carbonate species are adsorbed at potentials above 0.50 V.

Infrared Spectroscopy of Adsorbed Carbon Monoxide on Platinum/Nonacidic Zeolite Catalysts

Infrared spectroscopy of adsorbed carbon monoxide was investigated on a series of platinum (or palladium) zeolite catalysts plus platinum on silica. The platinum catalysts all showed a strong absorbance band around 2070 ± 20 cm−1 due to linear-bonded CO and a weaker band around 1850 ± 20 cm−1 due to bridge-bonded CO. Four platinum catalysts, Pt/K+ LTL, Pt/K+ MFI, Pt/K+ MTW, and Pt/K+ SiO2, had an additional absorbance near 1970 cm−1. The additional band is not observed for Pt/silica, Pt/La+3 LTL, or platinum supported on H+ LTL or H+ MFI, i.e., neutral and acidic supports. The additional band on the alkalized catalysts is speculated to result from an electrostatic attraction of linear-bonded CO with an alkali cation when Pt is on a nonacidic support. The electrostatic attraction increases the CO adsorption strength and lowers the stretching frequency. The additional linear CO absorption band was also observed on Pd/K+ LTL. The additional band, therefore, is not specific to platinum. Although the presence of this additional infrared band suggests a "new type of metal (catalytic) site," no relation was found between the catalytic activity or selectivity for n-hexane dehydrocyclization and the additional CO absorbance band.

Interfacing Surface Electrochemistry with Solid-State NMR. Characterization of Surface CO on Polycrystalline Platinum

A single-channel NMR probe and an electrochemical cell were built to interface surface electrochemistry to NMR. Using a 300 MHz solid-state NMR instrument with a spin-echo pulse sequence and an electrochemical cell configuration, 13C-enriched carbon monoxide adsorbates as a function of the electrode potential were monitored. The adsorbates were obtained via catalytic decomposition of 13C-enriched methanol on a high-surface-area electrode made of powdered platinum. Narrowing of the solid-like NMR peak of the surface CO at more positive potentials was observed. This has been interpreted to result from a transformation of a mixed, predominantly bridge- and linear-bonded CO population to essentially a linear-bonded CO. It has been shown that the 13CO adsorbate is stable in a natural ( 12C) methanol environment even at relatively high electrode potentials, which demonstrates that an exchange of surface CO with solution methanol is practically absent.

Electrode potential dependent NMR spectra of surface 13CO on polycrystalline platinum

The initial results of investigations of a solid/liquid interface under an external potential control using nuclear magnetic resonance are reported. The 13C nuclear magnetic resonance (NMR) spectra of CO formed on a polycrystalline platinum electrode via catalytic decomposition of methanol in aqueous media have been measured in a broad electrode potential range. The narrowing of the solid-like NMR peak at the most negative potentials to a single component at 0.225 V vs. the Ag/AgCl reference is interpreted as resulting from a transformation of a mixed (predominantly bridge- and linear-bonded) surface population to linear-bonded CO. Potential dependent T 1 measurements confirm the changes in bond configurations on the surface. Further oxidation produced solution carbon dioxide as evidenced by the solution-type NMR peak morphology. Our measurements represent the first successful attempt to study 13C adsorbates at the solid/liquid interface where the electrode potential was controlled via apotentiostat.

Characterization and Fourier transform infrared studies of the effects of TiO 2 crystal phases during CO oxidation on [formula omitted] catalysts

The catalytic oxidation of CO was investigated on Pt TiO 2 catalysts in an infrared cell reactor to study the influence of strong metal-support interaction effects on oxidation reactions and of using different crystal forms of TiO 2. Catalysts were prepared using rutile and anatase TiO 2 and were characterized by chemisorption, X-ray diffraction, and X-ray photoelectron spectroscopy. Hydrogen reduction at 500 °C suppresses CO and H 2 chemisorption and leads to changes in Pt 4f 7 2 electron binding energies for both phases of TiO 2. The anatase catalysts show an infrared absorption band around 2091 cm −1 which is assigned to linear-bonded CO; whereas, the rutile catalysts show an absorption doublet for adsorbed CO around 2094 and 2075 cm −1 and bands around 1840 cm −1 assigned to bridge-bonded CO. Both catalysts show the adsorption of CO 2 as a carbonate-like species which is demonstrated to be adsorbed onto the support. The effects of reduction temperature and of support material on CO oxidation activity were compared through temperature-programmed reaction experiments. The catalysts reduced at 200 °C show slightly higher activity and lower ignition temperatures than those reduced at 500 °C. The rutile-supported catalysts show much higher CO oxidation activity with lower ignition temperatures; the increased activity is speculated to result from a lower activation energy for oxygen desorption. A morphological model of metalsupport interactions involving oxygen transfer from the rutile support is proposed to coexist with the Langmuir-Hinshelwood reaction mechanism.

Temperature-programmed desorption of CO from Ni/SiO2 Application of analytic techniques for strongly interacting adsorbates

The interactions of CO with a Ni/SiO2 catalyst were studied using the temperature-programmed desorption (TPD) technique. Programmed heating following adsorption led to desorption of both CO and CO2. C and O were found to remain on the catalyst surface after TPD of CO; both were removed by subsequent H2 treatment. Four CO desorption peaks, designated β*, β1, β2, and β′, were assigned to a surface-type carbonyl, linear-bonded CO, bridge-bonded CO, and the recombination of C and O adatoms, respectively. CO2 desorption spectra showed a single desorption peak, P2, when the catalyst weight was low. A new chemical pathway, revealed by the appearance of another peak, P1, was opened for a higher weight of sample. The formation of P1 and P2 followed first-order and second-order kinetics, respectively.The strong readsorption properties of CO within the catalyst bed have made direct determination of kinetic parameters difficult. In a limited range of temperatures, however, a similarity of the CO desorption spectra to results from unsupported Ni was found. Numerical simulation of the TPD process for a flow system has shown that the differential bed assumption is reasonable, i.e. a uniform distribution of the adsorbate within the bed during desorption is approximated. With these observations serving as a basis, the catalyst weight, the amount adsorbed, and the heating rate were varied to obtain desorption energies by analysis of the TPD data for both CO and CO2.

FTIR studies of surface reaction dynamics: I. Temperature and concentration programming during CO oxidation on [formula omitted

A transient technique integrating FTIR spectroscopy with Temperature-Programmed Reaction (TPR) and Concentration-Programmed Reaction (CPR) using a small-volume infrared reactor, has been developed. Studies of CO oxidation on a Pt SiO 2 catalyst using these methods, have shown the occurrence of remarkable surface temperature behavior in regions of ignition and quenching. Extensive documentation of linear-bonded CO behavior is presented and the relevance of a Langmuir-Hinshelwood and other reaction models is discussed. Two different energy transport mechanisms are described which might lead to the observed catalyst temperature excursions.