CO Probe Research Articles

Identity of two types of strong Brønsted acid sites in mazzite revealed by CO probe: IR study and periodic DFT modeling

AbstractThe nature of catalytic Brønsted sites in mazzite is clarified by molecular modeling combined with spectroscopy. Density Functional Theory study for periodic models of high‐silica mazzite evidence that most stable bridging hydroxyls, noticeably binding CO probe, fall into two categories: Brønsted sites located in larger channels, characterized by higher OH frequency of bare hydroxyl with very large redshift upon CO interaction, and lower‐frequency sites located in smaller channels, showing lower redshift. This fully corresponds to two bands obtained for OH stretch in IR spectra. Very good agreement between theory and experiment found in this work not only confirms that Brønsted sites studied here belong to the strongest acid sites among known zeolites but also clarifies their identity in mazzite. Location of sites with exceptionally large red shift upon CO adsorption at 12‐T wide channel very well conforms to both intuitive expectations and predictions for other zeolites from former studies.

Nature of stable single atom Pt catalysts dispersed on anatase TiO2

Pt single atom catalysts anchored on the surface of anatase TiO2 have recently been shown to be stable through oxidation and reduction conditions and exhibit higher catalytic activity in CO oxidation than Pt metallic clusters. In this work we unravel the atomistic nature of the isolated Ptiso species on anatase TiO2 thanks to the combination of CO probe molecule Fourier Transform Infrared (FTIR) and temperature programmed desorption (TPD) experiments with an extensive set of Density Functional Theory (DFT) calculations. We used the stretching frequencies and adsorption energies of CO bound to Pt as a fingerprint of the specific structure of the stable Ptiso species. These consist of Pt atoms bound to two excess O atoms on the surface (PtO2). The excess oxygen atoms on the surface arise from the formation of surface hydroxyl groups, and provide a solid anchor to the oxide support, which explains the high thermal stability of the single-atom Pt catalyst through oxidative and reductive treatments. Beside the single atom Pt species, also models of metallic, Ptmetal, and oxidized, Ptox, sub-nanometer clusters have been considered. Comparisons between characteristics of Ptiso species and sub-nanometer Ptmetal and Ptox clusters demonstrate that the combination of CO vibrational frequency and adsorption energy can be an effective approach to differentiate these species.

Open Metal Sites in the Metal–Organic Framework CPO-27-Cu: Detection of Regular and Defect Copper Species by CO and NO Probe Molecules

The open copper metal sites in CPO-27-Cu were studied by means of IR spectroscopy of adsorbed CO and NO, and density functional theory calculations. Very low Lewis acidity of the Cu2+ sites was established by CO (IR band at 2153–2149 cm–1). Variable-temperature IR experiments indicate adsorption enthalpy of ca. −20 kJ mol–1. It was also found that CO is a sensitive probe of the occupation of the neighboring copper sites. In contrast to the general expectations, NO is very weakly adsorbed on the Cu2+ sites (−14.5 kJ mol–1, IR band at 1888 cm–1). The effect is attributed to the particular Cu2+ ion coordination and electronic state, leading to a large Jan–Teller deformation and low effective charge, preventing significant charge transfer effects between the metal center and the guest molecules as well as any significant electrostatic interactions. Thus, dominating are van der Waals interactions which position the adsorbed molecule relatively far away at about 2.7–3.0 A. Adsorption of CO also revealed that a ...

Pentameric PdAu and PdPt nanoparticles on the MgO(1 0 0) surface and their CO and O2 adsorption properties

The surface mode of the Birmingham Cluster Genetic Algorithm (S-BCGA), which performs an unbiased global optimisation search for clusters adsorbed on a surface, has been employed for the global optimisation of noble metal pentamers on an MgO(1 0 0) substrate. The effect of element identity and alloying in surface-bound neutral subnanometre particles is calculated by energetic analysis of all compositions of supported 5-atom PdAu and PdPt clusters. Our results show that the binding strengths of the component elements to the surface are in the order Pt > Pd > Au. In addition, alloying Pd with Au and Pt is favorable for this size since excess energy calculations show a preference for bimetallic clusters for both cases. Furthermore, the electronic behaviour, which is intermediate between molecular systems and bulk metals allows tuning of the characteristics of particles in the subnanometre size range. The adsorption of CO and O2 probe molecules are also modelled and it is found that CO and O2 adsorption leads to a weakening of the cluster–surface interaction.

Influence of Tetraalkylammonium Compounds on Photocatalytic and Physical Properties of TiO2

A wide range of experimental data are reported for the first time on the TiO2 prepared by hydrolysis of highly concentrated Ti(OiPr)4 in water solutions of quaternary ammonium compounds (QACs). These TiO2 materials have been shown to be photocatalytically active under visible light irradiation (LED, 450 nm) using acetone as a model substrate oxidized in the gas phase. Five-fold increase in activity in comparison with the commercial photocatalyst KRONOClean 7000 is achieved. Colloidal solutions of hydrolyzed Ti(OiPr)4 have been studied by SAXS method suggesting the way in which QACs solutions may influence the final composition of TiO2. Phase composition, morphology, texture and surface properties of the modified TiO2 have been studied using XRD, BET, SEM and low-temperature FTIR with CO probe. The surface elemental composition has been investigated by XPS method. Additional low-energy levels and high concentration of acid surface sites originated from N/C-doping, are likely to be the main reasons for exceptional photocatalytic performance of these samples.

Investigation of the acidic nature of MCM-68 zeolite based on the adsorption of CO and bulky probe molecules

The aluminosilicate MCM-68 has an MSE topology, a unique pore system, and solid acid characteristics. For these reasons, it has potential applications as a highly efficient catalyst for paraffin cracking, methanol-to-olefin/dimethyl ether-to-olefin reactions, and the alkylation of aromatic compounds. In this work, the acidic properties of both the parent MCM-68 and dealuminated versions were evaluated using infrared spectroscopy in conjunction with the adsorption of CO and/or 2,4,6-trimethylpyridine (collidine). Notably, the relatively bulky collidine molecules could not enter the 12-ring pore-mouths of the MCM-68 at room temperature but were able to do so at 403 K, due to the excitation of pore-mouth breathing vibrations that allowed increased diffusion at higher temperatures. During CO adsorption study, two types of Lewis acid sites and one type of Brønsted acid sites were identified on the parent MCM-68. After dealumination by acid treatment, the Brønsted and stronger Lewis acid sites, especially those on the external surfaces of the MCM-68 particles, were preferentially removed.

Utilising the interface interaction on tris(hydroxymethy)aminomethane-capped carbon dots to enhance the sensitivity and selectivity towards the detection of Co(II) ions

In this paper, a facile and green thermal carbonisation approach was adopted for the synthesis of carbon dots (CDs) from α-cellulose. The conditions during the synthesis have been optimised to produce CDs with highest fluorescence intensity. Later, tris(hydroxymethy)aminomethane (TRIS) was introduced to the isolated CDs as capping agent on the interface of the CDs. TRIS was chosen due to the well reported capability to interact with various metal ions to form complexes. Particularly in this study, TRIS has enhanced the specificity and sensitivity features of CDs to detect cobalt (II) ions, (Co(II) ions). The UV–vis absorption spectroscopy study performed has found that the CDs can effectively interact with TRIS, while the assembled CDs/TRIS system later can form complex with Co(II) ions. In this case, TRIS has acted as the bridge to bring CDs and Co(II) ions to a close proximity in distance, which subsequently promoted the energy transfer due to the good overlapping in the emission of the CDs with the absorbance of the Co(II) ions. The process was observed clearly by the concentration dependent fluorescence quenching in the presence of increasing amount of Co(II) ions. This has been demonstrated for the first time and the system has been suggested to be used as a single shot detection probe for Co(II) ions in aqueous media. The probe was found to be effective for the detection of Co(II) ions with a linear response up to 1.87 mM and a limit of detection of 16.84 μM. This study has presented some new insights on the surface interface interaction between CDs and its capping agent and demonstrated its potential application for metal ions sensing.

Nature of Sintering‐Resistant, Single‐Atom Ru Species Dispersed on Zirconia‐Based Catalysts: A DFT and FTIR Study of CO Adsorption

AbstractRecent studies on the upgrade of cellulosic biomass to renewable chemicals and fuels based on Ru/ZrO2 catalysts have shown that the catalyst contains atomically dispersed, highly stable Ru atoms. FTIR spectra after CO dosage reveal a complex manifold of bands resulting from Ru‐CO carbonyl species. The nature of the atomically dispersed Ru species on monoclinic zirconia, and the origin of their thermal stability are the object of this investigation. Combining density functional theory (DFT) calculations on model systems of both tetragonal and monoclinic ZrO2, with novel experimental data, we provide a basis for the identification of the monoatomic Ru species. Various candidates are explored based on DFT calculations of their intrinsic stability and of the vibrational properties of adsorbed CO probe molecules. The results allow us to discard a number of possible structures, and restrict the analysis to a few potential candidates. Most likely, the atomically dispersed sites result from the interaction of Ru atoms with one OH group of the surface with elimination of H2 by condensation, leaving RuO species strongly bound to the zirconia surface. CO binds strongly to the RuO units forming (RuO)(CO)2 geminal complexes with characteristic signatures in terms of CO vibrational frequency. Despite the formal positive oxidation state of the Ru species, large negative shifts are found in the CO stretching frequency compared with the free CO molecule.

Fine speciation of active sites in zeolites by a CO probe: Dynamics and IR frequencies

AbstractThe influence of system dynamics onto infrared spectra calculated and measured for probe molecules weakly interacting with active sites is discussed. OH stretching frequencies for Brønsted sites in Y zeolite and their shift on CO adsorption (measured by IR spectroscopy at ca. 170 K) are simulated from AIMD trajectory, based on BO DFT calculations. We present here very good agreement between experimental and computed OH frequency red‐shift on CO adsorption in dealuminated zeolite Y. It is shown that extracting nonharmonic OH frequencies from ab initio molecular dynamics trajectory provides better estimate of experimental IR results than standard vibrational analyses due to realistic computational model and better correspondence with experimental conditions.

CO Adsorption on Graphite-like ZnO Bilayers Supported on Cu(111), Ag(111), and Au(111) Surfaces

Graphitic-like ZnO bilayer films deposited on coinage metals Cu(111), Ag(111), and Au(111) have been studied by density functional theory calculations including dispersion corrections. The scope is to compare on an equal footing the properties of the three systems and in particular the nature of the metal/oxide interface. To this end we have considered the adsorption of a CO probe molecule and the vibrational shifts induced by adsorption on ZnO/Cu(111), ZnO/Ag(111), and ZnO/Au(111) compared to adsorption on the unsupported ZnO bilayer and on the wurtzite ZnO (1010) surface. We find that while the interaction of ZnO with Ag and Au supports is dominated by dispersion interactions with little or no charge transfer at the interface, in the case of Cu a moderate electron transfer occurs toward the ZnO bilayer. As a consequence, while the stretching frequency of CO on ZnO/Au is blue-shifted, that on ZnO/Cu is red-shifted compared to free CO. CO on ZnO/Ag is intermediate. In all three cases, however, the ZnO bi...

Catalyst Architecture for Stable Single Atom Dispersion Enables Site-Specific Spectroscopic and Reactivity Measurements of CO Adsorbed to Pt Atoms, Oxidized Pt Clusters, and Metallic Pt Clusters on TiO2.

Oxide-supported precious metal nanoparticles are widely used industrial catalysts. Due to expense and rarity, developing synthetic protocols that reduce precious metal nanoparticle size and stabilize dispersed species is essential. Supported atomically dispersed, single precious metal atoms represent the most efficient metal utilization geometry, although debate regarding the catalytic activity of supported single precious atom species has arisen from difficulty in synthesizing homogeneous and stable single atom dispersions, and a lack of site-specific characterization approaches. We propose a catalyst architecture and characterization approach to overcome these limitations, by depositing ∼1 precious metal atom per support particle and characterizing structures by correlating scanning transmission electron microscopy imaging and CO probe molecule infrared spectroscopy. This is demonstrated for Pt supported on anatase TiO2. In these structures, isolated Pt atoms, Ptiso, remain stable through various conditions, and spectroscopic evidence suggests Ptiso species exist in homogeneous local environments. Comparing Ptiso to ∼1 nm preoxidized (Ptox) and prereduced (Ptmetal) Pt clusters on TiO2, we identify unique spectroscopic signatures of CO bound to each site and find CO adsorption energy is ordered: Ptiso ≪ Ptmetal < Ptox. Ptiso species exhibited a 2-fold greater turnover frequency for CO oxidation than 1 nm Ptmetal clusters but share an identical reaction mechanism. We propose the active catalytic sites are cationic interfacial Pt atoms bonded to TiO2 and that Ptiso exhibits optimal reactivity because every atom is exposed for catalysis and forms an interfacial site with TiO2. This approach should be generally useful for studying the behavior of supported precious metal atoms.

Design of Highly Efficient Pt-SnO2 Hydrogenation Nanocatalysts using Pt@Sn Core–Shell Nanoparticles

In this work, Pt-SnO2 heteroaggregate nanocatalysts were synthesized by in situ transformation of Pt@Sn core–shell nanoparticles and their catalytic performance for hydrogenation of various substituted nitroaromatics was investigated. The Pt@Sn nanoparticles were prepared by a one-step method, and the alumina-supported Pt@Sn nanoparticles were further transformed in situ into Pt-SnO2 heteroaggregate nanostructures by calcination. The structures of Pt@Sn and Pt-SnO2 nanomaterials were characterized, and FT-IR with CO probes, HRTEM, XRD, and XPS characterizations revealed that the as-synthesized Pt@Sn nanoparticles were core@shell-like structures with Sn-rich shells and Pt-rich cores and the obtained Pt-SnO2 heteroaggregate nanostructures consisted of close-contact pure Pt and SnO2 phases. The Pt-SnO2/Al2O3 nanostructures demonstrated a better catalytic performance for hydrogenation of various substituted nitroaromatics relative to individual Pt/Al2O3 nanocatalysts. Theoretical calculations suggested that P...

A new fluorescent PET sensor probe for Co2+ion detection: computational, logic device and living cell imaging applications

A new probe,2exhibit quenching with Co2+(∼80% at 634 nm) while2·Co2+ensemble exhibit enhancement with NO3−(∼82% at 632.5 nm). On–Off–On behavior of2(Co2+and NO3−ions) the function of a sequential XNOR gate and can be utilized in live cell imaging.

Detection of CO and HCN in Pluto’s atmosphere with ALMA

Observations of the Pluto-Charon system, acquired with the ALMA interferometer on June 12–13, 2015, have led to the detection of the CO(3-2) and HCN(4-3) rotational transitions from Pluto (including the hyperfine structure of HCN), providing a strong confirmation of the presence of CO, and the first observation of HCN in Pluto’s atmosphere. The CO and HCN lines probe Pluto’s atmosphere up to ∼450 km and ∼900 km altitude, respectively, with a large contribution due to limb emission. The CO detection yields (i) a much improved determination of the CO mole fraction, as 515 ± 40 ppm for a 12 μbar surface pressure (ii) strong constraints on Pluto’s mean atmospheric dayside temperature profile over ∼50–400 km, with clear evidence for a well-marked temperature decrease (i.e., mesosphere) above the 30–50 km stratopause and a best-determined temperature of 70 ± 2 K at 300 km, somewhat lower than previously estimated from stellar occultations (81 ± 6 K), and in agreement with recent inferences from New Horizons / Alice solar occultation data. The HCN line shape implies a high abundance of this species in the upper atmosphere, with a mole fraction >1.5 × 10−5 above 450 km and a value of 4 × 10−5 near 800 km. Assuming HCN at saturation, this would require a warm (>92 K) upper atmosphere layer; while this is not ruled out by the CO emission, it is inconsistent with the Alice-measured CH4 and N2 line-of-sight column densities. Taken together, the large HCN abundance and the cold upper atmosphere imply supersaturation of HCN to a degree (7–8 orders of magnitude) hitherto unseen in planetary atmospheres, probably due to a lack of condensation nuclei above the haze region and the slow kinetics of condensation at the low pressure and temperature conditions of Pluto’s upper atmosphere. HCN is also present in the bottom ∼100 km of the atmosphere, with a 10−8–10−7 mole fraction; this implies either HCN saturation or undersaturation there, depending on the precise stratopause temperature. The HCN column is (1.6 ± 0.4)× 1014 cm−2, suggesting a surface-referred vertically-integrated net production rate of ∼2 × 107 cm−2 s−1. Although HCN rotational line cooling affects Pluto’s atmosphere heat budget, the amounts determined in this study are insufficient to explain the well-marked mesosphere and upper atmosphere’s ∼70 K temperature, which if controlled by HCN cooling would require HCN mole fractions of (3–7) ×10−4 over 400–800 km. We finally report an upper limit on the HC3N column density (<2 × 1013 cm−2) and on the HC15N / HC14N ratio (<1/125).

A colorimetric and fluorescent turn-on probe for carbon monoxide and imaging in living cells

A colorimetric and fluorescent dual probe for CO is designed and synthesized. The probe is capable of sensitive and specific detection of CO with naked-eye through colorimetric assay in phosphate buffer from colorless to yellow, which also features 75-fold turn-on fluorescence enhancement upon exciting at 480nm with the detection limit of 26.3nM. The probe shows great potential for sensing intracellular CO without the interference in living cell imaging. On the other hand, the probe could also be utilized as an efficient and convenient visual indicator for CO gas in air by naked eyes.

A DFT study of the acid–base properties of anatase TiO2 and tetragonal ZrO2 by adsorption of CO and CO2 probe molecules

We have performed a comparative study of the acid–base characteristics of the surfaces of anatase TiO2 and tetragonal ZrO2. To this end we performed DFT+U calculations on CO and CO2 probe molecules adsorbed both on terraces and steps of the two oxides. For titania, CO adsorption results in a moderate adsorption energy (about −0.3eV) and in a positive shift of the CO stretching frequency (about +40cm−1), typical of Lewis acid sites, with no clear difference in the acidity between terraces or steps. For zirconia we found a similar CO binding energy as for titania, and a CO vibrational shift that depends on the location of the Zr cation: negligible on terraces, similar to TiO2 on steps. We conclude that the acidic properties are similar in the two oxide surfaces. Things are different for CO2 adsorption. On titania the interaction is weak and surface carbonates compete with physisorbed CO2, indicating a weak basic character. On the contrary, on zirconia three types of stable carbonates have been identified. Their vibrational frequencies are consistent with IR measurements reported in the literature. The most stable species forms on steps of the t-ZrO2 surface and consists of a CO32− unit which lies flat on the surface with the O atoms pointing towards three Zr ions. The species forms spontaneously by extraction of a lattice oxygen by an incoming CO2 molecule. The different reactivity points towards a much more pronounced basic character of zirconia compared to titania, at least if measured by CO2 adsorption.

The role of MgO in the performance of Pd/SiO2/cordierite monolith catalyst for the hydrogenation of 2-ethyl-anthraquinone

Highly dispersed Pd/SiO2/cordierite monolith catalysts (PSC) were prepared to investigate the influence of support alkalinity on 2-ethyl-anthraquinone (eAQ) hydrogenation. The support alkalinity was adjusted by the MgO content in SiO2 washcoat. The behavior of the PSC catalysts was tested by liquid phase hydrogenation of eAQ in a continuous trickle bed reactor. PSC catalyst with 2wt.% MgO (Mg-2 sample) exhibited excellent for hydrogenation of eAQ. The support alkalinity directly influenced the electronic properties of the Pd nanoparticles as proved by Fourier transformed infrared spectroscopy with CO probe molecule and X-ray photoelectron spectroscopy. The electron density at lower binding energy of the supported Pd particles increased with the increasing alkalinity. PSC catalysts with alkali modifiers exhibited higher hydrogenation efficiency for benefiting the activation of CO group in eAQ and the adsorption of eAQ via the enhanced interaction of lone pair of CO electrons. However, increased support alkalinity contributed to higher Pd loss due to the weak coating strength. Besides, the active quinone selectivity decreased with the progressive MgO addition due to the tautomerization of 2-ethylanthrahydroquinone to 2-ethyloxoanthrone, which is the precursor of other degradations. Consequently, a volcano shape curve between hydrogenation efficiency (H2O2 yield) and MgO content was obtained.

Cu SSZ-13 zeolite catalyst on metallic foam support for SCR of NOx with ammonia: Catalyst layering and characterisation of active sites

A particular aim of this work was to develop a method of catalyst layering on the surface of metallic catalyst carriers used in structured reactors. The CuSSZ-13 catalyst was chosen as a prospective catalyst for SCR of NOx converter for exhaust from biogas engines. For the first time in the literature the method of in situ synthesis CuSSZ-13 catalyst was elaborated and tested for precalcined Kanthal foam. The CuSSZ-13 catalyst prepared in such a way demonstrated very high activity in NH3-SCR of NO with ammonia and high hydrothermal stability in comparison with analogically prepared CuZSM-5 used as a reference catalyst. The active centres working during SCR at various temperatures were investigated by in situ FTIR experiments with NH3 and CO probe molecules. The structure-activity relationships found for catalyst working at high temperature show that the Lewis acidic Cu+ sites are available exclusively for NO and Brønsted acidic sites for NH3 sorption above 400°C. Furthermore, high hydrothermal stability of the CuSSZ-13 catalyst is correlated to the favourable location of Al atoms in the zeolite lattice which prevents dealumination and also to the resulted from this stabilization of copper Cu+ active sites.

Can the state of platinum species be unambiguously determined by the stretching frequency of an adsorbed CO probe molecule?

The paper addresses possible ambiguities in the determination of the state of platinum species by the stretching frequency of a CO probe, which is a common technique for characterization of platinum-containing catalytic systems. We present a comprehensive comparison of the available experimental data with our theoretical modeling (density functional) results of pertinent systems - platinum surfaces, nanoparticles and clusters as well as reduced or oxidized platinum moieties on a ceria support. Our results for CO adsorbed on-top on metallic Pt(0), with C-O vibrational frequencies in the region 2018-2077 cm(-1), suggest that a decrease of the coordination number of the platinum atom, to which CO is bound, by one lowers the CO frequency by about 7 cm(-1). This trend corroborates the Kappers-van der Maas correlation derived from the analysis of the experimental stretching frequency of CO adsorbed on platinum-containing samples on different supports. We also analyzed the effect of the charge of platinum species on the CO frequency. Based on the calculated vibrational frequencies of CO in various model systems, we concluded that the actual state of the platinum species may be mistaken based only on the measured value of the C-O vibrational frequency due to overlapping regions of frequencies corresponding to different types of species. In order to identify the actual state of platinum species one has to combine this powerful technique with other approaches.

A new water-soluble heteronuclear PdII–AuI pincer complex as two-photon luminescent probe for biological Co2+ detection

A new water-soluble heteronuclear PdII–AuI pincer complex was synthesized and investigated for biological Co2+ detection.