DRIFT Spectroscopy Research Articles

Regulation of Al distributions and Cu2+ locations in SSZ-13 zeolites for NH3-SCR of NO by different alkali metal cations

Different types of alkali metal cations (Li+, Na+, K+ and Cs+) were used as counter-cations to synthesize SSZ-13 for improving its catalytic performance for selective catalytic reduction (SCR) of NO by NH3. The Al distribution and Cu2+ locations in the framework of SSZ-13 were investigated by DR UV–vis, 27Al and 29Si MAS NMR, ESR, NH3– and CO-adsorbed DRIFT spectroscopies. It is shown that paired Al species and distorted AlO4- tetrahedra both increase with decreasing radii of alkali metal cations, which leads to an increase of isolated Cu2+ species and an enhancement of NH3-SCR catalytic performance. Surprisingly, some Cu(OH)+ species was transformed into isolated Cu2+ species when the sample was treated at 800 °C for 12 h in low-content H2O/air despite of occurrence of a slight dealumination in the framework. The isolated Cu2+ species is demonstrated to show higher catalytic activity than Cu(OH)+ species and Cu2+ species adsorbed on Al2O3 species.

Ni-exsolved La1-xCaxNiO3 perovskites for improving CO2 methanation

This work investigates the promoting effect of Ca on CO2 hydrogenation to methane in the highly exsolved Ni particles of La1-xCaxNiO3 perovskite catalysts. TEM images presented that metallic Ni particles are exsolved to the surface under a reducing atmosphere. This exsolution behavior is observed until the Ca ratio (x) becomes 0.4, suggesting that the highly dispersed Ni particles formed by the exsolution can actively adsorb and dissociate H2 molecules. In-situ DRIFT spectroscopy reveals that the surface Ca-sites effectively adsorb CO2 molecules to form reactive carbonates species. XPS analyses and CO methanation tests further show that adding Ca negatively charges the adsorption sites and enhances the reactivity of CO. This surface coordination is optimized by incorporating 0.4 of Ca, presenting facile hydrogenation of CO molecules to CH4. Thus, incorporating 0.4 of Ca into the A-site of LaNiO3 demonstrated the optimized promoting effect on CO2 methanation.

Dynamics of Pd Dopant Atoms inside Au Nanoclusters during Catalytic CO Oxidation.

Doping gold nanoclusters with palladium has been reported to increase their catalytic activity and stability. PdAu24 nanoclusters, with the Pd dopant atom located at the center of the Au cluster core, were supported on titania and applied in catalytic CO oxidation, showing significantly higher activity than supported monometallic Au25 nanoclusters. After pretreatment, operando DRIFTS spectroscopy detected CO adsorbed on Pd during CO oxidation, indicating migration of the Pd dopant atom from the Au cluster core to the cluster surface. Increasing the number of Pd dopant atoms in the Au structure led to incorporation of Pd mostly in the S–(M–S)n protecting staples, as evidenced by in situ XAFS. A combination of oxidative and reductive thermal pretreatment resulted in the formation of isolated Pd surface sites within the Au surface. The combined analysis of in situ XAFS, operando DRIFTS, and ex situ XPS thus revealed the structural evolution of bimetallic PdAu nanoclusters, yielding a Pd single-site catalyst of 2.7 nm average particle size with improved CO oxidation activity.

LDH-GO composites as catalysts for the oxidative removal of indigo carmine dye from wastewater

Herein, we present the results of our researches focused on obtaining composites with enhanced affinity for indigo carmine (IC) by combining two different 2D materials e.g. graphene oxide (GO) and a Cerium modified layered double hydroxide (LDH), which could act as catalysts for the oxidative removal of IC, both under ultrasonic irradiation and under conventional stirring, using H2O2 as oxidation agent. Three Mg3Al0.75Ce0.25 LDH-GO composites bearing different concentrations of GO in the range of 5-15 wt.% abbreviated as HT3Ce-xGO where x stands for the concentration of GO (x=5,10,15% wt. GO) have been prepared and characterized by XRD, SEM, Raman and DRIFT spectroscopy. The XRD patterns of HT3Ce-xGO solids have revealed the formation of nanocomposites with fine particles of CeO2 (Scherrer dimensions around 3 nm) embedded in the 2D layered structure of LDH-GO. CeO2 phase was favored by the increase in GO content for all investigated composites. The size decrease of the solid particles with the increase of GO content was proved both by SEM and DRIFT analyses. Raman spectra proved the incorporation of GO in all composites. The results of the catalytic tests showed, without any doubt the activation effect of the ultrasonic irradiation which allowed doubling the dye removal percentage (DR%) in the first 30 minutes of reaction time. For the most active catalyst, HT3Ce-15GO, the COD after 30 minutes reaction time under ultrasonic irradiation was 221 mgO2/L, while TOC was 93 mgC/L marking a decay of 61.6% for COD and 46.2% for TOC.

Niobia-based magnetic nanocomposites: Design and application in direct glucose dehydration to HMF

Niobia-based magnetic nanocomposites were prepared by covering magnetite nanoparticle cores (Fe3O4, MNP) with either Nb2O5 or Nb2O5-SiO2 shells using a two-step procedure. In the first step magnetite nanoparticles were prepared by the coprecipitation method. The second step involved their coverage with either Nb2O5 shells, through a precipitation method, or with Nb2O5-SiO2 shells, through a sol-gel protocol followed by precipitation in the presence of the CTAB surfactant. The obtained materials were exhaustively characterised by X-ray diffraction, Mössbauer spectroscopy, magnetic measurements, ICP-OES, DRIFT and Raman spectroscopy, and CO2 - and NH3-TPD measurements, and investigated for glucose dehydration to HMF. The catalytic performances were directly correlated to the nature of the supported niobia phases, which, in turn, has been dictated by the niobia content and the preparation route. The high selectivity to HMF was correlated with to the large pseudohexagonal niobium oxide (TT-Nb2O5) phase while the catalytic activity was directly correlated to the small nanoparticles size. A proper combination of these features led to an optimum catalytic system for the selective production of HMF through glucose dehydration. A third important feature making the developed catalyst promising is its magnetic property, ensured by the magnetite nanoparticles core. This allowed its easy separation from the reaction products.

Enhanced Catalytic Performance of Cr/MOR for Ethane Dehydrogenation Through Dealumination

Dehydrogenation of ethane to ethylene assisted by CO2 was investigated over CrOx catalysts supported on MOR zeolites. The catalysts were characterized by XRD, N2 adsorption, SEM, 27Al and 29Si MAS NMR, XPS, DRIFTS, H2-TPR and laser Raman spectroscopy. The catalysts exhibit enhanced activity and stability after MOR dealumination, with an optimal ethane conversion of ~ 38% and ethylene yield of ~ 33%. Characterization results indicate that the improved catalytic performance derives from the increased silanol nests of MOR support created by acidic dealumination, which are in favor of dispersion and stabilization of CrOx species, resulting in more reducible Cr(VI) considered as the active site for dehydrogenation.

Synthesis strategies towards amorphous porous carbons with selective oxygen functionalization for the application as reference material

Manipulation of carbon surface functionalization and especially of oxygen surface groups has been demonstrated to be key for obtaining high performance materials in a multitude of applications. Although control of carbon surface chemistry offers large potential in many technical relevant applications, qualitative and quantitative analysis of surface oxides for amorphous and porous carbons remains challenging. In this study, we attempt selective, organic chemistry-based functionalization of a polymer-derived porous model carbon featuring high oxygen loadings, with the aim to establish analytical standards for temperature programmed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). In this context, quantitative chemical reduction of an oxidized carbon material with lithium aluminum hydride (LiAlH4) is utilized as the key for the synthesis of carbon materials with defined ensembles of oxygen surface groups (hydroxyl groups and ethers). Based on this reduction strategy, selective LiAlH4 reduction based on protection group chemistry leads to aldehydes, ketones and quinones as surface functional groups and chemical grafting is studied for the selective introduction of phenyl esters, methyl ethers and carboxylic acids. All materials are evaluated with respect to the applicability as analytical standard by DRIFT spectroscopy, TPD, X-ray photoelectron spectroscopy and titration methods.

Electrochemical preparation of Cu/Cu2O-Cu(BDC) metal-organic framework electrodes for photoelectrocatalytic reduction of CO2

Abstract Thin films of the metal-organic framework (MOF) Cu(BDC) were electrochemically grown by anodic deposition of the ligand 1,4-benzenedicarboxylate (1,4-BDC) on metallic copper to form Cu/Cu2O-Cu(BDC) electrode. The construction of the electrode was optimized by investigating parameters such as current density, time of anodization, and temperature that directly affect its stability and photoactivity. The best performing electrode was prepared when a current density of 2.5 mA cm−2 was applied for 6.5 min at 110 °C. A methanol concentration of 234 μmol L-1 was produced from the photoelectron reduction of CO2 under UV–vis irradiation for 3 h, an applied potential of +0.10 V, and 0.1 mol L-1 aqueous sodium sulfate solution saturated with CO2 as supporting electrolyte. The rate of CO2 reduction to methanol was found to be ∼20 times that obtained using Cu/Cu2O electrode alone presumably due to preconcentration of dissolved CO2 in the MOF. The capture of CO2 in the MOF surface and/or cavities was confirmed by ATR and DRIFT spectroscopy.

K-doping effect in the kinetics of CO2 capture at high temperature over lithium silicates obtained from rice husks: In situ/operando techniques

K-doped lithium silicates were synthesized as CO2 sorbents using rice husks as silica source. Two different doping methods were used to prepare the solids, incipient wetness impregnation and co-impregnation. Noticeable differences were observed between the sorbents obtained by each route. Unlike the co-impregnated sample, a KLi3SiO4 phase was detected by XRD in the doped-samples prepared by incipient wetness impregnation. This phase was associated with a porous layer deposition over the sorbent particle surface, which was observed by SEM. The effect of CO2 partial pressure and sorption temperature was evaluated for the solids. A remarkable improvement of the capture efficiency was observed in the doped-samples, especially at low temperature and CO2 partial pressure. The kinetics of the samples was studied using the double exponential model and operando Raman and DRIFT spectroscopies. Different reactivity properties were found depending on the synthesis method employed. The complete analysis of the results allowed us to propose a transformation mechanism for the solids during the sorption step, highlighting the improvement of surface reaction and volumetric diffusion processes in the CO2 capture step.

Interface excess on Sb-doped TiO2 photocatalysts and its influence on photocatalytic activity

Titanium dioxide (TiO2) is a well-known photocatalyst that has been used in photocatalysis and has been studied as an alternative for artificial photosynthesis cells. Many works have tried doping this material with different elements to increase its photoactivity; however, a consequence of doping that has not received much attention is the segregation at the nanoparticle interfaces, such as the surface and the grain boundary (GB). In this study, Sb-doped TiO2 nanoparticles have been produced by the polymeric precursor method, with the compositional range varying from 0 to 2 mol% Sb. The surface excess was measured by the surface lixiviation method, and the GB excess was calculated considering the total amount of Sb and its solubility in the TiO2 bulk. The results showed that Sb interfacial segregation at both surface and GB increased the specific surface areas, reduced the crystallite sizes, increased the presence of the rutile phase, and changed the CO2 adsorption, evidenced by DRIFT spectroscopy. The photoactivity was tested by the degradation of acetaminophen (ACT) under UV irradiation, revealing an increase of ~53% of activity for 0.05 mol% Sb-doped TiO2 compared to undoped TiO2.

Photocatalytic degradation mechanism of benzene over ZnWO4: Revealing the synergistic effects of Na-doping and oxygen vacancies

Metal ion doping and vacancy-engineering are effective strategies to promote photocatalytic performance. ZnWO4 is an important inorganic photocatalytic material. Herein, ZnWO4 with Na-doping and oxygen vacancies (OVs) was prepared by a facile one-step hydrothermal strategy via adding Na2WO4. Due to the synergistic effects of Na-doping and OVs, the photocatalytic benzene removal rate of the ZnWO4 samples is much higher than that of pure ZnWO4. According to the characterization results, the Na-doping and OVs not only promote the migration of electrons but also activate H2O and O2 molecules to generate a large number of reactive radicals. The results of in situ DRIFT spectroscopy and DFT calculations clearly demonstrate that the Na-doping and OVs enhance the adsorption and activation of reactants and intermediates. The photocatalytic benzene oxidation mechanism is proposed. Benzene is completely oxidized following the order of phenol, catechol (quinol), benzoquinone, and small molecule organics such as acetic acid, eventually producing H2O and CO2. The work could provide new mechanistic understanding for photocatalytic VOCs decomposition.

Chemoresistive CO2 Gas Sensors Based On La2O2CO3: Sensing Mechanism Insights Provided by Operando Characterization.

Our previous studies demonstrated that rare-earth oxycarbonates Ln2O2CO3 (Ln = La, Nd, and Sm) and rare-earth oxides Ln2O3 (Ln = Nd, Sm, Gd, Dy, Er, and Yb) are sensitive to CO2 and that hexagonal La2O2CO3 is the best among them in terms of sensitivity, stability, and selectivity. In this study, we have conducted a comprehensive operando characterization on a hexagonal La2O2CO3 based sensor for the basic understanding of the sensing mechanism. This was done by performing under actual operating conditions simultaneous DC resistance and work function changes measurements, AC impedance spectroscopy measurements, and simultaneous DC resistance and DRIFT spectroscopy measurements. The results demonstrate that the double Schottky barriers at grain-grain boundaries are dominant contribution to sensor resistance; there is a competitive adsorption between carbonate species and hydroxyl groups, which depends on both CO2 concentration and humidity and leads to the change in height of the Schottky barriers. Finally, we propose a reaction model stating that there are three types of adsorbates, -CO32-, -OH-, and -O2-, and the relative concentration of which is controlled by a reaction with ambient humidity and CO2. This model is able to consistently explain all our experimental findings.

Characterization of the sensitive material for a resistive NOx gas dosimeter by DRIFT spectroscopy

Resistive NOx gas dosimeters measure continuously the integrated NOx concentration, i.e., the NOx dose. The impedance of potassium-manganese based NOx storage catalyst films decrease at 350 °C linearly with the NOx dose. The slope of the signal is proportional to the actual NOx concentration. The dosimeter principle is based on the sorption of NOx on the sensitive material. During NOx absence, the sensor signal remains constant, i.e. no desorption occurs. When a certain sorption state is reached, the sorbed species has to be desorbed thermally. To study the sorption processes on the NOx storage material, Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) was used. Different material compositions were investigated by DRIFTS on powdered catalysts and operando DRIFTS on NOx dosimeters themselves. On Al2O3 (catalyst support) no NOx sorbs. Manganese addition (Mn-Al2O3) leads to few detectable nitrites on Aluminum-sites during NOx exposure. On Potassium-loaded alumina (K-Al2O3), nitrites and nitrates form. The comparison of NO2 and NO proves that NO2 is sorbed directly as nitrate species and NO cannot be strongly sorbed as nitrate on K-Al2O3. The complete material composition K/Mn-Al2O3 shows strong nitrate bands during NO and NO2 exposure. An interplay of potassium and manganese when forming nitrates was found. Operando DRIFTS confirms the direct relation between formed nitrates and complex impedance. Furthermore, it is proven that during thermal regeneration, sorbed nitrites and nitrates desorb completely and the sensor signal returns to its initial value. On the whole, the sorption behavior of the powders and the dosimeter behavior can be explained.

Acid mine drainage formation and arsenic mobility under strongly acidic conditions: Importance of soluble phases, iron oxyhydroxides/oxides and nature of oxidation layer on pyrite

Acid mine drainage (AMD) formation and toxic arsenic (As) pollution are serious environmental problems encountered worldwide. In this study, we investigated the crucial roles played by common secondary mineral phases formed during the natural weathering of pyrite-bearing wastes—soluble salts (melanterite, FeSO4·7H2O) and metal oxides (hematite, Fe2O3)—on AMD formation and As mobility under acidic conditions (pH 1.5–4) prevalent in historic tailings storage facilities, pyrite-bearing rock dumps and AMD-contaminated soils and sediments. Our results using a pyrite-rich natural geological material containing arsenopyrite (FeAsS) showed that melanterite and hematite both directly—by supplying H+ and/or oxidants (Fe3+)—and indirectly—via changes in the nature of oxidation layer formed on pyrite—influenced pyrite oxidation dynamics. Based on SEM-EDS, DRIFT spectroscopy and XPS results, the oxidation layer on pyrite was mainly composed of ferric arsenate and K-Jarosite when melanterite was abundant with/without hematite but changed to Fe-oxyhydroxide/oxide and scorodite when melanterite was low and hematite was present. This study also observed the formation of a mechanically ‘strong’ coating on pyrite that suppressed the mineral’s oxidation. Finally, As mobility under acidic conditions was limited by its precipitation as ferric arsenate, scorodite, or a Fe/Al arsenate phase, including its strong adsorption to Fe-oxyhydroxides/oxides.

NO reduction by CO on Ce-Fe mixed oxides and gold nanoparticles

Gold nanoparticles were synthesized by a colloidal method and deposited on a Ce0.8Fe0.2O2-δ mixed oxide. The resulting material was characterized by High-Resolution Scanning Transmission Electron Microscopy, Temperature-Programmed Reduction, Mössbauer Spectroscopy, Oxygen Storage Capacity, Adsorption and Temperature-Programmed Desorption (NO and CO) and DRIFT Spectroscopy in order to elucidate the influence of gold nanoparticles and oxygen vacancies of Ce0.8Fe0.2O2-δ support on the selective catalytic reduction of NO by CO. Ce0.8Fe0.2O2-δ support presented a higher NO conversion (95% at 250 °C; and 99% at 350 °C) than Au/Ce0.8Fe0.2O2-δ catalyst (48% at 250 °C). We propose that gold deposition changed the support surface sites by blocking oxygen vacancies, which are the dissociation centers of NO molecules, decreasing thereby the catalytic activity of the gold-based catalyst. On the other hand, gold nanoparticles improved the reducibility and the oxygen storage capacity of the mixed oxide because isolated Fe3+ (in OFeCe3 sites) become more reducible, which could be evidenced by the increased oxidation of CO to CO2 at room temperature. On Au/Ce0.8Fe0.2O2-δ, the oxygen needed for CO oxidation can be supplied by the mixed oxide crystalline lattice, possibly from interface sites between gold nanoparticles and the mixed oxide, once the oxygen from NO dissociation becomes less abundant.

Transportable Mononuclear Metal Atoms as Building Blocks for Bottom‐up Material Fabrication: Pt1(0) and Au1(0) Atoms in Stock Solutions

AbstractPremade and readily transportable mononuclear zero‐valent metal atoms in stock solvents offer the potential capability for bottom‐up designable metallic material fabrication. This work demonstrates a potentially universal and practical strategy to prepare weakly shielded mononuclear metal atoms in a volatile solvent, which also constitutes a strippable ligand to the metal atoms. Herein, M1(0) (M=Pt, Au) atoms are prepared by reducing H2PtCl6 and HAuCl4 with ethanol in ethylene glycol diethyl ether (EGDE) under mild temperatures. The diether oxygens of EGDE, H+, and Cl− coordinating to the M1(0) atoms, make the M1(0) atoms stable in the solvent under low pH. However, the M1(0) atoms can be released either by evaporation of HCl and EGDE or through neutralization of the HCl with a base. The premade M1(0) atoms can be used as primary building blocks to construct designable materials. To demonstrate a potential application, the M1(0) atoms are conveniently transported onto the surface of TiO2 through impregnation followed by mild heating. The atomic dispersion and reduced state of the metal atoms are well preserved, as verified by HAADF‐STEM images, XPS, and DRIFT spectroscopy.

Production of high-performance biochar using a simple and low-cost method: Optimization of pyrolysis parameters and evaluation for water treatment

Elephant grass is a plant that grows easily and rapidly, with low nutrient requirements. One of the possible applications of this raw biomass is as a renewable source for simple and low-cost production of carbon materials. The objective of this work was to use elephant grass biomass to produce biochar intended for the treatment of contaminated water. Biochars were produced with variation of the pyrolysis parameters, using three different temperatures (350, 600, and 900 °C) and two different residence times (30 and 120 min). The products were characterized by DRIFT spectroscopy, XRD, Raman imaging, EPR, TGA, SEM, TEM, nitrogen adsorption at 77 K, potentiometric titration to obtain the pKa values, and elemental CHN analysis. Continuous flow adsorption tests were used to evaluate the potential of the material for practical applications. The infrared spectra and pKa values revealed structural changes of the carboxylic groups (pKa 3–6), forming lactones (pKa 6–9) or phenols (pKa >9). The diffractograms, low H/C and O/C ratios, and Raman spectra indicated the formation of aromatic and graphitic structures in the materials. Environmentally persistent free radicals, mainly formed by constituents of lignin, were centered near to carbon atoms in samples produced at higher temperatures. Continuous flow adsorption tests showed that the sample with the highest surface area (∼550 m² g−1) and the largest pore volume (∼0.34 cm³ g−1) achieved a removal efficiency of 96.5 % and presented good reusability, indicating its suitability as a promising material for the treatment of water contaminated with aromatic pollutants.

Synergy Effect of Au and SiO2 Modification on SnO2 Sensor Properties in VOCs Detection in Humid Air.

Nanocomposites based on Au- and SiO2-modified SnO2 were studied as sensitive materials for ethanol and benzene detection in dry (RH = 1%) and humid (RH = 20%) air. Modification of SnO2 by amorphous SiO2 (13 mol.%) was effectuated by hydrothermal synthesis; modification by Au nanoparticles (1 wt.%) was carried out via impregnation by citrate-stabilized Au sol. The composition of the samples was determined by X-ray fluorescent spectroscopy and energy-dispersive X-ray spectroscopy. The microstructure was characterized by XRD, HRTEM, and low-temperature nitrogen adsorption. The surface groups were investigated by XPS, TPR-H2, and FTIR spectroscopy. DRIFT spectroscopy was performed to investigate the interaction between ethanol and the surface of the synthesized materials. Studies of the sensor properties have shown that in all cases the most sensitive is the SnO2/SiO2-Au nanocomposite. This material retains high sensitivity even in a humid atmosphere. The obtained results are discussed in terms of the synergistic effect of two modifiers (Au and SiO2) in the formation of sensor properties of SnO2/SiO2–Au nanocomposites.

Dehydration of methanol and ethanol over silica-supported heteropoly acids in the gas phase: Surface-type versus bulk-type catalysis mechanism

Dehydration of MeOH to dimethyl ether and EtOH to diethyl ether and ethene was studied at the gas-solid interface in the presence of bulk and SiO2-supported Keggin heteropoly acids (HPAs) H3PW12O40 (PW) and H4SiW12O40 (SiW) as catalysts. The catalysts were prepared by HPA impregnation from water and MeOH. Their acid strength, texture and structural integrity was characterised using NH3 adsorption calorimetry, BET, XRD and DRIFT spectroscopy. The strength of acid sites in HPA/SiO2 catalysts increased monotonously with HPA loading. In the dehydration of MeOH and EtOH, the turnover reaction rate for PW catalysts was higher than for SiW catalysts in agreement with their acid strength. HPA catalysts prepared from water and MeOH had a very close acid strength and showed similar activities in alcohol dehydration. The steady-state catalyst activity was found to correlate with the density of catalyst proton surface sites rather than with the HPA loading. This indicates that alcohol dehydration occurred via a mechanism of surface-type HPA catalysis at the gas-solid interface rather than a bulk-type (pseudo-homogeneous) catalysis.

Understanding the role of surface Lewis acid sites of Sn modified Pd/Al2O3 catalyst in the chemoselective reductive N-acetylation of nitrobenzene

N-Aryl acetamides synthesis from the corresponding nitro compounds via a reductive N-acetylation was achieved over Pd supported on Sn modified Al2O3 catalyst using H2, acetic anhydride as acetylating agent in aqueous media at ambient temperature. The pyridine-IR data demonstrated a majority of Lewis acid sites compared to Bronsted acid sites on the catalyst surface. Pyridine-IR and CO pulse chemisorption results emphasized a combination of surface palladium in conjunction with Lewis acid sites were responsible for the high activity of 2wt%Pd/5wt%Sn–Al2O3 over 2wt%Pd/Al2O3 catalyst with consistent activity and selectivity for four recycles. The bulk and surface properties of the catalysts were characterized by BET-SA, XRD, XPS, TPD of NH3, H2-TPR, CO pulse chemisorption, TEM and the promotional effect of surface Lewis acid sites are rationalized by in situ pyridine adsorbed DRIFT spectroscopy.