Subsequent CO Adsorption Research Articles

Fluorescence excitation/emission matrices as a tool to monitor the removal of organic micropollutants from wastewater effluents by adsorption onto activated carbon

Monitoring the removal of organic micropollutants (OMPs) in advanced wastewater treatment facilities requires expensive and time-consuming analytical methods that cannot be installed online. Spectroscopic techniques such as fluorescence excitation/emission spectroscopy were demonstrated to offer the potential for monitoring OMPs removal in conventional wastewater treatment plants or ozonation pilots but their application to activated carbon (AC) adsorption processes was only investigated at lab scale and not in real treatment facilities. In this study, indexes from fluorescence emission/excitation matrices (EEMs) were used to find correlations with the removal of 28 OMPs from a large-scale AC pilot in fluidized bed employed for wastewater advanced treatment, as well as from batch experiments. Differences in OMPs removal could be observed depending on the operational conditions (i.e. pilot or batch experiments, contact time, type of AC) and the physico-chemical properties of the molecules. 7 PARAFAC components were derived from the fluorescence EEMs of 60 samples obtained before and after adsorption. Positive correlations were obtained between the removal of fluorescence indexes and most OMPs, and correlation coefficients were much higher than the ones obtained with UV254, confirming the interesting potential of fluorescence spectroscopy to accurately monitor adsorption performances at the industrial scale. The highest correlation coefficients were obtained for OMPs having the best removals while the ones that were refractory to adsorption, as well as to interactions with DOM, exhibited weak correlations. These results suggest that interactions between OMPs and fluorescing DOM and their subsequent co-adsorption onto AC were at the origin of the correlations found. Lower correlations were also found for the most biodegradable OMPs, which indicated that the occurrence of biological effects could make the monitoring of these compounds more challenging.

Density Functional Theory Calculations of Oxygen Vacancy Formation and Subsequent Molecular Adsorption on Oxide Surfaces

The surface oxygen vacancy formation energy (EOvac) is an important parameter in determining the catalytic activity of metal oxides. Estimating these energies can therefore lead to data-driven design of promising catalyst candidates. In the present study, we determine EOvac for various insulating and semiconducting oxides. Statistical investigations indicate that the band gap, bulk formation energy, and electron affinity are factors that strongly influence EOvac. Electrons enter defect states after O desorption, and these states can be in the valence band, mid-gap, or in the conduction band. Subsequent adsorption of O2, NO, CO, CO2, and H2 molecules on an O-deficient surface is also investigated. These molecules become preferentially adsorbed at the defect sites, and EOvac is identified as the dominant factor that determines the adsorption mode as well as a descriptor that shows good correlation with the adsorption energy.

CO oxidation by oxygen of the catalyst and by gas-phase oxygen over (0.5–15)%CoO/ZrO2

CO adsorption on (0.5–15)%CoO/ZrО2 catalysts has been investigated by temperature-programmed desorption and IR spectroscopy. At 20°С, carbon monoxide forms carbonyl and monodentate carbonate complexes on Co 2+ O 2- clusters located on the surface of crystallites of tetragonal ZrO2. With an increasing CoO content of the clusters, the amount of these complexes increases and the temperature of carbonate decomposition, accompanied by CO2 desorption, decreases from 400 to 304°С. On the 5%CoO/ZrО2 sample, the carbonyls formed on the Со2+ and Со+ cations and Со0 atoms decompose at 20, 90, and 200–220°С, respectively, releasing CO. At 20°С, they are oxidized by oxygen to monodentate carbonates, which decompose at 180°С. Adsorbed oxygen decreases the temperature of their decomposition on oxidation sites by ~40°C, and the sample remains in an oxidized state ensuring the possibility of subsequent CO adsorption and oxidation. The rate of the oxidation of 5%CoO/ZrО2 containing adsorbed CO by oxygen is higher than the rate of the oxidation of the same sample reduced by carbon monoxide, because the latter reaction is an activated one. In view of the properties of the complexes, it can be concluded that the carbonates decomposing at 180°С are involved in CO oxidation by oxygen from the gas phase in the presence of hydrogen, a process occurring at 50–200°С. The rate-limiting step of this process the decomposition of the carbonates, which is characterized by an activation energy of 77–94 kJ/mol.

CO oxidation with oxygen of the catalyst and gas-phase oxygen over (0.5−15)%СоО/СеО2

The CO adsorption species on Co3O4 and (0.5-15%)CoO/CeO2 catalysts have been investigated by temperature-programmed desorption and IR spectroscopy. At 20°C, the largest amount of CO is adsorbed on the 5%CoO/CeO2 sample to form, on Com 2+On 2+ clusters, hydrogen-containing, bidentate, and monodentate carbonate complexes, whose decomposition is accompanied by CO2 desorption at 300 and 450°C (1.1 × 1020 g–1). The formation of the carbonates is accompanied by the formation of Co+ cations and Co0, on which carbonyls form. The latter decompose at 20, 90, and 170°C to release CO (2.7 × 1019 g–1). Part of the carbonyls oxidizes to CO2 upon oxygen adsorption, and the CO2 undergoes desorption at 20°C. Adsorbed oxygen decreases the decomposition temperature of the H-containing and bidentate carbonates from 300 to 100-170°C and maintains the sample in the oxidized state, which is active in subsequent CO adsorption and oxidation. CO oxidation by oxygen of the catalyst diminishes the activity of the sample in these processes and increases the decomposition temperature of the carbonate complexes. Taking into account the properties of the adsorption complexes, we concluded that the H-containing and bidentate carbonates are involved in CO oxidation by oxygen of the catalyst at ~170°C under isothermal conditions. The rate limiting step is the decomposition of the carbonates, a process whose activation energy is 65-74 kJ/mol.

Metal adatoms generated by the co-play of melamine assembly and subsequent CO adsorption.

Molecular self-assembly films are expected to tailor the surface process by the periodic nanostructures and add-on functional groups. In this work, a molecular network of melamine with featured pores of subnanometer size is prepared on the Au(111) surface, and is found to be able to trap the gold adatoms and concomitant single vacancies generated under the impingement of CO molecules at room temperature. DFT calculations suggest that the strong CO-Au adatom interaction as well as the high adhesion of the Au adatom inside the melamine pore could well be the driving force behind such process. This study not only sheds light onto the interactions between gasses and the metal surface that is covered by molecular self-assembly films, but also provides a novel route to manipulate the monoatomic surface species which is of catalytic interest.

Oxidation of co with catalyst oxygen and with oxygen from the gas phase over CuO/CeO2 according to TPD and IR spectroscopic data

The forms of CO adsorption on the 5 and 10% CuO/CeO2 samples were studied by temperature-programmed desorption and IR spectroscopy. It was established that CO forms bridging and mono- and bidentate carbonate complexes on CuO clusters at 20°C; these complexes are decomposed with the desorption of CO2 at T max = 160 and 240–350°C, respectively. The Cu+ cations, on which Cu+-CO carbonyls are formed, are produced simultaneously with carbonates. They are decomposed at 110°C in a vacuum with the desorption of CO and oxidized at 20°C in an atmosphere of oxygen to form bridging carbonates. These latter are decomposed in a range of 20–150°C with the release of CO2. Adsorbed oxygen decreases the decomposition temperature of the carbonates formed upon the adsorption of CO by 50°C and keeps the sample in an oxidized state, which is active with respect to the subsequent adsorption and oxidation of CO. The oxidation of CO by the catalyst oxygen decreases the activity of the sample in these processes and increases the thermal stability of carbonate complexes. Based on the properties of the adsorption complexes of CO, a conclusion was made that the bridging carbonates participate in the reaction of carbon monoxide oxidation by the catalyst oxygen and oxygen from a gas phase in a range of 20–150°C. The decomposition of the carbonates with an activation energy of 64 kJ/mol is the rate-limiting step.

Microwave-assisted synthesis and characterization of Cu-containing AlPO 4-5 and SAPO-5

The microwave-assisted hydrothermal synthesis of Cu-containing AFI structures AlPO 4-5 and SAPO-5 in good product quality was successfully accomplished. The modification of texture, acidity and redox properties caused by incorporation of Cu was examined using XRD, SEM, TG/DTA, N 2 adsorption, UV–vis- and FTIR spectroscopy. Copper impregnated AlPO 4-5 and SAPO-5 served as reference samples. Pyridine and CO were used as probe molecules to characterize acidity and nature of Cu species. XRD patterns of the calcined, template-free samples confirmed the crystallinity and the absence of any crystalline copper phase (Cu, Cu 2O, CuO). Surface areas and pore volumes indicate the absence of pore blocking by amorphous material or agglomerated Cu species. Brønsted acid sites were created in CuAPO-5 confirming isomorphous substitution of Cu 2+ into the lattice. Cu 2+/Cu + with Lewis acid character could be identified by FTIR spectroscopy using pyridine as probe molecule. Pre-reduction with CO and H 2 at various temperatures allowed a differentiation of Cu sites. An essential part of the Cu is located on non-lattice positions with high dispersion in form of isolated Cu 2+. Reduction with CO revealed the existence of Cu 2+ sites bearing oxygen (e.g., Cu 2+–O–Cu 2+). Reduction with H 2 and subsequent CO adsorption leads to Cu aquacarbonyls.

Theoretical Analysis of the CO Oxidation Process on Neutral and Single Ionized Au18 Clusters

AbstractExperiments on the reactivity of CO for Au nanoclusters have shown a local maximum in the adsorption of the first molecule for Au18 and its cation, whereas O2 adsorption has been observed in Au18-1. In this work we present a theoretical analysis of the preferential sites for the adsorption of the CO and O2 molecules on neutral and single ionized Au18 clusters with C2v symmetry, which has been shown both theoretical and experimentally, to be the most stable isomer of the Au18 cluster. We report the results of the calculation for the binding energies of CO and O2 for non-equivalent sites and compare with the available experimental values. The oxidation mechanism is studied in first instance by the subsequent adsorption of the CO on the O2 molecule, which was previously adsorbed on the respective gold cluster. The study is based on a DFT-GGA calculation with the PW91 functional.

Towards tuning of surface properties by atomic and molecular adsorption on boron-terminated cubic boron nitride (111) surface: A first-principles study

We present a theoretical investigation for the adsorptions of triangular nitrogen trimer radical (N 3) at both the hollow ( H3) and fourfold coordinated top ( T4) sites on boron-terminated (111) (B(111)) surface of cubic boron nitride (c-BN) in terms of structure, adsorption energy, band structure and work function. For the first time, we explore the H3 and T4 adsorption mechanisms. Moreover, we study the subsequent co-adsorption of N 3, boron and hydrogen atoms on the same adsorption site on the B(111) surface, which will form either pyramid quantum cluster or nearly planar adsorbate. We find that the surface band structure varies substantially depending on the types of terminated surface formed (changing from metallic to semi-conducting), and consequently, the surface work function changes. These results indicate the electronic characteristics of the B(111) surface can be tuned readily using chemical co-adsorption, suggesting its potential for chemical sensing applications.

Isolated Co2+ and [Co−O−Co]2+ Species in Na-MOR Exchanged with Cobalt to Various Extents: An FTIR Characterization by CO Adsorption of Oxidized and Prereduced Samples

A commercial Na-MOR was exchanged to various extents with cobalt. The sodium and cobalt contents were determined by atomic absorption. Samples were characterized by UV−vis and by FTIR. The FTIR results with CO show that Co-MOR heated in O2 at 793 K and evacuated at the same temperature contained isolated Co2+ (86 to 100% of cobalt detected by CO adsorption at RT) and [Co−O−Co]2+. After exposure of samples to CO at RT, FTIR showed that only a minute fraction of [Co−O−Co]2+ underwent reduction yielding [(CO)nCo+−Co+(CO)n], with n = 2 or 3, and CO2. After exposure to CO at increasing temperature up to 623 K, the subsequent adsorption of CO at RT yielded increasing amounts of [(CO)nCo+−Co+(CO)n] and CO2. Whereas isolated Co2+ did not undergo reduction, Co2+ in [Co−O−Co]2+ reduced to Co+. On evacuation at RT, [(CO)3Co+−Co+(CO)3] completely and reversibly transformed into [(CO)2Co+−Co+(CO)2]. On evacuation at increasing temperature, [(CO)2Co+−Co+(CO)2] progressively disappeared and transformed into a stable bri...

State of gold on an Au/Al 2O 3 catalyst subjected to different pre-treatments: An FTIR study

An Au/Al 2O 3 sample was prepared by deposition–precipitation and characterized by different techniques (XRD, TEM, FTIR spectroscopy of adsorbed CO and catalytic test). IR spectroscopy of adsorbed CO on a sample evacuated at ambient temperature reveals no gold sites. The results were explained by the fact that gold on this sample was present as Au 3+ cations and these sites (as well as the Al 3+ sites) were blocked by water. When the sample was activated at 473 K, CO adsorption at 100 K produced: (i) carbonyl bands associated with the support and (ii) a band at 2167 cm −1, stable towards evacuation and assigned to Au +–CO species. Evacuation of the sample at 573 K led to auto-reduction of the Au + sites. Subsequent CO adsorption resulted in formation of two types of Au 0–CO species (bands at 2112 and 2105 cm −1). Only one kind of Au 0 sites (respective carbonyl band at 2105 cm −1) were detected with samples activated at 673 K. Re-oxidation of the sample at 673 K in O 2 atmosphere resulted in oxidation of all accessible metallic gold and appearance of (i) a small amount of isolated Au + sites (respective carbonyl band at 2175 cm −1) and (ii) another kind of sites (respective carbonyl band at 2139 cm −1, disappearing during evacuation) associated with Au + cations on the surface of the metal particles. Both kinds of cations were stable at 573 K but gold cations were auto-reduced at 673 K. Comparison of the FTIR results with catalytic studies allowed some conclusions on the nature of the active sites in the CO oxidation reaction.

Origin and Activity of Oxidized Gold in Water-Gas-Shift Catalysis

As a promising route for large-scale H2 production, the water-gas-shift reaction (WGS, CO + H(2)O-->CO(2) + H(2)) on ceria-supported Au catalysts is of enormous potential in efforts to move towards a hydrogen economy. Recent research suggests that this reaction is in fact catalyzed by Au cations instead of the conventionally regarded metallic Au particles. Here density functional theory calculations demonstrate that the presence of empty localized nonbonding f states in CeO2 permits the oxidation of Au, enabling subsequent CO adsorption. A feasible reaction pathway leading to H2 production is identified, and it is concluded that four to six atom Au clusters at the O-vacancy sites of ceria catalyze the WGS reaction.

Surface Dynamics of Coadsorbed CO and D2O on Pt(100) in Ultrahigh Vacuum as Studied by Time-Resolved Infrared Reflection Absorption Spectroscopy

Changes in the properties of CO adsorbed at saturation coverages on Pt(100) induced by subsequent coadsorption of fixed amounts of D2O at 105 K in ultrahigh vacuum (UHV) were monitored by time-resolved infrared reflection absorption spectroscopy (tr-IRAS). The linear- and bridge-bonded CO stretching features were found to change in intensity and shift toward lower energies as a function of time at fixed CO and D2O coverages. Also observed was the development of multiple features in both CO spectral regions depending on the amount of D2O on the surface. These findings indicate that, under the conditions of these experiments, the interfacial dynamics are relatively slow, on the order of minutes, involving a gradual rearrangement of adsorbed CO and D2O on the surface to yield surface solvated CO, as has been suggested in the literature (Kizhakevariam et al. J. Chem. Phys. 1994, 100, 6750). This factor should be considered when comparing, quantitatively, shifts induced by water coadsorption with CO on Pt single crystals in UHV with CO adsorption on the same surfaces in electrochemical environments.

Adsorption of CO and C 2H 4 on Rh-loaded thin-film praseodymium oxide

A praseodymium oxide thin-film was deposited on Ru(0 0 0 1) by vapor depositing Pr in 2 × 10 −7 Torr O 2 while the Ru was at 700 K. The film was approximately 5 nm thick and did not produce a LEED pattern. The adsorption and reaction of CO and C 2H 4 adsorbed on Rh supported on the PrO X film were studied by TPD and SXPS. The CO initially reacted with oxygen in the substrate to produce CO 2. After this oxygen was removed, the CO adsorbed non-dissociatively in a manner similar to what is seen on Rh(1 1 1). C 2H 4 adsorbed on the Rh particles and underwent progressive dehydrogenation to produce H 2 during TPD. The C from the C 2H 4 reacted with the O in PrO X to produce CO. CO dissociation on the Rh particles could be promoted by reducing the PrO X with C 2H 4 before CO exposure. The CO dissociation occurred over a narrow range of Pr reduction and occurred only at high degrees of reduction. No CO dissociation was seen for films with a Pr 3+ content of up to 50%. Extensive treatment in C 2H 4 poisoned subsequent CO adsorption due to the build-up of C on the Rh particles.

On the nature of Pdn+surface carbonyl and nitrosyl complexes formed on Pd-promoted tungstated zirconia catalyst

Adsorption of CO on Pd/WO3-ZrO2 sample at 85 K results in formation of carbonyls typical of the support as well as of Pd3+-CO species (2212 cm−1). The latter are masked by the strong Zr4+-CO band. Even at 90 K the Pd3+ sites are reduced to Pd2+ (the respective carbonyls absorbing at 2184 cm−1) and, at higher temperatures, to Pd+ (carbonyl bands at 2145 and 2139 cm−1) and Pd0. Adsorption of NO on a CO precovered sample results in formation of Pd2+-NO complexes (1850 cm−1) that are stable towards evacuation. Subsequent CO adsorption leads to the appearance of mixed Pd2+(CO)(NO) complexes (νCO at 2150 cm−1 and νNO at 1803 cm−1). Decrease of the CO equilibrium pressure and evacuation result in decomposition of these species to CO and Pd2+-NO nitrosyls. The existence of some mixed complexes with the participation of Pd3+ cations is also proposed.

Adsorption of CO on ultrathin Cr layers on Ru(0 0 0 1)

We studied the adsorption of CO on ultrathin Cr layers on Ru(0 0 0 1) as a function of Cr coverage and deposition temperature. The preparation of the Cr layers was characterized by Auger electron spectroscopy, low energy electron diffraction, and the subsequent CO adsorption itself. At 90 K Cr grows statistically on Ru(0 0 0 1). For Cr deposition at 700 K, a pseudomorphic monolayer is formed; thereafter Cr grows in three dimensional islands and the formation of a highly ordered bcc(1 1 0) Cr phase is observed. For Cr deposition at 1000 K we propose the formation of a stable surface alloy. Temperature programmed desorption of CO after dosing onto the Cr layers prepared at 700 and 1000 K indicates an enhanced reactivity of the layers towards CO dissociation upon heating as compared to Ru(0 0 0 1). In both cases a high temperature CO desorption state (above 700 K) is observed that is attributed to recombinative desorption of CO; its intensity increases up to a nominal Cr coverage of 1 ML. For the Cr layers prepared at 700 K, this state is observed already at the lowest Cr coverage studied (0.12 ML); for 1 ML Cr, 47% of the saturated CO layer decompose upon heating. For the Cr layers prepared at 1000 K (surface alloy) significant decomposition is observed only for Cr coverages larger than 0.5 ML; a maximum fraction of 18% of the saturated CO layer decomposes upon heating. Thus, the Cr layer prepared at 700 K is more reactive towards CO dissociation than the one prepared at 1000 K.

Adsorption of Water and Methanol on a NaZSM-5 Zeolite. A temperature-programmed desorption (TPD) study

Using temperature-programmed desorption (TPD), we have investigated the desorption behavior after subsequent co-adsorption of methanol and water and after adsorption of their mixtures on a NaZSM-5 zeolite. The course of desorption indicates that a strong mutual displacement of both components occurs. However, on the strongest adsorption sites methanol is preferentially adsorbed, and already the addition of small amounts of methanol leads to a displacement of water. Our results support the idea of a subdivision of the pore space for adsorption of water/methanol mixtures. Above all, the experiments show that in the part of the pore space where both components are adsorbed, different sites are of importance which vary significantly in their interaction strength.

CO adsorption on the Pt/Rh(100) surface studied by high-resolution photoemission

Submonolayers of platinum have been deposited on an Rh(100) substrate. Platinum growth and subsequent CO adsorption on the bimetallic surface has been studied by high-resolution photoemission applying synchrotron radiation. Platinum was found to grow as small, single-layer islands on Rh(100). The CO adsorption site on both metals is very sensitive to compositional changes in the bimetallic surface. A site switch from on-top to bridge sites in the c(2×2) CO overlayer on Rh(100) has been observed in the presence of 0.2 monolayers (ML) of platinum. The CO saturation coverage is found to decrease dramatically with increasing platinum coverages. 0.9 ML of platinum on Rh(100) binds approximately half the CO bound by Pt(100). We explain this in terms of a reduced COPt bond strength through a ‘ligand effect’.

Characterization of Model Three-Way Catalysts: III. Infrared Study of the Surface Composition of Platinum–Rhodium Ceria–Alumina Catalysts

The surface composition of model and commercial bimetallic PtRh catalysts supported on ceria–alumina was tentatively determined by FTIR spectroscopy using the successive adsorption of NO at 473 K and CO at 298 K method which was previously applied to the case of PtRh/Al2O3 catalysts. The study of monometallic model catalysts shows that the adsorption of NO at 473 K on rhodium gives an intense band at 1912 cm−1 which is not modified by the presence of ceria and therefore can be used to quantify the number of surface rhodium atoms. However, the chemisorption properties of platinum toward the subsequent adsorption of CO at 298 K are strongly modified and the quantification of the surface platinum atoms could not be directly measured. However, their number could be indirectly obtained by combining the NO adsorption for rhodium and hydrogen volumetric adsorption for the total number of surface metal atoms, assuming that NO adsorption does not induce an important segregation of rhodium for bimetallic particles. Thus, for the fresh bimetallic model catalyst, the same composition was obtained at the surface and in the bulk. After aging at 1273 K, no rhodium was detected at the surface. This absence of surface rhodium atoms was confirmed by the data obtained from the direct adsorption of CO on platinum at room temperature, which gave the same number of surface atoms as hydrogen chemisorption. Similar results have been obtained with two commercial three-way catalysts. In particular, after aging at 1273 K, rhodium was practically not detected on the surface.

Heats of adsorption and desorption of CO 2, CH 4, SO 2, O 2 AND N 2 on microporous carbons

Flow adsorption experiments have been carried out on CH 4, O 2, N 2 and H 2O with active carbons at a range of pressures and temperatures. The equipment used was a new model Microscal Flow Adsorption Microcalorimeter in which the adsorption and the associated heat evolution could be measured simultaneously with the aid of appropriate down stream detectors. The flow methods permitted to study the sequential adsorption and desorption of the gases, and repetition of the cycles to an assessment of the adsorption reversibility. The effect of temperature on the reversibility was investigated up to 220 °C. The pressures used for the adsorption of the permanent gases ranged from atmospheric to 50 bar g. The use of the flow adsorption microcalorimeter methods have led to the discovery and a confirmation of the following surface properties of active carbons. 1. (1) The relatively high affinity of O 2 for the carbon surfaces compared with that of N 2. 2. (2) Strong effect of pressure on the heats of sorption of CH 4 and N 2. 3. (3) Partly irreversible adsorption of CO 2 from helium and its activation of the adsorption of O 2. 4. (4) Partly irreversible adsorption of SO 2 from helium and its activation of the subsequent adsorption of CO 2.