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In situ Raman spectroscopy at temperatures up to 450 °C is used to probe the structural and redox properties of Ce1-xZrxO2-δ solids (x = 0-0.8) prepared by the citrate sol-gel and coprecipitation with urea methods. The anionic sublattice structure of the solids is dependent on the preparation route. The composition effects exhibited by the Raman spectra are adequate for characterizing the phases present and/or eventual phase segregations. For x = 0.5 the pseudocubic t″ phase occurs for the solid prepared by the citrate sol-gel method, while phase segregation (cubic, tetragonal) is evidenced for the corresponding material prepared by the coprecipitation with urea method. A larger extent of defects and interstitial O atoms is evidenced for the materials prepared by the citrate sol-gel method. The well-known "defect" ("D") band around 600 cm-1 for CeO2 as well as for Ce1-xZrxO2-δ consists of at least two components: "D1" above 600 cm-1 and "D2" below 600 cm-1. Doping of Ce0.8Zr0.2O2-δ with rare earth cations (La3+, Nd3+, Y3+, Pr3+) results in strengthening of the "D2" band that, however, is found to be insensitive under reducing conditions of flowing 5% H2/He at 450 °C. A novel approach based on sequential in situ Raman spectra under alternating oxidizing (20% O2/He) and reducing (5% H2/He) gas atmospheres showed that the "D1" band is selectively attenuated under reducing conditions at 450 °C and is therefore assigned to a metal-oxygen vibrational mode involving interstitial oxygen atoms that can be delivered under suitable conditions. A reversible temperature-dependent evolution of the anionic sublattice structures of Ce1-xZrxO2-δ solids is evidenced by in situ Raman spectroscopy. The results are corroborated by powder XRD and oxygen storage capacity measurements, and observed structure/function relationships are discussed. It is shown that at low temperatures (e.g., 450 °C) the function of oxygen release and refill is based on a mechanism involving oxygen atoms in interstitial sites rather than on defects induced by hetrovalent M4+→ RE3+ doping, the latter improving the pertinent function at high (e.g., >600 °C) temperatures. © 2017 American Chemical Society. 121 14 7931 7943

Carbon dioxide reforming of methane to synthesis gas in the 550-750°C range over 5 wt% Ni/Ce1-xMxO2-δ (M = Zr4+, Pr3+) solids has been investigated with respect to the effects of support chemical composition and reaction temperature on the amount, reactivity (towards H2 and O2) and relative contribution of CH4 and CO2 activation routes towards "carbon" formation. For these "carbon" characterisation studies, various transient isothermal and temperature-programmed oxidation (TPO) and hydrogenation (TPH) experiments coupled with the use of 13CO and 13CO2 isotope gases were conducted. TPO following dry reforming (5%13CO2/5%12CH4/45%Ar/45%He) demonstrated that the relative amount of the various kinds of "carbon" formed via the CH4 and CO2 activation routes was strongly dependent on reaction temperature and support chemical composition. At 550°C, the ratio of 12CO2 to 13CO2 of the 12C-containing and 13C-containing inactive "carbon" formed was 0.4, 0.27 and 0.19, whereas at 750°C was 1.07, 1.06 and 0.29, respectively, for the 5 wt% Ni supported on Ce0.8Zr0.2O2, Ce0.8Pr0.2O2 and Ce0.5Zr0.5O2 carriers. The origin of "carbon" formation via the CO2 activation route was illustrated to be the Boudouard reaction (2CO-s → CO2(g) + C-s + s) through a transient isotopic experiment with a feed gas containing 13CO and 12CH4. It was also found that CO-s derived from the direct dissociation of CO2 and the CH4 activation route can lead to a number of different kinds of "carbon" which depends on support chemical composition. The present 5 wt% Ni/Ce0.8Pr0.2O2 catalytic system exhibited CO2 conversion of 84%, H2-yield of 48%, and H2/CO ratio of 1.04 after 50 h of dry reforming of methane at 750°C (20% CH4, 20% CO2, He GHSV = 30,000 h-1) with a relatively low amount (17.5 mg C/gcat or 1.75 wt%) of accumulated inactive "carbon". The support chemical composition was found to influence the nickel particle size, which in turn influenced the origin, kinetics and the reactivity of "carbon" deposition under dry reforming reaction conditions. © 2015 Elsevier B.V. 259 150 164 Cited By :21

Investigation of new efficient pathways for CO2 sequestration is of great significance for the mitigation of climate change. Ultramafic rocks are considered among the most promising lithotypes for the safe storage of CO2 via mineral carbonation. This paper investigates a powerful method for the optimization of the ex situ carbonation of pyroxenitic rocks, which comprise part of ultramafic lithologies occurring in ophiolite complexes. The ball milling process was applied to a sample of pyroxenite from the Troodos ophiolite (Cyprus) for the first time, in order to create novel nanomaterials with enhanced CO2 storage capacity. The goal was to accelerate the kinetics of rock-fluid reactions during the carbonation process. The starting rock material and the ball-milled samples were characterized using a variety of methodologies. The experimental results imply that only a few hours of wet ball milling with ethanol as process control agent can substantially increase the CO2 storage capacity of pyroxenites. Through temperature-programmed desorption of CO2 (CO2-TPD) experiments, we show that the optimum milling conditions are 4 h of ball milling with 50 wt% ethanol, leading to an increase of the CO2 uptake of the studied rock material by 41 times. This notable increase designates that pyroxenites are very promising lithologies for CO2 storage via ex situ carbonation, and that ball milling can be an effective preparation technique for this process, providing an efficient and secure carbon storage solution. © 2015 Society of Chemical Industry and John Wiley & Sons, Ltd. 5 5 577 591 Cited By :2

A series of 0.5 wt% Pt/Ce 0.5 La 0.5 O 2 − δ (Ce:La = 1:1) catalysts, the supports of which were prepared by different methods, namely: (i) sol–gel using citrate or oxalate as complexing agent, (ii) pechini, and (iii) urea co-precipitation, were investigated for the first time towards the water–gas shift (WGS) reaction in the 250–350 °C range and 1 atm total pressure. Towards a better understanding of the effect of support synthesis method on the intrinsic kinetic rate of WGS expressed per gram of catalyst (μmol CO g −1 s −1 ) or per length of the perimeter of Pt-support interface (μmol CO cm −1 s −1 ), a suite of various characterisation methods such as: in situ Raman, temperature-programmed techniques (TPD-H 2 , TPD-NH 3 , TPD-CO 2 ), powder XRD, and oxygen storage capacity (OSC) measurements were applied. The intrinsic kinetic rate of WGS (μmol CO g −1 s −1 ) was correlated with the concentration of the active “carbon-containing” (C-pool) and “hydrogen-containing” (H-pool) reaction intermediates formed within a reactive zone (Δ x , A) around each Pt nanoparticle (1.2–1.5 nm), parameters that were estimated via SSITKA and non steady-state transient isotopic and titration with water operando experiments. The urea co-precipitation method (U) resulted in the formation of a Ce 1 − x La x O 2 − δ solid solution with different composition (Ce:La atom ratio) than that formed by the other synthesis methods, which may be the main reason for Pt/Ce 0.5 La 0.5 O 2 − δ (U) to exhibit the highest by far CO conversion and kinetic rate towards the WGS compared to the other supported Pt catalysts. The same method (U) resulted in the formation of La 2 O 3 as opposed to the other methods. However, this was not considered as the main reason for explaining the higher activity of Pt supported on Ce 0.5 La 0.5 O 2 − δ (U) compared to the other carriers. The Ce 0.5 La 0.5 O 2 − δ (U) was also found to possess the highest surface acidity and basicity compared to the other supports but lower OSC (μmol g −1 ) (by more than 30% in the 250–550 °C range) than Ce 0.5 La 0.5 O 2 − δ prepared by the citrate sol–gel method, in harmony with the lower content of O vacancies in Ce 0.5 La 0.5 O 2 − δ (U) as evidenced by Raman studies. Transient DRIFTS formate (HCOO–) decomposition kinetic experiments towards CO 2 and H 2 formation have illustrated the importance of the presence of Pt and support composition.

The goal of this study is to propose a cost-effective method for the optimization of the ex situ carbonation of basaltic rocks. The ball milling process was applied to a sample of olivine basalt from the Troodos ophiolite complex (Cyprus) for the first time, in order to fabricate novel nanomaterials for CO2 storage. The purpose was to accelerate the kinetics of rock–fluid reactions during the carbonation procedure. Various methodologies were used for the characterization of the starting rock material and the ball-milled samples. Preliminary results reveal that only a few hours of wet ball milling with ethanol as process control agent can induce significant changes to olivine basalt towards improvement of its performance for CO2 storage. Specifically, CO2 uptake measurements via the use of the temperature-programmed desorption (TPD) technique indicate that 4h of ball milling with 50wt.% ethanol can lead to an enhancement of the carbonation of olivine basalt by 295%. The experimental results strongly suggest that (i) olivine basalts have important CO2-storage capacity and are very promising lithotypes for ex situ carbonation, and (ii) the ball milling process provides hopes for its use at an industrial scale as a preparation technique for the safe and permanent ex situ storage of CO2. •The ball milling process was used to create novel nanomaterials for CO2 capture.•The experiments were carried out using a sample of olivine basalt.•Olivine basalt shows higher CO2 uptake compared to the reference forsterite.•The pore volume principally controls the extent of carbonation.•Ball milling results in an enhancement of the carbonation of olivine basalt by 295%. 273 220 229 220-229

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The selective catalytic reduction of NOx by H2 (H2-SCR) under strongly oxidizing conditions (520 ppm NOx/1% H2/5% O2/10% CO2/He; NO:NO2–4:1–9:1) in the 150–600 °C range has been studied over 3 wt-% W-promoted CeO2–ZrO2 solids (85 wt-% CeO2-15 wt-% ZrO2 (CeZr), and 17 wt-% CeO2–83 wt-% Zr (ZrCe) synthesised by a proprietary method) for the first time. The highest NOx conversion ( X NO x = 54 % ) was obtained on the W-ZrCe (Zr-rich) solid at 300 °C (GHSV of 51,000 h−1), whereas N2-selectivity was in the 77–92%-range over both W-ZrCe (Zr-rich) and W-CeZr (Ce-rich) catalysts. Significantly higher integral specific rates (RNO, μmol NO m−2 min−1) were estimated on the W-ZrCe (Zr-rich) catalyst compared to the W-CeZr (Ce-rich) one in the 250–350 °C range. The formation of adsorbed NOx under 0.1% NO/10% O2/He gas treatment at 25 °C followed by H2/O2-TPSR experiments revealed that at least two different kinds of active NOx of low concentration (4–7 μmol g−1) were formed on both catalysts, whereas other inactive (spectator) NOx species formed were of larger concentration (>160 μmol g−1). UV–vis/DRS studies revealed that deposition of 3 wt-% W on ZrCe (Zr-rich) mixed metal oxide following calcination at 600 °C resulted in the formation of both polymeric WOx and WO3 clusters, whereas on CeZr (Ce-rich) only the latter phase (W6+) was seen. Large differences in the concentration (μmol m−2) and strength of surface acid sites between the W-CeZr and W-ZrCe solids were revealed after performing NH3-TPD and NH3-DRIFTS. These results were found to correlate with the specific H2-SCR rate (μmol m−2 min) obtained for the two solids. In particular, the surface acid sites on W-ZrCe and W-CeZr solids were found to be 5.96 and 2.76 μmol m−2, respectively, whereas the specific reaction rate was 0.14 and 0.046 μmol m−2 min at 300 and 250 °C, at which maximum rates were observed, respectively.

Platinum nanoparticles ( d Pt = 1.0–1.2 nm) supported on single CeO 2 and La 2 O 3 metal oxides and Ce 0.8 La 0.2 O 2− δ solid solution were prepared to investigate for the first time the effect of La 3+ -doping of ceria on important mechanistic and kinetic aspects of the water-gas shift (WGS) reaction, namely: (i) the concentration and chemical structure of active adsorbed reaction intermediates present in the C-path and H-path of WGS at 250 and 300 °C, (ii) the chemical nature of inactive species formed during WGS, and (iii) the prevailing mechanistic path among “redox” and “associative” both proposed in the literature. For this, steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with in situ DRIFTS and mass spectrometry were performed to follow the H-path (use of D 2 O) and C-path (use of 13 CO) of the WGS. In addition, other transient isotopic experiments using operando methodology (use of DRFTS and mass spectrometry) were designed to follow with time on stream the reactivity toward water of the various adsorbed species formed under WGS. It is proposed that on Pt/Ce 1− x La x O 2− δ ( x = 0.0, 0.2 and 1.0) the WGS reaction follows both the “redox” and “associative” mechanisms but the extent of participation of each mechanism to the overall WGS reaction rate depends on the support chemical composition. The WGS kinetic rate (μmol CO g −1 s −1 ) increased by a factor of 2.0 and 2.8 at 300 °C on 0.5 wt% Pt supported on Ce 0.8 La 0.2 O 2− δ compared to CeO 2 and La 2 O 3 , respectively. This was explained by (i) the larger concentration of active surface intermediates formed around each Pt nanoparticle (larger extent of reactive zone) and (ii) the higher reactivity of sites ( k , s −1 ) responsible for CO 2 and H 2 formation on Pt/Ce 0.8 La 0.2 O 2− δ compared to Pt/CeO 2 and Pt/La 2 O 3 . Active OH groups is suggested to be formed on defect sites (Ce 3+ □ s ) of Ce 0.8 La 0.2 O 2− δ as a consequence of the introduction of La 3+ into the ceria lattice, the latter enhancing the concentration of labile oxygen and its surface mobility, important characteristics of the “redox” mechanism.

Pt nanoparticles (1.0-1.4nm size) supported on Ce1-xLaxO2-δ (x=0.0, 0.2, 05, 0.8 and 1.0) carriers, the latter prepared by the citrate sol-gel method, were tested toward the water-gas shift (WGS) reaction in the 200-400°C range. A deep insight into the effect of Ce/La atom ratio of support chemical composition on the catalytic performance (CO conversion vs. temperature and stability) and kinetic rates of Pt-loaded catalysts was realized after employing HAADF/STEM, in situ Raman and DRIFT spectroscopies under different gas atmospheres, temperature-programmed surface reaction (TPSR) in He and O2/He gas atmospheres following WGS reaction, CO-TPD, in situ UV-vis/DRS, oxygen storage capacity measurements, and transient 18O-isotopic exchange studies followed by WGS reaction. It was found that doping of ceria with 20at.% La3+ has increased significantly the catalytic activity of 0.5wt% Pt/Ce0.8La0.2O2-δ solid in the 250-350°C range, whereas addition of 50-80at.% La3+ in ceria caused a negative effect on the CO conversion with respect to pure ceria. It was found that the Ce/La atom ratio in Ce1-xLaxO2-δ influences the catalytic site reactivity (k) along the Pt-support interface. The optimum La3+-dopant concentration of 20at.% (Ce/La=4/1) used in Pt/Ce0.8La0.2O2 compared to the worst one of 80at.% (Pt/Ce0.2La0.8O2-δ, Ce/La=1/4) correlates with (i) the higher specific kinetic rate per length of Pt-support interface (μmol COcm-1s-1), (ii) the higher concentration of oxygen vacant sites, (iii) the lower amount (μmol/g-1) of "carbon" accumulated during WGS and best stability with time on stream, (iv) the lower apparent activation energy (kcalmol-1) of WGS reaction, (v) the lower degree toward Pt oxidation (largest Pt2+/Pt4+ ratio), (vi) the lower Ce1-xLaxO2-δ support energy band gap, and (vii) the lower mobility of surface lattice oxygen. © 2013 Elsevier B.V. 140-141 333 347 Cited By :39

Pt nanoparticles (1.2–2.0 nm size) supported on Ce1–xTixO2−δ (x = 0, 0.2, 0.5, 0.8, and 1.0) carriers synthesized by the citrate sol–gel method were tested toward the water–gas shift (WGS) reaction in the 200–350 °C range. A deep insight into the effect of two structural parameters, the chemical composition of support (Ce/Ti atom ratio), and the Pt particle size on the catalytic performance of Pt-loaded catalysts was realized after employing in situ X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and HAADF/STEM, scanning electron microscopy (SEM), in situ Raman and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopies under different gas atmospheres, H2 temperature-programmed reduction (H2-TPR), and temperature-programmed desorption (NH3-TPD and CO2-TPD) techniques. The 0.5 wt % Pt/Ce0.8Ti0.2O2−δ solid (dPt = 1.7 nm) was found to be by far the best catalyst among all the other solids investigated. In particular, at 250 °C the CO conversion over Pt/Ce0.8Ti0.2O2−δ was increased by a factor of 2.5 and 1.9 compared to Pt/TiO2 and Pt/CeO2, respectively. The catalytic superiority of the Pt/Ce0.8Ti0.2O2−δ solid is the result of the support’s (i) robust morphology preserved during the WGS reaction, (ii) moderate acidity and basicity, and (iii) better reducibility at lower temperatures and the significant reduction of “coking” on the Pt surface and of carbonate accumulation on the Ce0.8Ti0.2O2−δ support. Several of these properties largely influenced the reactivity of sites (k, s–1) at the Pt–support interface. In particular, the specific WGS reaction rate at 200 °C expressed per length of the Pt–support interface (μmol CO cm–1 s–1) was found to be 2.2 and 4.6 times larger on Pt supported on Ce0.8Ti0.2O2−δ (Ti4+-doped CeO2) compared to TiO2 and CeO2 alone, respectively., The European Regional Development Fund, the Republic of Cyprus, the Research Promotion Foundation of Cyprus, and the Research Committee of the University of Cyprus are gratefully acknowledged for their financial support through the project TEXNO/0308(BE)/05. S.B. acknowledges financial support from the COST Action CM1104.

The present work reports on the effects of oxy-chlorine gas treatment (use of Cl 2/O 2/He gas mixture) applied on a 5wt% Pd/20wt% CeO 2-Al 2O 3 catalyst towards Pd redispersion. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HR-TEM), temperature programmed reduction in H 2 (H 2-TPR) and in situ diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) NO chemisorption techniques were employed before and after use of the oxy-chlorine gas treatment to critically evaluate its efficiency. The composition (xvol% Cl 2/18vol% O 2/He), temperature, and time on stream were investigated. The first two parameters were found to largely dictate optimum Pd redispersion, namely the use of a 2vol% Cl 2/18vol% O 2/He gas mixture at 500°C for 1h, where a significant reduction of an initial Pd mean particle size of 17.3-7.5nm was obtained. XPS studies revealed that after oxy-chlorine gas treatment followed by H 2 reduction at 500°C, complete elimination of Cl from the Pd surface was achieved. The oxygen storage capacity (OSC) of the catalyst measured following different oxy-chlorine gas treatments was found to significantly increase. Catalytic activity towards CO oxidation along with in situ DRIFTS NO chemisorption studies proved the large effect of the oxy-chlorine gas treatment on increasing the CO oxidation rate and the extent of NO chemisorption. Alternative treatment in oxygen gas atmosphere at high temperatures (500-850°C) followed by H 2 reduction (300-500°C) applied over the same catalyst failed to cause significant redispersion of Pd as observed with the oxy-chlorine gas treatment. © 2011 Elsevier B.V. 111-112 349 359 Cited By :5

•Novel nanomaterials were produced via ball milling (BM) of quarry waste material.•Various morphological and structural modifications occurred after the BM process.•The optimum conditions are 20h of BM with 50wt% ethanol as process control agent.•The BM process increased the CO2 uptake of mafic quarry wastes by a factor of ∼120.•The presence of augite nanocrystals after BM is linked to the increased CO2 uptake. The use of solid wastes rich in Ca2+, Mg2+ and Fe2+ for the ex situ sequestration of CO2 could provide an economically and technologically viable option for the reduction of anthropogenic CO2 emissions. Here, we investigate for the first time the potential use of waste materials from mafic rock quarries as a feedstock for the sequestration of CO2 by ex situ mineral carbonation. The experimental procedure was performed on a waste material from a dolerite quarry operating in the Troodos ophiolite (Cyprus). The ball milling process was applied to this quarry waste in order to create nanomaterials with enhanced CO2 adsorption capacity. Through CO2 chemisorption followed by temperature-programmed desorption (CO2-TPD) experiments, the optimum ball milling conditions were found (20h of wet ball milling with 50wt% ethanol as process control agent), leading to an enhancement of the CO2-storage capacity of the waste material by a factor of ∼120. This enhancement of CO2 uptake is closely related to (i) the reduction of particle size to the nanoscale, (ii) the structural disordering of the constituent silicate minerals, and (iii) the presence of augite nanocrystals after ball milling. The experimental results suggest that waste materials from dolerite quarries may indeed be used as feedstock for the ex situ mineralization of CO2. They also strongly demonstrate that ball milling is a very promising technique for optimizing the ex situ carbonation of this waste material thus the proposed methodology could be a fundamental part of a future carbon sequestration strategy. 16 361 370 361-370