Deposition-precipitation With Urea Research Articles

Turning trash into treasure: Gold nanoparticles (from e-waste) supported on TiO2 as catalyst for the oxidation of CO

Currently, electronic waste has become an environmental and public health issue. In this way, the United Nations has established goals within the 2030 agenda that aim to solve the problem surrounding this waste. In the effort to seek new alternatives to contribute to the agenda, this work presents the synthesis of the gold nanoparticles (Au-NPs) supported on TiO2 from electronic waste using the Deposition-Precipitation with Urea (DPU) method. Structural and morphological characterization by SEM and HR-TEM confirmed the successful synthesis of the well-distributed Au-NPs catalyst on TiO2 support. The results of the temperature-programmed reduction (TPR) test showed that the material can be reduced (activated) at a lower temperature compared to other reported materials. This characteristic is attributed to the presence of NiO or some other Ni species obtained from the gold coatings separation methodology, which could act as H2 spill-over agent on TiO2. Additionally, the UV–vis diffuse reflectance spectroscopy (UV–vis DRS) showed an increase in intensity (in terms of the F(R) function) and redshift of the Localized Surface Plasmon Resonance (LSPR) with increasing temperature. Overall, these results highlight the possibility of synthesizing Au/TiO2 NPs from electronic waste recycling with catalytic activity towards CO oxidation, adding significant value to this material.

Unveiling the structural behavior of bimetallic AuCu/TiO2 catalysts in the CO oxidation: A combined in-situ spectroscopic and theoretical study

Bimetallic AuCu/TiO2 catalysts were thoroughly studied during the CO oxidation reaction via three in-situ spectroscopic techniques (FTIR, UV–Vis and Raman) and density functional theory (DFT) calculations. Samples were synthesized through the deposition–precipitation with urea (DPU) method, and ex-situ characterized by several techniques. The superior catalytic activity displayed by the AuCu/TiO2 sample was associated with the presence of nanoparticles in the form of heterodimers composed of the Au and CuXO phases in close interaction with the TiO2 support. In-situ DRIFTS results indicated that gold nanoparticles (NPs) presented some restructuration in the monometallic sample, while this event did not occur in the bimetallic one. The better stability on the AuCu/TiO2 catalyst was related to the partial migration of CuXO species to the surface of the NPs, also confirmed through in-situ UV–Vis spectroscopy. The favorable role played by the TiO2 reducibility was established through in-situ Raman spectroscopy. Moreover, findings obtained by DFT confirmed the experimentally evidenced structural modifications.

Catalytic oxidation of carbon monoxide at low temperature using AuRh/TiO2 bimetallic catalysts: Effect of the synthesis method

AuRh nanoparticles supported on TiO2 P25 were synthesized by two sequential deposition methods: sequential deposition-precipitation with urea (DPU) and impregnation (IMP)-DPU. Segregated nanoparticles (RhcoreAushell ball-cup-type, and Janus-type arrangements) were observed in gold-rhodium bimetallic catalysts, but mainly ball-cup-type and Janus-type nanoparticles were found on catalysts prepared by IMP-DPU. XPS analysis of gold-rhodium bimetallic catalysts, before and after reaction allowed concluding that gold-rhodium interaction on metallic state was an important factor to avoid the oxidation of rhodium during CO oxidation. However, this effect of stabilization of the rhodium metallic phase by gold does not fully explain the promoting effect observed for some bimetallic catalysts on CO oxidation. Some structural aspects such as the atomic arrangement of the bimetallic nanoparticles and the type of active sites, determined by DRIFTS-CO, were also considered. TPR and UV–visible DR spectroscopy characterizations also showed differences in the gold-rhodium interactions.

Insights into CO oxidation on Au/TiO2-HMor zeolite catalysts at low temperature

The effect of combining TiO2 and mordenite zeolite (HMOR), employed as support of gold nanoparticles, on the CO oxidation reaction at low temperature is studied. The amount of TiO2 encapsulated into HMOR was varied and the catalyst efficiency was investigated. The deposition-precipitation with urea (DPU) method was used to deposit gold nanoparticles; likewise, the synthesis of monometallic catalysts based on TiO2 and HMOR is reported. The synthesized materials were characterized by X-ray diffraction (XRD), nitrogen adsorption, X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). The addition of TiO2 influenced the properties of the TiO2-HMOR composite, and its catalytic performance in the CO oxidation from 20°C. It was established that the 5Au/(28)TiO2-HMOR composite was the most active catalyst at lower temperatures, which was ascribed to the close contact among the components of the TiO2-HMOR composite, gold dispersion, gold and TiO2 loadings, and Au and Ti species present in the catalysts.

Outstanding synergistic effect of Au–Ir/Al2O3 catalysts on the total oxidation of propane

Bimetallic Au–Ir catalysts supported on ɣ-Al2O3 were synthesized by a sequential deposition-precipitation with urea (DPU) method. These materials displayed outstanding synergistic performance in the propane oxidation reaction at low reaction temperatures. A T50 of 150 °C was obtained for a bimetallic Au–Ir catalyst (Au loading of 3 wt % and Ir loading of 5 wt %), while for their gold and iridium monometallic counterparts it was of 370 and 190 °C, respectively. The synthesized materials were characterized by XRD, HRTEM-HAADF, H2-TPR, O2-TPD, UV–vis, and pyridine thermo-desorption, followed by FTIR and XPS, showing distinct characteristics that were correlated with their catalytic activity. Outcomes revealed a strong interaction among the gold and iridium particles when the catalysts were activated in H2 and that the thermal treatment atmosphere (hydrogen or air) modified the active sites; likewise, gold-iridium and monometallic iridium catalysts supported on ɣ-Al2O3 showed considerable endurance to activity loss because they hardly presented metal sintering at high reaction temperatures. Moreover, XPS results revealed electron transfer between Au–Ir (Au0 and Ir2+ species). Only the bimetallic system presented Lewis and Brønsted acidity (2.95 and 0.25 μ-mol m−2, respectively), which may have been responsible for enhancing the total oxidation of C3H8 at a low temperature. Likewise, the 3Au–5Ir/Al2O3 catalyst exhibited high stability and reusability during five reaction cycles and using a GHSV of 41,000 h−1.

Influence of the preparation method and silver content on the nature of active sites in Ag/CeO2 catalysts used for propylene oxidation

Silver supported on ceria solids (1 and 4 wt% Ag) are prepared by three different methods: Impregnation–Reduction with Citrate (IRC), Deposition–Precipitation with Urea (DPU), and Wetness Impregnation (WI). The catalysts are tested in the total oxidation of propylene. Presence of Ag2+ with Ag+ improves the catalytic activity. The solid prepared by the (IRC) method with a silver loading of 4 wt%, shows a better catalytic activity compared to the other solids. H2-Temperature Programmed Reduction (TPR) and Electron Paramagnetic Resonance (EPR) techniques confirm that the presence of three different redox couples: Ag2+/Ag+, Ag2+/Ag0, and Ag+/Ag0 in the (IRC) solid containing 4 wt% of silver is responsible for its superior catalytic performance.

Highly efficient photocatalytic hydrogen evolution by using Rh as co-catalyst in the Cu/TiO2 system

Cu/TiO2 was modified by adding Rh as co-catalyst and used as a highly efficient photocatalyst for the hydrogen evolution reaction. A low amount of Rh was loaded onto Cu/TiO2 by the deposition-precipitation with urea (DPU) method to observe the effect on the hydrogen production displayed by different samples. The Rh–Cu/TiO2 oxide structure exhibited a remarkably high photocatalytic hydrogen evolution performance, which was about twofold higher than that of the Cu/TiO2 monometallic photocatalyst. This outstanding performance was due to the efficient charge carrier transfer as well as to the delayed electron-hole recombination rate caused by the addition of Rh. The influence of the different parameters of the photocatalyst synthesis and reaction conditions on the photocatalytic activity was investigated in detail. Hydrogen evolution was studied using methanol, ethanol, 2-propanol and butanol as scavengers with an alcohol:water ratio of 20:80. The methanol-water system, which showed the highest hydrogen production, was studied under 254, 365 and 450 nm irradiation; Rh–Cu/TiO2 showed high photocatalytic activity with H2 production rates of 9260, 5500, and 1940 μmol h−1 g−1, respectively. The Cu–Rh/TiO2 photocatalyst was active under visible light irritation due to its strong light absorption in the visible region, low band gap value and ability to reduce the electron (e−) and hole (h+) recombination.

Bimetallic Ni–Zn/TiO2 catalysts for selective hydrogenation of alkyne and alkadiene impurities from alkenes stream

This study investigates the alternative of replacing noble metals by base metals as catalysts for selective hydrogenation of polyunsaturated hydrocarbons. Nickel catalysts are active for this type of reaction, but poorly selective. However, former calculations anticipated that Ni–Zn alloys could have higher selectivity to alkenes than monometallic Ni depending on the Ni/Zn atomic ratio in the alloy. In this contribution, Ni–Zn alloy nanoparticles supported on TiO2 are synthesized as catalysts for selective hydrogenation of acetylene and butadiene. The designed catalysts with 0.5 wt% Ni loading and various Ni/Zn nominal ratios are prepared by deposition–precipitation with urea (DPU). STEM-HAADF imaging coupled with EDS analysis reveals that Ni–Zn bimetallic particles are formed after reduction treatment at 450 °C. Alloying Ni with Zn leads to a slight increase in the average particle size and a broadening of the size distribution compared to monometallic Ni. However, the average Ni/Zn atomic ratio measured by EDS in the bimetallic particles is always higher than the nominal one, which could be due to Zn evaporation under the beam. The results of acetylene hydrogenation show that although the catalytic activity is slightly reduced after alloying Ni with Zn, the selectivity to ethylene is enhanced from 50% up to 85%, at the expense of the formation of oligomers (coupling products). However, the bimetallic Ni–Zn catalysts suffer from progressive deactivation in this reaction. During butadiene hydrogenation performed in the presence of an excess of propene, the bimetallic Ni–Zn/TiO2 catalysts are significantly more stable, with a high and constant selectivity to butenes (> 95%), compared with Ni/TiO2, which deactivates rapidly in the first hours. Some hypotheses concerning the observed differences in catalytic stability are discussed.

Synthesis and characterization of highly dispersed bimetallic Au-Rh nanoparticles supported on titanate nanotubes for CO oxidation reaction at low temperature.

Low-temperature CO oxidation was carried out by using rhodium incorporated into titanate nanotubes (Rh/NTs) prepared by the sol-gel and hydrothermal methods; otherwise, gold nanoparticles were deposited homogeneously onto the Rh/NT surface through the deposition-precipitation with urea (DPU) method. The Au-Rh/NT sample exhibited high metal dispersion (55%), outstanding CO oxidation at low temperature, and better resistance to deactivation than the monometallic Rh/NT and Au/NT samples. The characterization of bimetallic samples, with particle sizes from 1 to 3 nm, revealed the remarkable presence of interacting Au and Rh species in metallic state. In this way, Au0 and Rh0 were answerable for the higher catalytic activity observed in the bimetallic samples. The interaction between Au and Rh in the nanoparticles of Au-Rh/NT promoted a synergistic effect on the CO oxidation reaction, explained by the creation of new CO adsorption sites.

A novel approach for the preparation of silver nanoparticles supported on titanate nanotubes and bentonite-application in the synthesis of heterocyclic compound derivatives

The present work aims to study the effect of the type of support on the properties of silver nanoparticles by choosing two kinds of supports with similar characteristics, i.e. titanate nanotubes (TiNT) and Bentonite brought from the northwestern region of Maghnia in Algeria. The titanate nanotubes were prepared by conventional hydrothermal treatment; as for Bentonite, it was first purified, acidified and then doped with silver particles. The Deposition Precipitation with Urea (DPU) method was selected for the preparation of metalized titanate nanotubes (TiNT) as well as Bentonite at 2 wt% of Ag. The prepared materials were characterized by different methods, namely RD/UV–vis, FTIR, BET, XRD, TEM, SEM and EDS. The characterization results showed that titanate nanotubes exhibit a hollow structure with inner and outer diameters generally equal to 3 and 9 nm, respectively. However, silver nanoparticles, with an average size of 2.1 nm, were well distributed over titanate nanotubes and Bentonite. The catalytic activity of the prepared catalysts was investigated by performing the one-pot three-component synthesis of isoxazol-5(4 H)-ones in water, at room temperature. An excellent yield (≈90%) was obtained for the synthesis of isoxazol-5-(4 H)–ones; this yield generally depends on silver content.

Insight into surface properties of O2 plasma activated Au/TiO2 prepared by DPU in CO oxidation

Abstract Driven by excellent CO oxidation capability of gold nanoparticles at room temperature, a TiO2 supported Au nanoparticles (Au/TiO2) is synthesized by deposition-precipitation with urea (DPU). To solve the sintering issue of gold nanoparticles during activation process, atmospheric-pressure cold O2 plasma is employed to activate the Au/TiO2 catalysts and compared with the conventional calcination. The plasma activated one (S-P) shows poorer activity compared to the calcined one (S-C). However, S-P with further TPD treatment (S-PT) shows the best catalytic activity among above-mentioned samples. To elucidate the plasma effect, TEM, XPS, UV–vis spectroscopy and in situ DRIFTS techniques are used for catalyst characterization. Based on TEM results, S-P sample has smaller particle size and narrower size distribution compared to S-C sample, while the high content of low-coordinated Au atoms and the poisoning nitrate species over S-P are revealed by in situ DRIFTS. Upon further TPD treatment, the poisoning species occupying the active sites is desorbed from catalyst surface and the improvements in Au particle size and size distribution are maintained. The mechanism for plasma effect on activation of Au/TiO2 prepared by DPU is also discussed.

Bimetallic Fe-Ni/SiO2 catalysts for furfural hydrogenation: Identification of the interplay between Fe and Ni during deposition-precipitation and thermal treatments

Supported Fe-Ni catalysts have been reported for their activity and selectivity in the hydrogenation of unsaturated organic molecules. However, the control of the size and composition of the bimetallic nanoparticles remains a bottleneck when oxide-supported catalysts are prepared by impregnation, and alternative procedures should be investigated. Starting with Ni(II) and Fe(II) sulfates as precursor salts, deposition-precipitation with urea (DPU) on SiO2 in an inert atmosphere initially leads to the formation of an ill-crystallized Fe-containing Ni(II) 1:1 phyllosilicate, which reduces under hydrogen at 700 °C into bimetallic fcc Fe-Ni nanoparticles of 5.4 nm in average. Compared with the composition of the DPU solution (50 Fe at %, 50 Ni at %), an excess of Ni is detected on the catalyst (38 Fe at %, 62 Ni at %), due to the preferential reaction of Ni2+ ions with silica. In situ X-ray absorption spectroscopy and 57Fe Mössbauer spectroscopy show that the reduction of Fe ions to the metallic state is triggered by the formation of reduced Ni centers above 350 °C, and, from then, proceeds progressively, resulting in an excess of Fe in the outer shells of the bimetallic particles. The composition of individual Fe-Ni particles evidences a standard deviation of 8%. The bimetallic Fe-Ni/SiO2 catalyst gives high yields in furfuryl alcohol in the hydrogenation of furfural, in contrast with an analog Ni/SiO2 catalyst that favours side-reactions of etherification, hydrogenolysis and hydrogenation of the furan ring.

Cerium phosphate-supported Au catalysts for CO oxidation

LaPO4 and hydroxyapatite (Ca10(PO4)6(OH)2) are typical metal phosphates recently found to be useful for making supported metal or metal oxide catalysts, but CePO4 (also belonging to the metal phosphate family) has been rarely used to make supported catalysts. It would be interesting to develop CePO4-supported catalysts and explore their catalytic applications. Herein, hexagonal CePO4 nanorods (denoted as CePO4-H), hexagonal CePO4 nanowires (CePO4-HNW), monoclinic CePO4 nanoparticles (CePO4-M), and monoclinic CePO4 nanowires (CePO4-MNW) prepared by different methods were used to support gold via deposition-precipitation with urea (DPU). The gold contents of these catalysts were all around 1 wt%. The catalytic activities of these Au/CePO4 catalysts in CO oxidation were found to follow the sequence of Au/CePO4-MNW > Au/CePO4-HNW > Au/CePO4-M > Au/CePO4-H. These catalysts were characterized by inductively coupled plasma-optical emission spectroscopy (ICP-OES), N2 adsorption–desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), oxygen temperature-programmed desorption (O2-TPD), and CO2 temperature-programmed desorption (CO2-TPD) to find possible correlations between the physicochemical properties and catalytic activities of these catalysts.

Au/MOx (M = Zn, Ti) nanocomposites as highly efficient catalytic filters for chemical gas sensing at room temperature and in humid atmosphere

Metal oxide supported gold nanoparticles (Au/MOx) have been prepared using deposition-precipitation with urea (DPU) method on commercial ZnO and TiO2 powders. These Au/MOx nanocomposites have been characterized by X-ray powder diffraction, high resolution transmission electron microscopy, nitrogen adsorption-desorption isotherms, and inductively coupled plasma atom emission spectroscopy. They have been used at room temperature as catalytic filters for CO oxidation in humid atmosphere (RH 40%). The CO removal efficiency has been evaluated using metal oxide gas sensors (MOS) device. The catalytic performances assessment of these Au/MOx nanocomposites highlights the efficiency of the Au/ZnO system. An integrated sensing device comprising the gas sensor and its catalytic filter has been developed. In that case, a plastic holder filled with low amounts (50mg) of Au/ZnO catalytic filter is simply placed above the MOS sensor. This system allows the improvement of the selectivity of the MOS sensors towards other gases, i.e. propane. The presented setting can be applied as one of valuable addition for multi-array sensor devices.

Novel non-noble bimetallic Cu-Zn/TiO2 catalysts for selective hydrogenation of butadiene

Oxide-supported copper catalysts are active for the selective hydrogenation of polyunsaturated hydrocarbons with high selectivity to alkenes formation, but a low catalytic stability limits its application. The goal of the work was to investigate whether alloying Cu with Zn will improve catalyst stability, and whether such a non-noble bimetallic catalyst could compete with noble metal-based catalysts for this type of reaction. TiO2-supported mono-metallic Cu and Zn and bimetallic Cu-Zn (Cu/Zn atomic ratio of 1 and 3) were prepared by deposition-precipitation with urea (DPU) and deposition-precipitation at pH≈8 (DP8). Small metal nanoparticles (<2nm) with uniform particle sizes were obtained from DP8 samples, while two different particle size ranges (∼3nm and >15nm) were found in the DPU samples after calcination at 400°C and then reduction at 350°C. The bimetallic character of the nanoparticles was attested by XRD and STEM-HAADF coupled with EDS analysis, as Cu3Zn alloy was found in DPU CuZn 1:1/TO2 sample and an “intermediate” Cu-Zn alloy (Cu0.9Zn0.1) was found in DPU CuZn 3:1/TiO2 sample. UV-visible spectroscopy showed that alloying of copper with zinc inhibits the reoxidation of copper by contact with air. The catalytic results showed that alloying Cu with Zn slightly decreased the activity in butadiene selective hydrogenation, but had almost no effect on the selectivity to alkenes. The important result is that the bimetallic Cu-Zn/TiO2 catalysts displayed much higher stability under isothermal reaction than Cu/TiO2, which deactivated rapidly in the first few hours. This higher stability was ascribed to the formation of a lower amount of carbonaceous species on the bimetallic catalysts, as revealed by TGA analyses. Moreover, the DP8 Cu-Zn/TiO2 samples showed lower T100% but higher stability than the corresponding DPU samples. Surprisingly, change in the composition of the Cu-Zn alloy was observed during the catalytic reaction at 105°C, with a transition from Cu3Zn to Cu0.7Zn0.3 alloy for the DPU CuZn 1:1/TiO2 sample.

Highly efficient catalytic reductive degradation of various organic dyes by Au/CeO 2 -TiO 2 nano-hybrid

Highly improved catalytic reductive degradation of different organic dyes, in the presence of excess NaBH4 over Au/CeO2-TiO2 nano-hybrid as the catalyst is reported in this study. CeO2-TiO2 nanocomposite was prepared by a facile co-precipitation method using ultra-high dilute aqueous solutions. Small amount of Au (only 1 wt%) was loaded onto the nanocomposite material by deposition-precipitation with urea (DPU) method to fabricate the ternary Au/CeO2-TiO2 nano-hybrid. The catalysts were characterized by the representative techniques like XRD, BET surface area, ICP-AES, UV-Vis diffuse reflectance spectroscopy, TEM and XPS. The Au/CeO2-TiO2 nano-hybrid along with NaBH4 exhibited remarkable catalytic activities towards all the probed dyes, namely Methylene Blue, Methyl Orange, Congo Red, Rhodamine B and Malachite Green, with a degradation efficiency of ∼100% in a short reaction time. The degradation reaction followed pseudo-first-order kinetics with respect to the concentration of the dye. Different parameters that affect the rate of the reaction are discussed. A plausible mechanism for methylene blue degradation has also been proposed.

Exploration of the preparation of Cu/TiO2 catalysts by deposition–precipitation with urea for selective hydrogenation of unsaturated hydrocarbons

Deposition–precipitation with urea (DPu) was used to deposit copper on TiO2 and prepare metal Cu/TiO2 catalysts with different Cu loadings from 0.6 to 4.2wt%. Cu2+ complexes adsorb on TiO2 at pH lower than PZCTiO2 (around 6), i.e., when the titania surface is supposed to be positively charged. To attempt to explain these results, a study on the deposition behavior of Cu2+ was carried out as a function of the pH of the preparation solution and the Cu loading. The samples were characterized by XRD, UV–visible, TPR and electron microscopy leading to a chemical mechanism of copper deposition: the presence of some negatively charged Ti–O sites on the TiO2 surface even at low pH could be responsible for the Cu2+ complex adsorption, but it has not been possible to discard the occurrence of a surface reaction with OHs of titania. In fine, these adsorbed Cu2+ species formed Ti–O–Cu bonding, and were proposed to act as nucleation sites for the formation of Cu2(OH)3NO3 particles onto the support. As DPu proceeded, these particles grew according to Ostwald ripening mechanism, and became unstable and transformed into CuO depending on the final Cu loading and DPu duration. As a result, the size of Cu0 nanoparticles obtained after reduction at 350°C strongly depends on the Cu loading and the DPu time: very small Cu0 particles (<1nm) in the low loaded samples (<1wt% Cu) resulting from the presence of small particles of Cu2(OH)3NO3 after preparation, moderate Cu0 particle size (∼3nm) in samples initially containing larger particles of Cu2(OH)3NO3 and additional large particles (>10nm) in samples containing CuO. The Cu catalysts exhibit interesting potential for selective hydrogenation of butadiene at relatively low temperature with good activity and high selectivity. The highest activity and stability were obtained with the samples containing small metal Cu particles, i.e., the samples exempt of CuO after preparation.

Au/LaPO4 nanowires: Synthesis, characterization, and catalytic CO oxidation

Au/M-P-O (M=Mg, Al, Ca, Fe, Co, Zn, La) catalysts were prepared by loading ca. 1wt.% gold onto commercial metal phosphate supports by deposition-precipitation with urea (DPU), and tested in CO oxidation. Au/La-P-O showed the highest activity, achieving complete CO conversion at 70°C. Au/Ca-P-O was the second most active, achieving complete CO conversion at 155°C. The remaining catalysts were not active below 200°C. In view of the results, we additionally prepared Au/LaPO4 nanowires by loading ca. 1wt.% gold onto LaPO4 nanowires via DPU. The Au/LaPO4 nanowires catalyst was even more active, achieving complete CO conversion at 15°C. The catalysts were characterized by N2 adsorption-desorption, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption of O2 or CO2 (O2-TPD, CO2-TPD). Reasons for the higher activity of Au/LaPO4 nanowires were discussed.

Enhanced Interactions between Gold and MnO2 Nanowires for Water Oxidation: A Comparison of Different Chemical and Physical Preparation Methods

Oxygen evolution reaction (OER) has been considered as the bottleneck step in water electrolysis to simultaneously produce hydrogen and oxygen. The earth abundant first-row metal oxides such as manganese oxide have been found very active after anchoring nanoparticular noble metals such as Au, Pd and Pt onto its surface. Recent reports have demonstrated that Au can increase the turnover frequency (TOF) of MnOx more than 10 times for the OER due to the unique local interaction between Au and MnOx. Here, we conducted a detailed comparative study of different preparation methods on the OER activity of Au/MnO2 nanocomposites, including physical sputtering (PS), deposition–precipitation with urea (DPU), DP with NaOH (DPN) and deposition–reduction (DR) with NaBH4. Through carefully controlling the preparation conditions, we find chemical preparation methods (i.e., DPU and DPN) can achieve uniform growth of monodispersed Au nanoparticles on MnO2 nanowires (Au/MnO2) same as sputtering; moreover, the as-prepared Au–MnO2 by DPU and DPN shows stronger interaction than that by sputtering, thereby achieving higher OER performance. Delicate chemical valence change and conductivity improvement induced by such interaction are further quantified by conventional XPS, resistivity and impedance measurements.

The effect of metal composition on the performance of Ir–Au/TiO2 catalysts for citral hydrogenation.

The effect of gold addition to iridium catalysts and the nature of active sites for citral hydrogenation were investigated over Ir–Au/TiO2 catalysts. All samples (Au/TiO2, Ir/TiO2 and Ir–Au/TiO2) were prepared by deposition-precipitation with urea (DPU). Bimetallic catalysts were synthetized by co-deposition at different Ir/Au atomic ratios (3, 1, 0.3). The catalysts were characterized by ICP, BET, H2–TPR, H2-Chemisorption, TEM, DRIFTS and XPS techniques. A partial coverage of iridium sites by gold atoms takes place as the gold amount in the catalyst increases as shown by chemisorption and XPS measurements. The latter also evidenced a trend consistent with modification of the electronic environment of iridium due to interaction with gold atoms. Also, compared to Ir/TiO2, the amount of Irδ+ species was higher in the bimetallic catalysts reaching a maximum in Ir–Au (1) sample. Characterization of the catalytic surface using DRIFTS of adsorbed CO evidenced a shift towards higher wavenumbers as a function of the gold content indicating, as the XPS results, a modification of the adsorption site. The catalytic activity for citral hydrogenation increased as a function of the gold content. The selectivity to unsaturated alcohol is related to the amount of Irδ+ species which in turns depends on the catalyst composition. For the most active sample Ir–Au (1), a suitable Irδ+/Ir0 ratio is already obtained when the catalyst is reduced at 573K and does not vary with reduction temperature.