Physicochemical and functional properties of modified tin dioxide

Specimens of tin dioxide with modifying Sb and Pt additives are synthesized. Their physicochemical properties (specific surface area, porosity, and conductivity), chemisorption and catalytic activity in the model reaction of CO oxidation are studied. A considerable chemisorption of CO on SnO₂ and SnO₂-SbO _x is observed at 150–180°C. The oxidation of CO in the flow of gases starts in the same temperature range. An addition of platinum leads to a significant increase in the rate of CO oxidation, the reaction starts at 80°C. It is proposed that the process proceeds at the SnO₂/Pt interface.     

Microwave-assisted hydrothermal synthesis of Pt/SnO2 gas sensor for CO detection

In this paper, the Pt/SnO₂ nanostructures were prepared via a facile one-step microwave assisted hydrothermal route. The structure of the introduced Pt/SnO₂ and its gas-sensing properties toward CO were investigated. The results from the TEM test reveal that Pt grows on the SnO₂ nanostructure, which was not found for bulk in this situ method, constructing Pt/SnO₂. The results indicated that the sensor using 3.0 wt% Pt/SnO₂ to 100 ppm carbon monoxide performed a superior sensing properties compared to 1.5 wt% and 4.5 wt% Pt/SnO₂ at 225 °C. The response time of 3.0 wt% sensor is 16 s to 100 ppm CO at 225 °C. Such enhanced gas sensing performances could be attributed to the chemical and electrical factors. In view of chemical factors, the presence of Pt facilitates the surface reaction, which will improve the gas sensing properties. With respect to the electrical factors, the Pt/SnO₂ plays roles in increasing the sensor’s response due to its characteristic configuration. In addition, the one-step in situ microwave assisted process provides a promising and versatile choice for the preparation of gas sensing materials.     

Performance and kinetic study on Pd/OMS-2 catalyst for CO catalytic oxidation: effect of preparation method

Manganese oxide octahedral molecular sieves (OMS-2) synthesized from hydrothermal (H-OMS-2), reflux (R-OMS-2), co-precipitation (C-OMS-2), and solid phase (S-OMS-2) methods were impregnated with palladium and used for CO catalytic oxidation. Preparation methods presented an obvious effect on the morphology and catalytic activity of Pd/OMS-2 catalysts for CO oxidation. The hydrothermal synthesized OMS-2 (Pd/H-OMS-2) exhibited more ordered nanorod structure and higher crystallinity than Pd/R-OMS-2, Pd/C-OMS-2, and Pd/S-OMS-2. Further surface analysis indicated that different preparation methods of synthesizing OMS-2 and the impregnation process followed have a significant effect on the chemical states of Mn and O over the final Pd/OMS-2 products. The kinetics studies showed the trend of apparent activation energy (E _a) over different catalysts: Pd/H-OMS-2 (18.19 kJ/mol) < Pd/R-OMS-2 (21.56 kJ/mol) < Pd/C-OMS-2 (22.57 kJ/mol) < Pd/S-OMS-2 (29.44 kJ/mol). Over 99 % of the CO conversion was obtained at 35 °C by the optimal Pd/H-OMS-2 catalyst.     

Direct oxidation of methane at low temperature using Pt/C,Pd/C,Pt/C-ATO and Pd/C-ATO electrocatalysts prepared by sodium borohydride reduction process

The main objective of this paper was to characterize the voltammetric profiles of the Pt/C,Pt/C-ATO,Pd/C and Pd/CATO electrocatalysts and study their catalytic activities for methane oxidation in an acidic electrolyte at 25 ℃ and in a direct methane proton exchange membrane fuel cell at 80 ℃. The electrocatalysts prepared also were characterized by X-ray diffraction( XRD) and transmission electron microscopy( TEM). The diffractograms of the Pt/C and Pt/C-ATO electrocatalysts show four peaks associated with Pt face-centered cubic( fcc) structure,and the diffractograms of Pd/C and Pd/C-ATO show four peaks associated with Pd face-centered cubic( fcc) structure. For Pt/C-ATO and Pd/C-ATO,characteristic peaks of cassiterite( SnO_2) phase are observed,which are associated with Sb-doped SnO_2( ATO) used as supports for electrocatalysts. Cyclic voltammograms( CV) of all electrocatalysts after adsorption of methane show that there is a current increase during the anodic scan. However,this effect is more pronounced for Pt/C-ATO and Pd/C-ATO. This process is related to the oxidation of the adsorbed species through the bifunctional mechanism,where ATO provides oxygenated species for the oxidation of CO or HCO intermediates adsorbed in Pt or Pd sites. From in situ ATR-FTIR( Attenuated Total Reflectance-Fourier Transform Infrared) experiments for all electrocatalysts prepared the formation of HCO or CO intermediates are observed,which indicates the production of carbon dioxide. Polarization curves at 80 ℃in a direct methane fuel cell( DMEFC) show that Pd/C and Pt/C electroacatalysts have superior performance to Pd/C-ATO and Pt/C-ATO in methane oxidation.     

Platinum-Tin/Tin Oxide/CNT Catalysts for High-Performance Electrocatalytic Ethanol Oxidation

Ethanol is a promising liquid clean energy source in the energy conversion field. However, the self-poisoning caused by the strongly adsorbed reaction intermediates (typically, CO) is a critical problem in ethanol oxidation reaction. To address this issue, we proposed a joint use of two strategies, alloying of Pt with other metals and building Pt/metal-oxide interfaces, to achieve high-performance electrocatalytic ethanol oxidation. For this, a well-designed synthetic route combining wet impregnation with a two-step thermal treatment process was established to construct PtSn/SnO_x interfaces on carbon nanotubes. Using this route, the alloying of Pt−Sn and formation of PtSn−SnO_x interfaces can simultaneously be achieved, and the coverage of SnO_x thin films on PtSn alloy nanoparticles can be facilely tuned by the strong interaction between Pt and SnO_x. The results revealed that the partial coverage of SnO_x species not only retained the active sites, but also enhanced the CO anti-poisoning ability of the catalyst. Consequently, the H−PtSn/SnO_x/CNT-2 catalyst with an optimized PtSn−SnO_x interface showed significantly improved performances toward the ethanol oxidation reaction (825 mA mg_Pt⁻¹). This study provides deep insights into the structure-performance relationship of PtSn/metal oxide composite catalysts, which would be helpful for the future design and fabrication of high-performance Pt-based ethanol oxidation reaction catalysts.     

Studies of the roles of Sn or Fe on γ-Al2O3-supported Pt catalysts by CO adsorption microcalorimetry and dehydrogenation reaction of C4 alkanes

CO adsorption microcalorimetry was employed in the study of γ-Al₂O₃-supported Pt, Pt-Sn and Pt-Fe catalysts. The results indicated that the initial differential heat of CO adsorption of the Pt/γ-Al₂O₃ catalyst was 125 kJ/mol. As CO coverage increased, the differential heat of adsorption decreased. At higher coverages, the differential heat of adsorption decreased significantly. 60% of the differential heat of CO adsorption on the Pt/γ-N₂O₃ catalyst was higher than 100 kJ/mol. No significant effect on the initial differential heat was found after adding Sn and Fe to the Pt/γ-Al₂O₃ catalyst. The amount of strong CO adsorption sites decreased, while the portion of CO adsorption sites with differential heat of 60–110 kJ/mol increased after increasing the Sn or Fe content. This indicates that the surface adsorption energy was changed by adding Sn or Fe to Pt/γ-N₂O₃. The distribution of differential heat of CO adsorption on the Pt-Sn(C)/γ-Al₂O₃ catalyst was broad and homogeneous. Comparison of the dehydrogenation performance of C₄ alkanes with the number of CO adsorption sites with differential heat of 60–110 kJ/mol showed a good correlation. These results indicate that the surface Pt centers with differential heats of 60–110 kJ/mol for CO adsorption possess superior activity for the dehydrogenation of alkanes. Project supported by FORD and the National Natural Science Foundation of China (Grant No. 09412302) and the Transcentury Training Program Foundation for the Talents by The State Education Commission of China.     

Sensor Performance of Nanostructured TiO<Subscript>2</Subscript>–SnO<Subscript>2</Subscript> Films Prepared by an Aqueous Sol–Gel Process

Nanostructured TiO₂–SnO₂ thin films and powders were prepared by a facile aqueous particulate sol–gel route. The prepared sols showed a narrow particle size distribution with hydrodynamic diameter in the range 17.2–19.3 nm. Moreover, the sols were stable over 5 months, since the constant zeta potential was measured during this period. The effect of Sn:Ti molar ratio was studied on the crystallisation behaviour of the products. X-ray diffraction analysis revealed that the powders were crystallised at the low temperature of 400 °C containing anatase-TiO₂, rutile-TiO₂ and cassiterite-SnO₂ phases, depending on annealing temperature and Sn:Ti molar ratio. Furthermore, it was found that SnO₂ retarded the anatase to rutile transformation up to 800 °C. The activation energy of crystallite growth was calculated in the range 0.96–6.87 kJ/mol. Transmission electron microscope image showed that one of the smallest crystallite sizes was obtained for TiO₂–SnO₂ binary mixed oxide, being 3 nm at 600 °C. Field emission scanning electron microscope analysis revealed that the deposited thin films had nanostructured morphology with the average grain size in the range 20–40 nm at 600 °C. Thin films produced under optimized conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO gas at low operating temperature of 200 °C, resulting in increased thermal stability of sensing films as well as a decrease in their power consumption.     

Selective catalytic reduction of NO with CO on Pt/W–Ce–Zr catalysts

Various Pt catalysts (Pt/ZrO₂, Pt/CeO₂, Pt/CeZrO, Pt/WO₃/ZrO₂ and Pt/WO₃/CeZrO) were prepared and characterized, and their catalytic reduction reactions of NO by CO, with or without the presence of excess oxygen, were investigated. The results of temperature-programmed experiments showed that CO could be easily oxidized over Pt/CeO₂ and Pt/CeZrO while the introduction of WO₃ into the catalyst (Pt/WO₃/CeZrO) inhibited the reduction of catalyst surface; NO could not dissociate over those catalysts in oxidized state but after CO reduction at a low temperature, NO dissociation took place only over Pt/CeO₂ and Pt/CeZrO catalysts. For NO + CO reaction, those easily reduced catalysts Pt/CeO₂ and Pt/CeZrO exhibited better catalytic performances, and NO could be rapidly converted below 350 °C. For the reaction with the presence of excess O₂, the NO conversions were significantly inhibited, but better NO conversions were obtained over the tungstate-contained catalysts when compared with Pt/CeO₂ and Pt/CeZrO. The higher activities of Pt/W–Ce–Zr catalysts were attributed to their high acidities.     

Photo–thermo Catalytic Oxidation over a TiO2-WO3-Supported Platinum Catalyst

Photo–thermo catalysis, which integrates photocatalysis on semiconductors with thermocatalysis on supported nonplasmonic metals, has emerged as an attractive approach to improve catalytic performance. However, an understanding of the mechanisms in operation is missing from both the thermo- and photocatalytic perspectives. Deep insights into photo–thermo catalysis are achieved via the catalytic oxidation of propane (C₃H₈) over a Pt/TiO₂-WO₃ catalyst that severely suffers from oxygen poisoning at high O₂/C₃H₈ ratios. After introducing UV/Vis light, the reaction temperature required to achieve 70 % conversion of C₃H₈ lowers to a record-breaking 90 °C from 324 °C and the apparent activation energy drops from 130 kJ mol⁻¹ to 11 kJ mol⁻¹. Furthermore, the reaction order of O₂ is −1.4 in dark but reverses to 0.1 under light, thereby suppressing oxygen poisoning of the Pt catalyst. An underlying mechanism is proposed based on direct evidence of the in-situ-captured reaction intermediates.     

Influence of NO on the Reduction of NO2 with CO over Pt/SiO2 in the Presence of O2

Reduction of NO2 with CO in the presence of NO and excess oxygen, a model mixture for flue gas, over a 0.1% Pt/SiO2 catalyst was studied. The related reaction mechanisms, such as oxidation of CO and NO, were discussed. It was found that there was a narrow temperature window （180-190 ℃） for the reduction of NO2 by CO. When the temperature was lower than the lower limit of the window, the reduction hardly occurred, while when the temperature was higher than the upper limit of the window, the direct oxidation of CO by O2 occurred and thereby NO2 could not be effectively reduced by CO. The presence of NO shifted the window to higher temperatures owing to the inhibition effect of NO on the activation of O2 on Pt, which made it possible to reduce NO2 by CO in flue gas.     

Photo–thermo Catalytic Oxidation over a TiO2‐WO3‐Supported Platinum Catalyst

Photo–thermo catalysis, which integrates photocatalysis on semiconductors with thermocatalysis on supported nonplasmonic metals, has emerged as an attractive approach to improve catalytic performance. However, an understanding of the mechanisms in operation is missing from both the thermo‐ and photocatalytic perspectives. Deep insights into photo–thermo catalysis are achieved via the catalytic oxidation of propane (C₃H₈) over a Pt/TiO₂‐WO₃ catalyst that severely suffers from oxygen poisoning at high O₂/C₃H₈ ratios. After introducing UV/Vis light, the reaction temperature required to achieve 70 % conversion of C₃H₈ lowers to a record‐breaking 90 °C from 324 °C and the apparent activation energy drops from 130 kJ mol^?1 to 11 kJ mol^?1. Furthermore, the reaction order of O₂ is ?1.4 in dark but reverses to 0.1 under light, thereby suppressing oxygen poisoning of the Pt catalyst. An underlying mechanism is proposed based on direct evidence of the in‐situ‐captured reaction intermediates.     

Kinetics and mechanism of the low-temperature oxidation of carbon monoxide with oxygen on a PdCl<Subscript>2</Subscript>–CuCl<Subscript>2</Subscript>/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst

A systematic study of the kinetics of the low-temperature oxidation of carbon monoxide with oxygen on a PdCl₂–CuCl₂/γ-Al₂O₃ supported catalyst was carried out over a wide range of the partial pressures of oxygen, water, and CO in order to test hypotheses on the reaction mechanism. It was shown that, as the temperature was increased from 20 to 38°C, rate of formation of CO₂ decreased and the apparent activation energy was about–40 kJ/mol. The hypotheses of different degrees of complexity concerning the reaction mechanism were formulated based on physicochemical data and a Langmuir–Hinshelwood model. Mechanisms in which carbon dioxide is formed on the interaction of the surface Pd(I) and Pd(II) complexes that include carbon monoxide and water with the surface complex of Cu(I) that coordinates oxygen were recognized as the most probable.     

Pt-SnO2 / C催化剂对乙醇和吸附态CO的电催化氧化

由于乙醇最有可能成为直接甲醇燃料电池（DMFC）的替代燃料，因此近年来。对乙醇的电化学氧化及直接乙醇燃料电池的研究已引起人们的很大兴趣。甲醇毒性较大并且易透过Nafion膜进入阴极造成阴极的混合电位而影响DMFC的阴极性能．这是制约DMFC走向实用化的主要问题之一。因此人们在致力于研究直接甲醇燃料电池的同时．也寻求其它的小分子醇作为甲醇的替代燃料。乙醇是除甲醇以外最简单的醇．它来源广泛．无毒，是可再生和环保型能源．并且也有较高的能量密度和反应活性。但是乙醇在电极上的完全氧化因涉及到C-C键的断裂要比甲醇困难．阳极反应动力学过程也比较缓慢。到目前为止铂基催化剂仍然是乙醇氧化最好的催化剂．虽然也有使用非铂催化剂研究乙醇的电氧化，但催化活性远不如铂基催化剂高。     

Hydrogenation of nitrate in water over bimetallic Pd-Ag/Al2O3 catalysts

Hydrogenation of nitrate (NO₃ ⁻) in water was studied with 0.8 ×10⁻³–3.2 ×10⁻³ mol/dm³ of reactant in the temperature range of 293–313 K over palladium promoted Ag catalysts. Pd-Ag catalysts with a low ratio of Ag/Pd were characterized by high efficiency in the reduction of nitrates. The degradation of nitrates followed approximately first order decay and the estimated apparent activation energy was about 4 kJ/mol.     

Stereochemical nonrigidity of the square complex (PPh3)2Pt(HgGePh3)(GePh3)

³¹P, ¹⁹⁵Pt and ¹⁹⁹Hg NMR spectra of complex (PPh₃)₂Pt(HgGePh₃)(GePh₃) (I) have been studied. The spectra at temperatures below ?40°C prove that (I) is a cis-isomer with the square-planar coordination of the Pt atom. The reversibility of temperature dependences of spectra, insensitivity of line shape to the solvent, concentration and presence of free phosphine establish the fluxional behaviour of (I). The activation parameters of the intramolecular rearrangement which is realized, most probably, through a digonal twist, are: Δ^≠₂₉₈ = 51.5 ± 2.9 kJ/mol, ΔH^≠ = 59.3 ± 2.9 kJ/mol, ΔS^≠ = 26.2 ± 9.7 J/mol. K.     

Synthesis,characterization and electrocatalytic activity for ethanol oxidation of carbon supported Pt,Pt–Rh,Pt–SnO2 and Pt–Rh–SnO2 nanoclusters

Nanoclusters of Pt, Pt–Rh, Pt–SnO₂ and Pt–Rh–SnO₂ were successfully synthesized by polyol method and deposited on high-area carbon. HRTEM and XRD analysis revealed two phases in the ternary Pt–Rh–SnO₂/C catalyst: solid solution of Rh in Pt and SnO₂. The activity of Pt–Rh–SnO₂/C for ethanol oxidation was found to be much higher than Pt/C and Pt–Rh/C and also superior to Pt–SnO₂/C. Quasi steady-state measurements at various temperatures (30–60 °C), ethanol concentrations (0.01–1 M) and H₂SO₄ concentrations (0.02–0.5 M) showed that Pt–Rh–SnO₂/C is about 20 times more active than Pt/C in the potential range of interest for the fuel cell application.     

CO oxidation with oxygen of the catalyst and gas-phase oxygen over (0.5−15)%СоО/СеО<Subscript>2</Subscript>

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 Co_m²⁺O_n²⁺ 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.     

CO oxidation by oxygen of the catalyst and by gas-phase oxygen over (0.5–15)%CoO/ZrO<Subscript>2</Subscript>

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 _m ²⁺ O _n ^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.     

Mechanism of CO oxidation in excess H2 over CuO/CeO2 catalysts: ESR and TPD studies

The oxidation of CO with oxygen over (0.25–6.4)% CuO/CeO₂ catalysts in excess H₂ is studied. CO conversion increases and the temperature range of the reaction decreases by 100 K as the CuO content is raised. The maximal CO conversion, 98.5%, is achieved on 6.4% CuO/CeO₂ at 150°C. At T > 150°C, the CO conversion decreases as a result of the deactivation of part of the active sites because of the dissociative adsorption of hydrogen. CO is efficiently adsorbed on the oxidized catalyst to form CO-Cu⁺ carbonyls on Cu₂O clusters and is oxidized by the oxygen of these clusters, whereas it is neither adsorbed nor oxidized on Cu⁰ of the reduced catalysts. The activity of the catalysts is recovered after the dissociative adsorption of O₂ on Cu⁰ at T ～ 150°C. The activation energies of CO, CO₂, and H₂O desorption are estimated, and the activation energy of CO adsorption yielding CO-Cu⁺ carbonyls is calculated in the framework of the Langmuir-Hinshelwood model.     

Mild Synthesis of Pt/SnO2/Graphene Nanocomposites with Remarkably Enhanced Ethanol Electro‐oxidation Activity and Durability

We have designed a new Pt/SnO₂/graphene nanomaterial by using L ‐arginine as a linker; this material shows the unique Pt‐around‐SnO₂ structure. The Sn²⁺ cations reduce graphene oxide (GO), leading to the in situ formation of SnO₂/graphene hybrids. L ‐Arginine is used as a linker and protector to induce the in situ growth of Pt nanoparticles (NPs) connected with SnO₂ NPs and impede the agglomeration of Pt NPs. The obtained Pt/SnO₂/graphene composites exhibit superior electrocatalytic activity and stability for the ethanol oxidation reaction as compared with the commercial Pt/C catalyst owing to the close‐connected structure between the Pt NPs and SnO₂ NPs. This work should have a great impact on the rational design of future metal–metal oxide nanostructures with high catalytic activity and stability for fuel cell systems.