Hydrogen production from ethanol over bimetallic Rh-M/CeO2 (M = Pd or Pt)

The reaction of ethanol for the production of hydrogen has been studied over a series of metal supported CeO₂ catalysts. The study is conducted by TPD, steady state reaction, XPS, TEM, and infrared spectroscopy. TPD gave evidence for the role of Rh in dissociating the carbon–carbon bond needed for efficient production of hydrogen molecules. IR of CO adsorption at 90 K revealed that Rh particles are most likely in very small clusters as evidenced by a single OC–Rh IR band at 2020 cm⁻¹. TEM did not show conclusive evidence for the presence of the metal on-top of the CeO₂ support, yet the Rh-Pd/CeO₂ used catalyst has features that might be attributed to epitaxial growth of the noble metal along the (1 1 1) surface of the CeO₂ support. Considerable reconstruction of the CeO₂ support is seen for the used catalysts, in addition. Reforming of ethanol to hydrogen using (3 moles of water per mole of ethanol) was very efficient particularly above 650 K where hydrogen selectivity reaches 60 vol.%. At these temperatures hydrogen production from reforming of methane takes place.     

CO oxidation on Pd/CeO2–ZrO2 catalysts

The promotive effects of cerium oxide on commercial three-way catalysts (TWCs) for purification of motor exhaust gases have been widely investigated in recent years. This work shows the cooperative effects of CeO₂–Pd on the kinetics of CO oxidation over Pd/CeO₂–ZrO₂. Under reducing-to-moderately oxidizing conditions, a zero-order O₂ pressure dependence is found which can be interpreted on the basis of a mechanism involving a reaction between CO adsorbed on Pd and surface oxygen from the support. The high oxygen-exchange capability of the CeO₂–ZrO₂ support, as determined from temperature-programmed reduction/oxygen uptake measurements is suggested as being responsible for such a catalytic behavior.     

Photo-catalytic production of hydrogen form ethanol over M/TiO2 catalysts (M = Pd, Pt or Rh)

The photo-catalytic production of hydrogen from liquid ethanol, a renewable bio-fuel, over Rh/TiO₂, Pd/TiO₂ and Pt/TiO₂, anatase, has been studied. In the absence of the metal, TiO₂ shows negligible production of molecular hydrogen. The addition of Pd or Pt dramatically increases the production of hydrogen and a quantum yield of about 10% is reached at 350 K. On the contrary, the Rh doped TiO₂ is far less active. The low activity of Rh compared to that of Pd and Pt is not due to poor dispersion or low available Rh sites on the surface, as analyzed by XPS and TEM. For all three catalysts, TEM shows most particles with a size less than 10 nm. XPS results show that while the state of Pd and Pt particles in the as-prepared catalysts was mostly metallic that of the Rh was composed of non-negligible contribution of Rh cations. The extent of reaction of a series of alcohols was also studied, for comparison, on Pt/TiO₂. It was found that the reaction is governed by the solvation of the alcohol. In that regard, the production of molecular hydrogen over Pt/TiO₂ showed the following trend: methanol ≈ ethanol > propanol ≈ isopropanol > n-butanol.     

CO2 adsorption and activation over γ-Al2O3-supported transition metal dimers: A density functional study

Catalytic conversion of CO₂ to liquid fuels has the benefit of reducing CO₂ emission. Adsorption and activation of CO₂ on the catalyst surface are key steps of the conversion. Herein, we used density functional theory (DFT) slab calculations to study CO₂ adsorption and activation over the γ-Al₂O₃-supported 3d transition metal dimers (M₂/γ-Al₂O₃, M = Sc–Cu). CO₂ was found to adsorb on M₂/γ-Al₂O₃ negatively charged and in a bent configuration, indicating partial activation of CO₂. Our results showed that both the metal dimer and the γ-Al₂O₃ support contribute to the activation of the adsorbed CO₂. The presence of a metal dimer enhances the interaction of CO₂ with the substrate. Consequently, the adsorption energy of CO₂ on M₂/γ-Al₂O₃ is significantly higher than that on the γ-Al₂O₃ surface without the metal dimer. The decreasing binding strength of CO₂ on M₂/γ-Al₂O₃ as M₂ changes from Sc₂ to Cu₂ was attributed to decreasing electron-donation by the supported metal dimers. Hydroxylation of the support surface reduces the amount of charge transferred to CO₂ for the same metal dimer and weakens the CO₂ chemisorption bonds. Highly dispersed metal particles maintained at a small size are expected to exhibit good activity toward CO₂ adsorption and activation.     

H2 production through steam reforming of ethanol over Pt/ZrO2, Pt/CeO2 and Pt/CeZrO2 catalysts

The effect of the support nature and metal dispersion on the performance of Pt catalysts during steam reforming of ethanol was studied. H₂ and CO production was facilitated over Pt/CeO₂ and Pt/CeZrO₂, whereas the acetaldehyde and ethene formation was favored on Pt/ZrO₂. According to the reaction mechanism, determined by temperature-programmed desorption (TPD) and Diffuse Reflectance Infrared Spectroscopy (DRIFTS) analysis, some reaction pathways are favored depending on the support nature, which can explain the differences observed on the resulting product distribution.     

Two-dimensional M2CO2/MoS2 (M = Ti,Zr and Hf) van der Waals heterostructures for overall water splitting: A density functional theory study

Because of the wide applications for photocatalysis, electronics and optoelectronics, two-dimensional (2D) van der Waals (vdW) heterojunctions have attracted substantial attentions. In this work, we carried out a systematic investigation on the geometry structures, electronic properties, optical properties and carrier mobilities of the M₂CO₂-II (M = Ti, Zr and Hf) and MoS₂ heterostructures with a hybridized density functional theory (DFT). The BʹCʹ stacked M₂CO₂-II/MoS₂ heterostructure with a short interlayer distance is energetically favorable. All M₂CO₂-II/MoS₂ bilayer and sandwich-like structures are indirect semiconductors. The maximum of valence band (VBM) and conduction band minimum (CBM) of M₂CO₂-II/MoS₂ heterostructures are dominated by different layers, implying the spatial separation of photogenerated electron-hole pairs. Three M₂CO₂-II/MoS₂ heterostructures, including Zr₂CO₂-II/MoS₂ –S-BʹCʹ, Hf₂CO₂-II/MoS₂–B-BʹCʹ, and Hf₂CO₂-II/MoS₂–S-BʹCʹ, are considered as the promising candidates for overall water splitting due to their appropriate band structures, suppressed recombination of photogenerated electron-hole pairs, enhanced optical absorption and carrier mobilities. In addition, the gaint hole mobility makes Ti₂CO₂-II/MoS₂ heterostructure a potential candidate for the application for 2D electronic devices.     

Comparative study of Au/ZrO2 catalysts in CO oxidation and 1,3-butadiene hydrogenation

This work investigates the effects of Au³⁺/Au⁰ ratio or distribution of gold oxidation states in Au/ZrO₂ catalysts of different gold loadings (0.01–0.76% Au) on CO oxidation and 1,3-butadiene hydrogenation by regulating the temperature of catalyst calcination (393–673 K) and pre-reduction with hydrogen (473–523 K). The catalysts were prepared by deposition–precipitation and were characterized with elemental analysis, nitrogen adsorption/desorption, TEM, XPS and TPR. The catalytic data showed that the exposed metallic Au⁰ atoms at the surface of Au particles were not the only catalytic sites for the two reactions, isolated Au³⁺ ions at the surface of ZrO₂, such as those in the catalysts containing no more than 0.08% Au were more active by TOF. For 0.76% Au/ZrO₂ catalysts having coexisting Au³⁺ and Au⁰, the catalytic activity changed differently with varying the Au³⁺/Au⁰ ratio in the two reactions. The highest activity for the CO oxidation reaction was observed over the catalyst of Au³⁺/Au⁰ = 0.33. However, catalyst with a higher Au³⁺/Au⁰ ratio showed always a higher activity for the hydrogenation reaction; co-existance of Au⁰ with Au³⁺ ions lowered the catalyst activity. Moreover, the coexisting Au particles changed the product selectivity of 1,3-butadiene hydrogenation to favor the formation of more trans-2-butene and butane. It is thus suggested that for better control of the catalytic performance of Au catalyst the effect of Au³⁺/Au⁰ ratio on catalytic reactions should be investigated in combination with the particle size effect of Au.     

Methane combustion over Pd/ZrO2/SiC,Pd/CeO2/SiC,and Pd/Zr0.5Ce0.5O2/SiC catalysts

The performances of different promoters (CeO₂, ZrO₂ and Ce_0.5Zr_0.5O₂ solid solution) modified Pd/SiC catalysts for methane combustion are studied. XRD and XPS results showed that Zr⁴⁺ could be incorporated into the CeO₂ lattice to form Zr_0.5Ce_0.5O₂ solid solution. The catalytic activities of Pd/CeO₂/SiC and Pd/ZrO₂/SiC are lower than that of Pd/Zr_0.5Ce_0.5O₂/SiC. The Pd/Zr_0.5Ce_0.5O₂/SiC catalyst can ignite the reaction at 240 °C and obtain a methane conversion of 100% at 340 °C, and keep 100% methane conversion after 10 reaction cycles. These results indicate that active metallic nanoparticles are well stabilized on the SiC surface while the promoters serve as oxygen reservoir and retain good redox properties.     

Effects of ZrO2/Al2O3 properties on the catalytic activity of Pd catalysts for methane combustion and CO oxidation

Zirconia supported on alumina was prepared and characterized by BET surface area, X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), temperature programmed desorption (TPD), and pulse reaction. 0.2% Pd/ZrO₂/Al₂O₃ catalyst were prepared by incipient wetness impregnation of supports with aqueous solution of Pd(NO₃)₂. The effects of support properties on catalytic activity for methane combustion and CO oxidation were investigated. The results show that ZrO₂ is highly dispersed on the surface of Al₂O₃ up to 10 wt.% ZrO₂, beyond this value tetragonal ZrO₂ is formed. The presence of a small amount of ZrO₂ can increase the surface area, pore volume and acidity of support. CO–TPD results show that the increase of CO adsorption capacity and the activation of CO bond after the presence of ZrO₂ lead to the increase of catalytic activity of Pd catalyst for CO oxidation. CO pulse reaction results indicate that the lattice oxygen of support can be activated at lower temperature following the presence of ZrO₂, but it does not accelerate the activity of 0.2% Pd/ZrO₂/Al₂O₃ for methane combustion. 0.2% Pd/ZrO₂/Al₂O₃ dried at 120 °C shows highest activity for CH₄ combustion, and the activity can be further enhanced following the repeat run. The increase of treatment temperature and pre-reduction can decrease the activity of catalyst for CH₄ combustion.     

Lean NOx reduction with CO + H2 mixtures over Pt/Al2O3 and Pd/Al2O3 catalysts

The reduction of NO_x by hydrogen under lean burn conditions over Pt/Al₂O₃ is strongly poisoned by carbon monoxide. This is due to the strong adsorption and subsequent high coverage of CO, which significantly increases the temperature required to initiate the reaction. Even relatively small concentrations of CO dramatically reduce the maximum NO_x conversions achievable. In contrast, the presence of CO has a pronounced promoting influence in the case of Pd/Al₂O₃. In this case, although pure H₂ and pure CO are ineffective for NO_x reduction under lean burn conditions, H₂/CO mixtures are very effective. With a realistic (1:3) H₂:CO ratio, typical of actual exhaust gas, Pd/Al₂O₃ is significantly more active than Pt/Al₂O₃, delivering 45% NO_x conversion at 160 °C, compared to >15% for Pt/Al₂O₃ under identical conditions. The nature of the support is also critically important, with Pd/Al₂O₃ being much more active than Pd/SiO₂. Possible mechanisms for the improved performance of Pd/Al₂O₃ in the presence of H₂+CO are discussed.     

Guaiacol derivatives and inhibiting species adsorption over MoS2 and CoMoS catalysts under HDO conditions: A DFT study

We have investigated using DFT calculations the η¹ adsorption of guaiacol, phenol, anisole and phenol over CoMoS and MoS₂ phases under HDO conditions. This adsorption mode should lead to a direct deoxygenation (DDO) reaction, which is low hydrogen-consuming. The most stable mode is an adsorption through the OH group of the molecule. The calculation of adsorption Gibbs free energies of inhibiting compounds (H₂O, H₂S, and CO) which can be present under reaction conditions shows that these molecules adsorb more strongly than oxygenated compounds, which suggests that CO will be a major inhibitor of the HDO process of real feeds.     

The effect of CeO2 structure on the activity of supported Pd catalysts used for methane steam reforming

Palladium (Pd) supported on CeO₂-promoted γ-Al₂O₃ with various CeO₂ (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO₂ structure and dispersion on the γ-Al₂O₃ support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO₂ structure. Pd is present on the oxidized CeO₂-promoted catalysts as Pd⁰, Pd⁺ and Pd²⁺, at ratios strongly dependent on CeO₂ structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO₂ and is agglomerated as large clusters (particles in 10–20 nm range) on amorphous CeO₂. FTIR spectra of adsorbed CO revealed that after pre-treatment under H₂ or in the presence of amorphous CeO₂, partial encapsulation of Pd particles occurs. CeO₂ structure influences the CH₄ steam reforming reaction rates. Crystalline CeO₂ and dispersed Pd favor high reaction rates (low activation energy). The presence of CeO₂ as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/γ-Al₂O₃ or on amorphous (reduced) CeO₂/Al₂O₃ catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO₂/γ-Al₂O₃ than on Pd/γ-Al₂O₃, which is attributed to a catalytic synergism between Pd and CeO₂. The low rates on the reduced Pd/CeO₂/Al₂O₃ catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration.     

Comparison of H2 production from ethanol and ethylene glycol on M/Pt(1 1 1) (M = Ni, Fe, Ti) bimetallic surfaces

The reactions of ethylene glycol and ethanol have been studied on Fe/Pt(1 1 1) and Ti/Pt(1 1 1) bimetallic surfaces utilizing temperature programmed desorption (TPD). These results are compared to our previous studies on Ni/Pt(1 1 1) to illustrate the trend in the reforming activity on 3d-Pt bimetallic surfaces. The oxygenates decomposed on these surfaces to produce mainly H₂ and CO. The bimetallic surfaces were prepared by thermal evaporation of Fe or Ti onto Pt(1 1 1), using Auger electron spectroscopy (AES) to monitor surface compositions. Surfaces prepared by deposition of a monolayer of Fe or Ti on Pt(1 1 1), designated Fe–Pt–Pt(1 1 1) or Ti–Pt–Pt(1 1 1), displayed higher reforming activity for both ethylene glycol and ethanol than the corresponding subsurface monolayer Pt–Fe–Pt(1 1 1) and Pt–Ti–Pt(1 1 1) structures or clean Pt(1 1 1). The reforming yield increased as the surface d-band center, calculated from density functional theory (DFT), shifted closer to the Fermi level. The reforming selectivity of oxygenates, especially ethanol, began to decrease as the d-band center shifted closer to the Fermi level. Combining results in the current work with previous studies on Ni/Pt(1 1 1), a general criterion can be formulated for selecting 3d-Pt bimetallic surfaces with desirable reforming activity and selectivity.     

Shape effect of microstructured CeO2 with various morphologies on CO catalytic oxidation

Novel 3D-like CeO₂ microflowers and microspheres were synthesized by facile precipitation methods and tested for CO oxidation. Characterization by UV-Vis, PL, XPS and H₂-TPR revealed that 3D-like ceria show great superiority for favoring formation of oxygen vacancies, which in turn leads to higher oxygen mobility and reducibility. A direct relationship between the band gap of ceria and its catalytic activity towards CO oxidation was established. The activity and thermal stability of 3D-like ceria were much better than that of CeO₂ nanoparticles. The results indicated that the highly active ceria could be obtained by turning their shapes with a high concentration of oxygen vacancies.     

Complexes with pyrazine-tetrathiafulvalene-dithiolate (pztdt) ligand [M(pztdt)2], M = Ni,Pd, Pt; Synthesis and characterisation

The synthesis and characterisation of three metal bisdithiolene complexes based on the recently reported highly extended ligand incorporating fused tetrathiafulvalene and pyrazine moieties, [M(pztdt)₂], M = Ni, Pd, Pt and pztdt = pyrazinetetrathiafulvalenedithiolate, are reported. These complexes were obtained under anaerobic conditions as crystals of dianionic tetrabutylammonium salts. The crystal structures of Ni and Pd complexes as determined by single crystal X-ray diffraction confirm the ability of these molecules to established interactions with their neighbours through the pyrazine moieties. These dianionic complexes easily undergo oxidations to their monoanionic and neutral states, with a narrow range of stability for the monoanionic and an extended one for the neutral forms that is rather insoluble in common solvents.     

Cerium–terbium mixed oxides as alternative components for three-way catalysts: a comparative study of Pt/CeTbOx and Pt/CeO2 model systems

By using a combination of oxygen buffering capacity (OBC) and oxygen storage capacity (OSC) measurements, the redox behaviour of a Pt/CeTbO_x catalyst is compared to that of a classic model TWC system: Pt/CeO₂. The results reported here show that the redox efficiency of the Pt/CeTbO_x catalyst is much better, especially at low temperature operation conditions such as those occurring during the cold start of engines. The catalyst containing terbium also shows lower ‘light-off’ temperatures for both methane and carbon monoxide oxidation.