Direct dehydrogenation of n-butane over Pt/Sn/M/γ-Al2O3 catalysts: Effect of third metal (M) addition

A series of Pt/Sn/M/γ-Al₂O₃ catalysts with different third metal (M = Zn, In, Y, Bi, and Ga) were prepared by a sequential impregnation method for use in the direct dehydrogenation of n-butane to n-butene and 1,3-butadiene. In the direct dehydrogenation of n-butane, Pt/Sn/Zn/γ-Al₂O₃ catalyst showed the best catalytic performance. Catalytic performance decreased in the order of Pt/Sn/Zn/γ-Al₂O₃ > Pt/Sn/In/γ-Al₂O₃ > Pt/Sn/γ-Al₂O₃ > Pt/Sn/Y/γ-Al₂O₃ > Pt/Sn/Bi/γ-Al₂O₃ > Pt/Sn/Ga/γ-Al₂O₃. The catalytic performance increased with increasing metal–support interaction and Pt surface area of the catalyst.     

Differences in coke burning-off from Pt–Sn/Al2O3 catalyst with oxygen or ozone

Pt–Sn/γ-Al₂O₃ catalysts with different Sn loadings were prepared by incipient wetness coimpregnation of γ-Al₂O₃ with H₂PtCl₆ and SnCl₂. The Pt–Sn interaction was tested by temperature-programmed reduction and the catalytic activity was measured by cyclohexane dehydrogenation. The catalysts were coked by cyclopentane at 500 °C and totally or partially decoked with O₂ at 450 °C or O₃ at 125 °C. Coke deposits were studied by TPO and the catalytic activity of coked catalysts, partially or totally regenerated, by cyclohexane dehydrogenation.The TPO with O₃ shows that coke combustion with O₃ starts at a low temperature and has a maximum at 150 °C, that is a compensation between the increase of the burning rate and the rate of O₃ decomposition when increasing the temperature. Meanwhile O₂ burns coke with a maximum at 500 °C. When performing partial decoking with O₃ (125 °C) the remaining coke is more oxygenated and easier to burn than the coke that remains after decoking with O₂ (450 °C).After burning with O₃ the dehydrogenation activity of the fresh catalyst is recovered, while after burning with O₂ the activity is higher than that of the fresh catalyst. The burning with O₃ practically does not change the original Pt–Sn interaction while the burning with O₂ produces a decrease in the interaction, producing free Pt sites with higher dehydrogenation capacity.The differences in coke combustion with O₃ and O₂ are due to the different form of generation of activated oxygen, the species that oxidizes the coke. O₃ is activated by the γ-Al₂O₃ support at low temperatures firstly eliminating coke from the support while O₂ is activated by Pt at temperatures higher than 450 °C and the coke removal starts on the metal. Then, the recovery of the Pt catalytic activity as a function of coke elimination is faster with O₂ than with O₃.     

Catalytic oxidation of toluene over copper and manganese based catalysts: Effect of water vapor

Copper and manganese based catalysts with different supports were prepared by impregnation method for toluene oxidation in the presence of water vapor. Their catalytic activity was tested in the absence and presence of water vapor. The results showed that the activity of catalysts CuMn(y)O_x/γ-Al₂O₃ was higher than that of catalysts CuO_x/γ-Al₂O₃ and MnO_x/γ-Al₂O₃. The presence of water vapor had a negative effect on catalytic activity due to the competition of water molecules with toluene molecules for adsorption on surface active sites. The durability to water vapor followed the order: CuMn(1)O_x/Cordierite > CuMn(1)O_x/TiO₂ > CuMn(1)O_x/γ-Al₂O₃.     

Preparation and catalytic behavior of second metal Ni supported on a novel conductive structured Cu/γ-Al2O3/Al catalysts through electrolysis on steam reforming of dimethyl ether

Electrochemical treatment was employed to improve the electric conductivity of γ-Al₂O₃/Al. Optimal conditions were found to be 0.5 M KCl solution along with potential of 4 V for 7.5 min. The modified γ-Al₂O₃/Al support showed higher catalytic activity at low temperature because of its bigger specific surface area and more acid amount than γ-Al₂O₃/Al. Moreover, Ni was easily loaded on the modified Cu/γ-Al₂O₃/Al catalyst through electrolysis because of the high electric conductivity. The novel Ni/Cu/γ-Al₂O₃/Al catalyst also exhibited excellent stability for 40 h at 623 K with 100% conversion and 70% H₂ yield in steam reforming of dimethyl ether.     

Transient study of the dry reforming of methane over Pt supported on different γ-Al2O3

CO₂ reforming of methane has been studied over Pt/Al₂O₃ model catalysts in a temperature range of 600–800 °C using steady-state and transient methods (Transient Response Method (TRM) and DRIFT-MS). Pt-supported catalysts were prepared using two different alumina (γ-Al₂O₃(S) Sasol-Puralox and a synthesized γ-Al₂O₃(N) with nanofibrous structure). Catalysts and supports were characterized by conventional methods (XRD, TEM, A_BET, XPS) before and after reaction. Pt⁰ species are present in the catalysts, with a higher relative contribution for the catalyst that has a nanostructured support. Pt/γ-Al₂O₃(N) catalyst presented the best performance in reactivity and showed a low rate of carbon formation and a minimal water production. From TRM and DRIFT-MS results it can be concluded that, when CO₂ and CH₄ are fed separately into the reaction system, they are activated over the catalytic surface. Besides, when both reactants are fed contemporaneously the presence of CH_X species promotes the CO₂ activation that is responsible for the reforming reaction.     

Study of Ni and Pt catalysts supported on α-Al2O3 and ZrO2 applied in methane reforming with CO2

Ni and Pt catalysts supported on α-Al₂O_3, α-Al₂O₃-ZrO₂ and ZrO₂ were studied in the dry reforming of methane to produce synthesis gas. All catalytic systems presented well activity levels with TOF (s⁻¹) values between 1 and 3, being Ni based catalysts more active than Pt based catalysts. The selectivity measured at 650 °C, expressed by the molar ratio H₂/CO reached values near to 1. Concerning stability, Pt/ZrO₂, Pt/α-Al₂O₃-ZrO₂ and Ni/α-Al₂O₃-ZrO₂ systems clearly show lower deactivation levels than Ni/ZrO₂ and Ni or Pt catalysts supported on α-Al₂O₃. The lowest deactivation levels observed in Ni and Pt supported on α-Al₂O₃-ZrO₂, compared with Ni and Pt supported on α-Al₂O₃ can be explained by an inhibition of reactions leading to carbon deposition in systems having ZrO₂. These results suggest that ZrO₂ promotes the gasification of adsorbed intermediates, which are precursors of carbon formation and responsible for the main deactivation mechanism in dry reforming reaction.     

n-decane dehydrogenation on bimetallic PtSn and PtGe catalysts prepared by dip-coating

The catalytic performance of Pt, PtSn and PtGe supported on γ-Al₂O₃ (γ-A) deposited by dipcoating of spheres of α-Al₂O₃ (α-A) was studied in the n-decane dehydrogenation. The effect of Sn and Ge addition to Pt on the activity and selectivity was analyzed. The catalytic characterization was carried out using cyclohexane dehydrogenation (CHD), cyclopentane hydrogenolysis (CPH), temperature-programmed reduction (TPR), hydrogen chemisorption, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and scanning electronic microscopy (SEM). Pt(0.5)Sn/γ-A/α-A catalyst had the best catalytic performance and showed a low electronic interaction between the metals, with a surface segregation of Sn and the presence of oxidized Sn stabilized on the support. PtGe catalysts presented strong interactions with probable alloy formation. The catalytic performance of these catalysts is comparable to that reported in the patents.     

Effect of Ce addition on Cr/γ-Al2O3 catalysts for methane catalytic combustion

Cerium modified and chromium-based catalysts using nano-γ-Al₂O₃ as the carrier were prepared via incipient wetness impregnation method and investigated for the catalytic combustion of methane (CH₄). The Cr-based catalysts promoted with 3 wt.% Ce displayed the most effective catalytic activity among all catalysts investigated. In addition, Ce significantly improved the catalytic performance of CH₄ combustion by increasing the amount of reaction site [CrO₄]_ads species on the surface of Cr-based catalysts. Introduction of Ce content also restrained the deactivation of catalysts at high calcination temperature. Cr-based catalysts modified with cerium seem to be a promising cheap and low-temperature catalyst for CH₄ combustion.     

Modification of alumina support with TiO2-P25 in CO2 reforming of CH4

Catalyst activity and stability for CO₂ reforming of CH₄ depends specifically upon the support and the active metal. A side reaction of dry reforming of methane is the decomposition to carbon that covers the Ni particles causing catalyst deactivation. Hence, an appropriate combination of Ni with support is needed to allow for long term stable operation. In this paper, CO₂ reforming of CH₄ is studied by investigating the effect of addition of TiO₂-P25 separately to γ-Al₂O₃ and α-Al₂O₃ supports used for nickel based catalyst. The reforming reactions are performed using (CO₂:CH₄) feed ratio of 1:1 and reaction temperature range of 500–800 °C. Both fresh and used catalysts are characterized by SEM and TGA techniques. It is found when α-Al₂O₃ support is modified with 20 wt% TiO₂-P25, the catalyst activity and stability is enhanced. The conversion rates of CH₄ and CO₂ without and with 20 wt% TiO₂-P25, respectively, are changed from 72.3% to 76.7% and 73.3% to 81.2%, respectively, and, most importantly, carbon formation is reduced from 28.1 to 12.8, respectively. However, when γ-Al₂O₃ support is modified with TiO₂-P25, the catalyst activity is enhanced with simultaneous increase in carbon formation.     

Strong improvement on CH4 oxidation over Pt/γ-Al2O3 catalysts

A very strong promotion effect of the presence of 1000 ppmV C₃H₈ in the reaction feed on CH₄–O₂ reaction was found over unsulfated 1%Pt/γ-Al₂O₃ catalyst. This promotion was further increased on pre-sulfated 1%Pt/γ-Al₂O₃. The promoting effect of pre-sulfation on the activity of 1%Pt/γ-Al₂O₃ for propane combustion results in a further improvement on methane combustion due to propane combustion heat which is generated at lower temperatures, activating methane combustion over pre-sulfated 1%Pt/γ-Al₂O₃ at even lower temperatures relative to unsulfated 1%Pt/γ-Al₂O₃. These results suggest that small amounts of propane in the gas feed during CH₄–O₂ reaction over a pre-sulfated Pt/γ-Al₂O₃ catalyst may eliminate methane emissions at low temperatures from lean-burn NGV exhausts without being deactivated by sulfur poisoning as Pd supported catalysts.     

Methane conversion to higher hydrocarbons in a dielectric-barrier discharge reactor with Pt/γ-Al2O3 catalyst

Plasma catalytic reactions were applied to the conversion of methane to C₂, C₃ or higher hydrocarbons in a dielectric-barrier discharge (DBD) reactor at atmospheric pressure. Methane conversion was increased with the increase of Pt loading on γ-Al₂O₃. The highest C₂H₆ selectivity was 50.3% when 3 wt% Pt/γ-Al₂O₃ catalyst was calcined at 573 K. Methane conversion was increased with the increase of the catalyst weight in DBD reactor. The major products were C₂H₆ and C₃H₈, which were independent of catalyst weight in the presence of catalyst.     

Some Insights into the Preparation of Pt/γ-Al2O3 Catalysts for the Enantioselective Hydrogenation of α-Ketoesters

Pt/γ-Al₂O₃ catalysts were prepared by two different impregnation methods and characterized by XRD, TEM, and CO chemisorption. The Pt particle sizes ranged in 2.4–23.3 nm for these 5.0 wt% Pt/γ-Al₂O₃ catalysts. The catalysts were also characterized by FT-IR spectroscopy using CO as a probe molecule before and after the chiral modification with cinchonidine. Two IR bands (2078 and 2060 cm^-1) due to CO linearly adsorbed on the Pt/γ-Al₂O₃ catalyst, calcined at 500 °C before reduction in sodium formate solution were observed, whereas only one IR band at ～2070 cm^-1 was observed for other catalysts. A red shift of the IR band was observed after chiral modification of all the catalysts, except the one with the largest Pt particle size and lowest Pt dispersion. The catalytic performance of the cinchonidine-modified Pt/γ-Al₂O₃ catalysts was tested for the enantioselective hydrogenations of ethyl pyruvate and ethyl 2-oxo-4-phenylbutyrate (EOPB). A 95% ee value was obtained for the ethyl pyruvate hydrogenation and about 83% ee was achieved for the enantioselective hydrogenation of EOPB under the optimized preparation and reaction conditions. It is deduced that the interaction of Pt with γ-Al₂O₃ is a crucial factor for obtaining high activity and that the adsorption abilities (adsorption of reactant, solvent and chiral modifier molecules) of the catalyst surface affect the catalytic performance significantly.     

Role of support in CO2 reforming of CH4 over a Ni/γ-Al2O3 catalyst

A catalyst of 10% Ni/γ-Al₂O₃ for CO₂/CH₄ reforming was prepared and characterized by TPR, TPD, XPS, XRD and activity measurements. XPS and TPR showed that Ni mainly exists in the form of NiAl₂O₄ in the calcined catalyst and is hard to reduce below 650°C, indicating a strong interaction between metal and support. Reduction of the calcined catalyst results in fine particles of Ni⁰, with an average diameter of about 20 nm as determined by XRD. The uptake of H on the reduced catalyst measured by H₂-TPD is 4.2–4.6 mole per mole of Ni species and does not depend on the reduction degree of Ni species. This provides a convincing piece of evidence for the occurrence of hydrogen spillover in the reduced catalyst. Only reduced catalysts present good activity, but the degree of nickel reduction has almost no effect on the reforming activity. This seems to suggest that Ni⁰ is vital for the reforming activity, but γ-Al₂O₃ is also involved in CO₂/CH₄ reforming and contributes even more. Based on the mechanism proposed by Bradford et al. and on our observations, a mechanistic model has been proposed to elucidate the role of γ-Al₂O₃ in CO₂/CH₄ reforming.     

Catalytic synthesis of methanethiol from methanol and carbon disulfide over KW/Al2O3 catalysts

A series of KW/γ-Al₂O₃ catalysts with varying K/W mole ratio were prepared for the synthesis of methanethiol from carbon disulfide and methanol, and characterized by N₂ adsorption–desorption, XRD and NH₃/CO₂-TPD techniques. Experimental results showed that the acidic and basic property of the catalyst plays a key role on the catalytic performance. It is shown that the conversion of CH₃OH is chiefly related to the acid sites, while the base sites of catalysts are favorable for the selectivity toward CH₃SH and hydrocarbons, but the strong base sites will restrain the selectivity toward CH₃SH. When the K/W mole ratio is K/W = 2/1 and the reaction temperature is at 603 K, the conversion of CH₃OH and the selectivity toward CH₃SH are 98.3 and 56.2%, respectively.     

Improved activity of Pt/γ-Al2O3 catalysts for CH4O2 reaction under lean conditions due to sulfation and catalyst loading

The catalytic oxidation of low concentration of methane (2000 ppmV) in excess oxygen was investigated over unsulfated and pre-sulfated Pt/γ-Al₂O₃ catalysts. Over unsulfated samples, catalyst activity increased slightly with Pt concentration and with catalyst loading in the reactor. This enhancement was stronger over pre-sulfated catalyst relative to unsulfated samples. The results suggest that the new catalytic sites generated during sulfation (formed by the interaction between surface support sulfates and highly oxidized Pt atom at the edge of the platinum particle) may activate the almost non-polar C–H bonds of methane through a dissociate adsorption of CH₄, thus the increase in the sulfated catalyst loading results in the increase in the number of the catalytic sites generated during sulfation. The probability of the initial C–H activation of CH₄ increases and may lead to the observed increase in the oxidation rate for CH₄O₂ reaction.     

Strong promotional effects of Li,K, Rb and Cs on the Pt-catalysed reduction of NO by propene

The catalytic activity and selectivity of Pt dispersed on γ-Al₂O₃ for the reduction of NO by propene is promoted extremely strongly by Li, K, Rb and Cs alkalis in a wide temperature range ca. 450–800 K. Remarkable and unprecedented effects on both activity and selectivity are found. The best promotion effects are achieved by Rb-promotion, for which rate increases as high as 420-, 280- and 25-fold are obtained for the formation rates of N₂, CO₂ and N₂O, respectively, in comparison with the performance of un-promoted (alkali-free) Pt/γ-Al₂O₃. From the other hand, the selectivity towards N₂ is improved from ∼20% over the alkali-free unpromoted Pt catalyst, to >90% over the optimally alkali-promoted catalyst. These effects are understandable in terms of the effect of alkali promoter on the relative adsorption strengths of reactant species. Alkali promotes the adsorption, and consequently the dissociation, of electronegative adsorbates (NO) and inhibits the adsorption of electropositive adsorbates (propene), on a catalyst surface predominantly covered by propene and its fragments.     

XPS and EXAFS study of supported PtSn catalysts obtained by surface organometallic chemistry on metals: Application to the isobutane dehydrogenation

In this work, well defined alumina and silica supported Pt and PtSn catalysts were prepared by surface organometallic reactions and were characterized by TEM, XPS and EXAFS. These catalysts were tested in the catalytic dehydrogenation of isobutane. XPS results show that tin is found under the form of Sn(0) and Sn(II,IV), being the percentage of Sn(0) lower for alumina supported than for silica supported catalysts. Tin modified platinum catalysts, always show a decrease of approximately 1 eV in the BE of Pt, what would be indicative of an electron charge transfer from tin to platinum. When the concentration of Sn(0) is high enough, in our case Sn(0)/Pt ∼ 0.3, EXAFS experiments demonstrated the existence of a PtSn alloy diluting metallic Pt atoms, for both PtSn/γ-Al₂O₃ and PtSn/SiO₂. This PtSn alloy seems to be not active in the dehydrogenation reaction; however, it is very important for selectivity and stability, inhibiting cracking and coke formation reactions. The ensemble of our catalytic, XPS and EXAFS results, show that bimetallic PtSn/γ-Al₂O₃ catalysts, prepared via SOMC/M techniques, can be submitted to several sequential reaction–regeneration cycles, recovering the same level of initial activity each time and that the nature of the catalytic surface remains practically without modifications.     

Syngas production via CO2 reforming of methane using Co-Sr-Al catalyst

The effect of addition of strontium in Co based catalysts during CO₂ reforming of methane was investigated in the temperature range 500–700 °C. The Co/γ-Al₂O₃ supported catalysts with strontium as a promoter (0–2.25 wt%) were prepared by incipient wet impregnation method. Numerous techniques such as N₂ adsorption–desorption isotherm, H₂ temperature-programmed reduction (TPR), temperature-programmed desorption (TPD), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Transmission Electron Microscopy (TEM), pulse chemisorption and temperature-programmed oxidation (TPO) were applied for characterization of fresh and spent catalysts. The results of characterizations and catalyst activity test revealed that introduction of Sr in Co/γ-Al₂O₃ catalyst had significant effect on stability and coke suppression. The Sr addition improves the metal–support interaction as well as enhances the Lewis basicity of the catalyst. The improvement in basicity helps the chemisorption and dissociation of CO₂ over the catalyst which in turn reduces carbon deposition.     

Improving the dispersion of CeO2 on γ-Al2O3 to enhance the catalytic performances of CuO/CeO2/γ-Al2O3 catalysts for NO removal by CO

Acetic acid (HAc) aqueous was used as solvent in wetness impregnation to prepare CeO₂-modified γ-Al₂O₃ support. With the help of HAc, the dispersion of CeO₂ on γ-Al₂O₃ is significantly improved and the size of CeO₂ nanoparticles can be controlled through tuning the concentration of HAc aqueous. XPS analysis shows that the percentages of Ce^3 + in CeO₂ nanoparticles will vary with the size. Then we load CuO on the as-prepared CeO₂-modified γ-Al₂O₃ support and choose NO reduction with CO as a probe reaction to investigate the influences of impregnation solvent on the catalytic properties. The results demonstrate that the CuO/CeO₂/γ-Al₂O₃ prepared in the solvent with volume ratio of 20:1 (H₂O:HAc) has the highest activity in NO + CO reaction. Combing the structural characterizations and catalytic performances, we think that the size of the CeO₂ nanoparticles should be a key factor that affects the activities of CuO/CeO₂/γ-Al₂O₃. Furthermore, CuO dispersed on CeO₂ nanoparticles with an average size of ca. 5 nm should be the highest active sites for NO + CO reaction.