On the role of size controlled Pt particles in nanostructured Pt-containing Al2O3 catalysts for partial oxidation of methane

The effect of the Pt loadings and particles sizes on the stability of Pt(x wt%)/Al₂O₃ catalysts were investigated in the partial oxidation of methane (POM) reaction. The Al₂O₃ support was prepared by sol-gel method and different Pt loadings, varying from 0.5 to 2.0 wt% were incorporated to alumina through the incipient wetness impregnation method. The physicochemical features of the catalysts were determined by XRD, ICP-OES, Nitrogen-sorption, UV–Visible, H₂-TPR, CO-DRIFTS, SEM-EDS, XPS and HRTEM techniques. The metal dispersion was evaluated in the cyclohexane dehydrogenation reaction. Lower Pt loadings resulted in well dispersed Pt^o nanoparticles with an enhanced activity in cyclohexane dehydrogenation and POM reactions. With increasing Pt loading to 2.0 wt%, the Pt nanoparticles of the Pt(2.0 wt%)/Al₂O₃ showed a methane conversion of 63% in 24 h of time on stream, and the catalyst was very selective to H₂ and CO. Based on the HRTEM, XPS and Raman spectroscopy techniques, an increment in the Pt loadings evidenced an enrichment of Pt^o clusters on the surface, however, no heavy carbon deposits formation was observed.     

Preferential CO oxidation on Pt–Cu/Al2O3 catalysts with low Pt loadings

This work demonstrates that a small loading of Pt (0.2–0.5 wt.%) selectively located at the top of the Cu/Al₂O₃ solid is enough to obtain active catalysts for the COPrOx reaction. The characterizations of the catalysts prepared in this work using different Pt:Cu ratios show that the phases formed strongly depend on the Cu and Pt loadings. For the samples with 4 wt.% of Cu, the formation of defective CuAl₂O₄-like species is predominant, with Cu²⁺ occupying distorted sites in the alumina surface. For the samples with 8 wt.% of Cu, CuO and metallic Cu in the form of small crystals are observed besides Cu²⁺. For both 0.2 and 0.5 wt.% of Pt, some proportion of Cu–Pt alloy is found which increased with the Cu content. Metallic Pt particles are only observed by EXAFS for the samples with a higher loading of the noble metal.     

Evaluation of catalyst activity for release of hydrogen from liquid organic hydrogen carriers

This contribution investigate the effect of parameters for production of hydrogen by catalytic dehydrogenation of perhydrodibenzyltoluene (H18-DBT). The sensitivity of the dehydrogenation reaction to temperature (290–320 °C) is justified by an increase in degree of dehydrogenation (DoD) from 40 to 90% when using 1 wt % Pt/Al₂O₃ catalyst. However, the increase in temperature increases the hydrogen production rate and decreases the hydrogen purity by increasing the formation of by-products. In addition, the DoD of 96% is obtained when 2 wt % Pt/Al₂O₃ is used at 320 °C. The DoD obtained for Pd, Pt, and Pt–Pd catalysts is 11, 82 and 6%, respectively. Therefore, Pd is not a metal of choice for dehydrogenation of H18-DBT, in both monometallic and bimetallic system. The ab-initio density functional theory (DFT) calculations are consistent with this observation. Furthermore, dehydrogenation of H18-DBT followed 1st order reaction kinetics and the activation energies for 1 wt % Pt/Al₂O₃, 1 wt % Pd/Al₂O₃ and 1:1 wt % Pt–Pd/Al₂O₃ catalysts are: 205, 84 and 66 kJ/mol, respectively.     

Tuning the alloy degree for Pd-M/Al2O3 (M=Co/ Ni /Cu) bimetallic catalysts to enhance the activity and selectivity of dodecahydro-N-ethylcarbazole dehydrogenation

Hydrogen is a promising candidate to substitute the fossil fuels. However, the efficient hydrogen storage technologies restrict the commercial applications. Developing new catalysts with high activity and selectivity is important for the dehydrogenation reaction in N-ethylcarbazole/dodecahydro-N-ethylcarbazole (NECZ/12H-NECZ) hydrogen storage system. In this work, a series of Pd-M/Al₂O₃ (M = Co, Ni and Cu) bimetallic catalysts are synthesized successfully and show good performance in the dehydrogenation reaction of 12H-NECZ than the commercial Pd/Al₂O₃ catalyst. The Pd₁Co₁/Al₂O₃ catalyst (Practical Pd content = 2.4136 wt%) showed the highest catalytic performance with 95.34% H₂ release amount, TOF of 230.5 min⁻¹ and 85.4% selectivity of NECZ. Combined with the characterization analysis, it can be proposed that the dehydrogenation performance of 12H-NECZ is dependent on the alloy phases, reasonable electronic structures and nanoparticle size of catalysts. The fine-tuned alloy degree and appropriate nanoparticle size of Pd₁Co₁/Al₂O₃ bring the 17.7% increase of H₂ release amount and 99.5% increase of NECZ selectivity than those of Pd/Al₂O₃. For the bimetallic catalysts, the enhancement of selectivity of NECZ is mainly from the increase of the kinetic constant of rate-limiting step.     

Wet-impregnated bimetallic Pd-Ni catalysts with enhanced activity for dehydrogenation of perhydro-N-propylcarbazole

Palladium/platinum-based catalysts are widely used in the dehydrogenation process of Liquid Organic Hydrogen Carriers (LOHCs). The cost of noble metal has become a main drawback for LOHCs large-scale application. Partial replacement of Pd/Pt by other transition metals can be an effective solution. In this paper, we synthesize the bimetallic Pd–Ni catalyst by incipient wet impregnation and the catalytic dehydrogenation performance of perhydro-N-propylcarbazole (12H-NPCZ) as a LOHC candidate. Ni and Pd were impregnated on mesoporous alumina to obtain both monometallic and bimetallic catalysts, i.e. Pd/Al₂O₃, Ni/Al₂O₃ and Pd–Ni/Al₂O₃ (Pd:Ni = 1:1) with total metal loading of 5 wt%, respectively. The above catalysts were characterized by N₂-adsorption/desorption, H₂-temperature programmed reduction, X-Ray diffraction, X-Ray photoelectron spectroscopy, Inductively coupled plasma - optical emission spectrometer, CO pulse adsorption and Transmission electron microscopy. The catalytic dehydrogenation results indicated that the bimetallic Pd–Ni/Al₂O₃ showed best catalytic activity, followed by Pd/Al₂O₃, commercial Pd/Al₂O₃ and Ni/Al₂O₃. Notably, the catalytic activity of bimetallic was well maintained after 5 cycles at 200 °C with no degradation, indicating this bimetallic catalyst has potential prospect for large-scale application.     

Dehydrogenation of methylcyclohexane over Pt/V2O5 and Pt/Y2O3 for hydrogen delivery applications

Dehydrogenation of methylcyclohexane (MCH) for hydrogen transportation and delivery application was carried out over 3 wt% Pt/V₂O₅ and 3 wt% Pt/Y₂O₃ catalyst. The catalytic activity was tested using a spray-pulse mode of reactor. Effective dehydrogenation of MCH under spray-pulse mode of reactant injection was observed. In terms of hydrogen evolution rate at 60 min from start of reaction the activity of 958 mmol/g/min was obtained at temperature of 350 °C. Nearly 100% selectivity toward hydrogen was obtained. A relatively high conversion of 98% was observed with 3 wt% Pt/Y₂O₃ at 60 min using an advanced spray-pulse reactor system. The catalysts were characterized using x-ray diffraction pattern (XRD), CO-chemisorption metal analysis, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analysis.     

A experimental study on the dehydrogenation performance of dodecahydro-N-ethylcarbazole on M/TiO2 catalysts

Liquid organic hydrogen carrier (LOHC) is considered as a promising candidate for large-scale hydrogen storage. In this work, we found that Pt/TiO₂ catalysts exhibited better catalytic activity and selectivity compared to Pd/TiO₂ and commercial Pd/Al₂O₃ catalysts in the dehydrogenation of dodecahydro-N-ethylcarbazole (12H-NECZ) at 453 K. The catalytic activity of the noble metal catalysts followed the trend of Pt/TiO₂ > Pd/TiO₂ > Rh/TiO₂ > Au/TiO₂ > Ru/TiO₂. Compared with the commercial Pd/Al₂O₃, Pt/TiO₂ greatly improved the selectivity and conversion rate, the reaction time was also shortened. In addition, kinetics calculation was carried out to obtain fundamental reaction parameters. It was found that the third step of 4H-NECZ dehydrogenation to NECZ was the rate-limiting step of the entire dehydrogenation reaction for all catalysts.     

Preparation of Pt supported on mesoporous Mg–Al oxide catalysts for efficient dehydrogenation of methylcyclohexane

Hydrogen energy, characterizing by high-energy density, non-pollution and renewability, is regarded as an ideal clean green energy, and the chemical hydrogen storage is an optimal strategy to realize its large-scale utilization. In this study, to enhance the hydrogen evolution rate in the dehydrogenation of methylcyclohexane (MCH), Pt supported on Mg–Al oxide catalysts were prepared and the effects of the co-precipitation reaction time during the preparation of Mg–Al hydrotalcite on their structural properties were studied in detail. The results showed that both the pore diameter and Pt dispersion were increased after prolonging the precipitation reaction time. During the dehydrogenation of MCH, these resultant catalysts presented high activity and good stability: hydrogen evolution rate reached up to 1892 mmol·g_Pt^?1 min^?1 at 623 K and the conversion was still held at 92% after 218 h. Of course, a slight decrease on the conversion during the dehydrogenation reaction was also observed, which was mainly attributed to the aggregation of Pt particles at high temperature.     

Pt/CeO2 catalyst synthesized by combustion method for dehydrogenation of perhydro-dibenzyltoluene as liquid organic hydrogen carrier: Effect of pore size and metal dispersion

Liquid organic hydrogen carrier (LOHC) is a chemical hydrogen storage method that stores hydrogen in the form of liquid organics. Dibenzyltoluene (DBT) is a promising LOHC material due to its high storage density, low ignitability, and low cost. In this study, Pt/Al₂O₃ and Pt/CeO₂ catalysts are synthesized using a combustion nanocatalyst synthesis method called the glycine nitrate process (GNP) to obtain high catalytic activity for the dehydrogenation of perhydro-dibenzyltoluene (H18-DBT). Pt/CeO₂ exhibits much faster dehydrogenation than Pt/Al₂O₃, 80.5%/2.5 h versus 3.5%/2.5 h. To investigate the causes of the difference in the dehydrogenation rates, microstructural characterization by N₂ physisorption, CO chemisorption and transmission electron microscopy analysis are conducted, and the catalytic activities are evaluated at various liquid hourly space velocities (LHSVs). The differences in dehydrogenation can be attributed to the mass transport of liquid H18-DBT into the catalyst pores being slow due to the small pores in Pt/Al₂O₃, which is a rarely addressed issue for other LOHC materials. This is because many LOHC materials are dehydrogenated at the gas phase, which has higher diffusivity than that of the liquid phase. Pt/CeO₂ synthesized by the GNP is also compared with a commercial Pt/Al₂O₃ catalyst. The commercial Pt/Al₂O₃ catalyst shows a dehydrogenation of 17.8%/2.5 h, which is much slower than that of Pt/CeO₂ synthesized by the GNP, at 80.5%/2.5 h.     

High hydrogen selectivity and anti-carbon ability by cracking of RP-3 jet fuel over Pt/ZrO2–TiO2–Al2O3 catalyst modified by MxOy (M = Ba,Sr and Ce) promoters

In order to improve the anti-carbon property and obtain higher H₂ yields, the promoters (BaO, SrO and CeO₂) were introduced into Pt/ZrO₂–TiO₂–Al₂O₃ catalyst. The activity of theses catalysts were investigated in the cracking reactions of RP-3 jet fuel under high temperature and high pressure conditions. The physicochemical characteristics of the catalysts were detected by Temperature programmed oxidation, Raman spectrum, N₂ adsorption–desorption, Transmission electron microscope, NH₃-temperature programmed desorption and NH₃-infrared spectroscopy techniques. It was found that the addition of BaO, SrO and CeO₂ promoted the well dispersion of Pt, stimulated the dehydrogenation reactions, and consequently higher hydrogen yields over modified catalysts were obtained. Moreover, the acid sites have been partially neutralized by these promoters, thus the total amount of acid sites as well as the Lewis acid sites decreased over the modified catalysts. The modified acidic properties inhibited the hydrogen transfer reactions and alkenes oligomerization reactions, resulting in the obvious decrease of carbon deposit. Therefore, the catalysts exhibited remarkable anti-carbon property after modifying by BaO, SrO and CeO₂. The valuable information in this work may be helpful to develop highly efficient catalysts for the advanced aircrafts.     

Solvent-free synthesis of Rh/meso-Al2O3 via mechanochemistry for hydrolytic dehydrogenation of ammonia borane

An effective strategy synthesis of Rh/meso-Al₂O₃ catalysts was demonstrated by mechanochemistry for hydrolytic dehydrogenation of ammonia borane (AB). These catalysts are characterized systematically by N₂ adsorption-desorption isotherms, X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results show that the turnover frequency (TOF) and activation energy (E_a) are 246.8 mol_H2·mol_Rh^?1·min^?1 and 47.9 kJ mol^?1 for hydrolytic dehydrogenation of at 298 K catalyzed by Rh/Al₂O₃-CTAB-400, obviously higher than those previously reported catalysts. Furthermore, catalyst Rh/Al₂O₃-CTAB-400 can be recycled by simple centrifugal separation and the catalytic activity is still well maintained after five cycles. In addition, a plausible mechanism for hydrolytic dehydrogenation of AB has also been proposed. This mechanochemical synthesis method exhibits great application prospects for the preparation of heterogeneous catalysts.     

The effects of alumina morphology and Pt electron property on reversible hydrogenation and dehydrogenation of dibenzyltoluene as a liquid organic hydrogen carrier

The morphologies and the electron property of catalysts play the very important roles in the hydrogenation and dehydrogenation of liquid organic hydrogen carriers (LOHCs) such as dibenzyltoluene (DBT). The different morphologies and pore structures of γ-Al₂O₃ and Mo_xC doped γ-Al₂O₃ were synthesized as the supports for Pt catalysts. After analyzing of various characterizations and catalytic testing, it was found that the large surface area and the mesoporous structure of catalysts are beneficial to both DBT hydrogenation and perhydro-dibenzyltoluene (H18-DBT) dehydrogenation. The doping of Mo_xC promoted the formation of the smaller Pt nanoparticles and increased Pt dispersion. The forming Pt–Mo structure is beneficial to hydrogen spillover which suppress the formation of by-product. The high Pt dispersion of 0.1 wt% Mo_xC doped Pt/Al₂O₃ catalyst plays the positive roles in increasing H18-DBT dehydrogenation activity.     

Hydrogen production through autothermal reforming of CH4: Efficiency and action mode of noble (M = Pt,Pd) and non-noble (M = Re,Mo, Sn) metal additives in the composition of Ni-M/Ce0.5Zr0.5O2/Al2O3 catalysts

Hydrogen production through autothermal reforming of methane (ATR of CH₄) over promoted Ni catalysts was studied. The control of the ability to self-activation and activity of Ni-M/Ce_0.5Zr_0.5O₂/Al₂O₃ catalysts was achieved by tuning their reducibility through the application of different types (M = Pt, Pd, Re, Mo or Sn) and content (molar ratio M/Ni = 0.003, 0.01 or 0.03) of additive. The comparison of the efficiency and action mode of noble (M = Pt, Pd) and non-noble (M = Re, Mo, Sn) metal additives in the composition of Ni-M/Ce_0.5Zr_0.5O₂/Al₂O₃ catalysts was performed using X-ray fluorescence analysis, N₂ adsorption, X-ray diffraction, high-resolution transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction with hydrogen, and thermal analysis. The composition-characteristics-activity correlations were determined. It was shown that the introduction of a promoter does not affect the textural and structural properties of catalysts but influences their reducibility and performance in ATR of CH₄. At the similar dispersion of NiO active component (11 ± 2 nm), the Ni²⁺ reduction is intensified in the following order of additives: Mo < Sn < Re ≤ Pd < Pt. It was found that for the activation of Ni and Ni–Sn catalysts before ATR of CH₄ tests, the pre-reduction is required. On the contrary, the introduction of Pt, Pd and Re additives leads to the self-activation of catalysts under the reaction conditions and an increase of the H₂ yield due to the enhanced reducibility of Ni²⁺. The efficient and stable catalyst for hydrogen production has been developed: in ATR of CH₄ at 850 °C over an optimum 10Ni-0.9Re/Ce_0.5Zr_0.5O₂/Al₂O₃ catalyst the H₂ yield of 70% is attained. The designed catalyst has enhanced stability against oxidation and sintering of Ni active component as well as high resistance to coking.     

Ordered mesoporous Ni–Mg–Al2O3 as an effective catalyst for CO2 reforming of CH4

The Ni based catalysts have been considered as promising candidates for the CO₂ reforming of CH₄ (CRM). However, they have suffered from two challenging issues of sintering and carbon accumulation. In order to overcome these drawbacks, a series of ordered mesoporous Ni-xMg-Al₂O₃ catalysts (x was the mole ratio of Mg/(Mg + Al)) with different Mg contents were synthesized by an improved one-pot evaporation-induced self-assembly method. The effect of Mg on the physicochemical property and catalytic performance of Ni-xMg-Al₂O₃ catalysts for CRM was investigated. The catalysts were characterized by XRD, H₂-TPR, XPS, TEM, NH₃-TPD, and N₂ adsorption-desorption at low temperature. The results showed that the introduction of Mg into the Ni–Al₂O₃ maintained well the ordered mesoporous structure and enhanced the interaction between Ni and Al₂O₃, which could effectively restrict the thermal agglomeration of Ni nanoparticles. In addition, the acid sites were decreased with the introduction of Mg, which was beneficial for resistance to carbon accumulation, and then improving the CRM performance. Among Ni-xMg-Al₂O₃ catalysts, Ni–3Mg–Al₂O₃ presented the highest catalytic activity and stability. Under the conditions of 750 °C and GHSV = 32000 mL g^?1 h^?1, the conversion of CH₄ and CO₂ could reach 81.97% and 89.11% without deactivation for 20 h.     

Ni/Al2O3 catalysts for CO2 methanation: Effect of silica and nickel loading

Samples containing from 1 to 33 wt.% of NiO on silica and alumina doped with silica (1 and 20 wt.% silica in the support) have been prepared and characterized by BET, XRD, FT-IR, UV–vis–NIR, FE-SEM, EDXS, and TPR techniques. Catalysts have been pre-reduced in situ before catalytic experiments and data have been compared with Ni/Al₂O₃ reference sample. Characterization results showed that SiO₂ support has a low Ni dispersion ability mainly producing segregated NiO particles and a small amount of dispersed Ni²⁺ in exchange sites. Instead, for the Si-doped alumina a “surface spinel monolayer phase” is formed by increasing Ni loading and, only when the support surface is completely covered by this layer, NiO is formed. Moreover, H₂-TPR results indicated that NiO particles are more easily reduced compared to Ni species. Low loading Ni/SiO₂ catalysts show high selectivity and moderate activity for RWGS (reverse Water Gas Shift) reaction, likely mainly due to nickel species dispersed in silica exchange sites, as evidenced by visible spectroscopy. High loading Ni/SiO₂ catalysts show both methanation and RWGS but evident short-term deactivation for methanation, attributed to large, segregated Ni metal particles, covered by a carbon veil. Ni on alumina -rich carriers, where nickel disperses forming a surface spinel phase, show high activity and selectivity for methanation, and short-term catalyst stability as well. This activity is attributed to small nickel clusters or metal particles interacting with alumina, formed upon reaction. The addition of SiO₂ in Al₂O₃ support decreases the activity of Ni catalysts in CO₂ methanation, because it reduces the ability of the support to disperse nickel in form of the surface spinel phase, thus reducing the amount of Ni clusters in the reduced catalysts.     

Influence of metal-support interactions on reaction pathways over Ni/CeZrOx–Al2O3 catalysts for ethanol steam reforming

This work investigates selective Ni locations over Ni/CeZrO_x–Al₂O₃ catalysts at different Ni loading contents and their influences on reaction pathways in ethanol steam reforming (ESR). Depending on the Ni loading contents, the added Ni selectively interacts with CeZrO_x–Al₂O₃, resulting in the stepwise locations of Ni over CeZrO_x–Al₂O₃. This behavior induces a remarkable difference in hydrogen production and coke formation in ESR. The selective interaction between Ni and CeZrO_x for 10-wt.% Ni generates more oxygen vacancies in the CeZrO_x lattice. The Ni sites near the oxygen vacancies enhance reforming via steam activation, resulting in the highest hydrogen production rate of 1863.0 μmol/g_cat·min. In contrast, for 15 and 20-wt.% Ni, excessive Ni is additionally deposited on Al₂O₃ after the saturation of Ni–CeZrO_x interactions. These Ni sites on Al₂O₃ accelerate coking from the ethylene produced on the acidic sites, resulting in a high coke amount of 19.1 mg_c/g_cat·h (20Ni/CZ-Al).     

Two-dimensional Mo2C: An efficient promoter for hydrogen storage and release from a liquid organic hydrogen carrier

Two-dimensional Mo₂C (2D-Mo₂C) is reported for the first time as an effective promoter of a Pt/Al₂O₃ catalyst for both the hydrogenation and dehydrogenation of the liquid organic hydrogen carrier (LOHC) pair, dibenzyltoluene (DBT) and perhydro-dibenzyltoluene (H18-DBT), respectively. Addition of 6.2 wt% 2D-Mo₂C to a Pt/Al₂O₃ catalyst resulted in a significant increase in both the degree of hydrogenation and dehydrogenation compared to the unpromoted catalyst. An analysis of the initial (120 min) perhydro-DBT dehydrogenation kinetics in the temperature range of 270–330 °C, resulted in a reduction in apparent activation energy from 119.5 ± 3.8 kJ/mol for the Pt/Al₂O₃ catalyst to 110.4 ± 5.6 kJ/mol for the 6.2 wt% 2D-Mo₂C/Pt/Al₂O₃ catalyst. The 6.2 wt% 2D-Mo₂C/Pt/Al₂O₃ catalyst was also more stable than the unpromoted catalyst over several consecutive cycles of hydrogenation and dehydrogenation. Catalyst characterization showed that addition of 2D-Mo₂C resulted in an increase in particle size and electron density of the Pt, which enhanced both the hydrogenation and dehydrogenation reactions, despite the fact that the 2D-Mo₂C alone was inactive for both reactions.     

Modulating Fischer-Tropsch synthesis performance on the Co3O4@SixAly catalysts by tuning metal-support interaction and acidity

ZIF-67 derived catalysts for Fischer-Tropsch synthesis have attracted much attention in recent years, while there is still a potential to improve their activity and selectivity. In this work, we prepared Si/Al co-immobilized Co₃O₄@Si_xAl_y catalysts by in-situ doping tetraethylorthosilicate and aluminum nitrate during the synthesis of ZIF-67. The effects of different Si/Al ratios on the metal-support interaction, acidity and FTS performance were explored. Results indicated that the Co₃O₄@Si₀Al₄ catalyst exhibited the best FTS performance with the CO conversion as high as 79.9% and CTY (cobalt time yield) value of 19.5 × 10^?5 mol_CO·g_Co^?1·s^?1, which was ascribed to the moderate metal-support interaction and the most active Co sites. Meanwhile, the Co₃O₄@Si₃Al₁ and Co₃O₄@Si₂Al₂ catalysts exhibited higher iso-paraffin and olefin selectivity due to more acidic sites.     

Hydrogen production from acetic acid decomposition as bio-oil model molecule over supported metal catalysts

Acetic acid decomposition to produce hydrogen was studied over Pd/Al₂O₃, Pt/Al₂O₃, Ni/Al₂O_3, and Co/Al₂O₃ catalysts. Pd/Al₂O₃ and Pt/Al₂O₃ systems exhibited high levels of conversion and hydrogen selectivity, with Pt/Al₂O₃ showing a hydrogen selectivity of 51.3% at 973 K. This behavior was influenced by the high dispersion and small particle size of Pt as well as the dissociative adsorption of acetic acid (acetate species) as exhibited by Pt/Al₂O₃ and Pd/Al₂O₃ systems. Additionally, Ni/Al₂O₃ and Co/Al₂O₃ were less active and presented low selectivity to hydrogen. These catalysts exhibited low dissociation of acetic acid on their surfaces, therefore hindering acetic acid transformation and hydrogen generation. However, when Ni/Al₂O₃ and Co/Al₂O₃ were reduced at 973 K, the conversion of acetic acid and hydrogen formation increased favorably. Co/Al₂O₃ showed less deactivation during time on stream. Deposited carbon on catalysts corresponded to the formation of carbon filaments for Pd/Al₂O₃ and Co/Al₂O₃ and of carbon nanotubes in the case of Ni/Al₂O_3.     

Structure-dependent activity and stability of Al2O3 supported Rh catalysts for the steam reforming of n-dodecane

Steam reforming of liquid hydrocarbon fuels is an appealing way for the production of hydrogen. In this work, the Rh/Al₂O₃ catalysts with nanorod (NR), nanofiber (NF) and sponge-shaped (SP) alumina supports were successfully designed for the steam reforming of n-dodecane as a surrogate compound for diesel/jet fuels. The catalysts before and after reaction were well characterized by using ICP, XRD, N₂ adsorption, TEM, HAADF-STEM, H₂-TPR, CO chemisorption, NH₃-TPD, CO₂-TPD, XPS, Al²⁷ NMR and TG. The results confirmed that the dispersion and surface structure of Rh species is quite dependent on the enclosed various morphologies. Rh/Al₂O₃-NR possesses highly dispersed, uniform and accessible Rh particles with the highest percentage of surface electron deficient Rh⁰ active species, which due to the unique properties of Al₂O₃ nanorod including high crystallinity, relatively large alumina particle size, thermal stability, and large pore volume and size. As a consequent, Rh/Al₂O₃-NR catalyst exhibited superior catalytic activity towards steam reforming reactions and hydrogen production rate over other two catalysts. Especially, Rh/Al₂O₃-NR catalyst showed the highest hydrogen production rate of 87,600 mmol g_fuel^?1 g_Rh^?1min^?1 among any Rh-based catalysts and other noble metal-based catalysts to date. After long-term reaction, a significant deactivation occurred on Rh/Al₂O₃–NF and Rh/Al₂O₃-SP catalysts, due to aggregation and sintering of Rh metal particles, coke deposition and poor hydrothermal stability of nanofibrous structure. In contrast, the Rh/Al₂O₃-NR catalyst shows excellent reforming stability with negligible coke formation. No significantly sintering and aggregation of the Rh particles is observed after long-term reaction. Such great catalyst stability can be explained by the role of hydrothermal stable nanorod alumina support, which not only provides a unique environment for the stabilization of uniform and small-size Rh particles but also affords strong surface basic sites.