Hydrogen production via steam reforming of methanol over Cu/(Ce,Gd)O2−x catalysts

Hydrogen production via steam reforming of methanol is carried out over Cu/(Ce,Gd)O_2−x catalysts at 210–600 °C. The CO content in reformate is about 1% at 210–270 °C, which are the typical temperature for hydrogen production via steam reforming of methanol. Largest H₂ yield and CO₂ selectivity and smallest CO content are obtained at 240 °C. The formation rate of CO increases with increasing temperature. The average formation rate of CO becomes larger than that of CO₂ at about 450 °C. The H₂ yield, the CO₂ selectivity and the CO content become constant at about 550 °C. At 240 °C, the smallest CO content is obtained with a catalyst weight of 0.5 g and a Cu content of 3 wt%. The H₂ yield, defined as H₂/(CO + CO₂) in formation rates, at 240 °C is always 3 and not affected by the variations of either the catalyst weight or the Cu content.     

Assessing the potential of xNi-yMg-Al2O3 catalysts prepared by EISA-one-pot synthesis towards CO2 methanation: An overall study

Mesoporous xNi-yMg-Al₂O₃ catalysts prepared by combined evaporation induced self-assembly (EISA) and one-pot techniques were tested in CO₂ methanation reaction. All calcined/reduced materials were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), N₂ physisorption, thermogravimetric analysis (TGA), CO₂ adsorption, H₂ temperature programmed reduction (H₂-TPR) and transmission electron microscopy (TEM). The effects of Mg and Ni loadings on the catalysts properties and performances were systematically studied. Higher Mg contents enhanced methanation performances due to more favourable metallic interactions between the Ni, Mg and Al species. In addition, higher Ni contents led to better selectivity to CH₄ by enhancing methane formation that involves H₂ dissociation on Ni⁰ sites. The mesoporous 5Ni–Al₂O₃ catalyst obtained by the EISA-one-pot technique was significantly more active than silica-based catalysts with same 5 wt% Ni content supported on USY zeolite and SBA-15. Moreover, the performances of the most promising 15Ni–7Mg–Al₂O₃ mesoporous material were similar to those of a commercial 25Ni/γ-Al₂O₃ catalyst in spite of its reduced nickel content.     

Coke-resistance enhancement of mesoporous γ-Al2O3 and MgO-supported Ni-based catalysts for sustainable hydrogen generation via steam reforming of acetic acid

Highly ordered mesoporous γ-Al₂O₃ particles and MgO materials were synthesized by evaporation induced self-assembly (EISA) and template-free hydrothermal co-precipitation routes, respectively. Ni, Ni–MgO, and Ni–La₂O₃-containing catalysts were prepared using a wet-impregnation method. The synthesized catalysts were characterized by N₂ adsorption–desorption, XRD, SEM-EDS, DRIFTS, XPS, TGA-DTA, and Raman spectroscopy analysis. The mesoporous γ-Al₂O₃ catalyst support exhibited a high surface area of 245 m²/g and average pore volume of 0.481 cm³/g. The DRIFTS results indicate the existence of large Lewis's acid regions in the pure γ-Al₂O₃ and metal-containing catalysts. Catalytic activity tests of pure materials and metal-containing catalysts were carried out at the reaction temperature of 750 °C and a feed molar ratio of AA/H₂O/Ar:1/2.5/2 over 3 h. Complete conversion of acetic acid and 81.75% hydrogen selectivity were obtained over the catalyst 5Ni@γ-Al₂O₃. The temperature and feed molar ratio had a noticeable impact on H₂ selectivity and acetic acid conversion. Increasing the water proportion in the feed composition from 2.5 to 10 considerably improved the catalytic activity by increasing hydrogen selectivity from 81.75% to 91%. Although the Ni-based γ-Al₂O₃-supported catalysts exhibited higher activity performance compared to the Ni-based MgO-supported catalysts, they were not as resistant to coke formation as were MgO-supported catalysts. The introduction of MgO and La₂O₃ into the Ni@γ-Al₂O₃ and Ni@MgO catalysts' structures played a significant role in lowering the carbon formation (from 37.15% to 17.6%–12.44% and 12.17%, respectively) and improving the thermal stability of the catalysts by decreasing the agglomeration of acidic sites and reinforcing the adsorption of CO₂ on the catalysts' surfaces. Therefore, coke deposition was reduced, and catalyst lifetime was improved.     

Production of H2- and CO-rich syngas from the CO2 gasification of cow manure over (Sr/Mg)-promoted-Ni/Al2O3 catalysts

The valorization of cow manure (CM), as bio-waste, under a CO₂ atmosphere could be an attractive strategy for tackling the environmental problems related to waste management and CO₂ emission and producing valuable syngas. For this purpose, highly loaded Ni–Al₂O₃ catalysts with alkaline-earth metals (Mg and Sr) were synthesized and applied to the gasification of CM under CO₂. The lowest yields of bio-oil (16.98 wt %) and coke (0.34 wt %) and the highest yield of syngas (55.09 wt %) were obtained from the catalytic decomposition of hydrocarbons when Sr was incorporated into Ni/Al₂O₃ (SN-AO). The highest selectivity for H₂ (34.23 vol %) and CO (37.16 vol %) were obtained applying SN-AO followed by Mg-promoted Ni/Al₂O₃ (MN-AO) and Ni/Al₂O₃ (N-AO) catalysts. With increasing gasification temperature from 750 °C to 850 °C, the syngas yield (from 55.09 to 70.17 wt %) and H₂ concentration (from 34.23 to 38.03 vol %) increased considerably because of the endothermic gasification process. The yield and selectivity of syngas (H₂ and CO) increased under CO₂ compared to those obtained under N₂, indicating the high potential of CO₂ for the thermal decomposition and dehydrogenation of the volatile matter.     

Moderate-pressure conversion of H2 and CO2 to methanol via adsorption enhanced hydrogenation

CO₂ hydrogenation to methanol plays an increasingly important role in fields of chemical engineering, energy generation and H₂/CO₂ utilization, and the prohibitive costs result partially from the high operating pressures required for practical application. Thus, a hybrid catalyst/adsorbent consisting of Cu-ZnO-ZrO₂ supported on hydrotalcite (named CZZ@HT) was synthesized, characterized and analyzed in this study with the intent that the adsorbent hydrotalcite would enhance the local concentration of CO₂ and assist in catalyst dispersion. The as-prepared CZZ@HT catalyst containing 43.4 wt% of CuO-ZnO-ZrO₂ in the form of well dispersed nanoparticles possessed a considerable BET surface area and external surface area after reduction. A remarkable copper dispersion of 58.7% was thereby achieved. This reduced catalyst displayed elevated uptakes of H₂O and CO₂ at 473 K compared to the reference adsorbent-free catalyst and presented enhanced adsorption capacities of CO₂ at reaction temperatures due to collective effects of physisorption and chemisorption. Catalysis experiments on a fixed bed reactor using the rCZZ@HT catalyst showed a methanol selectivity of 83.4% and a S_MeOH/S_CO ratio of 5.0 in products. A control experiment in which hydrotalcite was replaced with quartz (named rCZZ&QS) showed considerably lower conversion at low pressure and demonstrated the enhancing effect of the hydrotalcite support. The new catalyst could achieve the same methanol productivity as the control catalyst at 2.45 MPa lower reaction pressure. This lower pressure corresponds to a ∼61.3% savings in energy consumption for compression. Accordingly, the CZZ@HT is a promising candidate for CO₂ hydrogenation to methanol at moderate pressures.     

Alumina-supported cobalt catalysts in the hydrogenation of CO2 at atmospheric pressure

20 wt.% cobalt catalysts supported on pure and 5 wt.% silica-containing alumina have been prepared and characterized by X-Ray Diffraction, IR and DR-UV-vis-NIR spectroscopies and Field-Emission Scanning Electron Microscopy (FE-SEM). The presence of a cobalt-containing surface spinel phase Co_3-xAl_xO₄ and, for the silica-containing sample, of a segregated Co₃O₄ phase is evident. These catalysts have been tested in CO₂ hydrogenation at atmospheric pressure in steady state conditions in the temperature range 523–773 K. Both catalysts are active in CO₂ hydrogenation to methane (methanation) and to CO (reverse Water Gas Shift, rWGS). CO₂ hydrogenation activity is higher on freshly pre-reduced silica-free Co/Al₂O₃, while selectivity to methane is slightly higher on the silica-containing sample. Spent catalysts contain clustered or amorphous cobalt metal centers as active sites for methanation. The silica-containing catalyst shows slow deactivation in CO₂ hydrogenation upon 13 h experiments, with quite stable or even slightly increasing rWGS activity and decreasing CH₄ selectivity. This confirms previous data suggesting that, over cobalt catalysts, sites for methanation are metal centers prone to deactivation by carbon deposition. However, in contrast with what happens with unsupported and silica-supported cobalt catalysts, where deactivation is very fast, over these alumina-based catalysts carbon deposition and deactivation occur much more slowly. Sites for rWGS are unreduced cobalt centers which do not undergo such a deactivation phenomenon.     

Effect of surface properties controlled by Ce addition on CO2 methanation over Ni/Ce/Al2O3 catalyst

Ce-promoted Ni/Al₂O₃ catalysts with Ce contents of 0, 5, 10, 15, and 20 wt% were investigated for CO₂ methanation. Ni/15Ce/Al₂O₃ showed good selectivity and catalytic performance in CO₂ methanation and remained stable at 350 °C for 80 h with minor fluctuations. Interactions between Ni and the Ce/Al₂O₃ support was characterized using X-ray diffraction, temperature-programmed reduction of H₂, temperature-programmed desorption of CO₂, X-ray photoelectron spectroscopy, Raman spectroscopy, and thermogravimetric analysis. Addition of Ce did not increase the catalytic surface area, which can significantly enhance the heterogeneous catalytic activity. However, XPS analysis showed that the Ce on the Ni/Al₂O₃ catalyst changed the surface electron states of Ni, Ce, and O. Additionally, CO₂ adsorption/desorption was confirmed to be related to the amount of Ce present on Ni/Al₂O₃ by TGA and CO₂-TPD. The Ce addition thus played an important role in determining the CO₂ adsorption, desorption, and conversion.     

Carbon dioxide methanation over Ni catalysts prepared by reduction of NixMg3‒xAl hydrotalcite-like compounds: Influence of Ni:Mg molar ratio

A series of supported Ni catalysts have been prepared from Ni_xMg_3?xAl hydrotalcite-like compounds (HTlcs) and the influence of Ni:Mg molar ratio on the structural property and catalytic activity for CO₂ methanation is investigated. The catalysts were characterized by N₂ physical adsorption, X-ray powder diffraction (XRD), temperature-programmed reduction (H₂-TPR), temperature-programmed desorption (CO₂-TPD), H₂ chemisorption, scanning electronic microscopy (SEM), scanning transmission electronic microscopy (STEM), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). By reducing HTlcs at 800 °C, well dispersed Ni particles with average size of 5–10 nm are formed. The Ni crystal size decreases with the decrease of Ni:Mg ratio, attributable to the strong interaction between nickel and magnesium oxides. Among the catalysts, Ni₂Mg₁Al-HT shows the highest activity, giving ～93% CO₂ conversion and >99% CH₄ selectivity at 275 °C and SV = 5000 mL g^?1 h^?1. Meanwhile, this catalyst exhibits good stability without obvious sintering and coking. The high activity is related to the large amount of surface Ni⁰ species and medium basic sites. From CO₂-TPD and DRIFTS, it is inferred that CO₂ adsorbs on the medium basic sites, i.e., Ni–Mg(Al)O interface, forming monodentate carbonate. In situ DRIFTS reveals that monodentate carbonate, monodentate formate, and adsorbed CO are the main intermediate species, suggesting that the reaction may proceed via the formate formation route.     

Hydroxyapatite decorated TiO2 as efficient photocatalyst for selective reduction of CO2 with H2O into CH4

Efficient catalysts with high selectivity in products are highly desirable for photocatalytic CO₂ reduction. In this work, hydroxyapatite (HAP) decorated TiO₂ (HAP/TiO₂) were successfully fabricated via in-situ deposition of Ca(OH)₂ on rutile TiO₂ followed by a facile hydrothermal reaction. Comparing with TiO₂, HAP/TiO₂ exhibited significant enhancement (ca. 40 times) toward photocatalytic CO₂ reduction in the presence of H₂O with a >95% selectivity of CH₄. The characterizations revealed HAP possessed Lewis basic sites (O²⁻ in -PO₄^3- groups) and Lewis acidic sites (Ca²⁺ or OH⁻ vacancies), where Lewis basic sites could enhance the adsorption/activation of CO₂ and Lewis acidic sites facilitated the adsorption/dissociation of H₂O respectively, thus promoting the photocatalytic reduction and oxidation half-reactions of CO₂ and H₂O over Pt/TiO₂. The formation of much more stable intermediates over HAP/TiO₂ would be responsible for the high selectivity of CH₄. Moreover, photoelectrochemical and electrochemical characterizations revealed HAP could also promote the charge separation of TiO₂ and the charge transfer between TiO₂ and adsorbed species. The findings demonstrate HAP has a great potential as efficient assistant for photocatalytic CO₂ reduction with H₂O and will stimulate us to design novel semiconductor-based materials with tuned Lewis acidic and Lewis basic sites to achieve highly efficient photocatalysts.     

Cu+-ZrO2 interfacial sites with highly dispersed copper nanoparticles derived from Cu@UiO-67 hybrid for efficient CO2 hydrogenation to methanol

A series of Cu@ZrO₂-U framework catalysts derived from Cu@UiO-67 precursors with adjustable copper loadings were constructed by the deposition-precipitation method, and the catalytic activities of methanol synthesis via CO₂ hydrogenation were investigated. The optimized 20-Cu@ZrO₂-U catalyst showed the best catalytic activity. At 3 MPa and 260 °C, the space-time yield of CH₃OH (STY_CH3OH) reached 2.28 mmol_CH3OH/(g_cat·h), which was 3.5 times higher than that of 20-Cu/ZrO₂. The catalyst of 20-Cu@ZrO₂-U also showed good stability during the 100-h time on stream test. The catalysts were further characterized by XRD, N₂ sorption, TEM, XPS, H₂-TPR, CO₂/H₂-TPD and in situ DRIFTS. The characterization results showed that the stable ZrO₂ framework derived from UiO-67 is propitious to the confine of copper nanoparticles and formation of Cu⁺-ZrO₂ interfacial sites, which should be responsible for the excellent performance of methanol synthesis. Moreover, in situ DRIFTS was used to probe that the methanol synthesis via CO₂ hydrogenation over 20-Cu@ZrO₂-U follows a HCOO1-intermediated reaction pathway.     

Characterization and evaluation of mesoporous high surface area promoted Ni- Al2O3 catalysts in CO2 methanation

In this research, mesoporous high surface area Ni-Al₂O₃ catalysts promoted with different transition metals (Cr, Fe, Mn, Cu, and Co) were synthesized via ultrasound-assisted co-precipitation method and their performance was explored in CO₂ methanation process. The promoters can affect the textural and catalytic properties of the Ni-Al₂O₃ catalysts to some extent. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed reduction with hydrogen (H₂-TPR), and N₂ adsorption-desorption (BET) were used for the characterization of the prepared samples. From the BET results, it was found that the incorporation of 5 wt% of the promoter into Ni-Al₂O₃ catalysts caused a decrease in the surface area, NiAl₂O₄ crystalline size and an increase in the mean pore diameter and total pore volume. Among the samples, the catalyst modified by Mn, exhibited the higher catalytic activity and selectivity towards CH₄, especially at low temperatures (200–350 °C). These results could be explained by highest Ni active sites dispersion of this catalyst and enhancement of the catalyst reducibility at the low temperatures. The effect of Mn content was also evaluated and the results revealed that the Ni-Al₂O₃ sample modified with 3 wt% Mn with the highest BET surface area and the lowest crystalline size possessed the best catalytic performance. To further investigate the influence of ultrasonic irradiation, the optimal catalyst was prepared with a conventional co-precipitation method and its textural and catalytic properties were compared with those obtained for the catalyst prepared with the ultrasonic assisted coprecipitation method. Also, 25Ni-3Mn-Al₂O₃ catalyst showed a stable performance at 350 °C for 10 h in the CO₂ methanation reaction.     

Platinum supported catalysts for carbon monoxide preferential oxidation: Study of support influence

The aim of this work is to study the influence of the addition of different oxides to an alumina support, on surface acidity and platinum reducibility in platinum-based catalysts, as well as their effect on the activity and selectivity in CO preferential oxidation, in presence of hydrogen. A correlation between surface acidity and acid strength of surface sites and metal reducibility was obtained, being Pt-support interaction a function of the acid sites concentration under a particular temperature range. In platinum supported on alumina catalysts, CO oxidation follows a Langmuir-Hinshelwood mechanism, where O₂ and CO compete in the adsorption on the same type of active sites. It is noteworthy that the addition of La₂O₃ modifies the reaction mechanism. In this case, CO is not only adsorbed on the Pt active sites but also on La₂O₃, forming bridge bonded carbonates which leads to high reactivity at low temperatures. An increase on temperature produces CO desorption from Pt surface sites and favours oxygen adsorption producing CO₂. CO oxidation with surface hydroxyl groups was activated producing simultaneously CO₂ and H₂.     

CO2 hydrogenation to methanol over CuOZnOTiO2ZrO2 catalyst prepared by a facile solid-state route: The significant influence of assistant complexing agents

A series of CuOZnOTiO₂ZrO₂ catalysts were prepared by a facile solid-state route, and the effects of adding citric acid or oxalic acid on the physicochemical properties and catalytic performance for CO₂ hydrogenation to methanol were investigated. The catalysts were characterized by several techniques, including X-ray diffraction (XRD), N₂ adsorption, transmission electron microscopy (TEM), reactive N₂O adsorption, X-ray photoelectron spectroscopy (XPS), atomic absorption spectroscopy (AAS), temperature-programmed reduction with H₂ (H₂-TPR), and adsorption of CO₂ followed by temperature programmed desorption (CO₂-TPD). The results reveal that the addition of assistant complexing agents can improve the CuO dispersion in the catalysts and increase the metallic Cu surface area, and consequently greatly increase the CO₂ conversion and methanol yield, with oxalic acid being a more effective assistant complexing agent than citric acid.     

Methane tri-reforming for synthesis gas production using Ni/CeZrO2/MgAl2O4 catalysts: Effect of Zr/Ce molar ratio

Three Ni/CeZrO₂/MgAl₂O₄ catalysts synthesized using different Zr/Ce molar ratios (0.25, 1, and 4) were studied for methane tri-reforming. The catalysts were characterized using XRD, ²⁷Al-NMR, H₂-TPR, CO₂-TPD, XPS, and in situ techniques (XPD and XANES). The addition of CeZrO₂ at Zr/Ce = 0.25 on the MgAl₂O₄ spinel support considerably reduced the amount of carbon deposits, because the methane decomposition reaction was attenuated by the presence of less agglomerated Ni⁰ species produced after the reduction process. The highest CO₂ adsorption capacity (basicity) was associated with the participation of medium-strength basic sites, which facilitated coke gasification and led to higher CO₂ conversions. A syngas with quality (H₂/CO ratio) of 1.8 was produced, suitable for use in Fischer-Tropsch reactions.     

Deactivation and in-situ regeneration of Dy-doped Ni/SiO2 catalyst in CO2 reforming of methanol

The self-regeneration of Ni-based catalysts has been considered as a promising approach to maintain not only a continuous but also economical process. However, the effect of catalyst nature, operating temperature, amount and types of carbon deposits on the effectiveness of the in-situ regeneration is still not well-investigated. Therefore, in this work, the self-regeneration ability of the undoped and Dy-doped Ni/SiO₂ catalysts, which were prepared by the same impregnation method, were examined in the dry reforming of methanol. The physicochemical properties of the fresh and spent catalysts were analyzed by various techniques such as X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H₂-TPR), oxygen temperature-programmed desorption (O₂-TPD), transmission electron microscopy (TEM), N₂-BET isothermal adsorption. The nature and chemical reactivity of coke deposits formed during dry reforming at various temperatures (550, 600, and 650 °C) and the regeneration possibility of used catalysts through CO₂ gasification at these temperatures were investigated by the in-situ temperature-programmed gasification by CO₂ (TPCO₂). The Dy additive significantly improves the dispersion of the nickel active sites of Ni/SiO₂ catalyst, as demonstrated by the decreased Ni crystal size as well as the increased specific surface area and reduction degree of the catalyst. Furthermore, Dy promotion increases the quantity of oxygen vacancies and the nature of oxygen species, thereby improving the catalyst activity and stability. Specifically, methanol conversion dropped from 93% to 96%–61% and 31% for undoped Ni/SiO₂ at 600 °C and 650 °C, respectively and from about 99% to 87% (at 600 °C) and 52% (at 650 °C) for Dy-doped catalyst.     

Tuning activity of Pt/FeOx/TiO2 catalysts synthesized through selective-electrostatic adsorption for hydrogen purification by prox reaction

Hydrogen purification by removing CO traces was studied via the preferential CO oxidation (PROX) reaction using highly dispersed Pt catalysts supported on dual oxide FeO_x/TiO₂. These catalysts were prepared by the strong electrostatic adsorption (SEA) method by varying the pH of synthesis and the calcination temperature. By measuring the point of zero charge (PZC) of the support components, it was possible to determine the pH in which Pt can be selectively deposited onto one of the support components, obtaining Pt dispersion values above 90%. The selective SEA of a Pt precursor onto the co-support (FeO_x) was achieved at a synthesis pH between the PZCs of the support components (i.e., TiO₂ PZC = 5.2 and Fe₂O₃ PZC = 6.9) by using a Pt anionic complex. The catalytic activity for the PROX reaction, expressed in terms of the CO conversion, O₂ selectivity to CO₂, apparent activation energy, and turnover frequency, confirmed that the SEA prepared catalysts were active and selective for the PROX reaction. XPS and TPR results of the Pt/FeO_x/TiO₂ catalysts showed the formation of Pt-FeO_x interfaces, called as (Pt-FeO_x)_i interfacial sites, which enhanced the stability and catalytic activity for the PROX reaction. The concentration of these sites can be controlled by the synthesis conditions used, mainly pH and to a lower extent the calcination temperature.     

Current advances in syngas (CO + H2) production through bi-reforming of methane using various catalysts: A review

Today, bi - reforming of methane is considered as an emerging replacement for the generation of high-grade synthesis gas (H₂:CO = 2.0), and also as an encouraging renewable energy substitute for fossil fuel resources. For achieving high conversion levels of CH₄, H₂O, and CO₂ in this process, appropriate operation variables such as pressure, temperature and molar feed constitution are prerequisites for the high yield of synthesis gas. One of the biggest stumbling blocks for the methane reforming reaction is the sudden deactivation of catalysts, which is attributed to the sintering and coke formation on active sites. Consequently, it is worthwhile to choose promising catalysts that demonstrate excellent stability, high activity and selectivity during the production of syngas. This review describes the characterisation and synthesis of various catalysts used in the bi-reforming process, such as Ni-based catalysts with MgO, MgO–Al₂O₃, ZrO_2, CeO₂, SiO₂ as catalytic supports. In summary, the addition of a Ni/SBA-15 catalyst showed greater catalytic reactivity than nickel celites; however, both samples deactivated strongly on stream. Ce-promoted catalysts were more found to more favourable than Ni/MgAl₂O₄ catalyst alone in the bi-reforming reaction due to their inherent capability of removing amorphous coke from the catalyst surface. Also, Lanthanum promoted catalysts exhibited greater nickel dispersion than Ni/MgAl₂O₄ catalyst due to enhanced interaction between the metal and support. Furthermore, La₂O₃ addition was found to improve the selectivity, activity, sintering and coking resistance of Ni implanted within SiO₂. Non-noble metal-based carbide catalysts were considered to be active and stable catalysts for bi-reforming reactions. Interestingly, a five-fold increase in the coking resistance of the nickel catalyst with Al₂O₃ support was observed with incorporation of Cr, La₂O₃ and Ba for a continuous reaction time of 140 h. Bi-reforming for 200 h with Ni-γAl₂O₃ catalyst promoted 98.3% conversion of CH₄ and CO₂ conversion of around 82.4%. Addition of MgO to the Ni catalyst formed stable MgAl₂O₄ spinel phase at high temperatures and was quite effective in preventing coke formation due to enhancement in the basicity on the surface of catalyst. Additionally, the distribution of perovskite oxides over 20 wt % silicon carbide-modified with aluminium oxide supports promoted catalytic activity. NdCOO₃ catalysts were found to be promising candidates for longer bi-reforming operations.     

Catalysts for H2 production using the ethanol steam reforming (a review)

This review aims to provide an overview of the main catalytic studies of H₂ production by ethanol steam reforming (ESR). The reaction is endothermic and produces H₂, CO₂, CH₄, CO and coke. The conversion and H₂ selectivity of these products depended greatly of the physicochemical properties of the catalysts, active metal, promoters, temperature, long-term reaction, water/ethanol ratio, space velocity, contact time, and presence of O₂. Initial total conversion has been reported in all catalysts evaluated between 300 and 850 °C. The noble catalysts with high selectivity to H₂ (more than 80%) were: Rh, Ru, Pd and Ir and non-noble metal catalysts were: Ni, Co and Cu. The support materials include CeO₂, ZnO, MgO, Al₂O₃, zeolites-Y, TiO₂, SiO₂, La₂O₂CO₃, CeO₂–ZrO₂ and hydrotalcites. The impregnation method produced the best noble metal catalysts in terms of selectivity and conversion. The decrease of coke was related with the presence of basic sites on the support.     

Soft-templated NiO–CeO2 mixed oxides for biogas upgrading by direct CO2 methanation

The catalytic performance in the direct CO₂ methanation of a model biogas is investigated on NiO–CeO₂ nanostructured mixed oxides synthesized by the soft-template procedure with different Ni/Ce molar ratios. The samples are thoroughly characterized by means of ICP-AES, XRD, TEM and HR-TEM, N₂ physisorption at −196 °C, and H₂-TPR. They result to be constituted of CeO₂ rounded nanocrystals and of polycrystalline needle-like NiO particles. After a H₂-treatment at 400 °C for 1 h, the surface basic properties and the metal surface area are also assessed using CO₂ adsorption microcalorimetry and H₂-pulse chemisorption measurements, respectively. At increasing Ni content the Ni⁰ surface area increases, while the opposite occurs for the number of basic sites. Using a CO₂/CH₄/H₂ feed, at 11,000 cm³ h⁻¹ g_cat⁻¹, CO₂ conversions in the 83–89 mol% range and methane selectivities >99.5 mol% are reached at 275 °C and atmospheric pressure, highlighting the very good performances of the investigated catalysts.     

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.