Search

Reverse water gas shift (RWGS) competes with methanation as a direct pathway in the CO2recycling route, with methanation being a dominant process in the low-temperature window and RWGS at higher temperatures. This work showcases the design of multi-component catalysts for a full-temperature-range RWGS behavior by suppressing the methanation reaction at low temperatures. The addition of alkali promoters (Na, K, and Cs) to the reference Ni/CeO2catalyst allows identifying a clear trend in RWGS activation promotion in both low- and high-temperature ranges. Our characterization data evidence changes in the electronic, structural, and textural properties of the reference catalyst when promoted with selected dopants. Such modifications are crucial to displaying an advanced RWGS performance. Among the studied promoters, Cs leads to a more substantial impact on the catalytic activity. Beyond the improved CO selectivity, our best performing catalyst maintains high conversion levels for long-term runs in cyclable temperature ranges, showcasing the versatility of this catalyst for different operating conditions. All in all, this work provides an illustrative example of the impact of promoters on fine-tuning the selectivity of a CO2conversion process, opening new opportunities for CO2utilization strategies enabled by multi-component catalysts.

Pt catalysts supported on N-doped activated carbons for a novel in-situHDO of guaiacol strategy with H2O as a hydrogen source.

The carbothermal synthesis of monometallic and bimetallic molybdenum carbide and copper, supported on high surface area graphite (H), has been studied by in situXRD, XPS, D2-TPD, TEM/STEM, TG-mass spectrometry, and N2adsorption. The catalysts were prepared using H2at 600 °C or 700 °C and tested in the hydrogenation of CO2to methanol. Molybdenum carbide and oxycarbide phases were obtained, as well as hydride species, at 600 °C on both monometallic MoxC/H and bimetallic CuMoxC/H in a similar proportion. Upon increasing the temperature up to 700 °C, the formation of metallic Mo is favourable. Although this is observed on supported MoxC and CuMoxC, the bimetallic sample is less affected by the formation of the hydride, and molybdenum carbide is also observed upon treatment at 700 °C. With regards to the catalytic performance, supported monometallic copper was not active, but copper increased the activity and selectivity of the molybdenum carbide. The yield of methanol per catalyst's weight increases upon increasing the copper loading, indicating that a cooperation reaction takes place between the smallest Cu particles in contact with the molybdenum phase. The catalysts synthesized at 700 °C are less active and less selective to methanol favouring the reverse water gas shift under the studied conditions. Interestingly, the catalysts are stable under the reaction conditions, and the detected phases by XRD of the spent catalysts suggest that the hydride species favoured transformations involving MoOxCyHz↔ β-Mo2C.

The hydrogenation of nitroarenes to produce the corresponding amines using dihydrogen as reducing agent has an important industrial role, since it allows to obtain important added-value products. This reaction needs the help of a catalyst to proceed. Many catalysts have been already tested and studied. Most of them are based on noble metals supported on metal oxides. These catalysts perform well, but they are expensive and thus, alternative systems are needed. In this context, cobalt-based catalysts have emerged as adequate alternatives, despite cobalt nanoparticles per seare not very active for this reaction. A way to improve the catalytic activity of cobalt nanoparticles is by supporting them on a support with functional groups that are able to change their intrinsic properties and to enhance their catalytic properties. In this sense, N-containing carbons are promising candidates to be used as support, since nitrogen functionalities may modify the catalytic properties of cobalt. In this work, cobalt nanoparticles supported on N-doped activated carbons have been prepared and studied as catalysts for the hydrogenation of 1-chloro-4-nitrobencene to the corresponding chloro-aniline. It is demonstrated that the catalytic activity is enhanced by the presence of nitrogen species in the support. When the temperature of the catalyst activation treatment (reduction under flowing hydrogen) is increased, the catalytic activity increases drastically in the presence of nitrogen functionalities on the support. The catalysts have been characterised by transmission electron microscopy (TEM), temperature-programmed reduction (TPR), X-ray diffraction, X-ray photoelectron spectroscopi (XPS) and N2adsorption at 77 K. It has been found that the enhanced catalytic activity was due to two different factors, namely the interaction of the cobalt particles with the nitrogen functional groups (forming Co4N), and the development of mesopores in the support during the activation process that increases the accessibility of reactants to the active sites.

Understanding/visualizing the established interactions between gases and adsorbents is mandatory to implement better performance materials in adsorption/separation processes. Here we report the unique behavior of a rare example of a hemilabile chiral three-dimensional metal–organic framework (MOF) with an unprecedented qtz-e-type topology, with formula CuII2(S,S)-hismox·5H2O (1) (hismox = bis[(S)-histidine]oxalyl diamide). 1exhibits a continuous and reversible breathing behavior, based on the hemilability of carboxylate groups from l-histidine. In situ powder (PXRD) and single crystal X-ray diffraction (SCXRD) using synchrotron radiation allowed us to unveil the crystal structures of four different host–guest adsorbates (Ar@1, N2@1, CO2@1, and C3H6@1), rationalize the breathing motion, and unravel the mechanisms governing the adsorption of these gases. Then this information was transferred to implement efficient separations of mixtures of industrial and environmental relevance, CO2/N2, CO2/CH4, and C3H8/C3H6, using 1in packed columns as the stationary phase and dispersed in a mixed matrix membrane.

Materials able to produce the reduction of nitrate from water without the need of a metal catalyst and with avoiding the use of gaseous hydrogen have been developed by combining the synergistic properties of titania and two conducting polymers. Polymerization of aniline and pyrrol on titanium dioxide in the presence of two different oxidants/dopants (iron trichloride or potassium persulfate) has been evaluated. The resulting hybrid materials have good thermal stability imparted by the titania counterpart, and a considerable conductivity provided by the conducting polymers. The capability of the hybrid materials of reducing aqueous nitrate has been assessed and compared to the catalytic hydrogenation of nitrate using a platinum catalyst supported on these hybrid synthesized materials. The mechanism of nitrate abatement implies adsorption of nitrate on the polymer by ion exchange with the dopant anion, followed by the reduction of nitrate. The electron transfer from titania to the conducting polymer in the hybrid material favors the reductive ability of the polymer, in such a way that nitrate is selectively reduced with a very low production of undesirable side products. The obtained results show that the activity and selectivity of the catalytic reduction of nitrate with dihydrogen in the presence of a platinum catalyst supported on the hybrid materials are considerably lower than those of the metal-free nanocomposites.

A series of carbon molecular sieves (CMSs) has been prepared, either as powders or monoliths, from petroleum pitch using potassium hydroxide as the activating agent. The CMS monoliths are prepared without the use of a binder based on the self‐sintering ability of the mesophase pitch. Characterization results show that these CMSs combine a large apparent surface area (up to ca. 3100 m2g−1) together with a well‐developed narrow microporosity (Vnup to ca. 1.4 cm3g−1). The materials exhibit high adsorption capacities for CO2at 1 bar and 273 K (up to ca. 380 mg CO2g sorbent−1). To our knowledge, this is the best result obtained for CO2adsorption using carbon‐based materials. Furthermore, although the CO2adsorption capacity for activated carbons has usually been considered lower than that of zeolites, the reported values exceed the total amount adsorbed on traditional 13X and 5A zeolites (ca. 230 mg and 180 mg CO2g sorbent−1, respectively), under identical experimental conditions. Additionally, the narrow pore openings found in the CMS samples (ca. 0.4 nm) allows for the selective adsorption of CO2from molecules of similar dimensions (e.g., CH4and N2).

Catalytic adsorption of SO 2on activated carbon materials provides an appropriate alternative for the control of low concentration emissions of this air pollutant. The surface complexes formed upon SO 2adsorption at 30 °C were studied by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The effect of the addition of O 2and the presence of copper as catalyst were studied. Copper assisted the oxygen transfer to the carbon matrix. For the Cu-impregnated carbon sample, the presence of O 2favored SO 2adsorption by increasing the breakthrough time, the adsorption capacity and the formation of sulfur and oxygen complexes of higher thermal stabilities, which mainly desorb as SO 2and CO 2.

Simultaneous adsorption of NO and SO2on activated carbon provides an adequate alternative to control the low concentration emissions of these air pollutants. The surface complexes formed during the NO and SO2adsorptions at 30 °C were studied by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The effects of the addition of copper supported on the carbon and O2on the gas stream were studied. The presence of copper enhanced the chemical adsorption of both NO and SO2. In the simultaneous adsorption of NO and SO2, SO2inhibited the adsorption of NO, while SO2adsorption was notably improved. The reaction carried out under a NO/SO2/O2gas mixture showed a synergic effect increasing the formation of surface complexes. The addition of copper assisted the oxygen transfer to the carbon matrix.

Abstract The effect of different preparation parameters (pyrolysis temperature, process time, benzene concentration and residence time) on the properties of carbon molecular sieves obtained by pyrolytic carbon deposition from benzene on a microporous activated carbon, has been studied. For a fixed process time of 2 h, the increase of the pyrolysis temperature from 525 to 700∘C produced a strong increase of the CO2/CH4 selectivity, reaching a maximum value of 13 for pyrolysis at 700∘C. Selectivity also increased with process time (up to 5 h, with the pyrolysis temperature fixed at 650 and 675∘C), yielding values as high as 28. The obtained results have allowed to find the optimal preparation parameters for the preparation of CMS with high selectivity.

Infrared spectra of CO alone and CO followed by the introduction of 2-butenal (crotonaldehyde) have been recorded for Pt/CeO₂–SiO₂ and Zn doped Pt/CeO₂–SiO₂ catalysts which had been pre-reduced at 473, 623 and 773 K. Samples treated at the highest reduction temperature showed the greatest selectivity toward the unsaturated alcohol although no clear spectroscopic evidence for the formation of an SMSI state was obtained. Although 773 K reduced Zn containing catalysts showed the greatest yield, there was no IR spectroscopic evidence to support the formation of significant quantities of a Pt–Zn alloyed phase or that the addition of Zn enhanced Pt-support interactions beyond increasing the amount of surface reduced ceria. Results show that, contrary to popular opinion, the development of an SMSI state which enhances the density of support-metal interface sites is not essential in order to increase the yield of unsaturated alcohol and the presence of reduced cationic sites at the interface zone may be the crucial factor. Although IR spectra in the νCO region were dominated by dipole coupling effects, enhanced intensity of carbonyl bands in the 2030–1950 cm−1 region for 773 K reduced samples and for Zn containing samples reduced at lower temperatures suggests that sites responsible for these carbonyl bands may be the active sites in the selective hydrogenation reaction.

Microcalorimetric measurements of the adsorption of H2and C2H4were carried out at 300 K on a Pt–Zn/X-zeolite catalyst (Pt : Zn atomic ratio equal to 1 : 1). The initial heats of H2and C2H4adsorption were equal to 75 and 122 kJ/mol, respectively, and these values are weaker than the values of 90 and 155 kJ/mol typically observed for supported Pt catalysts. Reaction kinetics measurements for isobutane dehydrogenation over the Pt–Zn/X-zeolite catalyst were carried out at temperatures from 673 to 773 K, at isobutane pressures from 0.01 to 0.04 atm, and at hydrogen pressures from 0.1 to 0.7 atm. The catalyst shows high activity and selectivity for dehydrogenation of isobutane to isobutylene. The reaction kinetics can be described with a Horiuti–Polanyi reaction scheme. DFT calculations were carried out for the adsorption of ethylene on slabs of Pt(111), Pt3Zn(111) and PtZn(011). Results from these calculations indicate that addition of Zn to Pt weakens the binding energies of π-bonded ethylene, di-σ-bonded ethylene, and ethylidyne species on atop, bridged, and three-fold Pt sites, respectively. These effects are most significant for the bonding of ethylidyne species, and they are least significant for π-bonded ethylene species. Results from DFT calculations for the adsorption of formaldehyde show that addition of Zn to Pt weakens the di-σ-bonding at Pt–Pt sites; however, this weakening effect of Zn on formaldehyde adsorption is less significant than the effect on ethylene adsorption. Moreover, the preferred location for adsorption of formaldehyde on PtZn(011) is a Pt–Zn site, whereas the preferred location for adsorption of ethylene is a Pt–Pt site. Thus, formaldehyde is adsorbed more strongly by 53 kJ/mol on PtZn(011) compared to the di-σ-adsorption of ethylene, whereas formaldehyde and ethylene adsorb in the di-σ-forms with comparable energies on Pt(111). This preferred adsorption of formaldehyde compared to ethylene on PtZn(011) may be at least partially responsible for the enhanced selectivity of Pt–Zn-based catalysts for hydrogenation of C=O groups compared to C=C bonds in α,β-unsaturated aldehydes.

Magnesia-supported bimetallic Pt–Sn catalysts with various metal contents and Pt/Sn atomic ratios were prepared using the bimetallic complex cis-[PtCl(SnCl₃)(PPh₃)₂] or the monometallic complex cis-[PtCl₂(PPh₃)₂] and SnCl₂ as metal precursors. Catalysts were characterised by X-ray diffraction, transmission electron microscopy techniques and X-ray photoelectron spectroscopy. The bulk and surface composition of the catalysts was related to the catalytic behaviour in the vapour phase hydrogenation of crotonaldehyde (but-2-enal). It is shown that the various preparation methods produce catalysts with distinct properties and catalytic behaviour. For catalysts prepared with the bimetallic complex, the highest selectivity for the unsaturated alcohol (but-2-en-1-ol) was achieved for the catalyst containing larger bimetallic particles, which are associated with a higher surface concentration of oxidised tin species. The catalyst prepared with separate metal precursors contains a higher amount of residual chlorides, which significantly affect the catalytic activity but only slightly alter the selectivity for the hydrogenation of the carbonyl bond.

Alumina- and silica-supported bimetallic Pt–Sn catalysts with different Pt/Sn atomic ratios have been prepared by using the bimetallic complex cis-[PtCl(SnCl₃)(PPh₃)₂], the monometallic complex cis-[PtCl₂(PPh₃)₂] and SnCl₂ as metal precursors. Catalysts have been characterised by X-ray diffraction and transmission electron microscopy techniques, including energy-dispersive X-ray microanalysis, electron diffraction experiments and lattice-fringe microanalysis. The phase composition of the different catalysts has been related to the catalytic behaviour in the vapour phase hydrogenation of crotonaldehyde (but-2-enal). Results show that catalysts containing platinum–tin alloy phases with large particles are more selective towards the hydrogenation of the carbonyl bond to yield crotyl alcohol. The presence of oxidised tin species on the surface of the large alloy particles promotes the hydrogenation of the carbonyl bond. In situ diffuse reflectance infrared spectroscopy studies of crotonaldehyde adsorption under He or H₂ reveal differences between bimetallic catalysts and monometallic, and these differences have been related to the different catalytic behaviour of the catalysts.

The platinum-ceria interaction, induced after high temperature reduction of platinum based catalysts, has been used to improve the selectivity towards crotyl alcohol in the gas phase hydrogenation of crotonaldehyde. Three catalysts have been used in this study, Pt/SiO2, Pt/CeO2, and Pt/CeO2/SiO2. They have been characterized by X-ray diffraction and temperature-programmed reduction, and their catalytic behavior in the gas phase benzene hydrogenation and crotonaldehyde hydrogenation reactions have been determined after reduction treatments in flowing hydrogen at 473 and 773 K. Results obtained in the gas phase hydrogenation of crotonaldehyde show that the presence of ceria clearly promotes the hydrogenation of the carbonyl bond when the sample is reduced at high temperature, this being attributed to the presence of oxygen vacancies or Ce3+cations in the neighbourhood of platinum particles, induced by the thermal treatment under hydrogen. The promoting effect is more important for Pt/CeO2; crotonaldehyde hydrogenation on this catalyst yields up to 80% crotyl alcohol after reduction at 773 K, whereas it produces 100% butanal after reduction at only 473 K.

Pt/activated carbon catalysts were prepared by using [Pt(NH3)4]Cl2 as the metal precursor, and characterized by a number of techniques. The selectivity of these catalysts towards crotyl alcohol in the gas-phase hydrogenation of crotonaldehyde was considerably increased when the activated carbon support was slightly oxidized before the metal incorporation.

A carbon‐supported platinum catalyst, prepared by the impregnation method followed by reduction under hydrogen, is highly reusable in the hydrogen‐transfer reduction of aromatic ketones, using 2‐propanol as the hydrogen donor. Up to six different aromatic ketones can be effectively reduced using the same catalyst sample.

Textural characterization of porous solids is commonly carried out by N2adsorption at 77 K. Additionally, CO2adsorption at 273 K and sub-atmospheric pressure was proposed as a complement to N2for the more complete characterization of the narrow microporosity. However, and contrary to the general assumption, CO2adsorption may be kinetically restricted in narrow pores (<0.3−0.4 nm), in spite of the high-temperature (273 K) at which the measurement is carried out, thus giving rise to an erroneous characterization.