A. Bueno-López

Relationship between surface area and crystal size of pure and doped cerium oxides

Abstract Pure and Zr, La or Pr-doped cerium oxides were characterised by transmission electron microscopy (TEM), N 2 adsorption-desorption at −196 °C and X-ray diffraction (XRD). For crystal sizes calculation, the Scherrer and Williamson-Hall equations were compared, and the relationship between surface area and crystal size was critically discussed. It was demonstrated that the Williamson-Hall equation must be used instead of the Scherrer equation to calculate crystal sizes, since the latter equation underestimated the crystal size while the former one provided suitable values by taking into account the strain contribution to line broadening. The miscalculation of crystal size by using the Scherrer equation affected both pure and doped cerium oxides, but the size underestimation became more important for high dopant loading. Under the conditions of the BET surface area measurements, N 2 molecules were adsorbed on the surface of the single crystals identified by XRD.

Diesel soot combustion catalysts: review of active phases

The most relevant information about the different active phases that have been studied for the catalytic combustion of soot is reviewed and discussed in this article. Many catalysts have been reported to accelerate soot combustion, including formulations with noble metals, alkaline metals and alkaline earth metals, transition metals that can accomplish redox cycles (V, Mn, Co, Cu, Fe, etc.), and internal transition metals. Platinum catalysts are among those of most interest for practical applications, and an important feature of these catalysts is that sulphur-resistant platinum formulations have been prepared. Some metal oxide-based catalysts also appear to be promising candidates for soot combustion in practical applications, including ceria-based formulations and mixed oxides with perovskite and spinel structures. Some of these metal oxide catalysts produce highly reactive active oxygen species that promote efficient soot combustion. Thermal stability is an important requirement for a soot combustion catalyst, which precludes the practical utilisation of several potential catalysts such as most alkaline metal catalysts, molten salts, and metal chlorides. Some noble metal catalysts are also unstable due to the formation of volatile oxides (ruthenium, iridium, and osmium).

Contributions of surface and bulk heterogeneities to the NO oxidation activities of ceria–zirconia catalysts with composition Ce0.76Zr0.24O2 prepared by different methods

The study of the catalytic activity towards NO oxidation to NO(2) was approached by using ceria-zirconia mixed oxides with the same nominal composition (Ce(0.76)Zr(0.24)O(2)) but prepared by different routes of synthesis: coprecipitation, solid combustion synthesis with urea, citrate complexation route, reversed microemulsion and template synthesis. The characterisation of the catalysts was performed by N(2) adsorption at -196 °C, XRD, Raman Spectroscopy, H(2)-TPR and XPS in order to ascertain the relationships between their catalytic activities and their bulk and surface properties. The results showed that the preparation method is critical for the physico-chemical properties of the mixed oxides, exhibiting very different BET surface areas, crystalline phase/s contributions and bulk oxygen mobility. The distribution of Ce and Zr on the surface with regard to the bulk is very much influenced by the preparation method as well. The NO(2) production from NO oxidation was shown to be mostly correlated with the Ce/Zr surface atomic ratio and the proportion of Ce(4+) (presumably in a doped cubic phase) in the uppermost layers.

NOx reduction by potassium-containing coal pellets. Discussing lifetime test profiles

The activity of potassium-containing coal pellets for NOx reduction in an oxygen-rich environment was investigated at 350 °C, during lifetime tests, until the sample was completely consumed. A complex reaction profile is exhibited by the samples, consisting of four different steps. As the potassium content increases, the selectivity towards NOx reduction, the amounts of NOx reduced per gram of sample (ash-free basis), and the maximum NOx conversion reached progressively increase. The highest potassium content sample can remove a considerable amount of NOx, 3.1 g/g free-ash sample.

Potential of Cu–saponite catalysts for soot combustion

H– and Na–saponite supports have been prepared by several synthesis approaches. 5% Cu/saponite catalysts have been prepared and tested for soot combustion in a NOx + O2 + N2 gas flow and with soot and catalyst mixed in loose contact mode. XRD, FT-IR, N2 adsorption and TEM characterization results revealed that the use of either surfactant or microwaves during the synthesis led to delamination of the saponite support, yielding high surface area and small crystallite size materials. The degree of delamination affected further copper oxide dispersion and soot combustion capacity of the Cu/saponite catalysts. All Cu/saponite catalysts were active for soot combustion, and the NO2-assisted mechanism seemed to prevail. The best activity was achieved with copper oxide supported on a Na–saponite prepared at pH 13 and with surfactant. This best activity was attributed to the efficient copper oxide dispersion on the high surface area delaminated saponite (603 m2 g−1) and to the presence of Na. Copper oxide reduction in H2-TPR experiments occurred at lower temperature for the Na-containing catalysts than for the H-containing counterparts, and all Cu/Na–saponite catalysts were more active for soot combustion than the corresponding Cu/H–saponite catalysts.

BETA Zeolite Thin Films Supported on Honeycomb Monoliths with Tunable Properties as Hydrocarbon Traps under Cold‐Start Conditions

A high percentage of hydrocarbon (HC) emissions from gasoline vehicles occur during the cold-start period. Among the alternatives proposed to reduce these HC emissions, the use of zeolites before the three-way catalyst (TWC) is thought to be very effective. Zeolites are the preferred adsorbents for this application; however, to avoid high pressure drops, supported zeolites are needed. In this work, two coating methods (dip-coating and in situ crystallization) are optimized to prepare BETA zeolite thin films supported on honeycomb monoliths with tunable properties. The important effect of the density of the thin film in the final performance as a HC trap is demonstrated. A highly effective HC trap is prepared showing 100% toluene retention, accomplishing the desired performance as a HC trap, desorbing propene at temperatures close to 300 °C, and remaining stable after cycling. The use of this material before the TWC is very promising, and works towards achieving the sustainability and environmental protection goals.

Rapid-Scan Operando Infrared Spectroscopy

Novel methodology, referred to as rapid‐scan operando, has been demonstrated to be a highly powerful tool for studying reaction mechanisms in heterogeneous catalysis, because it combines rapid scan for IR monitoring in the milliseconds time frame and operando methodology. As proof of concept, the NOx–CO and NOx–H2 reactions were studied as model catalyzed reactions with practical interest for removal of NOx in diesel exhausts. Rapid‐scan operando experiments confirmed state‐of‐the‐art knowledge concerning the reaction mechanisms and, more importantly, allowed us to elucidate, for the first time, the different roles of the surface hydroxy groups depending on the reductant used (CO or H2). Moreover, this new tool was used to distinguish the behavior of carbonates and nitrites under reaction conditions that could not be monitored by conventional IR spectroscopy approaches owing to overlap of their absorbance bands.

Methanol Electro-Oxidation on Carbon-Supported PtRu Nanowires

One of the key objectives in fuel cell technology is to improve the alcohol oxidation efficiency of Pt-based catalysts. A series of carbon-supported PtRu nanowires with different concentrations of Pt and Ru were prepared for application in methanol oxidation in acid media. The physicochemical properties and electrocatalytic activity of these catalysts during methanol oxidation are function on their structure, morphology and composition. A Pt60Ru40/C catalyst shows the best behaviour towards methanol electro-oxidation allowing decrease the onset potential approximately 0.2 V respect to others PtRu/C synthesised nanowires. The structural modification of Pt by Ru and synergetic character of RuPt are main factors that could contribute to reduction of energy necessary for electro-oxidation process. The Pt and PtRu nanowires have different sizes and distribution on the substrate. The average crystallite sizes, found by XRD, are in the 4.6-5.9 nm range and the lattice parameter is between 0.3903-0.3908 nm. Small differences with the values of the Pt/C catalyst were found. The XPS results show a prevailing presence of metallic Pt and Ru4+ species.

PROJECT-BASED LEARNING IN CHEMISTRY: THE ROAD FROM HIGHER EDUCATION TO APPLIED RESEARCH

With the Formative Evaluation model founded on project based learning that we propose in this work a student may develop, throughout the duration of practical laboratory teaching which comprises a significant number of hours invested in laboratory demonstrations, a given “project” with concise teaching objectives and solid formative background (as determined by the corresponding teaching staff) that involves the development of diverse skills. By using the term “project” we mean a set of coordinated and intertwined activities of both theoretical and practical character with specific objectives that aim to potentiate the learning of knowledge and skills in the student that may be of great help in his/her development, personal realization, and future professional practice. From a broad perspective, they may be considered small research projects.