Unusual catalytic properties of high-energetic-facet polar metal oxides

Conspectus Heterogeneous catalysis is an area of great importance not only in chemical industries but also in energy conversion and environmental technologies. It is well-established that the specific surface morphology and structure of solid catalysts exert remarkable effects on catalytic performances, since most physical and chemical processes take place on the surface during catalytic reactions. Different from the widely studied faceted metallic nanoparticles, metal oxides give more complicated structures and surface features. Great progress has been achieved in controlling the shape and exposed facets of transition metal oxides during nanocrystal growth, usually by using surface-directing agents (SDAs). However, the effects of exposed facets remain controversial among researchers. It should be noted that high-energetic facets, especially polar facets, tend to lower their surface energy via different relaxation processes, such as surface reconstruction, redox change, adsorption of countercharged species, etc. These processes can subsequently lead to surface defect formation and break the surface stoichiometry, and the resulting changes in electronic configurations and charge migration properties all play important roles in heterogeneous catalysis. Because different materials prefer different relaxation methods, various surface features are created, and different techniques are required to investigate the different features from facet to facet. Conventional characterization techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, etc. appear to be insufficient to elucidate the underlying principles of the facet effects. Consequently, an increasing number of novel techniques have been developed to differentiate the surface features, enabling greater understanding of the effects of facets on heterogeneous catalysis. In this Account, on the basis of previous studies by our own group, we will focus on the effects of tailored facets on heterogeneous catalysis introduced by engineered simple binary metal oxide nanomaterials primarily with exposed polar facets, in combination with detailed surface studies using a range of new characterization techniques. As a result, fundamental principles of the effects of facets are elucidated, and the structure–activity correlations are demonstrated. The surface features introduced by different relaxation processes are also investigated using a range of characterization techniques. For example, electron paramagnetic resonance spectroscopy is used to detect the oxygen vacancies, while probe-assisted solid-state NMR spectroscopy is shown to be facet-sensitive and able to evaluate the surface acidity. It is also shown that such different features influence the heterogeneous catalytic performances in different ways. With the help of first-principles density functional theory calculations, unique properties of the faceted metal oxides are discussed and unraveled. Besides, other materials such as transition metal chalcogenides and layered double hydroxides are also briefly discussed with regard to their application in facet-dependent catalysis studies.</p

Facet-dependent photocatalysis of nanosize semiconductive metal oxides and progress of their characterization

Semiconductive metal oxides are of great importance in environmental remediation and electronics because of their ability to generate charge carriers when excited with appropriate light energy. The electronic structure, light absorption and charge transport properties of the metal oxides have made possible their applications as photocatalysts. Recently, facet-engineering by morphology control has been intensively studied as an efficient approach to further enhance their photocatalytic performance. However, various processing steps and post-treatments used during the preparation of facet-engineered particles may generate different surface active sites which may affect their photocatalysis. Moreover, many traditional techniques (PL, EPR, XPS and Raman) used for materials characterization (oxygen vacancy, hydroxyl group, cation…etc.) are not truly surface specific but the analyses range from top few layers to bulk. Accordingly, they can only provide very limited information on the chemical states of the surface active features and distributions among facets, causing difficulty to unambiguously correlate facet-dependent results with activity. As a result, this always leads to different interpretations amongst researchers during the past decades. In this article, we will review on the controversies generated among researchers, when they correlated the performance of two most popular photocatalysts, ZnO and TiO2 with their facet activities based on characterization from the traditional techniques. As there are shortcomings of these techniques in producing truly facet-dependent features, some results can be misleading and with no cross-literature comparison. This review is also focussed on the new capability of probe-molecule-assisted NMR which allows a genuine differentiation of surface active sites from various facets. This surface-fingerprint technique has been demonstrated to provide both qualitative (chemical shift) and quantitative (peak intensity) information on the concentration and distribution of truly surface features. In light of the new technique this article will revisit the facet-dependent photocatalytic properties and shed light on these issues

Structure-reactivity relationship in catalytic hydrogenation of heterocyclic compounds over ruthenium black; Part B: Effect of carbon substitution by heteroatom

The effect of the type of heteroatom in the structure on the recyclability of possible candidate compounds for application as LOC (liquid organic carriers) was studied by comparing the rate and selectivity obtained in hydrogenation of carbazole, dibenzothiophene, dibenzofuran and fluorene. The effect of a partial saturation of the compound on its hydrogenation yield and reaction pathway was also considered by studying hydrogenation of 1,2,3,4-tetrahydrocarbazole. Using Ru black catalyst, the rate of hydrogenation was found to decrease in order; dibenzofuran. &gt;. 1,2,3,4-tetrahydrocarbazole. &gt;. carbazole. &gt;&gt;. fluorene. No reaction was observed in the hydrogenation of dibenzothiophene under the conditions studied which was attributed to the immediate poisoning of ruthenium metal by sulphur compounds. The rate of hydrogenation of fluorene was around 3 times lower as compared to carbazole and over 8 times lower as compared to that of dibenzofuran under the same reaction conditions. With the exeption of S containing dibenzothiophene, the presence of the heteroatom in the structure was found to be beneficial in terms of increasing the rate of hydrogen loading step. Additionally, a higher reaction rate was obtained in the hydrogenation of the partially saturated 1,2,3,4-tetrahydrocarbazole as compared to the substrate carbazole. The structure and stability of intermediates was found to be significantly influenced by the type and presence of a heteroatom in the structure. A stable octahydro-intermediate was observed only with N-heterocycles, whereas a stable hexahydro-intermediate was produced in the polyaromatic hydrocarbon-fluorene. Additionally, the theoretically obtained lowest total enthalpies using DFT calculations agreed well with the stable intermediates observed experimentally in the hydrogenation of fluorene. Theoretical DFT differences in enthalpies also indicated the products of hydrogenolysis of perhydro-dibenzofuran to be the most favourable products of its hydrogenation, which agreed well with the experimental observations. Overall, taking into account the recyclability of LOC, substitution of carbon with a N heteroatom was demonstrated to be one of the promising approaches to improve the kinetics of the hydrogen loading step

Facet-dependent photocatalysis of nanosize semiconductive metal oxides and progress of their characterization

Semiconductive metal oxides are of great importance in environmental remediation and electronics because of their ability to generate charge carriers when excited with appropriate light energy. The electronic structure, light absorption and charge transport properties of the metal oxides have made possible their applications as photocatalysts. Recently, facet-engineering by morphology control has been intensively studied as an efficient approach to further enhance their photocatalytic performance. However, various processing steps and post-treatments used during the preparation of facet-engineered particles may generate different surface active sites which may affect their photocatalysis. Moreover, many traditional techniques (PL, EPR, XPS and Raman) used for materials characterization (oxygen vacancy, hydroxyl group, cation…etc.) are not truly surface specific but the analyses range from top few layers to bulk. Accordingly, they can only provide very limited information on the chemical states of the surface active features and distributions among facets, causing difficulty to unambiguously correlate facet-dependent results with activity. As a result, this always leads to different interpretations amongst researchers during the past decades. In this article, we will review on the controversies generated among researchers, when they correlated the performance of two most popular photocatalysts, ZnO and TiO2 with their facet activities based on characterization from the traditional techniques. As there are shortcomings of these techniques in producing truly facet-dependent features, some results can be misleading and with no cross-literature comparison. This review is also focussed on the new capability of probe-molecule-assisted NMR which allows a genuine differentiation of surface active sites from various facets. This surface-fingerprint technique has been demonstrated to provide both qualitative (chemical shift) and quantitative (peak intensity) information on the concentration and distribution of truly surface features. In light of the new technique this article will revisit the facet-dependent photocatalytic properties and shed light on these issues

Recent progress and strategies for enhancing photocatalytic water splitting

Solar light-driven water splitting provides a promising way to store and use abundant solar energy in the form of gaseous hydrogen which is the cleanest chemical fuel for mankind; therefore this field has been attracting increasing attention over the past decades. The fundamental steps for efficient photocatalyst for water splitting include uptake of photons of targeted energy range by appropriate electronic band structure, excited electrons and holes (excitons) migration, as well as recombination and selective conversion excited electrons for H+ reduction to H2 and holes and OH− to O2 on catalyst surface. Each step if not efficiently taken place could hamper the overall photocatalytic activity. Numerous semiconductors with appropriate bandgaps have mainly been developed as candidates for effective solar energy capture, whereas at present, their low quantum efficiency still remains as the major obstacle in further applications. In this minireview, we will disentangle the progress to develop photocatalysts with good photon uptake from photocatalytic water splitting performance. In accordance with the thermodynamic and kinetic considerations of the photocatalytic water splitting reaction, different strategies for improving the fundamental processes have been briefly reviewed. Some recent advances in facilitating charge carriers separation have also been presented. Photocatalytic water splitting at elevated temperatures is emphasized as a novel approach to suppress photo-excitons recombination on catalyst surface owing to adsorption of enhanced concentration of ionic species including H+ and OH− to create their local polarization to the excitons. Stronger polarization to hinder the excitons recombination can also be obtained by using polar-faceted support materials to the active phase of semiconductor. It is clearly demonstrated in this minireview that such high temperature–promoted photocatalytic water splitting systems could open up a new direction and provide a new innovation to this field

Facet-dependent photocatalysis of nanosize semiconductive metal oxides and progress of their characterization

Semiconductive metal oxides are of great importance in environmental remediation and electronics because of their ability to generate charge carriers when excited with appropriate light energy. The electronic structure, light absorption and charge transport properties of the metal oxides have made possible their applications as photocatalysts. Recently, facet-engineering by morphology control has been intensively studied as an efficient approach to further enhance their photocatalytic performance. However, various processing steps and post-treatments used during the preparation of facet-engineered particles may generate different surface active sites which may affect their photocatalysis. Moreover, many traditional techniques (PL, EPR, XPS and Raman) used for materials characterization (oxygen vacancy, hydroxyl group, cation…etc.) are not truly surface specific but the analyses range from top few layers to bulk. Accordingly, they can only provide very limited information on the chemical states of the surface active features and distributions among facets, causing difficulty to unambiguously correlate facet-dependent results with activity. As a result, this always leads to different interpretations amongst researchers during the past decades. In this article, we will review on the controversies generated among researchers, when they correlated the performance of two most popular photocatalysts, ZnO and TiO2 with their facet activities based on characterization from the traditional techniques. As there are shortcomings of these techniques in producing truly facet-dependent features, some results can be misleading and with no cross-literature comparison. This review is also focussed on the new capability of probe-molecule-assisted NMR which allows a genuine differentiation of surface active sites from various facets. This surface-fingerprint technique has been demonstrated to provide both qualitative (chemical shift) and quantitative (peak intensity) information on the concentration and distribution of truly surface features. In light of the new technique this article will revisit the facet-dependent photocatalytic properties and shed light on these issues