Supported copper–titanium oxide catalysts have been studied by IR spectroscopy, XRD, UV–VIS diffuse reflectance spectroscopy, ESR, TPR with hydrogen and EXAFS. Different states of Cu2+ ions have been found on the surface of TiO2. The ratio between these states depends on the copper concentration and catalyst preparation method. Chain structures are formed due to the interaction of Cu2+ ions with TiO2. Their concentration and geometry of their nearest oxygen environment are determined by the structure of anatase faces. Some of these surface-stabilized Cu2+ ions are nucleation centers of oxide clusters. A noticeable growth of bulk CuO phase is observed after the completion of the formation of chain structures and oxide clusters. A quantitative estimation of the concentration of these copper forms in the catalysts is reported. For supported copper–titanium oxide catalysts, chain forms of Cu2+ ions are shown to be the most active forms of copper in NO SCR with ammonia and methane oxidation.

Investigation of the state of copper in supported copper–titanium oxide catalysts

Synthesized silicas modified with alumina, titania, and zirconia (about 13% wt) were used as supports for dispersing nanosized CuO phase. All the prepared catalysts, containing about 1 mmolcu gcat-1 possessed high surface areas (230–430 m2gcat-1) and homogeneous coverage of the relevant support, as revealed by SEM-EDS analysis. The nature of the support and its acidity directed the CuO deposition modifying the dimensions of the CuO aggregates and the ratio between highly and scarcely interacting copper species with support, as revealed by complementary analyses. The redox character of the CuO phase was studied realizing cycles of programmed temperature reduction/oxidation (TPR-TPO) which gave the extent of CuO reduction and CuO re-oxidation. Deconvolution of the reduction profiles permitted identifying different copper species which presence depended on the support nature. Attempts were made to individuate relations between the properties of the CuO species and catalytic activity in NOx reduction with ethene (HC-SCR process) in highly oxidant atmosphere. The CuO phase deposited on the most acidic supports showed the best activity and selectivity in the NOxreduction.

Bulk and Surface Properties of Dispersed CuO Phases in Relation with Activity of NOx Reduction

Copper has been investigated as a promising alternative for SCR reactions. However, most Cu-based catalysts supported on zeolites, alumina or titania produced a certain amount of undesired N2O by-product. Herein, we report a strategy to tune the redox properties of copper oxides via confinement within titanate nanotubes (Cu/TNTs) and investigate their SCR activity as well as the N2 selectivity. The Cu/TNT catalyst showed an excellent NO reduction performance (above 90%) in the temperature window of 300–450 °C and the N2 selectivity could exceed 98% among the whole reaction temperature range of 150–470 °C, with a negligibly low concentration of N2O being detected. After systematic characterizations, the tuning of the chemical state of copper and oxygen, the remarkable adsorption capability, the accelerated oxidation of Cu+ to Cu2+, the lower level of NH3 oxidation and ultimately the tuning of redox properties were discovered. This work could provide a new approach to design SCR catalysts with superior catalytic reduction performance as well as excellent N2 selectivity.

Design strategies for SCR catalysts with improved N2 selectivity: the significance of nano-confining effects by titanate nanotubes

Using TiO2 as carrier, CuO/TiO2 catalysts with different CuO loading were prepared by the impregnation method. The catalytic activities in NO+CO reaction were examined with a micro-reactor gas chromatography reaction system and the methods of TPR, XPS and NO-TPD. It was found that the catalytic activities were affected by pretreatment atmosphere, i.e. H2 atmosphere > reduction–reoxidation > 10%CO/He > reaction gas (fresh sample). NO decomposition was better by low-valence Cu species than by high-valence Cu species, i.e. Cu0>Cu+>Cu2+. The XPS results indicated that Cu species on CuO/TiO2 were Cu0, Cu+, normal Cu2+(Cu2+(I)) and chain-structured Cu2+(Cu2+(II)) as –Cu–O–Ti–O–. The activities of Cu2+(II) were much higher than that of Cu2+(I), but both species were very unstable in the reaction atmosphere and easily reduced by CO, which accounted for the variable activities of fresh catalysts with increasing reaction temperature. In NO+CO reaction, the redox process was a cycle of Cu+–Cu2+(I) at low reaction temperature but was a cycle of Cu0–Cu+ at high reaction temperature. As shown by NO-TPD, high catalytic activities could be attributed to the following factors, e.g. oxygen caves on the catalyst’s surface after pretreatment with H2 and reduction–reoxidation, formation of Cu0 after pretreatment with H2, and increment of Cu species dispersion and formation of Cu2+(II) after pretreatment with reduction–reoxidation.

Effect of Pretreatment Atmosphere on CuO/TiO2 Activities in NO + CO Reaction

Two types of ESR spectrum (A and B) of exchange-coupled Cu2+ ions have been found for Cu-Ti-O catalysts at 77 K and 300 K. In associates A and B, the Cu2+ ions form a system with orbital ordering. The difference between the spectra is due to the difference between the ground states of adjacent Cu2+ ions in the associates: the ground states are \(d_{x^2 } \) and \(d_{y^2 } \) for A type associates and \(d_{x^2 - y^2 } \) and \(d_{z^2 - x^2 } \) for B type associates. The copper associates lie on the surface of the TiO2 (anatase) support microparticles.

Ordering of Cu(II) ions in supported copper-titanium oxide catalysts

The activity of Cu−Ti oxide catalysts has been studied in the selective catalytic reduction of NO with ammonia. We have elucidated how the nature ofstarting titania affects the catalyst activity in this reaction.

Effect of the nature of titania on the activity of Cu−Ti−O catalysts for selective catalytic reduction of nitrogen oxides with ammonia

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Effect of the nature of titania on the activity of Cu−Ti−O catalysts for selective catalytic reduction of nitrogen oxides with ammonia