CO2 Self-Poisoning and Its Mitigation in CuO Catalyzed CO Oxidation: Determining and Speeding up the Rate-Determining Step

Cu-based catalysts are promising for CO oxidation applications with catalyst deactivation being a major barrier. We start with a CuO/Al2O3 catalyst and find that while the CO conversion decreases, CO2 accumulates and the average Cu chemical state stays the same. It suggests CO2 self-poisoning, i.e., CO2 desorption is the rate-determining step. Subsequently, experiments are performed to prove this hypothesis by showing (1) CO2 adsorption inhibits O2 adsorption, (2) complete desorption of CO2 regenerate the catalyst, (3) pre-adsorbed CO2 quenches catalyst activity which recovers during the reaction and (4) the apparent activation energy is consistent with CO2 desorption. It is further evidenced by using a stronger CO2 adsorbing support CeO2 to speed up CO2 desorption from the CuO sites resulting in a superior CuO/CeO2 catalyst. It provides an example for experimentally deciding and speeding up the rate-determining step in a catalytic reaction.

Active Reaction Control of Cu Redox State Based on Real-Time Feedback from In Situ Synchrotron Measurements

We achieve a target material state by using a recursive algorithm to control the material reaction based on real-time feedback on the system chemistry from in situ X-ray absorption spectroscopy. Without human intervention, the algorithm controlled O₂:H₂ gas partial pressures to approach a target average Cu oxidation state of 1+ for γ-Al₂O₃-supported Cu. This approach represents a new paradigm in autonomation for materials discovery and synthesis optimization; instead of iterating the parameters following the conclusion of each of a series of reactions, the iteration cycle has been scaled down to time points during an individual reaction. Application of the proof-of-concept illustrated here, using a feedback loop to couple in situ material characterization and the reaction conditions via a decision-making algorithm, can be readily envisaged in optimizing and understanding a broad range of systems including catalysis.

A density functional theory study on reduction-induced structural transformation of copper-oxide-based oxygen carrier

A clear understanding of the structural transformation of copper-oxide-based oxygen carriers accompanying their reduction by fuels helps to design more efficient oxygen carriers for chemical looping combustion. Herein, density functional theory calculations have been performed on the bulk CuO, CuO(111) surface, and (CuO)₃₂ cluster models with the same number of CuO molecular units to investigate structural transformation accompanying the reduction. The results showed that the averaged reaction energies of desorbing an oxygen molecule from the bulk and surface models are roughly the same [246.2 kJ/(mol O₂) and 245.9 kJ/(mol O₂), respectively]. The slab model does not significantly lower the overall reaction energy compared to the bulk model. In contrast, the averaged reaction energy using the cluster model is significantly lower [127.5 kJ/(mol O₂)] than that of bulk and slab models. The key structural difference is the obvious Cu-Cu bond formation in the cluster model, which would result in nucleation of a metallic Cu phase. The results also showed that different states can be reached by desorbing different number oxygen atoms in a single step, corresponding to different reaction rates, when the system reaches the same level of reduction. These results demonstrate the complexity of reactions involving solid state materials and are consistent with the structural diversity observed experimentally. This study illustrates the importance of particle sizes and reaction conditions in the formation of suboxides during CuO reduction.

Reshaping CuO on silica to generate a highly active Cu/SiO2 catalyst

Reduction–oxidation treatment [RO] is effective to reshape CuO within a confined area on SiO₂ support, forming highly dispersed nano CuO particles. The “shape” of CuO can be memorized during the activation process, resulting in the formation of a specific Cu metal and thus demonstrating enhanced catalytic activity.

Catalyst for the degradation of 1,1-dimethylhydrazine and its by-product N-nitrosodimethylamine in propellant wastewater

A three-component metal catalyst was prepared and used in the process of catalytic wet peroxide oxidation (CWPO) for the degradation of unsymmetrical dimethylhydrazine (UDMH) in propellant wastewater with H₂O₂.

Unusual Physical and Chemical Properties of Cu in Ce1-xCuxO2 Oxides

The structural and electronic properties of Ce(1-x)Cu(x)O(2) nano systems prepared by a reverse microemulsion method were characterized with synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, Raman spectroscopy, and density functional calculations. The Cu atoms embedded in ceria had an oxidation state higher than those of the cations in Cu(2)O or CuO. The lattice of the Ce(1)(-x)Cu(x)O(2) systems still adopted a fluorite-type structure, but it was highly distorted with multiple cation-oxygen distances with respect to the single cation-oxygen bond distance seen in pure ceria. The doping of CeO(2) with copper introduced a large strain into the oxide lattice and favored the formation of O vacancies, leading to a Ce(1-x)Cu(x)O(2-y) stoichiometry for our materials. Cu approached the planar geometry characteristic of Cu(II) oxides, but with a strongly perturbed local order. The chemical activities of the Ce(1-x)Cu(x)O(2) nanoparticles were tested using the reactions with H(2) and O(2) as probes. During the reduction in hydrogen, an induction time was observed and became shorter after raising the reaction temperature. The fraction of copper that could be reduced in the Ce(1-x)Cu(x)O(2) oxides also depended strongly on the reaction temperature. A comparison with data for the reduction of pure copper oxides indicated that the copper embedded in ceria was much more difficult to reduce. The reduction of the Ce(1-x)Cu(x)O(2) nanoparticles was rather reversible, without the generation of a significant amount of CuO or Cu(2)O phases during reoxidation. This reversible process demonstrates the unusual structural and chemical properties of the Cu-doped ceria materials.

Reduction of CuO and Cu2O with H2: H embedding and kinetic effects in the formation of suboxides

Time-resolved X-ray diffraction, X-ray absorption fine structure, and first-principles density functional calculations were used to investigate the reaction of CuO and Cu(2)O with H(2) in detail. The mechanism for the reduction of CuO is complex, involving an induction period and the embedding of H into the bulk of the oxide. The in-situ experiments show that, under a normal supply of hydrogen, CuO reduces directly to metallic Cu without formation of an intermediate or suboxide (i.e., no Cu(4)O(3) or Cu(2)O). The reduction of CuO is easier than the reduction of Cu(2)O. The apparent activation energy for the reduction of CuO is about 14.5 kcal/mol, while the value is 27.4 kcal/mol for Cu(2)O. During the reduction of CuO, the system can reach metastable states (MS) and react with hydrogen instead of forming Cu(2)O. To see the formation of Cu(2)O, one has to limit the flow of hydrogen, slowing the rate of reduction to allow a MS --> Cu(2)O transformation. These results show the importance of kinetic effects for the formation of well-defined suboxides during a reduction process and the activation of oxide catalysts.

Preparation of highly dispersed CuO catalysts on oxide supports for de-NO(x) reactions

CuO based catalysts dispersed on silica-alumina supports at low (0.56 wt.%) and high (13 wt.%) Al(2)O(3) content were prepared by adsorption method with or without ultrasound treatment. The catalysts obtained were studied in their bulk (atomic absorption, X-ray diffraction, temperature programmed reduction) and surface (N(2) adsorption and X-ray photoelectron spectroscopy) properties. Significant differences between the series of catalysts prepared over the two supports in terms of size of the CuO aggregates and of their redox properties were evidenced. All the catalysts were tested in the selective catalytic reduction of NO(x) using C(2)H(4) as reducing species (HC-SCR process) in highly oxidant atmosphere. The CuO-catalysts prepared using ultrasounds were the most active. Moreover, they displayed a peculiar activity being able to activate NO both by reducing it to N(2), in larger extent, and by oxidizing it to NO(2).

In situ EXAFS analysis of the temperature-programmed reduction of Cu-ZSM-5

High resolution in situ EXAFS during temperature-programmed reduction was performed on Cu-ZSM-5 to elucidate the state of copper under reaction conditions. Improvements in hardware and software allowed rapid acquisition of both XANES and EXAFS data during reduction, in particular, allowing observation of characteristic preedge features from various Cu oxidation states. EXAFS fitting and factor analysis of the normalized XANES edge were performed in an attempt to determine the number and type of Cu species present. The data suggests that initially only Cu(2+) is present in two different locations on the zeolite; both states reduce to Cu(1+) in both H(2) and CO, but under different conditions. Under H(2) conditions, migration of Cu(1+) to Cu(0) clusters is observed at 450 degrees C, while no metallic state is observed during CO reduction.