Reduction Half-cycle Research Articles

Low‐T CO Oxidation over Cu−CHA Catalysts in Presence of NH3: Probing the Mobility of CuII Ions and the Role of Multinuclear CuII Species

AbstractWe use dry CO oxidation to probe CuII dimers in Cu−CHA catalysts in the presence of ammonia. Samples with different Cu/Al and Si/Al ratios were exposed to CO in He at 200 °C after either one of two catalyst pre‐treatments: (i) pre‐oxidization in 8 % O2/He at 550 °C for 1 hour; (ii) pre‐oxidation in 8 % O2/He at 550 °C for 1 hour followed by NH3 saturation at 200 °C in O2. Pre‐adsorbed NH3 was necessary to observe a significant CO2 evolution, the rate of CO2 formation being second order in the CuIIOH fraction of the CuII sites. This is direct evidence that NH3, not involved in the CO oxidation chemistry, favours the formation of CuII dimers with removable oxygen by ligating with and mobilizing isolated CuIIOH ions, otherwise inactive. These findings are relevant for the low‐temperature NH3−SCR redox mechanism over Cu−CHA, in which CuII dimers may play a role in parallel or in alternative to the isolated CuII sites so far proposed for the reduction half cycle of standard SCR.

In Situ Spectroscopic Studies on the Redox Cycle of NH3−SCR over Cu−CHA Zeolites

AbstractThe selective catalytic reduction of NO with ammonia (NH3−SCR) catalyzed by Cu−CHA zeolites is thoroughly investigated using in situ spectroscopic experiments combined with on‐line mass spectroscopy (MS) under steady‐state NH3−SCR conditions and transient conditions for Cu(II)/Cu(I) redox cycles. Quantitative analysis of the in situ XANES spectra of Cu−CHA under steady‐state conditions of NH3−SCR show that NH3‐coordinated Cu(II) species is the dominant Cu species at low temperatures (100–150 °C). At higher temperatures, Cu(II) species and [Cu(NH3)2]+ complex coexist, possibly because the rate of the Cu(II)→Cu(I) reduction step is comparable to that of the Cu(I)→Cu(II) oxidation step. In situ XANES, IR/MS, and UV‐vis/MS experiments on the reduction half cycle demonstrate that the reduction of Cu(II) species occurs via the reaction of NH3‐liganded Cu(II) with NO to yield N2 and H2O. For the oxidation half cycle, in situ XANES experiments of Cu(I) oxidation in 10 % O2 at 200 °C indicate that an increased density in CHA zeolite exhibits a higher oxidation rate. In situ UV‐vis experiments of Cu(I) reoxidation using different mixtures of oxidant feed gas demonstrate the key role of O2 in the oxidation cycle. It is suggested that the reoxidation of Cu(I) to Cu(II) species occurs with only O2 as the oxidant, and a high Cu density in CHA zeolite promotes SCR activity by enhancing the oxidative activation of Cu(I) to Cu(II) during the catalytic cycle.

Computational Insights into the Mechanism of the Selective Catalytic Reduction of NO x : Fe- versus Cu-Exchanged Zeolite Catalysts.

We computationally investigate the mechanism of the reduction half-cycle of the selective catalytic reduction of nitrogen oxides with ammonia. We compare both Fe- and Cu-exchanged zeolite catalysts and aim at exploring all accessible reaction pathways. From our calculations, a comprehensive picture emerges that unifies several previous mechanistic proposals. We find that both for Fe and for Cu catalysts different reaction pathways are feasible but some of the possible reaction pathways differ in these two cases. Our computational results provide a basis for the interpretation of in situ spectroscopic investigations that can possibly distinguish the different mechanistic pathways.

Pressure-induced deactivation of core-shell nanomaterials for catalyst-assisted chemical looping

Catalyst-assisted chemical looping dry reforming is a promising technology for CO-rich syngas production with maximized CO2 utilization, especially when performed auto-thermally, over core-shell nanomaterials in a double-zone reactor bed with first a Fe/Zr@Zr-Ni@Zr bifunctional catalyst and next a Fe/Zr@Zr oxygen storage material. Understanding the origin of the material deactivation under high-pressure conditions is essential to advance this technology towards industrial application. Therefore, pressure-induced material deactivation was studied through a series of on-site assessments (steady-state CH4 reforming and prolonged redox cycling at 750 °C and 1–10 bar) and ex-situ characterization (STEM-EDX, XRD and N2 adsorption-desorption).At high pressure, both Fe/Zr@Zr-Ni@Zr and Fe/Zr@Zr show reaction-related deactivation, the origin of which can be ascribed to carbon deposition and particle sintering, respectively. During regular catalyst-assisted dry reforming over Fe/Zr@Zr-Ni@Zr, the rise of pressure decreases CH4 conversion and increases carbon deposition on the Ni surface. Rapidly growing carbon filaments destroy the core-shell structure, resulting in segregation of the Ni-based particles from the catalyst bulk with concomitant severe sintering. During H2/CO2 redox cycling of Fe/Zr@Zr, an increased pressure decreases the time-averaged space-time yield of CO. In the reduction half-cycle, high pressure prolongs the existence of the FeO intermediate phase in the transformation of Fe/Zr@Zr, which decreases the material’s melting point, leading to fast sintering. Adding a small amount of O2 makes the chemical looping dry reforming process auto-thermal, which is advantageous in eliminating carbon. Nevertheless, deactivation of the double-zone reactor bed still occurs and is mainly ascribed to particle sintering, following similar principles as mentioned above.For both regular and auto-thermal catalyst-assisted chemical looping dry reforming, the decreasing ability for CO2 utilization is naturally due to the deactivation of the oxygen storage material, but also controlled by the stability of the bifunctional catalyst. The latter determines the reduction capacity of the gas product mixture in the reduction half-cycle, thereby affecting the achievable reduction degree of the oxygen storage material.

Catalyst-assisted chemical looping auto-thermal dry reforming: Spatial structuring effects on process efficiency

Catalyst-assisted chemical looping auto-thermal dry reforming (CCAR) is an environment-friendly energy conversion process, performed over a reactor bed with double function, composed of a catalyst and an oxygen storage material (OSM). It converts CH4 and CO2 into industrial syngas, while simultaneously utilizing CO2 from the atmosphere. Two reactor bed configurations were tested, based on the concept of double- and single-zone distribution of catalyst and OSM. Combinations of core-shell structured materials were applied, such as Ni/ZrO2@ZrO2 catalyst, Fe2O3/ZrO2@ZrO2 OSM and Fe/Zr@Zr-Ni@Zr bifunctional catalyst, to assess the spatial structuring at both reactor bed and pellet scale. Samples from different reactor beds were characterized before and after use by ex- or in-situ XRD, N2 adsorption, XPS and STEM-EDX. 25 redox cycles of CCAR were performed to investigate the effect of spatial structuring on the activity and stability. The Fe/Zr@Zr-Ni@Zr bifunctional catalyst possesses higher activity and stability for catalytic CH4 conversion in the reduction half-cycle than the Ni/ZrO2@ZrO2 catalyst due to its small Ni particle size (<3 nm), high carbon resistance and thermal stability of the eccentric core-shell structure. A double-zone bed with Ni/ZrO2@ZrO2 catalyst and Fe2O3/ZrO2@ZrO2 OSM has a similar activity as a single-zone bed for producing syngas in the reduction half-cycle, but it presents a higher CO yield in the re-oxidation half-cycle. This is due to the full use of the oxygen storage capacity of the OSM achieved by complete reduction of Fe3O4 and the avoiding of Ni-Fe alloy formation during the reduction half-cycle. A double-zone bed with bifunctional catalyst and OSM combines both advantages. It provides the highest activity and stability for auto-thermal dry reforming and the highest oxygen storage capacity for CO2 utilization, offering a promising reactor technology for the CCAR process.

Selective Catalytic Reduction over Cu/SSZ-13: Linking Homo- and Heterogeneous Catalysis.

Active centers in Cu/SSZ-13 selective catalytic reduction (SCR) catalysts have been recently identified as isolated Cu2+ and [CuII(OH)]+ ions. A redox reaction mechanism has also been established, where Cu ions cycle between CuI and CuII oxidation states during SCR reaction. While the mechanism for the reduction half-cycle (CuII → CuI) is reasonably well-understood, that for the oxidation half-cycle (CuI → CuII) remains an unsettled debate. Herein we report detailed reaction kinetics on low-temperature standard NH3-SCR, supplemented by DFT calculations, as strong evidence that the low-temperature oxidation half-cycle occurs with the participation of two isolated CuI ions via formation of a transient [CuI(NH3)2]+-O2-[CuI(NH3)2]+ intermediate. The feasibility of this reaction mechanism is confirmed from DFT calculations, and the simulated energy barrier and rate constants are consistent with experimental findings. Significantly, the low-temperature standard SCR mechanism proposed here provides full consistency with low-temperature SCR kinetics.

CO2 conversion to CO by auto-thermal catalyst-assisted chemical looping

A bifunctional 9wt.%NiO-16wt.%Fe2O3/MgAl2O4 material was prepared for CO2 conversion to CO by auto-thermal catalyst-assisted chemical looping. This process is designed to maximize CO2 conversion. The generation of CO from CO2 was investigated between 873K and 1023K. The high endothermicity of methane dry reforming and the material deactivation by coke deposition were avoided by the simultaneous feeding of CH4, CO2 and O2 in a 1:1:0.5 molar ratio during the reduction half-cycle of chemical looping. In this half-cycle, interaction of Ni with Fe leads to Ni-Fe alloy formation. The resulting Ni-based catalyst converts CH4+CO2+O2 into a mixture of CO and H2, which both reduce Fe3O4, producing CO2 and H2O. In the CO2 re-oxidation half-cycle, CO is produced and the Ni-Fe alloy decomposes into Ni and Fe3O4. The reduction capacity (Rc) of the gas mixture strongly depends on the ratio Rc between reducing and oxidizing gases. Based on thermodynamic calculations, high conversion of Fe3O4 to reduced state can be reached if Rc>2 and T>873K. During prolonged auto-thermal chemical looping at 1023K, the 9wt.%NiO–16wt.%Fe2O3/MgAl2O4 suffers from deactivation in the first five cycles, after which a more stable operation is established. Based on TEM measurements, sintering was found to be the main cause for the initial decrease of CO production.

Performance characteristics of a miniaturised chemical looping steam reformer for hydrogen enrichment of fuels

Abstract A miniaturised chemical looping steam reformer (μ-CLSR) is being developed at the University of Newcastle (Australia) for onboard hydrogen enrichment of fuels in internal combustion engines. Experimental results from the first prototype highlighted an imbalance between the rates of redox reactions. A new configuration has been developed to resolve this shortcoming. The objective of this study is to examine the effectiveness of the new configuration in handling the imbalance between redox rates. Experiments were conducted on magnetite and wuestite metal oxides under methane and steam environments in the new configuration. Results indicate that if micro-reactors within the reformer are operated in parallel during the oxidation half cycle and in sequence during the reduction half cycle, the imbalance can be effectively managed. This combined with manifold switching of reacting gases at 48 s intervals enables metal oxides to achieve a 30% fractional conversion at 900 °C; resulting in a product gas stream with 96% H2 purity.

Redox Characteristics of Fe–Ni/SiO2Bimetallic Oxygen Carriers in CO under Conditions Pertinent to Chemical Looping Combustion

This study is concerned with the reduction/oxidation (redox) behavior of bimetallic oxygen carries of Ni and Fe under conditions pertinent to chemical looping combustion (CLC). Bimetallic oxygen carriers of Ni- and Fe-based metal oxides with different mixture ratios were developed through the incipient wet impregnation (IWP) method by employing SiO2 as the support. The redox properties of metal oxide mixtures were determined in a magnetic balanced thermogravimetric analyzer (TGA) over a range of temperatures between 500 and 950 °C at atmospheric pressure under alternating reduction and oxidation half cycles. The reduction half cycle was carried out in the presence of CO, CO2, and N2 mixtures with different volume fractions of CO (typically 5, 10, and 70%), while the oxidation half cycle was conducted in normal air. Oxygen carriers exhibited temperature-dependent reactivity during both reduction and oxidation half cycles. The reactivity of monospecies Ni-based oxides on the silica support under the reducin...

Redox doping behaviour of poly(3,4-ethylenedithiothiophene) – The counterion effect

Poly(3,4-ethylenedithiothiophene) – PEDTT, an alkylene sulphur derivative of PEDOT, presents itself as an interesting polymer with a number of disparate redox and chromic properties compared to its close analogue – PEDOT. In this study we present the results of an investigation into the electrochemical doping process of PEDTT, using four different electrolyte solutions, differing in anion content of the chosen salt. The results show that the anion identity plays a key role in the redox reactions accompanying these processes in what could be interpreted as anion ionochromism. In situ UV–Vis spectroelectrochemical experiments reveal an intriguing double electrochromic transition of PEDTT films during their oxidative doping, going from golden-yellow through green to pomegranate – a quality not so common within the family of electroactive conjugated polymers. The evolution of each UV–Vis spectrum over a potential range indicates that different redox states of the polymer are responsible for the chromatic changes. In the reduction half-cycle, the dedoping process of PEDTT appears to follow a path dissimilar to the p-doping one, featuring only one, direct electrochromic transition of the film’s colour, bypassing the green state, and a distinct two-step bleaching process of doping-induced charge carrier bands. The observed electrochemical and spectral phenomena have been accredited to the specific redox behaviour of doping-induced radical cation and cationic defect states interacting with the dithioalkylene sulphur atom.

In-situ ESR spectroelectrochemical studies of overoxidation behaviour of poly(3,4-butylenedioxythiophene)

The behaviour of poly(3,4-butylenedioxythiophene) (PBuDOT), a relative of poly(3,4-ethylenedioxythipohnene) PEDOT within the poly(3,4-alkylenedioxythiophene) family, has been investigated at potentials above its electrochemical stability threshold using in situ ESR spectroelectrochemistry. The aim was to investigate the effect of electrochemical overoxidation on the charge carrying species, namely polarons, normally generated and annihilated during reversible redox doping and dedoping reactions, by determining the potential dependencies of spectroscopic parameters of the ESR spectra of the polymer over a selected potential range. Specific features of the trends of these dependencies allowed also for an evaluation of presence of the second type of charge carrying species—diamagnetic bipolarons and the effects of their interactions with polarons at different potentials. Around 1.5 V, where the boundary of electrochemical stability of the polymer lies, sharp drop of the concentration of paramagnetic centres has been observed together with a transitory narrowing of the ESR line. These changes were found to be irreversible as evidenced by the course of subsequent reduction half-cycle, which differed from the one for a not overoxidised polymer, observed in previous studies. Aided by the results of electrochemical studies it was concluded that the overoxidation process leads to a degradation of the polymer most probably due decrease of the conjugation length of the main chain π-bond through cross-linking or addition reactions. While the electrochemical results pointed to a non-complete degradation of the polymer, the specific parameters of the ESR line in the reduction half-cycle indicate that the remaining spins are confined to isolated segments of a partially degraded polymer where their behaviour resembles oligomer-like radicals.

Stabilization of a Subvalent Oxidation State of Bismuth in N,N-Dimethylthioformamide Solution: An EXAFS, UV−Vis, IR, and Cyclic Voltammetry Study

At the dissolution of anhydrous bismuth(III) trifluoromethanesulfonate in N,N-dimethylthioformamide (DMTF) a deep red-orange complex, lambda(max) = 457 nm, is formed. Bismuth(III) is reduced by the solvent to a low-valent oxidation state stabilized by the sulfur-coordinating solvent DMTF. The obtained complex is weakly solvated seen by a low EXAFS amplitude and a slightly higher absorption energy of the L(III) edge than of the DMTF-solvated bismuth(III) ion. The EXAFS data reveal a dimeric bismuth complex solvated by a single DMTF molecule, which sulfur atom bridges the bismuth atoms. The Bi-S bond distance is 2.543(2) A, and the Bi...Bi distance is 3.929(7) A giving a Bi-S-Bi angle of 101.2(4) degrees. The very low number of coordinated solvent molecule shows that the lone electron pairs of the reduced bismuth ions are stereochemically active. Cyclic voltammetry investigations provide evidence that at least one bismuth atom in the dimer exists in an oxidation state lower than +III, seen by two peaks at approximately -0.36 and -0.57 V in the reduction half-cycle. The absence of EPR signals excludes the presence of bismuth(II) radicals.

Electrochemical insertion of lithium and sodium into (MoO 2) 2P 2O 7

Insertion of lithium and sodium into phosphate (MoO 2) 2P 2O 7 was investigated electrochemically to determine the usefulness as a possible cathode for ion-transfer secondary batteries. Specific charges of up to 250 mA h g −1 were obtained for A/(MoO 2) 2P 2O 7 (A: Li, Na) cells with liquid organic electrolytes in the first reduction half-cycle at room temperature. Intercalation processes under constant current densities of 0.2 mA cm −2 were reversible within the range of composition 0.85< x<4.0 for lithium and 0.5< x<3.1 for sodium in A x (MoO 2) 2P 2O 7 (A: Li, Na), respectively. Structural changes induced by lithium or sodium intercalation were followed by ex situ X-ray diffraction measurements, and the phase change from the crystal to the amorphous was observed in both cases.

Electrochemical insertion of lithium, sodium, and magnesium in molybdenum(VI) oxide

The electrochemical insertion of divalent magnesium cations into orthorhombic molybdenum(VI) oxide was studied with regard to their use as electroactive species in ion-transfer battery systems and compared to the insertion of Li + and Na +. Specific charges of up to 300 and 240 Ah kg −1 were obtained in organic, propylene carbonate-based electrolytes for the lithium and sodium insertion, respectively, in the first reduction half-cycle. Reversible Mg 2+ insertion could be demonstrated in a room temperature molten salt electrolyte consisting of 3 wt.% MgCl 2, 41 wt.% 1-ethyl-3-methylimidazolium chloride, and 56 wt.% AlCl 3. Specific charges of up to 160 Ah kg −1 were obtained with MoO 3 in the first reduction half-cycle. The Mg 2+ insertion process can be enhanced using an organic electrolyte with traces of H 2O. In 1 M Mg(ClO 4) 2/acetonitrile with 3 mol% of H 2O, specific charges of up to 210 Ah kg −1 were measured in the first reduction.