Limei Cao

Conversion of inert cryptomelane-type manganese oxide into a highly efficient oxygen evolution catalyst via limited Ir doping

The oxygen evolution reaction (OER) is a critical half reaction for energy storage techniques and is regarded as a major challenge due to its sluggish kinetics and complex reaction mechanism. The traditional OER catalysts, such as IrO2, RuO2 and their binary or ternary oxides, have finite large-scale commercial applications due to their significant cost and rareness. Here, we hydrothermally synthesized cry-Ir by doping Ir into non-OER active cryptomelane-type manganese oxide to significantly reduce the Ir mass ratio by 60.3% from 85.7% in IrO2 to 34% in the developed catalyst, along with higher OER performance with a lower onset potential and 10 times higher specific mass activity. The special tunnel structure of cryptomelane plays an important role in promoting its OER activity through facilitating water molecular insertion into the tunnel. We combined Raman, XPS and TEM mapping to confirm that no IrO2 composite is present on the cry-Ir surface. The XPS and XAS spectra indicate substitution of Ir4+ on the Mn3+ site and the presence of more 5d states in the Ir site compared to IrO2. The differences in VBS spectra between cry-Ir, IrO2 and cry-Mn indicate that the electronic structure of Ir sites is modified when Ir substitutes Mn3+ sites. Thus, this special tunnel structure and modified Ir electronic structure in cry-Ir are responsible for the outstanding OER performance. Our studies provide an approach for designing effective Ir-based OER catalysts whilst significantly reducing the consumption of precious elements.

OER activity manipulated by IrO6 coordination geometry: an insight from pyrochlore iridates.

The anodic reaction of oxygen evolution reaction (OER), an important point for electrolysis, however, remains the obstacle due to its complicated reaction at electrochemical interfaces. Iridium oxide (IrO2) is the only currently known 5d transition metal oxide possessing admirable OER activity. Tremendous efforts have been carried out to enhance the activity of iridium oxides. Unfortunately there lies a gap in understanding what factors responsible for the activity in doped IrO2 or the novel crystal structure. Based on two metallic pyrochlores (Bi2Ir2O7 and Pb2Ir2O6.5) and IrO2. It has been found that there exists a strong correlation between the specific OER activity and IrO6 coordination geometry. The more distortion in IrO6 geometry ascends the activity of Ir sites, and generates activity order of Pb-Ir > IrO2 > Bi-Ir. Our characterizations reveal that distorted IrO6 in Pb-Ir induces a disappearance of J = 1/2 subbands in valence band, while Bi-Ir and IrO2 resist this nature probe. The performed DFT calculations indicated the distortion in IrO6 geometry can optimize binding strength between Ir-5d and O-2p due to broader d band width. Based on this insight, enhancement in OER activity is obtained by effects that change IrO6 octahedral geometry through doping or utilizing structural manipulation with nature of distorted octahedral coordination.

Pt-Doped NiFe2O4 Spinel as a Highly Efficient Catalyst for H2 Selective Catalytic Reduction of NO at Room Temperature

H2 selective catalytic reduction (H2-SCR) has been proposed as a promising technology for controlling NOx emission because hydrogen is clean and does not emit greenhouse gases. We demonstrate that Pt doped into a nickel ferrite spinel structure can afford a high catalytic activity of H2-SCR. A superior NO conversion of 96% can be achieved by employing a novel NiFe1.95Pt0.05O4 spinel-type catalyst at 60 °C. This novel catalyst is different from traditional H2-SCR catalysts, which focus on the role of metallic Pt species and neglect the effect of oxidized Pt states in the reduction of NO. The obtained Raman and XPS spectra indicate that Pt in the spinel lattice has different valence states with Pt(2+) occupying the tetrahedral sites and Pt(4+) residing in the octahedral ones. These oxidation states of Pt enhance the back-donation process, and the lack of filling electrons of the 5d band causes Pt to more readily hybridize with the 5σ orbital of the NO molecule, especially for octahedral Pt(4+), which enhances the NO chemisorption on the Pt sites. We also performed DFT calculations to confirm the enhancement of adsorption of NO onto Pt sites when doped into the Ni-Fe spinel structure. The prepared Pt/Ni-Fe catalysts indicate that increasing the dispersity of Pt on the surfaces of the individual Ni-Fe spinel-type catalysts can efficiently promote the H2-SCR activity. Our demonstration provides new insight into designing advanced catalysts for H2-SCR.

Circulating Regeneration and Resource Recovery of Flue Gas Desulfurization Residuals using a Membrane Electroreactor: From Lab Concept to Commercial Scale

Desulfurization residuals (using NaOH sorbent) were regenerated electrochemically, and at the same time sulfur in the flue gas was recovered as H(2)SO(4) and H(2) was produced as a clean energy. Since industrialization should always be the final goal to pursue for lab technologies and the evolution of pilot- and full-scale commercial reactors has taken place relatively slowly, this paper is aimed to develop an electroreactor on a sufficiently large scale to evaluate the application potential of the proposed regeneration process. The following key design parameters are discussed: (1) voltage distributions over electrode, membrane, and electrolyte; and (2) scaling up correlation based on lab-scale reactor operation parameters. Thereafter, in the developed reactor, the desulfurization residuals using NaOH sorbent from a semidry flue gas desulfurization (FGD) facility of a power plant in Shandong Province were regenerated and it is significant to note that the electrochemical efficiency of the designed reactor is comparable to that of the chlor-alkali industry, showing that the technology is environmentally friendly and economically feasible. If this technology is to be employed for FGD, the facility could be a profit-generating manufacturing part instead of a currently money-consuming burden for the plants.

Development and field-scale optimization of a honeycomb zeolite rotor concentrator/recuperative oxidizer for the abatement of volatile organic carbons from semiconductor industry.

The combined concentrator/oxidizer system has been proposed as an effective physical-chemical option and proven to be a viable solution that enables Volatile Organic Carbons (VOCs) emitters to comply with the regulations. In this work, a field scale honeycomb zeolite rotor concentrator combined with a recuperative oxidizer was developed and applied for the treatment of the VOC waste gas. The research shows the following: (1) for the adsorption rotor, zeolite is a more appropriate material than Granular Activated Carbon (GAC). The designing and operation parameters of the concentrator were discussed in detail including the size and the optimal rotation speed of rotor. Also the developed rotor performances was evaluated in the field; (2) Direct Fired Thermal Oxidizer (DFTO), Recuperative Oxidizer (RO), Regenerative Thermal Oxidizer (RTO) and Regenerative Catalytic oxidizer (RCO) are the available incinerators and the RO was selected as the oxidizer in this work; (3) The overall performance of the developed rotor/oxidizer was explored in a field scale under varying conditions; (4) The energy saving strategy was fulfilled by reducing heat loss from the oxidizer and recovering heat from the exhaust gas. Data shows that the developed rotor/oxidizer could remove over 95% VOCs with reasonable cost and this could be helpful for similar plants when considering VOC abatement.

Highly active and stable synergistic Ir–IrO2 electro-catalyst for oxygen evolution reaction

ABSTRACT The design of efficient and stable electrocatalysts is still crucial to realize oxygen evolution reaction (OER) in acid and corrosive environment. Inspired by the metal/metal oxides catalysts, we hydrothermally synthesized Ir–IrO2 composites duly confirmed by X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive spectrometer (EDS), X-ray absorption data, and transmission electron microscope results. A low onset over-potential of merely 234 mV and a Tafel slope of 53 mV dec−1 were obtained for the prepared catalyst Ir–IrO2_0.5AF. In addition, Ir–IrO2_0.5AF catalyst retained high activity for long time under constant potential polarization. The enhanced performance is attributed to the introduction of lower valence Ir as hydrogen acceptor: hydrogen transfers from –OOH intermediate in OER to adjacent H acceptor site, forming intermediate with relatively lower Gibbs free energy, and resulting in higher activity. The present study emphasizes on important role of the lower valence composition in launching H acceptors and providing wide opportunities toward rational designing of efficient and stable electro-catalysts.

Rational Manipulation of IrO2 Lattice Strain on α-MnO2 Nanorods as a Highly Efficient Water Splitting Catalyst

Developing more efficient and stable oxygen evolution reaction (OER) catalysts is critical for future energy conversion and storage technologies. We demonstrate that inducing a lattice strain in IrO2 crystal structure due to interface lattice mismatch enables an enhancement of the OER catalytic activity. The lattice strain is obtained by the direct growth of IrO2 nanoparticles on a specially exposed surface of α-MnO2 nanorods via a simple two-step hydrothermal synthesis. Interestingly, the prepared hydride OER activity increases with a lower IrO2 grown mass, which offers an opportunity to reduce the usage of precious iridium and ultimately obtains a specific mass activity of 3.7 times than that of IrO2 prepared under the same conditions and exhibits equivalent stability. The lattice mismatch in the underlying interface induces the formation of lattice strain in IrO2 rather than the charge transfer between the materials. The lattice strain changes are in good agreement with the order of the OER activity. Our experimental results indicate that using the special exposed surface substrates or tuning the supporting morphology structure can manipulate the catalyst materials lattice strain for the design of more efficient OER catalysts.

Limited Ce doped Ni–Co as a highly efficient catalyst for H2-SCR of NO with good resistance to SO2 and H2O at low temperature

The Ni1−xCexCo1.95Pd0.05O4 catalysts doped with limited Ce were fabricated using a sol–gel method and employed to investigate the SO2-resistance in the selective catalytic reduction of NO by hydrogen (H2-SCR). The results indicated that the inclusion of a limited amount of cerium not only effectively enhanced the catalytic activity of Ni1−xCexCo1.95Pd0.05O4 catalysts but also improved their resistance to SO2. Notably, when the Ce doping was 0.07 at%, the catalyst showed high sulfur resistance and exhibited a broader temperature window of 140–350 °C with a maximum NOX conversion of 98% at 190 °C. Also, addition of 5% H2O into the feed gas slightly decreased the deNOX performance of all catalysts, the NOX conversion nearly recovered to the initial level with H2O withdrawal, and the resistance to H2O also increased gradually in correspondence to the increase in Ce doping. Meanwhile, an increase of GHSV from 4800 to 9600 mL h−1 g−1, the NO/H2 ratios from 1 : 10 to 1 : 1 and the O2 content from 0% to 6%, gradually decreased the removal efficiency of NO. The XPD and XRS analysis depicted fine cubic spinel-type structures and Ce bearing two chemical valence states doped in the spinels. In the H2-TPR and NH3-TPD analysis, the Ce doping resulted in the temperature shift of reductive peaks and weak acid sites to lower temperature and the increase of strong acid sites. These results enhanced the redox property and led to a positive effect on SCR reaction, which were consistent with the SCR activity data.

Highly Efficient Oxygen Evolution Activity of Ca2IrO4 in an Acidic Environment due to Its Crystal Configuration

We systematically characterized the perovskite-like oxides Ca2IrO4 and CaIrO3 as the oxygen evolution reaction (OER) catalysts. Compared with IrO2, universally accepted as the current state-of-the-art OER catalyst, Ca2IrO4 showed an excellent OER catalytic activity in an acidic environment, whereas CaIrO3 did not. X-ray photoelectron spectroscopy (XPS) spectra showed that the oxidation of iridium in Ca2IrO4 and CaIrO3 was slightly beyond +4. X-ray absorption near-edge structure (XANES) spectra illustrated that the IrO6 octahedral geometric structure in Ca2IrO4 and CaIrO3 showed differences. The IrO6 octahedral is asymmetrically distorted and uniformly compressed in Ca2IrO4 and CaIrO3. Therefore, we propose that the IrO6 octahedral distortion plays an important role in the OER activities.