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The replacement of precious metals (Rh, Pd, and Pt) in three-way catalysts with inexpensive and earth-abundant metal alternatives is an ongoing challenge. In this research, we examined various quaternary metal catalysts by selecting from six 3d transition metals, i.e., Cr, Mn, Fe, Co, Ni, and Cu, equimolar amounts (0.1 mol each), which were prepared on the Al

In this study, several nanometer-thick transition metal overlayers (M) were formed on an Fe–Cr–Al metal (SUS) foil by pulsed cathodic arc-plasma deposition. The thin-film materials prepared, exhibiting a full-coverage M overlayer (M/SUS), were applied as catalysts for NH3 oxidation in the presence of excess O2 (300 ppm NH3, 8% O2, and He balance). Under dry conditions (0% H2O in the gas feed), the observed NH3 oxidation activity decreased in the order of Ir > Pt > Co > Pd > Cu > Rh when the product selectivity was not taken into consideration. The activities of the materials composed of Co and Cu decreased significantly in the presence of 10% H2O, while those of the catalysts containing Pt and Ir remained nearly unchanged. Based on the kinetic analyses and X-ray photoelectron spectrometry experiments conducted, it was determined that the contrasting effect of H2O on the activity of M/SUS was related to the ease of oxidation of the non-precious metals studied. The resulting hydrophilic oxide surface was more susceptible to the competitive adsorption of H2O and thus inhibited the NH3 reaction than a metallic surface. The negative effect of H2O was evident by the decrease in the NH3 concentration in the gas feed. Notably, the metallic surface of the Pt overlayer was stable even under oxidizing reaction conditions; therefore, the oxidation of NH3 was less susceptible to the effects of high H2O concentrations.

A series of tetravalent metal pyrophosphates (MP2O7; M = Si, Ti, and Zr) were studied as support materials for rhodium (Rh) catalysts to elucidate the effect of the M–O–P bond covalency on the inte...

A nanometric Rh overlayer on a Fe − Cr − Al metal foil prepared by pulsed arc-plasma deposition showed simultaneous removal of NO, CO and C3H6 in a simulated three-way catalyst reaction under stoichiometric conditions, whereas the conversions of NO and C3H6 decreased under lean conditions. The decrease of C3H6 conversion despite of excess O2 was caused by the preferential adsorption of O2 on the surface of Rh overlayer, inhibiting C3H6 adsorption and oxidation. Very small amount of Pt deposited on Rh overlayer can act as adsorption sites of C3H6, and the activity of Rh overlayer for C3H6 oxidation under lean conditions was successfully improved.

The redox behavior of Pd/CeO2–ZrO2 (Pd/CZ) model three-way catalysts with different particle sizes was studied by in situ diffuse reflectance spectroscopy under the lean-rich perturbation condition. To estimate the contribution of three-phase boundary (TPB) between Pd, CZ, and gas phase, a kinetic analysis of the Pd redox was performed using time-resolved reflectance data. Because the Pd oxidation upon rich-to-lean switching was strongly retarded by oxygen storage in CZ, the ratio of apparent initial rate constants of the Pd redox, kR→L/kL→R, was significantly correlated with the presence of TPB. When both Pd and CZ particles are so small that most Pd is close to TPB, the retardation effect is most obvious (kR→L/kL→R ∼ 0.3). Increasing particle sizes of not only Pd but also CZ reduces the abundance of TPB and thus the retardation effect is less pronounced (kR→L/kL→R > 1). The rate constant ratio provides a useful index of the degradation of TPB to estimate the extent of deactivation of the three-way catalyst via thermal sintering.

RF reflectometry is a promising technique for spin qubit readout, suitable for large-scale integrated qubit systems by combination with multiplexing techniques and gate-based readout. However, one of the challenges in such systems would be that the accuracy of RF readout of individual qubits can be degraded by crosstalk among dense RF readout lines. In this study, we propose a mixed-mode RF reflectometry to reduce the effect of the crosstalk and verify its effectiveness by electromagnetic field simulations. The results of the simulations show the possibility of suppressing the influence of crosstalk by using mixed modes.

Alkaline earth vanadates (Ae–V: Ae = Ca, Sr, and Ba) were supported on mesoporous SiO2 by a wet impregnation method. The catalytic activity of the prepared materials for the decomposition of SO3 in...

Expectations for industrial implementation of direct conversion of CH4 to CH3OH are growing with the increasing demand for energy-efficient chemical processes. In this study, catalytic production of CH3OH by direct oxidation of CH4 with O2 was performed using Cu zeolite catalysts in a CH4/O2/H2O flow reaction. Among the various Cu zeolites investigated, the Cu-MOR catalyst exhibited relatively high CH3OH production with a turnover number of 7.4 molCH3OH molCu−1 over 24 h (CH4 conversion: 0.011%). The catalytically active Cu species and catalytic cycle were investigated by in situ simultaneous X-ray absorption and infrared spectroscopy. The results suggest that the key to the catalytic cycle over Cu-MOR is the redox of Cu(I)/Cu(II) species. Furthermore, H2O-adsorption-included dynamic Cu species are revealed to be catalytically active.

Direct oxidation of CH4 to CH3OH using O2 is challenging because of the high stability of CH4 and the relatively high reactivity of CH3OH. Here, Cu-CHA zeolites are tested for the direct oxidation of CH4. Catalytic production of CH3OH in a CH4–O2–H2O flow reaction is improved using CHA type zeolites compared to other zeolites including MOR, BEA, MFI, and FAU zeolites reported previously. In situ X-ray absorption fine structure (XAFS) spectroscopy reveals that the high catalytic activity of Cu-CHA is derived from its redox properties, particularly, the high reducibility of Cu2+ involved in CH4 activation. The reaction gas concentrations are varied to find the optimized reaction conditions on Cu-CHA. In addition, the reaction mechanism of the direct CH4 oxidation on Cu-CHA is investigated based on not only the effects of gas concentrations but also the isotope gas effects using CD4, 18O2, and D2O. It is suggested that the rate-determining step is the C–H activation of CH4, and the selectivity of CH3OH is determined by the oxidation rate of CH3OH, which is affected by the O2 and OH group activation rate on Cu-CHA.

Direct design of low temperature oxidative coupling of methane (OCM) catalysts is proposed via machine learning and data mining. 58 OCM catalysts are experimentally synthesized and evaluated. The collected 58 sets of data are then classified by unsupervised machine learning in a multi-dimensional space where an active catalyst group for low temperature OCM is identified. Data mining then identifies the physical rule within the group. Catalysts satisfying such a physical rule are designed where 2 undiscovered low temperature OCM catalysts are found and experimentally validated. Thus, machine learning and data mining reveal the hidden physical rule behind the catalysis leading to the direct design of catalysts. Hence, machine learning and data mining open up the insight on a powerful strategy for designing catalysts.

In the present study, we prepared a 12 nm thick Ir overlayer via pulsed cathodic arc plasma deposition on a 50 μm thick Fe-Cr-Al metal (SUS) foil. Using this thin-film catalyst made NH3-O2 reactions more environmentally benign due to a much lower selectivity for undesirable N2O ( 70-fold greater than that observed with conventional Ir nanoparticle catalysts supported on γ-Al2O3. We found that the high-TOF NH3 oxidation was associated with the stability of the metallic Ir surface against oxidation by excess O2 present in simulated diesel exhaust. Additionally, we found that the Ir overlayer structure was thermally unstable at reaction temperatures ≥400 °C and at which point the Ir surface coverage dropped significantly; however, thermal deterioration was substantially mitigated by inserting a 250 nm thick Zr buffer layer between the Ir overlayer and the SUS foil substrate (Ir/Zr/SUS). Although N2O formation was suppressed by NH3 oxidation over Ir/Zr/SUS, other undesired byproducts (i.e., NO and NO2) were readily converted to N2 by coupling with a V2O5-WO3/TiO2 catalyst in a second reactor for selective catalytic reduction by NH3. These results demonstrated that this tandem reactor configuration converted NH3 to N2 with nearly complete selectivity at a range of 200-600 °C in the presence of excess O2 (8%) and H2O (10%).

The thermal deactivation of Pd/CeO2-ZrO2 (Pd/CZ) three-way catalysts was studied via nanoscale structural characterization and catalytic kinetic analysis to obtain a fundamental modeling concept for predicting the real catalyst lifetime. The catalysts were engine-aged at 600-1100 °C and used for chassis dynamometer driving test cycles. Observations using an electron microscope and chemisorption experiments showed that the Pd particle size significantly changed in the range of 10-550 nm as a function of aging temperatures. The deactivated catalyst structure was modeled using different-sized hemispherical Pd particles that were in intimate contact with the support surface. Therefore, Pd/CZ contained two types of surface Pd sites residing on the surface of a hemisphere (Pds) and circular periphery of the Pd/CZ interface (Pdb), whereas a reference catalyst, Pd/Al2O3, contained only Pds. In all Pd particle sizes investigated herein, Pd/CZ exhibited higher reaction rates than Pd/Al2O3, which nonlinearly increased with increasing slope as the weight-based number of surface-exposed Pd atoms ([Pds] + [Pdb]) increased. This finding contrasted with that of Pd/Al2O3, where the reaction rate linearly increased with [Pds]. When the Pds sites in both catalysts were equivalent in terms of their specific activities, the activity difference between Pd/CZ and Pd/Al2O3 corresponded to the contribution from Pdb, where oxygen storage/release to/from CZ played a key role. This contribution linearly increased with [Pdb] and therefore decreased with Pd sintering. Although both Pds and Pdb sites showed nearly constant turnover frequencies despite the difference in the Pd particle size, the values for Pdb were more than 2 orders of magnitude greater than those for Pds when assuming a single-atom width one-dimensional Pdb row model. These results suggest that the thermal deterioration of the three-phase boundary site, where Pd, CZ, and the gas phase meet, determines the activity under surface-controlled conditions.

A platinum-group-metal-free catalyst comprising Fe-Ni alloy nanoparticles on a γ-Al2O3 support was investigated for use in three-way catalytic converters, with particular attention being paid to it...

Multicomponent 3d transition metal nanoparticles supported on Al2O3 were prepared using a complex polymerization process and a post H2-reduction treatment at 900 °C. Catalysts in a binary system we...

The changes in the oxidation state of supported Pd catalysts during light-off of NO–CO–C3H6–O2 reactions were evaluated during temperature ramp up and down cycles (10 °C min–1) under fuel-rich (air...

A pulsed cathodic arc-plasma deposition of a few-nanometer-thick Pt(111) overlayer on a 50 μm-thick Fe–Cr–Al metal foil produced a thin-film catalyst; this catalyst exhibited high activity for low-...

Single-phase quaternary spinel solid solutions, Cu0.05Ni0.95AlyCr2–yO4 (0 ≤ y ≤ 2.0), were prepared over the whole range of y by a polymerized complex method to study as platinum group metal-free three-way catalysts (TWC). Most conventional binary and/or ternary spinel oxides lose their NO reduction activity in the presence of water vapor and/or after high-temperature aging. In contrast, the present quaternary system with y = 1.8, which was aged at 900 °C for 25 h, preserved high activity even under a wet gas stream (5% H2O) simulating real TWC conditions. Comprehensive structural analyses via X-ray absorption fine structure and X-ray Rietveld analysis showed that, in the quaternary system, Cu and Cr prefer to occupy the tetrahedral site and the octahedral site, respectively, whereas Ni and Al are distributed across both sites. The partial replacement of Cr by Al increased the specific surface area from 7 m2 g–1 (y = 0) to 36 m2 g–1 (y = 1.8), which is a common feature of the NiAl2O4-based spinel platform...

The influence of high-temperature H2 reduction treatment on Rh and Pd catalysts supported on Al2O3 was studied in relation to thermal aging in air. After air-aging at ≥900 °C, the Rh/Al2O3 catalyst was more strongly deactivated compared with the Pd/Al2O3 catalyst. As has been widely recognized, the solid-state reactions between Rh oxide and Al2O3 decreased the active surface area and stabilized inactive Rh3+ species. The activity was restored by the postreduction treatment with 20% H2/He at 200 °C, whereas a striking enhancement of activity was achieved by the reduction at 800–1100 °C, where substantial deactivation occurred for Pd/Al2O3. A mechanistic interpretation is proposed based on local structural characterization, which explains these contrasting thermal behaviors. The high-temperature reduction treatment produced active and thermostable Rh metal nanoparticles, which were highly dispersed on Al2O3. The observed dispersion (as high as ∼20% after reduction at 1000 °C) is among the highest for suppor...