Ru-Ti Oxide Based Catalysts for HCl Oxidation: The Favorable Oxygen Species and Influence of Ce Additive

Ce-doped TiO2 supported RuO2 as efficient catalysts for the oxidation of HCl to Cl2

Reducing the cost of RuO2/TiO2 catalysts is still one of the urgent challenges in catalytic HCl oxidation. In the present work, a Ce-doped TiO2 supported RuO2 catalyst with a low Ru loading was developed, showing a high activity in the catalytic oxidation of HCl to Cl2. The results on some extensive characterizations of both Ce-doped TiO2 carriers and their supported RuO2 catalysts show that the doping of Ce into TiO2 can effectively change the lattice parameters of TiO2 to improve the dispersion of the active RuO2 species on the carrier, which facilitates the production of surface Ru species to expose more active sites for boosting the catalytic performance even under some harsh reaction conditions. This work provides some scientific basis and technical support for chlorine recycling.

Unraveling the Effects of Reducing and Oxidizing Pretreatments and Humidity on the Surface Chemistry of the Ru/CeO2 Catalyst during Propane Oxidation

This work investigates the surface chemistry of the Ru/CeO2 catalyst under varying pretreatment conditions and during the oxidation of propane, focusing on both dry and humid environments. Our results show that the Ru/CeO2 catalyst calcined in O2 at 500 °C initiates propane oxidation at 200 °C, achieves high conversion rates above 400 °C, and demonstrates almost no change in activity in the presence of water vapor across the entire studied temperature range of 200-500 °C. Prereduction of the oxidized Ru/CeO2 catalyst in H2 significantly enhances its activity, though this enhancement diminishes at higher temperatures. Adding water to the reaction mixture boosts the low-temperature activity of the prereduced catalyst but decreases it at 300-400 °C. Several ex-situ analytical techniques in combination with the in-situ NAP-XPS analysis reveal that while exposed to oxygen, Ru nanoparticles on the ceria surface oxidize to form RuO2 below 200 °C and volatile RuO x (x > 2) at higher temperatures. The presence of water vapor in the reaction mixture leads to the transformation of RuO2 into ruthenium hydroxide at 200 °C, which, in turn, facilitates propane oxidation. At higher temperatures, the water does not have much influence on the oxidation state of Ru but slightly inhibits its evaporation from the surface. It is also demonstrated that Ru in the Ru/CeO2 catalyst exists predominantly in the Run+ (n > 4) oxidation states at typical VOC oxidation temperatures rather than the expected Ru4+ state.

Active Probing of a RuO2/CZ Catalyst Surface as a Tool for Bridging the Gap Between CO Oxidation Catalytic Tests in a Model and Realistic Exhaust Gas Stream

Herein, we present a paper that attempts to bridge the gap between CO oxidation catalytic tests performed in a model stream and a more realistic exhaust gas stream by incorporating characterization methods that allow for active probing of the catalyst surface. The results have shown that it is not just the abundance of a given type of species on the surface that impacts the activity of a system but also the ease of extraction of ions from their surface (time-of-flight secondary ion mass spectrometry) and the response of the support to change in the feed composition (dynamic in situ X-ray diffraction (XRD) with variable atmosphere). The study utilizes the method of doping a catalyst (RuO2/CZ) with a small amount of alkali-metal (K+ or Na+) carbonates in order to slightly modify its surface to gain insight into parameters that may cause discrepancies between model stream activity and complex stream activity. The most pronounced difference is that in the model stream, which contains only CO and O2 in helium, both alkali ions improve the activity of the system at temperatures around 175 °C, whereas in the complex stream, which mimics the exhaust stream from a diesel engine under oxygen lean conditions, the K+-doped catalyst is slightly worse than RuO2 /CZ and RuO2 + Na+/CZ and much worse in propane combustion. The total hydrogen consumption values (temperature-programmed reduction) and the Oads/Olatt ratios (X-ray photoelectron spectroscopy) both place the RuO2 + K+/CZ system between the other two and hence provided no reason for the unusual behavior of the K+-doped catalyst. In contrast, both in situ XRD measurement tests and ToF SIMS results show a pronounced difference between the RuO2 + K+/CZ catalyst and the other two systems, which indicates that the interaction of the surfaces with the reagents might be the cause of the discrepancy. The CO2-TPD results show that this system retains more CO2, i.e., the product, at adsorption sites, which might reduce the adsorption of other reagents, i.e., oxygen ions, CO, and propane, hence lowering the overall activity of the system.

CeO2 nanorods supported CuOx-RuOx bimetallic catalysts for low temperature CO oxidation

Bimetallic catalysts often outperform monometallic catalysts due to changeable structural orientation, synergistic effects, and integration of two different metal or metal oxide properties. Here, a series of CeO2 nanorods (NR) supported bimetallic CuOx and RuOx catalysts (Cu: Ru ratios of 9:1, 7:3, and 5:5) were prepared using a wet impregnation method. In situ DRIFTS, H2 temperature programmed reduction (H2-TPR), CO temperature programmed desorption (CO-TPD), and other characterization techniques were used to investigate the effect of the Cu:Ru ratio on the activity of low-temperature CO oxidation. Among three catalysts, CeO2 NR supported 7 wt% Cu-3 wt% Ru catalyst after a reduction activation treatment showed the best performance with 100 % CO conversion at 166 °C and the lowest activation energy of 18.37 kJ mol-1. Raman and XPS profiles revealed that the origin of the superior performance is at least partially related to the high surface oxygen vacancy concentration and other distinct oxygen species (physi-/chemi-sorbed oxygen and bulk lattice oxygen), leading to outstanding adsorption and oxidation property of CO.

Strong metal-support interactions between atomically dispersed Ru and CrO x for improved durability of chlorobenzene oxidation

In this work, two single-atom catalysts (SACs) with atomically dispersed RuO₂ supported on CrO _x were successfully synthesized with a simple reduction strategy for the efficient catalytic oxidation of chlorobenzene (CB). With characterizations like Cs-corrected STEM, XPS, H₂-TPR, and O₂-TPD, the structure-activity relationship is addressed. The noble metal precursor Ru³⁺ was anchored with different oxygen species and exposed facets based on the physicochemical properties of catalyst supports. Based on the analysis results, the Ru³⁺ precursor could be mainly anchored into the surface lattice oxygen of Cr₂O₃-M over high-index facets (223) and adsorbed oxygen of Cr₂O₃-P over low-index facets (104), where the precursor Ru³⁺ was all oxidized to RuO₂ when being anchored with the oxygen species of Cr₂O₃-M and Cr₂O₃-P, respectively according to XPS analysis. There is a stronger metal-support interaction (SMSI) between Ru ions and the surface lattice oxygen of Cr₂O₃-M, according to H₂-TPR and O₂-TPD characterizations. Further, the catalytic performance for CB combustion at a high space velocity of 120 000 mL (g^-1 h^-1) was tested, and 1RuCr₂O₃-M performed better than 1RuCr₂O₃-P in both durability and activity. This could be attributed to the SMSI between single-atom Ru and the lattice oxygen of the 1RuCr₂O₃-M catalyst and the abundant active sites from the exposed high-index facets. The study provided a novel synthesis strategy for Ru-based SACs with SMSI effect, and the good durability of the catalyst (1RuCr₂O₃-M) extended the great potential for practical application.

Insight into the effects of calcination temperature on the structure and performance of RuO2/TiO2 in the Deacon process

With an appropriate calcination temperature for preparing a rutile-TiO2 supported RuO2 catalyst, rich surface RuO2 species can be formed on TiO2, leading to its high activity in the oxidation of HCl.

Turning Carbon Dioxide and Ethane into Ethanol by Solar-Driven Heterogeneous Photocatalysis over RuO2- and NiO-co-Doped SrTiO3

The current work focused on the sunlight-driven thermo-photocatalytic reduction of carbon dioxide (CO2), the primary greenhouse gas, by ethane (C2H6), the second most abundant element in shale gas, aiming at the generation of ethanol (EtOH), a renewable fuel. To promote this process, a hybrid catalyst was prepared and properly characterized, comprising of strontium titanate (SrTiO3) co-doped with ruthenium oxide (RuO2) and nickel oxide (NiO). The photocatalytic activity towards EtOH production was assessed in batch-mode and at gas-phase, under the influence of different conditions: (i) dopant loading; (ii) temperature; (iii) optical radiation wavelength; (vi) consecutive uses; and (v) electron scavenger addition. From the results here obtained, it was found that: (i) the functionalization of the SrTiO3 with RuO2 and NiO allows the visible light harvest and narrows the band gap energy (ca. 14–20%); (ii) the selectivity towards EtOH depends on the presence of Ni and irradiation; (iii) the catalyst photoresponse is mainly due to the visible photons; (iv) the photocatalyst loses > 50% efficiency right after the 2nd use; (v) the reaction mechanism is based on the photogenerated electron-hole pair charge separation; and (vi) a maximum yield of 64 μmol EtOH gcat−1 was obtained after 45-min (85 μmol EtOH gcat−1 h−1) of simulated solar irradiation (1000 W m−2) at 200 °C, using 0.4 g L−1 of SrTiO3:RuO2:NiO (0.8 wt.% Ru) with [CO2]:[C2H6] and [Ru]:[Ni] molar ratios of 1:3 and 1:1, respectively. Notwithstanding, despite its exploratory nature, this study offers an alternative route to solar fuels’ synthesis from the underutilized C2H6 and CO2.

Insights into the sintering resistance of RuO2/TiO2–SiO2 in the Deacon process: role of SiO2

RuO₂ nanoparticles are still formed on the surface of TiO₂ to prevent the thermal sintering because of the geometric effects of SiO₂ and the resultant RuO₂/TiO₂–SiO₂ catalyst has an improved stability in the Deacon process.

Editorial: Special Issue on “Emerging Trends in TiO2 Photocatalysis and Applications”

It is not an exaggerated fact that the semiconductor titanium dioxide (TiO2) has been evolved as a prototypical material to understand the photocatalytic process and has been demonstrated for various photocatalytic applications such as pollutants degradation, water splitting, heavy metal reduction, CO2 conversion, N2 fixation, bacterial disinfection, etc [...]

HCl Removal Using Calcined Ca–Mg–Al Layered Double Hydroxide in the Presence of CO2 at Medium–High Temperature

This present work aimed to investigate the influence of CO2 on HCl removal using calcined Ca–Mg–Al layered double hydroxides (CaMgAl-LDHs) at medium–high temperature (400–800 °C) in a fixed-bed reactor. It was revealed that a moderate CO2 concentration (~6%) in the flue gas of the municipal solid-waste incinerators could reduce the HCl capacity of the CaMgAl-layered double oxides (CaMgAl-LDOs). The highest capacity for HCl removal was observed over the CaMgAl-LDOs at 600 °C. However, sintering was also detected when the reaction temperature was below the calcination temperature (600 °C). Moreover, the decreasing HCl adsorption capacity of CaMgAl-LDOs was attributed to the existence of CO2 in the flue gas, which could efficiently inhibit the decomposition of carbonates as well as the conversion into metal chloride during the HCl removal process.