Showing papers by "Alfons Baiker published in 2021"

Continuous dimethyl carbonate synthesis from CO2 and methanol over BixCe1−xOδ monoliths: Effect of bismuth doping on population of oxygen vacancies, activity, and reaction pathway

Abstract: We evaluated bismuth doped cerium oxide catalysts for the continuous synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide in the absence of a dehydrating agent. BixCe1−xOδ nanocomposites of various compositions (x = 0.06–0.24) were coated on a ceramic honeycomb and their structural and catalytic properties were examined. The incorporation of Bi species into the CeO2 lattice facilitated controlling of the surface population of oxygen vacancies, which is shown to play a crucial role in the mechanism of this reaction and is an important parameter for the design of ceria-based catalysts. The DMC production rate of the BixCe1−xOδ catalysts was found to be strongly enhanced with increasing Ov concentration. The concentration of oxygen vacancies exhibited a maximum for Bi0.12Ce0.88Oδ, which afforded the highest DMC production rate. Long-term tests showed stable activity and selectivity of this catalyst over 45 h on-stream at 140 °C and a gas-hourly space velocity of 2,880 mL·gcat−1·h−1. In-situ modulation excitation diffuse reflection Fourier transform infrared spectroscopy and first-principle calculations indicate that the DMC synthesis occurs through reaction of a bidentate carbonate intermediate with the activated methoxy (−OCH3) species. The activation of CO2 to form the bidentate carbonate intermediate on the oxygen vacancy sites is identified as highest energy barrier in the reaction pathway and thus is likely the rate-determining step.

Synergistic Effects of Ternary PdO-CeO2-OMS-2 Catalyst Afford High Catalytic Performance and Stability in the Reduction of NO with CO.

Abstract: We developed a robust ternary PdO-CeO2-OMS-2 catalyst with excellent catalytic performance in the selective reduction of NO with CO using a strategy based on combining components that synergistically interact leading to an effective abatement of these toxic gases. The catalyst affords 100% selectivity to N2 at the nearly full conversion of NO and CO at 250 °C, high stability in the presence of H2O, and a remarkable SO2 tolerance. To unravel the origin of the excellent catalytic performance, the structural and chemical properties of the PdO-CeO2-OMS-2 nanocomposite were analyzed in the as-prepared and used state of the catalyst, employing a series of pertinent characterization methods and specific catalytic tests. The experimental as well as theoretical results, based on density-functional theory calculations suggest that CO and NO follow different reaction pathways, CO is preferentially adsorbed and oxidized at Pd sites (PdII and Pd0), while NO decomposes on the ceria surface. Lattice oxygen vacancies at the interfacial perimeter of PdO-CeO2 and PdO-OMS-2, and the diffusion of oxygen and oxygen vacancies are proposed to play a critical role in this multicenter reaction system.

Formation and Location of Pt Single Sites Induced by Pentacoordinated Al Species on Amorphous Silica-Alumina.

Abstract: Alumina and its mixed oxides are popular industrial supports for emerging supported metal catalysts. Pentacoordinated Al (AlV) species are identified as key surface sites for anchoring and stabilizing metal single-site catalysts; however, AlV is rare in conventional amorphous silica-alumina (ASA). Recently, we have developed AlV-enriched ASA, which was applied as a support for the synthesis of Pt single-site catalysts in this work. Each preparation stage and the interaction between Pt and surface Al species were explored by 1H and 27Al solid-state nuclear magnetic resonance spectroscopy, and the formation of the dominant Pt single sites on the surface of AlV-enriched ASA was confirmed by high-angle annular dark-field imaging scanning transmission electron microscopy and energy dispersive spectroscopy line scanning. On the surface of supports without a significant AlV population (Pt/Al2O3 and Pt/SiO2), mainly Pt nanoparticles were formed. This indicates that AlV contributes to the strong metal-support interaction to stabilize the Pt single sites on Pt/ASA, which was characterized by diffuse reflectance infrared Fourier transform spectroscopy combined with CO adsorption, X-ray photoelectron spectroscopy, and electron energy loss spectroscopy. Pt single sites supported on AlV-enriched ASA exhibit excellent chemoselectivity in the hydrogenation of C═O groups, affording 2-3-fold higher yields compared to those of Pt nanoparticles supported on Al2O3 and SiO2.

Bimetallic AuPd@CeO2 nanoparticles supported on potassium titanate nanobelts : a highly efficient catalyst for the reduction of NO with CO

Abstract: A nanocomposite consisting of bimetallic AuPd nanoparticles, which were modified with CeO2 (AuPd@CeO2), and deposited on potassium titanate nanobelts (KTN) as support, is shown to exhibit outstanding catalytic performance in the selective catalytic reduction of NO with CO. Transmission electron microscopy and energy dispersive X-Ray elemental mapping indicated that the AuPd nanoparticles surrounded by CeO2 were well-mixed forming an alloy. The potassium titanate support consisted of 1–3 µm long and 8–14 nm wide nanobelts. The AuPd@CeO2/KTN catalyst showed full NO conversion at 100 % selectivity to N2 at a gas-hourly space velocity (GHSV) of 15,000 h−1 and 200 °C. The outstanding performance of the AuPd@CeO2/KNT catalyst is attributed to favorable synergies between its components. Corresponding monometallic Au catalysts supported on KTN (Au@CeO2/KNT), as well as bimetallic AuPd supported on TiO2 (AuPd@CeO2/TiO2), showed inferior catalytic performance, indicating the absence of a beneficial synergy between the different components. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with modulation excitation spectroscopy (MES) proved that alloying of Au with Pd enhances the ability to adsorb CO and NO on the surface in an on-top configuration and that the deposition of the bimetallic AuPd nanoparticles on KTN facilitates the crucial formation of isocyanate (-NCO) species, resulting in high conversion and selectivity.

Potassium Titanate Nanobelts: A Unique Support for Au and AuRh Nanoparticles in the Catalytic Reduction of NO with CO

Abstract: The catalytic reduction of NO with CO was examined over monometallic Au and bimetallic AuRh nanoparticles supported on potassium titanate (K2Ti8O17) nanobelts (KTN) and TiO2. The highly crystalline KTN, prepared from TiO2 by a hydrothermal process using KOH, were several μm long and ca. 8 nm wide. Both Au and bimetallic AuRh catalysts showed a striking enhancement of the catalytic performance with KTN as support, compared to corresponding TiO2‐supported catalysts. The reasons for this behavior could be traced back using in situ diffuse reflectance infrared Fourier transform spectroscopy in tandem with modulation excitation spectroscopy, which proved that the KTN support promotes the formation of surface nitrate (−NO3−), carbonate (−CO32−), and isocyanide (−NCO) species. In particular, the combination of KTN with bimetallic AuRh nanoparticles results in the facile formation of isocyanide, which is a pivotal intermediate for producing dinitrogen molecules via the reaction with NO; NCO(a)+NO(a)→N2+CO2.

Engineering the Distinct Structure Interface of Subnano-alumina Domains on Silica for Acidic Amorphous Silica-Alumina toward Biorefining.

Abstract: Amorphous silica-aluminas (ASAs) are important solid catalysts and supports for many industrially essential and sustainable processes, such as hydrocarbon transformation and biorefining. However, the wide distribution of acid strength on ASAs often results in undesired side reactions, lowering the product selectivity. Here we developed a strategy for the synthesis of a unique class of ASAs with unvarying strength of Bronsted acid sites (BAS) and Lewis acid sites (LAS) using double-flame-spray pyrolysis. Structural characterization using high-resolution transmission electron microscopy (TEM) and solid-state nuclear magnetic resonance (NMR) spectroscopy showed that the uniform acidity is due to a distinct nanostructure, characterized by a uniform interface of silica-alumina and homogeneously dispersed alumina domains. The BAS population density of as-prepared ASAs is up to 6 times higher than that obtained by classical methods. The BAS/LAS ratio, as well as the population densities of BAS and LAS of these ASAs, could be tuned in a broad range. In cyclohexanol dehydration, the uniform Bronsted acid strength provides a high selectivity to cyclohexene and a nearly linear correlation between acid site densities and cyclohexanol conversion. Moreover, the concerted action of these BAS and LAS leads to an excellent bifunctional Bronsted-Lewis acid catalyst for glucose dehydration, affording a superior 5-hydroxymethylfurfural yield.

Revealing Brønsted Acidic Bridging SiOHAl Groups on Amorphous Silica–Alumina by Ultrahigh Field Solid-State NMR

Abstract: Amorphous silica-aluminas (ASAs) are important acidic catalysts and supports for many industrially essential and sustainable processes. The identification of surface acid sites with their local structures on ASAs is of critical importance for tuning their catalytic properties but still remains a great challenge and is under debate. Here, ultrahigh magnetic field (35.2 T) 27Al-{1H} D-HMQC (dipolar-mediated heteronuclear multiple-quantum correlation) two-dimensional NMR experiments demonstrate two types of Bronsted acid sites in ASA catalysts. In addition to the known pseudobridging silanol acid sites, the use of ultrahigh field NMR provides the first direct experimental evidence for the existence of bridging silanol (BS: SiOHAl) acid sites in ASAs, which has been hotly debated in the past few decades. This discovery provides new opportunities for scientists and engineers to develop and apply ASAs in various reaction processes due to the significance of BS in chemical and fuel productions based on its strong Bronsted acidity.