Catalytic Synergies Research Articles

Novel insight into the effect of relative humidity on the acetone photocatalytic decomposition using Ag−CeO2 under vacuum ultraviolet illumination: Catalytic synergies, performance, and mechanistic interpretation

The present work reports on the impact of relative humidity (RH) on the process of acetone oxidation in the absence and presence of Ag − CeO2 photocatalysts, including 185 nm vacuum UV (VUV) photolysis, ozonation, ozonation + 254 nm UV photolysis, and 254 nm UV photolysis treatment processes. The photocatalysts were evaluated by Raman, SEM, XRD, HRTEM, EPR, EDS, and XPS techniques. The information from characterization tests revealed that modification of ceria structure by the introduction of Ag nanoparticles increases the photocatalyst oxygen storage ability, which promotes the oxygen vacancies formation, resulting in the lattice oxygen activation. The attained data demonstrated the dual role of RH in the process of acetone oxidation under VUV irradiation. Compared to the dry condition, the introduction of RH = 25, 45, and 65% to the VUV photolysis system enhances the conversion of acetone remarkably from 40% to 60, 68, and 72%, respectively. However, the acetone oxidation did not change significantly in the presence of RH = 25–65% over the Ag − CeO2/VUV system. This observation confirms the dual role of RH; while RH boosted the gas phase photooxidation reactions, it poisoned the gas–solid interface, leading to an inhibitory effect on the catalytic oxidation reactions. Moreover, the results showed that the lower level of relative humidity (e.g., RH = 5%) over the Ag − CeO2 under VUV light not only enhances the acetone oxidation to 88% but also, through the participation of the catalytic ozonation, decomposed the formed O3 and increased the selectivity of the reaction towards complete oxidation up to 97%. The acetone oxidation mechanism was studied where the EPR results show that both.OH and.O2– contribute to acetone oxidation. Finally, the Ag − CeO2/VUV reusability was assessed in a way that the in-situ procedure for photocatalyst regeneration was utilized to inhibit the catalyst deactivation.

Catalytic Synergies in Bimetallic RuPt Single–Atom Catalysts via Speciation Control (Adv. Funct. Mater. 52/2022)

Catalytic Synergies in Bimetallic RuPt Single–Atom Catalysts via Speciation Control (Adv. Funct. Mater. 52/2022)

Catalytic Synergies in Bimetallic RuPt Single–Atom Catalysts via Speciation Control

AbstractBimetallic single–atom catalysts (b–SACs) have recently gained prominence by virtue of the unique catalytic cooperative interactions they can exhibit, intertwining electronic and geometric effects. To date, research efforts have exclusively focused on direct mechanisms such as electron density transfer or sequential reactivity. Herein, the first study on indirect, coordination–induced catalytic synergies in carbon‐supported RuPt SACs is conducted. To this end, a holistic approach is developed, combining i) precision synthesis, ii) advanced characterization, iii) exploration of single–site adsorption properties via the hydrogen evolution reaction, and iv) modeling through density functional theory. Despite the lack of both intermetallic coordination in the first or second shell and charge redistribution effects, the RuPt SACs exhibit a H2 formation rate enhanced up to 15–fold compared with their monometallic counterparts. To unfold the origin of the intermetallic cooperativity, modifications of the structural and catalytic properties induced by the integration of a second metal species are investigated. Thus, Pt atoms are found to selectively occupy the most energeticallyfavorable cavities in the support, prompting Ru atoms to assume a distinct, more active, configuration. This contribution unveils a novel principle of bimetallic cooperativity, demonstrating the key role of integrative experimental and computational analyses in studying b–SACs.

Catalytic Synergies in Bimetallic Ru-Pt Single-Atom Catalysts via Speciation Control

Catalytic Synergies in Bimetallic Ru-Pt Single-Atom Catalysts via Speciation Control

Controlled Formation of Dimers and Spatially Isolated Atoms in Bimetallic Au-Ru Catalysts via Carbon-Host Functionalization.

The introduction of a foreign metal atom in the coordination environment of single-atom catalysts constitutes an exciting frontier of active-site engineering, generating bimetallic low-nuclearity catalysts often exhibiting unique catalytic synergies. To date, the exploration of their full scope is thwarted by (i) the lack of synthetic techniques with control over intermetallic coordination, and (ii) the challenging characterization of these materials. Herein, carbon-host functionalization is presented as a strategy to selectively generate Au-Ru dimers and isolated sites by simple incipient wetness impregnation, as corroborated by careful X-ray absorption spectroscopy analysis. The distinct catalytic fingerprints are unveiled via the hydrogen evolution reaction, employed as a probe for proton adsorption properties. Intriguingly, the virtually inactive Au atoms enhance the reaction kinetics of their Ru counterparts already when spatially isolated, by shifting the proton adsorption free energy closer to neutrality. Remarkably, the effect is magnified by a factor of 2 in dimers. These results exemplify the relevance of controlling intermetallic coordination for the rational design of bimetallic low-nuclearity catalysts.

Intensification of persulfate-mediated elimination of bisphenol A by a spinel cobalt ferrite-anchored g-C3N4 S-scheme photocatalyst: Catalytic synergies and mechanistic interpretation

This study was designed to enhance wastewater treatment by the UVC/persulfate process via its coupling with spinel cobalt ferrite (SCF) nanoparticles anchoring graphitic carbon nitride (g-C3N4, GCN). The performance of SCF@GCN was assessed against bisphenol A (BPA) degradation as a model recalcitrant contaminant. Characterization tests confirmed the successful preparation of SCF@GCN with high photocatalytic activity and excellent capability in PS activation compared to plain SCF or pristine GCN. The results strongly pointed out that SCF@GCN combined with UVC and PS is capable to rapidly degrade BPA with a remarkable synergistic effect. Under optimum conditions (pH: 9.0, SCF@GCN: 0.3 g/L and PS: 4.0 mM), complete elimination of BPA (40 mg/L) was achieved in 15 min with excellent degradation kinetics (0.32 min−1). Scavenging experiments ascertained the generation of h+, 1O2 , O2•− , SO4•−, HO• species in SCF@GCN/PS/UVC system, indicating that both radical and non-radical mechanisms were involved during the degradation process. Furthermore, the developed process displayed a satisfactory performance for effective treating BPA-contaminated wastewater samples. Over 95% BPA was removed by the system after five catalyst reuse cycles, suggesting a cost-effectiveness potential in the reusability of SCF@GCN. A comprehensive mechanistic pathway for BPA degradation was proposed based on the reactive species and intermediates identified: A S-scheme heterojunction mechanism of photo-generated electron/hole pairs transfer in the SCF@GCN catalyst was found. Overall, our results present a promising technique with synergistic character for intensification of wastewater treatment.

Copper-alloy catalysts: structural characterization and catalytic synergies

Recent progress in the development of copper-alloy catalysts is highlighted, focusing on the structural and mechanistic characterizations of the catalysts in different catalytic reactions, and challenges and opportunities in future research.

Dynamic Core-Shell and Alloy Structures of Multimetallic Nanomaterials and Their Catalytic Synergies.

ConspectusMultimetallic nanomaterials containing noble metals (NM) and non-noble 3d-transition metals (3d-TMs) exhibit unique catalytic properties as a result of the synergistic combination of NMs and 3d-TMs in the nanostructure. The exploration of such a synergy depends heavily on the understanding of the atomic-scale structural details of NMs and 3d-TMs in the nanomaterials. This has attracted a great deal of recent interest in the field of catalysis science, especially concerning the core-shell and alloy nanostructures. A rarely asked question of fundamental significance is how the core-shell and alloy structural arrangements of atoms in the multimetallic nanomaterials dynamically change under reaction conditions, including reaction temperature, surface adsorbate, chemical environment, applied electrochemical potential, etc. The dynamic evolution of the core-shell/alloy structures under the reaction conditions plays a crucial role in the catalytic performance of the multimetallic nanocatalysts.This Account focuses on the dynamic structure characteristics for several different types of composition-tunable alloy and core-shell nanomaterials, including phase-segregated, elemental-enriched, dynamically evolved, and structurally different core-shell structures. In addition to outlining core-shell/alloy structure formation via processes such as seed-mediated growth, thermochemical calcination, adsorbate-induced evolution, chemical dealloying, underpotential deposition/galvanic displacement, etc., this Account will highlight the progress in understanding the dynamic core-shell/alloy structures under chemical or catalytic reaction conditions, which has become an important focal point of the research fronts in catalysis and electrocatalysis. The employment of advanced techniques, especially in situ/operando synchrotron high-energy X-ray diffraction and pair distribution function analyses, has provided significant insights into the dynamic evolution processes of NM/3d-TM nanocatalysts under electrocatalytic or fuel cell operating conditions. Examples will highlight Pt- or Pd-based nanoparticles and nanowires alloyed with various 3d-TMs with a focus on their structural evolution under reaction conditions. While the dynamic process is complex, the ability to gain an insight into the evolution of core-shell and alloy structures under the catalytic reaction condition is essential for advancing the design of multimetallic nanocatalysts. This Account serves as a springboard from fundamental understanding of the core-shell and alloy structural dynamics to the various applications of nanostructured catalysts/electrocatalysts, especially in the fronts of energy and environmental sustainability.

Shedding light on the catalytic synergies between Fe(II) and PMS in vacuum UV (VUV/Fe/PMS) photoreactors for accelerated elimination of pharmaceuticals: The case of metformin

In this work, an efficient VUV/Fe/PMS process was developed to degrade pharmaceutical compounds in contaminated waters. The effect of addition of low Fe(II) and PMS amounts individually and simultaneously was scrutinized for treating metformin (MET) as a model contaminant. Co-addition of 0.05 mg/L Fe(II) and 20 mg/L PMS to the bench-scale VUV photoreactor emitting both UV254 and UV185 photons, respectively, produced both HO∙ and SO4•- which had a considerable synergistic effect (30%) for the degradation of MET. Furthermore, adding Fe(II) and PMS to the VUV photoreactor resulted to a decreased electrical energy consumption by 3.5 times. The optimum treatment pH was near neutral, while a high degree of MET oxidation was reached even in the presence of common water anions. The complete degradation and mineralization of 5 mg/L MET in tap water was achieved in a continuous-flow VUV/Fe(II)/PMS mode at a hydraulic retention time of 60 min. The findings suggest that the VUV/Fe(II)/PMS process could be an effective and efficient advanced process for oxidation of emerging water contaminants.

Organic-inorganic multifunctional hybrid catalyst giving catalytic synergies in cooperative coupling between CO2 and propylene oxide to propylene carbonate

Abstract An organic-inorganic hybrid catalyst was synthesized by silylation of silanol (Si OH) groups with ammonium bromide group on the mesopore wall of mesoporous zeolite having a hierarchically nanoporous architecture, conferring both electrophilic and nucleophilic functions due to the acid sites of the zeolite and bromide in the functional group, respectively. In the coupling of carbon dioxide and propylene oxide for production of propylene carbonate, the hybrid catalyst exhibited a synergistic enhancement in activity leading to a two-fold higher product yield than the catalysts having a single catalytic function. Comprehensive reaction studies revealed that the catalytic synergy could only be obtained by achieving close proximity between the ammonium bromide group and acid sites on the zeolite surface. The acidity was also crucial, where no synergy was observed for a similarly designed organic-inorganic hybrid catalyst constructed on weakly acidic ordered mesoporous silica SBA-15. Comparison of the activation energy of the catalysts supported the observation that the zeolite-based hybrid catalyst with stronger acidity gave rise to a lower activation energy (37.3 kJ mol−1) than the SBA-15-based hybrid catalyst with weaker acidity (51.3 kJ mol−1). The mesoporous zeolite-based hybrid catalyst was very active under neat conditions without additional organic solvent, and showed good recyclability without significant loss of the initial activity for up to five runs.

Bifunctional Interface of Au and Cu for Improved CO2 Electroreduction.

Gold is known currently as the most active single-element electrocatalyst for CO2 electroreduction reaction to CO. In this work, we combine Au with a second metal element, Cu, to reduce the amount of precious metal content by increasing the surface-to-mass ratio and to achieve comparable activity to Au-based catalysts. In particular, we demonstrate that the introduction of a Au-Cu bifunctional "interface" is more beneficial than a simple and conventional homogeneous alloying of Au and Cu in stabilizing the key intermediate species, *COOH. The main advantages of the proposed metal-metal bifunctional interfacial catalyst over the bimetallic alloys include that (1) utilization of active materials is improved, and (2) intrinsic properties of metals are less affected in bifunctional catalysts than in alloys, which can then facilitate a rational bifunctional design. These results demonstrate for the first time the importance of metal-metal interfaces and morphology, rather than the simple mixing of the two metals homogeneously, for enhanced catalytic synergies.

An in situ methylation of toluene using syngas over bifunctional mixture of Cr2O3/ZnO and HZSM-5

The in situ methylation of toluene using syngas (in situ methylation hereafter) was studied to produce xylenes over bifunctional catalysts comprising Cr2O3/ZnO (CrZ) and HZSM-5 (SiO2/Al2O3=30) in a fixed-bed down-flow reactor at 460psig. In this study, three basic aspects in the in situ methylation were investigated: (1) the catalytic activity of CrZ as the methanol synthesis catalyst with respect to the equilibrium conversion, (2) the evaluation of catalytic synergies in the in situ methylation over bifunctional catalyst, and (3) the effect of the ratio of catalytic functions regarding the ratio of methanol synthesis catalyst to HZSM-5, and their intimacy. The CrZ catalyst exhibited very low CO conversion to methanol in the methanol synthesis reaction, which is controlled by the equilibrium. However, the bifunctional mixture of CrZ and HZSM-5 catalysts exhibited significant catalytic synergies in the in situ methylation in terms of catalytic activity and product selectivities, with about a 10-times-higher CO conversion rate, and 2- to 3-times-higher toluene conversion and xylene production rates compared to the monofunctional catalysis of methanol synthesis and toluene disproportionation, respectively. The in situ methylation also gave rise to higher catalytic performance than in typical toluene methylation conducted with a molar feed ratio of methanol/toluene of 0.5. The catalytic synergy was manifested when both metallic and acidic functions were brought into close intimacy with a weight ratio of CrZ/HZSM-5 close to one in the in situ methylation.

Selective aerobic oxidation of para-xylene in sub- and supercritical water. Part 1. Comparison with ortho-xylene and the role of the catalyst

The selective, continuous, aerobic oxidations of para-xylene (pX) and ortho-xylene (oX) were performed in an identical fashion in supercritical water. The xylenes were oxidized without a catalyst and with hydrobromic acid, cobalt(II) and manganese(II) bromide catalysts. The conversions and yields to phthalic acid (OA) from oX were always significantly higher than those for terephthalic acid (TA) from pX. The formation of CO2 was significantly higher for pX than oX despite the higher conversions to oX. These results are unexpected because the literature teaches that thermal and catalytic decarboxylation is much higher for OA than TA. The superior yields from oX are consistent with a lower steady-state concentration of hydroxyl radicals, OH˙ due to the internal, concerted attack of the peroxides with the oX methyl group. This mechanism forms the phthalide directly from o-tolualdehyde (oTOL) which is consistent with the observation that ortho-toluic acid (OTA) is much lower in oX than para-toluic acid, PTA, in pX oxidation. This mechanism also lowers the steady-state concentration of aromatic acids consistent with the observed lower benzoic acid and CO2 yields. Overall, the results suggest that the metal catalysts can play more than one role, thereby opening up the opportunity for discovering new catalytic synergies which are explored in our next paper, Part 2 of this series.

Comparative atomic-scale analysis of promotional effects by late 3d-transition metals in MoS 2 hydrotreating catalysts

The promotion of the activity of MoS 2-based hydrotreating catalysts by various first-row transition metals exhibits a typical variation referred to as a volcano plot. Co and Ni are seen to substantially promote the catalytic activity of MoS 2, whereas the neighboring first-row metals promote reactivity to a much lesser extent, or not at all. In order to provide a better atomistic understanding of the catalytic synergies, we perform here a comparative scanning tunneling microscopy (STM) analysis of the atomic-scale structure and morphology of MoS 2 nanoclusters doped with the first-row transition metals: Fe, Co, Ni, and Cu. We reveal that addition of all four dopant metals results in the formation of mixed-metal “Co–Mo–S”-type structures shaped as single-layer hexagonally truncated triangular MoS 2-like nanoclusters. The modification of the preferred nanocluster equilibrium morphology is explained as a direct consequence of a favored substitution of dopant metal atoms into the S-edges of MoS 2. The degree of truncation and the edge dispersion are, however, found to depend greatly on the type of dopant atom since the relative length of the dopant-stabilized edges decreases with the number of valence shell electrons of the dopant transition metal. A comparison of the observed atomic structure and morphology with the hydrotreating activity measured for industrial-style prepared Me–Mo–S catalysts (Me = Fe, Co, Ni, and Cu) supported on carbon reveals that two parameters are relevant to describe the promotional behavior: (i) a geometric parameter, which relates to the relative number of promoted and unpromoted sites in the Me–Mo–S nanoclusters, and (ii) a more conventional parameter relating to bonding and adsorption strength, i.e., describing the intrinsic activity of the particular Me-doped S-edge.

Preparation and Characterization of Co-Mo Model Sulfide Catalysts for Hydrodesulfurization

Development of highly active hydrodesulfurization (HDS) catalysts is one of the most urgent problems in the petroleum industry. Better characterization and understanding of the nature of HDS catalysts on the molecular scale are of great importance for the rational design of highly active HDS catalysts. Our approaches for this purpose involve the fabrication of model catalysts using metal carbonyls to overcome the difficulties caused by the heterogeneity of practical catalysts. One of our new approaches uses the synthesis of intrazeolite homogeneous Mo, Co and Co-Mo sulfide clusters with well defined structures. The local structure of intrazeolite Mo sulfide dimer clusters, Mo2S4, depends on the composition of the host zeolite. Thermally stabilized Co2Mo2S6 binary sulfide clusters, which show catalytic synergies between Co and Mo for thiophene HDS, with a thiocubane type structure are formed in zeolite. The intrinsic HDS activity of Co sulfide clusters depends on the host zeolite. The hydrode-selenium reaction of selenophene, combined with an extended X-ray absorption fine structure (EXAFS) study, over Mo2S4 and MoS2 clusters suggests a microscopic HDS reaction mechanism. More practical modeling of Co-Mo sulfide catalysts using a selective preparation method of CoMoS phases supported on refractory oxides is established to understand the nature of practical HDS catalysts. No effects of the support are found with the CoMoS phase supported on Al2O3, TiO2 and ZrO2, whereas the CoMoS phase supported on SiO2 shows a higher intrinsic activity. The fraction of the CoMoS phase accessible to NO adsorption is elucidated, based on the number of the CoMoS phase in the model catalyst, suggesting a new model of the CoMoS structure. The maximum potential HDS activity of Co(Ni)-Mo(W) catalysts is evaluated by using Co(CO)3NO as a probe molecule. Such model catalysts provide important information about the effects of the catalyst preparation and additives, the catalyst structure and the fundamental aspects of HDS catalysts such as microscopic reaction mechanisms and structure-reactivity relationship.

XAFS study of zirconia-supported copper catalysts for the NO–CO reaction: Deactivation, rejuvenation and stabilization of Cu species

A Cu/ZrO2 (1 wt.% Cu) catalyst shows high activity for the NO–CO reaction, even at low temperature (100–200 °C). However, the low-temperature activity is easily lost during prolonged reaction at 250 °C. The activity of the deactivated Cu/ZrO2 catalyst is completely restored by NO treatment (1% in He) at >250 °C or O2 treatment (5% in He) at 500 °C. The addition of iron (2 wt.%) to Cu/ZrO2 causes not only a dramatic increase in the low-temperature activity but also high stability against deactivation. An X-ray absorption fine structure (XAFS) study has been conducted to reveal the local structure and chemical state of the Cu species in Cu/ZrO2 and Cu–Fe/ZrO2 during the reaction and rejuvenation. X-ray absorption near-edge structure (XANES) spectra and, in particular, their first derivatives are shown to be very effective for characterization of the Cu species. It is revealed that the deactivation of Cu/ZrO2 is a consequence of reduction and sintering of highly dispersed Cu+ species, derived from Cu2+(Oct) species in an octahedral symmetry, to Cu metal particles. The Cu metal particles in a deactivated Cu/ZrO2 catalyst are transformed into the original Cu2+(Oct) species by the NO treatment at 250 °C via highly dispersed Cu+ species. On the other hand, O2 treatments bring about successive formation of Cu2O at 150 °C and CuO at 250 °C. The Cu2+(Oct) species are reformed at >400 °C. The XAFS study of Cu–Fe/ZrO2 suggests that the high stability of highly dispersed Cu+ species is a consequence of direct interaction of Cu species with iron oxide clusters. On the basis of kinetics and XAFS results, the catalytic synergies between Cu and Fe are inferred to be two-fold: stabilization of highly dispersed and catalytically active Cu+ species during the reaction and simultaneous participation of both Cu and Fe in the reaction.