One-Pot Surfactant-Free Synthesis of Transition Metal/ZnO Nanocomposites for Catalytic Hydrogenation of CO₂ to Methanol

Abstract

Catalytic hydrogenation of CO2 to methanol is an important chemical process owing to its contribution in alleviating the impacts of the greenhouse effect and in realizing the requirement for renewable energy sources. Owing to their excellent synergic functionalities and unique optoelectronic as well as catalytic properties, transition metal/ZnO (M/ZnO) nanocomposites have been widely used as catalysts for this reaction in recent years. Development of size-controlled synthesis of metal/oxide complexes is highly desirable. Further, because it is extremely difficult to achieve the strong-metal-support-interaction (SMSI) effect when the M/ZnO nanocomposites are prepared via physical methods, the use of chemical methods is more favorable for the fabrication of multi-component catalysts. However, because of the requirement for an extra H2 reduction step to obtain the active metallic phase (M) and surfactants to control the size of nanoparticles, most M/ZnO nanocomposites undergo two-or multi-step synthesis, which is disadvantageous for the stable catalytic performance of the M/ZnO nanocomposites. In this work, we demonstrate facile one-pot synthesis of M/ZnO (M = Pd, Au, Ag, and Cu) nanocomposites in refluxed ethylene glycol as a solvent, without using any surfactants. During the synthesis process, Pd and ZnO species can stabilize each other from further aggregation by reducing their individual surface energies, thereby achieving size control of particles. Besides, NaHCO3 serves as a size-control tool for Pd nanoparticles by adjusting the alkaline conditions. Ethylene glycol serves as a mild reducing agent and solvent owing to its capacity to reduce Pd ions to generate Pd crystals. The nucleation and growth of Pd particles are achieved by thermal reduction, while the ZnO nanocrystals are formed by thermal decomposition of Zn(OAc)2. X-ray diffraction patterns of the M/ZnO and ZnO were analyzed to study the phase of the nanocomposites, and the results show that no impurity phase was detected. Transmission electron microscopy (TEM) was used to study the morphology and structural properties. In addition, X-ray photoelectron spectroscopy analysis was performed to further confirm the formation of M/ZnO hybrid materials, and the results confirm SMSI between Pd and ZnO. Inductively coupled plasma mass spectrometry was used to check the actual elemental compositions, and the results show that the detected atomic ratios of Pd/Zn were consistent with the values in the theoretical recipe. To investigate the effects of the Pd/Zn molar ratios and the added amount of NaHCO3 on Pd size, the average sizes of Pd particles were calculated, and the results were confirmed by TEM observation. The Cu/ZnO/Al2O3 composite is a widely known catalyst for hydrogenation of CO2 to methanol, and other M/ZnO composites are also catalytic for this reaction. Therefore, different M/ZnO hybrids were further studied as catalysts for hydrogenation of CO2 to methanol, among which Pd/ZnO (1 : 9) demonstrated the best performance (30% CO2 conversion, 69% meth...

Tuning Thermal Catalytic Enhancement in Doped MnO2–Au Nano-Heterojunctions

“…In nanocatalysis, in addition to the metal NPs, semiconductors can be used as supports in order to avoid NP aggregation, facilitate catalyst recovery, provide additional catalytic sites due to metal–support interactions, and facilitate the interaction or activation of substrates or intermediates. − However, to our knowledge, few works mentioned the metal–semiconductor heterojunction interactions towards thermal catalysis. Similar to photocatalysis, it has been established in thermal catalysis that semiconductors can contribute to prolong the lifetimes of thermally excited carriers, and potentially boost their catalytic properties …”

Strong Metal–Support Interaction over PtPdCu/Al2O3 toward Hydrogenation of Nitrobenzene

“…Confirmed by STEM-EDS analysis (Figures 2E and S3F), the covering layer is Al 2 O 3 , which is an indication of SMSI between PtPdCu nanoparticles and the Al 2 O 3 support. 21,30 It has been reported that, after being subjected to thermal treatment, an enhanced metal−support interaction between the nanoparticles and the substrate may take place, which has an effect on catalytic activity. 21,30 Differently, PtPdCu nanoparticles in the PtPdCu-300 catalyst are entirely encapsulated with an Al 2 O 3 overlayer (Figures 2F and S3I), indicating more significant migration of Al 2 O 3 onto the metal surface.…”

“…21,30 It has been reported that, after being subjected to thermal treatment, an enhanced metal−support interaction between the nanoparticles and the substrate may take place, which has an effect on catalytic activity. 21,30 Differently, PtPdCu nanoparticles in the PtPdCu-300 catalyst are entirely encapsulated with an Al 2 O 3 overlayer (Figures 2F and S3I), indicating more significant migration of Al 2 O 3 onto the metal surface.…”

Coordination Environment Dependent Surface Cu State for CO2 Hydrogenation to Methanol

“…Heterogeneous catalysts for methanol synthesis reactions mainly include Cu-based, noble metal-based (e.g., Au, Ag, Pd, and Pt), and oxide-based catalysts. − Among them, Cu-based catalysts hold great potential for practical application due to the advantages of low cost and ideal activity under relatively mild conditions. It is generally believed that the reactivity of Cu catalysts is strongly associated with the use of supports such as ZnO, , ZrO 2 , − Al 2 O 3 , and CeO 2 . , The metal oxide supports can regulate the chemical environment of active Cu and the Cu-metal oxide interfaces usually act as active sites for methanol synthesis from CO 2 hydrogenation. Metal–organic frameworks (MOFs) are a new class of porous materials created by the hybrid self-assembly of metal ions, metal clusters, and organic ligands, which have become one of the most striking materials in the field of catalysis, due to their advantages of adjustable pore size, large specific surface area, and high porosity. − MOF materials can anchor and confine metal ions/clusters in channels, cages, or defects, and the coordination microenvironment can tailor the chemical state of active metal, thus generating catalytic reactivity .…”

“…8−13 Among them, Cu-based catalysts hold great potential for practical application due to the advantages of low cost and ideal activity under relatively mild conditions. It is generally believed that the reactivity of Cu catalysts is strongly associated with the use of supports such as ZnO, 14,15 ZrO 2 , 16−18 Al 2 O 3 , 19 and CeO 2 . 20,21 The metal oxide supports can regulate the chemical environment of active Cu and the Cu-metal oxide interfaces usually act as active sites for methanol synthesis from CO 2 hydrogenation.…”