Catalysts For Methanol Oxidation Research Articles

Soft XAS as an in situ technique for the study of heterogeneous catalysts.

Soft X-ray absorption in situ studies of heterogeneous catalysts have been applied to areas such as copper methanol oxidation catalysts and cobalt Fischer-Tropsch type catalysts over a period of around two decades. The technique has the potential to offer several advantages for studying heterogeneous catalysts against hard X-ray XAS in: the systems that can be studied (includes elements such as C, N, O), the potential for surface sensitivity (crucial for catalysts, where reactions occur at surfaces) and the information content of the resulting spectra. Nevertheless, it is technically challenging and the necessary hardware has only been developed and evolved in a few specific groups worldwide. This perspective will introduce the technique in the context of other competing spectroscopies, summarise the development of hardware and the challenges that have been overcome in experimental terms, along with the outcome and impact on different fields within catalysis. Additionally, anticipated future trends and directions will be discussed.

Al-doped Fe2O3 as a support for molybdenum oxide methanol oxidation catalysts.

We have made high surface area catalysts for the selective oxidation of methanol to formaldehyde. This is done in two ways - (i) by doping haematite with Al ions, to increase the surface area of the material, but which itself is unselective and (ii) by surface coating with Mo which induces high selectivity. Temperature programmed desorption (TPD) of methanol shows little difference in surface chemistry of the doped haematite from the undoped material, with the main products being CO2 and CO, but shifted to somewhat higher desorption temperature. However, when Mo is dosed onto the haematite surface, the chemistry changes completely to show mainly the selective product, formaldehyde, with no CO2 production, and this is little changed up to 10% Al loading. But at 15 wt% Al, the chemistry changes to indicate the presence of a strongly acidic function at the surface, with additional dimethyl ether and CO/CO2 production characteristic of the presence of alumina. Structurally, X-ray diffraction (XRD) shows little change over the range 0-20% Al doping, except for some small lattice contraction, while the surface area increases from around 20 to 100 m2 g-1. Using X-ray absorption spectroscopy (XAS) it is clear that, at 5% loading, the Al is incorporated into the Fe2O3 corundum lattice, which has the same structure as α-alumina. By 10% loading then it appears that the alumina starts to nano-crystallise within the haematite lattice into the γ form. At higher loadings, there is evidence of phase separation into separate Al-doped haematite and γ-alumina. If we add 1 monolayer equivalent of Mo to the surface there is already high selectivity to formaldehyde, but little change in structure, because that monolayer is isolated at the surface. However, when three monolayers equivalent of Mo is added, we then see aluminium molybdate type signatures in the XANES spectra at 5% Al loading and above. These appear to be in a sub-surface layer with Fe molybdate, which we interpret as due to Al substitution into ferric molybdate layers immediately beneath the topmost surface layer of molybdena. It seems like the separate γ-alumina phase is not covered by molybdena and is responsible for the appearance of the acid function products in the TPD.

Heteroatom doped 3D graphene aerogel supported catalysts for formic acid and methanol oxidation

In this study, nitrogen (N) and boron (B) heteroatom doped graphene aerogel support materials have been employed for the dispersion of platinum (Pt) nanoparticles to improve their electrocatalytic activities for formic acid and methanol oxidation. Pt nanoparticles dispersed on the heteroatom doped graphene aerogel (GA) support materials by a microwave heating method. The as-prepared catalysts were characterized by a variety of means such as SEM, EDS, ICP-MS, TEM, XRD, BET and XPS. The electrocatalytic activities, stability and impedance of the synthesized catalysts were investigated for formic acid and methanol oxidation using electrochemical measurements. The 3D graphene aerogels have higher capacitive currents than the Vulcan XC-72 in the double layer region. The results of electrochemical chronoamperometry tests reveal that Pt/BGA shows the best stability for methanol oxidation and also exhibited superior electrocatalytic activity towards the oxidation of methanol in cyclic voltammetry. In addition to, heteroatom doped GA supported catalysts higher activity compared to the Vulcan XC-72 supported catalyst for formic acid oxidation.

Electrocatalytic Activity and Stability of 3D Ordered N‐doped Hierarchically Porous Carbon Supported Pt Catalyst for Methanol Oxidation and Oxygen Reduction Reactions

AbstractHerein, nitrogen‐doped ordered hierarchical porous carbon (N‐OHPC) is fabricated via a templating method by applying silica colloidal crystal as template and polypyrrole (PPy) as carbon source. The as‐prepared N‐OHPC possesses ordered interconnected macropores, rich mesopores and large BET surface area (825.6 m2 g−1). Pt/N‐OHPC is then prepared by depositing Pt nanoparticles (19.3 wt%) on N‐OHPC as catalyst towards methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). The Pt/N‐OHPC containing small and uniform Pt nanoparticles (∼1.7 nm) exhibits good electrocatalytic performance and better durability for MOR and ORR than the commercial Pt/C catalyst.

Stability of Iron-Molybdate Catalysts for Selective Oxidation of Methanol to Formaldehyde: Influence of Preparation Method

Iron molybdate/molybdenum oxide catalysts with varying content of Mo (Mo/Fe = 1.6 and 2.0) were synthesized by a mild hydrothermal method and structurally characterized by XRD, XPS, Raman spectroscopy, SEM–EDX, BET and ICP-OES. The stability of the prepared catalysts in selective oxidation of methanol to formaldehyde was investigated by catalytic activity measurements for up to 100 h on stream in a laboratory fixed-bed reactor (5% MeOH, 10% O2 in N2, temp. = 380–407 °C). Excess MoO3 present in the catalyst volatilized under reaction conditions, which lead to an initial loss of activity. Interestingly, the structure of the excess MoO3 significantly affected the stability of the catalyst. By using low temperature hydrothermal synthesis, catalysts with the thermodynamically metastable hexagonal h-MoO3 phase was synthesized, which yielded relatively large crystals (2–10 µm), with correspondingly low surface area to volume ratio. The rate of volatilization of MoO3 from these crystals was comparatively low, which stabilized the catalysts. It was furthermore shown that heat-treatment of a spent catalyst, subject to significant depletion of MoO3, reactivated the catalyst, likely due to migration of Mo from the bulk of the iron molybdate crystals to the surface region. Fe2(MoO4)3/MoO3 catalysts for selective oxidation of methanol were synthesized by hydrothermal synthesis forming large hexagonal-MoO3 crystals. Significantly lower rate of catalyst deactivation due to volatilization of MoO3 under reaction conditions was observed for the large h-MoO3 compared to smaller crystals of thermodynamically stable α-MoO3.

Synthesis of Hollow Pt-Ni Nanoboxes for Highly Efficient Methanol Oxidation

In direct methanol fuel cell technology, highly stable electrochemical catalysts are critically important for their practical utilization at the commercial scale. In this study, sub ~10 nm hollow Pt-Ni (1:1 at. ratio) nanoboxes supported on functionalized Vulcan carbon (Pt-Ni/C-R2) were synthesized through a facile method for the efficient electrooxidation of methanol. Two reaction procedures, namely, a simultaneous reduction and a modified sequential reduction method using a reverse microemulsion (RME) method, were adopted to synthesize solid Pt-Ni NPs and hollow nanoboxes, respectively. To correlate the alloy composition and surface structure with the enhanced catalytic activity, the results were compared with the nanocatalyst synthesized using a conventional NaBH4 reduction method. The calculated electroactive surface area for the Pt-Ni/C-R2 nanoboxes was 190.8 m2.g−1, which is significantly higher compared to that of the Pt-Ni nanocatalyst (96.4 m2.g−1) synthesized by a conventional reduction method. Hollow nanoboxes showed 34% and 44% increases in mass activity and rate of methanol oxidation reaction, respectively, compared to solid NPs. These results support the nanoreactor confinement effect of the hollow nanoboxes. The experimental results were supported by Density Functional Theory (DFT) studies, which revealed that the lowest CO poisoning of the Pt1Ni1 catalyst among all Ptm-Nin mixing ratios may account for the enhanced methanol oxidation. The synthesized hollow Pt-Ni/C (R2) nanoboxes may prove to be a valuable and highly efficient catalysts for the electrochemical oxidation of methanol due to their low cost, numerous catalytically active sites, low carbon monoxide poisoning, large electroactive surface area and long-term stability.

A new durable and high performance platinum supported on Ag–Ni-porous coordination polymer as an anodic DMFC nano-electrocatalyst: DFT and experimental investigation

Due to the poor performance and intermediates poisoning of available catalysts in direct methanol fuel cells (DMFC), the researcher is confronted with a considerable challenge for obtaining modified electrocatalyst. Ag–Ni porous coordination polymer (ANP) as a new electrocatalyst supporter was synthesized by a hydrothermal method. To achieve favorable electrocatalyst for DMFC systems, platinum nanoparticles was deposited upon ANP by an electrochemical method and platinum supported on Ag–Ni porous coordination polymer (Pt-ANP) was formed. Fourier transform infrared spectroscopy (FTIR) analysis ensured correct synthesized of ANP and Pt-ANP. In addition, the morphologies investigation of ANP and Pt-ANP were carried out by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). The FE-SEM images indicate that the platinum nanoparticles have been greatly deposited on ANP surface. Electrochemical behaviors of prepared catalyst for methanol oxidation were evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry (CA) techniques. Electrochemical cyclic voltammetry tests (CV) indicate that the forward peak current density of Pt-ANP is about 105 mA/cm2 which it is 33% more than the forward peak current density of pure Pt catalyst (70.21 mA/cm2). Moreover, electrochemical surface area (ECSA) of Pt-ANP is 26.42 m2/gPt. In addition, density functional theory (DFT) computations show that with the deposition of Pt upon ANP, the HOMO-LOMO energy gap of ANP has been decreased which they are suitable for electrochemical reactions. Theoretical results are greatly in accordance with the experiments. Based on the results, Pt-ANP could be a superior electrocatalyst for methanol oxidation.

Methanol electro-oxidation on Cu@Pt/C core-shell catalyst derived from Cu-MOF

Core-shell structured Cu@Pt/C catalyst was fabricated by a simple three-step procedure involving hydrothermal synthesis of copper-centered metal organic framework (Cu-MOF), carbon-supported copper nanoparticles (Cu/C) by direct carbonization at 600℃ and partial galvanic replacement of Cu particle layers by Pt in a chloroplatinate solution. The characterization of Cu@Pt/C catalyst is achieved by XRD (X-ray powder diffraction), SEM (scanning electron microscopy) and voltammetry techniques. Cu@Pt/C catalyst has shown much larger electrochemical active surface area (ECSA, 74.3 m2.g−1) than commercial Pt/C catalyst (50.4 m2.g−1). Furthermore, nearly four-fold enhancement of activity of Cu@Pt/C catalyst for methanol oxidation has been observed. Moreover, the charge transfer resistance of methanol oxidation for Cu@Pt/C catalyst is 22.8 Ω cm-2, lower than that for Pt/C catalyst (73.7 Ω cm-2). Additionally, Cu@Pt/C catalyst exhibits a superior catalytic stability. The electronic effect modification and large ECSA may be collectively responsible for the enhanced catalytic activity of Cu@Pt/C over Pt/C.

Cellulose nanocrystal/clay based macroion nanogel as support for stable platinum catalyst for electrochemical oxidation of methanol in alkaline medium

Nanogels are well known for their ability to encapsulate metal nanoparticles. However, only few reports exist on the application of such metal nanoparticle-loaded nanogels as electrocatalyst. Here, the formation of a self-assembled macroion nanogel-based complex using sustainable nanomaterials and its functional application as catalyst support for methanol oxidation in alkaline medium is reported. Physico-chemical and morphological investigations of the nanogel showed the formation of micro-nano particles, suggesting the presence of good ionic interactions. Prepared complex was utilized to host catalytically active platinum nanoparticles. On calcination, Pt-loaded nanogels showed good activity towards methanol oxidation in alkaline medium. Further, based on linear sweep voltammetry, it was proposed that for the current system reported here, reaction intermediates formed in methanol oxidation during the cyclic voltammetry analysis get removed from the catalyst surface during the forward scan, instead of the reverse scan. The proposed hypothesis was further supported by impedance analysis and the potential range for removal of intermediates was determined, which was found to be 0.1 V – 0.2 V in alkaline medium during the forward scan. Thus, it has been successfully demonstrated that such catalytic nanoparticle-encapsulated complexes have good electrochemical activity.

Co-Electroless Deposition of Pt-M Bimetallic Alloyed Catalysts for Methanol Oxidation

Direct methanol fuel cells exhibit great promise as portable sources of power. A primary issue, however, is that the platinum catalyst used is readily poisoned by CO that is formed during the methanol oxidation reaction. Further, reducing the amount of Pt used is crucial to reducing the cost of direct methanol fuel cells. A straightforward way to address both issues is to synthesize bimetallic catalysts with Pt and a secondary metal that provides the necessary –OH groups to remove CO from Pt. Co-precipitation and co-impregnation methods have traditionally been used for the synthesis of metallic and bimetallic catalysts. These methods, however, afford little control over nanoparticle composition, size, and spacing. Electroless deposition solves these issues by allowing precise compositional, size, and, when paired with strong electrostatic adsorption, spacing control. The Monnier group has developed a continuous deposition method that produces composition-controlled bimetallic alloyed catalysts on a seeded support.1 Mehrabadi et al. 2 demonstrated previously that Ru deposited by electroless deposition onto a Pt/C support was superior to commercial PtRu catalysts for methanol oxidation. Bimetallic and trimetallic catalyst syntheses have also been demonstrated. In this work, we synthesize Pt-Co and Pt-Ni bimetallic catalysts with controlled compositions. We find that specific compositions (Pt3Ni and Pt2Co) exhibit significantly improved mass activity. This talk will discuss the synthesis and characterization of these bimetallic alloyed catalysts as well as the activity for methanol oxidation observed. Tate, G., Kenvin, A., Diao, W., Monnier, J. R., “Preparation of Pt-containing bimetallic and trimetallic catalysts using continuous electroless deposition methods,” Catalysis Today, (2018)Tavakoli Mehrabadi, B.A., Catalysis Today, https://doi.org/10.1016/j.cattod.2018.11.042

Operando XAS/XRD and Raman Spectroscopic Study of Structural Changes of the Iron Molybdate Catalyst during Selective Oxidation of Methanol

AbstractThe structural changes of an iron molybdate/molybdenum oxide (Mo/Fe=2.0) catalyst for the selective oxidation of methanol to formaldehyde were studied using combined operando X‐ray absorption spectroscopy (XAS) and X‐ray diffraction (XRD) as well as operando Raman spectroscopy. Under operating conditions, the Mo K‐edge XANES spectra showed a transition from a mixture of α‐MoO3 and Fe2(MoO4)3 towards only Fe2(MoO4)3. XRD and Raman spectroscopy also showed disappearance of the α‐MoO3 phase with time on stream. The results evidenced that the α‐MoO3 component evaporated completely, while the Fe2(MoO4)3 component remained stable. This was linked to a decrease in catalytic activity. Further studies unraveled that the rate of α‐MoO3 evaporation increased with increasing MeOH concentration, decreasing O2 concentration and increasing temperature. The simultaneous measurements of catalytic activity and spectroscopy allowed to derive a structure‐activity relationship showing that α‐MoO3 evaporation needs to be prevented to optimize MoO3‐based catalysts for selective oxidation of methanol.

Sol-Gel Synthesis of Ruthenium Oxide Nanowires To Enhance Methanol Oxidation in Supported Platinum Nanoparticle Catalysts.

A template-directed, sol–gel synthesis is utilized to produce crystalline RuO2 nanowires. Crystalline nanowires with a diameter of 128 ± 15 nm were synthesized after treating the nanowires at 600 °C in air. Analysis of these nanowires by X-ray powder diffraction revealed the major crystalline phase to be tetragonal RuO2 with a small quantity of metallic ruthenium present. Further analysis of the nanowire structures by high-resolution transmission electron microscopy reveals that they are polycrystalline and are composed of interconnected, highly crystalline, nanoparticles having an average size of ∼25 nm. Uniform 3 nm Pt nanoparticles were dispersed on the surface of RuO2 nanowires using an ambient, solution-based technique yielding a hybrid catalyst for methanol oxidation. Linear sweep voltammograms (LSVs) and chronoamperometry performed in the presence of methanol in an acidic electrolyte revealed a significant enhancement in the onset potential, mass activity, and long-term stability compared with analogous Pt nanoparticles supported on commercially available Vulcan XC-72R carbon nanoparticles. Formic acid oxidation LSVs and CO stripping voltammetry revealed that the RuO2-supported Pt nanoparticles exhibit significantly higher CO tolerance, which leads to higher catalytic stability over a period of several hours. X-ray photoelectron spectroscopy results suggest that crystalline RuO2 leads to less-significant oxidation of the Pt surface relative to more widely studied hydrous RuO2 supports, thereby increasing catalytic performance.

Transition Metal Ions Assisted Trimerization of Polyisocyanurate and Their Pyrolyzed Derivatives as Synergistic Electrocatalysts for Oxidation of Alcohols

We demonstrate the trimerization of polyisocyanurate nanoporous foams by reacting aromatic isocyanate in the presence of transition metal salts (NiCl2·2H2O and CoCl2·6H2O) as catalysts. The metal ions were found to accelerate the gelation of the polyisocyanurate networks. The nickel–cobalt–polyisocyanurate (Ni-Co-PIR-RT) hybrid monoliths were pyrolyzed under inert atmosphere at varying temperatures (from 600 to 1000 °C) to yield oxide and metallic forms of Ni and Co with significant carbon content. Pyrolysis of Ni-Co-PIR-RT at 600 °C (Ni-Co-PIR-600) resulted in the complete reduction of Ni and Co ions to metallic Ni and Co, whereas samples pyrolyzed at 800 °C (Ni-Co-PIR-800) resulted in the partial transformation of the metallic Ni and Co to its respective oxides. The complete transformation of metallic Co to CoO was obtained at 1000 °C (Ni-Co-PIR-1000) with a significant amount of metallic Ni and NiO. The Ni-PIR-1000 sample displayed minimal activity toward methanol oxidation reaction (MOR) with an onset potential and maximum current density of 1.44 V (vs reversible hydrogen electrode) and 0.6 mA/cm2 respectively, whereas the Co-PIR-1000 sample displayed oxygen evolution reaction (OER) at an onset potential of 1.33 V with no evident activity toward MOR in 0.1 M KOH with 0.2 M CH3OH. On the other hand, under the same conditions Ni-Co-PIR-1000 displayed 5 manifold increase in current density and reduction in onset potential by 100 mV toward MOR (with an onset potential of 1.34 V and current density 4 mA/cm2) relative to Ni-PIR-1000. The synergistic and superior activity of Ni-Co-PIR-1000 can be attributed to the combined presence of Ni, NiO, and CoO. Here, (1) Ni and NiO act as the catalysts for methanol oxidation by electrochemically transforming to NiOOH (active centers for methanol oxidation) via Ni(OH)2, (2) the presence of CoO facilitates the electrochemical generation of atomic oxygen ([O]) (an intermediate in oxygen evolution reaction and a well-known oxidizing agent) by transforming to CoOOH, the [O] formed here may be responsible for the formation of NiOOH by chemically reacting with Ni(OH)2 resulting in increased activity toward MOR.

PtIr alloy nanowire assembly on carbon cloth as advanced anode catalysts for methanol oxidation

The interconnecred PtIr alloy nanowires were uniformly deposited on carbon cloth via One-step wet chemistry method, which diameter is averaged to be 5 nm with a length of 50–200 nm. The carbon cloth supported PtIr nanowire assembly (PtIr NA/CC) shows a larger electrochemical active surface area (ECSA) due to its 3D nanostructure and a high CO-resistance as a result from the synergistic effect of PtIr alloy. The PtIr NA/CC exhibits an extremely high mass activity and a reliable long-term stability toward methanol oxidation reaction (MOR). The superior catalytic performance on MOR can match and even surpass those best Pt-based nanowires reported recently in the literature.

Fabrication and electrochemical kinetics studies of reduced carbon quantum dots- supported palladium nanoparticles as bifunctional catalysts in methanol oxidation and hydrogen evolution reactions

Glassy carbon electrode, GCE, is decorated with an electrochemically-deposited layer of reduced carbon quantum dots, RCQDs. The Pd nanoparticles are then electrodeposited on RCQDs and the Pd-NPs/RCQDs/GCE is fabricated. The fabricated electrodes are characterized by some characterization techniques. The electrocatalytic performance of the electrodes for methanol oxidation reaction, MOR, and hydrogen evolution reaction, HER, are investigated using different electrochemical measurement techniques in alkaline and acidic media, respectivly. Cyclic voltammetry, CV, Linear sweep voltammetry, LSV, Chronoamperometry, CA, and electrochemical impedance spectroscopy, EIS, measurements are used to evaluate the bifunctional catalytic behavior of the Pd-NPs/RCQDs/GCE. All results are compared with Pd-NPs/GCE and the effect of decorating the GCE substrate with RCQDs is discussed. The results indicate that modification of GCE with a layer of RCQDs increases the active surface area from 0.12 for Pd-NPs/GCE to 0.22 cm2 for Pd-NPs/RCQDs/GCE. Also Eonset and EIS parameters are calculated.

Pd nanoparticles immobilized on MIL-53(Al) as highly effective bifunctional catalysts for oxidation of liquid methanol to methyl formate

A series of Pd/MIL-53(Al) heterogeneous bifunctional catalysts with different Pd contents were prepared by an impregnation method. The prepared metal–organic frameworks MIL-53(Al) and catalysts were characterized by XRD, SEM, HRTEM, FT-IR and N2 adsorption/desorption techniques. The results showed that MIL-53(Al) was synthesized successfully, and the structure was unchanged during and after the preparation of the catalysts. The Pd nanoparticles (NPs) with an average particle size of 4.6 nm were uniformly dispersed on the MIL-53(Al). The catalyst exhibited good catalytic activity in the selective oxidation of liquid methanol to methyl formate. Under the conditions of 150 °C, 2 MPa O2 and solvent-free for 5 h, the conversion of methanol could reach 60.3%, and the selectivity of methyl formate was up to 62.2%. In addition, the Pd/MIL-53(Al) bifunctional catalyst exhibited excellent stability and maintained high catalytic activity after five cycles.

One-step efficiently coupling ultrafine Pt–Ni2P nanoparticles as robust catalysts for methanol and ethanol electro-oxidation in fuel cells reaction

Multi-components valid interaction is significant to maximize the catalyst performance in fuel cells catalyst system. Herein, we demonstrate the successful fabrication of nano-structured Pt–Ni2P-Graphene hybrid by a simple one-step approach and their active and stable performance for alcohols oxidation in fuel cells reaction. The hybrid crystal structure formation with a slightly reduced lattice parameter and a close interaction for Pt and Ni2P nanoparticles are demonstrated by spectrum analysis; a strong electronic effect is found by a large negative shift of Pt 4f spectrum. The more and the less of Ni2P in the system gives lower catalytic ability; 20% Pt–Ni2P-Graphene demonstrates the best CO anti-poisoning ability and alcohols fuel electro-oxidation performance. Specifically, the mass activity and specific activity are about 3 and 2 times higher than that of Pt/Graphene catalyst for methanol oxidation, and about 2.7 and 1.8 times improvement for ethanol oxidation. The high catalytic activity, rapid kinetics and excellent catalytic stability can be attributed to the Pt and Ni2P hybridization formed in the one-step hydrothermal approach that imparts them high anti-poisoning ability and large electrochemical active surface area. This fabrication approach can also be extended to other analogues catalyst system for fuel cells catalysis.

S,N dual-doped carbon nanotubes as substrate to enhance the methanol oxidation performance of NiO nanoparticles

Surface atomic engineering of substrates provides an efficient way to further improve the catalytic activity and stability of coupled catalysts for methanol oxidation. In this paper, S,N dual-doped carbon nanotubes (S,N-CNTs) with high doping levels of S (4.78 at%) and N (3.58 at%) were generated by laser irradiation, and used as substrate to in-situ load NiO nanoparticles (NPs). S and N atoms in CNTs provided abundant anchor sites for NiO NPs to restrain their growth, strengthen the interaction between NiO NPs and CNTs, and promote the interfacial electron transportation. Therefore, electrochemical tests demonstrated that NiO/S,N-CNTs shown a high primary methanol oxidation activity of 2200 mA/mg, a long-term durability with 65.8% of mass activity maintained after 40000s, and an excellent methanol saturation concentration of 13 M. This work provides a facile strategy to fabricate S,N-CNTs and helps to understand the effect of heteroatoms in carbonaceous substrates on boosting the electroactivity of coupled catalysts.

The effect of Ag and CeO2 distribution on SBA-15 surface on the activity of Ag–CeO2/SBA-15 catalysts in CO and methanol oxidation

The series of catalysts representing the mesoporous ordered silica SBA-15 with supported silver or silver and ceria nanoparticles were synthesized by incipient wetness impregnation, characterized by the low-temperature N2 adsorption, XRD, TEM HR, TPR-H2 methods, and tested in the low-temperature CO oxidation and total oxidation of methanol. It was found that two types of silver particles are formed: small particles (<3 nm) inside the pores of the SBA-15 and larger particles (3–15 nm) on the external surface of SBA-15. High activity of the prepared catalyst in oxidation of CO and methanol is attributed to both small sizes of silver particles and the Ag–CeO2 interaction. The distribution of both silver and ceria on the SBA-15 surface determines their interaction and, hence, enhances the catalytic activity in oxidative reactions due to the cooperation of active oxidative sites of silver and ceria at the Ag–CeO2 interface.

Preparation of two series of VxSiBeta zeolite catalysts with V centres in framework and extra-framework positions and their application in selective oxidation of methanol

Two series of V-containing Beta zeolites have been prepared by contacting of the aqueous NH4VO3 solution with two SiBeta zeolites at pH = 2.5 and 6, respectively. Because of the presence in NH4VO3 solution at pH = 2.5 mainly monomeric VO2+ ions, vanadium have been easily incorporated into framework of SiBeta zeolite as pseudo-tetrahedral non hydroxylated (SiO)3V = O and hydroxylated (SiO)2(OH)V = O species. The moderate nucleophilicity of the basic vanadyl oxygen of the latter species play important role in methanol oxidation toward formaldehyde. The selectivity toward formaldehyde on this series of VxSiBeta(I) increases with the amount of pseudo-tetrahedral hydroxylated (SiO)2(HO)V = O species, which act as either redox or acidic and basic centres. In contrast at pH = 6, the aqueous NH4VO3 solution is expected to contain both mononuclear and polynuclear V ions, thus it is more difficult to incorporate V species in SiBeta at this condition as shown by FT-IR and NMR data. The absence of pseudo-tetrahedral V species in framework position of V0.6SiBeta(II) is probably responsible for lack of activity of this catalyst in methanol oxidation. The appearance of this species in the series of VxSiBeta(II) zeolites for high V content leads to their activity in methanol oxidation toward formaldehyde.