Hierarchical SAPO-34 Catalysts as Host for Cu Active Sites.

Cu-containing hierarchical SAPO-34 catalysts were synthesized by the bottom-up method using different mesoporogen templates: CTAB encapsulated within ordered mesoporous silica nanoparticles (MSNs) and sucrose. A high fraction of the Cu centers exchanged in the hierarchical SAPO-34 architecture with high mesopore surface area and volume was achieved when CTAB was embedded within ordered mesoporous silica nanoparticles. Physicochemical characterization was performed by using structural and spectroscopic techniques to elucidate the properties of hierarchical SAPO-34 before and after Cu introduction. The speciation of the Cu sites, investigated by DR UV-Vis, and the results of the catalytic tests indicated that the synergy between the textural properties of the hierarchical SAPO-34 framework, the high Cu loading, and the coordination and localization of the Cu sites in the hierarchical architecture is the key point to obtaining good preliminary results in the NO selective catalytic reduction with hydrocarbons (HC-SCR).

Pinpointing basic sites formed upon incorporation of iron in hierarchical SAPO-11 using catalytic model reactions.

By utilizing previously established catalytic model reactions, a method for probing the topological location of transition metal sites incorporated in hierarchical silicoaluminophosphates (SAPOs) is presented. For the first time, iron(III)-incorporated hierarchical SAPO-11 (FeCTAB-11) was prepared and thoroughly characterized with conventional iron(III)-incorporated SAPO-11 (FeSAPO-11) as a reference. Initially, inductively coupled plasma - mass spectrometry (ICP-MS) indicated that the FeSAPOs contained similar amounts of metal (∼2.0 wt%), while N2-physisorption confirmed the bimodal porosity of the hierarchical FeCTAB-11. Furthermore, X-ray absorption spectroscopy (XAS) revealed that iron(III) was isomorphously incorporated into both SAPO-11 samples, whereas CO2-temperature programmed desorption (TPD) revealed the first reported presence of strong basic sites in the vicinity of a transition metal incorporated into a SAPO framework. The location of the basic sites, and thus the incorporated iron, was subsequently probed by studying the products of the base-catalyzed vapor phase isomerization of cyclohexanone oxime (Beckmann rearrangement, BMR) model reaction. Through an increased lifetime for the base-catalyzed production of aniline, the incorporated iron for FeCTAB-11 was found to be located in highly accessible mesopores, whereas the conventional FeSAPO-11 had incorporated iron located in its micropores. Lastly, the methanol-to-hydrocarbons (MTH) model reaction showed that both FeSAPOs only had Brønsted acid sites in the micropores of the structures. This was used to verify the pore connectivity of the hierarchical FeCTAB-11 by utilizing the base-catalyzed BMR mechanism's dependency on acid sites.

Improved lifetime and stability of copper species in hierarchical, copper-incorporated CuSAPO-34 verified by catalytic model reactions.

The first successful synthesis of hierarchical CuSAPO-34 (3.9 wt% Cu) is reported using the polymer Pluronic F127 as a mesoporous structure directing agent (SDA). X-Ray absorption spectroscopy (XAS) revealed single site Cu2+ with 4 nearest oxygen neighbours at 1.96 Å. A catalytic model reaction, the selective reduction of NO with different sized hydrocarbons as reductants, explained that Cu2+ is accessible and reactive in both micro- and mesopores of the hierarchical CuSAPO-34. The presence of mesopores resulted in superior lifetime of the hierarchical CuSAPO-34 in the catalytic model reaction, selective oxidation of propene.

XAS investigation of silica aerogel supported cobalt rhenium catalysts for ammonia decomposition.

The implementation of ammonia as a hydrogen vector relies on the development of active catalysts to release hydrogen on-demand at low temperatures. As an alternative to ruthenium-based catalysts, herein we report the high activity of silica aerogel supported cobalt rhenium catalysts. XANES/EXAFS studies undertaken at reaction conditions in the presence of the ammonia feed reveal that the cobalt and rhenium components of the catalyst which had been pre-reduced are initially re-oxidised prior to their subsequent reduction to metallic and bimetallic species before catalytic activity is observed. A synergistic effect is apparent in which this re-reduction step occurs at considerably lower temperatures than for the corresponding monometallic counterpart materials. The rate of hydrogen production via ammonia decomposition was determined to be 0.007 molH2 gcat-1 h-1 at 450 °C. The current study indicates that reduced Co species are crucial for the development of catalytic activity.

An in situ XAS study of high surface-area IrO2 produced by the polymeric precursor synthesis.

Iridium oxide powders with a surface area of more than 1 m2 g-1 (4 m2 g-2 from the H-UPD charge) and iridium-oxide crystallites less than 10 nm across were synthesized by heat treating gels formed from citric acid, ethylene glycol and dihydrogen hexachloroiridate(iv) in air. The characteristics of the resulting material was found to be strongly dependent on the heat-treatment step in the synthesis. A single heat-treatment of the gel resulted in a material with a substantial fraction of elemental iridium metal, i.e. iridium in oxidation state zero (Ir0). Post-synthesis modification of the powder by potential cycling resulted in oxidation peaks consistent with the conversion of the metal phase to iridium oxide. Linear combination of the near-edge part of the X-ray absorption data (X-ray absorption near-edge spectroscopy, XANES) collected in situ during potential cycling and an analysis of the extended X-ray fine-structure (EXAFS) part of the spectrum showed that the overall metal fraction was not significantly affected by the cycling. The oxidation of the metal phase is therefore limited to a thin layer of oxide at the metal surface, and a significant part of the iridium is left inactive. A modification of the heat treatment procedure of the sample resulted in iridium oxide containing only insignificant amounts of elemental iridium metal.

Strategies for the analysis of the elemental metal fraction of Ir and Ru oxides via XRD, XANES, and EXAFS.

Iridium and ruthenium oxide are active electrocatalysts for oxygen evolution. The relation between preparation method, structure, and behavior of mixed oxides of iridium and ruthenium are of interest in order to obtain active and stable catalysts. In this work the structure of mixed Ru-Ir oxides synthesized by the polymeric precursor method, which involves the formation of a gel containing the metal precursors and subsequent heat-treatment in air, was studied for the IrxRu1-xO2 system. An in-depth analysis of X-ray diffraction (XRD) and X-ray absorption (XAS) data, including EXAFS and linear combination of XANES, shows that the polymeric precursor synthesis method is capable of providing an intimate mixing of Ir and Ru in the catalyst. In addition to the oxide phase, metal phases, i.e. with Ru or Ir or both in oxidation state zero (Ir(fcc) and Ru(hcp)), were also found in the product materials. Facing complex structures such as some of those synthesized here, we have shown that a representation of shells with more than one atom type are efficiently represented using mixed sites, i.e. including scattering contributions from several elements in a site corresponding to a partial occupancy of the site by these elements, this method forming a very efficient basis for analyzing EXAFS data.

An In Situ XAS Study of the Cobalt Rhenium Catalyst for Ammonia Synthesis.

A cobalt rhenium catalyst active for ammonia synthesis at 400 °C and ambient pressure was studied using in situ XAS to elucidate the reducibility and local environment of the two metals during reaction conditions. The ammonia reactivity is greatly affected by the gas mixture used in the pre-treatment step. Following H2/Ar pre-treatment, a subsequent 20 min induction period is also observed before ammonia production occurs whereas ammonia production commences immediately following comparable H2/N2 pre-treatment. In situ XAS at the Co K-edge and Re LIII-edge show that cobalt initiates reduction, undergoing reduction between 225 and 300 °C, whereas reduction of rhenium starts at 300 °C. The reduction of rhenium is near complete below 400 °C, as also confirmed by H2-TPR measurements. A synergistic co-metal effect is observed for the cobalt rhenium system, as complete reduction of both cobalt and rhenium independently requires higher temperatures. The phases present in the cobalt rhenium catalyst during ammonia production following both pre-treatments are largely bimetallic Co-Re phases, and also monometallic Co and Re phases. The presence of nitrogen during the reduction step strongly promotes mixing of the two metals, and the bimetallic Co-Re phase is believed to be a pre-requisite for activity.

Identification of synergistic Cu/V redox pair in VCu:AlPO-5; a comparison with VCu:ZSM-5.

Vanadium(V) and copper(II) were co-deposited into AlPO-5 and H-ZSM-5 three-dimensional microporous carriers to yield VCu:AlPO-5 and VCu:ZSM-5. The materials, along with copper analogues were tested for the selective oxidation of propene, and the catalysts perform in the following order: VCu:AlPO-5 > Cu:AlPO-5 > VCu:ZSM-5 > Cu:ZSM-5. Acrolein was selectively formed over VCu:AlPO-5 and Cu:AlPO-5 over a very wide range from 300 to 450 °C, whereas VCu:ZSM-5 displays a limited temperature window for acrolein formation (300-350 °C). Hence, the choice of carrier and presence of vanadium as a co-cation greatly affects the acrolein selectivity and activity window. The vanadium and copper reduction events were monitored by in situ X-ray Absorption Spectroscopy (XAS) during C3H6-TPR (1.11%) to 450 °C. The Cu(II)/(I) redox pair initiates reduction of V(V) → V(IV) in VCu:AlPO-5 and VCu:ZSM-5 at 375 °C. Metallic copper is the major valence fraction above 400 °C in both samples while vanadium is present as V(IV)/V(III) species. In the monometallic copper analogues Cu(I) is the major valence fraction above 350 °C, hence synergistic effects between the Cu/V pair causes hyper-reduction of copper. EXAFS shows that copper and vanadium are in close proximity in VCu:AlPO-5 only, being linked by bridging oxygens (Cu-O-V) believed to interact with propene. By contrast, propene adsorbs on Brønsted sites in VCu:ZSM-5 inhibiting acrolein formation at elevated temperatures, as confirmed by DRIFTS. We believe the reactive Cu/V pair in neutral AlPO-5 generates extralattice oxygens favouring acrolein formation over a wide temperature range.

The Methylococcus capsulatus (Bath) secreted protein, MopE*, binds both reduced and oxidized copper.

Under copper limiting growth conditions the methanotrophic bacterium Methylococcus capsulatus (Bath) secrets essentially only one protein, MopE*, to the medium. MopE* is a copper-binding protein whose structure has been determined by X-ray crystallography. The structure of MopE* revealed a unique high affinity copper binding site consisting of two histidine imidazoles and one kynurenine, the latter an oxidation product of Trp130. In this study, we demonstrate that the copper ion coordinated by this strong binding site is in the Cu(I) state when MopE* is isolated from the growth medium of M. capsulatus. The conclusion is based on X-ray Near Edge Absorption spectroscopy (XANES), and Electron Paramagnetic Resonance (EPR) studies. EPR analyses demonstrated that MopE*, in addition to the strong copper-binding site, also binds Cu(II) at two weaker binding sites. Both Cu(II) binding sites have properties typical of non-blue type II Cu (II) centres, and the strongest of the two Cu(II) sites is characterised by a relative high hyperfine coupling of copper (A(||) =20 mT). Immobilized metal affinity chromatography binding studies suggests that residues in the N-terminal part of MopE* are involved in forming binding site(s) for Cu(II) ions. Our results support the hypothesis that MopE plays an important role in copper uptake, possibly making use of both its high (Cu(I) and low Cu(II) affinity properties.

In situ XAS and IR studies on Cu:SAPO-5 and Cu:SAPO-11: the contributory role of monomeric linear copper(i) species in the selective catalytic reduction of NOx by propene.

Cu:SAPO-5 and Cu:SAPO-11 were prepared by conventional and hydrothermal ion exchange. Copper incorporation is increased six-fold by hydrothermal ion exchange relative to conventional methods. In all cases, the amount of copper taken up by SAPO-11 is superior to uptake in SAPO-5. Copper is divalent and in tetragonally-distorted octahedral environments in the as-prepared samples independent of the method of incorporation for both systems. The local structures about the metal and the valence states associated with the different steps in the selective catalytic reduction of NO(x) in the presence of propene (SCR-HC) have been investigated using X-ray absorption spectroscopy (XAS). For both the Cu:SAPO-5 and Cu:SAPO-11 systems, heating in helium partially autoreduces copper(ii) to copper(i). Following activation in oxygen, propene causes further reduction to copper(i) in all four samples as shown by the evolution of an intense pre-edge diagnostic feature. XANES analysis reveal this to be characteristic of monomeric linear two coordinate copper(i) species. This is a prime example of a pre-edge peak with such a high intensity being observed in the solid state. This is supported by IR where peaks attributed to bidentate copper were observed for Cu:SAPO-11/HT. For all four samples NO(x) partially reoxidises the copper(i) formed in the helium and propene steps. Ion exchanged Cu:SAPO-5 and Cu:SAPO-11 exhibit low activity in reducing NO(x) by propene in an oxygen rich environment. The role of the copper ion during NO adsorption was studied using in situ infra red spectroscopy. The activity of copper exchanged materials is governed by both the degree of reducibility of copper(ii) and the ease of reversing the valence states with the structural characteristics of the parent materials playing a crucial role.