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

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

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 (

COx-free hydrogen production from ammonia – mimicking the activity of Ru catalysts with unsupported Co-Re alloys

On-demand production of hydrogen from ammonia is a challenge limiting the implementation of ammonia as a long term hydrogen vector to overcome the difficulties associated with hydrogen storage. Herein, we present the development of catalysts for the on-demand production of hydrogen from ammonia by combining metals with high and low N-adatom adsorption energies. In this way, cobalt-rhenium (Co-Re) catalysts show high activity mimicking that of ruthenium. EXAFS/XANES analyses demonstrate that the bimetallic Co-Re contribution is responsible for the activity and the stability of the catalysts in consecutive runs with no observable formation of nitrides (Co-N and Re-N) occurring under the ammonia atmosphere. While cobalt is partially re-oxidised under ammonia, re-reduction in the presence of rhenium is observed at higher temperatures, coinciding with the on-set of catalytic activity which is accompanied by minor structural changes. These results provide insight for the development of highly active alloy based ammonia decomposition catalysts

On the Promoting Effect of Water during NO_<i>x</i> Removal over Single-Site Copper in Hydrophobic Silica APD-Aerogels

Single-site copper cations incorporated into hydrophobic silica APD-aerogels (2–8 wt %) are highly active for the Selective Catalytic Reduction of NO_<i>x</i> with C₃H₆ as reducing agent (SCR-HC-deNO_<i>x</i>) in the range 300–450 °C, reaching conversions up to 67% at 450 °C. In contrast to reported behavior of zeolite type matrixes, water in the feed has a promoting effect on the deNO_<i>x</i> activity of the hydrophobic Cu-aerogels, making these systems promising commercial candidates in the 300–450 °C activity windows under realistic conditions. The Cu-aerogels were compared to Cu-xerogel, a hydrophilic denser gel analogue, and Cu-ZSM-5, an established deNO_<i>x</i> catalyst featuring poor hydrothermal stability. This study aims to elucidate the origins of deNO_<i>x</i> activity and the effect of water by correlating copper speciation with surface species and gel nature at different reaction stages in dry and wet feed. X-ray Absorption Spectroscopy (XAS) and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) measurements have been performed in situ, while monitoring reactor effluents during the experiments. Reversibility of the Cu²⁺/Cu⁺ redox pair was confirmed in the Cu-aerogel during and after wet redox cycling. This was not the case for the Cu-xerogel, or the Cu-ZSM-5, where the Cu²⁺/Cu⁺ redox pair and surroundings were reversibly and irreversibly affected by wet feed, respectively. Exclusive to Cu-aerogels was the formation of Brønsted acidic silanol cluster surrounding copper by multihydroxyl interaction with water. These silanol clusters are capable of storing reaction components and key intermediates, which we believe is responsible for enhancing the catalytic removal of NO_<i>x</i> in Cu-aerogels

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

Evaluation of surfactant templates for one-pot hydrothermal synthesis of hierarchical SAPO-5

Hierarchical SAPO-5 molecular sieves were synthesized with three different mesopore structure-directing agents (meso-SDAs) (cetyltrimethylammonium bromide (CTAB), myristyltrimethylammonium bromide (MTMAB) and [3-(trimethoxysilyl)propyl] dimethyloctadecylammonium chloride (TPOAC)) based on a soft-template hydrothermal synthesis procedure. To investigate the modified porosity of the hierarchical SAPO-5s, they were characterized thoroughly with the results being compared to the conventional microporous SAPO-5. Nitrogen sorption measurements revealed considerable hysteresis loops for the hierarchical SAPO-5s as well as larger mesopore volumes (�0.15 cm3 g-1) compared to the conventional SAPO-5 (0.05 cm3 g-1). The relative number of acid sites for each sample was calculated from FTIR adsorption data and was in the order of C-SAPO-5>HCTAB>H-MTMAB>H-TPOAC. The hierarchical SAPO-5s had a significantly increased lifetime (>150 h) in the methanol to hydrocarbons (MTH) model reaction compared to the conventional SAPO-5 (<10 h), with TPOAC producing the most stable catalyst and MTMAB producing the catalyst with the largest product distribution. The modified porosity of the hierarchical SAPO-5s was additionally verified by an enhanced lifetime and increased production of large products in a shape selective process as well as a lower rate of coke formation compared to the conventional SAPO-5

Comparing CuAPO-5 with Cu : ZSM-5 in the selective catalytic reduction of NOx: An in situ study

The CuAPO-5 and Cu:ZSM-5 systems are compared in the selective catalytic reduction (SCR) of NOx. Within the Cu:ZSM-5 system materials with high and low metal contents were prepared by hydrothermal and conventional ion exchange, respectively. X-ray absorption spectroscopy (XAS) was used to follow changes in the valence state and the local environment of copper as the different materials reacted with the individual components of this particular deNOx process. This was effected by measuring the XAS spectra after each consecutive stage of the treatment with propene and NOx. In the CuAPO-5 system the reduction of copper(II) by propene to copper(I) is only partial and metallic copper or copper oxides are formed during the NOx and propene treatments. This can explain the relatively low activity (16-18%) of both the low and high copper content CuAPO-5 samples toward NOx reduction. By contrast, in the Cu:ZSM-5 system, copper(II) is reduced to copper(I) by propene and subsequently reoxidised by NOx. Copper is only partially reoxidised in Cu: ZSM-5 prepared by hydrothermal ion exchange (45% deNO(x) conversion), but the amount of copper exchanged by this method (133%) is significantly higher than in the conventional ion exchanged preparations (35%). The final propene treatment establishes that the Cu(I)/Cu(II) redox pair is formed in both Cu:ZSM-5 samples when reducing NOx. Previous studies suggest that dimeric copper species, in addition to isolated copper ions, are responsible for the activity of high copper content Cu:ZSM-5. We find no evidence of copper clustering in this system with the protocol used here

Strategies for the analysis of the metallic 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

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