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Co3O4 nanoparticles were supported on different TiO2 polymorphs, namely, rutile, anatase, and a 15 : 85 mixture of rutile and anatase (also known as P25), via incipient wetness impregnation. The Co3O4/TiO2 catalysts were evaluated in the preferential oxidation of CO (CO-PrOx) in a H2-rich gas environment and characterised in situ using PXRD and magnetometry. Our results show that supporting Co3O4 on P25 resulted in better catalytic performance, that is, a higher maximum CO conversion to CO2 of 72.7% at 200 °C was achieved than on rutile (60.7%) and anatase (51.5%). However, the degree of reduction (DoR) of Co3O4 to Co0 was highest on P25 (91.9% at 450 °C), with no CoTiO3 detected in the spent catalyst. The DoR of Co3O4 was lowest on anatase (76.4%), with the presence of TixOy-encapsulated CoOx nanoparticles and bulk CoTiO3 (13.8%) also confirmed in the spent catalyst. Relatively low amounts of CoTiO3 (8.9%) were detected in the spent rutile-supported catalyst, while a higher DoR (85.9%) was reached under reaction conditions. The Co0 nanoparticles formed on P25 and rutile existed in the fcc and hcp crystal phases, while only fcc Co0 was detected on anatase. Furthermore, undesired CH4 formation took place over the Co0 present in the P25- and rutile-supported catalysts, while CH4 was not formed over the Co0 on anatase possibly due to encapsulation by TixOy species. For the first time, this study revealed the influence of different TiO2 polymorphs (used as catalyst supports) on the chemical and crystal phase transformations of Co3O4, which in turn affect its activity and selectivity during CO-PrOx.

The discovery of new materials for efficient transformation of carbon dioxide (CO2) into desired fuel can revolutionize large-scale renewable energy storage and mitigate environmental damage due to carbon emissions. In this work, we discovered an operando generated stable Ni–In kinetic phase that selectively converts CO2 to methanol (CTM) at low pressure compared to the state-of-the-art materials. The catalytic nature of a well-known methanation catalyst, nickel, has been tuned with the introduction of inactive indium, which enhances the CTM process. The remarkable change in the mechanistic pathways toward methanol production has been mapped by operando diffuse reflectance infrared Fourier transform spectroscopy analysis, corroborated by first-principles calculations. The ordered arrangement and pronounced electronegativity difference between metals are attributed to the complete shift in mechanism. The approach and findings of this work provide a unique advance toward the next-generation catalyst discovery for going beyond the state-of-the-art in CO2 reduction technologies.

The replacement of HgCl2/C with Au/C as a catalyst for acetylene hydrochlorination represents a significant reduction in the environmental impact of this industrial process. Under reaction conditions atomically dispersed cationic Au species are the catalytic active site, representing a large-scale application of heterogeneous single-site catalysts. While the metal nuclearity and oxidation state under operating conditions has been investigated in catalysts prepared from aqua regia and thiosulphate, limited studies have focused on the ligand environment surrounding the metal centre. We now report K-edge soft X-ray absorption spectroscopy of the Cl and S ligand species used to stabilise these isolated cationic Au centres in the harsh reaction conditions. We demonstrate the presence of three distinct Cl species in the materials; inorganic Cl−, Au–Cl, and C–Cl and how these species evolve during reaction. Direct evidence of Au–S interactions is confirmed in catalysts prepared using thiosulfate precursors which show high stability towards reduction to inactive metal nanoparticles. This stability was clear during gas switching experiments, where exposure to C2H2 alone did not dramatically alter the Au electronic structure and consequently did not deactivate the thiosulfate catalyst., In situ chlorine and sulphur XAS shows a dynamic ligand environment around cationic Au single-sites during acetylene hydrochlorination.

We have studied the effect of different supports (CeO2, ZrO2, SiC, SiO2 and Al2O3) on the catalytic performance and phase stability of Co3O4 nanoparticles during the preferential oxidation of CO (CO-PrOx) under different H2-rich gas environments and temperatures. Our results show that Co3O4/ZrO2 has superior CO oxidation activity, but transforms to Co0 and consequently forms CH4 at relatively low temperatures. The least reduced and least methanation active catalyst (Co3O4/Al2O3) also exhibits the lowest CO oxidation activity. Co-feeding H2O and CO2 suppresses CO oxidation over Co3O4/ZrO2 and Co3O4/SiC, but also suppresses Co0 and CH4 formation. In conclusion, weak nanoparticle-support interactions (as in Co3O4/ZrO2) favour high CO oxidation activity possibly via the Mars-van Krevelen mechanism. However, stronger interactions (as in Co3O4/Al2O3) help minimise Co0 and CH4 formation. Therefore, this work reveals the bi-functional role required of supports used in CO-PrOx, i.e., to enhance catalytic performance and improve the phase stability of Co3O4.

Techniques employing synchrotron radiation (SR) have had a major and growing impact on catalytic science. They have made key contributions to our understanding of structural properties of catalytic...

Operando X-ray absorption spectroscopy (XAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS) provide complementary information on the catalyst structure, surface reaction mechanisms and activity relationships. The powerful combination of the techniques has been the driving force to design and engineer suitable spectroscopic operando reactors that can mitigate limitations inherent to conventional reaction cells and facilitate experiments under kinetic regimes. Microreactors have recently emerged as effective spectroscopic operando cells due to their plug-flow type operation with no dead volume and negligible mass and heat transfer resistances. Here we present a novel microfabricated reactor that can be used for both operando XAS and DRIFTS studies. The reactor has a glass-silicon-glass sandwich-like structure with a reaction channel (3000 μm × 600 μm; width × depth) packed with a catalyst bed (ca. 25 mg) and placed sideways to the X-ray beam, while the infrared beam illuminates the catalyst bed from the top. The outlet of the reactor is connected to MS for continuous monitoring of the reactor effluent. The feasibility of the microreactor is demonstrated by conducting two reactions: i) combustion of methane over 2 wt% Pd/Al2O3 studied by operando XAS at the Pd K-edge and ii) CO oxidation over 1 wt% Pt/Al2O3 catalyst studied by operando DRIFTS. The former shows that palladium is in an oxidised state at all studied temperatures, 250, 300, 350, 400 °C and the latter shows the presence of linearly adsorbed CO on the platinum surface. Furthermore, temperature-resolved reduction of palladium catalyst with methane and CO oxidation over platinum catalyst are also studied. Based on these results, the catalyst structure and surface reaction dynamics are discussed, which demonstrate not only the applicability and versatility of the microreactor for combined operando XAS and DRIFTS studies, but also illustrate the unique advantages of the microreactor for high space velocity and transient response experiments.

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.

Mechanochemistry offers a solventless, ‘waste free’ route to preparing metal oxide catalysts, however, there is limited information on the chemical steps involved. In this work, the perovskite LaMnO3 has been successfully synthesized via mechanochemistry from metal oxide powders, La2O3 and Mn2O3, at room temperature, using a planetary ball mill. Separate ex situ ‘time slices’ were taken during the milling procedure to provide insights into the underlying chemistry. The crystalline material was assessed using XRD, which identified 100% perovskite phase after 3 h of milling. Conversely, characterization by X-ray absorption spectroscopy (XAS) at both the Mn K-edge and La L3-edge provides a very different picture. The XAS data shows that there are significant structural alterations as early as 30 min of milling, with the La precursor dispersed over Mn2O3. Increasing milling time then allows for mechanical activation of both precursors and the formation of powdered LaMnO3, with no calcination step required. The XAS highlights that there is a significant amount of amorphous, oxygen deficient, content even when XRD has identified 100% perovskite phase. The samples were tested for the decomposition of the environmental pollutant N2O; at a milling time of 3 h, the LaMnO3 catalyst displays a much early onset production of N2 compared to a traditional sol–gel synthesized LaMnO3, resulting from increased oxygen deficiency at the surface, confirmed by XPS and STEM-EELS. This is an encouraging sign that mechanochemical routes can be harnessed to provide a sustainable route to preparing mixed metal oxide catalysts with enhanced catalytic performance.

The utilization of operando spectroscopy has allowed us to watch the dynamic nature of supported metal nanoparticles. However, the realization that subtle changes to environmental conditions affect the form of the catalyst necessitates that we assess the structure of the catalyst across the reactant/product gradient that exists across a fixed bed reactor. In this study, we have performed spatial profiling of a Pd/Al2O3 catalyst during NH3 oxidation, simultaneously collecting mass spectrometry and X-ray absorption spectroscopy data at discrete axial positions along the length of the catalyst bed. The spatial analysis has provided unique insights into the structure–activity relationships that govern selective NH3 oxidation—(i) our data is consistent with the presence of PdNx after the spectroscopic signatures for bulk PdNx disappear and that there is a direct correlation to the presence of this structure and the selectivity toward N2; (ii) at high temperatures, ≥400 °C, we propose that there are two simultaneous reaction pathways—the oxidation of NH3 to NOx by PdO and the subsequent catalytic reduction of NOx by NH3 to produce N2. The results in this study confirm the structural and catalytic diversity that exists during catalysis and the need for such an understanding if improvements to important emission control technologies, such as the selective catalytic oxidation of NH3, are to be made.

We demonstrate a modified sol-immobilization procedure using (MeOH)x/(H2O)1-x solvent mixtures to prepare Pd/TiO2 catalysts that are able to reduce the formation of acid catalyzed products, e. g. ethers, for the hydrogenation of furfural. Transmission electron microscopy found a significant increase in polyvinyl alcohol (PVA) deposition at the metal-support interface and temperature programmed reduction found a reduced uptake of hydrogen, compared to an established Pd/TiO2 preparation. We propose that the additional PVA hinders hydrogen spillover onto the TiO2 support and limits the formation of Brønsted acid sites, required to produce ethers. Elsewhere, the new preparation route was able to successfully anchor colloidal Pd to the TiO2 surface, without the need for acidification. This work demonstrates the potential for minimizing process steps as well as optimizing catalyst selectivity – both important objectives for sustainable chemistry.

There is much about the action of bismuth within heterogeneous catalysis that still require a deeper understanding. We observed that, when Bi was added to AuPd bimetallic nanoparticles (NPs) supported on activated carbon, Bi affected the activity and significantly alters the selectivity in two model liquid phase reactions, namely the oxidation of cinnamyl alcohol and the hydrogenation of cinnamaldehyde. A combination of transmission electron microscopy and X-ray absorption spectroscopy provided a detailed characterization of trimetallic AuPdBi systems. We propose that the introduction of bismuth on AuPd NPs results in a partial blockage of most active sites, limiting the occurrence of consecutive reactions.

The utilization of operando spectroscopy has allowed us to watch the dynamic nature of supported metal nanoparticles. However, the realization that subtle changes to environmental conditions affect the form of the catalyst necessitates that we assess the structure of the catalyst across the reactant/product gradient that exists across a fixed bed reactor. In this study, we have performed spatial profiling of a Pd/Al2O3 catalyst during NH3 oxidation, simultaneously collecting mass spectrometry and X-ray absorption spectroscopy data at discrete axial positions along the length of the catalyst bed. The spatial analysis has provided unique insights into the structure–activity relationships that govern selective NH3 oxidation—(i) our data is consistent with the presence of PdNx after the spectroscopic signatures for bulk PdNx disappear and that there is a direct correlation to the presence of this structure and the selectivity toward N2; (ii) at high temperatures, ≥400 °C, we propose that there are two simultaneous reaction pathways—the oxidation of NH3 to NOx by PdO and the subsequent catalytic reduction of NOx by NH3 to produce N2. The results in this study confirm the structural and catalytic diversity that exists during catalysis and the need for such an understanding if improvements to important emission control technologies, such as the selective catalytic oxidation of NH3, are to be made.

The use of mechanochemistry to prepare catalytic materials is of significant interest; it offers an environmentally beneficial, solvent-free, route and produces highly complex structures of mixed amorphous and crystalline phases. This study reports on the effect of milling atmosphere, either air or argon, on mechanochemically prepared LaMnO3 and the catalytic performance towards N2O decomposition (deN2O). In this work, high energy resolution fluorescence detection (HERFD), X-ray absorption near edge structure (XANES), X-ray emission, and X-ray photoelectron spectroscopy (XPS) have been used to probe the electronic structural properties of the mechanochemically prepared materials. Moreover, in situ studies using near ambient pressure (NAP)-XPS, to follow the materials during catalysis, and high pressure energy dispersive EXAFS studies, to mimic the preparation conditions, have also been performed. The studies show that there are clear differences between the air and argon milled samples, with the most pronounced changes observed using NAP-XPS. The XPS results find increased levels of active adsorbed oxygen species, linked to the presence of surface oxide vacancies, for the sample prepared in argon. Furthermore, the argon milled LaMnO3 shows improved catalytic activity towards deN2O at lower temperatures compared to the air milled and sol–gel synthesised LaMnO3. Assessing this improved catalytic behaviour during deN2O of argon milled LaMnO3 by in situ NAP-XPS suggests increased interaction of N2O at room temperature within the O 1s region. This study further demonstrates the complexity of mechanochemically prepared materials and through careful choice of characterisation methods how their properties can be understood.

This themed issue includes a collection of articles on Synchrotron Radiation Techniques in Catalytic Science.

The use of mechanochemistry to prepare catalytic materials is of significant interest; it offers an environmentally beneficial, solvent-free, route and produces highly complex structures of mixed amorphous and crystalline phases. This study reports on the effect of milling atmosphere, either air or argon, on mechanochemically prepared LaMnO3 and the catalytic performance towards N2O decomposition (deN2O). In this work, high energy resolution fluorescence detection (HERFD), X-ray absorption near edge structure (XANES), X-ray emission, and X-ray photoelectron spectroscopy (XPS) have been used to probe the electronic structural properties of the mechanochemically prepared materials. Moreover, in situ studies using near ambient pressure (NAP)-XPS, to follow the materials during catalysis, and high pressure energy dispersive EXAFS studies, to mimic the preparation conditions, have also been performed. The studies show that there are clear differences between the air and argon milled samples, with the most pronounced changes observed using NAP-XPS. The XPS results find increased levels of active adsorbed oxygen species, linked to the presence of surface oxide vacancies, for the sample prepared in argon. Furthermore, the argon milled LaMnO3 shows improved catalytic activity towards deN2O at lower temperatures compared to the air milled and sol-gel synthesised LaMnO3. Assessing this improved catalytic behaviour during deN2O of argon milled LaMnO3 by in situ NAP-XPS suggests increased interaction of N2O at room temperature within the O 1s region. This study further demonstrates the complexity of mechanochemically prepared materials and through careful choice of characterisation methods how their properties can be understood.

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 CO

Creating a new chemical ecosystem based on platform chemicals derived from waste biomass has significant challenges: catalysts need to be able to convert these highly functionalized molecules to specific target chemicals and they need to be economical - not relying on large quantities of precious metals - and maintain activity over many cycles. Herein, we demonstrate how Pd/NiO is able to direct the selectivity of furfural hydrogenation and maintain performance at low Pd loading by a unique dual-site mechanism. Sol-immobilization was used to prepare 1 wt % Pd nanoparticles supported on NiO and TiO2, with the Pd/NiO catalyst showing enhanced activity with a significantly different selectivity profile; Pd/NiO favors tetrahydrofurfuryl alcohol (72%), whereas Pd/TiO2 produces furfuryl alcohol as the major product (68%). Density functional theory studies evidenced significant differences on the adsorption of furfural on both NiO and Pd surfaces. On the basis of this observation we hypothesized that the role of Pd was to dissociate hydrogen, with the NiO surface adsorbing furfural. This dual-site hydrogenation mechanism was supported by comparing the performance of 0.1 wt % Pd/NiO and 0.1 wt % Pd/TiO2. In this study, the 0.1 and 1 wt % Pd/NiO catalysts had comparable activities, whereas there was a 10-fold reduction in performance for 0.1 wt % Pd/TiO2. When TiO2 is used as the support, the Pd nanoparticles are responsible for both hydrogen dissociation and furfural adsorption and the activity is strongly correlated with the effective metal surface area. This work has significant implications for the upgrading of bioderived feedstocks, suggesting alternative ways for promoting selective transformations and reducing the reliance on precious metals.

Iron-catalysed cross-coupling is undergoing explosive development, but mechanistic understanding lags far behind synthetic methodology. Here, we find that the activity of iron–diphosphine pre-catalysts in the Negishi coupling of benzyl halides is strongly dependent on the diphosphine, but the ligand does not appear to be coordinated to the iron during turnover. This was determined using time-resolved in operando X-ray absorption fine structure spectroscopy employing a custom-made flow cell and confirmed by 31P NMR spectroscopy. While the diphosphine ligands tested are all able to coordinate to iron(ii), in the presence of excess zinc(ii)—as in the catalytic reaction—they coordinate predominantly to the zinc. Furthermore, combined synthetic and kinetic investigations implicate the formation of a putative mixed Fe–Zn(dpbz) species before the rate-limiting step of catalysis. These unexpected findings may not only impact the field of iron-catalysed Negishi cross-coupling, but potentially beyond to reactions catalysed by other transition metal/diphosphine complexes. Despite the many recent developments in iron-catalysed cross-couplings, the mechanistic understanding of these reactions is lacking compared to the more studied palladium and nickel variants. Here, the authors find that during iron-catalysed Negishi reactions the diphosphine ligand predominately binds to the zinc—rather than the iron—centre.

Efficient oxidation catalysts are important in many current industrial processes, including the selective oxidation of methanol to formaldehyde. Vanadium-containing catalysts have been shown to be effective selective oxidation catalysts for certain reactions, and research continues to examine their applicability to other reactions of interest. Several VOx/Fe2O3 shell–core catalysts with varying VOx coverage have been produced to investigate the stability of VOx monolayers and their selectivity for methanol oxidation. Catalyst formation proceeds via a clear progression of distinct surface species produced during catalyst calcination. At 300 °C the selective VOx overlayer has formed; by 500 °C a sandwich layer of FeVO4 arises between the VOx shell and the Fe2O3 core, inhibiting iron cation participation in the catalysis and enhancing catalyst selectivity. The resulting catalysts, comprising a shell–subshell–core system of VOx/FeVO4/Fe2O3, possess good catalytic activity and selectivity to formaldehyde. Electronic supplementary material The online version of this article (10.1007/s11244-017-0873-2) contains supplementary material, which is available to authorized users.

The frontispiece shows a novel in situ method of generating metal nanoparticles within porous molecular frameworks, thereby affording highly active and selective single-site heterogeneous catalysts for industrially significant aerobic oxidations. The Communication on page 1226 by Robert Raja and Andy Hor et al. highlights the importance of spectroscopic and kinetic studies in establishing structureproperty correlations that are fundamental in the design of sustainable heterogeneous catalysts.

Mechanochemistry offers a solventless, 'waste free' route to preparing metal oxide catalysts, however, there is limited information on the chemical steps involved. In this work, the perovskite LaMnO

We present a comparison of Mo, V and Nb oxides as shell materials atop haematite cores used for selective methanol oxidation. While Mo and V both yield high selectivity to formaldehyde, Nb does not. Very different reactivity patterns are seen for Nb, which mainly shows dehydrogenation (to CO) and dehydration (to DME), indicating the lack of a complete shell, while Raman spectroscopy shows that the Mo and V formation process is not followed by NbOx. We suggest this is due to the large differences in mobility within the solid materials during formation, NbOx requiring significantly higher (and deleterious) calcination temperatures to allow sufficient mobility for shell completion.

The interaction between Pd and TiO2 for promoting photocatalytic activity was investigated by tailoring the size of Pd nanoparticles and monitoring the photocatalytic activity of methanol photo-reforming reaction for hydrogen gas production. We show that at 0.6 wt% Pd loading, the catalyst with highly dispersed nanoparticles obtained at 1 °C temperature exhibits superior photocatalytic activity for hydrogen gas production. At different weights of Pd loading, tailoring two sets of catalysts with different structural properties provides correlation between the changes in the Pd local structures and the rate of hydrogen production. The impact of controlling the structural properties of metal nanoparticles on influencing H2 production outweighs the effect of metal loading variation. The differences of Pd/TiO2 activity at the different metal loadings were correlated with the changes in the Pd local structure consequently affecting the electronic transfer and photocatalytic efficiency.

Different supporting procedures were followed to alter the nanoparticle–support interactions (NPSI) in two Co3O4/Al2O3 catalysts, prepared using the reverse micelle technique. The catalysts were tested in the dry preferential oxidation of carbon monoxide (CO-PrOx) while their phase stability was monitored using four complementary in situ techniques, viz., magnet-based characterization, PXRD, and combined XAS/DRIFTS, as well as quasi in situ XPS, respectively. The catalyst with weak NPSI achieved higher CO2 yields and selectivities at temperatures below 225 °C compared to the sample with strong NPSI. However, relatively high degrees of reduction of Co3O4 to metallic Co were reached between 250 and 350 °C for the same catalyst. The presence of metallic Co led to the undesired formation of CH4, reaching a yield of over 90% above 300 °C. The catalyst with strong NPSI formed very low amounts of metallic Co (less than 1%) and CH4 (yield of up to 20%) even at 350 °C. When the temperature was decreased from 350 to 50 °C under the reaction gas, both catalysts were slightly reoxidized and gradually regained their CO oxidation activity, while the formation of CH4 diminished. The present study shows a strong relationship between catalyst performance (i.e., activity and selectivity) and phase stability, both of which are affected by the strength of the NPSI. When using a metal oxide as the active CO-PrOx catalyst, it is important for it to have significant reduction resistance to avoid the formation of undesired products, e.g., CH4. However, the metal oxide should also be reducible (especially on the surface) to allow for a complete conversion of CO to CO2 via the Mars–van Krevelen mechanism.

The interaction between Pd and TiO2 for promoting photocatalytic activity was investigated by tailoring the size of Pd nanoparticles and monitoring the photocatalytic activity of methanol photo-reforming reaction for hydrogen gas production. We show that at 0.6 wt% Pd loading, the catalyst with highly dispersed nanoparticles obtained at 1 °C temperature exhibits superior photocatalytic activity for hydrogen gas production. At different weights of Pd loading, tailoring two sets of catalysts with different structural properties provides correlation between the changes in the Pd local structures and the rate of hydrogen production. The impact of controlling the structural properties of metal nanoparticles on influencing H2 production outweighs the effect of metal loading variation. The differences of Pd/TiO2 activity at the different metal loadings were correlated with the changes in the Pd local structure consequently affecting the electronic transfer and photocatalytic efficiency.

The structure of a highly active pyridine-alkoxide iridium water oxidation catalyst (WOC) is examined by X-ray absorption spectroscopy (XAS). A detailed comparison with IrO2 points to a rigid molecular unit of low nuclearity, with the best analysis suggesting the existence of a novel tetrameric iridium-oxo cubane as the resting state.

Sol-immobilization is increasingly used to achieve supported metal nanoparticles (NPs) with controllable size and shape; it affords a high degree of control of the metal particle size and yields a narrow particle size distribution. Using state-of-the-art beamlines, we demonstrate how X-ray absorption fine structure (XAFS) techniques are now able to provide accurate structural information on nano-sized colloidal Au solutions at μM concentrations. This study demonstrates: (i) the size of Au colloids can be accurately tuned by adjusting the temperature of reduction, (ii) Au concentration, from 50 μM to 1000 μM, has little influence on the average size of colloidal Au NPs in solution and (iii) the immobilization step is responsible for significant growth in Au particle size, which is further exacerbated at increased Au concentrations. The work presented demonstrates that an increased understanding of the primary steps in sol-immobilization allows improved optimization of materials for catalytic applications.

Spatially resolved, combined energy dispersive EXAFS (EDE) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements have been performed over a fixed catalyst bed of Pd/γ-Al 2 O 3 during kinetic oscillations of CO oxidation. The kinetic oscillations of CO oxidation over Pd (or for that matter Pt or Rh) catalysts are a complicated phenomenon that require characterisation techniques with high time resolution and spatial resolution in order to make links between catalyst structure and surface reactivity. By measuring the extent of Pd oxidation at the nanoparticle surface, from Pd K-edge EDE, and matching this with the CO coverage, from DRIFTS spectra, at multiple positions of the fixed bed reactor it is found that the majority of the catalyst undergoes a sharp transition from the CO poisoned catalyst to the highly active, oxidised Pd surface. This transition occurs initially at the end of the catalyst bed, nearest the outlet, and propagates upstream with increasing temperature of the reactor. The oscillations in Pd surface oxide formation and CO coverage are observed only in the first ∼1 mm of the bed, which gives rise to oscillations in CO 2 and O 2 concentrations observed by end-pipe mass spectrometry after the light-off temperature. The catalyst initially exists as less active, CO poisoned metallic Pd nanoparticles before light-off which transition to a highly active state after light-off when the Pd nanoparticle surface becomes dominated by chemisorbed oxygen. Kinetic oscillations only occur at the front of the catalyst bed where there is sufficient concentration of CO in the gas phase to compete with O 2 for adsorption sites at the catalyst surface. We demonstrate the complex nature of the evolving catalyst structure and surface reactivity during catalytic operation and the need for spatially resolved operando methods for understanding and optimising catalyst technologies.

There remains considerable debate over the active form of gold under operating conditions of a recently validated gold catalyst for acetylene hydrochlorination. We have performed an in situ x-ray absorption fine structure study of gold/carbon (Au/C) catalysts under acetylene hydrochlorination reaction conditions and show that highly active catalysts comprise single-site cationic Au entities whose activity correlates with the ratio of Au(I):Au(III) present. We demonstrate that these Au/C catalysts are supported analogs of single-site homogeneous Au catalysts and propose a mechanism, supported by computational modeling, based on a redox couple of Au(I)-Au(III) species.\ud View Full Text

Aqueous phase reforming of glycerol was studied over a series of γ-Al2O3 supported metal nanoparticle catalysts for hydrogen production in a batch reactor. Of the metals studied, Pt/Al2O3 was found to be the most active catalyst under the conditions tested. A further systematic study on the impact of reaction parameters, including stirring speed, pressure, temperature, and substrate/metal molar ratio, was conducted and the optimum conditions for hydrogen production (and kinetic regime) were determined as 240 °C, 42 bar, 1000 rpm, and substrate/metal molar ratio ≥ 4100 for a 10 wt% glycerol feed. The glycerol conversion and hydrogen yield achieved at these conditions were 18% and 17%, respectively, with negligible CO and CH4 formation. Analysis of the spent catalyst using FTIR provides an indication that the reaction pathway includes glycerol dehydrogenation and dehydration steps in the liquid phase in addition to typical reforming and water gas shift reactions in the gas phase.

A MoOx/Al2O3 catalyst was synthesised and tested for oxidative (ODP) and non-oxidative (DP) dehydrogenation of propane in a reaction cycle of ODP followed by DP and a second ODP run. Characterisation results show that the fresh catalyst contains highly dispersed Mo oxide species in the +6 oxidation state with tetrahedral coordination as [MoVIO4]2&minus, moieties. In situ X-ray Absorption Spectroscopy (XAS) shows that [MoVIO4]2&minus, is present during the first ODP run of the reaction cycle and is reduced to MoIVO2 in the following DP run. The reduced species are partly re-oxidised in the subsequent second ODP run of the reaction cycle. The partly re-oxidised species exhibit oxidation and coordination states that are lower than 6 but higher than 4 and are referred to as MoxOy. These species significantly improved propene formation (relatively 27% higher) in the second ODP run at similar propane conversion activity. Accordingly, the initial tetrahedral [MoVIO4]2&minus, present during the first ODP run of the reaction cycle is active for propane conversion, however, it is unselective for propene. The reduced MoIVO2 species are relatively less active and selective for DP. It is suggested that the MoxOy species generated by the reaction cycle are active and selective for ODP. The vibrational spectroscopic data indicate that the retained surface species are amorphous carbon deposits with a higher proportion of aromatic/olefinic like species.

Different supporting procedures were followed to alter the nanoparticle–support interactions (NPSI) in two Co3O4/Al2O3 catalysts, prepared using the reverse micelle technique. The catalysts were tested in the dry preferential oxidation of carbon monoxide (CO-PrOx) while their phase stability was monitored using four complementary in situ techniques, viz., magnet-based characterization, PXRD, and combined XAS/DRIFTS, as well as quasi in situ XPS, respectively. The catalyst with weak NPSI achieved higher CO2 yields and selectivities at temperatures below 225 °C compared to the sample with strong NPSI. However, relatively high degrees of reduction of Co3O4 to metallic Co were reached between 250 and 350 °C for the same catalyst. The presence of metallic Co led to the undesired formation of CH4, reaching a yield of over 90% above 300 °C. The catalyst with strong NPSI formed very low amounts of metallic Co (less than 1%) and CH4 (yield of up to 20%) even at 350 °C. When the temperature was decreased from 350 to 50 °C under the reaction gas, both catalysts were slightly reoxidized and gradually regained their CO oxidation activity, while the formation of CH4 diminished. The present study shows a strong relationship between catalyst performance (i.e., activity and selectivity) and phase stability, both of which are affected by the strength of the NPSI. When using a metal oxide as the active CO-PrOx catalyst, it is important for it to have significant reduction resistance to avoid the formation of undesired products, e.g., CH4. However, the metal oxide should also be reducible (especially on the surface) to allow for a complete conversion of CO to CO2 via the Mars–van Krevelen mechanism.

The selective catalytic oxidation of NH3 to N2 presents a promising solution for the abatement of unused NH3-based reductants from diesel exhaust after treatment. Supported Pd nanoparticle catalysts show selectivity to N2 rather than NOx, which is investigated in this work. The link between Pd nanoparticle structure and surface reactivity was found using operando X-ray absorption fine structure spectroscopy, diffuse reflectance infrared Fourier-transformed spectroscopy and on-line mass spectrometry. Nitrogen insertion into the metallic Pd nanoparticle structure at low temperatures (2 selectivity, whereas the unfavourable formation of NO is linked to adsorbed nitrates, which form at the surface of bulk PdO nanoparticles at high temperatures (>280 °C). Our work demonstrates the ability of combined operando spectroscopy and density functional theory calculations to characterize a previously unidentified PdNx species, and clarify the selectivity-directing structure of supported Pd catalysts for the selective catalytic oxidation of NH3 to N2.

The interaction of CO with an attapulgite-supported Cu(II)Cl2 catalyst has been examined in a micro-reactor arrangement. CO exposure to the dried, as-received catalyst at elevated temperatures leads to the formation of CO2 as the only identifiable product. However, phosgene production can be induced by a catalyst pre-treatment where the supported Cu(II)Cl2 sample is exposed to a diluted stream of chlorine. Subsequent CO exposure at  370C then leads to phosgene production. In order to investigate the origins of this atypical set of reaction characteristics, a series of x-ray absorption experiments were performed that were supplemented by DFT calculations. XANES measurements establish that at the elevated temperatures connected with phosgene formation, the catalyst is comprised of Cu+ and a small amount of Cu2+. Moreover, the data show that unique to the chlorine pre-treated sample, CO exposure at elevated temperature results in a short-lived oxidation of the copper. On the basis of calculated CO adsorption energies, DFT calculations indicate that a mixed Cu+/Cu2+ catalyst is required to support CO chemisorption.

Correction for ‘In situ spectroscopic investigations of MoOx/Fe2O3 catalysts for the selective oxidation of methanol’ by Catherine Brookes et al., Catal. Sci. Technol., 2016, 6, 722–730.

Controlling the structural properties of nanoparticle catalysts within a microporous framework is a major challenge. Using in situ X-ray absorption fine structure (XAFS) spectroscopy we detail the influence of activation parameters on the nature of ruthenium particles that are located within the confines of a nanoporous aluminophosphate (RuAlPO-5) architecture. These in situ studies confirm that controlled annealing conditions can tailor the formation of specific ruthenium species, which alter the catalytic performance towards the oxidation of cyclohexane to KA oil (a 1 : 1 mixture of cyclohexanone and cyclohexanol), the precursor for Nylon-6 and Nylon-6,6.

The hydrogenation of lactic acid to form 1,2-propanediol has been investigated using Ru nanoparticles supported on carbon as a catalyst. Two series of catalysts which were prepared by wet impregnation and sol-immobilization were investigated. Their activity was contrasted with that of a standard commercial Ru/C catalyst (all catalysts comprise 5 wt % Ru). The catalyst prepared using sol-immobilization was found to be more active than the wet impregnation materials. In addition, the catalyst made by sol-immobilization was initially more active than the standard commercial catalyst. However, when reacted for an extended time or with successive reuse cycles, the sol-immobilized catalyst became less active, whereas the standard commercial catalyst became steadily more active. Furthermore, both catalysts exhibited an induction period during the first 1000 s of reaction. Detailed scanning transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray absorption fine structure analysis data, when correlated with the catalytic performance results, showed that the high activity can be ascribed to highly dispersed Ru nanoparticles. Although the sol-immobilization method achieved these optimal discrete Ru nanoparticles immediately, as can be expected from this preparation methodology, the materials were unstable upon reuse. In addition, surface lactide species were detected on these particles using X-ray photoelectron spectroscopy, which could contribute to their deactivation. The commercial Ru/C catalysts, on the other hand, required treatment under reaction conditions to change from raft-like morphologies to the desired small nanoparticle morphology, during which time the catalytic performance progressively improved.

A series of zinc oxide and copper(0) colloidal nanocatalysts, produced by a one-pot synthesis, are shown to catalyze the hydrogenation of carbon dioxide to methanol. The catalysts are produced by the reaction between diethyl zinc and bis(carboxylato/phosphinato)copper(II) precursors. The reaction leads to the formation of a precatalyst solution, characterized using various spectroscopic (NMR, UV–vis spectroscopy) and X-ray diffraction/absorption (powder XRD, EXAFS, XANES) techniques. The combined characterization methods indicate that the precatalyst solution contains copper(0) nanoparticles and a mixture of diethyl zinc and an ethyl zinc stearate cluster compound [Et4Zn5(stearate)6]. The catalysts are applied, at 523 K with a 50 bar total pressure of a 3:1 mixture of H2/CO2, in the solution phase, quasi-homogeneous, hydrogenation of carbon dioxide, and they show high activities (>55 mmol/gZnOCu/h of methanol). The postreaction catalyst solution is characterized using a range of spectroscopies, X-ray diffraction techniques, and transmission electron microscopy (TEM). These analyses show the formation of a mixture of zinc oxide nanoparticles, of size 2–7 nm and small copper nanoparticles. The catalyst composition can be easily adjusted, and the influence of the relative loadings of ZnO/Cu, the precursor complexes and the total catalyst concentration on the catalytic activity are all investigated. The optimum system, comprising a 55:45 loading of ZnO/Cu, shows equivalent activity to a commercial, activated methanol synthesis catalyst. These findings indicate that using diethyl zinc to reduce copper precursors in situ leads to catalysts with excellent activities for the production of methanol from carbon dioxide

A controlled surface reaction (CSR) technique has been successfully employed to prepare a series of CeOx modified Pt/Al2O3 catalysts, offering a unique system to specifically probe the relationship between Ce and Pt without any bulk CeO2 present. Ce L3 edge X-ray absorption near edge structure (XANES) analysis was used to ascertain the oxidation state of the Ce in the catalyst materials in atmospheres of air, H2 (g), and CO (g) at room temperature. The XANES data showed that the Ce was present as both Ce3+ and Ce4+ oxidation states in an atmosphere of air, becoming predominantly present as Ce3+ in H2 and CO. The results indicate the role of Pt in the process, and show that with the absence of bulk CeO2, changes in Ce oxidation state can be observed at non-elevated temperatures. The CeOx/Pt/Al2O3 catalysts were tested for their performance toward the water gas shift (WGS) reaction and showed improved performance compared to the unmodified Pt/Al2O3, even at very low concentrations of Ce (∼0.35 wt-%).

The carbon–carbon coupling via electrochemical reduction of carbon dioxide represents the biggest challenge for using this route as platform for chemicals synthesis. Here we show that nanostructured iron (III) oxyhydroxide on nitrogen-doped carbon enables high Faraday efficiency (97.4%) and selectivity to acetic acid (61%) at very-low potential (−0.5 V vs silver/silver chloride). Using a combination of electron microscopy, operando X-ray spectroscopy techniques and density functional theory simulations, we correlate the activity to acetic acid at this potential to the formation of nitrogen-coordinated iron (II) sites as single atoms or polyatomic species at the interface between iron oxyhydroxide and the nitrogen-doped carbon. The evolution of hydrogen is correlated to the formation of metallic iron and observed as dominant reaction path over iron oxyhydroxide on oxygen-doped carbon in the overall range of negative potential investigated, whereas over iron oxyhydroxide on nitrogen-doped carbon it becomes important only at more negative potentials., Trapping carbon dioxide within usable chemicals is a promising means to mitigate climate change, yet electrochemical C–C couplings are challenging to perform. Here, the authors prepared iron oxyhydroxides on nitrogen-doped carbon that efficiently convert carbon dioxide to acetic acid.

A new class of tunable crystalline solid supports for metal complexes has been synthesized and used for ethylene polymerization. We developed a family of high surface area, highly dispersible layered double hydroxides (AMO–LDHs) which, on treatment with alkyl aluminum activators, are able to support metallocene and nonmetallocene complexes to make active catalyst systems for the slurry polymerization of ethylene. We show that the chemical composition of the AMO–LDH support can dramatically affect catalyst activity, polymer morphology, and polymer microstructure. A Zr K-edge EXAFS study of these active catalysts has enabled us to observe a metallocene-derived single-center catalytic species in close proximity to the support.

Funding: Marie Curie Individual Fellowship project ArtOxiZymes to AGJ (contract no. H2020-MSCA-IF-2014-657755), EPSRC Critical mass grant ‘Clean catalysis for sustainable development’ (EP/J018139/1) and Sasol (CASE studentship to P.J.D.), EPSRC (EP/K014854/1). Artificial metalloenzymes (ArMs) are hybrid catalysts that offer a unique opportunity to combine the superior performance of natural protein structures with the unnatural reactivity of transition-metal catalytic centers. Therefore, they provide the prospect of highly selective and active catalytic chemical conversions for which natural enzymes are unavailable. Herein, we show how by rationally combining robust site-specific phosphine bioconjugation methods and a lipid-binding protein (SCP-2L), an artificial rhodium hydroformylase was developed that displays remarkable activities and selectivities for the biphasic production of long-chain linear aldehydes under benign aqueous conditions. Overall, this study demonstrates that judiciously chosen protein-binding scaffolds can be adapted to obtain metalloenzymes that provide the reactivity of the introduced metal center combined with specifically intended product selectivity. Publisher PDF

We report a detailed density functional theory (DFT) study in conjunction with extended X-ray absorption fine structure (EXAFS) experiments on the geometrical and local electronic properties of Cu adatoms and Cu(ii) ions in presence of water molecules and of CuO nanoclusters on the CeO

Three recurring hypotheses are often used to explain the effect of non‐thermal plasmas (NTPs) on NTP catalytic hybrid reactions; namely, modification or heating of the catalyst or creation of new reaction pathways by plasma‐produced species. NTP‐assisted methane (CH4) oxidation over Pd/Al2O3 was investigated by direct monitoring of the X‐ray absorption fine structure of the catalyst, coupled with end‐of‐pipe mass spectrometry. This in situ study revealed that the catalyst did not undergo any significant structural changes under NTP conditions. However, the NTP did lead to an increase in the temperature of the Pd nanoparticles; although this temperature rise was insufficient to activate the thermal CH4 oxidation reaction. The contribution of a lower activation barrier alternative reaction pathway involving the formation of CH3(g) from electron impact reactions is proposed.

The pyrochlore solid solution (Na0.33Ce0.67)(2)-(Ir1-xRux)(2)O-7 (0

The catalytic performance of a series of 1 wt % Pd/C catalysts prepared by the sol-immobilization method has been studied in the liquid-phase hydrogenation of furfural. The temperature range studied was 25&ndash, 75 &deg, C, keeping the H2 pressure constant at 5 bar. The effect of the catalyst preparation using different capping agents containing oxygen or nitrogen groups was assessed. Polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and poly (diallyldimethylammonium chloride) (PDDA) were chosen. The catalysts were characterized by ultraviolet-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The characterization data suggest that the different capping agents affected the initial activity of the catalysts by adjusting the available Pd surface sites, without producing a significant change in the Pd particle size. The different activity of the three catalysts followed the trend: PdPVA/C &gt, PdPDDA/C &gt, PdPVP/C. In terms of selectivity to furfuryl alcohol, the opposite trend has been observed: PdPVP/C &gt, PdPVA/C. The different reactivity has been ascribed to the different shielding effect of the three ligands used, they influence the adsorption of the reactant on Pd active sites.

The development of efficient photocatalytic routines for producing hydrogen is of great importance as society moves away from energy sources derived from fossil fuels. Recent studies have identified that the addition of metal nanoparticles to TiO2 greatly enhances the photocatalytic performance of these materials towards the reforming of alcohols for hydrogen production. The core-shell structured Au-Pd bimetallic nanoparticle supported on TiO2 has being of interest as it exhibited extremely high quantum efficiencies for hydrogen production. However, the effect of shell composition and thickness on photocatalytic performance remains unclear. Here we report the synthesis of core-shell structured AuPd NPs with the controlled deposition of one and two monolayers (ML) equivalent of Pd onto Au NPs by colloidal and photodeposition methods. We have determined the shell composition and thickness of the nanoparticles by a combination of X-ray absorption fine structure and X-ray photoelectron spectroscopy. Photocatalytic ethanol reforming showed that the core-shell structured Au-Pd promoters supported on TiO2 exhibit enhanced activity compared to that of monometallic Au and Pd as promoters, whilst the core-shell Au-Pd promoters containing one ML equivalent Pd provide the optimum reactivity. © the Owner Societies.