|
|
Open Access
Abstract: 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.
|
Feb 2021
|
|
B18-Core EXAFS
|
Santhosh K.
Matam
,
Caitlin
Moffat
,
Pip
Hellier
,
Michael
Bowker
,
Ian P.
Silverwood
,
C. Richard A.
Catlow
,
S. David
Jackson
,
James
Craswell
,
Peter P.
Wells
,
Stewart F.
Parker
,
Emma K.
Gibson
Diamond Proposal Number(s):
[10306]
Open Access
Abstract: 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− moieties. In situ X-ray Absorption Spectroscopy (XAS) shows that [MoVIO4]2− 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− 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.
|
Nov 2020
|
|
B18-Core EXAFS
|
B.
Venezia
,
E.
Cao
,
Santhosh K.
Matam
,
C.
Waldron
,
G.
Cibin
,
E. K.
Gibson
,
S.
Golunski
,
P. P.
Wells
,
I.
Silverwood
,
C. R. A.
Catlow
,
G.
Sankar
,
A.
Gavriilidis
Diamond Proposal Number(s):
[19359]
Open Access
Abstract: 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.
|
Oct 2020
|
|
B18-Core EXAFS
|
Diamond Proposal Number(s):
[15151, 18431]
Open Access
Abstract: The technique of inelastic neutron scattering (INS) is used to investigate how hydrogen is partitioned within a series of Na and S promoted iron-based Fischer-Tropsch-to-olefin catalysts. Two reaction test regimes are examined. First, reaction testing at elevated temperature and pressure demonstrate how Na/S additions enhance short chain olefin selectivity and reduce methane formation under industrially relevant reaction conditions. For a fixed level of Na incorporation (2000 ppm), sulfur concentrations of ≤ 100 ppm result in only a modest improvement in olefin selectivity. However, for sulfur values of ≥ 100 ppm there is a noticeable and systematic increase in C2-C4 olefin selectivity; rising from ∼30.0 % to 35.2% at 250 ppm. Second, using ambient pressure CO hydrogenation as a test reaction in INS and micro-reactor configurations, catalyst samples are further analysed by TPR, TPO, XRD and S K-edge XANES. INS shows the formation of a hydrocarbonaceous overlayer to be significantly attenuated by the presence of the promoters, with increasing S levels significantly reducing the intensity of the sp2 and sp3 hybridised ν(C-H) modes of the overlayer, albeit to differing degrees. A probable role for how this combination of promoters is perturbing the form of the hydrocarbonaceous overlayer to subsequently moderate the product distribution is considered.
|
Oct 2020
|
|
B18-Core EXAFS
E01-JEM ARM 200CF
E02-JEM ARM 300CF
|
Liqun
Kang
,
Bolun
Wang
,
Adam
Thetford
,
Ke
Wu
,
Mohsen
Danaie
,
Qian
He
,
Emma
Gibson
,
Ling-dong
Sun
,
Hiroyuki
Asakura
,
Richard
Catlow
,
Feng Ryan
Wang
Diamond Proposal Number(s):
[16966, 17559, 18909, 19246, 19318, 20643, 20847, 17377, 15151, 14239]
Open Access
Abstract: Ru(II) compounds are widely used in catalysis, photocatalysis and medical applications. They are usually obtained in reductive environment as molecular O 2 can oxidize Ru(II) to Ru(III) and Ru(IV). Here we report the design, identification and evolution of an air‐stable surface ‐[bipy‐Ru(II)(CO) 2 Cl 2 ] site that is covalently mounted onto a polyphenylene framework. Such Ru(II) site was obtained by reduction of ‐[bipy‐Ru(III)Cl 4 ] ‐ with simultaneous ligand exchange from Cl ‐ to CO. This structural evolution was witnessed by a combination of in situ X‐ray and infrared spectroscopy studies. The ‐[bipy‐Ru(II)(CO) 2 Cl 2 ] site enables oxidation of CO with a turnover frequency of 0.73 × 10 ‐2 s ‐1 at 462 K, while the Ru(III) site is completely inert. This work contributes to the studies of structure‐activity relationship by demonstrating a practical control over both geometric and electronic structures of single‐site catalysts at molecular level, which can be further applied in other single site catalyst researches.
|
Sep 2020
|
|
B18-Core EXAFS
E01-JEM ARM 200CF
E02-JEM ARM 300CF
I11-High Resolution Powder Diffraction
I20-Scanning-X-ray spectroscopy (XAS/XES)
|
Liqun
Kang
,
Bolun
Wang
,
Qiming
Bing
,
Michal
Zalibera
,
Robert
Büchel
,
Ruoyu
Xu
,
Qiming
Wang
,
Yiyun
Liu
,
Diego
Gianolio
,
Chiu C.
Tang
,
Emma K.
Gibson
,
Mohsen
Danaie
,
Christopher
Allen
,
Ke
Wu
,
Sushila
Marlow
,
Ling-dong
Sun
,
Qian
He
,
Shaoliang
Guan
,
Anton
Savitsky
,
Juan J.
Velasco-vélez
,
June
Callison
,
Christopher W. M.
Kay
,
Sotiris E.
Pratsinis
,
Wolfgang
Lubitz
,
Jing-yao
Liu
,
Feng Ryan
Wang
Diamond Proposal Number(s):
[15151, 15763, 16966, 17377, 19072, 19246, 20939, 17559, 24285, 19318, 19850]
Open Access
Abstract: Supported atomic metal sites have discrete molecular orbitals. Precise control over the energies of these sites is key to achieving novel reaction pathways with superior selectivity. Here, we achieve selective oxygen (O2) activation by utilising a framework of cerium (Ce) cations to reduce the energy of 3d orbitals of isolated copper (Cu) sites. Operando X-ray absorption spectroscopy, electron paramagnetic resonance and density-functional theory simulations are used to demonstrate that a [Cu(I)O2]3− site selectively adsorbs molecular O2, forming a rarely reported electrophilic η2-O2 species at 298 K. Assisted by neighbouring Ce(III) cations, η2-O2 is finally reduced to two O2−, that create two Cu–O–Ce oxo-bridges at 453 K. The isolated Cu(I)/(II) sites are ten times more active in CO oxidation than CuO clusters, showing a turnover frequency of 0.028 ± 0.003 s−1 at 373 K and 0.01 bar PCO. The unique electronic structure of [Cu(I)O2]3− site suggests its potential in selective oxidation.
|
Aug 2020
|
|
B18-Core EXAFS
|
Grazia
Malta
,
Simon A.
Kondrat
,
Simon J.
Freakley
,
David J.
Morgan
,
Emma K.
Gibson
,
Peter P.
Wells
,
Matteo
Aramini
,
Diego
Gianolio
,
Paul B. J.
Thompson
,
Peter
Johnston
,
Graham J.
Hutchings
Diamond Proposal Number(s):
[15214]
Open Access
Abstract: 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.
|
Jun 2020
|
|
B18-Core EXAFS
|
Bingqiao
Xie
,
Roong Jien
Wong
,
Tze Hao
Tan
,
Michael
Higham
,
Emma K.
Gibson
,
Donato
Decarolis
,
June
Callison
,
Kondo-francois
Aguey-zinsou
,
Michael
Bowker
,
C. Richard A.
Catlow
,
Jason
Scott
,
Rose
Amal
Diamond Proposal Number(s):
[19850]
Open Access
Abstract: Although photoexcitation has been employed to unlock the low-temperature equilibrium regimes of thermal catalysis, mechanism underlining potential interplay between electron excitations and surface chemical processes remains elusive. Here, we report an associative zinc oxide band-gap excitation and copper plasmonic excitation that can cooperatively promote methanol-production at the copper-zinc oxide interfacial perimeter of copper/zinc oxide/alumina (CZA) catalyst. Conversely, selective excitation of individual components only leads to the promotion of carbon monoxide production. Accompanied by the variation in surface copper oxidation state and local electronic structure of zinc, electrons originating from the zinc oxide excitation and copper plasmonic excitation serve to activate surface adsorbates, catalysing key elementary processes (namely formate conversion and hydrogen molecule activation), thus providing one explanation for the observed photothermal activity. These observations give valuable insights into the key elementary processes occurring on the surface of the CZA catalyst under light-heat dual activation.
|
Mar 2020
|
|
B18-Core EXAFS
I18-Microfocus Spectroscopy
|
Christopher
Hardacre
,
Andrew M.
Beale
,
Emma K.
Gibson
,
Josephine B. M.
Goodall
,
Alex
Goguet
,
Simon A.
Kondrat
,
Grazia
Malta
,
Cristina
Stere
,
Peter P.
Wells
,
Graham J.
Hutchings
,
C. Richard A.
Catlow
Diamond Proposal Number(s):
[12986, 10306, 11398, 15214, 12601, 10242, 12064, 12499, 14440]
Abstract: 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 systems
and of structural changes during the operation of a catalytic process.
They can also improve our understanding of electronic and vibrational
properties, which can contribute to the understanding of mechanisms.
SR techniques are now key components of the experimental tool box of
the catalytic scientist.
|
Feb 2020
|
|
B18-Core EXAFS
|
Jessica C.
Mcglynn
,
Torben
Dankwort
,
Lorenz
Kienle
,
Nuno A. G.
Bandeira
,
James P.
Fraser
,
Emma K.
Gibson
,
Irene
Cascallana-matias
,
Katalin
Kamarás
,
Mark D.
Symes
,
Harry N.
Miras
,
Alexey Y.
Ganin
Diamond Proposal Number(s):
[14239]
Open Access
Abstract: The electrochemical generation of hydrogen is a key enabling technology for the production of sustainable fuels. Transition metal chalcogenides show considerable promise as catalysts for this reaction, but to date there are very few reports of tellurides in this context, and none of these transition metal telluride catalysts are especially active. Here, we show that the catalytic performance of metallic 1T′-MoTe2 is improved dramatically when the electrode is held at cathodic bias. As a result, the overpotential required to maintain a current density of 10 mA cm−2 decreases from 320 mV to just 178 mV. We show that this rapid and reversible activation process has its origins in adsorption of H onto Te sites on the surface of 1T′-MoTe2. This activation process highlights the importance of subtle changes in the electronic structure of an electrode material and how these can influence the subsequent electrocatalytic activity that is displayed.
|
Oct 2019
|
|
