The nickel/zirconia (Ni/ZrO 2 ) interface plays a key role in the performance of the anode of sol... more The nickel/zirconia (Ni/ZrO 2 ) interface plays a key role in the performance of the anode of solid oxide fuel cells (SOFC) and it is therefore important to understand the interaction between nickel nanoparticles and the ZrO 2 surface. Here, we have described the interaction of five Ni n (n = 1-5) clusters with the (111) surface of cubic zirconia, c-ZrO 2 (111), using spin polarized density functional theory (DFT) calculations with inclusion of long-range dispersion forces. We have systematically evaluated the geometric and electronic structure of different cluster configurations and sizes and shown how the clusters interact with the oxygen and zirconium surface atoms. The cluster-surface interaction is characterized by a charge transfer from the Ni clusters to the surface. From calculations of the hopping rate and clustering energies, we have demonstrated that Ni atoms prefer to aggregate rather than wet the surface and we would therefore suggest that modifications in the synthesis could be needed to modify the coalescence of the supported metal particles of this catalytic system.
Cobalt nanoparticles play an important role as a catalyst in the Fischer-Tropsch synthesis. Durin... more Cobalt nanoparticles play an important role as a catalyst in the Fischer-Tropsch synthesis. During the reaction process, cobalt nanoparticles can become oxidized leading to the formation of two phases: CoO rock-salt and Co 3 O 4 cubic spinel. Experimentally, it is possible to evaluate the phase change and follow the catalyst degradation by measuring the magnetic moment, as each material presents a different magnetic structure. It is therefore important to develop a fundamental description, at the atomic scale, of cobalt and its oxide phases which we have done here using density functional theory with the Dudarev approach to account for the on-site Coulomb interactions (DFT+U). We have explored different U eff values, ranging from 0 to 5 eV, and found that U eff = 3.0 eV describes most appropriately the mechanical properties, as well as the electronic and magnetic structures of Co, CoO and Co 3 O 4 . We have considered a ferromagnetic ordering for the metallic phase and the antiferromagnetic structure for the oxide phases. Our results support the interpretation of the catalytic performance of metallic cobalt as it transforms into its oxidized phases under experimental conditions.
This is t h e a u t h o r's v e r sio n of a w o r k t h a t w a s s u b mi t t e d t o / a c c e... more This is t h e a u t h o r's v e r sio n of a w o r k t h a t w a s s u b mi t t e d t o / a c c e p t e d fo r p u blic a tio n. Cit a tio n fo r fin al p u blis h e d ve r sio n: F a n g, H ui h u a n g, Rol d a n M a r ti n ez, Alb e r t o ORCID
We have calculated the adsorption of the reducing agent hydrazine (N 2 H 4 ) on copper surfaces u... more We have calculated the adsorption of the reducing agent hydrazine (N 2 H 4 ) on copper surfaces using density functional theory calculations with a correction for the long-range interactions (DFT-D2). We have modeled the perfect and a number of defective Cu(100), (110), and (111) surfaces, which are found in the experimentally produced structures of copper nanoparticles. We have studied adsorption of hydrazine at three types of defects in the surfaces, i.e., monatomic steps, Cu adatoms, and Cu vacancies. Several lowenergy adsorption structures for hydrazine on each perfect and defective surface have been identified and compared. Our calculations reveal that hydrazine bridges surface copper atoms, with the molecule twisted from the gauche toward an eclipsed conformation, except on the adatom (100) and vacancycontaining ( ) and ( ) surfaces, where it adsorbs through one nitrogen atom in gauche and trans conformations, respectively. The strongest adsorption energy is found on the stepped (110) surface, where hydrazine bridges between the copper atoms on the step edge and the terrace, as it stabilizes the low-coordinated copper atoms. Our results show that, although the (110) surface contains a number of low-coordinated atoms that enhance the surface-molecule interactions, the addition of defects on the more stable ( ) and ( ) surfaces provides sites that enable hydrazine binding to almost the same extent. This study also confirms general observations of surface adsorption trends in terms of d-band center and binding energy as a function of coordination number, i.e., the stronger the molecular adsorption, the higher the d-band shifts at low-coordinated sites.
We have employed density functional theory calculations to analyze the possible tautomerization o... more We have employed density functional theory calculations to analyze the possible tautomerization of phenol mediated by three diff erent Lewis acid sites at the external (10) surface of zeolite MFI. A silicon atom of the silanol group was substituted by Sc, Fe, and Ga metal atoms, which adopted a formal charge of 3+. This substituted silanol was dehydrated in order to form three-coordinated Lewis acid sites. The tautomerization of phenol involves the adsorption of the molecule on the Lewis site, the dissociation of the phenolic O−H bond and the transfer of the proton to the zeolite framework. This proton is transferred from the zeolite to the C atom at ortho positions to the phenolic O atom, thus generating the tautomer. The acidity of the substituted Lewis sites follows a strength order of Ga < Fe < Sc, where the Sc substitution provides the lowest energy barriers: 33 kJ/mol for the dissociation of the O−H bond, and 32 kJ/mol for the formation of the C−H bond, calculated with the GGA functional PBE including Grimme's dispersion corrections. We observed that the GGA functional PBE produces binding energies and energy barriers with a diff erence of less than 13 kJ/mol compared to the meta-GGA TPSS and rev-TPSS and the hybrid-GGA HSE06.
Solid Oxide Fuel Cell (SOFC) devices convert chemical energy into electrical energy by oxidizing ... more Solid Oxide Fuel Cell (SOFC) devices convert chemical energy into electrical energy by oxidizing fuels such as hydrogen, natural gas, ethanol or hydrocarbon mixtures, which highlights the capability of SOFCs as candidates for applications in stationary power generation. The main species used as fuel in the SOFC are H2 and CO which are oxidized at the anode [1]. Instead of pure H2 or CO, H2/CO mixtures can be used leading to more complicated mechanisms since other processes such as the water-gas shift reaction (CO + H2O ↔ CO2 + H2) are involved. Robust materials are needed for SOFC since the working temperature is approximately equal to 800˚C. For example, a suitable anode is Ni/YSZ (nickel – yttria stabilized zirconia) since it has a high catalytic activity, mechanical and chemical stability and compatibility with the electrolyte under working conditions [2]. It is well known that the performance of the Ni/YSZ cermet depends on the microstructure and the distribution of Ni and YSZ phases in the cermet [3]. Additionally, this performance depends on the key reactions occurring at the triple phase boundary where the gas phase, Ni particles and YSZ surface meet. Thus, it is relevant to understand, at the atomic scale, the structure of Ni/YSZ and its interaction with the gas phase, and in particular CO2 and H2. To simulate the interaction of CO2 and H2 with Ni-YSZ, we have used electronic structure calculations based on the Density Functional Theory using the Vienna Ab-initio Simulation Package (VASP). We have studied the deposition of Nin (n=4-10) clusters on the YSZ(111) followed by the adsorption of molecules on the optimised Nin/YSZ(111) structures and calculated the geometric and electronic structure at the most stable adsorption site. We have also analysed the reaction path of the reverse water-gas shift reaction (CO2 + H2 ↔ CO + H2O) on three different Nin/YSZ(111) (n = 5, 6 and 10) interfaces, considering two possible pathways intermediates: hydrocarboxyl or formate. Finally, we have observed that both CO2 and H2 prefer to adsorb on the Nin cluster rather than on the surface. On the metal cluster the CO2 molecule is activated and the H2 is dissociated upon adsorption, whereas charge is transferred from the cluster to the molecules. We have determined two intermediate states and three intermediate states for the carboxylic acid and formate intermediate pathway, respectively, on the three Nin/YSZ(111) (n = 5, 6 and 10) interfaces. References 1. J. Hanna, W. Y. Lee, Y. Shi and A. F. Ghoniem, Prog. Energy Combust. Sci., 2014, 40, 74–111. 2. Kim, S. et al. Solid State Ionics, (2007), 178, 1304–1309. 3. Kim, S. et al. Solid State Ionics, (2006), 177, 931–938.
Controlling the selectivity of a reaction by rational designing of catalyst is an important and c... more Controlling the selectivity of a reaction by rational designing of catalyst is an important and challenging topic in catalysis research. In this article, we report a strategy to tune the product selectivity during aqueous phase hydrogenolysis of glycerol using gaseous H 2 . Ru/TiO 2 is an active catalyst for the hydrogenolysis of glycerol, however it promotes the hydrogenolysis of CÀ C bonds resulting in large quantities of C2 and C1 products. On the other hand, Pd/TiO 2 and Pt/TiO 2 catalysts are very selective for the hydrogenolysis of CÀ O bonds producing mainly C3 products (1,2 and 1,3 propanediols), however they are much less active compared to the Ru catalysts. In this article, we report that by combining Ru with Pt or Pd in a bimetallic nanoparticle, we can develop new catalysts that are both active and selective for CÀ O hydrogenolysis. A physical mixture of two monometallic catalysts does not show this enhanced selectivity for CÀ O hydrogenolysis, proving that intimate mixing of the two metals in a nanoparticle is crucial to tune the selectivity. All the monometallic and bimetallic catalysts have been characterised by microscopic and spectroscopic methods to understand their structural features. DFT studies were also done to rationalise the observed difference in the catalytic properties between monometallic and bimetallic catalysts.
The adsorption and dissociation of carbon monoxide (CO) have been studied on 81 Transition Metal ... more The adsorption and dissociation of carbon monoxide (CO) have been studied on 81 Transition Metal (TM) surfaces, with TMs having body centred cubic (bcc), face centred cubic (fcc), or hexagonal close-packed (hcp) crystalline structures. For each surface, CO, C, and O adsorptions, and C+O co-adsorptions were studied by density functional theory calculations on suited slab models, using Perdew-Burke-Ernzerhof functional with Grimme’s D3 correction for dispersive forces. CO dissociation activation and reaction energies, ΔE, were determined. The values, including zero point energy, were used to capture chemical trends along groups, d-series, and crystallographic phases concerning CO adsorption and dissociation, while simulated infrared (IR) spectroscopies and thermodynamic phase diagrams are provided. Late fcc TMs are found to adsorb CO weakly, perpendicularly, and are IR-visible, opposite to early bcc TMs, while hcp cases are distributed along these two extremes. The d-band centre, εd, ...
Surface cavities formed by molecularly rigid polymers of intrinsic microporosity affect catalytic... more Surface cavities formed by molecularly rigid polymers of intrinsic microporosity affect catalytic processes. Palladium nanoparticles of typically 3 nm diameter are formed in an intrinsically microporous polyamine (PIM-EA-TB) by borohydride reduction. These particles are shown to indirectly catalyse the oxidative colour change of indicator dye 3,5,3 0 ,5 0 -tetramethylbenzidine (TMB) in the presence of formic acid via formation of H 2 O 2 . Investigation reveals that oxygen reduction on the palladium is rate limiting with optimised H 2 O 2 production at approximately pH 3 to 4, and first order in formate, followed by purely homogeneous TMB oxidation. The H 2 O 2 production is therefore studied separately as a nanozyme-like catalytic process equivalent to formate oxidase reactivity, linked to the molecularly rigid polyamine host (PIM-EA-TB) providing ammonium sites (in molecularly rigid surface cavities) that enhance both (i) 2-electron formate oxidation and (ii) 2-electron oxygen reduction to H 2 O 2 . Beneficial effects of hydrophobic ClO 4 -anions are noted as indirect evidence for the effect of ammonium sites in surface cavities. A computational DFT model for the artificial formate oxidase reactivity is developed to underpin and illustrate the hypothesis of PIM-EA-TB as an active catalyst component with implications for future nanozyme sensor development.
Controlling the selectivity of a reaction by rational designing of catalyst is an important and c... more Controlling the selectivity of a reaction by rational designing of catalyst is an important and challenging topic in catalysis research. In this article, we report a strategy to tune the product selectivity during aqueous phase hydrogenolysis of glycerol using gaseous H 2 . Ru/TiO 2 is an active catalyst for the hydrogenolysis of glycerol, however it promotes the hydrogenolysis of CÀ C bonds resulting in large quantities of C2 and C1 products. On the other hand, Pd/TiO 2 and Pt/TiO 2 catalysts are very selective for the hydrogenolysis of CÀ O bonds producing mainly C3 products (1,2 and 1,3 propanediols), however they are much less active compared to the Ru catalysts. In this article, we report that by combining Ru with Pt or Pd in a bimetallic nanoparticle, we can develop new catalysts that are both active and selective for CÀ O hydrogenolysis. A physical mixture of two monometallic catalysts does not show this enhanced selectivity for CÀ O hydrogenolysis, proving that intimate mixing of the two metals in a nanoparticle is crucial to tune the selectivity. All the monometallic and bimetallic catalysts have been characterised by microscopic and spectroscopic methods to understand their structural features. DFT studies were also done to rationalise the observed difference in the catalytic properties between monometallic and bimetallic catalysts.
Tungsten carbides display excellent performance in many heterogeneously processes because of thei... more Tungsten carbides display excellent performance in many heterogeneously processes because of their “Pt-like” catalytic properties. Preparation of tungsten carbides with controllable phase composition intently relevant to their catalytic behaviors is essential yet challenging. In this study, a structure-tuning tungsten carbide embedding in the carbon spheres (WxC@CS) was fabricated via carburization of organic-inorganic hybrid precursors, exhibiting high activity and promising regioselectivity towards CO bond hydrogenolysis. The rational tuning of the structure of WxC@CS was explored intensively. The catalytic hydrogenolysis performance is compared with those of a commercial-available WC transforming from inert phase to a composition-dependence active catalyst by structure reconstruction. The combined catalytic and characteristic analysis illustrates that the phenol production was dependent on the C-defect sites. Subsequently, an intimate connection between the phenol yield and the ...
This review summaries recent development across electro-, photoelectro- and bioelectro-catalyst d... more This review summaries recent development across electro-, photoelectro- and bioelectro-catalyst developments for multi-carbon products from CO2. It also explores the role of device design and operating conditions in enabling C–C bond generation.
Cit a tio n fo r fin al p u blis h e d ve r sio n: B a rloc c o, Il a ri a, B ello mi, Silvio, D ... more Cit a tio n fo r fin al p u blis h e d ve r sio n: B a rloc c o, Il a ri a, B ello mi, Silvio, D el g a d o, Ju a n J., C h e n , Xiao w ei, P r a ti, L a u r a , Di mi t r a t o s, N ikol a o s, Rol d a n, Alb e r t o a n d Villa, Alb e r t o 2 0 2 1. E n h a n ci n g a c tivity, s el e c tivity a n d s t a bility of p all a di u m c a t alys t s in fo r mi c a ci d d e c o m p o si tio n: Effe c t of s u p p o r t fu n c tio n aliz a tio n. C a t alysi s Tod ay 3 8 2 , p p . 6 1-7 0.
Herein, we report the synthesis of preformed bimetallic Pd‐Au nanoparticles supported on carbon n... more Herein, we report the synthesis of preformed bimetallic Pd‐Au nanoparticles supported on carbon nanofibers with different Pd : Au atomic ratio (nominal molar ratio: 8–2, 6–4, 4–6, 2–8) and the corresponding Pd and Au monometallic catalysts by sol immobilization method. The obtained materials were characterized thoroughly by Transmission Electron Microscopy (TEM), X‐ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectroscopy (ICP‐OES). The catalytic performances of the Pd‐Au catalysts were evaluated in the aqueous phase dehydrogenation of formic acid (FA) at room temperature obtaining enhanced activity, stability and selectivity compared to the monometallic systems. In particular, Pd6Au4 and Pd8Au2 showed the best combination of catalytic properties, i. e., high selectivity to H2 and improved catalytic stability. Density functional theory (DFT) calculations on Pd15, Au15 and Pd9Au6 clusters supported on a carbon sheet were then simulated to provi...
A combination of experiments and Density functional theory (DFT) calculations demonstrated that t... more A combination of experiments and Density functional theory (DFT) calculations demonstrated that the single vacancies present on the graphitic layers are the only active sites for FA dehydrogenation.
The incorporation of Ni single atoms into carbon paper produces self-standing electrode for effic... more The incorporation of Ni single atoms into carbon paper produces self-standing electrode for efficient CO2reduction to CO in water.
A nitrogen‐stabilized single‐atom catalyst containing low‐valence zinc atoms (Znδ+‐NC) is reporte... more A nitrogen‐stabilized single‐atom catalyst containing low‐valence zinc atoms (Znδ+‐NC) is reported. It contains saturated four‐coordinate (Zn‐N4) and unsaturated three‐coordinate (Zn‐N3) sites. The latter makes Zn a low‐valence state, as deduced from X‐ray photoelectron spectroscopy, X‐ray absorption spectroscopy, electron paramagnetic resonance, and density functional theory. Znδ+‐NC catalyzes electrochemical reduction of CO2 to CO with near‐unity selectivity in water at an overpotential as low as 310 mV. A current density up to 1 A cm−2 can be achieved together with high CO selectivity of >95 % using Znδ+‐NC in a flow cell. Calculations suggest that the unsaturated Zn‐N3 could dramatically reduce the energy barrier by stabilizing the COOH* intermediate owing to the electron‐rich environment of Zn. This work sheds light on the relationship among coordination number, valence state, and catalytic performance and achieves high current densities relevant for industrial applications.
This supplementary material contains: Images of a commercial transmission cell for UV-Vis. Ge... more This supplementary material contains: Images of a commercial transmission cell for UV-Vis. Geometries in XYZ of optimised complexes employed in this work. A complementary theory-experiment IR spectrum of [PdCl 3 Ap] 2+ .
In recent years, research efforts have focused on the development of safe and efficient H 2 gener... more In recent years, research efforts have focused on the development of safe and efficient H 2 generation/storage materials toward a fuel-cell-based H 2 economy as a long-term solution in the near future. Herein, we report the development of Pd nanoparticles supported on carbon nanofibers (CNFs) via sol-immobilisation and impregnation techniques. Thorough characterisation has been carried out by means of XRD, XPS, SEM-EDX, TEM, and BET. The catalysts have been evaluated for the catalytic decomposition of formic acid (HCOOH), which has been identified as a safe and convenient H 2 carrier under mild conditions. The influence of preparation method was investigated and catalysts prepared by the sol-immobilisation method showed higher catalytic performance (Pd SI /CNF) than their analogues prepared by the impregnation method (Pd IMP /CNF). A high turnover frequency (TOF) of 979 h -1 for Pd SI /CNF and high selectivity (>99.99%) was obtained at 30 • C for the additive-free formic acid decomposition. Comparison with a Pd/AC (activated charcoal) catalyst synthesised with sol-immobilisation method using as a support activated charcoal (AC) showed an increase of catalytic activity by a factor of four, demonstrating the improved performance by choosing CNFs as the preferred choice of support for the deposition of preformed colloidal Pd nanoparticles.
The production of chemicals from renewable resources, resulting in the establishment of biorefine... more The production of chemicals from renewable resources, resulting in the establishment of biorefineries, represents a challenge of increasing importance. Here we show that succinic acid, a C4 compound increasingly being produced on a kiloton scale by the microbial fermentation of sugar, can be selectively converted into a variety of important chemicals. Optimal performance in terms of activity, selectivity and reusability is observed with Al 2 O 3-supported Pd nanoparticles, which mediate the selective, hydrogenative lactonization of succinic acid to γ-butyrolactone at >90% selectivity, even at high levels of conversion (<70%). Through a variety of kinetic, spectroscopic and microscopic studies, preliminary structure−activity relationships are presented, and the roles of the reaction conditions, the choice of metal and the nature of the support in terms of guiding the overall process selectivity, are also investigated. On a broader level, these studies demonstrate the suitability of succinic acid to act as a platform for renewable chemical production in future biorefineries.
The nickel/zirconia (Ni/ZrO 2 ) interface plays a key role in the performance of the anode of sol... more The nickel/zirconia (Ni/ZrO 2 ) interface plays a key role in the performance of the anode of solid oxide fuel cells (SOFC) and it is therefore important to understand the interaction between nickel nanoparticles and the ZrO 2 surface. Here, we have described the interaction of five Ni n (n = 1-5) clusters with the (111) surface of cubic zirconia, c-ZrO 2 (111), using spin polarized density functional theory (DFT) calculations with inclusion of long-range dispersion forces. We have systematically evaluated the geometric and electronic structure of different cluster configurations and sizes and shown how the clusters interact with the oxygen and zirconium surface atoms. The cluster-surface interaction is characterized by a charge transfer from the Ni clusters to the surface. From calculations of the hopping rate and clustering energies, we have demonstrated that Ni atoms prefer to aggregate rather than wet the surface and we would therefore suggest that modifications in the synthesis could be needed to modify the coalescence of the supported metal particles of this catalytic system.
Cobalt nanoparticles play an important role as a catalyst in the Fischer-Tropsch synthesis. Durin... more Cobalt nanoparticles play an important role as a catalyst in the Fischer-Tropsch synthesis. During the reaction process, cobalt nanoparticles can become oxidized leading to the formation of two phases: CoO rock-salt and Co 3 O 4 cubic spinel. Experimentally, it is possible to evaluate the phase change and follow the catalyst degradation by measuring the magnetic moment, as each material presents a different magnetic structure. It is therefore important to develop a fundamental description, at the atomic scale, of cobalt and its oxide phases which we have done here using density functional theory with the Dudarev approach to account for the on-site Coulomb interactions (DFT+U). We have explored different U eff values, ranging from 0 to 5 eV, and found that U eff = 3.0 eV describes most appropriately the mechanical properties, as well as the electronic and magnetic structures of Co, CoO and Co 3 O 4 . We have considered a ferromagnetic ordering for the metallic phase and the antiferromagnetic structure for the oxide phases. Our results support the interpretation of the catalytic performance of metallic cobalt as it transforms into its oxidized phases under experimental conditions.
This is t h e a u t h o r's v e r sio n of a w o r k t h a t w a s s u b mi t t e d t o / a c c e... more This is t h e a u t h o r's v e r sio n of a w o r k t h a t w a s s u b mi t t e d t o / a c c e p t e d fo r p u blic a tio n. Cit a tio n fo r fin al p u blis h e d ve r sio n: F a n g, H ui h u a n g, Rol d a n M a r ti n ez, Alb e r t o ORCID
We have calculated the adsorption of the reducing agent hydrazine (N 2 H 4 ) on copper surfaces u... more We have calculated the adsorption of the reducing agent hydrazine (N 2 H 4 ) on copper surfaces using density functional theory calculations with a correction for the long-range interactions (DFT-D2). We have modeled the perfect and a number of defective Cu(100), (110), and (111) surfaces, which are found in the experimentally produced structures of copper nanoparticles. We have studied adsorption of hydrazine at three types of defects in the surfaces, i.e., monatomic steps, Cu adatoms, and Cu vacancies. Several lowenergy adsorption structures for hydrazine on each perfect and defective surface have been identified and compared. Our calculations reveal that hydrazine bridges surface copper atoms, with the molecule twisted from the gauche toward an eclipsed conformation, except on the adatom (100) and vacancycontaining ( ) and ( ) surfaces, where it adsorbs through one nitrogen atom in gauche and trans conformations, respectively. The strongest adsorption energy is found on the stepped (110) surface, where hydrazine bridges between the copper atoms on the step edge and the terrace, as it stabilizes the low-coordinated copper atoms. Our results show that, although the (110) surface contains a number of low-coordinated atoms that enhance the surface-molecule interactions, the addition of defects on the more stable ( ) and ( ) surfaces provides sites that enable hydrazine binding to almost the same extent. This study also confirms general observations of surface adsorption trends in terms of d-band center and binding energy as a function of coordination number, i.e., the stronger the molecular adsorption, the higher the d-band shifts at low-coordinated sites.
We have employed density functional theory calculations to analyze the possible tautomerization o... more We have employed density functional theory calculations to analyze the possible tautomerization of phenol mediated by three diff erent Lewis acid sites at the external (10) surface of zeolite MFI. A silicon atom of the silanol group was substituted by Sc, Fe, and Ga metal atoms, which adopted a formal charge of 3+. This substituted silanol was dehydrated in order to form three-coordinated Lewis acid sites. The tautomerization of phenol involves the adsorption of the molecule on the Lewis site, the dissociation of the phenolic O−H bond and the transfer of the proton to the zeolite framework. This proton is transferred from the zeolite to the C atom at ortho positions to the phenolic O atom, thus generating the tautomer. The acidity of the substituted Lewis sites follows a strength order of Ga < Fe < Sc, where the Sc substitution provides the lowest energy barriers: 33 kJ/mol for the dissociation of the O−H bond, and 32 kJ/mol for the formation of the C−H bond, calculated with the GGA functional PBE including Grimme's dispersion corrections. We observed that the GGA functional PBE produces binding energies and energy barriers with a diff erence of less than 13 kJ/mol compared to the meta-GGA TPSS and rev-TPSS and the hybrid-GGA HSE06.
Solid Oxide Fuel Cell (SOFC) devices convert chemical energy into electrical energy by oxidizing ... more Solid Oxide Fuel Cell (SOFC) devices convert chemical energy into electrical energy by oxidizing fuels such as hydrogen, natural gas, ethanol or hydrocarbon mixtures, which highlights the capability of SOFCs as candidates for applications in stationary power generation. The main species used as fuel in the SOFC are H2 and CO which are oxidized at the anode [1]. Instead of pure H2 or CO, H2/CO mixtures can be used leading to more complicated mechanisms since other processes such as the water-gas shift reaction (CO + H2O ↔ CO2 + H2) are involved. Robust materials are needed for SOFC since the working temperature is approximately equal to 800˚C. For example, a suitable anode is Ni/YSZ (nickel – yttria stabilized zirconia) since it has a high catalytic activity, mechanical and chemical stability and compatibility with the electrolyte under working conditions [2]. It is well known that the performance of the Ni/YSZ cermet depends on the microstructure and the distribution of Ni and YSZ phases in the cermet [3]. Additionally, this performance depends on the key reactions occurring at the triple phase boundary where the gas phase, Ni particles and YSZ surface meet. Thus, it is relevant to understand, at the atomic scale, the structure of Ni/YSZ and its interaction with the gas phase, and in particular CO2 and H2. To simulate the interaction of CO2 and H2 with Ni-YSZ, we have used electronic structure calculations based on the Density Functional Theory using the Vienna Ab-initio Simulation Package (VASP). We have studied the deposition of Nin (n=4-10) clusters on the YSZ(111) followed by the adsorption of molecules on the optimised Nin/YSZ(111) structures and calculated the geometric and electronic structure at the most stable adsorption site. We have also analysed the reaction path of the reverse water-gas shift reaction (CO2 + H2 ↔ CO + H2O) on three different Nin/YSZ(111) (n = 5, 6 and 10) interfaces, considering two possible pathways intermediates: hydrocarboxyl or formate. Finally, we have observed that both CO2 and H2 prefer to adsorb on the Nin cluster rather than on the surface. On the metal cluster the CO2 molecule is activated and the H2 is dissociated upon adsorption, whereas charge is transferred from the cluster to the molecules. We have determined two intermediate states and three intermediate states for the carboxylic acid and formate intermediate pathway, respectively, on the three Nin/YSZ(111) (n = 5, 6 and 10) interfaces. References 1. J. Hanna, W. Y. Lee, Y. Shi and A. F. Ghoniem, Prog. Energy Combust. Sci., 2014, 40, 74–111. 2. Kim, S. et al. Solid State Ionics, (2007), 178, 1304–1309. 3. Kim, S. et al. Solid State Ionics, (2006), 177, 931–938.
Controlling the selectivity of a reaction by rational designing of catalyst is an important and c... more Controlling the selectivity of a reaction by rational designing of catalyst is an important and challenging topic in catalysis research. In this article, we report a strategy to tune the product selectivity during aqueous phase hydrogenolysis of glycerol using gaseous H 2 . Ru/TiO 2 is an active catalyst for the hydrogenolysis of glycerol, however it promotes the hydrogenolysis of CÀ C bonds resulting in large quantities of C2 and C1 products. On the other hand, Pd/TiO 2 and Pt/TiO 2 catalysts are very selective for the hydrogenolysis of CÀ O bonds producing mainly C3 products (1,2 and 1,3 propanediols), however they are much less active compared to the Ru catalysts. In this article, we report that by combining Ru with Pt or Pd in a bimetallic nanoparticle, we can develop new catalysts that are both active and selective for CÀ O hydrogenolysis. A physical mixture of two monometallic catalysts does not show this enhanced selectivity for CÀ O hydrogenolysis, proving that intimate mixing of the two metals in a nanoparticle is crucial to tune the selectivity. All the monometallic and bimetallic catalysts have been characterised by microscopic and spectroscopic methods to understand their structural features. DFT studies were also done to rationalise the observed difference in the catalytic properties between monometallic and bimetallic catalysts.
The adsorption and dissociation of carbon monoxide (CO) have been studied on 81 Transition Metal ... more The adsorption and dissociation of carbon monoxide (CO) have been studied on 81 Transition Metal (TM) surfaces, with TMs having body centred cubic (bcc), face centred cubic (fcc), or hexagonal close-packed (hcp) crystalline structures. For each surface, CO, C, and O adsorptions, and C+O co-adsorptions were studied by density functional theory calculations on suited slab models, using Perdew-Burke-Ernzerhof functional with Grimme’s D3 correction for dispersive forces. CO dissociation activation and reaction energies, ΔE, were determined. The values, including zero point energy, were used to capture chemical trends along groups, d-series, and crystallographic phases concerning CO adsorption and dissociation, while simulated infrared (IR) spectroscopies and thermodynamic phase diagrams are provided. Late fcc TMs are found to adsorb CO weakly, perpendicularly, and are IR-visible, opposite to early bcc TMs, while hcp cases are distributed along these two extremes. The d-band centre, εd, ...
Surface cavities formed by molecularly rigid polymers of intrinsic microporosity affect catalytic... more Surface cavities formed by molecularly rigid polymers of intrinsic microporosity affect catalytic processes. Palladium nanoparticles of typically 3 nm diameter are formed in an intrinsically microporous polyamine (PIM-EA-TB) by borohydride reduction. These particles are shown to indirectly catalyse the oxidative colour change of indicator dye 3,5,3 0 ,5 0 -tetramethylbenzidine (TMB) in the presence of formic acid via formation of H 2 O 2 . Investigation reveals that oxygen reduction on the palladium is rate limiting with optimised H 2 O 2 production at approximately pH 3 to 4, and first order in formate, followed by purely homogeneous TMB oxidation. The H 2 O 2 production is therefore studied separately as a nanozyme-like catalytic process equivalent to formate oxidase reactivity, linked to the molecularly rigid polyamine host (PIM-EA-TB) providing ammonium sites (in molecularly rigid surface cavities) that enhance both (i) 2-electron formate oxidation and (ii) 2-electron oxygen reduction to H 2 O 2 . Beneficial effects of hydrophobic ClO 4 -anions are noted as indirect evidence for the effect of ammonium sites in surface cavities. A computational DFT model for the artificial formate oxidase reactivity is developed to underpin and illustrate the hypothesis of PIM-EA-TB as an active catalyst component with implications for future nanozyme sensor development.
Controlling the selectivity of a reaction by rational designing of catalyst is an important and c... more Controlling the selectivity of a reaction by rational designing of catalyst is an important and challenging topic in catalysis research. In this article, we report a strategy to tune the product selectivity during aqueous phase hydrogenolysis of glycerol using gaseous H 2 . Ru/TiO 2 is an active catalyst for the hydrogenolysis of glycerol, however it promotes the hydrogenolysis of CÀ C bonds resulting in large quantities of C2 and C1 products. On the other hand, Pd/TiO 2 and Pt/TiO 2 catalysts are very selective for the hydrogenolysis of CÀ O bonds producing mainly C3 products (1,2 and 1,3 propanediols), however they are much less active compared to the Ru catalysts. In this article, we report that by combining Ru with Pt or Pd in a bimetallic nanoparticle, we can develop new catalysts that are both active and selective for CÀ O hydrogenolysis. A physical mixture of two monometallic catalysts does not show this enhanced selectivity for CÀ O hydrogenolysis, proving that intimate mixing of the two metals in a nanoparticle is crucial to tune the selectivity. All the monometallic and bimetallic catalysts have been characterised by microscopic and spectroscopic methods to understand their structural features. DFT studies were also done to rationalise the observed difference in the catalytic properties between monometallic and bimetallic catalysts.
Tungsten carbides display excellent performance in many heterogeneously processes because of thei... more Tungsten carbides display excellent performance in many heterogeneously processes because of their “Pt-like” catalytic properties. Preparation of tungsten carbides with controllable phase composition intently relevant to their catalytic behaviors is essential yet challenging. In this study, a structure-tuning tungsten carbide embedding in the carbon spheres (WxC@CS) was fabricated via carburization of organic-inorganic hybrid precursors, exhibiting high activity and promising regioselectivity towards CO bond hydrogenolysis. The rational tuning of the structure of WxC@CS was explored intensively. The catalytic hydrogenolysis performance is compared with those of a commercial-available WC transforming from inert phase to a composition-dependence active catalyst by structure reconstruction. The combined catalytic and characteristic analysis illustrates that the phenol production was dependent on the C-defect sites. Subsequently, an intimate connection between the phenol yield and the ...
This review summaries recent development across electro-, photoelectro- and bioelectro-catalyst d... more This review summaries recent development across electro-, photoelectro- and bioelectro-catalyst developments for multi-carbon products from CO2. It also explores the role of device design and operating conditions in enabling C–C bond generation.
Cit a tio n fo r fin al p u blis h e d ve r sio n: B a rloc c o, Il a ri a, B ello mi, Silvio, D ... more Cit a tio n fo r fin al p u blis h e d ve r sio n: B a rloc c o, Il a ri a, B ello mi, Silvio, D el g a d o, Ju a n J., C h e n , Xiao w ei, P r a ti, L a u r a , Di mi t r a t o s, N ikol a o s, Rol d a n, Alb e r t o a n d Villa, Alb e r t o 2 0 2 1. E n h a n ci n g a c tivity, s el e c tivity a n d s t a bility of p all a di u m c a t alys t s in fo r mi c a ci d d e c o m p o si tio n: Effe c t of s u p p o r t fu n c tio n aliz a tio n. C a t alysi s Tod ay 3 8 2 , p p . 6 1-7 0.
Herein, we report the synthesis of preformed bimetallic Pd‐Au nanoparticles supported on carbon n... more Herein, we report the synthesis of preformed bimetallic Pd‐Au nanoparticles supported on carbon nanofibers with different Pd : Au atomic ratio (nominal molar ratio: 8–2, 6–4, 4–6, 2–8) and the corresponding Pd and Au monometallic catalysts by sol immobilization method. The obtained materials were characterized thoroughly by Transmission Electron Microscopy (TEM), X‐ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectroscopy (ICP‐OES). The catalytic performances of the Pd‐Au catalysts were evaluated in the aqueous phase dehydrogenation of formic acid (FA) at room temperature obtaining enhanced activity, stability and selectivity compared to the monometallic systems. In particular, Pd6Au4 and Pd8Au2 showed the best combination of catalytic properties, i. e., high selectivity to H2 and improved catalytic stability. Density functional theory (DFT) calculations on Pd15, Au15 and Pd9Au6 clusters supported on a carbon sheet were then simulated to provi...
A combination of experiments and Density functional theory (DFT) calculations demonstrated that t... more A combination of experiments and Density functional theory (DFT) calculations demonstrated that the single vacancies present on the graphitic layers are the only active sites for FA dehydrogenation.
The incorporation of Ni single atoms into carbon paper produces self-standing electrode for effic... more The incorporation of Ni single atoms into carbon paper produces self-standing electrode for efficient CO2reduction to CO in water.
A nitrogen‐stabilized single‐atom catalyst containing low‐valence zinc atoms (Znδ+‐NC) is reporte... more A nitrogen‐stabilized single‐atom catalyst containing low‐valence zinc atoms (Znδ+‐NC) is reported. It contains saturated four‐coordinate (Zn‐N4) and unsaturated three‐coordinate (Zn‐N3) sites. The latter makes Zn a low‐valence state, as deduced from X‐ray photoelectron spectroscopy, X‐ray absorption spectroscopy, electron paramagnetic resonance, and density functional theory. Znδ+‐NC catalyzes electrochemical reduction of CO2 to CO with near‐unity selectivity in water at an overpotential as low as 310 mV. A current density up to 1 A cm−2 can be achieved together with high CO selectivity of >95 % using Znδ+‐NC in a flow cell. Calculations suggest that the unsaturated Zn‐N3 could dramatically reduce the energy barrier by stabilizing the COOH* intermediate owing to the electron‐rich environment of Zn. This work sheds light on the relationship among coordination number, valence state, and catalytic performance and achieves high current densities relevant for industrial applications.
This supplementary material contains: Images of a commercial transmission cell for UV-Vis. Ge... more This supplementary material contains: Images of a commercial transmission cell for UV-Vis. Geometries in XYZ of optimised complexes employed in this work. A complementary theory-experiment IR spectrum of [PdCl 3 Ap] 2+ .
In recent years, research efforts have focused on the development of safe and efficient H 2 gener... more In recent years, research efforts have focused on the development of safe and efficient H 2 generation/storage materials toward a fuel-cell-based H 2 economy as a long-term solution in the near future. Herein, we report the development of Pd nanoparticles supported on carbon nanofibers (CNFs) via sol-immobilisation and impregnation techniques. Thorough characterisation has been carried out by means of XRD, XPS, SEM-EDX, TEM, and BET. The catalysts have been evaluated for the catalytic decomposition of formic acid (HCOOH), which has been identified as a safe and convenient H 2 carrier under mild conditions. The influence of preparation method was investigated and catalysts prepared by the sol-immobilisation method showed higher catalytic performance (Pd SI /CNF) than their analogues prepared by the impregnation method (Pd IMP /CNF). A high turnover frequency (TOF) of 979 h -1 for Pd SI /CNF and high selectivity (>99.99%) was obtained at 30 • C for the additive-free formic acid decomposition. Comparison with a Pd/AC (activated charcoal) catalyst synthesised with sol-immobilisation method using as a support activated charcoal (AC) showed an increase of catalytic activity by a factor of four, demonstrating the improved performance by choosing CNFs as the preferred choice of support for the deposition of preformed colloidal Pd nanoparticles.
The production of chemicals from renewable resources, resulting in the establishment of biorefine... more The production of chemicals from renewable resources, resulting in the establishment of biorefineries, represents a challenge of increasing importance. Here we show that succinic acid, a C4 compound increasingly being produced on a kiloton scale by the microbial fermentation of sugar, can be selectively converted into a variety of important chemicals. Optimal performance in terms of activity, selectivity and reusability is observed with Al 2 O 3-supported Pd nanoparticles, which mediate the selective, hydrogenative lactonization of succinic acid to γ-butyrolactone at >90% selectivity, even at high levels of conversion (<70%). Through a variety of kinetic, spectroscopic and microscopic studies, preliminary structure−activity relationships are presented, and the roles of the reaction conditions, the choice of metal and the nature of the support in terms of guiding the overall process selectivity, are also investigated. On a broader level, these studies demonstrate the suitability of succinic acid to act as a platform for renewable chemical production in future biorefineries.
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Papers by Alberto Roldan