Yitao Dai, Chao Li, Yanbin Shen, Jian Xu, Hans Niemantsverdriet, Yongwang Li, Flemming Besenbacher, Nina Lock, Ren Su, Nature Communications 9 (2018) Article number 60
Solar-driven photocatalysis has attracted significant attention in water splitting, CO2 reduction and organic synthesis. The syntheses of valuable azo- and azoxyaromatic dyes via selective photoreduction of nitroaromatic compounds have been realised using supported plasmonic metal nanoparticles at elevated temperatures (≥90 °C); however, the high cost, low efficiency and poor selectivity of such catalyst systems at room temperature limit their application. Here we demonstrate that the inexpensive graphitic C3N4 is an efficient photocatalyst for selective syntheses of a series of azo- and azoxy-aromatic compounds from their corresponding nitroaromatics under either purple (410 nm) or blue light (450 nm) excitation. The high efficiency and high selectivity towards azo- and azoxy-aromatic compounds can be attributed to the weakly bound photogenerated surface adsorbed H-atoms and a favourable N-N coupling reaction. The results reveal financial and environmental potential of photocatalysis for mass production of valuable chemicals.
Yunzhe Jiao, Ryan Sharpe, Tingbin Lim, Hans Niemantsverdriet, Jose Gracia, JACS 139 (2017) 16604–16608
The oxygen evolution complex (OEC) of photosystem II (PSII) is intrinsically more active than any synthetic alternative for the oxygen evolution reaction (OER). A crucial question to solve for the progress of artificial photosynthesis is to understand the influential interactions during water oxidation in PSII. We study the principles of interatomic electron transfer steps in OER, with emphasis on exchange interactions, revealing the influence of delocalizing ferromagnetic spin potentials during the catalytic process. The OEC is found to be an exchange coupled mixed-valence electron-spin acceptor where its orbital physics determine the unique activity of PSII. The two unpaired electrons needed in the triplet O2 molecule interact with the high spin state of the catalyst via exchange interactions; the optimal ferromagnetic catalyst and the resulting radical intermediates are spin paired. As a result, the active center of the CaMn4O5 cofactor, stimulated by the driving potential provided by photons, works as a spin valve to accelerate the formation and release of O2 from diamagnetic H2O.
Xianchi Jin, Chao Li, Chenbiao Xu, Dawei Guan, Ajin Cheruvathur, Yi Wang, Jian Xu, Dong Wei, Hongwei Xiang, J.W. (Hans) Niemantsverdriet, Yongwang Li, Qing Guo, Zhibo Ma, Ren Su, Xueming Yang, J. Catal. 354 (2017) 37–45
Polyol conversion to value-added products is of great interest for the bio-diesel industry. Photocatalytic oxidation processes may offer a green approach for polyol conversion; however the lack of comprehensive mechanistic understanding from an interdisciplinary perspective limits or even misleads the design of highly selective and efficient photocatalysts for such process. Here we have studied the photocatalytic polyol conversion on pristine TiO2 and metal (Au, Pd, and Pt) nanoparticles (NPs) decorated TiO2 using ethylene glycol (EG) as the model compound. We have developed a mechanistic picture at molecular level by coupling in-situ surface science study on rutile (110) surface with in-situ vibrational-mass spectrometry study on TiO2 nanopowders. The C-C bond cleavage was found to be the only pathway in EG photo-conversion under deaerated conditions, leading to the formation of formaldehyde and hydrogen. We rationalized that the desorption of the surface adsorbed H (Hads) to be the rate determining step (RDS), making pristine TiO2 a poor photocatalyst that only catalyze the EG conversion at very low surface coverages. The addition of metal NPs on TiO2 surface promotes the desorption of Hads significantly, thus leading to an enhanced C-C bond cleavage performance at higher surface coverages that is more applicable.
Xi Liu, Chenghua Zhang, Yongwang Li, J. W. Niemantsverdriet, Jakob B. Wagner, and Thomas W. Hansen, ACS Catal. 7 (2017) 4867−4875
Structural evolution of iron nanoparticles involving the formation and growth of iron carbide nuclei in the iron nanoparticle was directly visualized at the atomic level, using environmental transmission electron microscopy (TEM) under reactive conditions mimicking Fischer−Tropsch synthesis. Formation of the iron carbide nuclei and surface reconstruction of the iron nanoparticle play an essential role in carburization of the iron nanoparticle and consequent formation of Fe5C2. Identification of carbide and oxide intermediates evidenced by high-resolution TEM images, electron diffraction patterns and electron energy-loss spectra provides a detailed picture from initial activation to final degradation of iron under synthesis gas.
Mengzhu Li, Yiou Wang, Pei Tang, Nanhong Xie, Yunxuan Zhao, Xi Liu, Gang Hu, Jinglin Xie, Yufei Zhao, Junwang Tang, Tierui Zhang, and Ding Ma, Chem. Mater. 29 (2017) 2769–2776
Band gap opening and engineering make up one of the primary goals of developing novel materials for photocatalytic hydrogen generation. We report here a facile synthesis of graphene decorated with in-plane boron nitride domains via control of both the doping sequence of heteroatoms and the oxygen content of the graphene precursor, showing significant differences in the doping pattern compared with B and/or N singly doped or co-doped graphene. We uncover that the formation of BN domains in graphene is critical for engineering the band gap and delivering an improved activity for photocatalytic hydrogen generation in the absence of any photosensitizer. This work paves the way for the rational design and construction of graphene-based photocatalysts for efficient photocatalysis.
F.M. Sapountzi, M.N. Tsampas, H.O.A. Fredriksson, J.M. Gracia, J.W. Niemantsverdriet, International Journal of Hydrogen Energy 42 (2017) 10762–10774
This study investigates the production of hydrogen from the electrochemical reforming of short-chain alcohols (methanol, ethanol, iso-propanol) and their mixtures. High surface gas diffusion Pt/C electrodes were interfaced to a Nafion polymeric membrane. The assembly separated the two chambers of an electrochemical reactor, which were filled with anolyte (alcohol + H2O or alcohol + H2SO4) and catholyte (H2SO4) aqueous solutions. The half-reactions, which take place upon polarization, are the alcohol electrooxidation and the hydrogen evolution reaction at the anode and cathode, respectively. A standard Ag/AgCl reference electrode was introduced for monitoring the individual anodic and cathodic overpotentials. Our results show that roughly 75% of the total potential losses are due to sluggish kinetics of the alcohol electrooxidation reaction. Anodic overpotential becomes larger as the number of C-atoms in the alcohol increases, while a slight dependence on the pH was observed upon changing the acidity of the anolyte solution. In the case of alcohol mixtures, it is the largest alcohol that dictates the overall cell performance.
Yunzhe Jiao, Marta Serrano Torne, Jose Gracia, J. W. (Hans) Niemantsverdriet and Piet W. N. M. van Leeuwen, Catal. Sci. Technol. 7 (2017) 1404-1414
Twelve commercially available bisphosphine ligands have been evaluated in rhodium-catalyzed hydroformylation reactions. All ligands exhibited high chemoselectivities for aldehyde formation. The highest enantioselectivity (53% ee) of styrene hydroformylation was achieved with (S)-BTFM-Garphos (L7) substituted with electron withdrawing substituents. High pressure NMR (HP-NMR) spectroscopy and in situ high pressure IR spectroscopy (HP-IR) were used to study the resting states of the catalyst species in the reactions. The ligand effect on the structures of the observable species was examined. Both electronic and steric factors were considered to contribute to the performance of the various ligands. The results showed that decreasing the phosphine basicity increased the enantioselectivity, while in the systems studied here the steric character plays a less important role than the electronic features in achieving good regioselectivities.
Xiao Zhang, Xiaobing Zhu, Lili Lin, Siyu Yao, Mengtao Zhang, Xi Liu, Xiaoping Wang, Yong-Wang Li, Chuan Shi, and Ding Ma, ACS Catal. 7 (2017) 912–918
Cu-oxide catalysts have a tendency to deactivate dramatically in reverse water gas shift (RWGS) reaction, because of the aggregation of supported copper particles at high temperatures. Herein, β-Mo2C, which is a typical type of transition-metal carbide, has been demonstrated to be capable of dispersing and stabilizing copper particles. Cu/β-Mo2C catalysts exhibit good catalytic activity and stability for the RWGS reaction. Under relatively high weight hourly space velocity (WHSV = 300 000 mL/g/h), the optimized 1 wt % Cu/β-Mo2C exhibits superior activity over traditional oxide-supported Pt- and Cu-based catalysts. The activity was well-maintained in a 40 h stability test, and the catalyst shows stable reactivity in a six-cycle start-up cool-down experiment. Detailed structure characterizations demonstrate that the strong interaction between Cu and β-Mo2C effectively promotes the dispersion of supported copper and prevents the aggregation of Cu particles, which accounts for the extraordinary activity and stability for the RWGS reaction.
Ryan Sharpe, Tingbin Lim, Yunzhe Jiao, J. W. (Hans) Niemantsverdriet, Jose Gracia, ChemCatChem 8 (2016) 37628-3768
We have studied, by using ab initio calculations, the electronic properties of electro-catalysts for the oxygen evolution reaction (OER) with polarised density of states caused by localised spins in the d shell. Oxygen is a molecule in the triplet state (i.e. the outer electrons have parallel spins), which means that the spins localised in the p shell (↑O=O↑), the d shell and the conduction band electrons (t2gnegm) will couple through exchange interactions, which we think will provide favourable conditions for the OER. We compare the perovskites CaCu3Fe4O12 (CCF) and Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) with RuO2. CCF and BSCF both have fluctuating electronic structures accessible at room temperature that are linked to conducting spin-polarised density of states, equivalent to the paramagnetic state of covalent transition metal oxides with fine charge conductivity. CCF and BSCF both possess a considerable number of unpaired electrons localised in the inner d shell, high-spin configurations and competing inter-atomic exchange interactions. As a first approximation, the average fluctuation of the magnetisation in the metal atoms correlates linearly with the OER onset potential for the studied compositions. By linking the dynamics of the localised inner-electron spins to the conduction spins through exchange interactions, we can predict that other perovskites, such as Sr2Fe0.75Co0.25MoO6, will be OER active at room temperature, as they have similar electronic properties to CCF and BSCF.
C. J. Weststrate, Ionel M. Ciobîcă, Jan van de Loosdrecht, and J. W. Niemantsverdriet , J. Phys. Chem. C 120 (2016) 29210–29224
Experiments that provide insight into the elementary reaction steps of CxHy adsorbates are of crucial importance to better understand the chemistry of chain growth in Fischer–Tropsch synthesis (FTS). In the present study we use a combination of experimental and theoretical tools to explore the reactivity of C2Hx and C3Hx adsorbates derived from ethene and propene on the close-packed surface of cobalt. Adsorption studies show that both alkenes adsorb with a high sticking coefficient. Surface hydrogen does not affect the sticking coefficient but reduces the adsorption capacity of both ethene and propene by 50% and suppresses decomposition. On the other hand, even subsaturation quantities of COad strongly suppress alkene adsorption. Partial alkene dehydrogenation occurs at low surface temperature and predominantly yields acetylene and propyne. Ethylidyne and propylidyne can be formed as well, but only when the adsorbate coverage is high. Translated to FTS, the stable, hydrogen-lean adsorbates such as alkynes and alkylidynes will have long residence times on the surface and are therefore feasible intermediates for chain growth. The comparatively lower desorption barrier for propene relative to ethene can to a large extent be attributed to the higher stability of the molecule in the gas phase, where hyperconjugation of the double bond with σ bonds in the adjacent methyl group provides additional stability to propene. The higher desorption barrier for ethene can potentially contribute to the anomalously low C2Hx production rate that is typically observed in cobalt-catalyzed FTS.
Foteini M. Sapountzi, Jose M. Gracia, C.J. (Kees-Jan) Weststrate, Hans O.A. Fredriksson, J.W. (Hans) Niemantsverdriet, Progress in Energy and Combustion Science 58 (2017) 1–35
Water electrolysis is the most promising method for efficient production of high purity hydrogen (and oxygen), while the required power input for the electrolysis process can be provided by renewable sources (e.g. solar or wind). The thus produced hydrogen can be used either directly as a fuel or as a reducing agent in chemical processes, such as in Fischer–Tropsch synthesis. Water splitting can be realized both at low temperatures (typically below 100 °C) and at high temperatures (steam water electrolysis at 500–1000 °C), while different ionic agents can be electrochemically transferred during the electrolysis process (OH−, H+, O2−). Singular requirements apply in each of the electrolysis technologies (alkaline, polymer electrolyte membrane and solid oxide electrolysis) for ensuring high electrocatalytic activity and long-term stability. The aim of the present article is to provide a brief overview on the effect of the nature and structure of the catalyst–electrode materials on the electrolyzer's performance. Past findings and recent progress in the development of efficient anode and cathode materials appropriate for large-scale water electrolysis are presented. The current trends, limitations and perspectives for future developments are summarized for the diverse electrolysis technologies of water splitting, while the case of CO2/H2O co-electrolysis (for synthesis gas production) is also discussed.
See also nieuwsflash
Tingbin Lim, J. W. (Hans) Niemantsverdriet, Jose Gracia, ChemCatChem 8 (2016) 2968–2974
We have performed an in-depth ab initio study of the magnetic structure within the most active perovskites for the oxygen evolution reaction. In all cases, the ground state exhibits an extended antiferromagnetic coupling in the unit cell. Layered antiparallel alignment of the magnetic moments appears to be related to their electrocatalytic activity. All the perovskites calculated within this paper show space-separated charge-transport channels depending on the spin orientation. Comparing the electronic structures with the reported activities, we find a direct correlation between the magnetic accumulation on the spin channels in the bulk material and the catalytic activity. We discuss the possible implications of such observations in terms of magnetic interactions. During oxygen evolution in water electrolysis, reactants and products do not preserve spin. For triplet state oxygen to evolve, the catalyst at the anode can speed up the reaction if it is able to balance the magnetism of the oxygen molecule by extracting electrons with an opposite magnetic moment, conserving the overall spin.
Understanding the co-catalyst/semiconductor interaction is of key importance for the design and synthesis of next generation photocatalytic materials for efficient hydrogen production and environmental clean-up applications. Here we investigate preformed Pd nanoparticles (NPs) supported on a series of anatase TiO2 having well-controlled but varying degrees of crystallinity and crystallite size, and explore their photocatalytic performance for H2 production and phenol decomposition. Whilst tuning the anatase crystallite size significantly influences the photocatalytic performance, varying the TiO2 crystallinity shows a negligible effect. Interestingly, the optimum quantum efficiency (~78%) for H2 evolution is achieved with anatase having medium crystallite size (~16 nm), whereas for phenol decomposition, a promotional effect is only observed for anatase with larger crystallite sizes (> 20 nm). Surface radical species and radical densities study reveal that the photogenerated charge carriers have been trapped at different sites depending on the crystallite size of anatase. Whilst the excited electrons are only trapped in bulk lattice sites in small anatase (<16 nm), larger anatase particles provide extra surface sites for charge trapping, which benefit charge storage and transportation to Pd surface sites, leading to a more efficient utilisation of charge carriers for photocatalysis. Additionally, Pd supported on medium-size anatase (~16 nm) hinders the formation of O2•- radicals on TiO2 surfaces, thus preventing unwanted re-oxidation of photogenerated H2.
Collaboration withCardiff University, Aarhus University,
C.J. Weststrate, J. van de Loosdrecht, J.W. Niemantsverdriet, J. Catal. 342 (2016) 1
The present article summarizes experimental findings of the interaction of CO with single crystal surfaces of cobalt. We first provide a quantitative study of non-dissociative CO adsorption on Co(0001) and establish a quantitative correlation between θCO and adsorption site occupation. In light of these findings we revisit the structure of previously reported ordered CO/Co(0001) adsorbate layers. Measurements of the CO coverage at equilibrium conditions are used to derive a phase diagram for CO on Co(0001). For low temperature Fischer-Tropsch synthesis conditions the CO coverage is predicted to be ≈0.5 ML, a value that hardly changes with pCO. The CO desorption temperature found in temperature programmed desorption is practically structure-independent, despite structure-dependent heats of adsorption reported in the literature. This mismatch is attributed to a structure-dependent pre-exponential factor for desorption. IR spectra reported throughout this study provide a reference point for IR studies on cobalt catalysts. Results for CO adsorbed on flat and defect-rich Co surfaces as well as particular, CO adsorbed on top sites, and in addition affect the distribution of COad over the various possible adsorption sites.
C.J. Weststrate, P. van Helden, J.W. (Hans) Niemantsverdriet, Catalysis Today 275 (2016) 100
The current paper presents a mechanistic view on important steps in the Fischer-Tropsch synthesis on cobalt catalysts, inspired by surface science studies. By revisiting the relation between activity and selectivity that results from the ASF assumption we highlight that knowledge about the number of growing chains as well as their residence time (∼growth rate) is of crucial importance to sketch a physically realistic scenario for FTS. This motivates further investigations into the microscopic scenario for FTS chain growth on fcc cobalt nanoparticles, by looking into the reaction mechanism in relation to surface structure and by determining the activation energies for key elementary steps. Such studies indicate that the modest activity of Co FTS catalysts might very well be attributable to the difficulty to remove chemisorbed oxygen from the metallic surface, rather than to dissociation of CO, which was found to proceed readily at step edge sites. Chain growth is envisaged to take place on the close-packed surfaces, with chain initiation via CH + CH to form acetylene, followed by hydrogenation to form ethylidyne, C-CH3, a reaction that is shown to be promoted by co-adsorbed CO. Ethylidyne then couples with CH to form propyne, HC-C-CH3, etc. We propose that a fairly large number of surface sites is involved in the growth of a single chain. In such a “growth ensemble” multiple active step sites produce CHx monomer species that spill over onto the same close-packed coupling terrace, where one or only a few chains grow at the same time. In such a scenario diffusion of hydrocarbonaceous surface species is an essential step in the overall reaction sequence. We explore which factors need to be taken into account when considering of CxHy species under realistic reaction conditions. In addition, we note that the coupling reaction itself, via CH + C-CnH2n+1, is a source of growing chain mobility.
Editor’s choice = open access!
Using scanning tunneling microscopy (STM), we characterize the atomic-scale details of ultrathin films of iron carbide (FexCy) on Au(111) synthesized as a potential model system for the active iron carbide phase in iron Fischer–Tropsch synthesis (FTS) catalysts. The experiments show that room-temperature exposure of Fe islands gas to C2H4 deposited on the clean Au(111) surface results in partly converted Fe/FexCy islands. Multistep flash-heating treatment of the partly converted Fe/FexCy islands at 523 and 773 K results in pure highly crystalline FexCy islands with in-plane nearest-neighbor distances of 0.315 ± 0.005 nm. On the basis of the atom-resolved STM data, we propose that C2H4 dissociates at Fe island edges, after which the carbon diffuses inward into the interstitial region between the Fe and the Au substrate to form an FexCy surface that may be a good starting point for the investigation of iron carbide surfaces present under FTS conditions.
Aarhus University, Syngaschem BV
SynCat@Beijing scientists have (co-)authored several publications in collaboration with other laboratories all around the world.
Ryan Sharpe, Jon Counsell, Michael Bowker, Surf. Sci. 656 (2017) 60–65
Yibin Bu, C. J. Weststrate, J. W. Niemantsverdriet, and Hans O. A. Fredriksson, ACS Catal. 6 (2016) 7994–800
Blenerhassitt E. Buitendach, Elizabeth Erasmus, J. W. (Hans) Niemantsverdriet, and Jannie C. Swarts, Molecules 21 (2016) 1427
Blenerhassitt E. Buitendach, Elizabeth Erasmus, Marilé Landman, J. W. (Hans) Niemantsverdriet, and Jannie C. Swarts, Inorg. Chem. 55 (2016) 1992
Qian Yanga, Wilm Jones, Peter P. Wells, David Morgan, Lichun Dong, Baoshan Hu, Nikolaos Dimitratos, Mingdong Dong, Mike Bowker, Flemming Besenbacher, Ren Su, Appl. Catal. A 518 (2016) 213
G. Li, R. Su, J. Rao, J. Wu, P. Rudolf, G.R. Blake, R.A. de Groot, F. Besenbacher, and T.T.M. Palstra, J. Mater. Chem. A 4 (2016) 209
Ren Su, Flemming Besenbacher, Graham Hutchings; In: J. C. Colmenares, Y.-J. Xu (Eds.) Heterogeneous Photocatalysis, Green Chemistry and Sustainable Technology, Berlin Heidelberg, Springer, 2016, p. 109
Xi Liu, Viktoria Fabos, Stuart Taylor, David W. Knight, Keith Whiston, Graham J. Hutchings, Chem. - Eur. J. 22 (2016) 12290–12294
Tianfu Zhang, Stephen M. Driver, Stephanie J. Pratt, Stephen J. Jenkins, David A. King, Surf. Sci. 648 (2016) 10–13
Ying Wei, Ling Zhang and Zhijian Tian, RSC Adv. 66 (2016) 61915-61919
"Cu Model Catalyst Dynamics and CO Oxidation Kinetics Studied by Simultaneous in Situ UV–Vis and Mass Spectroscopy"
Yibin Bu, J. W. Hans Niemantsverdriet, and Hans O. A. Fredriksson, ACS Catal. 6 (2015) 2867