An official website of the United States government
Understanding the origin of enhanced catalytic activity is critical to heterogeneous catalyst design. This is especially important for non-noble metal-based catalysts, notably metal oxides, which have recently emerged as viable candidates for numerous thermal catalytic processes. For thermal catalytic reduction/hydrogenation using metal oxide nanoparticles, enhanced catalytic performance is typically attributed to an increased surface area and the presence of oxygen vacancies. Concomitantly, the treatments that induce oxygen vacancies also impact other material properties, such as the microstrain, crystallinity, oxidation state, and particle shape. Herein, multivariate statistical analysis is used to disentangle the impact of material properties of CuO nanoparticles on catalytic rates for nitroaromatic and methylene blue reduction. The impact of the microstrain, shape, and Cu(0) atomic percent is demonstrated for these reactions; furthermore, a protocol for correlating material property parameters to catalytic efficiency is presented, and the importance of catalyst design for these broadly utilized probe reactions is highlighted.
more »
« less
Kinetic Analysis of the Non‐Monotonic Response of Ethene Hydrogenation Rates to Ceria Surface Reduction
Abstract The precise effect of oxide understoichiometry on bulk oxide catalytic properties continues to remain a subject of intense investigation. Of specific interest in this regard is the role of oxygen vacancies present on bulk ceria catalysts that have recently been reported to represent a more cost‐effective alternative to the more toxic and expensive catalysts used industrially for the selective hydrogenation of acetylene to ethylene. Contrasting claims as to the effect of surface reduction on hydrogenation rates exist in the open literature, with vacancy formation attributed, in separate studies, either a favorable or a deleterious role in effecting hydrogenation turnovers. We report here the non‐monotonic behavior of ethene hydrogenation rates that subsumes both of these trends as a function of degree of surface reduction over a sufficiently large range of pre‐reduction temperatures. Steady state transient kinetic and isotopic exchange data combined with in‐situ titration experiments suggest that this non‐monotonic trend can be attributed not to a change in either the kinetic relevance of specific elementary steps or the hydrogenation mechanism, but rather to site requirements that stipulate the need for two distinct, proximal sites. We also show that the sensitivity of hydrogenation rates to surface reduction can be altered by varying ceria surface termination, with the more open (110) and (100) surfaces exhibiting a less asymmetric effect of surface reduction on ethene hydrogenation rates.
more »
« less
- Award ID(s):
- 1916133
- PAR ID:
- 10495267
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- ChemCatChem
- Volume:
- 16
- Issue:
- 13
- ISSN:
- 1867-3880
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Ceria (CeO 2 ) has recently been found to catalyze the selective hydrogenation of alkynes, which has stimulated much discussion on the catalytic mechanism on various facets of reducible oxides. In this work, H 2 dissociation and acetylene hydrogenation on bare and Ni doped CeO 2 (110) surfaces are investigated using density functional theory (DFT). Similar to that on the CeO 2 (111) surface, our results suggest that catalysis is facilitated by frustrated Lewis pairs (FLPs) formed by oxygen vacancies (O v s) on the oxide surfaces. On bare CeO 2 (110) with a single O v (CeO 2 (110)–O v ), two surface Ce cations with one non-adjacent O anion are shown to form (Ce 3+ –Ce 4+ )/O quasi-FLPs, while for the Ni doped CeO 2 (110) surface with one (Ni–CeO 2 (110)–O v ) or two (Ni–CeO 2 (110)–2O v ) O v s, one Ce and a non-adjacent O counterions are found to form a mono-Ce/O FLP. DFT calculations indicate that Ce/O FLPs facilitate the H 2 dissociation via a heterolytic mechanism, while the resulting surface O–H and Ce–H species catalyze the subsequent acetylene hydrogenation. With CeO 2 (110)–O v and Ni–CeO 2 (110)–2O v , our DFT calculations suggest that the first hydrogenation step is the rate-determining step with a barrier of 0.43 and 0.40 eV, respectively. For Ni–CeO 2 (110)–O v , the reaction is shown to be controlled by the H 2 dissociation with a barrier of 0.41 eV. These barriers are significantly lower than that (about 0.7 eV) on CeO 2 (111), explaining the experimentally observed higher catalytic efficiency of the (110) facet of ceria. The change of the rate-determining step is attributed to the different electronic properties of Ce in the Ce/O FLPs – the Ce f states closer to the Fermi level not only facilitate the heterolytic dissociation of H 2 but also lead to a higher barrier of acetylene hydrogenation.more » « less
-
Abstract Oxide supports with well‐defined shapes enable investigations on the effects of surface structure on metal–support interactions and correlations to catalytic activity and selectivity. Here, a modified atomic layer deposition technique was developed to achieve ultra‐low loadings (8–16 ppm) of Pt on shaped ceria nanocrystals. Using octahedra and cubes, which expose exclusively (111) and (100) surfaces, respectively, the effect of CeO2surface facet on Pt‐CeO2interactions under reducing conditions was revealed. Strong electronic interactions result in electron‐deficient Pt species on CeO2(111) after reduction, which increased the stability of the atomically dispersed Pt. This afforded significantly higher NMR signal enhancement in parahydrogen‐induced polarization experiments compared with the electron‐rich platinum on CeO2(100), and a factor of two higher pairwise selectivity (6.1 %) in the hydrogenation of propene than any previously reported monometallic heterogeneous Pt catalyst.more » « less
-
Ceria and its solid solutions play a vital role in several industrial processes and devices. These include solar energy-to-fuel conversion, solid oxide fuel and electrolyzer cells, memristors, chemical looping combustion, automotive 3-way catalysts, catalytic surface coatings, supercapacitors and recently, electrostrictive devices. An attractive feature of ceria is the possibility of tuning defect-chemistry to increase the effectiveness of the materials in application areas. Years of study have revealed many features of the long-range, macroscopic characteristics of ceria and its derivatives. In this review we focus on an area of ceria defect chemistry which has received comparatively little attention – defect-induced local distortions and short-range associates. These features are non-periodic in nature and hence not readily detected by conventional X-ray powder diffraction. We compile the relevant literature data obtained by thermodynamic analysis, Raman spectroscopy, and X-ray absorption fine structure (XAFS) spectroscopy. Each of these techniques provides insight into material behavior without reliance on long-range periodic symmetry. From thermodynamic analyses, association of defects is inferred. From XAFS, an element-specific probe, local structure around selected atomic species is obtained, whereas from Raman spectroscopy, local symmetry breaking and vibrational changes in bonding patterns is detected. We note that, for undoped ceria and its solid solutions, the relationship between short range order and cation–oxygen-vacancy coordination remains a subject of active debate. Beyond collating the sometimes contradictory data in the literature, we strengthen this review by reporting new spectroscopy results and analysis. We contribute to this debate by introducing additional data and analysis, with the expectation that increasing our fundamental understanding of this relationship will lead to an ability to predict and tailor the defect-chemistry of ceria-based materials for practical applications.more » « less
-
Small nanoparticles of ceria deposited on a powder of CuO display a very high selectivity for the production of methanol via CO2 hydrogenation. CeO2/CuO catalysts with ceria loadings of 5%, 20%, and 50% were investigated. Among these, the system with 5% CeOx showed the best catalytic performance at temperatures between 200 and 350 °C. The evolution of this system under reaction conditions was studied using a combination of environmental transmission electron microscopy (E-TEM), in situ X-ray absorption spectroscopy (XAS), and time-resolved X-ray diffraction (TR-XRD). For 5% CeOx/Cu, the in situ studies pointed to a full conversion of CuO into metallic copper, with a complete transformation of Ce4+ into Ce3+. Images from E-TEM showed drastic changes in the morphology of the catalyst when it was exposed to H2, CO2, and CO2/H2 mixtures. Under a CO2/H2 feed, there was a redispersion of the ceria particles that was detected by E-TEM and in situ TR-XRD. These morphological changes were made possible by the inverse oxide/metal configuration and facilitate the binding and selective conversion of CO2 to methanol.more » « less
