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•WO3/Fe2O3 exhibited high activity for the NH3-SCR of NOx.•The existed WO3 inhibited the crystallization of the Fe2O3 phase.•The addition of WO3 leads to increased Lewis acid sites ...and proper redox property.•The formation of inactive nitrate species was inhibited over WO3/Fe2O3.
A novel WO3-doped Fe2O3 catalyst was investigated for the selective catalytic reduction of NOx by NH3 (NH3-SCR). It was found that the WO3/Fe2O3 catalyst exhibited high NH3-SCR activity in a wide range of operating temperatures and high resistance against H2O and SO2. The highly dispersed WO3 acted as both “chemical” and “structural” promoters, which inhibited the crystallization of the Fe2O3 phase, leading to the high surface area. The synergetic effect between WO3 and Fe2O3 makes WO3/Fe2O3 catalyst remain a proper redox property, effectively weaken the unselective catalytic oxidation of NH3, thus resulting in the enhanced NH3-SCR performance. In particular, introducing WO3 to Fe2O3 resulted in the increased number of Lewis acid sites, thus preventing the formation of nitrate and leaving more active sites available for the adsorption and activation of NH3. All of these factors, collectively, accounted for the superior deNOx performance of WO3/Fe2O3 catalyst.
Electrocatalytic CO2 conversion into fuel is a prospective strategy for the sustainable energy production. However, still many parts of the catalyst such as low catalytic activity, selectivity, and ...stability are challenging. Herein, a hierarchical hexagonal Zn catalyst showed highly efficient and, more importantly, stable performance as an electrocatalyst for selectively producing CO. Moreover, we found that its high selectivity for CO is attributed to morphology. In electrochemical analysis, Zn (101) facet is favorable to CO formation whereas Zn (002) facet favors the H2 evolution during CO2 electrolysis. Indeed, DFT calculations showed that (101) facet lowers a reduction potential for CO2 to CO by more effectively stabilizing a .COOH intermediate than (002) facet. This further suggests that tuning the crystal structure to control (101)/(002) facet ratio of Zn can be considered as a key design principle to achieve a desirable product from Zn catalyst.
CO2 conversion: A design strategy for efficient carbon dioxide reduction is suggested using a well‐synthesized hierarchical hexagonal Zn catalyst which shows highly selective and, more importantly, stable performance towards carbon monoxide production (see picture). The manipulation of the Zn crystal structure and its facet ratio (101)/(002) can be used as a key control factor for product selectivity.
Nitrogen (N)-doped carbon materials were shown in recent studies to have promising catalytic activity for oxygen reduction reaction (ORR) as a metal-free alternative to platinum, but the underlying ...molecular mechanism or even the active sites for high catalytic efficiency are still missing or controversial both experimentally and theoretically. We report here the results of periodic density functional theory (DFT) calculations about the ORR at the edge of a graphene nanoribbon (GNR). The edge structure and doped-N near the edge are shown to enhance the oxygen adsorption, the first electron transfer, and also the selectivity toward the four-electron, rather than the two-electron, reduction pathway. We find that the outermost graphitic nitrogen site in particular gives the most desirable characteristics for improved ORR activity, and hence the active site. However, the latter graphitic nitrogen becomes pyridinic-like in the next electron and proton transfer reaction via the ring-opening of a cyclic C-N bond. This inter-conversion between the graphitic and pyridinic sites within a catalytic cycle may reconcile the controversy whether the pyridinic, graphitic, or both nitrogens are active sites.
Catalysis is a key technology for the synthesis of renewable fuels through electrochemical reduction of CO2. However, successful CO2 reduction still suffers from the lack of affordable catalyst ...design and understanding the factors governing catalysis. Herein, we demonstrate that the CO2 conversion selectivity on Sn (or SnOx/Sn) electrodes is correlated to the native oxygen content at the subsurface. Electrochemical analyses show that the reduced Sn electrode with abundant oxygen species effectively stabilizes a CO2.− intermediate rather than the clean Sn surface, and consequently results in enhanced formate production in the CO2 reduction. Based on this design strategy, a hierarchical Sn dendrite electrode with high oxygen content, consisting of a multi‐branched conifer‐like structure with an enlarged surface area, was synthesized. The electrode exhibits a superior formate production rate (228.6 μmol h−1 cm−2) at −1.36 VRHE without any considerable catalytic degradation over 18 h of operation.
Not exactly what it says on the tin: Rational design principles for tin electrodes to be used in selective CO2 reduction to formate are suggested using hierarchical tin dendrite electrodes (multi‐branched conifer‐like structure) that show remarkable activity and stability. The initial oxygen content of the tin electrode is set as “selectivity descriptor” and the architecture is manipulated to maximize the number of active sites.
The catalytic removal of nitrogen oxide (NO
x
) under lean-burn conditions is one of the most important targets in catalysis research. Some lean-NO
x
control technologies such as the direct ...decomposition of NO
x
, NO
x
storage-reduction (NSR), and selective catalytic reduction (SCR) using different reducing agents (diesel soot, NH
3
, or hydrocarbon) are described. The reaction mechanism of NSR, which is the most promising technology, together with some novel NSR catalysts is discussed. Some mechanisms of SCR of NO
x
by hydrocarbon (HC-SCR) were classified into two categories: one is the adsorption/dissociation mechanism, and the other is the oxidation-reduction mechanism. Based on the discussion of the reaction mechanism, the influence of some factors (catalyst support, metal loading, calcination temperature, catalyst preparation method, oxygen, reducing agents, water, and sulfur) on the activity of HC-SCR catalyst is discussed. It seems that Ag/Al
2
O
3
catalyst offers the most promising for SCR of NO
x
by hydrocarbon due to the exciting promotion effect of H
2
. Plasma or microwave assisted zeolite-based catalyst may also lead to a new approach for HC-SCR of NO
x
. Combinatorial catalysis, which was developed recently for discovering the practical combined catalyst quickly, also was introduced. Finally, future research directions in the area of lean-NO
x
is proposed.
Carbon nanotubes (CNTs), either single wall carbon nanotubes (SWNTs) or multiwall carbon nanotubes (MWNTs), can improve the thermoelectric properties of poly(3,4-ethylenedioxythiophene) ...poly(styrenesulfonate) (PEDOT:PSS), but it requires addition of 3040 wt% CNTs. We report that the figure of merit (ZT) value of PEDOT:PSS thin film for thermoelectric property is increased about 10 times by incorporating 2 wt% of graphene. PEDOT:PSS thin films containing 1, 2, 3 wt% graphene are prepared by solution spin coating method. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy analyses identified the strong interactions which facilitated the dispersion between graphene and PEDOT:PSS. The uniformly distributed graphene increased the interfacial area by 210 times as compared with CNT based on the same weight. The power factor and ZT value of PEDOT:PSS thin film containing 2 wt% graphene was 11.09 W mK
2
and 2.1 10
2
, respectively. This enhancement arises from the facilitated carrier transfer between PEDOT:PSS and graphene as well as the high electron mobility of graphene (200000 cm
2
V
1
s
1
). Furthermore the porous structure of the thin film decreases the thermal conductivity resulting in a high ZT value, which is higher by 20% than that for a PEDOT:PSS thin film containing 35 wt% SWNTs.
High performance thermoelectric (PEDOT:PSS):graphene nanocomposite thin films were fabricated with a small amount of graphene. This ZT value is the larger than that of pristine PEDOT:PSS thin film by 10 times.
N-doped carbon materials are considered as next-generation oxygen reduction reaction (ORR) catalysts for fuel cells due to their prolonged stability and low cost. However, the underlying mechanism of ...these catalysts has been only insufficiently identified, preventing the rational design of high-performing catalysts. Here, we show that the first electron is transferred into O2 molecules at the outer Helmholtz plane (ET-OHP) over a long range. This is in sharp contrast to the conventional belief that O2 adsorption must precede the ET step and thus that the active site must possess as good an O2 binding character as that which occurs on metallic catalysts. Based on the ET-OHP mechanism, the location of the electrode potential dominantly characterizes the ORR activity. Accordingly, we demonstrate that the electrode potential can be elevated by reducing the graphene size and/or including metal impurities, thereby enhancing the ORR activity, which can be transferred into single-cell operations with superior stability.
Graphene has been highlighted recently as a promising material for energy conversion due to its unique properties deriving from a two-dimensional layered structure of sp super(2)-hybridized carbon. ...Herein, N-doped graphene (NGr) is developed for its application in oxygen reduction reactions (ORRs) in acidic media, and additional doping of B or P into the NGr is attempted to enhance the ORR performance. The NGr exhibits an onset potential of 0.84 V and a mass activity of 0.45 mA mg super(-1) at 0.75 V. However, the B, N- (BNGr) and P, N-doped graphene (PNGr) show onset potentials of 0.86 and 0.87 V, and mass activities of 0.53 and 0.80 mA mg super(-1), respectively, which are correspondingly 1.2 and 1.8 times higher than those of the NGr. Moreover, an additional doping of B or P effectively reduces the production of H sub(2)O sub(2) in the ORRs, and shows much higher stability than that of Pt/C in acidic media. It is proposed that the improvement in the ORR activity results from the enhanced asymmetry of the spin density or electron transfer on the basal plane of the graphene, and the decrease in the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the graphene through additional doping of B or P.
Heteroatom (nitrogen and sulfur)-doped carbons were synthesized via the pyrolysis of composites composed of iron chloride, cobalt chloride and five different amino acids (alanine, cysteine, glycine, ...niacine and valine), and their electrocatalytic activity towards oxygen reduction reactions (ORR) compared with each other for fuel cell applications. In all of the prepared catalysts, carbon was doped by nitrogen, and, in particular, a catalyst synthesized from cysteine was dual-doped with nitrogen and sulfur. Among all the catalysts, the dual-doped carbon showed the highest onset potential (0.55 V, vs. Ag/AgCl) and electrochemical activity in acidic media, - 0.2 mA (at 0.2 V, vs. Ag/AgCl), which is about 43% of that of commercial Pt/C (40 wt%). XPS revealed that sulfur was doped in the carbon as sulfate or sulfonate, and it is surmised that not only nitrogen doping but also sulfur doping of carbon plays a key role in improving its electrocatalytic activity towards ORR.
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