Constructing Metal(II)-Sulfate Site Catalysts toward Low Overpotential Carbon Dioxide Electroreduction to Fuel Chemicals.

Precise regulation of the active site structure is an important means to enhance the activity and selectivity of catalysts in CO2 electroreduction. Here, we creatively introduce anionic groups, which can not only stabilize metal sites with strong coordination ability but also have rich interactions with protons at active sites to modify the electronic structure and proton transfer process of catalysts. This strategy helps to convert CO2 into fuel chemicals at low overpotentials. As a typical example, a composite catalyst, CuO/Cu-NSO4/CN, with highly dispersed Cu(II)-SO4 sites has been reported, in which CO2 electroreduction to formate occurs at a low overpotential with a high Faradaic efficiency (-0.5 V vs. RHE, FEformate=87.4 %). Pure HCOOH is produced with an energy conversion efficiency of 44.3 % at a cell voltage of 2.8 V. Theoretical modeling demonstrates that sulfate promotes CO2 transformation into a carboxyl intermediate followed by HCOOH generation, whose mechanism is significantly different from that of the traditional process via a formate intermediate for HCOOH production.

Structure Sensitivity of Metal Catalysts Revealed by Interpretable Machine Learning and First-Principles Calculations.

The nature of the active sites and their structure sensitivity are the keys to rational design of efficient catalysts but have been debated for almost one century in heterogeneous catalysis. Though the Brønsted-Evans-Polanyi (BEP) relationship along with linear scaling relation has long been used to study the reactivity, explicit geometry, and composition properties are absent in this relationship, a fact that prevents its exploration in structure sensitivity of supported catalysts. In this work, based on interpretable multitask symbolic regression and a comprehensive first-principles data set, we discovered a structure descriptor, the topological under-coordinated number mediated by number of valence electrons and the lattice constant, to successfully address the structure sensitivity of metal catalysts. The database used for training, testing, and transferability investigation includes bond-breaking barriers of 20 distinct chemical bonds over 10 transition metals, two metal crystallographic phases, and 17 different facets. The resulting 2D descriptor composing the structure term and the reaction energy term shows great accuracy to predict the reaction barriers and generalizability over the data set with diverse chemical bonds in symmetry, bond order, and steric hindrance. The theory is physical and concise, providing a constructive strategy not only to understand the structure sensitivity but also to decipher the entangled geometric and electronic effects of metal catalysts. The insights revealed are valuable for the rational design of the site-specific metal catalysts.

Tungsten-Iron-Ruthenium Ternary Alloy Immobilized into the Inner Nickel Foam for High-Current-Density Water Oxidation.

The pursuit of highly-active and stable catalysts in anodic oxygen evolution reaction (OER) is desirable for high-current-density water electrolysis toward industrial hydrogen production. Herein, a straightforward yet feasible method to prepare WFeRu ternary alloying catalyst on nickel foam is demonstrated, whereby the foreign W, Fe, and Ru metal atoms diffuse into the Ni foam resulting in the formation of inner immobilized ternary alloy. Thanks to the synergistic impact of foreign metal atoms and structural robustness of inner immobilized alloying catalyst, the well-designed WFeRu@NF self-standing anode exhibits superior OER activities. It only requires overpotentials of 245 and 346 mV to attain current densities of 20 and 500 mA cm-2, respectively. Moreover, the as-prepared ternary alloying catalyst also exhibits a long-term stability at a high-current-density of 500 mA cm-2 for over 45 h, evidencing the inner-immobilization strategy is promising for the development of highly active and stable metal-based catalysts for high-density-current water oxidation process.

Crystal-Phase Engineering in Heterogeneous Catalysis.

The performance of a chemical reaction is critically dependent on the electronic and/or geometric structures of a material in heterogeneous catalysis. Over the past century, the Sabatier principle has already provided a conceptual framework for optimal catalyst design by adjusting the electronic structure of the catalytic material via a change in composition. Beyond composition, it is essential to recognize that the geometric atomic structures of a catalyst, encompassing terraces, edges, steps, kinks, and corners, have a substantial impact on the activity and selectivity of a chemical reaction. Crystal-phase engineering has the capacity to bring about substantial alterations in the electronic and geometric configurations of a catalyst, enabling control over coordination numbers, morphological features, and the arrangement of surface atoms. Modulating the crystallographic phase is therefore an important strategy for improving the stability, activity, and selectivity of catalytic materials. Nonetheless, a complete understanding of how the performance depends on the crystal phase of a catalyst remains elusive, primarily due to the absence of a molecular-level view of active sites across various crystal phases. In this review, we primarily focus on assessing the dependence of catalytic performance on crystal phases to elucidate the challenges and complexities inherent in heterogeneous catalysis, ultimately aiming for improved catalyst design.

An early warning signal for grassland degradation on the Qinghai-Tibetan Plateau.

Intense grazing may lead to grassland degradation on the Qinghai-Tibetan Plateau, but it is difficult to predict where this will occur and to quantify it. Based on a process-based ecosystem model, we define a productivity-based stocking rate threshold that induces extreme grassland degradation to assess whether and where the current grazing activity in the region is sustainable. We find that the current stocking rate is below the threshold in ~80% of grassland areas, but in 55% of these grasslands the stocking rate exceeds half the threshold. According to our model projections, positive effects of climate change including elevated CO2 can partly offset negative effects of grazing across nearly 70% of grasslands on the Plateau, but only in areas below the stocking rate threshold. Our analysis suggests that stocking rate that does not exceed 60% (within 50% to 70%) of the threshold may balance human demands with grassland protection in the face of climate change.

Boosting CO hydrogenation towards C2+ hydrocarbons over interfacial TiO2-x/Ni catalysts.

Considerable attention has been drawn to tune the geometric and electronic structure of interfacial catalysts via modulating strong metal-support interactions (SMSI). Herein, we report the construction of a series of TiO2-x/Ni catalysts, where disordered TiO2-x overlayers immobilized onto the surface of Ni nanoparticles (~20 nm) are successfully engineered with SMSI effect. The optimal TiO2-x/Ni catalyst shows a CO conversion of ~19.8% in Fischer-Tropsch synthesis (FTS) process under atmospheric pressure at 220 °C. More importantly, ~64.6% of the product is C2+ paraffins, which is in sharp contrast to the result of the conventional Ni catalyst with the main product being methane. A combination study of advanced electron microscopy, multiple in-situ spectroscopic characterizations, and density functional theory calculations indicates the presence of Niδ-/TiO2-x interfacial sites, which could bind carbon atom strongly, inhibit methane formation and facilitate the C-C chain propagation, lead to the production of C2+ hydrocarbon on Ni surface.

Atomically dispersed Ir/α-MoC catalyst with high metal loading and thermal stability for water-promoted hydrogenation reaction.

Synthesis of atomically dispersed catalysts with high metal loading and thermal stability is challenging but particularly valuable for industrial application in heterogeneous catalysis. Here, we report a facile synthesis of a thermally stable atomically dispersed Ir/α-MoC catalyst with metal loading as high as 4 wt%, an unusually high value for carbide supported metal catalysts. The strong interaction between Ir and the α-MoC substrate enables high dispersion of Ir on the α-MoC surface, and modulates the electronic structure of the supported Ir species. Using quinoline hydrogenation as a model reaction, we demonstrate that this atomically dispersed Ir/α-MoC catalyst exhibits remarkable reactivity, selectivity and stability, for which the presence of high-density isolated Ir atoms is the key to achieving high metal-normalized activity and mass-specific activity. We also show that the water-promoted quinoline hydrogenation mechanism is preferred over the Ir/α-MoC, and contributes to high selectivity towards 1,2,3,4-tetrahydroquinoline. The present work demonstrates a new strategy in constructing a high-loading atomically dispersed catalyst for the hydrogenation reaction.

Hydroxyl improving the activity, selectivity and stability of supported Ni single atoms for selective semi-hydrogenation.

Atomically dispersed metal catalysts with high atomic utilization and selectivity have been widely studied for acetylene semi-hydrogenation in excess ethylene among others. Further improvements of activity and selectivity, in addition to stability and loading, remain elusive due to competitive adsorption and desorption between reactants and products, hydrogen activation, partial hydrogenation etc. on limited site available. Herein, comprehensive density functional theory calculations have been used to explore the new strategy by introducing an appropriate ligand to stabilize the active single atom, improving the activity and selectivity on oxide supports. We find that the hydroxyl group can stabilize Ni single atoms significantly by forming Ni1(OH)2 complexes on anatase TiO2(101), whose unique electronic and geometric properties enable high performance in acetylene semi-hydrogenation. Specifically, Ni1(OH)2/TiO2(101) shows favorable acetylene adsorption and promotes the heterolytic dissociation of H2 achieving high catalytic activity, and it simultaneously weakens the ethylene bonding to facilitate subsequent desorption showing high ethylene selectivity. Hydroxyl stabilization of single metal atoms on oxide supports and promotion of the catalytic activity are sensitive to transition metal and the oxide supports. Compared to Co, Rh, Ir, Pd, Pt, Cu, Ag and Au, and anatase ZrO2, IrO2 and NbO2 surfaces, the optimum interactions between Ni, O and Ti and resulted high activity, selectivity and stability make Ni1(OH)2/TiO2(101) a promising catalyst in acetylene hydrogenation. Our work provides valuable guidelines for utilization of ligands in the rational design of stable and efficient atomically dispersed catalysts.

Identifying dominant environmental predictors of freshwater wetland methane fluxes across diurnal to seasonal time scales.

While wetlands are the largest natural source of methane (CH4 ) to the atmosphere, they represent a large source of uncertainty in the global CH4 budget due to the complex biogeochemical controls on CH4 dynamics. Here we present, to our knowledge, the first multi-site synthesis of how predictors of CH4 fluxes (FCH4) in freshwater wetlands vary across wetland types at diel, multiday (synoptic), and seasonal time scales. We used several statistical approaches (correlation analysis, generalized additive modeling, mutual information, and random forests) in a wavelet-based multi-resolution framework to assess the importance of environmental predictors, nonlinearities and lags on FCH4 across 23 eddy covariance sites. Seasonally, soil and air temperature were dominant predictors of FCH4 at sites with smaller seasonal variation in water table depth (WTD). In contrast, WTD was the dominant predictor for wetlands with smaller variations in temperature (e.g., seasonal tropical/subtropical wetlands). Changes in seasonal FCH4 lagged fluctuations in WTD by ~17 ± 11 days, and lagged air and soil temperature by median values of 8 ± 16 and 5 ± 15 days, respectively. Temperature and WTD were also dominant predictors at the multiday scale. Atmospheric pressure (PA) was another important multiday scale predictor for peat-dominated sites, with drops in PA coinciding with synchronous releases of CH4 . At the diel scale, synchronous relationships with latent heat flux and vapor pressure deficit suggest that physical processes controlling evaporation and boundary layer mixing exert similar controls on CH4 volatilization, and suggest the influence of pressurized ventilation in aerenchymatous vegetation. In addition, 1- to 4-h lagged relationships with ecosystem photosynthesis indicate recent carbon substrates, such as root exudates, may also control FCH4. By addressing issues of scale, asynchrony, and nonlinearity, this work improves understanding of the predictors and timing of wetland FCH4 that can inform future studies and models, and help constrain wetland CH4 emissions.

Structure Sensitivity of Au-TiO2 Strong Metal-Support Interactions.

Strong metal-support interactions (SMSI) is an important concept in heterogeneous catalysis. Herein, we demonstrate that the Au-TiO2 SMSI of Au/TiO2 catalysts sensitively depends on both Au nanoparticle (NP) sizes and TiO2 facets. Au NPs of ca. 5 nm are more facile undergo Au-TiO2 SMSI than those of ca. 2 nm, while TiO2 {001} and {100} facets are more facile than TiO2 {101} facets. The resulting capsulating TiO2-x overlayers on Au NPs exhibit an average oxidation state between +3 and +4 and a Au-to-TiO2-x charge transfer, which, combined with calculations, determines the Ti:O ratio as ca. 6:11. Both TiO2-x overlayers and TiO2-x -Au interface exhibit easier lattice oxygen activation and higher intrinsic activity in catalyzing low-temperature CO oxidation than the starting Au-TiO2 interface. These results advance fundamental understanding of SMSI and demonstrate engineering of metal NP size and oxide facet as an effective strategy to tune the SMSI for efficient catalysis.

Global response of terrestrial gross primary productivity to climate extremes.

Extreme climate events undoubtedly have essential impacts on ecosystem gross primary productivity (GPP), but the global spatio-temporal patterns of GPP responses to climate extremes are unclear. In this study, we analyzed the responses of GPP to temperature and precipitation extremes during historical (1901-2016) and future (2006-2100) periods using climate extreme indices (CEIs) developed by the Expert Team on Climate Change Detection and Indices. Eight temperature-related CEIs and eight precipitation-related CEIs were used for this analysis, along with three future greenhouse gas concentration trajectory scenarios generated by the IPCC: RCP 2.6, RCP 4.5, and RCP 8.5. Our results show that under RCP 4.5 and RCP 8.5, most climate extremes are increasing from the historical period into the future, indicating a warming globe with more frequent and more intense extreme climate events. But the increasing rate is only persistently enhanced with time under scenario RCP 8.5. GPP shows a continuous negative relationship with cold CEIs and positive relationship with wet CEIs from the historical period into the future. In all zonal scales, the changed magnitude of GPP responds strongly to extreme value-related temperature extremes under different scenarios. However, the precipitation-related extremes with the strongest GPP response are various in different regions. In the future, GPP is most sensitive to temperature extremes in upper northern latitudes and in high-altitude regions (e.g., Qinghai-Tibet Plateau) and to precipitation extremes in the tropical zone. This study may provide a basis for predicting how GPP responds to climate extremes and explaining the underlying changes in the carbon cycle.

Critical land change information enhances the understanding of carbon balance in the United States.

Large-scale terrestrial carbon (C) estimating studies using methods such as atmospheric inversion, biogeochemical modeling, and field inventories have produced different results. The goal of this study was to integrate fine-scale processes including land use and land cover change into a large-scale ecosystem framework. We analyzed the terrestrial C budget of the conterminous United States from 1971 to 2015 at 1-km resolution using an enhanced dynamic global vegetation model and comprehensive land cover change data. Effects of atmospheric CO2 fertilization, nitrogen deposition, climate, wildland fire, harvest, and land use/land cover change (LUCC) were considered. We estimate annual C losses from cropland harvest, forest clearcut and thinning, fire, and LUCC were 436.8, 117.9, 10.5, and 10.4 TgC/year, respectively. C stored in ecosystems increased from 119,494 to 127,157 TgC between 1971 and 2015, indicating a mean annual net C sink of 170.3 TgC/year. Although ecosystem net primary production increased by approximately 12.3 TgC/year, most of it was offset by increased C loss from harvest and natural disturbance and increased ecosystem respiration related to forest aging. As a result, the strength of the overall ecosystem C sink did not increase over time. Our modeled results indicate the conterminous US C sink was about 30% smaller than previous modeling studies, but converged more closely with inventory data.

Surpassing the single-atom catalytic activity limit through paired Pt-O-Pt ensemble built from isolated Pt1 atoms.

Despite the maximized metal dispersion offered by single-atom catalysts, further improvement of intrinsic activity can be hindered by the lack of neighboring metal atoms in these systems. Here we report the use of isolated Pt1 atoms on ceria as "seeds" to develop a Pt-O-Pt ensemble, which is well-represented by a Pt8O14 model cluster that retains 100% metal dispersion. The Pt atom in the ensemble is 100-1000 times more active than their single-atom Pt1/CeO2 parent in catalyzing the low-temperature CO oxidation under oxygen-rich conditions. Rather than the Pt-O-Ce interfacial catalysis, the stable catalytic unit is the Pt-O-Pt site itself without participation of oxygen from the 10-30 nm-size ceria support. Similar Pt-O-Pt sites can be built on various ceria and even alumina, distinguishable by facile activation of oxygen through the paired Pt-O-Pt atoms. Extending this design to other reaction systems is a likely outcome of the findings reported here.

Effects of 21st-century climate, land use, and disturbances on ecosystem carbon balance in California.

Terrestrial ecosystems are an important sink for atmospheric carbon dioxide (CO2 ), sequestering ~30% of annual anthropogenic emissions and slowing the rise of atmospheric CO2 . However, the future direction and magnitude of the land sink is highly uncertain. We examined how historical and projected changes in climate, land use, and ecosystem disturbances affect the carbon balance of terrestrial ecosystems in California over the period 2001-2100. We modeled 32 unique scenarios, spanning 4 land use and 2 radiative forcing scenarios as simulated by four global climate models. Between 2001 and 2015, carbon storage in California's terrestrial ecosystems declined by -188.4 Tg C, with a mean annual flux ranging from a source of -89.8 Tg C/year to a sink of 60.1 Tg C/year. The large variability in the magnitude of the state's carbon source/sink was primarily attributable to interannual variability in weather and climate, which affected the rate of carbon uptake in vegetation and the rate of ecosystem respiration. Under nearly all future scenarios, carbon storage in terrestrial ecosystems was projected to decline, with an average loss of -9.4% (-432.3 Tg C) by the year 2100 from current stocks. However, uncertainty in the magnitude of carbon loss was high, with individual scenario projections ranging from -916.2 to 121.2 Tg C and was largely driven by differences in future climate conditions projected by climate models. Moving from a high to a low radiative forcing scenario reduced net ecosystem carbon loss by 21% and when combined with reductions in land-use change (i.e., moving from a high to a low land-use scenario), net carbon losses were reduced by 55% on average. However, reconciling large uncertainties associated with the effect of increasing atmospheric CO2 is needed to better constrain models used to establish baseline conditions from which ecosystem-based climate mitigation strategies can be evaluated.

Understanding the Impact of Defects on Catalytic CO Oxidation of LaFeO3-Supported Rh, Pd, and Pt Single-Atom Catalysts.

Understanding the intrinsic catalytic properties of perovskite materials can accelerate the development of highly active and abundant complex oxide catalysts. Here, we performed a first-principles density functional theory study combined with a microkinetics analysis to comprehensively investigate the influence of defects on catalytic CO oxidation of LaFeO3 catalysts containing single atoms of Rh, Pd, and Pt. La defects and subsurface O vacancies considerably affect the local electronic structure of these single atoms adsorbed at the surface or replacing Fe in the surface of the perovskite. As a consequence, not only the stability of the introduced single atoms is enhanced but also the CO and O2 adsorption energies are modified. This also affects the barriers for CO oxidation. Uniquely, we find that the presence of La defects results in a much higher CO oxidation rate for the doped perovskite surface. A linear correlation between the activation barrier for CO oxidation and the surface O vacancy formation energy for these models is identified. Additionally, the presence of subsurface O vacancies only slightly promotes CO oxidation on the LaFeO3 surface with an adsorbed Rh atom. Our findings suggest that the introduction of La defects in LaFeO3-based environmental catalysts could be a promising strategy toward improved oxidation performance. The insights revealed herein guide the design of the perovskite-based three-way catalyst through compositional variation.

Highly Active and Stable CH4 Oxidation by Substitution of Ce4+ by Two Pd2+ Ions in CeO2(111).

Methane (CH4) combustion is an increasingly important reaction for environmental protection, for which Pd/CeO2 has emerged as the preferred catalyst. There is a lack of understanding of the nature of the active site in these catalysts. Here, we use density functional theory to understand the role of doping of Pd in the ceria surface for generating sites highly active toward the C-H bonds in CH4. Specifically, we demonstrate that two Pd2+ ions can substitute one Ce4+ ion, resulting in a very stable structure containing a highly coordinated unsaturated Pd cation that can strongly adsorb CH4 and dissociate the first C-H bond with a low energy barrier. An important aspect of the high activity of the stabilized isolated Pd cation is its ability to form a strong σ-complex with CH4, which leads to effective activation of CH4. We show that also other transition metals like Pt, Rh, and Ni can give rise to similar structures with high activity toward C-H bond dissociation. These insights provide us with a novel structural view of solid solutions of transition metals such as Pt, Pd, Ni, and Rh in CeO2, known to exhibit high activity in CH4 combustion.

Optimum Particle Size for Gold-Catalyzed CO Oxidation.

The structure sensitivity of gold-catalyzed CO oxidation is presented by analyzing in detail the dependence of CO oxidation rate on particle size. Clusters with less than 14 gold atoms adopt a planar structure, whereas larger ones adopt a three-dimensional structure. The CO and O2 adsorption properties depend strongly on particle structure and size. All of the reaction barriers relevant to CO oxidation display linear scaling relationships with CO and O2 binding strengths as main reactivity descriptors. Planar and three-dimensional gold clusters exhibit different linear scaling relationship due to different surface topologies and different coordination numbers of the surface atoms. On the basis of these linear scaling relationships, first-principles microkinetics simulations were conducted to determine CO oxidation rates and possible rate-determining step of Au particles. Planar Au9 and three-dimensional Au79 clusters present the highest CO oxidation rates for planar and three-dimensional clusters, respectively. The planar Au9 cluster is much more active than the optimum Au79 cluster. A common feature of optimum CO oxidation performance is the intermediate binding strengths of CO and O2, resulting in intermediate coverages of CO, O2, and O. Both these optimum particles present lower performance than maximum Sabatier performance, indicating that there is sufficient room for improvement of gold catalysts for CO oxidation.

A Linear Scaling Relation for CO Oxidation on CeO2-Supported Pd.

Resolving the structure and composition of supported nanoparticles under reaction conditions remains a challenge in heterogeneous catalysis. Advanced configurational sampling methods at the density functional theory level are used to identify stable structures of a Pd8 cluster on ceria (CeO2) in the absence and presence of O2. A Monte Carlo method in the Gibbs ensemble predicts Pd-oxide particles to be stable on CeO2 during CO oxidation. Computed potential energy diagrams for CO oxidation reaction cycles are used as input for microkinetics simulations. Pd-oxide exhibits a much higher CO oxidation activity than metallic Pd on CeO2. This work presents for the first time a scaling relation for a CeO2-supported metal nanoparticle catalyst in CO oxidation: a higher oxidation degree of the Pd cluster weakens CO binding and facilitates the rate-determining CO oxidation step with a ceria O atom. Our approach provides a new strategy to model supported nanoparticle catalysts.

Stable Pd-Doped Ceria Structures for CH4 Activation and CO Oxidation.

Doping CeO2 with Pd atoms has been associated with catalytic CO oxidation, but current surface models do not allow CO adsorption. Here, we report a new structure of Pd-doped CeO2(111), in which Pd adopts a square planar configuration instead of the previously assumed octahedral configuration. Oxygen removal from this doped structure is favorable. The resulting defective Pd-doped CeO2 surface is active for CO oxidation and is also able to cleave the first C-H bond in methane. We show how the moderate CO adsorption energy and dynamic features of the Pd atom upon CO adsorption and CO oxidation contribute to a low-barrier catalytic cycle for CO oxidation. These structures, which are also observed for Ni and Pt, can lead to a more open coordination environment around the doped-transition-metal center. These thermally stable structures are relevant to the development of single-atom catalysts.

Theoretical Study of Ripening Mechanisms of Pd Clusters on Ceria.

We carried out density functional theory calculations to investigate the ripening of Pd clusters on CeO2(111). Starting from stable Pd n clusters (n = 1-21), we compared how these clusters can grow through Ostwald ripening and coalescence. As Pd atoms have mobility higher than that of Pd n clusters on the CeO2(111) surface, Ostwald ripening is predicted to be the dominant sintering mechanism. Particle coalescence is possible only for clusters with less than 5 Pd atoms. These ripening mechanisms are facilitated by adsorbed CO through lowering barriers for the cluster diffusion, detachment of a Pd atom from clusters, and transformation of initial planar clusters.