Volcano-shaped Relationship Research Articles

Optimizing the Intermediates Adsorption by Manipulating the Second Coordination Shell of Ir Single Atoms for Efficient Water Oxidation.

The precise regulation of single-atom catalysts (SACs) with the desired local chemical environment is vital to elucidate the relationship between the SACs structure and the catalytic performance. The debate on the effect of the local coordination environment is quite complicated even for the SACs with the same composition and chemical nature, calling for increased attention on the regulation of second coordination shell. For oxide-supported SACs, it remains a significant challenge to precisely manipulate the second coordination shell of single atoms supported on oxides due to the structural robustness of oxides. Here, Ir single atoms are anchored on NiO supports via different bonding strategies, resulting in the diverse Ir-O-Ni coordination numbers for Ir sites. Specifically, Ir1/NiO, Ir1-NiO, and Ir1@NiO SACs with increasing Ir-O-Ni coordination numbers of 3, 4, and 5 were synthesized, respectively. We found that the activity of the three samples towards oxygen evolution reaction (OER) exhibited a volcano-shaped relationship with the Ir-O-Ni coordination number, with Ir1-NiO showing the lowest overpotential of 225 mV at 10 mA cm-2. Mechanism investigations indicate that the moderate coordination number of Ir-O-Ni in Ir1-NiO creates the higher occupied Ir dz2 orbital, weakening the adsorption strength for *OOH intermediates and thereby enhancing the OER activity.

Influence of the La/Ce ratio on La-Ce oxides promoted by Sr in the methane oxidative coupling reaction

The high methane availability and its significant role as a greenhouse gas have led the scientific community to pursue methods for its use. This paper proposes the usage of the oxidative coupling of methane (OCM) as a promising path to transform methane directly into value-added hydrocarbons, mostly ethylene, which serves as a crucial compound in the petrochemical sector. An efficient OCM catalyst should contain substantial basicity and oxygen vacancies for providing surface electrophilic oxygen species, such as superoxides and peroxides, instrumental for boosting selectivity towards C2 products. Mixed lanthanum-cerium oxide (La-Ce) catalysts have emerged as strong candidates in OCM due to their high thermal stability, oxygen mobility, and high alkalinity. In this study, they were prepared through a surfactant-assisted hydrothermal method with different La/Ce ratios for fine-tuning their basic properties and promoting oxygen mobility on the surface of the catalysts. Samples followed a volcano-shaped relationship between C2 yield and La content, with optimal performance at a La/Ce molar ratio of 2.1, attributed to the interplay between the high amount of basic sites and oxygen vacancies, increasing the presence of superoxide species over lattice oxygen. Moreover, the Sr-promoted catalyst showed high density of strong basic sites while preserving the reactive oxygen species, achieving 20 % CH4 conversion, with 57 % C2 compounds selectivity at 750 °C and GHSV of 18.000 mL.gcat−1.h−1.

Modulating Electron Transfer between Pt and MOF Support through Pd Doping Promotes Nanozyme Catalytic Efficiency.

Electron transfer is considered to be a typical parameter that affects the catalytic activity of nanozymes. However, there is still controversy regarding whether higher or lower electron transfer numbers are beneficial for improving the catalytic activity of nanozymes. To address this issue, we propose the introduction of Pd doping as an important electron regulation strategy to tune electron transfer between Pt and ZIF-8 carriers (PtxPd1@ZIF-8). We observe a volcano-shaped relationship between the electron transfer number and catalytic activity, reaching its peak at Pt4Pd1@ZIF-8. Mechanism studies indicate that as the electron transfer number from Pt to ZIF-8 carriers increases, the d-band center of the active site Pt increases, reducing the occupancy of antibonding states and enhancing the adsorption capacity of the key intermediate (*O). However, a further increase in the adsorption of *O energy makes it difficult to desorb and participate in the next reaction, thus exhibiting volcanic activity. The optimized Pt4Pd1@ZIF-8 nanozyme is applied to develop an immunoassay for the detection of zearalenone, achieving a detection limit of 0.01 μg/L, which is 6 times higher than that of the traditional enzyme-linked immunosorbent assay. This work not only reveals the potential regulatory mechanism of electron transfer on the catalytic activity of nanozymes but also improves the performance of nanozyme-based biosensors.

Unraveling volcano trend in OER of metal–organic frameworks with asymmetric configuration through energy band engineering

Local symmetry breaking of catalysts has emerged as an effective strategy for finely tuning oxygen evolution reaction (OER) activity, yet the fundamental comprehension regarding asymmetric structure-activity relationships remains limited. Here, we propose the energy band engineering to bridge the correlation between established asymmetric electronic structure and adsorption/desorption ability of reaction intermediates within metal-organic frameworks (MOFs). The deliberate synthesis of CoM-MOFs (M=Cu, Ni, and Fe) with distinct coordination microenvironments enables the customization of asymmetric Co-O-M electronic structure. A volcano-shaped relationship can be revealed between calculated OER overpotential and average d‐band center (Ed) energy level for both active Co sites and substituted M. The CoFe-MOF, located close to the summit, showcases the balanced reaction intermediate behavior and thus for enhanced OER activity. This work presents a promising approach to thoroughly understand asymmetric electronic structure-activity relationships from the perspective of energy band engineering and further guide the discovery of high-efficiency MOF-based OER catalysts.

First-Principles Study of Bimetallic Pairs Embedded on Graphene Co-Doped with N and O for N2 Electroreduction.

The electrocatalytic nitrogen reduction reaction (NRR) is considered a viable alternative to the Haber-Bosch process for ammonia synthesis, and the design of highly active and selective catalysts is crucial for the industrialization of the NRR. Dual-atom catalysts (DACs) with dual active sites offer flexible active sites and synergistic effects between atoms, providing more possibilities for the tuning of catalytic performance. In this study, we designed 48 graphene-based DACs with N4O2 coordination (MM'@N4O2-G) using density functional theory. Through a series of screening strategies, we explored the reaction mechanisms of the NRR for eight catalysts in depth and revealed the "acceptance-donation" mechanism between the active sites and the N2 molecules through electronic structure analysis. The study found that the limiting potential of the catalysts exhibited a volcano-shaped relationship with the d-band center of the active sites, indicating that the synergistic effect between the bimetallic components can regulate the d-band center position of the active metal M, thereby controlling the reaction activity. Furthermore, we investigated the selectivity of the eight DACs and identified five potential NRR catalysts. Among them, MoCo@N4O2-G showed the best NRR performance, with a limiting potential of -0.20 V. This study provides theoretical insights for the design and development of efficient NRR electrocatalysts.

Size-dependent catalytic activity for CO oxidation over sub-nano-Au clusters.

Gold (Au) nanocatalysts present outstanding activity for many reactions and have long attracted much attention, but the size effect of sub-nano-clusters on catalytic activity lacks systematic research. Using CO oxidation as a probe reaction, the size-dependent catalytic capability of sub-nano-Au clusters was explored. The global-minimum (GM) structures of AuN (N = 2-300, <2.5 nm) were obtained utilizing revised particle swarm optimization (RPSO) combined with density functional theory (DFT) calculations and the Gupta empirical potential. Geometric structural descriptors built a bridge among geometric features, adsorption energy, and the CO oxidation rate of each site of any given sub-nano-Au clusters, making it possible for high-throughput evaluation of the adsorption energy and catalytic activity of the whole sub-nano-Au cluster. The activity per unit mass of sub-nano-Au clusters shows a volcano-shaped relationship with the cluster size, where the sub-nano-Au clusters with a 0.75 nm diameter possess the highest CO2 formation rate per unit mass. The Edge and Kink sites have a higher turnover frequency (approximately 106) than the Face sites (approximately 102), which contribute the most to CO2 formation. The weak adsorption of CO and O2 was found to be a crucial factor determining the inferior activity of the Face site to the Kink and Edge sites. The adsorption process rather than the surface reaction step becomes the rate-determining step on the Face site, attributed to the decreased activity per unit mass of sub-nano-Au clusters. This work provides an in-depth mechanistic understanding of size-dependent catalytic activity for Au clusters at the sub-nano level.

Synergy of Co0-Co2+ in cobalt-based catalysts for CO2 hydrogenation: Quantifying via reduced and exposed atoms fraction

Cobalt (Co)-based catalysts exhibit exceptional versatility in CO2 hydrogenation, yielding valuable fuels and chemicals like methane, methanol, and ethanol. Generally, the Co0–Co2+ synergy plays a vital role in promoting this process, yet a quantifying descriptor remains absent. This study presents a series of CoAl-CO32− layered double hydroxide (LDH) derived catalysts synthesized through coprecipitation. By tuning precipitation pH and reduction temperature, various Co-based catalysts with distinct reduction degrees and dispersions were obtained. By introducing the descriptor "fraction of reduced and surface exposed Co (FCo,r,s)", the Co0–Co2+ synergy is effectively quantified for CO2 hydrogenation. The balanced Co0–Co2+ synergy in catalysts yields a moderate FCo,r,s, resulting in high turnover frequencies (TOFs) for CO2 conversion via HCOO intermediate formed at the Co0–Co2+ interfaces. The HCOO species can be hydrogenated to form methane and methanol, as well as coupled with CHx intermediate followed by hydrogenation to form ethanol. Conversely, catalysts with excessively high or low FCo,r,s display poor Co0–Co2+ synergy and consequently lower TOFs. A clear volcano-shaped relationship between FCo,r,s and TOFs for CO2 conversion, methane, methanol, and ethanol formation underscores its significance as a key determinant of Co-based catalysts' CO2 hydrogenation performance. The catalyst with optimized Co0–Co2+ synergy (precipitated at pH 8.5 and reduced at 750 oC) possesses a high CO2 conversion rate of 26.8 mmol gcat−1 h−1 at a low temperature (i.e., 180 oC).

Metal–support interface engineering of Ni catalysts for improved H2 storage performance: Grafting alkyltriethoxysilane onto commercial alumina

Tuning metal–support interface in supported Ni catalysts is a promising approach for overcoming the agglomeration of Ni particles and the low reducibility of Ni species. This study describes a facile two-step method to produce active Ni catalysts for the hydrogenation of benzyltoluene. In the first step, alkyltriethoxysilanes (CnTES, n = 1, 3, and 8) were grafted onto the surface of commercial Al2O3. The maximum Si/Al ratio of the prepared CnTES-on-Al2O3 materials was determined to be approximately 2.9 mol.%, which can vary depending on the surface properties of Al2O3. The second step was H2 reduction of the Ni precursor loaded on the CnTES-on-Al2O3 materials to remove alkyl substituents from the grafted CnTES and form Ni particles on the surface of silica grafted onto alumina (SGA). The hydrogenation performance of the obtained Ni/SGA_CnTES catalysts was improved with increasing Si/Al ratio owing to higher Ni dispersion and more abundant metallic Ni. The activity of the Ni/SGA_2.9CnTES (Si/Al = 2.9 mol.%) catalysts exhibited a volcano-shaped relationship with the length of the alkyl substituent of grafted CnTES, reaching the maximum for C3TES. Notably, Ni/SGA_2.9C3TES showed excellent ability for the adsorption of H2 and substrate, as well as high stability, resulting from the reduced agglomeration of Ni particles and the modulated metal–support interface. Therefore, the demonstrated synthesis approach can render Ni catalysts servable to promote the commercialization of liquid organic hydrogen carrier system.

Steering CO2 electroreduction pathway toward ethanol via surface-bounded hydroxyl species-induced noncovalent interaction

Selective electroreduction of carbon dioxide (CO2RR) into ethanol at an industrially relevant current density is highly desired. However, it is challenging because the competing ethylene production pathway is generally more thermodynamically favored. Herein, we achieve a selective and productive ethanol production over a porous CuO catalyst that presents a high ethanol Faradaic efficiency (FE) of 44.1 ± 1.0% and an ethanol-to-ethylene ratio of 1.2 at a large ethanol partial current density of 501.0 ± 15.0 mA cm-2, in addition to an extraordinary FE of 90.6 ± 3.4% for multicarbon products. Intriguingly, we found a volcano-shaped relationship between ethanol selectivity and nanocavity size of porous CuO catalyst in the range of 0 to 20 nm. Mechanistic studies indicate that the increased coverage of surface-bounded hydroxyl species (*OH) associated with the nanocavity size-dependent confinement effect contributes to the remarkable ethanol selectivity, which preferentially favors the *CHCOH hydrogenation to *CHCHOH (ethanol pathway) via yielding the noncovalent interaction. Our findings provide insights in favoring the ethanol formation pathway, which paves the path toward rational design of ethanol-oriented catalysts.

Built-in electric field-assisted W-C3/X-C3 van der Waals heterogeneous single-atom catalysts for enhanced electrocatalytic nitrogen reduction

Electrocatalytic nitrogen reduction reaction (eNRR) is considered an alternative approach to the Haber-Bosch process for green ammonia synthesis. However, developing NRR electrocatalysts with high activity and selectivity remains a great challenge. Here, we designed a new type of van der Waals heterogeneous single-atom catalysts (vdW-SACs): W-C3/X-C3 (X = TM, Be, B, C, N, O, P, S, and Cl). By means of large-scale density functional theory (DFT) calculations, we built up the full profile of the stability, NRR activity, and selectivity of these vdW-SACs. Five structures exhibit lower overpotential for NRR than the previously reported monolayer W1C3. A volcano-shaped relationship between ΔG∗N and UL was obtained, and particularly, the W-C3/P-C3 locates right on the peak of the volcano with the highest activity. The built-in electric field induced by the asymmetric active sites (W and P) adjusts the binding strength between reactants and catalysts surface and achieves the lowest overpotential. The electronic structure analysis discloses that the high degree of pd hybridization between W 5d orbitals and N2π* antibonding orbitals in W-C3/P-C3 promotes the activation of intermediates. These new insights may open up opportunities for exploring advanced electrocatalysts for NRR and beyond by regulating the built-in electric field of vdW-SACs.

Atomically Incorporating Ni into Mesoporous CeO2 Matrix via Synchronous Spray-Pyrolysis as Efficient Noble-Metal-Free Catalyst for Low-Temperature CO Oxidation

Low-temperature catalytic CO oxidation is an important chemical process in versatile applications, such as the H2 utilization for low-temperature H2 air fuel cells. Pt-group metal catalysts are efficient but highly cost-consuming. This work demonstrates an excellent and sixpenny catalyst with earth-abundant Ni and Ce, in which Ni ions are atomically incorporated into the CeO2 matrix (Ni-Ce-Ox) by synchronous spray-pyrolysis (SSP) of mixture nitrates of Ni and Ce. The Ni-Ce-Ox catalyst presents a mesoporous structure. Revealed by a model reaction of 1% CO, 1% O2, and 98% balance He at a space velocity of 13,200 mL/gcat/h, Ni-Ce-Ox catalysts display a typical volcano-shaped relationship between reactivity and Ni incorporation amount. The optimized Ni incorporation appears with a high Ni/Ce atomic ratio of 0.25, endowing the T50 (temperature corresponding to a CO conversion of 50%), which is lower-shifted by 165 °C than that of pristine CeO2 (266 °C). The density functional theory (DFT) calculations further indicate that the much-reduced oxygen vacancy formation energy at Ni-Ce single-atom sites boosted the adsorption activation of the CO molecule and therefore promoted the CO oxidation process. Besides, the2 Ni-Ce-Ox from the SSP method presents better performance than the counterparts from immersion and hydrothermal methods. This work paves a way to access efficient noble-metal-free catalysts for low-temperature CO oxidation.

Metal-Organic Framework-Derived Atomically Dispersed Co-N-C Electrocatalyst for Efficient Oxygen Reduction Reaction

In this work, an atomically dispersed cobalt-nitrogen-carbon (Co-N-C) catalyst is prepared for the oxygen reduction reaction (ORR) by using a metal-organic framework (MOF) as a self-sacrifice template under high-temperature pyrolysis. Spherical aberration-corrected electron microscopy is employed to confirm the atomic dispersion of high-density Co atoms on the nitrogen-doped carbon scaffold. The X-ray photoelectron spectroscopy results verify the existence of Co-N-C active sites and their content changes with the Co content. The electrochemical results show that the electrocatalytic activity shows a volcano-shaped relationship, which increases with the Co content from 0 to 0.99 wt.% and then decreases when the presence of Co nanoparticles at 1.61 wt.%. The atomically dispersed Co-N-C catalyst with Co content of 0.99 wt.% shows an onset potential of 0.96 V vs. reversible hydrogen electrode (RHE) and a half-wave potential of 0.89 V vs. RHE toward ORR. The excellent ORR activity is attributed to the high density of the Co-N-C sites with high intrinsic activity and high specific surface area to expose more active sites.

Breaking the Volcano-Shaped Relationship for Highly Efficient Electrocatalytic Nitrogen Reduction: A Computational Guideline.

The volcano-shaped relationship is very common in electrocatalytic nitrogen reduction reaction (e-NRR) and is usually caused by the competition between the first and last hydrogenation steps. How to break such a relationship to further improve the catalytic performance remains a great challenge. Herein, using first-principles calculations, we investigate a range of transition-metal (TM)-doped Cu-based single-atom alloys (TM1-Cu(111)) as catalysts for e-NRR. When the adsorption of N2 on the catalysts is strong enough, the inert N2 molecules can be effectively activated for the first hydrogenation step. Meanwhile, the last hydrogenation step is not affected by the scaling relationship and remains easy on all of the catalysts due to the unstable top-site adsorption of NH2, resulting in the break of the volcano-shaped relationship in e-NRR. Thus, only the first hydrogenation step is identified as the potential determining step. Four TM1-Cu(111) catalysts (TM = Re, W, Tc, and Mo) are selected as promising catalysts with limiting potential ranging from -0.38 to -0.56 V, showing outstanding e-NRR activity. Besides, the four catalysts also inhibit the competing hydrogen evolution reaction and long-term stability. Our work provides a guideline for breaking the volcano-shaped relationship in e-NRR and significant in the rational design of highly efficient electrocatalysts.

Waste-Derived Copper-Lead Electrocatalysts for CO2 Reduction.

It remains a real challenge to control the selectivity of the electrocatalytic CO2 reduction (eCO2R) reaction to valuable chemicals and fuels. Most of the electrocatalysts are made of non-renewable metal resources, which hampers their large-scale implementation. Here, we report the preparation of bimetallic copper-lead (CuPb) electrocatalysts from industrial metallurgical waste. The metal ions were extracted from the metallurgical waste through simple chemical treatment with ammonium chloride, and CuxPby electrocatalysts with tunable compositions were fabricated through electrodeposition at varying cathodic potentials. X-ray spectroscopy techniques showed that the pristine electrocatalysts consist of Cu0, Cu1+ and Pb2+ domains, and no evidence for alloy formation was found. We found a volcano-shape relationship between eCO2R selectivity toward two electron products, such as CO, and the elemental ratio of Cu and Pb. A maximum Faradaic efficiency towards CO was found for Cu9.00Pb1.00, which was four times higher than that of pure Cu, under the same electrocatalytic conditions. In situ Raman spectroscopy revealed that the optimal amount of Pb effectively improved the reducibility of the pristine Cu1+ and Pb2+ domains to metallic Cu and Pb, which boosted the selectivity towards CO by synergistic effects. This work provides a framework of thinking to design and tune the selectivity of bimetallic electrocatalysts for CO2 reduction through valorization of metallurgical waste.

Computational design of ternary NiO/MPt interface active sites for H2O dissociation

The search for highly active and cost-effective catalysts of hydrogen evolution reaction is necessary to reduce energy losses in water-alkali electrolyser. Based on bifunctional effect, ternary NiO/MPt111 (M = Mn, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu) interface sites were built to study the HER activity trend by DFT calculations. Transition metal atoms tuned the electronic properties of interface sites, regulated the H2O adsorption activation and lowered the H2O dissociation energy barrier. Especially, on NiO/NiPt111 and NiO/OsPt111 interface sites the energy barriers were largely reduced to be 0.33 and 0.35 eV compared to NiO/Pt111 (0.55 eV) and Pt111 (0.92 eV). Volcano shape relationship between OH∗ adsorption energy ΔEOH∗ and energy barriers indicated ΔEOH∗ could be used as a HER activity descriptor for oxide-metal interface active sites. The present work indicated a potential interface-engineering strategy for designing interface sites with multimetallic composition and the tuned electronic structures to achieve excellent HER activity.

Size effects of Ag nanoparticle for N2 photofixation over Ag/g-C3N4: Built-in electric fields determine photocatalytic performance

One of the main characteristics of metal Ag on semiconductor photocatalysts is Ag particle size effect, which determines its dispersion and electronic structure. In this work, we have studied Ag particles with mean diameters of 9.3–33.2 nm on N 2 photofixation catalyst g-C 3 N 4 by a method of ultra-low temperature reduced-deposit. Characterizations and N 2 photofixation results show that the largest photoelectron life- time (τ = 0.29 ms) is obtained over the middle-sized Ag particles. Taking the activity of N 2 photofixation as an example, the NH 3 production rate shows a volcanic trend with the decrease of Ag particles size. The highest activity was gained over Ag4%/C 3 N 4 (−40) catalyst prepared at the temperature of −40 °C (particle size 13.2 nm). So, the N 2 photofixation rate has a volcano-shape relationship with Ag NPs (nano-particle) size indicated in this work. 4 Ag ( + ) + 3 N 2 H 4 = Ag + N 2 + 2 N 2 H 6 ( 2+ ) (The by-products in the preparation process were later separated by washing with water and methanol). Controllable Ag NPs size and built-in electric field between. • The particle size of Ag was designed and tuned. • The powder catalyst Ag 0 /g-C 3 N 4 was prepared by low temperature-reduced-deposited method. • The key factor of the relationship between interfacial contact area and photocatalytic performance was proposed. • A volcano-shape relationship was found between the N 2 photofixation rate and NPs size.

Oxygen Reduction Reaction Catalytic Activities of Pure Ni-Based Perovskite-Related Structure Oxides

Complex transition-metal oxides offer an interesting class of materials for use as oxygen reduction reaction (ORR) catalysts. A recent study proposed a volcano-shape relationship between ORR activi...

Energy density-enhancement mechanism and design principles for heteroatom-doped carbon supercapacitors

Abstract Carbon-based pseudo-supercapacitors are one of the most promising electrical energy storage devices with high power density and long cycle life which is strongly desired for electric vehicles but their capacitance needs to be improved. Here, a new approach and design principle to enhance the energy density have been developed with the density functional theory methods. The results reveal that compared with pure carbon, the energy density could be enhanced significantly via heteroatom-doping. An intrinsic descriptor is discovered to establish a volcano-shaped relationship that correlates the capacitance with the heteroatom-doping structures of carbon nanomaterials, from which the best electrode structures are identified, which are in good agreement with the experimental results. Th strategies for further enhancing energy densities are proposed for rational design and fabrication of high-energy-density supercapacitors.

Higher gold atom efficiency over Au-Pd/TS-1 alloy catalysts for the direct propylene epoxidation with H2 and O2

Bimetallic nanoparticles (NPs) have shown amazing charm for their unique structures and catalytic activity. Herein, AuPd alloy NPs with cuboctahedron structure supported on titanium silicalite-1 (denoted as Au-Pd/TS-1) were prepared by the alcohol reduction method, which exhibited higher gold atom efficiency for the direct propylene epoxidation than the traditional Au/TS-1-DP (prepared by the deposition precipitation method). A combination of experimental characterization with density functional theory (DFT) calculations indicates that the charge transfer from Pd to Au atoms promotes the formation of H2O2 from O2 and H2, and consequently improves the gold atom efficiency of Au-Pd/TS-1 alloy catalysts. The volcano-shaped relationship was established between Pd/Au molar ratio and catalytic activity. Au0.68Pd0.32/TS-1 showed the highest gold atom efficiency of ~1000 gPO h−1 gAu−1 at 200 °C. The insights reported here may pave the way for unraveling the catalyst structure-catalytic activity relationship of supported nano-gold catalysts.

Unraveling the composition-activity relationship of Pt[sbnd]Ru binary alloy for hydrogen oxidation reaction in alkaline media

The research and development of hydrogen oxidation reaction catalysts in alkaline media is a prerequisite for the commercialization of alkaline polymer electrolyte fuel cell. PtRu bimetallic catalyst was found to be more active towards HOR than Pt in alkaline, but the composition-activity relationship of PtRu alloys is still lacking. Herein, a series of PtRu alloy planar electrodes with 14 different compositions were prepared in a high-throughput fashion using an improved magnetron sputtering method. The atomic ratio of Pt was controlled from 6.7% to 99.7% within the samples. The corresponding electro-catalytic activities towards the HOR in alkaline were systematically measured by rotating disc electrode method. A volcano-shape relationship between the HOR exchange current density and the Ru atomic fraction was revealed, and maximum activity was observed at ca. 55% content of Ru. The enhancement in electro-catalytic activity is attributed to a reduction in the electronic charge density of Pt upon Ru doping.