Bifunctional Oxygen Electrocatalysis on Mixed Metal Phthalocyanine-Modified Carbon Nanotubes Prepared via Pyrolysis.

Non-precious-metal catalysts are promising alternatives for Pt-based cathode materials in low-temperature fuel cells, which is of great environmental importance. Here, we have investigated the bifunctional electrocatalytic activity toward the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) of mixed metal (FeNi; FeMn; FeCo) phthalocyanine-modified multiwalled carbon nanotubes (MWCNTs) prepared by a simple pyrolysis method. Among the bimetallic catalysts containing nitrogen derived from corresponding metal phthalocyanines, we report the excellent ORR activity of FeCoN-MWCNT and FeMnN-MWCNT catalysts with the ORR onset potential of 0.93 V and FeNiN-MWCNT catalyst for the OER having EOER = 1.58 V at 10 mA cm-2. The surface morphology, structure, and elemental composition of the prepared catalysts were examined with scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The FeCoN-MWCNT and FeMnN-MWCNT catalysts were prepared as cathodes and tested in anion-exchange membrane fuel cells (AEMFCs). Both catalysts displayed remarkable AEMFC performance with a peak power density as high as 692 mW cm-2 for FeCoN-MWCNT.

Benchmarking Catalysts for Formic Acid/Formate Electrooxidation.

Energy production and consumption without the use of fossil fuels are amongst the biggest challenges currently facing humankind and the scientific community. Huge efforts have been invested in creating technologies that enable closed carbon or carbon neutral fuel cycles, limiting CO2 emissions into the atmosphere. Formic acid/formate (FA) has attracted intense interest as a liquid fuel over the last half century, giving rise to a plethora of studies on catalysts for its efficient electrocatalytic oxidation for usage in fuel cells. However, new catalysts and catalytic systems are often difficult to compare because of the variability in conditions and catalyst parameters examined. In this review, we discuss the extensive literature on FA electrooxidation using platinum, palladium and non-platinum group metal-based catalysts, the conditions typically employed in formate electrooxidation and the main electrochemical parameters for the comparison of anodic electrocatalysts to be applied in a FA fuel cell. We focused on the electrocatalytic performance in terms of onset potential and peak current density obtained during cyclic voltammetry measurements and on catalyst stability. Moreover, we handpicked a list of the most relevant examples that can be used for benchmarking and referencing future developments in the field.

Copper Metal-Organic Framework Surface Catalysis: Catalyst Poisoning, IR Spectroscopic, and Kinetic Evidence Addressing the Nature and Number of the Catalytically Active Sites En Route to Improved Applications.

The metal-organic framework (MOF) H3[(Cu4Cl)3-(BTTri)8, H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene] (CuBTTri) is a precatalyst for biomedically relevant nitric oxide (NO) release from S-nitrosoglutathione (GSNO). The questions of the number and nature of the catalytically most active, kinetically dominant sites are addressed. Also addressed is whether or not the well-defined structural geometry of MOFs (as solid-state analogues of molecular compounds) can be used to generate specific, testable hypotheses about, for example, if intrapore vs exterior surface metal sites are more catalytically active. Studies of the initial catalytic rate vs CuBTTri particle external surface area to interior volume ratio show that intrapore copper sites are inactive within the experimental error (≤1.7 × 10-5% of the observed catalytic activity)-restated, the traditional MOF intrapore metal site catalysis hypothesis is disproven for the current system. All observed catalysis occurs at exterior surface Cu sites, within the experimental error. Fourier transform infrared (FT-IR) analysis of CN--poisoned CuBTTri reveals just two detectable Cu sites at a ca. ≥0.5% detection limit, those that bind three or one CN- ("Cu(CN)3" and "CuCN"), corresponding to the CN- binding expected for exterior surface, 3-coordinate (Cusurface) and intrapore, 5-coordinate (Cupore) sites predicted by the idealized, metal-terminated crystal structure. Two-coordinate Cu defect sites are ruled out at the ≥0.5% FT-IR detection limit as such defect sites would have been detectable by the FT-IR studies of the CN--poisoned catalyst. Size-selective poisoning studies of CuBTTri exterior surface sites reveal that 1.3 (±0.4)% of total copper in 0.6 ± 0.4 µm particles is active. That counting of active sites yields a normalized turnover frequency (TOF), TOFnorm = (4.9 ± 1.2) × 10-2 mol NO (mol Cusurface)-1 s-1 (in water, at 20 min, 25 °C, 1 mM GSNO, 30% loss of GSNO, and 1.3 ± 0.4 mol % Cusurface)-a value ∼100× higher than the TOF calculated without active site counting. Overall, Ockham's razor interpretation of the data is that exterior surface, Cusurface sites are the catalytically most active sites present at a 1.3 (±0.4)% level of total Cu.

Electrochemically Driven Water-Oxidation Catalysis Beginning with Six Exemplary Cobalt Polyoxometalates: Is It Molecular, Homogeneous Catalysis or Electrode-Bound, Heterogeneous CoO x Catalysis?

A series of six exemplary cobalt-polyoxometalate (Co-POM) precatalysts have been examined to determine if they are molecular water-oxidation catalysts (WOCatalysts) or if, instead, they actually form heterogeneous, electrode-bound CoO x as the true WOCatalyst under electrochemically driven water-oxidation catalysis (WOCatalysis) conditions. Specifically, WOCatalysis derived from the following six Co-POMs has been examined at pH 5.8, 8.0, and 9.0: [Co4(H2O)2(PW9O34)2]10- (Co4P2W18), [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]16- (Co9P5W27), [ ßß-Co4(H2O)2(P2W15O56)2]16- (Co4P4W30), [Co(H2O)PW11O39]5- (CoPW11), [α1-Co(H2O)P2W17O61]8- (α1-CoP2W17), and [α2-Co(H2O)P2W17O61]8- (α2-CoP2W17). The amount of Co(II)aq in 500 µM solutions of each Co-POM was measured after 3 h of aging as well as from t = 0 for pH = 5.8 and 8.0 by µM sensitive Co(II)aq-induced 31P NMR line broadening and at pH = 9.0 by cathodic stripping. The amount of detectable Co(II)aq after 3 h for the six Co-POMs ranges from ∼0.25 to ∼90% of the total cobalt initially present in the Co-POM. For 12 out of 18 total Co-POM and different pH cases, the amount Co(II)aq detected after 3 h forms heterogeneous CoO x able to account for ≥100% of the observed WOCatalysis activity. However, under 0.1 M NaPi, pH 5.8 conditions for CoPW11 and α1-CoP2W17 where ∼1.5% and 0.25% Co(II)aq is detectable, the measured Co(II)aq cannot account for the observed WOCatalysis. The implication is that these two Co-POMs are primarily molecular, Co-POM-based, WOCatalysts under electrochemically driven, pH 5.8, phosphate-buffer conditions. Even for the single most stable Co-POM, α1-CoP2W17, CoO x is still an estimated ∼76× faster WOCatalyst at pH = 5.8 and an estimated ∼740× faster WOCatalyst at pH = 8.

Alcohol Solvent Effects in the Synthesis of Co3O4 Metal-Oxide Nanoparticles: Disproof of a Surface-Ligand Thermodynamic Effect en Route to Alternative Kinetic and Thermodynamic Explanations.

The synthesis of Co3O4 core nanoparticles from cobalt acetate is explored in alcohol solvents plus limited water using O2 as oxidant and NH4OH as the base, all in comparison to controls in water alone employing the otherwise identical synthetic procedure. Syntheses in EtOH or t-BuOH cosolvents with limited water yield phase-pure and size-controlled (3 ± 1 nm) Co3O4-core nanoparticles. In marked contrast, the synthesis in water alone yields mixed phases of Co3O4 and ß-Co(OH)2 with a very large particle-size range (14-400 nm). Importantly, acidic reductive digestion of the Co3O4 particles followed by 1H NMR on the resultant solution yields no detectable EtOH in nanoparticles prepared in EtOH, nor any detectable t-BuOH in nanoparticles prepared in t-BuOH (∼5% detection limits for each alcohol), despite the dramatic effect of each alcohol cosolvent on the resultant cobalt-oxide product. Instead, in both cases HOAc is detected and quantified, indicative of OAc- as a surface ligand-and not EtO- or t-BuO- as the surface ligand. The resultant ROH cosolvent-derived particles were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, plus elemental analysis to arrive at an approximate, average molecular formula in the case of the particles prepared in EtOH, {[Co3O4(C2H3O2)]-[(NH4+)0.3(H+0.7)]+·(H2O)}∼216. The key finding is that, because EtOH and t-BuOH have a substantial effect on the phase- and size-dispersion of the cobalt-oxide nanoparticle product, yet the intact alcohol does not show up in the final Co3O4 nanoparticle product, the effect of these alcohols cannot be a surface-ligand thermodynamic effect on the net nanoparticle formation reaction. A careful search of the literature provided scattered, but consistent, literature in which anions or other additives have large effects on metal-oxide nanoparticle formation reactions, yet also do not show up in the nanoparticle products-that is, where the observed effects are again not due to binding by that anion or other additive in a surface-ligand thermodynamic effect on the overall reaction. Alternative hypotheses are provided as to the origin of ROH solvent effects on metal-oxide nanoparticles.

Cobalt Polyoxometalate Co4V2W18O68(10-): A Critical Investigation of Its Synthesis, Purity, and Observed (51)V Quadrupolar NMR.

The vanadium-containing cobalt polyoxometalate (Co-POM) Co4V2W18O68(10-) (hereafter Co4V2W18) has been reported to be a stable, homogeneous water-oxidation catalyst, one with a claimed record turnover frequency that is also reportedly 200-fold faster than its phosphorus congener, Co4P2W18O68(10-). The claimed superior water-oxidation catalysis activity of the vanadium congener, Co4V2W18, rests squarely on the reported synthesis of Co4V2W18, its purity, and its stability in both the solid-state and in solution. Attempts to repeat the preparation of Co4V2W18 by either of two literature syntheses, along with the other studies reported herein, led to the discovery of multiple, convoluted problems in the prior literature of Co4V2W18. The three most serious of those problems proved to be the prior misunderstanding of the quadrupolar (herein (51)V) NMR peak widths in complexes that also contain paramagnetic metals such as Co(II), the incorrect assignment of a -506.8 ppm (51)V NMR to Co4V2W18, and then the use of that -506.8 peak to argue for the stability of Co4V2W18 in solution. The results are reported in a somewhat historical, "story" fashion en route to elucidating and fully supporting the 11 insights and take-home messages listed in the Summary and Conclusions section.