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Pyridinium and its substituted derivatives are effective and stable homogeneous electrocatalysts for the aqueous multiple-electron, multiple-proton reduction of carbon dioxide to products such as formic acid, formaldehyde, and methanol. Importantly, high faradaic yields for methanol have been observed in both electrochemical and photoelectrochemical systems at low reaction overpotentials. Herein, we report the detailed mechanism of pyridinium-catalyzed CO(2) reduction to methanol. At metal electrodes, formic acid and formaldehyde were observed to be intermediate products along the pathway to the 6e(-)-reduced product of methanol, with the pyridinium radical playing a role in the reduction of both intermediate products. It has previously been thought that metal-derived multielectron transfer was necessary to achieve highly reduced products such as methanol. Surprisingly, this simple organic molecule is found to be capable of reducing many different chemical species en route to methanol through six sequential electron transfers instead of metal-based multielectron transfer. We show evidence for the mechanism of the reduction proceeding through various coordinative interactions between the pyridinium radical and carbon dioxide, formaldehyde, and related species. This suggests an inner-sphere-type electron transfer from the pyridinium radical to the substrate for various mechanistic steps where the pyridinium radical covalently binds to intermediates and radical species. These mechanistic insights should aid the development of more efficient and selective catalysts for the reduction of carbon dioxide to the desired products.

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Cu is currently used as a replacement for aluminum in IC interconnections. One of the challenges of integrated circuit ͑IC͒ interconnection technology is the planarization process that involves fine copper lines patterning; the most perspective technique for Cu lines patterning is chemical mechanical planarization ͑CMP͒. CMP is applicable only once a given metal-slurry system can provide certain requirements. Different media, such as ammonium hydroxide (NH 4 OH), peroxide (H 2 O 2 ), ferric nitrite ͓Fe(NO 3 ) 3 ͔, and nitric acid (HNO 3 ), with and without the presence of inhibitors, were suggested as potential candidates for copper CMP slurries. In this work we studied the compatibility of these media with CMP requirements. Our work indicated that these slurry solutions ͑basic and acidic media͒ do not provide sufficient conditions for a conventional CMP application. The Cu is actively dissolved in all of these solutions and in some of them with high dissolution rate, without a rapid repassivation. The sharp acceleration in Cu etching rate under abrasive abrading can be attributed only to a mechanical Cu removal. It was found that the active dissolution of Cu is conducted through deep intergranular penetrations which may result in the deterioration of thin Cu connectors. This study also shows that the use of benzotriazole or other inhibitors commonly used for reducing the etching rate of the actively dissolved Cu cannot provide the needed conditions for a rapid passive film growth on the copper metal. The electrochemical behavior of Cu in these solutions, morphology of dissolved copper metal surface, and mechanism of copper CMP are discussed.

Electrochemical studies of copper in peroxide solution for chemical mechanical planarization applications was conducted in a solution media containing Na 2 SO 4 as an inactive and highly conductive electrolyte salt. The low current values observed during anodic potentiodynamic polarization of copper in peroxide based solutions can be mistakenly interpreted as a development of passivity at the copper surface. Cathodic pretreatment of the copper surface followed by anodic potentiodynamic polarization reveals the formation of an anodic peak at potentials of 0.2-0.3 V SCE . The formation of a dense deposition film of copper oxides at potentials below 0.4 V is attributed to increase in the solution pH at the electrode-electrolyte interface to values above pH 5.5. However, further positive shift in the applied potential decreases the cathodic reduction rate of H 2 O 2 , leading to a decrease in the solution pH values at the electrode/electrolyte interface and consequently, decreases the deposit formation rates covering the copper surface. The addition of benzotriazole ͑BTA͒ to peroxide solutions was also studied, and it was revealed that BTA cannot provide the protection needed to the copper surface in this system.