Chapter
13
Electrochemical Studies of Carbon Dioxide and Sodium Formate in Aqueous Solutions M . H . Miles and A . N . Fletcher
Downloaded by MONASH UNIV on November 25, 2015 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch013
Chemistry Division, Research Department, Naval Weapons Center, China Lake, CA 93555 +
Protonated species such as H O must be minimized in order to obtain favorable conditions for the electro chemical reduction of carbon dioxide or sodium formate in aqueous solutions. Nearly neutral electrolytes that do not function as effective proton donors or acceptors provide an extra 0.3 V potential window for cathodic reactions between the RHE potential and the actual reduction of water molecules. Investigations of NaCOOH on 30 different electrode materials show that the electrochemical reactions of formate are controlled mainly by the adsorption of HCO - on the electrode surface. Appreciable formate adsorption occurs only for Rh, Pd, Ir, Pt, and Au. Evidence for CO reduction was observed on precious metals and their alloys such as Pt, Ir, Pd, Pt-Ru alloy and Ru-Ir alloy. The catalytic activity found for precious metals in nearly neutral electrolytes may reflect their favorable adsorption of both carbon dioxide molecules and formate ions. 3
2
2
The
conversion C0
2
o f carbon d i o x i d e and water i n t o methanol and oxygen + 2 H 0 2
CH3OH + 3/2 0 +
(1)
2
r e q u i r e s an i n p u t of energy (ΔΗ° = 719.238 k j / m o l ) . However, t h e thermodynamic p o t e n t i a l f o r t h i s r e a c t i o n (E° = -1.976 V) i s a c t u a l l y l e s s than t h a t f o r the e l e c t r o l y s i s of water (E° = -1.2288 V ) . The major source of i n t e r e s t i n t h i s r e a c t i o n i s as a means o f con v e r t i n g carbon d i o x i d e i n t o o r g a n i c compounds and p o r t a b l e f u e l s (1-3). The e l e c t r o c h e m i c a l r e d u c t i o n o f C 0 i n aqueous s o l u t i o n s u s i n g metal e l e c t r o d e s g e n e r a l l y y i e l d s f o r m i c a c i d and formate i o n s as the main p r o d u c t s ( 2 , 3 ) . The major e x p e r i m e n t a l d i f f i c u l t y i n the c a t h o d i c r e a c t i o n i s t h e f u r t h e r r e d u c t i o n of f o r m i c a c i d t o methanol ( 3 , 4 ) . P r e v i o u s s t u d i e s of the f o r m i c a c i d r e d u c t i o n s t e p i n a c i d i c s o l u t i o n s have shown t h a t H 0 r a t h e r than HCOOH i s reduced ( 4 ) . U n d i s s o c i a t e d HCOOH m o l e c u l e s a i d t h e r e d u c t i o n o f 2
+
3
This chapter is not subject to U.S. copyright. Published 1988, American Chemical Society
In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
172
CATALYTIC ACTIVATION OF CARBON DIOXIDE +
Η3
