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I have no idea how to write this equilibrated reversible step of a surface reaction: equilibrated revversible step. Is there a circled leftright harpoon in some package? or, how can I put that tiny circle in the middle of the leftright harpoon?

  • If I understand it right this is an arrow for a quasi-equilibrated reaction? And it seems that a circle is used more often than an ellipse? – cgnieder May 9 '16 at 9:40
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You can use the chemformula package for your chemical reactions. (I'd recommend using a dedicated package for those, anyway.) Since v4.15 it has an arrow type for this:

\documentclass{article}
\usepackage{chemformula}[2016/05/11]

\begin{document}

\ch{ CO + "*" <=o> CO "*" }

\end{document}

enter image description here

With versions earlier than v4.15 you can still define a new arrow type using TikZ:

\documentclass{article}

\usepackage{chemformula}

\NewChemArrow{<=o>}{
  \draw[chemarrow,-left cf]
    ([yshift=.15ex]cf_arrow_start) -- ([yshift=.15ex]cf_arrow_end) ;
  \draw[chemarrow,left cf-]
    ([yshift=-.15ex]cf_arrow_start) -- ([yshift=-.15ex]cf_arrow_end) ;
  \draw[chemarrow] (cf_arrow_mid) circle (.3ex) ;
}

\begin{document}

\ch{ CO + "*" <=o> CO "*" }

\end{document}

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