Reactivity of formic acid (HCOOH) with H atoms on cold surfaces of interstellar interest

Henda Chaabouni,Marco Minissale,Saoud Baouche,Stephan Diana

doi:10.1051/0004-6361/201936411

Abstract

Context. Formic acid (HCOOH) is the simplest organic carboxylic acid in chemical synthesis and the significant species in interstellar chemistry. HCOOH has been abundantly detected in interstellar ices, dense molecular clouds and star-forming regions. Aims. Laboratory hydrogenation experiments of HCOOH molecules with H atoms were performed with two cryogenic ultra-high vacuum devices on amorphous solid water ices, and highly oriented pyrolytic graphite surfaces. The aim of this work is to study the reactivity of HCOOH molecules with H atoms at low surface temperature 10 K, low surface coverage of one monolayer to three layers, and low H-atom flux of about 3.0 × 1012 molecule cm−2 s−1. Methods. HCOOH and H beams were deposited on cold surfaces held at 10 K, and the condensed films were analyzed by in-situ Reflection Absorption InfraRed Spectroscopy and temperature programmed desorption (TPD) mass spectrometry technique by heating the sample from 10 to 200 K. Results. Using the temperature programmed during exposure desorption technique, we highlight the possible dimerization of HCOOH molecules at low surface temperatures between 10 and 100 K. In our HCOOH+H experiments, we evaluated a consumption of 20–30% of formic acid by comparing the TPD curves at m/z 46 of pure and H-exposed HCOOH ice. Conclusions. The hydrogenation HCOOH+H reaction is efficient at low surface temperatures. The main products identified experimentally are carbon dioxide (CO2) and water (H2O) molecules. CO bearing species CH3OH, and H2CO are also detected mainly on graphite surfaces. A chemical surface reaction route for the HCOOH+H system is proposed to explain the product formation.

Highlights

Formic acid (HCOOH), the simplest organic acid, is thought to play a major role both in atmospheric and interstellar environments, as well as human body (Cao et al 2014) as intermediate in chemical synthesis of organic molecules and pre-biotic species
We experimentally study the reactivity of hydrogenation reaction of the smallest acid compound (HCOOH) molecules adsorbed on graphite and amorphous water ice surfaces with a typical H atom-flux of 3.0 × 1012 molecule cm−2 s−1, at low surface temperatures of 10 K and low surface coverage of HCOOH ice varying from one to three layers
Our present experiments performed in the sub-monolayer regime of 1–3 ML showed that the reactivity of HCOOH with H atoms at low surface temperature (10 K) depends on the substrate used in this work

Summary

Introduction

Formic acid (HCOOH), the simplest organic acid, is thought to play a major role both in atmospheric and interstellar environments, as well as human body (Cao et al 2014) as intermediate in chemical synthesis of organic molecules and pre-biotic species. HCOOH is the smallest organic acid which has been detected in cold dark interstellar clouds L134 N with a relative abundance of 10−10 with respect to H2 (Irvine et al 1990). It has been observed in the gas phase towards high-and low-mass star-forming regions (Woods et al 1983; Bisschop et al 2007a; Lefloch et al 2017), and towards L1544 (Vastel et al 2014) with a fractional gas-phase abundance [HCOOH]/[H2] = 10−11–10−9. A recent astrochemical model of Skouteris et al (2018) shows a new gas-phase scheme of reactions for the formation of formic acid involving ethanol (CH3CH2OH) as a parent molecule

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